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Versions: (draft-sipos-dtn-tcpclv4) 00 01

Delay Tolerant Networking                                       B. Sipos
Internet-Draft                                           RKF Engineering
Obsoletes: RFC7242 (if approved)                               M. Demmer
Intended status: Standards Track                             UC Berkeley
Expires: May 31, 2017                                             J. Ott
                                                        Aalto University
                                                            S. Perreault
                                                       November 27, 2016


   Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
                       draft-ietf-dtn-tcpclv4-01

Abstract

   This document describes a revised protocol for the TCP-based
   convergence layer for Delay-Tolerant Networking (DTN).  The protocol
   revision is based on implementation issues in the original [RFC7242]
   and updates to the Bundle Protocol contents, encodings, and
   convergence layer requirements in [I-D.ietf-dtn-bpbis].

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 http://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 May 31, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 . . . . . . . . . . .   6
     3.2.  Example Message Exchange  . . . . . . . . . . . . . . . .   6
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Contact Header  . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Validation and Parameter Negotiation  . . . . . . . . . .  10
   5.  Established Session Operation . . . . . . . . . . . . . . . .  11
     5.1.  Message Type Codes  . . . . . . . . . . . . . . . . . . .  11
     5.2.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  12
       5.2.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  12
       5.2.2.  Message Rejection (REJECT)  . . . . . . . . . . . . .  13
     5.3.  Session Security  . . . . . . . . . . . . . . . . . . . .  14
       5.3.1.  TLS Handshake Result  . . . . . . . . . . . . . . . .  14
       5.3.2.  Example TLS Initiation  . . . . . . . . . . . . . . .  15
     5.4.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  15
       5.4.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  16
       5.4.2.  Bundle Length (LENGTH)  . . . . . . . . . . . . . . .  16
       5.4.3.  Bundle Data Transmission (DATA_SEGMENT) . . . . . . .  17
       5.4.4.  Bundle Acknowledgments (ACK_SEGMENT)  . . . . . . . .  18
       5.4.5.  Bundle Refusal (REFUSE_BUNDLE)  . . . . . . . . . . .  19
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  21
     6.1.  Shutdown Message (SHUTDOWN) . . . . . . . . . . . . . . .  21
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  23
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  25
     8.3.  Message Types . . . . . . . . . . . . . . . . . . . . . .  26
     8.4.  REFUSE_BUNDLE Reason Codes  . . . . . . . . . . . . . . .  26
     8.5.  SHUTDOWN Reason Codes . . . . . . . . . . . . . . . . . .  27
     8.6.  REJECT Reason Codes . . . . . . . . . . . . . . . . . . .  27
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     10.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30




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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 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 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            | ---> 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



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   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",
   "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 that are interpreted to be specific
   to the operation of the TCPCL protocol, as described below.

   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 when a bundle node initiates a TCP connection to be
      established 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" refer 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.

   Transmission:  Transmission refers to the procedures and mechanisms
      (described below) for conveyance of a bundle from one node to
      another.



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3.  General Protocol Description

   The service of this protocol is the transmission of DTN bundles over
   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.

   First, one node establishes a TCPCL session to the other by
   initiating a TCP connection.  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 in one or more logical segments of data.  Each logical data
   segment consists of a DATA_SEGMENT message header, a count of the
   length of the segment, and finally the octet range of the bundle
   data.  The choice of the length to use for segments is an
   implementation matter (but must be within the Segment MRU size 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 DATA_SEGMENT
   message header.

   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 (ACK_SEGMENT).  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 ACK_SEGMENT code followed by an count containing the
   cumulative length of the bundle that has been received.  The sending
   node MAY transmit multiple DATA_SEGMENT messages without necessarily
   waiting for the corresponding ACK_SEGMENT responses.  This enables
   pipelining of messages on a channel.  In addition, there is no
   explicit flow control on the TCPCL layer.





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   Another feature is that a receiver MAY interrupt the transmission of
   a bundle at any point in time by replying with a REFUSE_BUNDLE
   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 liveness information.

   Finally, before sessions close, a SHUTDOWN message is sent to the
   session peer.  After sending a SHUTDOWN message, the sender of this
   message MAY send further acknowledgments (ACK_SEGMENT or
   REFUSE_BUNDLE) but no further data messages (DATA_SEGMENT).  A
   SHUTDOWN message MAY also be used to refuse a session setup by a
   peer.

