Delay Tolerant Networking                                       B. Sipos
Internet-Draft                                           RKF Engineering
Obsoletes: 7242 (if approved)                                  M. Demmer
Intended status: Standards Track                             UC Berkeley
Expires: May 17, June 13, 2018                                            J. Ott
                                                        Aalto University
                                                            S. Perreault
                                                       November 13,
                                                       December 10, 2017

   Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4


   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 Protocl 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

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   This Internet-Draft will expire on May 17, June 13, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Definitions Specific to the TCPCL Protocol  . . . . . . .   4
   3.  General Protocol Description  . . . . . . . . . . . . . . . .   5
     3.1.  Bidirectional Use of  TCPCL Sessions Session Overview  . . . . . . . . . . .   7 . . . . . .   6
     3.2.  Example Message Exchange  . . . . . . . . . . . . . . . .   7
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Contact Header  . . . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  Header Extension Items  . . . . . . . . . . . . . . .  12
     4.2.  Validation and Parameter Negotiation  . . . . . . . . . .  13
     4.3.  Session Security  . . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  TLS Handshake Result  . . . . . . . . . . . . . . . .  15
       4.3.2.  Example TLS Initiation  . . . . . . . . . . . . . . .  15
   5.  Established Session Operation . . . . . . . . . . . . . . . .  16
     5.1.  Message Type Codes  . . . . . . . . . . . . . . . . . . .  16
     5.2.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  18  17
       5.2.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  18  17
       5.2.2.  Message Rejection (MSG_REJECT)  . . . . . . . . . . .  18
     5.3.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  19
       5.3.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  20  19
       5.3.2.  Transfer Initialization (XFER_INIT) . . . . . . . . .  20
       5.3.3.  Data Transmission (XFER_SEGMENT)  . . . . . . . . . .  21
       5.3.4.  Data Acknowledgments (XFER_ACK) . . . . . . . . . . .  22
       5.3.5.  Transfer Refusal (XFER_REFUSE)  . . . . . . . . . . .  23
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  25
     6.1.  Shutdown Message (SHUTDOWN) . . . . . . . . . . . . . . .  26  25
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  28
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  29  28
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30  29
     8.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  30
     8.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  31  30
     8.3.  Header Extension Types  . . . . . . . . . . . . . . . . .  31
     8.4.  Message Types . . . . . . . . . . . . . . . . . . . . . .  32  31
     8.5.  XFER_REFUSE Reason Codes  . . . . . . . . . . . . . . . .  32
     8.6.  SHUTDOWN Reason Codes . . . . . . . . . . . . . . . . . .  33
     8.7.  MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . .  34

   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  34
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     10.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  35  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36  37

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"

   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 revised Bundle Protocol (BP)
   [I-D.ietf-dtn-bpbis], an application-layer protocol that is used to
   construct a store-and- forward overlay network.  As described in the
   Bundle Protocol specification [I-D.ietf-dtn-bpbis], it 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 TCPCL.

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

         |     DTN Application     | -\
         +-------------------------|   |
         |  Bundle Protocol (BP)   |   -> Application Layer
         +-------------------------+   |
         | TCP Conv. Layer (TCPCL) | -/   |
         +-------------------------+   |
         |     TLS (optional)      | ---> Presentation Layer -/
         |          TCP            | ---> Transport Layer
         |           IP       IPv4/IPv6         | ---> Network Layer
         |   Link-Layer Protocol   | ---> Link Layer
         |    Physical Medium      | ---> Physical 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.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.1.  Definitions Specific to the TCPCL Protocol

   This section contains definitions that are interpreted to be specific
   to the operation of the TCPCL protocol, as described below.

   TCPCL Node:  A TCPCL node refers to either side of an 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:  A TCP connection 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.  Therefore, a TCPCL session is
      initiated after a bundle node establishes a TCP connection to for
      the purposes of bundle communication.  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:  The session parameters 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 which two bundle nodes exchange and negotiate the
      values to be used for a given session is described in Section 4.2.


   Transfer:  Transfer refers to the procedures and mechanisms
      (described below) for conveyance of an individual bundle from one
      node to another.  Each transfer within TCPCLv4 is identified by a
      Transfer ID number which is unique only to a single direction
      within a single Session.

   Reason Codes:  The TCPCL uses numeric codes to encode specific
      reasons for individual failure/error message types.  This limits
      the expressiveness of TCPCL error encodings, but simplifies the
      encoding of errors and allows an application policy to attempt
      recovery from expected 'failure' modes (e.g. if a Session cannot
      be established with USE_TLS disabled because of a Contact Failure
      shutdown, a re-attempt can be made with USE_TLS enabled).

