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Versions: (draft-sipos-dtn-tcpclv4) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Delay-Tolerant Networking                                       B. Sipos
Internet-Draft                                           RKF Engineering
Intended status: Standards Track                               M. Demmer
Expires: April 29, 2021                                      UC Berkeley
                                                                  J. Ott
                                                        Aalto University
                                                            S. Perreault
                                                        October 26, 2020


   Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
                       draft-ietf-dtn-tcpclv4-22

Abstract

   This document describes a TCP-based convergence layer (TCPCL) for
   Delay-Tolerant Networking (DTN).  This version of the TCPCL protocol
   resolves implementation issues in the earlier TCPCL Version 3 of
   RFC7242 and updates to the Bundle Protocol (BP) contents, encodings,
   and convergence layer requirements in BP 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.
   This version of TCPCL also includes security and extensibility
   mechanisms.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 29, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Definitions Specific to the TCPCL Protocol  . . . . . . .   5
   3.  General Protocol Description  . . . . . . . . . . . . . . . .   9
     3.1.  Convergence Layer Services  . . . . . . . . . . . . . . .   9
     3.2.  TCPCL Session Overview  . . . . . . . . . . . . . . . . .  11
     3.3.  TCPCL States and Transitions  . . . . . . . . . . . . . .  13
     3.4.  PKIX Environments and CA Policy . . . . . . . . . . . . .  19
     3.5.  Session Keeping Policies  . . . . . . . . . . . . . . . .  20
     3.6.  Transfer Segmentation Policies  . . . . . . . . . . . . .  21
     3.7.  Example Message Exchange  . . . . . . . . . . . . . . . .  22
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .  23
     4.1.  TCP Connection  . . . . . . . . . . . . . . . . . . . . .  24
     4.2.  Contact Header  . . . . . . . . . . . . . . . . . . . . .  25
     4.3.  Contact Validation and Negotiation  . . . . . . . . . . .  26
     4.4.  Session Security  . . . . . . . . . . . . . . . . . . . .  27
       4.4.1.  Entity Identification . . . . . . . . . . . . . . . .  28
       4.4.2.  TLS Handshake . . . . . . . . . . . . . . . . . . . .  29
       4.4.3.  TLS Authentication  . . . . . . . . . . . . . . . . .  30
       4.4.4.  Example TLS Initiation  . . . . . . . . . . . . . . .  32
     4.5.  Message Header  . . . . . . . . . . . . . . . . . . . . .  33
     4.6.  Session Initialization Message (SESS_INIT)  . . . . . . .  34
     4.7.  Session Parameter Negotiation . . . . . . . . . . . . . .  36
     4.8.  Session Extension Items . . . . . . . . . . . . . . . . .  37
   5.  Established Session Operation . . . . . . . . . . . . . . . .  38
     5.1.  Upkeep and Status Messages  . . . . . . . . . . . . . . .  38
       5.1.1.  Session Upkeep (KEEPALIVE)  . . . . . . . . . . . . .  38
       5.1.2.  Message Rejection (MSG_REJECT)  . . . . . . . . . . .  39
     5.2.  Bundle Transfer . . . . . . . . . . . . . . . . . . . . .  40
       5.2.1.  Bundle Transfer ID  . . . . . . . . . . . . . . . . .  41
       5.2.2.  Data Transmission (XFER_SEGMENT)  . . . . . . . . . .  41
       5.2.3.  Data Acknowledgments (XFER_ACK) . . . . . . . . . . .  43
       5.2.4.  Transfer Refusal (XFER_REFUSE)  . . . . . . . . . . .  44
       5.2.5.  Transfer Extension Items  . . . . . . . . . . . . . .  47
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .  49



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     6.1.  Session Termination Message (SESS_TERM) . . . . . . . . .  49
     6.2.  Idle Session Shutdown . . . . . . . . . . . . . . . . . .  52
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  52
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  52
     8.1.  Threat: Passive Leak of Node Data . . . . . . . . . . . .  53
     8.2.  Threat: Passive Leak of Bundle Data . . . . . . . . . . .  53
     8.3.  Threat: TCPCL Version Downgrade . . . . . . . . . . . . .  53
     8.4.  Threat: Transport Security Stripping  . . . . . . . . . .  53
     8.5.  Threat: Weak TLS Configurations . . . . . . . . . . . . .  54
     8.6.  Threat: Certificate Validation Vulnerabilities  . . . . .  54
     8.7.  Threat: Symmetric Key Limits  . . . . . . . . . . . . . .  54
     8.8.  Threat: BP Node Impersonation . . . . . . . . . . . . . .  54
     8.9.  Threat: Denial of Service . . . . . . . . . . . . . . . .  55
     8.10. Alternate Uses of TLS . . . . . . . . . . . . . . . . . .  56
       8.10.1.  TLS Without Authentication . . . . . . . . . . . . .  56
       8.10.2.  Non-Certificate TLS Use  . . . . . . . . . . . . . .  56
     8.11. Predictability of Transfer IDs  . . . . . . . . . . . . .  56
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  57
     9.1.  Port Number . . . . . . . . . . . . . . . . . . . . . . .  57
     9.2.  Protocol Versions . . . . . . . . . . . . . . . . . . . .  57
     9.3.  Session Extension Types . . . . . . . . . . . . . . . . .  58
     9.4.  Transfer Extension Types  . . . . . . . . . . . . . . . .  59
     9.5.  Message Types . . . . . . . . . . . . . . . . . . . . . .  60
     9.6.  XFER_REFUSE Reason Codes  . . . . . . . . . . . . . . . .  61
     9.7.  SESS_TERM Reason Codes  . . . . . . . . . . . . . . . . .  62
     9.8.  MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . .  63
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  64
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  64
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  64
     11.2.  Informative References . . . . . . . . . . . . . . . . .  66
   Appendix A.  Significant changes from RFC7242 . . . . . . . . . .  67
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  69

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



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   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 [I-D.ietf-dtn-bpbis].  This includes the
      concept of bundle fragmentation or bundle encapsulation.  The
      TCPCL transfers bundles as opaque data blocks.





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   o  Mechanisms for locating or identifying other bundle entities
      (peers) within a network or across an internet.  The mapping of
      Node ID to potential convergence layer (CL) protocol and network
      address is left to implementation and configuration of the BP
      Agent and its various potential routing strategies.

   o  Logic for routing bundles along a path toward a bundle's endpoint.
      This CL protocol is involved only in transporting bundles between
      adjacent nodes in a routing sequence.

   o  Policies or mechanisms for issuing Public Key Infrastructure Using
      X.509 (PKIX) certificates; provisioning, deploying, or accessing
      certificates and private keys; deploying or accessing certificate
      revocation lists (CRLs); or configuring security parameters on an
      individual entity or across a network.

   o  Uses of TLS which are not based on PKIX certificate authentication
      (see Section 8.10.2) or in which authentication of both entities
      is not possible (see Section 8.10.1).

   Any TCPCL implementation requires a BP agent to perform those above
   listed functions in order to perform end-to-end bundle delivery.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.1.  Definitions Specific to the TCPCL Protocol

   This section contains definitions specific to the TCPCL protocol.

   Network Byte Order:  Most significant byte first, a.k.a., big endian.
      All of the integer encodings in this protocol SHALL be transmitted
      in network byte order.

   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:





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

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

      *  A TCPCL Entity MAY passively 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 associated with that TCP connection.

