Delay Tolerant Networking                                       B. Sipos
Internet-Draft                                           RKF Engineering
Obsoletes: 7242 (if approved)                                  M. Demmer
Intended status: Standards Track                             UC Berkeley
Expires: May 9, September 1, 2019                                        J. Ott
                                                        Aalto University
                                                            S. Perreault
                                                        November 5, 2018
                                                       February 28, 2019

   Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
                       draft-ietf-dtn-tcpclv4-10
                       draft-ietf-dtn-tcpclv4-11

Abstract

   This document describes a revised protocol for the TCP-based
   convergence layer (TCPCL) for Delay-Tolerant Networking (DTN).  The
   protocol revision is based on implementation issues in the original
   TCPCL Version 3 of RFC7242 and updates to the Bundle Protocol
   contents, encodings, and convergence layer requirements in Bundle
   Protocol Version 7.  Specifically, the TCPCLv4 uses CBOR-encoded BPv7
   bundles as its service data unit being transported and provides a
   reliable transport of such bundles.  Several new IANA registries are
   defined for TCPCLv4 which define some behaviors inherited from
   TCPCLv3 but with updated encodings and/or semantics.

Status of This Memo

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   This Internet-Draft will expire on May 9, September 1, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Convergence Layer Services  . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Definitions Specific to the TCPCL Protocol  . . . . . . .   6
   3.  General Protocol Description  . . . . . . . . . . . . . . . .   9
     3.1.  TCPCL Session Overview  . . . . . . . . . . . . . . . . .   9
     3.2.  TCPCL States and Transitions  . . . . . . . . . . . . . .  11
     3.3.  Transfer Segmentation Policies  . . . . . . . . . . . . .  16
     3.4.  Example Message Exchange  . . . . . . . . . . . . . . . .  17
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .  19
     4.1.  TCP Connection  . . . . . . . . . . . . . . . . . . . . .  19
     4.2.  Contact Header  . . . . . . . . . . . . . . . . . . . . .  19
     4.3.  Contact Validation and Negotiation  . . . . . . . . . . .  20
     4.4.  Session Security  . . . . . . . . . . . . . . . . . . . .  21
       4.4.1.  TLS Handshake Result  . . . . . . . . . . . . . . . .  22
       4.4.2.  Example TLS Initiation  . . . . . . . . . . . . . . .  22
     4.5.  Message Type Codes  . . . . . . . . . . . . . . . . . . .  23
     4.6.  Session Initialization Message (SESS_INIT)  . . . . . . .  24
       4.6.1.
     4.7.  Session Extension Items . Parameter Negotiation . . . . . . . . . . . . . .  26
     4.7.
     4.8.  Session Parameter Negotiation Extension Items . . . . . . . . . . . . . . . . .  27
   5.  Established Session Operation . . . . . . . . . . . . . . . .  28
     5.1.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  28
       5.1.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  28
       5.1.2.  Message Rejection (MSG_REJECT)  . . . . . . . . . . .  29
     5.2.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  30
       5.2.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  30  31
       5.2.2.  Transfer Initialization (XFER_INIT)  Data Transmission (XFER_SEGMENT)  . . . . . . . . . .  31
       5.2.3.  Data Transmission (XFER_SEGMENT) Acknowledgments (XFER_ACK) . . . . . . . . . .  34 .  33
       5.2.4.  Data Acknowledgments (XFER_ACK)  Transfer Refusal (XFER_REFUSE)  . . . . . . . . . . .  35  34
       5.2.5.  Transfer Refusal (XFER_REFUSE) Extension Items  . . . . . . . . . . . . .  36 .  37
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  38
     6.1.  Session Termination Message (SESS_TERM) . . . . . . . . .  38  39
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  41
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  41
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  41  42
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  43
     9.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  43
     9.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  43  44
     9.3.  Session Extension Types . . . . . . . . . . . . . . . . .  44
     9.4.  Transfer Extension Types  . . . . . . . . . . . . . . . .  44  45
     9.5.  Message Types . . . . . . . . . . . . . . . . . . . . . .  45  46
     9.6.  XFER_REFUSE Reason Codes  . . . . . . . . . . . . . . . .  46
     9.7.  SESS_TERM Reason Codes  . . . . . . . . . . . . . . . . .  47
     9.8.  MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . .  48
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  49
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     11.2.  Informative References . . . . . . . . . . . . . . . . .  49  50
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

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 Version 7 (BPv7)
   [I-D.ietf-dtn-bpbis], an application-layer protocol that is used to
   construct a store-and-forward overlay network.  BPv7 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 TCP Convergence Layer Version 4
   (TCPCLv4).  For the remainder of this document, the abbreviation "BP"
   without the version suffix refers to BPv7.  For the remainder of this
   document, the abbreviation "TCPCL" without the version suffix refers
   to TCPCLv4.

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

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

        Figure 1: The Locations of the Bundle Protocol and the TCP
       Convergence-Layer Protocol above the Internet Protocol Stack

   This document describes the format of the protocol data units passed
   between entities participating in TCPCL communications.  This
   document does not address:

   o  The format of protocol data units of the Bundle Protocol, as those
      are defined elsewhere in [RFC5050] and [I-D.ietf-dtn-bpbis].  This
      includes the concept of bundle fragmentation or bundle
      encapsulation.  The TCPCL transfers bundles as opaque data blocks.

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

1.1.  Convergence Layer Services

   This version of the TCPCL provides the following services to support
   the overlaying Bundle Protocol agent.  In all cases, this is not an
   API defintion but a logical description of how the CL may interact
   with the BP agent.  Each of these interactions may be associated with
   any number of additional metadata items as necessary to support the
   operation of the CL or BP agent.

   Attempt Session  The TCPCL allows a BP agent to pre-emptively attempt
      to establish a TCPCL session with a peer entity.  Each session
      attempt can send a different set of session negotiation parameters
      as directed by the BP agent.

   Terminate Session  The TCPCL allows a BP agent to pre-emptively
      terminate an established TCPCL session with a peer entity.  The
      terminate request is on a per-session basis.

   Session State Changed  The TCPCL supports indication when the session
      state changes.  The top-level session states indicated are:

      Contact Negotating:  A TCP connection has been made (as either
         active or passive entity) and contact negotiation has begun.

      Session Negotiating:  Contact negotation has been completed
         (including possible TLS use) and session negotiation has begun.

      Established:  The session has been fully established and is ready
         for its first transfer.

      Closing:  The entity received a SESS_TERM message and is in the
         closing state.

      Terminated:  The session has finished normal termination
         sequencing..

      Failed:  The session ended without normal termination sequencing.

   Session Idle Changed  The TCPCL supports indication when the live/
      idle sub-state changes.  This occurs only when the top-level
      session state is Established.  Because TCPCL transmits serially
      over a TCP connection, it suffers from "head of queue blocking"
      this indication provides information about when a session is
      available for immediate transfer start.

   Begin Transmission  The principal purpose of the TCPCL is to allow a
      BP agent to transmit bundle data over an established TCPCL
      session.  Transmission request is on a per-session basis, the CL
      does not necessarily perform any per-session or inter-session
      queueing.  Any queueing of transmissions is the obligation of the
      BP agent.

   Transmission Success  The TCPCL supports positive indication when a
      bundle has been fully transferred to a peer entity.

   Transmission Intermediate Progress  The TCPCL supports positive
      indication of intermediate progress of transferr to a peer entity.
      This intermediate progress is at the granularity of each
      transferred segment.

   Transmission Failure  The TCPCL supports positive indication of
      certain reasons for bundle transmission failure, notably when the
      peer entity rejects the bundle or when a TCPCL session ends before
      transferr success.  The TCPCL itself does not have a notion of
      transfer timeout.

   Reception Initialized  The TCPCL supports indication to the reciver
      just before any transmssion data is sent.  This corresponds to
      reception of the XFER_INIT message. XFER_SEGMENT message with the START flag set.

   Interrupt Reception  The TCPCL allows a BP agent to interrupt an
      individual transfer before it has fully completed (successfully or
      not).  Interruption can occur any time after the reception is
      initialized.

   Reception Success  The TCPCL supports positive indication when a
      bundle has been fully transferred from a peer entity.

   Reception Intermediate Progress  The TCPCL supports positive
      indication of intermediate progress of transfer from the peer
      entity.  This intermediate progress is at the granularity of each
      transferred segment.  Intermediate reception indication allows a
      BP agent the chance to inspect bundle header contents before the
      entire bundle is available, and thus supports the "Reception
      Interruption" capability.

   Reception Failure  The TCPCL supports positive indication of certain
      reasons for reception failure, notably when the local entity
      rejects an attempted transfer for some local policy reason or when
      a TCPCL session ends before transfer success.  The TCPCL itself
      does not have a notion of transfer timeout.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.1.  Definitions Specific to the TCPCL Protocol

   This section contains definitions specific to the TCPCL protocol.

   TCPCL Entity:  This is the notional TCPCL application that initiates
      TCPCL sessions.  This design, implementation, configuration, and
      specific behavior of such an entity is outside of the scope of
      this document.  However, the concept of an entity has utility
      within the scope of this document as the container and initiator
      of TCPCL sessions.  The relationship between a TCPCL entity and
      TCPCL sessions is defined as follows:

         A TCPCL Entity MAY actively initiate any number of TCPCL
         Sessions and should do so whenever the entity is the initial
         transmitter of information to another entity in the network.

         A TCPCL Entity MAY support zero or more passive listening
         elements that listen for connection requests from other TCPCL
         Entities operating on other entitys in the network.

         A TCPCL Entity MAY passivley initiate any number of TCPCL
         Sessions from requests received by its passive listening
         element(s) if the entity uses such elements.

      These relationships are illustrated in Figure 2.  For most TCPCL
      behavior within a session, the two entities are symmetric and
      there is no protocol distinction between them.  Some specific
      behavior, particularly during session establishment, distinguishes
      between the active entity and the passive entity.  For the
      remainder of this document, the term "entity" without the prefix
      "TCPCL" refers to a TCPCL entity.

   TCP Connection:  The term Connection in this specification
      exclusively refers to a TCP connection and any and all behaviors,
      sessions, and other states association with that TCP connection.

   TCPCL Session:  A TCPCL session (as opposed to a TCP connection) is a
      TCPCL communication relationship between two TCPCL entities.
      Within a single TCPCL session there are two possible transfer
      streams; one in each direction, with one stream from each entity
      being the outbound stream and the other being the inbound stream.
      The lifetime of a TCPCL session is bound to the lifetime of an
      underlying TCP connection.  A TCPCL session is terminated when the
      TCP connection ends, due either to one or both entities actively
      terminating the TCP connection or due to network errors causing a
      failure of the TCP connection.  For the remainder of this
      document, the term "session" without the prefix "TCPCL" refers to
      a TCPCL session.

