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Delay-Tolerant Networking TCP Convergence-Layer Protocol Version 4
RFC 9174

Document Type RFC - Proposed Standard (January 2022)
Authors Brian Sipos , Michael Demmer , Joerg Ott , Simon Perreault
Last updated 2022-01-31
Replaces draft-sipos-dtn-tcpclv4
Stream Internet Engineering Task Force (IETF)
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Reviews
Additional resources GitHub Repository
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Stream WG state Submitted to IESG for Publication
Document shepherd Edward J. Birrane
Shepherd write-up Show Last changed 2019-08-07
IESG IESG state RFC 9174 (Proposed Standard)
Action Holders
(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Magnus Westerlund
Send notices to Edward Birrane <edward.birrane@jhuapl.edu>
IANA IANA review state Version Changed - Review Needed
IANA action state In Progress
IANA expert review state Expert Reviews OK
IANA expert review comments Yes, this is approved. However, I ask the authors to add a reference to the ASN.1 specification in Section 9.9 to be clearly specify object identifier: [X.680] ITU-T, "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680, ISO/IEC 8824-1:2015, August 2015, <https://www.itu.int/rec/T-REC-X.680-201508-I/en>.
RFC 9174


Internet Engineering Task Force (IETF)                          B. Sipos
Request for Comments: 9174                               RKF Engineering
Category: Standards Track                                      M. Demmer
ISSN: 2070-1721                                                         
                                                                  J. Ott
                                          Technical University of Munich
                                                            S. Perreault
                                                                 LogMeIn
                                                            January 2022

   Delay-Tolerant Networking TCP Convergence-Layer Protocol Version 4

Abstract

   This document describes a TCP convergence layer (TCPCL) for Delay-
   Tolerant Networking (DTN).  This version of the TCPCL protocol
   resolves implementation issues in the earlier TCPCL version 3 as
   defined in RFC 7242 and provides updates to the Bundle Protocol (BP)
   contents, encodings, and convergence-layer requirements in BP version
   7 (BPv7).  Specifically, TCPCLv4 uses BPv7 bundles encoded by the
   Concise Binary Object Representation (CBOR) as its service data unit
   being transported and provides a reliable transport of such bundles.
   This TCPCL version also includes security and extensibility
   mechanisms.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9174.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Scope
   2.  Requirements Language
     2.1.  Definitions Specific to the TCPCL Protocol
   3.  General Protocol Description
     3.1.  Convergence-Layer Services
     3.2.  TCPCL Session Overview
     3.3.  TCPCL States and Transitions
     3.4.  PKIX Environments and CA Policy
     3.5.  Session-Keeping Policies
     3.6.  Transfer Segmentation Policies
     3.7.  Example Message Exchange
   4.  Session Establishment
     4.1.  TCP Connection
     4.2.  Contact Header
     4.3.  Contact Validation and Negotiation
     4.4.  Session Security
       4.4.1.  Entity Identification
       4.4.2.  Certificate Profile for the TCPCL
       4.4.3.  TLS Handshake
       4.4.4.  TLS Authentication
       4.4.5.  Policy Recommendations
       4.4.6.  Example TLS Initiation
     4.5.  Message Header
     4.6.  Session Initialization Message (SESS_INIT)
     4.7.  Session Parameter Negotiation
     4.8.  Session Extension Items
   5.  Established Session Operation
     5.1.  Upkeep and Status Messages
       5.1.1.  Session Upkeep (KEEPALIVE)
       5.1.2.  Message Rejection (MSG_REJECT)
     5.2.  Bundle Transfer
       5.2.1.  Bundle Transfer ID
       5.2.2.  Data Transmission (XFER_SEGMENT)
       5.2.3.  Data Acknowledgments (XFER_ACK)
       5.2.4.  Transfer Refusal (XFER_REFUSE)
       5.2.5.  Transfer Extension Items
   6.  Session Termination
     6.1.  Session Termination Message (SESS_TERM)
     6.2.  Idle Session Termination
   7.  Security Considerations
     7.1.  Threat: Passive Leak of Node Data
     7.2.  Threat: Passive Leak of Bundle Data
     7.3.  Threat: TCPCL Version Downgrade
     7.4.  Threat: Transport Security Stripping
     7.5.  Threat: Weak TLS Configurations
     7.6.  Threat: Untrusted End-Entity Certificate
     7.7.  Threat: Certificate Validation Vulnerabilities
     7.8.  Threat: Symmetric Key Limits
     7.9.  Threat: BP Node Impersonation
     7.10. Threat: Denial of Service
     7.11. Mandatory-to-Implement TLS
     7.12. Alternate Uses of TLS
       7.12.1.  TLS without Authentication
       7.12.2.  Non-certificate TLS Use
     7.13. Predictability of Transfer IDs
   8.  IANA Considerations
     8.1.  Port Number
     8.2.  Protocol Versions
     8.3.  Session Extension Types
     8.4.  Transfer Extension Types
     8.5.  Message Types
     8.6.  XFER_REFUSE Reason Codes
     8.7.  SESS_TERM Reason Codes
     8.8.  MSG_REJECT Reason Codes
     8.9.  Object Identifier for PKIX Module Identifier
     8.10. Object Identifier for PKIX Other Name Forms
     8.11. Object Identifier for PKIX Extended Key Usage
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Significant Changes from RFC 7242
   Appendix B.  ASN.1 Module
   Appendix C.  Example of the BundleEID Other Name Form
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document describes the TCP convergence-layer protocol for Delay-
   Tolerant Networking (DTN).  DTN 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 Networking 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)
   [RFC9171], an application-layer protocol that is used to construct a
   store-and-forward overlay network.  BPv7 requires the services of a
   "convergence-layer adapter" (CLA) to send and receive bundles using
   the service of some "native" link, network, or Internet protocol.
   This document describes one such convergence-layer adapter that uses
   the well-known Transmission Control Protocol (TCP).  This convergence
   layer is referred to as TCP Convergence Layer version 4 (TCPCLv4).
   For the remainder of this document,

   *  the abbreviation "BP" without the version suffix refers to BPv7.

   *  the abbreviation "TCPCL" without the version suffix refers to
      TCPCLv4.

   The locations of the TCPCL and the Bundle Protocol 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 the TCPCL as
   its convergence layer, both BP and the 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:

   *  The format of protocol data units of the Bundle Protocol, as those
      are defined elsewhere in [RFC9171].  This includes the concept of
      bundle fragmentation or bundle encapsulation.  The TCPCL transfers
      bundles as opaque data blocks.

   *  Mechanisms for locating or identifying other bundle entities
      (peers) within a network or across an internet.  The mapping of a
      node ID to a potential convergence layer (CL) protocol and network
      address is left to implementation and configuration of the BP
      Agent (BPA) and its various potential routing strategies, as is
      the mapping of a DNS name and/or address to a choice of an end-
      entity certificate to authenticate a node to its peers.

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

   *  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.

   *  Uses of TLS that are not based on PKIX certificate authentication
      (see Section 7.12.2) or in which authentication of both entities
      is not possible (see Section 7.12.1).

   Any TCPCL implementation requires a BPA 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:  Here, "network byte order" means 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:

      *  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 to either (1) one or both entities actively
      closing the TCP connection or (2) 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 BPAs, 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
      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 unidirectional 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 BPA that transfers are fully decoded as bundles.  Each
      unidirectional 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 the 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 transfer segment is 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.

