| Delay Tolerant Networking | B. Sipos |
| Internet-Draft | RKF Engineering |
| Obsoletes: 7242 (if approved) | M. Demmer |
| Intended status: Standards Track | UC Berkeley |
| Expires: December 26, 2018 | J. Ott |
| Aalto University | |
| S. Perreault | |
| June 24, 2018 |
Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
draft-ietf-dtn-tcpclv4-09
This document describes a revised protocol for the TCP-based convergence layer (TCPCL) for Delay-Tolerant Networking (DTN). The protocol revision is based on implementation issues in the original TCPCL Version 3 of [RFC7242] and updates to the Bundle Protocol contents, encodings, and convergence layer requirements in Bundle Protocol Version 7. Specifically, the TCPCLv4 uses CBOR-encoded BPv7 bundles as its service data unit being transported and provides a reliable transport of such bundles. Several new IANA registries are defined for TCPCLv4 which define some behaviors inherited from TCPCLv3 but with updated encodings and/or semantics.
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 December 26, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document describes the TCP-based convergence-layer protocol for Delay-Tolerant Networking. Delay-Tolerant Networking is an end-to- end architecture providing communications in and/or through highly stressed environments, including those with intermittent connectivity, long and/or variable delays, and high bit error rates. More detailed descriptions of the rationale and capabilities of these networks can be found in "Delay-Tolerant Network Architecture" [RFC4838].
An important goal of the DTN architecture is to accommodate a wide range of networking technologies and environments. The protocol used for DTN communications is the Bundle Protocol Version 7 (BPv7) [I-D.ietf-dtn-bpbis], an application-layer protocol that is used to construct a store-and-forward overlay network. BPv7 requires the services of a "convergence-layer adapter" (CLA) to send and receive bundles using the service of some "native" link, network, or Internet protocol. This document describes one such convergence-layer adapter that uses the well-known Transmission Control Protocol (TCP). This convergence layer is referred to as TCP Convergence Layer Version 4 (TCPCLv4). For the remainder of this document, the abbreviation "BP" without the version suffix refers to BPv7. For the remainder of this document, the abbreviation "TCPCL" without the version suffix refers to TCPCLv4.
The locations of the TCPCL and the BP in the Internet model protocol stack (described in [RFC1122]) are shown in Figure 1. In particular, when BP is using TCP as its bearer with TCPCL as its convergence layer, both BP and TCPCL reside at the application layer of the Internet model.
+-------------------------+
| DTN Application | -\
+-------------------------| |
| Bundle Protocol (BP) | -> Application Layer
+-------------------------+ |
| TCP Conv. Layer (TCPCL) | |
+-------------------------+ |
| TLS (optional) | -/
+-------------------------+
| TCP | ---> Transport Layer
+-------------------------+
| IPv4/IPv6 | ---> Network Layer
+-------------------------+
| Link-Layer Protocol | ---> Link Layer
+-------------------------+
Figure 1: The Locations of the Bundle Protocol and the TCP Convergence-Layer Protocol above the Internet Protocol Stack
This document describes the format of the protocol data units passed between entities participating in TCPCL communications. This document does not address:
This version of the TCPCL provides the following services to support the overlaying Bundle Protocol agent. In all cases, this is not an API defintion but a logical description of how the CL may interact with the BP agent. Each of these interactions may be associated with any number of additional metadata items as necessary to support the operation of the CL or BP agent.
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].
This section contains definitions specific to the TCPCL protocol. Figure 3.
These relationships are illustrated in
Figure 2. For most TCPCL behavior within a session, the two entities are symmetric and there is no protocol distinction between them. Some specific behavior, particularly during session establishment, distinguishes between the active entity and the passive entity. For the remainder of this document, the term "entity" without the prefix "TCPCL" refers to a TCPCL entity.The relationship between connections, sessions, and streams is shown in
+--------------------------------------------+ | TCPCL Entity | | | +----------------+ | +--------------------------------+ | | |-+ | | Actively Inititated Session #1 +------------->| Other | | | +--------------------------------+ | | TCPCL Entity's | | | ... | | Passive | | | +--------------------------------+ | | Listener | | | | Actively Inititated Session #n +------------->| | | | +--------------------------------+ | +----------------+ | | | +-----------------+ | +---------------------------+ | | +---| +---------------------------+ | +----------------+ | | | | Optional Passive | | | |-+ | | +-| Listener(s) +<-------------+ | | | | +---------------------------+ | | | | | | | | Other | | | | +---------------------------------+ | | TCPCL Entity's | | | +--->| Passively Inititated Session #1 +-------->| Active | | | | +---------------------------------+ | | Initiator(s) | | | | | | | | | | +---------------------------------+ | | | | | +--->| Passively Inititated Session #n +-------->| | | | +---------------------------------+ | +----------------+ | | | +-----------------+ +--------------------------------------------+
Figure 2: The relationships between TCPCL entities
+----------------------------+ +--------------------------+ | TCPCL Session | | TCPCL "Other" Session | | | | | | +-----------------------+ | | +---------------------+ | | | TCP Connection | | | | TCP Connection | | | | | | | | | | | | +-------------------+ | | | | +-----------------+ | | | | | Optional Inbound | | | | | | Peer Outbound | | | | | | Transfer Stream |<-[Seg]--[Seg]--[Seg]-| | Transfer Stream | | | | | | ----- | | | | | | ----- | | | | | | RECEIVER | | | | | | SENDER | | | | | +-------------------+ | | | | +-----------------+ | | | | | | | | | | | | +-------------------+ | | | | +-----------------+ | | | | | Optional Outbound | | | | | | Peer Inbound | | | | | | Transfer Stream |------[Seg]---[Seg]---->| Transfer Stream | | | | | | ----- | | | | | | ----- | | | | | | SENDER | | | | | | RECEIVER | | | | | +-------------------+ | | | | +-----------------+ | | | +-----------------------+ | | +---------------------+ | +----------------------------+ +--------------------------+
Figure 3: The relationship within a TCPCL Session of its two streams
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.
