Delay Tolerant Networking                                       B. Sipos
Internet-Draft                                           RKF Engineering
Obsoletes: 7242 (if approved)                                  M. Demmer
Intended status: Standards Track                             UC Berkeley
Expires: October 2, 2019 February 21, 2020                                        J. Ott
                                                        Aalto University
                                                            S. Perreault
                                                          March 31,
                                                         August 20, 2019

   Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
                       draft-ietf-dtn-tcpclv4-12
                       draft-ietf-dtn-tcpclv4-13

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 2, 2019. February 21, 2020.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Convergence Layer Services  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6   5
     2.1.  Definitions Specific to the TCPCL Protocol  . . . . . . .   6   5
   3.  General Protocol Description  . . . . . . . . . . . . . . . .   9   8
     3.1.  Convergence Layer Services  . . . . . . . . . . . . . . .   8
     3.2.  TCPCL Session Overview  . . . . . . . . . . . . . . . . .   9
     3.2.  10
     3.3.  TCPCL States and Transitions  . . . . . . . . . . . . . .  11
     3.3.  12
     3.4.  Transfer Segmentation Policies  . . . . . . . . . . . . .  16
     3.4.  18
     3.5.  Example Message Exchange  . . . . . . . . . . . . . . . .  17  19
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .  19  21
     4.1.  TCP Connection  . . . . . . . . . . . . . . . . . . . . .  19  21
     4.2.  Contact Header  . . . . . . . . . . . . . . . . . . . . .  19  22
     4.3.  Contact Validation and Negotiation  . . . . . . . . . . .  20  23
     4.4.  Session Security  . . . . . . . . . . . . . . . . . . . .  21  24
       4.4.1.  TLS Handshake Result . . . . . . . . . . . . . . . .  22 . . . .  24
       4.4.2.  TLS Authentication  . . . . . . . . . . . . . . . . .  25
       4.4.3.  Example TLS Initiation  . . . . . . . . . . . . . . .  22  26
     4.5.  Message Type Codes Header  . . . . . . . . . . . . . . . . . . .  23 . .  27
     4.6.  Session Initialization Message (SESS_INIT)  . . . . . . .  24  28
     4.7.  Session Parameter Negotiation . . . . . . . . . . . . . .  26  30
     4.8.  Session Extension Items . . . . . . . . . . . . . . . . .  27  31
   5.  Established Session Operation . . . . . . . . . . . . . . . .  28  32
     5.1.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  28  32
       5.1.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  28  32
       5.1.2.  Message Rejection (MSG_REJECT)  . . . . . . . . . . .  29  33
     5.2.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  30  34
       5.2.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  30  35
       5.2.2.  Data Transmission (XFER_SEGMENT)  . . . . . . . . . .  31  35
       5.2.3.  Data Acknowledgments (XFER_ACK) . . . . . . . . . . .  33  37
       5.2.4.  Transfer Refusal (XFER_REFUSE)  . . . . . . . . . . .  34  38
       5.2.5.  Transfer Extension Items  . . . . . . . . . . . . . .  36  41
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  38  43
     6.1.  Session Termination Message (SESS_TERM) . . . . . . . . .  38  43
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  40  45
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  40  45
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  41  46
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42  47
     9.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  42  47
     9.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  43  48
     9.3.  Session Extension Types . . . . . . . . . . . . . . . . .  43  48
     9.4.  Transfer Extension Types  . . . . . . . . . . . . . . . .  44  49
     9.5.  Message Types . . . . . . . . . . . . . . . . . . . . . .  45  50
     9.6.  XFER_REFUSE Reason Codes  . . . . . . . . . . . . . . . .  45  50
     9.7.  SESS_TERM Reason Codes  . . . . . . . . . . . . . . . . .  46  51
     9.8.  MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . .  47  52
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48  53
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48  53
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  48  53
     11.2.  Informative References . . . . . . . . . . . . . . . . .  49  54
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  49  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50  56

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

1.1.  Scope

   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
      (peers) within a network or across an internet.

1.1.  Convergence Layer Services

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

   Attempt Session  The TCPCL allows a BP agent to pre-emptively attempt
      to establish Agent and its various
      potential routing strategies.

   o  Logic for routing bundles along a TCPCL session with path toward a peer entity.  Each session
      attempt can send bundle's endpoint.
      This CL protocol is involved only in transporting bundles between
      adjacent nodes in a different set of session negotiation routing sequence.

   o  Policies or mechanisms for assigning X.509 certificates,
      provisioning or deploying certificates and private keys, or
      configuring security parameters
      as directed by the on an individual BP agent.

   Terminate Session  The node or across
      a network.

   Any TCPCL allows implementation requires 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: perform those above
   listed functions in order to perform end-to-end bundle delivery.

2.  Requirements Language

   The session has been fully established key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and is ready
         for its first transfer.

      Closing:  The entity received a SESS_TERM message
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and is only when, they appear in all
   capitals, as shown here.

2.1.  Definitions Specific to 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. Protocol

   This occurs only when section contains definitions specific to the top-level
      session state TCPCL protocol.

   TCPCL Entity:  This is Established.  Because the notional TCPCL transmits serially
      over a TCP connection, it suffers from "head application that initiates
      TCPCL sessions.  This design, implementation, configuration, and
      specific behavior 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 such an established TCPCL
      session.  Transmission request entity is on a per-session basis, the CL
      does not necessarily perform any per-session or inter-session
      queueing.  Any queueing outside of transmissions is the obligation scope of
      this document.  However, 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 concept of intermediate progress an entity has utility
      within the scope of transferr to a peer entity.
      This intermediate progress is at this document as the granularity container and initiator
      of each
      transferred segment.

   Transmission Failure TCPCL sessions.  The relationship between a TCPCL supports positive indication of
      certain reasons for bundle transmission failure, notably when the
      peer entity rejects the bundle or when a and
      TCPCL session ends before
      transferr success.  The sessions is defined as follows:

         A TCPCL itself does not have a notion Entity MAY actively initiate any number of
      transfer timeout.

   Reception Initialized  The TCPCL supports indication to
         Sessions and should do so whenever the reciver
      just before any transmssion data entity is sent.  This corresponds to
      reception of the XFER_SEGMENT message with the START flag set.

   Interrupt Reception  The TCPCL allows a BP agent initial
         transmitter of information to interrupt an
      individual transfer before it has fully completed (successfully or
      not).  Interruption can occur any time after another entity in 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 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  Convergence Layer Services

   This version 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 provides the session negotiation is
   used following services to set parameters of support
   the TCPCL session.  One of these parameters overlaying Bundle Protocol agent.  In all cases, this is not an
   API defintion but a singleton endpoint identifier for each node (not logical description of how the singleton
   Endpoint Identifier (EID) CL can interact
   with the BP agent.  Each of these interactions can be associated with
   any application running on the node) number of additional metadata items as necessary to
   denote support the bundle-layer identity
   operation of each DTN node.  This is used the CL or BP agent.

   Attempt Session:  The TCPCL allows a BP agent to
   assist in routing and forwarding messages (e.g. pre-emptively
      attempt to prevent loops).

   Once negotiated, the parameters of establish a TCPCL session cannot change and
   if there is with a desire by either peer to transfer data under entity.  Each
      session attempt can send a different set of session negotiation
      parameters then as directed by the BP agent.