   Session parameters:  These are a set of values used to affect the
      operation of the TCPCL for a given session.  The manner in which
      these parameters are conveyed to the bundle entity and thereby to
      the TCPCL is implementation dependent.  However, the mechanism by
      which two entities exchange and negotiate the values to be used
      for a given session is described in Section 4.3.

   Transfer Stream:  A Transfer stream is a uni-directional user-data
      path within a TCPCL Session.  Messages sent over a transfer stream
      are serialized, meaning that one set of user data must complete
      its transmission prior to another set of user data being
      transmitted over the same transfer stream.  Each uni-directional
      stream has a single sender entity and a single receiver entity.

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

   Transfer Segment:  A subset of a transfer of user data being
      communicated over a trasnfer stream.

   Idle Session:  A TCPCL session is idle while the only messages being
      transmitted or received are KEEPALIVE messages.

   Live Session:  A TCPCL session is live while any messages are being
      transmitted or received.

   Reason Codes:  The TCPCL uses numeric codes to encode specific
      reasons for individual failure/error message types.

   The relationship between connections, sessions, and streams is shown
   in Figure 3.

+--------------------------------------------+
|                 TCPCL Entity               |
|                                            |      +----------------+
|   +--------------------------------+       |      |                |-+
|   | Actively Inititated Session #1 +------------->| Other          | |
|   +--------------------------------+       |      | TCPCL Entity's | |
|                  ...                       |      | Passive        | |
|   +--------------------------------+       |      | Listener       | |
|   | Actively Inititated Session #n +------------->|                | |
|   +--------------------------------+       |      +----------------+ |
|                                            |       +-----------------+
|      +---------------------------+         |
|  +---| +---------------------------+       |      +----------------+
|  |   | | Optional Passive          |       |      |                |-+
|  |   +-| Listener(s)               +<-------------+                | |
|  |     +---------------------------+       |      |                | |
|  |                                         |      | Other          | |
|  |    +---------------------------------+  |      | TCPCL Entity's | |
|  +--->| Passively Inititated Session #1 +-------->| Active         | |
|  |    +---------------------------------+  |      | Initiator(s)   | |
|  |                                         |      |                | |
|  |    +---------------------------------+  |      |                | |
|  +--->| Passively Inititated Session #n +-------->|                | |
|       +---------------------------------+  |      +----------------+ |
|                                            |       +-----------------+
+--------------------------------------------+

            Figure 2: The relationships between TCPCL entities

+----------------------------+              +--------------------------+
|      TCPCL Session         |              |  TCPCL "Other" Session   |
|                            |              |                          |
| +-----------------------+  |              |  +---------------------+ |
| |   TCP Connection      |  |              |  |    TCP Connection   | |
| |                       |  |              |  |                     | |
| | +-------------------+ |  |              |  | +-----------------+ | |
| | | Optional Inbound  | |  |              |  | |  Peer Outbound  | | |
| | | Transfer Stream   |<-[Seg]--[Seg]--[Seg]-| | Transfer Stream | | |
| | |       -----       | |  |              |  | |       -----     | | |
| | |     RECEIVER      | |  |              |  | |      SENDER     | | |
| | +-------------------+ |  |              |  | +-----------------+ | |
| |                       |  |              |  |                     | |
| | +-------------------+ |  |              |  | +-----------------+ | |
| | | Optional Outbound | |  |              |  | |  Peer Inbound   | | |
| | | Transfer Stream   |------[Seg]---[Seg]---->| Transfer Stream | | |
| | |       -----       | |  |              |  | |       -----     | | |
| | |      SENDER       | |  |              |  | |     RECEIVER    | | |
| | +-------------------+ |  |              |  | +-----------------+ | |
| +-----------------------+  |              |  +---------------------+ |
+----------------------------+              +--------------------------+

   Figure 3: The relationship within a TCPCL Session of its two streams

3.  General Protocol Description

   The service of this protocol is the transmission of DTN bundles via
   the Transmission Control Protocol (TCP).  This document specifies the
   encapsulation of bundles, procedures for TCP setup and teardown, and
   a set of messages and node requirements.  The general operation of
   the protocol is as follows.

3.1.  TCPCL Session Overview

   First, one node establishes a TCPCL session to the other by
   initiating a TCP connection in accordance with [RFC0793].  After
   setup of the TCP connection is complete, an initial contact header is
   exchanged in both directions to establish a shared TCPCL version and
   possibly initiate TLS security.  Once contact negotiation is
   complete, TCPCL messaging is available and the session negotiation is
   used to set parameters of the TCPCL session.  One of these parameters
   is 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 negotiated, the parameters of a TCPCL session cannot change and
   if there is a desire by either peer to transfer data under different
   parameters then a new session must be established.  This makes CL
   logic simpler but relies on the assumption that establishing a TCP
   connection is lightweight enough that TCP connection overhead is
   negligable compared to TCPCL data sizes.

   Once the TCPCL session is established and configured in this way,
   bundles can be transferred in either direction.  Each transfer is
   performed by an initialization (XFER_INIT) message followed by one or
   more sequence of logical segments of data within an
   XFER_SEGMENT message. messages.  Multiple bundles can be transmitted
   consecutively in a single direction on a single TCPCL connection.
   Segments from different bundles are never interleaved.  Bundle
   interleaving can be accomplished by fragmentation at the BP layer or
   by establishing multiple TCPCL sessions between the same peers.

   A feature of this protocol is for the receiving node to send
   acknowledgment (XFER_ACK) messages as bundle data segments arrive . arrive.
   The rationale behind these acknowledgments is to enable the sender
   node to determine how much of the bundle has been received, so that
   in case the session is interrupted, it can perform reactive
   fragmentation to avoid re-sending the already transmitted part of the
   bundle.  In addition, there is no explicit flow control on the TCPCL
   layer.

   A TCPCL receiver can interrupt the transmission of a bundle at any
   point in time by replying with a XFER_REFUSE message, which causes
   the sender to stop transmission of the associated bundle (if it
   hasn't already finished transmission) Note: This enables a cross-
   layer optimization in that it allows a receiver that detects that it
   already has received a certain bundle to interrupt transmission as
   early as possible and thus save transmission capacity for other
   bundles.

   For sessions that are idle, a KEEPALIVE message is sent at a
   negotiated interval.  This is used to convey node live-ness
   information during otherwise message-less time intervals.

   A SESS_TERM message is used to start the closing of a TCPCL session
   (see Section 6.1).  During shutdown sequencing, in-progress transfers
   can be completed but no new transfers can be initiated.  A SESS_TERM
   message can also be used to refuse a session setup by a peer (see
   Section 4.3).  It is an implementation matter to determine whether or
   not to close a TCPCL session while there are no transfers queued or
   in-progress.

   Once a session is established established, TCPCL is a symmetric
   protocol between the peers.  Both sides can start sending data
   segments in a session, and one side's bundle transfer does not have
   to complete before the other side can start sending data segments on
   its own.  Hence, the protocol allows for a bi-directional mode of
   communication.  Note that in the case of concurrent bidirectional
   transmission, acknowledgment segments MAY be interleaved with data
   segments.

3.2.  TCPCL States and Transitions

   The states of a nominal TCPCL session (i.e. without session failures)
   are indicated in Figure 4.

     +-------+
     | START |
     +-------+
         |
     TCP Establishment
         |
         V
   +-----------+            +---------------------+
   |    TCP    |----------->|  Contact / Session  |
   | Connected |            |     Negotiation     |
   +-----------+            +---------------------+
                                       |
          +-----Session Parameters-----+
          |         Negotiated
          V
   +-------------+                     +-------------+
   | Established |----New Transfer---->| Established |
   |   Session   |                     |   Session   |
   |    Idle     |<---Transfers Done---|     Live    |
   +-------------+                     +-------------+
         |                                    |
         +------------------------------------+
         |
   SESS_TERM Exchange
         |
         V
   +-------------+
   | Established |                    +-------------+
   |   Session   |----Transfers------>|     TCP     |
   |   Ending    |      Done          | Terminating |
   +-------------+                    +-------------+
                                              |
        +------------Close Message------------+
        |
        V
    +-------+
    |  END  |
    +-------+

               Figure 4: Top-level states of a TCPCL session

   Notes on Established Session states:

      Session "Live" means transmitting or reeiving over a transfer
      stream.

      Session "Idle" means no transmission/reception over a transfer
      stream.

      Session "Closing" means no new transfers will be allowed.

   The contact negotiation sequencing is performed either as the active
   or passive peer, and is illustrated in Figure 5 and Figure 6
   respectively which both share the data validation and analyze final
   states of Figure 7.

   +-------+
   | START |-----TCP-----+
   +-------+  Connecting |
                         V
                   +-----------+       +---------+
                   | Connected |--OK-->| Send CH |--OK-->[PCH]
                   +-----------+       +---------+
                         |                  |
                       Error              Error
                         |                  |
                         V                  |
                     [TCPTERM]<-------------+

                Figure 5: Contact Initiation as Active peer

   +-------+
   | START |-----TCP----->[PCH]
   +-------+   Connected

               Figure 6: Contact Initiation as Passive peer
                     +-------->[TCPTERM]<----------+
                     |                           |
                  Timeout                      Error
                  or Error                       |
                     |                           |
   +-------+     +---------+    Contact     +----------+
   | START |---->| Waiting |---- Header --->| Validate |
   +-------+     +---------+    Received    +----------+
                                                 |
                     +---------------------------+
                     |
                     V
                +---------+
      +--Error--| Analyze |---No TLS---->[SI]
      |         |         |               ^
      |         +---------+               |
      |              |                    |
      V             TLS                   |
   [TCPTERM]     Negotiated               |
      ^              |                    |
      |              V                    |
      |         +-----------+             |
      |         | Establish |---Success---+
      +--Error--|    TLS    |
                +-----------+

               Figure 7: Processing of Contact Header (PCH)

   The session negotiation sequencing is performed either as the active
   or passive peer, and is illustrated in Figure 8 and Figure 9
   respectively which both share the data validation and analyze final
   states of Figure 10.

   +-------+    TCPCL
   | START |--Messaging--+
   +-------+  Available  |
                         V
                +----------------+
                | Send SESS_INIT |--OK-->[PSI]
                +----------------+
                         |
                       Error
                         |
                         V
                     [SESSTERM]

                Figure 8: Session Initiation as Active peer

   +-------+     TCPCL
   | START |---Messaging-->[PSI]
   +-------+   Available

               Figure 9: Session Initiation as Passive peer

                     +------->[SESSTERM]<--------+
                     |                           |
                  Timeout                      Error
                  or Error                       |
                     |                           |
   +-------+     +---------+                +----------+
   | START |---->| Waiting |---SESS_INIT--->| Validate |
   +-------+     +---------+   Received     +----------+
                                                 |
                     +---------------------------+
                     |
                     V
                +---------+     +--------------+
      +--Error--| Analyze |---->| Established  |
      |         |         |     | Session Idle |
      |         +---------+     +--------------+
      V
   [SESSTERM]

             Figure 10: Processing of Session Initiation (PSI)

   Transfers can occur after a session is established and it's not in
   the ending state.  Each transfer occurs within a single logical
   transfer stream between a sender and a receiver, as illustrated in
   Figure 11 and Figure 12 respectively.