3.1.  Bidirectional Use of TCPCL Sessions

   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 DATA_SEGMENT
   messages without necessarily waiting for the corresponding
   ACK_SEGMENT 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 DATA_SEGMENT
   messages for different bundles without necessarily waiting for
   ACK_SEGMENT 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      |
       +-------------------------+         +-------------------------+

       +-------------------------+
       |         LENGTH          | ->
       |     Transfer ID [I1]    |
       |    Total Length [L1]    |
       +-------------------------+
       +-------------------------+
       |   DATA_SEGMENT (start)  | ->
       |     Transfer ID [I1]    |
       |       Length [L1]       |
       |  Bundle Data 0..(L1-1)  |
       +-------------------------+
       +-------------------------+         +-------------------------+
       |     DATA_SEGMENT        | ->   <- |   ACK_SEGMENT (start)   |
       |     Transfer ID [I1]    |         |     Transfer ID [I1]    |
       |       Length   [L2]     |         |        Length   [L1]    |
       |Bundle Data L1..(L1+L2-1)|         +-------------------------+
       +-------------------------+
       +-------------------------+         +-------------------------+
       |    DATA_SEGMENT (end)   | ->   <- |       ACK_SEGMENT       |
       |     Transfer ID [I1]    |         |     Transfer ID [I1]    |
       |        Length   [L3]    |         |      Length   [L1+L2]   |
       |Bundle Data              |         +-------------------------+
       |    (L1+L2)..(L1+L2+L3-1)|
       +-------------------------+
                                           +-------------------------+
                                        <- |    ACK_SEGMENT (end)    |
                                           |     Transfer ID [I1]    |
                                           |     Length   [L1+L2+L3] |
                                           +-------------------------+

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

   Figure 2: A Simple Visual 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.

   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.  Port number 4556 has been
   assigned by IANA as the well-known port number for the TCP
   convergence layer.  Other port numbers MAY be used per local
   configuration.  Determining a peer's port number (if different from
   the well-known TCPCL port) is up to the implementation.

   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
   message.

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.



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

                      Figure 3: Contact Header Format

   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 longest
      allowable interval in seconds between KEEPALIVE messages received
      in this session.

   Segment MRU:  A 64-bit unsigned integer indicating the largest
      allowable single-segment data payload size to be received in this
      session.  Any DATA_SEGMENT sent to this peer SHALL have a data
      payload no longer than the peer's Segment MRU.  The two endpoints
      of a single session MAY have different Segment MRUs, and no
      relation between the two is required.





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   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
      endpoints 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 is a special case which indicates the lack of EID rather
      than a truly empty EID.  A non-zero-length EID Data contains 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>.

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

                       Table 1: Contact Header 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.

   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




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   its operation to conform to the older version of the protocol.  This
   decision 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.

   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 5.3) begins immediately following
      the contact header exchange.

   Once this process of parameter negotiation is completed, the protocol
   defines no additional mechanism to change the parameters of an
   established session; to effect such a change, the session MUST be
   terminated and a new session established.

5.  Established Session Operation

   This section describes the protocol operation for the duration of an
   established session, including the mechanism for transmitting bundles
   over the session.

5.1.  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
   +-+-+-+-+-+-+-+-+
   | type  | flags |
   +-+-+-+-+-+-+-+-+

             Figure 4: Format of the One-Octet Message Header

   type: Indicates the type of the message as per Table 2 below.



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   flags: Optional flags defined based on message type.

   The types and values for the message type code are as follows.

   +---------------+------+--------------------------------------------+
   | Type          | Code | Description                                |
   +---------------+------+--------------------------------------------+
   | DATA_SEGMENT  | 0x1  | Indicates the transmission of a segment of |
   |               |      | bundle data, as described in Section       |
   |               |      | 5.4.3.                                     |
   |               |      |                                            |
   | ACK_SEGMENT   | 0x2  | Acknowledges reception of a data segment,  |
   |               |      | as described in Section 5.4.4.             |
   |               |      |                                            |
   | REFUSE_BUNDLE | 0x3  | Indicates that the transmission of the     |
   |               |      | current bundle SHALL be stopped, as        |
   |               |      | described in Section 5.4.5.                |
   |               |      |                                            |
   | KEEPALIVE     | 0x4  | KEEPALIVE message for the session, as      |
   |               |      | described in Section 5.2.1.                |
   |               |      |                                            |
   | SHUTDOWN      | 0x5  | Indicates that one of the nodes            |
   |               |      | participating in the session wishes to     |
   |               |      | cleanly terminate the session, as          |
   |               |      | described in Section 6.                    |
   |               |      |                                            |
   | LENGTH        | 0x6  | Contains the length (in octets) of the     |
   |               |      | next bundle, as described in Section       |
   |               |      | 5.4.2.                                     |
   |               |      |                                            |
   | REJECT        | TBD  | Contains a TCPCL message rejection, as     |
   |               |      | described in Section 5.2.2.                |
   +---------------+------+--------------------------------------------+

                       Table 2: 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
   messages.



