One document matched: draft-ietf-dccp-spec-02.txt
Differences from draft-ietf-dccp-spec-01.txt
Internet Engineering Task Force
INTERNET-DRAFT Eddie Kohler
draft-ietf-dccp-spec-02.txt Mark Handley
Sally Floyd
ICIR
Jitendra Padhye
Microsoft Research
9 May 2003
Expires: November 2003
Datagram Congestion Control Protocol (DCCP)
Status of this Document
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC 2026]. 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
Abstract
This document specifies the Datagram Congestion Control
Protocol (DCCP), which implements a congestion-controlled,
unreliable flow of datagrams suitable for use by applications
such as streaming media.
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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes since draft-ietf-dccp-spec-01.txt:
* Revise definition of when packets are reported as received,
due to ECN Nonce verification problems with the previous
definition and options.
* Replace Receive Buffer Drops with Data Dropped.
* Remove Data Discarded in favor of Data Dropped with Drop
State 0.
* Remove Buffer Closed in favor of Data Dropped with Drop
State 4.
* Add Initial Sequence Number setting guidelines.
* Add sections on retransmission of Requests, and a table to
the state diagram.
* Made the 4-bit Reserved field in the DCCP generic header
available for use by CCIDs.
* Refine description of CCID 1.
* Add Middlebox Considerations.
* Change Identification option to allow middleboxes to change
port numbers, DCCP options, and/or packet data without
disrupting the connection.
* Specify that Ignored should be sent only on packets with
Acknowledgement Numbers.
* Add Aggression Penalty Reset Reason.
* Add Payload Checksum option.
* Add Elapsed Time option (formerly specific to CCID 3).
* Timestamp Echo option can omit Elapsed Time, or provide a
two-byte Elapsed Time value. Elapsed Time is measured in
tenths of milliseconds, not microseconds.
* Clean up DCCP-Move and feature-negotiation options
discussions.
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* Confirm(Connection Nonce) sends no data.
* Ack Vector implementation supports ECN Nonce Echo.
* Add CSlen and Partial Checksumming Design Motivation.
* Clarify that Ack Vectors may be sent even if Use Ack Vector
is false.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . 6
2. Design Rationale. . . . . . . . . . . . . . . . . . . . 7
3. Concepts and Terminology. . . . . . . . . . . . . . . . 8
3.1. Anatomy of a DCCP Connection . . . . . . . . . . . . 8
3.2. Congestion Control . . . . . . . . . . . . . . . . . 9
3.3. Connection Initiation and Termination. . . . . . . . 9
3.4. Features . . . . . . . . . . . . . . . . . . . . . . 10
4. DCCP Packets. . . . . . . . . . . . . . . . . . . . . . 10
4.1. Examples of DCCP Congestion Control. . . . . . . . . 12
4.1.1. DCCP with TCP-like Congestion Control . . . . . . 12
4.1.2. DCCP with TFRC Congestion Control . . . . . . . . 14
5. Packet Formats. . . . . . . . . . . . . . . . . . . . . 15
5.1. Generic Packet Header. . . . . . . . . . . . . . . . 15
5.2. Sequence Number Validity . . . . . . . . . . . . . . 18
5.3. DCCP State Diagram . . . . . . . . . . . . . . . . . 19
5.4. DCCP-Request Packet Format . . . . . . . . . . . . . 20
5.5. DCCP-Response Packet Format. . . . . . . . . . . . . 22
5.6. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packet
Formats . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.7. DCCP-CloseReq and DCCP-Close Packet Format . . . . . 25
5.8. DCCP-Reset Packet Format . . . . . . . . . . . . . . 26
5.9. DCCP-Move Packet Format. . . . . . . . . . . . . . . 27
6. Options and Features. . . . . . . . . . . . . . . . . . 29
6.1. Padding Option . . . . . . . . . . . . . . . . . . . 30
6.2. Ignored Option . . . . . . . . . . . . . . . . . . . 30
6.3. Feature Negotiation. . . . . . . . . . . . . . . . . 31
6.3.1. Feature Numbers . . . . . . . . . . . . . . . . . 32
6.3.2. Change Option . . . . . . . . . . . . . . . . . . 32
6.3.3. Prefer Option . . . . . . . . . . . . . . . . . . 33
6.3.4. Confirm Option. . . . . . . . . . . . . . . . . . 33
6.3.5. Example Negotiations. . . . . . . . . . . . . . . 33
6.3.6. Unknown Features. . . . . . . . . . . . . . . . . 34
6.3.7. State Diagram . . . . . . . . . . . . . . . . . . 34
6.4. Identification Options . . . . . . . . . . . . . . . 38
6.4.1. Identification Regime Feature . . . . . . . . . . 38
6.4.2. Connection Nonce Feature. . . . . . . . . . . . . 39
6.4.3. Identification Option . . . . . . . . . . . . . . 39
6.4.4. Challenge Option. . . . . . . . . . . . . . . . . 40
6.5. Init Cookie Option . . . . . . . . . . . . . . . . . 41
6.6. Timestamp Option . . . . . . . . . . . . . . . . . . 42
6.7. Elapsed Time Option. . . . . . . . . . . . . . . . . 42
6.8. Timestamp Echo Option. . . . . . . . . . . . . . . . 43
6.9. Loss Window Feature. . . . . . . . . . . . . . . . . 43
7. Congestion Control IDs. . . . . . . . . . . . . . . . . 44
7.1. Unspecified Sender-Based Congestion Control. . . . . 45
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7.2. TCP-like Congestion Control. . . . . . . . . . . . . 46
7.3. TFRC Congestion Control. . . . . . . . . . . . . . . 46
7.4. CCID-Specific Options and Features . . . . . . . . . 46
8. Acknowledgements. . . . . . . . . . . . . . . . . . . . 47
8.1. Acks of Acks and Unidirectional Connections. . . . . 48
8.2. Ack Piggybacking . . . . . . . . . . . . . . . . . . 49
8.3. Ack Ratio Feature. . . . . . . . . . . . . . . . . . 49
8.4. Use Ack Vector Feature . . . . . . . . . . . . . . . 50
8.5. Ack Vector Options . . . . . . . . . . . . . . . . . 50
8.5.1. Ack Vector Consistency. . . . . . . . . . . . . . 52
8.5.2. Ack Vector Coverage . . . . . . . . . . . . . . . 54
8.6. Slow Receiver Option . . . . . . . . . . . . . . . . 54
8.7. Data Dropped Option. . . . . . . . . . . . . . . . . 55
8.8. Payload Checksum Option. . . . . . . . . . . . . . . 57
8.9. Ack Vector Implementation Notes. . . . . . . . . . . 58
8.9.1. New Packets . . . . . . . . . . . . . . . . . . . 60
8.9.2. Sending Acknowledgements. . . . . . . . . . . . . 61
8.9.3. Clearing State. . . . . . . . . . . . . . . . . . 62
8.9.4. Processing Acknowledgements . . . . . . . . . . . 63
9. Explicit Congestion Notification. . . . . . . . . . . . 64
9.1. ECN Capable Feature. . . . . . . . . . . . . . . . . 64
9.2. ECN Nonces . . . . . . . . . . . . . . . . . . . . . 65
9.3. Other Aggression Penalties . . . . . . . . . . . . . 66
10. Multihoming and Mobility . . . . . . . . . . . . . . . 66
10.1. Mobility Capable Feature. . . . . . . . . . . . . . 67
10.2. Security. . . . . . . . . . . . . . . . . . . . . . 67
10.3. Congestion Control State. . . . . . . . . . . . . . 67
10.4. Loss During Transition. . . . . . . . . . . . . . . 68
11. Path MTU Discovery . . . . . . . . . . . . . . . . . . 68
12. Middlebox Considerations . . . . . . . . . . . . . . . 70
13. Abstract API . . . . . . . . . . . . . . . . . . . . . 71
14. Multiplexing Issues. . . . . . . . . . . . . . . . . . 71
15. DCCP and RTP . . . . . . . . . . . . . . . . . . . . . 72
16. Security Considerations. . . . . . . . . . . . . . . . 73
17. IANA Considerations. . . . . . . . . . . . . . . . . . 74
18. Design Motivation. . . . . . . . . . . . . . . . . . . 74
18.1. CSlen and Partial Checksumming. . . . . . . . . . . 74
19. Thanks . . . . . . . . . . . . . . . . . . . . . . . . 76
20. Normative References . . . . . . . . . . . . . . . . . 76
21. Informative References . . . . . . . . . . . . . . . . 76
22. Authors' Addresses . . . . . . . . . . . . . . . . . . 77
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1. Introduction
This document specifies the Datagram Congestion Control Protocol
(DCCP). DCCP provides the following features:
o An unreliable flow of datagrams, with acknowledgements.
o A reliable handshake for connection setup and teardown.
o Reliable negotiation of options, including negotiation of a
suitable congestion control mechanism.
o Mechanisms allowing a server to avoid holding any state for
unacknowledged connection attempts or already-finished
connections.
o Optional mechanisms that tell the sender, with high reliability,
which packets reached the receiver, and whether those packets were
ECN marked, corrupted, or dropped in the receive buffer.
o Congestion control incorporating Explicit Congestion Notification
(ECN) and the ECN Nonce, as per [RFC 3168] and [ECN NONCE].
o Path MTU discovery, as per [RFC 1191].
DCCP is intended for applications that require the flow-based
semantics of TCP, but which do not want TCP's in-order delivery and
reliability semantics, or which would like different congestion
control dynamics than TCP. Similarly, DCCP is intended for
applications that do not require features of SCTP [RFC 2960] such as
sequenced delivery within multiple streams.
Applications that could make use of DCCP include those with timing
constraints on the delivery of data such that reliable in-order
delivery, when combined with congestion control, is likely to result
in some information arriving at the receiver after it is no longer
of use. Such applications might include streaming media and
Internet telephony.
To date most such applications have used either TCP, with the
problems described above, or used UDP and implemented their own
congestion control mechanisms (or no congestion control at all). The
purpose of DCCP is to provide a standard way to implement congestion
control and congestion control negotiation for such applications.
One of the motivations for DCCP is to enable the use of ECN, along
with conformant end-to-end congestion control, for applications that
would otherwise be using UDP. In addition, DCCP implements reliable
connection setup, teardown, and feature negotiation.
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A DCCP connection contains acknowledgement traffic as well as data
traffic. Acknowledgements inform a sender whether its packets
arrived, and whether they were ECN marked. Acks are transmitted as
reliably as the congestion control mechanism in use requires,
possibly completely reliably.
Previous drafts of this specification called the protocol DCP, or
Datagram Control Protocol. The name was changed to make the acronym
sound less like "TCP".
2. Design Rationale
DCCP is intended to be used by applications that currently use UDP
without end-to-end congestion control. The desire is for many
applications to have little reason not to use DCCP instead of UDP,
once DCCP is deployed. Thus, DCCP was designed to have as little
overhead as possible, in terms both of the size of the packet header
and in terms of the state and CPU overhead required at the end
hosts.
