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EXPERIMENTAL

Internet Research Task Force (IRTF)                            M. Demmer
Request for Comments: 7242                                   UC Berkeley
Category: Experimental                                            J. Ott
ISSN: 2070-1721                                         Aalto University
                                                            S. Perreault

                                                               June 2014


        Delay-Tolerant Networking TCP Convergence-Layer Protocol

Abstract

   This document describes the protocol for the TCP-based convergence
   layer for Delay-Tolerant Networking (DTN).  It is the product of the
   IRTF's DTN Research Group (DTNRG).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Research Task
   Force (IRTF).  The IRTF publishes the results of Internet-related
   research and development activities.  These results might not be
   suitable for deployment.  This RFC represents the consensus of the
   Delay-Tolerant Networking Research Group of the Internet Research
   Task Force (IRTF).  Documents approved for publication by the IRSG
   are not a candidate for any level of Internet Standard; see Section 2
   of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7242.

Copyright Notice

   Copyright (c) 2014 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
   to this document.



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Table of Contents

   1. Introduction ....................................................2
   2. Definitions .....................................................4
      2.1. Definitions Specific to the TCPCL Protocol .................4
   3. General Protocol Description ....................................5
      3.1. Bidirectional Use of TCP Connection ........................6
      3.2. Example Message Exchange ...................................6
   4. Connection Establishment ........................................7
      4.1. Contact Header .............................................8
      4.2. Validation and Parameter Negotiation ......................10
   5. Established Connection Operation ...............................11
      5.1. Message Type Codes ........................................11
      5.2. Bundle Data Transmission (DATA_SEGMENT) ...................12
      5.3. Bundle Acknowledgments (ACK_SEGMENT) ......................13
      5.4. Bundle Refusal (REFUSE_BUNDLE) ............................14
      5.5. Bundle Length (LENGTH) ....................................15
      5.6. KEEPALIVE Feature (KEEPALIVE) .............................16
   6. Connection Termination .........................................17
      6.1. Shutdown Message (SHUTDOWN) ...............................17
      6.2. Idle Connection Shutdown ..................................18
   7. Security Considerations ........................................19
   8. IANA Considerations ............................................20
      8.1. Port Number ...............................................20
      8.2. Protocol Versions .........................................20
      8.3. Message Types .............................................20
      8.4. REFUSE_BUNDLE Reason Codes ................................21
      8.5. SHUTDOWN Reason Codes .....................................21
   9. Acknowledgments ................................................21
   10. References ....................................................21
      10.1. Normative References .....................................21
      10.2. Informative References ...................................21

1.  Introduction

   This document describes the TCP-based convergence-layer protocol for
   Delay-Tolerant Networking.  Delay-Tolerant Networking is an end-to-
   end architecture providing communications in and/or through highly
   stressed environments, including those with intermittent
   connectivity, long and/or variable delays, and high bit error rates.
   More detailed descriptions of the rationale and capabilities of these
   networks can be found in "Delay-Tolerant Network Architecture"
   [RFC4838].

   An important goal of the DTN architecture is to accommodate a wide
   range of networking technologies and environments.  The protocol used
   for DTN communications is the Bundle Protocol (BP) [RFC5050], an
   application-layer protocol that is used to construct a store-and-



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   forward overlay network.  As described in the Bundle Protocol
   specification [RFC5050], 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) | -/
      +-------------------------+
      |          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 in 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 [RFC5050].

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

   Note that this document describes version 3 of the protocol.
   Versions 0, 1, and 2 were never specified in an Internet-Draft, RFC,
   or any other public document.  These prior versions of the protocol
   were, however, implemented in the DTN reference implementation
   [DTNIMPL] in prior releases; hence, the current version number
   reflects the existence of those prior versions.



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   This is an experimental protocol produced within the IRTF's Delay-
   Tolerant Networking Research Group (DTNRG).  It represents the
   consensus of all active contributors to this group.  If this protocol
   is used on the Internet, IETF standard protocols for security and
   congestion control should be used.