   Terminate Session:  The TCPCL allows a new BP agent to pre-emptively
      terminate an established TCPCL session must be established.  This makes CL
   logic simpler but relies on the assumption that establishing with a TCP
   connection is lightweight enough that TCP connection overhead peer entity.  The
      terminate request is
   negligable compared to on a per-session basis.

   Session State Changed:  The TCPCL data sizes.

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

      Connecting:  A TCP connection is established and configured in this way,
   bundles can be transferred in either direction.  Each transfer is
   performed by an sequence of logical segments of data within
   XFER_SEGMENT 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 being established.  This state
         only applies to the same peers. active entity.

      Contact Negotating:  A feature of this protocol 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 the receiving node to send
   acknowledgment (XFER_ACK) messages as bundle data segments arrive. its first transfer.

      Ending:  The rationale behind these acknowledgments entity received a SESS_TERM message and is to enable the sender
   node to determine how much of the bundle has been received, so that in case the
         ending state.

      Terminated:  The session is interrupted, it can perform reactive
   fragmentation to avoid re-sending has finished normal termination
         sequencing.

      Failed:  The session ended without normal termination sequencing.

   Session Idle Changed:  The TCPCL supports indication when the already transmitted part live/
      idle sub-state of the
   bundle.  In addition, there session changes.  This occurs only when the
      top-level session state is no explicit flow control on "Established".  The session transitions
      from Idle to Live at the TCPCL
   layer.

   A TCPCL receiver can interrupt at the transmission start of a bundle at any
   point transfer in time by replying with a XFER_REFUSE message, which causes either
      transfer stream; the sender session transitions from Live to stop transmission of Idle at 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
      end of a certain bundle to interrupt transmission as
   early as possible and thus save transmission capacity for transfer when the other
   bundles.

   For sessions that are idle, a KEEPALIVE message is sent at transfer stream does not have an
      ongoing transfer.  Because TCPCL transmits serially over a
   negotiated interval.  This is used to convey node live-ness TCP
      connection, it suffers from "head of queue blocking" this
      indication provides information during otherwise message-less time intervals.

   A SESS_TERM message about when a session is used to start the closing available
      for immediate transfer start.

   Begin Transmission:  The principal purpose of a the 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 allow a session is
      BP agent to transmit bundle data over an established established, TCPCL
      session.  Transmission request is on a symmetric
   protocol between per-session basis, the peers.  Both sides can start sending data
   segments in a session, and one side's bundle transfer CL
      does not have
   to complete before necessarily perform any per-session or inter-session
      queueing.  Any queueing of transmissions is the other side can start sending data segments on
   its own.  Hence, the protocol allows for a bi-directional mode obligation of
   communication.  Note that in the case of concurrent bidirectional
   transmission, acknowledgment segments MAY be interleaved with data
   segments.

3.2.
      BP agent.

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

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

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

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

   Reception Initialized:  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 TCPCL supports indication to the reciver
      just before any transmssion data validation and analyze final
   states is sent.  This corresponds to
      reception 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     +----------+
   | the XFER_SEGMENT message with the 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) flag set.

   Interrupt Reception:  The session negotiation sequencing is performed either as the active TCPCL allows a BP agent to interrupt an
      individual transfer before it has fully completed (successfully or passive peer, and is illustrated in Figure 8 and Figure 9
   respectively which both share
      not).  Interruption can occur any time after the data validation and analyze final
   states of Figure 10.

   +-------+ reception is
      initialized.

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

                Figure 8: Session Initiation as Active supports positive indication when a
      bundle has been fully transferred from a peer

   +-------+ entity.

   Reception Intermediate Progress:  The TCPCL
   | START |---Messaging-->[PSI]
   +-------+   Available

               Figure 9: Session Initiation as Passive supports positive
      indication of intermediate progress of transfer from the peer

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

             Figure 10: Processing
      entity.  This intermediate progress is at the granularity of Session Initiation (PSI)

   Transfers can occur after each
      transferred segment.  Intermediate reception indication allows a session
      BP agent the chance to inspect bundle header contents before the
      entire bundle is established available, and it's not in thus supports the ending state.  Each "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 occurs within for some local policy reason or when
      a single logical TCPCL session ends before transfer stream between a sender and success.  The TCPCL itself
      does not have a receiver, as illustrated in
   Figure 11 and Figure 12 respectively.

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

                     Figure 11: Transfer sender states

   Notes on transfer sending:

      Pipelining notion of transfers can occur when transfer timeout.

3.2.  TCPCL Session Overview

   First, one node establishes a TCPCL session to the sending entity begins other by
   initiating a
      new transfer while TCP connection in accordance with [RFC0793].  After
   setup of the "Waiting for Ack" state.

                                            +-Receive XFER_SEGMENT-+
   +--------+                               |    Send XFER_ACK     |
   | Stream |                         +-------------+              |
   |  IDLE  |--Receive XFER_SEGMENT-->| In Progress |<-------------+
   +--------+                         +-------------+
                                            |
        +--------Sent Final 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 TCP connection is complete, an initial contact header is
   exchanged in both directions to determine the largest acceptable
   segment size, and a transmitting node can segment establish a transfer into any
   sizes smaller than shared TCPCL version and
   possibly initiate TLS security.  Once contact negotiation is
   complete, TCPCL messaging is available and the receiver's Segment MRU.  It session negotiation 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 set parameters of the TCPCL session.  One of these parameters
   is also advised a Node ID of each TCPCL Entity.  This is used to
   consider the Segment MRU assist in relation to chunking/packetization
   performed routing
   and forwarding messages by TLS, TCP, the BP Agent and any intermediate network-layer nodes.

   Minimum Overhead  For a simple network expected to exchange
      relatively small bundles, is part of the Segment MRU can be set to be
      identical to
   authentication capability provided by TLS.

   Once negotiated, the Transfer MRU which indicates that all transfers
      can be sent with parameters of a single data segment (i.e. no actual
      segmentation).  If the network is closed TCPCL session cannot change and all transmitters are
      known to follow
   if there is a single-segment transfer policy, then receivers
      can avoid the necessity of segment reassembly.  Because this desire by either peer to transfer data under different
   parameters then a new session must be established.  This makes CL
      operates over
   logic simpler but relies on the assumption that establishing a TCP stream, which suffers from a form of head-of-
      queue blocking between messages, while one node
   connection is transmitting a
      single XFER_SEGMENT message it lightweight enough that TCP connection overhead is not able
   negligable compared to transmit any
      XFER_ACK or XFER_REFUSE for any associated received transfers.

   Predictable Message Sizing  In situations where TCPCL data sizes.

   Once the maximum message
      size TCPCL session is desired to be well-controlled, the Segment MRU established and configured in this way,
   bundles can be set
      to the largest acceptable size (the message size less transferred in either direction.  Each transfer is
   performed by an sequence of logical segments of data within
   XFER_SEGMENT
      header size) and transmitters messages.  Multiple bundles can always segment be transmitted
   consecutively in a transfer into
      maximum-size chunks 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.
   There is no larger than fundamental limit on the Segment MRU.  This
      guarantees that any number of TCPCL sessions which a
   single XFER_SEGMENT will not monopolize node can establish beyond the limit imposed by the number of
   available (ephemeral) TCP stream for too long, which would prevent outgoing XFER_ACK and
      XFER_REFUSE associated with received transfers.

