]> RKF Engineering Solutions, LLC

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Aalto University

Department of Communications and Networking PO Box 13000 Aalto 02015 Finland jo@netlab.tkk.fi

Quebec QC Canada simon@per.reau.lt

Transport Delay Tolerant Networking This document describes a revised protocol for the TCP-based convergence layer for Delay-Tolerant Networking (DTN). The protocol revision is based on implementation issues in the original and updates to the Bundle Protocol contents, encodings, and convergence layer requirements in . The majority of this specification is unchanged from TCPCL version 3.

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" . 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 revsided Bundle Protocol (BP) , an application-layer protocol that is used to construct a store-and- forward overlay network. As described in the Bundle Protocol specification , it requires the services of a "convergence- layer adapter" (CLA) to send and receive bundles using the service of some "native" link, network, or Internet protocol. This document describes one such convergence-layer adapter that uses the well-known Transmission Control Protocol (TCP). This convergence layer is referred to as TCPCL. The locations of the TCPCL and the BP in the Internet model protocol stack 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) | -/ +-------------------------+ | TCP | ---> Transport Layer +-------------------------+ | IP | ---> Network Layer +-------------------------+ | Link-Layer Protocol | ---> Link Layer +-------------------------+ | Physical Medium | ---> Physical Layer +-------------------------+

This document describes the format of the protocol data units passed between entities participating in TCPCL communications. This document does not address: The format of protocol data units of the Bundle Protocol, as those are defined elsewhere in and . Mechanisms for locating or identifying other bundle nodes within an internet. Note that this document describes version 3 of the protocol. Versions 0, 1, and 2 were never specified in an Internet-Draft, RFC, or any other public document. These prior versions of the protocol were, however, implemented in the DTN reference implementation [DTNIMPL] in prior releases; hence, the current version number reflects the existence of those prior versions. This is an experimental protocol produced within the IRTF's Delay- Tolerant Networking Research Group (DTNRG). It represents the consensus of all active contributors to this group. If this protocol is used on the Internet, IETF standard protocols for security and congestion control should be used.

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 .

This section contains definitions that are interpreted to be specific to the operation of the TCPCL protocol, as described below. A TCP connection refers to a transport connection using TCP as the transport protocol. A TCPCL connection (as opposed to a TCP connection) is a TCPCL communication relationship between two bundle nodes. The lifetime of a TCPCL connection is bound to the lifetime of an underlying TCP connection. Therefore, a TCPCL connection is initiated when a bundle node initiates a TCP connection to be established for the purposes of bundle communication. A TCPCL connection is terminated when the TCP connection ends, due either to one or both nodes actively terminating the TCP connection or due to network errors causing a failure of the TCP connection. For the remainder of this document, the term "connection" without the prefix "TCPCL" refer to a TCPCL connection. The connection parameters are a set of values used to affect the operation of the TCPCL for a given connection. The manner in which these parameters are conveyed to the bundle node and thereby to the TCPCL is implementation dependent. However, the mechanism by which two bundle nodes exchange and negotiate the values to be used for a given session is described in . Transmission refers to the procedures and mechanisms (described below) for conveyance of a bundle from one node to another.

The service of this protocol is the transmission of DTN bundles over 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 connection to the other by initiating a TCP connection. After setup of the TCP connection is complete, an initial contact header is exchanged in both directions to set parameters of the TCPCL connection and exchange a singleton endpoint identifier for each node (not the singleton Endpoint Identifier (EID) of any application running on the node) to denote the bundle-layer identity of each DTN node. This is used to assist in routing and forwarding messages, e.g., to prevent loops. Once the TCPCL connection is established and configured in this way, bundles can be transmitted in either direction. Each bundle is transmitted in one or more logical segments of formatted bundle data. Each logical data segment consists of a DATA_SEGMENT message header, a Self-Delimiting Numeric Value (SDNV) as defined in containing the length of the segment, and finally the byte range of the bundle data. The choice of the length to use for segments is an implementation matter. The first segment for a bundle must set the 'start' flag, and the last one must set the 'end' flag in the DATA_SEGMENT message header. If multiple bundles are transmitted on a single TCPCL connection, they MUST be transmitted consecutively. Interleaving data segments from different bundles is not allowed. Bundle interleaving can be accomplished by fragmentation at the BP layer. An optional feature of the protocol is for the receiving node to send acknowledgments as bundle data segments arrive (ACK_SEGMENT). 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 connection is interrupted, it can perform reactive fragmentation to avoid re-sending the already transmitted part of the bundle. When acknowledgments are enabled, then for each data segment that is received, the receiving node sends an ACK_SEGMENT code followed by an SDNV containing the cumulative length of the bundle that has been received. The sending node may transmit multiple DATA_SEGMENT messages without necessarily waiting for the corresponding ACK_SEGMENT responses. This enables pipelining of messages on a channel. In addition, there is no explicit flow control on the TCPCL layer. Another optional feature is that a receiver may interrupt the transmission of a bundle at any point in time by replying with a REFUSE_BUNDLE message, which causes the sender to stop transmission of the current bundle, after completing transmission of a partially sent data segment. Note: This enables a cross-layer optimization in that it allows a receiver that detects that it already has received a certain bundle to interrupt transmission as early as possible and thus save transmission capacity for other bundles. For connections that are idle, a KEEPALIVE message may optionally be sent at a negotiated interval. This is used to convey liveness information. Finally, before connections close, a SHUTDOWN message is sent on the channel. After sending a SHUTDOWN message, the sender of this message may send further acknowledgments (ACK_SEGMENT or REFUSE_BUNDLE) but no further data messages (DATA_SEGMENT). A SHUTDOWN message may also be used to refuse a connection setup by a peer.

