Delay Tolerant Networking TCP Convergence Layer Protocol
draft-irtf-dtnrg-tcp-clayer-02

Delay Tolerant Networking Research                             M. Demmer
Group                                                        UC Berkeley
Internet-Draft                                                    J. Ott
Intended status: Experimental          Helsinki University of Technology
Expires: May 7, 2009                                    November 3, 2008

        Delay Tolerant Networking TCP Convergence Layer Protocol
                   draft-irtf-dtnrg-tcp-clayer-02.txt

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Abstract

   This document describes the protocol for the TCP-based Convergence
   Layer for Delay Tolerant Networking (DTN).

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Definitions Relating to the Bundle Protocol  . . . . . . .  4
     2.2.  Definitions specific to the TCPCL Protocol . . . . . . . .  5
   3.  General Protocol Description . . . . . . . . . . . . . . . . .  6
     3.1.  Example message exchange . . . . . . . . . . . . . . . . .  7
   4.  Connection Establishment . . . . . . . . . . . . . . . . . . .  8
     4.1.  Contact Header . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Validation and parameter negotiation . . . . . . . . . . . 11
   5.  Established Connection Operation . . . . . . . . . . . . . . . 12
     5.1.  Message Type Codes . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Bundle Data Transmission . . . . . . . . . . . . . . . . . 13
     5.3.  Bundle Acknowledgments . . . . . . . . . . . . . . . . . . 14
     5.4.  Bundle Refusal . . . . . . . . . . . . . . . . . . . . . . 15
     5.5.  Keepalive Messages . . . . . . . . . . . . . . . . . . . . 16
   6.  Connection Termination . . . . . . . . . . . . . . . . . . . . 16
     6.1.  Shutdown Message . . . . . . . . . . . . . . . . . . . . . 16
     6.2.  Idle Connection Shutdown . . . . . . . . . . . . . . . . . 18
   7.  Requirements notation  . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21

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1.  Introduction

   This document describes the TCP-based convergence layer protocol for
   Delay Tolerant Networking (TCPCL).  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 the Delay-Tolerant Network Architecture
   [refs.dtnarch] RFC.

   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 Bundling Protocol (BP)
   [refs.bundleproto], an application-layer protocol that is used to
   construct a store-and-forward overlay network.  As described in the
   bundle protocol specification, it requires the services of a
   "convergence layer adapter" (CLA) to send and receive bundles using
   an underlying 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, both the BP and the
   TCPCL reside above the transport layer, i.e., at the application
   layer.

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

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   between entities participating in TCPCL communications.  This
   document does not address:

        The format of protocol data units of the bundling protocol, as
        those are defined elsewhere [refs.bundleproto].

        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 any Internet Draft, RFC,
   or any other public document.  These prior versions of the protocol
   were, however, implemented in the DTN reference implementation
   [refs.dtnimpl], in prior releases, hence the current version number
   reflects the existence of those prior versions.

2.  Definitions

2.1.  Definitions Relating to the Bundle Protocol

   The following set of definitions are abbreviated versions of those
   which appear in the Bundle Protocol Specification [refs.bundleproto].
   To the extent in which terms appear in both documents, they are
   intended to have the same meaning.

   Bundle --  A bundle is a protocol data unit of the DTN bundle
        protocol.

   Bundle payload --  A bundle payload (or simply "payload") is the
        application data whose conveyance to the bundle's destination is
        the purpose for the transmission of a given bundle.

   Fragment --  A fragment is a bundle whose payload contains a range of
        bytes from another bundle's payload.

   Bundle node --  A bundle node (or simply a "node") is any entity that
        can send and/or receive bundles.  The particular instantiation
        of this entity is deliberately unconstrained, allowing for
        implementations in software libraries, long-running processes,
        or even hardware.  One component of the bundle node is the
        implementation of a convergence layer adapter.

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   Convergence layer adapter --  A convergence layer adapter (CLA) sends
        and receives bundles utilizing the services of some 'native'
        link, network, or internet protocol.  This document describes
        the manner in which a CLA sends and receives bundles when using
        the TCP protocol for inter-node communication.

