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Versions: (draft-sipos-dtn-tcpclv4) 00 01 02 03 04 05 06 07 08 09

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


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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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
     3.1.  TCPCL Session Overview  . . . . . . . . . . . . . . . . .   7
     3.2.  Example Message Exchange  . . . . . . . . . . . . . . . .   8
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .  10
     4.1.  TCP Connection  . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Contact Header  . . . . . . . . . . . . . . . . . . . . .  11
       4.2.1.  Header Extension Items  . . . . . . . . . . . . . . .  14
     4.3.  Validation and Parameter Negotiation  . . . . . . . . . .  15
       4.3.1.  Reactive Fragmentation Extension  . . . . . . . . . .  16
     4.4.  Session Security  . . . . . . . . . . . . . . . . . . . .  17
       4.4.1.  TLS Handshake Result  . . . . . . . . . . . . . . . .  18
       4.4.2.  Example TLS Initiation  . . . . . . . . . . . . . . .  18
   5.  Established Session Operation . . . . . . . . . . . . . . . .  19
     5.1.  Message Type Codes  . . . . . . . . . . . . . . . . . . .  19
     5.2.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  20
       5.2.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  20
       5.2.2.  Message Rejection (MSG_REJECT)  . . . . . . . . . . .  21
     5.3.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  22
       5.3.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  23
       5.3.2.  Transfer Initialization (XFER_INIT) . . . . . . . . .  23
       5.3.3.  Data Transmission (XFER_SEGMENT)  . . . . . . . . . .  24
       5.3.4.  Data Acknowledgments (XFER_ACK) . . . . . . . . . . .  26
       5.3.5.  Transfer Refusal (XFER_REFUSE)  . . . . . . . . . . .  27
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  29
     6.1.  Shutdown Message (SHUTDOWN) . . . . . . . . . . . . . . .  29
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  32
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  32
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
     8.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  34
     8.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  34
     8.3.  Header Extension Types  . . . . . . . . . . . . . . . . .  35
     8.4.  Message Types . . . . . . . . . . . . . . . . . . . . . .  35



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     8.5.  XFER_REFUSE Reason Codes  . . . . . . . . . . . . . . . .  36
     8.6.  SHUTDOWN Reason Codes . . . . . . . . . . . . . . . . . .  37
     8.7.  MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . .  38
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  38
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     10.2.  Informative References . . . . . . . . . . . . . . . . .  39
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

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.










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         +-------------------------+
         |     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 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.  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.  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.



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   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
      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.

   Transmission Intermediate Progress  The TCPCL supports positive
      indication of intermediate progress of transferr to a peer node.
      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 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.

   Reception Intermediate Progress  The TCPCL supports positive
      indication of intermediate progress of transfer from the peer



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      node.  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 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:  This term refers to either side of a negotiating or in-
      service TCPCL Session.  For most TCPCL behavior, the two nodes are
      symmetric and there is no protocol distinction between them.  Some
      specific behavior, particularly during negotiation, distinguishes
      between the connecting node and the connected-to node.  For the
      remainder of this document, the term "node" without the prefix
      "TCPCL" refers to a TCPCL node.

   TCP Connection:  This term refers to a transport connection using TCP
      as the transport protocol.

   TCPCL Session:  A TCPCL session (as opposed to a TCP connection) is a
      TCPCL communication relationship between two bundle nodes.  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 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 and thereby to
      the TCPCL is implementation dependent.  However, the mechanism by




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      which two bundle nodes exchange and negotiate the values to be
      used for a given session is described in Section 4.3.

   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.

   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 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.





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

   Note that 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.



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   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.












































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

       +-------------------------+
       |        XFER_INIT        | ->
       |     Transfer ID [I1]    |
       |    Total Length [L1]    |
       +-------------------------+
       +-------------------------+
       |   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] |
                                           +-------------------------+

       +-------------------------+         +-------------------------+
       |       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.



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   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 the routing algorithm selects a certain next-hop
   node.