There are specific messages for sending and receiving operations (in addition to connection setup/teardown). TCPCL is symmetric, i.e., both sides can start sending data segments in a connection, 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 following figure visually depicts the protocol exchange for a simple session, showing the connection establishment and the transmission of a single bundle split into three data segments (of lengths L1, L2, and L3) from Node A to Node B. Note that the sending node may transmit multiple DATA_SEGMENT messages without necessarily waiting for the corresponding ACK_SEGMENT responses. This enables pipelining of messages on a channel. Although this example only demonstrates a single bundle transmission, it is also possible to pipeline multiple DATA_SEGMENT messages for different bundles without necessarily waiting for ACK_SEGMENT messages to be returned for each one. However, interleaving data segments from different bundles is not allowed. No errors or rejections are shown in this example.

Node A Node B ====== ====== +-------------------------+ +-------------------------+ | Contact Header | -> <- | Contact Header | +-------------------------+ +-------------------------+ +-------------------------+ | DATA_SEGMENT (start) | -> | SDNV length [L1] | -> | Bundle Data 0..(L1-1) | -> +-------------------------+ +-------------------------+ +-------------------------+ | DATA_SEGMENT | -> <- | ACK_SEGMENT | | SDNV length [L2] | -> <- | SDNV length [L1] | |Bundle Data L1..(L1+L2-1)| -> +-------------------------+ +-------------------------+ +-------------------------+ +-------------------------+ | DATA_SEGMENT (end) | -> <- | ACK_SEGMENT | | SDNV length [L3] | -> <- | SDNV length [L1+L2] | |Bundle Data | -> +-------------------------+ | (L1+L2)..(L1+L2+L3-1)| +-------------------------+ +-------------------------+ <- | ACK_SEGMENT | <- | SDNV length [L1+L2+L3] | +-------------------------+ +-------------------------+ +-------------------------+ | SHUTDOWN | -> <- | SHUTDOWN | +-------------------------+ +-------------------------+

For bundle transmissions to occur using the TCPCL, a TCPCL connection must first be established between communicating nodes. It is up to the implementation to decide how and when connection setup is triggered. For example, some connections may be opened proactively and maintained for as long as is possible given the network conditions, while other connections 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 connection, a node must first establish a TCP connection with the intended peer node, typically by using the services provided by the operating system. Port number 4556 has been assigned by IANA as the well-known port number for the TCP convergence layer. Other port numbers MAY be used per local configuration. Determining a peer's port number (if different from the well-known TCPCL port) is up to the implementation. If the node is unable to establish a TCP connection for any reason, then it is an implementation matter to determine how to handle the connection failure. A node MAY decide to re-attempt to establish the connection. If it does so, it MUST NOT overwhelm its target with repeated connection attempts. Therefore, the node MUST retry the connection setup only after some delay (a 1-second minimum is RECOMMENDED), and it SHOULD use a (binary) exponential backoff mechanism to increase this delay in case of repeated failures. In case a SHUTDOWN message specifying a reconnection delay is received, that delay is used as the initial delay. The default initial delay SHOULD be at least 1 second but SHOULD be configurable since it will be application and network type dependent. The node MAY declare failure after one or more connection attempts and MAY attempt to find an alternate route for bundle data. Such decisions are up to the higher layer (i.e., the BP). Once a TCP connection is established, each node MUST immediately transmit a contact header over the TCP connection. The format of the contact header is described in . Upon receipt of the contact header, both nodes perform the validation and negotiation procedures defined in After receiving the contact header from the other node, either node MAY also refuse the connection by sending a SHUTDOWN message. If connection setup is refused, a reason MUST be included in the SHUTDOWN message.