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

+---------------------------+              +---------------------------+
|    "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
   TCP.  This document specifies the encapsulation of bundles,
   procedures for TCP setup and teardown, and a set of messages and
   entity requirements.  The general operation of the protocol is as
   follows.

3.1.  Convergence-Layer Services

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

   Attempt Session:  The TCPCL allows a BPA 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 BPA.

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

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

      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
         the 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.  When the session is established, the
         peer node ID (along with an indication of whether or not it was
         authenticated) and the negotiated session parameters (see
         Section 4.7) are also communicated to the BPA.

      Ending:  The entity sent a 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 BPA when the
      Live/Idle substate of the session changes.  This occurs only when
      the top-level session state is "Established".  The session
      transitions from Idle to Live 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 the 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
      BPA to transmit bundle data over an established TCPCL session.
      Transmission requests are done 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
      BPA.

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

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

   Transmission Failure:  The TCPCL entity indicates to the BPA 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 this status to the
      receiving BPA just before any transmission data is sent.  This
      corresponds to reception of the XFER_SEGMENT message with the
      START flag set to 1.

   Interrupt Reception:  The TCPCL entity allows a BPA 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 BPA when a
      bundle has been fully transferred from a peer entity.

   Reception Intermediate Progress:  The TCPCL entity indicates to the
      BPA the intermediate progress of a transfer from the peer entity.
      This intermediate progress is at the granularity of each
      transferred segment.  An indication of intermediate reception
      gives a BPA the chance to inspect bundle header contents before
      the entire bundle is available and thus supports the "Interrupt
      Reception" capability.

   Reception Failure:  The TCPCL entity indicates to the BPA 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 entity 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; each TCPCL entity is acting on
   behalf of a BPA having a node ID.  This is used to assist in routing
   and forwarding messages by the BPA and is part of the authentication
   capability provided by TLS.

   Once negotiated, the parameters of a TCPCL session cannot change; 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 that a
   single entity can establish, beyond the limit imposed by the number
   of available (ephemeral) TCP ports of the active entity.

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

   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 has already 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 entity liveness
   information during otherwise messageless 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
   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 the other entity
   responds to it, as illustrated by Figures 13 and 14 in the next
   subsection.  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, the 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 bidirectional 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.

              +-------+
              | 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.

   Contact negotiation involves exchanging a Contact Header ("CH" in
   Figures 5, 6, and 7) in both directions and deriving a negotiated
   state from the two headers.  The contact negotiation sequencing is
   performed as either the active or passive entity and is illustrated
   in Figures 5 and 6, respectively, which both share the data
   validation and negotiation of the Processing of Contact Header
   ("[PCH]") activity (Figure 7) and the "[TCPCLOSE]" activity, which
   indicates TCP connection close.  Successful negotiation results in
   one of the Session Initiation ("[SI]") activities being performed, as
   shown further below.  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

               +-----------+
               |  Peer CH  |
               | available |
               +-----------+
                     |
                Validate and
                 Negotiate
                     V
                +------------+
                | Negotiated |--Failure-->[TCPCLOSE]
                +------------+
                   |       |
                 No TLS    +----Negotiate---+      [ST]
                   |               TLS      |       ^
                   V                        |    Failure
                 +-----------+              V       |
                 |   TCPCL   |            +---------------+
                 | Messaging |<--Success--| TLS Handshake |
                 | 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 as either the active or passive entity and is illustrated
   in Figures 8 and 9, respectively (where "[PSI]" means "Processing of
   Session Initiation"), which both share the data validation and
   negotiation shown in 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

 +-----------+
 |   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
   Figures 11 and 12, respectively.

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

                     Figure 11: Transfer Sender States

      |  Note 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.

                +-----------+                   +---------+
                |  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 defines requirements regarding 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 regarding TCPCL certificate use are broad, to support
   two quite different PKIX environments:

   DTN-Aware CAs:  In the ideal case, the CA or CAs 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 the node ID of the peer.  The TCPCL
      authenticates the node ID whenever possible; this is preferred
      over lower-level PKIX identities.

   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 is not ideal, as it allows the
      vulnerabilities described in Section 7.9, but it still provides
      some amount of mutual authentication to take place for a TCPCL
      session.

   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 a PKIX certificate are
   provided 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.  Some security policy recommendations are given
   in Section 4.4.5, but these are meant as a starting point for
   tailoring.  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 [DTN-BIBECT] to ensure strong
   bundle security.

   By using the Server Name Indication (SNI) DNS name (see
   Section 4.4.3), a single passive entity can act as a convergence
   layer for multiple BPAs 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 a single DNS name and/or
   node ID, using the SNI value from the peer to select which
   certificate to use.  The logic for mapping an SNI DNS name to an end-
   entity certificate is an implementation matter and can involve
   correlating a DNS name with a node ID or other certificate
   attributes.

3.5.  Session-Keeping Policies

   This specification defines requirements regarding how to initiate,
   sustain, and terminate a TCPCL session but does not impose any
   requirements on how sessions need to be managed by a BPA.  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 that needs to be exchanged for a set
      of transfers (assuming that the KEEPALIVE size is significantly
      smaller than the transfer size) and allowing the session state to
      indicate peer connectivity.  Drawbacks include wasted network
      resources when a session is mostly idle or when network
      connectivity is inconsistent (which requires that failed sessions
      be reestablished), 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 needs to be sent.  Benefits of this policy
      include not wasting network resources on sessions that are idle
      for long periods of time and avoiding potential queueing issues as
      can be seen when using a single persistent session.  Drawbacks
      include the TCP and TLS overhead of establishing 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
      that needs to send a transfer; when that is not the case, the
      polling behavior discussed below needs to happen.  This policy can
      be augmented to keep the session established as long as any
      transfers are queued.

   Active-Only Polling Sessions:  When naming and/or addressing of one
      entity is variable (i.e., a 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 entity can set
   the Segment Maximum Receive Unit (MRU) in its SESS_INIT message to
   determine the largest acceptable segment size, and a transmitting
   entity 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 using the
   TCPCL protocol, but guidance given here can be used to steer policy
   toward performance goals.  Administrators are 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 entity is transmitting
      a single XFER_SEGMENT message it is not able to transmit any
      XFER_ACK or XFER_REFUSE messages 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 the
      XFER_SEGMENT header size) and transmitters can always segment a
      transfer into maximum-size chunks no larger than the Segment MRU.
      This guarantees that any single XFER_SEGMENT will not monopolize
      the TCP stream for too long, which would prevent outgoing XFER_ACK
      and XFER_REFUSE messages associated with received transfers.

   Dynamic Segmentation:  Even after negotiation of a Segment MRU for
      each receiving entity, the actual transfer segmentation only needs
      to guarantee that 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 entity.  Additionally, future session extension and
   transfer extension types can apply further nuance to transfer
   policies and policy negotiation.

3.7.  Example Message Exchange

   Figure 15 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 entity 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        | ->      +-------------------------+
        +-------------------------+      <- |        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 will be opened only when there is a bundle that
   is queued for transmission and the routing algorithm selects a
   certain next-hop node.