   TCPCL Session:  A TCPCL session (as opposed to a TCP connection) is a
      TCPCL communication relationship between two TCPCL entities.  A
      TCPCL session operates within a single underlying TCP connection
      and the lifetime of a TCPCL session is bound to the lifetime of
      that TCP connection.  A TCPCL session is terminated when the TCP
      connection ends, due either to one or both entities actively
      closing the TCP connection or due to network errors causing a
      failure of the TCP connection.  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 (see Figure 3).  From the
      perspective of a TCPCL session, the two transfer streams do not
      logically interact with each other.  The streams do operate over
      the same TCP connection and between the same BP agents, so there
      are logical relationships at those layers (message and bundle
      interleaving respectively).  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




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      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.  Transfers sent over a transfer
      stream are serialized, meaning that one transfer must complete its
      transmission prior to another transfer being started over the same
      transfer stream.  At the stream layer there is no logical
      relationship between transfers in that stream; it's only within
      the BP agent that transfers are fully decoded as bundles.  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 guaranteed to be unique only to a single direction within a
      single Session.

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

   Idle Session:  A TCPCL session is idle while there is no transmission
      in-progress in either direction.  While idle, the only messages
      being transmitted or received are KEEPALIVE messages.

   Live Session:  A TCPCL session is live while there is a transmission
      in-progress in either direction.

   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.

















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

            Figure 2: The relationships between TCPCL entities























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+---------------------------+              +---------------------------+
|    "Own" TCPCL Session    |              |   "Other" TCPCL Session   |
|                           |              |                           |
| +----------------------+  |              |  +----------------------+ |
| |   TCP Connection     |  |              |  |    TCP Connection    | |
| |                      |  |              |  |                      | |
| | +-----------------+  |  |   Messages   |  |  +-----------------+ | |
| | |   Own Inbound   |  +--------------------+  |  Peer Outbound  | | |
| | | Transfer Stream |                          | Transfer Stream | | |
| | |       -----     |<---[Seg]--[Seg]--[Seg]---|       -----     | | |
| | |     RECEIVER    |---[Ack]----[Ack]-------->|      SENDER     | | |
| | +-----------------+                          +-----------------+ | |
| |                                                                  | |
| | +-----------------+                          +-----------------+ | |
| | | Own Outbound    |-------[Seg]---[Seg]----->|  Peer Inbound   | | |
| | | Transfer Stream |<---[Ack]----[Ack]-[Ack]--| 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.  Convergence Layer Services

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

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






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

   Session State Changed:  The TCPCL entity indicates to the BP agent
      when the session state changes.  The top-level session states
      indicated are:

      Connecting:  A TCP connection is being established.  This state
         only applies to the active entity.

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

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

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

      Ending:  The entity sent SESS_TERM message and is in the ending
         state.

      Terminated:  The session has finished normal termination
         sequencing.

      Failed:  The session ended without normal termination sequencing.

   Session Idle Changed:  The TCPCL entity indicates to the BP agent
      when the live/idle sub-state of the session changes.  This occurs
      only when the top-level session state is "Established".  The
      session transitions from Idle to Live at the at the start of a
      transfer in either transfer stream; the session transitions from
      Live to Idle at the end of a transfer when the other transfer
      stream does not have an ongoing transfer.  Because TCPCL transmits
      serially over a TCP connection it suffers from "head of queue
      blocking," so a transfer in either direction can block an
      immediate start of a new transfer in the session.

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

   Transmission Success:  The TCPCL entity indicates to the BP agent
      when a bundle has been fully transferred to a peer entity.



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   Transmission Intermediate Progress:  The TCPCL entity indicates to
      the BP agent on intermediate progress of transfer to a peer
      entity.  This intermediate progress is at the granularity of each
      transferred segment.

   Transmission Failure:  The TCPCL entity indicates to the BP agent on
      certain reasons for bundle transmission failure, notably when the
      peer entity rejects the bundle or when a TCPCL session ends before
      transfer success.  The TCPCL itself does not have a notion of
      transfer timeout.

   Reception Initialized:  The TCPCL entity indicates to the receiving
      BP agent just before any transmission data is sent.  This
      corresponds to reception of the XFER_SEGMENT message with the
      START flag of 1.

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

   Reception Success:  The TCPCL entity indicates to the BP agent when a
      bundle has been fully transferred from a peer entity.

   Reception Intermediate Progress:  The TCPCL entity indicates to the
      BP agent on 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 entity indicates to the BP agent on
      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.

3.2.  TCPCL Session Overview

   First, one node establishes a TCPCL session to the other by
   initiating a TCP connection in accordance with [RFC0793].  After
   setup of the TCP connection is complete, an initial Contact Header is
   exchanged in both directions to establish a shared TCPCL version and
   negotiate the use of TLS security (as described in Section 4).  Once
   contact negotiation is complete, TCPCL messaging is available and the
   session negotiation is used to set parameters of the TCPCL session.
   One of these parameters is a Node ID that each TCPCL Entity is acting



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   as.  This is used to assist in routing and forwarding messages by the
   BP Agent and is part of the authentication capability provided by
   TLS.

   Once negotiated, the parameters of a TCPCL session cannot change and
   if there is a desire by either peer to transfer data under different
   parameters then a new session must be established.  This makes CL
   logic simpler but relies on the assumption that establishing a TCP
   connection is lightweight enough that TCP connection overhead is
   negligible compared to TCPCL data sizes.

   Once the TCPCL session is established and configured in this way,
   bundles can be transferred in either direction.  Each transfer is
   performed by segmenting the transfer data into one or more
   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 the same peers.
   There is no fundamental limit on the number of TCPCL sessions which a
   single node can establish beyond the limit imposed by the number of
   available (ephemeral) TCP ports of the active entity.

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

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

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

   A SESS_TERM message is used to initiate the ending of a TCPCL session
   (see Section 6.1).  During termination sequencing, in-progress
   transfers can be completed but no new transfers can be initiated.  A



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   SESS_TERM message can also be used to refuse a session setup by a
   peer (see Section 4.3).  Regardless of the reason, session
   termination is initiated by one of the entities and responded-to by
   the other as illustrated by Figure 13 and Figure 14.  Even when there
   are no transfers queued or in-progress, the session termination
   procedure allows each entity to distinguish between a clean end to a
   session and the TCP connection being closed because of some
   underlying network issue.

   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 normal TCPCL session (i.e., without session failures)
   are indicated in Figure 4.






























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

               Figure 4: Top-level states of a TCPCL session

   Notes on Established Session states:

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

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

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



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   Contact negotiation 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 entity, and is illustrated in Figure 5 and Figure 6
   respectively which both share the data validation and negotiation of
   the Processing of Contact Header "[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.  To avoid data loss, a Session
   Termination "[ST]" exchange allows cleanly finishing transfers before
   a session is ended.

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

               Figure 5: Contact Initiation as Active Entity

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

              Figure 6: Contact Initiation as Passive Entity











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   +-----------+
   |  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 negotiation 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 entity, and is illustrated
   in Figure 8 and Figure 9 respectively which both share the data
   validation and negotiation 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-->[ST]
   | Available |                   +---------+
   +-----------+                       |
                               Received SESS_INIT
                                       |
                                       V
                                     [PSI]

            Figure 8: Session Initiation [SI] as Active Entity









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

            Figure 9: Session Initiation [SI] as Passive Entity

   +----------------+
   | Peer SESS_INIT |
   |   available    |
   +----------------+
           |
      Validate and
       Negotiate
           V
      +------------+
      | Negotiated |---Failure--->[ST]
      +------------+
           |
        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.













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                                          +--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
   in the Ending state only in-progress transfers can be completed and
   no new transfers can be started.








