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IETF RFC 7401
Last modified on Thursday, April 9th, 2015
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Internet Engineering Task Force (IETF) R. Moskowitz, Ed.
Request for Comments: 7401 HTT Consulting
Obsoletes: 5201 T. Heer
Category: Standards Track Hirschmann Automation and Control
ISSN: 2070-1721 P. Jokela
Ericsson Research NomadicLab
T. Henderson
University of Washington
April 2015
Host Identity Protocol Version 2 (HIPv2)
 Abstract
This document specifies the details of the Host Identity Protocol
(HIP). HIP allows consenting hosts to securely establish and
maintain shared IP-layer state, allowing separation of the identifier
and locator roles of IP addresses, thereby enabling continuity of
communications across IP address changes. HIP is based on a Diffie-
Hellman key exchange, using public key identifiers from a new Host
Identity namespace for mutual peer authentication. The protocol is
designed to be resistant to denial-of-service (DoS) and man-in-the-
middle (MitM) attacks. When used together with another suitable
security protocol, such as the Encapsulating Security Payload (ESP),
it provides integrity protection and optional encryption for upper-
layer protocols, such as TCP and UDP.
This document obsoletes RFC 5201 and addresses the concerns raised by
the IESG, particularly that of crypto agility. It also incorporates
lessons learned from the implementations of RFC 5201.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7401.
Moskowitz, et al. Standards Track PAGE 1 
RFC 7401 HIPv2 April 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................5
1.1. A New Namespace and Identifiers ............................6
1.2. The HIP Base Exchange (BEX) ................................6
1.3. Memo Structure .............................................7
2. Terms and Definitions ...........................................7
2.1. Requirements Terminology ...................................7
2.2. Notation ...................................................8
2.3. Definitions ................................................8
3. Host Identity (HI) and Its Structure ............................9
3.1. Host Identity Tag (HIT) ...................................10
3.2. Generating a HIT from an HI ...............................11
4. Protocol Overview ..............................................12
4.1. Creating a HIP Association ................................12
4.1.1. HIP Puzzle Mechanism ...............................14
4.1.2. Puzzle Exchange ....................................15
4.1.3. Authenticated Diffie-Hellman Protocol with
DH Group Negotiation ...............................17
4.1.4. HIP Replay Protection ..............................18
4.1.5. Refusing a HIP Base Exchange .......................19
4.1.6. Aborting a HIP Base Exchange .......................20
4.1.7. HIP Downgrade Protection ...........................20
4.1.8. HIP Opportunistic Mode .............................21
4.2. Updating a HIP Association ................................24
4.3. Error Processing ..........................................24
4.4. HIP State Machine .........................................25
4.4.1. State Machine Terminology ..........................26
4.4.2. HIP States .........................................27
4.4.3. HIP State Processes ................................28
4.4.4. Simplified HIP State Diagram .......................35
Moskowitz, et al. Standards Track PAGE 2
RFC 7401 HIPv2 April 2015
4.5. User Data Considerations ..................................37
4.5.1. TCP and UDP Pseudo Header Computation for
User Data ..........................................37
4.5.2. Sending Data on HIP Packets ........................37
4.5.3. Transport Formats ..................................37
4.5.4. Reboot, Timeout, and Restart of HIP ................37
4.6. Certificate Distribution ..................................38
5. Packet Formats .................................................38
5.1. Payload Format ............................................38
5.1.1. Checksum ...........................................40
5.1.2. HIP Controls .......................................40
5.1.3. HIP Fragmentation Support ..........................40
5.2. HIP Parameters ............................................41
5.2.1. TLV Format .........................................44
5.2.2. Defining New Parameters ............................46
5.2.3. R1_COUNTER .........................................47
5.2.4. PUZZLE .............................................48
5.2.5. SOLUTION ...........................................49
5.2.6. DH_GROUP_LIST ......................................50
5.2.7. DIFFIE_HELLMAN .....................................51
5.2.8. HIP_CIPHER .........................................52
5.2.9. HOST_ID ............................................54
5.2.10. HIT_SUITE_LIST ....................................56
5.2.11. TRANSPORT_FORMAT_LIST .............................58
5.2.12. HIP_MAC ...........................................59
5.2.13. HIP_MAC_2 .........................................59
5.2.14. HIP_SIGNATURE .....................................60
5.2.15. HIP_SIGNATURE_2 ...................................61
5.2.16. SEQ ...............................................61
5.2.17. ACK ...............................................62
5.2.18. ENCRYPTED .........................................62
5.2.19. NOTIFICATION ......................................64
5.2.20. ECHO_REQUEST_SIGNED ...............................67
5.2.21. ECHO_REQUEST_UNSIGNED .............................68
5.2.22. ECHO_RESPONSE_SIGNED ..............................69
5.2.23. ECHO_RESPONSE_UNSIGNED ............................69
5.3. HIP Packets ...............................................70
5.3.1. I1 - the HIP Initiator Packet ......................71
5.3.2. R1 - the HIP Responder Packet ......................72
5.3.3. I2 - the Second HIP Initiator Packet ...............75
5.3.4. R2 - the Second HIP Responder Packet ...............76
5.3.5. UPDATE - the HIP Update Packet .....................77
5.3.6. NOTIFY - the HIP Notify Packet .....................78
5.3.7. CLOSE - the HIP Association Closing Packet .........78
5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet ..79
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5.4. ICMP Messages .............................................79
5.4.1. Invalid Version ....................................79
5.4.2. Other Problems with the HIP Header and
Packet Structure ...................................80
5.4.3. Invalid Puzzle Solution ............................80
5.4.4. Non-existing HIP Association .......................80
6. Packet Processing ..............................................80
6.1. Processing Outgoing Application Data ......................81
6.2. Processing Incoming Application Data ......................82
6.3. Solving the Puzzle ........................................83
6.4. HIP_MAC and SIGNATURE Calculation and Verification ........84
6.4.1. HMAC Calculation ...................................84
6.4.2. Signature Calculation ..............................87
6.5. HIP KEYMAT Generation .....................................89
6.6. Initiation of a HIP Base Exchange .........................90
6.6.1. Sending Multiple I1 Packets in Parallel ............91
6.6.2. Processing Incoming ICMP Protocol
Unreachable Messages ...............................92
6.7. Processing of Incoming I1 Packets .........................92
6.7.1. R1 Management ......................................94
6.7.2. Handling of Malformed Messages .....................94
6.8. Processing of Incoming R1 Packets .........................94
6.8.1. Handling of Malformed Messages .....................97
6.9. Processing of Incoming I2 Packets .........................97
6.9.1. Handling of Malformed Messages ....................100
6.10. Processing of Incoming R2 Packets .......................101
6.11. Sending UPDATE Packets ..................................101
6.12. Receiving UPDATE Packets ................................102
6.12.1. Handling a SEQ Parameter in a Received
UPDATE Message ...................................103
6.12.2. Handling an ACK Parameter in a Received
UPDATE Packet ....................................104
6.13. Processing of NOTIFY Packets ............................104
6.14. Processing of CLOSE Packets .............................105
6.15. Processing of CLOSE_ACK Packets .........................105
6.16. Handling State Loss .....................................105
7. HIP Policies ..................................................106
8. Security Considerations .......................................106
9. IANA Considerations ...........................................109
10. Differences from RFC 5201 ....................................113
11. References ...................................................117
11.1. Normative References ....................................117
11.2. Informative References ..................................119
Appendix A. Using Responder Puzzles ..............................122
Appendix B. Generating a Public Key Encoding from an HI ..........123
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RFC 7401 HIPv2 April 2015
Appendix C. Example Checksums for HIP Packets ....................123
C.1. IPv6 HIP Example (I1 Packet) ..............................124
C.2. IPv4 HIP Packet (I1 Packet) ...............................124
C.3. TCP Segment ...............................................125
Appendix D. ECDH and ECDSA 160-Bit Groups ........................125
Appendix E. HIT Suites and HIT Generation ........................125
Acknowledgments ..................................................127
Authors' Addresses ...............................................128
1. Introduction
This document specifies the details of the Host Identity Protocol
(HIP). A high-level description of the protocol and the underlying
architectural thinking is available in the separate HIP architecture
description [HIP-ARCH]. Briefly, the HIP architecture proposes an
alternative to the dual use of IP addresses as "locators" (routing
labels) and "identifiers" (endpoint, or host, identifiers). In HIP,
public cryptographic keys, of a public/private key pair, are used as
host identifiers, to which higher-layer protocols are bound instead
of an IP address. By using public keys (and their representations)
as host identifiers, dynamic changes to IP address sets can be
directly authenticated between hosts, and if desired, strong
authentication between hosts at the TCP/IP stack level can be
obtained.
This memo specifies the base HIP protocol ("base exchange") used
between hosts to establish an IP-layer communications context, called
a HIP association, prior to communications. It also defines a packet
format and procedures for updating and terminating an active HIP
association. Other elements of the HIP architecture are specified in
other documents, such as:
o "Using the Encapsulating Security Payload (ESP) Transport Format
with the Host Identity Protocol (HIP)" [RFC 7402]: how to use the
Encapsulating Security Payload (ESP) for integrity protection and
optional encryption
o "Host Mobility with the Host Identity Protocol" [HIP-HOST-MOB]:
how to support host mobility in HIP
o "Host Identity Protocol (HIP) Domain Name System (DNS) Extension"
[HIP-DNS-EXT]: how to extend DNS to contain Host Identity
information
o "Host Identity Protocol (HIP) Rendezvous Extension"
[HIP-REND-EXT]: using a rendezvous mechanism to contact mobile HIP
hosts
Moskowitz, et al. Standards Track PAGE 5
RFC 7401 HIPv2 April 2015
Since the HIP base exchange was first developed, there have been a
few advances in cryptography and attacks against cryptographic
systems. As a result, all cryptographic protocols need to be agile.
That is, the ability to switch from one cryptographic primitive to
another should be a part of such protocols. It is important to
support a reasonable set of mainstream algorithms to cater to
different use cases and allow moving away from algorithms that are
later discovered to be vulnerable. This update to the base exchange
includes this needed cryptographic agility while addressing the
downgrade attacks that such flexibility introduces. In addition,
Elliptic Curve support via Elliptic Curve DSA (ECDSA) and Elliptic
Curve Diffie-Hellman (ECDH) has been added.
1.1. A New Namespace and Identifiers
The Host Identity Protocol introduces a new namespace, the Host
Identity namespace. Some ramifications of this new namespace are
explained in the HIP architecture description [HIP-ARCH].
There are two main representations of the Host Identity, the full
Host Identity (HI) and the Host Identity Tag (HIT). The HI is a
public key and directly represents the Identity of a host. Since
there are different public key algorithms that can be used with
different key lengths, the HI, as such, is unsuitable for use as a
packet identifier, or as an index into the various state-related
implementation structures needed to support HIP. Consequently, a
hash of the HI, the Host Identity Tag (HIT), is used as the
operational representation. The HIT is 128 bits long and is used
in the HIP headers and to index the corresponding state in the
end hosts. The HIT has an important security property in that it
is self-certifying (see Section 3).
1.2. The HIP Base Exchange (BEX)
The HIP base exchange is a two-party cryptographic protocol used to
establish communications context between hosts. The base exchange is
a SIGMA-compliant [KRA03] four-packet exchange. The first party is
called the Initiator and the second party the Responder. The
protocol exchanges Diffie-Hellman [DIF76] keys in the 2nd and 3rd
packets, and authenticates the parties in the 3rd and 4th packets.
The four-packet design helps to make HIP resistant to DoS attacks.
It allows the Responder to stay stateless until the IP address and
the cryptographic puzzle are verified. The Responder starts the
puzzle exchange in the 2nd packet, with the Initiator completing it
in the 3rd packet before the Responder stores any state from the
exchange.
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RFC 7401 HIPv2 April 2015
The exchange can use the Diffie-Hellman output to encrypt the Host
Identity of the Initiator in the 3rd packet (although Aura, et al.
[AUR05] note that such operation may interfere with packet-inspecting
middleboxes), or the Host Identity may instead be sent unencrypted.
The Responder's Host Identity is not protected. It should be noted,
however, that both the Initiator's and the Responder's HITs are
transported as such (in cleartext) in the packets, allowing an
eavesdropper with a priori knowledge about the parties to identify
them by their HITs. Hence, encrypting the HI of any party does not
provide privacy against such an attacker.
Data packets start to flow after the 4th packet. The 3rd and 4th HIP
packets may carry a data payload in the future. However, the details
of this may be defined later.
An existing HIP association can be updated using the update mechanism
defined in this document, and when the association is no longer
needed, it can be closed using the defined closing mechanism.
Finally, HIP is designed as an end-to-end authentication and key
establishment protocol, to be used with Encapsulating Security
Payload (ESP) [RFC 7402] and other end-to-end security protocols. The
base protocol does not cover all the fine-grained policy control
found in Internet Key Exchange (IKE) [RFC 7296] that allows IKE to
support complex gateway policies. Thus, HIP is not a complete
replacement for IKE.
1.3. Memo Structure
The rest of this memo is structured as follows. Section 2 defines
the central keywords, notation, and terms used throughout the rest of
the document. Section 3 defines the structure of the Host Identity
and its various representations. Section 4 gives an overview of the
HIP base exchange protocol. Sections 5 and 6 define the detailed
packet formats and rules for packet processing. Finally, Sections 7,
8, and 9 discuss policy, security, and IANA considerations,
respectively.
2. Terms and Definitions
2.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
Moskowitz, et al. Standards Track PAGE 7
RFC 7401 HIPv2 April 2015
2.2. Notation
[x] indicates that x is optional.
{x} indicates that x is encrypted.
X(y) indicates that y is a parameter of X.
<x>i indicates that x exists i times.
--> signifies "Initiator to Responder" communication (requests).
<-- signifies "Responder to Initiator" communication (replies).
| signifies concatenation of information (e.g., X | Y is the
concatenation of X with Y).
Ltrunc (H(x), #K)
denotes the lowest-order #K bits of the result of the
hash function H on the input x.
2.3. Definitions
HIP base exchange (BEX): The handshake for establishing a new HIP
association.
Host Identity (HI): The public key of the signature algorithm that
represents the identity of the host. In HIP, a host proves its
identity by creating a signature with the private key belonging to
its HI (cf. Section 3).
Host Identity Tag (HIT): A shorthand for the HI in IPv6 format. It
is generated by hashing the HI (cf. Section 3.1).
HIT Suite: A HIT Suite groups all cryptographic algorithms that are
required to generate and use an HI and its HIT. In particular,
these algorithms are 1) the public key signature algorithm, 2) the
hash function, and 3) the truncation (cf. Appendix E).
HIP association: The shared state between two peers after completion
of the BEX.
HIP packet: A control packet carrying a HIP packet header relating
to the establishment, maintenance, or termination of the HIP
association.
Initiator: The host that initiates the BEX. This role is typically
forgotten once the BEX is completed.
Moskowitz, et al. Standards Track PAGE 8
RFC 7401 HIPv2 April 2015
Responder: The host that responds to the Initiator in the BEX. This
role is typically forgotten once the BEX is completed.
Responder's HIT hash algorithm (RHASH): The hash algorithm used for
various hash calculations in this document. The algorithm is the
same as is used to generate the Responder's HIT. The RHASH is the
hash function defined by the HIT Suite of the Responder's HIT
(cf. Section 5.2.10).
Length of the Responder's HIT hash algorithm (RHASH_len): The
natural output length of RHASH in bits.
Signed data: Data that is signed is protected by a digital signature
that was created by the sender of the data by using the private
key of its HI.
KDF: The Key Derivation Function (KDF) is used for deriving the
symmetric keys from the Diffie-Hellman key exchange.
KEYMAT: The keying material derived from the Diffie-Hellman key
exchange by using the KDF. Symmetric keys for encryption and
integrity protection of HIP packets and encrypted user data
packets are drawn from this keying material.
3. Host Identity (HI) and Its Structure
In this section, the properties of the Host Identity and Host
Identity Tag are discussed, and the exact format for them is defined.
In HIP, the public key of an asymmetric key pair is used as the Host
Identity (HI). Correspondingly, the host itself is defined as the
entity that holds the private key of the key pair. See the HIP
architecture specification [HIP-ARCH] for more details on the
difference between an identity and the corresponding identifier.
HIP implementations MUST support the Rivest Shamir Adleman [RSA]
public key algorithm and the Elliptic Curve Digital Signature
Algorithm (ECDSA) for generating the HI as defined in Section 5.2.9.
Additional algorithms MAY be supported.
A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
protocols to represent the Host Identity. The HIT is 128 bits long
and has the following three key properties: i) it is the same length
as an IPv6 address and can be used in fixed address-sized fields in
APIs and protocols; ii) it is self-certifying (i.e., given a HIT, it
is computationally hard to find a Host Identity key that matches the
HIT); and iii) the probability of a HIT collision between two hosts
Moskowitz, et al. Standards Track PAGE 9
RFC 7401 HIPv2 April 2015
is very low; hence, it is infeasible for an attacker to find a
collision with a HIT that is in use. For details on the security
properties of the HIT, see [HIP-ARCH].
The structure of the HIT is defined in [RFC 7343]. The HIT is an
Overlay Routable Cryptographic Hash Identifier (ORCHID) and consists
of three parts: first, an IANA-assigned prefix to distinguish it from
other IPv6 addresses; second, a four-bit encoding of the algorithms
that were used for generating the HI and the hashed representation of
HI; third, a 96-bit hashed representation of the Host Identity. The
encoding of the ORCHID generation algorithm and the exact algorithm
for generating the hashed representation are specified in Appendix E
and [RFC 7343].
Carrying HIs and HITs in the header of user data packets would
increase the overhead of packets. Thus, it is not expected that they
are carried in every packet, but other methods are used to map the
data packets to the corresponding HIs. In some cases, this makes it
possible to use HIP without any additional headers in the user data
packets. For example, if ESP is used to protect data traffic, the
Security Parameter Index (SPI) carried in the ESP header can be used
to map the encrypted data packet to the correct HIP association.
3.1. Host Identity Tag (HIT)
The Host Identity Tag is a 128-bit value -- a hashed encoding of the
Host Identifier. There are two advantages of using a hashed encoding
over the actual variable-sized Host Identity public key in protocols.
First, the fixed length of the HIT keeps packet sizes manageable and
eases protocol coding. Second, it presents a consistent format for
the protocol, independent of the underlying identity technology
in use.
RFC 7343 [RFC 7343] specifies 128-bit hash-based identifiers, called
ORCHIDs. Their prefix, allocated from the IPv6 address block, is
defined in [RFC 7343]. The Host Identity Tag is one type of ORCHID.
This document extends the original, experimental HIP specification
[RFC 5201] with measures to support crypto agility. One of these
measures allows different hash functions for creating a HIT. HIT
Suites group the sets of algorithms that are required to generate and
use a particular HIT. The Suites are encoded in HIT Suite IDs.
These HIT Suite IDs are transmitted in the ORCHID Generation
Algorithm (OGA) field in the ORCHID. With the HIT Suite ID in the
OGA ID field, a host can tell, from another host's HIT, whether it
supports the necessary hash and signature algorithms to establish a
HIP association with that host.
Moskowitz, et al. Standards Track PAGE 10
RFC 7401 HIPv2 April 2015
3.2. Generating a HIT from an HI
The HIT MUST be generated according to the ORCHID generation method
described in [RFC 7343] using a context ID value of 0xF0EF F02F BFF4
3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly
by the editor of this specification), and an input that encodes the
Host Identity field (see Section 5.2.9) present in a HIP payload
packet. The set of hash function, signature algorithm, and the
algorithm used for generating the HIT from the HI depends on the HIT
Suite (see Section 5.2.10) and is indicated by the four bits of the
OGA ID field in the ORCHID. Currently, truncated SHA-1, truncated
SHA-384, and truncated SHA-256 [FIPS.180-4.2012] are defined as
hashes for generating a HIT.
For identities that are either RSA, Digital Signature Algorithm (DSA)
[FIPS.186-4.2013], or Elliptic Curve DSA (ECDSA) public keys, the
ORCHID input consists of the public key encoding as specified for the
Host Identity field of the HOST_ID parameter (see Section 5.2.9).
This document defines four algorithm profiles: RSA, DSA, ECDSA, and
ECDSA_LOW. The ECDSA_LOW profile is meant for devices with low
computational capabilities. Hence, one of the following applies:
The RSA public key is encoded as defined in [RFC 3110], Section 2,
taking the exponent length (e_len), exponent (e), and modulus (n)
fields concatenated. The length (n_len) of the modulus (n) can be
determined from the total HI Length and the preceding HI fields
including the exponent (e). Thus, the data that serves as input
for the HIT generation has the same length as the HI. The fields
MUST be encoded in network byte order, as defined in [RFC 3110].
The DSA public key is encoded as defined in [RFC 2536], Section 2,
taking the fields T, Q, P, G, and Y, concatenated as input. Thus,
the data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
where T is the size parameter as defined in [RFC 2536]. The size
parameter T, affecting the field lengths, MUST be selected as the
minimum value that is long enough to accommodate P, G, and Y. The
fields MUST be encoded in network byte order, as defined in
[RFC 2536].
The ECDSA public keys are encoded as defined in Sections 4.2 and 6
of [RFC 6090].
In Appendix B, the public key encoding process is illustrated using
pseudo-code.
Moskowitz, et al. Standards Track PAGE 11
RFC 7401 HIPv2 April 2015
4. Protocol Overview
This section is a simplified overview of the HIP protocol operation,
and does not contain all the details of the packet formats or the
packet processing steps. Sections 5 and 6 describe in more detail
the packet formats and packet processing steps, respectively, and are
normative in case of any conflicts with this section.
The protocol number 139 has been assigned by IANA to the Host
Identity Protocol.
The HIP payload (Section 5.1) header could be carried in every IP
datagram. However, since HIP headers are relatively large
(40 bytes), it is desirable to 'compress' the HIP header so that the
HIP header only occurs in control packets used to establish or change
HIP association state. The actual method for header 'compression'
and for matching data packets with existing HIP associations (if any)
is defined in separate documents, describing transport formats and
methods. All HIP implementations MUST implement, at minimum, the ESP
transport format for HIP [RFC 7402].
4.1. Creating a HIP Association
By definition, the system initiating a HIP base exchange is the
Initiator, and the peer is the Responder. This distinction is
typically forgotten once the base exchange completes, and either
party can become the Initiator in future communications.
The HIP base exchange serves to manage the establishment of state
between an Initiator and a Responder. The first packet, I1,
initiates the exchange, and the last three packets, R1, I2, and R2,
constitute an authenticated Diffie-Hellman [DIF76] key exchange for
session-key generation. In the first two packets, the hosts agree on
a set of cryptographic identifiers and algorithms that are then used
in and after the exchange. During the Diffie-Hellman key exchange, a
piece of keying material is generated. The HIP association keys are
drawn from this keying material by using a Key Derivation Function
(KDF). If other cryptographic keys are needed, e.g., to be used with
ESP, they are expected to be drawn from the same keying material by
using the KDF.
The Initiator first sends a trigger packet, I1, to the Responder.
The packet contains the HIT of the Initiator and possibly the HIT of
the Responder, if it is known. Moreover, the I1 packet initializes
the negotiation of the Diffie-Hellman group that is used for
generating the keying material. Therefore, the I1 packet contains a
list of Diffie-Hellman Group IDs supported by the Initiator. Note
that in some cases it may be possible to replace this trigger packet
Moskowitz, et al. Standards Track PAGE 12
RFC 7401 HIPv2 April 2015
with some other form of a trigger, in which case the protocol starts
with the Responder sending the R1 packet. In such cases, another
mechanism to convey the Initiator's supported DH groups (e.g., by
using a default group) must be specified.
The second packet, R1, starts the actual authenticated Diffie-Hellman
exchange. It contains a puzzle -- a cryptographic challenge that the
Initiator must solve before continuing the exchange. The level of
difficulty of the puzzle can be adjusted based on the level of trust
with the Initiator, the current load, or other factors. In addition,
the R1 contains the Responder's Diffie-Hellman parameter and lists of
cryptographic algorithms supported by the Responder. Based on these
lists, the Initiator can continue, abort, or restart the base
exchange with a different selection of cryptographic algorithms.
Also, the R1 packet contains a signature that covers selected parts
of the message. Some fields are left outside the signature to
support pre-created R1s.
In the I2 packet, the Initiator MUST display the solution to the
received puzzle. Without a correct solution, the I2 message is
discarded. The I2 packet also contains a Diffie-Hellman parameter
that carries needed information for the Responder. The I2 packet is
signed by the Initiator.
The R2 packet acknowledges the receipt of the I2 packet and completes
the base exchange. The packet is signed by the Responder.
Moskowitz, et al. Standards Track PAGE 13
RFC 7401 HIPv2 April 2015
The base exchange is illustrated below in Figure 1. The term "key"
refers to the Host Identity public key, and "sig" represents a
signature using such a key. The packets contain other parameters not
shown in this figure.
Initiator Responder
I1: DH list
-------------------------->
select precomputed R1
R1: puzzle, DH, key, sig
<-------------------------
check sig remain stateless
solve puzzle
I2: solution, DH, {key}, sig
-------------------------->
compute DH check puzzle
check sig
R2: sig
<--------------------------
check sig compute DH
Figure 1
4.1.1. HIP Puzzle Mechanism
The purpose of the HIP puzzle mechanism is to protect the Responder
from a number of denial-of-service threats. It allows the Responder
to delay state creation until receiving the I2 packet. Furthermore,
the puzzle allows the Responder to use a fairly cheap calculation to
check that the Initiator is "sincere" in the sense that it has
churned enough CPU cycles in solving the puzzle.
The puzzle allows a Responder implementation to completely delay
association-specific state creation until a valid I2 packet is
received. An I2 packet without a valid puzzle solution can be
rejected immediately once the Responder has checked the solution.
The solution can be checked by computing only one hash function, and
invalid solutions can be rejected before state is created, and before
CPU-intensive public-key signature verification and Diffie-Hellman
key generation are performed. By varying the difficulty of the
puzzle, the Responder can frustrate CPU- or memory-targeted DoS
attacks.