   Dynamic Segmentation  Even after negotiation ports of a Segment MRU the passive peer.

   A feature of this protocol is for
      each receiving node, the actual transfer segmentation only needs receiving node to guarantee than any individual segment is no larger than that
      MRU.  In a situation where network "goodput" send
   acknowledgment (XFER_ACK) messages as bundle data segments arrive.
   The rationale behind these acknowledgments is dynamic, to enable the
      transfer segmentation size can also be dynamic in order sender
   node to control
      message transmission duration.

   Many other policies can be established determine how much of the bundle has been received, so that
   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 case the session is interrupted, it can apply further nuance perform reactive
   fragmentation to transfer
   policies and policy negotiation.

3.4.  Example Message Exchange

   The following figure depicts avoid re-sending the protocol exchange for a simple
   session, showing already transmitted part of the session establishment and
   bundle.  In addition, there is no explicit flow control on the TCPCL
   layer.

   A TCPCL receiver can interrupt 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 at any
   point in time by replying with a XFER_REFUSE message, which causes
   the sending node can transmit multiple XFER_SEGMENT
   messages without waiting for sender to stop transmission of the corresponding XFER_ACK responses. associated bundle (if it
   hasn't already finished transmission) Note: This enables pipelining of messages on a transfer stream.  Although
   this example only demonstrates cross-
   layer optimization in that it allows a single bundle transmission, receiver that detects that it is
   also possible
   already has received a certain bundle to pipeline multiple XFER_SEGMENT messages for
   different bundles without necessarily waiting interrupt transmission as
   early as possible and thus save transmission capacity for XFER_ACK messages 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 ending of a TCPCL session
   (see Section 6.1).  During shutdown sequencing, in-progress transfers
   can be returned for each one.  However, interleaving data segments
   from different bundles 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 allowed.

   No errors to close a TCPCL session while there are no transfers queued or rejections
   in-progress.

   Once a session is 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.

3.3.  TCPCL States and Transitions

   The states of a nominal TCPCL session (i.e. without session failures)
   are shown indicated in this example.

                Entity A                             Entity B
                ========                             ========
       +-------------------------+ Figure 4.

     +-------+
     |     Contact Header START | ->      +-------------------------+
       +-------------------------+      <-
     +-------+
         |
     TCP Establishment
         |
         V
   +-----------+            +---------------------+
   |    TCP    |----------->|  Contact Header / Session  |
                                           +-------------------------+
       +-------------------------+
   |        SESS_INIT Connected | ->      +-------------------------+
       +-------------------------+      <-            |        SESS_INIT     Negotiation     |
                                           +-------------------------+

       +-------------------------+
   +-----------+            +---------------------+
                                       |   XFER_SEGMENT (start)
          +-----Session Parameters-----+
          | ->         Negotiated
          V
   +-------------+                     +-------------+
   |     Transfer ID [I1] Established |----New Transfer---->| Established |
   |       Length [L1]   Session   |                     |  Bundle Data 0..(L1-1)   Session   |
       +-------------------------+
       +-------------------------+         +-------------------------+
   |     XFER_SEGMENT    Idle     |<---Transfers Done---|     Live    | ->   <-
   +-------------+                     +-------------+
         |     XFER_ACK (start)                                    |
         +------------------------------------+
         |     Transfer ID [I1]
   [SESSTERM] Exchange
         |
         V
   +-------------+
   |     Transfer ID [I1] Established |                    +-------------+
   |       Length   [L2]   Session   |----Transfers------>|     TCP     |
   |        Length   [L1]   Ending    |
       |Bundle Data L1..(L1+L2-1)|         +-------------------------+
       +-------------------------+
       +-------------------------+         +-------------------------+      Done          |    XFER_SEGMENT (end) Terminating | ->   <-
   +-------------+                    +-------------+
                                              |         XFER_ACK
        +----------TCP Close Message----------+
        |
        V
    +-------+
    |     Transfer ID [I1]  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 "Ending" means no new transfers will be allowed.

   Contact negotation involves exchanging a Contact Header (CH) in both
   directions and deriving a negotiated state from the two headers.  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 the "[PCH]" activity of Figure 7 and the "[TCPCLOSE]"
   activity which indicates TCP connection close.  Successful
   negotiation results in one of the Session Initiation "[SI]"
   activities being performed.

   +-------+
   | START |
   +-------+
       |
   TCP Connecting
       V
   +-----------+
   |    TCP    |            +---------+
   | Connected |--Send CH-->| Waiting |--Timeout-->[TCPCLOSE]
   +-----------+            +---------+
                                 |
                             Recevied CH
                                 V
                               [PCH]

                Figure 5: Contact Initiation as Active peer

   +-----------+             +---------+
   |   TCP     |--Wait for-->| Waiting |--Timeout-->[TCPCLOSE]
   | Connected |     CH      +---------+
   +-----------+                  |
                             Received CH
                                  V
                          +-----------------+
                          | Preparing reply |--Send CH-->[PSI]
                          +-----------------+

               Figure 6: Contact Initiation as Passive peer

   +-----------+
   |  Peer CH  |
   | available |
   +-----------+
         |
    Validate and
     Negotiate
         V
    +------------+
    | Negotiated |----Failure---->[TCPCLOSE]
    +------------+                      ^
       |       |                        |
     No TLS    +----Negotiate---+       |
       V               TLS      |    Failure
     +-----------+              V       |
     |   TCPCL   |            +---------------+
     | Messaging |<--Success--| TLS Finished |
     | Available |            +---------------+
     +-----------+

               Figure 7: Processing of Contact Header [PCH]

   Session negotation involves exchanging a session initialization
   (SESS_INIT) message in both directions and deriving a negotiated
   state from the two messages.  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.  The validation
   here includes certificate validation and authentication when TLS is
   used for the session.

   +-----------+
   |   TCPCL   |                   +---------+
   | Messaging |--Send SESS_INIT-->| Waiting |--Timeout-->[SESSTERM]
   | Available |                   +---------+
   +-----------+                       |
                               Recevied SESS_INIT
                                       |
                                       V
                                     [PSI]

             Figure 8: Session Initiation [SI] as Active peer

 +-----------+
 |   TCPCL   |                  +---------+
 | Messaging |----Wait for ---->| Waiting |--Timeout-->[SESSTERM]
 | Available |    SESS_INIT     +---------+
 +-----------+                       |
                             Recevied SESS_INIT
                                     |
                             +-----------------+
                             | Preparing reply |--Send SESS_INIT-->[PSI]
                             +-----------------+

             Figure 9: Session Initiation [SI] as Passive peer

   +----------------+
   | Peer SESS_INIT |
   |   available    |
   +----------------+
           |
      Validate and
       Negotiate
           V
      +------------+
      | Negotiated |---Failure--->[SESSTERM]
      +------------+
           |
        Success
           V
     +--------------+
     | Established  |
     | Session Idle |
     +--------------+

             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_SEGMENT--+
   +--------+                             |                     |
   | Stream |                       +-------------+             |
   |  Idle  |---Send XFER_SEGMENT-->| In Progress |<------------+
   +--------+                        +-------------+
                                          |
        +---------All segments sent-------+
        |
        V
   +---------+                       +--------+
   | Waiting |---- Receive Final---->| Stream |
   | for 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_SEGMENT-+
   +--------+                               |    Send XFER_ACK     |
   | Stream |                         +-------------+              |
   |  Idle  |--Receive XFER_SEGMENT-->| In Progress |<-------------+
   +--------+                         +-------------+
                                            |
        +--------Sent Final XFER_ACK--------+
        |
        V
   +--------+
   | Stream |
   |  Idle  |
   +--------+

                    Figure 12: Transfer receiver states

   Session termination involves one entity initiating the termination of
   the session and the other entity acknowledging the termination.  For
   either entity, it is the sending of the SESS_TERM message which
   transitions the session to the ending substate.  While a session is
   being terminated only in-progress transfers can be completed and no
   new transfers can be started.