3.  General Protocol Description

   The service of this protocol is the transmission of DTN bundles over
   TCP. 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. 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., 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 in by an initialization (XFER_INIT) message followed by one or
   more logical segments of data.  Each logical data
   segment consists of a within an XFER_SEGMENT message header and flags, a count
   of the length of the segment, and finally the octet range of the
   bundle data. message.  The
   choice of the length to use for segments is an implementation matter (but matter,
   but each segment must be within no larger than the Segment MRU size receiving node's maximum
   receive unit (MRU) (see the field "Segment MRU" of Section 4.1).  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.

   If multiple bundles are transmitted on a single TCPCL connection,
   they MUST be transmitted consecutively.  Interleaving data segments
   from different bundles is not allowed.  Bundle interleaving can be
   accomplished by fragmentation at the BP layer or by establishing
   multiple TCPCL sessions.

   A feature of this protocol is for the receiving node to send
   acknowledgments as bundle data segments arrive (XFER_ACK).  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.  For each data segment that is received, the receiving node
   sends an XFER_ACK message containing the cumulative length of the
   bundle that has been received.  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.  In addition, there is no explicit flow
   control on the TCPCL layer.

   Another feature is that a receiver MAY 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 current bundle,
   after completing transmission of a partially sent data segment.
   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 liveness

   Finally, before sessions close, a SHUTDOWN message is sent to the
   session peer.  A SHUTDOWN message MAY also be used to refuse a
   session setup by a peer (see Section 4.2).  After sending a SHUTDOWN
   message, the sender of this the message MAY send further acknowledgments
   (XFER_ACK or XFER_REFUSE) but no further data messages (XFER_SEGMENT).  A (XFER_INIT or
   XFER_SEGMENT).  After receving a SHUTDOWN message MAY
   also be used to refuse a session setup by a peer.

3.1.  Bidirectional Use of TCPCL Sessions and when no
   transfers are in-progress (i.e. have unacknowledged segemnts)

   There are specific messages for sending and receiving operations (in
   addition to session setup/teardown).  TCPCL is symmetric, i.e., 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 visually 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.
   Although this example only demonstrates a single bundle transmission,
   it is also possible to pipeline multiple XFER_SEGMENT messages for
   different bundles without necessarily waiting for XFER_ACK messages
   to be returned for each one.  However, interleaving data segments
   from different bundles is not allowed.

   No errors or rejections are shown in this example.

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

       |        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: A SL1e Visual An Example of the Flow of Protocol Messages on a Single TCP
                    Session between Two Nodes (A and B)

4.  Session Establishment

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

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

   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 well-known port 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 well-known registered TCPCL port) 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 only after some delay (a 1-second minimum is
   RECOMMENDED), 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 delay
   SHOULD be at least 1 second but SHOULD be configurable since it will
   be application and network type dependent.

   The node MAY declare failure after one or more connection attempts
   and MAY attempt to find an alternate route for bundle data.  Such
   decisions are up to the higher layer (i.e., the BP).

   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 4.1.

   Upon receipt of the contact header, both nodes perform the validation
   and negotiation procedures defined in Section 4.2

   After receiving the contact header from the other node, either node
   MAY also refuse the session by sending a SHUTDOWN message.  If
   session setup is refused, a reason MUST be included in the SHUTDOWN

4.1.  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.

   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 4.2 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

   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 longest
      allowable interval, in seconds, between any message being received
      in this session and a subsequent KEEPALIVE message being received.

   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
      data 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 an 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 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
      identical form as described in Section 4.1.1.

   | 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

4.1.1.  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 an
      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, an
      node SHALL skip over and ignore any item with an unkonwn Item

   Item Type:  A 16-bit unsigned integer field containing the type of
      the extension item.  Each type  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 extensions use of available Item Value data.
      Extension specification SHOULD avoid the use of large data
      exchanges within the TCPCLv4 contact header as no bundle transfers
      can begin until the a 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 Extention Item Format

   | 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

4.2.  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").  Otherwise, 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
   negotatiated parameters defined by this specification are described
   in the following paragraphs.

   Session Keepalive:  Negotiation of the Session Keepalive parameter is
      performed by taking the minimum of this two contact headers'
      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 (described in Section 5.2.1)
      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).