The Responder can remain stateless and drop most spoofed I2 packets
because puzzle calculation is based on the Initiator's Host Identity
Tag. The idea is that the Responder has a (perhaps varying) number
of pre-calculated R1 packets, and it selects one of these based on
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the information carried in the I1 packet. When the Responder then
later receives the I2 packet, it can verify that the puzzle has been
solved using the Initiator's HIT. This makes it impractical for the
attacker to first exchange one I1/R1 packet, and then generate a
large number of spoofed I2 packets that seemingly come from different
HITs. This method does not protect the Responder from an attacker
that uses fixed HITs, though. Against such an attacker, a viable
approach may be to create a piece of local state, and remember that
the puzzle check has previously failed. See Appendix A for one
possible implementation. Responder implementations SHOULD include
sufficient randomness in the puzzle values so that algorithmic
complexity attacks become impossible [CRO03].
The Responder can set the puzzle difficulty for the Initiator, based
on its level of trust of the Initiator. Because the puzzle is not
included in the signature calculation, the Responder can use
pre-calculated R1 packets and include the puzzle just before sending
the R1 to the Initiator. The Responder SHOULD use heuristics to
determine when it is under a denial-of-service attack, and set the
puzzle difficulty value #K appropriately, as explained later.
4.1.2. Puzzle Exchange
The Responder starts the puzzle exchange when it receives an I1
packet. The Responder supplies a random number #I, and requires the
Initiator to find a number #J. To select a proper #J, the Initiator
must create the concatenation of #I, the HITs of the parties, and #J,
and calculate a hash over this concatenation using the RHASH
algorithm. The lowest-order #K bits of the result MUST be zeros.
The value #K sets the difficulty of the puzzle.
To generate a proper number #J, the Initiator will have to generate a
number of #Js until one produces the hash target of zeros. The
Initiator SHOULD give up after exceeding the puzzle Lifetime in the
PUZZLE parameter (as described in Section 5.2.4). The Responder
needs to re-create the concatenation of #I, the HITs, and the
provided #J, and compute the hash once to prove that the Initiator
completed its assigned task.
To prevent precomputation attacks, the Responder MUST select the
number #I in such a way that the Initiator cannot guess it.
Furthermore, the construction MUST allow the Responder to verify that
the value #I was indeed selected by it and not by the Initiator. See
Appendix A for an example on how to implement this.
Using the Opaque data field in the PUZZLE (see Section 5.2.4) in an
ECHO_REQUEST_SIGNED (see Section 5.2.20) or in an
ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21), the Responder
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can include some data in R1 that the Initiator MUST copy unmodified
in the corresponding I2 packet. The Responder can use the opaque
data to transfer a piece of local state information to the Initiator
and back -- for example, to recognize that the I2 is a response to a
previously sent R1. The Responder can generate the opaque data in
various ways, e.g., using encryption or hashing with some secret, the
sent #I, and possibly using other related data. With the same
secret, the received #I (from the I2 packet), and the other related
data (if any), the Responder can verify that it has itself sent the
#I to the Initiator. The Responder MUST periodically change such a
secret.
It is RECOMMENDED that the Responder generates new secrets for the
puzzle and new R1s once every few minutes. Furthermore, it is
RECOMMENDED that the Responder is able to verify a valid puzzle
solution at least Lifetime seconds after the puzzle secret has been
deprecated. This time value guarantees that the puzzle is valid for
at least Lifetime and at most 2 * Lifetime seconds. This limits the
usability that an old, solved puzzle has to an attacker. Moreover,
it avoids problems with the validity of puzzles if the lifetime is
relatively short compared to the network delay and the time for
solving the puzzle.
The puzzle value #I and the solution #J are inputs for deriving the
keying material from the Diffie-Hellman key exchange (see
Section 6.5). Therefore, to ensure that the derived keying material
differs, a Responder SHOULD NOT use the same puzzle #I with the same
DH keys for the same Initiator twice. Such uniqueness can be
achieved, for example, by using a counter as an additional input for
generating #I. This counter can be increased for each processed I1
packet. The state of the counter can be transmitted in the Opaque
data field in the PUZZLE (see Section 5.2.4), in an
ECHO_REQUEST_SIGNED parameter (see Section 5.2.20), or in an
ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21) without the need
to establish state.
NOTE: The protocol developers explicitly considered whether R1 should
include a timestamp in order to protect the Initiator from replay
attacks. The decision was to NOT include a timestamp, to avoid
problems with global time synchronization.
NOTE: The protocol developers explicitly considered whether a memory-
bound function should be used for the puzzle instead of a CPU-bound
function. The decision was to not use memory-bound functions.
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4.1.3. Authenticated Diffie-Hellman Protocol with DH Group Negotiation
The packets R1, I2, and R2 implement a standard authenticated
Diffie-Hellman exchange. The Responder sends one of its public
Diffie-Hellman keys and its public authentication key, i.e., its Host
Identity, in R1. The signature in the R1 packet allows the Initiator
to verify that the R1 has been once generated by the Responder.
However, since the R1 is precomputed and therefore does not cover
association-specific information in the I1 packet, it does not
protect against replay attacks.
Before the actual authenticated Diffie-Hellman exchange, the
Initiator expresses its preference regarding its choice of the DH
groups in the I1 packet. The preference is expressed as a sorted
list of DH Group IDs. The I1 packet is not protected by a signature.
Therefore, this list is sent in an unauthenticated way to avoid
costly computations for processing the I1 packet at the Responder
side. Based on the preferences of the Initiator, the Responder sends
an R1 packet containing its most suitable public DH value. The
Responder also attaches a list of its own preferences to the R1 to
convey the basis for the DH group selection to the Initiator. This
list is carried in the signed part of the R1 packet. If the choice
of the DH group value in the R1 does not match the preferences of the
Initiator and the Responder, the Initiator can detect that the list
of DH Group IDs in the I1 was manipulated (see below for details).
If none of the DH Group IDs in the I1 packet are supported by the
Responder, the Responder selects the DH group most suitable for it,
regardless of the Initiator's preference. It then sends the R1
containing this DH group and its list of supported DH Group IDs to
the Initiator.
When the Initiator receives an R1, it receives one of the Responder's
public Diffie-Hellman values and the list of DH Group IDs supported
by the Responder. This list is covered by the signature in the R1
packet to avoid forgery. The Initiator compares the Group ID of the
public DH value in the R1 packet to the list of supported DH Group
IDs in the R1 packets and to its own preferences expressed in the
list of supported DH Group IDs. The Initiator continues the BEX only
if the Group ID of the public DH value of the Responder is the most
preferred of the IDs supported by both the Initiator and Responder.
Otherwise, the communication is subject to a downgrade attack, and
the Initiator MUST either restart the base exchange with a new I1
packet or abort the base exchange. If the Responder's choice of the
DH group is not supported by the Initiator, the Initiator MAY abort
the handshake or send a new I1 packet with a different list of
supported DH groups. However, the Initiator MUST verify the
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signature of the R1 packet before restarting or aborting the
handshake. It MUST silently ignore the R1 packet if the signature is
not valid.
If the preferences regarding the DH Group ID match, the Initiator
computes the Diffie-Hellman session key (Kij). The Initiator creates
a HIP association using keying material from the session key (see
Section 6.5) and may use the HIP association to encrypt its public
authentication key, i.e., the Host Identity. The resulting I2 packet
contains the Initiator's Diffie-Hellman key and its (optionally
encrypted) public authentication key. The signature of the I2
message covers all parameters of the signed parameter ranges (see
Section 5.2) in the packet without exceptions, as in the R1.
The Responder extracts the Initiator's Diffie-Hellman public key from
the I2 packet, computes the Diffie-Hellman session key, creates a
corresponding HIP association, and decrypts the Initiator's public
authentication key. It can then verify the signature using the
authentication key.
The final message, R2, completes the BEX and protects the Initiator
against replay attacks, because the Responder uses the shared key
from the Diffie-Hellman exchange to create a Hashed Message
Authentication Code (HMAC) and also uses the private key of its Host
Identity to sign the packet contents.
4.1.4. HIP Replay Protection
HIP includes the following mechanisms to protect against malicious
packet replays. Responders are protected against replays of I1
packets by virtue of the stateless response to I1 packets with
pre-signed R1 messages. Initiators are protected against R1 replays
by a monotonically increasing "R1 generation counter" included in
the R1. Responders are protected against replays of forged I2
packets by the puzzle mechanism (see Section 4.1.1 above), and
optional use of opaque data. Hosts are protected against replays of
R2 packets and UPDATEs by use of a less expensive HMAC verification
preceding the HIP signature verification.
The R1 generation counter is a monotonically increasing 64-bit
counter that may be initialized to any value. The scope of the
counter MAY be system-wide, but there SHOULD be a separate counter
for each Host Identity, if there is more than one local Host
Identity. The value of this counter SHOULD be preserved across
system reboots and invocations of the HIP base exchange. This
counter indicates the current generation of puzzles. Implementations
MUST accept puzzles from the current generation and MAY accept
puzzles from earlier generations. A system's local counter MUST be
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incremented at least as often as every time old R1s cease to be
valid. The local counter SHOULD never be decremented; otherwise, the
host exposes its peers to the replay of previously generated, higher-
numbered R1s.
A host may receive more than one R1, either due to sending multiple
I1 packets (see Section 6.6.1) or due to a replay of an old R1. When
sending multiple I1 packets to the same host, an Initiator SHOULD
wait for a small amount of time (a reasonable time may be
2 * expected RTT) after the first R1 reception to allow possibly
multiple R1s to arrive, and it SHOULD respond to an R1 among the set
with the largest R1 generation counter. If an Initiator is
processing an R1 or has already sent an I2 packet (still waiting for
the R2 packet) and it receives another R1 with a larger R1 generation
counter, it MAY elect to restart R1 processing with the fresher R1,
as if it were the first R1 to arrive.
The R1 generation counter may roll over or may become reset. It is
important for an Initiator to be robust to the loss of state about
the R1 generation counter of a peer or to a reset of the peer's
counter. It is recommended that, when choosing between multiple R1s,
the Initiator prefer to use the R1 that corresponds to the current R1
generation counter, but that if it is unable to make progress with
that R1, the Initiator may try the other R1s, beginning with the R1
packet with the highest counter.
4.1.5. Refusing a HIP Base Exchange
A HIP-aware host may choose not to accept a HIP base exchange. If
the host's policy is to only be an Initiator and policy allows the
establishment of a HIP association with the original Initiator, it
should begin its own HIP base exchange. A host MAY choose to have
such a policy since only the privacy of the Initiator's HI is
protected in the exchange. It should be noted that such behavior can
introduce the risk of a race condition if each host's policy is to
only be an Initiator, at which point the HIP base exchange will fail.
If the host's policy does not permit it to enter into a HIP exchange
with the Initiator, it should send an ICMP 'Destination Unreachable,
Administratively Prohibited' message. A more complex HIP packet is
not used here as it actually opens up more potential DoS attacks than
a simple ICMP message. A HIP NOTIFY message is not used because no
HIP association exists between the two hosts at that time.
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4.1.6. Aborting a HIP Base Exchange
Two HIP hosts may encounter situations in which they cannot complete
a HIP base exchange because of insufficient support for cryptographic
algorithms, in particular the HIT Suites and DH groups. After
receiving the R1 packet, the Initiator can determine whether the
Responder supports the required cryptographic operations to
successfully establish a HIP association. The Initiator can abort
the BEX silently after receiving an R1 packet that indicates an
unsupported set of algorithms. The specific conditions are described
below.
The R1 packet contains a signed list of HIT Suite IDs as supported by
the Responder. Therefore, the Initiator can determine whether its
source HIT is supported by the Responder. If the HIT Suite ID of the
Initiator's HIT is not contained in the list of HIT Suites in the R1,
the Initiator MAY abort the handshake silently or MAY restart the
handshake with a new I1 packet that contains a source HIT supported
by the Responder.
During the handshake, the Initiator and the Responder agree on a
single DH group. The Responder selects the DH group and its DH
public value in the R1 based on the list of DH Group IDs in the I1
packet. If the Responder supports none of the DH groups requested by
the Initiator, the Responder selects an arbitrary DH and replies with
an R1 containing its list of supported DH Group IDs. In such a case,
the Initiator receives an R1 packet containing the DH public value
for an unrequested DH group and also the Responder's DH group list in
the signed part of the R1 packet. At this point, the Initiator MAY
abort the handshake or MAY restart the handshake by sending a new I1
packet containing a selection of DH Group IDs that is supported by
the Responder.
4.1.7. HIP Downgrade Protection
In a downgrade attack, an attacker attempts to unnoticeably
manipulate the packets of an Initiator and/or a Responder to
influence the result of the cryptographic negotiations in the BEX in
its favor. As a result, the victims select weaker cryptographic
algorithms than they would otherwise have selected without the
attacker's interference. Downgrade attacks can only be successful if
they remain undetected by the victims and the victims falsely assume
a secure communication channel.
In HIP, almost all packet parameters related to cryptographic
negotiations are covered by signatures. These parameters cannot be
directly manipulated in a downgrade attack without invalidating the
signature. However, signed packets can be subject to replay attacks.
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In such a replay attack, the attacker could use an old BEX packet
with an outdated and weak selection of cryptographic algorithms and
replay it instead of a more recent packet with a collection of
stronger cryptographic algorithms. Signed packets that could be
subject to this replay attack are the R1 and I2 packet. However,
replayed R1 and I2 packets cannot be used to successfully establish a
HIP BEX because these packets also contain the public DH values of
the Initiator and the Responder. Old DH values from replayed packets
lead to invalid keying material and mismatching shared secrets
because the attacker is unable to derive valid keying material from
the DH public keys in the R1 and cannot generate a valid HMAC and
signature for a replayed I2.
In contrast to the first version of HIP [RFC 5201], version 2 of HIP
as defined in this document begins the negotiation of the DH groups
already in the first BEX packet, the I1. The I1 packet is, by
intention, not protected by a signature, to avoid CPU-intensive
cryptographic operations processing floods of I1 packets targeted at
the Responder. Hence, the list of DH Group IDs in the I1 packet is
vulnerable to forgery and manipulation. To thwart an unnoticed
manipulation of the I1 packet, the Responder chooses the DH group
deterministically and includes its own list of DH Group IDs in the
signed part of the R1 packet. The Initiator can detect an attempted
downgrade attack by comparing the list of DH Group IDs in the R1
packet to its own preferences in the I1 packet. If the choice of the
DH group in the R1 packet does not equal the best match of the two
lists (the highest-priority DH ID of the Responder that is present in
the Initiator's DH list), the Initiator can conclude that its list in
the I1 packet was altered by an attacker. In this case, the
Initiator can restart or abort the BEX. As mentioned before, the
detection of the downgrade attack is sufficient to prevent it.
4.1.8. HIP Opportunistic Mode
It is possible to initiate a HIP BEX even if the Responder's HI (and
HIT) is unknown. In this case, the initial I1 packet contains all
zeros as the destination HIT. This kind of connection setup is
called opportunistic mode.
The Responder may have multiple HITs due to multiple supported HIT
Suites. Since the Responder's HIT Suite in the opportunistic mode is
not determined by the destination HIT of the I1 packet, the Responder
can freely select a HIT of any HIT Suite. The complete set of HIT
Suites supported by the Initiator is not known to the Responder.
Therefore, the Responder SHOULD select its HIT from the same HIT
Suite as the Initiator's HIT (indicated by the HIT Suite information
in the OGA ID field of the Initiator's HIT) because this HIT Suite is
obviously supported by the Initiator. If the Responder selects a
Moskowitz, et al. Standards Track PAGE 21
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different HIT that is not supported by the Initiator, the Initiator
MAY restart the BEX with an I1 packet with a source HIT that is
contained in the list of the Responder's HIT Suites in the R1 packet.
Note that the Initiator cannot verify the signature of the R1 packet
if the Responder's HIT Suite is not supported. Therefore, the
Initiator MUST treat R1 packets with unsupported Responder HITs as
potentially forged and MUST NOT use any parameters from the
unverified R1 besides the HIT_SUITE_LIST. Moreover, an Initiator
that uses an unverified HIT_SUITE_LIST from an R1 packet to determine
a possible source HIT MUST verify that the HIT_SUITE_LIST in the
first unverified R1 packet matches the HIT_SUITE_LIST in the second
R1 packet for which the Initiator supports the signature algorithm.
The Initiator MUST restart the BEX with a new I1 packet for which the
algorithm was mentioned in the verifiable R1 if the two lists do not
match. This procedure is necessary to mitigate downgrade attacks.
There are both security and API issues involved with the
opportunistic mode. These issues are described in the remainder of
this section.
Given that the Responder's HI is not known by the Initiator, there
must be suitable API calls that allow the Initiator to request,
directly or indirectly, that the underlying system initiates the HIP
base exchange solely based on locators. The Responder's HI will be
tentatively available in the R1 packet, and in an authenticated form
once the R2 packet has been received and verified. Hence, the
Responder's HIT could be communicated to the application via new API
mechanisms. However, with a backwards-compatible API the application
sees only the locators used for the initial contact. Depending on
the desired semantics of the API, this can raise the following
issues:
o The actual locators may later change if an UPDATE message is used,
even if from the API perspective the association still appears to
be between two specific locators. However, the locator update is
still secure, and the association is still between the same nodes.
o Different associations between the same two locators may result in
connections to different nodes, if the implementation no longer
remembers which identifier the peer had in an earlier association.
This is possible when the peer's locator has changed for
legitimate reasons or when an attacker pretends to be a node that
has the peer's locator. Therefore, when using opportunistic mode,
HIP implementations MUST NOT place any expectation that the peer's
HI returned in the R1 message matches any HI previously seen from
that address.
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If the HIP implementation and application do not have the same
understanding of what constitutes an association, this may even
happen within the same association. For instance, an
implementation may not know when HIP state can be purged for
UDP-based applications.
In addition, the following security considerations apply. The
generation counter mechanism will be less efficient in protecting
against replays of the R1 packet, given that the Responder can choose
a replay that uses an arbitrary HI, not just the one given in the I1
packet.
More importantly, the opportunistic exchange is vulnerable to
man-in-the-middle attacks, because the Initiator does not have any
public key information about the peer. To assess the impacts of this
vulnerability, we compare it to vulnerabilities in current,
non-HIP-capable communications.
An attacker on the path between the two peers can insert itself as a
man-in-the-middle by providing its own identifier to the Initiator
and then initiating another HIP association towards the Responder.
For this to be possible, the Initiator must employ opportunistic
mode, and the Responder must be configured to accept a connection
from any HIP-enabled node.
An attacker outside the path will be unable to do so, given that it
cannot respond to the messages in the base exchange.
These security properties are characteristic also of communications
in the current Internet. A client contacting a server without
employing end-to-end security may find itself talking to the server
via a man-in-the-middle, assuming again that the server is willing to
talk to anyone.
If end-to-end security is in place, then the worst that can happen in
both the opportunistic HIP and non-HIP (normal IP) cases is denial-
of-service; an entity on the path can disrupt communications, but
will be unable to successfully insert itself as a man-in-the-middle.
However, once the opportunistic exchange has successfully completed,
HIP provides confidentiality and integrity protection for the
communications, and can securely change the locators of the
endpoints.
As a result, opportunistic mode in HIP offers a "better than nothing"
security model. Initially, a base exchange authenticated in the
opportunistic mode involves a leap of faith subject to man-in-the-
middle attacks, but subsequent datagrams related to the same HIP
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association cannot be compromised by a new man-in-the-middle attack.
Further, if the man-in-the-middle moves away from the path of the
active association, the attack would be exposed after the fact.
Thus, it can be stated that opportunistic mode in HIP is at least as
secure as unprotected IP-based communications.
4.2. Updating a HIP Association
A HIP association between two hosts may need to be updated over time.
Examples include the need to rekey expiring security associations,
add new security associations, or change IP addresses associated with
hosts. The UPDATE packet is used for those and other similar
purposes. This document only specifies the UPDATE packet format and
basic processing rules, with mandatory parameters. The actual usage
is defined in separate specifications.
HIP provides a general-purpose UPDATE packet, which can carry
multiple HIP parameters, for updating the HIP state between two
peers. The UPDATE mechanism has the following properties:
UPDATE messages carry a monotonically increasing sequence number
and are explicitly acknowledged by the peer. Lost UPDATEs or
acknowledgments may be recovered via retransmission. Multiple
UPDATE messages may be outstanding under certain circumstances.
UPDATE is protected by both HIP_MAC and HIP_SIGNATURE parameters,
since processing UPDATE signatures alone is a potential DoS attack
against intermediate systems.
UPDATE packets are explicitly acknowledged by the use of an
acknowledgment parameter that echoes an individual sequence number
received from the peer. A single UPDATE packet may contain both a
sequence number and one or more acknowledgment numbers (i.e.,
piggybacked acknowledgment(s) for the peer's UPDATE).
The UPDATE packet is defined in Section 5.3.5.
4.3. Error Processing
HIP error processing behavior depends on whether or not there exists
an active HIP association. In general, if a HIP association exists
between the sender and receiver of a packet causing an error
condition, the receiver SHOULD respond with a NOTIFY packet. On the
other hand, if there are no existing HIP associations between the
sender and receiver, or the receiver cannot reasonably determine the
identity of the sender, the receiver MAY respond with a suitable ICMP
message; see Section 5.4 for more details.
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The HIP protocol and state machine are designed to recover from one
of the parties crashing and losing its state. The following
scenarios describe the main use cases covered by the design.
No prior state between the two systems.
The system with data to send is the Initiator. The process
follows the standard four-packet base exchange, establishing
the HIP association.
The system with data to send has no state with the receiver, but
the receiver has a residual HIP association.
The system with data to send is the Initiator. The Initiator
acts as in no prior state, sending an I1 packet and receiving
an R1 packet. When the Responder receives a valid I2 packet,
the old association is 'discovered' and deleted, and the new
association is established.
The system with data to send has a HIP association, but the
receiver does not.
The system sends data on the outbound user data security
association. The receiver 'detects' the situation when it
receives a user data packet that it cannot match to any HIP
association. The receiving host MUST discard this packet.
The receiving host SHOULD send an ICMP packet, with the type
Parameter Problem, to inform the sender that the HIP
association does not exist (see Section 5.4), and it MAY
initiate a new HIP BEX. However, responding with these
optional mechanisms is implementation or policy dependent. If
the sending application doesn't expect a response, the system
could possibly send a large number of packets in this state, so
for this reason, the sending of one or more ICMP packets is
RECOMMENDED. However, any such responses MUST be rate-limited
to prevent abuse (see Section 5.4).
4.4. HIP State Machine
HIP itself has little state. In the HIP base exchange, there is an
Initiator and a Responder. Once the security associations (SAs) are
established, this distinction is lost. If the HIP state needs to be
re-established, the controlling parameters are which peer still has
state and which has a datagram to send to its peer. The following
state machine attempts to capture these processes.
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The state machine is symmetric and is presented in a single system
view, representing either an Initiator or a Responder. The state
machine is not a full representation of the processing logic.
Additional processing rules are presented in the packet definitions.
Hence, both are needed to completely implement HIP.
This document extends the state machine as defined in [RFC 5201] and
introduces a restart option to allow for the negotiation of
cryptographic algorithms. The extension to the previous state
machine in [RFC 5201] is a transition from state I1-SENT back again to
I1-SENT; namely, the restart option. An Initiator is required to
restart the HIP base exchange if the Responder does not support the
HIT Suite of the Initiator. In this case, the Initiator restarts the
HIP base exchange by sending a new I1 packet with a source HIT
supported by the Responder.
Implementors must understand that the state machine, as described
here, is informational. Specific implementations are free to
implement the actual processing logic differently. Section 6
describes the packet processing rules in more detail. This state
machine focuses on the HIP I1, R1, I2, and R2 packets only. New
states and state transitions may be introduced by mechanisms in other
specifications (such as mobility and multihoming).
4.4.1. State Machine Terminology
Unused Association Lifetime (UAL): Implementation-specific time for
which, if no packet is sent or received for this time interval, a
host MAY begin to tear down an active HIP association.
Maximum Segment Lifetime (MSL): Maximum time that a HIP packet is
expected to spend in the network. A default value of 2 minutes
has been borrowed from [RFC 793] because it is a prevailing
assumption for packet lifetimes.
Exchange Complete (EC): Time that the host spends at the R2-SENT
state before it moves to the ESTABLISHED state. The time is n *
I2 retransmission timeout, where n is about I2_RETRIES_MAX.
Receive ANYOTHER: Any received packet for which no state transitions
or processing rules are defined for a given state.
Moskowitz, et al. Standards Track PAGE 26
RFC 7401 HIPv2 April 2015
4.4.2. HIP States
+---------------------+---------------------------------------------+
| State | Explanation |
+---------------------+---------------------------------------------+
| UNASSOCIATED | State machine start |
| | |
| I1-SENT | Initiating base exchange |
| | |
| I2-SENT | Waiting to complete base exchange |
| | |
| R2-SENT | Waiting to complete base exchange |
| | |
| ESTABLISHED | HIP association established |
| | |
| CLOSING | HIP association closing, no data can be |
| | sent |
| | |
| CLOSED | HIP association closed, no data can be sent |
| | |
| E-FAILED | HIP base exchange failed |
+---------------------+---------------------------------------------+
Table 1: HIP States
Moskowitz, et al. Standards Track PAGE 27
RFC 7401 HIPv2 April 2015
4.4.3. HIP State Processes
System behavior in state UNASSOCIATED, Table 2.
