Delay Tolerant Networking Research M. Demmer
Group UC Berkeley
Internet-Draft J. Ott
Intended status: Experimental Helsinki University of
Expires: March 1, 2013 Technology
S. Perreault
Viagenie
August 28, 2012
Delay Tolerant Networking TCP Convergence Layer Protocol
draft-irtf-dtnrg-tcp-clayer-04.txt
Abstract
This document describes the protocol for the TCP-based Convergence
Layer for Delay Tolerant Networking (DTN).
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 1, 2013.
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definitions Relating to the Bundle Protocol . . . . . . . 4
2.2. Definitions specific to the TCPCL Protocol . . . . . . . . 5
3. General Protocol Description . . . . . . . . . . . . . . . . . 6
3.1. Example message exchange . . . . . . . . . . . . . . . . . 7
4. Connection Establishment . . . . . . . . . . . . . . . . . . . 8
4.1. Contact Header . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Validation and parameter negotiation . . . . . . . . . . . 11
5. Established Connection Operation . . . . . . . . . . . . . . . 12
5.1. Message Type Codes . . . . . . . . . . . . . . . . . . . . 12
5.2. Bundle Data Transmission . . . . . . . . . . . . . . . . . 13
5.3. Bundle Acknowledgments . . . . . . . . . . . . . . . . . . 14
5.4. Bundle Refusal . . . . . . . . . . . . . . . . . . . . . . 15
5.5. Bundle Length . . . . . . . . . . . . . . . . . . . . . . 16
5.6. Keepalive Messages . . . . . . . . . . . . . . . . . . . . 16
6. Connection Termination . . . . . . . . . . . . . . . . . . . . 17
6.1. Shutdown Message . . . . . . . . . . . . . . . . . . . . . 17
6.2. Idle Connection Shutdown . . . . . . . . . . . . . . . . . 19
7. Requirements notation . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
This document describes the TCP-based convergence layer protocol for
Delay Tolerant Networking (TCPCL). 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 the Delay-Tolerant Network Architecture
[refs.dtnarch] RFC.
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 Bundling Protocol (BP)
[refs.bundleproto], 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
+-------------------------+
Figure 1: The locations of the bundle protocol and the TCP
convergence layer protocol in the Internet protocol stack
This document describes the format of the protocol data units passed
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between entities participating in TCPCL communications. This
document does not address:
The format of protocol data units of the bundling protocol, as
those are defined elsewhere [refs.bundleproto].
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 any Internet Draft, RFC,
or any other public document. These prior versions of the protocol
were, however, implemented in the DTN reference implementation
[refs.dtnimpl], in prior releases, hence the current version number
reflects the existence of those prior versions.
2. Definitions
2.1. Definitions Relating to the Bundle Protocol
The following set of definitions are abbreviated versions of those
which appear in the Bundle Protocol Specification [refs.bundleproto].
To the extent in which terms appear in both documents, they are
intended to have the same meaning.
Bundle -- A bundle is a protocol data unit of the DTN bundle
protocol.
Bundle payload -- A bundle payload (or simply "payload") is the
application data whose conveyance to the bundle's destination is
the purpose for the transmission of a given bundle.
Fragment -- A fragment is a bundle whose payload contains a
contiguous subset of bytes from another bundle's payload.
Bundle node -- A bundle node (or simply a "node") is any entity that
can send and/or receive bundles. The particular instantiation
of this entity is deliberately unconstrained, allowing for
implementations in software libraries, long-running processes,
or even hardware. One component of the bundle node is the
implementation of a convergence layer adapter.
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Convergence layer adapter -- A convergence layer adapter (CLA) sends
and receives bundles utilizing the services of some 'native'
link, network, or internet protocol. This document describes
the manner in which a CLA sends and receives bundles when using
the TCP protocol for inter-node communication.
Self Describing Numeric Value -- A self describing numeric value
(SDNV) is a variable length encoding for integer values, defined
in the bundle protocol specification.
