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IETF RFC 9174
Last modified on Tuesday, February 1st, 2022
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Internet Engineering Task Force (IETF) B. Sipos
Request for Comments: 9174 RKF Engineering
Category: Standards Track M. Demmer
ISSN: 2070-1721
J. Ott
Technical University of Munich
S. Perreault
LogMeIn
January 2022
Delay-Tolerant Networking TCP Convergence-Layer Protocol Version 4
 Abstract
This document describes a TCP convergence layer (TCPCL) for Delay-
Tolerant Networking (DTN). This version of the TCPCL protocol
resolves implementation issues in the earlier TCPCL version 3 as
defined in RFC 7242 and provides updates to the Bundle Protocol (BP)
contents, encodings, and convergence-layer requirements in BP version
7 (BPv7). Specifically, TCPCLv4 uses BPv7 bundles encoded by the
Concise Binary Object Representation (CBOR) as its service data unit
being transported and provides a reliable transport of such bundles.
This TCPCL version also includes security and extensibility
mechanisms.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9174.
Copyright Notice
Copyright (c) 2022 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Scope
2. Requirements Language
2.1. Definitions Specific to the TCPCL Protocol
3. General Protocol Description
3.1. Convergence-Layer Services
3.2. TCPCL Session Overview
3.3. TCPCL States and Transitions
3.4. PKIX Environments and CA Policy
3.5. Session-Keeping Policies
3.6. Transfer Segmentation Policies
3.7. Example Message Exchange
4. Session Establishment
4.1. TCP Connection
4.2. Contact Header
4.3. Contact Validation and Negotiation
4.4. Session Security
4.4.1. Entity Identification
4.4.2. Certificate Profile for the TCPCL
4.4.3. TLS Handshake
4.4.4. TLS Authentication
4.4.5. Policy Recommendations
4.4.6. Example TLS Initiation
4.5. Message Header
4.6. Session Initialization Message (SESS_INIT)
4.7. Session Parameter Negotiation
4.8. Session Extension Items
5. Established Session Operation
5.1. Upkeep and Status Messages
5.1.1. Session Upkeep (KEEPALIVE)
5.1.2. Message Rejection (MSG_REJECT)
5.2. Bundle Transfer
5.2.1. Bundle Transfer ID
5.2.2. Data Transmission (XFER_SEGMENT)
5.2.3. Data Acknowledgments (XFER_ACK)
5.2.4. Transfer Refusal (XFER_REFUSE)
5.2.5. Transfer Extension Items
6. Session Termination
6.1. Session Termination Message (SESS_TERM)
6.2. Idle Session Termination
7. Security Considerations
7.1. Threat: Passive Leak of Node Data
7.2. Threat: Passive Leak of Bundle Data
7.3. Threat: TCPCL Version Downgrade
7.4. Threat: Transport Security Stripping
7.5. Threat: Weak TLS Configurations
7.6. Threat: Untrusted End-Entity Certificate
7.7. Threat: Certificate Validation Vulnerabilities
7.8. Threat: Symmetric Key Limits
7.9. Threat: BP Node Impersonation
7.10. Threat: Denial of Service
7.11. Mandatory-to-Implement TLS
7.12. Alternate Uses of TLS
7.12.1. TLS without Authentication
7.12.2. Non-certificate TLS Use
7.13. Predictability of Transfer IDs
8. IANA Considerations
8.1. Port Number
8.2. Protocol Versions
8.3. Session Extension Types
8.4. Transfer Extension Types
8.5. Message Types
8.6. XFER_REFUSE Reason Codes
8.7. SESS_TERM Reason Codes
8.8. MSG_REJECT Reason Codes
8.9. Object Identifier for PKIX Module Identifier
8.10. Object Identifier for PKIX Other Name Forms
8.11. Object Identifier for PKIX Extended Key Usage
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Significant Changes from RFC 7242
Appendix B. ASN.1 Module
Appendix C. Example of the BundleEID Other Name Form
Acknowledgments
Authors' Addresses
1. Introduction
This document describes the TCP convergence-layer protocol for Delay-
Tolerant Networking (DTN). DTN is an end-to-end architecture
providing communications in and/or through highly stressed
environments, including those with intermittent connectivity, long
and/or variable delays, and high bit error rates. More detailed
descriptions of the rationale and capabilities of these networks can
be found in "Delay-Tolerant Networking Architecture" [RFC 4838].
An important goal of the DTN architecture is to accommodate a wide
range of networking technologies and environments. The protocol used
for DTN communications is the Bundle Protocol version 7 (BPv7)
[RFC 9171], an application-layer protocol that is used to construct a
store-and-forward overlay network. BPv7 requires the services of a
"convergence-layer adapter" (CLA) to send and receive bundles using
the service of some "native" link, network, or Internet protocol.
This document describes one such convergence-layer adapter that uses
the well-known Transmission Control Protocol (TCP). This convergence
layer is referred to as TCP Convergence Layer version 4 (TCPCLv4).
For the remainder of this document,
* the abbreviation "BP" without the version suffix refers to BPv7.
* the abbreviation "TCPCL" without the version suffix refers to
TCPCLv4.
The locations of the TCPCL and the Bundle Protocol in the Internet
model protocol stack (described in [RFC 1122]) are shown in Figure 1.
In particular, when BP is using TCP as its bearer with the TCPCL as
its convergence layer, both BP and the TCPCL reside at the
application layer of the Internet model.
+-------------------------+
| DTN Application | -\
+-------------------------| |
| Bundle Protocol (BP) | -> Application Layer
+-------------------------+ |
| TCP Conv. Layer (TCPCL) | |
+-------------------------+ |
| TLS (optional) | -/
+-------------------------+
| TCP | ---> Transport Layer
+-------------------------+
| IPv4/IPv6 | ---> Network Layer
+-------------------------+
| Link-Layer Protocol | ---> Link Layer
+-------------------------+
Figure 1: The Locations of the Bundle Protocol and the TCP
Convergence-Layer Protocol above the Internet Protocol Stack
1.1. Scope
This document describes the format of the protocol data units passed
between entities participating in TCPCL communications. This
document does not address:
* The format of protocol data units of the Bundle Protocol, as those
are defined elsewhere in [RFC 9171]. This includes the concept of
bundle fragmentation or bundle encapsulation. The TCPCL transfers
bundles as opaque data blocks.
* Mechanisms for locating or identifying other bundle entities
(peers) within a network or across an internet. The mapping of a
node ID to a potential convergence layer (CL) protocol and network
address is left to implementation and configuration of the BP
Agent (BPA) and its various potential routing strategies, as is
the mapping of a DNS name and/or address to a choice of an end-
entity certificate to authenticate a node to its peers.
* Logic for routing bundles along a path toward a bundle's endpoint.
This CL protocol is involved only in transporting bundles between
adjacent entities in a routing sequence.
* Policies or mechanisms for issuing Public Key Infrastructure Using
X.509 (PKIX) certificates; provisioning, deploying, or accessing
certificates and private keys; deploying or accessing certificate
revocation lists (CRLs); or configuring security parameters on an
individual entity or across a network.
* Uses of TLS that are not based on PKIX certificate authentication
(see Section 7.12.2) or in which authentication of both entities
is not possible (see Section 7.12.1).
Any TCPCL implementation requires a BPA to perform those above-listed
functions in order to perform end-to-end bundle delivery.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
2.1. Definitions Specific to the TCPCL Protocol
This section contains definitions specific to the TCPCL protocol.
Network Byte Order: Here, "network byte order" means most
significant byte first, a.k.a. big endian. All of the integer
encodings in this protocol SHALL be transmitted in network byte
order.
TCPCL Entity: This is the notional TCPCL application that initiates
TCPCL sessions. This design, implementation, configuration, and
specific behavior of such an entity is outside of the scope of
this document. However, the concept of an entity has utility
within the scope of this document as the container and initiator
of TCPCL sessions. The relationship between a TCPCL entity and
TCPCL sessions is defined as follows:
* A TCPCL entity MAY actively initiate any number of TCPCL
sessions and should do so whenever the entity is the initial
transmitter of information to another entity in the network.
* A TCPCL entity MAY support zero or more passive listening
elements that listen for connection requests from other TCPCL
entities operating on other entities in the network.
* A TCPCL entity MAY passively initiate any number of TCPCL
sessions from requests received by its passive listening
element(s) if the entity uses such elements.
These relationships are illustrated in Figure 2. For most TCPCL
behavior within a session, the two entities are symmetric and
there is no protocol distinction between them. Some specific
behavior, particularly during session establishment, distinguishes
between the active entity and the passive entity. For the
remainder of this document, the term "entity" without the prefix
"TCPCL" refers to a TCPCL entity.
TCP Connection: The term "connection" in this specification
exclusively refers to a TCP connection and any and all behaviors,
sessions, and other states associated with that TCP connection.
TCPCL Session: A TCPCL session (as opposed to a TCP connection) is a
TCPCL communication relationship between two TCPCL entities. A
TCPCL session operates within a single underlying TCP connection,
and the lifetime of a TCPCL session is bound to the lifetime of
that TCP connection. A TCPCL session is terminated when the TCP
connection ends, due to either (1) one or both entities actively
closing the TCP connection or (2) network errors causing a failure
of the TCP connection. Within a single TCPCL session, there are
two possible transfer streams: one in each direction, with one
stream from each entity being the outbound stream and the other
being the inbound stream (see Figure 3). From the perspective of
a TCPCL session, the two transfer streams do not logically
interact with each other. The streams do operate over the same
TCP connection and between the same BPAs, so there are logical
relationships at those layers (message and bundle interleaving,
respectively). For the remainder of this document, the term
"session" without the prefix "TCPCL" refers to a TCPCL session.
Session Parameters: These are a set of values used to affect the
operation of the TCPCL for a given session. The manner in which
these parameters are conveyed to the bundle entity and thereby to
the TCPCL is implementation dependent. However, the mechanism by
which two entities exchange and negotiate the values to be used
for a given session is described in Section 4.3.
Transfer Stream: A transfer stream is a unidirectional user-data
path within a TCPCL session. Transfers sent over a transfer
stream are serialized, meaning that one transfer must complete its
transmission prior to another transfer being started over the same
transfer stream. At the stream layer, there is no logical
relationship between transfers in that stream; it's only within
the BPA that transfers are fully decoded as bundles. Each
unidirectional stream has a single sender entity and a single
receiver entity.
Transfer: This refers to the procedures and mechanisms for
conveyance of an individual bundle from one node to another. Each
transfer within the TCPCL is identified by a Transfer ID number,
which is guaranteed to be unique only to a single direction within
a single session.
Transfer Segment: A transfer segment is a subset of a transfer of
user data being communicated over a transfer stream.
Idle Session: A TCPCL session is idle while there is no transmission
in progress in either direction. While idle, the only messages
being transmitted or received are KEEPALIVE messages.
Live Session: A TCPCL session is live while there is a transmission
in progress in either direction.
Reason Codes: The TCPCL uses numeric codes to encode specific
reasons for individual failure/error message types.
The relationship between connections, sessions, and streams is shown
in Figure 3.
+--------------------------------------------+
| TCPCL Entity |
| | +----------------+
| +--------------------------------+ | | |-+
| | Actively Initiated Session #1 +------------->| Other | |
| +--------------------------------+ | | TCPCL Entity's | |
| ... | | Passive | |
| +--------------------------------+ | | Listener | |
| | Actively Initiated Session #n +------------->| | |
| +--------------------------------+ | +----------------+ |
| | +-----------------+
| +---------------------------+ |
| +---| +---------------------------+ | +----------------+
| | | | Optional Passive | | | |-+
| | +-| Listener(s) +<-------------+ | |
| | +---------------------------+ | | | |
| | | | Other | |
| | +---------------------------------+ | | TCPCL Entity's | |
| +--->| Passively Initiated Session #1 +-------->| Active | |
| | +---------------------------------+ | | Initiator(s) | |
| | | | | |
| | +---------------------------------+ | | | |
| +--->| Passively Initiated Session #n +-------->| | |
| +---------------------------------+ | +----------------+ |
| | +-----------------+
+--------------------------------------------+
Figure 2: The Relationships between TCPCL Entities
+---------------------------+ +---------------------------+
| "Own" TCPCL Session | | "Other" TCPCL Session |
| | | |
| +----------------------+ | | +----------------------+ |
| | TCP Connection | | | | TCP Connection | |
| | | | | | | |
| | +-----------------+ | | Messages | | +-----------------+ | |
| | | Own Inbound | +--------------------+ | Peer Outbound | | |
| | | Transfer Stream | | Transfer Stream | | |
| | | ----- |<---[Seg]--[Seg]--[Seg]---| ----- | | |
| | | RECEIVER |---[Ack]----[Ack]-------->| SENDER | | |
| | +-----------------+ +-----------------+ | |
| | | |
| | +-----------------+ +-----------------+ | |
| | | Own Outbound |-------[Seg]---[Seg]----->| Peer Inbound | | |
| | | Transfer Stream |<---[Ack]----[Ack]-[Ack]--| Transfer Stream | | |
| | | ----- | | ----- | | |
| | | SENDER | +--------------------+ | RECEIVER | | |
| | +-----------------+ | | | | +-----------------+ | |
| +-----------------------+ | | +---------------------+ |
+----------------------------+ +--------------------------+
Figure 3: The Relationship within a TCPCL Session of its Two Streams
3. General Protocol Description
The service of this protocol is the transmission of DTN bundles via
TCP. This document specifies the encapsulation of bundles,
procedures for TCP setup and teardown, and a set of messages and
entity requirements. The general operation of the protocol is as
follows.
3.1. Convergence-Layer Services
This version of the TCPCL protocol provides the following services to
support the overlaying BPA. In all cases, this is not an API
definition but a logical description of how the CL can interact with
the BPA. Each of these interactions can be associated with any
number of additional metadata items as necessary to support the
operation of the CL or BPA.
Attempt Session: The TCPCL allows a BPA to preemptively attempt to
establish a TCPCL session with a peer entity. Each session
attempt can send a different set of session negotiation parameters
as directed by the BPA.
Terminate Session: The TCPCL allows a BPA to preemptively terminate
an established TCPCL session with a peer entity. The terminate
request is done on a per-session basis.
Session State Changed: The TCPCL entity indicates to the BPA when
the session state changes. The top-level session states indicated
are as follows:
Connecting: A TCP connection is being established. This state
only applies to the active entity.
Contact Negotiating: A TCP connection has been made (as either
the active or passive entity), and contact negotiation has
begun.
Session Negotiating: Contact negotiation has been completed
(including possible TLS use), and session negotiation has
begun.