+----------------------------+--------------------------------------+
| Trigger | Action |
+----------------------------+--------------------------------------+
| User data to send, | Send I1 and go to I1-SENT |
| requiring a new HIP | |
| association | |
| | |
| Receive I1 | Send R1 and stay at UNASSOCIATED |
| | |
| Receive I2, process | If successful, send R2 and go to |
| | R2-SENT |
| | |
| | If fail, stay at UNASSOCIATED |
| | |
| Receive user data for an | Optionally send ICMP as defined in |
| unknown HIP association | Section 5.4 and stay at UNASSOCIATED |
| | |
| Receive CLOSE | Optionally send ICMP Parameter |
| | Problem and stay at UNASSOCIATED |
| | |
| Receive ANYOTHER | Drop and stay at UNASSOCIATED |
+----------------------------+--------------------------------------+
Table 2: UNASSOCIATED - Start State
Moskowitz, et al. Standards Track PAGE 28
RFC 7401 HIPv2 April 2015
System behavior in state I1-SENT, Table 3.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 from | If the local HIT is smaller than the peer |
| Responder | HIT, drop I1 and stay at I1-SENT (see |
| | Section 6.5 for HIT comparison) |
| | |
| | If the local HIT is greater than the peer |
| | HIT, send R1 and stay at I1-SENT |
| | |
| Receive I2, process | If successful, send R2 and go to R2-SENT |
| | |
| | If fail, stay at I1-SENT |
| | |
| Receive R1, process | If the HIT Suite of the local HIT is not |
| | supported by the peer, select supported |
| | local HIT, send I1, and stay at I1-SENT |
| | |
| | If successful, send I2 and go to I2-SENT |
| | |
| | If fail, stay at I1-SENT |
| | |
| Receive ANYOTHER | Drop and stay at I1-SENT |
| | |
| Timeout | Increment trial counter |
| | |
| | If counter is less than I1_RETRIES_MAX, |
| | send I1 and stay at I1-SENT |
| | |
| | If counter is greater than I1_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 3: I1-SENT - Initiating the HIP Base Exchange
Moskowitz, et al. Standards Track PAGE 29
RFC 7401 HIPv2 April 2015
System behavior in state I2-SENT, Table 4.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay at I2-SENT |
| | |
| Receive R1, process | If successful, send I2 and stay at I2-SENT |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive I2, process | If successful and local HIT is smaller than |
| | the peer HIT, drop I2 and stay at I2-SENT |
| | |
| | If successful and local HIT is greater than |
| | the peer HIT, send R2 and go to R2-SENT |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive R2, process | If successful, go to ESTABLISHED |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive CLOSE, | If successful, send CLOSE_ACK and go to |
| process | CLOSED |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive ANYOTHER | Drop and stay at I2-SENT |
| | |
| Timeout | Increment trial counter |
| | |
| | If counter is less than I2_RETRIES_MAX, |
| | send I2 and stay at I2-SENT |
| | |
| | If counter is greater than I2_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 4: I2-SENT - Waiting to Finish the HIP Base Exchange
Moskowitz, et al. Standards Track PAGE 30
RFC 7401 HIPv2 April 2015
System behavior in state R2-SENT, Table 5.
+------------------------+------------------------------------------+
| Trigger | Action |
+------------------------+------------------------------------------+
| Receive I1 | Send R1 and stay at R2-SENT |
| | |
| Receive I2, process | If successful, send R2 and stay at |
| | R2-SENT |
| | |
| | If fail, stay at R2-SENT |
| | |
| Receive R1 | Drop and stay at R2-SENT |
| | |
| Receive R2 | Drop and stay at R2-SENT |
| | |
| Receive data or UPDATE | Move to ESTABLISHED |
| | |
| Exchange Complete | Move to ESTABLISHED |
| Timeout | |
| | |
| Receive CLOSE, process | If successful, send CLOSE_ACK and go to |
| | CLOSED |
| | |
| | If fail, stay at ESTABLISHED |
| | |
| Receive CLOSE_ACK | Drop and stay at R2-SENT |
| | |
| Receive NOTIFY | Process and stay at R2-SENT |
+------------------------+------------------------------------------+
Table 5: R2-SENT - Waiting to Finish HIP
Moskowitz, et al. Standards Track PAGE 31
RFC 7401 HIPv2 April 2015
System behavior in state ESTABLISHED, Table 6.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay at ESTABLISHED |
| | |
| Receive I2 | Process with puzzle and possible Opaque |
| | data verification |
| | |
| | If successful, send R2, drop old HIP |
| | association, establish a new HIP |
| | association, and go to R2-SENT |
| | |
| | If fail, stay at ESTABLISHED |
| | |
| Receive R1 | Drop and stay at ESTABLISHED |
| | |
| Receive R2 | Drop and stay at ESTABLISHED |
| | |
| Receive user data | Process and stay at ESTABLISHED |
| for HIP association | |
| | |
| No packet | Send CLOSE and go to CLOSING |
| sent/received | |
| during UAL minutes | |
| | |
| Receive UPDATE | Process and stay at ESTABLISHED |
| | |
| Receive CLOSE, | If successful, send CLOSE_ACK and go to |
| process | CLOSED |
| | |
| | If fail, stay at ESTABLISHED |
| | |
| Receive CLOSE_ACK | Drop and stay at ESTABLISHED |
| | |
| Receive NOTIFY | Process and stay at ESTABLISHED |
+---------------------+---------------------------------------------+
Table 6: ESTABLISHED - HIP Association Established
Moskowitz, et al. Standards Track PAGE 32
RFC 7401 HIPv2 April 2015
System behavior in state CLOSING, Table 7.
+----------------------------+--------------------------------------+
| Trigger | Action |
+----------------------------+--------------------------------------+
| User data to send, | Send I1 and go to I1-SENT |
| requires the creation of | |
| another incarnation of the | |
| HIP association | |
| | |
| Receive I1 | Send R1 and stay at CLOSING |
| | |
| Receive I2, process | If successful, send R2 and go to |
| | R2-SENT |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive R1, process | If successful, send I2 and go to |
| | I2-SENT |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive CLOSE, process | If successful, send CLOSE_ACK, |
| | discard state, and go to CLOSED |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive CLOSE_ACK, process | If successful, discard state and go |
| | to UNASSOCIATED |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive ANYOTHER | Drop and stay at CLOSING |
| | |
| Timeout | Increment timeout sum and reset |
| | timer. If timeout sum is less than |
| | UAL+MSL minutes, retransmit CLOSE |
| | and stay at CLOSING. |
| | |
| | If timeout sum is greater than |
| | UAL+MSL minutes, go to UNASSOCIATED |
+----------------------------+--------------------------------------+
Table 7: CLOSING - HIP Association Has Not Been Used for UAL Minutes
Moskowitz, et al. Standards Track PAGE 33
RFC 7401 HIPv2 April 2015
System behavior in state CLOSED, Table 8.
+----------------------------------------+--------------------------+
| Trigger | Action |
+----------------------------------------+--------------------------+
| Datagram to send, requires the | Send I1 and stay at |
| creation of another incarnation of the | CLOSED |
| HIP association | |
| | |
| Receive I1 | Send R1 and stay at |
| | CLOSED |
| | |
| Receive I2, process | If successful, send R2 |
| | and go to R2-SENT |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive R1, process | If successful, send I2 |
| | and go to I2-SENT |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive CLOSE, process | If successful, send |
| | CLOSE_ACK and stay at |
| | CLOSED |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive CLOSE_ACK, process | If successful, discard |
| | state and go to |
| | UNASSOCIATED |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive ANYOTHER | Drop and stay at CLOSED |
| | |
| Timeout (UAL+2MSL) | Discard state and go to |
| | UNASSOCIATED |
+----------------------------------------+--------------------------+
Table 8: CLOSED - CLOSE_ACK Sent, Resending CLOSE_ACK if Necessary
Moskowitz, et al. Standards Track PAGE 34
RFC 7401 HIPv2 April 2015
System behavior in state E-FAILED, Table 9.
+-------------------------+-----------------------------------------+
| Trigger | Action |
+-------------------------+-----------------------------------------+
| Wait for | Go to UNASSOCIATED. Renegotiation is |
| implementation-specific | possible after moving to UNASSOCIATED |
| time | state. |
+-------------------------+-----------------------------------------+
Table 9: E-FAILED - HIP Failed to Establish Association with Peer
4.4.4. Simplified HIP State Diagram
The following diagram (Figure 2) shows the major state transitions.
Transitions based on received packets implicitly assume that the
packets are successfully authenticated or processed.
Moskowitz, et al. Standards Track PAGE 35
RFC 7401 HIPv2 April 2015
+--+ +----------------------------+
recv I1, send R1 | | | |
| v v |
+--------------+ recv I2, send R2 |
+----------------| UNASSOCIATED |----------------+ |
datagram | +--+ +--------------+ | |
to send, | | | Alg. not supported, | |
send I1 | | | send I1 | |
. v | v | |
. +---------+ recv I2, send R2 | |
+---->| I1-SENT |--------------------------------------+ | |
| +---------+ +----------------------+ | | |
| | recv R2, | recv I2, send R2 | | | |
| v send I2 | v v v |
| +---------+ | +---------+ |
| +--->| I2-SENT |----------+ +--------------| R2-SENT |<---+ |
| | +---------+ | +---------+ | |
| | | |recv R2 | data or| | |
| |recv R1, | | | EC timeout| | |
| |send I2 +--|-----------------+ | receive I2,| |
| | | | +-------------+ | send R2| |
| | | +------>| ESTABLISHED |<----------+ | |
| | | +-------------+ | |
| | | | | | receive I2, send R2 | |
| | +------------+ | +-------------------------------+ |
| | | +-----------+ | |
| | | no packet sent/received| +---+ | |
| | | for UAL min, send CLOSE| | |timeout | |
| | | v v |(UAL+MSL) | |
| | | +---------+ |retransmit | |
+--|----------|------------------------| CLOSING |-+CLOSE | |
| | +---------+ | |
| | | | | | | |
+----------|-------------------------+ | | +----------------+ |
| | +-----------+ +------------------|--+
| | |recv CLOSE, recv CLOSE_ACK | |
| +-------------+ |send CLOSE_ACK or timeout | |
| recv CLOSE, | | (UAL+MSL) | |
| send CLOSE_ACK v v | |
| +--------+ receive I2, send R2 | |
+---------------------| CLOSED |------------------------------+ |
+--------+ |
^ | | |
recv CLOSE, send CLOSE_ACK| | | timeout (UAL+2MSL) |
+-+ +------------------------------------+
Figure 2
Moskowitz, et al. Standards Track PAGE 36
RFC 7401 HIPv2 April 2015
4.5. User Data Considerations
4.5.1. TCP and UDP Pseudo Header Computation for User Data
When computing TCP and UDP checksums on user data packets that flow
through sockets bound to HITs, the IPv6 pseudo header format
[RFC 2460] MUST be used, even if the actual addresses in the header of
the packet are IPv4 addresses. Additionally, the HITs MUST be used
in place of the IPv6 addresses in the IPv6 pseudo header. Note that
the pseudo header for actual HIP payloads is computed differently;
see Section 5.1.1.
4.5.2. Sending Data on HIP Packets
Other documents may define how to include user data in various HIP
packets. However, currently the HIP header is a terminal header, and
not followed by any other headers.
4.5.3. Transport Formats
The actual data transmission format, used for user data after the HIP
base exchange, is not defined in this document. Such transport
formats and methods are described in separate specifications. All
HIP implementations MUST implement, at minimum, the ESP transport
format for HIP [RFC 7402]. The transport format to be chosen is
negotiated in the base exchange. The Responder expresses its
preference regarding the transport format in the
TRANSPORT_FORMAT_LIST in the R1 packet, and the Initiator selects one
transport format and adds the respective HIP parameter to the I2
packet.
4.5.4. Reboot, Timeout, and Restart of HIP
Simulating a loss of state is a potential DoS attack. The following
process has been crafted to manage state recovery without presenting
a DoS opportunity.
If a host reboots or the HIP association times out, it has lost its
HIP state. If the host that lost state has a datagram to send to the
peer, it simply restarts the HIP base exchange. After the base
exchange has completed, the Initiator can create a new payload
association and start sending data. The peer does not reset its
state until it receives a valid I2 packet.
If a system receives a user data packet that cannot be matched to any
existing HIP association, it is possible that it has lost the state
and its peer has not. It MAY send an ICMP packet with the Parameter
Problem type, and with the Pointer pointing to the referred
Moskowitz, et al. Standards Track PAGE 37
RFC 7401 HIPv2 April 2015
HIP-related association information. Reacting to such traffic
depends on the implementation and the environment where the
implementation is used.
If the host that apparently has lost its state decides to restart the
HIP base exchange, it sends an I1 packet to the peer. After the base
exchange has been completed successfully, the Initiator can create a
new HIP association, and the peer drops its old payload associations
and creates a new one.
4.6. Certificate Distribution
This document does not define how to use certificates or how to
transfer them between hosts. These functions are expected to be
defined in a future specification, as was done for HIP version 1 (see
[RFC 6253]). A parameter type value, meant to be used for carrying
certificates, is reserved, though: CERT, Type 768; see Section 5.2.
5. Packet Formats
5.1. Payload Format
All HIP packets start with a fixed header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Header Length |0| Packet Type |Version| RES.|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Controls |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ HIP Parameters /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Moskowitz, et al. Standards Track PAGE 38
RFC 7401 HIPv2 April 2015
The HIP header is logically an IPv6 extension header. However, this
document does not describe processing for Next Header values other
than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
Future documents MAY define behavior for other values. However,
current implementations MUST ignore trailing data if an unimplemented
Next Header value is received.
The Header Length field contains the combined length of the HIP
Header and HIP parameters in 8-byte units, excluding the first
8 bytes. Since all HIP headers MUST contain the sender's and
receiver's HIT fields, the minimum value for this field is 4, and
conversely, the maximum length of the HIP Parameters field is
(255 * 8) - 32 = 2008 bytes (see Section 5.1.3 regarding HIP
fragmentation). Note: this sets an additional limit for sizes of
parameters included in the Parameters field, independent of the
individual parameter maximum lengths.
The Packet Type indicates the HIP packet type. The individual packet
types are defined in the relevant sections. If a HIP host receives a
HIP packet that contains an unrecognized packet type, it MUST drop
the packet.
The HIP Version field is four bits. The version defined in this
document is 2. The version number is expected to be incremented only
if there are incompatible changes to the protocol. Most extensions
can be handled by defining new packet types, new parameter types, or
new Controls (see Section 5.1.2).
The following three bits are reserved for future use. They MUST be
zero when sent, and they MUST be ignored when handling a received
packet.
The two fixed bits in the header are reserved for SHIM6 compatibility
[RFC 5533], Section 5.3. For implementations adhering (only) to this
specification, they MUST be set as shown when sending and MUST be
ignored when receiving. This is to ensure optimal forward
compatibility. Note that for implementations that implement other
compatible specifications in addition to this specification, the
corresponding rules may well be different. For example, an
implementation that implements both this specification and the SHIM6
protocol may need to check these bits in order to determine how to
handle the packet.
The HIT fields are always 128 bits (16 bytes) long.
Moskowitz, et al. Standards Track PAGE 39
RFC 7401 HIPv2 April 2015
5.1.1. Checksum
Since the checksum covers the source and destination addresses in the
IP header, it MUST be recomputed on HIP-aware NAT devices.
If IPv6 is used to carry the HIP packet, the pseudo header [RFC 2460]
contains the source and destination IPv6 addresses, HIP packet length
in the pseudo header Length field, a zero field, and the HIP protocol
number (see Section 5.1) in the Next Header field. The Length field
is in bytes and can be calculated from the HIP Header Length field:
(HIP Header Length + 1) * 8.
In case of using IPv4, the IPv4 UDP pseudo header format [RFC 768] is
used. In the pseudo header, the source and destination addresses are
those used in the IP header, the zero field is obviously zero, the
protocol is the HIP protocol number (see Section 4), and the length
is calculated as in the IPv6 case.
5.1.2. HIP Controls
The HIP Controls field conveys information about the structure of the
packet and capabilities of the host.
The following fields have been defined:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | | | | | |A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A - Anonymous: If this is set, the sender's HI in this packet is
anonymous, i.e., one not listed in a directory. Anonymous HIs
SHOULD NOT be stored. This control is set in packets using
anonymous sender HIs. The peer receiving an anonymous HI in an R1
or I2 may choose to refuse it.
The rest of the fields are reserved for future use, and MUST be set
to zero in sent packets and MUST be ignored in received packets.
5.1.3. HIP Fragmentation Support
A HIP implementation MUST support IP fragmentation/reassembly.
Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
fragment generation is REQUIRED to be implemented in IPv4 (IPv4
stacks and networks will usually do this by default) and RECOMMENDED
to be implemented in IPv6. In IPv6 networks, the minimum MTU is
larger, 1280 bytes, than in IPv4 networks. The larger MTU size is
usually sufficient for most HIP packets, and therefore fragment
Moskowitz, et al. Standards Track PAGE 40
RFC 7401 HIPv2 April 2015
generation may not be needed. If it is expected that a host will
send HIP packets that are larger than the minimum IPv6 MTU, the
implementation MUST implement fragment generation even for IPv6.
In IPv4 networks, HIP packets may encounter low MTUs along their
routed path. Since basic HIP, as defined in this document, does not
provide a mechanism to use multiple IP datagrams for a single HIP
packet, support for path MTU discovery does not bring any value to
HIP in IPv4 networks. HIP-aware NAT devices SHOULD perform IPv4
reassembly/fragmentation for HIP packets.
All HIP implementations have to be careful while employing a
reassembly algorithm so that the algorithm is sufficiently resistant
to DoS attacks.
Certificate chains can cause the packet to be fragmented, and
fragmentation can open implementations to denial-of-service attacks
[KAU03]. "Hash and URL" schemes as defined in [RFC 6253] for HIP
version 1 may be used to avoid fragmentation and mitigate resulting
DoS attacks.
5.2. HIP Parameters
The HIP parameters carry information that is necessary for
establishing and maintaining a HIP association. For example, the
peer's public keys as well as the signaling for negotiating ciphers
and payload handling are encapsulated in HIP parameters. Additional
information, meaningful for end hosts or middleboxes, may also be
included in HIP parameters. The specification of the HIP parameters
and their mapping to HIP packets and packet types is flexible to
allow HIP extensions to define new parameters and new protocol
behavior.
In HIP packets, HIP parameters are ordered according to their numeric
type number and encoded in TLV format.
Moskowitz, et al. Standards Track PAGE 41
RFC 7401 HIPv2 April 2015
The following parameter types are currently defined.
+------------------------+-------+-----------+----------------------+
| TLV | Type | Length | Data |
+------------------------+-------+-----------+----------------------+
| R1_COUNTER | 129 | 12 | Puzzle generation |
| | | | counter |
| | | | |
| PUZZLE | 257 | 12 | #K and Random #I |
| | | | |
| SOLUTION | 321 | 20 | #K, Random #I and |
| | | | puzzle solution #J |
| | | | |
| SEQ | 385 | 4 | UPDATE packet ID |
| | | | number |
| | | | |
| ACK | 449 | variable | UPDATE packet ID |
| | | | number |
| | | | |
| DH_GROUP_LIST | 511 | variable | Ordered list of DH |
| | | | Group IDs supported |
| | | | by a host |
| | | | |
| DIFFIE_HELLMAN | 513 | variable | public key |
| | | | |
| HIP_CIPHER | 579 | variable | List of HIP |
| | | | encryption |
| | | | algorithms |
| | | | |
| ENCRYPTED | 641 | variable | Encrypted part of a |
| | | | HIP packet |
| | | | |
| HOST_ID | 705 | variable | Host Identity with |
| | | | Fully Qualified |
| | | | Domain Name (FQDN) |
| | | | or Network Access |
| | | | Identifier (NAI) |
| | | | |
| HIT_SUITE_LIST | 715 | variable | Ordered list of the |
| | | | HIT Suites supported |
| | | | by the Responder |
| | | | |
| CERT | 768 | variable | HI Certificate; used |
| | | | to transfer |
| | | | certificates. |
| | | | Specified in a |
| | | | separate document. |
| | | | |
Moskowitz, et al. Standards Track PAGE 42
RFC 7401 HIPv2 April 2015
| NOTIFICATION | 832 | variable | Informational data |
| | | | |
| ECHO_REQUEST_SIGNED | 897 | variable | Opaque data to be |
| | | | echoed back; signed |
| | | | |
| ECHO_RESPONSE_SIGNED | 961 | variable | Opaque data echoed |
| | | | back by request; |
| | | | signed |
| | | | |
| TRANSPORT_FORMAT_LIST | 2049 | Ordered | variable |
| | | list of | |
| | | preferred | |
| | | HIP | |
| | | transport | |
| | | type | |
| | | numbers | |
| | | | |
| HIP_MAC | 61505 | variable | HMAC-based message |
| | | | authentication code, |
| | | | with key material |
| | | | from KEYMAT |
| | | | |
| HIP_MAC_2 | 61569 | variable | HMAC-based message |
| | | | authentication code, |
| | | | with key material |
| | | | from KEYMAT. Unlike |
| | | | HIP_MAC, the HOST_ID |
| | | | parameter is |
| | | | included in |
| | | | HIP_MAC_2 |
| | | | calculation. |
| | | | |
| HIP_SIGNATURE_2 | 61633 | variable | Signature used in R1 |
| | | | packet |
| | | | |
| HIP_SIGNATURE | 61697 | variable | Signature of the |
| | | | packet |
| | | | |
| ECHO_REQUEST_UNSIGNED | 63661 | variable | Opaque data to be |
| | | | echoed back; after |
| | | | signature |
| | | | |
| ECHO_RESPONSE_UNSIGNED | 63425 | variable | Opaque data echoed |
| | | | back by request; |
| | | | after signature |
+------------------------+-------+-----------+----------------------+
Moskowitz, et al. Standards Track PAGE 43
RFC 7401 HIPv2 April 2015
As the ordering (from lowest to highest) of HIP parameters is
strictly enforced (see Section 5.2.1), the parameter type values for
existing parameters have been spaced to allow for future protocol
extensions.
The following parameter type number ranges are defined.
+---------------+---------------------------------------------------+
| Type Range | Purpose |
+---------------+---------------------------------------------------+
| 0 - 1023 | Handshake |
| | |
| 1024 - 2047 | Reserved |
| | |
| 2048 - 4095 | Parameters related to HIP transport formats |
| | |
| 4096 - 8191 | Signed parameters allocated through specification |
| | documents |
| | |
| 8192 - 32767 | Reserved |
| | |
| 32768 - 49151 | Reserved for Private Use. Signed parameters. |
| | |
| 49152 - 61439 | Reserved |
| | |
| 61440 - 62463 | Signatures and (signed) MACs |
| | |
| 62464 - 63487 | Parameters that are neither signed nor MACed |
| | |
| 63488 - 64511 | Rendezvous and relaying |
| | |
| 64512 - 65023 | Parameters that are neither signed nor MACed |
| | |
| 65024 - 65535 | Reserved |
+---------------+---------------------------------------------------+
The process for defining new parameters is described in Section 5.2.2
of this document.
The range between 32768 (2^15) and 49151 (2^15 + 2^14) is Reserved
for Private Use. Types from this range SHOULD be selected in a
random fashion to reduce the probability of collisions.
5.2.1. TLV Format
The TLV-encoded parameters are described in the following
subsections. The Type field value also describes the order of these
fields in the packet. The parameters MUST be included in the packet
Moskowitz, et al. Standards Track PAGE 44
RFC 7401 HIPv2 April 2015
so that their types form an increasing order. If multiple parameters
with the same type number are in one packet, the parameters with the
same type MUST be consecutive in the packet. If the order does not
follow this rule, the packet is considered to be malformed and it
MUST be discarded.
Parameters using type values from 2048 up to 4095 are related to
transport formats. Currently, one transport format is defined: the
ESP transport format [RFC 7402].
All of the encoded TLV parameters have a length (that includes the
Type and Length fields), which is a multiple of 8 bytes. When
needed, padding MUST be added to the end of the parameter so that the
total length is a multiple of 8 bytes. This rule ensures proper
alignment of data. Any added padding bytes MUST be zeroed by the
sender, and their values SHOULD NOT be checked by the receiver.
The Length field indicates the length of the Contents field (in
bytes). Consequently, the total length of the TLV parameter
(including Type, Length, Contents, and Padding) is related to the
Length field according to the following formula:
Total Length = 11 + Length - (Length + 3) % 8;
where % is the modulo operator.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |C| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Contents /
/ +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Type code for the parameter. 16 bits long, C-bit
being part of the Type code.
C Critical. One if this parameter is critical and
MUST be recognized by the recipient, zero otherwise.
The C-bit is considered to be a part of the Type
field. Consequently, critical parameters are always
odd, and non-critical ones have an even value.
Length Length of the Contents, in bytes, excluding Type,
Length, and Padding
Contents Parameter specific, defined by Type
Padding Padding, 0-7 bytes, added if needed
Moskowitz, et al. Standards Track PAGE 45
RFC 7401 HIPv2 April 2015
Critical parameters (indicated by the odd type number value) MUST be
recognized by the recipient. If a recipient encounters a critical
parameter that it does not recognize, it MUST NOT process the packet
any further. It MAY send an ICMP or NOTIFY, as defined in
Section 4.3.
Non-critical parameters MAY be safely ignored. If a recipient
encounters a non-critical parameter that it does not recognize, it
SHOULD proceed as if the parameter was not present in the received
packet.
5.2.2. Defining New Parameters
Future specifications may define new parameters as needed. When
defining new parameters, care must be taken to ensure that the
parameter type values are appropriate and leave suitable space for
other future extensions. One must remember that the parameters MUST
always be arranged in numerically increasing order by Type code,
thereby limiting the order of parameters (see Section 5.2.1).
The following rules MUST be followed when defining new parameters.
1. The low-order bit C of the Type code is used to distinguish
between critical and non-critical parameters. Hence, even
parameter type numbers indicate non-critical parameters while odd
parameter type numbers indicate critical parameters.
2. A new parameter MAY be critical only if an old implementation
that ignored it would cause security problems. In general, new
parameters SHOULD be defined as non-critical, and expect a reply
from the recipient.
3. If a system implements a new critical parameter, it MUST provide
the ability to set the associated feature off, such that the
critical parameter is not sent at all. The configuration option
MUST be well documented. Implementations operating in a mode
adhering to this specification MUST disable the sending of new
critical parameters by default. In other words, the management
interface MUST allow vanilla standards-only mode as a default
configuration setting, and MAY allow new critical payloads to be
configured on (and off).