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

          Figure 13: Session Termination [ST] from the Initiator

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

          Figure 14: Session Termination [ST] from the Responder

3.4.  PKIX Environments and CA Policy

   This specification gives requirements about how to use PKIX
   certificates issued by a Certificate Authority (CA), but does not
   define any mechanisms for how those certificates come to be.  The
   requirements about TCPCL certificate use are broad to support two
   quite different PKIX environments:

   DTN-Aware CAs:  In the ideal case, the CA(s) issuing certificates for
      TCPCL entities are aware of the end use of the certificate, have a
      mechanism for verifying ownership of a Node ID, and are issuing
      certificates directly for that Node ID.  In this environment, the
      ability to authenticate a peer entity Node ID directly avoids the
      need to authenticate a network name or address and then implicitly
      trust Node ID of the peer.  The TCPCL authenticates the Node ID
      whenever possible and this is preferred over lower-level PKIX
      identifiers.

   DTN-Ignorant CAs:  It is expected that Internet-scale "public" CAs
      will continue to focus on DNS names as the preferred PKIX
      identifier.  There are large infrastructures already in-place for
      managing network-level authentication and protocols to manage
      identity verification in those environments [RFC8555].  The TCPCL
      allows for this type of environment by authenticating a lower-
      level identifier for a peer and requiring the entity to trust that
      the Node ID given by the peer (during session initialization) is
      valid.  This situation not ideal, as it allows vulnerabilities
      described in Section 8.8, but still provides some amount of mutual
      authentication to take place for a TCPCL session.




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   Even within a single TCPCL session, each entity may operate within
   different PKI environments and with different identifier limitations.
   The requirements related to identifiers in in a PKIX certificate are
   in Section 4.4.1.

   It is important for interoperability that a TCPCL entity have its own
   security policy tailored to accommodate the peers with which it is
   expected to operate.  A strict TLS security policy is appropriate for
   a private network with a single shared CA.  Operation on the Internet
   (such as inter-site BP gateways) could trade more lax TCPCL security
   with the use of encrypted bundle encapsulation [I-D.ietf-dtn-bibect]
   to ensure strong bundle security.

3.5.  Session Keeping Policies

   This specification gives requirements about how to initiate, sustain,
   and terminate a TCPCL session but does not impose any requirements on
   how sessions need to be managed by a BP agent.  It is a network
   administration matter to determine an appropriate session keeping
   policy, but guidance given here can be used to steer policy toward
   performance goals.

   Persistent Session:  This policy preemptively establishes a single
      session to known entities in the network and keeps the session
      active using KEEPALIVEs.  Benefits of this policy include reducing
      the total amount of TCP data needing to be exchanged for a set of
      transfers (assuming KEEPALIVE size is significantly smaller than
      transfer size), and allowing the session state to indicate peer
      connectivity.  Drawbacks include wasted network resources when a
      session is mostly idle or when the network connectivity is
      inconsistent (which requires re-establishing failed sessions), and
      potential queueing issues when multiple transfers are requested
      simultaneously.  This policy assumes that there is agreement
      between pairs of entities as to which of the peers will initiate
      sessions; if there is no such agreement, there is potential for
      duplicate sessions to be established between peers.

   Ephemeral Sessions:  This policy only establishes a session when an
      outgoing transfer is needed to be sent.  Benefits of this policy
      include not wasting network resources on sessions which are idle
      for long periods of time, and avoids queueing issues of a
      persistent session.  Drawbacks include the TCP and TLS overhead of
      establish a new session for each transfer.  This policy assumes
      that each entity can function in a passive role to listen for
      session requests from any peer which needs to send a transfer;
      when that is not the case the Polling behavior below needs to
      happen.  This policy can be augmented to keep the session
      established as long as any transfers are queued.



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   Active-Only Polling Sessions:  When naming and/or addressing of one
      entity is variable (i.e. dynamically assigned IP address or domain
      name) or when firewall or routing rules prevent incoming TCP
      connections, that entity can only function in the active role.  In
      these cases, sessions also need to be established when an incoming
      transfer is expected from a peer or based on a periodic schedule.
      This polling behavior causes inefficiencies compared to as-needed
      ephemeral sessions.

   Many other policies can be established in a TCPCL network between the
   two extremes of single persistent sessions and only ephemeral
   sessions.  Different policies can be applied to each peer entity and
   to each bundle as it needs to be transferred (e.g for quality of
   service).  Additionally, future session extension types can apply
   further nuance to session policies and policy negotiation.

3.6.  Transfer Segmentation Policies

   Each TCPCL session allows a negotiated transfer segmentation policy
   to be applied in each transfer direction.  A receiving node can set
   the Segment MRU in its SESS_INIT message 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



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      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 TCP throughput 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 any
   particular node.  Additionally, future session extension and transfer
   extension types can apply further nuance to transfer policies and
   policy negotiation.

3.7.  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
                ========                             ========
       +-------------------------+
       |  Open TCP Connection    | ->      +-------------------------+
       +-------------------------+      <- |    Accept Connection    |
                                           +-------------------------+
       +-------------------------+
       |     Contact Header      | ->      +-------------------------+
       +-------------------------+      <- |     Contact Header      |
                                           +-------------------------+
       +-------------------------+
       |        SESS_INIT        | ->      +-------------------------+



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       +-------------------------+      <- |        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        |
                                           +-------------------------+
       +-------------------------+         +-------------------------+
       |        TCP Close        | ->   <- |        TCP Close        |
       +-------------------------+         +-------------------------+

    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 can be opened proactively and
   maintained for as long as is possible given the network conditions,
   while other sessions are be opened only when there is a bundle that




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   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 NOT retry
   the connection setup 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 (60 seconds) before signaling to the BP agent
   that a connection cannot be made.

   Once a TCP connection is established, the active entity SHALL
   immediately transmit its Contact Header.  Once a TCP connection is
   established, the passive entity SHALL wait for the peer's Contact
   Header.  If the passive entity does not receive a Contact Header
   after some implementation-defined time duration after TCP connection
   is established, the entity SHALL close the TCP connection.  Entities
   SHOULD choose a Contact Header reception timeout interval no longer
   than one minute (60 seconds).  Upon reception of a Contact Header,
   the passive entity SHALL transmit its Contact Header.  The ordering
   of the Contact Header exchange allows the passive entity to avoid
   allocating resources to a potential TCPCL session until after a valid
   Contact Header has been received from the active entity.  This
   ordering also allows the passive peer to adapt to alternate TCPCL
   protocol versions.

   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).



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4.2.  Contact Header

   This section describes the format of the Contact Header and the
   meaning of its fields.

   If an entity is capable of exchanging messages according to TLS 1.3
   [RFC8446] or any successors which are compatible with that TLS
   ClientHello, the the CAN_TLS flag within its Contact Header SHALL be
   set to 1.  This behavior prefers the use of TLS when possible, even
   if security policy does not allow or require authentication.  This
   follows the opportunistic security model of [RFC7435], though an
   active attacker could interfere with the exchange in such cases.

   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 set to 0 by the sender.  All reserved header flag bits
      SHALL be ignored by the receiver.




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

                       Table 1: Contact Header Flags

4.3.  Contact Validation and Negotiation

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

   If the magic string is not present or is not valid, the connection
   MUST be terminated.  The intent of the magic string is to provide
   some protection against an inadvertent TCP connection by a different
   protocol than the one described in this document.  To prevent a flood
   of repeated connections from a misconfigured application, a passive
   entity MAY deny new TCP connections from a specific peer address for
   a period of time after one or more connections fail to provide a
   decodable Contact Header.

   The first negotiation is on the TCPCL protocol version to use.  The
   active entity always sends its Contact Header first and waits for a
   response from the passive entity.  During contact initiation, the
   active TCPCL node SHALL send the highest TCPCL protocol version on a
   first session attempt for a TCPCL peer.  If the active entity
   receives a Contact Header with a lower protocol version than the one
   sent earlier on the TCP connection, the TCP connection SHALL be
   closed.  If the active entity receives a SESS_TERM message with
   reason of "Version Mismatch", that node MAY attempt further TCPCL
   sessions with the peer using earlier protocol version numbers in
   decreasing order.  Managing multi-TCPCL-session state such as this is
   an implementation matter.

