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IETF RFC 5973
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
Last modified on Wednesday, October 6th, 2010
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Internet Engineering Task Force (IETF) M. Stiemerling
Request for Comments: 5973 NEC
Category: Experimental H. Tschofenig
ISSN: 2070-1721 Nokia Siemens Networks
C. Aoun
Consultant
E. Davies
Folly Consulting
October 2010
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
 Abstract
This memo defines the NSIS Signaling Layer Protocol (NSLP) for
Network Address Translators (NATs) and firewalls. This NSLP allows
hosts to signal on the data path for NATs and firewalls to be
configured according to the needs of the application data flows. For
instance, it enables hosts behind NATs to obtain a publicly reachable
address and hosts behind firewalls to receive data traffic. The
overall architecture is given by the framework and requirements
defined by the Next Steps in Signaling (NSIS) working group. The
network scenarios, the protocol itself, and examples for path-coupled
signaling are given in this memo.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see 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 5973.
Stiemerling, et al. Experimental PAGE 1 
RFC 5973 NAT/FW NSIS NSLP October 2010
Copyright Notice
Copyright (c) 2010 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
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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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Stiemerling, et al. Experimental PAGE 2
RFC 5973 NAT/FW NSIS NSLP October 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Scope and Background . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology and Abbreviations . . . . . . . . . . . . . . 8
1.3. Notes on the Experimental Status . . . . . . . . . . . . . 10
1.4. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 10
1.5. General Scenario for NATFW Traversal . . . . . . . . . . . 11
2. Network Deployment Scenarios Using the NATFW NSLP . . . . . . 13
2.1. Firewall Traversal . . . . . . . . . . . . . . . . . . . . 13
2.2. NAT with Two Private Networks . . . . . . . . . . . . . . 14
2.3. NAT with Private Network on Sender Side . . . . . . . . . 15
2.4. NAT with Private Network on Receiver Side Scenario . . . . 15
2.5. Both End Hosts behind Twice-NATs . . . . . . . . . . . . . 16
2.6. Both End Hosts behind Same NAT . . . . . . . . . . . . . . 17
2.7. Multihomed Network with NAT . . . . . . . . . . . . . . . 18
2.8. Multihomed Network with Firewall . . . . . . . . . . . . . 18
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 19
3.1. Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 19
3.2. Basic Protocol Overview . . . . . . . . . . . . . . . . . 20
3.2.1. Signaling for Outbound Traffic . . . . . . . . . . . . 20
3.2.2. Signaling for Inbound Traffic . . . . . . . . . . . . 22
3.2.3. Signaling for Proxy Mode . . . . . . . . . . . . . . . 23
3.2.4. Blocking Traffic . . . . . . . . . . . . . . . . . . . 24
3.2.5. State and Error Maintenance . . . . . . . . . . . . . 24
3.2.6. Message Types . . . . . . . . . . . . . . . . . . . . 25
3.2.7. Classification of RESPONSE Messages . . . . . . . . . 25
3.2.8. NATFW NSLP Signaling Sessions . . . . . . . . . . . . 26
3.3. Basic Message Processing . . . . . . . . . . . . . . . . . 27
3.4. Calculation of Signaling Session Lifetime . . . . . . . . 27
3.5. Message Sequencing . . . . . . . . . . . . . . . . . . . . 31
3.6. Authentication, Authorization, and Policy Decisions . . . 32
3.7. Protocol Operations . . . . . . . . . . . . . . . . . . . 32
3.7.1. Creating Signaling Sessions . . . . . . . . . . . . . 32
3.7.2. Reserving External Addresses . . . . . . . . . . . . . 35
3.7.3. NATFW NSLP Signaling Session Refresh . . . . . . . . . 43
3.7.4. Deleting Signaling Sessions . . . . . . . . . . . . . 45
3.7.5. Reporting Asynchronous Events . . . . . . . . . . . . 46
3.7.6. Proxy Mode of Operation . . . . . . . . . . . . . . . 48
3.8. Demultiplexing at NATs . . . . . . . . . . . . . . . . . . 53
3.9. Reacting to Route Changes . . . . . . . . . . . . . . . . 54
3.10. Updating Policy Rules . . . . . . . . . . . . . . . . . . 55
4. NATFW NSLP Message Components . . . . . . . . . . . . . . . . 55
4.1. NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 56
4.2. NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.1. Signaling Session Lifetime Object . . . . . . . . . . 58
4.2.2. External Address Object . . . . . . . . . . . . . . . 58
4.2.3. External Binding Address Object . . . . . . . . . . . 59
Stiemerling, et al. Experimental PAGE 3
RFC 5973 NAT/FW NSIS NSLP October 2010
4.2.4. Extended Flow Information Object . . . . . . . . . . . 59
4.2.5. Information Code Object . . . . . . . . . . . . . . . 60
4.2.6. Nonce Object . . . . . . . . . . . . . . . . . . . . . 64
4.2.7. Message Sequence Number Object . . . . . . . . . . . . 64
4.2.8. Data Terminal Information Object . . . . . . . . . . . 64
4.2.9. ICMP Types Object . . . . . . . . . . . . . . . . . . 66
4.3. Message Formats . . . . . . . . . . . . . . . . . . . . . 67
4.3.1. CREATE . . . . . . . . . . . . . . . . . . . . . . . . 67
4.3.2. EXTERNAL . . . . . . . . . . . . . . . . . . . . . . . 68
4.3.3. RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 68
4.3.4. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 69
5. Security Considerations . . . . . . . . . . . . . . . . . . . 69
5.1. Authorization Framework . . . . . . . . . . . . . . . . . 70
5.1.1. Peer-to-Peer Relationship . . . . . . . . . . . . . . 70
5.1.2. Intra-Domain Relationship . . . . . . . . . . . . . . 71
5.1.3. End-to-Middle Relationship . . . . . . . . . . . . . . 72
5.2. Security Framework for the NAT/Firewall NSLP . . . . . . . 73
5.2.1. Security Protection between Neighboring NATFW NSLP
Nodes . . . . . . . . . . . . . . . . . . . . . . . . 73
5.2.2. Security Protection between Non-Neighboring NATFW
NSLP Nodes . . . . . . . . . . . . . . . . . . . . . . 74
5.3. Implementation of NATFW NSLP Security . . . . . . . . . . 75
6. IAB Considerations on UNSAF . . . . . . . . . . . . . . . . . 76
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 77
7.1. NATFW NSLP Message Type Registry . . . . . . . . . . . . . 77
7.2. NATFW NSLP Header Flag Registry . . . . . . . . . . . . . 77
7.3. NSLP Message Object Registry . . . . . . . . . . . . . . . 78
7.4. NSLP Response Code Registry . . . . . . . . . . . . . . . 78
7.5. NSLP IDs and Router Alert Option Values . . . . . . . . . 78
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 78
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 79
9.1. Normative References . . . . . . . . . . . . . . . . . . . 79
9.2. Informative References . . . . . . . . . . . . . . . . . . 79
Appendix A. Selecting Signaling Destination Addresses for
EXTERNAL . . . . . . . . . . . . . . . . . . . . . . 81
Appendix B. Usage of External Binding Addresses . . . . . . . . . 82
Appendix C. Applicability Statement on Data Receivers behind
Firewalls . . . . . . . . . . . . . . . . . . . . . . 83
Appendix D. Firewall and NAT Resources . . . . . . . . . . . . . 84
D.1. Wildcarding of Policy Rules . . . . . . . . . . . . . . . 84
D.2. Mapping to Firewall Rules . . . . . . . . . . . . . . . . 84
D.3. Mapping to NAT Bindings . . . . . . . . . . . . . . . . . 85
D.4. NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 85
Appendix E. Example for Receiver Proxy Case . . . . . . . . . . . 86
Stiemerling, et al. Experimental PAGE 4
RFC 5973 NAT/FW NSIS NSLP October 2010
1. Introduction
1.1. Scope and Background
Firewalls and Network Address Translators (NATs) have both been used
throughout the Internet for many years, and they will remain present
for the foreseeable future. Firewalls are used to protect networks
against certain types of attacks from internal networks and the
Internet, whereas NATs provide a virtual extension of the IP address
space. Both types of devices may be obstacles to some applications,
since they only allow traffic created by a limited set of
applications to traverse them, typically those that use protocols
with relatively predetermined and static properties (e.g., most HTTP
traffic, and other client/server applications). Other applications,
such as IP telephony and most other peer-to-peer applications, which
have more dynamic properties, create traffic that is unable to
traverse NATs and firewalls without assistance. In practice, the
traffic of many applications cannot traverse autonomous firewalls or
NATs, even when they have additional functionality that attempts to
restore the transparency of the network.
Several solutions to enable applications to traverse such entities
have been proposed and are currently in use. Typically, application-
level gateways (ALGs) have been integrated with the firewall or NAT
to configure the firewall or NAT dynamically. Another approach is
middlebox communication (MIDCOM). In this approach, ALGs external to
the firewall or NAT configure the corresponding entity via the MIDCOM
protocol [RFC 3303]. Several other work-around solutions are
available, such as Session Traversal Utilities for NAT (STUN)
[RFC 5389]. However, all of these approaches introduce other problems
that are generally hard to solve, such as dependencies on the type of
NAT implementation (full-cone, symmetric, etc.), or dependencies on
certain network topologies.
NAT and firewall (NATFW) signaling shares a property with Quality-of-
Service (QoS) signaling -- each must reach any device that is on the
data path and is involved in (respectively) NATFW or QoS treatment of
data packets. This means that for both NATFW and QoS it is
convenient if signaling travels path-coupled, i.e., the signaling
messages follow exactly the same path that the data packets take.
The Resource Reservation Protocol (RSVP) [RFC 2205] is an example of a
current QoS signaling protocol that is path-coupled. [rsvp-firewall]
proposes the use of RSVP as a firewall signaling protocol but does
not include NATs.
This memo defines a path-coupled signaling protocol for NAT and
firewall configuration within the framework of NSIS, called the NATFW
NSIS Signaling Layer Protocol (NSLP). The general requirements for
Stiemerling, et al. Experimental PAGE 5
RFC 5973 NAT/FW NSIS NSLP October 2010
NSIS are defined in [RFC 3726] and the general framework of NSIS is
outlined in [RFC 4080]. It introduces the split between an NSIS
transport layer and an NSIS signaling layer. The transport of NSLP
messages is handled by an NSIS Network Transport Layer Protocol
(NTLP, with General Internet Signaling Transport (GIST) [RFC 5971]
being the implementation of the abstract NTLP). The signaling logic
for QoS and NATFW signaling is implemented in the different NSLPs.
The QoS NSLP is defined in [RFC 5974].
The NATFW NSLP is designed to request the dynamic configuration of
NATs and/or firewalls along the data path. Dynamic configuration
includes enabling data flows to traverse these devices without being
obstructed, as well as blocking of particular data flows at inbound
firewalls. Enabling data flows requires the loading of firewall
rules with an action that allows the data flow packets to be
forwarded and NAT bindings to be created. The blocking of data flows
requires the loading of firewall rules with an action that will deny
forwarding of the data flow packets. A simplified example for
enabling data flows: a source host sends a NATFW NSLP signaling
message towards its data destination. This message follows the data
path. Every NATFW NSLP-enabled NAT/firewall along the data path
intercepts this message, processes it, and configures itself
accordingly. Thereafter, the actual data flow can traverse all these
configured firewalls/NATs.
It is necessary to distinguish between two different basic scenarios
when operating the NATFW NSLP, independent of the type of the
middleboxes to be configured.
1. Both the data sender and data receiver are NSIS NATFW NSLP aware.
This includes the cases in which the data sender is logically
decomposed from the initiator of the NSIS signaling (the so-
called NSIS initiator) or the data receiver logically decomposed
from the receiver of the NSIS signaling (the so-called NSIS
receiver), but both sides support NSIS. This scenario assumes
deployment of NSIS all over the Internet, or at least at all NATs
and firewalls. This scenario is used as a base assumption, if
not otherwise noted.
2. Only one end host or region of the network is NSIS NATFW NSLP
aware, either the data receiver or data sender. This scenario is
referred to as proxy mode.
The NATFW NSLP has two basic signaling messages that are sufficient
to cope with the various possible scenarios likely to be encountered
before and after widespread deployment of NSIS:
Stiemerling, et al. Experimental PAGE 6
RFC 5973 NAT/FW NSIS NSLP October 2010
CREATE message: Sent by the data sender for configuring a path
outbound from a data sender to a data receiver.
EXTERNAL message: Used by a data receiver to locate inbound NATs/
firewalls and prime them to expect inbound signaling and used at
NATs to pre-allocate a public address. This is used for data
receivers behind these devices to enable their reachability.
CREATE and EXTERNAL messages are sent by the NSIS initiator (NI)
towards the NSIS responder (NR). Both types of message are
acknowledged by a subsequent RESPONSE message. This RESPONSE message
is generated by the NR if the requested configuration can be
established; otherwise, the NR or any of the NSLP forwarders (NFs)
can also generate such a message if an error occurs. NFs and the NR
can also generate asynchronous messages to notify the NI, the so-
called NOTIFY messages.
If the data receiver resides in a private addressing realm or behind
a firewall, and it needs to preconfigure the edge-NAT/edge-firewall
to provide a (publicly) reachable address for use by the data sender,
a combination of EXTERNAL and CREATE messages is used.
During the introduction of NSIS, it is likely that one or the other
of the data sender and receiver will not be NSIS aware. In these
cases, the NATFW NSLP can utilize NSIS-aware middleboxes on the path
between the data sender and data receiver to provide proxy NATFW NSLP
services (i.e., the proxy mode). Typically, these boxes will be at
the boundaries of the realms in which the end hosts are located.
The CREATE and EXTERNAL messages create NATFW NSLP and NTLP state in
NSIS entities. NTLP state allows signaling messages to travel in the
forward (outbound) and the reverse (inbound) direction along the path
between a NAT/firewall NSLP sender and a corresponding receiver.
This state is managed using a soft-state mechanism, i.e., it expires
unless it is refreshed from time to time. The NAT bindings and
firewall rules being installed during the state setup are bound to
the particular signaling session. However, the exact local
implementation of the NAT bindings and firewall rules are NAT/
firewall specific and it is out of the scope of this memo.
This memo is structured as follows. Section 2 describes the network
environment for NATFW NSLP signaling. Section 3 defines the NATFW
signaling protocol and Section 4 defines the message components and
the overall messages used in the protocol. The remaining parts of
the main body of the document cover security considerations
Section 5, IAB considerations on UNilateral Self-Address Fixing
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RFC 5973 NAT/FW NSIS NSLP October 2010
(UNSAF) [RFC 3424] in Section 6, and IANA considerations in Section 7.
Please note that readers familiar with firewalls and NATs and their
possible location within networks can safely skip Section 2.
1.2. Terminology and Abbreviations
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].
This document uses a number of terms defined in [RFC 3726] and
[RFC 4080]. The following additional terms are used:
o Policy rule: A policy rule is "a basic building block of a policy-
based system. It is the binding of a set of actions to a set of
conditions - where the conditions are evaluated to determine
whether the actions are performed" [RFC 3198]. In the context of
NSIS NATFW NSLP, the conditions are the specification of a set of
packets to which the rule is applied. The set of actions always
contains just a single element per rule, and is limited to either
action "deny" or action "allow".
o Reserved policy rule: A policy rule stored at NATs or firewalls
for activation by a later, different signaling exchange. This
type of policy rule is kept in the NATFW NSLP and is not loaded
into the firewall or NAT engine, i.e., it does not affect the data
flow handling.
o Installed policy rule: A policy rule in operation at NATs or
firewalls. This type of rule is kept in the NATFW NSLP and is
loaded into the firewall or NAT engine, i.e., it is affecting the
data flow.
o Remembered policy rule: A policy rule stored at NATs and firewalls
for immediate use, as soon as the signaling exchange is
successfully completed.
o Firewall: A packet filtering device that matches packets against a
set of policy rules and applies the actions.
o Network Address Translator: Network Address Translation is a
method by which IP addresses are mapped from one IP address realm
to another, in an attempt to provide transparent routing between
hosts (see [RFC 2663]). Network Address Translators are devices
that perform this work by modifying packets passing through them.
o Data Receiver (DR): The node in the network that is receiving the
data packets of a flow.
Stiemerling, et al. Experimental PAGE 8
RFC 5973 NAT/FW NSIS NSLP October 2010
o Data Sender (DS): The node in the network that is sending the data
packets of a flow.
o NATFW NSLP peer (or simply "peer"): An NSIS NATFW NSLP node with
which an NTLP adjacency has been created as defined in [RFC 5971].
o NATFW NSLP signaling session (or simply "signaling session"): A
signaling session defines an association between the NI, NFs, and
the NR related to a data flow. All the NATFW NSLP peers on the
path, including the NI and the NR, use the same identifier to
refer to the state stored for the association. The same NI and NR
may have more than one signaling session active at any time. The
state for the NATFW NSLP consists of NSLP state and associated
policy rules at a middlebox.
o Edge-NAT: An edge-NAT is a NAT device with a globally routable IP
address that is reachable from the public Internet.
o Edge-firewall: An edge-firewall is a firewall device that is
located on the borderline of an administrative domain.
o Public Network: "A Global or Public Network is an address realm
with unique network addresses assigned by Internet Assigned
Numbers Authority (IANA) or an equivalent address registry. This
network is also referred as external network during NAT
discussions" [RFC 2663].
o Private/Local Network: "A private network is an address realm
independent of external network addresses. Private network may
also be referred alternately as Local Network. Transparent
routing between hosts in private realm and external realm is
facilitated by a NAT router" [RFC 2663].
o Public/Global IP address: An IP address located in the public
network according to Section 2.7 of [RFC 2663].
o Private/Local IP address: An IP address located in the private
network according to Section 2.8 of [RFC 2663].
o Signaling Destination Address (SDA): An IP address generally taken
from the public/global IP address range, although, the SDA may, in
certain circumstances, be part of the private/local IP address
range. This address is used in EXTERNAL signaling message
exchanges, if the data receiver's IP address is unknown.
Stiemerling, et al. Experimental PAGE 9
RFC 5973 NAT/FW NSIS NSLP October 2010
1.3. Notes on the Experimental Status
The same deployment issues and extensibility considerations described
in [RFC 5971] and [RFC 5978] also apply to this document.
1.4. Middleboxes
The term "middlebox" covers a range of devices and is well-defined in
[RFC 3234]: "A middlebox is defined as any intermediary device
performing functions other than the normal, standard functions of an
IP router on the datagram path between a source host and a
destination host". As such, middleboxes fall into a number of
categories with a wide range of functionality, not all of which is
pertinent to the NATFW NSLP. Middlebox categories in the scope of
this memo are firewalls that filter data packets against a set of
filter rules, and NATs that translate packet addresses from one
address realm to another address realm. Other categories of
middleboxes, such as QoS traffic shapers, are out of the scope of
this memo.
The term "NAT" used in this document is a placeholder for a range of
different NAT flavors. We consider the following types of NATs:
o Traditional NAT (basic NAT and NAPT)
o Bi-directional NAT
o Twice-NAT
o Multihomed NAT
For definitions and a detailed discussion about the characteristics
of each NAT type, please see [RFC 2663].
All types of middleboxes under consideration here use policy rules to
make a decision on data packet treatment. Policy rules consist of a
flow identifier that selects the packets to which the policy applies
and an associated action; data packets matching the flow identifier
are subjected to the policy rule action. A typical flow identifier
is the 5-tuple selector that matches the following fields of a packet
to configured values:
o Source and destination IP addresses
o Transport protocol number
o Transport source and destination port numbers
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RFC 5973 NAT/FW NSIS NSLP October 2010
Actions for firewalls are usually one or more of:
o Allow: forward data packet
o Deny: block data packet and discard it
o Other actions such as logging, diverting, duplicating, etc.