   +-----------+                   +---------+
   |  Session  |--Send SESS_TERM-->| Session |
   | Live/Idle |                   | Ending  |
   +-----------+                   +---------+

       Figure 13: Session Termination [SESSTERM] from the Initiator

   +-----------+                   +---------+
   |  Session  |--Send SESS_TERM-->| Session |
   | Live/Idle |                   | Ending  |
   +-----------+<------+           +---------+
         |             |
    Receive SESS_TERM  |
         |             |
         +-------------+

       Figure 14: Session Termination [SESSTERM] from the Responder

3.4.  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 the
   two extremes of minimum overhead (large MRU, single-segment) and
   predictable message sizing (small MRU, highly segmented).  Different
   policies can be applied to each transfer stream to and from from any
   particular node.  Additionally, future header and transfer extension
   types can apply further nuance to transfer policies and policy
   negotiation.

3.5.  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 can transmit multiple XFER_SEGMENT
   messages without 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_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 15: 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 back-off mechanism
   to increase this delay in case of repeated failures.  The upper limit
   on a re-attempt back-off is implementation defined but SHOULD be no
   longer than one minute before signaling to the BP agent that a
   connection cannot be made.

   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.  Because the TCPCL
   protocol version in use is part of the initial contact header, nodes
   using TCPCL version 4 can coexist on a network with nodes using
   earlier TCPCL versions (with some negotiation needed for
   interoperation as described in Section 4.3).

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 16: 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 TCPCL).

   Flags:  A one-octet field of single-bit flags, interpreted according
      to the descriptions in Table 1.  All reserved header flag bits
      SHALL be not set by the sender.  All reserved header flag bits
      SHALL be ignored by the receiver.

   +----------+--------+-----------------------------------------------+
   | Name     |     Transfer ID [I1] Code   | Description                                   |        Length   [L3]
   +----------+--------+-----------------------------------------------+
   | CAN_TLS  |      Length   [L1+L2] 0x01   |
       |Bundle Data If bit is set, indicates that the sending     |         +-------------------------+
   |    (L1+L2)..(L1+L2+L3-1)|
       +-------------------------+
                                           +-------------------------+
                                        <-          |      XFER_ACK (end)        | peer is capable of TLS security.              |     Transfer ID [I1]
   |          |     Length   [L1+L2+L3]        |
                                           +-------------------------+

       +-------------------------+                                               |       SESS_TERM
   | ->      +-------------------------+
       +-------------------------+      <- Reserved |        SESS_TERM others |
                                           +-------------------------+

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

                       Table 1: Contact Header Flags

4.3.  Contact Validation and Negotiation

   Upon reception of 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 contact header, each node follows the Registered Port number for following
   procedures to ensure the
   TCP convergence layer.  Other destination port numbers MAY be used
   per local configuration.  Determining a peer's destination port
   number (if different from validity of the registered TCPCL port number) is up session and to
   the implementation.  Any source port number MAY be used
   negotiate values for TCPCL
   sessions.  Typically an operating system assigned number in the TCP
   Ephemeral range (49152-65535) is used. session parameters.

   If the entity magic string is unable to establish a TCP connection for any reason,
   then it not present or is an implementation matter to determine how to handle the
   connection failure.  An entity MAY decide to re-attempt to establish not valid, the connection.  If it does so, it MUST NOT overwhelm its target with
   repeated connection attempts.  Therefore, the entity
   MUST retry be terminated.  The intent of the magic string is to provide
   some protection against an inadvertent TCP connection setup no earlier by a different
   protocol than some delay time from the last
   attempt, and it SHOULD use a (binary) exponential backoff mechanism
   to increase this delay one described in case this document.  To prevent a flood
   of repeated failures.

   Once connections from a TCP misconfigured application, an entity
   MAY elect to hold an invalid connection open and idle for some time
   before ending it.

   The first negotiation is established, each entity MUST immediately
   transmit a contact header over on the TCP connection. TCPCL protocol version to use.  The format of
   active node always sends its Contact Header first and waits for a
   response from the
   contact header is described passive node.  The active node can repeatedly
   attempt different protocol versions in Section 4.2.

4.2. descending order until the
   passive node accepts one with a corresponding Contact Header

   Once a TCP connection is established, both parties exchange reply.
   Only upon response of a contact
   header.  This section describes Contact Header from the format of passive node is the contact header
   TCPCL protocol version established and parameter negotiation begun.

   During contact initiation, the meaning of its fields.

   Upon receipt of active TCPCL node SHALL send the contact header, both entities perform
   highest TCPCL protocol version on a first session attempt for a TCPCL
   peer.  If the
   validation and negotiation procedures defined in Section 4.3.  After
   receiving active node receives a Contact Header with a different
   protocol version than the contact header from one sent earlier on the other entity, either entity MAY
   refuse TCP connection, the session by sending
   TCP connection SHALL be terminated.  If the active node receives a
   SESS_TERM message with an appropriate reason code.

   The format for of "Version Mismatch", that node MAY
   attempt further TCPCL sessions with the Contact Header is peer using earlier protocol
   version numbers in decreasing order.  Managing multi-TCPCL-session
   state such 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 this is an implementation matter.

   If the passive node receives a contact header containing a version
   that is greater than the use current version of each the TCPCL that the node
   implements, then the node SHALL shutdown the session with a reason
   code of these "Version mismatch".  If the passive node receives a contact
   header fields.

   The fields with a version that is lower than the version of the contact header are:

   magic:  A four-octet field protocol
   that always contains the octet sequence
      0x64 0x74 0x6E 0x21, i.e., node implements, the text string "dtn!" in US-ASCII (and
      UTF-8).

   Version:  A one-octet field value containing node MAY either terminate the value 4 (current
      version session
   (with a reason code of "Version mismatch") or the protocol).

   Flags:  A one-octet field of single-bit flags, interpreted according node MAY adapt its
   operation to conform to the descriptions in Table 1.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CAN_TLS  | 0x01   | If bit is set, indicates that older version of the sending     |
   |          |        | peer protocol.  The
   decision of version fall-back is capable 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 security.              |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                       Table 1: Contact Header Flags

4.3.  Contact Validation and Negotiation

   Upon reception of
   is used within the contact header, each node follows TCPCL it affects the following
   procedures entire session.  Once
   established, there is no mechanism available to ensure the validity of the downgrade a TCPCL
   session and to
   negotiate values for the session parameters. non-TLS operation.  If the magic string is not present or this is not valid, desired, the connection entire TCPCL
   session MUST be terminated. terminated and a new non-TLS-negotiated session
   established.