   If the passive entity receives a Contact Header containing a version
   that is not a version of the TCPCL that the entity implements, then
   the entity SHALL send its Contact Header and immediately terminate
   the session with a reason code of "Version mismatch".  If the passive
   entity receives a Contact Header with a version that is lower than
   the latest version of the protocol that the entity implements, the
   entity MAY either terminate the session (with a reason code of
   "Version mismatch") or adapt its operation to conform to the older




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   version of the protocol.  The decision of version fall-back is an
   implementation matter.

   The negotiated contact parameters defined by this specification are
   described in the following paragraphs.

   TCPCL Version:  Both Contact Headers of a successful contact
      negotiation have identical TCPCL Version numbers as described
      above.  Only upon response of a Contact Header from the passive
      entity is the TCPCL protocol version established and session
      negotiation begun.

   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
      negotiated value of Enable TLS is acceptable.  It can be a
      reasonable security policy to 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 entity 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.

4.4.  Session Security

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

   Once established, the lifetime of a TLS connection SHALL be bound to
   the lifetime of the underlying TCP connection.  Immediately prior to
   actively ending a TLS connection after TCPCL session termination, the
   peer which sent the original (non-reply) SESS_TERM message SHOULD
   follow the Closure Alert procedure of [RFC8446] to cleanly terminate
   the TLS connection.  Because each TCPCL message is either fixed-
   length or self-indicates its length, the lack of a TLS Closure Alert
   will not cause data truncation or corruption.

   Subsequent TCPCL session attempts to the same passive entity MAY
   attempt use the TLS connection resumption feature.  There is no
   guarantee that the passive entity will accept the request to resume a



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   TLS session, and the active entity cannot assume any resumption
   outcome.

4.4.1.  Entity Identification

   The TCPCL uses TLS for certificate exchange in both directions to
   identify each entity and to allow each entity to authenticate its
   peer.  Each certificate can potentially identify multiple entities
   and there is no problem using such a certificate as long as the
   identifiers are sufficient to meet authentication policy (as
   described in later sections) for the entity which presents it.

   Because the PKIX environment of each TCPCL entity are likely not
   controlled by the certificate end users (see Section 3.4), the TCPCL
   defines a prioritized list of what a certificate can identify about a
   TCPCL entity:

   Node ID:  The ideal certificate identity is the Node ID of the entity
      using the NODE-ID definition below.  When the Node ID is
      identified, there is no need for any lower-level identification to
      take place.

   DNS Name:  If CA policy forbids a certificate to contain an arbitrary
      NODE-ID but allows a DNS-ID to be identified then one or more
      stable DNS names can be identified in the certificate.  The use of
      wildcard DNS-ID is discouraged due to the complex rules for
      matching and dependence on implementation support for wildcard
      matching (see Section 6.4.3 of [RFC6125]).

   Network Address:  If no stable DNS name is available but a stable
      network address is available and CA policy allows a certificate to
      contain a IPADDR-ID (as defined below) then one or more network
      addresses can be identified in the certificate.

   When only a DNS-ID or IPADDR-ID can be identified by a certificate,
   it is implied that an entity which authenticates using that
   certificate is trusted to provide a valid Node ID in its SESS_INIT;
   the certificate itself does not actually authenticate that Node ID.

   The RECOMMENDED security policy of an entity is to validate a peer
   which authenticates its Node ID regardless of an authenticated DNS
   name or address, and only consider the host/address authentication in
   the absence of an authenticated Node ID.

   This specification defines a NODE-ID of a certificate as being the
   subjectAltName entry of type uniformResourceIdentifier whose value is
   a URI consistent with the requirements of [RFC3986] and the URI
   schemes of the IANA "Bundle Protocol URI Scheme Type" registry.  This



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   is similar to the URI-ID of [RFC6125] but does not require any
   structure to the scheme-specific-part of the URI.  Unless specified
   otherwise by the definition of the URI scheme being authenticated,
   URI matching of a NODE-ID SHALL use the URI comparison logic of
   [RFC3986] and scheme-based normalization of those schemes specified
   in [I-D.ietf-dtn-bpbis].  A URI scheme can refine this "exact match"
   logic with rules about how Node IDs within that scheme are to be
   compared with the certificate-authenticated NODE-ID.

   This specification defines a IPADDR-ID of a certificate as being the
   subjectAltName entry of type iPAddress whose value is encoded
   according to [RFC5280].

4.4.2.  TLS Handshake

   The use of TLS is negotiated using the Contact Header as described in
   Section 4.3.  After negotiating an Enable TLS parameter of true, and
   before any other TCPCL messages are sent within the session, the
   session entities SHALL begin a TLS handshake in accordance with
   [RFC8446].  By convention, this protocol uses the entity which
   initiated the underlying TCP connection (the active peer) as the
   "client" role of the TLS handshake request.

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

   The parameters within each TLS negotiation are implementation
   dependent but any TCPCL node SHALL follow all recommended practices
   of BCP 195 [RFC7525], or any updates or successors that become part
   of BCP 195.  Within each TLS handshake, the following requirements
   apply (using the rough order in which they occur):

   Client Hello:  When a resolved DNS name was used to establish the TCP
      connection, the TLS ClientHello SHOULD include a Server Name
      Indication (SNI) in accordance with [RFC6066].  When present, the
      "server_name" extension SHALL contain a "HostName" value taken
      from the DNS name (of the passive entity) which was resolved.
      Note: The 'HostName in the "server_name" extension is the network
      name for the passive entity, not the Node ID of that entity.

   Server Certificate:  The passive entity SHALL supply a certificate
      within the TLS handshake to allow authentication of its side of
      the session.  Unless prohibited by CA policy, the passive entity
      certificate SHALL contain a NODE-ID which authenticates the Node
      ID of the peer.  When assigned a stable DNS name, the passive
      entity certificate SHOULD contain a DNS-ID which authenticates
      that (fully qualified) name.  When assigned a stable network



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      address, the passive entity certificate MAY contain a IPADDR-ID
      which authenticates that address.  The passive entity MAY use the
      SNI DNS name to choose an appropriate server-side certificate
      which authenticates that DNS name.

   Certificate Request:  During TLS handshake, the passive entity SHALL
      request a client-side certificate.

   Client Certificate:  The active entity SHALL supply a certificate
      chain within the TLS handshake to allow authentication of its side
      of the session.  Unless prohibited by CA policy, the active entity
      certificate SHALL contain a NODE-ID which authenticates the Node
      ID of the peer.  When assigned a stable DNS name, the active
      entity certificate SHOULD contain a DNS-ID which authenticates
      that (fully qualified) name.  When assigned a stable network
      address, the active entity certificate MAY contain a IPADDR-ID
      which authenticates that address.

   All certificates supplied during TLS handshake SHALL conform to
   [RFC5280], or any updates or successors to that profile.  When a
   certificate is supplied during TLS handshake, the full certification
   chain SHOULD be included unless security policy indicates that is
   unnecessary.

   If a TLS handshake cannot negotiate a TLS connection, both entities
   of the TCPCL session SHALL close the TCP connection.  At this point
   the TCPCL session has not yet been established so there is no TCPCL
   session to terminate.

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

4.4.3.  TLS Authentication

   Using PKIX certificates exchanged during the TLS handshake, each of
   the entities can authenticate a peer Node ID directly or authenticate
   the peer DNS name or network address.  The logic for attempting
   authentication is separate from the logic for handling the result of
   authentication, which is based on local security policy.