Actions for NATs include (amongst many others):
o Change source IP address and transport port number to a globally
routable IP address and associated port number.
o Change destination IP address and transport port number to a
private IP address and associated port number.
It should be noted that a middlebox may contain two logical
representations of the policy rule. The policy rule has a
representation within the NATFW NSLP, comprising the message routing
information (MRI) of the NTLP and NSLP information (such as the rule
action). The other representation is the implementation of the NATFW
NSLP policy rule within the NAT and firewall engine of the particular
device. Refer to Appendix D for further details.
1.5. General Scenario for NATFW Traversal
The purpose of NSIS NATFW signaling is to enable communication
between endpoints across networks, even in the presence of NAT and
firewall middleboxes that have not been specially engineered to
facilitate communication with the application protocols used. This
removes the need to create and maintain application layer gateways
for specific protocols that have been commonly used to provide
transparency in previous generations of NAT and firewall middleboxes.
It is assumed that these middleboxes will be statically configured in
such a way that NSIS NATFW signaling messages themselves are allowed
to reach the locally installed NATFW NSLP daemon. NSIS NATFW NSLP
signaling is used to dynamically install additional policy rules in
all NATFW middleboxes along the data path that will allow
transmission of the application data flow(s). Firewalls are
configured to forward data packets matching the policy rule provided
by the NSLP signaling. NATs are configured to translate data packets
matching the policy rule provided by the NSLP signaling. An
additional capability, that is an exception to the primary goal of
NSIS NATFW signaling, is that the NATFW nodes can request blocking of
particular data flows instead of enabling these flows at inbound
firewalls.
Stiemerling, et al. Experimental PAGE 11
RFC 5973 NAT/FW NSIS NSLP October 2010
The basic high-level picture of NSIS usage is that end hosts are
located behind middleboxes, meaning that there is at least one
middlebox on the data path from the end host in a private network to
the external network (NATFW in Figure 1). Applications located at
these end hosts try to establish communication with corresponding
applications on other such end hosts. This communication
establishment may require that the applications contact an
application server that serves as a rendezvous point between both
parties to exchange their IP address and port(s). The local
applications trigger the NSIS entity at the local host to control
provisioning for middlebox traversal along the prospective data path
(e.g., via an API call). The NSIS entity, in turn, uses NSIS NATFW
NSLP signaling to establish policy rules along the data path,
allowing the data to travel from the sender to the receiver without
obstruction.
Application Application Server (0, 1, or more) Application
+----+ +----+ +----+
| +------------------------+ +------------------------+ |
+-+--+ +----+ +-+--+
| |
| NSIS Entities NSIS Entities |
+-+--+ +----+ +-----+ +-+--+
| +--------+ +----------------------------+ +-----+ |
+-+--+ +-+--+ +--+--+ +-+--+
| | ------ | |
| | //// \\\\\ | |
+-+--+ +-+--+ |/ | +-+--+ +-+--+
| | | | | Internet | | | | |
| +--------+ +-----+ +----+ +-----+ |
+----+ +----+ |\ | +----+ +----+
\\\\ /////
sender NATFW (1+) ------ NATFW (1+) receiver
Note that 1+ refers to one or more NATFW nodes.
Figure 1: Generic View of NSIS with NATs and/or Firewalls
For end-to-end NATFW signaling, it is necessary that each firewall
and each NAT along the path between the data sender and the data
receiver implements the NSIS NATFW NSLP. There might be several NATs
and FWs in various possible combinations on a path between two hosts.
Section 2 presents a number of likely scenarios with different
combinations of NATs and firewalls. However, the scenarios given in
the following sections are only examples and should not be treated as
limiting the scope of the NATFW NSLP.
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RFC 5973 NAT/FW NSIS NSLP October 2010
2. Network Deployment Scenarios Using the NATFW NSLP
This section introduces several scenarios for middlebox placement
within IP networks. Middleboxes are typically found at various
different locations, including at enterprise network borders, within
enterprise networks, as mobile phone network gateways, etc. Usually,
middleboxes are placed more towards the edge of networks than in
network cores. Firewalls and NATs may be found at these locations
either alone or combined; other categories of middleboxes may also be
found at such locations, possibly combined with the NATs and/or
firewalls.
NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
regular data path to the NSIS responder (NR). On the data path,
NATFW NSLP signaling messages reach different NSIS nodes that
implement the NATFW NSLP. Each NATFW NSLP node processes the
signaling messages according to Section 3 and, if necessary, installs
policy rules for subsequent data packets.
Each of the following sub-sections introduces a different scenario
for a different set of middleboxes and their ordering within the
topology. It is assumed that each middlebox implements the NSIS
NATFW NSLP signaling protocol.
2.1. Firewall Traversal
This section describes a scenario with firewalls only; NATs are not
involved. Each end host is behind a firewall. The firewalls are
connected via the public Internet. Figure 2 shows the topology. The
part labeled "public" is the Internet connecting both firewalls.
+----+ //----\\ +----+
NI -----| FW |---| |------| FW |--- NR
+----+ \\----// +----+
private public private
FW: Firewall
NI: NSIS Initiator
NR: NSIS Responder
Figure 2: Firewall Traversal Scenario
Each firewall on the data path must provide traversal service for
NATFW NSLP in order to permit the NSIS message to reach the other end
host. All firewalls process NSIS signaling and establish appropriate
policy rules, so that the required data packet flow can traverse
them.
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RFC 5973 NAT/FW NSIS NSLP October 2010
There are several very different ways to place firewalls in a network
topology. To distinguish firewalls located at network borders, such
as administrative domains, from others located internally, the term
edge-firewall is used. A similar distinction can be made for NATs,
with an edge-NAT fulfilling the equivalent role.
2.2. NAT with Two Private Networks
Figure 3 shows a scenario with NATs at both ends of the network.
Therefore, each application instance, the NSIS initiator and the NSIS
responder, are behind NATs. The outermost NAT, known as the edge-NAT
(MB2 and MB3), at each side is connected to the public Internet. The
NATs are generically labeled as MBX (for middlebox No. X), since
those devices certainly implement NAT functionality, but can
implement firewall functionality as well.
Only two middleboxes (MBs) are shown in Figure 3 at each side, but in
general, any number of MBs on each side must be considered.
+----+ +----+ //----\\ +----+ +----+
NI --| MB1|-----| MB2|---| |---| MB3|-----| MB4|--- NR
+----+ +----+ \\----// +----+ +----+
private public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 3: NAT with two Private Networks Scenario
Signaling traffic from the NI to the NR has to traverse all the
middleboxes on the path (MB1 to MB4, in this order), and all the
middleboxes must be configured properly to allow NSIS signaling to
traverse them. The NATFW signaling must configure all middleboxes
and consider any address translation that will result from this
configuration in further signaling. The sender (NI) has to know the
IP address of the receiver (NR) in advance, otherwise it will not be
possible to send any NSIS signaling messages towards the responder.
Note that this IP address is not the private IP address of the
responder but the NAT's public IP address (here MB3's IP address).
Instead, a NAT binding (including a public IP address) has to be
previously installed on the NAT MB3. This NAT binding subsequently
allows packets reaching the NAT to be forwarded to the receiver
within the private address realm. The receiver might have a number
of ways to learn its public IP address and port number (including the
NATFW NSLP) and might need to signal this information to the sender
using an application-level signaling protocol.
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RFC 5973 NAT/FW NSIS NSLP October 2010
2.3. NAT with Private Network on Sender Side
This scenario shows an application instance at the sending node that
is behind one or more NATs (shown as generic MB, see discussion in
Section 2.2). The receiver is located in the public Internet.
+----+ +----+ //----\\
NI --| MB |-----| MB |---| |--- NR
+----+ +----+ \\----//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 4: NAT with Private Network on Sender Side
The traffic from NI to NR has to traverse middleboxes only on the
sender's side. The receiver has a public IP address. The NI sends
its signaling message directly to the address of the NSIS responder.
Middleboxes along the path intercept the signaling messages and
configure accordingly.
The data sender does not necessarily know whether or not the receiver
is behind a NAT; hence, it is the receiving side that has to detect
whether or not it is behind a NAT.
2.4. NAT with Private Network on Receiver Side Scenario
The application instance receiving data is behind one or more NATs
shown as MB (see discussion in Section 2.2).
//----\\ +----+ +----+
NI ---| |---| MB |-----| MB |--- NR
\\----// +----+ +----+
public private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 5: NAT with Private Network on Receiver Scenario
Initially, the NSIS responder must determine its publicly reachable
IP address at the external middlebox and notify the NSIS initiator
about this address. One possibility is that an application-level
Stiemerling, et al. Experimental PAGE 15
RFC 5973 NAT/FW NSIS NSLP October 2010
protocol is used, meaning that the public IP address is signaled via
this protocol to the NI. Afterwards, the NI can start its signaling
towards the NR and therefore establish the path via the middleboxes
in the receiver side private network.
This scenario describes the use case for the EXTERNAL message of the
NATFW NSLP.
2.5. Both End Hosts behind Twice-NATs
This is a special case, where the main problem arises from the need
to detect that both end hosts are logically within the same address
space, but are also in two partitions of the address realm on either
side of a twice-NAT (see [RFC 2663] for a discussion of twice-NAT
functionality).
Sender and receiver are both within a single private address realm,
but the two partitions potentially have overlapping IP address
ranges. Figure 6 shows the arrangement of NATs.
public
+----+ +----+ //----\\
NI --| MB |--+--| MB |---| |
+----+ | +----+ \\----//
|
| +----+
+--| MB |------------ NR
+----+
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 6: NAT to Public, Sender and Receiver on Either Side of a
Twice-NAT Scenario
The middleboxes shown in Figure 6 are twice-NATs, i.e., they map IP
addresses and port numbers on both sides, meaning the mapping of
source and destination IP addresses at the private and public
interfaces.
This scenario requires the assistance of application-level entities,
such as a DNS server. The application-level entities must handle
requests that are based on symbolic names and configure the
middleboxes so that data packets are correctly forwarded from NI to
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RFC 5973 NAT/FW NSIS NSLP October 2010
NR. The configuration of those middleboxes may require other
middlebox communication protocols, such as MIDCOM [RFC 3303]. NSIS
signaling is not required in the twice-NAT only case, since
middleboxes of the twice-NAT type are normally configured by other
means. Nevertheless, NSIS signaling might be useful when there are
also firewalls on the path. In this case, NSIS will not configure
any policy rule at twice-NATs, but will configure policy rules at the
firewalls on the path. The NSIS signaling protocol must be at least
robust enough to survive this scenario. This requires that twice-
NATs must implement the NATFW NSLP also and participate in NATFW
signaling sessions, but they do not change the configuration of the
NAT, i.e., they only read the address mapping information out of the
NAT and translate the Message Routing Information (MRI, [RFC 5971])
within the NSLP and NTLP accordingly. For more information, see
Appendix D.4.
2.6. Both End Hosts behind Same NAT
When the NSIS initiator and NSIS responder are behind the same NAT
(thus, being in the same address realm, see Figure 7), they are most
likely not aware of this fact. As in Section 2.4, the NSIS responder
must determine its public IP address in advance and transfer it to
the NSIS initiator. Afterwards, the NSIS initiator can start sending
the signaling messages to the responder's public IP address. During
this process, a public IP address will be allocated for the NSIS
initiator at the same middlebox as for the responder. Now, the NSIS
signaling and the subsequent data packets will traverse the NAT
twice: from initiator to public IP address of responder (first time)
and from public IP address of responder to responder (second time).
NI public
\ +----+ //----\\
+-| MB |----| |
/ +----+ \\----//
NR
private
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 7: NAT to Public, Both Hosts behind Same NAT
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RFC 5973 NAT/FW NSIS NSLP October 2010
2.7. Multihomed Network with NAT
The previous sub-sections sketched network topologies where several
NATs and/or firewalls are ordered sequentially on the path. This
section describes a multihomed scenario with two NATs placed on
alternative paths to the public network.
+----+ //---\\
NI -------| MB |---| |
\ +----+ \\-+-//
\ |
\ +----- NR
\ |
\ +----+ //-+-\\
--| MB |---| |
+----+ \\---//
private public
MB: Middlebox
NI: NSIS Initiator
NR: NSIS Responder
Figure 8: Multihomed Network with Two NATs
Depending on the destination, either one or the other middlebox is
used for the data flow. Which middlebox is used, depends on local
policy or routing decisions. NATFW NSLP must be able to handle this
situation properly, see Section 3.7.2 for an extended discussion of
this topic with respect to NATs.
2.8. Multihomed Network with Firewall
This section describes a multihomed scenario with two firewalls
placed on alternative paths to the public network (Figure 9). The
routing in the private and public networks decides which firewall is
being taken for data flows. Depending on the data flow's direction,
either outbound or inbound, a different firewall could be traversed.
This is a challenge for the EXTERNAL message of the NATFW NSLP where
the NSIS responder is located behind these firewalls within the
private network. The EXTERNAL message is used to block a particular
data flow on an inbound firewall. NSIS must route the EXTERNAL
message inbound from NR to NI probably without knowing which path the
data traffic will take from NI to NR (see also Appendix C).
Stiemerling, et al. Experimental PAGE 18
RFC 5973 NAT/FW NSIS NSLP October 2010
+----+
NR -------| FW |\
\ +----+ \ //---\\
\ -| |-- NI
\ \\---//
\ +----+ |
--| FW |-------+
+----+
private
private public
FW: Firewall
NI: NSIS Initiator
NR: NSIS Responder
Figure 9: Multihomed Network with Two Firewalls
3. Protocol Description
This section defines messages, objects, and protocol semantics for
the NATFW NSLP.
3.1. Policy Rules
Policy rules, bound to a NATFW NSLP signaling session, are the
building blocks of middlebox devices considered in the NATFW NSLP.
For firewalls, the policy rule usually consists of a 5-tuple and an
action such as allow or deny. The information contained in the tuple
includes source/destination IP addresses, transport protocol, and
source/destination port numbers. For NATs, the policy rule consists
of the action 'translate this address' and further mapping
information, that might be, in the simplest case, internal IP address
and external IP address.
The NATFW NSLP carries, in conjunction with the NTLP's Message
Routing Information (MRI), the policy rules to be installed at NATFW
peers. This policy rule is an abstraction with respect to the real
policy rule to be installed at the respective firewall or NAT. It
conveys the initiator's request and must be mapped to the possible
configuration on the particular used NAT and/or firewall in use. For
pure firewalls, one or more filter rules must be created, and for
pure NATs, one or more NAT bindings must be created. In mixed
firewall and NAT boxes, the policy rule must be mapped to filter
rules and bindings observing the ordering of the firewall and NAT
engine. Depending on the ordering, NAT before firewall or vice
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RFC 5973 NAT/FW NSIS NSLP October 2010
versa, the firewall rules must carry public or private IP addresses.
However, the exact mapping depends on the implementation of the
firewall or NAT that is possibly different for each implementation.
The policy rule at the NATFW NSLP level comprises the message routing
information (MRI) part, carried in the NTLP, and the information
available in the NATFW NSLP. The information provided by the NSLP is
stored in the 'extend flow information' (NATFW_EFI) and 'data
terminal information' (NATFW_DTINFO) objects, and the message type.
Additional information, such as the external IP address and port
number, stored in the NAT or firewall, will be used as well. The MRI
carries the filter part of the NAT/firewall-level policy rule that is
to be installed.
The NATFW NSLP specifies two actions for the policy rules: deny and
allow. A policy rule with action set to deny will result in all
packets matching this rule to be dropped. A policy rule with action
set to allow will result in all packets matching this rule to be
forwarded.
3.2. Basic Protocol Overview
The NSIS NATFW NSLP is carried over the General Internet Signaling
Transport (GIST, the implementation of the NTLP) defined in
[RFC 5971]. NATFW NSLP messages are initiated by the NSIS initiator
(NI), handled by NSLP forwarders (NFs) and received by the NSIS
responder (NR). It is required that at least NI and NR implement
this NSLP, intermediate NFs only implement this NSLP when they
provide relevant middlebox functions. NSLP forwarders that do not
have any NATFW NSLP functions just forward these packets as they have
no interest in them.
3.2.1. Signaling for Outbound Traffic
A data sender (DS), intending to send data to a data receiver (DR),
has to start NATFW NSLP signaling. This causes the NI associated
with the DS to launch NSLP signaling towards the address of the DR
(see Figure 10). Although it is expected that the DS and the NATFW
NSLP NI will usually reside on the same host, this specification does
not rule out scenarios where the DS and NI reside on different hosts,
the so-called proxy mode (see Section 3.7.6).
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RFC 5973 NAT/FW NSIS NSLP October 2010
+-------+ +-------+ +-------+ +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR |
| |--->| NF1 |--->| NF2 |--->| |
+-------+ +-------+ +-------+ +-------+
========================================>
Data Traffic Direction (outbound)
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSLP forwarder 1
MB2 : Middlebox 2 and NSLP forwarder 2
Figure 10: General NSIS Signaling
The following list shows the normal sequence of NSLP events without
detailing the interaction with the NTLP and the interactions on the
NTLP level.
o NSIS initiators generate request messages (which are either CREATE
or EXTERNAL messages) and send these towards the NSIS responder.
This request message is the initial message that creates a new
NATFW NSLP signaling session. The NI and the NR will most likely
already share an application session before they start the NATFW
NSLP signaling session. Note well the difference between both
sessions.
o NSLP request messages are processed each time an NF with NATFW
NSLP support is traversed. Each NF that is intercepting a request
message and is accepting it for further treatment is joining the
particular NATFW NSLP signaling session. These nodes process the
message, check local policies for authorization and
authentication, possibly create policy rules, and forward the
signaling message to the next NSIS node. The request message is
forwarded until it reaches the NSIS responder.
o NSIS responders will check received messages and process them if
applicable. NSIS responders generate RESPONSE messages and send
them hop-by-hop back to the NI via the same chain of NFs
(traversal of the same NF chain is guaranteed through the
established reverse message routing state in the NTLP). The NR is
also joining the NATFW NSLP signaling session if the request
message is accepted.
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RFC 5973 NAT/FW NSIS NSLP October 2010
o The RESPONSE message is processed at each NF that has been
included in the prior NATFW NSLP signaling session setup.
o If the NI has received a successful RESPONSE message and if the
signaling NATFW NSLP session started with a CREATE message, the
data sender can start sending its data flow to the data receiver.
If the NI has received a successful RESPONSE message and if the
signaling NATFW NSLP session started with an EXTERNAL message, the
data receiver is ready to receive further CREATE messages.
Because NATFW NSLP signaling follows the data path from DS to DR,
this immediately enables communication between both hosts for
scenarios with only firewalls on the data path or NATs on the sender
side. For scenarios with NATs on the receiver side, certain problems
arise, as described in Section 2.4.
3.2.2. Signaling for Inbound Traffic
When the NR and the NI are located in different address realms and
the NR is located behind a NAT, the NI cannot signal to the NR
address directly. The DR/NR is not reachable from other NIs using
the private address of the NR and thus NATFW signaling messages
cannot be sent to the NR/DR's address. Therefore, the NR must first
obtain a NAT binding that provides an address that is reachable for
the NI. Once the NR has acquired a public IP address, it forwards
this information to the DS via a separate protocol. This
application-layer signaling, which is out of the scope of the NATFW
NSLP, may involve third parties that assist in exchanging these
messages.
The same holds partially true for NRs located behind firewalls that
block all traffic by default. In this case, NR must tell its inbound
firewalls of inbound NATFW NSLP signaling and corresponding data
traffic. Once the NR has informed the inbound firewalls, it can
start its application-level signaling to initiate communication with
the NI. This mechanism can be used by machines hosting services
behind firewalls as well. In this case, the NR informs the inbound
firewalls as described, but does not need to communicate this to the
NIs.