2.2. Definitions specific to the TCPCL Protocol
This section contains definitions that are interpreted to be specific
to the operation of the TCPCL protocol, as described below.
TCP Connection -- A TCP connection refers to a transport connection
using TCP as the transport protocol.
TCPCL Connection -- 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 one-to-one
with 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" shall
refer to a TCPCL connection.
Connection parameters -- 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 Section Section 4.2.
Transmission -- Transmission refers to the procedures and mechanisms
(described below) for conveyance of a bundle from one node to
another.
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3. General Protocol Description
This protocol provides bundle conveyance over a TCP connection and
specifies the encapsulation of bundles as well as procedures for TCP
connection setup and teardown. 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 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,
an SDNV 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.
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. Note that in the case of concurrent bidirectional
transmission, then ack segments may be interleaved with data
segments.
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
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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.
3.1. Example message exchange
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.
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Node A Node B
====== ======
+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+
+-------------------------+
| DATA_SEGMENT (start) | ->
| SDNV length [L1] | ->
| Bundle Data 0..L1 | ->
+-------------------------+
+-------------------------+ +-------------------------+
| DATA_SEGMENT | -> <- | ACK_SEGMENT |
| SDNV length [L2] | -> <- | SDNV length [L1] |
| Bundle Data L1..L2 | -> +-------------------------+
+-------------------------+
+-------------------------+ +-------------------------+
| DATA_SEGMENT (end) | -> <- | ACK_SEGMENT |
| SDNV length [L3] | -> <- | SDNV length [L1+L2] |
| Bundle Data L2..L3 | -> +-------------------------+
+-------------------------+
+-------------------------+
<- | ACK_SEGMENT |
<- | SDNV length [L1+L2+L3] |
+-------------------------+
+-------------------------+ +-------------------------+
| SHUTDOWN | -> <- | SHUTDOWN |
+-------------------------+ +-------------------------+
Figure 2: A simple visual example of the flow of protocol messages on
a single TCP session between two nodes (A and B)
4. Connection Establishment
For bundle transmissions to occur using the TCPCL, a TCPCL connection
must first be established between communicating nodes. The manner in
which a bundle node makes the decision to establish such a connection
is implementation-dependent. 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
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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, perhaps. 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 and it SHOULD use a
(binary) exponential backoff mechanism to increase this delay in case
of repeated failures.
The node MAY declare failure after one or more connection attempts
and MAY attempt to find an alternate route for bundle data. Such
decisions are up to the higher layer (i.e., the BP).
Once a TCP connection is established, each node MUST immediately
transmit a contact header over the TCP connection. The format of the
contact header is described in Section 4.1).
Upon receipt of the contact header, both nodes perform the validation
and negotiation procedures defined in Section 4.2
After receiving the contact header from the other node, either node
MAY also refuse the connection by sending a SHUTDOWN message. If
connection setup is refused a reason MUST be included in the SHUTDOWN
message.
4.1. Contact Header
Once a TCP connection is established, both parties exchange a contact
header. This section describes the format of the contact header and
the meaning of its fields.
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The format for the Contact Header is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| magic='dtn!' |
+---------------+---------------+---------------+---------------+
| version | flags | keepalive_interval |
+---------------+---------------+---------------+---------------+
| local EID length (SDNV) |
+---------------+---------------+---------------+---------------+
| |
+ local EID (variable) +
| |
+---------------+---------------+---------------+---------------+
Figure 3: Contact Header Format
The fields of the contact header are:
magic: A four byte field that always contains the byte sequence 0x64
0x74 0x6e 0x21, i.e. the text string "dtn!".
version: A one byte field value containing the current version of
the protocol.
flags: A one byte field containing optional connection flags. The
first four bits are unused and MUST be set to zero upon
transmission and MUST be ignored upon reception. The last four
bits are interpreted as shown in table Table 1 below.
keepalive_interval: A two byte integer field containing the number
of seconds between exchanges of keepalive messages on the
connection (see Section 5.6). This value is in network byte
order, as are all other multi-byte fields described in this
protocol.
local eid length: A variable length SDNV field containing the length
of the endpoint identifier (EID) for some singleton endpoint in
which the sending node is a member. A four byte SDNV is
depicted for clarity of the figure.
local EID: An octet string containing the EID of some singleton
endpoint in which the sending node is a member, in the canonical
format of <scheme name>:<scheme-specific part>. A eight byte
EID is shown the clarity of the figure.