   By using the SNI DNS name (see Section 4.4.2) a single passive entity
   can act as a convergence layer for multiple BP agents with distinct
   Node IDs.  When this "virtual host" behavior is used, the DNS name is
   used as the indication of which BP Node the active entity is
   attempting to communicate with.  A virtual host CL entity can be
   authenticated by a certificate containing all of the DNS names and/or
   Node IDs being hosted or by several certificates each authenticating



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   a single DNS name and/or Node ID, using the SNI value from the peer
   to select which certificate to use.

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

   The result of authenticating a peer value against a certificate claim
   is one of:

   Absent:  Indicating that the claim type is not present in the
      certificate and cannot be authenticated.

   Success:  Indicating the claim type is present and matches the peer
      (Node ID / DNS name / address) value.

   Failure:  Indicating the claim type is present but did not match the
      peer value.

   Either during or immediately after the TLS handshake, the active
   entity SHALL authenticate the DNS name (of the passive entity) used
   to initiate the TCP connection using any DNS-ID of the peer
   certificate.  If the DNS name authentication result is Failure or if
   the result is Absent and security policy requires an authenticated
   DNS name, the entity SHALL terminate the session (with a reason code
   of "Contact Failure").

   Either during or immediately after the TLS handshake, the active
   entity SHALL authenticate the IP address of the other side of the TCP
   connection using any IPADDR-ID of the peer certificate.  Either
   during or immediately after the TLS handshake, the passive entity
   SHALL authenticate the IP address of the other side of the TCP
   connection using any IPADDR-ID of the peer certificate.  If the
   address authentication result is Failure or if the result is Absent
   and security policy requires an authenticated network address, the
   entity SHALL terminate the session (with a reason code of "Contact
   Failure").

   Immediately before Session Parameter Negotiation, each side of the
   session SHALL authenticate the Node ID of the peer's SESS_INIT
   message using any NODE-ID of the peer certificate.  If the Node ID
   authentication result is Failure or if the result is Absent and
   security policy requires an authenticated Node ID, the entity SHALL
   terminate the session (with a reason code of "Contact Failure").





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4.4.4.  Example TLS Initiation

   A summary of a typical TLS use is shown in the sequence in Figure 17
   below.  In this example the active peer terminates the session but
   termination can be initiated from either peer.

                Entity A                             Entity B
               active peer                         passive peer

       +-------------------------+
       |  Open TCP Connection    | ->      +-------------------------+
       +-------------------------+      <- |    Accept Connection    |
                                           +-------------------------+
       +-------------------------+
       |     Contact Header      | ->      +-------------------------+
       +-------------------------+      <- |     Contact Header      |
                                           +-------------------------+

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

                         DNS name validation occurs.
                      Secured TCPCL messaging can begin.

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

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

       +-------------------------+
       |       SESS_TERM         | ->      +-------------------------+
       +-------------------------+      <- |        SESS_TERM        |
                                           +-------------------------+
       +-------------------------+
       |    TLS Closure Alert    | ->      +-------------------------+
       +-------------------------+      <- |    TLS Closure Alert    |
                                           +-------------------------+
       +-------------------------+         +-------------------------+
       |        TCP Close        | ->   <- |        TCP Close        |
       +-------------------------+         +-------------------------+

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



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4.5.  Message 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 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.

































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   +--------------+------+---------------------------------------------+
   | 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 6.1.                             |
   |              |      |                                             |
   | 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 19.











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

                        Figure 19: SESS_INIT Format

   The fields of the SESS_INIT message are:

   Keepalive Interval:  A 16-bit unsigned integer indicating the minimum
      interval, in seconds, to negotiate as the Session Keepalive using
      the method of Section 4.7.

   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.

   Node ID Length and Node ID Data:  Together these fields represent a
      variable-length text string.  The Node ID Length is a 16-bit
      unsigned integer indicating the number of octets of Node ID Data
      to follow.  A zero-length Node ID SHALL be used to indicate the
      lack of Node ID rather than a truly empty Node ID.  This case



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      allows an entity to avoid exposing Node ID information on an
      untrusted network.  A non-zero-length Node ID Data SHALL contain
      the UTF-8 encoded Node ID of the Entity which sent the SESS_INIT
      message.  Every Node ID SHALL be a URI consistent with the
      requirements of [RFC3986] and the URI schemes of the IANA "Bundle
      Protocol URI Scheme Type" registry.  The Node ID itself can be
      authenticated as described in Section 4.4.3.

   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
      multiplicity of these Session Extension Items is significant, as
      defined in the associated type specification(s).  If the content
      of the Session Extension Items data disagrees with the Session
      Extension Length (e.g., the last Item claims to use more octets
      than are present in the Session Extension Length), the reception
      of the SESS_INIT is considered to have failed.

   When the active entity initiates a TCPCL session, it is likely based
   on routing information which binds a Node ID to CL parameters.  If
   the active entity receives a SESS_INIT with different Node ID than
   was intended for the TCPCL session, the session MAY be allowed to be
   established.  If allowed, such a session SHALL be associated with the
   Node ID provided in the SESS_INIT message rather than any intended
   value.

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
   SESS_INIT it sent to the peer) with the preferences of the peer node
   (expressed in the SESS_INIT 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 identical to the
      Transfer MRU and Segment MRU, respectively, of the received
      SESS_INIT message.  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.  If
      either the Transfer MRU or Segment MRU is unacceptable, the entity



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      SHALL terminate the session with a reason code of "Contact
      Failure".

   Session Keepalive:  Negotiation of the Session Keepalive parameter is
      performed by taking the minimum of the two Keepalive Interval
      values from the two SESS_INIT messages.  The Session Keepalive
      interval is a parameter for the behavior described in
      Section 5.1.1.  If the Session Keepalive interval is unacceptable,
      the entity SHALL terminate the session with a reason code of
      "Contact Failure".  Note: a negotiated Session Keepalive of zero
      indicates that KEEPALIVEs are disabled.

   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 20.

   The fields of the Session Extension Item are:

   Item 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 set to 0 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 of 1, the entity SHALL close the TCPCL session with SESS_TERM
      reason code of "Contact Failure".  If the CRITICAL flag is 0, 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 create 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




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      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 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 (within the
   minimum imposed by the positive-value encoding), 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 (within the maximum imposed



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   by the fixed-size encoding), 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 can 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 entity
   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)

   This message type is not expected to be seen in a well-functioning
   session.  Its purpose is to aid in troubleshooting bad entity
   behavior by allowing the peer to observe why an entity is not
   responding as expected to its messages.

   If a TCPCL entity receives a message type which is unknown to it
   (possibly due to an unhandled protocol version mismatch or a
   incorrectly-negotiated session extension which defines a new message
   type), the entity SHALL send a MSG_REJECT message with a Reason Code
   of "Message Type Unknown" and close the TCP connection.  If a TCPCL
   entity receives a message type which is known but is inappropriate
   for the negotiated session parameters (possibly due to incorrectly-
   negotiated session extension), the entity SHALL send a MSG_REJECT
   message with a Reason Code of "Message Unsupported".  If a TCPCL
   entity receives a message which is inappropriate for the current
   session state (e.g., a SESS_INIT after the session has already been
   established or an XFER_ACK message with an unknown Transfer ID), the
   entity SHALL send a MSG_REJECT message with a Reason Code of "Message
   Unexpected".

   The format of a MSG_REJECT message is as follows in Figure 21.




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

                 Figure 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  |
   | Type        |      | Message Type code unknown to the TCPCL node. |
   | Unknown     |      |                                              |
   |             |      |                                              |
   | Message     | 0x02 | A message was received but               the |
   | Unsupported |      | TCPCL entity cannot comply with the message  |
   |             |      | contents.                                    |
   |             |      |                                              |
   | Message     | 0x03 | A message was received while the             |
   | Unexpected  |      | session is 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 NOT be larger than



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   the receiving node's maximum receive unit (MRU) (see the field
   Segment MRU of Section 4.2).  The first segment for a bundle is
   indicated by the 'START' flag and the last segment is indicated by
   the 'END' flag.