4.1.  TCP Connection

   To establish a TCPCL session, an entity MUST first establish a TCP
   connection with the intended peer entity, typically by using the
   services provided by the operating system.  Destination port number
   4556 has been assigned by IANA as the registered port number for the
   TCPCL; see Section 8.1.  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 left
   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 reattempt 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 backoff mechanism
   to increase this delay in the case of repeated failures.  The upper
   limit on a reattempt backoff is implementation defined but SHOULD be
   no longer than one minute (60 seconds) before signaling to the BPA
   that a connection cannot be made.

   Once a TCP connection is established, the active entity SHALL
   immediately transmit its Contact Header.  The passive entity SHALL
   wait for the active entity's Contact Header.  Upon reception of a
   Contact Header, the passive entity SHALL transmit its Contact Header.
   If either entity does not receive a Contact Header after some
   implementation-defined time duration after the TCP connection is
   established, the waiting entity SHALL close the TCP connection.
   Entities SHOULD choose a Contact Header reception timeout interval no
   longer than one minute (60 seconds).  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, entities using TCPCL version 4 can coexist on a
   network with entities using earlier TCPCL versions (with some
   negotiation needed for interoperation, as described in Section 4.3).

   Within this specification, when an entity is said to "close" a TCP
   connection the entity SHALL use the TCP FIN mechanism and not the RST
   mechanism.  However, either mechanism, when received, will cause a
   TCP connection to become closed.

4.2.  Contact Header

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

   If the entity is configured to enable the exchange of messages
   according to TLS 1.3 [RFC8446] or any successors that are compatible
   with that TLS ClientHello, the CAN_TLS flag within its Contact Header
   SHALL be set to 1.  The RECOMMENDED policy is to enable TLS for all
   sessions, even if security policy does not allow or require
   authentication.  This follows the "opportunistic security" model
   specified in [RFC7435], though an active attacker could interfere
   with the exchange in such cases (see Section 7.4).

   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 as follows:

   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 protocol).

   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.

     +==========+========+===========================================+
     | Name     | Code   | Description                               |
     +==========+========+===========================================+
     | CAN_TLS  | 0x01   | If this bit is set, it indicates that the |
     |          |        | sending peer has enabled TLS security.    |
     +----------+--------+-------------------------------------------+
     | Reserved | others |                                           |
     +----------+--------+-------------------------------------------+

                       Table 1: Contact Header Flags

4.3.  Contact Validation and Negotiation

   Upon reception of the Contact Header, each entity 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 attempts to determine which 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 entity 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 a reason code of "Version mismatch", that
   entity 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 protocol 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 version of the protocol.  The decision of
   version fallback 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 whether the
      negotiated value of Enable TLS is acceptable.  A reasonable
      security policy would require or disallow the use of TLS,
      depending upon the desired network flows.  The RECOMMENDED policy
      is to require TLS for all sessions, even if security policy does
      not allow or require authentication.  Because this state is
      negotiated over an unsecured medium, there is a risk of TLS
      Stripping as described in Section 7.4.

      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 a 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 the TLS negotiation
      feature described in Section 4.4 begins immediately following the
      Contact Header exchange.

4.4.  Session Security

   This version of the TCPCL protocol supports establishing a 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 that sent the original (non-reply) SESS_TERM message SHOULD
   follow the closure alert procedure provided in [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 to use the TLS session resumption feature.  There is no
   guarantee that the passive entity will accept the request to resume a
   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 that presents it.

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

   Node ID:  The ideal certificate identity is the node ID of the entity
      using the NODE-ID, as defined below.  When the node ID is
      identified, there is no need for any lower-level identification to
      be present (though it can still be present, and if so it is also
      validated).

   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-IDs 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 an IPADDR-ID (as defined below), then one or more network
      addresses can be identified in the certificate.

   This specification defines a NODE-ID of a certificate as being the
   subjectAltName entry of type otherName with a name form of BundleEID
   (see Section 4.4.2.1) and a value limited to a node ID.  An entity
   SHALL ignore any entry of type otherName with a name form of
   BundleEID and a value that is some URI other than a node ID.  The
   NODE-ID is similar to the URI-ID as defined in [RFC6125] but is
   restricted to a node ID rather than a URI with a qualified-name
   authority part.  Unless specified otherwise by the definition of the
   URI scheme being authenticated, URI matching of a NODE-ID SHALL use
   the URI comparison logic provided in [RFC3986] and scheme-based
   normalization of those schemes specified in [RFC9171].  A URI scheme
   can refine this "exact match" logic with rules regarding how node IDs
   within that scheme are to be compared with the certificate-
   authenticated NODE-ID.

   This specification reuses the DNS-ID definition in Section 1.8 of
   [RFC6125], which is the subjectAltName entry of type dNSName whose
   value is encoded according to [RFC5280].

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

4.4.2.  Certificate Profile for the TCPCL

   All end-entity certificates used by a TCPCL entity SHALL conform to
   [RFC5280], or any updates or successors to that profile.  When an
   end-entity certificate is supplied, the full certification chain
   SHOULD be included unless security policy indicates that is
   unnecessary.  An entity SHOULD omit the root CA certificate (the last
   item of the chain) when sending a certification chain, as the
   recipient already has the root CA to anchor its validation.

   The TCPCL requires version 3 certificates due to the extensions used
   by this profile.  TCPCL entities SHALL reject as invalid version 1
   and version 2 end-entity certificates.

   TCPCL entities SHALL accept certificates that contain an empty
   Subject field or contain a Subject without a Common Name.  Identity
   information in end-entity certificates is contained entirely in the
   subjectAltName extension as defined in Section 4.4.1 and discussed in
   the paragraphs below.

   All end-entity and CA certificates used for the TCPCL SHOULD contain
   both a subject key identifier and an authority key identifier
   extension in accordance with [RFC5280].  TCPCL entities SHOULD NOT
   rely on either a subject key identifier or an authority key
   identifier being present in any received certificate.  Including key
   identifiers simplifies the work of an entity that needs to assemble a
   certification chain.

   Unless prohibited by CA policy, a TCPCL end-entity certificate SHALL
   contain a NODE-ID that authenticates the node ID of the peer.  When
   assigned one or more stable DNS names, a TCPCL end-entity certificate
   SHOULD contain a DNS-ID that authenticates those (fully qualified)
   names.  When assigned one or more stable network addresses, a TCPCL
   end-entity certificate MAY contain an IPADDR-ID that authenticates
   those addresses.

   When allowed by CA policy, a Bundle Protocol Security (BPSec; see
   [RFC9172]) end-entity certificate SHOULD contain a PKIX Extended Key
   Usage (EKU) extension in accordance with Section 4.2.1.12 of
   [RFC5280].  When the PKIX EKU extension is present, it SHOULD contain
   the key purpose id-kp-bundleSecurity (see Section 4.4.2.1).  Although
   not specifically required by the TCPCL, some networks or TLS
   implementations assume that id-kp-clientAuth and id-kp-serverAuth
   need to be used for the client side and the server side of TLS
   authentication, respectively.  For interoperability, a TCPCL end-
   entity certificate MAY contain an EKU with both id-kp-clientAuth and
   id-kp-serverAuth values.

   When allowed by CA policy, a TCPCL end-entity certificate SHOULD
   contain a PKIX key usage extension in accordance with Section 4.2.1.3
   of [RFC5280].  The PKIX key usage bit that is consistent with TCPCL
   security using TLS 1.3 is digitalSignature.  The specific algorithms
   used during the TLS handshake will determine which of those key uses
   are exercised.  Earlier versions of TLS can mandate the use of the
   keyEncipherment bit or the keyAgreement bit.