4. See Section 9 for allocation rules regarding Type codes.
Moskowitz, et al. Standards Track PAGE 46
RFC 7401 HIPv2 April 2015
5.2.3. R1_COUNTER
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R1 generation counter, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 129
Length 12
R1 generation
counter The current generation of valid puzzles
The R1_COUNTER parameter contains a 64-bit unsigned integer in
network byte order, indicating the current generation of valid
puzzles. The sender SHOULD increment this counter periodically. It
is RECOMMENDED that the counter value is incremented at least as
often as old PUZZLE values are deprecated so that SOLUTIONs to them
are no longer accepted.
Support for the R1_COUNTER parameter is mandatory, although its
inclusion in the R1 packet is optional. It SHOULD be included in the
R1 (in which case it is covered by the signature), and if present in
the R1, it MUST be echoed (including the Reserved field verbatim) by
the Initiator in the I2 packet.
Moskowitz, et al. Standards Track PAGE 47
RFC 7401 HIPv2 April 2015
5.2.4. PUZZLE
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| #K, 1 byte | Lifetime | Opaque, 2 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random #I, RHASH_len / 8 bytes |
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 257
Length 4 + RHASH_len / 8
#K #K is the number of verified bits
Lifetime puzzle lifetime 2^(value - 32) seconds
Opaque data set by the Responder, indexing the puzzle
Random #I random number of size RHASH_len bits
Random #I is represented as an n-bit integer (where n is RHASH_len),
and #K and Lifetime as 8-bit integers, all in network byte order.
The PUZZLE parameter contains the puzzle difficulty #K and an n-bit
random integer #I. The Puzzle Lifetime indicates the time during
which the puzzle solution is valid, and sets a time limit that should
not be exceeded by the Initiator while it attempts to solve the
puzzle. The lifetime is indicated as a power of 2 using the formula
2^(Lifetime - 32) seconds. A puzzle MAY be augmented with an
ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in
the R1; the contents of the echo request are then echoed back in the
ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED parameter,
allowing the Responder to use the included information as a part of
its puzzle processing.
The Opaque and Random #I fields are not covered by the
HIP_SIGNATURE_2 parameter.
Moskowitz, et al. Standards Track PAGE 48
RFC 7401 HIPv2 April 2015
5.2.5. SOLUTION
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| #K, 1 byte | Reserved | Opaque, 2 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random #I, n bytes |
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Puzzle solution #J, RHASH_len / 8 bytes |
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 321
Length 4 + RHASH_len / 4
#K #K is the number of verified bits
Reserved zero when sent, ignored when received
Opaque copied unmodified from the received PUZZLE
parameter
Random #I random number of size RHASH_len bits
Puzzle solution #J random number of size RHASH_len bits
Random #I and Random #J are represented as n-bit unsigned integers
(where n is RHASH_len), and #K as an 8-bit unsigned integer, all in
network byte order.
The SOLUTION parameter contains a solution to a puzzle. It also
echoes back the random difficulty #K, the Opaque field, and the
puzzle integer #I.
Moskowitz, et al. Standards Track PAGE 49
RFC 7401 HIPv2 April 2015
5.2.6. DH_GROUP_LIST
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DH GROUP ID #1| DH GROUP ID #2| DH GROUP ID #3| DH GROUP ID #4|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DH GROUP ID #n| Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 511
Length number of DH Group IDs
DH GROUP ID identifies a DH GROUP ID supported by the host.
The list of IDs is ordered by preference of the
host. The possible DH Group IDs are defined
in the DIFFIE_HELLMAN parameter. Each DH
Group ID is one octet long.
The DH_GROUP_LIST parameter contains the list of supported DH Group
IDs of a host. The Initiator sends the DH_GROUP_LIST in the I1
packet, and the Responder sends its own list in the signed part of
the R1 packet. The DH Group IDs in the DH_GROUP_LIST are listed in
the order of their preference of the host sending the list. DH Group
IDs that are listed first are preferred over the DH Group IDs listed
later. The information in the DH_GROUP_LIST allows the Responder to
select the DH group preferred by itself and supported by the
Initiator. Based on the DH_GROUP_LIST in the R1 packet, the
Initiator can determine if the Responder has selected the best
possible choice based on the Initiator's and Responder's preferences.
If the Responder's choice differs from the best choice, the Initiator
can conclude that there was an attempted downgrade attack (see
Section 4.1.7).
When selecting the DH group for the DIFFIE_HELLMAN parameter in the
R1 packet, the Responder MUST select the first DH Group ID in its
DH_GROUP_LIST in the R1 packet that is compatible with one of the
Suite IDs in the Initiator's DH_GROUP_LIST in the I1 packet. The
Responder MUST NOT select any other DH Group ID that is contained in
both lists, because then a downgrade attack cannot be detected.
In general, hosts SHOULD prefer stronger groups over weaker ones if
the computation overhead is not prohibitively high for the intended
application.
Moskowitz, et al. Standards Track PAGE 50
RFC 7401 HIPv2 April 2015
5.2.7. DIFFIE_HELLMAN
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID | Public Value Length | Public Value /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 513
Length length in octets, excluding Type, Length, and
Padding
Group ID identifies values for p and g as well as the KDF
Public Value length of the following Public Value in octets
Length
Public Value the sender's public Diffie-Hellman key
A single DIFFIE_HELLMAN parameter may be included in selected HIP
packets based on the DH Group ID selected (Section 5.2.6). The
following Group IDs have been defined; values are assigned by this
document:
Group KDF Value
Reserved 0
DEPRECATED 1
DEPRECATED 2
1536-bit MODP group [RFC 3526] HKDF [RFC 5869] 3
3072-bit MODP group [RFC 3526] HKDF [RFC 5869] 4
DEPRECATED 5
DEPRECATED 6
NIST P-256 [RFC 5903] HKDF [RFC 5869] 7
NIST P-384 [RFC 5903] HKDF [RFC 5869] 8
NIST P-521 [RFC 5903] HKDF [RFC 5869] 9
SECP160R1 [SECG] HKDF [RFC 5869] 10
2048-bit MODP group [RFC 3526] HKDF [RFC 5869] 11
The MODP Diffie-Hellman groups are defined in [RFC 3526]. ECDH
groups 7-9 are defined in [RFC 5903] and [RFC 6090]. ECDH group 10
is covered in Appendix D. Any ECDH used with HIP MUST have a
co-factor of 1.
Moskowitz, et al. Standards Track PAGE 51
RFC 7401 HIPv2 April 2015
The Group ID also defines the key derivation function that is to be
used for deriving the symmetric keys for the HMAC and symmetric
encryption from the keying material from the Diffie-Hellman key
exchange (see Section 6.5).
A HIP implementation MUST implement Group ID 3. The 160-bit
SECP160R1 group can be used when lower security is enough (e.g., web
surfing) and when the equipment is not powerful enough (e.g., some
PDAs). Implementations SHOULD implement Group IDs 4 and 8.
To avoid unnecessary failures during the base exchange, the rest of
the groups SHOULD be implemented in hosts where resources are
adequate.
5.2.8. HIP_CIPHER
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cipher ID #1 | Cipher ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cipher ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 579
Length length in octets, excluding Type, Length, and
Padding
Cipher ID identifies the cipher algorithm to be used for
encrypting the contents of the ENCRYPTED parameter
Moskowitz, et al. Standards Track PAGE 52
RFC 7401 HIPv2 April 2015
The following Cipher IDs are defined:
Suite ID Value
RESERVED 0
NULL-ENCRYPT 1 ([RFC 2410])
AES-128-CBC 2 ([RFC 3602])
RESERVED 3 (unused value)
AES-256-CBC 4 ([RFC 3602])
The sender of a HIP_CIPHER parameter MUST make sure that there are no
more than six (6) Cipher IDs in one HIP_CIPHER parameter.
Conversely, a recipient MUST be prepared to handle received transport
parameters that contain more than six Cipher IDs by accepting the
first six Cipher IDs and dropping the rest. The limited number of
Cipher IDs sets the maximum size of the HIP_CIPHER parameter. As the
default configuration, the HIP_CIPHER parameter MUST contain at least
one of the mandatory Cipher IDs. There MAY be a configuration option
that allows the administrator to override this default.
The Responder lists supported and desired Cipher IDs in order of
preference in the R1, up to the maximum of six Cipher IDs. The
Initiator MUST choose only one of the corresponding Cipher IDs. This
Cipher ID will be used for generating the ENCRYPTED parameter.
Mandatory implementation: AES-128-CBC. Implementors SHOULD support
NULL-ENCRYPT for testing/debugging purposes but MUST NOT offer or
accept this value unless explicitly configured for testing/debugging
of HIP.
Moskowitz, et al. Standards Track PAGE 53
RFC 7401 HIPv2 April 2015
5.2.9. HOST_ID
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HI Length |DI-Type| DI Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Host Identity /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Domain Identifier /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 705
Length length in octets, excluding Type, Length, and
Padding
HI Length length of the Host Identity in octets
DI-Type type of the following Domain Identifier field
DI Length length of the Domain Identifier field in octets
Algorithm index to the employed algorithm
Host Identity actual Host Identity
Domain Identifier the identifier of the sender
The following DI-Types have been defined:
Type Value
none included 0
FQDN 1
NAI 2
FQDN Fully Qualified Domain Name, in binary format
NAI Network Access Identifier
The format for the FQDN is defined in RFC 1035 [RFC 1035],
Section 3.1. The format for the NAI is defined in [RFC 4282].
A host MAY optionally associate the Host Identity with a single
Domain Identifier in the HOST_ID parameter. If there is no Domain
Identifier, i.e., the DI-Type field is zero, the DI Length field is
set to zero as well.
Moskowitz, et al. Standards Track PAGE 54
RFC 7401 HIPv2 April 2015
The following HI Algorithms have been defined:
Algorithm profiles Values
RESERVED 0
DSA 3 [FIPS.186-4.2013] (RECOMMENDED)
RSA 5 [RFC 3447] (REQUIRED)
ECDSA 7 [RFC 4754] (REQUIRED)
ECDSA_LOW 9 [SECG] (RECOMMENDED)
For DSA, RSA, and ECDSA key types, profiles containing at least
112 bits of security strength (as defined by [NIST.800-131A.2011])
should be used. For RSA signature padding, the Probabilistic
Signature Scheme (PSS) method of padding [RFC 3447] MUST be used.
The Host Identity is derived from the DNSKEY format for RSA and DSA.
For these, the Public Key field of the RDATA part from RFC 4034
[RFC 4034] is used. For Elliptic Curve Cryptography (ECC), we
distinguish two different profiles: ECDSA and ECDSA_LOW. ECC
contains curves approved by NIST and defined in RFC 4754 [RFC 4754].
ECDSA_LOW is defined for devices with low computational capabilities
and uses shorter curves from the Standards for Efficient Cryptography
Group [SECG]. Any ECDSA used with HIP MUST have a co-factor of 1.
For ECDSA and ECDSA_LOW, Host Identities are represented by the
following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECC Curve | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Public Key |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ECC Curve Curve label
Public Key Represented in octet-string format [RFC 6090]
For hosts that implement ECDSA as the algorithm, the following ECC
curves are required:
Algorithm Curve Values
ECDSA RESERVED 0
ECDSA NIST P-256 1 [RFC 4754]
ECDSA NIST P-384 2 [RFC 4754]
Moskowitz, et al. Standards Track PAGE 55
RFC 7401 HIPv2 April 2015
For hosts that implement the ECDSA_LOW algorithm profile, the
following curve is required:
Algorithm Curve Values
ECDSA_LOW RESERVED 0
ECDSA_LOW SECP160R1 1 [SECG]
5.2.10. HIT_SUITE_LIST
The HIT_SUITE_LIST parameter contains a list of the supported HIT
Suite IDs of the Responder. The Responder sends the HIT_SUITE_LIST
in the signed part of the R1 packet. Based on the HIT_SUITE_LIST,
the Initiator can determine which source HIT Suite IDs are supported
by the Responder.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID #1 | ID #2 | ID #3 | ID #4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 715
Length number of HIT Suite IDs
ID identifies a HIT Suite ID supported by the host.
The list of IDs is ordered by preference of the
host. Each HIT Suite ID is one octet long. The
four higher-order bits of the ID field correspond
to the HIT Suite ID in the ORCHID OGA ID field. The
four lower-order bits are reserved and set to 0
by the sender. The reception of an ID with the
four lower-order bits not set to 0 SHOULD be
considered as an error that MAY result in a
NOTIFICATION of type UNSUPPORTED_HIT_SUITE.
The HIT Suite ID indexes a HIT Suite. HIT Suites are composed of
signature algorithms as defined in Section 5.2.9, and hash functions.
The ID field in the HIT_SUITE_LIST is defined as an eight-bit field,
as opposed to the four-bit HIT Suite ID and OGA ID field in the
ORCHID. This difference is a measure to accommodate larger HIT Suite
IDs if the 16 available values prove insufficient. In that case, one
of the 16 values, zero, will be used to indicate that four additional
bits of the ORCHID will be used to encode the HIT Suite ID. Hence,
Moskowitz, et al. Standards Track PAGE 56
RFC 7401 HIPv2 April 2015
the current four-bit HIT Suite IDs only use the four higher-order
bits in the ID field. Future documents may define the use of the
four lower-order bits in the ID field.
The following HIT Suite IDs are defined, and the relationship between
the four-bit ID value used in the OGA ID field and the eight-bit
encoding within the HIT_SUITE_LIST ID field is clarified:
HIT Suite Four-bit ID Eight-bit encoding
RESERVED 0 0x00
RSA,DSA/SHA-256 1 0x10 (REQUIRED)
ECDSA/SHA-384 2 0x20 (RECOMMENDED)
ECDSA_LOW/SHA-1 3 0x30 (RECOMMENDED)
The following table provides more detail on the above HIT Suite
combinations. The input for each generation algorithm is the
encoding of the HI as defined in Section 3.2. The output is 96 bits
long and is directly used in the ORCHID.
+-------+----------+--------------+------------+--------------------+
| Index | Hash | HMAC | Signature | Description |
| | function | | algorithm | |
| | | | family | |
+-------+----------+--------------+------------+--------------------+
| 0 | | | | Reserved |
| | | | | |
| 1 | SHA-256 | HMAC-SHA-256 | RSA, DSA | RSA or DSA HI |
| | | | | hashed with |
| | | | | SHA-256, truncated |
| | | | | to 96 bits |
| | | | | |
| 2 | SHA-384 | HMAC-SHA-384 | ECDSA | ECDSA HI hashed |
| | | | | with SHA-384, |
| | | | | truncated to 96 |
| | | | | bits |
| | | | | |
| 3 | SHA-1 | HMAC-SHA-1 | ECDSA_LOW | ECDSA_LOW HI |
| | | | | hashed with SHA-1, |
| | | | | truncated to 96 |
| | | | | bits |
+-------+----------+--------------+------------+--------------------+
Table 10: HIT Suites
Moskowitz, et al. Standards Track PAGE 57
RFC 7401 HIPv2 April 2015
The hash of the Responder as defined in the HIT Suite determines the
HMAC to be used for the RHASH function. The HMACs currently defined
here are HMAC-SHA-256 [RFC 4868], HMAC-SHA-384 [RFC 4868], and
HMAC-SHA-1 [RFC 2404].
5.2.11. TRANSPORT_FORMAT_LIST
The TRANSPORT_FORMAT_LIST parameter contains a list of the supported
HIP transport formats (TFs) of the Responder. The Responder sends
the TRANSPORT_FORMAT_LIST in the signed part of the R1 packet. Based
on the TRANSPORT_FORMAT_LIST, the Initiator chooses one suitable
transport format and includes the respective HIP transport format
parameter in its response packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TF type #1 | TF type #2 /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ TF type #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 2049
Length 2x number of TF types
TF Type identifies a transport format (TF) type supported
by the host. The TF type numbers correspond to
the HIP parameter type numbers of the respective
transport format parameters. The list of TF types
is ordered by preference of the sender.
The TF type numbers index the respective HIP parameters for the
transport formats in the type number range between 2050 and 4095.
The parameters and their use are defined in separate documents.
Currently, the only transport format defined is IPsec ESP [RFC 7402].
For each listed TF type, the sender of the TRANSPORT_FORMAT_LIST
parameter MUST include the respective transport format parameter in
the HIP packet. The receiver MUST ignore the TF type in the
TRANSPORT_FORMAT_LIST if no matching transport format parameter is
present in the packet.
Moskowitz, et al. Standards Track PAGE 58
RFC 7401 HIPv2 April 2015
5.2.12. HIP_MAC
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC |
/ /
/ +-------------------------------+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 61505
Length length in octets, excluding Type, Length, and
Padding
HMAC HMAC computed over the HIP packet, excluding the
HIP_MAC parameter and any following parameters,
such as HIP_SIGNATURE, HIP_SIGNATURE_2,
ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
The Checksum field MUST be set to zero, and the
HIP header length in the HIP common header MUST be
calculated not to cover any excluded parameters
when the HMAC is calculated. The size of the
HMAC is the natural size of the hash computation
output depending on the used hash function.
The HMAC uses RHASH as the hash algorithm. The calculation and
verification process is presented in Section 6.4.1.
5.2.13. HIP_MAC_2
HIP_MAC_2 is a MAC of the packet and the HI of the sender in the form
of a HOST_ID parameter when that parameter is not actually included
in the packet. The parameter structure is the same as the structure
shown in Section 5.2.12. The fields are as follows:
Type 61569
Length length in octets, excluding Type, Length, and
Padding
HMAC HMAC computed over the HIP packet, excluding the
HIP_MAC_2 parameter and any following parameters
such as HIP_SIGNATURE, HIP_SIGNATURE_2,
ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
and including an additional sender's HOST_ID
parameter during the HMAC calculation. The
Checksum field MUST be set to zero, and the HIP
Moskowitz, et al. Standards Track PAGE 59
RFC 7401 HIPv2 April 2015
header length in the HIP common header MUST be
calculated not to cover any excluded parameters
when the HMAC is calculated. The size of the
HMAC is the natural size of the hash computation
output depending on the used hash function.
The HMAC uses RHASH as the hash algorithm. The calculation and
verification process is presented in Section 6.4.1.
5.2.14. HIP_SIGNATURE
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIG alg | Signature /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 61697
Length length in octets, excluding Type, Length, and
Padding
SIG alg signature algorithm
Signature the signature is calculated over the HIP packet,
excluding the HIP_SIGNATURE parameter and any
parameters that follow the HIP_SIGNATURE
parameter. When the signature is calculated, the
Checksum field MUST be set to zero, and the HIP
header length in the HIP common header MUST be
calculated only up to the beginning of the
HIP_SIGNATURE parameter.
The signature algorithms are defined in Section 5.2.9. The signature
in the Signature field is encoded using the method depending on the
signature algorithm (e.g., according to [RFC 3110] in the case of RSA/
SHA-1, [RFC 5702] in the case of RSA/SHA-256, [RFC 2536] in the case of
DSA, or [RFC 6090] in the case of ECDSA).
HIP_SIGNATURE calculation and verification follow the process defined
in Section 6.4.2.
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RFC 7401 HIPv2 April 2015
5.2.15. HIP_SIGNATURE_2
HIP_SIGNATURE_2 excludes the variable parameters in the R1 packet to
allow R1 pre-creation. The parameter structure is the same as the
structure shown in Section 5.2.14. The fields are as follows:
Type 61633
Length length in octets, excluding Type, Length, and
Padding
SIG alg signature algorithm
Signature Within the R1 packet that contains the
HIP_SIGNATURE_2 parameter, the Initiator's HIT, the
Checksum field, and the Opaque and Random #I fields
in the PUZZLE parameter MUST be set to zero while
computing the HIP_SIGNATURE_2 signature. Further,
the HIP packet length in the HIP header MUST be
adjusted as if the HIP_SIGNATURE_2 was not in the
packet during the signature calculation, i.e., the
HIP packet length points to the beginning of
the HIP_SIGNATURE_2 parameter during signing and
verification.
Zeroing the Initiator's HIT makes it possible to create R1 packets
beforehand, to minimize the effects of possible DoS attacks. Zeroing
the Random #I and Opaque fields within the PUZZLE parameter allows
these fields to be populated dynamically on precomputed R1s.
Signature calculation and verification follow the process defined in
Section 6.4.2.
5.2.16. SEQ
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Update ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 385
Length 4
Update ID 32-bit sequence number
The Update ID is an unsigned number in network byte order,
initialized by a host to zero upon moving to ESTABLISHED state. The
Update ID has scope within a single HIP association, and not across
Moskowitz, et al. Standards Track PAGE 61
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multiple associations or multiple hosts. The Update ID is
incremented by one before each new UPDATE that is sent by the host;
the first UPDATE packet originated by a host has an Update ID of 0.
5.2.17. ACK
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| peer Update ID 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ peer Update ID n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 449
Length length in octets, excluding Type and Length
peer Update ID 32-bit sequence number corresponding to the
Update ID being ACKed
The ACK parameter includes one or more Update IDs that have been
received from the peer. The number of peer Update IDs can be
inferred from the length by dividing it by 4.
5.2.18. ENCRYPTED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV /
/ /
/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ Encrypted data /
/ /
/ +-------------------------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 641
Length length in octets, excluding Type, Length, and
Padding
Reserved zero when sent, ignored when received
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IV Initialization vector, if needed, otherwise
nonexistent. The length of the IV is inferred from
the HIP_CIPHER.
Encrypted The data is encrypted using the encryption algorithm
data defined in the HIP_CIPHER parameter.
The ENCRYPTED parameter encapsulates other parameters, the encrypted
data, which holds one or more HIP parameters in block encrypted form.
Consequently, the first fields in the encapsulated parameter(s) are
Type and Length of the first such parameter, allowing the contents to
be easily parsed after decryption.
The field labeled "Encrypted data" consists of the output of one or
more HIP parameters concatenated together that have been passed
through an encryption algorithm. Each of these inner parameters is
padded according to the rules of Section 5.2.1 for padding individual
parameters. As a result, the concatenated parameters will be a block
of data that is 8-byte aligned.
Some encryption algorithms require that the data to be encrypted must
be a multiple of the cipher algorithm block size. In this case, the
above block of data MUST include additional padding, as specified by
the encryption algorithm. The size of the extra padding is selected
so that the length of the unencrypted data block is a multiple of the
cipher block size. The encryption algorithm may specify padding
bytes other than zero; for example, AES [FIPS.197.2001] uses the
PKCS5 padding scheme (see Section 6.1.1 of [RFC 2898]) where the
remaining n bytes to fill the block each have the value of n. This
yields an "unencrypted data" block that is transformed to an
"encrypted data" block by the cipher suite. This extra padding added
to the set of parameters to satisfy the cipher block alignment rules
is not counted in HIP TLV Length fields, and this extra padding
should be removed by the cipher suite upon decryption.
Note that the length of the cipher suite output may be smaller or
larger than the length of the set of parameters to be encrypted,
since the encryption process may compress the data or add additional
padding to the data.
Once this encryption process is completed, the Encrypted data field
is ready for inclusion in the parameter. If necessary, additional
Padding for 8-byte alignment is then added according to the rules of
Section 5.2.1.
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5.2.19. NOTIFICATION
The NOTIFICATION parameter is used to transmit informational data,
such as error conditions and state transitions, to a HIP peer. A
NOTIFICATION parameter may appear in NOTIFY packets. The use of the
NOTIFICATION parameter in other packet types is for further study.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ Notification Data /
/ +---------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 832
Length length in octets, excluding Type, Length, and
Padding
Reserved zero when sent, ignored when received
Notify Message specifies the type of notification
Type
Notification informational or error data transmitted in
Data addition to the Notify Message Type. Values
for this field are type specific (see below).
Notification information can be error messages specifying why a HIP
Security Association could not be established. It can also be status
data that a HIP implementation wishes to communicate with a peer
process. The table below lists the notification messages and their
Notify Message Types. HIP packets MAY contain multiple NOTIFICATION
parameters if several problems exist or several independent pieces of
information must be transmitted.
To avoid certain types of attacks, a Responder SHOULD avoid sending a
NOTIFICATION to any host with which it has not successfully verified
a puzzle solution.
Notify Message Types in the range 0-16383 are intended for reporting
errors, and those in the range 16384-65535 are for other status
information. An implementation that receives a NOTIFY packet with a
Notify Message Type that indicates an error in response to a request
Moskowitz, et al. Standards Track PAGE 64
RFC 7401 HIPv2 April 2015
packet (e.g., I1, I2, UPDATE) SHOULD assume that the corresponding
request has failed entirely. Unrecognized error types MUST be
ignored, except that they SHOULD be logged.
As currently defined, Notify Message Type values 1-10 are used for
informing about errors in packet structures, and values 11-20 for
informing about problems in parameters.
Notification Data in NOTIFICATION parameters where the Notify Message
Type is in the status range MUST be ignored if not recognized.
Notify Message Types - Errors Value
----------------------------- -----
UNSUPPORTED_CRITICAL_PARAMETER_TYPE 1
Sent if the parameter type has the "critical" bit set and the
parameter type is not recognized. Notification Data contains the
two-octet parameter type.
INVALID_SYNTAX 7
Indicates that the HIP message received was invalid because some
type, length, or value was out of range or because the request
was otherwise malformed. To avoid a denial-of-service
attack using forged messages, this status may only be returned
for packets whose HIP_MAC (if present) and SIGNATURE have been
verified. This status MUST be sent in response to any error not
covered by one of the other status types and SHOULD NOT contain
details, to avoid leaking information to someone probing a node.
To aid debugging, more detailed error information SHOULD be
written to a console or log.
NO_DH_PROPOSAL_CHOSEN 14
None of the proposed Group IDs were acceptable.
INVALID_DH_CHOSEN 15
The DH Group ID field does not correspond to one offered
by the Responder.
NO_HIP_PROPOSAL_CHOSEN 16
None of the proposed HIT Suites or HIP Encryption Algorithms were
acceptable.