   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.  The sending entity originates a transfer ID and the
   receiving entity uses that same Transfer ID in acknowledgements.

   Transfer IDs from each node SHALL be unique within a single TCPCL
   session.  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.

   Although there is not a strict requirement for Transfer ID initial
   values or ordering (see Section 8.11), in the absence of any other
   mechanism for generating Transfer IDs an entity SHALL use the
   following algorithm: The initial Transfer ID from each node is zero
   and subsequent Transfer ID values are incremented from the prior
   Transfer ID value by one.

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 22.






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                     +------------------------------+
                     |       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 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 set to 0 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 to 1 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 associated single transfer.  The order and
      multiplicity of these transfer extension items is significant, as
      defined in the associated type specification(s).  If the content
      of the Transfer Extension Items data disagrees with the Transfer
      Extension Length (e.g., the last Item claims to use more octets
      than are present in the Transfer Extension Length), the reception
      of the XFER_SEGMENT is considered to have failed.



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   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 flag values in the two
   low-order bits, denoted 'START' and 'END' in Table 5.  The 'START'
   flag SHALL be set to 1 when transmitting the first segment of a
   transfer.  The 'END' flag SHALL be set to 1 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' flags
   SHALL be set to 1.

   Once a transfer of a bundle has commenced, the entity MUST only send
   segments containing sequential portions of that bundle until it sends
   a segment with the 'END' flag set to 1.  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 and fully
   processed 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 23.




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

                  Figure 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 set to 0 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 after the segment has been fully
   processed.  The flags portion of the XFER_ACK header SHALL be set to
   match the corresponding XFER_SEGMENT message being acknowledged
   (including flags not decodable to the entity).  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 entity sends an acknowledgment
   of length 100.  After the second segment is received, the entity
   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 entity



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   MAY transmit a XFER_REFUSE message.  As data segments and
   acknowledgments can 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 transfer receiver MAY send an XFER_REFUSE message as soon as it
   receives any XFER_SEGMENT message.  The transfer sender MUST be
   prepared for this and MUST associate the refusal with the correct
   bundle via the Transfer ID fields.

   The TCPCL itself does not have any required behavior to respond to an
   XFER_REFUSE based on its Reason Code; the refusal is passed up as an
   indication to the BP agent that the transfer has been refused.  If a
   transfer refusal has a Reason Code which is not decodable to the BP
   agent, the agent SHOULD treat the refusal as having an Unknown
   reason.

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

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

                 Figure 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.








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   +------------+------+-----------------------------------------------+
   | Name       | Code | Description                                   |
   +------------+------+-----------------------------------------------+
   | Unknown    | 0x00 | Reason for refusal is unknown or not          |
   |            |      | specified.                                    |
   |            |      |                                               |
   | Completed  | 0x01 | The receiver already has the complete bundle. |
   |            |      | The sender MAY consider the bundle as         |
   |            |      | completely received.                          |
   |            |      |                                               |
   | No         | 0x02 | The receiver's resources are exhausted. The   |
   | Resources  |      | sender SHOULD apply reactive bundle           |
   |            |      | fragmentation before retrying.                |
   |            |      |                                               |
   | Retransmit | 0x03 | The receiver has encountered a problem that   |
   |            |      | requires the bundle to be retransmitted in    |
   |            |      | its entirety.                                 |
   |            |      |                                               |
   | Not        | 0x04 | Some issue with the bundle data or the        |
   | Acceptable |      | transfer extension data was encountered. The  |
   |            |      | sender SHOULD NOT retry the same bundle with  |
   |            |      | the same extensions.                          |
   |            |      |                                               |
   | Extension  | 0x05 | A failure processing the Transfer Extension   |
   | Failure    |      | Items has occurred.                           |
   +------------+------+-----------------------------------------------+

                     Table 6: XFER_REFUSE Reason Codes

   The receiver MUST, for each transfer preceding the one to be refused,
   have either acknowledged all XFER_SEGMENT messages 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 can
   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
   does not receive a segment with the 'END' flag set to 1 for the
   aborted bundle.  The beginning of the next bundle is identified by



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   the 'START' flag 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 25.

   The fields of the Transfer Extension Item are:

   Item 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 set to 0 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 is 1, the entity SHALL refuse the transfer with an
      XFER_REFUSE reason code of "Extension Failure".  If the CRITICAL
      flag is 0, 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 creates 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 25: Transfer Extension Item Format








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   +----------+--------+-----------------------------------------------+
   | 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
   flags are 1) 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 26.

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

                 Figure 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 and send an XFER_REFUSE with a Reason
      Code of "Not Acceptable".






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6.  Session Termination

   This section describes the procedures for terminating a TCPCL
   session.  The purpose of terminating a session is to allow transfers
   to complete before the session is closed but not allow any new
   transfers to start.  A session state change is necessary for this to
   happen because transfers can be in-progress in either direction
   (transfer stream) within a session.  Waiting for a transfer to
   complete in one direction does not control or influence the
   possibility of a transfer in the other direction.  Either peer of a
   session can terminate an established session at any time.

6.1.  Session Termination Message (SESS_TERM)

   To cleanly terminate 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 flag of a SESS_TERM message SHALL be 0.  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 flag, which is set to 1 to indicate
   acknowledgement.

   Once a SESS_TERM message is sent the state of that TCPCL session
   changes to Ending.  While the session is in the Ending state, an
   entity MAY finish an in-progress transfer in either direction.  While
   the session is in the Ending state, an entity SHALL NOT begin any new
   outgoing transfer for the remainder of the session.  While the
   session is in the Ending state, an entity SHALL NOT accept any new
   incoming transfer for the remainder of the session.

   Instead of following a clean termination sequence, after transmitting
   a SESS_TERM message an entity MAY immediately close the associated
   TCP connection.  When performing an unclean termination, a receiving
   node SHOULD acknowledge all received XFER_SEGMENTs with an XFER_ACK
   before closing the TCP connection.  Not acknowledging received
   segments can result in unnecessary bundle or bundle fragment
   retransmission.  When performing an unclean termination, 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 close the TCP connection and
   actual closing of the connection (a "half-closed" state) MAY be
   ignored by the TCPCL entity.

   The TCPCL itself does not have any required behavior to respond to an
   SESS_TERM based on its Reason Code; the termination is passed up as



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   an indication to the BP agent that the session state has changed.  If
   a termination has a Reason Code which is not decodable to the BP
   agent, the agent SHOULD treat the termination as having an Unknown
   reason.

   The format of the SESS_TERM message is as follows in Figure 27.

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

                  Figure 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 set to 0 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













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   +--------------+------+---------------------------------------------+
   | 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 a    |
   | Failure      |      | Contact Header or SESS_INIT option.         |
   |              |      |                                             |
   | Resource     | 0x05 | The node has run into some resource limit   |
   | Exhaustion   |      | and cannot continue the session.            |
   +--------------+------+---------------------------------------------+

                      Table 9: SESS_TERM Reason Codes

   The earliest a TCPCL session termination MAY occur is immediately
   after transmission of a Contact Header (and prior to any further
   message transmit).  This can, for example, be used to notify that the
   entity 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.

   Termination of the TCP connection MAY occur prior to receiving the
   Contact header as discussed in Section 4.1.  If reception of the
   Contact Header itself somehow fails (e.g., an invalid "magic string"
   is received), an entity SHALL close the TCP connection without
   sending a SESS_TERM message.

   If a session is to be terminated before a protocol message has
   completed being sent, then the entity 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.







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6.2.  Idle Session Shutdown

   The protocol includes a provision for clean termination of idle
   sessions.  Determining the length of time to wait before 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 can
   exist.