4.1.  TCP Connection

   To establish a TCPCL session, a node MUST first establish 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 the Registered Port number for the
   TCP convergence layer.  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 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 node is unable to establish a TCP connection for any reason,
   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 the
   connection setup no earlier than some delay time from the last
   attempt, and it SHOULD use a (binary) exponential backoff mechanism
   to increase this delay in case of repeated failures.  In case a
   SHUTDOWN message specifying a reconnection delay is received, that
   delay is used as the initial delay.  The default initial re-attempt
   delay SHOULD be no shorter than 1 second and SHOULD be configurable
   since it will be application and network type dependent.

   Once a TCP connection is established, each node MUST immediately
   transmit a contact header over the TCP connection.  The format of the
   contact header is described in Section 4.2.

4.2.  Contact Header

   Once a TCP connection is established, both parties exchange a contact
   header.  This section describes the format of the contact header and
   the meaning of its fields.

   Upon receipt of the contact header, both nodes perform the validation
   and negotiation procedures defined in Section 4.3.  After receiving
   the contact header from the other node, either node MAY refuse the
   session by sending a SHUTDOWN message with an appropriate reason
   code.




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   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       |      Keepalive Interval       |
     +---------------+---------------+---------------+---------------+
     |                          Segment MRU...                       |
     +---------------+---------------+---------------+---------------+
     |                          contd.                               |
     +---------------+---------------+---------------+---------------+
     |                         Transfer MRU...                       |
     +---------------+---------------+---------------+---------------+
     |                          contd.                               |
     +---------------+---------------+---------------+---------------+
     |          EID Length           |             EID Data...       |
     +---------------+---------------+---------------+---------------+
     |                        EID Data contd.                        |
     +---------------+---------------+---------------+---------------+
     |                  Header Extension Length...                   |
     +---------------+---------------+---------------+---------------+
     |                          contd.                               |
     +---------------+---------------+---------------+---------------+
     |                   Header Extension Items...                   |
     +---------------+---------------+---------------+---------------+

                      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-




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      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 the peer's Segment MRU.  The two nodes of a
      single session MAY have different Segment MRUs, and no relation
      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
      nodes of a single session MAY have different Transfer MRUs, and no
      relation between the two is required.

   EID Length and EID Data:  Together these fields represent a variable-
      length text string.  The EID Length is a 16-bit unsigned integer
      indicating the number of octets of EID Data to follow.  A zero EID
      Length SHALL be used to indicate the lack of EID rather than a
      truly empty EID.  This case allows a node to avoid exposing EID
      information on 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 with [I-D.ietf-dtn-bpbis].

   Header Extension Length and Header Extension Items:  Together these
      fields represent protocol extension data not defined by this
      specification.  The Header Extension Length is the total number of
      octets to follow which are used to encode the Header Extension
      Item list.  The encoding of each Header Extension Item is within a
      consistent data container as described in Section 4.2.1.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CAN_TLS  | 0x01   | If bit is set, indicates that the sending     |
   |          |        | peer is capable of TLS security.              |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                       Table 1: Contact Header Flags





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4.2.1.  Header Extension Items

   Each of the Header Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated 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.  If a TCPCL node receives a
      Header Extension Item with an unknown Item Type and the CRITICAL
      flag set, the node SHALL close the TCPCL session with SHUTDOWN
      reason code of "Contact Failure".  If the CRITICAL flag is not
      set, a node 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 allocate an IANA registry for
      such codes (see 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 is interpreted
      according to the associated Item Type.  This specification places
      no restrictions on an extension's use of available Item Value
      data.  Extension specification SHOULD avoid the use of large data
      exchanges within the TCPCL contact header as no bundle transfers
      can begin until the 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 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... |
     +---------------+---------------+---------------+---------------+
     |    value contd.                                               |
     +---------------+---------------+---------------+---------------+

                  Figure 4: Header Extension Item Format










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   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CRITICAL | 0x01   | If bit is set, indicates that the receiving   |
   |          |        | peer must handle the extension item.          |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                   Table 2: Header Extension Item Flags

4.3.  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 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 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 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 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.



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   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 parameter
      for the behavior described in Section 5.2.1.