NATFW NSLP signaling supports this scenario by using the EXTERNAL
message.
1. The DR acquires a public address by signaling on the reverse path
(DR towards DS) and thus making itself available to other hosts.
This process of acquiring public addresses is called reservation.
During this process the DR reserves publicly reachable addresses
and ports suitable for further usage in application-level
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RFC 5973 NAT/FW NSIS NSLP October 2010
signaling and the publicly reachable address for further NATFW
NSLP signaling. However, the data traffic will not be allowed to
use this address/port initially (see next point). In the process
of reservation, the DR becomes the NI for the messages necessary
to obtain the publicly reachable IP address, i.e., the NI for
this specific NATFW NSLP signaling session.
2. Now on the side of the DS, the NI creates a new NATFW NSLP
signaling session and signals directly to the public IP address
of the DR. This public IP address is used as NR's address, as
the NI would do if there is no NAT in between, and creates policy
rules at middleboxes. Note, that the reservation will only allow
forwarding of signaling messages, but not data flow packets.
Policy rules allowing forwarding of data flow packets set up by
the prior EXTERNAL message signaling will be activated when the
signaling from NI towards NR is confirmed with a positive
RESPONSE message. The EXTERNAL message is described in
Section 3.7.2.
3.2.3. Signaling for Proxy Mode
administrative domain
----------------------------------\
|
+-------+ +-------+ +-------+ | +-------+
| DS/NI |<~~~| MB1/ |<~~~| MB2/ | | | DR |
| |--->| NF1 |--->| NR | | | |
+-------+ +-------+ +-------+ | +-------+
|
----------------------------------/
========================================>
Data Traffic Direction (outbound)
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSLP forwarder 1
MB2 : Middlebox 2 and NSLP responder
Figure 11: Proxy Mode Signaling for Data Sender
The above usage assumes that both ends of a communication support
NSIS, but fails when NSIS is only deployed at one end of the path.
In this case, only one of the sending side (see Figure 11) or
receiving side (see Figure 12) is NSIS aware and not both at the same
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RFC 5973 NAT/FW NSIS NSLP October 2010
time. NATFW NSLP supports both scenarios (i.e., either the DS or DR
does not support NSIS) by using a proxy mode, as described in
Section 3.7.6.
administrative domain
/ ----------------------------------
|
+-------+ | +-------+ +-------+ +-------+
| DS | | | MB2/ |~~~>| MB1/ |~~~>| DR |
| | | | NR |<---| NF1 |<---| |
+-------+ | +-------+ +-------+ +-------+
|
\----------------------------------
========================================>
Data Traffic Direction (inbound)
---> : NATFW NSLP request signaling
~~~> : NATFW NSLP response signaling
DS/NI : Data sender and NSIS initiator
DR/NR : Data receiver and NSIS responder
MB1 : Middlebox 1 and NSLP forwarder 1
MB2 : Middlebox 2 and NSLP responder
Figure 12: Proxy Mode Signaling for Data Receiver
3.2.4. Blocking Traffic
The basic functionality of the NATFW NSLP provides for opening
firewall pin holes and creating NAT bindings to enable data flows to
traverse these devices. Firewalls are normally expected to work on a
"deny-all" policy, meaning that traffic not explicitly matching any
firewall filter rule will be blocked. Similarly, the normal behavior
of NATs is to block all traffic that does not match any already
configured/installed binding or NATFW NSLP session. However, some
scenarios require support of firewalls having "allow-all" policies,
allowing data traffic to traverse the firewall unless it is blocked
explicitly. Data receivers can utilize NATFW NSLP's EXTERNAL message
with action set to "deny" to install policy rules at inbound
firewalls to block unwanted traffic.
3.2.5. State and Error Maintenance
The protocol works on a soft-state basis, meaning that whatever state
is installed or reserved on a middlebox will expire, and thus be
uninstalled or forgotten after a certain period of time. To prevent
premature removal of state that is needed for ongoing communication,
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RFC 5973 NAT/FW NSIS NSLP October 2010
the NATFW NI involved will have to specifically request a NATFW NSLP
signaling session extension. An explicit NATFW NSLP state deletion
capability is also provided by the protocol.
If the actions requested by a NATFW NSLP message cannot be carried
out, NFs and the NR must return a failure, such that appropriate
actions can be taken. They can do this either during the request
message handling (synchronously) by sending an error RESPONSE message
or at any time (asynchronously) by sending a NOTIFY notification
message.
The next sections define the NATFW NSLP message types and formats,
protocol operations, and policy rule operations.
3.2.6. Message Types
The protocol uses four messages types:
o CREATE: a request message used for creating, changing, refreshing,
and deleting NATFW NSLP signaling sessions, i.e., open the data
path from DS to DR.
o EXTERNAL: a request message used for reserving, changing,
refreshing, and deleting EXTERNAL NATFW NSLP signaling sessions.
EXTERNAL messages are forwarded to the edge-NAT or edge-firewall
and allow inbound CREATE messages to be forwarded to the NR.
Additionally, EXTERNAL messages reserve an external address and,
if applicable, port number at an edge-NAT.
o NOTIFY: an asynchronous message used by NATFW peers to alert other
NATFW peers about specific events (especially failures).
o RESPONSE: used as a response to CREATE and EXTERNAL request
messages.
3.2.7. Classification of RESPONSE Messages
RESPONSE messages will be generated synchronously to CREATE and
EXTERNAL messages by NSLP forwarders and responders to report success
or failure of operations or some information relating to the NATFW
NSLP signaling session or a node. RESPONSE messages MUST NOT be
generated for any other message, such as NOTIFY and RESPONSE.
All RESPONSE messages MUST carry a NATFW_INFO object that contains an
error class code and a response code (see Section 4.2.5). This
section defines terms for groups of RESPONSE messages depending on
the error class.
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RFC 5973 NAT/FW NSIS NSLP October 2010
o Successful RESPONSE: Messages carrying NATFW_INFO with error class
'Success' (2).
o Informational RESPONSE: Messages carrying NATFW_INFO with error
class 'Informational' (1) (only used with NOTIFY messages).
o Error RESPONSE: Messages carrying NATFW_INFO with error class
other than 'Success' or 'Informational'.
3.2.8. NATFW NSLP Signaling Sessions
A NATFW NSLP signaling session defines an association between the NI,
NFs, and the NR related to a data flow. This association is created
when the initial CREATE or EXTERNAL message is successfully received
at the NFs or the NR. There is signaling NATFW NSLP session state
stored at the NTLP layer and at the NATFW NSLP level. The NATFW NSLP
signaling session state for the NATFW NSLP comprises NSLP state and
the associated policy rules at a middlebox.
The NATFW NSLP signaling session is identified by the session ID
(plus other information at the NTLP level). The session ID is
generated by the NI before the initial CREATE or EXTERNAL message is
sent. The value of the session ID MUST be generated as a
cryptographically random number (see [RFC 4086]) by the NI, i.e., the
output MUST NOT be easily guessable by third parties. The session ID
is not stored in any NATFW NSLP message but passed on to the NTLP.
A NATFW NSLP signaling session has several conceptual states that
describe in what state a signaling session is at a given time. The
signaling session can have these states at a node:
o Pending: The NATFW NSLP signaling session has been created and the
node is waiting for a RESPONSE message to the CREATE or EXTERNAL
message. A NATFW NSLP signaling session in state 'Pending' MUST
be marked as 'Dead' if no corresponding RESPONSE message has been
received within the time of the locally granted NATFW NSLP
signaling session lifetime of the forwarded CREATE or EXTERNAL
message (as described in Section 3.4).
o Established: The NATFW NSLP signaling session is established, i.e,
the signaling has been successfully performed and the lifetime of
NATFW NSLP signaling session is counted from now on. A NATFW NSLP
signaling session in state 'Established' MUST be marked as 'Dead'
if no refresh message has been received within the time of the
locally granted NATFW NSLP signaling session lifetime of the
RESPONSE message (as described in Section 3.4).
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RFC 5973 NAT/FW NSIS NSLP October 2010
o Dead: Either the NATFW NSLP signaling session is timed out or the
node has received an error RESPONSE message for the NATFW NSLP
signaling session and the NATFW NSLP signaling session can be
deleted.
o Transitory: The node has received an asynchronous message, i.e., a
NOTIFY, and can delete the NATFW NSLP signaling session if needed
after some time. When a node has received a NOTIFY message, it
marks the signaling session as 'Transitory'. This signaling
session SHOULD NOT be deleted before a minimum hold time of 30
seconds, i.e., it can be removed after 30 seconds or more. This
hold time ensures that the existing signaling session can be
reused by the NI, e.g., a part of a signaling session that is not
affected by the route change can be reused once the updating
request message is received.
3.3. Basic Message Processing
All NATFW messages are subject to some basic message processing when
received at a node, independent of the message type. Initially, the
syntax of the NSLP message is checked and a RESPONSE message with an
appropriate error of class 'Protocol error' (3) code is generated if
a non-recoverable syntax error is detected. A recoverable error is,
for instance, when a node receives a message with reserved flags set
to values other than zero. This also refers to unknown NSLP objects
and their handling, according to Section 4.2. If a message is
delivered to the NATFW NSLP, this implies that the NTLP layer has
been able to correlate it with the session ID (SID) and MRI entries
in its database. There is therefore enough information to identify
the source of the message and routing information to route the
message back to the NI through an established chain of NTLP messaging
associations. The message is not further forwarded if any error in
the syntax is detected. The specific response codes stemming from
the processing of objects are described in the respective object
definition section (see Section 4). After passing this check, the
NATFW NSLP node performs authentication- and authorization-related
checks, described in Section 3.6. Further processing is executed
only if these tests have been successfully passed; otherwise, the
processing stops and an error RESPONSE is returned.
Further message processing stops whenever an error RESPONSE message
is generated, and the EXTERNAL or CREATE message is discarded.
3.4. Calculation of Signaling Session Lifetime
NATFW NSLP signaling sessions, and the corresponding policy rules
that may have been installed, are maintained via a soft-state
mechanism. Each signaling session is assigned a signaling session
Stiemerling, et al. Experimental PAGE 27
RFC 5973 NAT/FW NSIS NSLP October 2010
lifetime and the signaling session is kept alive as long as the
lifetime is valid. After the expiration of the signaling session
lifetime, signaling sessions and policy rules MUST be removed
automatically and resources bound to them MUST be freed as well.
Signaling session lifetime is handled at every NATFW NSLP node. The
NSLP forwarders and NSLP responder MUST NOT trigger signaling session
lifetime extension refresh messages (see Section 3.7.3): this is the
task of the NSIS initiator.
The NSIS initiator MUST choose a NATFW NSLP signaling session
lifetime value (expressed in seconds) before sending any message,
including the initial message that creates the NATFW NSLP signaling
session, to other NSLP nodes. It is RECOMMENDED that the NATFW NSLP
signaling session lifetime value is calculated based on:
o the number of lost refresh messages with which NFs should cope;
o the end-to-end delay between the NI and NR;
o network vulnerability due to NATFW NSLP signaling session
hijacking ([RFC 4081]), NATFW NSLP signaling session hijacking is
made easier when the NI does not explicitly remove the NATFW NSLP
signaling session;
o the user application's data exchange duration, in terms of time
and networking needs. This duration is modeled as R, with R the
message refresh period (in seconds);
o the load on the signaling plane. Short lifetimes imply more
frequent signaling messages;
o the acceptable time for a NATFW NSLP signaling session to be
present after it is no longer actually needed. For example, if
the existence of the NATFW NSLP signaling session implies a
monetary cost and teardown cannot be guaranteed, shorter lifetimes
would be preferable;
o the lease time of the NI's IP address. The lease time of the IP
address must be longer than the chosen NATFW NSLP signaling
session lifetime; otherwise, the IP address can be re-assigned to
a different node. This node may receive unwanted traffic,
although it never has requested a NAT/firewall configuration,
which might be an issue in environments with mobile hosts.
The RSVP specification [RFC 2205] provides an appropriate algorithm
for calculating the NATFW NSLP signaling session lifetime as well as
a means to avoid refresh message synchronization between NATFW NSLP
signaling sessions. [RFC 2205] recommends:
Stiemerling, et al. Experimental PAGE 28
RFC 5973 NAT/FW NSIS NSLP October 2010
1. The refresh message timer to be randomly set to a value in the
range [0.5R, 1.5R].
2. To avoid premature loss of state, lt (with lt being the NATFW
NSLP signaling session lifetime) must satisfy lt >= (K +
0.5)*1.5*R, where K is a small integer. Then, in the worst case,
K-1 successive messages may be lost without state being deleted.
Currently, K = 3 is suggested as the default. However, it may be
necessary to set a larger K value for hops with high loss rate.
Other algorithms could be used to define the relation between the
NATFW NSLP signaling session lifetime and the refresh message
period; the algorithm provided is only given as an example.
It is RECOMMENDED to use a refresh timer of 300 s (5 minutes), unless
the NI or the requesting application at the NI has other requirements
(e.g., flows lasting a very short time).
This requested NATFW NSLP signaling session lifetime value lt is
stored in the NATFW_LT object of the NSLP message.
NSLP forwarders and the NSLP responder can execute the following
behavior with respect to the requested lifetime handling:
Requested signaling session lifetime acceptable:
No changes to the NATFW NSLP signaling session lifetime values are
needed. The CREATE or EXTERNAL message is forwarded, if
applicable.
Signaling session lifetime can be lowered:
An NSLP forwarded or the NSLP responder MAY also lower the
requested NATFW NSLP signaling session lifetime to an acceptable
value (based on its local policies). If an NF changes the NATFW
NSLP signaling session lifetime value, it MUST store the new value
in the NATFW_LT object. The CREATE or EXTERNAL message is
forwarded.
Requested signaling session lifetime is too big:
An NSLP forwarded or the NSLP responder MAY reject the requested
NATFW NSLP signaling session lifetime value as being too big and
MUST generate an error RESPONSE message of class 'Signaling
session failure' (7) with response code 'Requested lifetime is too
big' (0x02) upon rejection. Lowering the lifetime is preferred
instead of generating an error message.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Requested signaling session lifetime is too small:
An NSLP forwarded or the NSLP responder MAY reject the requested
NATFW NSLP signaling session lifetime value as being to small and
MUST generate an error RESPONSE message of class 'Signaling
session failure' (7) with response code 'Requested lifetime is too
small' (0x10) upon rejection.
NFs or the NR MUST NOT increase the NATFW NSLP signaling session
lifetime value. Messages can be rejected on the basis of the NATFW
NSLP signaling session lifetime being too long when a NATFW NSLP
signaling session is first created and also on refreshes.
The NSLP responder generates a successful RESPONSE for the received
CREATE or EXTERNAL message, sets the NATFW NSLP signaling session
lifetime value in the NATFW_LT object to the above granted lifetime
and sends the message back towards NSLP initiator.
Each NSLP forwarder processes the RESPONSE message and reads and
stores the granted NATFW NSLP signaling session lifetime value. The
forwarders MUST accept the granted NATFW NSLP signaling session
lifetime, if the lifetime value is within the acceptable range. The
acceptable value refers to the value accepted by the NSLP forwarder
when processing the CREATE or EXTERNAL message. For received values
greater than the acceptable value, NSLP forwarders MUST generate a
RESPONSE message of class 'Signaling session failure' (7) with
response code 'Modified lifetime is too big' (0x11), including a
Signaling Session Lifetime object that carries the maximum acceptable
signaling session lifetime for this node. For received values lower
than the values acceptable by the node local policy, NSLP forwarders
MUST generate a RESPONSE message of class 'Signaling session failure'
(7) with response code 'Modified lifetime is too small' (0x12),
including a Signaling Session Lifetime object that carries the
minimum acceptable signaling session lifetime for this node. In both
cases, either 'Modified lifetime is too big' (0x11) or 'Modified
lifetime is too small' (0x12), the NF MUST generate a NOTIFY message
and send it outbound with the error class set to 'Informational' (1)
and with the response code set to 'NATFW signaling session
terminated' (0x05).
Figure 13 shows the procedure with an example, where an initiator
requests 60 seconds lifetime in the CREATE message and the lifetime
is shortened along the path by the forwarder to 20 seconds and by the
responder to 15 seconds. When the NSLP forwarder receives the
RESPONSE message with a NATFW NSLP signaling session lifetime value
of 15 seconds it checks whether this value is lower or equal to the
acceptable value.
Stiemerling, et al. Experimental PAGE 30
RFC 5973 NAT/FW NSIS NSLP October 2010
+-------+ CREATE(lt=60s) +-------------+ CREATE(lt=20s) +--------+
| |---------------->| NSLP |---------------->| |
| NI | | forwarder | | NR |
| |<----------------| check 15<20 |<----------------| |
+-------+ RESPONSE(lt=15s)+-------------+ RESPONSE(lt=15s)+--------+
lt = lifetime
Figure 13: Signaling Session Lifetime Setting Example
3.5. Message Sequencing
NATFW NSLP messages need to carry an identifier so that all nodes
along the path can distinguish messages sent at different points in
time. Messages can be lost along the path or duplicated. So, all
NATFW NSLP nodes should be able to identify messages that have been
received before (duplicated) or lost before (loss). For message
replay protection, it is necessary to keep information about messages
that have already been received and requires every NATFW NSLP message
to carry a message sequence number (MSN), see also Section 4.2.7.
The MSN MUST be set by the NI and MUST NOT be set or modified by any
other node. The initial value for the MSN MUST be generated randomly
and MUST be unique only within the NATFW NSLP signaling session for
which it is used. The NI MUST increment the MSN by one for every
message sent. Once the MSN has reached the maximum value, the next
value it takes is zero. All NATFW NSLP nodes MUST use the algorithm
defined in [RFC 1982] to detect MSN wrap-arounds.
NSLP forwarders and the responder store the MSN from the initial
CREATE or EXTERNAL packet that creates the NATFW NSLP signaling
session as the start value for the NATFW NSLP signaling session. NFs
and NRs MUST include the received MSN value in the corresponding
RESPONSE message that they generate.
When receiving a CREATE or EXTERNAL message, a NATFW NSLP node uses
the MSN given in the message to determine whether the state being
requested is different from the state already installed. The message
MUST be discarded if the received MSN value is equal to or lower than
the stored MSN value. Such a received MSN value can indicate a
duplicated and delayed message or replayed message. If the received
MSN value is greater than the already stored MSN value, the NATFW
NSLP MUST update its stored state accordingly, if permitted by all
security checks (see Section 3.6), and store the updated MSN value
accordingly.
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RFC 5973 NAT/FW NSIS NSLP October 2010
3.6. Authentication, Authorization, and Policy Decisions
NATFW NSLP nodes receiving signaling messages MUST first check
whether this message is authenticated and authorized to perform the
requested action. NATFW NSLP nodes requiring more information than
provided MUST generate an error RESPONSE of class 'Permanent failure'
(0x5) with response code 'Authentication failed' (0x01) or with
response code 'Authorization failed' (0x02).
The NATFW NSLP is expected to run in various environments, such as
IP-based telephone systems, enterprise networks, home networks, etc.
The requirements on authentication and authorization are quite
different between these use cases. While a home gateway, or an
Internet cafe, using NSIS may well be happy with a "NATFW signaling
coming from inside the network" policy for authorization of
signaling, enterprise networks are likely to require more strongly
authenticated/authorized signaling. This enterprise scenario may
require the use of an infrastructure and administratively assigned
identities to operate the NATFW NSLP.
Once the NI is authenticated and authorized, another step is
performed. The requested policy rule for the NATFW NSLP signaling
session is checked against a set of policy rules, i.e., whether the
requesting NI is allowed to request the policy rule to be loaded in
the device. If this fails, the NF or NR must send an error RESPONSE
of class 'Permanent failure' (5) and with response code
'Authorization failed' (0x02).