                                          +--Send XFER_DATA--+ XFER_SEGMENT--+
   +--------+                             |                     |
   | Stream |                       +-------------+             |
   |  Idle  |---Send XFER_INIT-->| XFER_SEGMENT-->| In Progress |<---------+ |<------------+
   +--------+                        +-------------+
                                          |
        +------All
        +---------All segments sent-------+
        |
        V
   +---------+                       +--------+
   | Waiting |---- Receive Final---->| Stream |
   | for Ack |         Ack       XFER_ACK        |  IDLE  |
   +---------+                       +--------+

                     Figure 11: Transfer sender states

   Notes on transfer sending:

      Pipelining of transfers can occur when the sending entity begins a
      new transfer while in the "Waiting for Ack" state.

                                            +-Receive XFER_DATA-+ XFER_SEGMENT-+
   +--------+                               |    Send Ack XFER_ACK     |
   | Stream |                         +-------------+              |
   |  IDLE  |--Receive XFER_INIT-->| XFER_SEGMENT-->| In Progress |<----------+ |<-------------+
   +--------+                         +-------------+
                                            |
        +---------Sent
        +--------Sent Final Ack---------+ XFER_ACK--------+
        |
        V
   +--------+
   | Stream |
   |  IDLE  |
   +--------+

                    Figure 12: Transfer receiver states

3.3.  Transfer Segmentation Policies

   Each TCPCL session allows a negotiated transfer segmentation polcy to
   be applied in each transfer direction.  A receiving node can set the
   Segment MRU in its contact header to determine the largest acceptable
   segment size, and a transmitting node can segment a transfer into any
   sizes smaller than the receiver's Segment MRU.  It is a network
   administration matter to determine an appropriate segmentation policy
   for entities operating TCPCL, but guidance given here can be used to
   steer policy toward performance goals.  It is also advised to
   consider the Segment MRU in relation to chunking/packetization
   performed by TLS, TCP, and any intermediate network-layer nodes.

   Minimum Overhead  For a simple network expected to exchange
      relatively small bundles, the Segment MRU can be set to be
      identical to the Transfer MRU which indicates that all transfers
      can be sent with a single data segment (i.e. no actual
      segmentation).  If the network is closed and all transmitters are
      known to follow a single-segment transfer policy, then receivers
      can avoid the necessity of segment reassembly.  Because this CL
      operates over a TCP stream, which suffers from a form of head-of-
      queue blocking between messages, while one node is transmitting a
      single XFER_SEGMENT message it is not able to transmit any
      XFER_ACK or XFER_REFUSE for any associated received transfers.

   Predictable Message Sizing  In situations where the maximum message
      size is desired to be well-controlled, the Segment MRU can be set
      to the largest acceptable size (the message size less XFER_SEGMENT
      header size) and transmitters can always segment a transfer into
      maximum-size chunks no larger than the Segment MRU.  This
      guarantees that any single XFER_SEGMENT will not monopolize the
      TCP stream for too long, which would prevent outgoing XFER_ACK and
      XFER_REFUSE associated with received transfers.

   Dynamic Segmentation  Even after negotiation of a Segment MRU for
      each receiving node, the actual transfer segmentation only needs
      to guarantee than any individual segment is no larger than that
      MRU.  In a situation where network "goodput" is dynamic, the
      transfer segmentation size can also be dynamic in order to control
      message transmission duration.

   Many other policies can be established in a TCPCL network between
   these two extremes.  Different policies can be applied to each
   direction to/from any particular node.  Additionally, future header
   and transfer extension types can apply further nuance to transfer
   policies and policy negotiation.

3.4.  Example Message Exchange

   The following figure depicts the protocol exchange for a simple
   session, showing the session establishment and the transmission of a
   single bundle split into three data segments (of lengths "L1", "L2",
   and "L3") from Entity A to Entity B.

   Note that the sending node MAY can transmit multiple XFER_SEGMENT
   messages without necessarily waiting for the corresponding XFER_ACK responses.
   This enables pipelining of messages on a transfer stream.  Although
   this example only demonstrates a single bundle transmission, it is
   also possible to pipeline multiple XFER_SEGMENT messages for
   different bundles without necessarily waiting for XFER_ACK messages
   to be returned for each one.  However, interleaving data segments
   from different bundles is not allowed.

   No errors or rejections are shown in this example.

                Entity A                             Entity B
                ========                             ========
       +-------------------------+
       |     Contact Header      | ->      +-------------------------+
       +-------------------------+      <- |     Contact Header      |
                                           +-------------------------+
       +-------------------------+
       |        SESS_INIT        | ->      +-------------------------+
       +-------------------------+      <- |        SESS_INIT        |
                                           +-------------------------+

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

       +-------------------------+
       |       SESS_TERM         | ->      +-------------------------+
       +-------------------------+      <- |        SESS_TERM        |
                                           +-------------------------+

    Figure 13: An example of the flow of protocol messages on a single
                     TCP Session between two entities

4.  Session Establishment

   For bundle transmissions to occur using the TCPCL, a TCPCL session
   MUST first be established between communicating entities.  It is up
   to the implementation to decide how and when session setup is
   triggered.  For example, some sessions MAY be opened proactively and
   maintained for as long as is possible given the network conditions,
   while other sessions MAY be opened only when there is a bundle that
   is queued for transmission and the routing algorithm selects a
   certain next-hop node.

4.1.  TCP Connection

   To establish a TCPCL session, an entity MUST first establish a TCP
   connection with the intended peer entity, typically by using the
   services provided by the operating system.  Destination port number
   4556 has been assigned by IANA as the Registered Port number for the
   TCP convergence layer.  Other destination port numbers MAY be used
   per local configuration.  Determining a peer's destination port
   number (if different from the registered TCPCL port number) is up to
   the implementation.  Any source port number MAY be used for TCPCL
   sessions.  Typically an operating system assigned number in the TCP
   Ephemeral range (49152-65535) is used.

   If the entity is unable to establish a TCP connection for any reason,
   then it is an implementation matter to determine how to handle the
   connection failure.  An entity 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 entity MUST retry the
   connection setup no earlier than some delay time from the last
   attempt, and it SHOULD use a (binary) exponential backoff mechanism
   to increase this delay in case of repeated failures.

   Once a TCP connection is established, each entity MUST immediately
   transmit a contact header over the TCP connection.  The format of the
   contact header is described in Section 4.2.

4.2.  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.

   Upon receipt of the contact header, both entities perform the
   validation and negotiation procedures defined in Section 4.3.  After
   receiving the contact header from the other entity, either entity MAY
   refuse the session by sending a SESS_TERM message with an appropriate
   reason code.

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

                     Figure 14: Contact Header Format

   See Section 4.3 for details on the use of each of these contact
   header fields.

   The fields of the contact header are:

   magic:  A four-octet field that always contains the octet sequence
      0x64 0x74 0x6e 0x21, i.e., the text string "dtn!" in US-ASCII (and
      UTF-8).

   Version:  A one-octet field value containing the value 4 (current
      version of the protocol).

   Flags:  A one-octet field of single-bit flags, interpreted according
      to the descriptions in Table 1.

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

                       Table 1: Contact Header Flags

4.3.  Contact Validation and Negotiation

   Upon reception of the contact header, each node follows the following
   procedures to ensure the validity of the TCPCL session and to
   negotiate values for the session parameters.

   If the magic string is not present or is not valid, the connection
   MUST be terminated.  The intent of the magic string is to provide
   some protection against an inadvertent TCP connection by a different
   protocol than the one described in this document.  To prevent a flood
   of repeated connections from a misconfigured application, an entity
   MAY elect to hold an invalid connection open and idle for some time
   before closing it.

   The first negotiation is on the TCPCL protocol version to use.  The
   active node always sends its Contact Header first and waits for a
   response from the passive node.  The active node can repeatedly
   attempt different protocol versions in descending order until the
   passive node accepts one with a corresponding Contact Header reply.
   Only upon response of a Contact Header from the passive node is the
   TCPCL protocol version established and parameter negotiation begun.

   During contact initiation, the active TCPCL node SHALL send the
   highest TCPCL protocol version on a first session attempt for a TCPCL
   peer.  If the active node receives a Contact Header with a different
   protocol version than the one sent earlier on the TCP connection, the
   TCP connection SHALL be terminated.  If the active node receives a
   SESS_TERM message with reason of "Version Mismatch", that node MAY
   attempt further TCPCL sessions with the peer using earlier protocol
   version numbers in decreasing order.  Managing multi-TCPCL-session
   state such as this is an implementation matter.

   If the passive node receives a contact header containing a version
   that is greater than the current version of the protocol that the
   node implements, then the node SHALL shutdown the session with a
   reason code of "Version mismatch".  If the passive node receives a
   contact header with a version that is lower than the version of the
   protocol that the node implements, the node MAY either terminate the
   session (with a reason code of "Version mismatch") or the node MAY
   adapt its operation to conform to the older version of the protocol.
   The decision of version fall-back is an implementation matter.

4.4.  Session Security

   This version of the TCPCL supports establishing a Transport Layer
   Security (TLS) session within an existing TCP connection.  When TLS
   is used within the TCPCL it affects the entire session.  Once
   established, there is no mechanism available to downgrade a TCPCL
   session to non-TLS operation.  If this is desired, the entire TCPCL
   session MUST be terminated and a new non-TLS-negotiated session
   established.

   The use of TLS is negotated using the Contact Header as described in
   Section 4.3.  After negotiating an Enable TLS parameter of true, and
   before any other TCPCL messages are sent within the session, the
   session entities SHALL begin a TLS handshake in accordance with
   [RFC5246].  The parameters within each TLS negotiation are
   implementation dependent but any TCPCL node SHALL follow all
   recommended practices of [BCP195], or any updates or successors that
   become part of [BCP195].  By convention, this protocol uses the node
   which initiated the underlying TCP connection as the "client" role of
   the TLS handshake request.

   The TLS handshake, if it occurs, is considered to be part of the
   contact negotiation before the TCPCL session itself is established.
   Specifics about sensitive data exposure are discussed in Section 8.