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RFC 7401 HIPv2 April 2015
INVALID_HIP_CIPHER_CHOSEN 17
The HIP_CIPHER Crypto ID does not correspond to one offered by
the Responder.
UNSUPPORTED_HIT_SUITE 20
Sent in response to an I1 or R1 packet for which the HIT Suite
is not supported.
AUTHENTICATION_FAILED 24
Sent in response to a HIP signature failure, except when
the signature verification fails in a NOTIFY message.
CHECKSUM_FAILED 26
Sent in response to a HIP checksum failure.
HIP_MAC_FAILED 28
Sent in response to a HIP HMAC failure.
ENCRYPTION_FAILED 32
The Responder could not successfully decrypt the
ENCRYPTED parameter.
INVALID_HIT 40
Sent in response to a failure to validate the peer's
HIT from the corresponding HI.
BLOCKED_BY_POLICY 42
The Responder is unwilling to set up an association
for some policy reason (e.g., the received HIT is NULL
and the policy does not allow opportunistic mode).
RESPONDER_BUSY_PLEASE_RETRY 44
The Responder is unwilling to set up an association, as it is
suffering under some kind of overload and has chosen to shed load
by rejecting the Initiator's request. The Initiator may retry;
however, the Initiator MUST find another (different) puzzle
solution for any such retries. Note that the Initiator may need
to obtain a new puzzle with a new I1/R1 exchange.
Moskowitz, et al. Standards Track PAGE 66
RFC 7401 HIPv2 April 2015
Notify Message Types - Status Value
----------------------------- -----
I2_ACKNOWLEDGEMENT 16384
The Responder has an I2 packet from the Initiator but had to
queue the I2 packet for processing. The puzzle was correctly
solved, and the Responder is willing to set up an association but
currently has a number of I2 packets in the processing queue.
The R2 packet is sent after the I2 packet was processed.
5.2.20. ECHO_REQUEST_SIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 897
Length length of the opaque data in octets
Opaque data opaque data, supposed to be meaningful only to
the node that sends ECHO_REQUEST_SIGNED and
receives a corresponding ECHO_RESPONSE_SIGNED or
ECHO_RESPONSE_UNSIGNED
The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
that the sender wants to get echoed back in the corresponding reply
packet.
The ECHO_REQUEST_SIGNED and corresponding echo response parameters
MAY be used for any purpose where a node wants to carry some state in
a request packet and get it back in a response packet. The
ECHO_REQUEST_SIGNED is covered by the HIP_MAC and SIGNATURE. A HIP
packet can contain only one ECHO_REQUEST_SIGNED parameter and MAY
contain multiple ECHO_REQUEST_UNSIGNED parameters. The
ECHO_REQUEST_SIGNED parameter MUST be responded to with an
ECHO_RESPONSE_SIGNED.
Moskowitz, et al. Standards Track PAGE 67
RFC 7401 HIPv2 April 2015
5.2.21. ECHO_REQUEST_UNSIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63661
Length length of the opaque data in octets
Opaque data opaque data, supposed to be meaningful only to
the node that sends ECHO_REQUEST_UNSIGNED and
receives a corresponding ECHO_RESPONSE_UNSIGNED
The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
that the sender wants to get echoed back in the corresponding reply
packet.
The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
MAY be used for any purpose where a node wants to carry some state in
a request packet and get it back in a response packet. The
ECHO_REQUEST_UNSIGNED is not covered by the HIP_MAC and SIGNATURE. A
HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
in HIP packets passing by. The creator of the ECHO_REQUEST_UNSIGNED
(end host or middlebox) has to create the Opaque field so that it can
later identify and remove the corresponding ECHO_RESPONSE_UNSIGNED
parameter.
The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
ECHO_RESPONSE_UNSIGNED parameter.
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RFC 7401 HIPv2 April 2015
5.2.22. ECHO_RESPONSE_SIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 961
Length length of the opaque data in octets
Opaque data opaque data, copied unmodified from the
ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
parameter that triggered this response
The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data
that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back.
The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED
parameter.
The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be
used for any purpose where a node wants to carry some state in a
request packet and get it back in a response packet. The
ECHO_RESPONSE_SIGNED is covered by the HIP_MAC and SIGNATURE.
5.2.23. ECHO_RESPONSE_UNSIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63425
Length length of the opaque data in octets
Opaque data opaque data, copied unmodified from the
ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
parameter that triggered this response
The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data
that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
wants to get echoed back. The opaque data is copied unmodified from
the corresponding echo request parameter.
Moskowitz, et al. Standards Track PAGE 69
RFC 7401 HIPv2 April 2015
The echo request and ECHO_RESPONSE_UNSIGNED parameters MAY be used
for any purpose where a node wants to carry some state in a request
packet and get it back in a response packet. The
ECHO_RESPONSE_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.
5.3. HIP Packets
There are eight basic HIP packets (see Table 11). Four are for the
HIP base exchange, one is for updating, one is for sending
notifications, and two are for closing a HIP association. Support
for the NOTIFY packet type is optional, but support for all other HIP
packet types listed below is mandatory.
+------------------+------------------------------------------------+
| Packet type | Packet name |
+------------------+------------------------------------------------+
| 1 | I1 - the HIP Initiator Packet |
| | |
| 2 | R1 - the HIP Responder Packet |
| | |
| 3 | I2 - the Second HIP Initiator Packet |
| | |
| 4 | R2 - the Second HIP Responder Packet |
| | |
| 16 | UPDATE - the HIP Update Packet |
| | |
| 17 | NOTIFY - the HIP Notify Packet |
| | |
| 18 | CLOSE - the HIP Association Closing Packet |
| | |
| 19 | CLOSE_ACK - the HIP Closing Acknowledgment |
| | Packet |
+------------------+------------------------------------------------+
Table 11: HIP Packets and Packet Type Values
Packets consist of the fixed header as described in Section 5.1,
followed by the parameters. The parameter part, in turn, consists of
zero or more TLV-coded parameters.
In addition to the base packets, other packet types may be defined
later in separate specifications. For example, support for mobility
and multihoming is not included in this specification.
See "Notation" (Section 2.2) for the notation used in the operations.
Moskowitz, et al. Standards Track PAGE 70
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In the future, an optional upper-layer payload MAY follow the HIP
header. The Next Header field in the header indicates if there is
additional data following the HIP header. The HIP packet, however,
MUST NOT be fragmented into multiple extension headers by setting the
Next Header field in a HIP header to the HIP protocol number. This
limits the size of the possible additional data in the packet.
5.3.1. I1 - the HIP Initiator Packet
The HIP header values for the I1 packet:
Header:
Packet Type = 1
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT, or NULL
IP ( HIP ( DH_GROUP_LIST ) )
The I1 packet contains the fixed HIP header and the Initiator's
DH_GROUP_LIST.
Valid control bits: None
The Initiator receives the Responder's HIT from either a DNS lookup
of the Responder's FQDN (see [HIP-DNS-EXT]), some other repository,
or a local table. If the Initiator does not know the Responder's
HIT, it may attempt to use opportunistic mode by using NULL (all
zeros) as the Responder's HIT. See also "HIP Opportunistic Mode"
(Section 4.1.8).
Since the I1 packet is so easy to spoof even if it were signed, no
attempt is made to add to its generation or processing cost.
The Initiator includes a DH_GROUP_LIST parameter in the I1 packet to
inform the Responder of its preferred DH Group IDs. Note that the
DH_GROUP_LIST in the I1 packet is not protected by a signature.
Implementations MUST be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
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RFC 7401 HIPv2 April 2015
5.3.2. R1 - the HIP Responder Packet
The HIP header values for the R1 packet:
Header:
Packet Type = 2
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( [ R1_COUNTER, ]
PUZZLE,
DIFFIE_HELLMAN,
HIP_CIPHER,
HOST_ID,
HIT_SUITE_LIST,
DH_GROUP_LIST,
[ ECHO_REQUEST_SIGNED, ]
TRANSPORT_FORMAT_LIST,
HIP_SIGNATURE_2 )
<, ECHO_REQUEST_UNSIGNED >i)
Valid control bits: A
If the Responder's HI is an anonymous one, the A control MUST be set.
The Initiator's HIT MUST match the one received in the I1 packet if
the R1 is a response to an I1. If the Responder has multiple HIs,
the Responder's HIT used MUST match the Initiator's request. If the
Initiator used opportunistic mode, the Responder may select freely
among its HIs. See also "HIP Opportunistic Mode" (Section 4.1.8).
The R1 packet generation counter is used to determine the currently
valid generation of puzzles. The value is increased periodically,
and it is RECOMMENDED that it is increased at least as often as
solutions to old puzzles are no longer accepted.
The puzzle contains a Random #I and the difficulty #K. The
difficulty #K indicates the number of lower-order bits, in the puzzle
hash result, that must be zeros; see Section 4.1.2. The Random #I is
not covered by the signature and must be zeroed during the signature
calculation, allowing the sender to select and set the #I into a
precomputed R1 packet just prior to sending it to the peer.
The Responder selects the DIFFIE_HELLMAN Group ID and Public Value
based on the Initiator's preference expressed in the DH_GROUP_LIST
parameter in the I1 packet. The Responder sends back its own
preference based on which it chose the DH public value as
Moskowitz, et al. Standards Track PAGE 72
RFC 7401 HIPv2 April 2015
DH_GROUP_LIST. This allows the Initiator to determine whether its
own DH_GROUP_LIST in the sent I1 packet was manipulated by an
attacker.
The Diffie-Hellman public value is ephemeral, and values SHOULD NOT
be reused across different HIP associations. Once the Responder has
received a valid response to an R1 packet, that Diffie-Hellman value
SHOULD be deprecated. It is possible that the Responder has sent the
same Diffie-Hellman value to different hosts simultaneously in
corresponding R1 packets, and those responses should also be
accepted. However, as a defense against I1 packet storms, an
implementation MAY propose, and reuse unless avoidable, the same
Diffie-Hellman value for a period of time -- for example, 15 minutes.
By using a small number of different puzzles for a given
Diffie-Hellman value, the R1 packets can be precomputed and delivered
as quickly as I1 packets arrive. A scavenger process should clean up
unused Diffie-Hellman values and puzzles.
Reusing Diffie-Hellman public values opens up the potential security
risk of more than one Initiator ending up with the same keying
material (due to faulty random number generators). Also, more than
one Initiator using the same Responder public key half may lead to
potentially easier cryptographic attacks and to imperfect forward
security.
However, these risks involved in reusing the same public value are
statistical; that is, the authors are not aware of any mechanism that
would allow manipulation of the protocol so that the risk of the
reuse of any given Responder Diffie-Hellman public key would differ
from the base probability. Consequently, it is RECOMMENDED that
Responders avoid reusing the same DH key with multiple Initiators,
but because the risk is considered statistical and not known to be
manipulable, the implementations MAY reuse a key in order to ease
resource-constrained implementations and to increase the probability
of successful communication with legitimate clients even under an I1
packet storm. In particular, when it is too expensive to generate
enough precomputed R1 packets to supply each potential Initiator with
a different DH key, the Responder MAY send the same DH key to several
Initiators, thereby creating the possibility of multiple legitimate
Initiators ending up using the same Responder-side public key.
However, as soon as the Responder knows that it will use a particular
DH key, it SHOULD stop offering it. This design is aimed to allow
resource-constrained Responders to offer services under I1 packet
storms and to simultaneously make the probability of DH key reuse
both statistical and as low as possible.
Moskowitz, et al. Standards Track PAGE 73
RFC 7401 HIPv2 April 2015
If the Responder uses the same DH key pair for multiple handshakes,
it must take care to avoid small subgroup attacks [RFC 2785]. To
avoid these attacks, when receiving the I2 message, the Responder
SHOULD validate the Initiator's DH public key as described in
[RFC 2785], Section 3.1. If the validation fails, the Responder MUST
NOT generate a DH shared key and MUST silently abort the HIP BEX.
The HIP_CIPHER parameter contains the encryption algorithms supported
by the Responder to encrypt the contents of the ENCRYPTED parameter,
in the order of preference. All implementations MUST support AES
[RFC 3602].
The HIT_SUITE_LIST parameter is an ordered list of the Responder's
preferred and supported HIT Suites. The list allows the Initiator to
determine whether its own source HIT matches any suite supported by
the Responder.
The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED parameters contain
data that the sender wants to receive unmodified in the corresponding
response packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
parameter. The R1 packet may contain zero or more
ECHO_REQUEST_UNSIGNED parameters as described in Section 5.2.21.
The TRANSPORT_FORMAT_LIST parameter is an ordered list of the
Responder's preferred and supported transport format types. The list
allows the Initiator and the Responder to agree on a common type for
payload protection. This parameter is described in Section 5.2.11.
The signature is calculated over the whole HIP packet as described in
Section 5.2.15. This allows the Responder to use precomputed R1s.
The Initiator SHOULD validate this signature. It MUST check that the
Responder's HI matches with the one expected, if any.
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RFC 7401 HIPv2 April 2015
5.3.3. I2 - the Second HIP Initiator Packet
The HIP header values for the I2 packet:
Header:
Packet Type = 3
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT
IP ( HIP ( [R1_COUNTER,]
SOLUTION,
DIFFIE_HELLMAN,
HIP_CIPHER,
ENCRYPTED { HOST_ID } or HOST_ID,
[ ECHO_RESPONSE_SIGNED, ]
TRANSPORT_FORMAT_LIST,
HIP_MAC,
HIP_SIGNATURE
<, ECHO_RESPONSE_UNSIGNED>i ) )
Valid control bits: A
The HITs used MUST match the ones used in the R1.
If the Initiator's HI is an anonymous one, the A control bit MUST
be set.
If present in the I1 packet, the Initiator MUST include an unmodified
copy of the R1_COUNTER parameter received in the corresponding R1
packet into the I2 packet.
The Solution contains the Random #I from R1 and the computed #J. The
low-order #K bits of the RHASH( #I | ... | #J ) MUST be zero.
The Diffie-Hellman value is ephemeral. If precomputed, a scavenger
process should clean up unused Diffie-Hellman values. The Responder
MAY reuse Diffie-Hellman values under some conditions as specified in
Section 5.3.2.
The HIP_CIPHER contains the single encryption suite selected by the
Initiator, that it uses to encrypt the ENCRYPTED parameters. The
chosen cipher MUST correspond to one of the ciphers offered by the
Responder in the R1. All implementations MUST support AES [RFC 3602].
The Initiator's HI MAY be encrypted using the HIP_CIPHER encryption
algorithm. The keying material is derived from the Diffie-Hellman
exchange as defined in Section 6.5.
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The ECHO_RESPONSE_SIGNED and ECHO_RESPONSE_UNSIGNED contain the
unmodified opaque data copied from the corresponding echo request
parameter(s).
The TRANSPORT_FORMAT_LIST contains the single transport format type
selected by the Initiator. The chosen type MUST correspond to one of
the types offered by the Responder in the R1. Currently, the only
transport format defined is the ESP transport format ([RFC 7402]).
The HMAC value in the HIP_MAC parameter is calculated over the whole
HIP packet, excluding any parameters after the HIP_MAC, as described
in Section 6.4.1. The Responder MUST validate the HIP_MAC.
The signature is calculated over the whole HIP packet, excluding any
parameters after the HIP_SIGNATURE, as described in Section 5.2.14.
The Responder MUST validate this signature. The Responder uses the
HI in the packet or an HI acquired by some other means for verifying
the signature.
5.3.4. R2 - the Second HIP Responder Packet
The HIP header values for the R2 packet:
Header:
Packet Type = 4
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( HIP_MAC_2, HIP_SIGNATURE ) )
Valid control bits: None
The HIP_MAC_2 is calculated over the whole HIP packet, with the
Responder's HOST_ID parameter concatenated with the HIP packet. The
HOST_ID parameter is removed after the HMAC calculation. The
procedure is described in Section 6.4.1.
The signature is calculated over the whole HIP packet.
The Initiator MUST validate both the HIP_MAC and the signature.
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5.3.5. UPDATE - the HIP Update Packet
The HIP header values for the UPDATE packet:
Header:
Packet Type = 16
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( [SEQ, ACK, ] HIP_MAC, HIP_SIGNATURE ) )
Valid control bits: None
The UPDATE packet contains mandatory HIP_MAC and HIP_SIGNATURE
parameters, and other optional parameters.
The UPDATE packet contains zero or one SEQ parameter. The presence
of a SEQ parameter indicates that the receiver MUST acknowledge the
UPDATE. An UPDATE that does not contain a SEQ but only an ACK
parameter is simply an acknowledgment of a previous UPDATE and itself
MUST NOT be acknowledged by a separate ACK parameter. Such UPDATE
packets containing only an ACK parameter do not require processing in
relative order to other UPDATE packets. An UPDATE packet without
either a SEQ or an ACK parameter is invalid; such unacknowledged
updates MUST instead use a NOTIFY packet.
An UPDATE packet contains zero or one ACK parameter. The ACK
parameter echoes the SEQ sequence number of the UPDATE packet being
ACKed. A host MAY choose to acknowledge more than one UPDATE packet
at a time; e.g., the ACK parameter may contain the last two SEQ
values received, for resilience against packet loss. ACK values are
not cumulative; each received unique SEQ value requires at least one
corresponding ACK value in reply. Received ACK parameters that are
redundant are ignored. Hosts MUST implement the processing of ACK
parameters with multiple SEQ sequence numbers even if they do not
implement sending ACK parameters with multiple SEQ sequence numbers.
The UPDATE packet may contain both a SEQ and an ACK parameter. In
this case, the ACK parameter is being piggybacked on an outgoing
UPDATE. In general, UPDATEs carrying SEQ SHOULD be ACKed upon
completion of the processing of the UPDATE. A host MAY choose to
hold the UPDATE carrying an ACK parameter for a short period of time
to allow for the possibility of piggybacking the ACK parameter, in a
manner similar to TCP delayed acknowledgments.
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A sender MAY choose to forego reliable transmission of a particular
UPDATE (e.g., it becomes overcome by events). The semantics are such
that the receiver MUST acknowledge the UPDATE, but the sender MAY
choose to not care about receiving the ACK parameter.
UPDATEs MAY be retransmitted without incrementing SEQ. If the same
subset of parameters is included in multiple UPDATEs with different
SEQs, the host MUST ensure that the receiver's processing of the
parameters multiple times will not result in a protocol error.
5.3.6. NOTIFY - the HIP Notify Packet
The NOTIFY packet MAY be used to provide information to a peer.
Typically, NOTIFY is used to indicate some type of protocol error or
negotiation failure. NOTIFY packets are unacknowledged. The
receiver can handle the packet only as informational, and SHOULD NOT
change its HIP state (see Section 4.4.2) based purely on a received
NOTIFY packet.
The HIP header values for the NOTIFY packet:
Header:
Packet Type = 17
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT, or zero if unknown
IP ( HIP (<NOTIFICATION>i, [HOST_ID, ] HIP_SIGNATURE) )
Valid control bits: None
The NOTIFY packet is used to carry one or more NOTIFICATION
parameters.
5.3.7. CLOSE - the HIP Association Closing Packet
The HIP header values for the CLOSE packet:
Header:
Packet Type = 18
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( ECHO_REQUEST_SIGNED, HIP_MAC, HIP_SIGNATURE ) )
Valid control bits: None
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The sender MUST include an ECHO_REQUEST_SIGNED used to validate
CLOSE_ACK received in response, and both a HIP_MAC and a signature
(calculated over the whole HIP packet).
The receiver peer MUST reply with a CLOSE_ACK containing an
ECHO_RESPONSE_SIGNED corresponding to the received
ECHO_REQUEST_SIGNED.
5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet
The HIP header values for the CLOSE_ACK packet:
Header:
Packet Type = 19
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( ECHO_RESPONSE_SIGNED, HIP_MAC, HIP_SIGNATURE ) )
Valid control bits: None
The sender MUST include both an HMAC and signature (calculated over
the whole HIP packet).
The receiver peer MUST validate the ECHO_RESPONSE_SIGNED and validate
both the HIP_MAC and the signature if the receiver has state for a
HIP association.
5.4. ICMP Messages
When a HIP implementation detects a problem with an incoming packet,
and it either cannot determine the identity of the sender of the
packet or does not have any existing HIP association with the sender
of the packet, it MAY respond with an ICMP packet. Any such replies
MUST be rate-limited as described in [RFC 4443]. In most cases, the
ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for
ICMPv6), with the Pointer pointing to the field that caused the ICMP
message to be generated.
5.4.1. Invalid Version
If a HIP implementation receives a HIP packet that has an
unrecognized HIP version number, it SHOULD respond, rate-limited,
with an ICMP packet with type Parameter Problem, with the Pointer
pointing to the Version/RES. byte in the HIP header.
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5.4.2. Other Problems with the HIP Header and Packet Structure
If a HIP implementation receives a HIP packet that has other
unrecoverable problems in the header or packet format, it MAY
respond, rate-limited, with an ICMP packet with type Parameter
Problem, with the Pointer pointing to the field that failed to pass
the format checks. However, an implementation MUST NOT send an ICMP
message if the checksum fails; instead, it MUST silently drop the
packet.
5.4.3. Invalid Puzzle Solution
If a HIP implementation receives an I2 packet that has an invalid
puzzle solution, the behavior depends on the underlying version of
IP. If IPv6 is used, the implementation SHOULD respond with an ICMP
packet with type Parameter Problem, with the Pointer pointing to the
beginning of the Puzzle solution #J field in the SOLUTION payload in
the HIP message.
If IPv4 is used, the implementation MAY respond with an ICMP packet
with the type Parameter Problem, copying enough bytes from the I2
message so that the SOLUTION parameter fits into the ICMP message,
with the Pointer pointing to the beginning of the Puzzle solution #J
field, as in the IPv6 case. Note, however, that the resulting ICMPv4
message exceeds the typical ICMPv4 message size as defined in
[RFC 792].
5.4.4. Non-existing HIP Association
If a HIP implementation receives a CLOSE or UPDATE packet, or any
other packet whose handling requires an existing association, that
has either a Receiver or Sender HIT that does not match with any
existing HIP association, the implementation MAY respond, rate-
limited, with an ICMP packet with the type Parameter Problem. The
Pointer of the ICMP Parameter Problem packet is set pointing to the
beginning of the first HIT that does not match.
A host MUST NOT reply with such an ICMP if it receives any of the
following messages: I1, R2, I2, R2, and NOTIFY packet. When
introducing new packet types, a specification SHOULD define the
appropriate rules for sending or not sending this kind of ICMP reply.
6. Packet Processing
Each host is assumed to have a single HIP implementation that manages
the host's HIP associations and handles requests for new ones. Each
HIP association is governed by a conceptual state machine, with
states defined above in Section 4.4. The HIP implementation can
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simultaneously maintain HIP associations with more than one host.
Furthermore, the HIP implementation may have more than one active HIP
association with another host; in this case, HIP associations are
distinguished by their respective HITs. It is not possible to have
more than one HIP association between any given pair of HITs.
Consequently, the only way for two hosts to have more than one
parallel association is to use different HITs, at least at one end.
The processing of packets depends on the state of the HIP
association(s) with respect to the authenticated or apparent
originator of the packet. A HIP implementation determines whether it
has an active association with the originator of the packet based on
the HITs. In the case of user data carried in a specific transport
format, the transport format document specifies how the incoming
packets are matched with the active associations.
6.1. Processing Outgoing Application Data
In a HIP host, an application can send application-level data using
an identifier specified via the underlying API. The API can be a
backwards-compatible API (see [RFC 5338]), using identifiers that look
similar to IP addresses, or a completely new API, providing enhanced
services related to Host Identities. Depending on the HIP
implementation, the identifier provided to the application may be
different; for example, it can be a HIT or an IP address.
The exact format and method for transferring the user data from the
source HIP host to the destination HIP host are defined in the
corresponding transport format document. The actual data is
transferred in the network using the appropriate source and
destination IP addresses.
In this document, conceptual processing rules are defined only for
the base case where both hosts have only single usable IP addresses;
the multi-address multihoming case is specified separately.
The following conceptual algorithm describes the steps that are
required for handling outgoing datagrams destined to a HIT.
1. If the datagram has a specified source address, it MUST be a HIT.
If it is not, the implementation MAY replace the source address
with a HIT. Otherwise, it MUST drop the packet.
2. If the datagram has an unspecified source address, the
implementation MUST choose a suitable source HIT for the
datagram. Selecting the source HIT is subject to local policy.
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3. If there is no active HIP association with the given <source,
destination> HIT pair, one MUST be created by running the base
exchange. While waiting for the base exchange to complete, the
implementation SHOULD queue at least one user data packet per HIP
association to be formed, and it MAY queue more than one.
4. Once there is an active HIP association for the given <source,
destination> HIT pair, the outgoing datagram is passed to
transport handling. The possible transport formats are defined
in separate documents, of which the ESP transport format for HIP
is mandatory for all HIP implementations.
5. Before sending the packet, the HITs in the datagram are replaced
with suitable IP addresses. For IPv6, the rules defined in
[RFC 6724] SHOULD be followed. Note that this HIT-to-IP-address
conversion step MAY also be performed at some other point in the
stack, e.g., before wrapping the packet into the output format.
6.2. Processing Incoming Application Data
The following conceptual algorithm describes the incoming datagram
handling when HITs are used at the receiving host as application-
level identifiers. More detailed steps for processing packets are
defined in corresponding transport format documents.
1. The incoming datagram is mapped to an existing HIP association,
typically using some information from the packet. For example,
such mapping may be based on the ESP Security Parameter Index
(SPI).
2. The specific transport format is unwrapped, in a way depending on
the transport format, yielding a packet that looks like a
standard (unencrypted) IP packet. If possible, this step SHOULD
also verify that the packet was indeed (once) sent by the remote
HIP host, as identified by the HIP association.
Depending on the used transport mode, the verification method can
vary. While the HI (as well as the HIT) is used as the higher-
layer identifier, the verification method has to verify that the
data packet was sent by the correct node identity and that the
actual identity maps to this particular HIT. When using the ESP
transport format [RFC 7402], the verification is done using the
SPI value in the data packet to find the corresponding SA with
associated HIT and key, and decrypting the packet with that
associated key.