   An example implementation of the this draft of TCPCLv4 has been
   created as a GitHub project [github-dtn-bpbis-tcpcl] and is intended
   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

   This section separates security considerations into threat categories
   based on guidance of BCP 72 [RFC3552].








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8.1.  Threat: Passive Leak of Node Data

   When used without TLS security, the TCPCL exposes the Node ID and
   other configuration data to passive eavesdroppers.  This occurs even
   when no transfers occur within a TCPCL session.  This can be avoided
   by always using TLS, even if authentication is not available (see
   Section 8.10).

8.2.  Threat: Passive Leak of Bundle Data

   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
   [I-D.ietf-dtn-bpsec] are to be used instead.

   When used without TLS security, the TCPCL exposes all bundle data to
   passive eavesdroppers.  This can be avoided by always using TLS, even
   if authentication is not available (see Section 8.10).

8.3.  Threat: TCPCL Version Downgrade

   When a TCPCL entity supports multiple versions of the protocol it is
   possible for a malicious or misconfigured peer to use an older
   version of TCPCL which does not support transport security.  A man-
   in-the-middle attacker can also manipulate a Contact Header to
   present a lower protocol version than desired.

   It is up to security policies within each TCPCL node to ensure that
   the negotiated TCPCL version meets transport security requirements.

8.4.  Threat: Transport Security Stripping

   When security policy allows non-TLS sessions, TCPCL does not protect
   against active network attackers.  It is possible for a man-in-the-
   middle attacker to set the CAN_TLS flag to 0 on either side of the
   Contact Header exchange.  This leads to the "SSL Stripping" attack
   described in [RFC7457].

   The purpose of the CAN_TLS flag is to allow the use of TCPCL on
   entities which simply do not have a TLS implementation available.
   When TLS is available on an entity, it is strongly encouraged that
   the security policy disallow non-TLS sessions.  This requires that
   the TLS handshake occurs, regardless of the policy-driven parameters
   of the handshake and policy-driven handling of the handshake outcome.

   The negotiated use of TLS is identical behavior to STARTTLS use in
   [RFC2595] and [RFC4511].




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8.5.  Threat: Weak TLS Configurations

   Even when using TLS to secure the TCPCL session, the actual
   ciphersuite negotiated between the TLS peers can be insecure.
   Recommendations for ciphersuite use are included in BCP 195
   [RFC7525].  It is up to security policies within each TCPCL node to
   ensure that the negotiated TLS ciphersuite meets transport security
   requirements.

8.6.  Threat: Certificate Validation Vulnerabilities

   Even when TLS itself is operating properly an attacker can attempt to
   exploit vulnerabilities within certificate check algorithms or
   configuration to establish a secure TCPCL session using an invalid
   certificate.  A BP agent treats the peer Node ID within a TCPCL
   session as authoritative and an invalid certificate exploit could
   lead to bundle data leaking and/or denial of service to the Node ID
   being impersonated.  There are many reasons, described in [RFC5280],
   why a certificate can fail to validate, including using the
   certificate outside of its valid time interval, using purposes for
   which it was not authorized, or using it after it has been revoked by
   its CA.  Validating a certificate is a complex task and can require
   network connectivity outside of the primary TCPCL network path(s) if
   a mechanism such as the Online Certificate Status Protocol (OCSP) is
   used by the CA.  The configuration and use of particular certificate
   validation methods are outside of the scope of this document.

8.7.  Threat: Symmetric Key Limits

   Even with a secure block cipher and securely-established session
   keys, there are limits to the amount of plaintext which can be safely
   encrypted with a given set of keys as described in [AEAD-LIMITS].
   When permitted by the negotiated TLS version (see [RFC8446]), it is
   advisable to take advantage of session key updates to avoid those
   limits.

8.8.  Threat: BP Node Impersonation

   The certificates exchanged by TLS enable authentication of peer DNS
   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 DNS name or Node ID of the peer (see Section 3.4).  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 in Section 4.4.2 bind the DNS name
   or node ID based on the contents of the certificate.





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   The DNS name validation is a weaker form of authentication, because
   even if a peer is operating on an authenticated network DNS 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
   addresses of nodes can be time-variable so binding a certificate to a
   Node ID is a more stable identity.

   Node ID validation ensures that the peer to which a bundle is
   transferred is in fact the node which the BP Agent expects it to be.
   It is a reasonable policy to skip DNS name validation if certificates
   can be guaranteed to validate the peer's Node ID.  In circumstances
   where certificates can only be issued to DNS names, Node ID
   validation is not possible but it could be reasonable to assume that
   a trusted host is not going to present an invalid Node ID.
   Determining of when a DNS name authentication can be trusted to
   validate a Node ID is also a policy matter outside the scope of this
   document.

8.9.  Threat: Denial of Service

   The behaviors described in this section all amount to a potential
   denial-of-service to a TCPCL entity.  The denial-of-service could be
   limited to an individual TCPCL session, could affect other well-
   behaving sessions on an entity, or could affect all sessions on a
   host.

   A malicious entity can continually establish TCPCL sessions and delay
   sending of protocol-required data to trigger timeouts.  The victim
   entity can block TCP connections from network peers which are thought
   to be incorrectly behaving within TCPCL.

   An entity can send a large amount of data over a TCPCL session,
   requiring the receiving entity to handle the data.  The victim entity
   can attempt to stop the flood of data by sending an XFER_REFUSE
   message, or forcibly terminate the session.

   There is the possibility of a "data dribble" attack in which an
   entity presents a very small Segment MRU which causes transfers to be
   split among an large number of very small segments and causes the
   segmentation overhead to overwhelm the actual bundle data segments.
   Similarly, an entity can present a very small Transfer MRU which will
   cause resources to be wasted on establishment and upkeep of a TCPCL
   session over which a bundle could never be transferred.  The victim
   entity can terminate the session during the negotiation of
   Section 4.7 if the MRUs are unacceptable.

   The keepalive mechanism can be abused to waste throughput within a
   network link which would otherwise be usable for bundle



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   transmissions.  Due to the quantization of the Keepalive Interval
   parameter the smallest Session Keepalive is one second, which should
   be long enough to not flood the link.  The victim entity can
   terminate the session during the negotiation of Section 4.7 if the
   Keepalive Interval is unacceptable.

   Finally, an attacker or a misconfigured entity can cause issues at
   the TCP connection which will cause unnecessary TCP retransmissions
   or connection resets, effectively denying the use of the overlying
   TCPCL session.

8.10.  Alternate Uses of TLS

   This specification makes use of PKIX certificate validation and
   authentication within TLS.  There are alternate uses of TLS which are
   not necessarily incompatible with the security goals of this
   specification, but are outside of the scope of this document.  The
   following subsections give examples of alternate TLS uses.

8.10.1.  TLS Without Authentication

   In environments where PKI is available but there are restrictions on
   the issuance of certificates (including the contents of
   certificates), it may be possible to make use of TLS in a way which
   authenticates only the passive entity of a TCPCL session or which
   does not authenticate either entity.  Using TLS in a way which does
   not successfully authenticate some claim of both peer entities of a
   TCPCL session is outside of the scope of this document but does have
   similar properties to the opportunistic security model of [RFC7435].

8.10.2.  Non-Certificate TLS Use

   In environments where PKI is unavailable, alternate uses of TLS which
   do not require certificates such as pre-shared key (PSK)
   authentication [RFC5489] and the use of raw public keys [RFC7250] are
   available and can be used to ensure confidentiality within TCPCL.
   Using non-PKI node authentication methods is outside of the scope of
   this document.