   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 of the
      negotated value of Enable TLS is acceptable.  If not, the node
      SHALL shutdown the session 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 negotiated Enable TLS value is true and acceptable then TLS
      negotiation feature (described in Section 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 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, a header extension type
   REACTIVE_FRAGMENT is defined to negotate the fragmentation
   capabilities 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 are outside 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 from a peer, all REACTIVE_FRAGMENT flags of that
   peer SHALL be considered to be not set.  The CRITICAL flag of the



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   REACTIVE_FRAGMENT item MAY be set to indicate that the peer node has
   to interpret and negotiate the reactive fragmentation capability.
   The order of the REACTIVE_FRAGMENT item within the extension items is
   not significant.  The Item Length of a REACTIVE_FRAGMENT item SHALL
   be a single octet.  The contents of the REACTIVE_FRAGMENT item shall
   be interpreted as 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 the
   CAN_GENERATE flag, the receving node SHALL treat partial received
   transfers as reactively fragmented bundles and use the partial
   transfer to reassemble future fragments of that bundle.

   +--------------+--------+-------------------------------------------+
   | Name         | Code   | Description                               |
   +--------------+--------+-------------------------------------------+
   | CAN_GENERATE | 0x01   | If bit is set, indicates that the sending |
   |              |        | node is capable of generating reactively  |
   |              |        | fragmented bundles.                       |
   |              |        |                                           |
   | CAN_RECEIVE  | 0x02   | If bit is set, indicates that the sending |
   |              |        | node is capable of receving and           |
   |              |        | reassembling reactively fragmented        |
   |              |        | bundles.                                  |
   |              |        |                                           |
   | Reserved     | others |
   +--------------+--------+-------------------------------------------+

                     Table 3: REACTIVE_FRAGMENT Flags

4.4.  Session Security

   This version of the TCPCL supports establishing a Transport Layer
   Security (TLS) session within an existing TCP connection.  When TLS
   is used within the TCPCL it affects the entire session.  Once
   established, there is no mechanism available to downgrade a TCPCL
   session to non-TLS operation.  If this is desired, the entire TCPCL
   session MUST be shutdown and a new non-TLS-negotiated session
   established.

   The use of TLS is negotated using 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 nodes SHALL begin a TLS handshake in accordance with



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   [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 node which initiated the underlying TCP connection
   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 about sensitive data exposure are discussed in Section 7.

4.4.1.  TLS Handshake Result

   If a TLS handshake cannot negotiate a TLS session, both nodes 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 be discarded.
   This re-exchange avoids a "man-in-the-middle" attack in identical
   fashion to [RFC2595].  Each re-exchange header CAN_TLS flag SHALL be
   identical to the original header CAN_TLS flag from the same node.
   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 of a typical CAN_TLS usage is shown in the sequence in
   Figure 5 below.






















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                 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: A simple visual example of TCPCL TLS Establishment between
                                 two nodes

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.  Message Type Codes

   After the initial exchange of a contact header, all messages
   transmitted over the session are identified by a one-octet header
   with the following structure:









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    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 4
      below.  Encoded values are listed in Section 8.4.

   +--------------+----------------------------------------------------+
   | Type         | Description                                        |
   +--------------+----------------------------------------------------+
   | XFER_INIT    | Contains the length (in octets) of the next        |
   |              | transfer, as described in Section 5.3.2.           |
   |              |                                                    |
   | XFER_SEGMENT | Indicates the transmission of a segment of bundle  |
   |              | data, as described in Section 5.3.3.               |
   |              |                                                    |
   | XFER_ACK     | Acknowledges reception of a data segment, as       |
   |              | described in Section 5.3.4.                        |
   |              |                                                    |
   | XFER_REFUSE  | Indicates that the transmission of the current     |
   |              | bundle SHALL be stopped, as described in Section   |
   |              | 5.3.5.                                             |
   |              |                                                    |
   | KEEPALIVE    | Used to keep TCPCL session active, as described in |
   |              | Section 5.2.1.                                     |
   |              |                                                    |
   | SHUTDOWN     | Indicates that one of the nodes participating in   |
   |              | the session wishes to cleanly terminate the        |
   |              | session, as described in Section 6.                |
   |              |                                                    |
   | MSG_REJECT   | Contains a TCPCL message rejection, as described   |
   |              | in Section 5.2.2.                                  |
   +--------------+----------------------------------------------------+

                       Table 4: TCPCL Message Types

5.2.  Upkeep and Status Messages

5.2.1.  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.