Established: The session has been fully established and is ready
for its first transfer. When the session is established, the
peer node ID (along with an indication of whether or not it was
authenticated) and the negotiated session parameters (see
Section 4.7) are also communicated to the BPA.
Ending: The entity sent a SESS_TERM message and is in the Ending
state.
Terminated: The session has finished normal termination
sequencing.
Failed: The session ended without normal termination sequencing.
Session Idle Changed: The TCPCL entity indicates to the BPA when the
Live/Idle substate of the session changes. This occurs only when
the top-level session state is "Established". The session
transitions from Idle to Live at the start of a transfer in either
transfer stream; the session transitions from Live to Idle at the
end of a transfer when the other transfer stream does not have an
ongoing transfer. Because the TCPCL transmits serially over a TCP
connection, it suffers from "head-of-queue blocking", so a
transfer in either direction can block an immediate start of a new
transfer in the session.
Begin Transmission: The principal purpose of the TCPCL is to allow a
BPA to transmit bundle data over an established TCPCL session.
Transmission requests are done on a per-session basis, and the CL
does not necessarily perform any per-session or inter-session
queueing. Any queueing of transmissions is the obligation of the
BPA.
Transmission Success: The TCPCL entity indicates to the BPA when a
bundle has been fully transferred to a peer entity.
Transmission Intermediate Progress: The TCPCL entity indicates to
the BPA the intermediate progress of a transfer to a peer entity.
This intermediate progress is at the granularity of each
transferred segment.
Transmission Failure: The TCPCL entity indicates to the BPA certain
reasons for bundle transmission failure, notably when the peer
entity rejects the bundle or when a TCPCL session ends before
transfer success. The TCPCL itself does not have a notion of
transfer timeout.
Reception Initialized: The TCPCL entity indicates this status to the
receiving BPA just before any transmission data is sent. This
corresponds to reception of the XFER_SEGMENT message with the
START flag set to 1.
Interrupt Reception: The TCPCL entity allows a BPA to interrupt an
individual transfer before it has fully completed (successfully or
not). Interruption can occur any time after the reception is
initialized.
Reception Success: The TCPCL entity indicates to the BPA when a
bundle has been fully transferred from a peer entity.
Reception Intermediate Progress: The TCPCL entity indicates to the
BPA the intermediate progress of a transfer from the peer entity.
This intermediate progress is at the granularity of each
transferred segment. An indication of intermediate reception
gives a BPA the chance to inspect bundle header contents before
the entire bundle is available and thus supports the "Interrupt
Reception" capability.
Reception Failure: The TCPCL entity indicates to the BPA certain
reasons for reception failure, notably when the local entity
rejects an attempted transfer for some local policy reason or when
a TCPCL session ends before transfer success. The TCPCL itself
does not have a notion of transfer timeout.
3.2. TCPCL Session Overview
First, one entity establishes a TCPCL session to the other by
initiating a TCP connection in accordance with [RFC 793]. After
setup of the TCP connection is complete, an initial Contact Header is
exchanged in both directions to establish a shared TCPCL version and
negotiate the use of TLS security (as described in Section 4). Once
contact negotiation is complete, TCPCL messaging is available and the
session negotiation is used to set parameters of the TCPCL session.
One of these parameters is a node ID; each TCPCL entity is acting on
behalf of a BPA having a node ID. This is used to assist in routing
and forwarding messages by the BPA and is part of the authentication
capability provided by TLS.
Once negotiated, the parameters of a TCPCL session cannot change; if
there is a desire by either peer to transfer data under different
parameters, then a new session must be established. This makes CL
logic simpler but relies on the assumption that establishing a TCP
connection is lightweight enough that TCP connection overhead is
negligible compared to TCPCL data sizes.
Once the TCPCL session is established and configured in this way,
bundles can be transferred in either direction. Each transfer is
performed by segmenting the transfer data into one or more
XFER_SEGMENT messages. Multiple bundles can be transmitted
consecutively in a single direction on a single TCPCL connection.
Segments from different bundles are never interleaved. Bundle
interleaving can be accomplished by fragmentation at the BP layer or
by establishing multiple TCPCL sessions between the same peers.
There is no fundamental limit on the number of TCPCL sessions that a
single entity can establish, beyond the limit imposed by the number
of available (ephemeral) TCP ports of the active entity.
One feature of this protocol is that the receiving entity can send
acknowledgment (XFER_ACK) messages as bundle data segments arrive.
The rationale behind these acknowledgments is to enable the
transmitting entity to determine how much of the bundle has been
received, so that if the session is interrupted, it can perform
reactive fragmentation to avoid resending the already-transmitted
part of the bundle. In addition, there is no explicit flow control
on the TCPCL.
A TCPCL receiver can interrupt the transmission of a bundle at any
point in time by replying with a XFER_REFUSE message, which causes
the sender to stop transmission of the associated bundle (if it
hasn't already finished transmission).
| Note: This enables a cross-layer optimization in that it allows
| a receiver that detects that it has already received a certain
| bundle to interrupt transmission as early as possible and thus
| save transmission capacity for other bundles.
For sessions that are idle, a KEEPALIVE message is sent at a
negotiated interval. This is used to convey entity liveness
information during otherwise messageless time intervals.
A SESS_TERM message is used to initiate the ending of a TCPCL session
(see Section 6.1). During termination sequencing, in-progress
transfers can be completed but no new transfers can be initiated. A
SESS_TERM message can also be used to refuse a session setup by a
peer (see Section 4.3). Regardless of the reason, session
termination is initiated by one of the entities and the other entity
responds to it, as illustrated by Figures 13 and 14 in the next
subsection. Even when there are no transfers queued or in progress,
the session termination procedure allows each entity to distinguish
between a clean end to a session and the TCP connection being closed
because of some underlying network issue.
Once a session is established, the TCPCL is a symmetric protocol
between the peers. Both sides can start sending data segments in a
session, and one side's bundle transfer does not have to complete
before the other side can start sending data segments on its own.
Hence, the protocol allows for a bidirectional mode of communication.
Note that in the case of concurrent bidirectional transmission,
acknowledgment segments MAY be interleaved with data segments.
3.3. TCPCL States and Transitions
The states of a normal TCPCL session (i.e., without session failures)
are indicated in Figure 4.
+-------+
| START |
+-------+
|
TCP Establishment
|
V
+-----------+ +---------------------+
| TCP |----------->| Contact / Session |
| Connected | | Negotiation |
+-----------+ +---------------------+
|
+-----Session Parameters-----+
| Negotiated
V
+-------------+ +-------------+
| Established |----New Transfer---->| Established |
| Session | | Session |
| Idle |<---Transfers Done---| Live |
+-------------+ +-------------+
| |
+------------------------------------+
|
V
+-------------+
| Established | +-------------+
| Session |----Transfers------>| TCP |
| Ending | Done | Terminating |
+-------------+ +-------------+
|
+----------TCP Close Message----------+
|
V
+-------+
| END |
+-------+
Figure 4: Top-Level States of a TCPCL Session
Notes on established session states:
* Session "Live" means transmitting or receiving over a transfer
stream.
* Session "Idle" means no transmission/reception over a transfer
stream.
* Session "Ending" means no new transfers will be allowed.
Contact negotiation involves exchanging a Contact Header ("CH" in
Figures 5, 6, and 7) in both directions and deriving a negotiated
state from the two headers. The contact negotiation sequencing is
performed as either the active or passive entity and is illustrated
in Figures 5 and 6, respectively, which both share the data
validation and negotiation of the Processing of Contact Header
("[PCH]") activity (Figure 7) and the "[TCPCLOSE]" activity, which
indicates TCP connection close. Successful negotiation results in
one of the Session Initiation ("[SI]") activities being performed, as
shown further below. To avoid data loss, a Session Termination
("[ST]") exchange allows cleanly finishing transfers before a session
is ended.
+-------+
| START |
+-------+
|
TCP Connecting
V
+-----------+
| TCP | +---------+
| Connected |--Send CH-->| Waiting |--Timeout-->[TCPCLOSE]
+-----------+ +---------+
|
Received CH
V
[PCH]
Figure 5: Contact Initiation as Active Entity
+-----------+ +---------+
| TCP |--Wait for-->| Waiting |--Timeout-->[TCPCLOSE]
| Connected | CH +---------+
+-----------+ |
Received CH
V
+-----------------+
| Preparing reply |--Send CH-->[PCH]
+-----------------+
Figure 6: Contact Initiation as Passive Entity
+-----------+
| Peer CH |
| available |
+-----------+
|
Validate and
Negotiate
V
+------------+
| Negotiated |--Failure-->[TCPCLOSE]
+------------+
| |
No TLS +----Negotiate---+ [ST]
| TLS | ^
V | Failure
+-----------+ V |
| TCPCL | +---------------+
| Messaging |<--Success--| TLS Handshake |
| Available | +---------------+
+-----------+
Figure 7: Processing of Contact Header [PCH]
Session negotiation involves exchanging a session initialization
(SESS_INIT) message in both directions and deriving a negotiated
state from the two messages. The session negotiation sequencing is
performed as either the active or passive entity and is illustrated
in Figures 8 and 9, respectively (where "[PSI]" means "Processing of
Session Initiation"), which both share the data validation and
negotiation shown in Figure 10. The validation here includes
certificate validation and authentication when TLS is used for the
session.
+-----------+
| TCPCL | +---------+
| Messaging |--Send SESS_INIT-->| Waiting |--Timeout-->[ST]
| Available | +---------+
+-----------+ |
Received SESS_INIT
|
V
[PSI]
Figure 8: Session Initiation [SI] as Active Entity
+-----------+
| TCPCL | +---------+
| Messaging |----Wait for ---->| Waiting |--Timeout-->[ST]
| Available | SESS_INIT +---------+
+-----------+ |
Received SESS_INIT
|
+-----------------+
| Preparing reply |--Send SESS_INIT-->[PSI]
+-----------------+
Figure 9: Session Initiation [SI] as Passive Entity
+----------------+
| Peer SESS_INIT |
| available |
+----------------+
|
Validate and
Negotiate
V
+------------+
| Negotiated |---Failure--->[ST]
+------------+
|
Success
V
+--------------+
| Established |
| Session Idle |
+--------------+
Figure 10: Processing of Session Initiation [PSI]
Transfers can occur after a session is established and it's not in
the Ending state. Each transfer occurs within a single logical
transfer stream between a sender and a receiver, as illustrated in
Figures 11 and 12, respectively.
+--Send XFER_SEGMENT--+
+--------+ | |
| Stream | +-------------+ |
| Idle |---Send XFER_SEGMENT-->| In Progress |<------------+
+--------+ +-------------+
|
+---------All segments sent-------+
|
V
+---------+ +--------+
| Waiting |---- Receive Final---->| Stream |
| for Ack | XFER_ACK | Idle |
+---------+ +--------+
Figure 11: Transfer Sender States
| Note on transfer sending: Pipelining of transfers can occur
| when the sending entity begins a new transfer while in the
| "Waiting for Ack" state.
+-Receive XFER_SEGMENT-+
+--------+ | Send XFER_ACK |
| Stream | +-------------+ |
| Idle |--Receive XFER_SEGMENT-->| In Progress |<-------------+
+--------+ +-------------+
|
+--------Sent Final XFER_ACK--------+
|
V
+--------+
| Stream |
| Idle |
+--------+
Figure 12: Transfer Receiver States
Session termination involves one entity initiating the termination of
the session and the other entity acknowledging the termination. For
either entity, it is the sending of the SESS_TERM message, which
transitions the session to the Ending substate. While a session is
in the Ending state, only in-progress transfers can be completed and
no new transfers can be started.
+-----------+ +---------+
| Session |--Send SESS_TERM-->| Session |
| Live/Idle | | Ending |
+-----------+ +---------+
Figure 13: Session Termination [ST] from the Initiator
+-----------+ +---------+
| Session |--Send SESS_TERM-->| Session |
| Live/Idle | | Ending |
+-----------+<------+ +---------+
| |
Receive SESS_TERM |
| |
+-------------+
Figure 14: Session Termination [ST] from the Responder
3.4. PKIX Environments and CA Policy
This specification defines requirements regarding how to use PKIX
certificates issued by a Certificate Authority (CA) but does not
define any mechanisms for how those certificates come to be. The
requirements regarding TCPCL certificate use are broad, to support
two quite different PKIX environments:
DTN-Aware CAs: In the ideal case, the CA or CAs issuing certificates
for TCPCL entities are aware of the end use of the certificate,
have a mechanism for verifying ownership of a node ID, and are
issuing certificates directly for that node ID. In this
environment, the ability to authenticate a peer entity node ID
directly avoids the need to authenticate a network name or address
and then implicitly trust the node ID of the peer. The TCPCL
authenticates the node ID whenever possible; this is preferred
over lower-level PKIX identities.
DTN-Ignorant CAs: It is expected that Internet-scale "public" CAs
will continue to focus on DNS names as the preferred PKIX
identifier. There are large infrastructures already in place for
managing network-level authentication and protocols to manage
identity verification in those environments [RFC 8555]. The TCPCL
allows for this type of environment by authenticating a lower-
level identifier for a peer and requiring the entity to trust that
the node ID given by the peer (during session initialization) is
valid. This situation is not ideal, as it allows the
vulnerabilities described in Section 7.9, but it still provides
some amount of mutual authentication to take place for a TCPCL
session.
Even within a single TCPCL session, each entity may operate within
different PKI environments and with different identifier limitations.
The requirements related to identifiers in a PKIX certificate are
provided in Section 4.4.1.
It is important for interoperability that a TCPCL entity have its own
security policy tailored to accommodate the peers with which it is
expected to operate. Some security policy recommendations are given
in Section 4.4.5, but these are meant as a starting point for
tailoring. A strict TLS security policy is appropriate for a private
network with a single shared CA. Operation on the Internet (such as
inter-site BP gateways) could trade more lax TCPCL security with the
use of encrypted bundle encapsulation [DTN-BIBECT] to ensure strong
bundle security.
By using the Server Name Indication (SNI) DNS name (see
Section 4.4.3), a single passive entity can act as a convergence
layer for multiple BPAs with distinct node IDs. When this "virtual
host" behavior is used, the DNS name is used as the indication of
which BP node the active entity is attempting to communicate with. A
virtual host CL entity can be authenticated by a certificate
containing all of the DNS names and/or node IDs being hosted or by
several certificates each authenticating a single DNS name and/or
node ID, using the SNI value from the peer to select which
certificate to use. The logic for mapping an SNI DNS name to an end-
entity certificate is an implementation matter and can involve
correlating a DNS name with a node ID or other certificate
attributes.
3.5. Session-Keeping Policies
This specification defines requirements regarding how to initiate,
sustain, and terminate a TCPCL session but does not impose any
requirements on how sessions need to be managed by a BPA. It is a
network administration matter to determine an appropriate session-
keeping policy, but guidance given here can be used to steer policy
toward performance goals.