   Enable TLS:  Negotiation of the Enable TLS parameter is performed by
      taking the logical AND of the two contact headers' CAN_TLS flags.
      If the negotiated Enable TLS value is true then TLS negotiation
      feature (described in Section 4.3) begins immediately following
      the contact header exchange.  The security policy on either node
      MAY forbid the establishment of a TCPCL session for any Enable TLS
      result (or for any combination of local or peer CAN_TLS flag), in
      which case the node SHALL shutdown the session with a reason code
      of "Contact Failure".  For example, one node may disallow TCPCL
      sessions without TLS, while a second node may disallow sessions
      with TLS.  Also note that this Contact Failure (of the header
      negotiation) is different than a TLS Failure (after an agreed-upon
      Enable TLS state).

   Once this process of parameter negotiation is completed (which
   includes a possible completed TLS handshakede 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.  Session Security

   This version of the TCPCL supports establishing a Transport Layer
   Security (TLS) session within an existing TCP connection.  Negotation
   of whether or not to initiate TLS within a TCPCL session is part of
   the contact header as described in Section 4.2.  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.

   When TLS is used within the TCPCL it affects the entire session.  By
   convention, this protocol uses the node which initiated the
   underlying TCP connection as the "client" role of the TLS handshake
   request.  Once a TLS session is established within TCPCL, there is no
   mechanism provided to end the TLS session and downgrade the session.
   If a non-TLS session is desired after a TLS session is started then
   the entire TCPCL session MUST be shutdown first.

   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 [RFC5246].  The
   parameters within each TLS negotation are implementation dependent
   but any TCPCL node SHOULD follow all recommended best practices of

4.3.1.  TLS Handshake Result

   If a TLS handshake cannot negotiate a TLS session, both nodes of the
   TCPCL session SHALL cause a TCPCL shutdown with reason "TLS Failure".

   After a TLS session is successfuly 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 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

4.3.2.  Example TLS Initiation

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

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

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

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

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

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

                       ... secured TCPCL messaging ...

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

   Figure 5: 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:

    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 3

   The message types defined in this specificaiton are listed in
   Table 3.  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 3: 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.

   As described in Section 4.1, one of the parameters in the contact
   header is the Keepalive Interval.  Both sides populate this field
   with their requested intervals (in seconds) between KEEPALIVE

   The format of a KEEPALIVE message is a one-octet message type code of
   KEEPALIVE (as described in Table 3) 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 for at least
   twice the Keepalive Interval, then either party MAY terminate the
   session by transmitting a one-octet SHUTDOWN message (as described in
   Section 6.1, with reason code "Idle Timeout") and by closing the

   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.

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 negotation 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:

                      |       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 4.

   Rejected Message Header:  The Rejected Message Header is a copy of
      the Message Header to which the MSG_REJECT message is sent as a

   | 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 4: MSG_REJECT Reason Codes

5.3.  Bundle Transfer

   All of the message in this section are directly associated with
   transfering 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 receving-side receiving-side Segment MRU
   (see Section 4.1).

   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.

5.3.1.  Bundle Transfer ID

   Each of the bundle transfer messages contains a Transfer ID number
   which is used to correlate messages originating from sender and
   receiver of a 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.

   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 immediately before transmission of
   any XFER_SEGMENT messages for each 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).

   A receiver MAY send a BUNDLE_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.

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.

                     |       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 5.

   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 5: 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 5.  The
   'START' bit MUST be set to one if it precedes the transmission of the
   first segment of a transfer.  The 'END' bit MUST be set to one when
   transmitting the last segment of a transfer.  In the case where an
   entire transfer is accomplished in a single segment, both the 'START'
   and 'END' bits MUST be set to one.

   Once a transfer of a bundle has commenced, the node MUST only send
   segments containing sequential portions of that bundle until it sends
   a segment with the 'END' bit set.  No interleaving of multiple
   transfers from the same node is possible within a single TCPCL
   session.  Simultaneous transfers between two nodes 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, a Bundle Protocol agent 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 5.

   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 receving receiving TCPCL endpoing 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_SEGEMNT DATA_SEGMENT
   message being acknowledged.  The acknowledged length of each XFER_ACK
   contains the sum of the data length fields of all XFER_SEGMENT
   messages received so far in the course of the indicated transfer.

   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)

   As bundles can be large, the

   The TCPCL supports an optional mechanism by which a receiving node MAY 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.

   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

   The fields of the XFER_REFUSE message are:

   Reason Code:  A one-octet refusal reason code interpreted according
      to the descriptions in Table 6.

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

   | Name       | 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 6: 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

   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.