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   The format of a KEEPALIVE message is a one-octet message type code of
   KEEPALIVE (as described in Table 2) with no additional data.  Both
   sides 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
   Table 2, with reason code "Idle Timeout") and by closing the session.

   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 (REJECT)

   If a TCPCL endpoint 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
   REJECT reply.

   The format of a REJECT message follows:

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

                    Figure 5: Format of REJECT Messages

   The Rejected Message Header is a copy of the Message Header to which
   the REJECT message is sent as a response.  The REJECT Reason Code is
   an 8-bit unsigned integer and indicates why the REJECT itself was
   sent.  The specified values of the reason code are:









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   +-------------+------+----------------------------------------------+
   | Name        | Code | Description                                  |
   +-------------+------+----------------------------------------------+
   | Message     | 0x01 | A message was received with a Message Type   |
   | Type        |      | code unknown to the TCPCL endpoint.          |
   | Unknown     |      |                                              |
   |             |      |                                              |
   | Message     | 0x02 | A message was received but the TCPCL         |
   | Unsupported |      | endpoint 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 3: REJECT Reason Codes

5.3.  Session Security

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

   When TLS is used within the TCPCL it affects the entire session.  By
   convention, this protocol uses the endpoint 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
   endpoints SHALL begin a TLS handshake in accordance with [RFC5246].
   The parameters within each TLS negotation are implementation
   dependent but any TCPCL endpoint SHOULD follow all recommended best
   practices of [RFC7525].

5.3.1.  TLS Handshake Result

   If a TLS handshake cannot negotiate a TLS session, both endpoints of
   the TCPCL session SHALL cause a TCPCL shutdown with reason "TLS
   negotiation failed".

   After a TLS session is successfuly established, both TCPCL endpoints
   SHALL re-exchange TCPCL Contact Header messages.  Any information



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   cached from the prior Contact Header exchange SHALL be discarded.
   This re-exchange avoids man-in-the-middle attack in identical fashion
   to [RFC2595].

5.3.2.  Example TLS Initiation

   A summary of a typical CAN_TLS usage is shown in the sequence in
   Figure 6 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 6: A simple visual example of TCPCL TLS Establishment between
                                 two nodes

5.4.  Bundle Transfer

   All of the message in this section are directly associated with
   tranfering a bundle between TCPCL endpoints.

   A single TCPCL transfer results in a bundle (handled by the
   convergence layer as opaque data) being exchanged from one endpoint
   to the other.  In TCPCL a transfer is accomplished by dividing a



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   single bundle up into "segments" based on the receving-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.

5.4.1.  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 LENGTH message and some number
   of DATA_SEGMENT and ACK_SEGMENT messages; all are correlated by the
   same Transfer ID.

   Transfer IDs from each endpoint SHALL be unique within a single TCPCL
   session.  The initial Transfer ID from each endpoint 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 endpoint SHALL terminate the session
   with SHUTDOWN reason code "Resource Exhaustion".

   For bidirectional bundle transfers, a TCPCL endpoint SHOULD NOT rely
   on any relation between Transfer IDs originating from each side of
   the TCPCL session.

5.4.2.  Bundle Length (LENGTH)

   The LENGTH 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 LENGTH 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 5.4.5 for details on when refusal based on LENGTH content is
   acceptable.

   The Total Bundle Length field within a LENGTH message SHALL be used
   as informative data by the receiver.  If, for whatever reason, the
   actual total length of bundle data received differs from the value
   indicated by the LENGTH message, the receiver SHOULD accept the full
   set of bundle data as valid.



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   The format of the LENGTH message is as follows:

                      +-----------------------------+
                      |       Message Header        |
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                      +-----------------------------+
                      |  Total bundle length (U64)  |
                      +-----------------------------+

                    Figure 7: Format of LENGTH Messages

   LENGTH messages SHALL be sent immediately before transmission of any
   DATA_SEGMENT messages.  LENGTH messages MUST NOT be sent unless the
   next DATA_SEGMENT message has the 'S' 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
   LENGTH message without waiting for the next DATA_SEGMENT message.
   The sender MUST be prepared for this and MUST associate the refusal
   with the correct bundle via the Transfer ID fields.

   Upon reception of a LENGTH message not immediately before the start
   of a starting DATA_SEGMENT the reciever SHALL send a REJECT message
   with a Reason Code of "Message Unexpected".

5.4.3.  Bundle Data Transmission (DATA_SEGMENT)

   Each bundle is transmitted in one or more data segments.  The format
   of a DATA_SEGMENT message follows in Figure 8 and its use of header
   flags is shown in Figure 9.

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

                 Figure 8: Format of DATA_SEGMENT Messages








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                                  4 5 6 7
                                 +-+-+-+-+
                                 |0|0|S|E|
                                 +-+-+-+-+

               Figure 9: Format of DATA_SEGMENT Header flags

   The flags portion of the message header octet contains two optional
   values in the two low-order bits, denoted 'S' and 'E' in Figure 9.
   The 'S' bit MUST be set to one if it precedes the transmission of the
   first segment of a transfer.  The 'E' 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 'S' and
   'E' bits MUST be set to one.

   Following the message header, the length field is a 64-bit unsigned
   integer containing the number of octets of bundle data that are
   transmitted in this segment.  Following the length are the actual
   data contents.

   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 'E' bit set.  No interleaving of multiple
   transfers from the same endpoint is possible (within a single TCPCL
   session).

5.4.4.  Bundle Acknowledgments (ACK_SEGMENT)

   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 ACK_SEGMENT message follows in Figure 10 and its use
   of header flags is the same as for DATA_SEGMENT (shown in Figure 9).
   The flags of an ACK_SEGMENT message SHALL be identical to the flags
   of the DATA_SEGMENT message for which it is a reply.









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

                 Figure 10: Format of ACK_SEGMENT Messages

   A receving TCPCL endpoing SHALL send an ACK_SEGMENT message in
   response to each received DATA_SEGMENT message.  The flags portion of
   the ACK_SEGMENT header SHALL be set to match the corresponding
   DATA_SEGEMNT message being acknowledged.  The acknowledged length of
   each ACK_SEGMENT contains the sum of the data length fields of all
   DATA_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.

5.4.5.  Bundle Refusal (REFUSE_BUNDLE)

   As bundles can be large, the TCPCL supports an optional mechanism by
   which a receiving node MAY indicate to the sender that it does not
   want to receive the corresponding bundle.

   To do so, upon receiving a LENGTH or DATA_SEGMENT message, the node
   MAY transmit a REFUSE_BUNDLE 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
   endpoint (which is supposed to represent a firm limitation of what
   the endpoint will accept) and sending of a REFUSE_BUNDLE message.  A
   REFUSE_BUNDLE can be used in cases where the agent's bundle storage
   is temporarily depleted or somehow constrained.  A REFUSE_BUNDLE 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 REFUSE_BUNDLE message is as follows:






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                      +-----------------------------+
                      |       Message Header        |
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                      +-----------------------------+

                Figure 11: Format of REFUSE_BUNDLE Messages

                                  4 5 6 7
                                 +-+-+-+-+
                                 | RCode |
                                 +-+-+-+-+

              Figure 12: Format of REFUSE_BUNDLE Header flags

   The RCode field, which stands for "reason code", contains a value
   indicating why the bundle was refused.  The following table contains
   semantics for some values.  Other values MAY be registered with IANA,
   as defined in Section 8.

   +------------+-------+----------------------------------------------+
   | Name       | RCode | Semantics                                    |
   +------------+-------+----------------------------------------------+
   | Unknown    | 0x0   | Reason for refusal is unknown or not         |
   |            |       | specified.                                   |
   |            |       |                                              |
   | Completed  | 0x1   | The receiver now has the complete bundle.    |
   |            |       | The sender MAY now consider the bundle as    |
   |            |       | completely received.                         |
   |            |       |                                              |
   | No         | 0x2   | The receiver's resources are exhausted. The  |
   | Resources  |       | sender SHOULD apply reactive bundle          |
   |            |       | fragmentation before retrying.               |
   |            |       |                                              |
   | Retransmit | 0x3   | The receiver has encountered a problem that  |
   |            |       | requires the bundle to be retransmitted in   |
   |            |       | its entirety.                                |
   +------------+-------+----------------------------------------------+

                    Table 4: REFUSE_BUNDLE Reason Codes

   The receiver MUST, for each transfer preceding the one to be refused,
   have either acknowledged all DATA_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 REFUSE_BUNDLE message, the
   sender MUST complete the transmission of any partially sent



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   DATA_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 DATA_SEGMENT for the same bundle
   after transmitting a REFUSE_BUNDLE message since messages MAY cross
   on the wire; if this happens, subsequent segments of the bundle
   SHOULD also be refused with a REFUSE_BUNDLE message.

   Note: If a bundle transmission is aborted in this way, the receiver
   MAY not receive a segment with the 'E' flag set to '1' for the
   aborted bundle.  The beginning of the next bundle is identified by
   the 'S' 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.