8.11.  Predictability of Transfer IDs

   The only requirement on Transfer IDs is that they be unique with each
   session from the sending peer only.  The trivial algorithm of the
   first transfer starting at zero and later transfers incrementing by
   one causes absolutely predictable Transfer IDs.  Even when a TCPCL
   session is not TLS secured and there is a man-in-the-middle attacker
   causing denial of service with XFER_REFUSE messages, it is not




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   possible to preemptively refuse a transfer so there is no benefit in
   having unpredictable Transfer IDs within a session.

9.  IANA Considerations

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

   Some of the registries 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 codepoints between TCPCLv3 and
   TCPCLv4 while preserving the semantics of some of the codepoints.

9.1.  Port Number

   Within the port registry of [IANA-PORTS], TCP port number 4556 has
   been previously assigned as the default port for the TCP convergence
   layer in [RFC7242].  This assignment is unchanged by TCPCL version 4,
   but the assignment reference is updated to this specification.  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.  The related assignments for UDP and DCCP
   port 4556 (both registered by [RFC7122]) are unchanged.

          +------------------------+----------------------------+
          | Parameter              | Value                      |
          +------------------------+----------------------------+
          | Service Name:          | dtn-bundle                 |
          |                        |                            |
          | Transport Protocol(s): | TCP                        |
          |                        |                            |
          | Assignee:              | IESG <iesg@ietf.org>       |
          |                        |                            |
          | Contact:               | IESG <iesg@ietf.org>       |
          |                        |                            |
          | Description:           | DTN Bundle TCP CL Protocol |
          |                        |                            |
          | Reference:             | This specification.        |
          |                        |                            |
          | Port Number:           | 4556                       |
          +------------------------+----------------------------+

9.2.  Protocol Versions

   IANA has created, under the "Bundle Protocol" registry [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   Numbers".  The version number table is updated to include this
   specification.  The registration procedure is RFC Required.



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        +-------+-------------+-----------------------------------+
        | 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 Session Extension Types" and initialize it with the contents of
   Table 10.  The registration procedure is 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.

   Specifications of new session extension types need to define the
   encoding of the Item Value data as well as any meaning or restriction
   on the number of or order of instances of the type within an
   extension item list.  Specifications need to define how the extension
   functions when no instance of the new extension type is received
   during session negotiation.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).












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               +----------------+--------------------------+
               | Code           | Session Extension Type   |
               +----------------+--------------------------+
               | 0x0000         | Reserved                 |
               |                |                          |
               | 0x0001--0x7FFF | Unassigned               |
               |                |                          |
               | 0x8000--0xFFFF | Private/Experimental Use |
               +----------------+--------------------------+

                  Table 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 Transfer Extension Types" and initialize it with the contents of
   Table 11.  The registration procedure is 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.

   Specifications of new transfer extension types need to define the
   encoding of the Item Value data as well as any meaning or restriction
   on the number of or order of instances of the type within an
   extension item list.  Specifications need to define how the extension
   functions when no instance of the new extension type is received in a
   transfer.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).
















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              +----------------+---------------------------+
              | Code           | Transfer Extension Type   |
              +----------------+---------------------------+
              | 0x0000         | Reserved                  |
              |                |                           |
              | 0x0001         | Transfer Length Extension |
              |                |                           |
              | 0x0002--0x7FFF | Unassigned                |
              |                |                           |
              | 0x8000--0xFFFF | Private/Experimental Use  |
              +----------------+---------------------------+

                  Table 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 Message Types" and initialize it with the contents of Table 12.
   The registration procedure is RFC 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.

   Specifications of new message types need to define the encoding of
   the message data as well as the purpose and relationship of the new
   message to existing session/transfer state within the baseline
   message sequencing.  The use of new message types need to be
   negotiated between TCPCL entities within a session (using the session
   extension mechanism) so that the receiving entity can properly decode
   all message types used in the session.

   Expert(s) are encouraged to favor new session/transfer extension
   types over new message types.  TCPCL messages are not self-
   delimiting, so care must be taken in introducing new message types.
   If an entity receives an unknown message type the only thing that can
   be done is to send a MSG_REJECT and close the TCP connection; not
   even a clean termination can be done at that point.











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                 +------------+--------------------------+
                 | 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--0xEF | Unassigned               |
                 |            |                          |
                 | 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                       Table 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 XFER_REFUSE Reason Codes" and initialize it with the contents of
   Table 13.  The registration procedure is 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.

   Specifications of new XFER_REFUSE reason codes need to define the
   meaning of the reason and disambiguate it with pre-existing reasons.
   Each refusal reason needs to be usable by the receiving BP Agent to
   make retransmission or re-routing decisions.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).




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                 +------------+--------------------------+
                 | Code       | Refusal Reason           |
                 +------------+--------------------------+
                 | 0x00       | Unknown                  |
                 |            |                          |
                 | 0x01       | Completed                |
                 |            |                          |
                 | 0x02       | No Resources             |
                 |            |                          |
                 | 0x03       | Retransmit               |
                 |            |                          |
                 | 0x04       | Not Acceptable           |
                 |            |                          |
                 | 0x05       | Extension Failure        |
                 |            |                          |
                 | 0x06--0xEF | Unassigned               |
                 |            |                          |
                 | 0xF0--0xFF | Private/Experimental Use |
                 +------------+--------------------------+

                    Table 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 SESS_TERM Reason Codes" and initialize it with the contents of
   Table 14.  The registration procedure is 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.

   Specifications of new SESS_TERM reason codes need to define the
   meaning of the reason and disambiguate it with pre-existing reasons.
   Each termination reason needs to be usable by the receiving BP Agent
   to make re-connection decisions.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).








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

                     Table 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 [IANA-BUNDLE],
   a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version
   4 MSG_REJECT Reason Codes" and initialize it with the contents of
   Table 15.  The registration procedure is 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.

   Specifications of new MSG_REJECT reason codes need to define the
   meaning of the reason and disambiguate it with pre-existing reasons.
   Each rejection reason needs to be usable by the receiving TCPCL
   Entity to make message sequencing and/or session termination
   decisions.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).







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

                     Table 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

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-26 (work in progress),
              July 2020.

   [IANA-BUNDLE]
              IANA, "Bundle Protocol",
              <https://www.iana.org/assignments/bundle/>.

   [IANA-PORTS]
              IANA, "Service Name and Transport Protocol Port Number
              Registry", <https://www.iana.org/assignments/service-
              names-port-numbers/>.

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







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   [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>.

   [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>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [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>.




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   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  Informative References

   [AEAD-LIMITS]
              Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", August 2017,
              <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.

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

   [I-D.ietf-dtn-bibect]
              Burleigh, S., "Bundle-in-Bundle Encapsulation", draft-
              ietf-dtn-bibect-03 (work in progress), February 2020.

   [I-D.ietf-dtn-bpsec]
              Birrane, E. and K. McKeever, "Bundle Protocol Security
              Specification", draft-ietf-dtn-bpsec-23 (work in
              progress), October 2020.

   [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>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511,
              DOI 10.17487/RFC4511, June 2006,
              <https://www.rfc-editor.org/info/rfc4511>.

   [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>.

   [RFC5489]  Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
              Transport Layer Security (TLS)", RFC 5489,
              DOI 10.17487/RFC5489, March 2009,
              <https://www.rfc-editor.org/info/rfc5489>.




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   [RFC7122]  Kruse, H., Jero, S., and S. Ostermann, "Datagram
              Convergence Layers for the Delay- and Disruption-Tolerant
              Networking (DTN) Bundle Protocol and Licklider
              Transmission Protocol (LTP)", RFC 7122,
              DOI 10.17487/RFC7122, March 2014,
              <https://www.rfc-editor.org/info/rfc7122>.

   [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>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [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>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

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.



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   o  Added 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
      (network byte order) fields.

   o  Renamed SHUTDOWN to SESS_TERM to deconflict term "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 connection security mechanism.

   o  Added Resource Exhaustion SESS_TERM reason code.







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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













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