   When allowed by CA policy, a TCPCL end-entity certificate SHOULD
   contain an Online Certificate Status Protocol (OCSP) URI within an
   authority information access extension in accordance with
   Section 4.2.2.1 of [RFC5280].

4.4.2.1.  PKIX OID Allocations

   This document defines a PKIX Other Name Form identifier, id-on-
   bundleEID, in Appendix B; this identifier can be used as the type-id
   in a subjectAltName entry of type otherName.  The BundleEID value
   associated with the otherName type-id id-on-bundleEID SHALL be a URI,
   encoded as an IA5String, with a scheme that is present in the IANA
   "Bundle Protocol URI Scheme Types" registry [IANA-BUNDLE].  Although
   this Other Name Form allows any endpoint ID to be present, the NODE-
   ID defined in Section 4.4.1 limits its use to contain only a node ID.

   This document defines a PKIX EKU key purpose, id-kp-bundleSecurity,
   in Appendix B; this purpose can be used to restrict a certificate's
   use.  The id-kp-bundleSecurity purpose can be combined with other
   purposes in the same certificate.

4.4.3.  TLS Handshake

   The use of TLS is negotiated via 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 that
   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 regarding exposure of sensitive data are discussed in
   Section 7.

   The parameters within each TLS negotiation are implementation
   dependent but any TCPCL entity SHALL follow all recommended practices
   specified in 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):

   ClientHello:  When a resolved DNS name was used to establish the TCP
      connection, the TLS ClientHello SHOULD include a "server_name"
      extension in accordance with [RFC6066].  When present, the
      server_name extension SHALL contain a "HostName" value taken from
      the DNS name (of the passive entity) that 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.  The supplied end-entity certificate SHALL conform to
      the profile described in Section 4.4.2.  The passive entity MAY
      use the SNI DNS name to choose an appropriate server-side
      certificate that authenticates that DNS name.

   Certificate Request:  During the 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.  The supplied end-entity certificate SHALL conform
      to the profile described in Section 4.4.2.

   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.

4.4.4.  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 handling
   certificates and certificate data is separated into the following
   phases:

   1.  Validating the certification path from the end-entity certificate
       up to a trusted root CA.

   2.  Validating the EKU and other properties of the end-entity
       certificate.

   3.  Authenticating identities from a valid end-entity certificate.

   4.  Applying security policy to the result of each identity type
       authentication.

   The result of validating a peer identity (see Section 4.4.1) against
   one or more types of certificate claims is one of the following:

   Absent:  Indicating that no such claims are present in the
      certificate and the identity cannot be authenticated.

   Success:  Indicating that one or more such claims are present and at
      least one matches the peer identity value.

   Failure:  Indicating that one or more such claims are present and
      none match the peer identity.

4.4.4.1.  Certificate Path and Purpose Validation

   For any peer end-entity certificate received during the TLS
   handshake, the entity SHALL perform the certification path validation
   described in [RFC5280] up to one of the entity's trusted CA
   certificates.  If enabled by local policy, the entity SHALL perform
   an OCSP check of each certificate providing OCSP authority
   information in accordance with [RFC6960].  If certificate validation
   fails or if security policy disallows a certificate for any reason,
   the entity SHALL fail the TLS handshake with a "bad_certificate"
   alert.  Leaving out part of the certification chain can cause the
   entity to fail to validate a certificate if the certificates that
   were left out are unknown to the entity (see Section 7.6).

   For the end-entity peer certificate received during the TLS
   handshake, the entity SHALL apply security policy to the key usage
   extension (if present) and EKU extension (if present) in accordance
   with Sections 4.2.1.12 and 4.2.1.3 of [RFC5280], respectively, and
   with the profile discussed in Section 4.4.2 of this document.

4.4.4.2.  Network-Level Authentication

   Either during or immediately after the TLS handshake, each entity, if
   required by security policy, SHALL validate the following certificate
   identifiers together in accordance with Section 6 of [RFC6125]:

   *  If the active entity resolved a DNS name (of the passive entity)
      in order to initiate the TCP connection, that DNS name SHALL be
      used as a DNS-ID reference identifier.

   *  The IP address of the other side of the TCP connection SHALL be
      used as an IPADDR-ID reference identifier.

   If the network-level identifier's authentication result is Failure or
   if the result is Absent and security policy requires an authenticated
   network-level identifier, the entity SHALL terminate the session
   (with a reason code of "Contact Failure").

4.4.4.3.  Node ID Authentication

   Immediately before session parameter negotiation, each entity, if
   required by security policy, SHALL validate the certificate NODE-ID
   in accordance with Section 6 of [RFC6125] using the node ID of the
   peer's SESS_INIT message as the NODE-ID reference identifier.  If the
   NODE-ID validation 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").

4.4.5.  Policy Recommendations

   A RECOMMENDED security policy encompasses the following:

   *  enabling the use of OCSP checking during the TLS handshake.

   *  instructing that, if an EKU extension is present, the extension
      needs to contain id-kp-bundleSecurity (Section 4.4.2.1) to be
      usable with TCPCL security.

   *  requiring a validated node ID (Section 4.4.4.3) and ignoring any
      network-level identifier (Section 4.4.4.2).

   This policy relies on and informs the certificate requirements
   provided in Section 4.4.3.  This policy assumes that a DTN-aware CA
   (see Section 3.4) will only issue a certificate for a node ID when it
   has verified that the private key holder actually controls the bundle
   node; this is needed to avoid the threat identified in Section 7.9.
   This policy requires that a certificate contain a NODE-ID and allows
   the certificate to also contain network-level identifiers.  A
   tailored policy on a more controlled network could relax the
   requirement on node ID validation and allow just network-level
   identifiers to authenticate a peer.

4.4.6.  Example TLS Initiation

   A summary of a typical TLS initiation 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-ID and IPADDR-ID authentication occurs.
                     Secured TCPCL messaging can begin.

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

                        NODE-ID authentication 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

4.5.  Message Header

   After the initial exchange of a Contact Header and (if TLS is
   negotiated to be used) the TLS handshake, 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 contains the following field:

   Message Type:  Indicates the type of the message as per Table 2
      below.  Encoded values are listed in Section 8.5.

       +==============+======+=====================================+
       | Name         | Code | Description                         |
       +==============+======+=====================================+
       | SESS_INIT    | 0x07 | Contains the session parameter      |
       |              |      | inputs from one of the entities, as |
       |              |      | described in Section 4.6.           |
       +--------------+------+-------------------------------------+
       | SESS_TERM    | 0x05 | Indicates that one of the entities  |
       |              |      | participating in the session wishes |
       |              |      | to cleanly terminate the session,   |
       |              |      | as described in Section 6.1.        |
       +--------------+------+-------------------------------------+
       | 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 the 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 shown in Figure 19.

                      +-----------------------------+
                      |       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 as follows:

   Keepalive Interval:  A 16-bit unsigned integer indicating the minimum
      interval, in seconds, to negotiate as the Session Keepalive using
      the method described in 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
      relationship 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 relationship 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 a node ID rather than a truly empty node ID.  This case
      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 that sent the SESS_INIT
      message.  Every node ID SHALL be a URI consistent with the
      requirements in [RFC3986] and the URI schemes of the IANA "Bundle
      Protocol URI Scheme Types" registry [IANA-BUNDLE].  The node ID
      itself can be authenticated as described in Section 4.4.4.