Once a TCP connection is established, both parties exchange a contact header. This section describes the format of the contact header and the meaning of its fields. The format for the Contact Header is as follows:

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+---------------+---------------+ | magic='dtn!' | +---------------+---------------+---------------+---------------+ | version | Option count (SDNV) | +---------------+---------------+---------------+---------------+ | Options list (sequence of TLV) | +---------------+---------------+---------------+---------------+

The fields of the contact header are: A four-byte field that always contains the byte sequence 0x64 0x74 0x6e 0x21, i.e., the text string "dtn!" in US-ASCII (and UTF-8). A one-byte field value containing the value 4 (current version of the protocol). A SDNV-encoded count of connection options to follow. A sequence of type-length-value (TLV) encoded connection options as shown in and described in Each option within a contact header represents the configuration of the peer which has sent the contact header. The manner in which options are configured and chosen for the various parameters in the contact header is implementation dependent.

Each of the contact header Options SHALL be encoded as a sequence of TLV data as shown in . An SDNV-encoded field which SHALL determine the encoding and interpretation of the option value. An SDNV-encoded field which SHALL define the length (in octets) of the Value Data to follow. The Value Length does not include the length of the Option Type or Value Length fields themselves. The encoded type-dependent data of the option. The order of items within the Option list SHALL not be significant. Each option type SHALL be present no more than one time in a Contact Header. Each option type SHALL be considered to have a default value if not present in a contact header. The option type defines how the particular Value Data is encoded and interpreted. If a TCPCL peer receives an Option Type which is unknown to it, the peer may send a SHUTDOWN and terminate the connection. Otherwise, the peer SHALL ignore the option and continue contact header processing.

+-----------------------+ | Option Type (SDNV) | +-----------------------+ | Value Length (SDNV) | +-----------------------+ | Value Data (variable) | +-----------------------+

Upon reception of the contact header, each node follows the following procedures to ensure the validity of the TCPCL connection and to negotiate values for the connection parameters. If the magic string is not present or is not valid, the connection MUST be terminated. The intent of the magic string is to provide some protection against an inadvertent TCP connection by a different protocol than the one described in this document. To prevent a flood of repeated connections from a misconfigured application, a node MAY elect to hold an invalid connection open and idle for some time before closing it. If a node receives a contact header containing a version that is greater than the current version of the protocol that the node implements, then the node SHALL terminate the connection. If a node receives a contact header with a version that is lower than the version of the protocol that the node implements, the node may either terminate the connection due to the version mismatch or may adapt its operation to conform to the older version of the protocol. This decision is an implementation matter. A node calculates the parameters for a TCPCL connection by negotiating the values from its own preferences (conveyed by the contact header it sent to the peer) with the preferences of the peer node (expressed in the contact header that it received from the peer). The negotatiated parameters defined by this specification are listed in and described in the following paragraphs. TypeCodeDescription Handle LENGTH0x1Determine how peer handles LENGTH messages. Acknowledge Intermediate0x2Determine how peer handles acknowledgment of non-final bundle segments. Acknowledge Bundle0x3Determine how peer handles acknowledgement of final bundle segments and bundle refusal. Keepalive0x4The maximum keepalive interval of the peer. Peer EID0x5The Endpoint ID of the peer. BP Versions0x6The set of BP versions supported by the peer. Segment MRU0x7The largest segment length supported by the peer. Handle TLS0x8Determine how peer handles STARTTLS messages. This parameter is a value enumerated by and encoded as a single octet. The value determines whether LENGTH messages (as described in ) are handled by this peer. If the value is IGNORE then the LENGTH message SHOULD NOT be sent to this peer. If the value is ALLOW then the LENGTH message SHOULD be sent to this peer. If the value is REQUIRE then the LENGTH message SHALL be sent to this peer. The default Handle LENGTH value is ALLOW. This parameter is a value enumerated by and encoded as a single octet. The value determines how any non-final ACK_SEGMENT (i.e. one with its E flag not set as described in ) is handled by this peer. If the value is IGNORE then any non-final ACK_SEGMENT message SHOULD NOT be sent to this peer. If the value is ALLOW then any non-final ACK_SEGMENT message SHOULD be sent to this peer. If the value is REQUIRE then any non-final ACK_SEGMENT message SHALL be sent to this peer. The default Acknowledge Intermediate value is ALLOW. This parameter is a value enumerated by and encoded as a single octet. The value determines how any final ACK_SEGMENT (i.e. one with its E flag set as described in ) is handled by this peer. This value also determines how any REFUSE_BUNDLE message is handled by this peer. If the value is IGNORE then any final ACK_SEGMENT or REFUSE_BUNDLE message SHOULD NOT be sent to this peer. If the value is ALLOW then any final ACK_SEGMENT or REFUSE_BUNDLE message SHOULD be sent to this peer. If the value is REQUIRE then any final ACK_SEGMENT or REFUSE_BUNDLE message SHALL be sent to this peer. The default Acknowledge Bundle value is ALLOW. This parameter is encoded as a two-octet unsigned integer in network byte order (U16). The value is the maximum keepalive time interval (in seconds) for this peer. The negotiated keepalive interval is set to the minimum value from both contact headers. If one or both contact headers contains the value zero, then the keepalive feature (described in ) is disabled. The default Keepalive value is zero. This parameter is a byte string containing the UTF-8 encoded EID of some singleton endpoint in which the sending node is a member, in the canonical format of <scheme name>:<scheme-specific part>. The default Peer EID value is the empty string. This parameter is a sequence of bytes, each containing an individual Bundle Protocol version number supported by this peer. The set of supported BP versions of the connection is the intersection of the BP versions indicated by both of the contact headers. If interoperating with a TCPCL Version 3 node, a TCPCL Version 4 node MAY assume that the TCPCL Version 3 node supports exactly one BP Version: 0x06 of . If there is no common supported BP version then the connection SHOULD be shutdown with reason "Contact Failure", as no possible bundle exchange can occur. Otherwise, the default BP Versions value is the single version 4 (this specification). This parameter is an SDNV-encoded integer. The value indicates the maximum Data Length within of DATA_SEGMENT which this peer will accept, or zero if there is no maximum length. Any DATA_SEGMENT send to this peer SHALL have a data payload no longer than this peer's maximum length. The default Segment MRU value is zero. NameCodeDescription IGNORE0x01The peer sending this option will ignore any such messages. ALLOW0x02The peer sending this option will process any such messages and act upon the data. REQUIRE0x03The peer sending this option will refuse to operate without such messages. Once this process of parameter negotiation is completed, the protocol defines no additional mechanism to change the parameters of an established connection; to effect such a change, the connection MUST be terminated and a new connection established.