4.4.1.  TLS Handshake

   The intent use of the magic string TLS is to provide
   some protection against an inadvertent TCP connection by a different
   protocol than negotated using the one Contact Header as described in this document.  To prevent a flood
   of repeated connections from a misconfigured application, an entity
   MAY elect to hold
   Section 4.3.  After negotiating an invalid connection open Enable TLS parameter of true, and idle for some time
   before closing it.

   The first negotiation is on the any other TCPCL protocol version to use.  The
   active node always sends its Contact Header first and waits for a
   response from messages are sent within the passive node.  The active node can repeatedly
   attempt different protocol versions session, the
   session entities SHALL begin a TLS handshake in descending order until accordance with TLS
   1.2 [RFC5246] or any successors that are compatible with TLS 1.2.  By
   convention, this protocol uses the
   passive node accepts one with a corresponding Contact Header reply.
   Only upon response which initiated the
   underlying TCP connection as the "client" role of a Contact Header from the passive node TLS handshake
   request.

   The TLS handshake, if it occurs, is considered to be part of the
   TCPCL protocol version established and parameter negotiation begun.

   During
   contact initiation, negotiation before the active TCPCL session itself is established.
   Specifics about sensitive data exposure are discussed in Section 8.

   The parameters within each TLS negotiation are implementation
   dependent but any TCPCL node SHALL send the
   highest TCPCL protocol version on a first session attempt for follow all recommended practices
   of BCP 195 [RFC7525], or any updates or successors that become part
   of BCP 195.  The TLS handshake SHOULD include a TCPCL
   peer.  If Server Name
   Indication from the active node receives a Contact Header with peer.  The TLS handshake SHALL request a different
   protocol version than the one sent earlier on
   client-side certificate to allow authentication of the TCP connection, active peer.
   The passive peer SHOULD supply a certificate within the
   TCP connection SHALL be terminated.  If TLS handshake
   to allow authentication of its side of the session.  The active node receives peer
   SHOULD supply a
   SESS_TERM message with reason certificate within the TLS handshake to allow
   authentication of "Version Mismatch", that node MAY
   attempt further TCPCL sessions its side of the session.  All certificates supplied
   during TLS handshake SHALL conform with the peer using earlier protocol
   version numbers in decreasing order.  Managing multi-TCPCL-session
   state such as this profile of [RFC5280].
   When a certificate is an implementation matter.

   If supplied during TLS handshake, the passive node receives a contact header containing a version full
   certification chain SHOULD be included unless security policy
   indicates that is greater than the current version unnecessary.

   If a TLS handshake cannot negotiate a TLS session, both entities of
   the protocol that the
   node implements, then the node TCPCL session SHALL shutdown close 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 a
   reason code of "Version mismatch".  If the passive node receives a
   contact header further TCP communication with an untrusted peer.

   After a version that TLS session is lower than the version of the
   protocol that the node implements, the node MAY either terminate successfully established, the
   session (with active peer
   SHALL send a reason code SESS_INIT message to begin session negotiation.  This
   session negotation and all subsequent messaging are secured.

4.4.2.  TLS Authentication

   Using X.509 certificates exchanged during the TLS handshake, each of "Version mismatch") or
   the node MAY
   adapt its operation to conform entities can attempt to authenticate its peer at the older version of network
   layer (host name and address) and at the protocol. application layer (BP Node
   ID).  The decision of version fall-back is an implementation matter.

4.4.  Session Security

   This version of Node ID exchanged in the TCPCL supports establishing a Transport Layer
   Security (TLS) session within an existing TCP connection.  When TLS Session Initialization is likely
   to be used within the TCPCL it affects by the entire session.  Once
   established, there BP agent for making transfer and routing decisions,
   so attempting host name validation is no mechanism available to downgrade a TCPCL
   session to non-TLS operation.  If this optional while attempting Node
   ID validation is desired, the entire TCPCL
   session MUST be terminated and a new non-TLS-negotiated session
   established. required.  The use of TLS logic for attempting validation is negotated using
   separate from the Contact Header as described in
   Section 4.3.  After negotiating an Enable TLS parameter logic for handling the result of true, and
   before validation, which
   is based on local security policy.

   Any certificate received during TLS handshake SHALL be validated up
   to one or more trusted certificate authority (CA) certificates.  If
   certificate validation fails or if security policy disallows a
   certificate for any other TCPCL messages are sent within reason, the session, entity SHOULD terminate the session entities SHALL begin
   (with a reason code of "Contact Failure").

   Immediately after the TLS handshake handshake, each side of the TCP connection
   SHOULD perform host name validation of its peer in accordance with
   [RFC5246].
   [RFC6125] unless it is not needed by security policy.  The parameters within each TLS negotiation are
   implementation dependent but any TCPCL node active
   peer SHALL follow all
   recommended practices of [BCP195], or any updates or successors that
   become part attempt to authenticate the host name (of the passive
   peer) used to initiate the TCP connection.  The active peer MAY
   attempt to authenticate the IP address of [BCP195].  By convention, this protocol uses the node
   which initiated other side of the underlying TCP connection as
   connection.  The passive peer SHALL attempt to authenticate the "client" role IP
   address of the TLS handshake request. other side of the TCP connection.  The TLS handshake, if it occurs, is considered passive peer
   MAY use the IP address to be part resolve one or more host names of the
   contact negotiation before
   active peer and attempt to authenticate those.  If host name
   validation fails (including failure because the certificate does not
   contain any DNS-ID), the entity SHOULD terminate 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 (with a TLS session, both entities
   reason code of "Contact Failure") unless security policy allows an
   unauthticated host.

   Immediately before Session Parameter Negotiation, each side of the TCPCL
   session SHALL terminate perform Node ID validation of its peer as described
   below.  Node ID validation SHALL succeed if the TCP connection.  At this point associated
   certificate contains a subjectAltName entry of type
   uniformResourceIdentifier whose value matches the Node ID of the
   TCPCL session has not yet been established so there is no TCPCL
   session to terminate. entity.  URI matching of Node IDs SHALL use the URI comparison
   logic of [RFC3986] and scheme-based normalization of those schemes
   specified in [I-D.ietf-dtn-bpbis].  This also avoids any potential security issues
   assoicated with further TCP communication with an untrusted peer.

   After a TLS session is successfully established, similar to the active peer
   SHALL send a SESS_INIT message URI-ID of

   [RFC6125] but does not require any structure to begin session negotiation.  This the scheme-specific-
   part of the URI.  If Node ID validation fails (including failure
   because the certificate does not contain any subjectAltName URI), the
   entity SHOULD terminate the session negotation and all subsequent messaging are secured.

4.4.2. (with a reason code of "Contact
   Failure") unless security policy allows an unauthticated node.

4.4.3.  Example TLS Initiation

   A summary of a typical CAN_TLS usage TLS use is shown in the sequence in Figure 15 17
   below.

                Entity A                             Entity B
                ========                             ========
               active peer                         passive peer

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

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

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

           ... secured

                         Host name validation occurs.
                      Secured TCPCL messaging, starting with messaging can begin.