   Session Extension Items Length and Session Extension Items list:  
      Together, these fields represent protocol extension data not
      defined by this specification.  The Session Extension Items Length
      is the total number of octets to follow that are used to encode
      the Session Extension Items 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 are significant, as
      defined in the associated type specification(s).  If the content
      of the Session Extension Items list disagrees with the Session
      Extension Items Length (e.g., the last item claims to use more or
      fewer octets than are indicated in the Session Extension Items
      Length), the reception of the SESS_INIT is considered to have
      failed.

   If an entity receives a peer node ID that is not authenticated (by
   the procedure described in Section 4.4.4.3), that node ID SHOULD NOT
   be used by a BPA for any discovery or routing functions.  Trusting an
   unauthenticated node ID can lead to the threat described in
   Section 7.9.

   When the active entity initiates a TCPCL session, it is likely based
   on routing information that binds a node ID to CL parameters used to
   initiate the session.  If the active entity receives a SESS_INIT with
   a 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
   entity (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 Transmission Unit (MTU)
      for whole transfers and individual segments is 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
      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, 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 as follows:

   Item Flags:  A one-octet field containing generic bit flags related
      to 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 set to 1, the entity SHALL terminate the TCPCL
      session with a 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 8.3).

   Item Length:  A 16-bit unsigned integer field containing the number
      of Item Value octets to follow.

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

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

                  Figure 20: Session Extension Item Format

         +==========+========+==================================+
         | Name     | Code   | Description                      |
         +==========+========+==================================+
         | CRITICAL | 0x01   | If this bit is set, it 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 4.7, a negotiated parameter of each session
   is the Session Keepalive Interval.  If the negotiated Session
   Keepalive is zero (i.e., one or both SESS_INIT messages contain 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 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).

   The chosen Keepalive Interval SHOULD NOT be too short, as TCP
   retransmissions may occur in the 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 a reason code of "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 that is unknown to it
   (possibly due to an unhandled protocol version mismatch or an
   incorrectly negotiated session extension that 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 that is known but is inappropriate for
   the negotiated session parameters (possibly due to an 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 that is inappropriate for the current
   session state (e.g., a SESS_INIT after the session has already been
   established or a 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 shown in Figure 21.

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

                  Figure 21: Format of MSG_REJECT Messages

   The fields of the MSG_REJECT message are as follows:

   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 Type | 0x01 | A message was received with a Message  |
     | Unknown      |      | Type code unknown to the TCPCL entity. |
     +--------------+------+----------------------------------------+
     | Message      | 0x02 | A message was received, but the TCPCL  |
     | Unsupported  |      | 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 discussed in this section are directly associated
   with transferring a bundle between TCPCL entities.

   A single TCPCL transfer results in the exchange of a bundle (handled
   by the convergence layer as opaque data) between two entities.  In
   the TCPCL, a transfer is accomplished by dividing a single bundle up
   into "segments" based on the receiving-side Segment MRU, which is
   defined in Section 4.6.  The choice of the length to use for segments
   is an implementation matter, but each segment MUST NOT be larger than
   the receiving entity's Segment MRU.  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 on the TCPCL
   itself.

   If multiple bundles are transmitted on a single TCPCL connection,
   they MUST be transmitted consecutively, without the 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 is not related to such concepts as bundle
   fragmentation.  Each invocation of the TCPCL by the BPA, 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 acknowledgments.

   Transfer IDs from each entity SHALL be unique within a single TCPCL
   session.  Upon exhaustion of the entire 64-bit Transfer ID space, the
   sending entity SHALL terminate the session with a SESS_TERM reason
   code of "Resource Exhaustion".  For bidirectional bundle transfers, a
   TCPCL entity SHOULD NOT rely on any relationship between Transfer IDs
   originating from each side of the TCPCL session.

   Although there is not a strict requirement for initial Transfer ID
   values or the ordering of Transfer IDs (see Section 7.13), in the
   absence of any other mechanism for generating Transfer IDs, an entity
   SHALL use the following algorithm: the initial Transfer ID from each
   entity 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 is shown in Figure 22.

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

                 Figure 22: Format of XFER_SEGMENT Messages

   The fields of the XFER_SEGMENT message are as follows:

   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 Items Length and Transfer Extension Items list:
      Together, these fields represent protocol extension data for this
      specification.  The Transfer Extension Items Length and Transfer
      Extension Items list SHALL only be present when the START flag is
      set to 1 on the message.  The Transfer Extension Items Length is
      the total number of octets to follow that are used to encode the
      Transfer Extension Items 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 are significant, as
      defined in the associated type specification(s).  If the content
      of the Transfer Extension Items list disagrees with the Transfer
      Extension Items Length (e.g., the last item claims to use more or
      fewer octets than are indicated in the Transfer Extension Items
      Length), the reception of the XFER_SEGMENT is considered to have
      failed.

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

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

    +==========+========+============================================+
    | Name     | Code   | Description                                |
    +==========+========+============================================+
    | END      | 0x01   | If this bit is set, it indicates that this |
    |          |        | is the last segment of the transfer.       |
    +----------+--------+--------------------------------------------+
    | START    | 0x02   | If this bit is set, it 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 flag and the END flag 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 entity 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 entity transmits
   acknowledgments of reception of data segments.

   The format of a XFER_ACK message is shown in Figure 23.

                      +-----------------------------+
                      |       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 as follows:

   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 that are being acknowledged.

   A receiving TCPCL entity SHALL send a 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 entity 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 entity 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 lengths 800 and 1800, respectively.

5.2.4.  Transfer Refusal (XFER_REFUSE)

   The TCPCL supports a mechanism by which a receiving entity can
   indicate to the sender that it does not want to receive the
   corresponding bundle.  To do so, upon receiving a XFER_SEGMENT
   message, the entity 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 relationship between the Transfer MRU of a TCPCL
   entity (which is supposed to represent a firm limitation of what the
   entity will accept) and the 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 a 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 related to
   responding to a XFER_REFUSE based on its reason code; the refusal is
   passed up as an indication to the BPA that the transfer has been
   refused.  If a transfer refusal has a reason code that is not
   decodable to the BPA, the agent SHOULD treat the refusal as having a
   reason code of "Unknown".

   The format of the XFER_REFUSE message is shown 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 as follows:

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

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

     +=============+======+==========================================+
     | Name        | Code | Description                              |
     +=============+======+==========================================+
     | Unknown     | 0x00 | The reason for refusal is unknown or is  |
     |             |      | 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.  |
     | Resources   |      | The 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        |
     | Failure     |      | Extension Items has occurred.            |
     +-------------+------+------------------------------------------+
     | Session     | 0x06 | The receiving entity is in the process   |
     | Terminating |      | of terminating the session.  The sender  |
     |             |      | MAY retry the same bundle at a later     |
     |             |      | time in a different session.             |
     +-------------+------+------------------------------------------+

                     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 subsequently
   commence transmission of any further segments of the refused bundle.
   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 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 as follows:

   Item Flags:  A one-octet field containing generic bit flags related
      to 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 entity
      receives a Transfer Extension Item with an unknown Item Type and
      the CRITICAL flag is 1, the entity SHALL refuse the transfer with
      a 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 8.4).