First, one node establishes a TCPCL session to the other by initiating a TCP connection in accordance with [RFC0793]. After setup of the TCP connection is complete, an initial contact header is exchanged in both directions to establish a shared TCPCL version and possibly initiate TLS security. 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 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 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 negligable 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 an initialization (XFER_INIT) message followed by one or more logical segments of data within an XFER_SEGMENT message. Multiple bundles can be transmitted consecutively 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.
A feature of this protocol is for the receiving node to send acknowledgment (XFER_ACK) messages as bundle data segments arrive . The rationale behind these acknowledgments is to enable the sender node to determine how much of the bundle has been received, so that in case the session is interrupted, it can perform reactive fragmentation to avoid re-sending the already transmitted part of the bundle. In addition, there is no explicit flow control on the TCPCL layer.
A TCPCL receiver can interrupt the transmission of a bundle at any point in time by replying with a XFER_REFUSE message, which causes the sender to stop transmission of the associated bundle (if it hasn't already finished transmission) Note: This enables a cross-layer optimization in that it allows a receiver that detects that it already has received a certain bundle to interrupt transmission as early as possible and thus save transmission capacity for other bundles.
For sessions that are idle, a KEEPALIVE message is sent at a negotiated interval. This is used to convey node live-ness information during otherwise message-less time intervals.
A SESS_TERM message is used to start the closing of a TCPCL session (see Section 6.1). During shutdown sequencing, in-progress transfers can be completed but no new transfers can be initiated. A SESS_TERM message can also be used to refuse a session setup by a peer (see Section 4.3). It is an implementation matter to determine whether or not to close a TCPCL session while there are no transfers queued or in-progress.
Once a session is established 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.
The states of a nominal TCPCL session (i.e. without session failures) are indicated in Figure 4.
+-------+
| START |
+-------+
|
TCP Establishment
|
V
+-----------+ +---------------------+
| TCP |----------->| Contact / Session |
| Connected | | Negotiation |
+-----------+ +---------------------+
|
+-----Session Parameters-----+
| Negotiated
V
+-------------+ +-------------+
| Established |----New Transfer---->| Established |
| Session | | Session |
| Idle |<---Transfers Done---| Live |
+-------------+ +-------------+
| |
+------------------------------------+
|
SESS_TERM Exchange
|
V
+-------------+
| Established | +-------------+
| Session |----Transfers------>| TCP |
| Ending | Done | Terminating |
+-------------+ +-------------+
|
+------------Close Message------------+
|
V
+-------+
| END |
+-------+
Figure 4: Top-level states of a TCPCL session
Notes on Established Session states:
The contact negotiation sequencing is performed either as the active or passive peer, and is illustrated in Figure 5 and Figure 6 respectively which both share the data validation and analyze final states of Figure 7.
+-------+
| START |-----TCP-----+
+-------+ Connecting |
V
+-----------+ +---------+
| Connected |--OK-->| Send CH |--OK-->[PCH]
+-----------+ +---------+
| |
Error Error
| |
V |
[TCPTERM]<-------------+
Figure 5: Contact Initiation as Active peer
+-------+ | START |-----TCP----->[PCH] +-------+ Connected
Figure 6: Contact Initiation as Passive peer
+-------->[TCPTERM]<----------+
| |
Timeout Error
or Error |
| |
+-------+ +---------+ Contact +----------+
| START |---->| Waiting |---- Header --->| Validate |
+-------+ +---------+ Received +----------+
|
+---------------------------+
|
V
+---------+
+--Error--| Analyze |---No TLS---->[SI]
| | | ^
| +---------+ |
| | |
V TLS |
[TCPTERM] Negotiated |
^ | |
| V |
| +-----------+ |
| | Establish |---Success---+
+--Error--| TLS |
+-----------+
Figure 7: Processing of Contact Header (PCH)
The session negotiation sequencing is performed either as the active or passive peer, and is illustrated in Figure 8 and Figure 9 respectively which both share the data validation and analyze final states of Figure 10.