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   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 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 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) 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 a one-octet SHUTDOWN message
   (as described in Section 6.1) with reason code "Idle Timeout.

5.2.2.  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 follows:











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

                  Figure 7: 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 5.

   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     | 0x01 | A message was received with a Message Type   |
   | Type        |      | code unknown to the TCPCL node.              |
   | Unknown     |      |                                              |
   |             |      |                                              |
   | Message     | 0x02 | A message was received but the TCPCL node    |
   | Unsupported |      | cannot comply with the message contents.     |
   |             |      |                                              |
   | Message     | 0x03 | A message was received while the session is  |
   | Unexpected  |      | in a state in which the message is not       |
   |             |      | expected.                                    |
   +-------------+------+----------------------------------------------+

                     Table 5: MSG_REJECT Reason Codes

5.3.  Bundle Transfer

   All of the messages in this section are directly associated with
   transferring a bundle between TCPCL nodes.

   A single TCPCL transfer results in a bundle (handled by the
   convergence layer as opaque data) being exchanged from one node to
   the other.  In TCPCL a transfer is accomplished by dividing a single
   bundle up into "segments" based on the receiving-side Segment MRU
   (see Section 4.2).  The choice of the length to use for segments is
   an implementation matter, but each segment MUST be no larger than the
   receiving node's maximum receive unit (MRU) (see the field "Segment



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   MRU" of Section 4.2).  The first segment for a bundle MUST set the
   'START' flag, and the last one MUST set the 'end' flag in the
   XFER_SEGMENT message 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 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 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 has no relation to concepts such as bundle
   fragmentation.  Each invocation of TCPCL by the bundle protocol
   agent, requesting transmission of a bundle (fragmentary or
   otherwise), results in the initiation of a single TCPCL transfer.
   Each transfer entails the sending of a XFER_INIT message and some
   number of XFER_SEGMENT and XFER_ACK messages; all are correlated by
   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 be incremented from the
   prior Transfer ID value by one.  Upon exhaustion of the entire 64-bit
   Transfer ID space, the sending node SHALL terminate the session with
   SHUTDOWN reason code "Resource Exhaustion".

   For bidirectional bundle transfers, a TCPCL node SHOULD NOT rely on
   any relation between Transfer IDs originating from each side 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 in the associated transfer.  The total length is
   formatted as a 64-bit unsigned integer.

   The purpose of the XFER_INIT message is to allow nodes to
   preemptively refuse bundles that would exceed their resources or to
   prepare storage on the receiving node for the upcoming bundle data.
   See Section 5.3.5 for details on when refusal based on XFER_INIT
   content is acceptable.




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   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 data received differs from the
   value indicated by the XFER_INIT message, the receiver SHOULD treat
   the transmitted data as invalid.

   The format of the XFER_INIT message is as follows:

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

                  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 the first message in a transfer
   sequence, before transmission of any XFER_SEGMENT messages for the
   same 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.  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.















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

                 Figure 9: 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.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | END      | 0x01   | If bit is set, indicates that this is the     |
   |          |        | last segment of the transfer.                 |
   |          |        |                                               |
   | START    | 0x02   | If bit is set, indicates that this is the     |
   |          |        | first segment of the transfer.                |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                        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.



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   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 MAY be achieved
   using multiple TCPCL sessions.

5.3.4.  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.

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

                  Figure 10: 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



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   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.  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:

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

                 Figure 11: Format of XFER_REFUSE Messages




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   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.

