Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
draft-ietf-dtn-tcpclv4-04

Abstract

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

Status of This Memo

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

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

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

This Internet-Draft will expire on June 13, 2018.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 2.1. Definitions Specific to the TCPCL Protocol
• 3. General Protocol Description
• 3.1. TCPCL Session Overview
• 3.2. Example Message Exchange
• 4. Session Establishment
• 4.1. Contact Header
• 4.1.1. Header Extension Items
• 4.2. Validation and Parameter Negotiation
• 4.3. Session Security
• 4.3.1. TLS Handshake Result
• 4.3.2. Example TLS Initiation
• 5. Established Session Operation
• 5.1. Message Type Codes
• 5.2. Upkeep and Status Messages
• 5.2.1. Session Upkeep (KEEPALIVE)
• 5.2.2. Message Rejection (MSG_REJECT)
• 5.3. Bundle Transfer
• 5.3.1. Bundle Transfer ID
• 5.3.2. Transfer Initialization (XFER_INIT)
• 5.3.3. Data Transmission (XFER_SEGMENT)
• 5.3.4. Data Acknowledgments (XFER_ACK)
• 5.3.5. Transfer Refusal (XFER_REFUSE)
• 6. Session Termination
• 6.1. Shutdown Message (SHUTDOWN)
• 6.2. Idle Session Shutdown
• 7. Security Considerations
• 8. IANA Considerations
• 8.1. Port Number
• 8.2. Protocol Versions
• 8.3. Header Extension Types
• 8.4. Message Types
• 8.5. XFER_REFUSE Reason Codes
• 8.6. SHUTDOWN Reason Codes
• 8.7. MSG_REJECT Reason Codes
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Significant changes from RFC7242
• Authors' Addresses

1. Introduction

This document describes the TCP-based convergence-layer protocol for Delay-Tolerant Networking. Delay-Tolerant Networking is an end-to- end architecture providing communications in and/or through highly stressed environments, including those with intermittent connectivity, long and/or variable delays, and high bit error rates. More detailed descriptions of the rationale and capabilities of these networks can be found in "Delay-Tolerant Network Architecture" [RFC4838].

An important goal of the DTN architecture is to accommodate a wide range of networking technologies and environments. The protocol used for DTN communications is the revised Bundle Protocol (BP) [I-D.ietf-dtn-bpbis], an application-layer protocol that is used to construct a store-and- forward overlay network. As described in the Bundle Protocol specification [I-D.ietf-dtn-bpbis], it requires the services of a "convergence- layer adapter" (CLA) to send and receive bundles using the service of some "native" link, network, or Internet protocol. This document describes one such convergence-layer adapter that uses the well-known Transmission Control Protocol (TCP). This convergence layer is referred to as TCPCL.

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

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

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

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

• The format of protocol data units of the Bundle Protocol, as those are defined elsewhere in [RFC5050] and [I-D.ietf-dtn-bpbis]. This includes the concept of bundle fragmentation or bundle encapsulation. The TCPCL transfers bundles as opaque data blocks.
• Mechanisms for locating or identifying other bundle nodes within an internet.

2. Requirements Language

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

2.1. Definitions Specific to the TCPCL Protocol

This section contains definitions that are interpreted to be specific to the operation of the TCPCL protocol, as described below.

TCPCL Node:

A TCPCL node refers to either side of an negotiating or in-service TCPCL Session. For most TCPCL behavior, the two nodes are symmetric and there is no protocol distinction between them. Some specific behavior, particularly during negotiation, distinguishes between the connecting node and the connected-to node. For the remainder of this document, the term "node" without the prefix "TCPCL" refers to a TCPCL node.

TCP Connection:

A TCP connection refers to a transport connection using TCP as the transport protocol.

TCPCL Session:

A TCPCL session (as opposed to a TCP connection) is a TCPCL communication relationship between two bundle nodes. The lifetime of a TCPCL session is bound to the lifetime of an underlying TCP connection. Therefore, a TCPCL session is initiated after a bundle node establishes a TCP connection to for the purposes of bundle communication. A TCPCL session is terminated when the TCP connection ends, due either to one or both nodes actively terminating the TCP connection or due to network errors causing a failure of the TCP connection. For the remainder of this document, the term "session" without the prefix "TCPCL" refers to a TCPCL session.