Persistent Session: This policy preemptively establishes a single
session to known entities in the network and keeps the session
active using KEEPALIVEs. Benefits of this policy include reducing
the total amount of TCP data that needs to be exchanged for a set
of transfers (assuming that the KEEPALIVE size is significantly
smaller than the transfer size) and allowing the session state to
indicate peer connectivity. Drawbacks include wasted network
resources when a session is mostly idle or when network
connectivity is inconsistent (which requires that failed sessions
be reestablished), and potential queueing issues when multiple
transfers are requested simultaneously. This policy assumes that
there is agreement between pairs of entities as to which of the
peers will initiate sessions; if there is no such agreement, there
is potential for duplicate sessions to be established between
peers.
Ephemeral Sessions: This policy only establishes a session when an
outgoing transfer needs to be sent. Benefits of this policy
include not wasting network resources on sessions that are idle
for long periods of time and avoiding potential queueing issues as
can be seen when using a single persistent session. Drawbacks
include the TCP and TLS overhead of establishing a new session for
each transfer. This policy assumes that each entity can function
in a passive role to listen for session requests from any peer
that needs to send a transfer; when that is not the case, the
polling behavior discussed below needs to happen. This policy can
be augmented to keep the session established as long as any
transfers are queued.
Active-Only Polling Sessions: When naming and/or addressing of one
entity is variable (i.e., a dynamically assigned IP address or
domain name) or when firewall or routing rules prevent incoming
TCP connections, that entity can only function in the active role.
In these cases, sessions also need to be established when an
incoming transfer is expected from a peer or based on a periodic
schedule. This polling behavior causes inefficiencies compared to
as-needed ephemeral sessions.
Many other policies can be established in a TCPCL network between the
two extremes of single persistent sessions and only ephemeral
sessions. Different policies can be applied to each peer entity and
to each bundle as it needs to be transferred (e.g., for quality of
service). Additionally, future session extension types can apply
further nuance to session policies and policy negotiation.
3.6. Transfer Segmentation Policies
Each TCPCL session allows a negotiated transfer segmentation policy
to be applied in each transfer direction. A receiving entity can set
the Segment Maximum Receive Unit (MRU) in its SESS_INIT message to
determine the largest acceptable segment size, and a transmitting
entity can segment a transfer into any sizes smaller than the
receiver's Segment MRU. It is a network administration matter to
determine an appropriate segmentation policy for entities using the
TCPCL protocol, but guidance given here can be used to steer policy
toward performance goals. Administrators are also advised to
consider the Segment MRU in relation to chunking/packetization
performed by TLS, TCP, and any intermediate network-layer nodes.
Minimum Overhead: For a simple network expected to exchange
relatively small bundles, the Segment MRU can be set to be
identical to the Transfer MRU, which indicates that all transfers
can be sent with a single data segment (i.e., no actual
segmentation). If the network is closed and all transmitters are
known to follow a single-segment transfer policy, then receivers
can avoid the necessity of segment reassembly. Because this CL
operates over a TCP stream, which suffers from a form of head-of-
queue blocking between messages, while one entity is transmitting
a single XFER_SEGMENT message it is not able to transmit any
XFER_ACK or XFER_REFUSE messages for any associated received
transfers.
Predictable Message Sizing: In situations where the maximum message
size is desired to be well controlled, the Segment MRU can be set
to the largest acceptable size (the message size less the
XFER_SEGMENT header size) and transmitters can always segment a
transfer into maximum-size chunks no larger than the Segment MRU.
This guarantees that any single XFER_SEGMENT will not monopolize
the TCP stream for too long, which would prevent outgoing XFER_ACK
and XFER_REFUSE messages associated with received transfers.
Dynamic Segmentation: Even after negotiation of a Segment MRU for
each receiving entity, the actual transfer segmentation only needs
to guarantee that any individual segment is no larger than that
MRU. In a situation where TCP throughput is dynamic, the transfer
segmentation size can also be dynamic in order to control message
transmission duration.
Many other policies can be established in a TCPCL network between the
two extremes of minimum overhead (large MRU, single segment) and
predictable message sizing (small MRU, highly segmented). Different
policies can be applied to each transfer stream to and from any
particular entity. Additionally, future session extension and
transfer extension types can apply further nuance to transfer
policies and policy negotiation.
3.7. Example Message Exchange
Figure 15 depicts the protocol exchange for a simple session, showing
the session establishment and the transmission of a single bundle
split into three data segments (of lengths "L1", "L2", and "L3") from
Entity A to Entity B.
Note that the sending entity can transmit multiple XFER_SEGMENT
messages without waiting for the corresponding XFER_ACK responses.
This enables pipelining of messages on a transfer stream. Although
this example only demonstrates a single bundle transmission, it is
also possible to pipeline multiple XFER_SEGMENT messages for
different bundles without necessarily waiting for XFER_ACK messages
to be returned for each one. However, interleaving data segments
from different bundles is not allowed.
No errors or rejections are shown in this example.
Entity A Entity B
======== ========
+-------------------------+
| Open TCP Connection | -> +-------------------------+
+-------------------------+ <- | Accept Connection |
+-------------------------+
+-------------------------+
| Contact Header | -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+
+-------------------------+
| XFER_SEGMENT (start) | ->
| Transfer ID [I1] |
| Length [L1] |
| Bundle Data 0..(L1-1) |
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT | -> <- | XFER_ACK (start) |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L2] | | Length [L1] |
|Bundle Data L1..(L1+L2-1)| +-------------------------+
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT (end) | -> <- | XFER_ACK |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L3] | | Length [L1+L2] |
|Bundle Data | +-------------------------+
| (L1+L2)..(L1+L2+L3-1)|
+-------------------------+
+-------------------------+
<- | XFER_ACK (end) |
| Transfer ID [I1] |
| Length [L1+L2+L3] |
+-------------------------+
+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+
+-------------------------+ +-------------------------+
| TCP Close | -> <- | TCP Close |
+-------------------------+ +-------------------------+
Figure 15: An Example of the Flow of Protocol Messages on a
Single TCP Session between Two Entities
4. Session Establishment
For bundle transmissions to occur using the TCPCL, a TCPCL session
MUST first be established between communicating entities. It is up
to the implementation to decide how and when session setup is
triggered. For example, some sessions can be opened proactively and
maintained for as long as is possible given the network conditions,
while other sessions will be opened only when there is a bundle that
is queued for transmission and the routing algorithm selects a
certain next-hop node.
4.1. TCP Connection
To establish a TCPCL session, an entity MUST first establish a TCP
connection with the intended peer entity, typically by using the
services provided by the operating system. Destination port number
4556 has been assigned by IANA as the registered port number for the
TCPCL; see Section 8.1. Other destination port numbers MAY be used
per local configuration. Determining a peer's destination port
number (if different from the registered TCPCL port number) is left
up to the implementation. Any source port number MAY be used for
TCPCL sessions. Typically, an operating system assigned number in
the TCP Ephemeral range (49152-65535) is used.
If the entity is unable to establish a TCP connection for any reason,
then it is an implementation matter to determine how to handle the
connection failure. An entity MAY decide to reattempt to establish
the connection. If it does so, it MUST NOT overwhelm its target with
repeated connection attempts. Therefore, the entity MUST NOT retry
the connection setup earlier than some delay time from the last
attempt, and it SHOULD use a (binary) exponential backoff mechanism
to increase this delay in the case of repeated failures. The upper
limit on a reattempt backoff is implementation defined but SHOULD be
no longer than one minute (60 seconds) before signaling to the BPA
that a connection cannot be made.
Once a TCP connection is established, the active entity SHALL
immediately transmit its Contact Header. The passive entity SHALL
wait for the active entity's Contact Header. Upon reception of a
Contact Header, the passive entity SHALL transmit its Contact Header.
If either entity does not receive a Contact Header after some
implementation-defined time duration after the TCP connection is
established, the waiting entity SHALL close the TCP connection.
Entities SHOULD choose a Contact Header reception timeout interval no
longer than one minute (60 seconds). The ordering of the Contact
Header exchange allows the passive entity to avoid allocating
resources to a potential TCPCL session until after a valid Contact
Header has been received from the active entity. This ordering also
allows the passive peer to adapt to alternate TCPCL protocol
versions.
The format of the Contact Header is described in Section 4.2.
Because the TCPCL protocol version in use is part of the initial
Contact Header, entities using TCPCL version 4 can coexist on a
network with entities using earlier TCPCL versions (with some
negotiation needed for interoperation, as described in Section 4.3).
Within this specification, when an entity is said to "close" a TCP
connection the entity SHALL use the TCP FIN mechanism and not the RST
mechanism. However, either mechanism, when received, will cause a
TCP connection to become closed.
4.2. Contact Header
This section describes the format of the Contact Header and the
meaning of its fields.
If the entity is configured to enable the exchange of messages
according to TLS 1.3 [RFC 8446] or any successors that are compatible
with that TLS ClientHello, the CAN_TLS flag within its Contact Header
SHALL be set to 1. The RECOMMENDED policy is to enable TLS for all
sessions, even if security policy does not allow or require
authentication. This follows the "opportunistic security" model
specified in [RFC 7435], though an active attacker could interfere
with the exchange in such cases (see Section 7.4).
Upon receipt of the Contact Header, both entities perform the
validation and negotiation procedures defined in Section 4.3. After
receiving the Contact Header from the other entity, either entity MAY
refuse the session by sending a SESS_TERM message with an appropriate
reason code.
The format for the Contact Header is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+---------------+---------------+---------------+---------------+
| magic='dtn!' |
+---------------+---------------+---------------+---------------+
| Version | Flags |
+---------------+---------------+
Figure 16: Contact Header Format
See Section 4.3 for details on the use of each of these Contact
Header fields.
The fields of the Contact Header are as follows:
magic: A four-octet field that always contains the octet sequence
0x64 0x74 0x6E 0x21, i.e., the text string "dtn!" in US-ASCII (and
UTF-8).
Version: A one-octet field value containing the value 4 (current
version of the TCPCL protocol).
Flags: A one-octet field of single-bit flags, interpreted according
to the descriptions in Table 1. All reserved header flag bits
SHALL be set to 0 by the sender. All reserved header flag bits
SHALL be ignored by the receiver.
+==========+========+===========================================+
| Name | Code | Description |
+==========+========+===========================================+
| CAN_TLS | 0x01 | If this bit is set, it indicates that the |
| | | sending peer has enabled TLS security. |
+----------+--------+-------------------------------------------+
| Reserved | others | |
+----------+--------+-------------------------------------------+
Table 1: Contact Header Flags
4.3. Contact Validation and Negotiation
Upon reception of the Contact Header, each entity follows the
following procedures to ensure the validity of the TCPCL session and
to negotiate values for the session parameters.
If the "magic string" is not present or is not valid, the connection
MUST be terminated. The intent of the magic string is to provide
some protection against an inadvertent TCP connection by a different
protocol than the one described in this document. To prevent a flood
of repeated connections from a misconfigured application, a passive
entity MAY deny new TCP connections from a specific peer address for
a period of time after one or more connections fail to provide a
decodable Contact Header.
The first negotiation attempts to determine which TCPCL protocol
version to use. The active entity always sends its Contact Header
first and waits for a response from the passive entity. During
contact initiation, the active TCPCL entity SHALL send the highest
TCPCL protocol version on a first session attempt for a TCPCL peer.
If the active entity receives a Contact Header with a lower protocol
version than the one sent earlier on the TCP connection, the TCP
connection SHALL be closed. If the active entity receives a
SESS_TERM message with a reason code of "Version mismatch", that
entity MAY attempt further TCPCL sessions with the peer using earlier
protocol version numbers in decreasing order. Managing multi-TCPCL-
session state such as this is an implementation matter.
If the passive entity receives a Contact Header containing a version
that is not a version of the TCPCL protocol that the entity
implements, then the entity SHALL send its Contact Header and
immediately terminate the session with a reason code of "Version
mismatch". If the passive entity receives a Contact Header with a
version that is lower than the latest version of the protocol that
the entity implements, the entity MAY either terminate the session
(with a reason code of "Version mismatch") or adapt its operation to
conform to the older version of the protocol. The decision of
version fallback is an implementation matter.
The negotiated contact parameters defined by this specification are
described in the following paragraphs.
TCPCL Version: Both Contact Headers of a successful contact
negotiation have identical TCPCL version numbers as described
above. Only upon response of a Contact Header from the passive
entity is the TCPCL protocol version established and session
negotiation begun.
Enable TLS: Negotiation of the Enable TLS parameter is performed by
taking the logical AND of the two Contact Headers' CAN_TLS flags.
A local security policy is then applied to determine whether the
negotiated value of Enable TLS is acceptable. A reasonable
security policy would require or disallow the use of TLS,
depending upon the desired network flows. The RECOMMENDED policy
is to require TLS for all sessions, even if security policy does
not allow or require authentication. Because this state is
negotiated over an unsecured medium, there is a risk of TLS
Stripping as described in Section 7.4.
If the Enable TLS state is unacceptable, the entity SHALL
terminate the session with a reason code of "Contact Failure".
Note that this "Contact Failure" reason is different than a
failure of a TLS handshake or TLS authentication after an agreed-
upon and acceptable Enable TLS state. If the negotiated Enable
TLS value is "true" and acceptable, then the TLS negotiation
feature described in Section 4.4 begins immediately following the
Contact Header exchange.
4.4. Session Security
This version of the TCPCL protocol supports establishing a TLS
session within an existing TCP connection. When TLS is used within
the TCPCL, it affects the entire session. Once TLS is established,
there is no mechanism available to downgrade the TCPCL session to
non-TLS operation.
Once established, the lifetime of a TLS connection SHALL be bound to
the lifetime of the underlying TCP connection. Immediately prior to
actively ending a TLS connection after TCPCL session termination, the
peer that sent the original (non-reply) SESS_TERM message SHOULD
follow the closure alert procedure provided in [RFC 8446] to cleanly
terminate the TLS connection. Because each TCPCL message is either
fixed length or self-indicates its length, the lack of a TLS closure
alert will not cause data truncation or corruption.
Subsequent TCPCL session attempts to the same passive entity MAY
attempt to use the TLS session resumption feature. There is no
guarantee that the passive entity will accept the request to resume a
TLS session, and the active entity cannot assume any resumption
outcome.
4.4.1. Entity Identification
The TCPCL uses TLS for certificate exchange in both directions to
identify each entity and to allow each entity to authenticate its
peer. Each certificate can potentially identify multiple entities,
and there is no problem using such a certificate as long as the
identifiers are sufficient to meet authentication policy (as
described in later sections) for the entity that presents it.
