Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4

3.1. Convergence Layer Services

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

Attempt Session:

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

Terminate Session:

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

Session State Changed:

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

Connecting:

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

Contact Negotiating:

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

Session Negotiating:

Contact negotiation has been completed (including possible TLS use) and session negotiation has begun.¶

Established:

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

Ending:

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

Terminated:

The session has finished normal termination sequencing.¶

Failed:

The session ended without normal termination sequencing.¶

Session Idle Changed:

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

Begin Transmission:

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

Transmission Success:

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

Transmission Intermediate Progress:

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

Transmission Failure:

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

Reception Initialized:

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

Interrupt Reception:

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

Reception Success:

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

Reception Intermediate Progress:

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

Reception Failure:

The TCPCL entity indicates to the BP agent on certain reasons for reception failure, notably when the local entity rejects an attempted transfer for some local policy reason or when a TCPCL session ends before transfer success. The TCPCL itself does not have a notion of transfer timeout.¶

3.2. TCPCL Session Overview

First, one 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 that each TCPCL Entity is acting as. This is used to assist in routing and forwarding messages by the BP Agent and is part of the authentication capability provided by TLS.¶

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

Once the TCPCL session is established and configured in this way, bundles can be transferred in either direction. Each transfer is performed by segmenting the transfer data into one or more XFER_SEGMENT messages. Multiple bundles can be transmitted consecutively in a single direction on a single TCPCL connection. Segments from different bundles are never interleaved. Bundle interleaving can be accomplished by fragmentation at the BP layer or by establishing multiple TCPCL sessions between the same peers. There is no fundamental limit on the number of TCPCL sessions which a single entity can establish beyond the limit imposed by the number of available (ephemeral) TCP ports of the active entity.¶

A feature of this protocol is for the receiving entity to 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 in case the session is interrupted, it can perform reactive fragmentation to avoid re-sending the already transmitted part of the bundle. In addition, there is no explicit flow control on the TCPCL layer.¶

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

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

A SESS_TERM message is used to initiate the ending of a TCPCL session (see Section 6.1). During termination sequencing, in-progress transfers can be completed but no new transfers can be initiated. A SESS_TERM message can also be used to refuse a session setup by a peer (see Section 4.3). Regardless of the reason, session termination is initiated by one of the entities and responded-to by the other as illustrated by Figure 13 and Figure 14. Even when there are no transfers queued or in-progress, the session termination procedure allows each entity to distinguish between a clean end to a session and the TCP connection being closed because of some underlying network issue.¶

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

3.3. TCPCL States and Transitions

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

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

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 both directions and deriving a negotiated state from the two headers. The contact negotiation sequencing is performed either as the active or passive entity, and is illustrated in Figure 5 and Figure 6 respectively which both share the data validation and negotiation of the Processing of Contact Header "[PCH]" activity of Figure 7 and the "[TCPCLOSE]" activity which indicates TCP connection close. Successful negotiation results in one of the Session Initiation "[SI]" activities being performed. To avoid data loss, a Session Termination "[ST]" exchange allows cleanly finishing transfers before a session is ended.¶

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

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

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

Session negotiation involves exchanging a session initialization (SESS_INIT) message in both directions and deriving a negotiated state from the two messages. The session negotiation sequencing is performed either as the active or passive entity, and is illustrated in Figure 8 and Figure 9 respectively which both share the data validation and negotiation of Figure 10. The validation here includes certificate validation and authentication when TLS is used for the session.¶

+-----------+
|   TCPCL   |                   +---------+
| Messaging |--Send SESS_INIT-->| Waiting |--Timeout-->[ST]
| Available |                   +---------+
+-----------+                       |
                            Received SESS_INIT
                                    |
                                    V
                                  [PSI]

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

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

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

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

Notes on transfer sending:¶

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

                                         +-Receive XFER_SEGMENT-+
+--------+                               |    Send XFER_ACK     |
| Stream |                         +-------------+              |
|  Idle  |--Receive XFER_SEGMENT-->| In Progress |<-------------+
+--------+                         +-------------+
                                         |
     +--------Sent Final XFER_ACK--------+
     |
     V
+--------+
| Stream |
|  Idle  |
+--------+

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

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

3.4. PKIX Environments and CA Policy

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

DTN-Aware CAs:

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

DTN-Ignorant CAs:

It is expected that Internet-scale "public" CAs will continue to focus on DNS names as the preferred PKIX identifier. There are large infrastructures already in-place for managing network-level authentication and protocols to manage identity verification in those environments [RFC8555]. The TCPCL allows for this type of environment by authenticating a lower-level identifier for a peer and requiring the entity to trust that the Node ID given by the peer (during session initialization) is valid. This situation not ideal, as it allows vulnerabilities described in Section 8.9, but 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 in a PKIX certificate are in Section 4.4.1.¶