4.4.1.  TLS Handshake Result

   If a TLS handshake cannot negotiate a TLS session, both entities of
   the TCPCL session SHALL terminate the TCP connection.  At this point
   the TCPCL session has not yet been established so there is no TCPCL
   session to terminate.  This also avoids any potential security issues
   assoicated with further TCP communication with an untrusted peer.

   After a TLS session is successfully established, the active peer
   SHALL send a SESS_INIT message to begin session negotiation.  This
   session negotation and all subsequent messaging are secured.

4.4.2.  Example TLS Initiation

   A summary of a typical CAN_TLS usage is shown in the sequence in
   Figure 15 below.

                Entity A                             Entity B
                ========                             ========

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

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

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

           ... secured TCPCL messaging, starting with SESS_INIT ...

       +-------------------------+         +-------------------------+
       |       SESS_TERM          | ->   <- |         SESS_TERM        |
       +-------------------------+         +-------------------------+

   Figure 15: A simple visual example of TCPCL TLS Establishment between
                               two entities

4.5.  Message Type Codes

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

                              0 1 2 3 4 5 6 7
                             +---------------+
                             | Message Type  |
                             +---------------+

                  Figure 16: Format of the Message Header

   The message header fields are as follows:

   Message Type:  Indicates the type of the message as per Table 2
      below.  Encoded values are listed in Section 9.5.

   +--------------+----------------------------------------------------+
   | Type         | Description                                        |
   +--------------+----------------------------------------------------+
   | SESS_INIT    | Contains the session parameter inputs from one of  |
   |              | the entities, as described in Section 4.6.         |
   |              |                                                    |
   | XFER_INIT    | Contains the length (in octets) of the next        |
   |              | transfer, as described in Section 5.2.2.           |
   |              |                                                    |
   | XFER_SEGMENT | Indicates the transmission of a segment of bundle  |
   |              | data, as described in Section 5.2.3. 5.2.2.               |
   |              |                                                    |
   | XFER_ACK     | Acknowledges reception of a data segment, as       |
   |              | described in Section 5.2.4. 5.2.3.                        |
   |              |                                                    |
   | XFER_REFUSE  | Indicates that the transmission of the current     |
   |              | bundle SHALL be stopped, as described in Section   |
   |              | 5.2.5. 5.2.4.                                             |
   |              |                                                    |
   | KEEPALIVE    | Used to keep TCPCL session active, as described in |
   |              | Section 5.1.1.                                     |
   |              |                                                    |
   | SESS_TERM    | Indicates that one of the entities participating   |
   |              | in the session wishes to cleanly terminate the     |
   |              | session, as described in Section 6.                |
   |              |                                                    |
   | MSG_REJECT   | Contains a TCPCL message rejection, as described   |
   |              | in Section 5.1.2.                                  |
   +--------------+----------------------------------------------------+

                       Table 2: TCPCL Message Types

4.6.  Session Initialization Message (SESS_INIT)

   Before a session is established and ready to transfer bundles, the
   session parameters are negotiated between the connected entities.
   The SESS_INIT message is used to convey the per-entity parameters
   which are used together to negotiate the per-session parameters. parameters as
   described in Section 4.7.

   The format of a SESS_INIT message is as follows in Figure 17.

                     +-------------------------------+
                     |        Message Header         |
                     +-------------------------------+
                     |    Keepalive Interval (U16)   |
                     +-------------------------------+
                     |       Segment MRU (U64)       |
                     +-------------------------------+
                     |      Transfer MRU (U64)       |
                     +-------------------------------+
                     |        EID Length (U16)       |
                     +-------------------------------+
                     |      EID Data (variable)      |
                     +-------------------------------+
                     | Session Extension Length (U64)| (U32)|
                     +-------------------------------+
                     | Session Extension Items (var.)|
                     +-------------------------------+

                        Figure 17: SESS_INIT Format

   The fields of the SESS_INIT message are:

   Keepalive Interval:  A 16-bit unsigned integer indicating the
      interval, in seconds, between any subsequent messages being
      transmitted by the peer.  The peer receiving this contact header
      uses this interval to determine how long to wait after any last-message last-
      message transmission and a necessary subsequent KEEPALIVE message
      transmission.

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

   Transfer MRU:  A 64-bit unsigned integer indicating the largest
      allowable total-
      bundle total-bundle data size to be received in this session.
      Any bundle transfer sent to this peer SHALL have a Total Bundle
      Length payload no longer than the peer's Transfer MRU.  This value
      can be used to perform proactive bundle fragmentation.  The two
      entities of a single session MAY have different Transfer MRUs, and
      no relation between the two is required.

   EID Length and EID Data:  Together these fields represent a variable-length variable-
      length text string.  The EID Length is a 16-bit unsigned integer
      indicating the number of octets of EID Data to follow.  A zero EID
      Length SHALL be used to indicate the lack of EID rather than a
      truly empty EID.  This case allows an entity to avoid exposing EID
      information on an untrusted network.  A non-zero-length EID Data
      SHALL contain the UTF-8 encoded EID of some singleton endpoint in
      which the sending entity is a member, in the canonical format of
      <scheme name>:<scheme-specific part>.  This EID encoding is
      consistent with [I-D.ietf-dtn-bpbis].

   Session Extension Length and Session Extension Items:  Together these
      fields represent protocol extension data not defined by this
      specification.  The Session Extension Length is the total number
      of octets to follow which are used to encode the Session Extension
      Item list.  The encoding of each Session Extension Item is within
      a consistent data container as described in Section 4.6.1. 4.8.  The full
      set of Session Extension Items apply for the duration of the TCPCL
      session to follow.  The order and mulitplicity of these Session
      Extension Items MAY be significant, as defined in the associated
      type specification(s).

4.6.1.

4.7.  Session Extension Items

   Each Parameter Negotiation

   An entity calculates the parameters for a TCPCL session by
   negotiating the values from its own preferences (conveyed by the
   contact header it sent to the peer) with the preferences of the Session Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated peer
   node (expressed in Figure 18.
   The fields of the Session Extension Item are:

   Flags:  A one-octet field containing generic bit flags about contact header that it received from the
      Item, which
   peer).  The negotiated parameters defined by this specification are listed
   described in Table 3.  If a TCPCL entity receives a
      Session Extension Item with an unknown Item Type and the CRITICAL
      flag set, following paragraphs.

   Transfer MTU and Segment MTU:  The maximum transmit unit (MTU) for
      whole transfers and individual segments are idententical to the entity SHALL close
      Transfer MRU and Segment MRU, respectively, of the TCPCL session recevied
      contact header.  A transmitting peer can send individual segments
      with SESS_TERM
      reason code of "Contact Failure".  If any size smaller than the CRITICAL flag Segment MTU, depending on local
      policy, dynamic network conditions, etc.  Determining the size of
      each transmitted segment is not
      set, an entity SHALL skip over and ignore any item with an unknown
      Item Type.

   Item Type:  A 16-bit unsigned integer field containing the type implementation matter.

   Session Keepalive:  Negotiation of the extension item.  This specification does not define any
      extension types directly, but does allocate an IANA registry for
      such codes (see Section 9.3).

   Item Length:  A 32-bit unsigned integer field containing Session Keepalive parameter is
      performed by taking the number minimum of Item Value octets to follow.

   Item Value:  A variable-length data field which this two contact headers'
      Keepalive Interval.  The Session Keepalive interval is interpreted
      according to a parameter
      for the associated Item Type.  This specification places
      no restrictions on an extension's use behavior described in Section 5.1.1.

   Enable TLS:  Negotiation of available Item Value
      data.  Extension specification SHOULD avoid the use Enable TLS parameter is performed by
      taking the logical AND of large data
      exchanges within the TCPCL two contact header as no bundle transfers
      can begin until headers' CAN_TLS flags.
      A local security policy is then applied to determine of the full contact exchange and negotiation has been
      completed.

                          1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+---------------+---------------+
     |  Item Flags   |           Item Type           | Item Length...|
     +---------------+---------------+---------------+---------------+
     |    length contd.                              | Item Value... |
     +---------------+---------------+---------------+---------------+
     |
      negotated value contd.                                               |
     +---------------+---------------+---------------+---------------+

                 Figure 18: Session Extension Item Format

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CRITICAL | 0x01   | If bit of Enable TLS is set, indicates that the receiving   |
   |          |        | peer must handle the extension item.          |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                   Table 3: Session Extension Item Flags

4.7.  Session Parameter Negotiation

   An entity calculates the parameters for acceptable.  It can be a TCPCL session by
   negotiating the values from its own preferences (conveyed by the
   contact header it sent to the peer) with the preferences of the peer
   node (expressed in the contact header that it received from the
   peer).  The negotiated parameters defined by this specification are
   described in the following paragraphs.

   Transfer MTU and Segment MTU:  The maximum transmit unit (MTU) for
      whole transfers and individual segments are idententical
      reasonable security policy to both require or disallow the
      Transfer MRU and Segment MRU, respectively, use of the recevied
      contact header.  A transmitting peer can send individual segments
      with any size smaller than the Segment MTU,
      TLS depending on local
      policy, dynamic upon the desired network conditions, etc.  Determining flows.  If the size of
      each transmitted segment Enable TLS
      state is an implementation matter.

   Session Keepalive:  Negotiation of unacceptable, the Session Keepalive parameter is
      performed by taking node SHALL terminate the minimum session with a
      reason code of "Contact Failure".  Note that this two contact headers'
      Keepalive Interval.  The Session Keepalive interval failure
      is different than a parameter
      for the behavior described in Section 5.1.1.

   Enable TLS:  Negotiation failure of TLS handshake after an agreed-upon
      and acceptable Enable TLS state.  If the negotiated Enable TLS parameter
      value is performed by
      taking the logical AND of true and acceptable then TLS negotiation feature
      (described in Section 4.4) begins immediately following the two
      contact headers' CAN_TLS flags.
      A local security policy header exchange.

   Once this process of parameter negotiation is then applied to determine completed (which
   includes a possible completed TLS handshake of the
      negotated value connection to use
   TLS), this protocol defines no additional mechanism to change the
   parameters of Enable TLS is acceptable.  It can an established session; to effect such a change, the
   TCPCL session MUST be terminated and a
      reasonable security policy to both require or disallow new session established.

4.8.  Session Extension Items

   Each of the use Session Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated in Figure 18.

   The fields of
      TLS depending upon the desired network flows. Session Extension Item are:

   Flags:  A one-octet field containing generic bit flags about the
      Item, which are listed in Table 3.  If a TCPCL entity receives a
      Session Extension Item with an unknown Item Type and the Enable TLS
      state is unacceptable, CRITICAL
      flag set, the node entity SHALL terminate close the TCPCL session with a SESS_TERM
      reason code of "Contact Failure".  Note that this contact failure  If the CRITICAL flag is different than a failure of TLS handshake after not
      set, an agreed-upon entity SHALL skip over and acceptable Enable TLS state. ignore any item with an unknown
      Item Type.