Because the PKIX environment of each TCPCL entity is likely not
controlled by the certificate end users (see Section 3.4), the TCPCL
defines a prioritized list of what a certificate can identify
regarding a TCPCL entity:
Node ID: The ideal certificate identity is the node ID of the entity
using the NODE-ID, as defined below. When the node ID is
identified, there is no need for any lower-level identification to
be present (though it can still be present, and if so it is also
validated).
DNS Name: If CA policy forbids a certificate to contain an arbitrary
NODE-ID but allows a DNS-ID to be identified, then one or more
stable DNS names can be identified in the certificate. The use of
wildcard DNS-IDs is discouraged due to the complex rules for
matching and dependence on implementation support for wildcard
matching (see Section 6.4.3 of [RFC 6125]).
Network Address: If no stable DNS name is available but a stable
network address is available and CA policy allows a certificate to
contain an IPADDR-ID (as defined below), then one or more network
addresses can be identified in the certificate.
This specification defines a NODE-ID of a certificate as being the
subjectAltName entry of type otherName with a name form of BundleEID
(see Section 4.4.2.1) and a value limited to a node ID. An entity
SHALL ignore any entry of type otherName with a name form of
BundleEID and a value that is some URI other than a node ID. The
NODE-ID is similar to the URI-ID as defined in [RFC 6125] but is
restricted to a node ID rather than a URI with a qualified-name
authority part. Unless specified otherwise by the definition of the
URI scheme being authenticated, URI matching of a NODE-ID SHALL use
the URI comparison logic provided in [RFC 3986] and scheme-based
normalization of those schemes specified in [RFC 9171]. A URI scheme
can refine this "exact match" logic with rules regarding how node IDs
within that scheme are to be compared with the certificate-
authenticated NODE-ID.
This specification reuses the DNS-ID definition in Section 1.8 of
[RFC 6125], which is the subjectAltName entry of type dNSName whose
value is encoded according to [RFC 5280].
This specification defines an IPADDR-ID of a certificate as being the
subjectAltName entry of type iPAddress whose value is encoded
according to [RFC 5280].
4.4.2. Certificate Profile for the TCPCL
All end-entity certificates used by a TCPCL entity SHALL conform to
[RFC 5280], or any updates or successors to that profile. When an
end-entity certificate is supplied, the full certification chain
SHOULD be included unless security policy indicates that is
unnecessary. An entity SHOULD omit the root CA certificate (the last
item of the chain) when sending a certification chain, as the
recipient already has the root CA to anchor its validation.
The TCPCL requires version 3 certificates due to the extensions used
by this profile. TCPCL entities SHALL reject as invalid version 1
and version 2 end-entity certificates.
TCPCL entities SHALL accept certificates that contain an empty
Subject field or contain a Subject without a Common Name. Identity
information in end-entity certificates is contained entirely in the
subjectAltName extension as defined in Section 4.4.1 and discussed in
the paragraphs below.
All end-entity and CA certificates used for the TCPCL SHOULD contain
both a subject key identifier and an authority key identifier
extension in accordance with [RFC 5280]. TCPCL entities SHOULD NOT
rely on either a subject key identifier or an authority key
identifier being present in any received certificate. Including key
identifiers simplifies the work of an entity that needs to assemble a
certification chain.
Unless prohibited by CA policy, a TCPCL end-entity certificate SHALL
contain a NODE-ID that authenticates the node ID of the peer. When
assigned one or more stable DNS names, a TCPCL end-entity certificate
SHOULD contain a DNS-ID that authenticates those (fully qualified)
names. When assigned one or more stable network addresses, a TCPCL
end-entity certificate MAY contain an IPADDR-ID that authenticates
those addresses.
When allowed by CA policy, a Bundle Protocol Security (BPSec; see
[RFC 9172]) end-entity certificate SHOULD contain a PKIX Extended Key
Usage (EKU) extension in accordance with Section 4.2.1.12 of
[RFC 5280]. When the PKIX EKU extension is present, it SHOULD contain
the key purpose id-kp-bundleSecurity (see Section 4.4.2.1). Although
not specifically required by the TCPCL, some networks or TLS
implementations assume that id-kp-clientAuth and id-kp-serverAuth
need to be used for the client side and the server side of TLS
authentication, respectively. For interoperability, a TCPCL end-
entity certificate MAY contain an EKU with both id-kp-clientAuth and
id-kp-serverAuth values.
When allowed by CA policy, a TCPCL end-entity certificate SHOULD
contain a PKIX key usage extension in accordance with Section 4.2.1.3
of [RFC 5280]. The PKIX key usage bit that is consistent with TCPCL
security using TLS 1.3 is digitalSignature. The specific algorithms
used during the TLS handshake will determine which of those key uses
are exercised. Earlier versions of TLS can mandate the use of the
keyEncipherment bit or the keyAgreement bit.
When allowed by CA policy, a TCPCL end-entity certificate SHOULD
contain an Online Certificate Status Protocol (OCSP) URI within an
authority information access extension in accordance with
Section 4.2.2.1 of [RFC 5280].
4.4.2.1. PKIX OID Allocations
This document defines a PKIX Other Name Form identifier, id-on-
bundleEID, in Appendix B; this identifier can be used as the type-id
in a subjectAltName entry of type otherName. The BundleEID value
associated with the otherName type-id id-on-bundleEID SHALL be a URI,
encoded as an IA5String, with a scheme that is present in the IANA
"Bundle Protocol URI Scheme Types" registry [IANA-BUNDLE]. Although
this Other Name Form allows any endpoint ID to be present, the NODE-
ID defined in Section 4.4.1 limits its use to contain only a node ID.
This document defines a PKIX EKU key purpose, id-kp-bundleSecurity,
in Appendix B; this purpose can be used to restrict a certificate's
use. The id-kp-bundleSecurity purpose can be combined with other
purposes in the same certificate.
4.4.3. TLS Handshake
The use of TLS is negotiated via the Contact Header, as described in
Section 4.3. After negotiating an Enable TLS parameter of "true",
and before any other TCPCL messages are sent within the session, the
session entities SHALL begin a TLS handshake in accordance with
[RFC 8446]. By convention, this protocol uses the entity that
initiated the underlying TCP connection (the active peer) as the
"client" role of the TLS handshake request.
The TLS handshake, if it occurs, is considered to be part of the
contact negotiation before the TCPCL session itself is established.
Specifics regarding exposure of sensitive data are discussed in
Section 7.
The parameters within each TLS negotiation are implementation
dependent but any TCPCL entity SHALL follow all recommended practices
specified in BCP 195 [RFC 7525], or any updates or successors that
become part of BCP 195. Within each TLS handshake, the following
requirements apply (using the rough order in which they occur):
ClientHello: When a resolved DNS name was used to establish the TCP
connection, the TLS ClientHello SHOULD include a "server_name"
extension in accordance with [RFC 6066]. When present, the
server_name extension SHALL contain a "HostName" value taken from
the DNS name (of the passive entity) that was resolved.
| Note: The "HostName" in the server_name extension is the
| network name for the passive entity, not the node ID of that
| entity.
Server Certificate: The passive entity SHALL supply a certificate
within the TLS handshake to allow authentication of its side of
the session. The supplied end-entity certificate SHALL conform to
the profile described in Section 4.4.2. The passive entity MAY
use the SNI DNS name to choose an appropriate server-side
certificate that authenticates that DNS name.
Certificate Request: During the TLS handshake, the passive entity
SHALL request a client-side certificate.
Client Certificate: The active entity SHALL supply a certificate
chain within the TLS handshake to allow authentication of its side
of the session. The supplied end-entity certificate SHALL conform
to the profile described in Section 4.4.2.
If a TLS handshake cannot negotiate a TLS connection, both entities
of the TCPCL session SHALL close the TCP connection. At this point,
the TCPCL session has not yet been established, so there is no TCPCL
session to terminate.
After a TLS connection is successfully established, the active entity
SHALL send a SESS_INIT message to begin session negotiation. This
session negotiation and all subsequent messaging are secured.
4.4.4. TLS Authentication
Using PKIX certificates exchanged during the TLS handshake, each of
the entities can authenticate a peer node ID directly or authenticate
the peer DNS name or network address. The logic for handling
certificates and certificate data is separated into the following
phases:
1. Validating the certification path from the end-entity certificate
up to a trusted root CA.
2. Validating the EKU and other properties of the end-entity
certificate.
3. Authenticating identities from a valid end-entity certificate.
4. Applying security policy to the result of each identity type
authentication.
The result of validating a peer identity (see Section 4.4.1) against
one or more types of certificate claims is one of the following:
Absent: Indicating that no such claims are present in the
certificate and the identity cannot be authenticated.
Success: Indicating that one or more such claims are present and at
least one matches the peer identity value.
Failure: Indicating that one or more such claims are present and
none match the peer identity.
4.4.4.1. Certificate Path and Purpose Validation
For any peer end-entity certificate received during the TLS
handshake, the entity SHALL perform the certification path validation
described in [RFC 5280] up to one of the entity's trusted CA
certificates. If enabled by local policy, the entity SHALL perform
an OCSP check of each certificate providing OCSP authority
information in accordance with [RFC 6960]. If certificate validation
fails or if security policy disallows a certificate for any reason,
the entity SHALL fail the TLS handshake with a "bad_certificate"
alert. Leaving out part of the certification chain can cause the
entity to fail to validate a certificate if the certificates that
were left out are unknown to the entity (see Section 7.6).
For the end-entity peer certificate received during the TLS
handshake, the entity SHALL apply security policy to the key usage
extension (if present) and EKU extension (if present) in accordance
with Sections 4.2.1.12 and 4.2.1.3 of [RFC 5280], respectively, and
with the profile discussed in Section 4.4.2 of this document.
4.4.4.2. Network-Level Authentication
Either during or immediately after the TLS handshake, each entity, if
required by security policy, SHALL validate the following certificate
identifiers together in accordance with Section 6 of [RFC 6125]:
* If the active entity resolved a DNS name (of the passive entity)
in order to initiate the TCP connection, that DNS name SHALL be
used as a DNS-ID reference identifier.
* The IP address of the other side of the TCP connection SHALL be
used as an IPADDR-ID reference identifier.
If the network-level identifier's authentication result is Failure or
if the result is Absent and security policy requires an authenticated
network-level identifier, the entity SHALL terminate the session
(with a reason code of "Contact Failure").
4.4.4.3. Node ID Authentication
Immediately before session parameter negotiation, each entity, if
required by security policy, SHALL validate the certificate NODE-ID
in accordance with Section 6 of [RFC 6125] using the node ID of the
peer's SESS_INIT message as the NODE-ID reference identifier. If the
NODE-ID validation result is Failure or if the result is Absent and
security policy requires an authenticated node ID, the entity SHALL
terminate the session (with a reason code of "Contact Failure").
4.4.5. Policy Recommendations
A RECOMMENDED security policy encompasses the following:
* enabling the use of OCSP checking during the TLS handshake.
* instructing that, if an EKU extension is present, the extension
needs to contain id-kp-bundleSecurity (Section 4.4.2.1) to be
usable with TCPCL security.
* requiring a validated node ID (Section 4.4.4.3) and ignoring any
network-level identifier (Section 4.4.4.2).
This policy relies on and informs the certificate requirements
provided in Section 4.4.3. This policy assumes that a DTN-aware CA
(see Section 3.4) will only issue a certificate for a node ID when it
has verified that the private key holder actually controls the bundle
node; this is needed to avoid the threat identified in Section 7.9.
This policy requires that a certificate contain a NODE-ID and allows
the certificate to also contain network-level identifiers. A
tailored policy on a more controlled network could relax the
requirement on node ID validation and allow just network-level
identifiers to authenticate a peer.
4.4.6. Example TLS Initiation
A summary of a typical TLS initiation is shown in the sequence in
Figure 17 below. In this example, the active peer terminates the
session, but termination can be initiated from either peer.
Entity A Entity B
active peer passive peer
+-------------------------+
| Open TCP Connection | -> +-------------------------+
+-------------------------+ <- | Accept Connection |
+-------------------------+
+-------------------------+
| Contact Header | -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+
DNS-ID and IPADDR-ID authentication occurs.
Secured TCPCL messaging can begin.
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+
NODE-ID authentication occurs.
Session is established, transfers can begin.
+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+
+-------------------------+
| TLS Closure Alert | -> +-------------------------+
+-------------------------+ <- | TLS Closure Alert |
+-------------------------+
+-------------------------+ +-------------------------+
| TCP Close | -> <- | TCP Close |
+-------------------------+ +-------------------------+
Figure 17: A Simple Visual Example of TCPCL TLS Establishment
between Two Entities
4.5. Message Header
After the initial exchange of a Contact Header and (if TLS is
negotiated to be used) the TLS handshake, all messages transmitted
over the session are identified by a one-octet header with the
following structure:
0 1 2 3 4 5 6 7
+---------------+
| Message Type |
+---------------+
Figure 18: Format of the Message Header
The Message Header contains the following field:
Message Type: Indicates the type of the message as per Table 2
below. Encoded values are listed in Section 8.5.
+==============+======+=====================================+
| Name | Code | Description |
+==============+======+=====================================+
| SESS_INIT | 0x07 | Contains the session parameter |
| | | inputs from one of the entities, as |
| | | described in Section 4.6. |
+--------------+------+-------------------------------------+
| SESS_TERM | 0x05 | Indicates that one of the entities |
| | | participating in the session wishes |
| | | to cleanly terminate the session, |
| | | as described in Section 6.1. |
+--------------+------+-------------------------------------+
| XFER_SEGMENT | 0x01 | Indicates the transmission of a |
| | | segment of bundle data, as |
| | | described in Section 5.2.2. |
+--------------+------+-------------------------------------+
| XFER_ACK | 0x02 | Acknowledges reception of a data |
| | | segment, as described in |
| | | Section 5.2.3. |
+--------------+------+-------------------------------------+
| XFER_REFUSE | 0x03 | Indicates that the transmission of |
| | | the current bundle SHALL be |
| | | stopped, as described in |
| | | Section 5.2.4. |
+--------------+------+-------------------------------------+
| KEEPALIVE | 0x04 | Used to keep the TCPCL session |
| | | active, as described in |
| | | Section 5.1.1. |
+--------------+------+-------------------------------------+
| MSG_REJECT | 0x06 | Contains a TCPCL message rejection, |
| | | as described in Section 5.1.2. |
+--------------+------+-------------------------------------+
Table 2: TCPCL Message Types
4.6. Session Initialization Message (SESS_INIT)
Before a session is established and ready to transfer bundles, the
session parameters are negotiated between the connected entities.
The SESS_INIT message is used to convey the per-entity parameters,
which are used together to negotiate the per-session parameters as
described in Section 4.7.