2.  Definitions

   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].

   The terms defined in Section 3.1 of [RFC5050] are used extensively in
   this document.

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 Connection --  A TCPCL connection (as opposed to a TCP
        connection) is a TCPCL communication relationship between two
        bundle nodes.  The lifetime of a TCPCL connection is bound to
        the lifetime of an underlying TCP connection.  Therefore, a
        TCPCL connection is initiated when a bundle node initiates a TCP
        connection to be established for the purposes of bundle
        communication.  A TCPCL connection 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 "connection" without the prefix "TCPCL" shall
        refer to a TCPCL connection.

   Connection parameters --  The connection parameters are a set of
        values used to affect the operation of the TCPCL for a given
        connection.  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 connection 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 connection 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 connection is established and configured in this way,
   bundles can be transmitted in either direction.  Each bundle is
   transmitted in one or more logical segments of formatted bundle data.
   Each logical data segment consists of a DATA_SEGMENT message header,
   a Self-Delimiting Numeric Value (SDNV) as defined in [RFC5050] (see
   also [RFC6256]) containing the length of the segment, and finally the
   byte range of the bundle data.  The choice of the length to use for
   segments is an implementation matter.  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.

   An optional feature of the 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 connection is interrupted, it can perform reactive
   fragmentation to avoid re-sending the already transmitted part of the
   bundle.

   When acknowledgments are enabled, then for each data segment that is
   received, the receiving node sends an ACK_SEGMENT code followed by an
   SDNV 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 optional 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 connections that are idle, a KEEPALIVE message may optionally be
   sent at a negotiated interval.  This is used to convey liveness
   information.

   Finally, before connections close, a SHUTDOWN message is sent on the
   channel.  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 connection setup by a
   peer.

3.1.  Bidirectional Use of TCP Connection

   There are specific messages for sending and receiving operations (in
   addition to connection setup/teardown).  TCPCL is symmetric, i.e.,
   both sides can start sending data segments in a connection, 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 connection 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







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   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.

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

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

        +-------------------------+
        |   DATA_SEGMENT (start)  | ->
        |    SDNV length [L1]     | ->
        |  Bundle Data 0..(L1-1)  | ->
        +-------------------------+
        +-------------------------+         +-------------------------+
        |     DATA_SEGMENT        | ->   <- |       ACK_SEGMENT       |
        |    SDNV length [L2]     | ->   <- |     SDNV length [L1]    |
        |Bundle Data L1..(L1+L2-1)| ->      +-------------------------+
        +-------------------------+
        +-------------------------+         +-------------------------+
        |    DATA_SEGMENT (end)   | ->   <- |       ACK_SEGMENT       |
        |     SDNV length [L3]    | ->   <- |   SDNV length [L1+L2]   |
        |Bundle Data              | ->      +-------------------------+
        |    (L1+L2)..(L1+L2+L3-1)|
        +-------------------------+
                                            +-------------------------+
                                         <- |       ACK_SEGMENT       |
                                         <- |  SDNV 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.  Connection Establishment

   For bundle transmissions to occur using the TCPCL, a TCPCL connection
   must first be established between communicating nodes.  It is up to
   the implementation to decide how and when connection setup is
   triggered.  For example, some connections may be opened proactively
   and maintained for as long as is possible given the network



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   conditions, while other connections 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 connection, 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 connection by sending a SHUTDOWN message.  If
   connection 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       |
   +---------------+---------------+---------------+---------------+
   |                     local EID length (SDNV)                   |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   +                      local EID (variable)                     +
   |                                                               |
   +---------------+---------------+---------------+---------------+

                      Figure 3: Contact Header Format

   The fields of the contact header are:

   magic:  A four-byte field that always contains the byte sequence 0x64
        0x74 0x6e 0x21, i.e., the text string "dtn!" in US-ASCII.