   Item Type:  A 16-bit unsigned integer field containing the type of
      the extension item.  This specification does not define any
      extension types directly, but does allocate an IANA registry for
      such codes (see Section 9.3).

   Item Length:  A 32-bit unsigned integer field containing the number
      of Item Value octets to follow.

   Item Value:  A variable-length data field which is interpreted
      according to the associated Item Type.  This specification places
      no restrictions on an extension's use of available Item Value
      data.  Extension specifications SHOULD avoid the use of large data
      lengths, as no bundle transfers can begin until the full extension
      data is sent.

                          1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +---------------+---------------+---------------+---------------+
     |  Item Flags   |           Item Type           | Item Length...|
     +---------------+---------------+---------------+---------------+
     |    length contd.                              | Item Value... |
     +---------------+---------------+---------------+---------------+
     |    value contd.                                               |
     +---------------+---------------+---------------+---------------+

                 Figure 18: Session Extension Item Format

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CRITICAL | 0x01   | If the negotiated Enable TLS
      value is true and acceptable then TLS negotiation feature
      (described in Section 4.4) begins immediately following the
      contact header exchange.

   Once this process of parameter negotiation bit is completed (which
   includes a possible completed TLS handshake of the connection to use
   TLS), this protocol defines no additional mechanism to change set, indicates that the
   parameters of an established session; to effect such a change, receiving   |
   |          |        | peer must handle the
   TCPCL session MUST be terminated and a new session established. extension item.          |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                   Table 3: Session Extension Item Flags

5.  Established Session Operation

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

5.1.  Upkeep and Status Messages

5.1.1.  Session Upkeep (KEEPALIVE)

   The protocol includes a provision for transmission of KEEPALIVE
   messages over the TCPCL session to help determine if the underlying
   TCP connection has been disrupted.

   As described in Section 4.3, a negotiated parameter of each session
   is the Session Keepalive interval.  If the negotiated Session
   Keepalive is zero (i.e. one or both contact headers contains a zero
   Keepalive Interval), then the keepalive feature is disabled.  There
   is no logical minimum value for the keepalive interval, but when used
   for many sessions on an open, shared network a short interval could
   lead to excessive traffic.  For shared network use, entities SHOULD
   choose a keepalive interval no shorter than 30 seconds.  There is no
   logical maximum value for the keepalive interval, but an idle TCP
   connection is liable for closure by the host operating system if the
   keepalive time is longer than tens-of-minutes.  Entities SHOULD
   choose a keepalive interval no longer than 10 minutes (600 seconds).

   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.

   The format of a KEEPALIVE message is a one-octet message type code of
   KEEPALIVE (as described in Table 2) with no additional data.  Both
   sides SHOULD SHALL 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 in a session
   after some implementation-defined time duration, then the node SHOULD SHALL
   terminate the session by transmitting a SESS_TERM message (as
   described in Section 6.1) with reason code "Idle Timeout".  If
   configurable, the idle timeout duration SHOULD be no shorter than
   twice the keepalive interval.  If not configurable, the idle timeout
   duration SHOULD be exactly twice the keepout interval.

5.1.2.  Message Rejection (MSG_REJECT)

   If a TCPCL node receives a message which is unknown to it (possibly
   due to an unhandled protocol mismatch) or is inappropriate for the
   current session state (e.g. a KEEPALIVE message received after
   contact header negotiation has disabled that feature), there is a
   protocol-level message to signal this condition in the form of a
   MSG_REJECT reply.

   The format of a MSG_REJECT message is as follows in Figure 19.

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

                 Figure 19: Format of MSG_REJECT Messages

   The fields of the MSG_REJECT message are:

   Reason Code:  A one-octet refusal reason code interpreted according
      to the descriptions in Table 4.

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

   +-------------+------+----------------------------------------------+
   | Name        | Code | Description                                  |
   +-------------+------+----------------------------------------------+
   | Message     | 0x01 | A message was received with a Message Type   |
   | Type        |      | code unknown to the TCPCL node.              |
   | Unknown     |      |                                              |
   |             |      |                                              |
   | Message     | 0x02 | A message was received but the TCPCL node    |
   | Unsupported |      | cannot comply with the message contents.     |
   |             |      |                                              |
   | Message     | 0x03 | A message was received while the session is  |
   | Unexpected  |      | in a state in which the message is not       |
   |             |      | expected.                                    |
   +-------------+------+----------------------------------------------+

                     Table 4: MSG_REJECT Reason Codes

5.2.  Bundle Transfer

   All of the messages in this section are directly associated with
   transferring a bundle between TCPCL entities.

   A single TCPCL transfer results in a bundle (handled by the
   convergence layer as opaque data) being exchanged from one node to
   the other.  In TCPCL a transfer is accomplished by dividing a single
   bundle up into "segments" based on the receiving-side Segment MRU
   (see Section 4.2).  The choice of the length to use for segments is
   an implementation matter, but each segment MUST be no larger than the
   receiving node's maximum receive unit (MRU) (see the field "Segment
   MRU" of Section 4.2).  The first segment for a bundle MUST set the
   'START' flag, and the last one MUST set the 'end' flag in the
   XFER_SEGMENT message flags.

   A single transfer (and by extension a single segment) SHALL NOT
   contain data of more than a single bundle.  This requirement is
   imposed on the agent using the TCPCL rather than TCPCL itself.

   If multiple bundles are transmitted on a single TCPCL connection,
   they MUST be transmitted consecutively without interleaving of
   segments from multiple bundles.

5.2.1.  Bundle Transfer ID

   Each of the bundle transfer messages contains a Transfer ID which is
   used to correlate messages (from both sides of a transfer) for each
   bundle.  A Transfer ID does not attempt to address uniqueness of the
   bundle data itself and has no relation to concepts such as bundle
   fragmentation.  Each invocation of TCPCL by the bundle protocol
   agent, requesting transmission of a bundle (fragmentary or
   otherwise), results in the initiation of a single TCPCL transfer.
   Each transfer entails the sending of a XFER_INIT message and sequence of some number of
   XFER_SEGMENT and XFER_ACK messages; all are correlated by the same
   Transfer ID.

   Transfer IDs from each node SHALL be unique within a single TCPCL
   session.  The initial Transfer ID from each node SHALL have value
   zero.  Subsequent Transfer ID values SHALL be incremented from the
   prior Transfer ID value by one.  Upon exhaustion of the entire 64-bit
   Transfer ID space, the sending node SHALL terminate the session with
   SESS_TERM reason code "Resource Exhaustion".

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

5.2.2.  Transfer Initialization (XFER_INIT)

   The XFER_INIT message contains the total length, in octets, of the
   bundle data in the associated transfer.  The total length is
   formatted as a 64-bit unsigned integer.

   The purpose of the XFER_INIT message is to allow entities to
   preemptively refuse bundles that would exceed their resources or to
   prepare storage on the receiving node for the upcoming  Data Transmission (XFER_SEGMENT)

   Each bundle data.
   See Section 5.2.5 for details on when refusal based on XFER_INIT
   content is acceptable.

   The Total Bundle Length field within a XFER_INIT message SHALL be
   treated as authoritative by the receiver.  If, for whatever reason,
   the actual total length of bundle data received differs from the
   value indicated by the XFER_INIT message, the receiver SHOULD treat
   the transmitted in one or more data as invalid. segments.  The format
   of the XFER_INIT a XFER_SEGMENT message is as follows in Figure 20.

                      +-----------------------------+

                     +------------------------------+
                     |       Message Header         |
                     +------------------------------+
                     |     Message Header Flags (U8)       |
                      +-----------------------------+
                     +------------------------------+
                     |      Transfer ID (U64)       |
                      +-----------------------------+
                     +------------------------------+
                     |     Transfer Extension       |
                     |  Total Bundle         Length (U64) (U32)         |
                      +-----------------------------+
                     |   (only for START segment)   |
                     +------------------------------+
                     |     Transfer Extension       |
                     |          Length         Items (var.)         |
                     |   (only for START segment)   |
                     +------------------------------+
                     |      Data length (U64)       |
                      +-----------------------------+
                     +------------------------------+
                     | Transfer Extension Items... Data contents (octet string) |
                      +-----------------------------+
                     +------------------------------+

                Figure 20: Format of XFER_INIT XFER_SEGMENT Messages

   The fields of the XFER_INIT XFER_SEGMENT message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 5.

   Transfer ID:  A 64-bit unsigned integer identifying the transfer
      about to begin.

   Total Bundle Length:  A 64-bit unsigned integer indicating the size
      of the data-to-be-transferred.
      being made.

   Transfer Extension Length and Transfer Extension Items:  Together
      these fields represent protocol extension data not defined by for this
      specification.  The Transfer Extension Length is the total number
      of octets to follow which are used to encode the Transfer
      Extension Item list.  The encoding of each and Transfer
      Extension Item
      is within a consistent data container as described in
      Section 5.2.2.1.  The full set of transfer extension items apply
      only to the assoicated single transfer.  The order and
      mulitplicity of these transfer extension items MAY be significant,
      as defined in the associated type specification(s).

   An XFER_INIT message fields SHALL only be sent as the first message in a transfer
   sequence, before transmission of any XFER_SEGMENT messages for the
   same Transfer ID.  XFER_INIT messages MUST NOT be sent unless the
   next XFER_SEGMENT message has present when the 'START' bit flag
      is set to "1" (i.e., just
   before the start of a new transfer).

5.2.2.1.  Transfer Extension Items

   Each of on the Transfer Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated in Figure 21. message.  The fields of the Transfer Extension Item are:

   Flags:  A one-octet field containing generic bit flags about Length is the
      Item, total
      number of octets to follow which are listed in Table 5.  If a TCPCL node receives a are used to encode the Transfer
      Extension Item with an unknown list.  The encoding of each Transfer Extension Item Type and the CRITICAL
      flag set, the node SHALL refuse the transfer with an XFER_REFUSE
      reason code
      is within a consistent data container as described in
      Section 5.2.5.  The full set of "Extension Failure".  If transfer extension items apply
      only to the CRITICAL flag is not
      set, an entity SHALL skip over assoicated single transfer.  The order and ignore any item with an unknown
      Item Type.

   Item Type:  A 16-bit unsigned integer field containing the type
      mulitplicity of
      the extension item.  This specification does not define any these transfer extension types directly, but does allocate an IANA registry for
      such codes (see Section 9.4).

   Item Length: items MAY be significant,
      as defined in the associated type specification(s).

   Data length:  A 32-bit 64-bit unsigned integer field containing indicating the number of Item Value
      octets in the Data contents to follow.