This section describes the protocol operation for the duration of an established connection, including the mechanisms for transmitting bundles over the connection.

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

0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | type | flags | +-+-+-+-+-+-+-+-+

type: Indicates the type of the message as per below. flags: Optional flags defined based on message type. The types and values for the message type code are as follows. TypeCodeDescription DATA_SEGMENT0x1Indicates the transmission of a segment of bundle data, as described in . ACK_SEGMENT0x2Acknowledges reception of a data segment, as described in . REFUSE_BUNDLE0x3Indicates that the transmission of the current bundle SHALL be stopped, as described in . KEEPALIVE0x4KEEPALIVE message for the connection, as described in . SHUTDOWN0x5Indicates that one of the nodes participating in the connection wishes to cleanly terminate the connection, as described in . LENGTH0x6Contains the length (in bytes) of the next bundle, as described in .

The protocol includes a provision for transmission of KEEPALIVE messages over the TCP connection to help determine if the connection has been disrupted. As described in , one of the parameters in the contact header is the keepalive_interval. Both sides populate this field with their requested intervals (in seconds) between KEEPALIVE messages. The format of a KEEPALIVE message is a one-byte 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 for at least twice the keepalive_interval, then either party MAY terminate the session by transmitting a one-byte SHUTDOWN message (as described in Table 2) and by closing the TCP connection. 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.

If a TCPCL endpoint receives a message which is uknown to it (possibly due to an unhandled protocol mismatch) or is inappropriate for the current connection state (e.g. a KEEPALIVE or LENGTH message received after feature negotation has disabled those features), there is a protocol-level message to signal this condition in the form of a REJECT reply. The format of a REJECT message follows:

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

The Rejected Message Header is a copy of the Message Header to which the REJECT message is sent as a response. The REJECT Reason Code indicates why the REJECT itself was sent. The specified values of the reason code are: NameCodeDescription Message Type Unknown0x01A message was received with a Message Type code unknown to the TCPCL endpoint. Message Unsupported0x02A message was received but the TCPCL endpoint cannot comply with the message contents. Message Unexpected0x03A message was received while the connection is in a state in which the message is not expected.