   Item Length:  A 16-bit unsigned integer field containing the number
      of Item Value octets to follow.

   Item Value:  A variable-length data field that 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

         +==========+========+==================================+
         | Name     | Code   | Description                      |
         +==========+========+==================================+
         | CRITICAL | 0x01   | If this bit is set, it 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 entity 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 regarding the potential length of
   the transfer.  The Transfer Length Extension SHALL use the IANA-
   assigned code point from Section 8.4.

   If a transfer occupies exactly one segment (i.e., both the START flag
   and the END flag 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 Extension data is shown in
   Figure 26.

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

            Figure 26: Format of Transfer Length Extension Data

   The Transfer Length Extension data contains the following field:

   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 a XFER_REFUSE with a reason
      code of "Not Acceptable".

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 TCP connection 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 entity 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
   acknowledgment.

   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.

   *  an entity SHALL NOT begin any new outgoing transfer for the
      remainder of the session.

   *  an entity SHALL NOT accept any new incoming transfer for the
      remainder of the session.

   If a new incoming transfer is attempted while in the Ending state,
   the receiving entity SHALL send a XFER_REFUSE with a reason code of
   "Session Terminating".

   There are circumstances where an entity has an urgent need to close a
   TCP connection associated with a TCPCL session, without waiting for
   transfers to complete but also in a way that doesn't force timeouts
   to occur -- for example, due to impending shutdown of the underlying
   data-link layer.  Instead of following a clean termination sequence,
   after transmitting a SESS_TERM message, an entity MAY perform an
   unclean termination by immediately closing the associated TCP
   connection.  When performing an unclean termination, an entity SHOULD
   acknowledge all received XFER_SEGMENTs with a XFER_ACK before closing
   the TCP connection.  Not acknowledging received segments can result
   in unnecessary bundle or bundle fragment retransmissions.  Any delay
   between a request to close the TCP connection and the actual closing
   of the connection (a "half-closed" state) MAY be ignored by the TCPCL
   entity.  If the underlying TCP connection is closed during a
   transmission (in either transfer stream), the transfer SHALL be
   indicated to the BPA as failed (see the transmission failure and
   reception failure indications defined in Section 3.1).

   The TCPCL itself does not have any required behavior related to
   responding to a SESS_TERM based on its reason code; the termination
   is passed up as an indication to the BPA that the session state has
   changed.  If a termination has a reason code that is not decodable to
   the BPA, the agent SHOULD treat the termination as having a reason
   code of "Unknown".

   The format of the SESS_TERM message is shown 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 as follows:

   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 this bit is set, it indicates that |
       |          |        | this message is an acknowledgment of  |
       |          |        | an earlier SESS_TERM message.         |
       +----------+--------+---------------------------------------+
       | Reserved | others |                                       |
       +----------+--------+---------------------------------------+

                          Table 8: SESS_TERM Flags

    +==============+======+==========================================+
    | Name         | Code | Description                              |
    +==============+======+==========================================+
    | Unknown      | 0x00 | A termination reason is not available.   |
    +--------------+------+------------------------------------------+
    | Idle timeout | 0x01 | The session is being terminated due to   |
    |              |      | idleness.                                |
    +--------------+------+------------------------------------------+
    | Version      | 0x02 | The entity cannot conform to the         |
    | mismatch     |      | specified TCPCL protocol version.        |
    +--------------+------+------------------------------------------+
    | Busy         | 0x03 | The entity is too busy to handle the     |
    |              |      | current session.                         |
    +--------------+------+------------------------------------------+
    | Contact      | 0x04 | The entity cannot interpret or negotiate |
    | Failure      |      | a Contact Header or SESS_INIT option.    |
    +--------------+------+------------------------------------------+
    | Resource     | 0x05 | The entity has run into some resource    |
    | Exhaustion   |      | limit 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 transmissions).  This can, for example, be used as a
   notification 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 the sending of a protocol
   message has completed, 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 another message.  This is
   particularly important when large segment sizes are being
   transmitted; either the entire XFER_SEGMENT is sent before a
   SESS_TERM message or the connection is simply terminated mid-
   XFER_SEGMENT.

6.2.  Idle Session Termination

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

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

7.  Security Considerations

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

7.1.  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 7.12).

7.2.  Threat: Passive Leak of Bundle Data

   The TCPCL can be used to provide point-to-point transport security,
   but it does not provide security of data at rest and does not
   guarantee end-to-end bundle security.  The bundle security mechanisms
   defined in [RFC9172] 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 7.12).

7.3.  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 the TCPCL protocol that does not support transport
   security.  An on-path attacker can also manipulate a Contact Header
   to present a lower protocol version than desired.

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

7.4.  Threat: Transport Security Stripping

   When security policy allows non-TLS sessions, the TCPCL does not
   protect against active network attackers.  It is possible for an on-
   path attacker to set the CAN_TLS flag to 0 on either side of the
   Contact Header exchange, which will cause the negotiation discussed
   in Section 4.3 to disable TLS.  This leads to the "SSL Stripping"
   attack described in [RFC7457].

   The purpose of the CAN_TLS flag is to allow the use of the TCPCL on
   entities that 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 occur, regardless of the policy-driven parameters
   of the handshake and policy-driven handling of the handshake outcome.

   One mechanism to mitigate the possibility of TLS Stripping is the use
   of DNS-based Authentication of Named Entities (DANE) [RFC6698] toward
   the passive peer.  This mechanism relies on DNS and is
   unidirectional, so it doesn't help with applying policy toward the
   active peer, but it can be useful in an environment using
   opportunistic security.  The configuration and use of DANE are
   outside of the scope of this document.

   The negotiated use of TLS is identical in behavior to the use of
   STARTTLS as described in [RFC2595], [RFC4511], and others.

7.5.  Threat: Weak TLS Configurations

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

7.6.  Threat: Untrusted End-Entity Certificate

   The authentication method discussed in Section 4.4.4 uses end-entity
   certificates chained to a trusted root CA.  During a TLS handshake,
   either entity can send a certificate set that does not contain the
   full chain, possibly excluding intermediate or root CAs.  In an
   environment where peers are known to already contain needed root and
   intermediate CAs, there is no need to include those CAs, but this
   carries the risk of an entity not actually having one of the needed
   CAs.

7.7.  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 BPA treats the peer node ID within a TCPCL session as
   authoritative, and exploitation via an invalid certificate could lead
   to bundle data leaking and/or denial of service to the node ID being
   impersonated.

   There are many reasons, as described in [RFC5280] and [RFC6125], 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 OCSP [RFC6960] is used by the CA.  The
   configuration and use of particular certificate validation methods
   are outside of the scope of this document.

7.8.  Threat: Symmetric Key Limits

   Even with a secure block cipher and securely established session
   keys, there are limits to the amount of plaintext that 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.

7.9.  Threat: BP Node Impersonation

   The certificates exchanged by TLS enable authentication of the peer
   DNS name and node ID, but it is possible that either a peer does not
   provide a valid certificate or the certificate does not validate
   either the DNS-ID/IPADDR-ID 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 as provided in
   Section 4.4.4 bind the DNS-ID/IPADDR-ID or NODE-ID based on the
   contents of the certificate.