+-------+ TCPCL
| START |--Messaging--+
+-------+ Available |
V
+----------------+
| Send SESS_INIT |--OK-->[PSI]
+----------------+
|
Error
|
V
[SESSTERM]
Figure 8: Session Initiation as Active peer
+-------+ TCPCL | START |---Messaging-->[PSI] +-------+ Available
Figure 9: Session Initiation as Passive peer
+------->[SESSTERM]<--------+
| |
Timeout Error
or Error |
| |
+-------+ +---------+ +----------+
| START |---->| Waiting |---SESS_INIT--->| Validate |
+-------+ +---------+ Received +----------+
|
+---------------------------+
|
V
+---------+ +--------------+
+--Error--| Analyze |---->| Established |
| | | | Session Idle |
| +---------+ +--------------+
V
[SESSTERM]
Figure 10: Processing of Session Initiation (PSI)
Transfers can occur after a session is established and it's not in the ending state. Each transfer occurs within a single logical transfer stream between a sender and a receiver, as illustrated in Figure 11 and Figure 12 respectively.
+--Send XFER_DATA--+
+--------+ | |
| Stream | +-------------+ |
| Idle |---Send XFER_INIT-->| In Progress |<---------+
+--------+ +-------------+
|
+------All segments sent-------+
|
V
+---------+ +--------+
| Waiting |---- Receive Final---->| Stream |
| for Ack | Ack | IDLE |
+---------+ +--------+
Figure 11: Transfer sender states
Notes on transfer sending:
+-Receive XFER_DATA-+
+--------+ | Send Ack |
| Stream | +-------------+ |
| IDLE |--Receive XFER_INIT-->| In Progress |<----------+
+--------+ +-------------+
|
+---------Sent Final Ack---------+
|
V
+--------+
| Stream |
| IDLE |
+--------+
Figure 12: Transfer receiver states
Each TCPCL session allows a negotiated transfer segmentation polcy to be applied in each transfer direction. A receiving node can set the Segment MRU in its contact header to determine the largest acceptable segment size, and a transmitting node can segment a transfer into any sizes smaller than the receiver's Segment MRU. It is a network administration matter to determine an appropriate segmentation policy for entities operating TCPCL, but guidance given here can be used to steer policy toward performance goals. It is also advised to consider the Segment MRU in relation to chunking/packetization performed by TLS, TCP, and any intermediate network-layer nodes.
Many other policies can be established in a TCPCL network between these two extremes. Different policies can be applied to each direction to/from any particular node. Additionally, future header and transfer extension types can apply further nuance to transfer policies and policy negotiation.
The following figure depicts the protocol exchange for a simple session, showing the session establishment and the transmission of a single bundle split into three data segments (of lengths "L1", "L2", and "L3") from Entity A to Entity B.
Note that the sending node MAY transmit multiple XFER_SEGMENT messages without necessarily waiting for the corresponding XFER_ACK responses. This enables pipelining of messages on a transfer stream. Although this example only demonstrates a single bundle transmission, it is also possible to pipeline multiple XFER_SEGMENT messages for different bundles without necessarily waiting for XFER_ACK messages to be returned for each one. However, interleaving data segments from different bundles is not allowed.
No errors or rejections are shown in this example.
Entity A Entity B
======== ========
+-------------------------+
| Contact Header | -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+
+-------------------------+
| XFER_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] |
+-------------------------+
+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+
Figure 13: An example of the flow of protocol messages on a single TCP Session between two entities
For bundle transmissions to occur using the TCPCL, a TCPCL session MUST first be established between communicating entities. It is up to the implementation to decide how and when session setup is triggered. For example, some sessions MAY be opened proactively and maintained for as long as is possible given the network conditions, while other sessions MAY be opened only when there is a bundle that is queued for transmission and the routing algorithm selects a certain next-hop node.
To establish a TCPCL session, an entity MUST first establish a TCP connection with the intended peer entity, typically by using the services provided by the operating system. Destination port number 4556 has been assigned by IANA as the Registered Port number for the TCP convergence layer. Other destination port numbers MAY be used per local configuration. Determining a peer's destination port number (if different from the registered TCPCL port number) is up to the implementation. Any source port number MAY be used for TCPCL sessions. Typically an operating system assigned number in the TCP Ephemeral range (49152-65535) is used.
If the entity is unable to establish a TCP connection for any reason, then it is an implementation matter to determine how to handle the connection failure. An entity MAY decide to re-attempt to establish the connection. If it does so, it MUST NOT overwhelm its target with repeated connection attempts. Therefore, the entity MUST retry the connection setup no earlier than some delay time from the last attempt, and it SHOULD use a (binary) exponential backoff mechanism to increase this delay in case of repeated failures.