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

3.5. Session Keeping Policies

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

Persistent Session:

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

Ephemeral Sessions:

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

Active-Only Polling Sessions:

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

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

3.6. Transfer Segmentation Policies

Each TCPCL session allows a negotiated transfer segmentation policy to be applied in each transfer direction. A receiving entity can set the Segment 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 operating TCPCL, but guidance given here can be used to steer policy toward performance goals. It is also advised to consider the Segment MRU in relation to chunking/packetization performed by TLS, TCP, and any intermediate network-layer nodes.¶

Minimum Overhead:

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

Predictable Message Sizing:

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

Dynamic Segmentation:

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

Many other policies can be established in a TCPCL network between the two extremes of minimum overhead (large MRU, single-segment) and predictable message sizing (small MRU, highly segmented). Different policies can be applied to each transfer stream to and from any particular entity. Additionally, future session extension and transfer extension types can apply further nuance to transfer policies and policy negotiation.¶

3.7. Example Message Exchange

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

Note that the sending 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        |
    +-------------------------+         +-------------------------+

4.1. TCP Connection

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

If the entity is unable to establish a TCP connection for any reason, then it is an implementation matter to determine how to handle the connection failure. An entity MAY decide to re-attempt to establish the connection. If it does so, it MUST NOT overwhelm its target with repeated connection attempts. Therefore, the entity MUST NOT retry the connection setup earlier than some delay time from the last attempt, and it SHOULD use a (binary) exponential back-off mechanism to increase this delay in case of repeated failures. The upper limit on a re-attempt back-off is implementation defined but SHOULD be no longer than one minute (60 seconds) before signaling to the BP agent that a connection cannot be made.¶

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

The format of the Contact Header is described in Section 4.2. Because the TCPCL protocol version in use is part of the initial Contact Header, 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. Either mechanism, however, 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 an entity is capable of exchanging messages according to TLS 1.3 [RFC8446] or any successors which are compatible with that TLS ClientHello, the the CAN_TLS flag within its Contact Header SHALL be set to 1. This behavior prefers the use of TLS when possible, even if security policy does not allow or require authentication. This follows the opportunistic security model of [RFC7435], though an active attacker could interfere with the exchange in such cases.¶

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

The format for the Contact Header is as follows:¶

                     1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
|                          magic='dtn!'                         |
+---------------+---------------+---------------+---------------+
|     Version   |   Flags       |
+---------------+---------------+

See Section 4.3 for details on the use of each of these Contact Header fields.¶

The fields of the Contact Header are:¶

magic:

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

Version:

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

Flags:

A one-octet field of single-bit flags, interpreted according to the descriptions in Table 1. All reserved header flag bits SHALL be set to 0 by the sender. All reserved header flag bits SHALL be ignored by the receiver.¶

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 is on the TCPCL protocol version to use. The active entity always sends its Contact Header first and waits for a response from the passive entity. During contact initiation, the active TCPCL 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 reason 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 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 fall-back is an implementation matter.¶

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

TCPCL Version:

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

Enable TLS:

Negotiation of the Enable TLS parameter is performed by taking the logical AND of the two Contact Headers' CAN_TLS flags. A local security policy is then applied to determine of the negotiated value of Enable TLS is acceptable. It can be a reasonable security policy to require or disallow the use of TLS depending upon the desired network flows. Because this state is negotiated over an unsecured medium, there is a risk of a TLS Stripping as described in Section 8. If the Enable TLS state is unacceptable, the entity SHALL terminate the session with a reason code of "Contact Failure". Note that this contact failure reason is different than a failure of TLS handshake or TLS authentication after an agreed-upon and acceptable Enable TLS state. If the negotiated Enable TLS value is true and acceptable then TLS negotiation feature (described in Section 4.4) begins immediately following the Contact Header exchange.¶

4.4. Session Security

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

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

Subsequent TCPCL session attempts to the same passive entity MAY attempt use the TLS connection resumption feature. There is no guarantee that the passive entity will accept the request to resume a TLS session, and the active entity cannot assume any resumption outcome.¶

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

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

The message header fields are as follows:¶

Message Type:

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

4.6. Session Initialization Message (SESS_INIT)

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

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

+-----------------------------+
|       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.)        |
+-----------------------------+

The fields of the SESS_INIT message are:¶

Keepalive Interval:

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

Segment MRU:

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

Transfer MRU:

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

Node ID Length and Node ID Data:

Together these fields represent a variable-length text string. The Node ID Length is a 16-bit unsigned integer indicating the number of octets of Node ID Data to follow. A zero-length Node ID SHALL be used to indicate the lack of Node ID rather than a truly empty Node ID. This case allows an entity to avoid exposing Node ID information on an untrusted network. A non-zero-length Node ID Data SHALL contain the UTF-8 encoded Node ID of the Entity which sent the SESS_INIT message. Every Node ID SHALL be a URI consistent with the requirements of [RFC3986] and the URI schemes of the IANA "Bundle Protocol URI Scheme Type" registry [IANA-BUNDLE]. The Node ID itself can be authenticated as described in Section 4.4.3.¶

Session Extension Length and Session Extension Items:

Together these fields represent protocol extension data not defined by this specification. The Session Extension Length is the total number of octets to follow which are used to encode the Session Extension Item list. The encoding of each Session Extension Item is within a consistent data container as described in Section 4.8. The full set of Session Extension Items apply for the duration of the TCPCL session to follow. The order and multiplicity of these Session Extension Items is significant, as defined in the associated type specification(s). If the content of the Session Extension Items data disagrees with the Session Extension Length (e.g., the last Item claims to use more octets than are present in the Session Extension Length), the reception of the SESS_INIT is considered to have failed.¶

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

4.7. Session Parameter Negotiation

An entity calculates the parameters for a TCPCL session by negotiating the values from its own preferences (conveyed by the SESS_INIT it sent to the peer) with the preferences of the peer 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 transmit unit (MTU) for whole transfers and individual segments are identical to the Transfer MRU and Segment MRU, respectively, of the received SESS_INIT message. A transmitting peer can send individual segments with any size smaller than the Segment MTU, depending on local policy, dynamic network conditions, etc. Determining the size of each transmitted segment is an implementation matter. If either the Transfer MRU or Segment MRU is unacceptable, the entity 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:¶

Item Flags:

A one-octet field containing generic bit flags about the Item, which are listed in Table 3. All reserved header flag bits SHALL be set to 0 by the sender. All reserved header flag bits SHALL be ignored by the receiver. If a TCPCL entity receives a Session Extension Item with an unknown Item Type and the CRITICAL flag of 1, the entity SHALL terminate the TCPCL session with SESS_TERM reason code of "Contact Failure". If the CRITICAL flag is 0, an entity SHALL skip over and ignore any item with an unknown Item Type.¶

Item Type:

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

Item Length:

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

Item Value:

A variable-length data field which is interpreted according to the associated Item Type. This specification places no restrictions on an extension's use of available Item Value data. Extension specifications SHOULD avoid the use of large data 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...                                 |
+---------------+---------------+---------------+---------------+

5.1. Upkeep and Status Messages

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

5.2. Bundle Transfer

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

A single TCPCL transfer results in a bundle (handled by the convergence layer as opaque data) being exchanged from one entity to the other. In TCPCL a transfer is accomplished by dividing a single bundle up into "segments" based on the receiving-side Segment MRU (see Section 4.2). The choice of the length to use for segments is an implementation matter, but each segment MUST NOT be larger than the receiving entity's maximum receive unit (MRU) (see the field Segment MRU of Section 4.2). The first segment for a bundle is indicated by the 'START' flag and the last segment is indicated by the 'END' flag.¶

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

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

+------------------------------+
|       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) |
+------------------------------+

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

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

                     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...                                 |
+---------------+---------------+---------------+---------------+

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

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 acknowledgement.¶

Once a SESS_TERM message is sent the state of that TCPCL session changes to Ending. While the session is in the Ending state, an entity MAY finish an in-progress transfer in either direction. While the session is in the Ending state, an entity SHALL NOT begin any new outgoing transfer for the remainder of the session. While the session is in the Ending state, an entity SHALL NOT accept any new incoming transfer for the remainder of the session. If a new incoming transfer is attempted while in the Ending state, the receiving entity SHALL send an 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 which 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 an XFER_ACK before closing the TCP connection. Not acknowledging received segments can result in unnecessary bundle or bundle fragment retransmission. Any delay between request to close the TCP connection and actual closing of the connection (a "half-closed" state) MAY be ignored by the TCPCL entity. If the underlying TCP connection is closed during a transmission (in either transfer stream), the transfer SHALL be indicated to the BP agent as failed (see the transmission failure and reception failure indications of Section 3.1).¶

The TCPCL itself does not have any required behavior to respond to an SESS_TERM based on its Reason Code; the termination is passed up as an indication to the BP agent that the session state has changed. If a termination has a Reason Code which is not decodable to the BP agent, the agent SHOULD treat the termination as having an Unknown reason.¶

