Delay Tolerant Networking Research Group M. Demmer
UC Berkeley
Internet-Draft J. Ott
Intended status: Experimental Helsinki University of Technology
Expires: April 19, 2007 October 16, 2006
Delay Tolerant Networking TCP Convergence Layer Protocol
draft-demmer-dtnrg-tcp-clayer-00.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 19, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the protocol for the TCP-based Convergence
Layer for Delay Tolerant Networking (DTN).
Demmer & Ott Expires April 19, 2007 [Page 1]
Internet-Draft DTN TCP Convergence Layer October 2006
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 Acknowledgements . . . . . . . . . . . . . . . . . 14
5.4. Bundle Refusal . . . . . . . . . . . . . . . . . . . . . . 15
5.5. Keepalive Messages . . . . . . . . . . . . . . . . . . . . 15
6. Connection Termination . . . . . . . . . . . . . . . . . . . . 16
6.1. Shutdown Message . . . . . . . . . . . . . . . . . . . . . 16
6.2. Idle Connection Shutdown . . . . . . . . . . . . . . . . . 17
7. Requirements notation . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
Demmer & Ott Expires April 19, 2007 [Page 2]
Internet-Draft DTN TCP Convergence Layer October 2006
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 [2]
Internet Draft.
An important goal of the DTN architecture is to accomodate a wide
range of networking technologies and environments. The protocol used
for DTN communications is the Bundling Protocol (BP) [3], an
application-layer protocol that is used to construct a store-and-
forward overlay network. As described in the bundle protocol
specification, BP requires the services of a "convergence layer
adapter" (CLA) to send and receive bundles using an underlying
internet protocol. This document describes one such convergence
layer adapter that uses the well-known Transmission Control Protocol
(TCP).
The locations of the TCPCL and BP in the Internet model protocol
stack are shown in Figure Figure 1. In particular, both the BP and
the TCPCL reside above the transport layer, i.e., at the application
layer.
+-------------------------+
| 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
between entities participating in TCPCL communications. This
Demmer & Ott Expires April 19, 2007 [Page 3]
Internet-Draft DTN TCP Convergence Layer October 2006
document does not address:
The format of protocol data units of the bundling protocol, as
those are defined elsewhere [3].
Mechanisms for locating or identifying other bundle nodes within
an internet.
Operational logic or procedures used to implement this protocol.
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 [5],
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 [3]. 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 range 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.
Demmer & Ott Expires April 19, 2007 [Page 4]
Internet-Draft DTN TCP Convergence Layer October 2006
Convergence layer adapter -- A convergence layer adapter (CLA) sends
and receives bundles utilizing the services of some 'native'
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.
Connection initiator -- The connection initiator is the bundle node
that causes the establishment of a new connection by creating a
new TCP connection (for example, by using the connect() call in
the BSD sockets API) and then following the procedures described
in Section 4.
Demmer & Ott Expires April 19, 2007 [Page 5]
Internet-Draft DTN TCP Convergence Layer October 2006
Connection acceptor -- The connection acceptor is the bundle node
that establishes a connection in response to an active
connection attempt by another bundle node (for example, by using
the listen() and accept() calls of the BSD sockets API) and then
following the procedures described in Section 4.
Transmission -- Transmission refers to the procedures and mechanisms
(described below) for conveyance of a bundle from one node to
another.
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 and teardown. The general operation of the protocol is as
follows:
First one node establishes a connection to the other by initiating a
TCP connection. At the beginning of the connection, an initial
contact header is exchanged in both directions to set parameters of
the connection and exchange a singleton endpoint identifier for each
node, to denote the bundle-layer identity of each DTN node.
Once the connection is established, 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
manner. The first segment for a bundle must set the 'start' flag and
the last must set the 'end' flag in the DATA_SEGMENT message header.
An optional feature of the protocol is for the receiving node to send
acknowledgements as bundle data segments arrive (ACK_SEGMENT). The
rationale behind these acknowledgements 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 acknowledgements 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
Demmer & Ott Expires April 19, 2007 [Page 6]
Internet-Draft DTN TCP Convergence Layer October 2006
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
negative acknowledgement (REFUSE_BUNDLE) which causes the sender to
stop transmission of the current bundle, after completing
transmission of all partially sent data segments. Note: This allows
a receiver that detects 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 acknowledgements (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.