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+----------+--------------------------------------------------------+
| Value | Meaning |
+----------+--------------------------------------------------------+
| 00000001 | Request acknowledgment of bundle segments. |
| 00000010 | Request enabling of reactive fragmentation. |
| 00000100 | Indicate support for bundle refusal. This flag MUST |
| | NOT be set to '1' unless support for acknowledgments |
| | is also indicated. |
| 00001000 | Request sending of LENGTH messages. |
+----------+--------------------------------------------------------+
Table 1: Contact Header Flags
The manner in which values are configured and chosen for the various
flags and parameters in the contact header is implementation
dependent.
4.2. Validation and parameter negotiation
Upon reception of the contact header, each node follows the following
procedures for ensuring 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 SHOULD interpret all fields and messages as
it would normally. 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) with the preferences of the peer node
(expressed in the contact header that it received). This negotiation
MUST proceed in the following manner:
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The segment acknowledgments enabled parameter is set to true iff
the corresponding flag is set in both contact headers.
The reactive fragmentation enabled parameter is set to true iff
the corresponding flag is set in both contact headers.
The bundle refusal capability may only be used iff both peers
indicate support for it in their contact header.
The keepalive_interval parameter 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 Section 5.6) is disabled.
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.
5. Established Connection Operation
This section describes the protocol operation for the duration of an
established connection, including the mechanisms for transmitting
bundles over the connection.
5.1. Message Type Codes
After the initial exchange of a contact header, all messages
transmitted over the connection are identified by a one octet header
with the following structure:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| type | flags |
+-+-+-+-+-+-+-+-+
type: Indicates the type of the message as per Table 2 below
flags: Optional flags defined on a per message type basis.
The types and values for the message type code are as follows.
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+----------------+------+-------------------------------------------+
| Type | Code | Comment |
+----------------+------+-------------------------------------------+
| DATA_SEGMENT | 0x1 | Indicates the transmission of a segment |
| | | of bundle data, described in Section 5.2. |
| | | |
| ACK_SEGMENT | 0x2 | Acknowledges reception of a data segment, |
| | | described in Section 5.3 |
| | | |
| REFUSE_BUNDLE | 0x3 | Indicates that the transmission of the |
| | | current bundle shall be stopped, |
| | | described in Section 5.4. |
| | | |
| KEEPALIVE | 0x4 | Keepalive message for the connection, |
| | | described in Section 5.6. |
| | | |
| SHUTDOWN | 0x5 | Indicates that one of the nodes |
| | | participating in the connection wishes to |
| | | cleanly terminate the connection, |
| | | described in Section 6. |
| | | |
| LENGTH | 0x6 | Contains the length (in bytes) of the |
| | | next bundle, described in Section 5.5. |
| | | |
+----------------+------+-------------------------------------------+
Table 2: TCPCL Header Types
5.2. Bundle Data Transmission
Each bundle is transmitted in one or more data segments. The format
of a data segment message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x1 |0|0|S|E| length ... | contents.... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of bundle data segment messages
The type portion of the message header contains the value 0x1.
The flags portion of the message header octet contains two optional
values in the two low-order bits, denoted 'S' and 'E' above. The 'S'
bit MUST be set to one iff 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.
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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. However, a node MUST be able to
receive a bundle that has been transmitted in any segment size.
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.
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.
5.3. Bundle Acknowledgments
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
segment acknowledgments are enabled (see Section 4.1). If so, then
the receiver MUST transmit a bundle acknowledgment header when it
successfully receives each data segment.