Once a TCP connection is established, each entity MUST immediately transmit a contact header over the TCP connection. The format of the contact header is described in Section 4.2.
Once a TCP connection is established, both parties exchange a contact header. This section describes the format of the contact header and the meaning of its fields.
Upon receipt of the contact header, both entities perform the validation and negotiation procedures defined in Section 4.3. After receiving the contact header from the other entity, either entity MAY refuse the session by sending a SESS_TERM message with an appropriate reason code.
The format for the Contact Header is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| magic='dtn!' |
+---------------+---------------+---------------+---------------+
| Version | Flags |
+---------------+---------------+
Figure 14: Contact Header Format
See Section 4.3 for details on the use of each of these contact header fields. The fields of the contact header are:
| Name | Code | Description |
|---|---|---|
| CAN_TLS | 0x01 | If bit is set, indicates that the sending peer is capable of TLS security. |
| Reserved | others |
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, an entity 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 SESS_TERM message with reason of "Version Mismatch", that node MAY attempt further TCPCL sessions with the peer using earlier protocol version numbers in decreasing order. Managing multi-TCPCL-session state such as this is an implementation matter.
If an entity 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 an entity 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") or 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.
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 established, there is no mechanism available to downgrade a TCPCL session to non-TLS operation. If this is desired, the entire TCPCL session MUST be terminated and a new non-TLS-negotiated session established.
The use of TLS is negotated using the Contact Header as described in Section 4.3. After negotiating an Enable TLS parameter of true, and before any other TCPCL messages are sent within the session, the session entities SHALL begin a TLS handshake in accordance with [RFC5246]. The parameters within each TLS negotiation are implementation dependent but any TCPCL node SHOULD follow all recommended best practices of [RFC7525]. By convention, this protocol uses the node which initiated the underlying TCP connection as the "client" role of the TLS handshake request.
The TLS handshake, if it occurs, is considered to be part of the contact negotiation before the TCPCL session itself is established. Specifics about sensitive data exposure are discussed in Section 8.
If a TLS handshake cannot negotiate a TLS session, both entities of the TCPCL session SHALL terminate the TCP connection. At this point the TCPCL session has not yet been established so there is no TCPCL session to terminate. This also avoids any potential security issues assoicated with further TCP communication with an untrusted peer.
After a TLS session is successfully established, the active peer SHALL send a SESS_INIT message to begin session negotiation. This session negotation and all subsequent messaging are secured.
A summary of a typical CAN_TLS usage is shown in the sequence in Figure 15 below.
Entity A Entity B
======== ========
+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+
+-------------------------+
| Contact Header | ->
+-------------------------+ +-------------------------+
<- | Contact Header |
+-------------------------+
+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+
... secured TCPCL messaging, starting with SESS_INIT ...
+-------------------------+ +-------------------------+
| SESS_TERM | -> <- | SESS_TERM |
+-------------------------+ +-------------------------+
Figure 15: A simple visual example of TCPCL TLS Establishment between two entities
After the initial exchange of a contact header, all messages transmitted over the session are identified by a one-octet header with the following structure:
0 1 2 3 4 5 6 7
+---------------+
| Message Type |
+---------------+
Figure 16: Format of the Message Header
The message header fields are as follows:
| Type | Description |
|---|---|
| SESS_INIT | Contains the session parameter inputs from one of the entities, as described in Section 4.6. |
| XFER_INIT | Contains the length (in octets) of the next transfer, as described in Section 5.2.2. |
| XFER_SEGMENT | Indicates the transmission of a segment of bundle data, as described in Section 5.2.3. |
| XFER_ACK | Acknowledges reception of a data segment, as described in Section 5.2.4. |
| XFER_REFUSE | Indicates that the transmission of the current bundle SHALL be stopped, as described in Section 5.2.5. |
| KEEPALIVE | Used to keep TCPCL session active, as described in Section 5.1.1. |
| SESS_TERM | Indicates that one of the entities participating in the session wishes to cleanly terminate the session, as described in Section 6. |
| MSG_REJECT | Contains a TCPCL message rejection, as described in Section 5.1.2. |
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.
The format of a SESS_INIT message is as follows in Figure 17.
+-------------------------------+ | Message Header | +-------------------------------+ | Keepalive Interval (U16) | +-------------------------------+ | Segment MRU (U64) | +-------------------------------+ | Transfer MRU (U64) | +-------------------------------+ | EID Length (U16) | +-------------------------------+ | EID Data (variable) | +-------------------------------+ | Session Extension Length (U64)| +-------------------------------+ | Session Extension Items (var.)| +-------------------------------+
Figure 17: SESS_INIT Format
Each of the Session Extension Items SHALL be encoded in an identical Type-Length-Value (TLV) container form as indicated in Figure 18. The fields of the Session Extension Item are:
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 18: Session Extension Item Format
| Name | Code | Description |
|---|---|---|
| CRITICAL | 0x01 | If bit is set, indicates that the receiving peer must handle the extension item. |
| Reserved | others |
An entity calculates the parameters for a TCPCL session by negotiating the values from its own preferences (conveyed by the contact header it sent to the peer) with the preferences of the peer node (expressed in the contact header that it received from the peer). The negotiated parameters defined by this specification are described in the following paragraphs.