   version:  A one-byte field value containing the value 3 (current
        version of the protocol).

   flags:  A one-byte field containing optional connection flags.  The
        first four bits are unused and MUST be set to zero upon
        transmission and MUST be ignored upon reception.  The last four
        bits are interpreted as shown in Table 1 below.

   keepalive_interval:  A two-byte integer field containing the number
        of seconds between exchanges of KEEPALIVE messages on the
        connection (see Section 5.6).  This value is in network byte
        order, as are all other multi-byte fields described in this
        protocol.

   local EID length:  A variable-length SDNV field containing the length
        of the endpoint identifier (EID) for some singleton endpoint in
        which the sending node is a member.  A four-byte SDNV is
        depicted for clarity of the figure.

   local EID:  A byte string containing the EID of some singleton
        endpoint in which the sending node is a member, in the canonical
        format of <scheme name>:<scheme-specific part>.  An eight-byte
        EID is shown for clarity of the figure.





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   +----------+--------------------------------------------------------+
   |  Value   | Meaning                                                |
   +----------+--------------------------------------------------------+
   | 00000001 | Request acknowledgment of bundle segments.             |
   | 00000010 | Request enabling of reactive fragmentation.            |
   | 00000100 | Indicate support for bundle refusal.  This flag MUST   |
   |          | NOT be set to '1' unless support for acknowledgments   |
   |          | is also indicated.                                     |
   | 00001000 | Request sending of LENGTH messages.                    |
   +----------+--------------------------------------------------------+

                       Table 1: Contact Header Flags

   The manner in which values are configured and chosen for the various
   flags and parameters in the contact header is implementation
   dependent.

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 connection and to
   negotiate values for the connection 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 SHOULD interpret all fields and messages as
   it would normally.  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 connection due to the
   version mismatch or may adapt 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 connection by
   negotiating the values from its own preferences (conveyed by the
   contact header it sent) with the preferences of the peer node
   (expressed in the contact header that it received).  This negotiation
   MUST proceed in the following manner:






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   o  The parameter for requesting acknowledgment of bundle segments is
      set to true iff the corresponding flag is set in both contact
      headers.

   o  The parameter for enabling reactive fragmentation is set to true
      iff the corresponding flag is set in both contact headers.

   o  The bundle refusal capability is set to true if the corresponding
      flag is set in both contact headers and if segment acknowledgment
      has been enabled.

   o  The keepalive_interval parameter is set to the minimum value from
      both contact headers.  If one or both contact headers contains the
      value zero, then the keepalive feature (described in Section 5.6)
      is disabled.

   o  The flag requesting sending of LENGTH messages is handled as
      described in Section 5.5.

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

5.  Established Connection Operation

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

5.1.  Message Type Codes

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

                             0 1 2 3 4 5 6 7
                            +-+-+-+-+-+-+-+-+
                            | type  | flags |
                            +-+-+-+-+-+-+-+-+

              Figure 4: Format of the One-Byte Message Header

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

   flags:  Optional flags defined based on message type.

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



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   +----------------+---------+----------------------------------------+
   |      Type      | Code    | Description                            |
   +----------------+---------+----------------------------------------+
   |                | 0x0     | Reserved.                              |
   |                |         |                                        |
   |  DATA_SEGMENT  | 0x1     | Indicates the transmission of a        |
   |                |         | segment of bundle data, as described   |
   |                |         | in Section 5.2.                        |
   |                |         |                                        |
   |  ACK_SEGMENT   | 0x2     | Acknowledges reception of a data       |
   |                |         | segment, as described in Section 5.3   |
   |                |         |                                        |
   | REFUSE_BUNDLE  | 0x3     | Indicates that the transmission of the |
   |                |         | current bundle shall be stopped, as    |
   |                |         | described in Section 5.4.              |
   |                |         |                                        |
   |   KEEPALIVE    | 0x4     | KEEPALIVE message for the connection,  |
   |                |         | as described in Section 5.6.           |
   |                |         |                                        |
   |    SHUTDOWN    | 0x5     | Indicates that one of the nodes        |
   |                |         | participating in the connection wishes |
   |                |         | to cleanly terminate the connection,   |
   |                |         | as described in Section 6.             |
   |                |         |                                        |
   |     LENGTH     | 0x6     | Contains the length (in bytes) of the  |
   |                |         | next bundle, as described in Section   |
   |                |         | 5.5.                                   |
   |                |         |                                        |
   |                | 0x7-0xf | Unassigned.                            |
   |                |         |                                        |
   +----------------+---------+----------------------------------------+