This version of the TCPCL supports establishing a connection-level Transport Layer Security (TLS) session within an existing TCPCL connection. When TLS is used within the TCPCL it affects the entire connection, and it can technically be initiated by either endpoint of the connection. By convention, this protocol uses the endpoint which initiated the underlying TCP connection as the initiator of the TLS session request. Once a TLS session is established within TCPCL, there is no mechanism provided to end the TLS session and downgrade the connection. If a non-TLS connection is desired after a TLS session is started then the entire TCPCL connection MUST be shutdown first.

A STARTTLS message SHOULD be sent by the TCP client immediately after reception of the TCPCL Contact Header from the server. Upon sending a STARTTLS message, the requester SHALL enter a waiting state. While in the waiting state, upon reception of a confirmation STARTTLS message the requestor SHALL begin a TLS handshake in accordance with . While in the waiting state, the recepiton of any message other than a STARTTLS reply MAY cause a connection shutdown depending upon security policy of the endpoint.

Upon reception of a STARTTLS message while not already within a TLS session and while not acting as a TLS requester and if the endpoint supports TLS connections, a STARTTLS message SHALL be sent in response. If an endpoint receives a STARTTLS message but cannot support a TLS connection (for any reason) then a REJECT message SHALL be sent in response containing a Reason Code of "Message Unsupported. Upon reception of a STARTTLS message while already within a TLS session, a REJECT message SHOULD be sent in response containing a Reason Code of "Message Unexpected". Upon sending a response STARTTLS message the responder SHALL begin a TLS handshake in accordance with .

If a TLS handshake cannot negotiate a TLS session, either endpoint of the TCPCL connection SHOULD cause a TCPCL shutdown with reason "TLS negotiation failed". After a TLS handshake failure, if the connection is not shutdown then either endpoint MAY request a new TLS handshake. Unless the TLS parameters change between two sequential handshakes, the subsequent handshake is likely to fail just as the earlier one. After a TLS session is successfuly established, both TCPCL endpoints SHALL re-exchange TCPCL Contact Header messages. Any information cached from the prior Contact Header exchange SHALL be discarded. This re-exchange avoids man-in-the-middle attack in identical fashon to .

A summary of a typical STARTTLS usage is shown in the sequence below where the client/requester role is represented by the prefix "C" and the server/responder role is represented by the prefix "S". Unordered or "simultaneous" actions are shown as "C/S".

Node A Node B ====== ====== +-------------------------+ | Open TCP Connnection | -> +-------------------------+ +-------------------------+ <- | Accept Connection | +-------------------------+ +-------------------------+ +-------------------------+ | Contact Header | -> <- | Contact Header | +-------------------------+ +-------------------------+ ... plaintext TCPCL messaging ... +-------------------------+ | STARTTLS | -> +-------------------------+ +-------------------------+ <- | STARTTLS | +-------------------------+ +-------------------------+ +-------------------------+ | TLS Negotiation | -> <- | TLS Negotiation | +-------------------------+ +-------------------------+ +-------------------------+ +-------------------------+ | Contact Header | -> <- | Contact Header | +-------------------------+ +-------------------------+ ... secured TCPCL messaging ... +-------------------------+ +-------------------------+ | SHUTDOWN | -> <- | SHUTDOWN | +-------------------------+ +-------------------------+

All of the message in this section are directly associated with tranfering a bundle between TCPCL endpoints. Each of the messages contains a Bundle ID number which is used to correlate messages originating from sender and receiver of a bundle. The Bundle ID provides a similar behaivior to a datagram sequence number, but there are no requirements on Bundle ID ordering or reuse. Bundle IDs SHOULD be unique within a limited scope dependant upon implementation needs. Sequential bundle transfers SHALL NOT use the same Bundle ID. Bundle ID numbers MAY be reused after a window of either count or time. Bundle ID reuse SHOULD take into account unacknowledged bundle segments if acknowledgement is used within a connection. For example, Bundle IDs in the range 1--50 inclusive can be used for sequential bundle transmissions in ascending order before recycling back to 1. This allows discrimination between 50 adjacent bundle transfers. A TCPCL endpoint SHALL support Bundle IDs at least between 0 and 2^14 (two-bytes encoded). A TCPCL endpoint MAY support larger Bundle IDs depending upon implementation needs. For bidirectional bundle transfers, a TCPCL endpoint SHOULD NOT rely on any relation between Bundle IDs originating from each side of the TCPCL connection. Upon reception of a Bundle ID not able to be handled by an endpoint, a REFUSE_BUNDLE message SHOULD be sent in response.

Each bundle is transmitted in one or more data segments. The format of a DATA_SEGMENT message follows in and its use of header flags is shown in .