3.7. Protocol Operations
This section defines the protocol operations including how to create
NATFW NSLP signaling sessions, maintain them, delete them, and how to
reserve addresses.
This section requires a good knowledge of the NTLP [RFC 5971] and the
message routing method mechanism and the associated message routing
information (MRI). The NATFW NSLP uses information from the MRI,
e.g., the destination and source ports, and the NATFW NSLP to
construct the policy rules used on the NATFW NSLP level. See also
Appendix D for further information about this.
3.7.1. Creating Signaling Sessions
Allowing two hosts to exchange data even in the presence of
middleboxes is realized in the NATFW NSLP by the use of the CREATE
message. The NI (either the data sender or a proxy) generates a
CREATE message as defined in Section 4.3.1 and hands it to the NTLP.
The NTLP forwards the whole message on the basis of the message
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RFC 5973 NAT/FW NSIS NSLP October 2010
routing information (MRI) towards the NR. Each NSLP forwarder along
the path that implements NATFW NSLP processes the NSLP message.
Forwarding is done hop-by-hop but may pass transparently through NSLP
forwarders that do not contain NATFW NSLP functionality and non-NSIS-
aware routers between NSLP hop way points. When the message reaches
the NR, the NR can accept the request or reject it. The NR generates
a response to CREATE and this response is transported hop-by-hop
towards the NI. NATFW NSLP forwarders may reject requests at any
time. Figure 14 sketches the message flow between the NI (DS in this
example), an NF (e.g., NAT), and an NR (DR in this example).
NI Private Network NF Public Internet NR
| | |
| CREATE | |
|----------------------------->| |
| | |
| | |
| | CREATE |
| |--------------------------->|
| | |
| | RESPONSE |
| RESPONSE |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 14: CREATE Message Flow with Success RESPONSE
There are several processing rules for a NATFW peer when generating
and receiving CREATE messages, since this message type is used for
creating new NATFW NSLP signaling sessions, updating existing ones,
and extending the lifetime and deleting NATFW NSLP signaling
sessions. The three latter functions operate in the same way for all
kinds of CREATE messages, and are therefore described in separate
sections:
o Extending the lifetime of NATFW NSLP signaling sessions is
described in Section 3.7.3.
o Deleting NATFW NSLP signaling sessions is described in
Section 3.7.4.
o Updating policy rules is described in Section 3.10.
For an initial CREATE message creating a new NATFW NSLP signaling
session, the processing of CREATE messages is different for every
NATFW node type:
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RFC 5973 NAT/FW NSIS NSLP October 2010
o NSLP initiator: An NI only generates CREATE messages and hands
them over to the NTLP. The NI should never receive CREATE
messages and MUST discard them.
o NATFW NSLP forwarder: NFs that are unable to forward the CREATE
message to the next hop MUST generate an error RESPONSE of class
'Permanent failure' (5) with response code 'Did not reach the NR'
(0x07). This case may occur if the NTLP layer cannot find a NATFW
NSLP peer, either another NF or the NR, and returns an error via
the GIST API (a timeout error reported by GIST). The NSLP message
processing at the NFs depends on the middlebox type:
* NAT: When the initial CREATE message is received at the public
side of the NAT, it looks for a reservation made in advance, by
using an EXTERNAL message (see Section 3.7.2). The matching
process considers the received MRI information and the stored
MRI information, as described in Section 3.8. If no matching
reservation can be found, i.e., no reservation has been made in
advance, the NSLP MUST return an error RESPONSE of class
'Signaling session failure' (7) with response code 'No
reservation found matching the MRI of the CREATE request'
(0x03). If there is a matching reservation, the NSLP stores
the data sender's address (and if applicable port number) as
part of the source IP address of the policy rule ('the
remembered policy rule') to be loaded, and forwards the message
with the destination IP address set to the internal (private in
most cases) address of the NR. When the initial CREATE message
is received at the private side, the NAT binding is allocated,
but not activated (see also Appendix D.3). An error RESPONSE
message is generated, if the requested policy rule cannot be
reserved right away, of class 'Signaling session failure' (7)
with response code 'Requested policy rule denied due to policy
conflict' (0x4). The MRI information is updated to reflect the
address, and if applicable port, translation. The NSLP message
is forwarded towards the NR with source IP address set to the
NAT's external address from the newly remembered binding.
* Firewall: When the initial CREATE message is received, the NSLP
just remembers the requested policy rule, but does not install
any policy rule. Afterwards, the message is forwarded towards
the NR. If the requested policy rule cannot be reserved right
away, an error RESPONSE message is generated, of class
'Signaling session failure' (7) with response code 'Requested
policy rule denied due to policy conflict' (0x4).
* Combined NAT and firewall: Processing at combined firewall and
NAT middleboxes is the same as in the NAT case. No policy
rules are installed. Implementations MUST take into account
Stiemerling, et al. Experimental PAGE 34
RFC 5973 NAT/FW NSIS NSLP October 2010
the order of packet processing in the firewall and NAT
functions within the device. This will be referred to as
"order of functions" and is generally different depending on
whether the packet arrives at the external or internal side of
the middlebox.
o NSLP receiver: NRs receiving initial CREATE messages MUST reply
with a success RESPONSE of class 'Success' (2) with response code
set to 'All successfully processed' (0x01), if they accept the
CREATE message. Otherwise, they MUST generate a RESPONSE message
with a suitable response code. RESPONSE messages are sent back
NSLP hop-by-hop towards the NI, irrespective of the response
codes, either success or error.
Remembered policy rules at middleboxes MUST be only installed upon
receiving a corresponding successful RESPONSE message with the same
SID as the CREATE message that caused them to be remembered. This is
a countermeasure to several problems, for example, wastage of
resources due to loading policy rules at intermediate NFs when the
CREATE message does not reach the final NR for some reason.
Processing of a RESPONSE message is different for every NSIS node
type:
o NSLP initiator: After receiving a successful RESPONSE, the data
path is configured and the DS can start sending its data to the
DR. After receiving an error RESPONSE message, the NI MAY try to
generate the CREATE message again or give up and report the
failure to the application, depending on the error condition.
o NSLP forwarder: NFs install the remembered policy rules, if a
successful RESPONSE message with matching SID is received. If an
ERROR RESPONSE message with matching SID is received, the NATFW
NSLP session is marked as 'Dead', no policy rule is installed and
the remembered rule is discarded.
o NSIS responder: The NR should never receive RESPONSE messages and
MUST silently drop any such messages received.
NFs and the NR can also tear down the CREATE session at any time by
generating a NOTIFY message with the appropriate response code set.
3.7.2. Reserving External Addresses
NSIS signaling is intended to travel end-to-end, even in the presence
of NATs and firewalls on-path. This works well in cases where the
data sender is itself behind a NAT or a firewall as described in
Section 3.7.1. For scenarios where the data receiver is located
Stiemerling, et al. Experimental PAGE 35
RFC 5973 NAT/FW NSIS NSLP October 2010
behind a NAT or a firewall and it needs to receive data flows from
outside its own network (usually referred to as inbound flows, see
Figure 5), the problem is more troublesome.
NSIS signaling, as well as subsequent data flows, are directed to a
particular destination IP address that must be known in advance and
reachable. Data receivers must tell the local NSIS infrastructure
(i.e., the inbound firewalls/NATs) about incoming NATFW NSLP
signaling and data flows before they can receive these flows. It is
necessary to differentiate between data receivers behind NATs and
behind firewalls to understand the further NATFW procedures. Data
receivers that are only behind firewalls already have a public IP
address and they need only to be reachable for NATFW signaling.
Unlike data receivers that are only behind firewalls, data receivers
behind NATs do not have public IP addresses; consequently, they are
not reachable for NATFW signaling by entities outside their
addressing realm.
The preceding discussion addresses the situation where a DR node that
wants to be reachable is unreachable because the NAT lacks a suitable
rule with the 'allow' action that would forward inbound data.
However, in certain scenarios, a node situated behind inbound
firewalls that do not block inbound data traffic (firewalls with
"default to allow") unless requested might wish to prevent traffic
being sent to it from specified addresses. In this case, NSIS NATFW
signaling can be used to achieve this by installing a policy rule
with its action set to 'deny' using the same mechanisms as for
'allow' rules.
The required result is obtained by sending an EXTERNAL message in the
inbound direction of the intended data flow. When using this
functionality, the NSIS initiator for the 'Reserve External Address'
signaling is typically the node that will become the DR for the
eventual data flow. To distinguish this initiator from the usual
case where the NI is associated with the DS, the NI is denoted by NI+
and the NSIS responder is similarly denoted by NR+.
Stiemerling, et al. Experimental PAGE 36
RFC 5973 NAT/FW NSIS NSLP October 2010
Public Internet Private Address
Space
Edge
NI(DS) NAT/FW NAT NR(DR)
NR+ NI+
| | | |
| | | |
| | | |
| | EXTERNAL[(DTInfo)] | EXTERNAL[(DTInfo)] |
| |<----------------------|<----------------------|
| | | |
| |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
| |---------------------->|---------------------->|
| | | |
| | | |
============================================================>
Data Traffic Direction
Figure 15: Reservation Message Flow for DR behind NAT or Firewall
Figure 15 shows the EXTERNAL message flow for enabling inbound NATFW
NSLP signaling messages. In this case, the roles of the different
NSIS entities are:
o The data receiver (DR) for the anticipated data traffic is the
NSIS initiator (NI+) for the EXTERNAL message, but becomes the
NSIS responder (NR) for following CREATE messages.
o The actual data sender (DS) will be the NSIS initiator (NI) for
later CREATE messages and may be the NSIS target of the signaling
(NR+).
o It may be necessary to use a signaling destination address (SDA)
as the actual target of the EXTERNAL message (NR+) if the DR is
located behind a NAT and the address of the DS is unknown. The
SDA is an arbitrary address in the outermost address realm on the
other side of the NAT from the DR. Typically, this will be a
suitable public IP address when the 'outside' realm is the public
Internet. This choice of address causes the EXTERNAL message to
be routed through the NATs towards the outermost realm and would
force interception of the message by the outermost NAT in the
network at the boundary between the private address and the public
address realm (the edge-NAT). It may also be intercepted by other
NATs and firewalls on the path to the edge-NAT.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Basically, there are two different signaling scenarios. Either
1. the DR behind the NAT/firewall knows the IP address of the DS in
advance, or
2. the address of the DS is not known in advance.
Case 1 requires the NATFW NSLP to request the path-coupled message
routing method (PC-MRM) from the NTLP. The EXTERNAL message MUST be
sent with PC-MRM (see Section 5.8.1 in [RFC 5971]) with the direction
set to 'upstream' (inbound). The handling of case 2 depends on the
situation of the DR: if the DR is solely located behind a firewall,
the EXTERNAL message MUST be sent with the PC-MRM, direction
'upstream' (inbound), and the data flow source IP address set to
'wildcard'. If the DR is located behind a NAT, the EXTERNAL message
MUST be sent with the loose-end message routing method (LE-MRM, see
Section 5.8.2 in [RFC 5971]), the destination-address set to the
signaling destination IP address (SDA, see also Appendix A). For
scenarios with the DR behind a firewall, special conditions apply
(see applicability statement in Appendix C). The data receiver is
challenged to determine whether it is solely located behind firewalls
or NATs in order to choose the right message routing method. This
decision can depend on a local configuration parameter, possibly
given through DHCP, or it could be discovered through other non-NSLP
related testing of the network configuration. The use of the PC-MRM
with the known data sender's IP address is RECOMMENDED. This gives
GIST the best possible handle to route the message 'upstream'
(outbound). The use of the LE-MRM, if and only if the data sender's
IP address is not known and the data receiver is behind a NAT, is
RECOMMENDED.
For case 2 with NAT, the NI+ (which could be on the data receiver DR
or on any other host within the private network) sends the EXTERNAL
message targeted to the signaling destination IP address. The
message routing for the EXTERNAL message is in the reverse direction
of the normal message routing used for path-coupled signaling where
the signaling is sent outbound (as opposed to inbound in this case).
When establishing NAT bindings (and a NATFW NSLP signaling session),
the signaling direction does not matter since the data path is
modified through route pinning due to the external IP address at the
NAT. Subsequent NSIS messages (and also data traffic) will travel
through the same NAT boxes. However, this is only valid for the NAT
boxes, but not for any intermediate firewall. That is the reason for
having a separate CREATE message enabling the reservations made with
EXTERNAL at the NATs and either enabling prior reservations or
creating new pinholes at the firewalls that are encountered on the
outbound path depending on whether the inbound and outbound routes
coincide.
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RFC 5973 NAT/FW NSIS NSLP October 2010
The EXTERNAL signaling message creates an NSIS NATFW signaling
session at any intermediate NSIS NATFW peer(s) encountered,
independent of the message routing method used. Furthermore, it has
to be ensured that the edge-NAT or edge-firewall device is discovered
as part of this process. The end host cannot be assumed to know this
device -- instead the NAT or firewall box itself is assumed to know
that it is located at the outer perimeter of the network. Forwarding
of the EXTERNAL message beyond this entity is not necessary, and MUST
be prohibited as it may provide information on the capabilities of
internal hosts. It should be noted, that it is the outermost NAT or
firewall that is the edge-device that must be found during this
discovery process. For instance, when there are a NAT and
(afterwards) a firewall on the outbound path at the network border,
the firewall is the edge-firewall. All messages must be forwarded to
the topology-wise outermost edge-device to ensure that this device
knows about the NATFW NSLP signaling sessions for incoming CREATE
messages. However, the NAT is still the edge-NAT because it has a
public globally routable IP address on its public side: this is not
affected by any firewall between the edge-NAT and the public network.
Possible edge arrangements are:
Public Net ----------------- Private net --------------
| Public Net|--|Edge-FW|--|FW|...|FW|--|DR|
| Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR|
| Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR|
The edge-NAT or edge-firewall device closest to the public realm
responds to the EXTERNAL request message with a successful RESPONSE
message. An edge-NAT includes a NATFW_EXTERNAL_IP object (see
Section 4.2.2), carrying the publicly reachable IP address, and if
applicable, a port number.
The NI+ can request each intermediate NAT (i.e., a NAT that is not
the edge-NAT) to include the external binding address (and if
applicable port number) in the external binding address object. The
external binding address object stores the external IP address (and
port) at the particular NAT. The NI+ has to include the external
binding address (see Section 4.2.3) object in the request message, if
it wishes to obtain the information.
There are several processing rules for a NATFW peer when generating
and receiving EXTERNAL messages, since this message type is used for
creating new reserve NATFW NSLP signaling sessions, updating
existing, extending the lifetime, and deleting NATFW NSLP signaling
Stiemerling, et al. Experimental PAGE 39
RFC 5973 NAT/FW NSIS NSLP October 2010
session. The three latter functions operate in the same way for all
kinds of CREATE and EXTERNAL messages, and are therefore described in
separate sections:
o Extending the lifetime of NATFW NSLP signaling sessions is
described in Section 3.7.3.
o Deleting NATFW NSLP signaling sessions is described in
Section 3.7.4.
o Updating policy rules is described in Section 3.10.
The NI+ MUST always include a NATFW_DTINFO object in the EXTERNAL
message. Especially, the LE-MRM does not include enough information
for some types of NATs (basically, those NATs that also translate
port numbers) to perform the address translation. This information
is provided in the NATFW_DTINFO (see Section 4.2.8). This
information MUST include at least the 'dst port number' and
'protocol' fields, in the NATFW_DTINFO object as these may be
required by NATs that are en route, depending on the type of the NAT.
All other fields MAY be set by the NI+ to restrict the set of
possible NIs. An edge-NAT will use the information provided in the
NATFW_DTINFO object to allow only a NATFW CREATE message with a
matching MRI to be forwarded. The MRI of the NATFW CREATE message
has to use the parameters set in NATFW_DTINFO object ('src IPv4/v6
address', 'src port number', 'protocol') as the source IP address/
port of the flow from DS to DR. A NAT requiring information carried
in the NATFW_DTINFO can generate a number of error RESPONSE messages
of class 'Signaling session failure' (7):
o 'Requested policy rule denied due to policy conflict' (0x04)
o 'Unknown policy rule action' (0x05)
o 'Requested rule action not applicable' (0x06)
o 'NATFW_DTINFO object is required' (0x07)
o 'Requested value in sub_ports field in NATFW_EFI not permitted'
(0x08)
o 'Requested IP protocol not supported' (0x09)
o 'Plain IP policy rules not permitted -- need transport layer
information' (0x0A)
o 'Source IP address range is too large' (0x0C)
Stiemerling, et al. Experimental PAGE 40
RFC 5973 NAT/FW NSIS NSLP October 2010
o 'Destination IP address range is too large' (0x0D)
o 'Source L4-port range is too large' (0x0E)
o 'Destination L4-port range is too large' (0x0F)
Processing of EXTERNAL messages is specific to the NSIS node type:
o NSLP initiator: NI+ only generate EXTERNAL messages. When the
data sender's address information is known in advance, the NI+ can
include a NATFW_DTINFO object in the EXTERNAL message, if not
anyway required to do so (see above). When the data sender's IP
address is not known, the NI+ MUST NOT include an IP address in
the NATFW_DTINFO object. The NI should never receive EXTERNAL
messages and MUST silently discard it.
o NSLP forwarder: The NSLP message processing at NFs depends on the
middlebox type:
* NAT: NATs check whether the message is received at the external
(public in most cases) address or at the internal (private)
address. If received at the external address, an NF MUST
generate an error RESPONSE of class 'Protocol error' (3) with
response code 'Received EXTERNAL request message on external
side' (0x0B). If received at the internal (private address)
and the NATFW_EFI object contains the action 'deny', an error
RESPONSE of class 'Protocol error' (3) with response code
'Requested rule action not applicable' (0x06) MUST be
generated. If received at the internal address, an IP address,
and if applicable, one or more ports, are reserved. If the
NATFW_EXTERNAL_BINDING object is present in the message, any
NAT that is not an edge-NAT MUST include the allocated external
IP address, and if applicable one or more ports, (the external
binding address) in the NATFW_EXTERNAL_BINDING object. If it
is an edge-NAT and there is no edge-firewall beyond, the NSLP
message is not forwarded any further and a successful RESPONSE
message is generated containing a NATFW_EXTERNAL_IP object
holding the translated address, and if applicable, port
information from the binding reserved as a result of the
EXTERNAL message. The edge-NAT MUST copy the
NATFW_EXTERNAL_BINDING object to response message, if the
object is included in the EXTERNAL message. The RESPONSE
message is sent back towards the NI+. If it is not an edge-
NAT, the NSLP message is forwarded further using the translated
IP address as signaling source IP address in the LE-MRM and
translated port in the NATFW_DTINFO object in the field 'DR
port number', i.e., the NATFW_DTINFO object is updated to
reflect the translated port number. The edge-NAT or any other
Stiemerling, et al. Experimental PAGE 41
RFC 5973 NAT/FW NSIS NSLP October 2010
NAT MUST reject EXTERNAL messages not carrying a NATFW_DTINFO
object or if the address information within this object is
invalid or is not compliant with local policies (e.g., the
information provided relates to a range of addresses
('wildcarded') but the edge-NAT requires exact information
about DS's IP address and port) with the above mentioned
response codes.
* Firewall: Non edge-firewalls remember the requested policy
rule, keep NATFW NSLP signaling session state, and forward the
message. Edge-firewalls stop forwarding the EXTERNAL message.
The policy rule is immediately loaded if the action in the
NATFW_EFI object is set to 'deny' and the node is an edge-
firewall. The policy rule is remembered, but not activated, if
the action in the NATFW_EFI object is set to 'allow'. In both
cases, a successful RESPONSE message is generated. If the
action is 'allow', and the NATFW_DTINFO object is included, and
the MRM is set to LE-MRM in the request, additionally a
NATFW_EXTERNAL_IP object is included in the RESPONSE message,
holding the translated address, and if applicable port,
information. This information is obtained from the
NATFW_DTINFO object's 'DR port number' and the source-address
of the LE-MRM. The edge-firewall MUST copy the
NATFW_EXTERNAL_BINDING object to response message, if the
object is included in the EXTERNAL message.