                       Table 2: TCPCL Message Types

5.2.  Bundle Data Transmission (DATA_SEGMENT)

   Each bundle is transmitted in one or more data segments.  The format
   of a DATA_SEGMENT message 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0x1  |0|0|S|E|   length ...    |  contents....               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: Format of DATA_SEGMENT Messages

   The type portion of the message header contains the value 0x1.



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   The flags portion of the message header byte contains two optional
   values in the two low-order bits, denoted 'S' and 'E' above.  The 'S'
   bit MUST be set to one if it precedes the transmission of the first
   segment of a new bundle.  The 'E' bit MUST be set to one when
   transmitting the last segment of a bundle.

   Following the message header, the length field is an SDNV containing
   the number of bytes of bundle data that are transmitted in this
   segment.  Following this length is the actual data contents.

   Determining the size of the segment is an implementation matter.  In
   particular, a node may, based on local policy or configuration, only
   ever transmit bundle data in a single segment, in which case both the
   'S' and 'E' bits MUST be set to one.

   In the Bundle Protocol specification [RFC5050], a single bundle
   comprises a primary bundle block, a payload block, and zero or more
   additional bundle blocks.  The relationship between the protocol
   blocks and the convergence-layer segments is an implementation-
   specific decision.  In particular, a segment MAY contain more than
   one protocol block; alternatively, a single protocol block (such as
   the payload) MAY be split into multiple segments.

   However, a single segment MUST NOT contain data of more than a single
   bundle.

   Once a transmission 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.

5.3.  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 offers an optional feature whereby a
   receiving node transmits acknowledgments of reception of data
   segments.  This feature is enabled if, and only if, during the
   exchange of contact headers, both parties set the flag to indicate
   that segment acknowledgments are enabled (see Section 4.1).  If so,
   then the receiver MUST transmit a bundle acknowledgment message when
   it successfully receives each data segment.




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   The format of a bundle acknowledgment 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0x2  |0|0|0|0|   acknowledged length ...                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: Format of ACK_SEGMENT Messages

   To transmit an acknowledgment, a node first transmits a message
   header with the ACK_SEGMENT type code and all flags set to zero, then
   transmits an SDNV containing the cumulative length in bytes of the
   received segment(s) of the current bundle.  The length MUST fall on a
   segment boundary.  That is, only full segments can be acknowledged.

   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.  Bundle Refusal (REFUSE_BUNDLE)

   As bundles may be large, the TCPCL supports an optional mechanisms 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 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 is implicitly identified by the sequence in which
   acknowledgements and refusals are received.

   The format of the REFUSE_BUNDLE message is as follows:

                               0 1 2 3 4 5 6 7
                              +-+-+-+-+-+-+-+-+
                              |  0x3  | RCode |
                              +-+-+-+-+-+-+-+-+

                Figure 7: Format of REFUSE_BUNDLE Messages

   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.