Session parameters:

The session parameters 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 node and thereby to the TCPCL is implementation dependent. However, the mechanism by which two bundle nodes exchange and negotiate the values to be used for a given session is described in Section 4.2.

Transfer:

Transfer refers to the procedures and mechanisms (described below) for conveyance of an individual bundle from one node to another. Each transfer within TCPCLv4 is identified by a Transfer ID number which is unique only to a single direction within a single Session.

Reason Codes:

The TCPCL uses numeric codes to encode specific reasons for individual failure/error message types. This limits the expressiveness of TCPCL error encodings, but simplifies the encoding of errors and allows an application policy to attempt recovery from expected 'failure' modes (e.g. if a Session cannot be established with USE_TLS disabled because of a Contact Failure shutdown, a re-attempt can be made with USE_TLS enabled).

3. General Protocol Description

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

3.1. TCPCL Session Overview

First, one node establishes a TCPCL session to the other by initiating a TCP connection in accordance with [RFC0793]. After setup of the TCP connection is complete, an initial contact header is exchanged in both directions to set parameters of the TCPCL session and exchange a singleton endpoint identifier for each node (not the singleton Endpoint Identifier (EID) of any application running on the node) to denote the bundle-layer identity of each DTN node. This is used to assist in routing and forwarding messages (e.g. to prevent loops).

Once the TCPCL session is established and configured in this way, bundles can be transferred in either direction. Each transfer is performed by an initialization (XFER_INIT) message followed by one or more logical segments of data within an XFER_SEGMENT message. The choice of the length to use for segments is an implementation matter, but each segment must be no larger than the receiving node's maximum receive unit (MRU) (see the field "Segment MRU" of Section 4.1). The first segment for a bundle MUST set the 'START' flag, and the last one MUST set the 'end' flag in the XFER_SEGMENT message flags.

If multiple bundles are transmitted on a single TCPCL connection, they MUST be transmitted consecutively. Interleaving data segments from different bundles is not allowed. Bundle interleaving can be accomplished by fragmentation at the BP layer or by establishing multiple TCPCL sessions.

A feature of this protocol is for the receiving node to send acknowledgments as bundle data segments arrive (XFER_ACK). The rationale behind these acknowledgments is to enable the sender node to determine how much of the bundle has been received, so that in case the session is interrupted, it can perform reactive fragmentation to avoid re-sending the already transmitted part of the bundle. For each data segment that is received, the receiving node sends an XFER_ACK message containing the cumulative length of the bundle that has been received. The sending node MAY transmit multiple XFER_SEGMENT messages without necessarily waiting for the corresponding XFER_ACK responses. This enables pipelining of messages on a channel. In addition, there is no explicit flow control on the TCPCL layer.

Another feature is that a receiver MAY 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 current bundle, after completing transmission of a partially sent data segment. Note: This enables a cross-layer optimization in that it allows a receiver that detects that it already has received a certain bundle to interrupt transmission as early as possible and thus save transmission capacity for other bundles.

For sessions that are idle, a KEEPALIVE message is sent at a negotiated interval. This is used to convey node liveness information.

Finally, before sessions close, a SHUTDOWN message is sent to the session peer. A SHUTDOWN message MAY also be used to refuse a session setup by a peer (see Section 4.2). After sending a SHUTDOWN message, the sender of the message MAY send further acknowledgments (XFER_ACK or XFER_REFUSE) but no further data messages (XFER_INIT or XFER_SEGMENT). After receving a SHUTDOWN message and when no transfers are in-progress (i.e. have unacknowledged segemnts)

There are specific messages for sending and receiving operations (in addition to session setup/teardown). TCPCL is symmetric, i.e., both sides can start sending data segments in a session, and one side's bundle transfer does not have to complete before the other side can start sending data segments on its own. Hence, the protocol allows for a bi-directional mode of communication. Note that in the case of concurrent bidirectional transmission, acknowledgment segments MAY be interleaved with data segments.

3.2. Example Message Exchange

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

Note that the sending node MAY transmit multiple XFER_SEGMENT messages without necessarily waiting for the corresponding XFER_ACK responses. This enables pipelining of messages on a channel. 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.