   The format of the SHUTDOWN message is as follows:

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

                  Figure 13: Format of SHUTDOWN Messages

                                  4 5 6 7
                                 +-+-+-+-+
                                 |0|0|R|D|
                                 +-+-+-+-+

                Figure 14: Format of SHUTDOWN Header flags




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   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 header flags and transmit a one-octet reason
   code immediately following the message header.  The specified values
   of the reason code are:

   +--------------+------+---------------------------------------------+
   | Name         | Code | Description                                 |
   +--------------+------+---------------------------------------------+
   | Idle timeout | 0x00 | The session is being closed due to          |
   |              |      | idleness.                                   |
   |              |      |                                             |
   | Version      | 0x01 | The node cannot conform to the specified    |
   | mismatch     |      | TCPCL protocol version.                     |
   |              |      |                                             |
   | Busy         | 0x02 | The node is too busy to handle the current  |
   |              |      | session.                                    |
   |              |      |                                             |
   | Contact      | 0x03 | The node cannot interpret or negotiate      |
   | Failure      |      | contact header option.                      |
   |              |      |                                             |
   | TLS failure  | 0x04 | The node failed to negotiate TLS session    |
   |              |      | and cannot continue the session.            |
   |              |      |                                             |
   | Resource     | 0x05 | The node has run into some resoure limit    |
   | Exhaustion   |      | and cannot continue the session.            |
   +--------------+------+---------------------------------------------+

                      Table 5: 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
   header 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.




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   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 another protocol message has
   completed being sent, then the node MUST NOT transmit the SHUTDOWN
   message but still SHOULD close the TCP connection.  This means that a
   SHUTDOWN cannot be used to preempt any other TCPCL messaging in-
   progress (particularly important when large segment sizes are being
   transmitted).

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 5).  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 6 in order.  These values avoid
   unnecessarily leaking endpoing 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, indating lack of EID.                |
   +--------------------+---------------------------------------------+

               Table 6: 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 REFUSE_BUNDLE 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]






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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.

     +------------------------+-------------------------------------+
     | 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 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  |
               +-------+-------------+---------------------+





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8.3.  Message Types

   IANA has created, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Message Types"
   and initialized it with the contents below.  The registration
   procedure is RFC Required.

                       +----------+---------------+
                       | Code     | Message Type  |
                       +----------+---------------+
                       | 0x0      | Reserved      |
                       |          |               |
                       | 0x1      | DATA_SEGMENT  |
                       |          |               |
                       | 0x2      | ACK_SEGMENT   |
                       |          |               |
                       | 0x3      | REFUSE_BUNDLE |
                       |          |               |
                       | 0x4      | KEEPALIVE     |
                       |          |               |
                       | 0x5      | SHUTDOWN      |
                       |          |               |
                       | 0x6      | LENGTH        |
                       |          |               |
                       | TBD      | REJECT        |
                       |          |               |
                       | TBD--0xf | Unassigned    |
                       +----------+---------------+

                            Message Type Codes

8.4.  REFUSE_BUNDLE Reason Codes

   IANA has created, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer REFUSE_BUNDLE
   Reason Codes" and initialized it with the contents of Table 3.  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 |
                 +----------+---------------------------+

                        REFUSE_BUNDLE Reason Codes

8.5.  SHUTDOWN Reason Codes

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

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

                           SHUTDOWN Reason Codes

8.6.  REJECT Reason Codes

   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 REJECT Reason
   Codes" and initialized it with the contents of Table 4.  The
   registration procedure is RFC Required.

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

                            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

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

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5246>.



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   [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, <http://www.rfc-editor.org/info/rfc7525>.

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol",
              draft-ietf-dtn-bpbis-06 (work in progress), October 2016.

   [refs.IANA-BP]
              IANA, "Bundle Protocol registry", May 2016.

10.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <http://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, <http://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,
              <http://www.rfc-editor.org/info/rfc6257>.

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

   [I-D.ietf-dtn-bpsec]
              Birrane, E. and K. McKeever, "Bundle Protocol Security
              Specification", draft-ietf-dtn-bpsec-03 (work in
              progress), October 2016.

Appendix A.  Significant changes from RFC7242

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

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





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

   o  Added a bundle transfer identification number to all bundle-
      related messages (LENGTH, DATA_SEGMENT, ACK_SEGMENT,
      REFUSE_BUNDLE).

   o  Use flags in ACK_SEGMENT to mirror flags from DATA_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 negotation failure SHUTDOWN reason code.

   o  Added 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
   1229 19th Street NW
   Wasington, DC  20036
   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








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   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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