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3. The IP addresses in the datagram are replaced with the HITs
associated with the HIP association. Note that this IP-address-
to-HIT conversion step MAY also be performed at some other point
in the stack.
4. The datagram is delivered to the upper layer (e.g., UDP or TCP).
When demultiplexing the datagram, the right upper-layer socket is
selected based on the HITs.
6.3. Solving the Puzzle
This subsection describes the details for solving the puzzle.
In the R1 packet, the values #I and #K are sent in network byte
order. Similarly, in the I2 packet, the values #I and #J are sent in
network byte order. The hash is created by concatenating, in network
byte order, the following data, in the following order and using the
RHASH algorithm:
n-bit random value #I (where n is RHASH_len), in network byte
order, as appearing in the R1 and I2 packets.
128-bit Initiator's HIT, in network byte order, as appearing in
the HIP Payload in the R1 and I2 packets.
128-bit Responder's HIT, in network byte order, as appearing in
the HIP Payload in the R1 and I2 packets.
n-bit random value #J (where n is RHASH_len), in network byte
order, as appearing in the I2 packet.
In a valid response puzzle, the #K low-order bits of the resulting
RHASH digest MUST be zero.
Notes:
i) The length of the data to be hashed is variable, depending on
the output length of the Responder's hash function RHASH.
ii) All the data in the hash input MUST be in network byte order.
iii) The orderings of the Initiator's and Responder's HITs are
different in the R1 and I2 packets; see Section 5.1. Care
must be taken to copy the values in the right order to the
hash input.
iv) For a puzzle #I, there may exist multiple valid puzzle
solutions #J.
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The following procedure describes the processing steps involved,
assuming that the Responder chooses to precompute the R1 packets:
Precomputation by the Responder:
Sets up the puzzle difficulty #K.
Creates a signed R1 and caches it.
Responder:
Selects a suitable cached R1.
Generates a random number #I.
Sends #I and #K in an R1.
Saves #I and #K for a delta time.
Initiator:
Generates repeated attempts to solve the puzzle until a matching
#J is found:
Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K ) == 0
Sends #I and #J in an I2.
Responder:
Verifies that the received #I is a saved one.
Finds the right #K based on #I.
Computes V := Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K )
Rejects if V != 0
Accepts if V == 0
6.4. HIP_MAC and SIGNATURE Calculation and Verification
The following subsections define the actions for processing HIP_MAC,
HIP_MAC_2, HIP_SIGNATURE, and HIP_SIGNATURE_2 parameters. The
HIP_MAC_2 parameter is contained in the R2 packet. The
HIP_SIGNATURE_2 parameter is contained in the R1 packet. The
HIP_SIGNATURE and HIP_MAC parameters are contained in other HIP
packets.
6.4.1. HMAC Calculation
The HMAC uses RHASH as the underlying hash function. The type of
RHASH depends on the HIT Suite of the Responder. Hence, HMAC-SHA-256
[RFC 4868] is used for HIT Suite RSA/DSA/SHA-256, HMAC-SHA-1 [RFC 2404]
is used for HIT Suite ECDSA_LOW/SHA-1, and HMAC-SHA-384 [RFC 4868] is
used for HIT Suite ECDSA/SHA-384.
The following process applies both to the HIP_MAC and HIP_MAC_2
parameters. When processing HIP_MAC_2, the difference is that the
HIP_MAC calculation includes a pseudo HOST_ID field containing the
Responder's information as sent in the R1 packet earlier.
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Both the Initiator and the Responder should take some care when
verifying or calculating the HIP_MAC_2. Specifically, the Initiator
has to preserve the HOST_ID exactly as it was received in the R1
packet until it receives the HIP_MAC_2 in the R2 packet.
The scope of the calculation for HIP_MAC is as follows:
HMAC: { HIP header | [ Parameters ] }
where Parameters include all of the packet's HIP parameters with type
values ranging from 1 to (HIP_MAC's type value - 1), and excluding
those parameters with type values greater than or equal to HIP_MAC's
type value.
During HIP_MAC calculation, the following apply:
o In the HIP header, the Checksum field is set to zero.
o In the HIP header, the Header Length field value is calculated to
the beginning of the HIP_MAC parameter.
Parameter order is described in Section 5.2.1.
The scope of the calculation for HIP_MAC_2 is as follows:
HIP_MAC_2: { HIP header | [ Parameters ] | HOST_ID }
where Parameters include all of the packet's HIP parameters with type
values from 1 to (HIP_MAC_2's type value - 1), and excluding those
parameters with type values greater than or equal to HIP_MAC_2's type
value.
During HIP_MAC_2 calculation, the following apply:
o In the HIP header, the Checksum field is set to zero.
o In the HIP header, the Header Length field value is calculated to
the beginning of the HIP_MAC_2 parameter and increased by the
length of the concatenated HOST_ID parameter length (including the
Type and Length fields).
o The HOST_ID parameter is exactly in the form it was received in
the R1 packet from the Responder.
Parameter order is described in Section 5.2.1, except that the
HOST_ID parameter in this calculation is added to the end.
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The HIP_MAC parameter is defined in Section 5.2.12 and the HIP_MAC_2
parameter in Section 5.2.13. The HMAC calculation and verification
process (the process applies both to HIP_MAC and HIP_MAC_2, except
where HIP_MAC_2 is mentioned separately) is as follows:
Packet sender:
1. Create the HIP packet, without the HIP_MAC, HIP_SIGNATURE,
HIP_SIGNATURE_2, or any other parameter with greater type value
than the HIP_MAC parameter has.
2. In case of HIP_MAC_2 calculation, add a HOST_ID (Responder)
parameter to the end of the packet.
3. Calculate the Header Length field in the HIP header, including
the added HOST_ID parameter in case of HIP_MAC_2.
4. Compute the HMAC using either the HIP-gl or HIP-lg integrity key
retrieved from KEYMAT as defined in Section 6.5.
5. In case of HIP_MAC_2, remove the HOST_ID parameter from the
packet.
6. Add the HIP_MAC parameter to the packet and any parameter with
greater type value than the HIP_MAC's (HIP_MAC_2's) that may
follow, including possible HIP_SIGNATURE or HIP_SIGNATURE_2
parameters.
7. Recalculate the Length field in the HIP header.
Packet receiver:
1. Verify the HIP Header Length field.
2. Remove the HIP_MAC or HIP_MAC_2 parameter, as well as all other
parameters that follow it with greater type value including
possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
contents if they are needed later.
3. In case of HIP_MAC_2, build and add a HOST_ID parameter (with
Responder information) to the packet. The HOST_ID parameter
should be identical to the one previously received from the
Responder.
4. Recalculate the HIP packet length in the HIP header and clear the
Checksum field (set it to all zeros). In case of HIP_MAC_2, the
length is calculated with the added HOST_ID parameter.
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5. Compute the HMAC using either the HIP-gl or HIP-lg integrity key
as defined in Section 6.5 and verify it against the received
HMAC.
6. Set the Checksum and Header Length fields in the HIP header to
original values. Note that the Checksum and Length fields
contain incorrect values after this step.
7. In case of HIP_MAC_2, remove the HOST_ID parameter from the
packet before further processing.
6.4.2. Signature Calculation
The following process applies both to the HIP_SIGNATURE and
HIP_SIGNATURE_2 parameters. When processing the HIP_SIGNATURE_2
parameter, the only difference is that instead of the HIP_SIGNATURE
parameter, the HIP_SIGNATURE_2 parameter is used, and the Initiator's
HIT and PUZZLE Opaque and Random #I fields are cleared (set to all
zeros) before computing the signature. The HIP_SIGNATURE parameter
is defined in Section 5.2.14 and the HIP_SIGNATURE_2 parameter in
Section 5.2.15.
The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2 is
as follows:
HIP_SIGNATURE: { HIP header | [ Parameters ] }
where Parameters include all of the packet's HIP parameters with type
values from 1 to (HIP_SIGNATURE's type value - 1).
During signature calculation, the following apply:
o In the HIP header, the Checksum field is set to zero.
o In the HIP header, the Header Length field value is calculated to
the beginning of the HIP_SIGNATURE parameter.
Parameter order is described in Section 5.2.1.
HIP_SIGNATURE_2: { HIP header | [ Parameters ] }
where Parameters include all of the packet's HIP parameters with type
values ranging from 1 to (HIP_SIGNATURE_2's type value - 1).
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During signature calculation, the following apply:
o In the HIP header, both the Checksum and the Receiver's HIT fields
are set to zero.
o In the HIP header, the Header Length field value is calculated to
the beginning of the HIP_SIGNATURE_2 parameter.
o The PUZZLE parameter's Opaque and Random #I fields are set to
zero.
Parameter order is described in Section 5.2.1.
The signature calculation and verification process (the process
applies both to HIP_SIGNATURE and HIP_SIGNATURE_2, except in the case
where HIP_SIGNATURE_2 is separately mentioned) is as follows:
Packet sender:
1. Create the HIP packet without the HIP_SIGNATURE parameter or any
other parameters that follow the HIP_SIGNATURE parameter.
2. Calculate the Length field and zero the Checksum field in the HIP
header. In case of HIP_SIGNATURE_2, set the Initiator's HIT
field in the HIP header as well as the PUZZLE parameter's Opaque
and Random #I fields to zero.
3. Compute the signature using the private key corresponding to the
Host Identifier (public key).
4. Add the HIP_SIGNATURE parameter to the packet.
5. Add any parameters that follow the HIP_SIGNATURE parameter.
6. Recalculate the Length field in the HIP header, and calculate the
Checksum field.
Packet receiver:
1. Verify the HIP Header Length field and checksum.
2. Save the contents of the HIP_SIGNATURE parameter and any other
parameters following the HIP_SIGNATURE parameter, and remove them
from the packet.
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3. Recalculate the HIP packet Length in the HIP header and clear the
Checksum field (set it to all zeros). In case of
HIP_SIGNATURE_2, set the Initiator's HIT field in the HIP header
as well as the PUZZLE parameter's Opaque and Random #I fields
to zero.
4. Compute the signature and verify it against the received
signature using the packet sender's Host Identity (public key).
5. Restore the original packet by adding removed parameters (in
step 2) and resetting the values that were set to zero (in
step 3).
The verification can use either the HI received from a HIP packet;
the HI retrieved from a DNS query, if the FQDN has been received in
the HOST_ID parameter; or an HI received by some other means.
6.5. HIP KEYMAT Generation
HIP keying material is derived from the Diffie-Hellman session key,
Kij, produced during the HIP base exchange (see Section 4.1.3). The
Initiator has Kij during the creation of the I2 packet, and the
Responder has Kij once it receives the I2 packet. This is why I2 can
already contain encrypted information.
The KEYMAT is derived by feeding Kij into the key derivation function
defined by the DH Group ID. Currently, the only key derivation
function defined in this document is the Hash-based Key Derivation
Function (HKDF) [RFC 5869] using the RHASH hash function. Other
documents may define new DH Group IDs and corresponding key
distribution functions.
In the following, we provide the details for deriving the keying
material using HKDF.
where
info = sort(HIT-I | HIT-R)
salt = #I | #J
Sort(HIT-I | HIT-R) is defined as the network byte order
concatenation of the two HITs, with the smaller HIT preceding the
larger HIT, resulting from the numeric comparison of the two HITs
interpreted as positive (unsigned) 128-bit integers in network byte
order. The #I and #J values are from the puzzle and its solution
that were exchanged in R1 and I2 messages when this HIP association
was set up. Both hosts have to store #I and #J values for the HIP
association for future use.
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The initial keys are drawn sequentially in the order that is
determined by the numeric comparison of the two HITs, with the
comparison method described in the previous paragraph. HOST_g
denotes the host with the greater HIT value, and HOST_l the host with
the lower HIT value.
The drawing order for the four initial keys is as follows:
HIP-gl encryption key for HOST_g's ENCRYPTED parameter
HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets
HIP-lg encryption key for HOST_l's ENCRYPTED parameter
HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets
The number of bits drawn for a given algorithm is the "natural" size
of the keys. For the mandatory algorithms, the following sizes
apply:
AES 128 or 256 bits
SHA-1 160 bits
SHA-256 256 bits
SHA-384 384 bits
NULL 0 bits
If other key sizes are used, they MUST be treated as different
encryption algorithms and defined separately.
6.6. Initiation of a HIP Base Exchange
An implementation may originate a HIP base exchange to another host
based on a local policy decision, usually triggered by an application
datagram, in much the same way that an IPsec IKE key exchange can
dynamically create a Security Association. Alternatively, a system
may initiate a HIP exchange if it has rebooted or timed out, or
otherwise lost its HIP state, as described in Section 4.5.4.
The implementation prepares an I1 packet and sends it to the IP
address that corresponds to the peer host. The IP address of the
peer host may be obtained via conventional mechanisms, such as DNS
lookup. The I1 packet contents are specified in Section 5.3.1. The
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selection of which source or destination Host Identity to use, if an
Initiator or Responder has more than one to choose from, is typically
a policy decision.
The following steps define the conceptual processing rules for
initiating a HIP base exchange:
1. The Initiator receives one or more of the Responder's HITs and
one or more addresses from either a DNS lookup of the Responder's
FQDN, some other repository, or a local database. If the
Initiator does not know the Responder's HIT, it may attempt
opportunistic mode by using NULL (all zeros) as the Responder's
HIT (see also "HIP Opportunistic Mode" (Section 4.1.8)). If the
Initiator can choose from multiple Responder HITs, it selects a
HIT for which the Initiator supports the HIT Suite.
2. The Initiator sends an I1 packet to one of the Responder's
addresses. The selection of which address to use is a local
policy decision.
3. The Initiator includes the DH_GROUP_LIST in the I1 packet. The
selection and order of DH Group IDs in the DH_GROUP_LIST MUST be
stored by the Initiator, because this list is needed for later R1
processing. In most cases, the preferences regarding the DH
groups will be static, so no per-association storage is
necessary.
4. Upon sending an I1 packet, the sender transitions to state
I1-SENT and starts a timer for which the timeout value SHOULD be
larger than the worst-case anticipated RTT. The sender SHOULD
also increment the trial counter associated with the I1.
5. Upon timeout, the sender SHOULD retransmit the I1 packet and
restart the timer, up to a maximum of I1_RETRIES_MAX tries.
6.6.1. Sending Multiple I1 Packets in Parallel
For the sake of minimizing the association establishment latency, an
implementation MAY send the same I1 packet to more than one of the
Responder's addresses. However, it MUST NOT send to more than three
(3) Responder addresses in parallel. Furthermore, upon timeout, the
implementation MUST refrain from sending the same I1 packet to
multiple addresses. That is, if it retries to initialize the
connection after a timeout, it MUST NOT send the I1 packet to more
than one destination address. These limitations are placed in order
to avoid congestion of the network, and potential DoS attacks that
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might occur, e.g., because someone's claim to have hundreds or
thousands of addresses could generate a huge number of I1 packets
from the Initiator.
As the Responder is not guaranteed to distinguish the duplicate I1
packets it receives at several of its addresses (because it avoids
storing states when it answers back an R1 packet), the Initiator may
receive several duplicate R1 packets.
The Initiator SHOULD then select the initial preferred destination
address using the source address of the selected received R1, and use
the preferred address as a source address for the I2 packet.
Processing rules for received R1s are discussed in Section 6.8.
6.6.2. Processing Incoming ICMP Protocol Unreachable Messages
A host may receive an ICMP 'Destination Protocol Unreachable' message
as a response to sending a HIP I1 packet. Such a packet may be an
indication that the peer does not support HIP, or it may be an
attempt to launch an attack by making the Initiator believe that the
Responder does not support HIP.
When a system receives an ICMP 'Destination Protocol Unreachable'
message while it is waiting for an R1 packet, it MUST NOT terminate
waiting. It MAY continue as if it had not received the ICMP message,
and send a few more I1 packets. Alternatively, it MAY take the ICMP
message as a hint that the peer most probably does not support HIP,
and return to state UNASSOCIATED earlier than otherwise. However, at
minimum, it MUST continue waiting for an R1 packet for a reasonable
time before returning to UNASSOCIATED.
6.7. Processing of Incoming I1 Packets
An implementation SHOULD reply to an I1 with an R1 packet, unless the
implementation is unable or unwilling to set up a HIP association.
If the implementation is unable to set up a HIP association, the host
SHOULD send an 'ICMP Destination Protocol Unreachable,
Administratively Prohibited' message to the I1 packet source IP
address. If the implementation is unwilling to set up a HIP
association, the host MAY ignore the I1 packet. This latter case may
occur during a DoS attack such as an I1 packet flood.
The implementation SHOULD be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
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A spoofed I1 packet can result in an R1 attack on a system. An R1
packet sender MUST have a mechanism to rate-limit R1 packets sent to
an address.
It is RECOMMENDED that the HIP state machine does not transition upon
sending an R1 packet.
The following steps define the conceptual processing rules for
responding to an I1 packet:
1. The Responder MUST check that the Responder's HIT in the received
I1 packet is either one of its own HITs or NULL. Otherwise, it
must drop the packet.
2. If the Responder is in ESTABLISHED state, the Responder MAY
respond to this with an R1 packet, prepare to drop an existing
HIP security association with the peer, and stay at ESTABLISHED
state.
3. If the Responder is in I1-SENT state, it MUST make a comparison
between the sender's HIT and its own (i.e., the receiver's) HIT.
If the sender's HIT is greater than its own HIT, it should drop
the I1 packet and stay at I1-SENT. If the sender's HIT is
smaller than its own HIT, it SHOULD send the R1 packet and stay
at I1-SENT. The HIT comparison is performed as defined in
Section 6.5.
4. If the implementation chooses to respond to the I1 packet with an
R1 packet, it creates a new R1 or selects a precomputed R1
according to the format described in Section 5.3.2. It creates
or chooses an R1 that contains its most preferred DH public value
that is also contained in the DH_GROUP_LIST in the I1 packet. If
no suitable DH Group ID was contained in the DH_GROUP_LIST in the
I1 packet, it sends an R1 with any suitable DH public key.
5. If the received Responder's HIT in the I1 is NULL, the Responder
selects a HIT with the same HIT Suite as the Initiator's HIT. If
this HIT Suite is not supported by the Responder, it SHOULD
select a REQUIRED HIT Suite from Section 5.2.10, which is
currently RSA/DSA/SHA-256. Other than that, selecting the HIT is
a local policy matter.
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6. The Responder expresses its supported HIP transport formats in
the TRANSPORT_FORMAT_LIST as described in Section 5.2.11. The
Responder MUST provide at least one payload transport format
type.
7. The Responder sends the R1 packet to the source IP address of the
I1 packet.
6.7.1. R1 Management
All compliant implementations MUST be able to produce R1 packets;
even if a device is configured by policy to only initiate
associations, it must be able to process I1s in cases of recovery
from loss of state or key exhaustion. An R1 packet MAY be
precomputed. An R1 packet MAY be reused for a short time period,
denoted here as "Delta T", which is implementation dependent, and
SHOULD be deprecated and not used once a valid response I2 packet has
been received from an Initiator. During an I1 message storm, an R1
packet MAY be reused beyond the normal Delta T. R1 information MUST
NOT be discarded until a time period "Delta S" (again, implementation
dependent) after the R1 packet is no longer being offered. Delta S
is the assumed maximum time needed for the last I2 packet in response
to the R1 packet to arrive back at the Responder.
Implementations that support multiple DH groups MAY precompute R1
packets for each supported group so that incoming I1 packets with
different DH Group IDs in the DH_GROUP_LIST can be served quickly.
An implementation MAY keep state about received I1 packets and match
the received I2 packets against the state, as discussed in
Section 4.1.1.
6.7.2. Handling of Malformed Messages
If an implementation receives a malformed I1 packet, it SHOULD NOT
respond with a NOTIFY message, as such a practice could open up a
potential denial-of-service threat. Instead, it MAY respond with an
ICMP packet, as defined in Section 5.4.
6.8. Processing of Incoming R1 Packets
A system receiving an R1 packet MUST first check to see if it has
sent an I1 packet to the originator of the R1 packet (i.e., it is in
state I1-SENT). If so, it SHOULD process the R1 as described below,
send an I2 packet, and transition to state I2-SENT, setting a timer
to protect the I2 packet. If the system is in state I2-SENT, it MAY
respond to the R1 packet if the R1 packet has a larger R1 generation
counter; if so, it should drop its state due to processing the
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previous R1 packet and start over from state I1-SENT. If the system
is in any other state with respect to that host, the system SHOULD
silently drop the R1 packet.
When sending multiple I1 packets, an Initiator SHOULD wait for a
small amount of time after the first R1 reception to allow possibly
multiple R1 packets to arrive, and it SHOULD respond to an R1 packet
among the set with the largest R1 generation counter.
The following steps define the conceptual processing rules for
responding to an R1 packet:
1. A system receiving an R1 MUST first check to see if it has sent
an I1 packet to the originator of the R1 packet (i.e., it has a
HIP association that is in state I1-SENT and that is associated
with the HITs in the R1). Unless the I1 packet was sent in
opportunistic mode (see Section 4.1.8), the IP addresses in the
received R1 packet SHOULD be ignored by the R1 processing and,
when looking up the right HIP association, the received R1
packet SHOULD be matched against the associations using only the
HITs. If a match exists, the system should process the R1
packet as described below.
2. Otherwise, if the system is in any state other than I1-SENT or
I2-SENT with respect to the HITs included in the R1 packet, it
SHOULD silently drop the R1 packet and remain in the current
state.
3. If the HIP association state is I1-SENT or I2-SENT, the received
Initiator's HIT MUST correspond to the HIT used in the original
I1. Also, the Responder's HIT MUST correspond to the one used
in the I1, unless the I1 packet contained a NULL HIT.
4. The system SHOULD validate the R1 signature before applying
further packet processing, according to Section 5.2.15.
5. If the HIP association state is I1-SENT, and multiple valid R1
packets are present, the system MUST select from among the R1
packets with the largest R1 generation counter.
6. The system MUST check that the Initiator's HIT Suite is
contained in the HIT_SUITE_LIST parameter in the R1 packet
(i.e., the Initiator's HIT Suite is supported by the Responder).
If the HIT Suite is supported by the Responder, the system
proceeds normally. Otherwise, the system MAY stay in state
I1-SENT and restart the BEX by sending a new I1 packet with an
Initiator HIT that is supported by the Responder and hence is
contained in the HIT_SUITE_LIST in the R1 packet. The system
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MAY abort the BEX if no suitable source HIT is available. The
system SHOULD wait for an acceptable time span to allow further
R1 packets with higher R1 generation counters or different HIT
and HIT Suites to arrive before restarting or aborting the BEX.
7. The system MUST check that the DH Group ID in the DIFFIE_HELLMAN
parameter in the R1 matches the first DH Group ID in the
Responder's DH_GROUP_LIST in the R1 packet, and also that this
Group ID corresponds to a value that was included in the
Initiator's DH_GROUP_LIST in the I1 packet. If the DH Group ID
of the DIFFIE_HELLMAN parameter does not express the Responder's
best choice, the Initiator can conclude that the DH_GROUP_LIST
in the I1 packet was adversely modified. In such a case, the
Initiator MAY send a new I1 packet; however, it SHOULD NOT
change its preference in the DH_GROUP_LIST in the new I1 packet.
Alternatively, the Initiator MAY abort the HIP base exchange.
8. If the HIP association state is I2-SENT, the system MAY re-enter
state I1-SENT and process the received R1 packet if it has a
larger R1 generation counter than the R1 packet responded to
previously.
9. The R1 packet may have the A-bit set -- in this case, the system
MAY choose to refuse it by dropping the R1 packet and returning
to state UNASSOCIATED. The system SHOULD consider dropping the
R1 packet only if it used a NULL HIT in the I1 packet. If the
A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be
stored permanently.
10. The system SHOULD attempt to validate the HIT against the
received Host Identity by using the received Host Identity to
construct a HIT and verify that it matches the Sender's HIT.
11. The system MUST store the received R1 generation counter for
future reference.
12. The system attempts to solve the puzzle in the R1 packet. The
system MUST terminate the search after exceeding the remaining
lifetime of the puzzle. If the puzzle is not successfully
solved, the implementation MAY either resend the I1 packet
within the retry bounds or abandon the HIP base exchange.
13. The system computes standard Diffie-Hellman keying material
according to the public value and Group ID provided in the
DIFFIE_HELLMAN parameter. The Diffie-Hellman keying material
Kij is used for key extraction as specified in Section 6.5.
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14. The system selects the HIP_CIPHER ID from the choices presented
in the R1 packet and uses the selected values subsequently when
generating and using encryption keys, and when sending the I2
packet. If the proposed alternatives are not acceptable to the
system, it may either resend an I1 within the retry bounds or
abandon the HIP base exchange.
15. The system chooses one suitable transport format from the
TRANSPORT_FORMAT_LIST and includes the respective transport
format parameter in the subsequent I2 packet.
16. The system initializes the remaining variables in the associated
state, including Update ID counters.
17. The system prepares and sends an I2 packet, as described in
Section 5.3.3.
18. The system SHOULD start a timer whose timeout value SHOULD be
larger than the worst-case anticipated RTT, and MUST increment a
trial counter associated with the I2 packet. The sender SHOULD
retransmit the I2 packet upon a timeout and restart the timer,
up to a maximum of I2_RETRIES_MAX tries.
19. If the system is in state I1-SENT, it SHALL transition to state
I2-SENT. If the system is in any other state, it remains in the
current state.
6.8.1. Handling of Malformed Messages
If an implementation receives a malformed R1 message, it MUST
silently drop the packet. Sending a NOTIFY or ICMP would not help,
as the sender of the R1 packet typically doesn't have any state. An
implementation SHOULD wait for some more time for a possibly well-
formed R1, after which it MAY try again by sending a new I1 packet.
6.9. Processing of Incoming I2 Packets
Upon receipt of an I2 packet, the system MAY perform initial checks
to determine whether the I2 packet corresponds to a recent R1 packet
that has been sent out, if the Responder keeps such state. For
example, the sender could check whether the I2 packet is from an
address or HIT for which the Responder has recently received an I1.