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

    +-------------------------+         +-------------------------+
    |       SHUTDOWN          | ->   <- |         SHUTDOWN        |
    +-------------------------+         +-------------------------+

Figure 2: An Example of the Flow of Protocol Messages on a Single TCP Session between Two Nodes (A and B)

4. Session Establishment

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

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

If the node is unable to establish a TCP connection for any reason, then it is an implementation matter to determine how to handle the connection failure. A node MAY decide to re-attempt to establish the connection. If it does so, it MUST NOT overwhelm its target with repeated connection attempts. Therefore, the node MUST retry the connection setup only after some delay (a 1-second minimum is RECOMMENDED), and it SHOULD use a (binary) exponential backoff mechanism to increase this delay in case of repeated failures. In case a SHUTDOWN message specifying a reconnection delay is received, that delay is used as the initial delay. The default initial delay SHOULD be at least 1 second but SHOULD be configurable since it will be application and network type dependent.

The node MAY declare failure after one or more connection attempts and MAY attempt to find an alternate route for bundle data. Such decisions are up to the higher layer (i.e., the BP).

Once a TCP connection is established, each node MUST immediately transmit a contact header over the TCP connection. The format of the contact header is described in Section 4.1.

Upon receipt of the contact header, both nodes perform the validation and negotiation procedures defined in Section 4.2

After receiving the contact header from the other node, either node MAY also refuse the session by sending a SHUTDOWN message. If session setup is refused, a reason MUST be included in the SHUTDOWN message.

4.1. Contact Header

Once a TCP connection is established, both parties exchange a contact header. This section describes the format of the contact header and the meaning of its fields.

The format for the Contact Header is as follows:

                     1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
|                          magic='dtn!'                         |
+---------------+---------------+---------------+---------------+
|     Version   |   Flags       |      Keepalive Interval       |
+---------------+---------------+---------------+---------------+
|                          Segment MRU...                       |
+---------------+---------------+---------------+---------------+
|                          contd.                               |
+---------------+---------------+---------------+---------------+
|                         Transfer MRU...                       |
+---------------+---------------+---------------+---------------+
|                          contd.                               |
+---------------+---------------+---------------+---------------+
|          EID Length           |             EID Data...       |
+---------------+---------------+---------------+---------------+
|                        EID Data contd.                        |
+---------------+---------------+---------------+---------------+
|                  Header Extension Length...                   |
+---------------+---------------+---------------+---------------+
|                          contd.                               |
+---------------+---------------+---------------+---------------+
|                   Header Extension Items...                   |
+---------------+---------------+---------------+---------------+

Figure 3: Contact Header Format

See Section 4.2 for details on the use of each of these contact header fields. The fields of the contact header are:

magic:

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

Version:

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

Flags:

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

Keepalive Interval:

A 16-bit unsigned integer indicating the longest allowable interval, in seconds, between any message being received in this session and a subsequent KEEPALIVE message being received.

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 nodes of a single session MAY have different Segment MRUs, and no relation between the two is required.

Transfer MRU:

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

EID Length and EID Data:

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

Header Extension Length Header Extension Items:

Together these fields represent protocol extension data not defined by this specification. The Header Extension Length is the total number of octets to follow which are used to encode the Header Extension Item list. The encoding of each Header Extension Item is identical form as described in Section 4.1.1.

4.1.1. Header Extension Items

Each of the Header Extension items SHALL be encoded in an identical Type-Length-Value (TLV) container form as indicated in Figure 4. The fields of the header extension item are:

Flags:

A one-octet field containing generic bit flags about the item, which are listed in Table 2. If a TCPCL node receives an extension item with an unknown Item Type and the CRITICAL flag set, the node SHALL close the TCPCL session with SHUTDOWN reason code of "Contact Failure". If the CRITICAL flag is not set, an node SHALL skip over and ignore any item with an unkonwn Item Type.

Item Type:

A 16-bit unsigned integer field containing the type of the extension item. This specification does not define any extension types directly, but does allocate an IANA registry for such codes (see Section 8.3).

Item Length:

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

Item Value:

A variable-length data field which is interpreted according to the associated Item Type. This specification places no restrictions on an extensions use of available Item Value data. Extension specification SHOULD avoid the use of large data exchanges within the TCPCLv4 contact header as no bundle transfers can begin until the a full contact exchange and negotiation has been completed.