   The DNS-ID/IPADDR-ID validation is a weaker form of authentication,
   because even if a peer is operating on an authenticated network DNS
   name or IP address 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 results in a more
   stable identity.

   NODE-ID validation ensures that the peer to which a bundle is
   transferred is in fact the node that the BPA expects it to be.  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 when a DNS-ID/IPADDR-ID authentication can be
   trusted to validate a node ID is also a policy matter outside of the
   scope of this document.

   One mitigation regarding arbitrary entities with valid PKIX
   certificates impersonating arbitrary node IDs is the use of the PKIX
   EKU key purpose id-kp-bundleSecurity (Section 4.4.2.1).  When this
   EKU is present in the certificate, it represents a stronger assertion
   that the private key holder should in fact be trusted to operate as a
   bundle node.

7.10.  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-
   behaved sessions on an entity, or could affect all sessions on a
   host.

   A malicious entity can trigger timeouts by continually establishing
   TCPCL sessions and delaying the sending of protocol-required data.
   The victim entity can block TCP connections from network peers that
   are thought to behave incorrectly within the 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 a XFER_REFUSE
   message or can forcibly terminate the session.

   A "data dribble" attack is also possible, in which an entity presents
   a very small Segment MRU that causes transfers to be split among a
   large number of very small segments and causes the resultant
   segmentation overhead to overwhelm the actual bundle data segments.
   Similarly, an entity can present a very small Transfer MRU that 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 parameter negotiation
   (Section 4.7) if the MRUs are unacceptable.

   An abusive entity could cause the keepalive mechanism to waste
   throughput within a network link that would otherwise be usable for
   bundle 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 parameter negotiation (Section 4.7)
   if the Keepalive Interval is unacceptable.

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

7.11.  Mandatory-to-Implement TLS

   Following IETF best current practice, TLS is mandatory to implement
   for all TCPCL implementations but TLS is optional to use for a given
   TCPCL session.  The policy recommendations in Sections 4.2 and 4.3
   both enable TLS and require TLS, but entities are permitted to
   disable and not require TLS based on local configuration.  The
   configuration to enable or require TLS for an entity or a session is
   outside of the scope of this document.  The configuration to disable
   TLS is different from the threat of TLS Stripping as described in
   Section 7.4.

7.12.  Alternate Uses of TLS

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

7.12.1.  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 that
   authenticates only the passive entity of a TCPCL session or that does
   not authenticate either entity.  Using TLS in a way that 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
   properties similar to the opportunistic security model [RFC7435].

7.12.2.  Non-certificate TLS Use

   In environments where PKI is unavailable, alternate uses of TLS that
   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 the TCPCL.
   Using non-PKI node authentication methods is outside of the scope of
   this document.

7.13.  Predictability of Transfer IDs

   The only requirement on Transfer IDs is that they be unique within
   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 an on-path attacker causing
   denial of service with XFER_REFUSE messages, it is not possible to
   preemptively refuse a transfer, so there is no benefit in having
   unpredictable Transfer IDs within a session.

8.  IANA Considerations

   Registration procedures referred to in this section (e.g., the RFC
   Required policy) are defined in [RFC8126].

   Some of the registries have been defined as version specific for
   TCPCLv4, and these registries reuse 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.

8.1.  Port Number

   Within the "Service Name and Transport Protocol Port Number Registry"
   [IANA-PORTS], TCP port number 4556 had previously been assigned as
   the default port for the TCPCL; see [RFC7242].  This assignment is
   unchanged by TCPCL version 4, but the assignment reference has been
   updated to point to this specification.  Each TCPCL entity identifies
   its TCPCL protocol version in its initial contact (see Sections 3.2
   and 8.2), so there is no ambiguity regarding 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                       |
          +------------------------+----------------------------+

                  Table 10: TCP Port Number for the TCPCL

8.2.  Protocol Versions

   IANA has registered the following value in the "Bundle Protocol TCP
   Convergence-Layer Version Numbers" registry [RFC7242].

               +=======+=============+====================+
               | Value | Description | Reference          |
               +=======+=============+====================+
               | 4     | TCPCLv4     | This specification |
               +-------+-------------+--------------------+

                    Table 11: New TCPCL Version Number

8.3.  Session Extension Types

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 Session
   Extension Types" registry and populated it with the contents of
   Table 12.  The registration procedure is Expert Review within the
   lower range 0x0001-0x7FFF.  Values in the range 0x8000-0xFFFF are
   reserved for Private or Experimental Use, which are not recorded by
   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.

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

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

                  Table 12: Session Extension Type Codes

8.4.  Transfer Extension Types

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 Transfer
   Extension Types" registry and populated it with the contents of
   Table 13.  The registration procedure is Expert Review within the
   lower range 0x0001-0x7FFF.  Values in the range 0x8000-0xFFFF are
   reserved for Private or Experimental Use, which are not recorded by
   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.

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

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

                 Table 13: Transfer Extension Type Codes

8.5.  Message Types

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 Message Types"
   registry and populated it with the contents of Table 14.  The
   registration procedure is RFC Required within the lower range
   0x01-0xEF.  Values in the range 0xF0-0xFF are reserved for Private or
   Experimental Use, which are not recorded by 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 needs 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.

   Experts 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.

         +===========+==========================================+
         | 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 | Reserved for Private or Experimental Use |
         +-----------+------------------------------------------+

                       Table 14: Message Type Codes

8.6.  XFER_REFUSE Reason Codes

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE
   Reason Codes" registry and populated it with the contents of
   Table 15.  The registration procedure is Specification Required
   within the lower range 0x00-0xEF.  Values in the range 0xF0-0xFF are
   reserved for Private or Experimental Use, which are not recorded by
   IANA.

   Specifications of new XFER_REFUSE reason codes need to define the
   meaning of the reason and disambiguate it from preexisting reasons.
   Each refusal reason needs to be usable by the receiving BPA to make
   retransmission or rerouting decisions.

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

         +===========+==========================================+
         | Code      | Refusal Reason                           |
         +===========+==========================================+
         | 0x00      | Unknown                                  |
         +-----------+------------------------------------------+
         | 0x01      | Completed                                |
         +-----------+------------------------------------------+
         | 0x02      | No Resources                             |
         +-----------+------------------------------------------+
         | 0x03      | Retransmit                               |
         +-----------+------------------------------------------+
         | 0x04      | Not Acceptable                           |
         +-----------+------------------------------------------+
         | 0x05      | Extension Failure                        |
         +-----------+------------------------------------------+
         | 0x06      | Session Terminating                      |
         +-----------+------------------------------------------+
         | 0x07-0xEF | Unassigned                               |
         +-----------+------------------------------------------+
         | 0xF0-0xFF | Reserved for Private or Experimental Use |
         +-----------+------------------------------------------+

                    Table 15: XFER_REFUSE Reason Codes

8.7.  SESS_TERM Reason Codes

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 SESS_TERM Reason
   Codes" registry and populated it with the contents of Table 16.  The
   registration procedure is Specification Required within the lower
   range 0x00-0xEF.  Values in the range 0xF0-0xFF are reserved for
   Private or Experimental Use, which are not recorded by IANA.

   Specifications of new SESS_TERM reason codes need to define the
   meaning of the reason and disambiguate it from preexisting reasons.
   Each termination reason needs to be usable by the receiving BPA to
   make reconnection decisions.