       +-------------------------+         +-------------------------+
       |        SESS_INIT ...        | ->   <- |        SESS_INIT        |
       +-------------------------+         +-------------------------+

                          Node ID validation occurs.
                  Session is established, transfers can begin.

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

   Figure 15: 17: A simple visual example of TCPCL TLS Establishment between
                               two entities

4.5.  Message Type Codes Header

   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: 18: 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.

   +--------------+------+---------------------------------------------+
   | Name         | Code | Description                                 |
   +--------------+------+---------------------------------------------+
   | SESS_INIT    | 0x07 | Contains the session parameter inputs from  |
   |              |      | one of the entities, as described in        |
   |              |      | Section 4.6.                                |
   |              |      |                                             |
   | SESS_TERM    | 0x05 | Indicates that one of the entities          |
   |              |      | participating in the session wishes to      |
   |              |      | cleanly terminate the session, as described |
   |              |      | in Section 6.                               |
   |              |      |                                             |
   | XFER_SEGMENT | 0x01 | Indicates the transmission of a segment of  |
   |              |      | bundle data, as described in Section 5.2.2. |
   |              |      |                                             |
   | XFER_ACK     | 0x02 | Acknowledges reception of a data segment,   |
   |              |      | as described in Section 5.2.3.              |
   |              |      |                                             |
   | XFER_REFUSE  | 0x03 | Indicates that the transmission of the      |
   |              |      | current bundle SHALL be stopped, as         |
   |              |      | described in Section 5.2.4.                 |
   |              |      |                                             |
   | KEEPALIVE    | 0x04 | Used to keep TCPCL session active, as       |
   |              |      | described in Section 5.1.1.                 |
   |              |      |                                             |
   | MSG_REJECT   | 0x06 | 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 as
   described in Section 4.7.

   The format of a SESS_INIT message is as follows in Figure 17. 19.

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

                        Figure 17: 19: 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 transmission and a necessary subsequent KEEPALIVE message
      transmission.

   Segment MRU:  A 64-bit unsigned integer indicating the largest
      allowable 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 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

   Node ID Length and EID Node ID Data:  Together these fields represent a variable-
      length
      variable-length text string.  The EID Node ID Length is a 16-bit
      unsigned integer indicating the number of octets of EID Node ID Data
      to follow.  A zero EID
      Length zero-length Node ID SHALL be used to indicate the
      lack of EID Node ID rather than a truly empty EID. Node ID.  This case
      allows an entity to avoid exposing EID Node ID information on an
      untrusted network.  A non-zero-length EID Node ID Data SHALL contain
      the UTF-8 encoded EID Node ID of some singleton endpoint in the Entity which sent the sending entity is SESS_INIT
      message.  Every Node ID SHALL be a member, in the canonical format of
      <scheme name>:<scheme-specific part>.  This EID encoding is URI consistent with the
      requirements of [RFC3986] and the URI schemes of
      [I-D.ietf-dtn-bpbis].  The Node ID itself can be authenticated as
      described in Section 4.4.2.

   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 4.8.  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).

4.7.  Session 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 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 to the
      Transfer MRU and Segment MRU, respectively, of the recevied
      contact header.  A transmitting peer can send individual segments
      with any size smaller than the Segment MTU, depending on local
      policy, dynamic network conditions, etc.  Determining the size of
      each transmitted segment is an implementation matter.

   Session Keepalive:  Negotiation of the Session Keepalive parameter is
      performed by taking the minimum of this two contact headers'
      Keepalive Interval.  The Session Keepalive interval is a parameter
      for the behavior described in Section 5.1.1.

   Enable TLS:  Negotiation of the Enable TLS parameter is performed by
      taking the logical AND of the two contact headers' CAN_TLS flags.
      A local security policy is then applied to determine of the
      negotated value of Enable TLS is acceptable.  It can be a
      reasonable security policy to both require or disallow the use of
      TLS depending upon the desired network flows.  Because this state
      is negotiated over an unsecured medium, there is a risk of a TLS
      Stripping as described in Section 8.  If the Enable TLS state is
      unacceptable, the node SHALL terminate the session with a reason
      code of "Contact Failure".  Note that this contact failure reason
      is different than a failure of TLS handshake or TLS authentication
      after an agreed-upon and acceptable Enable TLS state.  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 is completed (which
   includes a possible completed TLS handshake of the connection to use
   TLS), this protocol defines no additional mechanism to change the
   parameters of an established session; to effect such a change, the
   TCPCL session MUST be terminated and a new session established.

4.8.  Session Extension Items

   Each of the Session Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated in Figure 18. 20.

   The fields of the Session Extension Item are:

   Flags:  A one-octet field containing generic bit flags about the
      Item, which are listed in Table 3.  All reserved header flag bits
      SHALL be not set by the sender.  All reserved header flag bits
      SHALL be ignored by the receiver.  If a TCPCL entity receives a
      Session Extension Item with an unknown Item Type and the CRITICAL
      flag set, the entity SHALL close the TCPCL session with SESS_TERM
      reason code of "Contact 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 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 16-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...                                 |
     +---------------+---------------+---------------+---------------+

                 Figure 18: 20: Session Extension Item Format

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | CRITICAL | 0x01   | If bit is set, indicates that the receiving   |
   |          |        | peer must handle the 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 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 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 keepalive 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. 21.

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

                 Figure 19: 21: 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 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.  Data Transmission (XFER_SEGMENT)

   Each bundle is transmitted in one or more data segments.  The format
   of a XFER_SEGMENT message follows in Figure 20. 22.

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

                Figure 20: 22: Format of XFER_SEGMENT Messages

   The fields of the XFER_SEGMENT message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 5.  All reserved header
      flag bits SHALL be not set by the sender.  All reserved header
      flag bits SHALL be ignored by the receiver.

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

   Transfer Extension Length and Transfer Extension Items:  Together
      these fields represent protocol extension data for this
      specification.  The Transfer Extension Length and Transfer
      Extension Item fields SHALL only be present when the 'START' flag
      is set on the message.  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 Transfer Extension Item
      is within a consistent data container as described in
      Section 5.2.5.  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).

   Data length:  A 64-bit unsigned integer indicating the number of
      octets in the Data contents to follow.

   Data contents:  The variable-length data payload of the message.

   +----------+--------+-----------------------------------------------+
   | Name     | Code   | Description                                   |
   +----------+--------+-----------------------------------------------+
   | END      | 0x01   | If bit is set, indicates that this is the     |
   |          |        | last segment of the transfer.                 |
   |          |        |                                               |
   | START    | 0x02   | If bit is set, indicates that this is the     |
   |          |        | first segment of the transfer.                |
   |          |        |                                               |
   | Reserved | others |
   +----------+--------+-----------------------------------------------+

                        Table 5: XFER_SEGMENT Flags

   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 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 an XFER_ACK message follows in Figure 21. 23.

                      +-----------------------------+
                      |       Message Header        |
                      +-----------------------------+
                      |     Message Flags (U8)      |
                      +-----------------------------+
                      |      Transfer ID (U64)      |
                      +-----------------------------+
                      | Acknowledged length (U64)   |
                      +-----------------------------+

                  Figure 21: 23: Format of XFER_ACK Messages

   The fields of the XFER_ACK message are:

   Message Flags:  A one-octet field of single-bit flags, interpreted
      according to the descriptions in Table 5.  All reserved header
      flag bits SHALL be not set by the sender.  All reserved header
      flag bits SHALL be ignored by the receiver.