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

Figure 4: Header Extention Item Format

4.2. Validation and Parameter Negotiation

Upon reception of the contact header, each node follows the following procedures to ensure the validity of the TCPCL 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 node MAY elect to hold an invalid connection open and idle for some time before closing it.

A connecting TCPCL node SHALL send the highest TCPCL protocol version on a first session attempt for a TCPCL peer. If a connecting node receives a SHUTDOWN message with reason of "Version Mismatch", that node MAY attempt further TCPCL sessions with the peer using earlier protocol version numbers in decreasing order. Managing multi-TCPCL-session state such as this is an implementation matter.

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

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

Session Keepalive:

Negotiation of the Session Keepalive parameter is performed by taking the minimum of this two contact headers' Keepalive Interval. If the negotiated Session Keepalive is zero (i.e. one or both contact headers contains a zero Keepalive Interval), then the keepalive feature (described in Section 5.2.1) is disabled. There is no logical minimum value for the keepalive interval, but when used for many sessions on an open, shared network a short interval could lead to excessive traffic. For shared network use, nodes SHOULD choose a keepalive interval no shorter than 30 seconds. There is no logical maximum value for the keepalive interval, but an idle TCP connection is liable for closure by the host operating system if the keepalive time is longer than tens-of-minutes. Nodes SHOULD choose a keepalive interval no longer than 10 minutes (600 seconds).

Enable TLS:

Negotiation of the Enable TLS parameter is performed by taking the logical AND of the two contact headers' CAN_TLS flags. If the negotiated Enable TLS value is true then TLS negotiation feature (described in Section 4.3) begins immediately following the contact header exchange. The security policy on either node MAY forbid the establishment of a TCPCL session for any Enable TLS result (or for any combination of local or peer CAN_TLS flag), in which case the node SHALL shutdown the session with a reason code of "Contact Failure". For example, one node may disallow TCPCL sessions without TLS, while a second node may disallow sessions with TLS. Also note that this Contact Failure (of the header negotiation) is different than a TLS Failure (after an agreed-upon Enable TLS state).

Once this process of parameter negotiation is completed (which includes a possible completed TLS handshakede of the connection to use TLS), this protocol defines no additional mechanism to change the parameters of an established session; to effect such a change, the TCPCL session MUST be terminated and a new session established.

4.3. Session Security

This version of the TCPCL supports establishing a Transport Layer Security (TLS) session within an existing TCP connection. Negotation of whether or not to initiate TLS within a TCPCL session is part of the contact header as described in Section 4.2. The TLS handshake, if it occurs, is considered to be part of the contact negotiation before the TCPCL session itself is established. Specifics about sensitive data exposure are discussed in Section 7.

When TLS is used within the TCPCL it affects the entire session. By convention, this protocol uses the node which initiated the underlying TCP connection as the "client" role of the TLS handshake request. Once a TLS session is established within TCPCL, there is no mechanism provided to end the TLS session and downgrade the session. If a non-TLS session is desired after a TLS session is started then the entire TCPCL session MUST be shutdown first.

After negotiating an Enable TLS parameter of true, and before any other TCPCL messages are sent within the session, the session nodes SHALL begin a TLS handshake in accordance with [RFC5246]. The parameters within each TLS negotation are implementation dependent but any TCPCL node SHOULD follow all recommended best practices of [RFC7525].

4.3.1. TLS Handshake Result

If a TLS handshake cannot negotiate a TLS session, both nodes of the TCPCL session SHALL cause a TCPCL shutdown with reason "TLS Failure".

After a TLS session is successfuly established, both TCPCL nodes SHALL re-exchange TCPCL Contact Header messages. Any information cached from the prior Contact Header exchange SHALL be discarded. This re-exchange avoids man-in-the-middle attack in identical fashion to [RFC2595]. Each re-exchange header CAN_TLS flag SHALL be identical to the original header CAN_TLS flag from the same node. The CAN_TLS logic (TLS negotiation) SHALL NOT apply during header re-exchange. This reinforces the fact that there is no TLS downgrade mechanism.

4.3.2. Example TLS Initiation

A summary of a typical CAN_TLS usage is shown in the sequence in Figure 5 below.