No errors or rejections are shown in this example.
Demmer & Ott Expires April 19, 2007 [Page 7]
Internet-Draft DTN TCP Convergence Layer October 2006
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 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 dependant. 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
Demmer & Ott Expires April 19, 2007 [Page 8]
Internet-Draft DTN TCP Convergence Layer October 2006
connection with the intended peer node, typically by using the
services provided by the operating system. If the node is unable to
establish a TCP connection for any reason, then it is an
implementation manner to determine how to handle the failed
connection. For example, a node may decide to re-attempt to
establish the connection, perhaps after some delay or it may attempt
to find an alternate route for bundle data.
Note: If a node re-attempts a connection establishment, it SHOULD
ensure that it does not overwhelm its target with repeated connection
setup attempts and SHOULD use a (binary) exponential backoff to
determine the timeout after which to retry.
Once a TCP connection is established, the connection initiator MUST
immediately transmit a contact header over the TCP connection. The
connection acceptor MUST also 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.
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
Demmer & Ott Expires April 19, 2007 [Page 9]
Internet-Draft DTN TCP Convergence Layer October 2006
The fields of the contact header are:
magic: A four byte field that always contains the byte sequence 0x64
0x76 0x6e 0x21, i.e. the ASCII 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 five bits are unused and must be set to zero. The last
three bits are interpreted as follows:
keepalive_interval: A two byte integer field containing the number
of seconds between exchanges of keepalive messages on the
connection (see Section 5.5). 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 four byte EID
is shown the clarity of the figure.
+----------+--------------------------------------------------------+
| Value | Meaning |
+----------+--------------------------------------------------------+
| 00000001 | Request acknowledgement of bundle segments. |
| 00000010 | Request enabling of reactive fragmentation. |
| 00000100 | Indicate support for negative acknowledgements. This |
| | flags MUST NOT be used unless support for |
| | acknowledgements is also indicated. |
+----------+--------------------------------------------------------+
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.
Demmer & Ott Expires April 19, 2007 [Page 10]
Internet-Draft DTN TCP Convergence Layer October 2006
4.2. Validation and parameter negotiation
Upon reception of the contact header, both the connection initiator
and the connection acceptor follow the following procedures for
ensuring the validity of the 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 a
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 manner.
A node calculates the parameters for a 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 should proceed in the
following manner:
The segment acknowledgements 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.
Negative acknowledgements to interrupt transmission (actually:
refuse reception) of a bundle may only be used iff both peers
indicate support for negative acknowledements in their contact
header.
The keealive_interval parameter should be 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 5.5) is disabled.
Demmer & Ott Expires April 19, 2007 [Page 11]
Internet-Draft DTN TCP Convergence Layer October 2006
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 denoted 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.
+----------------+------+-------------------------------------------+
| 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, |
| | | decscribed in Section 5.4. |
| | | |
| KEEPALIVE | 0x4 | Keepalive message for the connection, |
| | | described in Section 5.5. |
| | | |
Demmer & Ott Expires April 19, 2007 [Page 12]
Internet-Draft DTN TCP Convergence Layer October 2006
| SHUTDOWN | 0x5 | Indicates that one of the nodes |
| | | participating in the connection wishes to |
| | | cleanly terminate the connection, |
| | | described in Section 6. |
| | | |
+----------------+------+-------------------------------------------+
Table 2: TCPCL Header Types
Open Issue: Currently, the message code implicitly identifies the
expected message length, possibly in combination with some flags.
This may limit backwards compatible extensibility: an "old" endpoint
may not know about a "new" message code or extension flag and hence
may not be able to skip the unknown (part of the) message, even
though it would otherwise be perfectly capable of interoperating (at
reduced functionality). Therefore, we may want to consider providing
an explicit length indication if the message length is more than a
single octet. Options include: (a) always including a length field,
(b) using one bit of the flags to indicate whether or not length
field is present (which then would be the case for all messages of
more than one byte length), and (c) relying on a new version number
for all extensions.