* Combined NAT and firewall: Processing at combined firewall and
NAT middleboxes is the same as in the NAT case.
o NSLP receiver: This type of message should never be received by
any NR+, and it MUST generate an error RESPONSE message of class
'Permanent failure' (5) with response code 'No edge-device here'
(0x06).
Processing of a RESPONSE message is different for every NSIS node
type:
o NSLP initiator: Upon receiving a successful RESPONSE message, the
NI+ can rely on the requested configuration for future inbound
NATFW NSLP signaling sessions. If the response contains a
NATFW_EXTERNAL_IP object, the NI can use IP address and port pairs
carried for further application signaling. After receiving an
error RESPONSE message, the NI+ MAY try to generate the EXTERNAL
message again or give up and report the failure to the
application, depending on the error condition.
Stiemerling, et al. Experimental PAGE 42
RFC 5973 NAT/FW NSIS NSLP October 2010
o NSLP forwarder: NFs simply forward this message as long as they
keep state for the requested reservation, if the RESPONSE message
contains NATFW_INFO object with class set to 'Success' (2). If
the RESPONSE message contains NATFW_INFO object with class set not
to 'Success' (2), the NATFW NSLP signaling session is marked as
'Dead'.
o NSIS responder: This type of message should never be received by
any NR+. The NF should never receive response messages and MUST
silently discard it.
NFs and the NR can also tear down the EXTERNAL session at any time by
generating a NOTIFY message with the appropriate response code set.
Reservations with action 'allow' made with EXTERNAL MUST be enabled
by a subsequent CREATE message. A reservation made with EXTERNAL
(independent of selected action) is kept alive as long as the NI+
refreshes the particular NATFW NSLP signaling session and it can be
reused for multiple, different CREATE messages. An NI+ may decide to
tear down a reservation immediately after receiving a CREATE message.
This implies that a new NATFW NSLP signaling session must be created
for each new CREATE message. The CREATE message does not re-use the
NATFW NSLP signaling session created by EXTERNAL.
Without using CREATE (see Section 3.7.1) or EXTERNAL in proxy mode
(see Section 3.7.6) no data traffic will be forwarded to the DR
beyond the edge-NAT or edge-firewall. The only function of EXTERNAL
is to ensure that subsequent CREATE messages traveling towards the NR
will be forwarded across the public-private boundary towards the DR.
Correlation of incoming CREATE messages to EXTERNAL reservation
states is described in Section 3.8.
3.7.3. NATFW NSLP Signaling Session Refresh
NATFW NSLP signaling sessions are maintained on a soft-state basis.
After a specified timeout, sessions and corresponding policy rules
are removed automatically by the middlebox, if they are not
refreshed. Soft-state is created by CREATE and EXTERNAL and the
maintenance of this state must be done by these messages. State
created by CREATE must be maintained by CREATE, state created by
EXTERNAL must be maintained by EXTERNAL. Refresh messages, are
messages carrying the same session ID as the initial message and a
NATFW_LT lifetime object with a lifetime greater than zero. Messages
with the same SID but which carry a different MRI are treated as
updates of the policy rules and are processed as defined in
Section 3.10. Every refresh CREATE or EXTERNAL message MUST be
acknowledged by an appropriate response message generated by the NR.
Upon reception by each NSLP forwarder, the state for the given
Stiemerling, et al. Experimental PAGE 43
RFC 5973 NAT/FW NSIS NSLP October 2010
session ID is extended by the NATFW NSLP signaling session refresh
period, a period of time calculated based on a proposed refresh
message period. The new (extended) lifetime of a NATFW NSLP
signaling session is calculated as current local time plus proposed
lifetime value (NATFW NSLP signaling session refresh period).
Section 3.4 defines the process of calculating lifetimes in detail.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime > 0] | |
|----------------------------->| |
| | |
| | |
| | CREATE[lifetime > 0] |
| |--------------------------->|
| | |
| | RESPONSE[Success/Error] |
| RESPONSE[Success/Error] |<---------------------------|
|<-----------------------------| |
| | |
| | |
Figure 16: Successful Refresh Message Flow, CREATE as Example
Processing of NATFW NSLP signaling session refresh CREATE and
EXTERNAL messages is different for every NSIS node type:
o NSLP initiator: The NI/NI+ can generate NATFW NSLP signaling
session refresh CREATE/EXTERNAL messages before the NATFW NSLP
signaling session times out. The rate at which the refresh
CREATE/EXTERNAL messages are sent and their relation to the NATFW
NSLP signaling session state lifetime is discussed further in
Section 3.4.
o NSLP forwarder: Processing of this message is independent of the
middlebox type and is as described in Section 3.4.
o NSLP responder: NRs accepting a NATFW NSLP signaling session
refresh CREATE/EXTERNAL message generate a successful RESPONSE
message, including the granted lifetime value of Section 3.4 in a
NATFW_LT object.
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RFC 5973 NAT/FW NSIS NSLP October 2010
3.7.4. Deleting Signaling Sessions
NATFW NSLP signaling sessions can be deleted at any time. NSLP
initiators can trigger this deletion by using a CREATE or EXTERNAL
messages with a lifetime value set to 0, as shown in Figure 17.
Whether a CREATE or EXTERNAL message type is use depends on how the
NATFW NSLP signaling session was created.
NI Public Internet NAT Private address NR
| | space |
| CREATE[lifetime=0] | |
|----------------------------->| |
| | |
| | CREATE[lifetime=0] |
| |--------------------------->|
| | |
Figure 17: Delete message flow, CREATE as Example
NSLP nodes receiving this message delete the NATFW NSLP signaling
session immediately. Policy rules associated with this particular
NATFW NSLP signaling session MUST be also deleted immediately. This
message is forwarded until it reaches the final NR. The CREATE/
EXTERNAL message with a lifetime value of 0, does not generate any
response, either positive or negative, since there is no NSIS state
left at the nodes along the path.
NSIS initiators can use CREATE/EXTERNAL message with lifetime set to
zero in an aggregated way, such that a single CREATE or EXTERNAL
message is terminating multiple NATFW NSLP signaling sessions. NIs
can follow this procedure if they like to aggregate NATFW NSLP
signaling session deletion requests: the NI uses the CREATE or
EXTERNAL message with the session ID set to zero and the MRI's
source-address set to its used IP address. All other fields of the
respective NATFW NSLP signaling sessions to be terminated are set as
well; otherwise, these fields are completely wildcarded. The NSLP
message is transferred to the NTLP requesting 'explicit routing' as
described in Sections 5.2.1 and 7.1.4. in [RFC 5971].
The outbound NF receiving such an aggregated CREATE or EXTERNAL
message MUST reject it with an error RESPONSE of class 'Permanent
failure' (5) with response code 'Authentication failed' (0x01) if the
authentication fails and with an error RESPONSE of class 'Permanent
failure' (5) with response code 'Authorization failed' (0x02) if the
authorization fails. Proof of ownership of NATFW NSLP signaling
sessions, as it is defined in this memo (see Section 5.2.1), is not
possible when using this aggregation for multiple session
Stiemerling, et al. Experimental PAGE 45
RFC 5973 NAT/FW NSIS NSLP October 2010
termination. However, the outbound NF can use the relationship
between the information of the received CREATE or EXTERNAL message
and the GIST messaging association where the request has been
received. The outbound NF MUST only accept this aggregated CREATE or
EXTERNAL message through already established GIST messaging
associations with the NI. The outbound NF MUST NOT propagate this
aggregated CREATE or EXTERNAL message but it MAY generate and forward
per NATFW NSLP signaling session CREATE or EXTERNAL messages.
3.7.5. Reporting Asynchronous Events
NATFW NSLP forwarders and NATFW NSLP responders must have the ability
to report asynchronous events to other NATFW NSLP nodes, especially
to allow reporting back to the NATFW NSLP initiator. Such
asynchronous events may be premature NATFW NSLP signaling session
termination, changes in local policies, route change or any other
reason that indicates change of the NATFW NSLP signaling session
state.
NFs and NRs may generate NOTIFY messages upon asynchronous events,
with a NATFW_INFO object indicating the reason for event. These
reasons can be carried in the NATFW_INFO object (class MUST be set to
'Informational' (1)) within the NOTIFY message. This list shows the
response codes and the associated actions to take at NFs and the NI:
o 'Route change: Possible route change on the outbound path' (0x01):
Follow instructions in Section 3.9. This MUST be sent inbound and
outbound, if the signaling session is any state except
'Transitory'. The NOTIFY message for signaling sessions in state
Transitory MUST be discarded, as the signaling session is anyhow
Transitory. The outbound NOTIFY message MUST be sent with
explicit routing by providing the SII-Handle to the NTLP.
o 'Re-authentication required' (0x02): The NI should re-send the
authentication. This MUST be sent inbound.
o 'NATFW node is going down soon' (0x03): The NI and other NFs
should be prepared for a service interruption at any time. This
message MAY be sent inbound and outbound.
o 'NATFW signaling session lifetime expired' (0x04): The NATFW
signaling session has expired and the signaling session is invalid
now. NFs MUST mark the signaling session as 'Dead'. This message
MAY be sent inbound and outbound.
Stiemerling, et al. Experimental PAGE 46
RFC 5973 NAT/FW NSIS NSLP October 2010
o 'NATFW signaling session terminated' (0x05): The NATFW signaling
session has been terminated for some reason and the signaling
session is invalid now. NFs MUST mark the signaling session as
'Dead'. This message MAY be sent inbound and outbound.
NOTIFY messages are always sent hop-by-hop inbound towards NI until
they reach NI or outbound towards the NR as indicated in the list
above.
The initial processing when receiving a NOTIFY message is the same
for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages
through already established NTLP messaging associations. The further
processing is different for each NATFW NSLP node type and depends on
the events notified:
o NSLP initiator: NIs analyze the notified event and behave
appropriately based on the event type. NIs MUST NOT generate
NOTIFY messages.
o NSLP forwarder: NFs analyze the notified event and behave based on
the above description per response code. NFs SHOULD generate
NOTIFY messages upon asynchronous events and forward them inbound
towards the NI or outbound towards the NR, depending on the
received direction, i.e., inbound messages MUST be forwarded
further inbound and outbound messages MUST be forwarded further
outbound. NFs MUST silently discard NOTIFY messages that have
been received outbound but are only allowed to be sent inbound,
e.g., 'Re-authentication required' (0x02).
o NSLP responder: NRs SHOULD generate NOTIFY messages upon
asynchronous events including a response code based on the
reported event. The NR MUST silently discard NOTIFY messages that
have been received outbound but are only allowed to be sent
inbound, e.g., 'Re-authentication required' (0x02).
NATFW NSLP forwarders, keeping multiple NATFW NSLP signaling sessions
at the same time, can experience problems when shutting down service
suddenly. This sudden shutdown can be as a result of local node
failure, for instance, due to a hardware failure. This NF generates
NOTIFY messages for each of the NATFW NSLP signaling sessions and
tries to send them inbound. Due to the number of NOTIFY messages to
be sent, the shutdown of the node may be unnecessarily prolonged,
since not all messages can be sent at the same time. This case can
be described as a NOTIFY storm, if a multitude of NATFW NSLP
signaling sessions is involved.
Stiemerling, et al. Experimental PAGE 47
RFC 5973 NAT/FW NSIS NSLP October 2010
To avoid the need for generating per NATFW NSLP signaling session
NOTIFY messages in such a scenario described or similar cases, NFs
SHOULD follow this procedure: the NF uses the NOTIFY message with the
session ID in the NTLP set to zero, with the MRI completely
wildcarded, using the 'explicit routing' as described in Sections
5.2.1 and 7.1.4 of [RFC 5971]. The inbound NF receiving this type of
NOTIFY immediately regards all NATFW NSLP signaling sessions from
that peer matching the MRI as void. This message will typically
result in multiple NOTIFY messages at the inbound NF, i.e., the NF
can generate per terminated NATFW NSLP signaling session a NOTIFY
message. However, an NF MAY also aggregate the NOTIFY messages as
described here.
3.7.6. Proxy Mode of Operation
Some migration scenarios need specialized support to cope with cases
where NSIS is only deployed in some areas of the Internet. End-to-
end signaling is going to fail without NSIS support at or near both
data sender and data receiver terminals. A proxy mode of operation
is needed. This proxy mode of operation must terminate the NATFW
NSLP signaling topologically-wise as close as possible to the
terminal for which it is proxying and proxy all messages. This NATFW
NSLP node doing the proxying of the signaling messages becomes either
the NI or the NR for the particular NATFW NSLP signaling session,
depending on whether it is the DS or DR that does not support NSIS.
Typically, the edge-NAT or the edge-firewall would be used to proxy
NATFW NSLP messages.
This proxy mode operation does not require any new CREATE or EXTERNAL
message type, but relies on extended CREATE and EXTERNAL message
types. They are called, respectively, CREATE-PROXY and EXTERNAL-
PROXY and are distinguished by setting the P flag in the NSLP header
to P=1. This flag instructs edge-NATs and edge-firewalls receiving
them to operate in proxy mode for the NATFW NSLP signaling session in
question. The semantics of the CREATE and EXTERNAL message types are
not changed and the behavior of the various node types is as defined
in Sections 3.7.1 and 3.7.2, except for the proxying node. The
following paragraphs describe the proxy mode operation for data
receivers behind middleboxes and data senders behind middleboxes.
3.7.6.1. Proxying for a Data Sender
The NATFW NSLP gives the NR the ability to install state on the
inbound path towards the data sender for outbound data packets, even
when only the receiving side is running NSIS (as shown in Figure 18).
The goal of the method described is to trigger the edge-NAT/
edge-firewall to generate a CREATE message on behalf of the data
receiver. In this case, an NR can signal towards the network border
Stiemerling, et al. Experimental PAGE 48
RFC 5973 NAT/FW NSIS NSLP October 2010
as it is performed in the standard EXTERNAL message handling scenario
as in Section 3.7.2. The message is forwarded until the edge-NAT/
edge-firewall is reached. A public IP address and port number is
reserved at an edge-NAT/edge-firewall. As shown in Figure 18, unlike
the standard EXTERNAL message handling case, the edge-NAT/
edge-firewall is triggered to send a CREATE message on a new reverse
path that traverse several firewalls or NATs. The new reverse path
for CREATE is necessary to handle routing asymmetries between the
edge-NAT/edge-firewall and the DR. It must be stressed that the
semantics of the CREATE and EXTERNAL messages are not changed, i.e.,
each is processed as described earlier.
DS Public Internet NAT/FW Private address DR
No NI NF space NR
NR+ NI+
| | EXTERNAL-PROXY[(DTInfo)] |
| |<------------------------- |
| | RESPONSE[Error/Success] |
| | ---------------------- > |
| | CREATE |
| | ------------------------> |
| | RESPONSE[Error/Success] |
| | <---------------------- |
| | |
Figure 18: EXTERNAL Triggering Sending of CREATE Message
A NATFW_NONCE object, carried in the EXTERNAL and CREATE message, is
used to build the relationship between received CREATEs at the
message initiator. An NI+ uses the presence of the NATFW_NONCE
object to correlate it to the particular EXTERNAL-PROXY. The absence
of a NONCE object indicates a CREATE initiated by the DS and not by
the edge-NAT. The two signaling sessions, i.e., the session for
EXTERNAL-PROXY and the session for CREATE, are not independent. The
primary session is the EXTERNAL-PROXY session. The CREATE session is
secondary to the EXTERNAL-PROXY session, i.e., the CREATE session is
valid as long as the EXTERNAL-PROXY session is the signaling states
'Established' or 'Transitory'. There is no CREATE session in any
other signaling state of the EXTERNAL-PROXY, i.e., 'Pending' or
'Dead'. This ensures fate-sharing between the two signaling
sessions.
These processing rules of EXTERNAL-PROXY messages are added to the
regular EXTERNAL processing:
Stiemerling, et al. Experimental PAGE 49
RFC 5973 NAT/FW NSIS NSLP October 2010
o NSLP initiator (NI+): The NI+ MUST take the session ID (SID) value
of the EXTERNAL-PROXY session as the nonce value of the
NATFW_NONCE object.
o NSLP forwarder being either edge-NAT or edge-firewall: When the NF
accepts an EXTERNAL-PROXY message, it generates a successful
RESPONSE message as if it were the NR, and it generates a CREATE
message as defined in Section 3.7.1 and includes a NATFW_NONCE
object having the same value as of the received NATFW_NONCE
object. The NF MUST NOT generate a CREATE-PROXY message. The NF
MUST refresh the CREATE message signaling session only if an
EXTERNAL-PROXY refresh message has been received first. This also
includes tearing down signaling sessions, i.e., the NF must tear
down the CREATE signaling session only if an EXTERNAL-PROXY
message with lifetime set to 0 has been received first.
The scenario described in this section challenges the data receiver
because it must make a correct assumption about the data sender's
ability to use NSIS NATFW NSLP signaling. It is possible for the DR
to make the wrong assumption in two different ways:
a) the DS is NSIS unaware but the DR assumes the DS to be NSIS
aware, and
b) the DS is NSIS aware but the DR assumes the DS to be NSIS
unaware.
Case a) will result in middleboxes blocking the data traffic, since
the DS will never send the expected CREATE message. Case b) will
result in the DR successfully requesting proxy mode support by the
edge-NAT or edge-firewall. The edge-NAT/edge-firewall will send
CREATE messages and DS will send CREATE messages as well. Both
CREATE messages are handled as separated NATFW NSLP signaling
sessions and therefore the common rules per NATFW NSLP signaling
session apply; the NATFW_NONCE object is used to differentiate CREATE
messages generated by the edge-NAT/edge-firewall from the NI-
initiated CREATE messages. It is the NR's responsibility to decide
whether to tear down the EXTERNAL-PROXY signaling sessions in the
case where the data sender's side is NSIS aware, but was incorrectly
assumed not to be so by the DR. It is RECOMMENDED that a DR behind
NATs use the proxy mode of operation by default, unless the DR knows
that the DS is NSIS aware. The DR MAY cache information about data
senders that it has found to be NSIS aware in past NATFW NSLP
signaling sessions.
Stiemerling, et al. Experimental PAGE 50
RFC 5973 NAT/FW NSIS NSLP October 2010
There is a possible race condition between the RESPONSE message to
the EXTERNAL-PROXY and the CREATE message generated by the edge-NAT.
The CREATE message can arrive earlier than the RESPONSE message. An
NI+ MUST accept CREATE messages generated by the edge-NAT even if the
RESPONSE message to the EXTERNAL-PROXY was not received.
3.7.6.2. Proxying for a Data Receiver
As with data receivers behind middleboxes, data senders behind
middleboxes can require proxy mode support. The issue here is that
there is no NSIS support at the data receiver's side and, by default,
there will be no response to CREATE messages. This scenario requires
the last NSIS NATFW NSLP-aware node to terminate the forwarding and
to proxy the response to the CREATE message, meaning that this node
is generating RESPONSE messages. This last node may be an edge-NAT/
edge-firewall, or any other NATFW NSLP peer, that detects that there
is no NR available (probably as a result of GIST timeouts but there
may be other triggers).
DS Private Address NAT/FW Public Internet NR
NI Space NF no NR
| | |
| CREATE-PROXY | |
|------------------------------>| |
| | |
| RESPONSE[SUCCESS/ERROR] | |
|<------------------------------| |
| | |
Figure 19: Proxy Mode CREATE Message Flow
The processing of CREATE-PROXY messages and RESPONSE messages is
similar to Section 3.7.1, except that forwarding is stopped at the
edge-NAT/edge-firewall. The edge-NAT/edge-firewall responds back to
NI according to the situation (error/success) and will be the NR for
future NATFW NSLP communication.