The format of a SESS_INIT message is shown in Figure 19.
+-----------------------------+
| Message Header |
+-----------------------------+
| Keepalive Interval (U16) |
+-----------------------------+
| Segment MRU (U64) |
+-----------------------------+
| Transfer MRU (U64) |
+-----------------------------+
| Node ID Length (U16) |
+-----------------------------+
| Node ID Data (variable) |
+-----------------------------+
| Session Extension |
| Items Length (U32) |
+-----------------------------+
| Session Extension |
| Items (var.) |
+-----------------------------+
Figure 19: SESS_INIT Format
The fields of the SESS_INIT message are as follows:
Keepalive Interval: A 16-bit unsigned integer indicating the minimum
interval, in seconds, to negotiate as the Session Keepalive using
the method described in Section 4.7.
Segment MRU: A 64-bit unsigned integer indicating the largest
allowable single-segment data payload size to be received in this
session. Any XFER_SEGMENT sent to this peer SHALL have a data
payload no longer than the peer's Segment MRU. The two entities
of a single session MAY have different Segment MRUs, and no
relationship between the two is required.
Transfer MRU: A 64-bit unsigned integer indicating the largest
allowable total-bundle data size to be received in this session.
Any bundle transfer sent to this peer SHALL have a Total Bundle
Length payload no longer than the peer's Transfer MRU. This value
can be used to perform proactive bundle fragmentation. The two
entities of a single session MAY have different Transfer MRUs, and
no relationship between the two is required.
Node ID Length and Node ID Data: Together, these fields represent a
variable-length text string. The Node ID Length is a 16-bit
unsigned integer indicating the number of octets of Node ID Data
to follow. A zero-length node ID SHALL be used to indicate the
lack of a node ID rather than a truly empty node ID. This case
allows an entity to avoid exposing node ID information on an
untrusted network. A non-zero-length Node ID Data SHALL contain
the UTF-8 encoded node ID of the entity that sent the SESS_INIT
message. Every node ID SHALL be a URI consistent with the
requirements in [RFC 3986] and the URI schemes of the IANA "Bundle
Protocol URI Scheme Types" registry [IANA-BUNDLE]. The node ID
itself can be authenticated as described in Section 4.4.4.
Session Extension Items Length and Session Extension Items list:
Together, these fields represent protocol extension data not
defined by this specification. The Session Extension Items Length
is the total number of octets to follow that are used to encode
the Session Extension Items list. The encoding of each Session
Extension Item is within a consistent data container as described
in Section 4.8. The full set of Session Extension Items apply for
the duration of the TCPCL session to follow. The order and
multiplicity of these Session Extension Items are significant, as
defined in the associated type specification(s). If the content
of the Session Extension Items list disagrees with the Session
Extension Items Length (e.g., the last item claims to use more or
fewer octets than are indicated in the Session Extension Items
Length), the reception of the SESS_INIT is considered to have
failed.
If an entity receives a peer node ID that is not authenticated (by
the procedure described in Section 4.4.4.3), that node ID SHOULD NOT
be used by a BPA for any discovery or routing functions. Trusting an
unauthenticated node ID can lead to the threat described in
Section 7.9.
When the active entity initiates a TCPCL session, it is likely based
on routing information that binds a node ID to CL parameters used to
initiate the session. If the active entity receives a SESS_INIT with
a different node ID than was intended for the TCPCL session, the
session MAY be allowed to be established. If allowed, such a session
SHALL be associated with the node ID provided in the SESS_INIT
message rather than any intended value.
4.7. Session Parameter Negotiation
An entity calculates the parameters for a TCPCL session by
negotiating the values from its own preferences (conveyed by the
SESS_INIT it sent to the peer) with the preferences of the peer
entity (expressed in the SESS_INIT that it received from the peer).
The negotiated parameters defined by this specification are described
in the following paragraphs.
Transfer MTU and Segment MTU: The Maximum Transmission Unit (MTU)
for whole transfers and individual segments is identical to the
Transfer MRU and Segment MRU, respectively, of the received
SESS_INIT message. A transmitting peer can send individual
segments with any size smaller than the Segment MTU, depending on
local policy, dynamic network conditions, etc. Determining the
size of each transmitted segment is an implementation matter. If
either the Transfer MRU or Segment MRU is unacceptable, the entity
SHALL terminate the session with a reason code of "Contact
Failure".
Session Keepalive: Negotiation of the Session Keepalive parameter is
performed by taking the minimum of the two Keepalive Interval
values from the two SESS_INIT messages. The Session Keepalive
Interval is a parameter for the behavior described in
Section 5.1.1. If the Session Keepalive Interval is unacceptable,
the entity SHALL terminate the session with a reason code of
"Contact Failure".
| Note: A negotiated Session Keepalive of zero indicates that
| KEEPALIVEs are disabled.
Once this process of parameter negotiation is completed, this
protocol defines no additional mechanism to change the parameters of
an established session; to effect such a change, the TCPCL session
MUST be terminated and a new session established.
4.8. Session Extension Items
Each of the Session Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in Figure 20.
The fields of the Session Extension Item are as follows:
Item Flags: A one-octet field containing generic bit flags related
to the Item, which are listed in Table 3. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver. If a TCPCL entity
receives a Session Extension Item with an unknown Item Type and
the CRITICAL flag set to 1, the entity SHALL terminate the TCPCL
session with a SESS_TERM reason code of "Contact Failure". If the
CRITICAL flag is 0, an entity SHALL skip over and ignore any item
with an unknown Item Type.
Item Type: A 16-bit unsigned integer field containing the type of
the extension item. This specification does not define any
extension types directly but does create an IANA registry for such
codes (see Section 8.3).
Item Length: A 16-bit unsigned integer field containing the number
of Item Value octets to follow.
Item Value: A variable-length data field that is interpreted
according to the associated Item Type. This specification places
no restrictions on an extension's use of available Item Value
data. Extension specifications SHOULD avoid the use of large data
lengths, as no bundle transfers can begin until the full extension
data is sent.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+
Figure 20: Session Extension Item Format
+==========+========+==================================+
| Name | Code | Description |
+==========+========+==================================+
| CRITICAL | 0x01 | If this bit is set, it indicates |
| | | that the receiving peer must |
| | | handle the extension item. |
+----------+--------+----------------------------------+
| Reserved | others | |
+----------+--------+----------------------------------+
Table 3: Session Extension Item Flags
5. Established Session Operation
This section describes the protocol operation for the duration of an
established session, including the mechanism for transmitting bundles
over the session.
5.1. Upkeep and Status Messages
5.1.1. Session Upkeep (KEEPALIVE)
The protocol includes a provision for transmission of KEEPALIVE
messages over the TCPCL session to help determine if the underlying
TCP connection has been disrupted.
As described in Section 4.7, a negotiated parameter of each session
is the Session Keepalive Interval. If the negotiated Session
Keepalive is zero (i.e., one or both SESS_INIT messages contain a
zero Keepalive Interval), then the keepalive feature is disabled.
There is no logical minimum value for the Keepalive Interval (within
the minimum imposed by the positive-value encoding), but when used
for many sessions on an open, shared network, a short interval could
lead to excessive traffic. For shared network use, entities SHOULD
choose a Keepalive Interval no shorter than 30 seconds. There is no
logical maximum value for the Keepalive Interval (within the maximum
imposed by the fixed-size encoding), but an idle TCP connection is
liable for closure by the host operating system if the keepalive time
is longer than tens of minutes. Entities SHOULD choose a Keepalive
Interval no longer than 10 minutes (600 seconds).
The chosen Keepalive Interval SHOULD NOT be too short, as TCP
retransmissions may occur in the case of packet loss. Those will
have to be triggered by a timeout (TCP retransmission timeout (RTO)),
which is dependent on the measured RTT for the TCP connection so that
KEEPALIVE messages can experience noticeable latency.
The format of a KEEPALIVE message is a one-octet Message Type code of
KEEPALIVE (as described in Table 2) with no additional data. Both
sides SHALL send a KEEPALIVE message whenever the negotiated interval
has elapsed with no transmission of any message (KEEPALIVE or other).
If no message (KEEPALIVE or other) has been received in a session
after some implementation-defined time duration, then the entity
SHALL terminate the session by transmitting a SESS_TERM message (as
described in Section 6.1) with a reason code of "Idle timeout". If
configurable, the idle timeout duration SHOULD be no shorter than
twice the Keepalive Interval. If not configurable, the idle timeout
duration SHOULD be exactly twice the Keepalive Interval.
5.1.2. Message Rejection (MSG_REJECT)
This message type is not expected to be seen in a well-functioning
session. Its purpose is to aid in troubleshooting bad entity
behavior by allowing the peer to observe why an entity is not
responding as expected to its messages.
If a TCPCL entity receives a message type that is unknown to it
(possibly due to an unhandled protocol version mismatch or an
incorrectly negotiated session extension that defines a new message
type), the entity SHALL send a MSG_REJECT message with a reason code
of "Message Type Unknown" and close the TCP connection. If a TCPCL
entity receives a message type that is known but is inappropriate for
the negotiated session parameters (possibly due to an incorrectly
negotiated session extension), the entity SHALL send a MSG_REJECT
message with a reason code of "Message Unsupported". If a TCPCL
entity receives a message that is inappropriate for the current
session state (e.g., a SESS_INIT after the session has already been
established or a XFER_ACK message with an unknown Transfer ID), the
entity SHALL send a MSG_REJECT message with a reason code of "Message
Unexpected".
The format of a MSG_REJECT message is shown in Figure 21.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+
Figure 21: Format of MSG_REJECT Messages
The fields of the MSG_REJECT message are as follows:
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 4.
Rejected Message Header: The Rejected Message Header is a copy of
the Message Header to which the MSG_REJECT message is sent as a
response.
+==============+======+========================================+
| Name | Code | Description |
+==============+======+========================================+
| Message Type | 0x01 | A message was received with a Message |
| Unknown | | Type code unknown to the TCPCL entity. |
+--------------+------+----------------------------------------+
| Message | 0x02 | A message was received, but the TCPCL |
| Unsupported | | entity cannot comply with the message |
| | | contents. |
+--------------+------+----------------------------------------+
| Message | 0x03 | A message was received while the |
| Unexpected | | session is in a state in which the |
| | | message is not expected. |
+--------------+------+----------------------------------------+
Table 4: MSG_REJECT Reason Codes
5.2. Bundle Transfer
All of the messages discussed in this section are directly associated
with transferring a bundle between TCPCL entities.
A single TCPCL transfer results in the exchange of a bundle (handled
by the convergence layer as opaque data) between two entities. In
the TCPCL, a transfer is accomplished by dividing a single bundle up
into "segments" based on the receiving-side Segment MRU, which is
defined in Section 4.6. The choice of the length to use for segments
is an implementation matter, but each segment MUST NOT be larger than
the receiving entity's Segment MRU. The first segment for a bundle
is indicated by the START flag, and the last segment is indicated by
the END flag.
A single transfer (and, by extension, a single segment) SHALL NOT
contain data of more than a single bundle. This requirement is
imposed on the agent using the TCPCL, rather than on the TCPCL
itself.
If multiple bundles are transmitted on a single TCPCL connection,
they MUST be transmitted consecutively, without the interleaving of
segments from multiple bundles.
5.2.1. Bundle Transfer ID
Each of the bundle transfer messages contains a Transfer ID, which is
used to correlate messages (from both sides of a transfer) for each
bundle. A Transfer ID does not attempt to address uniqueness of the
bundle data itself and is not related to such concepts as bundle
fragmentation. Each invocation of the TCPCL by the BPA, requesting
transmission of a bundle (fragmentary or otherwise), results in the
initiation of a single TCPCL transfer. Each transfer entails the
sending of a sequence of some number of XFER_SEGMENT and XFER_ACK
messages; all are correlated by the same Transfer ID. The sending
entity originates a Transfer ID, and the receiving entity uses that
same Transfer ID in acknowledgments.
Transfer IDs from each entity SHALL be unique within a single TCPCL
session. Upon exhaustion of the entire 64-bit Transfer ID space, the
sending entity SHALL terminate the session with a SESS_TERM reason
code of "Resource Exhaustion". For bidirectional bundle transfers, a
TCPCL entity SHOULD NOT rely on any relationship between Transfer IDs
originating from each side of the TCPCL session.
Although there is not a strict requirement for initial Transfer ID
values or the ordering of Transfer IDs (see Section 7.13), in the
absence of any other mechanism for generating Transfer IDs, an entity
SHALL use the following algorithm: the initial Transfer ID from each
entity is zero, and subsequent Transfer ID values are incremented
from the prior Transfer ID value by one.
5.2.2. Data Transmission (XFER_SEGMENT)
Each bundle is transmitted in one or more data segments. The format
of a XFER_SEGMENT message is shown in Figure 22.
+------------------------------+
| Message Header |
+------------------------------+
| Message Flags (U8) |
+------------------------------+
| Transfer ID (U64) |
+------------------------------+
| Transfer Extension |
| Items Length (U32) |
| (only for START segment) |
+------------------------------+
| Transfer Extension |
| Items (var.) |
| (only for START segment) |
+------------------------------+
| Data length (U64) |
+------------------------------+
| Data contents (octet string) |
+------------------------------+
Figure 22: Format of XFER_SEGMENT Messages
The fields of the XFER_SEGMENT message are as follows:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 5. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being made.
Transfer Extension Items Length and Transfer Extension Items list:
Together, these fields represent protocol extension data for this
specification. The Transfer Extension Items Length and Transfer
Extension Items list SHALL only be present when the START flag is
set to 1 on the message. The Transfer Extension Items Length is
the total number of octets to follow that are used to encode the
Transfer Extension Items list. The encoding of each Transfer
Extension Item is within a consistent data container, as described
in Section 5.2.5. The full set of Transfer Extension Items apply
only to the associated single transfer. The order and
multiplicity of these Transfer Extension Items are significant, as
defined in the associated type specification(s). If the content
of the Transfer Extension Items list disagrees with the Transfer
Extension Items Length (e.g., the last item claims to use more or
fewer octets than are indicated in the Transfer Extension Items
Length), the reception of the XFER_SEGMENT is considered to have
failed.
Data length: A 64-bit unsigned integer indicating the number of
octets in Data contents to follow.
Data contents: The variable-length data payload of the message.
+==========+========+============================================+
| Name | Code | Description |
+==========+========+============================================+
| END | 0x01 | If this bit is set, it indicates that this |
| | | is the last segment of the transfer. |
+----------+--------+--------------------------------------------+
| START | 0x02 | If this bit is set, it indicates that this |
| | | is the first segment of the transfer. |
+----------+--------+--------------------------------------------+
| Reserved | others | |
+----------+--------+--------------------------------------------+
Table 5: XFER_SEGMENT Flags
The flags portion of the message contains two flag values in the two
low-order bits, denoted START and END in Table 5. The START flag
SHALL be set to 1 when transmitting the first segment of a transfer.