The R1 packet may have had opaque data included that was echoed back
in the I2 packet. If the I2 packet is considered to be suspect, it
MAY be silently discarded by the system.
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Otherwise, the HIP implementation SHOULD process the I2 packet. This
includes validation of the puzzle solution, generating the
Diffie-Hellman key, possibly decrypting the Initiator's Host
Identity, verifying the signature, creating state, and finally
sending an R2 packet.
The following steps define the conceptual processing rules for
responding to an I2 packet:
1. The system MAY perform checks to verify that the I2 packet
corresponds to a recently sent R1 packet. Such checks are
implementation dependent. See Appendix A for a description of
an example implementation.
2. The system MUST check that the Responder's HIT corresponds to
one of its own HITs and MUST drop the packet otherwise.
3. The system MUST further check that the Initiator's HIT Suite is
supported. The Responder SHOULD silently drop I2 packets with
unsupported Initiator HITs.
4. If the system's state machine is in the R2-SENT state, the
system MAY check to see if the newly received I2 packet is
similar to the one that triggered moving to R2-SENT. If so, it
MAY retransmit a previously sent R2 packet and reset the R2-SENT
timer, and the state machine stays in R2-SENT.
5. If the system's state machine is in the I2-SENT state, the
system MUST make a comparison between its local and sender's
HITs (similar to the comparison method described in
Section 6.5). If the local HIT is smaller than the sender's
HIT, it should drop the I2 packet, use the peer Diffie-Hellman
key and nonce #I from the R1 packet received earlier, and get
the local Diffie-Hellman key and nonce #J from the I2 packet
sent to the peer earlier. Otherwise, the system should process
the received I2 packet and drop any previously derived
Diffie-Hellman keying material Kij it might have formed upon
sending the I2 packet previously. The peer Diffie-Hellman key
and the nonce #J are taken from the I2 packet that just arrived.
The local Diffie-Hellman key and the nonce #I are the ones that
were sent earlier in the R1 packet.
6. If the system's state machine is in the I1-SENT state, and the
HITs in the I2 packet match those used in the previously sent I1
packet, the system uses this received I2 packet as the basis for
the HIP association it was trying to form, and stops
retransmitting I1 packets (provided that the I2 packet passes
the additional checks below).
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7. If the system's state machine is in any state other than
R2-SENT, the system SHOULD check that the echoed R1 generation
counter in the I2 packet is within the acceptable range if the
counter is included. Implementations MUST accept puzzles from
the current generation and MAY accept puzzles from earlier
generations. If the generation counter in the newly received I2
packet is outside the accepted range, the I2 packet is stale
(and perhaps replayed) and SHOULD be dropped.
8. The system MUST validate the solution to the puzzle by computing
the hash described in Section 5.3.3 using the same RHASH
algorithm.
9. The I2 packet MUST have a single value in the HIP_CIPHER
parameter, which MUST match one of the values offered to the
Initiator in the R1 packet.
10. The system must derive Diffie-Hellman keying material Kij based
on the public value and Group ID in the DIFFIE_HELLMAN
parameter. This key is used to derive the HIP association keys,
as described in Section 6.5. If the Diffie-Hellman Group ID is
unsupported, the I2 packet is silently dropped.
11. The encrypted HOST_ID is decrypted by the Initiator's encryption
key defined in Section 6.5. If the decrypted data is not a
HOST_ID parameter, the I2 packet is silently dropped.
12. The implementation SHOULD also verify that the Initiator's HIT
in the I2 packet corresponds to the Host Identity sent in the I2
packet. (Note: some middleboxes may not be able to make this
verification.)
13. The system MUST process the TRANSPORT_FORMAT_LIST parameter.
Other documents specifying transport formats (e.g., [RFC 7402])
contain specifications for handling any specific transport
selected.
14. The system MUST verify the HIP_MAC according to the procedures
in Section 5.2.12.
15. The system MUST verify the HIP_SIGNATURE according to
Sections 5.2.14 and 5.3.3.
16. If the checks above are valid, then the system proceeds with
further I2 processing; otherwise, it discards the I2 and its
state machine remains in the same state.
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17. The I2 packet may have the A-bit set -- in this case, the system
MAY choose to refuse it by dropping the I2 and the state machine
returns to state UNASSOCIATED. If the A-bit is set, the
Initiator's HIT is anonymous and should not be stored
permanently.
18. The system initializes the remaining variables in the associated
state, including Update ID counters.
19. Upon successful processing of an I2 message when the system's
state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or
R2-SENT, an R2 packet is sent and the system's state machine
transitions to state R2-SENT.
20. Upon successful processing of an I2 packet when the system's
state machine is in state ESTABLISHED, the old HIP association
is dropped and a new one is installed, an R2 packet is sent, and
the system's state machine transitions to R2-SENT.
21. Upon the system's state machine transitioning to R2-SENT, the
system starts a timer. The state machine transitions to
ESTABLISHED if some data has been received on the incoming HIP
association, or an UPDATE packet has been received (or some
other packet that indicates that the peer system's state machine
has moved to ESTABLISHED). If the timer expires (allowing for a
maximal amount of retransmissions of I2 packets), the state
machine transitions to ESTABLISHED.
6.9.1. Handling of Malformed Messages
If an implementation receives a malformed I2 message, the behavior
SHOULD depend on how many checks the message has already passed. If
the puzzle solution in the message has already been checked, the
implementation SHOULD report the error by responding with a NOTIFY
packet. Otherwise, the implementation MAY respond with an ICMP
message as defined in Section 5.4.
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6.10. Processing of Incoming R2 Packets
An R2 packet received in state UNASSOCIATED, I1-SENT, or ESTABLISHED
results in the R2 packet being dropped and the state machine staying
in the same state. If an R2 packet is received in state I2-SENT, it
MUST be processed.
The following steps define the conceptual processing rules for an
incoming R2 packet:
1. If the system is in any state other than I2-SENT, the R2 packet
is silently dropped.
2. The system MUST verify that the HITs in use correspond to the
HITs that were received in the R1 packet that caused the
transition to the I1-SENT state.
3. The system MUST verify the HIP_MAC_2 according to the procedures
in Section 5.2.13.
4. The system MUST verify the HIP signature according to the
procedures in Section 5.2.14.
5. If any of the checks above fail, there is a high probability of
an ongoing man-in-the-middle or other security attack. The
system SHOULD act accordingly, based on its local policy.
6. Upon successful processing of the R2 packet, the state machine
transitions to state ESTABLISHED.
6.11. Sending UPDATE Packets
A host sends an UPDATE packet when it intends to update some
information related to a HIP association. There are a number of
possible scenarios when this can occur, e.g., mobility management and
rekeying of an existing ESP Security Association. The following
paragraphs define the conceptual rules for sending an UPDATE packet
to the peer. Additional steps can be defined in other documents
where the UPDATE packet is used.
The sequence of UPDATE messages is indicated by their SEQ parameter.
Before sending an UPDATE message, the system first determines whether
there are any outstanding UPDATE messages that may conflict with the
new UPDATE message under consideration. When multiple UPDATEs are
outstanding (not yet acknowledged), the sender must assume that such
UPDATEs may be processed in an arbitrary order by the receiver.
Therefore, any new UPDATEs that depend on a previous outstanding
UPDATE being successfully received and acknowledged MUST be postponed
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until reception of the necessary ACK(s) occurs. One way to prevent
any conflicts is to only allow one outstanding UPDATE at a time.
However, allowing multiple UPDATEs may improve the performance of
mobility and multihoming protocols.
The following steps define the conceptual processing rules for
sending UPDATE packets:
1. The first UPDATE packet is sent with an Update ID of zero.
Otherwise, the system increments its own Update ID value by one
before continuing the steps below.
2. The system creates an UPDATE packet that contains a SEQ parameter
with the current value of the Update ID. The UPDATE packet MAY
also include zero or more ACKs of the peer's Update ID(s) from
previously received UPDATE SEQ parameter(s).
3. The system sends the created UPDATE packet and starts an UPDATE
timer. The default value for the timer is 2 * RTT estimate. If
multiple UPDATEs are outstanding, multiple timers are in effect.
4. If the UPDATE timer expires, the UPDATE is resent. The UPDATE
can be resent UPDATE_RETRY_MAX times. The UPDATE timer SHOULD be
exponentially backed off for subsequent retransmissions. If no
acknowledgment is received from the peer after UPDATE_RETRY_MAX
times, the HIP association is considered to be broken and the
state machine SHOULD move from state ESTABLISHED to state CLOSING
as depicted in Section 4.4.4. The UPDATE timer is cancelled upon
receiving an ACK from the peer that acknowledges receipt of the
UPDATE.
6.12. Receiving UPDATE Packets
When a system receives an UPDATE packet, its processing depends on
the state of the HIP association and the presence and values of the
SEQ and ACK parameters. Typically, an UPDATE message also carries
optional parameters whose handling is defined in separate documents.
For each association, a host stores the peer's next expected
in-sequence Update ID ("peer Update ID"). Initially, this value is
zero. Update ID comparisons of "less than" and "greater than" are
performed with respect to a circular sequence number space. Hence, a
wraparound after 2^32 updates has to be expected and MUST be handled
accordingly.
The sender MAY send multiple outstanding UPDATE messages. These
messages are processed in the order in which they are received at the
receiver (i.e., no resequencing is performed). When processing
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UPDATEs out of order, the receiver MUST keep track of which UPDATEs
were previously processed, so that duplicates or retransmissions are
ACKed and not reprocessed. A receiver MAY choose to define a receive
window of Update IDs that it is willing to process at any given time,
and discard received UPDATEs falling outside of that window.
The following steps define the conceptual processing rules for
receiving UPDATE packets:
1. If there is no corresponding HIP association, the implementation
MAY reply with an ICMP Parameter Problem, as specified in
Section 5.4.4.
2. If the association is in the ESTABLISHED state and the SEQ (but
not ACK) parameter is present, the UPDATE is processed and
replied to as described in Section 6.12.1.
3. If the association is in the ESTABLISHED state and the ACK (but
not SEQ) parameter is present, the UPDATE is processed as
described in Section 6.12.2.
4. If the association is in the ESTABLISHED state and there are both
an ACK and SEQ in the UPDATE, the ACK is first processed as
described in Section 6.12.2, and then the rest of the UPDATE is
processed as described in Section 6.12.1.
6.12.1. Handling a SEQ Parameter in a Received UPDATE Message
The following steps define the conceptual processing rules for
handling a SEQ parameter in a received UPDATE packet:
1. If the Update ID in the received SEQ is not the next in the
sequence of Update IDs and is greater than the receiver's window
for new UPDATEs, the packet MUST be dropped.
2. If the Update ID in the received SEQ corresponds to an UPDATE
that has recently been processed, the packet is treated as a
retransmission. The HIP_MAC verification (next step) MUST NOT be
skipped. (A byte-by-byte comparison of the received packet and a
stored packet would be acceptable, though.) It is recommended
that a host caches UPDATE packets sent with ACKs to avoid the
cost of generating a new ACK packet to respond to a replayed
UPDATE. The system MUST acknowledge, again, such (apparent)
UPDATE message retransmissions but SHOULD also consider rate-
limiting such retransmission responses to guard against replay
attacks.
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3. The system MUST verify the HIP_MAC in the UPDATE packet. If the
verification fails, the packet MUST be dropped.
4. The system MAY verify the SIGNATURE in the UPDATE packet. If the
verification fails, the packet SHOULD be dropped and an error
message logged.
5. If a new SEQ parameter is being processed, the parameters in the
UPDATE are then processed. The system MUST record the Update ID
in the received SEQ parameter, for replay protection.
6. An UPDATE acknowledgment packet with the ACK parameter is
prepared and sent to the peer. This ACK parameter MAY be
included in a separate UPDATE or piggybacked in an UPDATE with
the SEQ parameter, as described in Section 5.3.5. The ACK
parameter MAY acknowledge more than one of the peer's Update IDs.
6.12.2. Handling an ACK Parameter in a Received UPDATE Packet
The following steps define the conceptual processing rules for
handling an ACK parameter in a received UPDATE packet:
1. The sequence number reported in the ACK must match with an UPDATE
packet sent earlier that has not already been acknowledged. If
no match is found or if the ACK does not acknowledge a new
UPDATE, then either the packet MUST be dropped if no SEQ
parameter is present, or the processing steps in Section 6.12.1
are followed.
2. The system MUST verify the HIP_MAC in the UPDATE packet. If the
verification fails, the packet MUST be dropped.
3. The system MAY verify the SIGNATURE in the UPDATE packet. If the
verification fails, the packet SHOULD be dropped and an error
message logged.
4. The corresponding UPDATE timer is stopped (see Section 6.11) so
that the now-acknowledged UPDATE is no longer retransmitted. If
multiple UPDATEs are acknowledged, multiple timers are stopped.
6.13. Processing of NOTIFY Packets
Processing of NOTIFY packets is OPTIONAL. If processed, any errors
in a received NOTIFICATION parameter SHOULD be logged. Received
errors MUST be considered only as informational, and the receiver
SHOULD NOT change its HIP state (see Section 4.4.2) purely based on
the received NOTIFY message.
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6.14. Processing of CLOSE Packets
When the host receives a CLOSE message, it responds with a CLOSE_ACK
message and moves to the CLOSED state. (The authenticity of the
CLOSE message is verified using both HIP_MAC and SIGNATURE.) This
processing applies whether or not the HIP association state is
CLOSING, in order to handle simultaneous CLOSE messages from both
ends that cross in flight.
The HIP association is not discarded before the host moves to the
UNASSOCIATED state.
Once the closing process has started, any new need to send data
packets triggers the creation and establishment of a new HIP
association, starting with sending an I1 packet.
If there is no corresponding HIP association, the CLOSE packet is
dropped.
6.15. Processing of CLOSE_ACK Packets
When a host receives a CLOSE_ACK message, it verifies that it is in
the CLOSING or CLOSED state and that the CLOSE_ACK was in response to
the CLOSE. A host can map CLOSE_ACK messages to CLOSE messages by
comparing the value of ECHO_REQUEST_SIGNED (in the CLOSE packet) to
the value of ECHO_RESPONSE_SIGNED (in the CLOSE_ACK packet).
The CLOSE_ACK contains the HIP_MAC and the SIGNATURE parameters for
verification. The state is discarded when the state changes to
UNASSOCIATED and, after that, the host MAY respond with an ICMP
Parameter Problem to an incoming CLOSE message (see Section 5.4.4).
6.16. Handling State Loss
In the case of a system crash and unanticipated state loss, the
system SHOULD delete the corresponding HIP state, including the
keying material. That is, the state SHOULD NOT be stored in
long-term storage. If the implementation does drop the state
(as RECOMMENDED), it MUST also drop the peer's R1 generation counter
value, unless a local policy explicitly defines that the value of
that particular host is stored. An implementation MUST NOT store a
peer's R1 generation counters by default, but storing R1 generation
counter values, if done, MUST be configured by explicit HITs.
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7. HIP Policies
There are a number of variables that will influence the HIP base
exchanges that each host must support. All HIP implementations MUST
support more than one simultaneous HI, at least one of which SHOULD
be reserved for anonymous usage. Although anonymous HIs will be
rarely used as Responders' HIs, they will be common for Initiators.
Support for more than two HIs is RECOMMENDED.
Initiators MAY use a different HI for different Responders to provide
basic privacy. Whether such private HIs are used repeatedly with the
same Responder, and how long these HIs are used, are decided by local
policy and depend on the privacy requirements of the Initiator.
The value of #K used in the HIP R1 must be chosen with care. Values
of #K that are too high will exclude clients with weak CPUs because
these devices cannot solve the puzzle within a reasonable amount of
time. #K should only be raised if a Responder is under high load,
i.e., it cannot process all incoming HIP handshakes any more. If a
Responder is not under high load, #K SHOULD be 0.
Responders that only respond to selected Initiators require an Access
Control List (ACL), representing for which hosts they accept HIP base
exchanges, and the preferred transport format and local lifetimes.
Wildcarding SHOULD be supported for such ACLs, and also for
Responders that offer public or anonymous services.
8. Security Considerations
HIP is designed to provide secure authentication of hosts. HIP also
attempts to limit the exposure of the host to various denial-of-
service and man-in-the-middle (MitM) attacks. In doing so, HIP
itself is subject to its own DoS and MitM attacks that potentially
could be more damaging to a host's ability to conduct business as
usual.
Denial-of-service attacks often take advantage of asymmetries in the
cost of starting an association. One example of such asymmetry is
the need of a Responder to store local state while a malicious
Initiator can stay stateless. HIP makes no attempt to increase the
cost of the start of state at the Initiator, but makes an effort to
reduce the cost for the Responder. This is accomplished by having
the Responder start the 3-way exchange instead of the Initiator,
making the HIP exchange 4 packets long. In doing this, the first
packet from the Responder, R1, becomes a 'stock' packet that the
Responder MAY use many times, until some Initiator has provided a
valid response to such an R1 packet. During an I1 packet storm, the
host may reuse the same DH value also, even if some Initiator has
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provided a valid response using that particular DH value. However,
such behavior is discouraged and should be avoided. Using the same
Diffie-Hellman values and random puzzle #I value has some risks.
This risk needs to be balanced against a potential storm of HIP I1
packets.
This shifting of the start of state cost to the Initiator in creating
the I2 HIP packet presents another DoS attack. The attacker can
spoof the I1 packet, and the Responder sends out the R1 HIP packet.
This could conceivably tie up the 'Initiator' with evaluating the R1
HIP packet, and creating the I2 packet. The defense against this
attack is to simply ignore any R1 packet where a corresponding I1
packet was not sent (as defined in Section 6.8, step 1).
The R1 packet is considerably larger than the I1 packet. This
asymmetry can be exploited in a reflection attack. A malicious
attacker could spoof the IP address of a victim and send a flood of
I1 messages to a powerful Responder. For each small I1 packet, the
Responder would send a larger R1 packet to the victim. The
difference in packet sizes can further amplify a flooding attack
against the victim. To avoid such reflection attacks, the Responder
SHOULD rate-limit the sending of R1 packets in general or SHOULD
rate-limit the sending of R1 packets to a specific IP address.
Floods of forged I2 packets form a second kind of DoS attack. Once
the attacking Initiator has solved the puzzle, it can send packets
with spoofed IP source addresses with either an invalid HIP signature
or invalid encrypted HIP payload (in the ENCRYPTED parameter). This
would take resources in the Responder's part to reach the point to
discover that the I2 packet cannot be completely processed. The
defense against this attack is that after N bad I2 packets with the
same puzzle solution, the Responder would discard any I2 packets that
contain the given solution. This will shut down the attack. The
attacker would have to request another R1 packet and use that to
launch a new attack. The Responder could increase the value of #K
while under attack. Keeping a list of solutions from malformed
packets requires that the Responder keeps state for these malformed
I2 packets. This state has to be kept until the R1 counter is
increased. As malformed packets are generally filtered by their
checksum before signature verification, only solutions in packets
that are forged to pass the checksum and puzzle are put into the
blacklist. In addition, a valid puzzle is required before a new list
entry is created. Hence, attackers that intend to flood the
blacklist must solve puzzles first.
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A third form of DoS attack is emulating the restart of state after a
reboot of one of the peers. A restarting host would send an I1
packet to the peers, which would respond with an R1 packet even if it
were in the ESTABLISHED state. If the I1 packet were spoofed, the
resulting R1 packet would be received unexpectedly by the spoofed
host and would be dropped, as in the first case above.
A fourth form of DoS attack is emulating the closing of the HIP
association. HIP relies on timers and a CLOSE/CLOSE_ACK handshake to
explicitly signal the end of a HIP association. Because both CLOSE
and CLOSE_ACK messages contain a HIP_MAC, an outsider cannot close a
connection. The presence of an additional SIGNATURE allows
middleboxes to inspect these messages and discard the associated
state (e.g., for firewalling, SPI-based NATing, etc.). However, the
optional behavior of replying to CLOSE with an ICMP Parameter Problem
packet (as described in Section 5.4.4) might allow an attacker
spoofing the source IP address to send CLOSE messages to launch
reflection attacks.
A fifth form of DoS attack is replaying R1s to cause the Initiator to
solve stale puzzles and become out of synchronization with the
Responder. The R1 generation counter is a monotonically increasing
counter designed to protect against this attack, as described in
Section 4.1.4.
Man-in-the-middle attacks are difficult to defend against, without
third-party authentication. A skillful MitM could easily handle all
parts of HIP, but HIP indirectly provides the following protection
from a MitM attack. If the Responder's HI is retrieved from a signed
DNS zone, a certificate, or through some other secure means, the
Initiator can use this to validate the R1 HIP packet.
Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
certificate, or otherwise securely available, the Responder can
retrieve the HI (after having got the I2 HIP packet) and verify that
the HI indeed can be trusted.
The HIP "opportunistic mode" concept has been introduced in this
document, but this document does not specify what the semantics of
such a connection setup are for applications. There are certain
concerns with opportunistic mode, as discussed in Section 4.1.8.
NOTIFY messages are used only for informational purposes, and they
are unacknowledged. A HIP implementation cannot rely solely on the
information received in a NOTIFY message because the packet may have
been replayed. An implementation SHOULD NOT change any state
information purely based on a received NOTIFY message.
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Since not all hosts will ever support HIP, ICMP 'Destination Protocol
Unreachable' messages are to be expected and may be used for a DoS
attack. Against an Initiator, the attack would look like the
Responder does not support HIP, but shortly after receiving the ICMP
message, the Initiator would receive a valid R1 HIP packet. Thus, to
protect against this attack, an Initiator SHOULD NOT react to an ICMP
message until a reasonable delta time to get the real Responder's R1
HIP packet. A similar attack against the Responder is more involved.
Normally, if an I1 message received by a Responder was a bogus one
sent by an attacker, the Responder may receive an ICMP message from
the IP address the R1 message was sent to. However, a sophisticated
attacker can try to take advantage of such behavior and try to break
up the HIP base exchange by sending such an ICMP message to the
Responder before the Initiator has a chance to send a valid I2
message. Hence, the Responder SHOULD NOT act on such an ICMP
message. Especially, it SHOULD NOT remove any minimal state created
when it sent the R1 HIP packet (if it did create one), but wait for
either a valid I2 HIP packet or the natural timeout (that is, if R1
packets are tracked at all). Likewise, the Initiator SHOULD ignore
any ICMP message while waiting for an R2 HIP packet, and SHOULD
delete any pending state only after a natural timeout.
9. IANA Considerations
IANA has reserved protocol number 139 for the Host Identity Protocol
and included it in the "IPv6 Extension Header Types" registry
[RFC 7045] and the "Assigned Internet Protocol Numbers" registry. The
reference in both of these registries has been updated from [RFC 5201]
to this specification.
The reference to the 128-bit value under the CGA Message Type
namespace [RFC 3972] of "0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA"
has been changed from [RFC 5201] to this specification.
The following changes to the "Host Identity Protocol (HIP)
Parameters" have been made. In many cases, the changes involved
updating the reference from [RFC 5201] to this specification, but
there are some differences as outlined below. Allocation terminology
is defined in [RFC 5226]; any existing references to "IETF Consensus"
can be replaced with "IETF Review" as per [RFC 5226].
Moskowitz, et al. Standards Track PAGE 109
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HIP Version
This document adds the value "2" to the existing registry. The
value of "1" has been left with a reference to [RFC 5201].
Packet Type
The 7-bit Packet Type field in a HIP protocol packet describes the
type of a HIP protocol message. It is defined in Section 5.1.
All existing values referring to [RFC 5201] have been updated to
refer to this specification. Other values have been left
unchanged.
HIT Suite ID
This specification creates a new registry for "HIT Suite ID".
This is different than the existing registry for "Suite ID", which
can be left unmodified for version 1 of the protocol ([RFC 5201]).
The registry has been closed to new registrations.
The four-bit HIT Suite ID uses the OGA ID field in the ORCHID to
express the type of the HIT. This document defines three HIT
Suites (see Section 5.2.10).
The HIT Suite ID is also carried in the four higher-order bits of
the ID field in the HIT_SUITE_LIST parameter. The four
lower-order bits are reserved for future extensions of the HIT
Suite ID space beyond 16 values.
For the time being, the HIT Suite uses only four bits because
these bits have to be carried in the HIT. Using more bits for the
HIT Suite ID reduces the cryptographic strength of the HIT. HIT
Suite IDs must be allocated carefully to avoid namespace
exhaustion. Moreover, deprecated IDs should be reused after an
appropriate time span. If 15 Suite IDs (the zero value is
initially reserved) prove to be insufficient and more HIT Suite
IDs are needed concurrently, more bits can be used for the HIT
Suite ID by using one HIT Suite ID (0) to indicate that more bits
should be used. The HIT_SUITE_LIST parameter already supports
8-bit HIT Suite IDs, should longer IDs be needed. However,
RFC 7343 [RFC 7343] does not presently support such an extension.
We suggest trying the rollover approach described in Appendix E
first. Possible extensions of the HIT Suite ID space to
accommodate eight bits and new HIT Suite IDs are defined through
IETF Review.
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Requests to register reused values should include a note that the
value is being reused after a deprecation period, to ensure
appropriate IETF review and approval.
Parameter Type
The 16-bit Type field in a HIP parameter describes the type of the
parameter. It is defined in Section 5.2.1. The current values
are defined in Sections 5.2.3 through 5.2.23. The existing
"Parameter Types" registry has been updated as follows.
A new value (129) for R1_COUNTER has been introduced, with a
reference to this specification, and the existing value (128) for
R1_COUNTER has been left in place with a reference to [RFC 5201].
This documents the change in value that has occurred in version 2
of this protocol. For clarity, the name for the value 128 has
been changed from "R1_COUNTER" to "R1_Counter (v1 only)".
A new value (579) for a new Parameter Type HIP_CIPHER has been
added, with reference to this specification. This Parameter Type
functionally replaces the HIP_TRANSFORM Parameter Type
(value 577), which has been left in the table with the existing
reference to [RFC 5201]. For clarity, the name for the
value 577 has been changed from "HIP_TRANSFORM" to
"HIP_TRANSFORM (v1 only)".