The NI can choose the proxy mode of operation although the DR is NSIS
aware. The CREATE-PROXY mode would not configure all NATs and
firewalls along the data path, since it is terminated at the edge-
device. Any device beyond this point will never receive any NATFW
NSLP signaling for this flow.
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RFC 5973 NAT/FW NSIS NSLP October 2010
3.7.6.3. Incremental Deployment Using the Proxy Mode
The above sections described the proxy mode for cases where the NATFW
NSLP is solely deployed at the network edges. However, the NATFW
NSLP might be incrementally deployed first in some network edges, but
later on also in other parts of the network. Using the proxy mode
only would prevent the NI from determining whether the other parts of
the network have also been upgraded to use the NATFW NSLP. One way
of determining whether the path from the NI to the NR is NATFW-NSLP-
capable is to use the regular CREATE message and to wait for a
successful response or an error response. This will lead to extra
messages being sent, as a CREATE message, in addition to the CREATE-
PROXY message (which is required anyhow), is sent from the NI.
The NATFW NSLP allows the usage of the proxy-mode and a further
probing of the path by the edge-NAT or edge-firewall. The NI can
request proxy-mode handling as described, and can set the E flag (see
Figure 20) to request the edge-NAT or edge-firewall to probe the
further path for NATFW NSLP enabled NFs or an NR.
The edge-NAT or edge-firewall MUST continue to send the CREATE-PROXY
or EXTERNAL-proxy towards the NR, if the received proxy-mode message
has the E flag set, in addition to the regular proxy mode handling.
The edge-NAT or edge-firewall relies on NTLP measures to determine
whether or not there is another NATFW NSLP reachable towards the NR.
A failed attempt to forward the request message to the NR will be
silently discarded. A successful attempt of forwarding the request
message to the NR will be acknowledged by the NR with a successful
response message, which is subject to the regular behavior described
in the proxy-mode sections.
3.7.6.4. Deployment Considerations for Edge-Devices
The proxy mode assumes that the edge-NAT or edge-firewall are
properly configured by network operator, i.e., the edge-device is
really the final NAT or firewall of that particular network. There
is currently no known way of letting the NATFW NSLP automatically
detect which of the NAT or firewalls are the actual edge of a
network. Therefore, it is important for the network operator to
configure the edge-NAT or edge-firewall and also to re-configure
these devices if they are not at the edge anymore. For instance, an
edge-NAT is located within an ISP and the ISP chooses to place
another NAT in front of this edge-NAT. In this case, the ISP needs
to reconfigure the old edge-NAT to be a regular NATFW NLSP NAT and to
configure the newly installed NAT to be the edge-NAT.
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RFC 5973 NAT/FW NSIS NSLP October 2010
3.8. Demultiplexing at NATs
Section 3.7.2 describes how NSIS nodes behind NATs can obtain a
publicly reachable IP address and port number at a NAT and how the
resulting mapping rule can be activated by using CREATE messages (see
Section 3.7.1). The information about the public IP address/port
number can be transmitted via an application-level signaling protocol
and/or third party to the communication partner that would like to
send data toward the host behind the NAT. However, NSIS signaling
flows are sent towards the address of the NAT at which this
particular IP address and port number is allocated and not directly
to the allocated IP address and port number. The NATFW NSLP
forwarder at this NAT needs to know how the incoming NSLP CREATE
messages are related to reserved addresses, meaning how to
demultiplex incoming NSIS CREATE messages.
The demultiplexing method uses information stored at the local NATFW
NSLP node and in the policy rule. The policy rule uses the LE-MRM
MRI source-address (see [RFC 5971]) as the flow destination IP address
and the network-layer-version (IP-ver) as IP version. The external
IP address at the NAT is stored as the external flow destination IP
address. All other parameters of the policy rule other than the flow
destination IP address are wildcarded if no NATFW_DTINFO object is
included in the EXTERNAL message. The LE-MRM MRI destination-address
MUST NOT be used in the policy rule, since it is solely a signaling
destination address.
If the NATFW_DTINFO object is included in the EXTERNAL message, the
policy rule is filled with further information. The 'dst port
number' field of the NATFW_DTINFO is stored as the flow destination
port number. The 'protocol' field is stored as the flow protocol.
The 'src port number' field is stored as the flow source port number.
The 'data sender's IPv4 address' is stored as the flow source IP
address. Note that some of these fields can contain wildcards.
When receiving a CREATE message at the NATFW NSLP, the NATFW NSLP
uses the flow information stored in the MRI to do the matching
process. This table shows the parameters to be compared against each
other. Note that not all parameters need be present in an MRI at the
same time.
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RFC 5973 NAT/FW NSIS NSLP October 2010
+-------------------------------+--------------------------------+
| Flow parameter (Policy Rule) | MRI parameter (CREATE message) |
+-------------------------------+--------------------------------+
| IP version | network-layer-version |
| Protocol | IP-protocol |
| source IP address (w) | source-address (w) |
| external IP address | destination-address |
| destination IP address (n/u) | N/A |
| source port number (w) | L4-source-port (w) |
| external port number (w) | L4-destination-port (w) |
| destination port number (n/u) | N/A |
| IPsec-SPI | ipsec-SPI |
+-------------------------------+--------------------------------+
Table entries marked with (w) can be wildcarded and
entries marked with (n/u) are not used for the matching.
Table 1
It should be noted that the Protocol/IP-protocol entries in Table 1
refer to the 'Protocol' field in the IPv4 header or the 'next header'
entry in the IPv6 header.
3.9. Reacting to Route Changes
The NATFW NSLP needs to react to route changes in the data path.
This assumes the capability to detect route changes, to perform NAT
and firewall configuration on the new path and possibly to tear down
NATFW NSLP signaling session state on the old path. The detection of
route changes is described in Section 7 of [RFC 5971], and the NATFW
NSLP relies on notifications about route changes by the NTLP. This
notification will be conveyed by the API between NTLP and NSLP, which
is out of the scope of this memo.
A NATFW NSLP node other than the NI or NI+ detecting a route change,
by means described in the NTLP specification or others, generates a
NOTIFY message indicating this change and sends this inbound towards
NI and outbound towards the NR (see also Section 3.7.5).
Intermediate NFs on the way to the NI can use this information to
decide later if their NATFW NSLP signaling session can be deleted
locally, if they do not receive an update within a certain time
period, as described in Section 3.2.8. It is important to consider
the transport limitations of NOTIFY messages as mandated in
Section 3.7.5.
The NI receiving this NOTIFY message MAY generate a new CREATE or
EXTERNAL message and send it towards the NATFW NSLP signaling
session's NI as for the initial message using the same session ID.
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RFC 5973 NAT/FW NSIS NSLP October 2010
All the remaining processing and message forwarding, such as NSLP
next-hop discovery, is subject to regular NSLP processing as
described in the particular sections. Normal routing will guide the
new CREATE or EXTERNAL message to the correct NFs along the changed
route. NFs that were on the original path receiving these new CREATE
or EXTERNAL messages (see also Section 3.10), can use the session ID
to update the existing NATFW NSLP signaling session; whereas NFs that
were not on the original path will create new state for this NATFW
NSLP signaling session. The next section describes how policy rules
are updated.
3.10. Updating Policy Rules
NSIS initiators can request an update of the installed/reserved
policy rules at any time within a NATFW NSLP signaling session.
Updates to policy rules can be required due to node mobility (NI is
moving from one IP address to another), route changes (this can
result in a different NAT mapping at a different NAT device), or the
wish of the NI to simply change the rule. NIs can update policy
rules in existing NATFW NSLP signaling sessions by sending an
appropriate CREATE or EXTERNAL message (similar to Section 3.4) with
modified message routing information (MRI) as compared with that
installed previously, but using the existing session ID to identify
the intended target of the update. With respect to authorization and
authentication, this update CREATE or EXTERNAL message is treated in
exactly the same way as any initial message. Therefore, any node
along in the NATFW NSLP signaling session can reject the update with
an error RESPONSE message, as defined in the previous sections.
The message processing and forwarding is executed as defined in the
particular sections. An NF or the NR receiving an update simply
replaces the installed policy rules installed in the firewall/NAT.
The local procedures on how to update the MRI in the firewall/NAT is
out of the scope of this memo.
4. NATFW NSLP Message Components
A NATFW NSLP message consists of an NSLP header and one or more
objects following the header. The NSLP header is carried in all
NATFW NSLP messages and objects are Type-Length-Value (TLV) encoded
using big endian (network ordered) binary data representations.
Header and objects are aligned to 32-bit boundaries and object
lengths that are not multiples of 32 bits must be padded to the next
higher 32-bit multiple.
The whole NSLP message is carried as payload of a NTLP message.
Note that the notation 0x is used to indicate hexadecimal numbers.
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4.1. NSLP Header
All GIST NSLP-Data objects for the NATFW NSLP MUST contain this
common header as the first 32 bits of the object (this is not the
same as the GIST Common Header). It contains two fields, the NSLP
message type and the P Flag, plus two reserved fields. The total
length is 32 bits. The layout of the NSLP header is defined by
Figure 20.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message type |P|E| reserved | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: Common NSLP Header
The reserved field MUST be set to zero in the NATFW NSLP header
before sending and MUST be ignored during processing of the header.
The defined messages types are:
o 0x1: CREATE
o 0x2: EXTERNAL
o 0x3: RESPONSE
o 0x4: NOTIFY
If a message with another type is received, an error RESPONSE of
class 'Protocol error' (3) with response code 'Illegal message type'
(0x01) MUST be generated.
The P flag indicates the usage of proxy mode. If the proxy mode is
used, it MUST be set to 1. Proxy mode MUST only be used in
combination with the message types CREATE and EXTERNAL. The P flag
MUST be ignored when processing messages with type RESPONSE or
NOTIFY.
The E flag indicates, in proxy mode, whether the edge-NAT or edge-
firewall MUST continue sending the message to the NR, i.e., end-to-
end. The E flag can only be set to 1 if the P flag is set;
otherwise, it MUST be ignored. The E flag MUST only be used in
combination with the message types CREATE and EXTERNAL. The E flag
MUST be ignored when processing messages with type RESPONSE or
NOTIFY.
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4.2. NSLP Objects
NATFW NSLP objects use a common header format defined by Figure 21.
The object header contains these fields: two flags, two reserved
bits, the NSLP object type, a reserved field of 4 bits, and the
object length. Its total length is 32 bits.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|B|r|r| Object Type |r|r|r|r| Object Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: Common NSLP Object Header
The object length field contains the total length of the object
without the object header. The unit is a word, consisting of 4
octets. The particular values of type and length for each NSLP
object are listed in the subsequent sections that define the NSLP
objects. An error RESPONSE of class 'Protocol error' (3) with
response code 'Wrong object length' (0x07) MUST be generated if the
length given in the object header is inconsistent with the type of
object specified or the message is shorter than implied by the object
length. The two leading bits of the NSLP object header are used to
signal the desired treatment for objects whose treatment has not been
defined in this memo (see [RFC 5971], Appendix A.2.1), i.e., the
Object Type has not been defined. NATFW NSLP uses a subset of the
categories defined in GIST:
o AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it MUST be rejected with an error RESPONSE of
class 'Protocol error' (3) with response code 'Unknown object
present' (0x06).
o AB=01 ("Optional"): If the object is not understood, it should be
deleted and then the rest of the message processed as usual.
o AB=10 ("Forward"): If the object is not understood, it should be
retained unchanged in any message forwarded as a result of message
processing, but not stored locally.
The combination AB=11 MUST NOT be used and an error RESPONSE of class
'Protocol error' (3) with response code 'Invalid Flag-Field
combination' (0x09) MUST be generated.
The following sections do not repeat the common NSLP object header,
they just list the type and the length.
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4.2.1. Signaling Session Lifetime Object
The signaling session lifetime object carries the requested or
granted lifetime of a NATFW NSLP signaling session measured in
seconds.
Type: NATFW_LT (0x00C)
Length: 1
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NATFW NSLP signaling session lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22: Signaling Session Lifetime Object
4.2.2. External Address Object
The external address object can be included in RESPONSE messages
(Section 4.3.3) only. It carries the publicly reachable IP address,
and if applicable port number, at an edge-NAT.
Type: NATFW_EXTERNAL_IP (0x00D)
Length: 2
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number |IP-Ver | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: External Address Object for IPv4 Addresses
Please note that the field 'port number' MUST be set to 0 if only an
IP address has been reserved, for instance, by a traditional NAT. A
port number of 0 MUST be ignored in processing this object.
IP-Ver (4 bits): The IP version number. This field MUST be set to 4.
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4.2.3. External Binding Address Object
The external binding address object can be included in RESPONSE
messages (Section 4.3.3) and EXTERNAL (Section 3.7.2) messages. It
carries one or multiple external binding addresses, and if applicable
port number, for multi-level NAT deployments (for an example, see
Section 2.5). The utilization of the information carried in this
object is described in Appendix B.
Type: NATFW_EXTERNAL_BINDING (0x00E)
Length: 1 + number of IPv4 addresses
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port number |IP-Ver | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// . . . //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: External Binding Address Object
Please note that the field 'port number' MUST be set to 0 if only an
IP address has been reserved, for instance, by a traditional NAT. A
port number of 0 MUST be ignored in processing this object.
IP-Ver (4 bits): The IP version number. This field MUST be set to 4.
4.2.4. Extended Flow Information Object
In general, flow information is kept in the message routing
information (MRI) of the NTLP. Nevertheless, some additional
information may be required for NSLP operations. The 'extended flow
information' object carries this additional information about the
action of the policy rule for firewalls/NATs and a potential
contiguous port.
Type: NATFW_EFI (0x00F)
Length: 1
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RFC 5973 NAT/FW NSIS NSLP October 2010
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rule action | sub_ports |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Extended Flow Information
This object has two fields, 'rule action' and 'sub_ports'. The 'rule
action' field has these meanings:
o 0x0001: Allow: A policy rule with this action allows data traffic
to traverse the middlebox and the NATFW NSLP MUST allow NSLP
signaling to be forwarded.
o 0x0002: Deny: A policy rule with this action blocks data traffic
from traversing the middlebox and the NATFW NSLP MUST NOT allow
NSLP signaling to be forwarded.
If the 'rule action' field contains neither 0x0001 nor 0x0002, an
error RESPONSE of class 'Signaling session failure' (7) with response
code 'Unknown policy rule action' (0x05) MUST be generated.
The 'sub_ports' field contains the number of contiguous transport
layer ports to which this rule applies. The default value of this
field is 0, i.e., only the port specified in the NTLP's MRM or
NATFW_DTINFO object is used for the policy rule. A value of 1
indicates that additionally to the port specified in the NTLP's MRM
(port1), a second port (port2) is used. This value of port 2 is
calculated as: port2 = port1 + 1. Other values than 0 or 1 MUST NOT
be used in this field and an error RESPONSE of class 'Signaling
session failure' (7) with response code 'Requested value in sub_ports
field in NATFW_EFI not permitted' (0x08) MUST be generated. These
two contiguous numbered ports can be used by legacy voice over IP
equipment. This legacy equipment assumes two adjacent port numbers
for its RTP/RTCP flows, respectively.
4.2.5. Information Code Object
This object carries the response code in RESPONSE messages, which
indicates either a successful or failed CREATE or EXTERNAL message
depending on the value of the 'response code' field. This object is
also carried in a NOTIFY message to specify the reason for the
notification.
Type: NATFW_INFO (0x010)
Length: 1
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resv. | Class | Response Code |r|r|r|r| Object Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Information Code Object
The field 'resv.' is reserved for future extensions and MUST be set
to zero when generating such an object and MUST be ignored when
receiving. The 'Object Type' field contains the type of the object
causing the error. The value of 'Object Type' is set to 0, if no
object is concerned. The leading fours bits marked with 'r' are
always set to zero and ignored. The 4-bit class field contains the
error class. The following classes are defined:
o 0: Reserved
o 1: Informational (NOTIFY only)
o 2: Success
o 3: Protocol error
o 4: Transient failure
o 5: Permanent failure
o 7: Signaling session failure
Within each error class a number of responses codes are defined as
follows.
o Informational:
* 0x01: Route change: possible route change on the outbound path.
* 0x02: Re-authentication required.
* 0x03: NATFW node is going down soon.
* 0x04: NATFW signaling session lifetime expired.
* 0x05: NATFW signaling session terminated.
o Success:
* 0x01: All successfully processed.
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o Protocol error:
* 0x01: Illegal message type: the type given in the Message Type
field of the NSLP header is unknown.
* 0x02: Wrong message length: the length given for the message in
the NSLP header does not match the length of the message data.
* 0x03: Bad flags value: an undefined flag or combination of
flags was set in the NSLP header.
* 0x04: Mandatory object missing: an object required in a message
of this type was missing.
* 0x05: Illegal object present: an object was present that must
not be used in a message of this type.
* 0x06: Unknown object present: an object of an unknown type was
present in the message.
* 0x07: Wrong object length: the length given for the object in
the object header did not match the length of the object data
present.
* 0x08: Unknown object field value: a field in an object had an
unknown value.
* 0x09: Invalid Flag-Field combination: An object contains an
invalid combination of flags and/or fields.
* 0x0A: Duplicate object present.
* 0x0B: Received EXTERNAL request message on external side.
o Transient failure:
* 0x01: Requested resources temporarily not available.
o Permanent failure:
* 0x01: Authentication failed.
* 0x02: Authorization failed.
* 0x04: Internal or system error.
* 0x06: No edge-device here.
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* 0x07: Did not reach the NR.
o Signaling session failure:
* 0x01: Session terminated asynchronously.
* 0x02: Requested lifetime is too big.
* 0x03: No reservation found matching the MRI of the CREATE
request.
* 0x04: Requested policy rule denied due to policy conflict.
* 0x05: Unknown policy rule action.
* 0x06: Requested rule action not applicable.
* 0x07: NATFW_DTINFO object is required.
* 0x08: Requested value in sub_ports field in NATFW_EFI not
permitted.
* 0x09: Requested IP protocol not supported.
* 0x0A: Plain IP policy rules not permitted -- need transport
layer information.
* 0x0B: ICMP type value not permitted.
* 0x0C: Source IP address range is too large.
* 0x0D: Destination IP address range is too large.
* 0x0E: Source L4-port range is too large.
* 0x0F: Destination L4-port range is too large.
* 0x10: Requested lifetime is too small.
* 0x11: Modified lifetime is too big.
* 0x12: Modified lifetime is too small.
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4.2.6. Nonce Object
This object carries the nonce value as described in Section 3.7.6.
Type: NATFW_NONCE (0x011)
Length: 1
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Nonce Object
4.2.7. Message Sequence Number Object
This object carries the MSN value as described in Section 3.5.
Type: NATFW_MSN (0x012)
Length: 1
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Message Sequence Number Object
4.2.8. Data Terminal Information Object
The 'data terminal information' object carries additional information
that MUST be included the EXTERNAL message. EXTERNAL messages are
transported by the NTLP using the Loose-End message routing method
(LE-MRM). The LE-MRM contains only the DR's IP address and a
signaling destination address (destination IP address). This
destination IP address is used for message routing only and is not
necessarily reflecting the address of the data sender. This object
contains information about (if applicable) DR's port number (the
destination port number), the DS's port number (the source port
number), the used transport protocol, the prefix length of the IP
address, and DS's IP address.
Type: NATFW_DTINFO (0x013)
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Length: variable. Maximum 3.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I|P|S| reserved | sender prefix | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: DR port number | DS port number :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: IPsec-SPI :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data sender's IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: Data Terminal IPv4 Address Object
The flags are:
o I: I=1 means that 'protocol' should be interpreted.
o P: P=1 means that 'dst port number' and 'src port number' are
present and should be interpreted.
o S: S=1 means that SPI is present and should be interpreted.