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

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

The fields of the SESS_TERM message are:¶

Message Flags:

A one-octet field of single-bit flags, interpreted according to the descriptions in Table 8. All reserved header flag bits SHALL be set to 0 by the sender. All reserved header flag bits SHALL be ignored by the receiver.¶

Reason Code:

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

The earliest a TCPCL session termination MAY occur is immediately after transmission of a Contact Header (and prior to any further message transmit). This can, for example, be used to notify that the entity is currently not able or willing to communicate. However, an entity MUST always send the Contact Header to its peer before sending a SESS_TERM message.¶

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

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

6.2. Idle Session Shutdown

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) has been received for at least that amount of time, then either entity MAY terminate the session by transmitting a SESS_TERM message indicating the reason code of "Idle timeout" (as described in Table 9).¶

8.6. Threat: Untrusted End-Entity Certificate

The profile in Section 4.4.3 uses end-entity certificates chained up to a trusted root CA. During TLS handshake, either entity can send a certificate set which 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 has a risk of an entity not actually having one of the needed CAs.¶

8.9. Threat: BP Node Impersonation

The certificates exchanged by TLS enable authentication of peer DNS name and Node ID, but it is possible that a peer either not provide a valid certificate or that the certificate does not validate either the DNS name or Node ID of the peer (see Section 3.4). Having a CA-validated certificate does not alone guarantee the identity of the network host or BP node from which the certificate is provided; additional validation procedures in Section 4.4.2 bind the DNS name or Node ID based on the contents of the certificate.¶

The DNS name validation is a weaker form of authentication, because even if a peer is operating on an authenticated network DNS name it can provide an invalid Node ID and cause bundles to be "leaked" to an invalid node. Especially in DTN environments, network names and addresses of nodes can be time-variable so binding a certificate to a Node ID is a more stable identity.¶

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

8.11. Alternate Uses of TLS

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

8.12. Predictability of Transfer IDs

The only requirement on Transfer IDs is that they be unique with each session from the sending peer only. The trivial algorithm of the first transfer starting at zero and later transfers incrementing by one causes absolutely predictable Transfer IDs. Even when a TCPCL session is not TLS secured and there is a man-in-the-middle attacker causing denial of service with XFER_REFUSE messages, it is not possible to preemptively refuse a transfer so there is no benefit in having unpredictable Transfer IDs within a session.¶

9.1. Port Number

Within the port registry of [IANA-PORTS], TCP port number 4556 has been previously assigned as the default port for the TCP convergence layer in [RFC7242]. This assignment is unchanged by TCPCL version 4, but the assignment reference is updated to this specification. Each TCPCL entity identifies its TCPCL protocol version in its initial contact (see Section 9.2), so there is no ambiguity about what protocol is being used. The related assignments for UDP and DCCP port 4556 (both registered by [RFC7122]) are unchanged.¶

9.2. Protocol Versions

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

9.3. Session Extension Types

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Session Extension Types" and initialize it with the contents of Table 12. The registration procedure is Expert Review within the lower range 0x0001--0x7FFF. Values in the range 0x8000--0xFFFF are reserved for use on private networks for functions not published to the IANA.¶

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

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

9.4. Transfer Extension Types

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Transfer Extension Types" and initialize it with the contents of Table 13. The registration procedure is Expert Review within the lower range 0x0001--0x7FFF. Values in the range 0x8000--0xFFFF are reserved for use on private networks for functions not published to the IANA.¶

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

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

9.5. Message Types

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Message Types" and initialize it with the contents of Table 14. The registration procedure is RFC Required within the lower range 0x01--0xEF. Values in the range 0xF0--0xFF are reserved for use on private networks for functions not published to the IANA.¶

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

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

9.6. XFER_REFUSE Reason Codes

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE Reason Codes" and initialize it with the contents of Table 15. The registration procedure is Specification Required within the lower range 0x00--0xEF. Values in the range 0xF0--0xFF are reserved for use on private networks for functions not published to the IANA.¶

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

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

9.7. SESS_TERM Reason Codes

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 SESS_TERM Reason Codes" and initialize it with the contents of Table 16. The registration procedure is Specification Required within the lower range 0x00--0xEF. Values in the range 0xF0--0xFF are reserved for use on private networks for functions not published to the IANA.¶

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

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

9.8. MSG_REJECT Reason Codes

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

IANA will create, under the "Bundle Protocol" registry [IANA-BUNDLE], a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 MSG_REJECT Reason Codes" and initialize it with the contents of Table 17. The registration procedure is Specification Required within the lower range 0x01--0xEF. Values in the range 0xF0--0xFF are reserved for use on private networks for functions not published to the IANA.¶

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

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