              Node A                              Node B
              ======                              ======
    
    +-------------------------+         
    |  Open TCP Connnection   | ->   
    +-------------------------+         +-------------------------+
                                     <- |   Accept Connection     |
                                        +-------------------------+
    
    +-------------------------+         +-------------------------+
    |     Contact Header      | ->   <- |     Contact Header      |
    +-------------------------+         +-------------------------+
    
    +-------------------------+         +-------------------------+
    |     TLS Negotiation     | ->   <- |     TLS Negotiation     |
    |       (as client)       |         |       (as server)       |
    +-------------------------+         +-------------------------+
    
    +-------------------------+         +-------------------------+
    |     Contact Header      | ->   <- |     Contact Header      |
    +-------------------------+         +-------------------------+
    
                    ... secured TCPCL messaging ...
    
    +-------------------------+         +-------------------------+
    |       SHUTDOWN          | ->   <- |         SHUTDOWN        |
    +-------------------------+         +-------------------------+

Figure 5: A simple visual example of TCPCL TLS Establishment between two nodes

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. Message Type Codes

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

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

Figure 6: Format of the Message Header

The message header fields are as follows:

Message Type:

Indicates the type of the message as per Table 3 below. Encoded values are listed in Section 8.4.

5.2. Upkeep and Status Messages

5.2.1. 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 4.1, one of the parameters in the contact header is the Keepalive Interval. Both sides populate this field with their requested intervals (in seconds) between KEEPALIVE messages.

The format of a KEEPALIVE message is a one-octet message type code of KEEPALIVE (as described in Table 3) with no additional data. Both sides SHOULD send a KEEPALIVE message whenever the negotiated interval has elapsed with no transmission of any message (KEEPALIVE or other).

If no message (KEEPALIVE or other) has been received for at least twice the Keepalive Interval, then either party MAY terminate the session by transmitting a one-octet SHUTDOWN message (as described in Section 6.1, with reason code "Idle Timeout") and by closing the session.

Note: The Keepalive Interval SHOULD not be chosen too short as TCP retransmissions MAY occur in case of packet loss. Those will have to be triggered by a timeout (TCP retransmission timeout (RTO)), which is dependent on the measured RTT for the TCP connection so that KEEPALIVE messages MAY experience noticeable latency.

5.2.2. Message Rejection (MSG_REJECT)

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

The format of a MSG_REJECT message follows:

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

Figure 7: Format of MSG_REJECT Messages

The fields of the MSG_REJECT message are:

Reason Code:

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

Rejected Message Header:

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

5.3. Bundle Transfer

All of the message in this section are directly associated with transfering a bundle between TCPCL nodes.

A single TCPCL transfer results in a bundle (handled by the convergence layer as opaque data) being exchanged from one node to the other. In TCPCL a transfer is accomplished by dividing a single bundle up into "segments" based on the receiving-side Segment MRU (see Section 4.1).

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

5.3.1. Bundle Transfer ID

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

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

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

5.3.2. Transfer Initialization (XFER_INIT)

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

The purpose of the XFER_INIT message is to allow nodes to preemptively refuse bundles that would exceed their resources or to prepare storage on the receiving node for the upcoming bundle data. See Section 5.3.5 for details on when refusal based on XFER_INIT content is acceptable.

The Total Bundle Length field within a XFER_INIT message SHALL be treated as authoritative by the receiver. If, for whatever reason, the actual total length of bundle data received differs from the value indicated by the XFER_INIT message, the receiver SHOULD treat the transmitted data as invalid.

The format of the XFER_INIT message is as follows:

+-----------------------------+
|       Message Header        |
+-----------------------------+
|      Transfer ID (U64)      |
+-----------------------------+
|  Total bundle length (U64)  |
+-----------------------------+

Figure 8: Format of XFER_INIT Messages

The fields of the XFER_INIT message are:

Transfer ID:

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

Total bundle length:

A 64-bit unsigned integer indicating the size of the data-to-be-transferred.

An XFER_INIT message SHALL be sent immediately before transmission of any XFER_SEGMENT messages for each Transfer ID. XFER_INIT messages MUST NOT be sent unless the next XFER_SEGMENT message has the 'START' bit set to "1" (i.e., just before the start of a new transfer).

A receiver MAY send a BUNDLE_REFUSE message as soon as it receives a XFER_INIT message without waiting for the next XFER_SEGMENT message. The sender MUST be prepared for this and MUST associate the refusal with the correct bundle via the Transfer ID fields.