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 MUST be
   transmitted by either node at any point following complete
   transmission of any other message.  A receiving node SHOULD
   acknowledge all received data segments before sending a SHUTDOWN
   message to end the session.  A transmitting node SHALL treat a
   SHUTDOWN message received mid-transfer (i.e. before the final
   acknowledgement) as a failure of the transfer.

   After transmitting a SHUTDOWN message, an node MAY immediately close
   the associated TCP connection.  Once the SHUTDOWN message is sent,
   any further received data on the TCP connection SHOULD be ignored.
   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:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 7.

   Reason Code:  A one-octet refusal reason code interpreted according
      to the descriptions in Table 8.  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 until further TCPCL sessions to the sending node.  The
      Reconnection Delay is present or absent as indicated by one of the

   | 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 7: SHUTDOWN Flags

   It is possible for a node to convey additional 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:

   | 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 resoure limit and      |
   | Exhaustion    | cannot continue the session.                      |

                      Table 8: SHUTDOWN Reason Codes

   It is also possible to convey a requested reconnection delay to
   indicate how long the other node MUST wait before attempting session
   re-establishment.  To do so, the node sets the 'D' bit in the message
   flags and then transmits an 16-bit unsigned integer specifying the
   requested delay, in seconds, following the message header (and
   optionally, the SHUTDOWN reason code).  The value 0 SHALL be
   interpreted as an infinite delay, i.e., that the connecting node MUST
   NOT re-establish the session.  In contrast, if the node does not wish
   to request a delay, it SHOULD omit the reconnection delay field (and
   set the 'D' bit to zero).

   A session shutdown MAY occur immediately after TCP connection
   establishment or reception of a contact header (and prior to any
   further data exchange).  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 either node terminates a session prematurely in this manner, it
   SHOULD send a SHUTDOWN message and MUST indicate a reason code unless
   the incoming connection did not include the magic string.  If the
   magic string was not present, a node SHOULD close the TCP connection
   without sending a SHUTDOWN message.  If a node does not want its peer
   to reopen a connection immediately, it SHOULD set the 'D' bit in the
   flags and include a reconnection delay to indicate when the peer is
   allowed to attempt another session setup.

   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
   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 termiated 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 bundle
   data (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 8).  After receiving a SHUTDOWN
   message in response, both sides MAY close the TCP connection.

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
   mandatory, an endpoint SHALL transmit initial unsecured contact
   header values indicated in Table 9 in order.  These values avoid
   unnecessarily leaking endpoing parameters and will be ignored when
   secure contact header re-exchange occurs.

   | 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, indating lack of EID.                |

               Table 9: 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

   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
   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 <> |
     |                        |                                     |
     | Contact:               | Simon Perreault <> |
     |                        |                                     |
     | 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 initialized it with the following table.  The
   registration procedure is RFC Required.

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

8.3.  Header Extension Types

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Header Extension Types" and initialized it with the contents of
   Table 10.  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--0x3fff | Unassigned               |
               |                |                          |
               | 0x8000--0xffff | Private/Experimental Use |

                   Table 10: Header Extension Type Codes

8.4.  Message Types

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Message Types" and initialized it with the contents of Table 11.  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 11: Message Type Codes

8.5.  XFER_REFUSE Reason Codes

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

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

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

                    Table 12: XFER_REFUSE Reason Codes

8.6.  SHUTDOWN Reason Codes

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

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

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

                      Table 13: SHUTDOWN Reason Codes

8.7.  MSG_REJECT Reason Codes

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   MSG_REJECT Reason Codes" and initialized it with the contents of
   Table 14.  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 14: REJECT Reason Codes

9.  Acknowledgments

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

10.  References

10.1.  Normative References

              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol",
              draft-ietf-dtn-bpbis-08 Protocol
              Version 7", draft-ietf-dtn-bpbis-10 (work in progress), August
              November 2017.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, DOI 10.17487/RFC5050, November
              2007, <>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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, <>.

10.2.  Informative References

              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,

   [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, <>.

   [RFC6257]  Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              DOI 10.17487/RFC6257, May 2011,

   [RFC7242]  Demmer, M., Ott, J., and S. Perreault, "Delay-Tolerant
              Networking TCP Convergence-Layer Protocol", RFC 7242,
              DOI 10.17487/RFC7242, June 2014,

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

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

   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-

   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

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

   o  Added contact negotation 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 SHUTDOWN reason code.

Authors' Addresses

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


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


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


   Simon Perreault
   Quebec, QC