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   +---------+---------------------------------------------------------+
   |  RCode  | Semantics                                               |
   +---------+---------------------------------------------------------+
   |   0x0   | Reason for refusal is unknown or not specified.         |
   |   0x1   | The receiver now has the complete bundle.  The sender   |
   |         | may now consider the bundle as completely received.     |
   |   0x2   | The receiver's resources are exhausted.  The sender     |
   |         | SHOULD apply reactive bundle fragmentation before       |
   |         | retrying.                                               |
   |   0x3   | The receiver has encountered a problem that requires    |
   |         | the bundle to be retransmitted in its entirety.         |
   | 0x4-0x7 | Unassigned.                                             |
   | 0x8-0xf | Reserved for future usage.                              |
   +---------+---------------------------------------------------------+

                    Table 3: REFUSE_BUNDLE Reason Codes

   The receiver MUST, for each bundle preceding the one to be refused,
   have either acknowledged all DATA_SEGMENTs or refused the bundle.
   This allows the sender to identify the bundles accepted and refused
   by means of a simple FIFO list of segments and acknowledgments.

   The bundle refusal MAY be sent before the entire data segment is
   received.  If a sender receives a REFUSE_BUNDLE message, the sender
   MUST complete the transmission of any partially sent DATA_SEGMENT
   message (so that the receiver stays in sync).  The sender MUST NOT
   commence transmission of any further segments of the rejected 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 bundle.

5.5.  Bundle Length (LENGTH)

   The LENGTH message contains the total length, in bytes, of the next
   bundle, formatted as an SDNV.  Its purpose is to allow nodes to
   preemptively refuse bundles that would exceed their resources.  It is
   an optimization.







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   The format of the LENGTH message 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0x6  |0|0|0|0|     total bundle length ...                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 8: Format of LENGTH Messages

   LENGTH messages MUST NOT be sent unless the corresponding flag bit is
   set in the contact header.  If the flag bit is set, LENGTH messages
   MAY be sent at the sender's discretion.  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 bundle).

   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 right bundle.

5.6.  KEEPALIVE Feature (KEEPALIVE)

   The protocol includes a provision for transmission of KEEPALIVE
   messages over the TCP connection to help determine if the 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.

   The format of a KEEPALIVE message is a one-byte 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-byte SHUTDOWN message (as described in
   Table 2) and by closing the TCP connection.

   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.



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6.  Connection Termination

   This section describes the procedures for ending a TCPCL connection.

6.1.  Shutdown Message (SHUTDOWN)

   To cleanly shut down a connection, a SHUTDOWN message MUST be
   transmitted by either node at any point following complete
   transmission of any other message.  In case acknowledgments have been
   negotiated, a node SHOULD acknowledge all received data segments
   first and then shut down the connection.

   The format of the SHUTDOWN message 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0x5  |0|0|R|D| reason (opt)  | reconnection delay (opt)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 9: Format of Bundle SHUTDOWN Messages

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

   +-----------+------------------+------------------------------------+
   |    Code   | Meaning          | Description                        |
   +-----------+------------------+------------------------------------+
   |    0x00   | Idle timeout     | The connection is being closed due |
   |           |                  | to idleness.                       |
   |           |                  |                                    |
   |    0x01   | Version mismatch | The node cannot conform to the     |
   |           |                  | specified TCPCL protocol version.  |
   |           |                  |                                    |
   |    0x02   | Busy             | The node is too busy to handle the |
   |           |                  | current connection.                |
   |           |                  |                                    |
   | 0x03-0xff |                  | Unassigned.                        |
   +-----------+------------------+------------------------------------+

                      Table 4: SHUTDOWN Reason Codes

   It is also possible to convey a requested reconnection delay to
   indicate how long the other node must wait before attempting
   connection re-establishment.  To do so, the node sets the 'D' bit in



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   the message header flags and then transmits an SDNV 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 connection.  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).  Note that in the figure above, the
   reconnection delay SDNV is represented as a two-byte field for
   convenience.

   A connection 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 connection 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 a node
   does not want its peer to reopen the 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 connection
   setup.