Once this process of parameter negotiation is completed (which includes a possible completed TLS handshake of the connection to use TLS), this protocol defines no additional mechanism to change the parameters of an established session; to effect such a change, the TCPCL session MUST be terminated and a new session established.
This section describes the protocol operation for the duration of an established session, including the mechanism for transmitting bundles over the session.
The protocol includes a provision for transmission of KEEPALIVE messages over the TCPCL session to help determine if the underlying TCP connection has been disrupted.
As described in Section 4.3, a negotiated parameter of each session is the Session Keepalive interval. If the negotiated Session Keepalive is zero (i.e. one or both contact headers contains a zero Keepalive Interval), then the keepalive feature is disabled. There is no logical minimum value for the keepalive interval, but when used for many sessions on an open, shared network a short interval could lead to excessive traffic. For shared network use, entities SHOULD choose a keepalive interval no shorter than 30 seconds. There is no logical maximum value for the keepalive interval, but an idle TCP connection is liable for closure by the host operating system if the keepalive time is longer than tens-of-minutes. Entities SHOULD choose a keepalive interval no longer than 10 minutes (600 seconds).
Note: The Keepalive Interval SHOULD NOT be chosen too short as TCP retransmissions MAY occur in case of packet loss. Those will have to be triggered by a timeout (TCP retransmission timeout (RTO)), which is dependent on the measured RTT for the TCP connection so that KEEPALIVE messages MAY experience noticeable latency.
The format of a KEEPALIVE message is a one-octet message type code of KEEPALIVE (as described in Table 2) with no additional data. Both sides 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 in a session after some implementation-defined time duration, then the node MAY terminate the session by transmitting a SESS_TERM message (as described in Section 6.1) with reason code "Idle Timeout.
If a TCPCL node receives a message which is unknown to it (possibly due to an unhandled protocol mismatch) or is inappropriate for the current session state (e.g. a KEEPALIVE message received after contact header negotiation has disabled that feature), there is a protocol-level message to signal this condition in the form of a MSG_REJECT reply.
The format of a MSG_REJECT message is as follows in Figure 19.
+-----------------------------+ | Message Header | +-----------------------------+ | Reason Code (U8) | +-----------------------------+ | Rejected Message Header | +-----------------------------+
Figure 19: Format of MSG_REJECT Messages
The fields of the MSG_REJECT message are:
| Name | Code | Description |
|---|---|---|
| Message Type Unknown | 0x01 | A message was received with a Message Type code unknown to the TCPCL node. |
| Message Unsupported | 0x02 | A message was received but the TCPCL node cannot comply with the message contents. |
| Message Unexpected | 0x03 | A message was received while the session is in a state in which the message is not expected. |
All of the messages in this section are directly associated with transferring a bundle between TCPCL entities.
A single TCPCL transfer results in a bundle (handled by the convergence layer as opaque data) being exchanged from one node to the other. In TCPCL a transfer is accomplished by dividing a single bundle up into "segments" based on the receiving-side Segment MRU (see Section 4.2). The choice of the length to use for segments is an implementation matter, but each segment MUST be no larger than the receiving node's maximum receive unit (MRU) (see the field "Segment MRU" of Section 4.2). The first segment for a bundle MUST set the 'START' flag, and the last one MUST set the 'end' flag in the XFER_SEGMENT message flags.
A single transfer (and by extension a single segment) SHALL NOT contain data of more than a single bundle. This requirement is imposed on the agent using the TCPCL rather than TCPCL itself.
If multiple bundles are transmitted on a single TCPCL connection, they MUST be transmitted consecutively without interleaving of segments from multiple bundles.
Each of the bundle transfer messages contains a Transfer ID which is used to correlate messages (from both sides of a transfer) for each bundle. A Transfer ID does not attempt to address uniqueness of the bundle data itself and has no relation to concepts such as bundle fragmentation. Each invocation of TCPCL by the bundle protocol agent, requesting transmission of a bundle (fragmentary or otherwise), results in the initiation of a single TCPCL transfer. Each transfer entails the sending of a 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 SESS_TERM reason code "Resource Exhaustion".
For bidirectional bundle transfers, a TCPCL node SHOULD NOT rely on any relation between Transfer IDs originating from each side of the TCPCL session.
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 entities to preemptively refuse bundles that would exceed their resources or to prepare storage on the receiving node for the upcoming bundle data. See Section 5.2.5 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 in Figure 20.