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

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

   A receiving TCPCL node SHALL send an XFER_ACK message in response 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 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 the indicated transfer.
   The 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 sending node transmits four segments of
   bundle data with lengths 100, 200, 500, and 1000, respectively.
   After receiving the first segment, the node sends an acknowledgment
   of length 100.  After the second segment is received, the node sends
   an acknowledgment of length 300.  The third and fourth
   acknowledgments are of length 800 and 1800, respectively.

5.2.4.  Transfer Refusal (XFER_REFUSE)

   The TCPCL supports a mechanism by which a receiving node can indicate
   to the sender that it does not want to receive the corresponding
   bundle.  To do so, upon receiving an XFER_SEGMENT message, the node
   MAY transmit a XFER_REFUSE message.  As data segments and
   acknowledgments MAY cross on the wire, the bundle that is being
   refused SHALL be identified by the Transfer ID of the refusal.

   There is no required relation between the Transfer MRU of a TCPCL
   node (which is supposed to represent a firm limitation of what the
   node will accept) and sending of 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 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 any
   XFER_SEGMENT message.  The sender MUST be prepared for this and MUST
   associate the refusal with the correct bundle via the Transfer ID
   fields.

   The format of the XFER_REFUSE message is as follows in Figure 22. 24.

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

                 Figure 22: 24: 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       | Code | Description                                   |
   +------------+------+-----------------------------------------------+
   | Unknown    | 0x00 | Reason for refusal is unknown or not          |
   |            |      | specified.                                    |
   |            |      |                                               |
   | Extension  | 0x01 | A failure processing the Transfer Extension   |
   | Failure    |      | Items ha occurred.                            |
   |            |      |                                               |
   | Completed  | 0x02 | The receiver already has the complete bundle. |
   |            |      | The sender MAY consider the bundle as         |
   |            |      | completely received.                          |
   |            |      |                                               |
   | No         | 0x03 | The receiver's resources are exhausted. The   |
   | Resources  |      | sender SHOULD apply reactive bundle           |
   |            |      | fragmentation before retrying.                |
   |            |      |                                               |
   | Retransmit | 0x04 | The receiver has encountered a problem that   |
   |            |      | requires the bundle to be retransmitted in    |
   |            |      | its entirety.                                 |
   +------------+------+-----------------------------------------------+

                     Table 6: 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.

   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 of any partially sent XFER_SEGMENT
   message.  There is no way to interrupt an individual TCPCL message
   partway through sending it.  The sender MUST NOT commence
   transmission of any further segments of the refused bundle
   subsequently.  Note, however, that this requirement does not ensure
   that an entity will not receive another XFER_SEGMENT for the same
   bundle after transmitting a XFER_REFUSE message since messages MAY
   cross on the wire; if this happens, subsequent segments of the bundle
   SHALL also be refused with a XFER_REFUSE message.

   Note: If a bundle transmission is aborted in this way, the receiver
   MAY not receive a segment with the 'END' flag set to '1' for the
   aborted bundle.  The beginning of the next bundle is identified by
   the 'START' bit set to '1', indicating the start of a new transfer,
   and with a distinct Transfer ID value.

5.2.5.  Transfer Extension Items

   Each of the Transfer Extension Items SHALL be encoded in an identical
   Type-Length-Value (TLV) container form as indicated in Figure 23. 25.

   The fields of the Transfer Extension Item are:

   Flags:  A one-octet field containing generic bit flags about the
      Item, which are listed in Table 7.  All reserved header flag bits
      SHALL be not set by the sender.  All reserved header flag bits
      SHALL be ignored by the receiver.  If a TCPCL node receives a
      Transfer 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 16-bit unsigned integer field containing the type of
      the extension item.  This specification allocates an IANA registry
      for such codes (see Section 9.4).

   Item Length:  A 16-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 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
     +---------------+---------------+---------------+---------------+
     |  Item Flags   |           Item Type           | Item Length...|
     +---------------+---------------+---------------+---------------+
     | length contd. | Item Value...                                 |
     +---------------+---------------+---------------+---------------+

                 Figure 23: 25: Transfer Extension Item Format

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

                  Table 7: Transfer Extension Item Flags

5.2.5.1.  Transfer Length Extension

   The purpose of the Transfer Length extension is to allow entities to
   preemptively refuse bundles that would exceed their resources or to
   prepare storage on the receiving node for the upcoming bundle data.

   Multiple Transfer Length extension items SHALL NOT occur within the
   same transfer.  The lack of a Transfer Length extension item in any
   transfer SHALL NOT imply anything about the potential length of the
   transfer.  The Transfer Length extension SHALL be assigned transfer
   extension type ID 0x0001.

   If a transfer occupies exactly one segment (i.e. both START and END
   bits are set) the Transfer Length extension SHOULD NOT be present.
   The extension does not provide any additional information for single-
   segment transfers.

   The format of the Transfer Length data is as follows in Figure 24. 26.

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

                 Figure 24: 26: Format of Transfer Length data

   The fields of the Transfer Length extension are:

   Total Length:  A 64-bit unsigned integer indicating the size of the
      data-to-be-transferred.  The Total Length field 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 Total Length value, the 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 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_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. 27.

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

                  Figure 25: 27: 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.  All reserved header
      flag bits SHALL be not set by the sender.  All reserved header
      flag bits SHALL be ignored by the receiver.

   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         | Code | Description                                 |
   +--------------+------+---------------------------------------------+
   | Unknown      | 0x00 | A termination reason is not available.      |
   |              |      |                                             |
   | Idle timeout | 0x01 | The session is being closed due to          |
   |              |      | idleness.                                   |
   |              |      |                                             |
   | Version      | 0x02 | The node cannot conform to the specified    |
   | mismatch     |      | TCPCL protocol version.                     |
   |              |      |                                             |
   | Busy         | 0x03 | The node is too busy to handle the current  |
   |              |      | session.                                    |
   |              |      |                                             |
   | Contact      | 0x04 | The node cannot interpret or negotiate      |
   | Failure      |      | contact header option.                      |
   |              |      |                                             |
   | Resource     | 0x05 | The node has run into some resource limit   |
   | Exhaustion   |      | and 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 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 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 ending 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 can
   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 it only runs on hosts which provide the
   Python "dbus" library.

8.  Security Considerations

   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 bundle security mechanisms defined in [RFC6257]
   [I-D.ietf-dtn-bpsec] are to be used instead.

   When negotating whether to use TLS security as part of the contact
   header exchange, it is possible for a man-in-the-middle attacker to
   unset the CAN_TLS flag on either side of the exchange.  This leads to
   the "SSL Stripping" attack described in [RFC7457].  If TLS is desired
   for use on any TCPCL network, it is strongly encouraged that the
   security policy disallow use of TCPCL when "Enable TLS" is negotiated
   to false.  This requires that the TLS handshake occurs, regardless of
   the policy-driven parameters of the handshake and policy-driven
   handling of the handshake outcome.

   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].