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

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

                     Table 16: SESS_TERM Reason Codes

8.8.  MSG_REJECT Reason Codes

   Under the "Bundle Protocol" registry [IANA-BUNDLE], IANA has created
   the "Bundle Protocol TCP Convergence-Layer Version 4 MSG_REJECT
   Reason Codes" registry and populated it with the contents of
   Table 17.  The registration procedure is Specification Required
   within the lower range 0x01-0xEF.  Values in the range 0xF0-0xFF are
   reserved for Private or Experimental Use, which are not recorded by
   IANA.

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

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

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

                    Table 17: MSG_REJECT Reason Codes

8.9.  Object Identifier for PKIX Module Identifier

   IANA has registered the following in the "SMI Security for PKIX
   Module Identifier" registry [IANA-SMI] for identifying the module
   described in Appendix B.

        +=========+=========================+====================+
        | Decimal | Description             | References         |
        +=========+=========================+====================+
        | 103     | id-mod-dtn-tcpclv4-2021 | This specification |
        +---------+-------------------------+--------------------+

                    Table 18: New SMI Security Module

8.10.  Object Identifier for PKIX Other Name Forms

   IANA has registered the following in the "SMI Security for PKIX Other
   Name Forms" registry [IANA-SMI] for identifying bundle endpoint IDs:

            +=========+=================+====================+
            | Decimal | Description     | References         |
            +=========+=================+====================+
            | 11      | id-on-bundleEID | This specification |
            +---------+-----------------+--------------------+

                    Table 19: New PKIX Other Name Form

   The formal structure of the associated Other Name Form is provided in
   Appendix B.  The use of this OID is defined in Sections 4.4.1 and
   4.4.2.

8.11.  Object Identifier for PKIX Extended Key Usage

   IANA has registered the following in the "SMI Security for PKIX
   Extended Key Purpose" registry [IANA-SMI] for securing BP bundles.

          +=========+======================+====================+
          | Decimal | Description          | References         |
          +=========+======================+====================+
          | 35      | id-kp-bundleSecurity | This specification |
          +---------+----------------------+--------------------+

                  Table 20: New PKIX Extended Key Purpose

   The formal definition of this EKU is provided in Appendix B.  The use
   of this OID is defined in Section 4.4.2.

9.  References

9.1.  Normative References

   [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/>.

   [IANA-SMI] IANA, "Structure of Management Information (SMI) Numbers
              (MIB Module Registrations)",
              <https://www.iana.org/assignments/smi-numbers/>.

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

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

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

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

   [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>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [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>.

   [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>.

   [RFC9171]  Burleigh, S., Fall, K., and E. Birrane, III, "Bundle
              Protocol Version 7", RFC 9171, DOI 10.17487/RFC9171,
              January 2022, <https://www.rfc-editor.org/info/rfc9171>.

   [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
              <https://www.itu.int/rec/T-REC-X.680-202102-I/en>.

9.2.  Informative References

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

   [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>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [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>.

   [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>.

   [RFC9172]  Birrane, III, E. and K. McKeever, "Bundle Protocol
              Security (BPSec)", RFC 9172, DOI 10.17487/RFC9172, January
              2022, <https://www.rfc-editor.org/info/rfc9172>.

   [DTN-BIBECT]
              Burleigh, S., "Bundle-in-Bundle Encapsulation", Work in
              Progress, Internet-Draft, draft-ietf-dtn-bibect-03, 18
              February 2020, <https://datatracker.ietf.org/doc/html/
              draft-ietf-dtn-bibect-03>.

Appendix A.  Significant Changes from RFC 7242

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

   *  Split Contact Header into pre-TLS protocol negotiation and
      SESS_INIT parameter negotiation.  The Contact Header is now fixed
      length.

   *  Changed Contact Header content to limit number of negotiated
      options.

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

   *  Renamed "endpoint ID" to "node ID" to conform with BPv7
      terminology.

   *  Added session extension capability.

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

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

   *  Pointed out that segments of all new IANA registries are reserved
      for private/experimental use.

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

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

   *  Added flags in XFER_ACK to mirror flags from XFER_SEGMENT.

   *  Removed all uses of Self-Delimiting Numeric Value (SDNV) fields
      and replaced with fixed-bit-length (network byte order) fields.

   *  Renamed SHUTDOWN to SESS_TERM to deconflict term "shutdown"
      related to TCP connections.

   *  Removed the notion of a reconnection delay parameter.

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

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

   *  Added TLS connection security mechanism.

   *  Added "Not Acceptable", "Extension Failure", and "Session
      Terminating" XFER_REFUSE reason codes.

   *  Added "Contact Failure" (contact negotiation failure) and
      "Resource Exhaustion" SESS_TERM reason codes.

Appendix B.  ASN.1 Module

   The following ASN.1 module formally specifies the BundleEID
   structure, its Other Name Form, and the bundleSecurity EKU, using
   ASN.1 syntax per [X.680].  This specification uses the ASN.1
   definitions from [RFC5912] with the 2002 ASN.1 notation used in that
   document.

   <CODE BEGINS>
   DTN-TCPCLv4-2021
     { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-dtn-tcpclv4-2021(103) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
     OTHER-NAME
     FROM PKIX1Implicit-2009 -- [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-implicit-02(59) }

     id-pkix
     FROM PKIX1Explicit-2009 -- [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51) } ;

   id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }

   id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

   DTNOtherNames OTHER-NAME ::= { on-bundleEID, ... }

   -- The otherName definition for BundleEID
   on-bundleEID OTHER-NAME ::= {
       BundleEID IDENTIFIED BY { id-on-bundleEID }
   }

   id-on-bundleEID OBJECT IDENTIFIER ::= { id-on 11 }

   -- Same encoding as GeneralName of uniformResourceIdentifier
   BundleEID ::= IA5String

   -- The Extended Key Usage key for bundle security
   id-kp-bundleSecurity OBJECT IDENTIFIER ::= { id-kp 35 }

   END
   <CODE ENDS>

Appendix C.  Example of the BundleEID Other Name Form

   This non-normative example demonstrates an otherName with a name form
   of BundleEID to encode the node ID "dtn://example/".

   The hexadecimal form of the DER encoding of the otherName is as
   follows:

   a01c06082b0601050507080ba010160e64746e3a2f2f6578616d706c652f

   And the text decoding in Figure 28 is an output of Peter Gutmann's
   "dumpasn1" program.

     0  28: [0] {
     2   8:   OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 11'
    12  16:   [0] {
    14  14:     IA5String 'dtn://example/'
          :     }
          :   }

             Figure 28: Visualized Decoding of the on-bundleEID

Acknowledgments

   This specification is based on comments regarding the implementation
   of [RFC7242] as provided by Scott Burleigh.

   The ASN.1 module and its Other Name Form are based on a
   recommendation provided by Russ Housley.

Authors' Addresses

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

   Email: brian.sipos+ietf@gmail.com

   Michael Demmer

   Email: demmer@gmail.com

   Jörg Ott
   Technical University of Munich
   Department of Informatics
   Chair of Connected Mobility
   Boltzmannstrasse 3
   DE-85748 Garching
   Germany

   Email: ott@in.tum.de

   Simon Perreault
   LogMeIn
   410 boulevard Charest Est
   Suite 250
   Quebec QC G1K 8G3
   Canada

   Email: simon.perreault@logmein.com