+-----------------------------+ | Message Header | +-----------------------------+ | Transfer ID (U64) | +-----------------------------+ | Total Bundle Length (U64) | +-----------------------------+ | Transfer Extension | | Length (U64) | +-----------------------------+ | Transfer Extension Items... | +-----------------------------+
Figure 20: Format of XFER_INIT Messages
The fields of the XFER_INIT message are:
An XFER_INIT message SHALL be sent as the first message in a transfer sequence, before transmission of any XFER_SEGMENT messages for the same 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).
Each of the Transfer Extension Items SHALL be encoded in an identical Type-Length-Value (TLV) container form as indicated in Figure 21. The fields of the Transfer Extension Item are:
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 21: Transfer Extension Item Format
| Name | Code | Description |
|---|---|---|
| CRITICAL | 0x01 | If bit is set, indicates that the receiving peer must handle the extension item. |
| Reserved | others |
Each bundle is transmitted in one or more data segments. The format of a XFER_SEGMENT message follows in Figure 22.
+------------------------------+ | Message Header | +------------------------------+ | Message Flags (U8) | +------------------------------+ | Transfer ID (U64) | +------------------------------+ | Data length (U64) | +------------------------------+ | Data contents (octet string) | +------------------------------+
Figure 22: Format of XFER_SEGMENT Messages
The fields of the XFER_SEGMENT message are:
| Name | Code | Description |
|---|---|---|
| END | 0x01 | If bit is set, indicates that this is the last segment of the transfer. |
| START | 0x02 | If bit is set, indicates that this is the first segment of the transfer. |
| Reserved | others |
The flags portion of the message contains two optional values in the two low-order bits, denoted 'START' and 'END' in Table 6. The 'START' bit MUST be set to one if it precedes the transmission of the first segment of a transfer. The 'END' bit MUST be set to one when transmitting the last segment of a transfer. In the case where an entire transfer is accomplished in a single segment, both the 'START' and 'END' bits MUST be set to one.
Once a transfer of a bundle has commenced, the node MUST only send segments containing sequential portions of that bundle until it sends a segment with the 'END' bit set. No interleaving of multiple transfers from the same node is possible within a single TCPCL session. Simultaneous transfers between two entities MAY be achieved using multiple TCPCL sessions.
Although the TCP transport provides reliable transfer of data between transport peers, the typical BSD sockets interface provides no means to inform a sending application of when the receiving application has processed some amount of transmitted data. Thus, after transmitting some data, the TCPCL needs an additional mechanism to determine whether the receiving agent has successfully received the segment. To this end, the TCPCL protocol provides feedback messaging whereby a receiving node transmits acknowledgments of reception of data segments.
The format of an XFER_ACK message follows in Figure 23.
+-----------------------------+ | Message Header | +-----------------------------+ | Message Flags (U8) | +-----------------------------+ | Transfer ID (U64) | +-----------------------------+ | Acknowledged length (U64) | +-----------------------------+
Figure 23: Format of XFER_ACK Messages
The fields of the XFER_ACK message are:
A receiving TCPCL node SHALL send an XFER_ACK message in response to each received XFER_SEGMENT message. The flags portion of the XFER_ACK header SHALL be set to match the corresponding DATA_SEGMENT message being acknowledged. The acknowledged length of each XFER_ACK contains the sum of the data length fields of all XFER_SEGMENT messages received so far in the course of the indicated transfer. The sending node MAY transmit multiple XFER_SEGMENT messages without necessarily waiting for the corresponding XFER_ACK responses. This enables pipelining of messages on a transfer stream.
For example, suppose the sending node transmits four segments of bundle data with lengths 100, 200, 500, and 1000, respectively. After receiving the first segment, the node sends an acknowledgment of length 100. After the second segment is received, the node sends an acknowledgment of length 300. The third and fourth acknowledgments are of length 800 and 1800, respectively.
The TCPCL supports a mechanism by which a receiving node can indicate to the sender that it does not want to receive the corresponding bundle. To do so, upon receiving 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.
A receiver MAY send an XFER_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.
The format of the XFER_REFUSE message is as follows in Figure 24.
+-----------------------------+ | Message Header | +-----------------------------+ | Reason Code (U8) | +-----------------------------+ | Transfer ID (U64) | +-----------------------------+
Figure 24: Format of XFER_REFUSE Messages
The fields of the XFER_REFUSE message are:
| Name | Semantics |
|---|---|
| Unknown | Reason for refusal is unknown or not specified. |
| Extension Failure | A failure processing the Transfer Extension Items ha occurred. |
| Completed | The receiver already has the complete bundle. The sender MAY consider the bundle as completely received. |
| No Resources | The receiver's resources are exhausted. The sender SHOULD apply reactive bundle fragmentation before retrying. |
| Retransmit | The receiver has encountered a problem that requires the bundle to be retransmitted in its entirety. |
The receiver MUST, for each transfer preceding the one to be refused, have either acknowledged all XFER_SEGMENTs or refused the bundle transfer.