5.3.3. Data Transmission (XFER_SEGMENT)

Each bundle is transmitted in one or more data segments. The format of a XFER_SEGMENT message follows in Figure 9.

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

Figure 9: Format of XFER_SEGMENT Messages

The fields of the XFER_SEGMENT message are:

Message Flags:

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

Transfer ID:

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

Data length:

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

Data contents:

The variable-length data payload of the message.

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

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

5.3.4. 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, a Bundle Protocol agent needs an additional mechanism to determine whether the receiving agent has successfully received the segment. To this end, the TCPCL protocol provides feedback messaging whereby a receiving node transmits acknowledgments of reception of data segments.

The format of an XFER_ACK message follows in Figure 10.

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

Figure 10: Format of XFER_ACK Messages

The fields of the XFER_ACK message are:

Message Flags:

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

Transfer ID:

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

Acknowledged length:

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

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

For example, suppose the sending node transmits four segments of bundle data with lengths 100, 200, 500, and 1000, respectively. After receiving the first segment, the node sends an acknowledgment of length 100. After the second segment is received, the node sends an acknowledgment of length 300. The third and fourth acknowledgments are of length 800 and 1800, respectively.

5.3.5. Transfer Refusal (XFER_REFUSE)

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

There is no required relation between the Transfer MRU of a TCPCL node (which is supposed to represent a firm limitation of what the node will accept) and sending of a XFER_REFUSE message. A XFER_REFUSE can be used in cases where the agent's bundle storage is temporarily depleted or somehow constrained. A XFER_REFUSE can also be used after the bundle header or any bundle data is inspected by an agent and determined to be unacceptable.

The format of the XFER_REFUSE message is as follows:

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

Figure 11: Format of XFER_REFUSE Messages

The fields of the XFER_REFUSE message are:

Reason Code:

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

Transfer ID:

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

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

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

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

6. Session Termination

This section describes the procedures for ending a TCPCL session.

6.1. Shutdown Message (SHUTDOWN)

To cleanly shut down a session, a SHUTDOWN message MUST be transmitted by either node at any point following complete transmission of any other message. A receiving node SHOULD acknowledge all received data segments before sending a SHUTDOWN message to end the session. A transmitting node SHALL treat a SHUTDOWN message received mid-transfer (i.e. before the final acknowledgement) as a failure of the transfer.

After transmitting a SHUTDOWN message, an node MAY immediately close the associated TCP connection. Once the SHUTDOWN message is sent, any further received data on the TCP connection SHOULD be ignored. Any delay between request to terminate the TCP connection and actual closing of the connection (a "half-closed" state) MAY be ignored by the TCPCL node.

The format of the SHUTDOWN message is as follows:

+-----------------------------------+
|          Message Header           |
+-----------------------------------+
|         Message Flags (U8)        |
+-----------------------------------+
|     Reason Code (optional U8)     |
+-----------------------------------+
| Reconnection Delay (optional U16) |
+-----------------------------------+

Figure 12: Format of SHUTDOWN Messages

The fields of the SHUTDOWN message are:

Message Flags:

A one-octet field of single-bit flags, interpreted according to the descriptions in Table 7.

Reason Code:

A one-octet refusal reason code interpreted according to the descriptions in Table 8. The Reason Code is present or absent as indicated by one of the flags.

Reconnection Delay:

A 16-bit unsigned integer indicating the desired delay until further TCPCL sessions to the sending node. The Reconnection Delay is present or absent as indicated by one of the flags.

It is possible for a node to convey additional information regarding the reason for session termination. To do so, the node MUST set the 'R' bit in the message flags and transmit a one-octet reason code immediately following the message header. The specified values of the reason code are:

It is also possible to convey a requested reconnection delay to indicate how long the other node MUST wait before attempting session re-establishment. To do so, the node sets the 'D' bit in the message flags and then transmits an 16-bit unsigned integer specifying the requested delay, in seconds, following the message header (and optionally, the SHUTDOWN reason code). The value 0 SHALL be interpreted as an infinite delay, i.e., that the connecting node MUST NOT re-establish the session. In contrast, if the node does not wish to request a delay, it SHOULD omit the reconnection delay field (and set the 'D' bit to zero).