   Item Value:  A

   Data contents:  The variable-length data field which is interpreted
      according to the associated Item Type.  This specification places
      no restrictions on an extension's use of available Item Value
      data.  Extension specification SHOULD avoid the use payload of large data
      exchanges within the XFER_INIT as the associated transfer cannot
      begin until the full initialization message is sent.

                          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
     +---------------+---------------+---------------+---------------+ message.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   |  Item Flags END      |           Item Type 0x01   | Item Length...|
     +---------------+---------------+---------------+---------------+ If bit is set, indicates that this is the     |    length contd.
   | Item Value...          |
     +---------------+---------------+---------------+---------------+        |    value contd. last segment of the transfer.                 |
     +---------------+---------------+---------------+---------------+

                 Figure 21: Transfer Extension Item Format

   +----------+--------+-----------------------------------------------+
   | Name          | Code        | Description                                               |
   +----------+--------+-----------------------------------------------+
   | CRITICAL START    | 0x01 0x02   | If bit is set, indicates that this is the receiving     |
   |          |        | peer must handle first segment of the extension item. transfer.                |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                        Table 5: Transfer Extension Item XFER_SEGMENT Flags

5.2.3.  Data Transmission (XFER_SEGMENT)

   Each

   The flags portion of the message contains two optional values in the
   two low-order bits, denoted 'START' and 'END' in Table 5.  The
   'START' bit MUST be set to one if it precedes the transmission of the
   first segment of a transfer.  The 'END' bit MUST be set to one when
   transmitting the last segment of a transfer.  In the case where an
   entire transfer is accomplished in a single segment, both the 'START'
   and 'END' bits MUST be set to one.

   Once a transfer of a bundle has commenced, the node MUST only send
   segments containing sequential portions of that bundle until it sends
   a segment with the 'END' bit set.  No interleaving of multiple
   transfers from the same node is possible within a single TCPCL
   session.  Simultaneous transfers between two entities MAY be achieved
   using multiple TCPCL sessions.

5.2.3.  Data Acknowledgments (XFER_ACK)

   Although the TCP transport provides reliable transfer of data between
   transport peers, the typical BSD sockets interface provides no means
   to inform a sending application of when the receiving application has
   processed some amount of transmitted in one or more data.  Thus, after transmitting
   some data, the TCPCL needs an additional mechanism to determine
   whether the receiving agent has successfully received the segment.
   To this end, the TCPCL protocol provides feedback messaging whereby a
   receiving node transmits acknowledgments of reception of data
   segments.

   The format of a XFER_SEGMENT an XFER_ACK message follows in Figure 22.

                     +------------------------------+ 21.

                      +-----------------------------+
                      |       Message Header        |
                     +------------------------------+
                      +-----------------------------+
                      |     Message Flags (U8)      |
                     +------------------------------+
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                     +------------------------------+
                      +-----------------------------+
                      |      Data Acknowledged length (U64)   |
                     +------------------------------+
                     | Data contents (octet string) |
                     +------------------------------+
                      +-----------------------------+

                  Figure 22: 21: Format of XFER_SEGMENT XFER_ACK Messages

   The fields of the XFER_SEGMENT XFER_ACK message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 6. 5.

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

   Data acknowledged.

   Acknowledged length:  A 64-bit unsigned integer indicating the total
      number of octets in the Data contents transfer which are being acknowledged.

   A receiving TCPCL node SHALL send an XFER_ACK message in response to follow.

   Data contents:
   each received XFER_SEGMENT message.  The variable-length data payload flags portion of the message.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | END      | 0x01   | If bit is set, indicates that this is
   XFER_ACK header SHALL be set to match the     |
   |          |        | last segment corresponding DATA_SEGMENT
   message being acknowledged.  The acknowledged length of each XFER_ACK
   contains the transfer.                 |
   |          |        |                                               |
   | START    | 0x02   | If bit is set, indicates that this is the     |
   |          |        | first segment sum of the transfer.                |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                        Table 6: XFER_SEGMENT Flags

   The flags portion data length fields of the message contains two optional values all XFER_SEGMENT
   messages received so far in the
   two low-order bits, denoted 'START' and 'END' in Table 6. course of the indicated transfer.
   The
   'START' bit MUST be set to one if it precedes sending node SHOULD transmit multiple XFER_SEGMENT messages
   without waiting for the corresponding XFER_ACK responses.  This
   enables pipelining of messages on a transfer stream.

   For example, suppose the transmission sending node transmits four segments of
   bundle data with lengths 100, 200, 500, and 1000, respectively.
   After receiving the first segment of a transfer.  The 'END' bit MUST be set to one when
   transmitting segment, the last segment node sends an acknowledgment
   of a transfer.  In length 100.  After the case where an
   entire transfer second segment is accomplished in a single segment, both received, the 'START' node sends
   an acknowledgment of length 300.  The third and 'END' bits MUST be set to one.

   Once a transfer fourth
   acknowledgments are of length 800 and 1800, respectively.

5.2.4.  Transfer Refusal (XFER_REFUSE)

   The TCPCL supports a bundle has commenced, the mechanism by which a receiving node MUST only send
   segments containing sequential portions of can indicate
   to the sender that bundle until it sends
   a segment with does not want to receive the 'END' bit set.  No interleaving of multiple
   transfers from corresponding
   bundle.  To do so, upon receiving an XFER_SEGMENT message, the same node is possible within
   MAY transmit a single TCPCL
   session.  Simultaneous transfers between two entities XFER_REFUSE message.  As data segments and
   acknowledgments MAY cross on the wire, the bundle that is being
   refused SHALL be achieved
   using multiple TCPCL sessions.

5.2.4.  Data Acknowledgments (XFER_ACK)

   Although identified by the TCP transport provides reliable transfer Transfer ID of data between
   transport peers, the typical BSD sockets interface provides refusal.

   There is no means required relation between the Transfer MRU of a TCPCL
   node (which is supposed to inform represent a sending application firm limitation of when what the receiving application has
   processed some amount
   node will accept) and sending of transmitted data.  Thus, a XFER_REFUSE message.  A
   XFER_REFUSE can be used in cases where the agent's bundle storage is
   temporarily depleted or somehow constrained.  A XFER_REFUSE can also
   be used after transmitting
   some data, the TCPCL needs bundle header or any bundle data is inspected by an additional mechanism
   agent and determined to determine
   whether be unacceptable.

   A receiver MAY send an XFER_REFUSE message as soon as it receives any
   XFER_SEGMENT message.  The sender MUST be prepared for this and MUST
   associate the receiving agent has successfully received refusal with the segment.
   To this end, correct bundle via the TCPCL protocol provides feedback messaging whereby a
   receiving node transmits acknowledgments of reception of data
   segments. Transfer ID
   fields.

   The format of an XFER_ACK the XFER_REFUSE message is as follows in Figure 23. 22.

                      +-----------------------------+
                      |       Message Header        |
                      +-----------------------------+
                      +-----------------------------+
                      |      Reason Code (U8)       |
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                      +-----------------------------+

                 Figure 22: Format of XFER_REFUSE Messages

   The fields of the XFER_REFUSE message are:

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

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

   +------------+------------------------------------------------------+
   | Name       | Semantics                                            |
   +------------+------------------------------------------------------+
   | Unknown    | Reason for refusal is unknown or not specified.      |
   |            |                                                      |
   | Extension  | A failure processing the Transfer Extension Items ha |
   | Failure    | occurred.                                            |
   |            |                                                      |
   | Completed  | The receiver already has the complete bundle. The    |
   |            | sender MAY consider the bundle as completely         |
   |            | received.                                            |
   |            |                                                      |
   | No         | The receiver's resources are exhausted. The sender   |
   | Resources  | SHOULD apply reactive bundle fragmentation before    |
   |            | retrying.                                            |     Message Flags (U8)
   |
                      +-----------------------------+            |      Transfer ID (U64)                                                      |
                      +-----------------------------+
   | Acknowledged length (U64) Retransmit |
                      +-----------------------------+

                  Figure 23: Format of XFER_ACK Messages The fields of receiver has encountered a problem that requires |
   |            | the XFER_ACK message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according bundle to the descriptions be retransmitted in its entirety.      |
   +------------+------------------------------------------------------+

                     Table 6.

   Transfer ID:  A 64-bit unsigned integer identifying the 6: XFER_REFUSE Reason Codes

   The receiver MUST, for each transfer
      being acknowledged.

   Acknowledged length:  A 64-bit unsigned integer indicating the total
      number of octets in preceding the transfer which are being acknowledged.

   A receiving TCPCL node SHALL send an XFER_ACK message in response one to
   each received XFER_SEGMENT message.  The flags portion of the
   XFER_ACK header SHALL be set to match the corresponding DATA_SEGMENT
   message being acknowledged.  The refused,
   have either acknowledged length of each XFER_ACK
   contains the sum of the data length fields of all XFER_SEGMENT
   messages received so far in the course of XFER_SEGMENTs or refused the indicated bundle
   transfer.

   The sending node MAY transmit multiple XFER_SEGMENT messages without
   necessarily waiting for the corresponding XFER_ACK responses.  This
   enables pipelining of messages on a transfer stream.

   For example, suppose the sending node transmits four segments of bundle transfer refusal MAY be sent before an entire data with lengths 100, 200, 500, and 1000, respectively.
   After receiving segment
   is received.  If a sender receives a XFER_REFUSE message, the first segment, sender
   MUST complete the node sends an acknowledgment transmission of length 100.  After the second segment any partially sent XFER_SEGMENT
   message.  There is received, the node sends no way to interrupt an acknowledgment of length 300. individual TCPCL message
   partway through sending it.  The third and fourth
   acknowledgments are sender MUST NOT commence
   transmission of any further segments of length 800 and 1800, respectively.

5.2.5.  Transfer Refusal (XFER_REFUSE)

   The TCPCL supports a mechanism by which a receiving node can indicate
   to the sender refused bundle
   subsequently.  Note, however, that it this requirement does not want to ensure
   that an entity will not receive the corresponding
   bundle.  To do so, upon receiving a XFER_INIT or another XFER_SEGMENT
   message, for the node MAY transmit same
   bundle after transmitting a XFER_REFUSE message.  As data
   segments and acknowledgments message since messages MAY
   cross on the wire, wire; if this happens, subsequent segments of the bundle that
   is being refused
   SHALL also be identified by the Transfer ID of the
   refusal.

   There is no required relation between the Transfer MRU of a TCPCL
   node (which is supposed to represent a firm limitation of what the
   node will accept) and sending of refused with a XFER_REFUSE message.  A
   XFER_REFUSE can be used in cases where the agent's

   Note: If a bundle storage transmission is
   temporarily depleted or somehow constrained.  A XFER_REFUSE can also
   be used after aborted in this way, the bundle header or any bundle data is inspected by an
   agent and determined to be unacceptable.