The bundle transfer refusal MAY be sent before an entire data segment is received. If a sender receives a XFER_REFUSE message, the sender MUST complete the transmission of any partially sent XFER_SEGMENT message. There is no way to interrupt an individual TCPCL message partway through sending it. The sender MUST NOT commence transmission of any further segments of the refused bundle subsequently. Note, however, that this requirement does not ensure that an entity will not receive another XFER_SEGMENT for the same bundle after transmitting a XFER_REFUSE message since messages MAY cross on the wire; if this happens, subsequent segments of the bundle 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.
This section describes the procedures for ending a TCPCL session.
To cleanly shut down a session, a SESS_TERM message SHALL be transmitted by either node at any point following complete transmission of any other message. Upon receiving a SESS_TERM message after not sending a SESS_TERM message in the same session, an entity SHOULD send a confirmation SESS_TERM message with identical content to the SESS_TERM for which it is confirming.
After sending a SESS_TERM message, an entity MAY continue a possible in-progress transfer in either direction. After sending a SESS_TERM message, an entity SHALL NOT begin any new outgoing transfer (i.e. send an XFER_INIT message) for the remainder of the session. After receving a SESS_TERM message, an entity SHALL NOT accept any new incoming transfer for the remainder of the session.
Instead of following a clean shutdown sequence, after transmitting a SESS_TERM message an entity MAY immediately close the associated TCP connection. When performing an unclean shutdown, a receiving node SHOULD acknowledge all received data segments before closing the TCP connection. When performing an unclean shutodwn, a transmitting node SHALL treat either sending or receiving a SESS_TERM message (i.e. before the final acknowledgment) as a failure of the transfer. Any delay between request to terminate the TCP connection and actual closing of the connection (a "half-closed" state) MAY be ignored by the TCPCL node.
The format of the SESS_TERM message is as follows in Figure 25.
+-----------------------------------+ | Message Header | +-----------------------------------+ | Message Flags (U8) | +-----------------------------------+ | Reason Code (optional U8) | +-----------------------------------+
Figure 25: Format of SESS_TERM Messages
The fields of the SESS_TERM message are:
| Name | Code | Description |
|---|---|---|
| R | 0x02 | If bit is set, indicates that a Reason Code field is present. |
| Reserved | others |
It is possible for an entity to convey optional 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:
| Name | Description |
|---|---|
| Idle timeout | The session is being closed due to idleness. |
| Version mismatch | The node cannot conform to the specified TCPCL protocol version. |
| Busy | The node is too busy to handle the current session. |
| Contact Failure | The node cannot interpret or negotiate contact header option. |
| Resource Exhaustion | The node has run into some resource limit and cannot continue the session. |
A session shutdown MAY occur immediately after transmission of a contact header (and prior to any further message transmit). This MAY, for example, be used to notify that the node is currently not able or willing to communicate. However, an entity MUST always send the contact header to its peer before sending a SESS_TERM message.
If reception of the contact header itself somehow fails (e.g. an invalid "magic string" is recevied), an entity SHOULD close the TCP connection without sending a SESS_TERM message. If the content of the Session Extension Items data disagrees with the Session Extension Length (i.e. the last Item claims to use more octets than are present in the Session Extension Length), the reception of the contact header is considered to have failed.
If a session is to be terminated before a protocol message has completed being sent, then the node MUST NOT transmit the SESS_TERM message but still 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 SESS_TERM message or the connection is simply terminated mid-XFER_SEGMENT.
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 TCPCL messages (other than KEEPALIVE messages) has been received for at least that amount of time, then either node MAY terminate the session by transmitting a SESS_TERM message indicating the reason code of "Idle timeout" (as described in Table 9).
[NOTE to the RFC Editor: please remove this section before publication, as well as the reference to [RFC7942] and [github-dtn-bpbis-tcpcl].]
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.
An example implementation of the this draft of TCPCLv4 has been created as a GitHub project [github-dtn-bpbis-tcpcl] and is intented to use as a proof-of-concept and as a possible source of interoperability testing. This example implementation uses D-Bus as the CL-BP Agent interface, so it only runs on hosts which provide the Python "dbus" library.