+-----------------------------+ | Message Header | +-----------------------------+ | Bundle ID (SDNV) | +-----------------------------+ | Data length (SDNV) | +-----------------------------+ | Data contents (byte string) | +-----------------------------+

4 5 6 7 +-+-+-+-+ |0|0|S|E| +-+-+-+-+

The type portion of the message header contains the value 0x1. The flags portion of the message header byte contains two optional values in the two low-order bits, denoted 'S' and 'E' above. The 'S' bit MUST be set to one if it precedes the transmission of the first segment of a new bundle. The 'E' bit MUST be set to one when transmitting the last segment of a bundle. Following the message header, the length field is an SDNV containing the number of bytes of bundle data that are transmitted in this segment. Following this length is the actual data contents. Determining the size of the segment is an implementation matter. In particular, a node may, based on local policy or configuration, only ever transmit bundle data in a single segment, in which case both the 'S' and 'E' bits MUST be set to one. In the Bundle Protocol specification , a single bundle comprises a primary bundle block, a payload block, and zero or more additional bundle blocks. The relationship between the protocol blocks and the convergence-layer segments is an implementation- specific decision. In particular, a segment MAY contain more than one protocol block; alternatively, a single protocol block (such as the payload) MAY be split into multiple segments. However, a single segment MUST NOT contain data of more than a single bundle. Once a transmission of a bundle has commenced, the node MUST only send segments containing sequential portions of that bundle until it sends a segment with the 'E' bit set.

Although the TCP transport provides reliable transfer of data between transport peers, the typical BSD sockets interface provides no means to inform a sending application of when the receiving application has processed some amount of transmitted data. Thus, after transmitting some data, a Bundle Protocol agent needs an additional mechanism to determine whether the receiving agent has successfully received the segment. To this end, the TCPCL protocol offers an optional feature whereby a receiving node transmits acknowledgments of reception of data segments. This feature is enabled if, and only if, during the exchange of contact headers, both parties set the flag to indicate that intermediate or final acknowledgments are enabled (see ). The format of an ACK_SEGMENT message follows in and its use of header flags is the same as for DATA_SEGMENT (shown in ). The flags of an ACK_SEGMENT message SHALL be identical to the flags of the DATA_SEGMENT message for which it is a reply.

+-----------------------------+ | Message Header | +-----------------------------+ | Bundle ID (SDNV) | +-----------------------------+ | Acknowledged length (SDNV) | +-----------------------------+

To transmit an acknowledgment, a node first transmits a message header with the ACK_SEGMENT type code and all flags set to zero, then transmits an SDNV containing the cumulative length in bytes of the received segment(s) of the current bundle. The length MUST fall on a segment boundary. That is, only full segments can be acknowledged. 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.

As bundles may be large, the TCPCL supports an optional mechanisms by which a receiving node may indicate to the sender that it does not want to receive the corresponding bundle. To do so, upon receiving a LENGTH or DATA_SEGMENT message, the node MAY transmit a REFUSE_BUNDLE message. As data segments and acknowledgments may cross on the wire, the bundle that is being refused SHALL be identified by the Bundle ID of the refusal. The format of the message is as follows:

+-----------------------------+ | Message Header | +-----------------------------+ | Bundle ID (SDNV) | +-----------------------------+

4 5 6 7 +-+-+-+-+ | RCode | +-+-+-+-+

The RCode field, which stands for "reason code", contains a value indicating why the bundle was refused. The following table contains semantics for some values. Other values may be registered with IANA, as defined in . NameRCodeSemantics Unknown0x0Reason for refusal is unknown or not specified. Completed0x1The receiver now has the complete bundle. The sender MAY now consider the bundle as completely received. No Resources0x2The receiver's resources are exhausted. The sender SHOULD apply reactive bundle fragmentation before retrying. Retransmit0x3The receiver has encountered a problem that requires the bundle to be retransmitted in its entirety. The receiver MUST, for each bundle preceding the one to be refused, have either acknowledged all DATA_SEGMENTs or refused the bundle. This allows the sender to identify the bundles accepted and refused by means of a simple FIFO list of segments and acknowledgments. The bundle refusal MAY be sent before the entire data segment is received. If a sender receives a REFUSE_BUNDLE message, the sender MUST complete the transmission of any partially sent DATA_SEGMENT message (so that the receiver stays in sync). The sender MUST NOT commence transmission of any further segments of the refused bundle subsequently. Note, however, that this requirement does not ensure that a node will not receive another DATA_SEGMENT for the same bundle after transmitting a REFUSE_BUNDLE message since messages may cross on the wire; if this happens, subsequent segments of the bundle SHOULD also be refused with a REFUSE_BUNDLE message. Note: If a bundle transmission is aborted in this way, the receiver may not receive a segment with the 'E' flag set to '1' for the aborted bundle. The beginning of the next bundle is identified by the 'S' bit set to '1', indicating the start of a new bundle.