   A receiver
   MAY send an XFER_REFUSE message as soon as it receives not receive a
   XFER_INIT message without waiting for segment with the next XFER_SEGMENT message.
   The sender MUST be prepared 'END' flag set to '1' for this and MUST associate the refusal
   with
   aborted bundle.  The beginning of the correct next bundle via is identified by
   the 'START' bit set to '1', indicating the start of a new transfer,
   and with a distinct Transfer ID fields.

   The format value.

5.2.5.  Transfer Extension Items

   Each of the XFER_REFUSE message is Transfer Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as follows indicated in Figure 24.

                      +-----------------------------+
                      |       Message Header        |
                      +-----------------------------+
                      |      Reason Code (U8)       |
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                      +-----------------------------+

                 Figure 24: Format of XFER_REFUSE Messages 23.

   The fields of the XFER_REFUSE message Transfer Extension Item are:

   Reason Code:

   Flags:  A one-octet refusal reason code interpreted according
      to field containing generic bit flags about the descriptions
      Item, which are listed in Table 7.  If a TCPCL node receives a
      Transfer ID: Extension Item with an unknown Item Type and the CRITICAL
      flag set, the node SHALL refuse the transfer with an XFER_REFUSE
      reason code of "Extension Failure".  If the CRITICAL flag is not
      set, an entity SHALL skip over and ignore any item with an unknown
      Item Type.

   Item Type:  A 64-bit 16-bit unsigned integer identifying field containing the type of
      the extension item.  This specification allocates an IANA registry
      for such codes (see Section 9.4).

   Item Length:  A 32-bit unsigned integer field containing the number
      of Item Value octets to follow.

   Item Value:  A variable-length data field which is interpreted
      according to the associated Item Type.  This specification places
      no restrictions on an extension's use of available Item Value
      data.  Extension specifications SHOULD avoid the use of large data
      lengths, as the associated transfer
      being refused.

   +------------+------------------------------------------------------+
   | Name       | Semantics                                            |
   +------------+------------------------------------------------------+ cannot begin until the full
      extension data is sent.

                          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
     +---------------+---------------+---------------+---------------+
     | Unknown  Item Flags   | Reason for refusal is unknown or not specified.           Item Type           | Item Length...|
     +---------------+---------------+---------------+---------------+
     |    length contd.                              | Item Value... |
     +---------------+---------------+---------------+---------------+
     | Extension    value contd.                                               | A failure processing the
     +---------------+---------------+---------------+---------------+

                 Figure 23: Transfer Extension Items ha |
   | Failure    | occurred.                                            |
   | Item Format

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Completed Description                                   | The receiver already has the complete bundle. The
   +----------+--------+-----------------------------------------------+
   | CRITICAL | 0x01   | sender MAY consider If bit is set, indicates that the bundle as completely         |
   |            | received.                                            |
   |            |                                                      |
   | No         | The receiver's resources are exhausted. The sender   |
   | Resources  | SHOULD apply reactive bundle fragmentation before    | receiving   |
   | retrying.          |        | peer must handle the extension item.          |
   |          | Retransmit        | The receiver has encountered a problem that requires                                               |
   | Reserved | the bundle to be retransmitted in its entirety. others |
   +------------+------------------------------------------------------+
   +----------+--------+-----------------------------------------------+

                  Table 7: XFER_REFUSE Reason Codes

   The receiver MUST, for each transfer preceding the one to be refused,
   have either acknowledged all XFER_SEGMENTs or refused the bundle
   transfer. Transfer Extension Item Flags

5.2.5.1.  Transfer Length Extension

   The bundle transfer refusal MAY be sent before an entire data segment
   is received.  If a sender receives a XFER_REFUSE message, the sender
   MUST complete the transmission purpose of any partially sent XFER_SEGMENT
   message.  There the Transfer Length extension is no way to interrupt an individual TCPCL message
   partway through sending it.  The sender MUST NOT commence
   transmission of any further segments of the refused bundle
   subsequently.  Note, however, that this requirement does not ensure allow entities to
   preemptively refuse bundles that an entity will not receive another XFER_SEGMENT would exceed their resources or to
   prepare storage on the receiving node for the same upcoming bundle after transmitting data.

   Multiple Transfer Length extension items SHALL NOT occurr within the
   same transfer.  The lack of a XFER_REFUSE message since messages MAY
   cross on Transfer Length extension item in any
   transfer SHALL NOT imply anything about the wire; if this happens, subsequent segments potential length of the bundle
   SHOULD also
   transfer.  The Transfer Length extension SHALL be refused with a XFER_REFUSE message.

   Note: If a bundle transmission assigned transfer
   extension type ID 0x0001.

   The format of the Transfer Length data is aborted as follows in this way, Figure 24.

                         +----------------------+
                         |  Total Length (U64)  |
                         +----------------------+

                 Figure 24: Format of Transfer Length data

   The fields of the receiver
   MAY not receive a segment with Transfer Length extension are:

   Total Length:  A 64-bit unsigned integer indicating the 'END' flag set to '1' for size of the
   aborted bundle.
      data-to-be-transferred.  The beginning of Total Length field SHALL be treated
      as authoritative by the next receiver.  If, for whatever reason, the
      actual total length of bundle is identified data received differs from the value
      indicated by the 'START' bit set to '1', indicating Total Length value, the start of a new transfer,
   and with a distinct Transfer ID value. receiver SHALL treat the
      transmitted data as invalid.

6.  Session Termination

   This section describes the procedures for ending a TCPCL session.

6.1.  Session Termination Message (SESS_TERM)

   To cleanly shut down a session, a SESS_TERM message SHALL be
   transmitted by either node at any point following complete
   transmission of any other message.  When sent to initiate a
   termination, the REPLY bit of a SESS_TERM message SHALL NOT be set.
   Upon receiving a SESS_TERM message after not sending a SESS_TERM
   message in the same session, an entity SHOULD SHALL send an acknowledging
   SESS_TERM message.  When sent to acknowledge a termination, a
   SESS_TERM message SHALL have identical data content from the message
   being acknowledged except for the REPLY bit, which is set to indicate
   acknowledgement.

   After sending a SESS_TERM message, an entity MAY continue a possible
   in-progress transfer in either direction.  After sending a SESS_TERM
   message, an entity SHALL NOT begin any new outgoing transfer (i.e.
   send an XFER_INIT XFER_SEGMENT message) for the remainder of the session.
   After receving a SESS_TERM message, an entity SHALL NOT accept any
   new incoming transfer for the remainder of the session.

   Instead of following a clean shutdown sequence, after transmitting a
   SESS_TERM message an entity MAY immediately close the associated TCP
   connection.  When performing an unclean shutdown, a receiving node
   SHOULD acknowledge all received data segments before closing the TCP
   connection.  Not acknowledging received segments can result in
   unnecessary retransmission.  When performing an unclean shutodwn, a
   transmitting node SHALL treat either sending or receiving a SESS_TERM
   message (i.e. before the final acknowledgment) as a failure of the
   transfer.  Any delay between request to terminate the TCP connection
   and actual closing of the connection (a "half-closed" state) MAY be
   ignored by the TCPCL node.

   The format of the SESS_TERM message is as follows in Figure 25.

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

                  Figure 25: Format of SESS_TERM Messages

   The fields of the SESS_TERM message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 8.

   Reason Code:  A one-octet refusal reason code interpreted according
      to the descriptions in Table 9.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | REPLY    | 0x01   | If bit is set, indicates that this message is |
   |          |        | an acknowledgement of an earlier SESS_TERM    |
   |          |        | message.                                      |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                         Table 8: SESS_TERM Flags

   +---------------+---------------------------------------------------+
   | Name          | Description                                       |
   +---------------+---------------------------------------------------+
   | Unknown       | A termination reason is not available.            |
   |               |                                                   |
   | Idle timeout  | The session is being closed due to idleness.      |
   |               |                                                   |
   | Version       | The node cannot conform to the specified TCPCL    |
   | mismatch      | protocol version.                                 |
   |               |                                                   |
   | Busy          | The node is too busy to handle the current        |
   |               | session.                                          |
   |               |                                                   |
   | Contact       | The node cannot interpret or negotiate contact    |
   | Failure       | header option.                                    |
   |               |                                                   |
   | Resource      | The node has run into some resource limit and     |
   | Exhaustion    | cannot continue the session.                      |
   +---------------+---------------------------------------------------+

                      Table 9: SESS_TERM Reason Codes

   A session shutdown MAY occur immediately after transmission of a
   contact header (and prior to any further message transmit).  This
   MAY, for example, be used to notify that the node is currently not
   able or willing to communicate.  However, an entity MUST always send
   the contact header to its peer before sending a SESS_TERM message.

   If reception of the contact header itself somehow fails (e.g. an
   invalid "magic string" is recevied), an entity SHOULD SHALL close the TCP
   connection without sending a SESS_TERM message.  If the content of
   the Session Extension Items data disagrees with the Session Extension
   Length (i.e. the last Item claims to use more octets than are present
   in the Session Extension Length), the reception of the contact header
   is considered to have failed.

   If a session is to be terminated before a protocol message has
   completed being sent, then the node MUST NOT transmit the SESS_TERM
   message but still SHOULD SHALL close the TCP connection.  Each TCPCL message
   is contiguous in the octet stream and has no ability to be cut short
   and/or preempted by an other message.  This is particularly important
   when large segment sizes are being transmitted; either entire
   XFER_SEGMENT is sent before a SESS_TERM message or the connection is
   simply terminated mid-XFER_SEGMENT.

6.2.  Idle Session Shutdown

   The protocol includes a provision for clean shutdown of idle
   sessions.  Determining the length of time to wait before closing idle
   sessions, if they are to be closed at all, is an implementation and
   configuration matter.

   If there is a configured time to close idle links and if no TCPCL
   messages (other than KEEPALIVE messages) has been received for at
   least that amount of time, then either node MAY terminate the session
   by transmitting a SESS_TERM message indicating the reason code of
   "Idle timeout" (as described in Table 9).

7.  Implementation Status

   [NOTE to the RFC Editor: please remove this section before
   publication, as well as the reference to [RFC7942] and
   [github-dtn-bpbis-tcpcl].]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   An example implementation of the this draft of TCPCLv4 has been
   created as a GitHub project [github-dtn-bpbis-tcpcl] and is intented
   to use as a proof-of-concept and as a possible source of
   interoperability testing.  This example implementation uses D-Bus as
   the CL-BP Agent interface, so it only runs on hosts which provide the
   Python "dbus" library.