   +------------+------------------------------------------------------+
   | Name       | Semantics                                            |
   +------------+------------------------------------------------------+
   | Unknown    | Reason for refusal is unknown or not specified.      |
   |            |                                                      |
   | Completed  | The receiver already has the complete bundle. The    |
   |            | sender MAY consider the bundle as completely         |
   |            | received.                                            |
   |            |                                                      |
   | No         | The receiver's resources are exhausted. The sender   |
   | Resources  | SHOULD apply reactive bundle fragmentation before    |
   |            | retrying.                                            |
   |            |                                                      |
   | Retransmit | The receiver has encountered a problem that requires |
   |            | the bundle to be retransmitted in its entirety.      |
   +------------+------------------------------------------------------+

                     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 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.



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6.  Session Termination

   This section describes the procedures for ending a TCPCL session.

6.1.  Shutdown Message (SHUTDOWN)

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

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

   Instead of following a clean shutdown sequence, after transmitting a
   SHUTDOWN message a node 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 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 message is as follows:

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

                  Figure 12: Format of SHUTDOWN Messages

   The fields of the SHUTDOWN message are:





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   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 the desired
      delay, in seconds, before re-attepmting a TCPCL session to the
      sending node.  The Reconnection Delay 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 Flags

   It is possible for a node 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:




















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   +---------------+---------------------------------------------------+
   | Name          | Description                                       |
   +---------------+---------------------------------------------------+
   | Idle timeout  | The session is being closed due to idleness.      |
   |               |                                                   |
   | Version       | The node cannot conform to the specified TCPCL    |
   | mismatch      | protocol version.                                 |
   |               |                                                   |
   | Busy          | The node is too busy to handle the current        |
   |               | session.                                          |
   |               |                                                   |
   | Contact       | The node cannot interpret or negotiate contact    |
   | Failure       | header option.                                    |
   |               |                                                   |
   | TLS Failure   | The node failed to negotiate TLS session and      |
   |               | cannot continue the session.                      |
   |               |                                                   |
   | Resource      | The node has run into some resource limit and     |
   | Exhaustion    | cannot continue the session.                      |
   +---------------+---------------------------------------------------+

                      Table 9: SHUTDOWN 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 MUST always send the
   contact header to its peer before sending a SHUTDOWN message.

   If reception of the contact header itself somehow fails (e.g. an
   invalid "magic string" is recevied), a node SHOULD close the TCP
   connection without sending a SHUTDOWN message.  If the content of the
   Header Extension Items data disagrees with the Header Extension
   Length (i.e. the last Item claims to use more octets than are present
   in the Header 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
   message but still SHOULD close the TCP connection.  Each TCPCL
   message is contiguous in the octet stream and has no ability to be



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   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 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 of time, then either node MAY terminate the session
   by transmitting a SHUTDOWN message indicating the reason code of
   "Idle timeout" (as described in Table 9).

7.  Security Considerations

   One security consideration for this protocol relates to the fact that
   nodes present their endpoint identifier as part of the contact header
   exchange.  It would be possible for a node 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 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.















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   +--------------------+---------------------------------------------+
   | Parameter          | Value                                       |
   +--------------------+---------------------------------------------+
   | Flags              | The USE_TLS flag is set.                    |
   |                    |                                             |
   | Keepalive Interval | Zero, indicating no keepalive.              |
   |                    |                                             |
   | Segment MRU        | Zero, indicating all segments are refused.  |
   |                    |                                             |
   | Transfer MRU       | Zero, indicating all transfers are refused. |
   |                    |                                             |
   | EID                | Empty, indicating lack of EID.              |
   +--------------------+---------------------------------------------+

              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 MAY send a large amount of data over a TCPCL
   session, requiring the receiving node 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 from continually establishing sessions and repeatedly
   trying to send copious amounts of bundle data.  A listening node 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 message quickly or with a
   delay, etc.

8.  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



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   these values between TCPCLv3 and TCPCLv4 while preserving the
   semantics of some values.

8.1.  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 identifies its TCPCL protocol
   version in its initial contact (see Section 8.2), so there is no
   ambiguity about what protocol is being used.