   If a connection is to be terminated before another protocol message
   has completed, then the node MUST NOT transmit the SHUTDOWN message
   but still SHOULD close the TCP connection.  In particular, if the
   connection is to be closed (for whatever reason) while a node is in
   the process of transmitting a bundle data segment, the receiving node
   is still expecting segment data and might erroneously interpret the
   SHUTDOWN message to be part of the data segment.

6.2.  Idle Connection Shutdown

   The protocol includes a provision for clean shutdown of idle TCP
   connections.  Determining the length of time to wait before closing
   idle connections, 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 connection by
   transmitting a SHUTDOWN message indicating the reason code of 'Idle
   timeout' (as described in Table 4).  After receiving a SHUTDOWN
   message in response, both sides may close the TCP connection.





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7.  Security Considerations

   One security consideration for this protocol relates to the fact that
   nodes present their endpoint identifier as part of the connection
   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 without further verification as a means to
   determine which bundles are transmitted over the connection, then the
   node that has falsified its identity may be able to obtain bundles
   that it should not have.  Therefore, a node SHALL NOT use the
   endpoint identifier conveyed in the TCPCL connection message to
   derive a peer node's identity unless it can ascertain it via other
   means.

   These concerns may be mitigated through the use of the Bundle
   Security Protocol [RFC6257].  In particular, the Bundle
   Authentication Block defines mechanism for secure exchange of bundles
   between DTN nodes.  Thus, an implementation could delay trusting the
   presented endpoint identifier until the node can securely validate
   that its peer is in fact the only member of the given singleton
   endpoint.

   In general, TCPCL does not provide any security services.  The
   mechanisms defined in [RFC6257] are to be used instead.

   Nothing in TCPCL prevents the use of the Transport Layer Security
   (TLS) protocol [RFC5246] to secure a connection.

   Another consideration for this protocol relates to denial-of-service
   attacks.  A node may send a large amount of data over a TCP
   connection, 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 connection.  This burden could cause denial of
   service on other, well-behaving connections.  There is also nothing
   to prevent a malicious node from continually establishing connections
   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.










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8.  IANA Considerations

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

8.1.  Port Number

   Port number 4556 has been assigned as the default port for the TCP
   convergence layer.

   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:

                    +-------+-------------+-----------+
                    | Value | Description | Reference |
                    +-------+-------------+-----------+
                    |   0   | Reserved    | [RFC7242] |
                    |   1   | Reserved    | [RFC7242] |
                    |   2   | Reserved    | [RFC7242] |
                    |   3   | TCPCL       | [RFC7242] |
                    | 4-255 | Unassigned  | [RFC7242] |
                    +-------+-------------+-----------+

   The registration procedure is RFC Required.

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 of Table 2.  The registration
   procedure is RFC Required.




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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.

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.

9.  Acknowledgments

   The authors would like to thank the following individuals who have
   participated in the drafting, review, and discussion of this memo:
   Alex McMahon, Brenton Walker, Darren Long, Dirk Kutscher, Elwyn
   Davies, Jean-Philippe Dionne, Joseph Ishac, Keith Scott, Kevin Fall,
   Lloyd Wood, Marc Blanchet, Peter Lovell, Scott Burleigh, Stephen
   Farrell, Vint Cerf, and William Ivancic.

10.  References

10.1.  Normative References

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

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

10.2.  Informative References

   [DTNIMPL]  DTNRG, "Delay-Tolerant Networking Reference
              Implementation", <https://sites.google.com/site/
              dtnresgroup/home/code>.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, April 2007.





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

   [RFC6256]  Eddy, W. and E. Davies, "Using Self-Delimiting Numeric
              Values in Protocols", RFC 6256, May 2011.

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

Authors' Addresses

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

   EMail: demmer@cs.berkeley.edu


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

   EMail: jo@netlab.tkk.fi


   Simon Perreault
   Quebec, QC
   Canada

   EMail: simon@per.reau.lt














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