The LENGTH message contains the total length, in bytes, of the next bundle, formatted as an SDNV. Its purpose is to allow nodes to preemptively refuse bundles that would exceed their resources. It is an optimization. The format of the LENGTH message is as follows:

+-----------------------------+ | Message Header | +-----------------------------+ | Bundle ID (SDNV) | +-----------------------------+ | Total bundle length (SDNV) | +-----------------------------+

LENGTH messages MUST NOT be sent unless the corresponding flag bit is set in the contact header. If the flag bit is set, LENGTH messages MAY be sent at the sender's discretion. LENGTH messages MUST NOT be sent unless the next DATA_SEGMENT message has the 'S' bit set to "1" (i.e., just before the start of a new bundle). A receiver MAY send a BUNDLE_REFUSE message as soon as it receives a LENGTH message without waiting for the next DATA_SEGMENT message. The sender MUST be prepared for this and MUST associate the refusal with the right bundle. Upon reception of a LENGTH message when either LENGTH has not been negotiated or not immediately before the start of a starting DATA_SEGMENT the reciever MAY send a REJECT message with a Reason Code of "Message Unexpected".

This section describes the procedures for ending a TCPCL connection.

To cleanly shut down a connection, a SHUTDOWN message MUST be transmitted by either node at any point following complete transmission of any other message. In case acknowledgments have been negotiated, a node SHOULD acknowledge all received data segments first and then shut down the connection. The format of the SHUTDOWN message is as follows:

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

4 5 6 7 +-+-+-+-+ |0|0|R|D| +-+-+-+-+

It is possible for a node to convey additional information regarding the reason for connection termination. To do so, the node MUST set the 'R' bit in the message header flags and transmit a one-byte reason code immediately following the message header. The specified values of the reason code are: NameCodeDescription Idle timeout0x00The connection is being closed due to idleness. Version mismatch0x01The node cannot conform to the specified TCPCL protocol version. Busy0x02The node is too busy to handle the current connection. Contact Failure0x03The node cannot interpret or negotiate contact header option. TLS failure0x04The node failed to negotiate TLS session and cannot continue the connection. It is also possible to convey a requested reconnection delay to indicate how long the other node must wait before attempting connection re-establishment. To do so, the node sets the 'D' bit in the message header flags and then transmits an SDNV specifying the requested delay, in seconds, following the message header (and optionally, the SHUTDOWN reason code). The value 0 SHALL be interpreted as an infinite delay, i.e., that the connecting node MUST NOT re-establish the connection. In contrast, if the node does not wish to request a delay, it SHOULD omit the reconnection delay field (and set the 'D' bit to zero). Note that in the figure above, the reconnection delay SDNV is represented as a two-byte field for convenience. A connection shutdown MAY occur immediately after TCP connection establishment or reception of a contact header (and prior to any further data exchange). This may, for example, be used to notify that the node is currently not able or willing to communicate. However, a node MUST always send the contact header to its peer before sending a SHUTDOWN message. If either node terminates a connection prematurely in this manner, it SHOULD send a SHUTDOWN message and MUST indicate a reason code unless the incoming connection did not include the magic string. If a node does not want its peer to reopen the connection immediately, it SHOULD set the 'D' bit in the flags and include a reconnection delay to indicate when the peer is allowed to attempt another connection setup. If a connection is to be terminated before another protocol message has completed, then the node MUST NOT transmit the SHUTDOWN message but still SHOULD close the TCP connection. In particular, if the connection is to be closed (for whatever reason) while a node is in the process of transmitting a bundle data segment, the receiving node is still expecting segment data and might erroneously interpret the SHUTDOWN message to be part of the data segment.

The protocol includes a provision for clean shutdown of idle TCP connections. Determining the length of time to wait before closing idle connections, if they are to be closed at all, is an implementation and configuration matter. If there is a configured time to close idle links and if no bundle data (other than KEEPALIVE messages) has been received for at least that amount of time, then either node MAY terminate the connection by transmitting a SHUTDOWN message indicating the reason code of 'Idle timeout' (as described in Table 4). After receiving a SHUTDOWN message in response, both sides may close the TCP connection.