   The certificates exchanged by TLS enable authentication of peer host
   name and Node ID, but it is possible that a peer either not provide a
   valid certificate or that the certificate does not validate either
   the host name or Node ID of the peer.  Having a CA-validated
   certificate does not alone guarantee the identity of the network host
   or BP node from which the certificate is provided; additional
   validation procedures bind the host name or node ID based on the
   contents of the certificate.  The host name validation is a weaker
   form of authentication, because even if a peer is operating on an
   authenticated network host name it can provide an invalid Node ID and
   cause bundles to be "leaked" to an invalid node.  Especially in DTN
   environments, network names and
   [I-D.ietf-dtn-bpsec] are to addresses of nodes can be used instead.

   Even when using TLS time-
   variable so binding a certificate to secure a Node ID is a more stable
   identity.  Node ID validation ensures that the TCPCL session, peer to which a bundle
   is transferred is in fact the actual
   ciphersuite negotiated between node which the TLS peers MAY BP Agent expects it to
   be.  It is a reasonable policy to skip host name validation if
   certificates can be insecure.  TLS guaranteed to validate the peer's Node ID.  In
   circumstances where certificates can only be used issued to perform authentication without data confidentiality,
   for example.  It network host
   names, Node ID validation is up to security policies within each TCPCL node not possible but it could be reasonable
   to
   ensure assume that the negotiated TLS ciphersuite meets transport security
   requirements.  This a trusted host is identical behavior not going to STARTTLS use in
   [RFC2595]. present an invalid Node
   ID.  Trusting an authenticated host name can be feasable on a network
   secured by a private CA but is not advisable on the Internet when
   using a variety of public CAs.

   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

   Registration procedures referred to in this section are as defined in
   [RFC8126].

   Some of the registries below are created new for TCPCLv4 but share
   code values with have been defined as version specific to
   TCPCLv4, and imports some or all codepoints from TCPCLv3.  This was
   done to disambiguate the use of these values codepoints between TCPCLv3 and
   TCPCLv4 while preserving the semantics of some values. of the codepoints.

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     | TCPCLv4     | 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. 10.  The registration procedure is RFC Required Expert Review 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           | Session Extension Type   |
               +----------------+--------------------------+
               | 0x0000         | Reserved                 |
               |                |                          |
               | 0x0001--0x7FFF | Unassigned               |
               |                |                          |
               | 0x8000--0xFFFF | Private/Experimental Use |
               +----------------+--------------------------+

                  Table 11: 10: 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. 11.  The registration procedure is RFC Required Expert Review 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           | Transfer Extension Type   |
              +----------------+---------------------------+
              | 0x0000         | Reserved                  |
              |                |                           |
              | 0x0001         | Transfer Length Extension |
              |                |                           |
              | 0x0002--0x7FFF | Unassigned                |
              |                |                           |
              | 0x8000--0xFFFF | Private/Experimental Use  |
              +----------------+---------------------------+

                  Table 12: 11: 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. 12.  The
   registration procedure is RFC Required.

                       +-----------+--------------+ Required within the lower range 0x01--
   0xEF.  Values in the range 0xF0--0xFF are reserved for use on private
   networks for functions not published to the IANA.

                 +------------+--------------------------+
                 | Code       | Message Type             |
                       +-----------+--------------+
                 +------------+--------------------------+
                 | 0x00       | Reserved                 |
                 |            |                          |
                 | 0x01       | XFER_SEGMENT             |
                 |            |                          |
                 | 0x02       | XFER_ACK                 |
                 |            |                          |
                 | 0x03       | XFER_REFUSE              |
                 |            |                          |
                 | 0x04       | KEEPALIVE                |
                 |            |                          |
                 | 0x05       | SESS_TERM                |
                 |            |                          |
                 | 0x06       | MSG_REJECT               |
                 |            |                          |
                 | 0x07       | SESS_INIT                |
                 |            |                          |
                 | 0x08--0xf 0x08--0xEF | Unassigned               |
                       +-----------+--------------+
                 |            |                          |
                 | 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                       Table 13: 12: 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. 13.  The registration procedure is RFC Required.

                +------------+---------------------------+ Specification Required
   within the lower range 0x00--0xEF.  Values in the range 0xF0--0xFF
   are reserved for use on private networks for functions not published
   to the IANA.

                 +------------+--------------------------+
                 | Code       | Refusal Reason           |
                +------------+---------------------------+
                 +------------+--------------------------+
                 | 0x00       | Unknown                  |
                 |            |                          |
                 | 0x01       | Extension Failure        |
                 |            |                          |
                 | 0x02       | Completed                |
                 |            |                          |
                 | 0x03       | No Resources             |
                 |            |                          |
                 | 0x04       | Retransmit               |
                 |            |                          |
                 | 0x05--0x07 0x05--0xEF | Unassigned               |
                 |            |                          |
                 | 0x08--0xFF | Reserved for future usage |
                +------------+---------------------------+ 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                    Table 14: 13: 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. 14.  The registration procedure is RFC Required.

                   +------------+---------------------+ Specification Required
   within the lower range 0x00--0xEF.  Values in the range 0xF0--0xFF
   are reserved for use on private networks for functions not published
   to the IANA.

                 +------------+--------------------------+
                 | Code       | Termination Reason       |
                   +------------+---------------------+
                 +------------+--------------------------+
                 | 0x00       | Unknown                  |
                 |            |                          |
                 | 0x01       | Idle timeout             |
                 |            |                          |
                 | 0x02       | Version mismatch         |
                 |            |                          |
                 | 0x03       | Busy                     |
                 |            |                          |
                 | 0x04       | Contact Failure          |
                 |            |                          |
                 | 0x05       | Resource Exhaustion      |
                 |            |                          |
                 | 0x06--0xFF 0x06--0xEF | Unassigned               |
                   +------------+---------------------+
                 |            |                          |
                 | 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                     Table 15: 14: 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. 15.  The registration procedure is RFC Required.

                   +-----------+----------------------+ Specification Required
   within the lower range 0x01--0xEF.  Values in the range 0xF0--0xFF
   are reserved for use on private networks for functions not published
   to the IANA.

                 +------------+--------------------------+
                 | Code       | Rejection Reason         |
                   +-----------+----------------------+
                 +------------+--------------------------+
                 | 0x00       | reserved                 |
                 |            |                          |
                 | 0x01       | Message Type Unknown     |
                 |            |                          |
                 | 0x02       | Message Unsupported      |
                 |            |                          |
                 | 0x03       | Message Unexpected       |
                 |            |                          |
                 | 0x04-0xFF 0x04--0xEF | Unassigned               |
                   +-----------+----------------------+
                 |            |                          |
                 | 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                     Table 16: 15: MSG_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., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-12 draft-ietf-dtn-bpbis-14 (work in progress),
              November 2018.
              August 2019.

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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-09 draft-ietf-dtn-bpsec-10 (work in
              progress), February April 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>.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
              February 2015, <https://www.rfc-editor.org/info/rfc7457>.

   [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 session option to negotiate maximum segment size (per each
      direction).

   o  Renamed "Endpoint ID" to "Node ID" to conform with BPv7
      terminology.

   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  Segments of all new IANA registries are reserved for private/
      experimental use.

   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_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". "shutdown"
      related to TCP connections.

   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: ott@in.tum.de

   Simon Perreault
   Quebec, QC
   Canada

   Email: simon@per.reau.lt