     +------------------------+-------------------------------------+
     | Parameter              | Value                               |
     +------------------------+-------------------------------------+
     | Service Name:          | dtn-bundle                          |
     |                        |                                     |
     | Transport Protocol(s): | TCP                                 |
     |                        |                                     |
     | Assignee:              | Simon Perreault <simon@per.reau.lt> |
     |                        |                                     |
     | Contact:               | Simon Perreault <simon@per.reau.lt> |
     |                        |                                     |
     | Description:           | DTN Bundle TCP CL Protocol          |
     |                        |                                     |
     | Reference:             | [RFC7242]                           |
     |                        |                                     |
     | Port Number:           | 4556                                |
     +------------------------+-------------------------------------+

8.2.  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.
















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               +-------+-------------+---------------------+
               | 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 Extension Types

   EDITOR NOTE: sub-registry to-be-created upon publication of this
   specification.

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Header Extension Types" and initialize it with the contents of
   Table 11.  The registration procedure is RFC Required within the
   lower range 0x0001--0x3fff.  Values in the range 0x8000--0xffff are
   reserved for use on private networks for functions not published to
   the IANA.

               +----------------+--------------------------+
               | Code           | Message Type             |
               +----------------+--------------------------+
               | 0x0000         | Reserved                 |
               |                |                          |
               | 0x0001         | REACTIVE_FRAGMENT        |
               |                |                          |
               | 0x0002--0x3fff | Unassigned               |
               |                |                          |
               | 0x8000--0xffff | Private/Experimental Use |
               +----------------+--------------------------+

                   Table 11: Header Extension Type Codes

8.4.  Message Types

   EDITOR NOTE: sub-registry to-be-created upon publication of this
   specification.




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   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Message Types" and initialize it with the contents of Table 12.  The
   registration procedure is RFC Required.

                       +-----------+--------------+
                       | Code      | Message Type |
                       +-----------+--------------+
                       | 0x00      | Reserved     |
                       |           |              |
                       | 0x01      | XFER_SEGMENT |
                       |           |              |
                       | 0x02      | XFER_ACK     |
                       |           |              |
                       | 0x03      | XFER_REFUSE  |
                       |           |              |
                       | 0x04      | KEEPALIVE    |
                       |           |              |
                       | 0x05      | SHUTDOWN     |
                       |           |              |
                       | 0x06      | XFER_INIT    |
                       |           |              |
                       | 0x07      | MSG_REJECT   |
                       |           |              |
                       | 0x08--0xf | Unassigned   |
                       +-----------+--------------+

                       Table 12: Message Type Codes

8.5.  XFER_REFUSE Reason Codes

   EDITOR NOTE: sub-registry to-be-created upon publication of this
   specification.

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   XFER_REFUSE Reason Codes" and initialize it with the contents of
   Table 13.  The registration procedure is RFC Required.













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                 +----------+---------------------------+
                 | Code     | Refusal Reason            |
                 +----------+---------------------------+
                 | 0x0      | Unknown                   |
                 |          |                           |
                 | 0x1      | Completed                 |
                 |          |                           |
                 | 0x2      | No Resources              |
                 |          |                           |
                 | 0x3      | Retransmit                |
                 |          |                           |
                 | 0x4--0x7 | Unassigned                |
                 |          |                           |
                 | 0x8--0xf | Reserved for future usage |
                 +----------+---------------------------+

                    Table 13: XFER_REFUSE Reason Codes

8.6.  SHUTDOWN Reason Codes

   EDITOR NOTE: sub-registry to-be-created upon publication of this
   specification.

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   SHUTDOWN Reason Codes" and initialize it with the contents of
   Table 14.  The registration procedure is RFC Required.

                   +------------+---------------------+
                   | Code       | Shutdown Reason     |
                   +------------+---------------------+
                   | 0x00       | Idle timeout        |
                   |            |                     |
                   | 0x01       | Version mismatch    |
                   |            |                     |
                   | 0x02       | Busy                |
                   |            |                     |
                   | 0x03       | Contact Failure     |
                   |            |                     |
                   | 0x04       | TLS failure         |
                   |            |                     |
                   | 0x05       | Resource Exhaustion |
                   |            |                     |
                   | 0x06--0xFF | Unassigned          |
                   +------------+---------------------+

                      Table 14: SHUTDOWN Reason Codes




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8.7.  MSG_REJECT Reason Codes

   EDITOR NOTE: sub-registry to-be-created upon publication of this
   specification.

