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rfc:rfc7242

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.

Demmer, et al. Experimental [Page 1] RFC 7242 DTN TCP Convergence Layer June 2014

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-

Demmer, et al. Experimental [Page 2] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 3] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 4] RFC 7242 DTN TCP Convergence Layer June 2014

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.

Demmer, et al. Experimental [Page 5] RFC 7242 DTN TCP Convergence Layer June 2014

 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

Demmer, et al. Experimental [Page 6] RFC 7242 DTN TCP Convergence Layer June 2014

 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

Demmer, et al. Experimental [Page 7] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 8] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 9] RFC 7242 DTN TCP Convergence Layer June 2014

 +----------+--------------------------------------------------------+
 |  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:

Demmer, et al. Experimental [Page 10] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 11] RFC 7242 DTN TCP Convergence Layer June 2014

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

Demmer, et al. Experimental [Page 12] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 13] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 14] RFC 7242 DTN TCP Convergence Layer June 2014

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

Demmer, et al. Experimental [Page 15] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 16] RFC 7242 DTN TCP Convergence Layer June 2014

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

Demmer, et al. Experimental [Page 17] RFC 7242 DTN TCP Convergence Layer June 2014

 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.

Demmer, et al. Experimental [Page 18] RFC 7242 DTN TCP Convergence Layer June 2014

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.

Demmer, et al. Experimental [Page 19] RFC 7242 DTN TCP Convergence Layer June 2014

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.

Demmer, et al. Experimental [Page 20] RFC 7242 DTN TCP Convergence Layer June 2014

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.

Demmer, et al. Experimental [Page 21] RFC 7242 DTN TCP Convergence Layer June 2014

 [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

Demmer, et al. Experimental [Page 22]

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