The END flag SHALL be set to 1 when transmitting the last segment of
a transfer. In the case where an entire transfer is accomplished in
a single segment, both the START flag and the END flag SHALL be set
to 1.
Once a transfer of a bundle has commenced, the entity MUST only send
segments containing sequential portions of that bundle until it sends
a segment with the END flag set to 1. No interleaving of multiple
transfers from the same entity is possible within a single TCPCL
session. Simultaneous transfers between two entities MAY be achieved
using multiple TCPCL sessions.
5.2.3. Data Acknowledgments (XFER_ACK)
Although the TCP transport provides reliable transfer of data between
transport peers, the typical BSD sockets interface provides no means
to inform a sending application of when the receiving application has
processed some amount of transmitted data. Thus, after transmitting
some data, the TCPCL needs an additional mechanism to determine
whether the receiving agent has successfully received and fully
processed the segment. To this end, the TCPCL protocol provides
feedback messaging whereby a receiving entity transmits
acknowledgments of reception of data segments.
The format of a XFER_ACK message is shown in Figure 23.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Acknowledged length (U64) |
+-----------------------------+
Figure 23: Format of XFER_ACK Messages
The fields of the XFER_ACK message are as follows:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 5. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being acknowledged.
Acknowledged length: A 64-bit unsigned integer indicating the total
number of octets in the transfer that are being acknowledged.
A receiving TCPCL entity SHALL send a XFER_ACK message in response to
each received XFER_SEGMENT message after the segment has been fully
processed. The flags portion of the XFER_ACK header SHALL be set to
match the corresponding XFER_SEGMENT message being acknowledged
(including flags not decodable to the entity). The acknowledged
length of each XFER_ACK contains the sum of the Data length fields of
all XFER_SEGMENT messages received so far in the course of the
indicated transfer. The sending entity SHOULD transmit multiple
XFER_SEGMENT messages without waiting for the corresponding XFER_ACK
responses. This enables pipelining of messages on a transfer stream.
For example, suppose the sending entity transmits four segments of
bundle data with lengths 100, 200, 500, and 1000, respectively.
After receiving the first segment, the entity sends an acknowledgment
of length 100. After the second segment is received, the entity
sends an acknowledgment of length 300. The third and fourth
acknowledgments are of lengths 800 and 1800, respectively.
5.2.4. Transfer Refusal (XFER_REFUSE)
The TCPCL supports a mechanism by which a receiving entity can
indicate to the sender that it does not want to receive the
corresponding bundle. To do so, upon receiving a XFER_SEGMENT
message, the entity MAY transmit a XFER_REFUSE message. As data
segments and acknowledgments can cross on the wire, the bundle that
is being refused SHALL be identified by the Transfer ID of the
refusal.
There is no required relationship between the Transfer MRU of a TCPCL
entity (which is supposed to represent a firm limitation of what the
entity will accept) and the sending of a XFER_REFUSE message. A
XFER_REFUSE can be used in cases where the agent's bundle storage is
temporarily depleted or somehow constrained. A XFER_REFUSE can also
be used after the bundle header or any bundle data is inspected by an
agent and determined to be unacceptable.
A transfer receiver MAY send a XFER_REFUSE message as soon as it
receives any XFER_SEGMENT message. The transfer sender MUST be
prepared for this and MUST associate the refusal with the correct
bundle via the Transfer ID fields.
The TCPCL itself does not have any required behavior related to
responding to a XFER_REFUSE based on its reason code; the refusal is
passed up as an indication to the BPA that the transfer has been
refused. If a transfer refusal has a reason code that is not
decodable to the BPA, the agent SHOULD treat the refusal as having a
reason code of "Unknown".
The format of the XFER_REFUSE message is shown in Figure 24.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
Figure 24: Format of XFER_REFUSE Messages
The fields of the XFER_REFUSE message are as follows:
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 6.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being refused.
+=============+======+==========================================+
| Name | Code | Description |
+=============+======+==========================================+
| Unknown | 0x00 | The reason for refusal is unknown or is |
| | | not specified. |
+-------------+------+------------------------------------------+
| Completed | 0x01 | The receiver already has the complete |
| | | bundle. The sender MAY consider the |
| | | bundle as completely received. |
+-------------+------+------------------------------------------+
| No | 0x02 | The receiver's resources are exhausted. |
| Resources | | The sender SHOULD apply reactive bundle |
| | | fragmentation before retrying. |
+-------------+------+------------------------------------------+
| Retransmit | 0x03 | The receiver has encountered a problem |
| | | that requires the bundle to be |
| | | retransmitted in its entirety. |
+-------------+------+------------------------------------------+
| Not | 0x04 | Some issue with the bundle data or the |
| Acceptable | | transfer extension data was encountered. |
| | | The sender SHOULD NOT retry the same |
| | | bundle with the same extensions. |
+-------------+------+------------------------------------------+
| Extension | 0x05 | A failure processing the Transfer |
| Failure | | Extension Items has occurred. |
+-------------+------+------------------------------------------+
| Session | 0x06 | The receiving entity is in the process |
| Terminating | | of terminating the session. The sender |
| | | MAY retry the same bundle at a later |
| | | time in a different session. |
+-------------+------+------------------------------------------+
Table 6: XFER_REFUSE Reason Codes
The receiver MUST, for each transfer preceding the one to be refused,
have either acknowledged all XFER_SEGMENT messages or refused the
bundle transfer.
The bundle transfer refusal MAY be sent before an entire data segment
is received. If a sender receives a XFER_REFUSE message, the sender
MUST complete the transmission of any partially sent XFER_SEGMENT
message. There is no way to interrupt an individual TCPCL message
partway through sending it. The sender MUST NOT subsequently
commence transmission of any further segments of the refused bundle.
Note, however, that this requirement does not ensure that an entity
will not receive another XFER_SEGMENT for the same bundle after
transmitting a XFER_REFUSE message, since messages can cross on the
wire; if this happens, subsequent segments of the bundle SHALL also
be refused with a XFER_REFUSE message.
| Note: If a bundle transmission is aborted in this way, the
| receiver does not receive a segment with the END flag set to 1
| for the aborted bundle. The beginning of the next bundle is
| identified by the START flag set to 1, indicating the start of
| a new transfer, and with a distinct Transfer ID value.
5.2.5. Transfer Extension Items
Each of the Transfer Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in Figure 25.
The fields of the Transfer Extension Item are as follows:
Item Flags: A one-octet field containing generic bit flags related
to the Item, which are listed in Table 7. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver. If a TCPCL entity
receives a Transfer Extension Item with an unknown Item Type and
the CRITICAL flag is 1, the entity SHALL refuse the transfer with
a XFER_REFUSE reason code of "Extension Failure". If the CRITICAL
flag is 0, an entity SHALL skip over and ignore any item with an
unknown Item Type.
Item Type: A 16-bit unsigned integer field containing the type of
the extension item. This specification creates an IANA registry
for such codes (see Section 8.4).
Item Length: A 16-bit unsigned integer field containing the number
of Item Value octets to follow.
Item Value: A variable-length data field that is interpreted
according to the associated Item Type. This specification places
no restrictions on an extension's use of available Item Value
data. Extension specifications SHOULD avoid the use of large data
lengths, as the associated transfer cannot begin until the full
extension data is sent.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+
Figure 25: Transfer Extension Item Format
+==========+========+==================================+
| Name | Code | Description |
+==========+========+==================================+
| CRITICAL | 0x01 | If this bit is set, it indicates |
| | | that the receiving peer must |
| | | handle the extension item. |
+----------+--------+----------------------------------+
| Reserved | others | |
+----------+--------+----------------------------------+
Table 7: Transfer Extension Item Flags
5.2.5.1. Transfer Length Extension
The purpose of the Transfer Length Extension is to allow entities to
preemptively refuse bundles that would exceed their resources or to
prepare storage on the receiving entity for the upcoming bundle data.
Multiple Transfer Length Extension Items SHALL NOT occur within the
same transfer. The lack of a Transfer Length Extension Item in any
transfer SHALL NOT imply anything regarding the potential length of
the transfer. The Transfer Length Extension SHALL use the IANA-
assigned code point from Section 8.4.
If a transfer occupies exactly one segment (i.e., both the START flag
and the END flag are 1), the Transfer Length Extension SHOULD NOT be
present. The extension does not provide any additional information
for single-segment transfers.
The format of the Transfer Length Extension data is shown in
Figure 26.
+----------------------+
| Total Length (U64) |
+----------------------+
Figure 26: Format of Transfer Length Extension Data
The Transfer Length Extension data contains the following field:
Total Length: A 64-bit unsigned integer indicating the size of the
data to be transferred. The Total Length field SHALL be treated
as authoritative by the receiver. If, for whatever reason, the
actual total length of bundle data received differs from the value
indicated by the Total Length value, the receiver SHALL treat the
transmitted data as invalid and send a XFER_REFUSE with a reason
code of "Not Acceptable".
6. Session Termination
This section describes the procedures for terminating a TCPCL
session. The purpose of terminating a session is to allow transfers
to complete before the TCP connection is closed but not allow any new
transfers to start. A session state change is necessary for this to
happen, because transfers can be in progress in either direction
(transfer stream) within a session. Waiting for a transfer to
complete in one direction does not control or influence the
possibility of a transfer in the other direction. Either peer of a
session can terminate an established session at any time.
6.1. Session Termination Message (SESS_TERM)
To cleanly terminate a session, a SESS_TERM message SHALL be
transmitted by either entity at any point following complete
transmission of any other message. When sent to initiate a
termination, the REPLY flag of a SESS_TERM message SHALL be 0. Upon
receiving a SESS_TERM message after not sending a SESS_TERM message
in the same session, an entity SHALL send an acknowledging SESS_TERM
message. When sent to acknowledge a termination, a SESS_TERM message
SHALL have identical data content from the message being acknowledged
except for the REPLY flag, which is set to 1 to indicate
acknowledgment.
Once a SESS_TERM message is sent, the state of that TCPCL session
changes to Ending. While the session is in the Ending state,
* an entity MAY finish an in-progress transfer in either direction.
* an entity SHALL NOT begin any new outgoing transfer for the
remainder of the session.
* an entity SHALL NOT accept any new incoming transfer for the
remainder of the session.
If a new incoming transfer is attempted while in the Ending state,
the receiving entity SHALL send a XFER_REFUSE with a reason code of
"Session Terminating".
There are circumstances where an entity has an urgent need to close a
TCP connection associated with a TCPCL session, without waiting for
transfers to complete but also in a way that doesn't force timeouts
to occur -- for example, due to impending shutdown of the underlying
data-link layer. Instead of following a clean termination sequence,
after transmitting a SESS_TERM message, an entity MAY perform an
unclean termination by immediately closing the associated TCP
connection. When performing an unclean termination, an entity SHOULD
acknowledge all received XFER_SEGMENTs with a XFER_ACK before closing
the TCP connection. Not acknowledging received segments can result
in unnecessary bundle or bundle fragment retransmissions. Any delay
between a request to close the TCP connection and the actual closing
of the connection (a "half-closed" state) MAY be ignored by the TCPCL
entity. If the underlying TCP connection is closed during a
transmission (in either transfer stream), the transfer SHALL be
indicated to the BPA as failed (see the transmission failure and
reception failure indications defined in Section 3.1).
The TCPCL itself does not have any required behavior related to
responding to a SESS_TERM based on its reason code; the termination
is passed up as an indication to the BPA that the session state has
changed. If a termination has a reason code that is not decodable to
the BPA, the agent SHOULD treat the termination as having a reason
code of "Unknown".
The format of the SESS_TERM message is shown in Figure 27.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
Figure 27: Format of SESS_TERM Messages
The fields of the SESS_TERM message are as follows:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 8. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 9.
+==========+========+=======================================+
| Name | Code | Description |
+==========+========+=======================================+
| REPLY | 0x01 | If this bit is set, it indicates that |
| | | this message is an acknowledgment of |
| | | an earlier SESS_TERM message. |
+----------+--------+---------------------------------------+
| Reserved | others | |
+----------+--------+---------------------------------------+
Table 8: SESS_TERM Flags
+==============+======+==========================================+
| Name | Code | Description |
+==============+======+==========================================+
| Unknown | 0x00 | A termination reason is not available. |
+--------------+------+------------------------------------------+
| Idle timeout | 0x01 | The session is being terminated due to |
| | | idleness. |
+--------------+------+------------------------------------------+
| Version | 0x02 | The entity cannot conform to the |
| mismatch | | specified TCPCL protocol version. |
+--------------+------+------------------------------------------+
| Busy | 0x03 | The entity is too busy to handle the |
| | | current session. |
+--------------+------+------------------------------------------+
| Contact | 0x04 | The entity cannot interpret or negotiate |
| Failure | | a Contact Header or SESS_INIT option. |
+--------------+------+------------------------------------------+
| Resource | 0x05 | The entity has run into some resource |
| Exhaustion | | limit and cannot continue the session. |
+--------------+------+------------------------------------------+
Table 9: SESS_TERM Reason Codes
The earliest a TCPCL session termination MAY occur is immediately
after transmission of a Contact Header (and prior to any further
message transmissions). This can, for example, be used as a
notification that the entity is currently not able or willing to
communicate. However, an entity MUST always send the Contact Header
to its peer before sending a SESS_TERM message.
Termination of the TCP connection MAY occur prior to receiving the
Contact Header as discussed in Section 4.1. If reception of the
Contact Header itself somehow fails (e.g., an invalid magic string is
received), an entity SHALL close the TCP connection without sending a
SESS_TERM message.
If a session is to be terminated before the sending of a protocol
message has completed, then the entity MUST NOT transmit the
SESS_TERM message but still SHALL close the TCP connection. Each
TCPCL message is contiguous in the octet stream and has no ability to
be cut short and/or preempted by another message. This is
particularly important when large segment sizes are being
transmitted; either the entire XFER_SEGMENT is sent before a
SESS_TERM message or the connection is simply terminated mid-
XFER_SEGMENT.
6.2. Idle Session Termination
The protocol includes a provision for clean termination of idle
sessions. Determining the length of time to wait before terminating
idle sessions, if they are to be terminated at all, is an
implementation and configuration matter.
If there is a configured time to terminate idle sessions and if no
TCPCL messages (other than KEEPALIVE messages) have been received for
at least that amount of time, then either entity MAY terminate the
session by transmitting a SESS_TERM message with a reason code of
"Idle timeout" (as described in Table 9).
7. Security Considerations
This section separates security considerations into threat categories
based on guidance provided in BCP 72 [RFC 3552].
7.1. Threat: Passive Leak of Node Data
When used without TLS security, the TCPCL exposes the node ID and
other configuration data to passive eavesdroppers. This occurs even
when no transfers occur within a TCPCL session. This can be avoided
by always using TLS, even if authentication is not available (see
Section 7.12).
7.2. Threat: Passive Leak of Bundle Data
The TCPCL can be used to provide point-to-point transport security,
but it does not provide security of data at rest and does not
guarantee end-to-end bundle security. The bundle security mechanisms
defined in [RFC 9172] are to be used instead.
When used without TLS security, the TCPCL exposes all bundle data to
passive eavesdroppers. This can be avoided by always using TLS, even
if authentication is not available (see Section 7.12).
7.3. Threat: TCPCL Version Downgrade
When a TCPCL entity supports multiple versions of the protocol, it is
possible for a malicious or misconfigured peer to use an older
version of the TCPCL protocol that does not support transport
security. An on-path attacker can also manipulate a Contact Header
to present a lower protocol version than desired.
It is up to security policies within each TCPCL entity to ensure that
the negotiated TCPCL version meets transport security requirements.
7.4. Threat: Transport Security Stripping
When security policy allows non-TLS sessions, the TCPCL does not
protect against active network attackers. It is possible for an on-
path attacker to set the CAN_TLS flag to 0 on either side of the
Contact Header exchange, which will cause the negotiation discussed
in Section 4.3 to disable TLS. This leads to the "SSL Stripping"
attack described in [RFC 7457].
The purpose of the CAN_TLS flag is to allow the use of the TCPCL on
entities that simply do not have a TLS implementation available.
When TLS is available on an entity, it is strongly encouraged that
the security policy disallow non-TLS sessions. This requires that
the TLS handshake occur, regardless of the policy-driven parameters
of the handshake and policy-driven handling of the handshake outcome.
One mechanism to mitigate the possibility of TLS Stripping is the use
of DNS-based Authentication of Named Entities (DANE) [RFC 6698] toward
the passive peer. This mechanism relies on DNS and is
unidirectional, so it doesn't help with applying policy toward the
active peer, but it can be useful in an environment using
opportunistic security. The configuration and use of DANE are
outside of the scope of this document.
The negotiated use of TLS is identical in behavior to the use of
STARTTLS as described in [RFC 2595], [RFC 4511], and others.
7.5. Threat: Weak TLS Configurations
Even when using TLS to secure the TCPCL session, the actual cipher
suite negotiated between the TLS peers can be insecure.
Recommendations for using cipher suites are included in BCP 195
[RFC 7525]. It is up to security policies within each TCPCL entity to
ensure that the negotiated TLS cipher suite meets transport security
requirements.
7.6. Threat: Untrusted End-Entity Certificate
The authentication method discussed in Section 4.4.4 uses end-entity
certificates chained to a trusted root CA. During a TLS handshake,
either entity can send a certificate set that does not contain the
full chain, possibly excluding intermediate or root CAs. In an
environment where peers are known to already contain needed root and
intermediate CAs, there is no need to include those CAs, but this
carries the risk of an entity not actually having one of the needed
CAs.
7.7. Threat: Certificate Validation Vulnerabilities
Even when TLS itself is operating properly, an attacker can attempt
to exploit vulnerabilities within certificate check algorithms or
configuration to establish a secure TCPCL session using an invalid
certificate. A BPA treats the peer node ID within a TCPCL session as
authoritative, and exploitation via an invalid certificate could lead
to bundle data leaking and/or denial of service to the node ID being
impersonated.
There are many reasons, as described in [RFC 5280] and [RFC 6125], why
a certificate can fail to validate, including using the certificate
outside of its valid time interval, using purposes for which it was
not authorized, or using it after it has been revoked by its CA.
Validating a certificate is a complex task and can require network
connectivity outside of the primary TCPCL network path(s) if a
mechanism such as OCSP [RFC 6960] is used by the CA. The
configuration and use of particular certificate validation methods
are outside of the scope of this document.
7.8. Threat: Symmetric Key Limits
Even with a secure block cipher and securely established session
keys, there are limits to the amount of plaintext that can be safely
encrypted with a given set of keys, as described in [AEAD-LIMITS].
When permitted by the negotiated TLS version (see [RFC 8446]), it is
advisable to take advantage of session key updates to avoid those
limits.
7.9. Threat: BP Node Impersonation
The certificates exchanged by TLS enable authentication of the peer
DNS name and node ID, but it is possible that either a peer does not
provide a valid certificate or the certificate does not validate
either the DNS-ID/IPADDR-ID or NODE-ID of the peer (see Section 3.4).
Having a CA-validated certificate does not alone guarantee the
identity of the network host or BP node from which the certificate is
provided; additional validation procedures as provided in
Section 4.4.4 bind the DNS-ID/IPADDR-ID or NODE-ID based on the
contents of the certificate.
The DNS-ID/IPADDR-ID validation is a weaker form of authentication,
because even if a peer is operating on an authenticated network DNS
name or IP address it can provide an invalid node ID and cause
bundles to be "leaked" to an invalid node. Especially in DTN
environments, network names and addresses of nodes can be time-
variable, so binding a certificate to a node ID results in a more
stable identity.
NODE-ID validation ensures that the peer to which a bundle is
transferred is in fact the node that the BPA expects it to be. In
circumstances where certificates can only be issued to DNS names,
node ID validation is not possible, but it could be reasonable to
assume that a trusted host is not going to present an invalid node
ID. Determining when a DNS-ID/IPADDR-ID authentication can be
trusted to validate a node ID is also a policy matter outside of the
scope of this document.
One mitigation regarding arbitrary entities with valid PKIX
certificates impersonating arbitrary node IDs is the use of the PKIX
EKU key purpose id-kp-bundleSecurity (Section 4.4.2.1). When this
EKU is present in the certificate, it represents a stronger assertion
that the private key holder should in fact be trusted to operate as a
bundle node.
7.10. Threat: Denial of Service
The behaviors described in this section all amount to a potential
denial of service to a TCPCL entity. The denial of service could be
limited to an individual TCPCL session, could affect other well-
behaved sessions on an entity, or could affect all sessions on a
host.
A malicious entity can trigger timeouts by continually establishing
TCPCL sessions and delaying the sending of protocol-required data.
The victim entity can block TCP connections from network peers that
are thought to behave incorrectly within the TCPCL.
An entity can send a large amount of data over a TCPCL session,
requiring the receiving entity to handle the data. The victim entity
can attempt to stop the flood of data by sending a XFER_REFUSE
message or can forcibly terminate the session.
A "data dribble" attack is also possible, in which an entity presents
a very small Segment MRU that causes transfers to be split among a
large number of very small segments and causes the resultant
segmentation overhead to overwhelm the actual bundle data segments.
Similarly, an entity can present a very small Transfer MRU that will
cause resources to be wasted on establishment and upkeep of a TCPCL
session over which a bundle could never be transferred. The victim
entity can terminate the session during parameter negotiation
(Section 4.7) if the MRUs are unacceptable.
An abusive entity could cause the keepalive mechanism to waste
throughput within a network link that would otherwise be usable for
bundle transmissions. Due to the quantization of the Keepalive
Interval parameter, the smallest Session Keepalive is one second,
which should be long enough to not flood the link. The victim entity
can terminate the session during parameter negotiation (Section 4.7)
if the Keepalive Interval is unacceptable.
Finally, an attacker or a misconfigured entity can cause issues at
the TCP connection that will cause unnecessary TCP retransmissions or
connection resets, effectively denying the use of the overlying TCPCL
session.
7.11. Mandatory-to-Implement TLS
Following IETF best current practice, TLS is mandatory to implement
for all TCPCL implementations but TLS is optional to use for a given
TCPCL session. The policy recommendations in Sections 4.2 and 4.3
both enable TLS and require TLS, but entities are permitted to
disable and not require TLS based on local configuration. The
configuration to enable or require TLS for an entity or a session is
outside of the scope of this document. The configuration to disable
TLS is different from the threat of TLS Stripping as described in
Section 7.4.
7.12. Alternate Uses of TLS
This specification makes use of PKIX certificate validation and
authentication within TLS. There are alternate uses of TLS that are
not necessarily incompatible with the security goals of this
specification but that are outside of the scope of this document.
The following subsections give examples of alternate TLS uses.
7.12.1. TLS without Authentication
In environments where PKI is available but there are restrictions on
the issuance of certificates (including the contents of
certificates), it may be possible to make use of TLS in a way that
authenticates only the passive entity of a TCPCL session or that does
not authenticate either entity. Using TLS in a way that does not
successfully authenticate some claim of both peer entities of a TCPCL
session is outside of the scope of this document but does have
properties similar to the opportunistic security model [RFC 7435].
7.12.2. Non-certificate TLS Use
In environments where PKI is unavailable, alternate uses of TLS that
do not require certificates such as pre-shared key (PSK)
authentication [RFC 5489] and the use of raw public keys [RFC 7250] are
available and can be used to ensure confidentiality within the TCPCL.
Using non-PKI node authentication methods is outside of the scope of
this document.
7.13. Predictability of Transfer IDs
The only requirement on Transfer IDs is that they be unique within
each session from the sending peer only. The trivial algorithm of
the first transfer starting at zero and later transfers incrementing
by one causes absolutely predictable Transfer IDs. Even when a TCPCL
session is not TLS secured and there is an on-path attacker causing
denial of service with XFER_REFUSE messages, it is not possible to
preemptively refuse a transfer, so there is no benefit in having
unpredictable Transfer IDs within a session.
8. IANA Considerations
Registration procedures referred to in this section (e.g., the RFC
Required policy) are defined in [RFC 8126].
Some of the registries have been defined as version specific for
TCPCLv4, and these registries reuse some or all codepoints from
TCPCLv3. This was done to disambiguate the use of these codepoints
between TCPCLv3 and TCPCLv4 while preserving the semantics of some of
the codepoints.
8.1. Port Number
Within the "Service Name and Transport Protocol Port Number Registry"
[IANA-PORTS], TCP port number 4556 had previously been assigned as
the default port for the TCPCL; see [RFC 7242]. This assignment is
unchanged by TCPCL version 4, but the assignment reference has been
updated to point to this specification. Each TCPCL entity identifies
its TCPCL protocol version in its initial contact (see Sections 3.2
and 8.2), so there is no ambiguity regarding what protocol is being
used. The related assignments for UDP and DCCP port 4556 (both
registered by [RFC 7122]) are unchanged.
+========================+============================+
| Parameter | Value |
+========================+============================+
| Service Name: | dtn-bundle |
+------------------------+----------------------------+
| Transport Protocol(s): | TCP |
+------------------------+----------------------------+
| Assignee: | IESG (iesg@ietf.org) |
+------------------------+----------------------------+
| Contact: | IESG (iesg@ietf.org) |
+------------------------+----------------------------+
| Description: | DTN Bundle TCP CL Protocol |
+------------------------+----------------------------+
| Reference: | This specification |
+------------------------+----------------------------+
| Port Number: | 4556 |
+------------------------+----------------------------+
Table 10: TCP Port Number for the TCPCL
8.2. Protocol Versions
IANA has registered the following value in the "Bundle Protocol TCP
Convergence-Layer Version Numbers" registry [RFC 7242].
+=======+=============+====================+
| Value | Description | Reference |
+=======+=============+====================+
| 4 | TCPCLv4 | This specification |
+-------+-------------+--------------------+
Table 11: New TCPCL Version Number
8.3. Session Extension Types
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 Session
Extension Types" registry and populated it with the contents of
Table 12. The registration procedure is Expert Review within the
lower range 0x0001-0x7FFF. Values in the range 0x8000-0xFFFF are
reserved for Private or Experimental Use, which are not recorded by
IANA.
Specifications of new session extension types need to define the
encoding of the Item Value data as well as any meaning or restriction
on the number of or order of instances of the type within an
extension item list. Specifications need to define how the extension
functions when no instance of the new extension type is received
during session negotiation.
Experts are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
esthetically displeasing or architecturally dubious).
+===============+==========================================+
| Code | Session Extension Type |
+===============+==========================================+
| 0x0000 | Reserved |
+---------------+------------------------------------------+
| 0x0001-0x7FFF | Unassigned |
+---------------+------------------------------------------+
| 0x8000-0xFFFF | Reserved for Private or Experimental Use |
+---------------+------------------------------------------+
Table 12: Session Extension Type Codes
8.4. Transfer Extension Types
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 Transfer
Extension Types" registry and populated it with the contents of
Table 13. The registration procedure is Expert Review within the
lower range 0x0001-0x7FFF. Values in the range 0x8000-0xFFFF are
reserved for Private or Experimental Use, which are not recorded by
IANA.
Specifications of new transfer extension types need to define the
encoding of the Item Value data as well as any meaning or restriction
on the number of or order of instances of the type within an
extension item list. Specifications need to define how the extension
functions when no instance of the new extension type is received in a
transfer.
Experts are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
esthetically displeasing or architecturally dubious).
+===============+==========================================+
| Code | Transfer Extension Type |
+===============+==========================================+
| 0x0000 | Reserved |
+---------------+------------------------------------------+
| 0x0001 | Transfer Length Extension |
+---------------+------------------------------------------+
| 0x0002-0x7FFF | Unassigned |
+---------------+------------------------------------------+
| 0x8000-0xFFFF | Reserved for Private or Experimental Use |
+---------------+------------------------------------------+
Table 13: Transfer Extension Type Codes
8.5. Message Types
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 Message Types"
registry and populated it with the contents of Table 14. The
registration procedure is RFC Required within the lower range
0x01-0xEF. Values in the range 0xF0-0xFF are reserved for Private or
Experimental Use, which are not recorded by IANA.
Specifications of new message types need to define the encoding of
the message data as well as the purpose and relationship of the new
message to existing session/transfer state within the baseline
message sequencing. The use of new message types needs to be
negotiated between TCPCL entities within a session (using the session
extension mechanism) so that the receiving entity can properly decode
all message types used in the session.
Experts are encouraged to favor new session/transfer extension types
over new message types. TCPCL messages are not self-delimiting, so
care must be taken in introducing new message types. If an entity
receives an unknown message type, the only thing that can be done is
to send a MSG_REJECT and close the TCP connection; not even a clean
termination can be done at that point.
+===========+==========================================+
| Code | Message Type |
+===========+==========================================+
| 0x00 | Reserved |
+-----------+------------------------------------------+
| 0x01 | XFER_SEGMENT |
+-----------+------------------------------------------+
| 0x02 | XFER_ACK |
+-----------+------------------------------------------+
| 0x03 | XFER_REFUSE |
+-----------+------------------------------------------+
| 0x04 | KEEPALIVE |
+-----------+------------------------------------------+
| 0x05 | SESS_TERM |
+-----------+------------------------------------------+
| 0x06 | MSG_REJECT |
+-----------+------------------------------------------+
| 0x07 | SESS_INIT |
+-----------+------------------------------------------+
| 0x08-0xEF | Unassigned |
+-----------+------------------------------------------+
| 0xF0-0xFF | Reserved for Private or Experimental Use |
+-----------+------------------------------------------+
Table 14: Message Type Codes
8.6. XFER_REFUSE Reason Codes
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE
Reason Codes" registry and populated it with the contents of
Table 15. The registration procedure is Specification Required
within the lower range 0x00-0xEF. Values in the range 0xF0-0xFF are
reserved for Private or Experimental Use, which are not recorded by
IANA.
Specifications of new XFER_REFUSE reason codes need to define the
meaning of the reason and disambiguate it from preexisting reasons.
Each refusal reason needs to be usable by the receiving BPA to make
retransmission or rerouting decisions.
Experts are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
esthetically displeasing or architecturally dubious).
+===========+==========================================+
| Code | Refusal Reason |
+===========+==========================================+
| 0x00 | Unknown |
+-----------+------------------------------------------+
| 0x01 | Completed |
+-----------+------------------------------------------+
| 0x02 | No Resources |
+-----------+------------------------------------------+
| 0x03 | Retransmit |
+-----------+------------------------------------------+
| 0x04 | Not Acceptable |
+-----------+------------------------------------------+
| 0x05 | Extension Failure |
+-----------+------------------------------------------+
| 0x06 | Session Terminating |
+-----------+------------------------------------------+
| 0x07-0xEF | Unassigned |
+-----------+------------------------------------------+
| 0xF0-0xFF | Reserved for Private or Experimental Use |
+-----------+------------------------------------------+
Table 15: XFER_REFUSE Reason Codes
8.7. SESS_TERM Reason Codes
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 SESS_TERM Reason
Codes" registry and populated it with the contents of Table 16. The
registration procedure is Specification Required within the lower
range 0x00-0xEF. Values in the range 0xF0-0xFF are reserved for
Private or Experimental Use, which are not recorded by IANA.
Specifications of new SESS_TERM reason codes need to define the
meaning of the reason and disambiguate it from preexisting reasons.
Each termination reason needs to be usable by the receiving BPA to
make reconnection decisions.
Experts are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
esthetically displeasing or architecturally dubious).
+===========+==========================================+
| Code | Termination Reason |
+===========+==========================================+
| 0x00 | Unknown |
+-----------+------------------------------------------+
| 0x01 | Idle timeout |
+-----------+------------------------------------------+
| 0x02 | Version mismatch |
+-----------+------------------------------------------+
| 0x03 | Busy |
+-----------+------------------------------------------+
| 0x04 | Contact Failure |
+-----------+------------------------------------------+
| 0x05 | Resource Exhaustion |
+-----------+------------------------------------------+
| 0x06-0xEF | Unassigned |
+-----------+------------------------------------------+
| 0xF0-0xFF | Reserved for Private or Experimental Use |
+-----------+------------------------------------------+
Table 16: SESS_TERM Reason Codes
8.8. MSG_REJECT Reason Codes
Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
the "Bundle Protocol TCP Convergence-Layer Version 4 MSG_REJECT
Reason Codes" registry and populated it with the contents of
Table 17. The registration procedure is Specification Required
within the lower range 0x01-0xEF. Values in the range 0xF0-0xFF are
reserved for Private or Experimental Use, which are not recorded by
IANA.
Specifications of new MSG_REJECT reason codes need to define the
meaning of the reason and disambiguate it from preexisting reasons.
Each rejection reason needs to be usable by the receiving TCPCL
entity to make message sequencing and/or session termination
decisions.
Experts are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
esthetically displeasing or architecturally dubious).
+===========+==========================================+
| Code | Rejection Reason |
+===========+==========================================+
| 0x00 | Reserved |
+-----------+------------------------------------------+
| 0x01 | Message Type Unknown |
+-----------+------------------------------------------+
| 0x02 | Message Unsupported |
+-----------+------------------------------------------+
| 0x03 | Message Unexpected |
+-----------+------------------------------------------+
| 0x04-0xEF | Unassigned |
+-----------+------------------------------------------+
| 0xF0-0xFF | Reserved for Private or Experimental Use |
+-----------+------------------------------------------+
Table 17: MSG_REJECT Reason Codes
8.9. Object Identifier for PKIX Module Identifier
IANA has registered the following in the "SMI Security for PKIX
Module Identifier" registry [IANA-SMI] for identifying the module
described in Appendix B.
+=========+=========================+====================+
| Decimal | Description | References |
+=========+=========================+====================+
| 103 | id-mod-dtn-tcpclv4-2021 | This specification |
+---------+-------------------------+--------------------+
Table 18: New SMI Security Module
8.10. Object Identifier for PKIX Other Name Forms
IANA has registered the following in the "SMI Security for PKIX Other
Name Forms" registry [IANA-SMI] for identifying bundle endpoint IDs:
+=========+=================+====================+
| Decimal | Description | References |
+=========+=================+====================+
| 11 | id-on-bundleEID | This specification |
+---------+-----------------+--------------------+
Table 19: New PKIX Other Name Form
The formal structure of the associated Other Name Form is provided in
Appendix B. The use of this OID is defined in Sections 4.4.1 and
4.4.2.
8.11. Object Identifier for PKIX Extended Key Usage
IANA has registered the following in the "SMI Security for PKIX
Extended Key Purpose" registry [IANA-SMI] for securing BP bundles.
+=========+======================+====================+
| Decimal | Description | References |
+=========+======================+====================+
| 35 | id-kp-bundleSecurity | This specification |
+---------+----------------------+--------------------+
Table 20: New PKIX Extended Key Purpose
The formal definition of this EKU is provided in Appendix B. The use
of this OID is defined in Section 4.4.2.
9. References
9.1. Normative References
[IANA-BUNDLE]
IANA, "Bundle Protocol",
<https://www.iana.org/assignments/bundle/>.
[IANA-PORTS]
IANA, "Service Name and Transport Protocol Port Number
Registry", <https://www.iana.org/assignments/service-
names-port-numbers/>.
[IANA-SMI] IANA, "Structure of Management Information (SMI) Numbers
(MIB Module Registrations)",
<https://www.iana.org/assignments/smi-numbers/>.
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC 793, September 1981,
<https://www.rfc-editor.org/info/RFC 793>.
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC 1122, October 1989,
<https://www.rfc-editor.org/info/RFC 1122>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<https://www.rfc-editor.org/info/RFC 3986>.
[RFC 5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC 5280, May 2008,
<https://www.rfc-editor.org/info/RFC 5280>.
[RFC 6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC 6066, January 2011,
<https://www.rfc-editor.org/info/RFC 6066>.
[RFC 6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC 6125, March
2011, <https://www.rfc-editor.org/info/RFC 6125>.
[RFC 6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC 6960, June 2013,
<https://www.rfc-editor.org/info/RFC 6960>.
[RFC 7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC 7525, May
2015, <https://www.rfc-editor.org/info/RFC 7525>.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC 8446, August 2018,
<https://www.rfc-editor.org/info/RFC 8446>.
[RFC 9171] Burleigh, S., Fall, K., and E. Birrane, III, "Bundle
Protocol Version 7", RFC 9171, DOI 10.17487/RFC 9171,
January 2022, <https://www.rfc-editor.org/info/RFC 9171>.
[X.680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
<https://www.itu.int/rec/T-REC-X.680-202102-I/en>.
9.2. Informative References
[AEAD-LIMITS]
Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", August 2017,
<https://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[RFC 2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC 2595, June 1999,
<https://www.rfc-editor.org/info/RFC 2595>.
[RFC 3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC 3552, July 2003,
<https://www.rfc-editor.org/info/RFC 3552>.
[RFC 4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511,
DOI 10.17487/RFC 4511, June 2006,
<https://www.rfc-editor.org/info/RFC 4511>.
[RFC 4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, DOI 10.17487/RFC 4838,
April 2007, <https://www.rfc-editor.org/info/RFC 4838>.
[RFC 5489] Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
Transport Layer Security (TLS)", RFC 5489,
DOI 10.17487/RFC 5489, March 2009,
<https://www.rfc-editor.org/info/RFC 5489>.
[RFC 5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC 5912, June 2010,
<https://www.rfc-editor.org/info/RFC 5912>.
[RFC 6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC 6698, August
2012, <https://www.rfc-editor.org/info/RFC 6698>.
[RFC 7122] Kruse, H., Jero, S., and S. Ostermann, "Datagram
Convergence Layers for the Delay- and Disruption-Tolerant
Networking (DTN) Bundle Protocol and Licklider
Transmission Protocol (LTP)", RFC 7122,
DOI 10.17487/RFC 7122, March 2014,
<https://www.rfc-editor.org/info/RFC 7122>.
[RFC 7242] Demmer, M., Ott, J., and S. Perreault, "Delay-Tolerant
Networking TCP Convergence-Layer Protocol", RFC 7242,
DOI 10.17487/RFC 7242, June 2014,
<https://www.rfc-editor.org/info/RFC 7242>.
[RFC 7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC 7250,
June 2014, <https://www.rfc-editor.org/info/RFC 7250>.
[RFC 7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC 7435,
December 2014, <https://www.rfc-editor.org/info/RFC 7435>.
[RFC 7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC 7457,
February 2015, <https://www.rfc-editor.org/info/RFC 7457>.
[RFC 8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC 8555, March 2019,
<https://www.rfc-editor.org/info/RFC 8555>.
[RFC 9172] Birrane, III, E. and K. McKeever, "Bundle Protocol
Security (BPSec)", RFC 9172, DOI 10.17487/RFC 9172, January
2022, <https://www.rfc-editor.org/info/RFC 9172>.
[DTN-BIBECT]
Burleigh, S., "Bundle-in-Bundle Encapsulation", Work in
Progress, Internet-Draft, draft-ietf-dtn-bibect-03, 18
February 2020, <https://datatracker.ietf.org/doc/html/
draft-ietf-dtn-bibect-03>.
Appendix A. Significant Changes from RFC 7242
The areas in which changes from [RFC 7242] have been made to existing
headers and messages are as follows:
* Split Contact Header into pre-TLS protocol negotiation and
SESS_INIT parameter negotiation. The Contact Header is now fixed
length.
* Changed Contact Header content to limit number of negotiated
options.
* Added session option to negotiate maximum segment size (per each
direction).
* Renamed "endpoint ID" to "node ID" to conform with BPv7
terminology.
* Added session extension capability.
* Added transfer extension capability. Moved transfer total length
into an extension item.
* Defined new IANA registries for message / type / reason codes to
allow renaming some codes for clarity.
* Pointed out that segments of all new IANA registries are reserved
for private/experimental use.
* Expanded Message Header to octet-aligned fields instead of bit-
packing.
* Added a bundle transfer identification number to all bundle-
related messages (XFER_SEGMENT, XFER_ACK, XFER_REFUSE).
* Added flags in XFER_ACK to mirror flags from XFER_SEGMENT.
* Removed all uses of Self-Delimiting Numeric Value (SDNV) fields
and replaced with fixed-bit-length (network byte order) fields.
* Renamed SHUTDOWN to SESS_TERM to deconflict term "shutdown"
related to TCP connections.
* Removed the notion of a reconnection delay parameter.
The areas in which extensions from [RFC 7242] have been made as new
messages and codes are as follows:
* Added MSG_REJECT message to indicate that an unknown or unhandled
message was received.
* Added TLS connection security mechanism.
* Added "Not Acceptable", "Extension Failure", and "Session
Terminating" XFER_REFUSE reason codes.
* Added "Contact Failure" (contact negotiation failure) and
"Resource Exhaustion" SESS_TERM reason codes.
Appendix B. ASN.1 Module
The following ASN.1 module formally specifies the BundleEID
structure, its Other Name Form, and the bundleSecurity EKU, using
ASN.1 syntax per [X.680]. This specification uses the ASN.1
definitions from [RFC 5912] with the 2002 ASN.1 notation used in that
document.
<CODE BEGINS>
DTN-TCPCLv4-2021
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-dtn-tcpclv4-2021(103) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
OTHER-NAME
FROM PKIX1Implicit-2009 -- [RFC 5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-implicit-02(59) }
id-pkix
FROM PKIX1Explicit-2009 -- [RFC 5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-explicit-02(51) } ;
id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }
DTNOtherNames OTHER-NAME ::= { on-bundleEID, ... }
-- The otherName definition for BundleEID
on-bundleEID OTHER-NAME ::= {
BundleEID IDENTIFIED BY { id-on-bundleEID }
}
id-on-bundleEID OBJECT IDENTIFIER ::= { id-on 11 }
-- Same encoding as GeneralName of uniformResourceIdentifier
BundleEID ::= IA5String
-- The Extended Key Usage key for bundle security
id-kp-bundleSecurity OBJECT IDENTIFIER ::= { id-kp 35 }
END
<CODE ENDS>
Appendix C. Example of the BundleEID Other Name Form
This non-normative example demonstrates an otherName with a name form
of BundleEID to encode the node ID "dtn://example/".
The hexadecimal form of the DER encoding of the otherName is as
follows:
a01c06082b0601050507080ba010160e64746e3a2f2f6578616d706c652f
And the text decoding in Figure 28 is an output of Peter Gutmann's
"dumpasn1" program.
0 28: [0] {
2 8: OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 11'
12 16: [0] {
14 14: IA5String 'dtn://example/'
: }
: }
Figure 28: Visualized Decoding of the on-bundleEID
Acknowledgments
This specification is based on comments regarding the implementation
of [RFC 7242] as provided by Scott Burleigh.
The ASN.1 module and its Other Name Form are based on a
recommendation provided by Russ Housley.
Authors' Addresses
Brian Sipos
RKF Engineering Solutions, LLC
7500 Old Georgetown Road
Suite 1275
Bethesda, MD 20814-6198
United States of America
Email: brian.sipos+ietf@gmail.com
Michael Demmer
Email: demmer@gmail.com
Jörg Ott
Technical University of Munich
Department of Informatics
Chair of Connected Mobility
Boltzmannstrasse 3
DE-85748 Garching
Germany
Email: ott@in.tum.de
Simon Perreault
LogMeIn
410 boulevard Charest Est
Suite 250
Quebec QC G1K 8G3
Canada
Email: simon.perreault@logmein.com
RFC TOTAL SIZE: 170267 bytes
PUBLICATION DATE: Tuesday, February 1st, 2022
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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