5.2. Bundle Data Transmission
Each bundle is transmitted in one or more data segments. The format
of a data segment 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x1 |0|0|S|E| length ... | contents.... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
Determining the segment size is an implementation manner. 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 tranmsmitted in any segment size.
Demmer & Ott Expires April 19, 2007 [Page 13]
Internet-Draft DTN TCP Convergence Layer October 2006
In the bundle protocol specification, a single bundle comprises of 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.
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 Acknowledgements
Although the TCP transport provides reliable transfer of data between
hosts, 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 acknowledgements 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 acknowledgements are enabled (see Section 4.1). If so, then
the receiver must transmit a bundle acknowledgement header when it
successfully receives each data segment.
The format of a bundle acknowledgement is:
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 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
To transmit an acknowledgement, 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. 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 acknowledgement of length 100. After the second
Demmer & Ott Expires April 19, 2007 [Page 14]
Internet-Draft DTN TCP Convergence Layer October 2006
segment is received, the node sends an acknowledgement of length 300.
The third and fourth acknowledgements 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
acknowledgements may cross on the wire, the data segment (and thus
the bundle) that is being refused is implicitly identified by the
sequence in which positive and negative acknowledgements are
received.
The receiver MUST have acknowledged (positively or negatively) all
other received DATA_SEGMENTs before the one to be refused so that the
sender can 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
on the wire; if this happens, subsequent segments of the bundle
SHOULD be refused with a REFUSE_BUNDLE message, too.
5.5. 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
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
Demmer & Ott Expires April 19, 2007 [Page 15]
Internet-Draft DTN TCP Convergence Layer October 2006
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.
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 can be
transmitted by either the initiator or the acceptor at any point
following complete transmission of any other message. In case
acknowledgements 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x3 |0|0|R|D| reason (opt) | reconnection delay (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It is possible for a node to convey additional information regarding
the reason for connection termination. To do so, the node sets the
'R' bit in the message header flags, and transmits a one-byte reason
code immediately following the message header. The specified values
of the reason code are:
+------+-------------------+----------------------------------------+
| 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. |
+------+-------------------+----------------------------------------+
Demmer & Ott Expires April 19, 2007 [Page 16]
Internet-Draft DTN TCP Convergence Layer October 2006
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 node should not ever
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 in representation.
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 a
the initiator that the node is currenly not capable of or willing to
communicate. Note, however, that 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 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 interrupted while a node is in the process of
transmitting a bundle data segment, then the node may identify that
the connection should be terminated before it has completed the
transmission of the data segment. Thus were the node to transmit the
SHUTDOWN message, the receiving node might erroneously interpret the
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
Demmer & Ott Expires April 19, 2007 [Page 17]
Internet-Draft DTN TCP Convergence Layer October 2006
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 [1].
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 [4]. 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,
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
There should be a well-known TCP port assignment for this protocol.
Demmer & Ott Expires April 19, 2007 [Page 18]
Internet-Draft DTN TCP Convergence Layer October 2006
10. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Cerf et al, V., "Delay-Tolerant Network Architecture", work in
progress, Internet Draft draft-irtf-dtnrg-arch-06.txt,
September 2006.
[3] Scott, K. and S. Burleigh, "Bundle Protocol Specification", work
in progress, Internet Draft
draft-irtf-dtnrg-bundle-security-02.txt, August 2006.
[4] Symington, S., Farrell, S., and H. Weiss, "Bundle Security
Protocol Specification", work in progress, Internet
Draft draft-irtf-dtnrg-bundle-security-02.txt, October 2006.
[5] DTNRG, "Delay Tolerant Networking Reference Implementation",
<http://www.dtnrg.org/Code>.
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
Joerg Ott
Helsinki University of Technology
Networking Laboratory
PO Box 3000
TKK 02015
Finland
Email: jo@netlab.tkk.fi
Demmer & Ott Expires April 19, 2007 [Page 19]
Internet-Draft DTN TCP Convergence Layer October 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Demmer & Ott Expires April 19, 2007 [Page 20]