A session shutdown MAY occur immediately after TCP connection establishment or reception of a contact header (and prior to any further data exchange). This MAY, for example, be used to notify that the node is currently not able or willing to communicate. However, a node MUST always send the contact header to its peer before sending a SHUTDOWN message.

If either node terminates a session prematurely in this manner, it SHOULD send a SHUTDOWN message and MUST indicate a reason code unless the incoming connection did not include the magic string. If the magic string was not present, a node SHOULD close the TCP connection without sending a SHUTDOWN message. If a node does not want its peer to reopen a connection immediately, it SHOULD set the 'D' bit in the flags and include a reconnection delay to indicate when the peer is allowed to attempt another session setup.

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

6.2. Idle Session Shutdown

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

If there is a configured time to close idle links and if no bundle data (other than KEEPALIVE messages) has been received for at least that amount of time, then either node MAY terminate the session by transmitting a SHUTDOWN message indicating the reason code of 'Idle timeout' (as described in Table 8).

7. Security Considerations

One security consideration for this protocol relates to the fact that nodes present their endpoint identifier as part of the contact header exchange. It would be possible for a node to fake this value and present the identity of a singleton endpoint in which the node is not a member, essentially masquerading as another DTN node. If this identifier is used outside of a TLS-secured session or without further verification as a means to determine which bundles are transmitted over the session, then the node that has falsified its identity would be able to obtain bundles that it otherwise would not have. Therefore, a node SHALL NOT use the EID value of an unsecured contact header to derive a peer node's identity unless it can corroborate it via other means. When TCPCL session security is mandatory, an endpoint SHALL transmit initial unsecured contact header values indicated in Table 9 in order. These values avoid unnecessarily leaking endpoing parameters and will be ignored when secure contact header re-exchange occurs.

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

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

Another consideration for this protocol relates to denial-of-service attacks. A node MAY send a large amount of data over a TCPCL session, requiring the receiving node to handle the data, attempt to stop the flood of data by sending a XFER_REFUSE message, or forcibly terminate the session. This burden could cause denial of service on other, well-behaving sessions. There is also nothing to prevent a malicious node from continually establishing sessions and repeatedly trying to send copious amounts of bundle data. A listening node MAY take countermeasures such as ignoring TCP SYN messages, closing TCP connections as soon as they are established, waiting before sending the contact header, sending a SHUTDOWN message quickly or with a delay, etc.

8. IANA Considerations

In this section, registration procedures are as defined in [RFC5226].

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

8.1. Port Number

Port number 4556 has been previously assigned as the default port for the TCP convergence layer in [RFC7242]. This assignment is unchanged by protocol version 4. Each TCPCL node identifies its TCPCL protocol version in its initial contact (see Section 8.2), so there is no ambiguity about what protocol is being used.

8.2. Protocol Versions

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

8.3. Header Extension Types

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

IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Header Extension Types" and initialized it with the contents of Table 10. The registration procedure is RFC Required within the lower range 0x0001--0x3fff. Values in the range 0x8000--0xffff are reserved for use on private networks for functions not published to the IANA.

8.4. Message Types

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

IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Message Types" and initialized it with the contents of Table 11. The registration procedure is RFC Required.

8.5. XFER_REFUSE Reason Codes

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

IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE Reason Codes" and initialized it with the contents of Table 12. The registration procedure is RFC Required.

8.6. SHUTDOWN Reason Codes

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

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

8.7. MSG_REJECT Reason Codes

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

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

9. Acknowledgments

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

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Significant changes from RFC7242

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

• Changed contact header content to limit number of negotiated options.
• Added contact option to negotiate maximum segment size (per each direction).
• Added contact header extension capability.
• Defined new IANA registries for message / type / reason codes to allow renaming some codes for clarity.
• Expanded Message Header to octet-aligned fields instead of bit-packing.
• Added a bundle transfer identification number to all bundle-related messages (XFER_INIT, XFER_SEGMENT, XFER_ACK, XFER_REFUSE).
• Use flags in XFER_ACK to mirror flags from XFER_SEGMENT.
• Removed all uses of SDNV fields and replaced with fixed-bit-length fields.

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

• Added contact negotation failure SHUTDOWN reason code.
• Added MSG_REJECT message to indicate an unknown or unhandled message was received.
• Added TLS session security mechanism.
• Added TLS failure SHUTDOWN reason code.

Authors' Addresses