One security consideration for this protocol relates to the fact that nodes present their endpoint identifier as part of the connection header exchange. It would be possible for a node to fake this value and present the identity of a singleton endpoint in which the node is not a member, essentially masquerading as another DTN node. If this identifier is used outside of a TLS-secured session or without further verification as a means to determine which bundles are transmitted over the connection, then the node that has falsified its identity may be able to obtain bundles that it should not have. Therefore, a node SHALL NOT use the endpoint identifier conveyed in the TCPCL connection message to derive a peer node's identity unless it can corroborate it via other means. These concerns may be mitigated through the use of the Bundle Security Protocol . In particular, the Bundle Authentication Block defines mechanism for secure exchange of bundles between DTN nodes. Thus, an implementation could delay trusting the presented endpoint identifier until the node can securely validate that its peer is in fact the only member of the given singleton endpoint. 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 and are to be used instead. Even when using TLS to secure the TCPCL connection, 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 . Another consideration for this protocol relates to denial-of-service attacks. A node may send a large amount of data over a TCP connection, requiring the receiving node to handle the data, attempt to stop the flood of data by sending a REFUSE_BUNDLE message, or forcibly terminate the connection. This burden could cause denial of service on other, well-behaving connections. There is also nothing to prevent a malicious node from continually establishing connections and repeatedly trying to send copious amounts of bundle data. A listening node MAY take countermeasures such as ignoring TCP SYN messages, closing TCP connections as soon as they are established, waiting before sending the contact header, sending a SHUTDOWN message quickly or with a delay, etc.

In this section, registration procedures are as defined in

Port number 4556 has been previously assigned as the default port for the TCP convergence layer in . This assignment is unchanged by protocol version 4. ParameterValue 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: Port Number:4556

IANA has created, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer Version Numbers" and initialized it with the following table. The registration procedure is RFC Required. ValueDescriptionReference 0Reserved 1Reserved 2Reserved 3TCPCL 4TCPCLbisThis specification. 5-255Unassigned

IANA has created, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer Message Types" and initialized it with the contents below. The registration procedure is RFC Required. CodeMessage Type 0x0Reserved 0x1DATA_SEGMENT 0x2ACK_SEGMENT 0x3REFUSE_BUNDLE 0x4KEEPALIVE 0x5SHUTDOWN 0x6LENGTH 0x7REJECT 0x8STARTTLS 0x9--0xfUnassigned

IANA will createe, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer Option Types" and initialize it with the contents below. The registration procedure is RFC Required for values less than 2^14 (16384). Values greater than or equal to 2^14 (16384) are to be used for experimental option types. CodeOption Type 0x0Reserved 0x1Handle LENGTH 0x2Acknowledge Intermediate 0x3Acknowledge Bundle 0x4Keepalive 0x5Peer EID 0x6BP Versions 0x7Segment MRU 0x8Handle TLS 0x9--0x3ffeUnassigned 0x3fff--Experimental

IANA has created, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer REFUSE_BUNDLE Reason Codes" and initialized it with the contents of Table 3. The registration procedure is RFC Required. CodeRefusal Reason 0x0Unknown 0x1Completed 0x2No Resources 0x3Retransmit 0x4--0x7Unassigned 0x8--0xfReserved for future usage

IANA has created, under the "Bundle Protocol" registry, a sub- registry titled "Bundle Protocol TCP Convergence-Layer SHUTDOWN Reason Codes" and initialized it with the contents of Table 4. The registration procedure is RFC Required. CodeShutdown Reason 0x00Idle timeout 0x01Version mismatch 0x02Busy 0x03Contact Failure 0x04TLS failure 0x05--0xFFUnassigned

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 REJECT Reason Codes" and initialized it with the contents of Table 4. The registration procedure is RFC Required. CodeRejection Reason 0x00reserved 0x01Message Type Unknown 0x02Message Unsupported 0x03Message Unexpected 0x04-0xFFUnassigned

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

IANA

The areas in which changes from have been made to existing messages are: Added a bundle identification number to all bundle-related messages (LENGTH, DATA_SEGMENT, ACK_SEGMENT, REFUSE_BUNDLE). Added bundle protocol version negotation to contact header. Use flags in ACK_SEGMENT to mirror flags from DATA_SEGMENT. The areas in which extensions from have been made as new messages and codes are: Changed contact header negotation from single-octet bit flags to extensible TLV-encoded options. Added contact option to negotiate maximum segment size (per each direction). Added contact option to negotiate acceptable Bundle Protoocl versions. Added REJECT message to indicate an unknown or unhandled message was received. Added STARTTLS message and connection security mechanism. Added TLS failure SHUTDOWN reason code.