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   MSG_REJECT Reason Codes" and initialize it with the contents of
   Table 15.  The registration procedure is RFC Required.

                   +-----------+----------------------+
                   | Code      | Rejection Reason     |
                   +-----------+----------------------+
                   | 0x00      | reserved             |
                   |           |                      |
                   | 0x01      | Message Type Unknown |
                   |           |                      |
                   | 0x02      | Message Unsupported  |
                   |           |                      |
                   | 0x03      | Message Unexpected   |
                   |           |                      |
                   | 0x04-0xFF | Unassigned           |
                   +-----------+----------------------+

                       Table 15: REJECT Reason Codes

9.  Acknowledgments

   This specification is based on comments on implementation of
   [RFC7242] provided from Scott Burleigh.

10.  References

10.1.  Normative References

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-10 (work in progress),
              November 2017.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.



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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, DOI 10.17487/RFC5050, November
              2007, <https://www.rfc-editor.org/info/rfc5050>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC7525]  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/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

10.2.  Informative References

   [I-D.ietf-dtn-bpsec]
              Birrane, E. and K. McKeever, "Bundle Protocol Security
              Specification", draft-ietf-dtn-bpsec-06 (work in
              progress), October 2017.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <https://www.rfc-editor.org/info/rfc2595>.

   [RFC4838]  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/RFC4838,
              April 2007, <https://www.rfc-editor.org/info/rfc4838>.

   [RFC6257]  Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              DOI 10.17487/RFC6257, May 2011,
              <https://www.rfc-editor.org/info/rfc6257>.






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   [RFC7242]  Demmer, M., Ott, J., and S. Perreault, "Delay-Tolerant
              Networking TCP Convergence-Layer Protocol", RFC 7242,
              DOI 10.17487/RFC7242, June 2014,
              <https://www.rfc-editor.org/info/rfc7242>.

Appendix A.  Significant changes from RFC7242

   The areas in which changes from [RFC7242] have been made to existing
   headers and messages are:

   o  Changed contact header content to limit number of negotiated
      options.

   o  Added contact option to negotiate maximum segment size (per each
      direction).

   o  Added contact header extension capability.

   o  Defined new IANA registries for message / type / reason codes to
      allow renaming some codes for clarity.

   o  Expanded Message Header to octet-aligned fields instead of bit-
      packing.

   o  Added a bundle transfer identification number to all bundle-
      related messages (XFER_INIT, XFER_SEGMENT, XFER_ACK, XFER_REFUSE).

   o  Use flags in XFER_ACK to mirror flags from XFER_SEGMENT.

   o  Removed all uses of SDNV fields and replaced with fixed-bit-length
      fields.

   The areas in which extensions from [RFC7242] have been made as new
   messages and codes are:

   o  Added contact negotiation failure SHUTDOWN reason code.

   o  Added MSG_REJECT message to indicate an unknown or unhandled
      message was received.

   o  Added TLS session security mechanism.

   o  Added TLS failure and Resource Exhaustion SHUTDOWN reason code.

   o  Added extension for reactive fragmentation negotiation
      (REACTIVE_FRAGMENT).





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Authors' Addresses

   Brian Sipos
   RKF Engineering Solutions, LLC
   7500 Old Georgetown Road
   Suite 1275
   Bethesda, MD  20814-6198
   US

   Email: BSipos@rkf-eng.com


   Michael Demmer
   University of California, Berkeley
   Computer Science Division
   445 Soda Hall
   Berkeley, CA  94720-1776
   US

   Email: demmer@cs.berkeley.edu


   Joerg Ott
   Aalto University
   Department of Communications and Networking
   PO Box 13000
   Aalto  02015
   Finland

   Email: jo@netlab.tkk.fi


   Simon Perreault
   Quebec, QC
   Canada

   Email: simon@per.reau.lt














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