A new value (715) for a new Parameter Type HIT_SUITE_LIST has been
added, with reference to this specification.
A new value (2049) for a new Parameter Type TRANSPORT_FORMAT_LIST
has been added, with reference to this specification.
The name of the HMAC Parameter Type (value 61505) has been changed
to HIP_MAC. The name of the HMAC_2 Parameter Type (value 61569)
has been changed to HIP_MAC_2. The reference has been changed to
this specification.
All other Parameter Types that reference [RFC 5201] have been
updated to refer to this specification, and Parameter Types that
reference other RFCs are unchanged.
The Type codes 32768 through 49151 (not 49141: a value corrected
from a previous version of this table) have been Reserved for
Private Use. Implementors SHOULD select types in a random fashion
from this range, thereby reducing the probability of collisions.
A method employing genuine randomness (such as flipping a coin)
SHOULD be used.
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Where the existing ranges once stated "First Come First Served
with Specification Required", this has been changed to
"Specification Required".
Group ID
The eight-bit Group ID values appear in the DIFFIE_HELLMAN
parameter and the DH_GROUP_LIST parameter and are defined in
Section 5.2.7. This registry has been updated based on the new
values specified in Section 5.2.7; values noted as being
DEPRECATED can be left in the table with reference to [RFC 5201].
New values are assigned through IETF Review.
HIP Cipher ID
The 16-bit Cipher ID values in a HIP_CIPHER parameter are defined
in Section 5.2.8. This is a new registry. New values from either
the reserved or unassigned space are assigned through IETF Review.
DI-Type
The four-bit DI-Type values in a HOST_ID parameter are defined in
Section 5.2.9. New values are assigned through IETF Review. All
existing values referring to [RFC 5201] have been updated to refer
to this specification.
HI Algorithm
The 16-bit Algorithm values in a HOST_ID parameter are defined in
Section 5.2.9. This is a new registry. New values from either
the reserved or unassigned space are assigned through IETF Review.
ECC Curve Label
When the HI Algorithm values in a HOST_ID parameter are defined to
the values of either "ECDSA" or "ECDSA_LOW", a new registry is
needed to maintain the values for the ECC Curve Label as defined
in Section 5.2.9. This might be handled by specifying two
algorithm-specific subregistries named "ECDSA Curve Label" and
"ECDSA_LOW Curve Label". New values are to be assigned through
IETF Review.
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Notify Message Type
The 16-bit Notify Message Type values in a NOTIFICATION parameter
are defined in Section 5.2.19.
Notify Message Type values 1-10 are used for informing about
errors in packet structures, values 11-20 for informing about
problems in parameters containing cryptographic related material,
and values 21-30 for informing about problems in authentication or
packet integrity verification. Parameter numbers above 30 can be
used for informing about other types of errors or events.
The existing registration procedures have been updated as follows.
The range from 1-50 can remain as "IETF Review". The range from
51-8191 has been marked as "Specification Required". Values
8192-16383 remain as "Reserved for Private Use". Values
16384-40959 have been marked as "Specification Required". Values
40960-65535 remain as "Reserved for Private Use".
The following updates to the values have been made to the existing
registry. All existing values referring to [RFC 5201] have been
updated to refer to this specification.
INVALID_HIP_TRANSFORM_CHOSEN has been renamed to
INVALID_HIP_CIPHER_CHOSEN with the same value (17).
A new value of 20 for the type UNSUPPORTED_HIT_SUITE has been
added.
HMAC_FAILED has been renamed to HIP_MAC_FAILED with the same
value (28).
SERVER_BUSY_PLEASE_RETRY has been renamed to
RESPONDER_BUSY_PLEASE_RETRY with the same value (44).
10. Differences from RFC 5201
This section summarizes the technical changes made from [RFC 5201].
This section is informational, intended to help implementors of the
previous protocol version. If any text in this section contradicts
text in other portions of this specification, the text found outside
of this section should be considered normative.
This document specifies the HIP Version 2 protocol, which is not
interoperable with the HIP Version 1 protocol specified in [RFC 5201].
The main technical changes are the inclusion of additional
cryptographic agility features, and an update of the mandatory and
optional algorithms, including Elliptic Curve support via the
Moskowitz, et al. Standards Track PAGE 113
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Elliptic Curve DSA (ECDSA) and Elliptic Curve Diffie-Hellman (ECDH)
algorithms. The mandatory cryptographic algorithm implementations
have been updated, such as replacing HMAC-SHA-1 with HMAC-SHA-256 and
the RSA/SHA-1 signature algorithm with RSASSA-PSS, and adding ECDSA
to RSA as mandatory public key types. This version of HIP is also
aligned with the ORCHID revision [RFC 7343].
The following changes have been made to the protocol operation.
o Section 4.1.3 describes the new process for Diffie-Hellman group
negotiation, an aspect of cryptographic agility. The Initiator
may express a preference for the choice of a DH group in the I1
packet and may suggest multiple possible choices. The Responder
replies with a preference based on local policy and the options
provided by the Initiator. The Initiator may restart the base
exchange if the option chosen by the Responder is unsuitable
(unsupported algorithms).
o Another aspect of cryptographic agility that has been added is the
ability to use different cryptographic hash functions to generate
the HIT. The Responder's HIT hash algorithm (RHASH) terminology
was introduced to support this. In addition, HIT Suites have been
introduced to group the set of cryptographic algorithms used
together for public key signature, hash function, and hash
truncation. The use of HIT Suites constrains the combinatorial
possibilities of algorithm selection for different functions. HIT
Suite IDs are related to the ORCHID OGA ID field ([RFC 7343]).
o The puzzle mechanism has been slightly changed, in that the #I
parameter depends on the HIT hash function (RHASH) selected, and
the specification now advises against reusing the same #I value to
the same Initiator; more details are provided in Sections 4.1.2
and 5.2.4).
o Section 4.1.4 was extended to cover details about R1 generation
counter rollover or reset.
o Section 4.1.6 was added to describe procedures for aborting a HIP
base exchange.
o Section 4.1.7 provides guidance on avoiding downgrade attacks on
the cryptographic algorithms.
o Section 4.1.8 on opportunistic mode has been updated to account
for cryptographic agility by adding HIT selection procedures.
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o The HIP KEYMAT generation has been updated as described in
Section 6.5 to make the key derivation function a negotiable
aspect of the protocol.
o Packet processing for the I1, R1, and I2 packets has been updated
to account for new parameter processing.
o This specification adds a requirement that hosts MUST support
processing of ACK parameters with several SEQ sequence numbers
even when they do not support sending such parameters.
o This document now clarifies that several ECHO_REQUEST_UNSIGNED
parameters may be present in an R1 and that several ECHO_RESPONSE
parameters may be present in an I2.
o Procedures for responding to version mismatches with an ICMP
Parameter Problem have been added.
o The security considerations section (Section 8) has been updated
to remove possible attacks no longer considered applicable.
o The use of the Anonymous bit for making the sender's Host Identity
anonymous is now supported in packets other than the R1 and I2.
o Support for the use of a NULL HIP CIPHER is explicitly limited to
debugging and testing HIP and is no longer a mandatory algorithm
to support.
The following changes have been made to the parameter types and
encodings (Section 5.2).
o Four new parameter types have been added: DH_GROUP_LIST,
HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST.
o Two parameter types have been renamed: HMAC has been renamed to
HIP_MAC, and HMAC2 has been renamed to HIP_MAC_2.
o One parameter type is deprecated: HIP_TRANSFORM. Functionally, it
has been replaced by the HIP_CIPHER but with slightly different
semantics (hashes have been removed and are now determined by
RHASH).
o The TRANSPORT_FORMAT_LIST parameter allows transports to be
negotiated with the list instead of by their order in the
HIP packet.
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o The type code for the R1_COUNTER has been changed from 128 to 129
to reflect that it is now considered a Critical parameter and must
be echoed when present in R1.
o The PUZZLE and SOLUTION parameter lengths are now variable and
dependent on the RHASH length.
o The Diffie-Hellman Group IDs supported have been updated.
o The HOST_ID parameter now requires specification of an Algorithm.
o The NOTIFICATION parameter supports new Notify Message Type
values.
o The HIP_SIGNATURE algorithm field has been changed from 8 bits to
16 bits to achieve alignment with the HOST_ID parameters.
o The specification clarifies that the SEQ parameter always contains
one update ID but that the ACK parameter may acknowledge several
update IDs.
o The restriction that only one ECHO_RESPONSE_UNSIGNED parameter
must be present in each HIP packet has been removed.
o The document creates a new type range allocation for parameters
that are only covered by a signature if a signature is present and
applies it to the newly created DH_GROUP_LIST parameter.
o The document clarifies that several NOTIFY parameters may be
present in a packet.
The following changes have been made to the packet contents
(Section 5.3).
o The I1 packet now carries the Initiator's DH_GROUP_LIST.
o The R1 packet now carries the HIP_CIPHER, HIT_SUITE_LIST,
DH_GROUP_LIST, and TRANSPORT_FORMAT_LIST parameters.
o The I2 packet now carries the HIP_CIPHER and TRANSPORT_FORMAT_LIST
parameters.
o This document clarifies that UPDATE packets that do not contain
either a SEQ or ACK parameter are invalid.
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11. References
11.1. Normative References
[FIPS.180-4.2012]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[NIST.800-131A.2011]
National Institute of Standards and Technology,
"Transitions: Recommendation for Transitioning the Use of
Cryptographic Algorithms and Key Lengths", NIST
SP 800-131A, January 2011, <http://csrc.nist.gov/
publications/nistpubs/800-131A/sp800-131A.pdf>.
[RFC 768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980, <http://www.rfc-editor.org/info/RFC 768>.
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981, <http://www.rfc-editor.org/
info/RFC 793>.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987,
<http://www.rfc-editor.org/info/RFC 1035>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998,
<http://www.rfc-editor.org/info/RFC 2404>.
[RFC 2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998,
<http://www.rfc-editor.org/info/RFC 2410>.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998,
<http://www.rfc-editor.org/info/RFC 2460>.
[RFC 2536] Eastlake 3rd, D., "DSA KEYs and SIGs in the Domain Name
System (DNS)", RFC 2536, March 1999,
<http://www.rfc-editor.org/info/RFC 2536>.
Moskowitz, et al. Standards Track PAGE 117
RFC 7401 HIPv2 April 2015
[RFC 3110] Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, May 2001,
<http://www.rfc-editor.org/info/RFC 3110>.
[RFC 3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003, <http://www.rfc-editor.org/
info/RFC 3526>.
[RFC 3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602,
September 2003, <http://www.rfc-editor.org/info/RFC 3602>.
[RFC 3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005, <http://www.rfc-editor.org/
info/RFC 3972>.
[RFC 4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005, <http://www.rfc-editor.org/
info/RFC 4034>.
[RFC 4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005,
<http://www.rfc-editor.org/info/RFC 4282>.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006,
<http://www.rfc-editor.org/info/RFC 4443>.
[RFC 4754] Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using
the Elliptic Curve Digital Signature Algorithm (ECDSA)",
RFC 4754, January 2007, <http://www.rfc-editor.org/
info/RFC 4754>.
[RFC 4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
May 2007, <http://www.rfc-editor.org/info/RFC 4868>.
[RFC 5702] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
and RRSIG Resource Records for DNSSEC", RFC 5702,
October 2009, <http://www.rfc-editor.org/info/RFC 5702>.
[RFC 6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012,
<http://www.rfc-editor.org/info/RFC 6724>.
Moskowitz, et al. Standards Track PAGE 118
RFC 7401 HIPv2 April 2015
[RFC 7343] Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
Routable Cryptographic Hash Identifiers Version 2
(ORCHIDv2)", RFC 7343, September 2014,
<http://www.rfc-editor.org/info/RFC 7343>.
[RFC 7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 7402, April 2015,
<http://www.rfc-editor.org/info/RFC 7402>.
11.2. Informative References
[AUR05] Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of the
HIP Base Exchange Protocol", in Proceedings of the 10th
Australasian Conference on Information Security and
Privacy, July 2005.
[CRO03] Crosby, S. and D. Wallach, "Denial of Service via
Algorithmic Complexity Attacks", in Proceedings of the
12th USENIX Security Symposium, Washington, D.C.,
August 2003.
[DIF76] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory
Volume IT-22, Number 6, pages 644-654, November 1976.
[FIPS.186-4.2013]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS PUB 186-4, July 2013,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-4.pdf>.
[FIPS.197.2001]
National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", FIPS PUB 197, November 2001,
<http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf>.
[HIP-ARCH] Moskowitz, R., Ed., and M. Komu, "Host Identity Protocol
Architecture", Work in Progress,
draft-ietf-hip-RFC 4423-bis-09, October 2014.
[HIP-DNS-EXT]
Laganier, J., "Host Identity Protocol (HIP) Domain Name
System (DNS) Extension", Work in Progress,
draft-ietf-hip-RFC 5205-bis-06, January 2015.
Moskowitz, et al. Standards Track PAGE 119
RFC 7401 HIPv2 April 2015
[HIP-HOST-MOB]
Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
with the Host Identity Protocol", Work in Progress,
draft-ietf-hip-RFC 5206-bis-08, January 2015.
[HIP-REND-EXT]
Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", Work in Progress,
draft-ietf-hip-RFC 5204-bis-05, December 2014.
[KAU03] Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
protection for UDP-based protocols", in Proceedings of the
10th ACM Conference on Computer and Communications
Security, October 2003.
[KRA03] Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE
Protocols", in Proceedings of CRYPTO 2003, pages 400-425,
August 2003.
[RFC 792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981, <http://www.rfc-editor.org/
info/RFC 792>.
[RFC 2785] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
Attacks on the Diffie-Hellman Key Agreement Method for
S/MIME", RFC 2785, March 2000,
<http://www.rfc-editor.org/info/RFC 2785>.
[RFC 2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000,
<http://www.rfc-editor.org/info/RFC 2898>.
[RFC 3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003,
<http://www.rfc-editor.org/info/RFC 3447>.
[RFC 3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004,
<http://www.rfc-editor.org/info/RFC 3849>.
[RFC 5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008,
<http://www.rfc-editor.org/info/RFC 5201>.
Moskowitz, et al. Standards Track PAGE 120
RFC 7401 HIPv2 April 2015
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008, <http://www.rfc-editor.org/info/RFC 5226>.
[RFC 5338] Henderson, T., Nikander, P., and M. Komu, "Using the Host
Identity Protocol with Legacy Applications", RFC 5338,
September 2008, <http://www.rfc-editor.org/info/RFC 5338>.
[RFC 5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009,
<http://www.rfc-editor.org/info/RFC 5533>.
[RFC 5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010,
<http://www.rfc-editor.org/info/RFC 5737>.
[RFC 5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, May 2010,
<http://www.rfc-editor.org/info/RFC 5869>.
[RFC 5903] Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
Prime (ECP Groups) for IKE and IKEv2", RFC 5903,
June 2010, <http://www.rfc-editor.org/info/RFC 5903>.
[RFC 6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011,
<http://www.rfc-editor.org/info/RFC 6090>.
[RFC 6253] Heer, T. and S. Varjonen, "Host Identity Protocol
Certificates", RFC 6253, May 2011,
<http://www.rfc-editor.org/info/RFC 6253>.
[RFC 7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045, December 2013,
<http://www.rfc-editor.org/info/RFC 7045>.
[RFC 7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014,
<http://www.rfc-editor.org/info/RFC 7296>.
[RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", Communications of the ACM 21 (2),
pp. 120-126, February 1978.
[SECG] SECG, "Recommended Elliptic Curve Domain Parameters",
SEC 2 Version 2.0, January 2010, <http://www.secg.org/>.
Moskowitz, et al. Standards Track PAGE 121
RFC 7401 HIPv2 April 2015
Appendix A. Using Responder Puzzles
As mentioned in Section 4.1.1, the Responder may delay state creation
and still reject most spoofed I2 packets by using a number of
pre-calculated R1 packets and a local selection function. This
appendix defines one possible implementation in detail. The purpose
of this appendix is to give the implementors an idea of how to
implement the mechanism. If the implementation is based on this
appendix, it MAY contain some local modification that makes an
attacker's task harder.
The Responder creates a secret value S, that it regenerates
periodically. The Responder needs to remember the two latest values
of S. Each time the S is regenerated, the R1 generation counter
value is incremented by one.
The Responder generates a pre-signed R1 packet. The signature for
pre-generated R1s must be recalculated when the Diffie-Hellman key is
recomputed or when the R1_COUNTER value changes due to S value
regeneration.
When the Initiator sends the I1 packet for initializing a connection,
the Responder receives the HIT and IP address from the packet, and
generates an #I value for the puzzle. The #I value is set to the
pre-signed R1 packet.
#I value calculation:
#I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), n)
where n = RHASH_len
The RHASH algorithm is the same as is used to generate the
Responder's HIT value.
From an incoming I2 packet, the Responder receives the required
information to validate the puzzle: HITs, IP addresses, and the
information of the used S value from the R1_COUNTER. Using these
values, the Responder can regenerate the #I, and verify it against
the #I received in the I2 packet. If the #I values match, it can
verify the solution using #I, #J, and difficulty #K. If the #I
values do not match, the I2 is dropped.
puzzle_check:
V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), #K )
if V != 0, drop the packet
If the puzzle solution is correct, the #I and #J values are stored
for later use. They are used as input material when keying material
is generated.
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RFC 7401 HIPv2 April 2015
Keeping state about failed puzzle solutions depends on the
implementation. Although it is possible for the Responder not to
keep any state information, it still may do so to protect itself
against certain attacks (see Section 4.1.1).
Appendix B. Generating a Public Key Encoding from an HI
The following pseudo-code illustrates the process to generate a
public key encoding from an HI for both RSA and DSA.
The symbol ":=" denotes assignment; the symbol "+=" denotes
appending. The pseudo-function "encode_in_network_byte_order" takes
two parameters, an integer (bignum) and a length in bytes, and
returns the integer encoded into a byte string of the given length.
switch ( HI.algorithm )
{
case RSA:
buffer := encode_in_network_byte_order ( HI.RSA.e_len,
( HI.RSA.e_len > 255 ) ? 3 : 1 )
buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )
break;
case DSA:
buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
8 * HI.DSA.T )
buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
8 * HI.DSA.T )
buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
8 * HI.DSA.T )
break;
}
Appendix C. Example Checksums for HIP Packets
The HIP checksum for HIP packets is specified in Section 5.1.1.
Checksums for TCP and UDP packets running over HIP-enabled security
associations are specified in Section 4.5.1. The examples below use
[RFC 3849] and [RFC 5737] addresses, and HITs with the prefix of
2001:20 followed by zeros, followed by a decimal 1 or 2,
respectively.
Moskowitz, et al. Standards Track PAGE 123
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The following example is defined only for testing the checksum
calculation.
C.1. IPv6 HIP Example (I1 Packet)
Source Address: 2001:db8::1
Destination Address: 2001:db8::2
Upper-Layer Packet Length: 48 0x30
Next Header: 139 0x8b
Payload Protocol: 59 0x3b
Header Length: 5 0x5
Packet Type: 1 0x1
Version: 2 0x2
Reserved: 1 0x1
Control: 0 0x0
Checksum: 6750 0x1a5e
Sender's HIT: 2001:20::1
Receiver's HIT: 2001:20::2
DH_GROUP_LIST type: 511 0x1ff
DH_GROUP_LIST length: 3 0x3
DH_GROUP_LIST Group IDs: 3,4,8
C.2. IPv4 HIP Packet (I1 Packet)
The IPv4 checksum value for the example I1 packet is shown below.
Source Address: 192.0.2.1
Destination Address: 192.0.2.2
Upper-Layer Packet Length: 48 0x30
Next Header: 139 0x8b
Payload Protocol: 59 0x3b
Header Length: 5 0x5
Packet Type: 1 0x1
Version: 2 0x2
Reserved: 1 0x1
Control: 0 0x0
Checksum: 61902 0xf1ce
Sender's HIT: 2001:20::1
Receiver's HIT: 2001:20::2
DH_GROUP_LIST type: 511 0x1ff
DH_GROUP_LIST length: 3 0x3
DH_GROUP_LIST Group IDs: 3,4,8
Moskowitz, et al. Standards Track PAGE 124
RFC 7401 HIPv2 April 2015
C.3. TCP Segment
Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
use the IPv6 pseudo header format [RFC 2460], with the HITs used in
place of the IPv6 addresses.
Sender's HIT: 2001:20::1
Receiver's HIT: 2001:20::2
Upper-Layer Packet Length: 20 0x14
Next Header: 6 0x06
Source port: 65500 0xffdc
Destination port: 22 0x0016
Sequence number: 1 0x00000001
Acknowledgment number: 0 0x00000000
Data offset: 5 0x5
Flags: SYN 0x02
Window size: 65535 0xffff
Checksum: 28586 0x6faa
Urgent pointer: 0 0x0000
Appendix D. ECDH and ECDSA 160-Bit Groups
The ECDH and ECDSA 160-bit group SECP160R1 is rated at 80 bits
symmetric strength. This was once considered appropriate for one
year of security. Today, these groups should be used only when the
host is not powerful enough (e.g., some embedded devices) and when
security requirements are low (e.g., long-term confidentiality is not
required).
Appendix E. HIT Suites and HIT Generation
The HIT as an ORCHID [RFC 7343] consists of three parts: A 28-bit
prefix, a 4-bit encoding of the ORCHID generation algorithm (OGA),
and a hash that includes the Host Identity and a context ID. The OGA
is an index pointing to the specific algorithm by which the public
key and the 96-bit hashed encoding are generated. The OGA is
protocol specific and is to be interpreted as defined below for all
protocols that use the same context ID as HIP. HIP groups sets of
valid combinations of signature and hash algorithms into HIT Suites.
These HIT Suites are addressed by an index, which is transmitted in
the OGA ID field of the ORCHID.
The set of used HIT Suites will be extended to counter the progress
in computation capabilities and vulnerabilities in the employed
algorithms. The intended use of the HIT Suites is to introduce a new
HIT Suite and phase out an old one before it becomes insecure. Since
the 4-bit OGA ID field only permits 15 HIT Suites to be used at the
same time (the HIT Suite with ID 0 is reserved), phased-out HIT
Moskowitz, et al. Standards Track PAGE 125
RFC 7401 HIPv2 April 2015
Suites must be reused at some point. In such a case, there will be a
rollover of the HIT Suite ID and the next newly introduced HIT Suite
will start with a lower HIT Suite index than the previously
introduced one. The rollover effectively deprecates the reused HIT
Suite. For a smooth transition, the HIT Suite should be deprecated a
considerable time before the HIT Suite index is reused.
Since the number of HIT Suites is tightly limited to 16, the HIT
Suites must be assigned carefully. Hence, sets of suitable
algorithms are grouped in a HIT Suite.
The HIT Suite of the Responder's HIT determines the RHASH and the
hash function to be used for the HMAC in HIP packets as well as the
signature algorithm family used for generating the HI. The list of
HIT Suites is defined in Table 10.
Moskowitz, et al. Standards Track PAGE 126
RFC 7401 HIPv2 April 2015
Acknowledgments
The drive to create HIP came to being after attending the MALLOC
meeting at the 43rd IETF meeting. Baiju Patel and Hilarie Orman
really gave the original author, Bob Moskowitz, the assist to get HIP
beyond 5 paragraphs of ideas. It has matured considerably since the
early versions thanks to extensive input from IETFers. Most
importantly, its design goals are articulated and are different from
other efforts in this direction. Particular mention goes to the
members of the NameSpace Research Group of the IRTF. Noel Chiappa
provided valuable input at early stages of discussions about
identifier handling and Keith Moore the impetus to provide
resolvability. Steve Deering provided encouragement to keep working,
as a solid proposal can act as a proof of ideas for a research group.
Many others contributed; extensive security tips were provided by
Steve Bellovin. Rob Austein kept the DNS parts on track. Paul
Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
for the Initiator to respond, but easy for the Responder to validate.
Bill Sommerfeld supplied the Birthday concept, which later evolved
into the R1 generation counter, to simplify reboot management. Erik
Nordmark supplied the CLOSE-mechanism for closing connections.
Rodney Thayer and Hugh Daniels provided extensive feedback. In the
early times of this document, John Gilmore kept Bob Moskowitz
challenged to provide something of value.
During the later stages of this document, when the editing baton was
transferred to Pekka Nikander, the input from the early implementors
was invaluable. Without having actual implementations, this document
would not be on the level it is now.
In the usual IETF fashion, a large number of people have contributed
to the actual text or ideas. The list of these people includes Jeff
Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Xin Gu, Rene
Hummen, Miika Komu, Mika Kousa, Julien Laganier, Andrew McGregor, Jan
Melen, Henrik Petander, Michael Richardson, Tim Shepard, Jorma Wall,
and Jukka Ylitalo. Our apologies to anyone whose name is missing.
Once the HIP Working Group was founded in early 2004, a number of
changes were introduced through the working group process. Most
notably, the original document was split in two, one containing the
base exchange and the other one defining how to use ESP. Some
modifications to the protocol proposed by Aura, et al. [AUR05] were
added at a later stage.
Moskowitz, et al. Standards Track PAGE 127
RFC 7401 HIPv2 April 2015
Authors' Addresses
Robert Moskowitz (editor)
HTT Consulting
Oak Park, MI
United States
EMail: rgm@labs.htt-consult.com
Tobias Heer
Hirschmann Automation and Control
Stuttgarter Strasse 45-51
Neckartenzlingen 72654
Germany
EMail: tobias.heer@belden.com
Petri Jokela
Ericsson Research NomadicLab
Jorvas FIN-02420
Finland
Phone: +358 9 299 1
EMail: petri.jokela@nomadiclab.com
Thomas R. Henderson
University of Washington
Campus Box 352500
Seattle, WA
United States
EMail: tomhend@u.washington.edu
Moskowitz, et al. Standards Track PAGE 128
RFC TOTAL SIZE: 309319 bytes
PUBLICATION DATE: Thursday, April 9th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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