One security consideration for this protocol relates to the fact that entities present their endpoint identifier as part of the contact header exchange. It would be possible for an entity to fake this value and present the identity of a singleton endpoint in which the node is not a member, essentially masquerading as another DTN node. If this identifier is used outside of a TLS-secured session or without further verification as a means to determine which bundles are transmitted over the session, then the node that has falsified its identity would be able to obtain bundles that it otherwise would not have. Therefore, an entity SHALL NOT use the EID value of an unsecured contact header to derive a peer node's identity unless it can corroborate it via other means. When TCPCL session security is mandated by a TCPCL peer, that peer SHALL transmit initial unsecured contact header values indicated in Table 10 in order. These values avoid unnecessarily leaking session parameters and will be ignored when secure contact header re-exchange occurs.
| Parameter | Value |
|---|---|
| Flags | The USE_TLS flag is set. |
| Keepalive Interval | Zero, indicating no keepalive. |
| Segment MRU | Zero, indicating all segments are refused. |
| Transfer MRU | Zero, indicating all transfers are refused. |
| EID | Empty, indicating lack of EID. |
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. An entity MAY send a large amount of data over a TCPCL session, requiring the receiving entity to handle the data, attempt to stop the flood of data by sending a XFER_REFUSE message, or forcibly terminate the session. This burden could cause denial of service on other, well-behaving sessions. There is also nothing to prevent a malicious entity from continually establishing sessions and repeatedly trying to send copious amounts of bundle data. A listening entity MAY take countermeasures such as ignoring TCP SYN messages, closing TCP connections as soon as they are established, waiting before sending the contact header, sending a SESS_TERM message quickly or with a delay, etc.
In this section, registration procedures are as defined in [RFC8126].
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.
Port number 4556 has been previously assigned as the default port for the TCP convergence layer in [RFC7242]. This assignment is unchanged by protocol version 4. Each TCPCL entity identifies its TCPCL protocol version in its initial contact (see Section 9.2), so there is no ambiguity about what protocol is being used.
| Parameter | Value |
|---|---|
| Service Name: | dtn-bundle |
| Transport Protocol(s): | TCP |
| Assignee: | Simon Perreault <simon@per.reau.lt> |
| Contact: | Simon Perreault <simon@per.reau.lt> |
| Description: | DTN Bundle TCP CL Protocol |
| Reference: | [RFC7242] |
| Port Number: | 4556 |
IANA has created, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer Version Numbers" and initialize it with the following table. The registration procedure is RFC Required.
| Value | Description | Reference |
|---|---|---|
| 0 | Reserved | [RFC7242] |
| 1 | Reserved | [RFC7242] |
| 2 | Reserved | [RFC7242] |
| 3 | TCPCL | [RFC7242] |
| 4 | TCPCLbis | This specification. |
| 5-255 | Unassigned |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Session Extension Types" and initialize it with the contents of Table 11. The registration procedure is RFC Required within the lower range 0x0001--0x7fff. Values in the range 0x8000--0xffff are reserved for use on private networks for functions not published to the IANA.
| Code | Message Type |
|---|---|
| 0x0000 | Reserved |
| 0x0001--0x7fff | Unassigned |
| 0x8000--0xffff | Private/Experimental Use |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Transfer Extension Types" and initialize it with the contents of Table 12. The registration procedure is RFC Required within the lower range 0x0001--0x7fff. Values in the range 0x8000--0xffff are reserved for use on private networks for functions not published to the IANA.
| Code | Message Type |
|---|---|
| 0x0000 | Reserved |
| 0x0001--0x7fff | Unassigned |
| 0x8000--0xffff | Private/Experimental Use |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 Message Types" and initialize it with the contents of Table 13. The registration procedure is RFC Required.
| Code | Message Type |
|---|---|
| 0x00 | Reserved |
| 0x01 | XFER_SEGMENT |
| 0x02 | XFER_ACK |
| 0x03 | XFER_REFUSE |
| 0x04 | KEEPALIVE |
| 0x05 | SESS_TERM |
| 0x06 | XFER_INIT |
| 0x07 | MSG_REJECT |
| 0x08--0xf | Unassigned |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE Reason Codes" and initialize it with the contents of Table 14. The registration procedure is RFC Required.
| Code | Refusal Reason |
|---|---|
| 0x0 | Unknown |
| 0x1 | Extension Failure |
| 0x2 | Completed |
| 0x3 | No Resources |
| 0x4 | Retransmit |
| 0x5--0x7 | Unassigned |
| 0x8--0xf | Reserved for future usage |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 SESS_TERM Reason Codes" and initialize it with the contents of Table 15. The registration procedure is RFC Required.
| Code | Shutdown Reason |
|---|---|
| 0x00 | Idle timeout |
| 0x01 | Version mismatch |
| 0x02 | Busy |
| 0x03 | Contact Failure |
| 0x04 | Resource Exhaustion |
| 0x05--0xFF | Unassigned |
EDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry, a sub-registry titled "Bundle Protocol TCP Convergence-Layer Version 4 MSG_REJECT Reason Codes" and initialize it with the contents of Table 16. The registration procedure is RFC Required.
| Code | Rejection Reason |
|---|---|
| 0x00 | reserved |
| 0x01 | Message Type Unknown |
| 0x02 | Message Unsupported |
| 0x03 | Message Unexpected |
| 0x04-0xFF | Unassigned |
This specification is based on comments on implementation of [RFC7242] provided from Scott Burleigh.
The areas in which changes from [RFC7242] have been made to existing headers and messages are:
The areas in which extensions from [RFC7242] have been made as new messages and codes are: