wdiff draft-ietf-dtn-tcpclv4-07.txt draft-ietf-dtn-tcpclv4-08.txt

Delay Tolerant Networking B. Sipos
Internet-Draft RKF Engineering
Obsoletes: 7242 (if approved) M. Demmer
Intended status: Standards Track UC Berkeley
Expires: September November 21, 2018 J. Ott
Aalto University
S. Perreault
Mar
May 20, 2018

Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
draft-ietf-dtn-tcpclv4-07
draft-ietf-dtn-tcpclv4-08

Abstract

This document describes a revised protocol for the TCP-based
convergence layer (TCPCL) for Delay-Tolerant Networking (DTN). The
protocol revision is based on implementation issues in the original
TCPCL Version 3 and updates to the Bundle Protocol contents,
encodings, and convergence layer requirements in Bundle Protocol
Version 7. Specifically, the TCPCLv4 uses CBOR-encoded BPv7 bundles
as its service data unit being transported and provides a reliable
transport of such bundles. Several new IANA registries are defined
for TCPCLv4 which define some behaviors inherited from TCPCLv3 but
with updated encodings and/or semantics.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on September November 21, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Convergence Layer Services . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
2.1. Definitions Specific to the TCPCL Protocol . . . . . . . 6
3. General Protocol Description . . . . . . . . . . . . . . . . 7 8
3.1. TCPCL Session Overview . . . . . . . . . . . . . . . . . 7 8
3.2. Transfer Segmentation Policies . . . . . . . . . . . . . 10
3.3. Example Message Exchange . . . . . . . . . . . . . . . . 8 11
4. Session Establishment . . . . . . . . . . . . . . . . . . . . 10 13
4.1. TCP Connection . . . . . . . . . . . . . . . . . . . . . 11 13
4.2. Contact Header . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Header Extension Items 13
4.3. Contact Validation and Negotiation . . . . . . . . . . . 14
4.4. Session Security . . . . 14
4.3. Validation and Parameter Negotiation . . . . . . . . . . 15
4.3.1. Reactive Fragmentation Extension . . . . . . 15
4.4.1. TLS Handshake Result . . . . 16
4.4. Session Security . . . . . . . . . . . . 16
4.4.2. Example TLS Initiation . . . . . . . . 17
4.4.1. TLS Handshake Result . . . . . . . 16
4.5. Message Type Codes . . . . . . . . . . . 18
4.4.2. Example TLS Initiation . . . . . . . . 17
4.6. Session Initialization Message (SESS_INIT) . . . . . . . 18
5. Established
4.6.1. Session Operation Extension Items . . . . . . . . . . . . . . . 20
4.7. Session Parameter Negotiation . 19
5.1. Message Type Codes . . . . . . . . . . . . . 21
5. Established Session Operation . . . . . . 19
5.2. . . . . . . . . . . 22
5.1. Upkeep and Status Messages . . . . . . . . . . . . . . . 20
5.2.1. 22
5.1.1. Session Upkeep (KEEPALIVE) . . . . . . . . . . . . . 20
5.2.2. 22
5.1.2. Message Rejection (MSG_REJECT) . . . . . . . . . . . 21
5.3. 23
5.2. Bundle Transfer . . . . . . . . . . . . . . . . . . . . . 22
5.3.1. 24
5.2.1. Bundle Transfer ID . . . . . . . . . . . . . . . . . 23
5.3.2. 24
5.2.2. Transfer Initialization (XFER_INIT) . . . . . . . . . 23
5.3.3. 25
5.2.3. Data Transmission (XFER_SEGMENT) . . . . . . . . . . 24
5.3.4. 28
5.2.4. Data Acknowledgments (XFER_ACK) . . . . . . . . . . . 26
5.3.5. 29
5.2.5. Transfer Refusal (XFER_REFUSE) . . . . . . . . . . . 27 30
6. Session Termination . . . . . . . . . . . . . . . . . . . . . 29 32
6.1. Shutdown Session Termination Message (SHUTDOWN) (SESS_TERM) . . . . . . . . . 32
6.2. Idle Session Shutdown . . . . . . 29
6.2. Idle Session Shutdown . . . . . . . . . . . . 35
7. Implementation Status . . . . . . 32
7. . . . . . . . . . . . . . . 35
8. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8. 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
8.1. 37
9.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 34
8.2. 37
9.2. Protocol Versions . . . . . . . . . . . . . . . . . . . . 34
8.3. Header 37
9.3. Session Extension Types . . . . . . . . . . . . . . . . . 35
8.4. 38
9.4. Transfer Extension Types . . . . . . . . . . . . . . . . 38
9.5. Message Types . . . . . . . . . . . . . . . . . . . . . . 35
8.5. 39
9.6. XFER_REFUSE Reason Codes . . . . . . . . . . . . . . . . 36
8.6. SHUTDOWN 40
9.7. SESS_TERM Reason Codes . . . . . . . . . . . . . . . . . . 37
8.7. 41
9.8. MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . . 38
9. 42
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38
10. 42
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. 42
11.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. 42
11.2. Informative References . . . . . . . . . . . . . . . . . 39 43
Appendix A. Significant changes from RFC7242 . . . . . . . . . . 40 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 45

1. Introduction

This document describes the TCP-based convergence-layer protocol for
Delay-Tolerant Networking. Delay-Tolerant Networking 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 Network Architecture"
[RFC4838].

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)
[I-D.ietf-dtn-bpbis], 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. For the remainder of this
document, the abbreviation "TCPCL" without the version suffix refers
to TCPCLv4.

The locations of the TCPCL and the BP in the Internet model protocol
stack (described in [RFC1122]) are shown in Figure 1. In particular,
when BP is using TCP as its bearer with TCPCL as its convergence
layer, both BP and 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

This document describes the format of the protocol data units passed
between entities participating in TCPCL communications. This
document does not address:

o The format of protocol data units of the Bundle Protocol, as those
are defined elsewhere in [RFC5050] and [I-D.ietf-dtn-bpbis]. This
includes the concept of bundle fragmentation or bundle
encapsulation. The TCPCL transfers bundles as opaque data blocks.

o Mechanisms for locating or identifying other bundle nodes entities
within an internet.

1.1. Convergence Layer Services

This version of the TCPCL provides the following services to support
the overlaying Bundle Protocol agent:

Attempt Session The TCPCL allows a BP agent to pre-emptively attempt
to establish a TCPCL session with a peer node. entity. Each session
attempt can send a different set of contact header parameters as
directed by the BP agent.

Shutdown Session The TCPCL allows a BP agent to pre-emptively
shutdown an established TCPCL session with a peer node. entity. The
shutdown request is on a per-session basis.

Session is Started The TCPCL supports indication when a new TCP
connection has been started (as either client or server) before
the TCPCL handshake has begun.

Session is Established The TCPCL supports indication when a new
session has been fully established and is ready for its first
transfer.

Session is Shutdown The TCPCL supports indication when an
established session has been ended by normal exchange of SHUTDOWN SESS_TERM
messages with all transfers completed.

Session is Failed The TCPCL supports indication when a session
fails, either during contact negotiation, TLS negotiation, or
after establishement for any reason other than normal shutdown.

Begin Transmission The principal purpose of the TCPCL is to allow a
BP agent to transmit bundle data over an established TCPCL
session. Transmission request is on a per-session basis, the CL
does not necessarily perform any per-session or inter-session
queueing. Any queueing of transmissions is the obligation of the
BP agent.

Transmission Availability Because TCPCL transmits serially over a
TCP connection, it suffers from "head of queue blocking" and
supports indication of when an established session is live-but-
idle (i.e. available for immediate transfer start) or live-and-
not-idle.

Transmission Success The TCPCL supports positive indication when a
bundle has been fully transferred to a peer node. entity.

Transmission Intermediate Progress The TCPCL supports positive
indication of intermediate progress of transferr to a peer node. entity.
This intermediate progress is at the granularity of each
transferred segment.

Transmission Failure The TCPCL supports positive indication of
certain reasons for bundle transmission failure, notably when the
peer node entity rejects the bundle or when a TCPCL session ends before
transferr success. The TCPCL itself does not have a notion of
transfer timeout.

Interrupt Reception The TCPCL allows a BP agent to interrupt an
individual transfer before it has fully completed (successfully or
not).

Reception Success The TCPCL supports positive indication when a
bundle has been fully transferred from a peer node. entity.

Reception Intermediate Progress The TCPCL supports positive
indication of intermediate progress of transfer from the peer
node.
entity. This intermediate progress is at the granularity of each
transferred segment. Intermediate reception indication allows a
BP agent the chance to inspect bundle header contents before the
entire bundle is available, and thus supports the "Reception
Interruption" capability.

Reception Failure The TCPCL supports positive indication of certain
reasons for reception failure, notably when the local node 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.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2.1. Definitions Specific to the TCPCL Protocol

This section contains definitions specific to the TCPCL protocol.

TCPCL Node: Entity: This term refers to either side of a negotiating or in-
service is the notional TCPCL Session. For most application that initiates
TCPCL behavior, the two nodes are
symmetric sessions. This design, implementation, configuration, and there is no protocol
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 entitys in the network.

A TCPCL Entity MAY passivley initiate any number of TCPCL
Sessions from requests received by its passive listening
element(s) if the entity uses such elements.

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 negotiation, session establishment,
distinguishes between the connecting node active entity and the connected-to node. passive entity.
For the remainder of this document, the term "node" "entity" without the
prefix "TCPCL" refers to a TCPCL node. entity.

TCP Connection: This The term Connection in this specification
exclusively refers to a transport TCP connection using and any and all behaviors,
sessions, and other states association with that TCP
as the transport protocol. connection.

TCPCL Session: A TCPCL session (as opposed to a TCP connection) is a
TCPCL communication relationship between two bundle nodes. TCPCL entities.
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.
The lifetime of a TCPCL session is bound to the lifetime of an
underlying TCP connection. A TCPCL session is terminated when the
TCP connection ends, due either to one or both nodes entities actively
terminating the TCP connection or due to network errors causing a
failure of the TCP connection. 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 node entity and thereby to
the TCPCL is implementation dependent. However, the mechanism by
which two bundle nodes 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 uni-directional user-data
path within a TCPCL Session. Messages sent over a transfer stream
are serialized, meaning that one set of user data must complete
its transmission prior to another set of user data being
transmitted over the same transfer stream. Each uni-directional
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 TCPCL is identified by a Transfer ID number which
is unique only to a single direction within a single Session.

Transfer Segment: A subset of a transfer of user data being
communicated over a trasnfer stream.

Idle Session: A TCPCL session is idle while the only messages being
transmitted or received are KEEPALIVE messages.

Live Session: A TCPCL session is live while any messages are being
transmitted or received.

Reason Codes: The TCPCL uses numeric codes to encode specific
reasons for individual failure/error message types.

3. General Protocol Description

The service of this protocol relationship between connections, sessions, and streams is the transmission of DTN shown
in Figure 2.

+----------------------------+ +--------------------------+
| TCPCL Session | | TCPCL "Other" Session |
| | | |
| +-----------------------+ | | +---------------------+ |
| | TCP Connection | | | | TCP Connection | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Inbound | | | | | | Peer Outbound | | |
| | | Transfer Stream |<-[Seg]--[Seg]--[Seg]-| | Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | RECEIVER | | | | | | SENDER | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Outbound | | | | | | Peer Inbound | | |
| | | Transfer Stream |------[Seg]---[Seg]---->| Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | SENDER | | | | | | RECEIVER | | |
| | +-------------------+ | | | | +-----------------+ | |
| +-----------------------+ | | +---------------------+ |
+----------------------------+ +--------------------------+

Figure 2: 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
the Transmission Control Protocol (TCP). This document specifies the
encapsulation of bundles, procedures for TCP setup and teardown, and
a set of messages and node requirements. The general operation of
the protocol is as follows.

3.1. TCPCL Session Overview

First, one node establishes a TCPCL session to the other by
initiating a TCP connection in accordance with [RFC0793]. After
setup of the TCP connection is complete, an initial contact header is
exchanged in both directions to set parameters of the TCPCL session
and exchange a singleton endpoint identifier for each node (not the
singleton Endpoint Identifier (EID) of any application running on the
node) to denote the bundle-layer identity of each DTN node. This is
used to assist in routing and forwarding messages (e.g. to prevent
loops).

Once the TCPCL session is established and configured in this way,
bundles can be transferred in either direction. Each transfer is
performed by an initialization (XFER_INIT) message followed by one or
more logical segments of data within an XFER_SEGMENT message.
Multiple bundles can be transmitted consecutively 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.

A feature of this protocol is for the receiving node to send
acknowledgment (XFER_ACK) messages as bundle data segments arrive .
The rationale behind these acknowledgments is to enable the sender
node to determine how much of the bundle has been received, so that
in case the session is interrupted, it can perform reactive
fragmentation to avoid re-sending the already transmitted part of the
bundle. In addition, there is no explicit flow control on the TCPCL
layer.

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
already has 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 node live-ness
information during otherwise message-less time intervals.

A SHUTDOWN SESS_TERM message is used to start the closing of a TCPCL session
(see Section 6.1). During shutdown sequencing, in-progress transfers
can be completed but no new transfers can be initiated. A SHUTDOWN SESS_TERM
message can also be used to refuse a session setup by a peer (see
Section 4.3). It is an implementation matter to determine whether or
not to close a TCPCL session while there are no transfers queued or
in-progress.

TCPCL is a symmetric protocol between the peers of a session. 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 bi-directional mode of communication. Note that in the case of
concurrent bidirectional transmission, acknowledgment segments MAY be
interleaved with data segments.

3.2. Example Message Exchange

The following figure depicts the protocol exchange for a simple
session, showing the Transfer Segmentation Policies

Each TCPCL session establishment and the transmission of allows a
single bundle split into three data segments (of lengths "L1", "L2",
and "L3") from Node A negotiated transfer segmentation polcy to Node B.

Note that the sending
be applied in each transfer direction. A receiving node MAY transmit multiple XFER_SEGMENT
messages without necessarily waiting for the corresponding XFER_ACK
responses. This enables pipelining of messages on a channel.

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.

Node A Node B
====== ======
+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+

+-------------------------+ +-------------------------+
| SHUTDOWN | -> <- | SHUTDOWN |
+-------------------------+ +-------------------------+

Figure 2: An Example of the Flow of Protocol Messages on a Single TCP
Session between Two Nodes (A and B)

4. Session Establishment

For bundle transmissions to occur using the TCPCL, a TCPCL session
MUST first be established between communicating nodes. It is up to
the implementation to decide how and when session setup is triggered.

For example, some sessions MAY be opened proactively and maintained
for as long as is possible given the network conditions, while other
sessions MAY be opened only when there is a bundle that is queued for
transmission and can set the routing algorithm selects a certain next-hop
node.

4.1. TCP Connection

To establish a TCPCL session,
Segment MRU in its contact header to determine the largest acceptable
segment size, and a transmitting node MUST first establish can segment a TCP
connection with the intended peer node, typically by using the
services provided by the operating system. Destination port number
4556 has been assigned by IANA as transfer into any
sizes smaller than the Registered Port number receiver's Segment MRU. It is a network
administration matter to determine an appropriate segmentation policy
for the
TCP convergence layer. Other destination port numbers MAY entities operating TCPCL, but guidance given here can be used
per local configuration. Determining to
steer policy toward performance goals.

Minimum Overhead For a peer's destination port
number (if different from the registered TCPCL port number) is up simple network expected to exchange
relatively small bundles, the implementation. Any source port number MAY Segment MRU can be used for TCPCL
sessions. Typically an operating system assigned number in set to be
identical to the TCP
Ephemeral range (49152-65535) is used. Transfer MRU which indicates that all transfers
can be sent with a single data segment (i.e. no actual
segmentation). If the node network is unable closed and all transmitters are
known to establish follow a TCP connection for any reason, single-segment transfer policy, then it is an implementation matter to determine how to handle the
connection failure. A node MAY decide to re-attempt to establish the
connection. If it does so, it MUST NOT overwhelm its target with
repeated connection attempts. Therefore, the node MUST retry receivers
can avoid the
connection setup no earlier than some delay time necessity of segment reassembly. Because this CL
operates over a TCP stream, which suffers from the last
attempt, and it SHOULD use a (binary) exponential backoff mechanism
to increase this delay in case form of repeated failures. In case head-of-
queue blocking between messages, while one node is transmitting a
SHUTDOWN
single XFER_SEGMENT message specifying a reconnection delay it is received, that
delay not able to transmit any
XFER_ACK or XFER_REFUSE for any associated received transfers.

Predictable Message Sizing In situations where the maximum message
size is used as desired to be well-controlled, the initial delay. The default initial re-attempt
delay SHOULD Segment MRU can be set
to the largest acceptable size (the message size less XFER_SEGMENT
header size) and transmitters can always segment a transfer into
maximum-size chunks no shorter larger than 1 second and SHOULD be configurable
since it the Segment MRU. This
guarantees that any single XFER_SEGMENT will be application and network type dependent.

Once a TCP connection is established, each node MUST immediately
transmit a contact header over not monopolize the
TCP connection. The format stream for too long, which would prevent outgoing XFER_ACK and
XFER_REFUSE associated with received transfers.

Dynamic Segmentation Even after negotiation of a Segment MRU for
each receiving node, the
contact header actual transfer segmentation only needs
to guarantee than any individual segment is described in Section 4.2.

4.2. Contact Header

Once no larger than that
MRU. In a TCP connection situation where network "goodput" is established, both parties exchange dynamic, the
transfer segmentation size can also be dynamic in order to control
message transmission duration.

Many other policies can be established in a contact
header. This section describes TCPCL network between
these two extremes. Different policies can be applied to each
direction to/from any particular node. Additionally, future header
and transfer extension types can apply further nuance to transfer
policies and policy negotiation.

3.3. Example Message Exchange

The following figure depicts the format of protocol exchange for a simple
session, showing the contact header session establishment and the meaning of its fields.

Upon receipt transmission of the contact header, both nodes perform the validation a
single bundle split into three data segments (of lengths "L1", "L2",
and negotiation procedures defined in Section 4.3. After receiving
the contact header "L3") from Entity A to Entity B.

Note that the other node, either sending node MAY refuse transmit multiple XFER_SEGMENT
messages without necessarily waiting for the
session by sending corresponding XFER_ACK
responses. This enables pipelining of messages on a SHUTDOWN message with an appropriate reason
code.

The format channel.
Although this example only demonstrates a single bundle transmission,
it is also possible to pipeline multiple XFER_SEGMENT messages for the
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
======== ========
+-------------------------+
| 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!' -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+

Figure 3: 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:

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 protocol).

Flags: A one-octet field of single-bit flags, interpreted according
to the descriptions in Table 1.

Keepalive Interval: A 16-bit unsigned integer indicating the
interval, in seconds, between any subsequent messages being
transmitted by the peer. The peer receiving this contact header
uses this interval to determine how long to wait after any last-
message transmission and a necessary subsequent KEEPALIVE message
transmission.

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 Length [L1+L2+L3] |
+-------------------------+

+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+

Figure 3: An example of the peer's Segment MRU. The two nodes flow of protocol messages on a single session MAY have different Segment MRUs, and no relation TCP
Session 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 entities

4. Session Establishment

For bundle transfer sent transmissions to this peer SHALL have a Total Bundle
Length payload no longer than occur using the peer's Transfer MRU. This value
can be used to perform proactive bundle fragmentation. The two
nodes of TCPCL, a single TCPCL session MAY have different Transfer MRUs, and no
relation
MUST first be established between communicating entities. It is up
to the two implementation to decide how and when session setup is required.

EID Length
triggered. For example, some sessions MAY be opened proactively and EID Data: Together these fields represent a variable-
length text string. The EID Length
maintained for as long as is a 16-bit unsigned integer
indicating possible given the number of octets of EID Data to follow. A zero EID
Length SHALL network conditions,
while other sessions MAY be used to indicate opened only when there is a bundle that
is queued for transmission and the lack of EID rather than routing algorithm selects a
truly empty EID. This case allows
certain next-hop node.

4.1. TCP Connection

To establish a node to avoid exposing EID
information on TCPCL session, an untrusted network. A non-zero-length EID Data
SHALL contain the UTF-8 encoded EID of some singleton endpoint in
which the sending node is a member, in the canonical format of
<scheme name>:<scheme-specific part>. This EID encoding is
consistent entity MUST first establish a TCP
connection with [I-D.ietf-dtn-bpbis].

Header Extension Length and Header Extension Items: Together these
fields represent protocol extension data not defined the intended peer entity, typically by this
specification. The Header Extension Length is using the total
services provided by the operating system. Destination port number of
octets to follow which are used to encode
4556 has been assigned by IANA as the Header Extension
Item list. The encoding of each Header Extension Item is within Registered Port number for the
TCP convergence layer. Other destination port numbers MAY be used
per local configuration. Determining a
consistent data container as described in Section 4.2.1.

Table 1: Contact Header Flags

4.2.1. Header Extension Items

Each of up to
the Header Extension Items SHALL implementation. Any source port number MAY be encoded in used for TCPCL
sessions. Typically an identical
Type-Length-Value (TLV) container form as indicated operating system assigned number in Figure 4. The
fields of the Header Extension Item are:

Flags: A one-octet field containing generic bit flags about the
Item, which are listed in Table 2. TCP
Ephemeral range (49152-65535) is used.

If the entity is unable to establish a TCPCL node receives a
Header Extension Item TCP connection for any reason,
then it is an implementation matter to determine how to handle the
connection failure. An entity MAY decide to re-attempt to establish
the connection. If it does so, it MUST NOT overwhelm its target with an unknown Item Type and
repeated connection attempts. Therefore, the CRITICAL
flag set, entity MUST retry the node SHALL close
connection setup no earlier than some delay time from the TCPCL session with SHUTDOWN
reason code last
attempt, and it SHOULD use a (binary) exponential backoff mechanism
to increase this delay in case of "Contact Failure". If the CRITICAL flag repeated failures.

Once a TCP connection is not
set, established, each entity MUST immediately
transmit a node SHALL skip contact header over and ignore any item with an unknown
Item Type.

Item Type: A 16-bit unsigned integer field containing the type TCP connection. The format of the extension item. This specification does not define any
extension types directly, but does allocate an IANA registry for
such codes (see
contact header is described in Section 8.3).

Item Length: A 32-bit unsigned integer field containing the number
of Item Value octets to follow.

Item Value: A variable-length data field which 4.2.

4.2. Contact Header

Once a TCP connection is interpreted
according to the associated Item Type. established, both parties exchange a contact
header. This specification places
no restrictions on an extension's use section describes the format of available Item Value
data. Extension specification SHOULD avoid the use contact header and
the meaning of its fields.

Upon receipt of large data
exchanges within the TCPCL contact header, both entities perform the
validation and negotiation procedures defined in Section 4.3. After
receiving the contact header as no bundle transfers
can begin until 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 full contact exchange and negotiation has been
completed. 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
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type magic='dtn!' | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+ Version | value contd. Flags |
+---------------+---------------+---------------+---------------+
+---------------+---------------+

Figure 4: Contact Header Extension Item Format

See Section 4.3 for details on the use of each of these contact
header fields. The fields of the contact header are:

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 protocol).

Flags: A one-octet field of single-bit flags, interpreted according
to the descriptions in Table 1.

Table 2: 1: Contact Header Extension Item Flags

4.3. Contact Validation and Parameter Negotiation

Upon reception of the contact header, each node 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 node an entity
MAY elect to hold an invalid connection open and idle for some time
before closing it.

A connecting TCPCL node SHALL send the highest TCPCL protocol version
on a first session attempt for a TCPCL peer. If a connecting node
receives a SHUTDOWN SESS_TERM message with reason of "Version Mismatch", that
node 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 a node an entity receives a contact header containing a version that is
greater than the current version of the protocol that the node
implements, then the node SHALL shutdown the session with a reason
code of "Version mismatch". If a node an entity receives a contact header
with a version that is lower than the version of the protocol that
the node implements, the node MAY either terminate the session (with a
reason code of "Version mismatch") or the node MAY adapt its
operation to conform to the older version of the protocol. The
decision of version fall-back is an implementation matter.

A node calculates the parameters for a TCPCL session by negotiating
the values from its own preferences (conveyed by the contact header
it sent to the peer) with the preferences of the peer node (expressed
in the contact header 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 transmit unit (MTU) for
whole transfers and individual segments are idententical to the
Transfer MRU and Segment MRU, respectively, session (with
a reason code of "Version mismatch") or the recevied
contact header. A transmitting peer can send individual segments
with any size smaller than node MAY adapt its
operation to conform to the Segment MTU, depending on local
policy, dynamic network conditions, etc. Determining older version of the size protocol. The
decision of
each transmitted segment version fall-back is an implementation matter.

4.4. Session Keepalive: Negotiation Security

This version of the Session Keepalive parameter TCPCL supports establishing a Transport Layer
Security (TLS) session within an existing TCP connection. When TLS
is
performed by taking used within the minimum of TCPCL it affects the entire session. Once
established, there is no mechanism available to downgrade a TCPCL
session to non-TLS operation. If this two contact headers'
Keepalive Interval. The Session Keepalive interval is desired, the entire TCPCL
session MUST be shutdown and a parameter
for new non-TLS-negotiated session
established.

The use of TLS is negotated using the behavior Contact Header as described in
Section 5.2.1. 4.3. After negotiating an Enable TLS: Negotiation TLS parameter of true, and
before any other TCPCL messages are sent within the Enable session, the
session entities SHALL begin a TLS parameter is performed by
taking handshake in accordance with
[RFC5246]. The parameters within each TLS negotiation are
implementation dependent but any TCPCL node SHOULD follow all
recommended best practices of [RFC7525]. By convention, this
protocol uses the logical AND node which initiated the underlying TCP connection
as the "client" role of the two contact headers' CAN_TLS flags.
A local security policy TLS handshake request.

The TLS handshake, if it occurs, is then applied considered to determine be part of the
negotated value of Enable TLS
contact negotiation before the TCPCL session itself is acceptable. established.
Specifics about sensitive data exposure are discussed in Section 8.

4.4.1. TLS Handshake Result

If not, a TLS handshake cannot negotiate a TLS session, both entities of
the node TCPCL session SHALL start a TCPCL shutdown the session with a reason code of "Contact "TLS
Failure". Note that this contact failure is different than

After a "TLS
Failure" after an agreed-upon and acceptable Enable TLS state. If
the negotiated Enable TLS value session is true and acceptable then TLS
negotiation feature (described successfully established, both TCPCL entities
SHALL re-exchange TCPCL Contact Header messages. Any information
cached from the prior Contact Header exchange SHALL be discarded.
This re-exchange avoids a "man-in-the-middle" attack in Section 4.4) begins immediately
following identical
fashion to [RFC2595]. Each re-exchange header CAN_TLS flag SHALL be
identical to the contact original header CAN_TLS flag from the same node.
The CAN_TLS logic (TLS negotiation) SHALL NOT apply during header re-
exchange.

Once this process of parameter negotiation This reinforces the fact that there is completed (which
includes no TLS downgrade
mechanism.

4.4.2. Example TLS Initiation

A summary of a possible completed typical CAN_TLS usage is shown in the sequence in
Figure 5 below.

Entity A Entity B
======== ========

+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+

+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+

+-------------------------+ +-------------------------+
| TLS handshake Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+

... secured TCPCL messaging, starting with SESS_INIT ...

+-------------------------+ +-------------------------+
| SESS_TERM | -> <- | SESS_TERM |
+-------------------------+ +-------------------------+

Figure 5: A simple visual example of TCPCL TLS Establishment between
two entities

4.5. Message Type Codes

After the connection to use
TLS), this protocol defines no additional mechanism to change the
parameters initial exchange of an established session; to effect such a change, contact header, all messages
transmitted over the
TCPCL session MUST be terminated and a new session established.

4.3.1. Reactive Fragmentation Extension

In order to allow BP agents to use this reliable convergence layer to
perform reactive fragmentation, are identified by a one-octet header extension type
REACTIVE_FRAGMENT is defined to negotate
with the fragmentation
capabilities following structure:

0 1 2 3 4 5 6 7
+---------------+
| Message Type |
+---------------+

Figure 6: Format of the node sending the extension item. If either node
does not send a REACTIVE_FRAGMENT item then no reactive fragmentation
is allowed to be initiated within that session. Reactive
fragmentation is performed after a failed transfer, so it necessarily
spans more than a single TCPCL session. In fact, follow-on bundle
fragments may be sent via an entirely different convergence layer.
For these reasons, details of how reactive fragmentation and
reassembly takes place Message Header

The message header fields are outside as follows:

Message Type: Indicates the scope of this specification.

Within a single contact header there SHALL be no more than one item
with an extension type of REACTIVE_FRAGMENT. If no REACTIVE_FRAGMENT
item is received the message as per Table 2
below. Encoded values are listed in Section 9.5.

+--------------+----------------------------------------------------+
| Type | Description |
+--------------+----------------------------------------------------+
| SESS_INIT | Contains the session parameter inputs from a peer, all REACTIVE_FRAGMENT flags of that
peer SHALL be considered to be not set. The CRITICAL flag one of |
| | the
REACTIVE_FRAGMENT item MAY be set to indicate that the peer node has
to interpret and negotiate entities, as described in Section 4.6. |
| | |
| XFER_INIT | Contains the reactive fragmentation capability.
The order length (in octets) of the REACTIVE_FRAGMENT item within next |
| | transfer, as described in Section 5.2.2. |
| | |
| XFER_SEGMENT | Indicates the extension items is
not significant. The Item Length transmission of a REACTIVE_FRAGMENT item SHALL
be a single octet. The contents segment of the REACTIVE_FRAGMENT item shall
be interpreted bundle |
| | data, as described in Section 5.2.3. |
| | |
| XFER_ACK | Acknowledges reception of a bit mask, with flags interpreted according to
Table 3.

When a transfer-sending node has set the CAN_GENERATE flag and the
peer node has set the CAN_RECEIVE flag, the sending node SHALL use
acknowledged data segment information to reactively fragment a failed
transfer within some later transfers. When a transfer-receving node
has set the CAN_RECEIVE flag and the peer node has set segment, as |
| | described in Section 5.2.4. |
| | |
| XFER_REFUSE | Indicates that the
CAN_GENERATE flag, transmission of the receving node current |
| | bundle SHALL treat partial received
transfers be stopped, as reactively fragmented bundles and use the partial
transfer described in Section |
| | 5.2.5. |
| | |
| KEEPALIVE | Used to reassemble future fragments of that bundle.

Table 2: TCPCL Message Types

4.6. Session Initialization Message (SESS_INIT)

Before a session is set, indicates that established and ready to transfer bundles, the sending |
| | | node
session parameters are negotiated between the connected entities.
The SESS_INIT message is capable used to convey the per-entity parameters
which are used together to negotiate the per-session parameters.

The format of receving and |
| a SESS_INIT message is as follows in Figure 7.

Table 3: REACTIVE_FRAGMENT Flags

4.4. Session Security

This version of Extension Items (var.)|
+-------------------------------+

Figure 7: SESS_INIT Format

A 16-bit unsigned integer indicating the TCPCL supports establishing a Transport Layer
Security (TLS) session within an existing TCP connection. When TLS
is used within interval, in seconds,
between any subsequent messages being transmitted by the TCPCL it affects peer.
The peer receiving this contact header uses this interval to
determine how long to wait after any last-message transmission and
a necessary subsequent KEEPALIVE message transmission.

A 64-bit unsigned integer indicating the entire largest allowable single-
segment data payload size to be received in this session. Once
established, there is no mechanism available Any
XFER_SEGMENT sent to downgrade this peer SHALL have a TCPCL data payload no longer
than the peer's Segment MRU. The two entities of a single session
MAY have different Segment MRUs, and no relation between the two
is required.

A 64-bit unsigned integer indicating the largest allowable total-
bundle data size to non-TLS operation. If be received in this is desired, session. Any bundle
transfer sent to this peer SHALL have a Total Bundle Length
payload no longer than the entire TCPCL peer's Transfer MRU. This value can be
used to perform proactive bundle fragmentation. The two entities
of a single session MUST be shutdown MAY have different Transfer MRUs, and no
relation between the two is required.

Together these fields represent a new non-TLS-negotiated session
established. variable-length text string.
The use of TLS EID Length is negotated using a 16-bit unsigned integer indicating the Contact Header as described in
Section 4.3. After negotiating an Enable TLS parameter number
of true, and
before any other TCPCL messages are sent within the session, the
session nodes octets of EID Data to follow. A zero EID Length SHALL begin a TLS handshake in accordance with

[RFC5246]. The parameters within each TLS negotiation are
implementation dependent but any TCPCL node SHOULD follow all
recommended best practices be used
to indicate the lack of [RFC7525]. By convention, this
protocol uses EID rather than a truly empty EID. This
case allows an entity to avoid exposing EID information on an
untrusted network. A non-zero-length EID Data SHALL contain the node
UTF-8 encoded EID of some singleton endpoint in which initiated the underlying TCP connection
as sending
entity is a member, in the "client" role canonical format of the TLS handshake request. <scheme
name>:<scheme-specific part>. This EID encoding is consistent
with [I-D.ietf-dtn-bpbis].

Together these fields represent protocol extension data not
defined by this specification. The TLS handshake, if it occurs, Session Extension Length is considered to be part of
the
contact negotiation before total number of octets to follow which are used to encode the TCPCL session itself
Session Extension Item list. The encoding of each Session
Extension Item is established.
Specifics about sensitive data exposure are discussed in Section 7.

4.4.1. TLS Handshake Result

If a TLS handshake cannot negotiate within a TLS session, both nodes consistent data container as described
in Section 4.6.1. The full set of Session Extension Items apply
for the duration of the TCPCL session SHALL start a TCPCL shutdown with reason "TLS Failure".

After a TLS session is successfully established, both TCPCL nodes
SHALL re-exchange TCPCL Contact Header messages. Any information
cached from the prior Contact Header exchange SHALL to follow. The order and
mulitplicity of these Session Extension Items MAY be discarded.
This re-exchange avoids a "man-in-the-middle" attack significant,
as defined in identical
fashion to [RFC2595]. the associated type specification(s).

4.6.1. Session Extension Items

Each re-exchange header CAN_TLS flag of the Session Extension Items SHALL be encoded in an identical to the original header CAN_TLS flag from the same node.
Type-Length-Value (TLV) container form as indicated in Figure 8. The CAN_TLS logic (TLS negotiation) SHALL NOT apply during header re-
exchange. This reinforces the fact that there is no TLS downgrade
mechanism.

4.4.2. Example TLS Initiation

A summary
fields of a typical CAN_TLS usage is shown in the sequence in
Figure 5 below.

Node A Node B
====== ======

+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+

+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+

+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+

+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+

... secured TCPCL messaging ...

+-------------------------+ +-------------------------+
| SHUTDOWN | -> <- | SHUTDOWN |
+-------------------------+ +-------------------------+

Figure 5: Session Extension Item are:

Flags: A simple visual example of one-octet field containing generic bit flags about the
Item, which are listed in Table 3. If a TCPCL TLS Establishment between
two nodes

5. Established entity receives a
Session Operation

This section describes Extension Item with an unknown Item Type and the protocol operation for CRITICAL
flag set, the duration entity SHALL close the TCPCL session with SESS_TERM
reason code of "Contact Failure". If the CRITICAL flag is not
set, an
established session, including entity SHALL skip over and ignore any item with an unknown
Item Type.

Item Type: A 16-bit unsigned integer field containing the mechanism type of
the extension item. This specification does not define any
extension types directly, but does allocate an IANA registry for transmitting bundles
over
such codes (see Section 9.3).

Item Length: A 32-bit unsigned integer field containing the session.

5.1. Message Type Codes

After number
of Item Value octets to follow.

Item Value: A variable-length data field which is interpreted
according to the initial exchange associated Item Type. This specification places
no restrictions on an extension's use of a contact header, all messages
transmitted over available Item Value
data. Extension specification SHOULD avoid the session are identified by a one-octet use of large data
exchanges within the TCPCL contact header
with as no bundle transfers
can begin until the following structure: full contact exchange and negotiation has been
completed.

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
+---------------+
| Message Type |
+---------------+

Figure 6: Format of the Message Header

The message header fields are as follows:

Message Type: Indicates the type of the message as per Table 8 9 0 1 2 3 4
below. Encoded values are listed in Section 8.4.

+--------------+----------------------------------------------------+ 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Description Item Length...|
+---------------+---------------+---------------+---------------+
|
+--------------+----------------------------------------------------+ length contd. | XFER_INIT Item Value... | Contains the length (in octets) of the next
+---------------+---------------+---------------+---------------+
| value contd. |
+---------------+---------------+---------------+---------------+

Figure 8: Session Extension Item Format

Table 3: Session Extension Item Flags

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
contact header it sent to the peer) with the preferences of the peer
node (expressed in the contact header 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 transmit unit (MTU) for
whole transfers and individual segments are idententical to the
Transfer MRU and Segment MRU, respectively, of the recevied
contact header. 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.

Session Keepalive: Negotiation of the Session Keepalive parameter is
performed by taking the minimum of this two contact headers'
Keepalive Interval. The Session Keepalive interval is a data segment, as |
| | parameter
for the behavior described in Section 5.3.4. |
| | |
| XFER_REFUSE | Indicates that 5.1.1.

Enable TLS: Negotiation of the transmission Enable TLS parameter is performed by
taking the logical AND of the current |
| | bundle SHALL be stopped, as described in Section |
| | 5.3.5. |
| | |
| KEEPALIVE | Used two contact headers' CAN_TLS flags.
A local security policy is then applied to keep TCPCL session active, as described in |
| | Section 5.2.1. |
| | |
| SHUTDOWN | Indicates that one determine of the nodes participating in |
| |
negotated value of Enable TLS is acceptable. If not, the node
SHALL shutdown the session wishes to cleanly terminate with a reason code of "Contact
Failure". Note that this contact failure is different than a "TLS
Failure" after an agreed-upon and acceptable Enable TLS state. If
the |
| | session, as described negotiated Enable TLS value is true and acceptable then TLS
negotiation feature (described in Section 6. |
| | |
| MSG_REJECT | Contains 4.4) begins immediately
following the contact header exchange.

Once this process of parameter negotiation is completed (which
includes a possible completed TLS handshake of the connection to use
TLS), this protocol defines no additional mechanism to change the
parameters of an established session; to effect such a change, the
TCPCL message rejection, as described |
| | in Section 5.2.2. |
+--------------+----------------------------------------------------+

Table 4: TCPCL Message Types

5.2. session MUST be terminated and a new session established.

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

5.2.1.

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 4.3, a negotiated parameter of each session
is the Session Keepalive interval. If the negotiated Session
Keepalive is zero (i.e. one or both contact headers contains a zero
Keepalive Interval), then the keepalive feature is disabled. There
is no logical minimum value for the keepalive interval, but when used
for many sessions on an open, shared network a short interval could
lead to excessive traffic. For shared network use, nodes entities SHOULD
choose a keepalive interval no shorter than 30 seconds. There is no
logical maximum value for the keepalive interval, but an idle TCP
connection is liable for closure by the host operating system if the
keepalive time is longer than tens-of-minutes. Nodes Entities SHOULD
choose a keepalive interval no longer than 10 minutes (600 seconds).

Note: The Keepalive Interval SHOULD NOT be chosen too short as TCP
retransmissions MAY occur in 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 MAY experience noticeable latency.

The format of a KEEPALIVE message is a one-octet message type code of
KEEPALIVE (as described in Table 4) 2) with no additional data. Both
sides SHOULD 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 node MAY
terminate the session by transmitting by transmitting a SESS_TERM message (as
described in Section 6.1) with reason code "Idle Timeout.

5.1.2. Message Rejection (MSG_REJECT)

If a TCPCL node receives a message which is unknown to it (possibly
due to an unhandled protocol mismatch) or is inappropriate for the
current session state (e.g. a KEEPALIVE message received after
contact header negotiation has disabled that feature), there is a
protocol-level message to signal this condition in the form of a
MSG_REJECT reply.

The format of a MSG_REJECT message is as follows in Figure 9.

+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+

Figure 9: Format of MSG_REJECT Messages

The fields of the MSG_REJECT message are:

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.

Table 4: MSG_REJECT Reason Codes

5.2. Bundle Transfer

All of the messages in Section 6.1) this section are directly associated with reason code "Idle Timeout.

5.2.2. Message Rejection (MSG_REJECT)

If
transferring a bundle between TCPCL node receives entities.

A single TCPCL transfer results in a message which is unknown to it (possibly
due bundle (handled by the
convergence layer as opaque data) being exchanged from one node to an unhandled protocol mismatch) or is inappropriate for
the
current session state (e.g. other. In TCPCL a KEEPALIVE message received after
contact header negotiation has disabled that feature), there transfer is accomplished by dividing a
protocol-level message single
bundle up into "segments" based on the receiving-side Segment MRU
(see Section 4.2). The choice of the length to signal this condition in use for segments is
an implementation matter, but each segment MUST be no larger than the form
receiving node's maximum receive unit (MRU) (see the field "Segment
MRU" of a
MSG_REJECT reply. Section 4.2). The format of first segment for a MSG_REJECT bundle MUST set the
'START' flag, and the last one MUST set the 'end' flag in the
XFER_SEGMENT message follows:

+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+

Figure 7: Format of MSG_REJECT Messages

The fields flags.

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 MSG_REJECT message are:

Reason Code: A one-octet refusal reason code interpreted according
to agent using the descriptions in Table 5.

Rejected Message Header: The Rejected Message Header is TCPCL rather than TCPCL itself.

If multiple bundles are transmitted on a copy single TCPCL connection,
they MUST be transmitted consecutively without interleaving of
segments from multiple bundles.

5.2.1. Bundle Transfer ID

Each of the Message Header to bundle transfer messages contains a Transfer ID which the MSG_REJECT message is sent as
used to correlate messages (from both sides of a
response.

+-------------+------+----------------------------------------------+
| Name | Code | Description |
+-------------+------+----------------------------------------------+
| Message | 0x01 | transfer) for each
bundle. A message was received with a Message Type |
| Type | | code unknown Transfer ID does not attempt to address uniqueness of the
bundle data itself and has no relation to concepts such as bundle
fragmentation. Each invocation of TCPCL node. |
| Unknown | | |
| | | |
| Message | 0x02 | A message was received but by the bundle protocol
agent, requesting transmission of a bundle (fragmentary or
otherwise), results in the initiation of a single TCPCL node |
| Unsupported | | cannot comply with transfer.
Each transfer entails the sending of a XFER_INIT message contents. |
| | | |
| Message | 0x03 | A message was received while and some
number of XFER_SEGMENT and XFER_ACK messages; all are correlated by
the session is |
| Unexpected | | in same Transfer ID.

Transfer IDs from each node SHALL be unique within a state in which single TCPCL
session. The initial Transfer ID from each node SHALL have value
zero. Subsequent Transfer ID values SHALL be incremented from the message is not |
| | | expected. |
+-------------+------+----------------------------------------------+

Table 5: MSG_REJECT Reason Codes

5.3. Bundle
prior Transfer

All ID value by one. Upon exhaustion of the messages in this section are directly associated entire 64-bit
Transfer ID space, the sending node SHALL terminate the session with
transferring a
SESS_TERM reason code "Resource Exhaustion".

For bidirectional bundle between transfers, a TCPCL nodes.

A single node SHOULD NOT rely on
any relation between Transfer IDs originating from each side of the
TCPCL transfer results session.

5.2.2. Transfer Initialization (XFER_INIT)

The XFER_INIT message contains the total length, in a bundle (handled by octets, of the
convergence layer as opaque data) being exchanged from one node to
bundle data in the other. In TCPCL a transfer associated transfer. The total length is accomplished by dividing
formatted as a single
bundle up into "segments" based on the receiving-side Segment MRU
(see Section 4.2). 64-bit unsigned integer.

The choice purpose of the length to use for segments XFER_INIT message is
an implementation matter, but each segment MUST be no larger than to allow entities to
preemptively refuse bundles that would exceed their resources or to
prepare storage on the receiving node's maximum receive unit (MRU) (see node for the field "Segment
MRU" of upcoming bundle data.
See Section 4.2). The first segment 5.2.5 for details on when refusal based on XFER_INIT
content is acceptable.

The Total Bundle Length field within a XFER_INIT message SHALL be
treated as authoritative by the receiver. If, for whatever reason,
the actual total length of bundle MUST set data received differs from the
value indicated by the
'START' flag, and XFER_INIT message, the last one MUST set receiver SHOULD treat
the 'end' flag transmitted data as invalid.

The format of the XFER_INIT message is as follows in Figure 10.

+-----------------------------+
| Message Header |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Total Bundle Length (U64) |
+-----------------------------+
| Transfer Extension |
| Length (U64) |
+-----------------------------+
| Transfer Extension Items... |
+-----------------------------+

Figure 10: Format of XFER_INIT Messages

The fields of the
XFER_SEGMENT XFER_INIT message flags. are:

Transfer ID: A single 64-bit unsigned integer identifying the transfer (and by
about to begin.

Total Bundle Length: A 64-bit unsigned integer indicating the size
of the data-to-be-transferred.

Transfer Extension Length and Transfer Extension Items: Together
these fields represent protocol extension a single segment) SHALL NOT
contain data of more than a single bundle. This requirement not defined by this
specification. The Transfer Extension Length is
imposed on the agent using the TCPCL rather than TCPCL itself.

If multiple bundles are transmitted on a single TCPCL connection,
they MUST be transmitted consecutively without interleaving of
segments from multiple bundles.

5.3.1. Bundle Transfer ID

Each total number
of the bundle transfer messages contains a Transfer ID octets to follow which is are used to correlate messages (from both sides encode the Transfer
Extension Item list. The encoding of a transfer) for each
bundle. A Transfer ID does not attempt to address uniqueness of the
bundle Extension Item
is within a consistent data itself and has no relation to concepts such container as bundle
fragmentation. Each invocation of TCPCL by the bundle protocol
agent, requesting transmission of a bundle (fragmentary or
otherwise), results described in the initiation
Section 5.2.2.1. The full set of a transfer extension items apply
only to the assoicated single TCPCL transfer.
Each The order and
mulitplicity of these transfer entails extension items MAY be significant,
as defined in the sending of a associated type specification(s).

An XFER_INIT message and some
number SHALL be sent as the first message in a transfer
sequence, before transmission of any XFER_SEGMENT and XFER_ACK messages; all are correlated by messages for the
same Transfer ID.

Transfer IDs from each node SHALL be unique within a single TCPCL
session. The initial Transfer ID from each node SHALL have value
zero. Subsequent Transfer ID values SHALL XFER_INIT messages MUST NOT be incremented from the
prior Transfer ID value by one. Upon exhaustion of sent unless the entire 64-bit
Transfer ID space,
next XFER_SEGMENT message has the sending node SHALL terminate 'START' bit set to "1" (i.e., just
before the session with
SHUTDOWN reason code "Resource Exhaustion".

For bidirectional bundle transfers, start of a TCPCL node SHOULD NOT rely on
any relation between new transfer).

5.2.2.1. Transfer IDs originating from each side Extension Items

Each of the
TCPCL session.

5.3.2. Transfer Initialization (XFER_INIT)

The XFER_INIT message contains the total length, in octets, of the
bundle data Extension Items SHALL be encoded in the associated transfer. The total length is
formatted an identical
Type-Length-Value (TLV) container form as a 64-bit unsigned integer. indicated in Figure 11.
The purpose fields of the XFER_INIT message is to allow nodes to
preemptively refuse bundles that would exceed their resources or to
prepare storage on Transfer Extension Item are:

Flags: A one-octet field containing generic bit flags about the
Item, which are listed in Table 5. If a TCPCL node receives a
Transfer Extension Item with an unknown Item Type and the CRITICAL
flag set, the receiving node for SHALL close the upcoming bundle data.
See Section 5.3.5 for details on when refusal based on XFER_INIT
content TCPCL session with SESS_TERM
reason code of "Contact Failure". If the CRITICAL flag is acceptable.

The Total Bundle Length field within a XFER_INIT message not
set, an entity SHALL be
treated as authoritative by skip over and ignore any item with an unknown
Item Type.

Item Type: A 16-bit unsigned integer field containing the receiver. If, type of
the extension item. This specification does not define any
extension types directly, but does allocate an IANA registry for whatever reason,
such codes (see Section 9.4).

Item Length: A 32-bit unsigned integer field containing the actual total length number
of bundle Item Value octets to follow.

Item Value: A variable-length data received differs from the
value indicated by the XFER_INIT message, field which is interpreted
according to the receiver associated Item Type. This specification places
no restrictions on an extension's use of available Item Value
data. Extension specification SHOULD treat avoid the transmitted data as invalid.

The format use of large data
exchanges within the XFER_INIT as the associated transfer cannot
begin until the full initialization message is as follows:

+-----------------------------+ 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... | Message Header
+---------------+---------------+---------------+---------------+
|
+-----------------------------+ value contd. |
+---------------+---------------+---------------+---------------+

Figure 11: Transfer ID (U64) Extension Item Format

+----------+--------+-----------------------------------------------+
|
+-----------------------------+ Name | Total Bundle Length (U64) Code |
+-----------------------------+

Figure 8: Format of XFER_INIT Messages

The fields of the XFER_INIT message are:

Transfer ID: A 64-bit unsigned integer identifying the transfer
about to begin.

Total Bundle Length: A 64-bit unsigned integer indicating the size
of the data-to-be-transferred.

An XFER_INIT message SHALL be sent as Description |
+----------+--------+-----------------------------------------------+
| CRITICAL | 0x01 | If bit is set, indicates that the first message in a transfer
sequence, before transmission of any XFER_SEGMENT messages for receiving |
| | | peer must handle the
same extension item. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+

Table 5: Transfer ID. XFER_INIT messages MUST NOT be sent unless the
next XFER_SEGMENT message has the 'START' bit set to "1" (i.e., just
before the start of a new transfer).

5.3.3. Extension Item Flags

5.2.3. Data Transmission (XFER_SEGMENT)

Each bundle is transmitted in one or more data segments. The format
of a XFER_SEGMENT message follows in Figure 9. 12.

+------------------------------+
| Message Header |
+------------------------------+
| Message Flags (U8) |
+------------------------------+
| Transfer ID (U64) |
+------------------------------+
| Data length (U64) |
+------------------------------+
| Data contents (octet string) |
+------------------------------+

Figure 9: 12: Format of XFER_SEGMENT Messages

The fields of the XFER_SEGMENT message are:

Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 6.

Transfer ID: A 64-bit unsigned integer identifying the transfer
being made.

Data length: A 64-bit unsigned integer indicating the number of
octets in the Data contents to follow.

Data contents: The variable-length data payload of the message.

Table 6: XFER_SEGMENT Flags

The flags portion of the message contains two optional values in the
two low-order bits, denoted 'START' and 'END' in Table 6. The
'START' bit MUST be set to one if it precedes the transmission of the
first segment of a transfer. The 'END' bit MUST be set to one when
transmitting the last segment of a transfer. In the case where an
entire transfer is accomplished in a single segment, both the 'START'
and 'END' bits MUST be set to one.

Once a transfer of a bundle has commenced, the node MUST only send
segments containing sequential portions of that bundle until it sends
a segment with the 'END' bit set. No interleaving of multiple
transfers from the same node is possible within a single TCPCL
session. Simultaneous transfers between two nodes entities MAY be achieved
using multiple TCPCL sessions.

5.3.4.

5.2.4. 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 the segment.
To this end, the TCPCL protocol provides feedback messaging whereby a
receiving node transmits acknowledgments of reception of data
segments.

The format of an XFER_ACK message follows in Figure 10. 13.

+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Acknowledged length (U64) |
+-----------------------------+

Figure 10: 13: Format of XFER_ACK Messages

The fields of the XFER_ACK message are:

Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 6.

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 which are being acknowledged.

A receiving TCPCL node SHALL send an XFER_ACK message in response to
each received XFER_SEGMENT message. The flags portion of the
XFER_ACK header SHALL be set to match the corresponding DATA_SEGMENT
message being acknowledged. 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 node MAY transmit multiple XFER_SEGMENT messages without
necessarily waiting for the corresponding XFER_ACK responses. This
enables pipelining of messages on a channel.

For example, suppose the sending node transmits four segments of
bundle data with lengths 100, 200, 500, and 1000, respectively.
After receiving the first segment, the node sends an acknowledgment
of length 100. After the second segment is received, the node sends
an acknowledgment of length 300. The third and fourth
acknowledgments are of length 800 and 1800, respectively.

5.3.5.

5.2.5. Transfer Refusal (XFER_REFUSE)

The TCPCL supports a mechanism by which a receiving node can indicate
to the sender that it does not want to receive the corresponding
bundle. To do so, upon receiving a XFER_INIT or XFER_SEGMENT
message, the node MAY transmit a XFER_REFUSE message. As data
segments and acknowledgments MAY cross on the wire, the bundle that
is being refused SHALL be identified by the Transfer ID of the
refusal.

There is no required relation between the Transfer MRU of a TCPCL
node (which is supposed to represent a firm limitation of what the
node will accept) and 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 receiver MAY send an XFER_REFUSE message as soon as it receives a
XFER_INIT message without waiting for the next XFER_SEGMENT message.
The sender MUST be prepared for this and MUST associate the refusal
with the correct bundle via the Transfer ID fields.

The format of the XFER_REFUSE message is as follows: follows in Figure 14.

+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+

Figure 11: 14: Format of XFER_REFUSE Messages

The fields of the XFER_REFUSE message are:

Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 7.

Transfer ID: A 64-bit unsigned integer identifying the transfer
being refused.

Table 7: XFER_REFUSE Reason Codes

The receiver MUST, for each transfer preceding the one to be refused,
have either acknowledged all XFER_SEGMENTs 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 commence
transmission of any further segments of the refused bundle
subsequently. Note, however, that this requirement does not ensure
that a node an entity will not receive another XFER_SEGMENT for the same
bundle after transmitting a XFER_REFUSE message since messages MAY
cross on the wire; if this happens, subsequent segments of the bundle
SHOULD also be refused with a XFER_REFUSE message.

Note: If a bundle transmission is aborted in this way, the receiver
MAY 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' bit set to '1', indicating the start of a new transfer,
and with a distinct Transfer ID value.

6. Session Termination

This section describes the procedures for ending a TCPCL session.

6.1. Shutdown Session Termination Message (SHUTDOWN) (SESS_TERM)

To cleanly shut down a session, a SHUTDOWN SESS_TERM message SHALL be
transmitted by either node at any point following complete
transmission of any other message. Upon receiving a SHUTDOWN SESS_TERM
message after not sending a SHUTDOWN SESS_TERM message in the same session, a node an
entity SHOULD send a confirmation SHUTDOWN SESS_TERM message with identical
content to the SHUTDOWN SESS_TERM for which it is confirming.

After sending a SHUTDOWN SESS_TERM message, a node an entity MAY continue a possible in-
progress
in-progress transfer in either direction. After sending a SHUTDOWN SESS_TERM
message, a node an entity SHALL NOT begin any new outgoing transfer (i.e.
send an XFER_INIT message) for the remainder of the session. After
receving a SHUTDOWN SESS_TERM message, a node an entity SHALL NOT accept any new
incoming transfer for the remainder of the session.

Instead of following a clean shutdown sequence, after transmitting a
SHUTDOWN
SESS_TERM message a node an entity MAY immediately close the associated TCP
connection. When performing an unclean shutdown, a receiving node
SHOULD acknowledge all received data segments before closing the TCP
connection. When performing an unclean shutodwn, a transmitting node
SHALL treat either sending or receiving a SHUTDOWN SESS_TERM message (i.e.
before the final acknowledgment) as a failure of the transfer. Any
delay between request to terminate the TCP connection and actual
closing of the connection (a "half-closed" state) MAY be ignored by
the TCPCL node.

The format of the SHUTDOWN SESS_TERM message is as follows: follows in Figure 15.

+-----------------------------------+
| Message Header |
+-----------------------------------+
| Message Flags (U8) |
+-----------------------------------+
| Reason Code (optional U8) |
+-----------------------------------+
| Reconnection Delay (optional U16) |
+-----------------------------------+

Figure 12: 15: Format of SHUTDOWN SESS_TERM Messages

The fields of the SHUTDOWN SESS_TERM message are:

Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 8.

Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 9. The Reason Code is present or
absent as indicated by one of the flags.

Reconnection Delay: A 16-bit unsigned integer indicating
according to the desired
delay, descriptions in seconds, before re-attepmting a TCPCL session Table 8.

Reason Code: A one-octet refusal reason code interpreted according
to the
sending node. descriptions in Table 9. The Reconnection Delay Reason Code is present or
absent as indicated by one of the flags.

+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| D | 0x01 | If bit is set, indicates that a Reconnection |
| | | Delay field is present. |
| | | |
| R | 0x02 | If bit is set, indicates that a Reason Code |
| | | field is present. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+

Table 8: SHUTDOWN SESS_TERM Flags

It is possible for a node an entity to convey optional information regarding
the reason for session termination. To do so, the node MUST set the
'R' bit in the message flags and transmit a one-octet reason code
immediately following the message header. The specified values of
the reason code are:

Table 9: SHUTDOWN SESS_TERM Reason Codes

If a node does not want its peer to reopen a connection immediately,
it SHALL set the 'D' bit in the flags and include a reconnection
delay to indicate when the peer is allowed to attempt another session
setup. The Reconnection Delay value 0 SHALL be interpreted as an
infinite delay, i.e., that the connecting node MUST NOT re-establish
the session.

A session shutdown MAY occur immediately after transmission of a
contact header (and prior to any further message transmit). This
MAY, for example, be used to notify that the node is currently not
able or willing to communicate. However, a node an entity MUST always send
the contact header to its peer before sending a SHUTDOWN SESS_TERM message.

If reception of the contact header itself somehow fails (e.g. an
invalid "magic string" is recevied), a node an entity SHOULD close the TCP
connection without sending a SHUTDOWN SESS_TERM message. If the content of
the
Header Session Extension Items data disagrees with the Header Session Extension
Length (i.e. the last Item claims to use more octets than are present
in the Header Session Extension Length), the reception of the contact header
is considered to have failed.

If a session is to be terminated before a protocol message has
completed being sent, then the node MUST NOT transmit the SHUTDOWN SESS_TERM
message but still SHOULD 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 an other message. This is particularly
important when large segment sizes are being transmitted; either
entire XFER_SEGMENT is sent before a SHUTDOWN SESS_TERM message or the
connection is simply terminated mid-XFER_SEGMENT.

6.2. Idle Session Shutdown

The protocol includes a provision for clean shutdown of idle
sessions. Determining the length of time to wait before closing idle
sessions, if they are to be closed at all, is an implementation and
configuration matter.

If there is a configured time to close idle links and if no TCPCL
messages (other than KEEPALIVE messages) has been received for at
least that amount received for at
least that amount of time, then either node MAY terminate the session
by transmitting a SESS_TERM message indicating the reason code of
"Idle timeout" (as described in Table 9).

7. Implementation Status

[NOTE to the RFC Editor: please remove this section before
publication, as well as the reference to [RFC7942] and
[github-dtn-bpbis-tcpcl].]

This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.

An example implementation of the this draft of TCPCLv4 has been
created as a GitHub project [github-dtn-bpbis-tcpcl] and is intented
to use as a proof-of-concept and as a possible source of time, then either node MAY terminate
interoperability testing. This example implementation uses D-Bus as
the session
by transmitting a SHUTDOWN message indicating CL-BP Agent interface, so it only runs on hosts which provide the reason code of
"Idle timeout" (as described in Table 9).

7.
Python "dbus" library.

8. Security Considerations

One security consideration for this protocol relates to the fact that
nodes
entities present their endpoint identifier as part of the contact
header exchange. It would be possible for a node an entity to fake this
value and present the identity of a singleton endpoint in which the
node is not a member, essentially masquerading as another DTN node.
If this identifier is used outside of a TLS-secured session or
without further verification as a means to determine which bundles
are transmitted over the session, then the node that has falsified
its identity would be able to obtain bundles that it otherwise would
not have. Therefore, a node an entity SHALL NOT use the EID value of an
unsecured contact header to derive a peer node's identity unless it
can corroborate it via other means. When TCPCL session security is
mandated by a TCPCL peer, that peer SHALL transmit initial unsecured
contact header values indicated in Table 10 in order. These values
avoid unnecessarily leaking session parameters and will be ignored
when secure contact header re-exchange occurs.

Table 10: Recommended Unsecured Contact Header

TCPCL can be used to provide point-to-point transport security, but
does not provide security of data-at-rest and does not guarantee end-
to-end bundle security. The mechanisms defined in [RFC6257] and
[I-D.ietf-dtn-bpsec] are to be used instead.

Even when using TLS to secure the TCPCL session, the actual
ciphersuite negotiated between the TLS peers MAY be insecure. TLS
can be used to perform authentication without data confidentiality,
for example. It is up to security policies within each TCPCL node to
ensure that the negotiated TLS ciphersuite meets transport security
requirements. This is identical behavior to STARTTLS use in
[RFC2595].

Another consideration for this protocol relates to denial-of-service
attacks. A node An entity MAY send a large amount of data over a TCPCL
session, requiring the receiving node entity to handle the data, attempt
to stop the flood of data by sending a XFER_REFUSE message, or
forcibly terminate the session. This burden could cause denial of
service on other, well-behaving sessions. There is also nothing to
prevent a malicious node entity from continually establishing sessions and
repeatedly trying to send copious amounts of bundle data. A
listening node entity MAY take countermeasures such as ignoring TCP SYN
messages, closing TCP connections as soon as they are established,
waiting before sending the contact header, sending a SHUTDOWN SESS_TERM
message quickly or with a delay, etc.

9. IANA Considerations

In this section, registration procedures are as defined in [RFC5226].

Some of the registries below are created new for TCPCLv4 but share
code values with TCPCLv3. This was done to disambiguate the use of
these values between TCPCLv3 and TCPCLv4 while preserving the
semantics of some values.

8.1.

9.1. Port Number

Port number 4556 has been previously assigned as the default port for
the TCP convergence layer in [RFC7242]. This assignment is unchanged
by protocol version 4. Each TCPCL node entity identifies its TCPCL
protocol version in its initial contact (see Section 8.2), 9.2), so there
is no ambiguity about what protocol is being used.

8.2.

9.2. Protocol Versions

IANA has created, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version
Numbers" and initialize it with the following table. The
registration procedure is RFC Required.

+-------+-------------+---------------------+
| Value | Description | Reference |
+-------+-------------+---------------------+
| 0 | Reserved | [RFC7242] |
| | | |
| 1 | Reserved | [RFC7242] |
| | | |
| 2 | Reserved | [RFC7242] |
| | | |
| 3 | TCPCL | [RFC7242] |
| | | |
| 4 | TCPCLbis | This specification. |
| | | |
| 5-255 | Unassigned |
+-------+-------------+---------------------+

8.3. Header

9.3. Session Extension Types