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The format of a bundle acknowledgment 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x2 |0|0|0|0| acknowledged length ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Format of bundle acknowledgement messages
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 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.
5.4. Bundle Refusal
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 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 is implicitly identified by the sequence in which
acknowledgements and refusals are received.
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 rejected 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
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on the wire; if this happens, subsequent segments of the bundle
SHOULD be refused with a REFUSE_BUNDLE message, too.
Note: If a bundle transmission if 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.
5.5. Bundle Length
The format of the LENGTH message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x6 |0|0|0|0| total bundle length ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Format of LENGTH messages
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.
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 receiver MUST be prepared for this and MUST associate the refusal
with the right bundle.
5.6. Keepalive Messages
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 Section 4.1, one of the parameters in the contact
header is the keepalive_interval. Both sides populate this field
with their requested intervals (in seconds) between keepalive
messages.
The format of a keepalive message is a one byte message type code of
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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 message type code of SHUTDOWN (as
described in Table 2) and 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 RTO) which is dependent on the
measured RTT for the TCP connection so that keepalive message may
experience noticeable latency.
6. Connection Termination
This section describes the procedures for ending a TCPCL connection.
6.1. Shutdown Message
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, it is advisable to acknowledge all received data segments
first and then shut down the connection.
The format of the shutdown message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x5 |0|0|R|D| reason (opt) | reconnection delay (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of bundle shutdown messages
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:
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+------+-------------------+----------------------------------------+
| Code | Meaning | Comment |
+------+-------------------+----------------------------------------+
| 0x00 | Idle timeout | The connection is being closed due to |
| | | idleness. |
| | | |
| 0x01 | Version mismatch | The node cannot conform to the |
| | | specified TCPCL protocol version. |
| | | |
| 0x02 | Busy | The node is too busy to handle the |
| | | current connection. |
+------+-------------------+----------------------------------------+
Table 3: Shutdown Reason Codes
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, 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 delay field (and set the
'D' bit to zero). Note that in the figure above, a two octet SDNV is
shown for convenience of the presentation.
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 capable of or willing to communicate.
However, a node MUST always send the contact header to its peer
first.
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 re-open 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, receiving node is
still expecting segment data and might erroneously interpret the
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SHUTDOWN message to be part of the data segment.
6.2. Idle Connection Shutdown
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, then 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 above). After receiving a SHUTDOWN message in
response, both sides may close the TCP connection.
7. Requirements notation
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].
8. Security Considerations
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 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.
These concerns may be mitigated through the use of the Bundle
Security Protocols [refs.dtnsecurity]. In particular, the Bundle
Authentication Header 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.
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 either handle the data,
attempt to stop the flood of data by sending a REFUSE_BUNDLE message,
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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.
9. IANA Considerations
Port number 4556 has been assigned as the default port for the TCP
convergence layer.
10. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[refs.bundleproto]
Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
[refs.dtnarch]
Cerf et al, V., "Delay-Tolerant Network Architecture",
RFC 4838, April 2007.
[refs.dtnimpl]
DTNRG, "Delay Tolerant Networking Reference
Implementation", <http://www.dtnrg.org/Code>.
[refs.dtnsecurity]
Symington, S., Farrell, S., and H. Weiss, "Bundle Security
Protocol Specification", Internet Draft, work in
progress draft-irtf-dtnrg-bundle-security-03.txt,
April 2007.
Authors' Addresses
Michael J. Demmer
University of California, Berkeley
Computer Science Division
445 Soda Hall
Berkeley, CA 94720-1776
US
Email: demmer@cs.berkeley.edu
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Joerg Ott
Helsinki University of Technology
Department of Communications and Networking
PO Box 3000
TKK 02015
Finland
Email: jo@netlab.tkk.fi
Simon Perreault
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
Phone: +1 418 656 9254
Email: simon.perreault@viagenie.ca
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