   Self Describing Numeric Value --  A self describing numeric value
        (SDNV) is a variable length encoding for integer values, defined
        in the bundle protocol specification.

2.2.  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 one-to-one
        with 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 Section 4.2.

   Connection initiator --  The connection initiator is the bundle node
        that causes the establishment of a new connection by creating a
        new TCP connection (for example, by using the connect() call in
        the BSD sockets API) and then following the procedures described
        in Section 4.

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   Connection acceptor --  The connection acceptor is the bundle node
        that establishes a connection in response to an active
        connection attempt by another bundle node (for example, by using
        the listen() and accept() calls of the BSD sockets API) and then
        following the procedures described in Section 4.

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

3.  General Protocol Description

   This protocol provides bundle conveyance over a TCP connection and
   specifies the encapsulation of bundles as well as procedures for TCP
   connection setup and teardown.  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 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,
   an SDNV 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.

   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.

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   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.  Note that in the case of concurrent bidirectional
   transmission, then ack segments may be interleaved with data
   segments.

   Another optional feature is that a receiver may interrupt the
   transmission of a bundle at any point in time by replying with a
   negative acknowledgment (REFUSE_BUNDLE) 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.  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
   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      |
      +-------------------------+         +-------------------------+

      +-------------------------+
      |   DATA_SEGMENT (start)  | ->
      |    SDNV length [L1]     | ->
      |    Bundle Data 0..L1    | ->
      +-------------------------+
      +-------------------------+         +-------------------------+
      |     DATA_SEGMENT        | ->   <- |       ACK_SEGMENT       |
      |    SDNV length [L2]     | ->   <- |     SDNV length [L1]    |
      |    Bundle Data L1..L2   | ->      +-------------------------+
      +-------------------------+
      +-------------------------+         +-------------------------+
      |    DATA_SEGMENT (end)   | ->   <- |       ACK_SEGMENT       |
      |     SDNV length [L3]    | ->   <- |   SDNV length [L1+L2]   |
      |    Bundle Data L2..L3   | ->      +-------------------------+
      +-------------------------+
                                          +-------------------------+
                                       <- |       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.  The manner in
   which a bundle node makes the decision to establish such a connection
   is implementation-dependent.  For example, some connections may be
   opened proactively and maintained for as long as is possible given
   the network 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

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   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, perhaps.  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 and it SHOULD use a
   (binary) exponential backoff mechanism to increase this delay in case
   of repeated failures.

   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, the connection initiator MUST
   immediately transmit a contact header over the TCP connection.  The
   connection acceptor MUST also 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!".

   version:  A one byte field value containing the current version of
        the protocol.

   flags:  A one byte field containing optional connection flags.  The
        first five bits are unused and MUST be set to zero upon
        transmission and MUST be ignored upon reception.  The last three
        bits are interpreted as shown in table 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 5.5).  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:  An octet 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>.  A eight byte
        EID is shown the 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 negative acknowledgments.  This   |
   |          | flag MUST NOT be set to '1' unless support for         |
   |          | acknowledgments is also indicated.                     |
   +----------+--------------------------------------------------------+

                       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, both the TCPCL connection
   initiator and the TCPCL connection acceptor follow the following
   procedures for ensuring 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 a
   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
   should proceed in the following manner:

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        The segment acknowledgments enabled parameter is set to true iff
        the corresponding flag is set in both contact headers.

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

        Negative acknowledgments to interrupt transmission (actually:
        refuse reception) of a bundle may only be used iff both peers
        indicate support for negative acknowledgments in their contact
        header.

        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 5.5) is disabled.

   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 octet header
   with the following structure:

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

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

   flags:  Optional flags defined on a per message type basis.