The SPI field is only present if S is set. The port numbers are only
present if P is set. The flags P and S MUST NOT be set at the same
time. An error RESPONSE of class 'Protocol error' (3) with response
code 'Invalid Flag-Field combination' (0x09) MUST be generated if
they are both set. If either P or S is set, I MUST be set as well
and the protocol field MUST carry the particular protocol. An error
RESPONSE of class 'Protocol error' (3) with response code 'Invalid
Flag-Field combination' (0x09) MUST be generated if S or P is set but
I is not set.
The fields MUST be interpreted according to these rules:
o (data) sender prefix: This parameter indicates the prefix length
of the 'data sender's IP address' in bits. For instance, a full
IPv4 address requires 'sender prefix' to be set to 32. A value of
0 indicates an IP address wildcard.
o protocol: The IP protocol field. This field MUST be interpreted
if I=1; otherwise, it MUST be set to 0 and MUST be ignored.
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RFC 5973 NAT/FW NSIS NSLP October 2010
o DR port number: The port number at the data receiver (DR), i.e.,
the destination port. A value of 0 indicates a port wildcard,
i.e., the destination port number is not known. Any other value
indicates the destination port number.
o DS port number: The port number at the data sender (DS), i.e., the
source port. A value of 0 indicates a port wildcard, i.e., the
source port number is not known. Any other value indicates the
source port number.
o data sender's IPv4 address: The source IP address of the data
sender. This field MUST be set to zero if no IP address is
provided, i.e., a complete wildcard is desired (see the dest
prefix field above).
4.2.9. ICMP Types Object
The 'ICMP types' object contains additional information needed to
configure a NAT of firewall with rules to control ICMP traffic. The
object contains a number of values of the ICMP Type field for which a
filter action should be set up:
Type: NATFW_ICMP_TYPES (0x014)
Length: Variable = ((Number of Types carried + 1) + 3) DIV 4
Where DIV is an integer division.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count | Type | Type | ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ | Type | (Padding) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: ICMP Types Object
The fields MUST be interpreted according to these rules:
count: 8-bit integer specifying the number of 'Type' entries in
the object.
type: 8-bit field specifying an ICMP Type value to which this rule
applies.
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RFC 5973 NAT/FW NSIS NSLP October 2010
padding: Sufficient 0 bits to pad out the last word so that the
total size of the object is an even multiple of words. Ignored on
reception.
4.3. Message Formats
This section defines the content of each NATFW NSLP message type.
The message types are defined in Section 4.1.
Basically, each message is constructed of an NSLP header and one or
more NSLP objects. The order of objects is not defined, meaning that
objects may occur in any sequence. Objects are marked either with
mandatory (M) or optional (O). Where (M) implies that this
particular object MUST be included within the message and where (O)
implies that this particular object is OPTIONAL within the message.
Objects defined in this memo always carry the flag combination AB=00
in the NSLP object header. An error RESPONSE message of class
'Protocol error' (3) with response code 'Mandatory object missing'
(0x04) MUST be generated if a mandatory declared object is missing.
An error RESPONSE message of class 'Protocol error' (3) with response
code 'Illegal object present' (0x05) MUST be generated if an object
was present that must not be used in a message of this type. An
error RESPONSE message of class 'Protocol error' (3) with response
code 'Duplicate object present' (0x0A) MUST be generated if an object
appears more than once in a message.
Each section elaborates the required settings and parameters to be
set by the NSLP for the NTLP, for instance, how the message routing
information is set.
4.3.1. CREATE
The CREATE message is used to create NATFW NSLP signaling sessions
and to create policy rules. Furthermore, CREATE messages are used to
refresh NATFW NSLP signaling sessions and to delete them.
The CREATE message carries these objects:
o Signaling Session Lifetime object (M)
o Extended flow information object (M)
o Message sequence number object (M)
o Nonce object (M) if P flag set to 1 in the NSLP header, otherwise
(O)
o ICMP Types Object (O)
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The message routing information in the NTLP MUST be set to DS as
source IP address and DR as destination IP address. All other
parameters MUST be set according to the required policy rule. CREATE
messages MUST be transported by using the path-coupled MRM with the
direction set to 'downstream' (outbound).
4.3.2. EXTERNAL
The EXTERNAL message is used to a) reserve an external IP address/
port at NATs, b) to notify firewalls about NSIS capable DRs, or c) to
block incoming data traffic at inbound firewalls.
The EXTERNAL message carries these objects:
o Signaling Session Lifetime object (M)
o Message sequence number object (M)
o Extended flow information object (M)
o Data terminal information object (M)
o Nonce object (M) if P flag set to 1 in the NSLP header, otherwise
(O)
o ICMP Types Object (O)
o External binding address object (O)
The selected message routing method of the EXTERNAL message depends
on a number of considerations. Section 3.7.2 describes exhaustively
how to select the correct method. EXTERNAL messages can be
transported via the path-coupled message routing method (PC-MRM) or
via the loose-end message routing method (LE-MRM). In the case of
PC-MRM, the source-address is set to the DS's address and the
destination-address is set to the DR's address, the direction is set
to inbound. In the case of LE-MRM, the destination-address is set to
the DR's address or to the signaling destination IP address. The
source-address is set to the DS's address.
4.3.3. RESPONSE
RESPONSE messages are responses to CREATE and EXTERNAL messages.
RESPONSE messages MUST NOT be generated for any other message, such
as NOTIFY and RESPONSE.
The RESPONSE message for the class 'Success' (2) carries these
objects:
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RFC 5973 NAT/FW NSIS NSLP October 2010
o Signaling Session Lifetime object (M)
o Message sequence number object (M)
o Information code object (M)
o External address object (O)
o External binding address object (O)
The RESPONSE message for other classes than 'Success' (2) carries
these objects:
o Message sequence number object (M)
o Information code object (M)
o Signaling Session Lifetime object (O)
This message is routed towards the NI hop-by-hop, using existing NTLP
messaging associations. The MRM used for this message MUST be the
same as MRM used by the corresponding CREATE or EXTERNAL message.
4.3.4. NOTIFY
The NOTIFY messages is used to report asynchronous events happening
along the signaled path to other NATFW NSLP nodes.
The NOTIFY message carries this object:
o Information code object (M)
The NOTIFY message is routed towards the next NF, NI, or NR hop-by-
hop using the existing inbound or outbound node messaging association
entry within the node's Message Routing State table. The MRM used
for this message MUST be the same as MRM used by the corresponding
CREATE or EXTERNAL message.
5. Security Considerations
Security is of major concern particularly in the case of firewall
traversal. This section provides security considerations for the
NAT/firewall traversal and is organized as follows.
In Section 5.1, we describe how the participating entities relate to
each other from a security point of view. That subsection also
motivates a particular authorization model.
Stiemerling, et al. Experimental PAGE 69
RFC 5973 NAT/FW NSIS NSLP October 2010
Security threats that focus on NSIS in general are described in
[RFC 4081] and they are applicable to this document as well.
Finally, we illustrate how the security requirements that were
created based on the security threats can be fulfilled by specific
security mechanisms. These aspects will be elaborated in
Section 5.2.
5.1. Authorization Framework
The NATFW NSLP is a protocol that may involve a number of NSIS nodes
and is, as such, not a two-party protocol. Figures 1 and 2 of
[RFC 4081] already depict the possible set of communication patterns.
In this section, we will re-evaluate these communication patterns
with respect to the NATFW NSLP protocol interaction.
The security solutions for providing authorization have a direct
impact on the treatment of different NSLPs. As it can be seen from
the QoS NSLP [RFC 5974] and the corresponding Diameter QoS work
[RFC 5866], accounting and charging seems to play an important role
for QoS reservations, whereas monetary aspects might only indirectly
effect authorization decisions for NAT and firewall signaling.
Hence, there are differences in the semantics of authorization
handling between QoS and NATFW signaling. A NATFW-aware node will
most likely want to authorize the entity (e.g., user or machine)
requesting the establishment of pinholes or NAT bindings. The
outcome of the authorization decision is either allowed or
disallowed, whereas a QoS authorization decision might indicate that
a different set of QoS parameters is authorized (see [RFC 5866] as an
example).
5.1.1. Peer-to-Peer Relationship
Starting with the simplest scenario, it is assumed that neighboring
nodes are able to authenticate each other and to establish keying
material to protect the signaling message communication. The nodes
will have to authorize each other, additionally to the
authentication. We use the term 'Security Context' as a placeholder
for referring to the entire security procedure, the necessary
infrastructure that needs to be in place in order for this to work
(e.g., key management) and the established security-related state.
The required long-term keys (symmetric or asymmetric keys) used for
authentication either are made available using an out-of-band
mechanism between the two NSIS NATFW nodes or are dynamically
established using mechanisms not further specified in this document.
Note that the deployment environment will most likely have an impact
on the choice of credentials being used. The choice of these
credentials used is also outside the scope of this document.
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RFC 5973 NAT/FW NSIS NSLP October 2010
+------------------------+ +-------------------------+
|Network A | | Network B|
| +---------+ +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | Security | box 2 | | |
| | +---------+ Context +---------+ | |
| | Security | | Security | |
| | Context | | Context | |
| | | | | |
| +--+---+ | | +--+---+ |
| | Host | | | | Host | |
| | A | | | | B | |
| +------+ | | +------+ |
+------------------------+ +-------------------------+
Figure 31: Peer-to-Peer Relationship
Figure 31 shows a possible relationship between participating NSIS-
aware nodes. Host A might be, for example, a host in an enterprise
network that has keying material established (e.g., a shared secret)
with the company's firewall (Middlebox 1). The network administrator
of Network A (company network) has created access control lists for
Host A (or whatever identifiers a particular company wants to use).
Exactly the same procedure might also be used between Host B and
Middlebox 2 in Network B. For the communication between Middlebox 1
and Middlebox 2 a security context is also assumed in order to allow
authentication, authorization, and signaling message protection to be
successful.
5.1.2. Intra-Domain Relationship
In larger corporations, for example, a middlebox is used to protect
individual departments. In many cases, the entire enterprise is
controlled by a single (or a small number of) security department(s),
which give instructions to the department administrators. In such a
scenario, the previously discussed peer-to-peer relationship might be
prevalent. Sometimes it might be necessary to preserve
authentication and authorization information within the network. As
a possible solution, a centralized approach could be used, whereby an
interaction between the individual middleboxes and a central entity
(for example, a policy decision point - PDP) takes place. As an
alternative, individual middleboxes exchange the authorization
decision with another middlebox within the same trust domain.
Individual middleboxes within an administrative domain may exploit
their relationship instead of requesting authentication and
authorization of the signaling initiator again and again. Figure 32
illustrates a network structure that uses a centralized entity.
Stiemerling, et al. Experimental PAGE 71
RFC 5973 NAT/FW NSIS NSLP October 2010
+-----------------------------------------------------------+
| Network A |
| +---------+ +---------+
| +----///--------+ Middle- +------///------++ Middle- +---
| | Security | box 2 | Security | box 2 |
| | Context +----+----+ Context +----+----+
| +----+----+ | | |
| | Middle- +--------+ +---------+ | |
| | box 1 | | | | |
| +----+----+ | | | |
| | Security | +----+-----+ | |
| | Context | | Policy | | |
| +--+---+ +-----------+ Decision +----------+ |
| | Host | | Point | |
| | A | +----------+ |
| +------+ |
+-----------------------------------------------------------+
Figure 32: Intra-Domain Relationship
The interaction between individual middleboxes and a policy decision
point (or AAA server) is outside the scope of this document.
5.1.3. End-to-Middle Relationship
The peer-to-peer relationship between neighboring NSIS NATFW NSLP
nodes might not always be sufficient. Network B might require
additional authorization of the signaling message initiator (in
addition to the authorization of the neighboring node). If
authentication and authorization information is not attached to the
initial signaling message then the signaling message arriving at
Middlebox 2 would result in an error message being created, which
indicates the additional authorization requirement. In many cases,
the signaling message initiator might already be aware of the
additionally required authorization before the signaling message
exchange is executed.
Figure 33 shows this scenario.
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+--------------------+ +---------------------+
| Network A | |Network B |
| | Security | |
| +---------+ Context +---------+ |
| +-///-+ Middle- +---///////----+ Middle- +-///-+ |
| | | box 1 | +-------+ box 2 | | |
| | +---------+ | +---------+ | |
| |Security | | | Security | |
| |Context | | | Context |
| | | | | | |
| +--+---+ | | | +--+---+ |
| | Host +----///----+------+ | | Host | |
| | A | | Security | | B | |
| +------+ | Context | +------+ |
+--------------------+ +---------------------+
Figure 33: End-to-Middle Relationship
5.2. Security Framework for the NAT/Firewall NSLP
The following list of security requirements has been created to
ensure proper secure operation of the NATFW NSLP.
5.2.1. Security Protection between Neighboring NATFW NSLP Nodes
Based on the analyzed threats, it is RECOMMENDED to provide, between
neighboring NATFW NSLP nodes, the following mechanisms:
o data origin authentication,
o replay protection,
o integrity protection, and,
o optionally, confidentiality protection
It is RECOMMENDED to use the authentication and key exchange security
mechanisms provided in [RFC 5971] between neighboring nodes when
sending NATFW signaling messages. The proposed security mechanisms
of GIST provide support for authentication and key exchange in
addition to denial-of-service protection. Depending on the chosen
security protocol, support for multiple authentication protocols
might be provided. If security between neighboring nodes is desired,
then the usage of C-MODE with a secure transport protocol for the
delivery of most NSIS messages with the usage of D-MODE only to
discover the next NATFW NSLP-aware node along the path is highly
RECOMMENDED. See [RFC 5971] for the definitions of C-MODE and D-MODE.
Almost all security threats at the NATFW NSLP-layer can be prevented
Stiemerling, et al. Experimental PAGE 73
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by using a mutually authenticated Transport Layer secured connection
and by relying on authorization by the neighboring NATFW NSLP
entities.
The NATFW NSLP relies on an established security association between
neighboring peers to prevent unauthorized nodes from modifying or
deleting installed state. Between non-neighboring nodes the session
ID (SID) carried in the NTLP is used to show ownership of a NATFW
NSLP signaling session. The session ID MUST be generated in a random
way and thereby prevents an off-path adversary from mounting targeted
attacks. Hence, an adversary would have to learn the randomly
generated session ID to perform an attack. In a mobility environment
a former on-path node that is now off-path can perform an attack.
Messages for a particular NATFW NSLP signaling session are handled by
the NTLP to the NATFW NSLP for further processing. Messages carrying
a different session ID not associated with any NATFW NSLP are subject
to the regular processing for new NATFW NSLP signaling sessions.
5.2.2. Security Protection between Non-Neighboring NATFW NSLP Nodes
Based on the security threats and the listed requirements, it was
noted that some threats also demand authentication and authorization
of a NATFW signaling entity (including the initiator) towards a non-
neighboring node. This mechanism mainly demands entity
authentication. The most important information exchanged at the
NATFW NSLP is information related to the establishment for firewall
pinholes and NAT bindings. This information can, however, not be
protected over multiple NSIS NATFW NSLP hops since this information
might change depending on the capability of each individual NATFW
NSLP node.
Some scenarios might also benefit from the usage of authorization
tokens. Their purpose is to associate two different signaling
protocols (e.g., SIP and NSIS) and their authorization decision.
These tokens are obtained by non-NSIS protocols, such as SIP or as
part of network access authentication. When a NAT or firewall along
the path receives the token it might be verified locally or passed to
the AAA infrastructure. Examples of authorization tokens can be
found in RFC 3520 [RFC 3520] and RFC 3521 [RFC 3521]. Figure 34 shows
an example of this protocol interaction.
An authorization token is provided by the SIP proxy, which acts as
the assertion generating entity and gets delivered to the end host
with proper authentication and authorization. When the NATFW
signaling message is transmitted towards the network, the
authorization token is attached to the signaling messages to refer to
the previous authorization decision. The assertion-verifying entity
needs to process the token or it might be necessary to interact with
Stiemerling, et al. Experimental PAGE 74
RFC 5973 NAT/FW NSIS NSLP October 2010
the assertion-granting entity using HTTP (or other protocols). As a
result of a successfully authorization by a NATFW NSLP node, the
requested action is executed and later a RESPONSE message is
generated.
+----------------+ Trust Relationship +----------------+
| +------------+ |<.......................>| +------------+ |
| | Protocol | | | | Assertion | |
| | requesting | | HTTP, SIP Request | | Granting | |
| | authz | |------------------------>| | Entity | |
| | assertions | |<------------------------| +------------+ |
| +------------+ | Artifact/Assertion | Entity Cecil |
| ^ | +----------------+
| | | ^ ^|
| | | . || HTTP,
| | | Trust . || other
| API Access | Relationship. || protocols
| | | . ||
| | | . ||
| | | v |v
| v | +----------------+
| +------------+ | | +------------+ |
| | Protocol | | NSIS NATFW CREATE + | | Assertion | |
| | using authz| | Assertion/Artifact | | Verifying | |
| | assertion | | ----------------------- | | Entity | |
| +------------+ | | +------------+ |
| Entity Alice | <---------------------- | Entity Bob |
+----------------+ RESPONSE +----------------+
Figure 34: Authorization Token Usage
Threats against the usage of authorization tokens have been mentioned
in [RFC 4081]. Hence, it is required to provide confidentiality
protection to avoid allowing an eavesdropper to learn the token and
to use it in another NATFW NSLP signaling session (replay attack).
The token itself also needs to be protected against tempering.
5.3. Implementation of NATFW NSLP Security
The prior sections describe how to secure the NATFW NSLP in the
presence of established trust between the various players and the
particular relationships (e.g., intra-domain, end-to-middle, or peer-
to-peer). However, in typical Internet deployments there is no
established trust, other than granting access to a network, but not
between various sites in the Internet. Furthermore, the NATFW NSLP
may be incrementally deployed with a widely varying ability to be
able to use authentication and authorization services.
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The NATFW NSLP offers a way to keep the authentication and
authorization at the "edge" of the network. The local edge network
can deploy and use any type of Authentication and Authorization (AA)
scheme without the need to have AA technology match with other edges
in the Internet (assuming that firewalls and NATs are deployed at the
edges of the network and not somewhere in the cores).
Each network edge that has the NATFW NSLP deployed can use the
EXTERNAL request message to allow a secure access to the network.
Using the EXTERNAL request message does allow the DR to open the
firewall/NAT on the receiver's side. However, the edge-devices
should not allow the firewall/NAT to be opened up completely (i.e.,
should not apply an allow-all policy), but should let DRs reserve
very specific policies. For instance, a DR can request reservation
of an 'allow' policy rule for an incoming TCP connection for a Jabber
file transfer. This reserved policy (see Figure 15) rule must be
activated by matching the CREATE request message (see Figure 15).
This mechanism allows for the authentication and authorization issues
to be managed locally at the particular edge-network. In the reverse
direction, the CREATE request message can be handled independently on
the DS side with respect to authentication and authorization.
The usage described in the above paragraph is further simplified for
an incremental deployment: there is no requirement to activate a
reserved policy rule with a CREATE request message. This is
completely handled by the EXTERNAL-PROXY request message and the
associated CREATE request message. Both of them are handled by the
local authentication and authorization scheme.
6. IAB Considerations on UNSAF
UNilateral Self-Address Fixing (UNSAF) is described in [RFC 3424] as a
process at originating endpoints that attempts to determine or fix
the address (and port) by which they are known to another endpoint.
UNSAF proposals, such as STUN [RFC 5389] are considered as a general
class of workarounds for NAT traversal and as solutions for scenarios
with no middlebox communication.