8.  Security Considerations

   One security consideration for this protocol relates to the fact that
   entities present their endpoint identifier as part of the contact
   header exchange.  It would be possible for an entity to fake this
   value and present the identity of a singleton endpoint in which the
   node is not a member, essentially masquerading as another DTN node.
   If this identifier is used outside of a TLS-secured session or
   without further verification as a means to determine which bundles
   are transmitted over the session, then the node that has falsified
   its identity would be able to obtain bundles that it otherwise would
   not have.  Therefore, an entity SHALL NOT use the EID value of an
   unsecured contact header to derive a peer node's identity unless it
   can corroborate it via other means.  When TCPCL session security is
   mandated by a TCPCL peer, that peer SHALL transmit initial unsecured
   contact header values indicated in Table 10 in order.  These values
   avoid unnecessarily leaking session parameters and will be ignored
   when secure contact header re-exchange occurs.

   +--------------------+---------------------------------------------+
   | Parameter          | Value                                       |
   +--------------------+---------------------------------------------+
   | Flags              | The USE_TLS flag is set.                    |
   |                    |                                             |
   | Keepalive Interval | Zero, indicating no keepalive.              |
   |                    |                                             |
   | Segment MRU        | Zero, indicating all segments are refused.  |
   |                    |                                             |
   | Transfer MRU       | Zero, indicating all transfers are refused. |
   |                    |                                             |
   | EID                | Empty, indicating lack of EID.              |
   +--------------------+---------------------------------------------+

              Table 10: Recommended Unsecured Contact Header

   TCPCL can be used to provide point-to-point transport security, but
   does not provide security of data-at-rest and does not guarantee end-
   to-end bundle security.  The mechanisms defined in [RFC6257] and
   [I-D.ietf-dtn-bpsec] are to be used instead.

   Even when using TLS to secure the TCPCL session, the actual
   ciphersuite negotiated between the TLS peers MAY be insecure.  TLS
   can be used to perform authentication without data confidentiality,
   for example.  It is up to security policies within each TCPCL node to
   ensure that the negotiated TLS ciphersuite meets transport security
   requirements.  This is identical behavior to STARTTLS use in
   [RFC2595].

   Another consideration for this protocol relates to denial-of-service
   attacks.  An entity MAY send a large amount of data over a TCPCL
   session, requiring the receiving entity to handle the data, attempt
   to stop the flood of data by sending a XFER_REFUSE message, or
   forcibly terminate the session.  This burden could cause denial of
   service on other, well-behaving sessions.  There is also nothing to
   prevent a malicious entity from continually establishing sessions and
   repeatedly trying to send copious amounts of bundle data.  A
   listening entity 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 SESS_TERM
   message quickly or with a delay, etc.

9.  IANA Considerations

   In this section, registration procedures are as defined in [RFC8126].

   Some of the registries below are created new for TCPCLv4 but share
   code values with TCPCLv3.  This was done to disambiguate the use of
   these values between TCPCLv3 and TCPCLv4 while preserving the
   semantics of some values.

9.1.  Port Number

   Port number 4556 has been previously assigned as the default port for
   the TCP convergence layer in [RFC7242].  This assignment is unchanged
   by protocol version 4.  Each TCPCL entity identifies its TCPCL
   protocol version in its initial contact (see Section 9.2), so there
   is no ambiguity about what protocol is being used.

     +------------------------+-------------------------------------+
     | Parameter              | Value                               |
     +------------------------+-------------------------------------+
     | Service Name:          | dtn-bundle                          |
     |                        |                                     |
     | Transport Protocol(s): | TCP                                 |
     |                        |                                     |
     | Assignee:              | Simon Perreault <simon@per.reau.lt> |
     |                        |                                     |
     | Contact:               | Simon Perreault <simon@per.reau.lt> |
     |                        |                                     |
     | Description:           | DTN Bundle TCP CL Protocol          |
     |                        |                                     |
     | Reference:             | [RFC7242]                           |
     |                        |                                     |
     | Port Number:           | 4556                                |
     +------------------------+-------------------------------------+

9.2.  Protocol Versions

   IANA has created, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version
   Numbers" and initialize it with the following table.  The
   registration procedure is RFC Required.

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

9.3.  Session Extension Types

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Session Extension Types" and initialize it with the contents of
   Table 11.  The registration procedure is RFC Required within the
   lower range 0x0001--0x7fff.  Values in the range 0x8000--0xffff are
   reserved for use on private networks for functions not published to
   the IANA.

               +----------------+--------------------------+
               | Code           | Message Type             |
               +----------------+--------------------------+
               | 0x0000         | Reserved                 |
               |                |                          |
               | 0x0001--0x7fff | Unassigned               |
               |                |                          |
               | 0x8000--0xffff | Private/Experimental Use |
               +----------------+--------------------------+

                  Table 11: Session Extension Type Codes

9.4.  Transfer Extension Types

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   Transfer Extension Types" and initialize it with the contents of
   Table 12.  The registration procedure is RFC Required within the
   lower range 0x0001--0x7fff.  Values in the range 0x8000--0xffff are
   reserved for use on private networks for functions not published to
   the IANA.

               +----------------+--------------------------+

              +----------------+---------------------------+
              | Code           | Message Type              |
               +----------------+--------------------------+
              +----------------+---------------------------+
              | 0x0000         | Reserved                  |
              |                |                           |
              | 0x0001--0x7fff 0x0001         | Transfer Length Extension |
              |                |                           |
              | 0x0002--0x7fff | Unassigned                |
              |                |                           |
              | 0x8000--0xffff | Private/Experimental Use  |
               +----------------+--------------------------+
              +----------------+---------------------------+

                  Table 12: Transfer Extension Type Codes

9.5.  Message Types

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

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

                       +-----------+--------------+
                       | Code      | Message Type |
                       +-----------+--------------+
                       | 0x00      | Reserved     |
                       |           |              |
                       | 0x01      | XFER_SEGMENT |
                       |           |              |
                       | 0x02      | XFER_ACK     |
                       |           |              |
                       | 0x03      | XFER_REFUSE  |
                       |           |              |
                       | 0x04      | KEEPALIVE    |
                       |           |              |
                       | 0x05      | SESS_TERM    |
                       |           |              |
                       | 0x06      | XFER_INIT    |
                       |           |              |
                       | 0x07      | MSG_REJECT   |
                       |           |              |
                       | 0x08--0xf 0x07--0xf | Unassigned   |
                       +-----------+--------------+

                       Table 13: Message Type Codes

9.6.  XFER_REFUSE Reason Codes

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   XFER_REFUSE Reason Codes" and initialize it with the contents of
   Table 14.  The registration procedure is RFC Required.

                 +----------+---------------------------+
                 | Code     | Refusal Reason            |
                 +----------+---------------------------+
                 | 0x0      | Unknown                   |
                 |          |                           |
                 | 0x1      | Extension Failure         |
                 |          |                           |
                 | 0x2      | Completed                 |
                 |          |                           |
                 | 0x3      | No Resources              |
                 |          |                           |
                 | 0x4      | Retransmit                |
                 |          |                           |
                 | 0x5--0x7 | Unassigned                |
                 |          |                           |
                 | 0x8--0xf | Reserved for future usage |
                 +----------+---------------------------+

                    Table 14: XFER_REFUSE Reason Codes

9.7.  SESS_TERM Reason Codes

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   SESS_TERM Reason Codes" and initialize it with the contents of
   Table 15.  The registration procedure is RFC Required.

                   +------------+---------------------+
                   | Code       | Shutdown Reason     |
                   +------------+---------------------+
                   | 0x00       | Unknown             |
                   |            |                     |
                   | 0x01       | Idle timeout        |
                   |            |                     |
                   | 0x02       | Version mismatch    |
                   |            |                     |
                   | 0x03       | Busy                |
                   |            |                     |
                   | 0x04       | Contact Failure     |
                   |            |                     |
                   | 0x05       | Resource Exhaustion |
                   |            |                     |
                   | 0x06--0xFF | Unassigned          |
                   +------------+---------------------+

                     Table 15: SESS_TERM Reason Codes

9.8.  MSG_REJECT Reason Codes

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

   IANA will create, under the "Bundle Protocol" registry, a sub-
   registry titled "Bundle Protocol TCP Convergence-Layer Version 4
   MSG_REJECT Reason Codes" and initialize it with the contents of
   Table 16.  The registration procedure is RFC Required.

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

                       Table 16: REJECT Reason Codes

10.  Acknowledgments

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

11.  References

11.1.  Normative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015.

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K. K., and E. Birrane, "Bundle Protocol
              Version 7",
              draft-ietf-dtn-bpbis-11 draft-ietf-dtn-bpbis-12 (work in progress), May
              November 2018.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

11.2.  Informative References

   [github-dtn-bpbis-tcpcl]
              Sipos, B., "TCPCL Example Implementation",
              <https://github.com/BSipos-RKF/dtn-bpbis-tcpcl/tree/
              develop>.

   [I-D.ietf-dtn-bpsec]
              Birrane, E. and K. McKeever, "Bundle Protocol Security
              Specification", draft-ietf-dtn-bpsec-08 draft-ietf-dtn-bpsec-09 (work in
              progress), October 2018. February 2019.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <https://www.rfc-editor.org/info/rfc2595>.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
              April 2007, <https://www.rfc-editor.org/info/rfc4838>.

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

   [RFC6257]  Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              DOI 10.17487/RFC6257, May 2011,
              <https://www.rfc-editor.org/info/rfc6257>.

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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

Appendix A.  Significant changes from RFC7242

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

   o  Split contact header into pre-TLS protocol negotiation and
      SESS_INIT parameter negotiation.  The contact header is now fixed-
      length.

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

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

   o  Added session extension capability.

   o  Added transfer extension capability.  Moved transfer total length
      into an extension item.

   o  Defined new IANA registries for message / type / reason codes to
      allow renaming some codes for clarity.

   o  Expanded Message Header to octet-aligned fields instead of bit-
      packing.

   o  Added a bundle transfer identification number to all bundle-
      related messages (XFER_INIT, XFER_SEGMENT, (XFER_SEGMENT, XFER_ACK, XFER_REFUSE).

   o  Use flags in XFER_ACK to mirror flags from XFER_SEGMENT.

   o  Removed all uses of SDNV fields and replaced with fixed-bit-length
      fields.

   o  Renamed SHUTDOWN to SESS_TERM to deconflict term "shutdown".

   o  Removed the notion of a re-connection delay parameter.

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

   o  Added contact negotiation failure SESS_TERM reason code.

   o  Added MSG_REJECT message to indicate an unknown or unhandled
      message was received.

   o  Added TLS session security mechanism.

   o  Added Resource Exhaustion SESS_TERM reason code.

Authors' Addresses
   Brian Sipos
   RKF Engineering Solutions, LLC
   7500 Old Georgetown Road
   Suite 1275
   Bethesda, MD  20814-6198
   United States of America

   Email: BSipos@rkf-eng.com

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

   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