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

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   +----------------+------+-------------------------------------------+
   |      Type      | Code | Comment                                   |
   +----------------+------+-------------------------------------------+
   |  DATA_SEGMENT  | 0x1  | Indicates the transmission of a segment   |
   |                |      | of bundle data, described in Section 5.2. |
   |                |      |                                           |
   |   ACK_SEGMENT  | 0x2  | Acknowledges reception of a data segment, |
   |                |      | described in Section 5.3                  |
   |                |      |                                           |
   |  REFUSE_BUNDLE | 0x3  | Indicates that the transmission of the    |
   |                |      | current bundle shall be stopped,          |
   |                |      | described in Section 5.4.                 |
   |                |      |                                           |
   |    KEEPALIVE   | 0x4  | Keepalive message for the connection,     |
   |                |      | described in Section 5.5.                 |
   |                |      |                                           |
   |    SHUTDOWN    | 0x5  | Indicates that one of the nodes           |
   |                |      | participating in the connection wishes to |
   |                |      | cleanly terminate the connection,         |
   |                |      | described in Section 6.                   |
   |                |      |                                           |
   +----------------+------+-------------------------------------------+

                        Table 2: TCPCL Header Types

5.2.  Bundle Data Transmission

   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 4: Format of bundle data segment messages

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

   The flags portion of the message header octet contains two optional
   values in the two low-order bits, denoted 'S' and 'E' above.  The 'S'
   bit MUST be set to one iff 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.

   Determining the size of the segment is an implementation matter.  In
   particular, a node may, based on local policy or configuration, only

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   ever transmit bundle data in a single segment, in which case both the
   'S' and 'E' bits MUST be set to one.  However, a node MUST be able to
   receive a bundle that has been transmitted in any segment size.

   In the bundle protocol specification, 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.

   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.

5.3.  Bundle Acknowledgments

   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 header 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 5: Format of bundle acknowledgement 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 of the received
   segment(s) of the current bundle.

   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

   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 data segment (and thus the
   bundle) that is being refused is implicitly identified by the
   sequence in which positive and negative acknowledgments are received.

   The receiver MUST have acknowledged (positively or negatively) all
   other received DATA_SEGMENTs before the one to be refused so that the
   sender can 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

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   SHOULD be refused with a REFUSE_BUNDLE message, too.

   Note: If a bundle transmission if 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.  Keepalive Messages

   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 message type code of SHUTDOWN (as
   described in Table 2) and 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 RTO) which is dependent on the
   measured RTT for the TCP connection so that keepalive message may
   experience noticeable latency.

6.  Connection Termination

   This section describes the procedures for ending a TCPCL connection.

6.1.  Shutdown Message

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

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   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  0x3  |0|0|R|D| reason (opt)  | reconnection delay (opt)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 6: 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           | Comment                                |
   +------+-------------------+----------------------------------------+
   | 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.                    |
   +------+-------------------+----------------------------------------+

                      Table 3: 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
   the message header flags, 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 delay field (and set the
   'D' bit to zero).  Note that in the figure above, a two octet SDNV is
   shown for convenience of the presentation.

   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 a
   the initiator that the node is currently not capable of or willing to
   communicate.  However, a node MUST always send the contact header to

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   its peer first.

   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 re-open 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, 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, then 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 above).  After receiving a SHUTDOWN message in
   response, both sides may close the TCP connection.

7.  Requirements notation

   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].

8.  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

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   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.

   These concerns may be mitigated through the use of the Bundle
   Security Protocols [refs.dtnsecurity].  In particular, the Bundle
   Authentication Header 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.

   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 either 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.

9.  IANA Considerations

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

10.  References

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

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

   [refs.dtnarch]
              Cerf et al, V., "Delay-Tolerant Network Architecture",
              RFC 4838, April 2007.

   [refs.dtnimpl]
              DTNRG, "Delay Tolerant Networking Reference
              Implementation", <http://www.dtnrg.org/Code>.

   [refs.dtnsecurity]
              Symington, S., Farrell, S., and H. Weiss, "Bundle Security

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              Protocol Specification", Internet Draft, work in
              progress draft-irtf-dtnrg-bundle-security-03.txt,
              April 2007.

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
   Helsinki University of Technology
   Department of Communications and Networking
   PO Box 3000
   TKK  02015
   Finland

   Email: jo@netlab.tkk.fi

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