This memo specifies a path-coupled middlebox communication protocol,
i.e., the NSIS NATFW NSLP. NSIS in general and the NATFW NSLP are
not intended as a short-term workaround, but more as a long-term
solution for middlebox communication. In NSIS, endpoints are
involved in allocating, maintaining, and deleting addresses and ports
at the middlebox. However, the full control of addresses and ports
at the middlebox is at the NATFW NSLP daemon located at the
respective NAT.
Stiemerling, et al. Experimental PAGE 76
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Therefore, this document addresses the UNSAF considerations in
[RFC 3424] by proposing a long-term alternative solution.
7. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
NATFW NSLP, in accordance with BCP 26, RFC 5226 [RFC 5226].
The NATFW NSLP requires IANA to create a number of new registries:
o NATFW NSLP Message Types
o NATFW NSLP Header Flags
o NSLP Response Codes
It also requires registration of new values in a number of
registries:
o NSLP Message Objects
o NSLP Identifiers (under GIST Parameters)
o Router Alert Option Values (IPv4 and IPv6)
7.1. NATFW NSLP Message Type Registry
The NATFW NSLP Message Type is an 8-bit value. The allocation of
values for new message types requires IETF Review. Updates and
deletion of values from the registry are not possible. This
specification defines four NATFW NSLP message types, which form the
initial contents of this registry. IANA has added these four NATFW
NSLP Message Types: CREATE (0x1), EXTERNAL (0x2), RESPONSE (0x3), and
NOTIFY (0x4). 0x0 is Reserved. Each registry entry consists of
value, description, and reference.
7.2. NATFW NSLP Header Flag Registry
NATFW NSLP messages have a message-specific 8-bit flags/reserved
field in their header. The registration of flags is subject to IANA
registration. The allocation of values for flag types requires IETF
Review. Updates and deletion of values from the registry are not
possible. This specification defines only two flags in Section 4.1,
the P flag (bit 8) and the E flag (bit 9). Each registry entry
consists of value, bit position, description (containing the section
number), and reference.
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7.3. NSLP Message Object Registry
In Section 4.2 this document defines 9 objects for the NATFW NSLP:
NATFW_LT, NATFW_EXTERNAL_IP, NATFW_EXTERNAL_BINDING, NATFW_EFI,
NATFW_INFO, NATFW_NONCE, NATFW_MSN, NATFW_DTINFO, NATFW_ICMP_TYPES.
IANA has assigned values for them from the NSLP Message Objects
registry.
7.4. NSLP Response Code Registry
In addition, this document defines a number of Response Codes for the
NATFW NSLP. These can be found in Section 4.2.5 and have been
assigned values from the NSLP Response Code registry. The allocation
of new values for Response Codes requires IETF Review. IANA has
assigned values for them as given in Section 4.2.5 for the error
class and also for the number of responses values per error class.
Each registry entry consists of response code, value, description,
and reference.
7.5. NSLP IDs and Router Alert Option Values
GIST NSLPID
This specification defines an NSLP for use with GIST and thus
requires an assigned NSLP identifier. IANA has added one new value
(33) to the NSLP Identifiers (NSLPID) registry defined in [RFC 5971]
for the NATFW NSLP.
IPv4 and IPv6 Router Alert Option (RAO) value
The GIST specification also requires that each NSLP-ID be associated
with specific Router Alert Option (RAO) value. For the purposes of
the NATFW NSLP, a single IPv4 RAO value (65) and a single IPv6 RAO
value (68) have been allocated.
8. Acknowledgments
We would like to thank the following individuals for their
contributions to this document at different stages:
o Marcus Brunner and Henning Schulzrinne for their work on IETF
documents that led us to start with this document;
o Miquel Martin for his large contribution on the initial version of
this document and one of the first prototype implementations;
o Srinath Thiruvengadam and Ali Fessi work for their work on the
NAT/firewall threats document;
Stiemerling, et al. Experimental PAGE 78
RFC 5973 NAT/FW NSIS NSLP October 2010
o Henning Peters for his comments and suggestions;
o Ben Campbell as Gen-ART reviewer;
o and the NSIS working group.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[RFC 1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC 4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
9.2. Informative References
[RFC 4080] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework",
RFC 4080, June 2005.
[RFC 3726] Brunner, M., "Requirements for Signaling Protocols",
RFC 3726, April 2004.
[RFC 5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling", RFC 5974, October 2010.
[RFC 5866] Sun, D., McCann, P., Tschofenig, H., Tsou, T., Doria, A.,
and G. Zorn, "Diameter Quality-of-Service Application",
RFC 5866, May 2010.
[RFC 5978] Manner, J., Bless, R., Loughney, J., and E. Davies, "Using
and Extending the NSIS Protocol Family", RFC 5978,
October 2010.
[RFC 3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
Stiemerling, et al. Experimental PAGE 79
RFC 5973 NAT/FW NSIS NSLP October 2010
[RFC 4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for
Next Steps in Signaling (NSIS)", RFC 4081, June 2005.
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC 3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC 2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC 3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC 5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC 3198] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J., and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, November 2001.
[RFC 3520] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh,
"Session Authorization Policy Element", RFC 3520,
April 2003.
[RFC 3521] Hamer, L-N., Gage, B., and H. Shieh, "Framework for
Session Set-up with Media Authorization", RFC 3521,
April 2003.
[rsvp-firewall]
Roedig, U., Goertz, M., Karten, M., and R. Steinmetz,
"RSVP as firewall Signalling Protocol", Proceedings of the
6th IEEE Symposium on Computers and Communications,
Hammamet, Tunisia, pp. 57 to 62, IEEE Computer Society
Press, July 2001.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Appendix A. Selecting Signaling Destination Addresses for EXTERNAL
As with all other message types, EXTERNAL messages need a reachable
IP address of the data sender on the GIST level. For the path-
coupled MRM, the source-address of GIST is the reachable IP address
(i.e., the real IP address of the data sender, or a wildcard). While
this is straightforward, it is not necessarily so for the loose-end
MRM. Many applications do not provide the IP address of the
communication counterpart, i.e., either the data sender or both a
data sender and receiver. For the EXTERNAL messages, the case of
data sender is of interest only. The rest of this section gives
informational guidance about determining a good destination-address
of the LE-MRM in GIST for EXTERNAL messages.
This signaling destination address (SDA, the destination-address in
GIST) can be the data sender, but for applications that do not
provide an address upfront, the destination IP address has to be
chosen independently, as it is unknown at the time when the NATFW
NSLP signaling has to start. Choosing the 'correct' destination IP
address may be difficult and it is possible that there is no 'right
answer' for all applications relying on the NATFW NSLP.
Whenever possible, it is RECOMMENDED to chose the data sender's IP
address as the SDA. It is necessary to differentiate between the
received IP addresses on the data sender. Some application-level
signaling protocols (e.g., SIP) have the ability to transfer multiple
contact IP addresses of the data sender. For instance, private IP
addresses, public IP addresses at a NAT, and public IP addresses at a
relay. It is RECOMMENDED to use all non-private IP addresses as
SDAs.
A different SDA must be chosen, if the IP address of the data sender
is unknown. This can have multiple reasons: the application-level
signaling protocol cannot determine any data sender IP address at
this point in time or the data receiver is server behind a NAT, i.e.,
accepting inbound packets from any host. In this case, the NATFW
NSLP can be instructed to use the public IP address of an application
server or any other node. Choosing the SDA in this case is out of
the scope of the NATFW NSLP and depends on the application's choice.
The local network can provide a network-SDA, i.e., an SDA that is
only meaningful to the local network. This will ensure that GIST
packets with destination-address set to this network-SDA are going to
be routed to an edge-NAT or edge-firewall.
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Appendix B. Usage of External Binding Addresses
The NATFW_EXTERNAL_BINDING object carries information, which has a
different utility to the information carried within the
NATFW_EXTERNAL_IP object. The NATFW_EXTERNAL_IP object has the
public IP address and potentially port numbers that can be used by
the application at the NI to be reachable via the public Internet.
However, there are cases in which various NIs are located behind the
same public NAT, but are subject to a multi-level NAT deployment, as
shown in Figure 35. They can use their public IP address port
assigned to them to communicate between each other (e.g., NI with NR1
and NR2) but they are forced to send their traffic through the edge-
NAT, even though there is a shorter way possible.
NI --192.168.0/24-- NAT1--10.0.0.0/8--NAT2 Internet (public IP)
|
NR1--192.168.0/24-- NAT3--
|
NR2 10.1.2.3
Figure 35: Multi-Level NAT Scenario
Figure 35 shows an example that is explored here:
1. NI -> NR1: Both NI and NR1 send EXTERNAL messages towards NAT2
and get an external address+port binding. Then, they exchange
that external binding and all traffic gets pinned to NAT2 instead
of taking the shortest path by NAT1 to NAT3 directly. However,
to do that, NR1 and NI both need to be aware that they also have
the address on the external side of NAT1 and NAT3, respectively.
If ICE is deployed and there is actually a STUN server in the
10/8 network configured, it is possible to get the shorter path
to work. The NATFW NSLP provides all external addresses in the
NATFW_EXTERNAL_BINDING towards the public network it could allow
for optimizations.
2. For the case NI -> NR2 is even more obvious. Pinning this to
NAT2 is an important fallback, but allowing for trying for a
direct path between NAT1 and NAT3 might be worth it.
Please note that if there are overlapping address domains between NR
and the public Internet, the regular routing will not necessary allow
sending the packet to the right domain.
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Appendix C. Applicability Statement on Data Receivers behind Firewalls
Section 3.7.2 describes how data receivers behind middleboxes can
instruct inbound firewalls/NATs to forward NATFW NSLP signaling
towards them. Finding an inbound edge-NAT in an address environment
with NAT'ed addresses is quite easy. It is only required to find
some edge-NAT, as the data traffic will be route-pinned to the NAT.
Locating the appropriate edge-firewall with the PC-MRM sent inbound
is difficult. For cases with a single, symmetric route from the
Internet to the data receiver, it is quite easy; simply follow the
default route in the inbound direction.
+------+ Data Flow
+-------| EFW1 +----------+ <===========
| +------+ ,--+--.
+--+--+ / \
NI+-----| FW1 | (Internet )----NR+/NI/DS
NR +--+--+ \ /
| +------+ `--+--'
+-------| EFW2 +----------+
+------+
~~~~~~~~~~~~~~~~~~~~~>
Signaling Flow
Figure 36: Data Receiver behind Multiple Firewalls
Located in Parallel
When a data receiver, and thus NR, is located in a network site that
is multihomed with several independently firewalled connections to
the public Internet (as shown in Figure 36), the specific firewall
through which the data traffic will be routed has to be ascertained.
NATFW NSLP signaling messages sent from the NI+/NR during the
EXTERNAL message exchange towards the NR+ must be routed by the NTLP
to the edge-firewall that will be passed by the data traffic as well.
The NTLP would need to be aware about the routing within the Internet
to determine the path between the DS and DR. Out of this, the NTLP
could determine which of the edge-firewalls, either EFW1 or EFW2,
must be selected to forward the NATFW NSLP signaling. Signaling to
the wrong edge-firewall, as shown in Figure 36, would install the
NATFW NSLP policy rules at the wrong device. This causes either a
blocked data flow (when the policy rule is 'allow') or an ongoing
attack (when the policy rule is 'deny'). Requiring the NTLP to know
all about the routing within the Internet is definitely a tough
challenge and usually not possible. In a case as described, the NTLP
must basically give up and return an error to the NSLP level,
indicating that the next hop discovery is not possible.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Appendix D. Firewall and NAT Resources
This section gives some examples on how NATFW NSLP policy rules could
be mapped to real firewall or NAT resources. The firewall rules and
NAT bindings are described in a natural way, i.e., in a way that one
will find in common implementations.
D.1. Wildcarding of Policy Rules
The policy rule/MRI to be installed can be wildcarded to some degree.
Wildcarding applies to IP address, transport layer port numbers, and
the IP payload (or next header in IPv6). Processing of wildcarding
splits into the NTLP and the NATFW NSLP layer. The processing at the
NTLP layer is independent of the NSLP layer processing and per-layer
constraints apply. For wildcarding in the NTLP, see Section 5.8 of
[RFC 5971].
Wildcarding at the NATFW NSLP level is always a node local policy
decision. A signaling message carrying a wildcarded MRI (and thus
policy rule) arriving at an NSLP node can be rejected if the local
policy does not allow the request. For instance, take an MRI with IP
addresses set (not wildcarded), transport protocol TCP, and TCP port
numbers completely wildcarded. If the local policy allows only
requests for TCP with all ports set and not wildcarded, the request
is going to be rejected.
D.2. Mapping to Firewall Rules
This section describes how a NSLP policy rule signaled with a CREATE
message is mapped to a firewall rule. The MRI is set as follows:
o network-layer-version=IPv4
o source-address=192.0.2.100, prefix-length=32
o destination-address=192.0.50.5, prefix-length=32
o IP-protocol=UDP
o L4-source-port=34543, L4-destination-port=23198
The NATFW_EFI object is set to action=allow and sub_ports=0.
The resulting policy rule (firewall rule) to be installed might look
like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198.
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RFC 5973 NAT/FW NSIS NSLP October 2010
D.3. Mapping to NAT Bindings
This section describes how a NSLP policy rule signaled with an
EXTERNAL message is mapped to a NAT binding. It is assumed that the
EXTERNAL message is sent by a NI+ located behind a NAT and does
contain a NATFW_DTINFO object. The MRI is set following using the
signaling destination address, since the IP address of the real data
sender is not known:
o network-layer-version=IPv4
o source-address= 192.168.5.100
o destination-address=SDA
o IP-protocol=UDP
The NATFW_EFI object is set to action=allow and sub_ports=0. The
NATFW_DTINFO object contains these parameters:
o P=1
o dest prefix=0
o protocol=UDP
o dst port number = 20230, src port number=0
o src IP=0.0.0.0
The edge-NAT allocates the external IP 192.0.2.79 and port 45000.
The resulting policy rule (NAT binding) to be installed could look
like: translate udp from any to 192.0.2.79 port=45000 to
192.168.5.100 port=20230.
D.4. NSLP Handling of Twice-NAT
The dynamic configuration of twice-NATs requires application-level
support, as stated in Section 2.5. The NATFW NSLP cannot be used for
configuring twice-NATs if application-level support is needed.
Assuming application-level support performing the configuration of
the twice-NAT and the NATFW NSLP being installed at this devices, the
NATFW NSLP must be able to traverse it. The NSLP is probably able to
traverse the twice-NAT, as is any other data traffic, but the flow
information stored in the NTLP's MRI will be invalidated through the
translation of source and destination IP addresses. The NATFW NSLP
implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP
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RFC 5973 NAT/FW NSIS NSLP October 2010
signaling messages as any other NATFW NSLP node does. For the given
signaling flow, the NATFW NSLP node MUST look up the corresponding IP
address translation and modify the NTLP/NSLP signaling accordingly.
The modification results in an updated MRI with respect to the source
and destination IP addresses.
Appendix E. Example for Receiver Proxy Case
This section gives an example on how to use the NATFW NLSP for a
receiver behind a NAT, where only the receiving side is NATFW NSLP
enabled. We assume FTP as the application to show a working example.
An FTP server is located behind a NAT, as shown in Figure 5, and uses
the NATFW NSLP to allocate NAT bindings for the control and data
channel of the FTP protocol. The information about where to reach
the server is communicated by a separate protocol (e.g., email, chat)
to the DS side.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Public Internet Private Address
Space
FTP Client FTP Server
DS NAT NI+
| | |
| | EXTERNAL |
| |<---------------------------|(1)
| | |
| |RESPONSE[Success] |
| |--------------------------->|(2)
| |CREATE |
| |--------------------------->|(3)
| |RESPONSE[Success] |
| |<---------------------------|(4)
| | |
| | <Use port=XYZ, IP=a.b.c.d> |
|<=======================================================|(5)
|FTP control port=XYZ | FTP control port=21 |
|~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(6)
| | |
| FTP control/get X | FTP control/get X |
|~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(7)
| | EXTERNAL |
| |<---------------------------|(8)
| | |
| |RESPONSE[Success] |
| |--------------------------->|(9)
| |CREATE |
| |--------------------------->|(10)
| |RESPONSE[Success] |
| |<---------------------------|(11)
| | |
| Use port=FOO, IP=a.b.c.d | Use port=FOO, IP=a.b.c.d |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~|<~~~~~~~~~~~~~~~~~~~~~~~~~~~|(12)
| | |
|FTP data to port=FOO | FTP data to port=20 |
|~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(13)
Figure 37: Flow Chart
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RFC 5973 NAT/FW NSIS NSLP October 2010
1. EXTERNAL request message sent to NAT, with these objects:
signaling session lifetime, extended flow information object
(rule action=allow, sub_ports=0), message sequence number
object, nonce object (carrying nonce for CREATE), and the data
terminal information object (I/P-flags set, sender prefix=0,
protocol=TCP, DR port number = 21, DS's IP address=0); using the
LE-MRM. This is used to allocate the external binding for the
FTP control channel (TCP, port 21).
2. Successful RESPONSE sent to NI+, with these objects: signaling
session lifetime, message sequence number object, information
code object ('Success':2), external address object (port=XYZ,
IPv4 addr=a.b.c.d).
3. The NAT sends a CREATE towards NI+, with these objects:
signaling session lifetime, extended flow information object
(rule action=allow, sub_ports=0), message sequence number
object, nonce object (with copied value from (1)); using the PC-
MRM (src-IP=a.b.c.d, src-port=XYZ, dst-IP=NI+, dst-port=21,
downstream).
4. Successful RESPONSE sent to NAT, with these objects: signaling
session lifetime, message sequence number object, information
code object ('Success':2).
5. The application at NI+ sends external NAT binding information to
the other end, i.e., the FTP client at the DS.
6. The FTP client connects the FTP control channel to port=XYZ,
IP=a.b.c.d.
7. The FTP client sends a get command for file X.
8. EXTERNAL request message sent to NAT, with these objects:
signaling session lifetime, extended flow information object
(rule action=allow, sub_ports=0), message sequence number
object, nonce object (carrying nonce for CREATE), and the data
terminal information object (I/P-flags set, sender prefix=32,
protocol=TCP, DR port number = 20, DS's IP address=DS-IP); using
the LE-MRM. This is used to allocate the external binding for
the FTP data channel (TCP, port 22).
9. Successful RESPONSE sent to NI+, with these objects: signaling
session lifetime, message sequence number object, information
code object ('Success':2), external address object (port=FOO,
IPv4 addr=a.b.c.d).
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RFC 5973 NAT/FW NSIS NSLP October 2010
10. The NAT sends a CREATE towards NI+, with these objects:
signaling session lifetime, extended flow information object
(rule action=allow, sub_ports=0), message sequence number
object, nonce object (with copied value from (1)); using the PC-
MRM (src-IP=a.b.c.d, src-port=FOO, dst-IP=NI+, dst-port=20,
downstream).
11. Successful RESPONSE sent to NAT, with these objects: signaling
session lifetime, message sequence number object, information
code object ('Success':2).
12. The FTP server responses with port=FOO and IP=a.b.c.d.
13. The FTP clients connects the data channel to port=FOO and
IP=a.b.c.d.
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RFC 5973 NAT/FW NSIS NSLP October 2010
Authors' Addresses
Martin Stiemerling
NEC Europe Ltd. and University of Goettingen
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 4342 113
EMail: Martin.Stiemerling@neclab.eu
URI: http://www.stiemerling.org
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
EMail: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.priv.at
Cedric Aoun
Consultant
Paris, France
EMail: cedaoun@yahoo.fr
Elwyn Davies
Folly Consulting
Soham
UK
Phone: +44 7889 488 335
EMail: elwynd@dial.pipex.com
Stiemerling, et al. Experimental PAGE 90
NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
RFC TOTAL SIZE: 219962 bytes
PUBLICATION DATE: Wednesday, October 6th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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