This desire for minimal overhead results in the design decision to
include only the minimal necessary functionality in DCCP, leaving
other functionality, such as FEC or semi-reliability, to be layered
on top of DCCP as desired. The desire for minimal overhead is also
one of the reasons to propose DCCP instead of just proposing an
unreliable version of SCTP for applications currently using UDP.
A second motivation behind the design of DCCP is to allow
applications to choose an alternative to the current TCP-style
congestion control that halves the congestion window in response to
a congestion indication. DCCP lets applications choose between
several forms of congestion control. One choice, TCP-like
congestion control, halves the congestion window in response to a
packet drop or mark, as in TCP. A second alternative, TFRC (TCP-
Friendly Rate Control, a form of equation-based congestion control),
minimizes abrupt changes in the sending rate while maintaining
longer-term fairness with TCP.
In proposing a new transport protocol, it is necessary to justify
the design decision not to require the use of the Congestion
Manager, as well as the design decision to add a new transport
protocol to the current family of UDP, TCP, and SCTP. The
Congestion Manager [RFC3124] allows multiple concurrent streams
between the same sender and receiver to share congestion control.
However, the current Congestion Manager can only be used by
applications that have their own end-to-end feedback about packet
losses, and this is not the case for many of the applications
currently using UDP. In addition, the current Congestion Manager
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does not lend itself to the use of forms of TFRC where the state
about past packet drops or marks is maintained at the receiver
rather than at the sender. While DCCP should be able to make use of
CM where desired by the application, we do not see any benefit in
making the deployment of DCCP contingent on the deployment of CM
itself.
3. Concepts and Terminology
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 [RFC 2119].
3.1. Anatomy of a DCCP Connection
Each DCCP connection runs between two endpoints, which we often name
DCCP A and DCCP B. Data may pass over the connection in either or
both directions. The DCCP connection between DCCP A and DCCP B
consists of four sets of packets, as follows:
(1) Data packets from DCCP A to DCCP B.
(2) Acknowledgements from DCCP B to DCCP A.
(3) Data packets from DCCP B to DCCP A.
(4) Acknowledgements from DCCP A to DCCP B.
We use the following terms to refer to subsets and endpoints of a
DCCP connection.
Subflows
A subflow consists of either data or acknowledgement packets,
sent in one direction. Each of the four sets of packets above is
a subflow. (Subflows may overlap to some extent, since
acknowledgements may be piggybacked on data packets.)
Sequences
A sequence consists of all packets sent in one direction,
regardless of whether they are data or acknowledgements. The
sets 1+4 and 2+3, above, are sequences. Each packet on a
sequence has a different sequence number.
Half-connections
A half-connection consists of the data packets sent in one
direction, plus the corresponding acknowledgements. The sets 1+2
and 3+4, above, are half-connections. Half-connections are named
after the direction of data flow, so the A-to-B half-connection
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contains the data packets from A to B and the acknowledgements
from B to A.
HC-Sender and HC-Receiver
In the context of a single half-connection, the HC-Sender is the
endpoint sending data, while the HC-Receiver is the endpoint
sending acknowledgements. For example, in the A-to-B half-
connection, DCCP A is the HC-Sender and DCCP B is the HC-
Receiver.
3.2. Congestion Control
Each half-connection is managed by a congestion control mechanism.
The endpoints negotiate these mechanisms at connection setup; the
mechanisms for the two half-connections need not be the same.
Conformant congestion control mechanisms correspond to single-byte
congestion control identifiers, or CCIDs. The CCID for a half-
connection describes how the HC-Sender limits data packet rates; how
it maintains necessary parameters, such as congestion windows; how
the HC-Receiver sends congestion feedback via acknowledgements; and
how it manages the acknowledgement rate. Section 7 introduces the
currently allocated CCIDs, which are defined in separate profile
documents.
3.3. Connection Initiation and Termination
Every DCCP connection is actively initiated by one DCCP, which
connects to a DCCP socket in the passive listening state. We refer
to the active endpoint as "the client" and the passive endpoint as
"the server". Most of the DCCP specification is indifferent to
whether a DCCP is client or server. However, only the server may
generate a DCCP-CloseReq packet. (A DCCP-CloseReq packet forces the
receiving DCCP to close the connection and maintain connection state
for a reasonable time, allowing old packets to clear the network.)
This means that the client cannot force the server to maintain
connection state after the connection is closed.
DCCP does not support TCP-style simultaneous open. In particular, a
host MUST NOT respond to a DCCP-Request packet with a DCCP-Response
packet unless the destination port specified in the DCCP-Request
corresponds to a local socket opened for listening.
DCCP does not support half-open connections either. That is, DCCP
shuts down both half-connections as a unit. However, DCCP SHOULD
allow applications to declare that they are no longer interested in
receiving data. This would allow DCCP implementations to streamline
state for certain half-connections. See Section 8.7, on the Data
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Dropped option---and particularly its Drop State 4---for more
information.
3.4. Features
DCCP uses a generic mechanism to negotiate connection properties,
such as the CCIDs active on the two half-connections. These
properties are called features. (We reserve the term "option" for a
collection of bytes in some DCCP header.) A feature name, such as
"CCID", generally corresponds to two features, one per half-
connection. For instance, there are two CCIDs per connection. The
endpoint in charge of a particular feature is called its feature
location.
The Change, Prefer, and Confirm options negotiate feature values.
Change is sent to a feature location, asking it to change its value
for the feature. The feature location may respond with Prefer, which
asks the other endpoint to Change again with different values, or it
may change the feature value and acknowledge the request with
Confirm. Retransmissions make feature negotiation reliable. Section
6.3 describes these options further.
4. DCCP Packets
DCCP has nine different packet types:
o DCCP-Request
o DCCP-Response
o DCCP-Data
o DCCP-Ack
o DCCP-DataAck
o DCCP-CloseReq
o DCCP-Close
o DCCP-Reset
o DCCP-Move
Only the first eight types commonly occur. The DCCP-Move packet is
used to support multihoming and mobility.
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The progress of a typical DCCP connection is as follows. (This
description is informative, not normative.)
(1) The client sends the server a DCCP-Request packet specifying the
client and server ports, the service being requested, and any
features being negotiated, including the CCID that the client
would like the server to use. The client may optionally
piggyback some data on the DCCP-Request packet---an application-
level request, say---which the server may ignore.
(2) The server sends the client a DCCP-Response packet indicating
that it is willing to communicate with the client. The response
indicates any features and options that the server agrees to,
begins or continues other feature negotiations if desired, and
optionally includes an Init Cookie that wraps up all this
information and which must be returned by the client for the
connection to complete.
(3) The client sends the server a DCCP-Ack packet that acknowledges
the DCCP-Response packet. This acknowledges the server's initial
sequence number and returns the Init Cookie if there was one in
the DCCP-Response. It may also continue feature negotiation.
(4) Next comes zero or more DCCP-Ack exchanges as required to
finalize feature negotiation. The client may piggyback an
application-level request on its final ack, producing a DCCP-
DataAck packet.
(5) The server and client then exchange DCCP-Data packets, DCCP-Ack
packets acknowledging that data, and, optionally, DCCP-DataAck
packets containing piggybacked data and acknowledgements. If the
client has no data to send, then the server will send DCCP-Data
and DCCP-DataAck packets, while the client will send DCCP-Acks
exclusively.
(6) The server sends a DCCP-CloseReq packet requesting a close.
(7) The client sends a DCCP-Close packet acknowledging the close.
(8) The server sends a DCCP-Reset packet whose Reason field is set
to "Closed", and clears its connection state.
(9) The client receives the DCCP-Reset packet and holds state for a
reasonable interval of time to allow any remaining packets to
clear the network.
An alternative connection closedown sequence is initiated by the
client:
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(6) The client sends a DCCP-Close packet closing the connection.
(7) The server sends a DCCP-Reset packet with Reason field set to
"Closed" and clears its connection state.
(8) The client receives the DCCP-Reset packet and holds state for a
reasonable interval of time to allow any remaining packets to
clear the network.
This arrangement of setup and teardown handshakes permits the server
to decline to hold any state until the handshake with the client has
completed, and ensures that the client must hold the TimeWait state
at connection closedown.
4.1. Examples of DCCP Congestion Control
Before giving the detailed specifications of DCCP, we present two
more detailed examples showing DCCP congestion control in operation.
Again, these examples are informative, not normative.
4.1.1. DCCP with TCP-like Congestion Control
The first example is of a connection where both half-connections use
TCP-like Congestion Control, specified by CCID 2 [CCID 2 PROFILE].
In this example, the client sends an application-level request to
the server, and the server responds with a stream of data packets.
This example is of a connection using ECN.
(1) The client sends the DCCP-Request, which includes a Change
option asking the server to use CCID 2 for the server's data
packets, and a Prefer option informing the server that the
client would like to use CCID 2 for the its data packets.
(2) The server sends a DCCP-Response, including a Confirm option
indicating that the server agrees to use CCID 2 for its data
packets, and a Change option indicating that the server agrees
to the client's suggestion of CCID 2 for the client's data
packets.
(3) The client responds with a DCCP-DataAck acknowledging the
server's initial sequence number, and including a Confirm option
finalizing the negotiation of the client-to-server CCID, and an
application-level request for data. We will not discuss the
client-to-server half-connection further in this example.
(4) The server sends DCCP-Data packets, where the number of packets
sent is governed by a congestion window, as in TCP. The details
of the congestion window are defined in the profile for CCID 2,
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which is a separate document [CCID 2 PROFILE]. The server also
sends Ack Ratio feature options specifying the number of server
data packets to be covered by an Ack packet from the client.
Some of these data packets are DCCP-DataAcks acknowledging
packets from the client.
Each DCCP-Data and DCCP-DataAck packet is sent as ECN-Capable,
with either the ECT(0) or the ECT(1) codepoint set, as described
in [ECN NONCE].
(5) The client sends a DCCP-Ack packet acknowledging the data
packets for every Ack Ratio data packets transmitted by the
server. Each DCCP-Ack packet uses a sequence number and
contains an Ack Vector, as defined in Section 8 on
Acknowledgements. These packets also include Confirm options
answering any Ack Ratio requests from the server.
The client's DCCP-Acks are also sent as ECN-Capable, with either
ECT(0) or ECT(1). The client's Ack Vector echoes the accumulated
ECN Nonce for the server's packets.
(6) The server continues sending DCCP-Data packets as controlled by
the congestion window. Upon receiving DCCP-Ack packets, the
server examines the Ack Vector to learn about marked or dropped
data packets, and adjusts its congestion window accordingly, as
described in [CCID 2 PROFILE]. Because this is unreliable
transfer, the server does not retransmit dropped packets.
(7) Because DCCP-Ack packets use sequence numbers, the server has
direct information about the fraction of loss or marked DCCP-Ack
packets. The server responds to lost or marked DCCP-Ack packets
by modifying the Ack Ratio sent to the client, as described in
[CCID 2 PROFILE]. Under certain conditions, the server must
acknowledge some of the client's acknowledgements; see Section
8.1 for more information.
(8) The server estimates round-trip times and calculates a TimeOut
(TO) value much as the RTO (Retransmit Timeout) is calculated in
TCP. Again, the specification for this is in [CCID 2 PROFILE].
The TO is used to determine when a new DCCP-Data packet can be
transmitted when the server has been limited by the congestion
window and no feedback has been received from the client.
(9) The DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets to close
the connection are as in the example above.
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4.1.2. DCCP with TFRC Congestion Control
This example is of a connection where both half-connections use TFRC
Congestion Control, specified by CCID 3 [CCID 3 PROFILE].
(1) The DCCP-Request and DCCP-Response packets specifying the use of
CCID 3 and the initial DCCP-DataAck packet are similar to those
in the CCID 2 example above.
(2) The server sends DCCP-Data packets, where the number of packets
sent is governed by an allowed transmit rate, as in TFRC. The
details of the allowed transmit rate are defined in the profile
for CCID 3, which is a separate document [CCID 3 PROFILE]. Each
DCCP-Data packet has a sequence number and a window counter
value.
Some of these data packets are DCCP-DataAck packets
acknowledging packets from the client, but for simplicity we
will not discuss the half-connection of data from the client to
the server in this example.
The use of ECN follows TCP-like Congestion Control, above, and
is described further in [CCID 3 PROFILE].
(3) The receiver sends DCCP-Ack packets at least once per round-trip
time acknowledging the data packets, unless the server is
sending at a rate of less than one packet per RTT, as specified
by [CCID 3 PROFILE]. These acknowledgements may be piggybacked
on data packets, producing DCCP-DataAck packets. Each DCCP-Ack
packet uses a sequence number and identifies the most recent
packet received from the server. Each DCCP-Ack packet includes
feedback about the loss event rate calculated by the client, as
specified by [CCID 3 PROFILE].
(4) The server continues sending DCCP-Data packets as controlled by
the allowed transmit rate. Upon receiving DCCP-Ack packets, the
server updates its allowed transmit rate as specified by [CCID 3
PROFILE].
(5) The server estimates round-trip times and calculates a TimeOut
(TO) value much as the RTO (Retransmit Timeout) is calculated in
TCP. Again, the specification for this is in [CCID 3 PROFILE].
(6) The DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets to close
the connection are as in the examples above.
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5. Packet Formats
5.1. Generic Packet Header
All DCCP packets begin with a generic DCCP packet header:
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | CCval | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | # NDP | Cslen | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source and Destination Ports: 16 bits each
These fields identify the connection, similar to the
corresponding fields in TCP and UDP. The Source Port represents
the relevant port on the endpoint that sent this packet, the
Destination Port the relevant port on the other endpoint.
Type: 4 bits
The type field specifies the type of the DCCP message. The
following values are defined:
0 DCCP-Request packet.
1 DCCP-Response packet.
2 DCCP-Data packet.
3 DCCP-Ack packet.
4 DCCP-DataAck packet.
5 DCCP-CloseReq packet.
6 DCCP-Close packet.
7 DCCP-Reset packet.
8 DCCP-Move packet.
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CCval: 4 bits
This field is reserved for use by the sending CCID. In
particular, the A-to-B CCID's sender, which is active at DCCP A,
MAY send information to the receiver at DCCP B by encoding that
information in CCval. DCCP proper MUST ignore the field. If the
relevant CCID does not specify its value, it SHOULD be set to
zero.
Sequence Number: 24 bits
The sequence number field is initialized by a DCCP-Request or
DCCP-Response packet, and increases by one (modulo 16777216)
with every packet sent. The receiver uses this information to
determine whether packet losses have occurred. Even packets
containing no data update the sequence number. Sequence numbers
also provide some protection against old and malicious packets;
see Section 5.2 on sequence number validity.
Very-high-rate DCCPs may need protection against wrapped
sequence numbers. For example, a 10 Gb/s flow of 1500-byte DCCP
packets will send 2^24 packets in about 20 seconds. This is a
long time, in terms of likely round-trip times that could
possibly achieve such a sustained rate, but it is not without
risk. Despite this, we leave the design of mechanisms to protect
against wrapped sequence numbers for future work. In particular,
if it is decided that very large packet sizes are better than
very large congestion windows for very-high-bandwidth flows,
then 24 bits may be enough.
The two subflows' initial sequence numbers are set by the first
DCCP-Request and DCCP-Response packets sent, and SHOULD be
chosen as for TCP. In particular, initial sequence number choice
MUST include a random or pseudorandom component to make it
harder for attackers to complete sequence number attacks [RFC
1948]. The initial sequence number chosen for a given connection
identifier (source address and port plus destination address and
port) SHOULD increase over time, as TCP suggests [RFC 793], to
prevent inappropriate delivery of old packets.
Data Offset: 8 bits
The offset from the start of the DCCP header to the beginning of
the packet's payload, measured in 32-bit words.
Number of Non-Data Packets (# NDP): 4 bits
DCCP sets this field to the number of non-data packets it has
sent so far on its sequence, modulo 16. A non-data packet is
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simply any packet not containing user data; DCCP-Ack, DCCP-
Close, DCCP-CloseReq, and DCCP-Reset are always non-data
packets, while DCCP-Request, DCCP-Response, and DCCP-Move might
or might not be. When sending a non-data packet, DCCP increments
the # NDP counter before storing its value in the packet header.
This field can help the receiving DCCP decide whether a lost
packet contained any user data. (An application may want to know
when it has lost data. DCCP could report every packet loss as a
potential data loss, but that would cause false loss reports
when non-data packets were lost.) For example, say that packet
10 had # NDP set to 5; packet 11 was lost; and packet 12 had #
NDP set to 5. Then the receiving DCCP could deduce that packet
11 contained data, since # NDP did not change. Likewise, if #
NDP had gone up to 6 (and packet 12 contained user data), then
packet 11 must not have contained any data.
Checksum Length (Cslen): 4 bits
The checksum length field specifies what parts of the packet are
covered by the checksum field. The checksum always covers at
least the DCCP header, DCCP options, and a pseudoheader taken
from the network-layer header (described under Checksum below).
If the checksum length field is zero, that is all the checksum
covers. If the field is 15, the checksum covers the packet's
payload as well, possibly with 8 bits of zero padding on the
right to pad the payload to an even number of bytes. Values
between 1 and 14, inclusive, indicate that the checksum
additionally covers that number of initial 32-bit words of the
packet's payload, padded on the right with zeros as necessary.
Values other than 15 specify that corruption is acceptable in
some or all of the DCCP packet's payload. In fact, DCCP cannot
even detect corruption there, unless the Payload Checksum option
is used (Section 8.8). The meaning of values other than 0 and 15
should be considered experimental.
Section 18.1 further discusses the motivation of, and issues
related to, partial checksums. The checksum length field was
inspired by UDP-Lite [UDP-LITE].
Checksum: 16 bits
DCCP uses the TCP/IP checksum algorithm. The checksum field
equals the 16 bit one's complement of the one's complement sum
of all 16 bit words in the DCCP header, DCCP options, a
pseudoheader taken from the network-layer header, and, depending
on the value of the checksum length field, some or all of the
payload. When calculating the checksum, the checksum field
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itself is treated as 0. If a packet contains an odd number of
header and text bytes to be checksummed, 8 zero bits are added
on the right to form a 16 bit word for checksum purposes. The
pad byte is not transmitted as part of the packet.
The pseudoheader is calculated as for TCP. For IPv4, it is 96
bits long, and consists of the IPv4 source and destination
addresses, the IP protocol number for DCCP (padded on the left
with 8 zero bits), and the DCCP length as a 16-bit quantity (the
length of the DCCP header with options, plus the length of any
data); see Section 3.1 of [RFC 793]. For IPv6, it is 320 bits
long, and consists of the IPv6 source and destination addresses,
the DCCP length as a 32-bit quantity, and the IP protocol number
for DCCP (padded on the left with 24 zero bits); see Section 8.1
of [RFC 2460].
Packets with invalid checksums MUST be ignored. In particular,
their options MUST NOT be processed.
5.2. Sequence Number Validity
DCCP endpoints SHOULD ignore packets with invalid sequence numbers,
which may arise if the network delivers a very old packet or an
attacker attempts to hijack a connection. TCP solves this problem
with its window. In DCCP, however, sequence numbers change with each
packet sent, even pure acknowledgements. Thus, a loss event that
dropped many consecutive packets could cause two DCCPs to get out of
sync relative to any window.
DCCP uses Loss Window and Identification mechanisms to determine
whether a given packet's sequence number is valid. Each HC-Sender
gives the corresponding HC-Receiver a loss window width W; see
Section 6.9. This reflects how many packets the sender expects to be
in flight. Only the sender can anticipate this number. One good
guideline is to set it to about 3 or 4 times the maximum number of
packets the sender expects to send in any round-trip time. Too-small
values increase the risk of the endpoints getting out sync after
bursts of loss; too-large values increase the risk of connection
hijacking. W defaults to 1000. The Identification mechanism is used
to get back into sync when more than W consecutive packets are lost.
The HC-Receiver sets up a loss window of W consecutive sequence
numbers containing GSN, the Greatest Sequence Number it has received
on any valid packet from the sender. ("Consecutive" and "greatest"
are measured in circular sequence space. The receiver may center the
loss window on GSN, or arrange it asymmetrically.) Sequence numbers
outside this loss window are invalid. Packets with invalid sequence
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numbers are themselves invalid, unless both of the following
conditions are true:
(1) No valid packet has been received recently (for instance, within
at least one round-trip time).
(2) The packet includes a correct Identification or Challenge option
(see Section 6.4.3).
The receiving DCCP SHOULD ignore invalid packets. In particular, it
SHOULD NOT pass any enclosed data to the application, update its
congestion control or feature state, or close the connection.
However, the receiving DCCP MAY send a DCCP-Ack packet to the
sender, as allowed by the congestion control mechanism in use. This
packet SHOULD contain the last received valid sequence number and a
Challenge option (Section 6.4.4). The other DCCP will send an
Identification option to resync.
A DCCP endpoint MAY implement rate limits to reduce the likelihood
of denial-of-service attack. In particular, it MAY ignore all
packets with bad sequence numbers---even those containing
Identification or Challenge options---for some amount of time, on
the order of one round-trip time, after receiving a packet with an
invalid Identification or Challenge option; and it MAY rate-limit
the Challenge options it sends.
5.3. DCCP State Diagram
In this section we present a DCCP state diagram showing how a DCCP
connection should progress, and the proper responses for packets or
timeout events in various connection states. The state diagram is
illustrative; the text should be considered definitive.
+----------------------------------+
| Figure omitted from text version |
+----------------------------------+
All receive events on the diagram represent receipt of valid
packets. For example, receiving a Reset with a bad Acknowledgement
Number SHOULD NOT cause DCCP to transition to the Time-Wait state.
DCCP implementations MAY send Acks as described above, or "Invalid
Packet" Resets, in response to invalid packets; any such responses
SHOULD be rate-limited.
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Otherwise-valid packets without explicit transitions in the state
diagram SHOULD be treated according to the table below. Particular
actions are "OK", meaning the packet MUST be processed according to
this document; "Rst", meaning the receiver SHOULD either ignore the
packet or respond with a (rate-limited) Reset; and "-", meaning the
packet SHOULD be ignored. Entries may take the form "Old/New",
where "Old" applies to old packets and "New" to new packets (whose
sequence numbers are greater than the largest sequence number seen
so far). The table respecifies some transitions listed in the state
diagram---for instance, those for receiving packets in the TIME-WAIT
state. In these cases, prefer the action listed in the diagram. For
example, in the TIME-WAIT case, prefer sending rate-limited Resets
when valid packets are received; the table would allow ignoring
them. However, either action would be acceptable.
Data/Ack/
DataAck/
State Request Response Move CloseReq Close Reset
------------- -------- -------- -------- -------- -------- --------
CLOSED Rst Rst Rst Rst Rst OK
LISTEN OK Rst Rst(1) Rst Rst OK
REQUEST Rst OK Rst Rst Rst OK
RESPOND -/OK Rst Rst/OK Rst OK OK
OPEN (server) -/Rst Rst OK Rst OK OK
OPEN (client) Rst -/Rst OK OK OK OK
SERVER-CLOSE -/Rst Rst OK Rst OK OK
CLIENT-CLOSE Rst -/Rst OK OK OK OK
TIME-WAIT Rst Rst Rst Rst Rst OK
Notes: (1) Data/Ack/DataAck with valid Init Cookie OK.
The Open state does not signify that a DCCP connection is ready for
data transfer. In particular, incomplete feature negotiations might
prevent data transfer. Feature negotiation takes place in parallel
with the state transitions on this diagram.
Only the server may take the transition from the OPEN state to the
SERVER-CLOSE state. (The server is the DCCP endpoint that began in
the LISTEN state.) Similarly, only the client must transition to
CLIENT-CLOSE after receiving a CloseReq packet.
5.4. DCCP-Request Packet Format
A DCCP connection is initiated by sending a DCCP-Request packet. The
format of a DCCP request packet is:
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0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Name |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Name: 32 bits
The Service Name field describes the service to which the sender
is trying to connect. Service Names are 32-bit numbers allocated
by IANA; they are meant to correspond to application services
and protocols, such as FTP and HTTP, and are not intended to be
DCCP-specific. With Service Names, stateful middleboxes, such as
firewalls, can identify the application running on a nonstandard
port (assuming the DCCP header has not been encrypted). A
Service Name of zero is a wildcard, matching any service. The
host operating system MAY force every DCCP socket, both actively
and passively opened, to specify a nonzero Service Name.
Connection requests MUST fail if the Destination Port on the
receiver has a different Service Name from that given in the
packet, and both Service Names are nonzero. In this case, the
receiver will respond with a DCCP-Reset packet (with Reason set
to "Bad Service Name"). A server or stateful middlebox MAY also
send a "Bad Service Name" DCCP-Reset in response to packets with
Service Name value 0.
Options
DCCP-Request packets will usually include a "Change(Connection
Nonce)" option, to inform the server of the client's connection
nonce; see Section 6.4.
The client MAY send new DCCP-Request packets if no response is
received after some timeout. Each retransmission MUST increment the
Sequence Number, and possibly # NDP, by one. The retransmission
strategy SHOULD be similar to that for retransmitting TCP SYNs.
A client MAY decide to give up after some number of DCCP-Requests.
If so, it MAY send a DCCP-Reset packet to the server, to clean up
state in case one or more of the Requests actually arrived. The
DCCP-Reset SHOULD have Reason set to "Closed".
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5.5. DCCP-Response Packet Format
In the second phase of the three-way handshake, the server sends a
DCCP-Response message to the client. In this phase, a server will
often specify the options it would like to use, either from among
those the client requested, or in addition to those. Among these
options is the congestion control mechanism the server expects to
use.
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=1 (DCCP-Response) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledgement Number: 24 bits
The Acknowledgement Number field, which appears in several
packet types, acknowledges the greatest valid sequence number
received so far on this connection. ("Greatest" is, of course,
measured in circular sequence space.) In the case of a DCCP-
Response packet, the acknowledgement number field will equal the
sequence number from the DCCP-Request. Acknowledgement numbers
make no attempt to provide precise information about which
packets have arrived; options such as the Ack Vector do this.
Some care is required in defining when a packet is "received"
for purposes of acknowledgement. All valid packets received by a
DCCP stack MUST be acknowledged as "received", even if their
payloads were dropped (due to receive buffer overflow or payload
corruption, for example). The receiving DCCP MUST have processed
the options on every packet it reports as "received". The Data
Dropped option (Section 8.7) helps the sending application
determine when packet payloads were dropped by the receiving
DCCP. This issue is discussed in somewhat more detail in
Section 8.5.
Reserved: 8 bits
The version of DCCP specified here SHOULD set this field to all
zeroes on generated packets, and ignore its value on received
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packets.
Options
The Data Dropped and Init Cookie options are particularly useful
for DCCP-Response packets (Sections 8.7 and 6.5). In addition,
DCCP-Response, or early DCCP-Data or DCCP-Ack packets, will
often include "Confirm(Connection Nonce)" and "Change(Connection
Nonce)" options, to further negotiate connection nonces (Section
6.4), as well as options to negotiate CCIDs and other relevant
features.
The receiver MAY respond to a DCCP-Request packet with a DCCP-Reset
packet to refuse the connection. Relevant Reset Reasons for refusing
a connection include "Connection Refused", when the DCCP-Request's
Destination Port did not correspond to a DCCP port open for
listening; "Bad Service Name", when the DCCP-Request's Service Name
did not correspond to the service name registered with the
Destination Port; and "Too Busy", when the server is currently too
busy to respond to requests. The server SHOULD limit the rate at
which it generates these resets.
The receiver SHOULD NOT retransmit DCCP-Response packets; the sender
will retransmit the DCCP-Request if necessary. The responder will
detect that the retransmitted DCCP-Request applies to an existing
connection because of its Source and Destination Ports. Every valid
DCCP-Request received MUST elicit a new DCCP-Response, unless the
responder can guarantee that the requestor has received at least one
Response already. (For instance, if the responder has received a
valid DCCP-Data or DCCP-Ack packet from the requestor, then it knows
the newly received Request is old, and SHOULD be ignored.) Each new
DCCP-Response MUST increment the Sequence Number, and possibly #
NDP, by one.
5.6. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packet Formats
The payload of a DCCP connection is sent in DCCP-Data and DCCP-
DataAck packets, while DCCP-Ack packets are used for
acknowledgements when there is no payload to be sent. DCCP-Data
packets look like this:
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0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-Ack packets dispense with the data, but contain an
acknowledgement number:
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-DataAck packets contain both data and an acknowledgement
number: acknowledgement information is piggybacked on a data packet.
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-Ack and DCCP-DataAck packets often include additional
acknowledgement options, such as Ack Vector, as required by the
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congestion control mechanism in use.
DCCP A sends DCCP-Data and DCCP-DataAck packets to DCCP B due to
application events on host A. These packets are congestion-
controlled by the CCID for the A-to-B half-connection. In contrast,
DCCP-Ack packets sent by DCCP A are controlled by the CCID for the
B-to-A half-connection. Generally, DCCP A will piggyback
acknowledgement information on data packets when acceptable,
creating DCCP-DataAck packets. DCCP-Ack packets are used when there
is no data to send from DCCP A to DCCP B, or when the link from A to
B is so congested that sending data would be inappropriate.
Section 8, below, describes acknowledgements in DCCP.
A DCCP-Data or DCCP-DataAck packet may contain no data bytes if the
application sends a zero-length datagram. Such zero-length datagrams
MUST be reported to the receiving application.
5.7. DCCP-CloseReq and DCCP-Close Packet Format
The DCCP-CloseReq and DCCP-Close packets have the same format.
However, only the server can send a DCCP-CloseReq packet. Either
client or server may send a DCCP-Close packet. The receiver of a
valid DCCP-Close packet SHOULD respond with a DCCP-Reset packet,
with Reason set to "Closed"; the endpoint that originally sent the
DCCP-Close will hold TimeWait state. The receiver of a valid DCCP-
CloseReq packet SHOULD respond with a DCCP-Close packet; that
receiving endpoint will expect to hold TimeWait state after later
receiving a DCCP-Reset. See the state diagram in 5.3 for more
information.
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=5 or 6 (DCCP-Close or CloseReq) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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5.8. DCCP-Reset Packet Format
DCCP-Reset packets unconditionally shut down a connection. Every
connection shutdown sequence ends with a DCCP-Reset, but resets may
be sent for other reasons, including bad port numbers, bad option
behavior, incorrect ECN Nonce Echoes, and so forth. The reason for a
reset is represented by an eight-bit number, the Reason field, and
24 bits of additional data.
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason | Data 1 | Data 2 | Data 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reason: 8 bits
The Reason field represents the reason that the sender reset the
DCCP connection.
Data 1, Data 2, and Data 3: 8 bits each
The Data fields provide additional information about why the
sender reset the DCCP connection. The meanings of these fields
depend on the value of Reason.
The following Reasons are currently defined. The "Data" columns
describe what the Data fields should contain for a given Reason. In
those columns, N/A means the Data field SHOULD be set to 0 by the
sender of the DCCP-Reset, and ignored by its receiver.
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Section
Reason Name Data 1 Data 2 Data 3 Reference
------ ---- ------ ------ ------ ---------
0 Unspecified N/A N/A N/A
1 Closed N/A N/A N/A 4
2 Invalid Packet packet N/A N/A 5.3
type
3 Option Error option option data
number (if any)
4 Feature Error feature feature data
number (if any)
5 Connection Refused N/A N/A N/A 5.5
6 Bad Service Name N/A N/A N/A 5.4
7 Too Busy N/A N/A N/A 5.5
8 Bad Init Cookie N/A N/A N/A 6.5
9 Invalid Move N/A N/A N/A 5.9
10 Unanswered Challenge N/A N/A N/A 6.4.4
11 Fruitless Negotiation feature feature data 6.3.7
number (optional)
12 Aggression Penalty N/A N/A N/A 9.2
5.9. DCCP-Move Packet Format
The DCCP-Move packet type is part of DCCP's support for multihoming
and mobility, which is described further in Section 10. DCCP A sends
a DCCP-Move packet to DCCP B after changing its address and/or port
number. The DCCP-Move packet requests that DCCP B start sending
packets to the new address and port number. The old address and port
are stored explicitly in the DCCP-Move header; the new address and
port come from the packet's network header and generic DCCP header.
The old address's type is indicated explicitly by an Old Address
Family field. The Sequence Number and Acknowledgement Number fields
and a mandatory Identification option provide some protection
against hijacked connections. See Section 10 for more on security
and DCCP's mobility support.
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0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (12 bytes) /
/ with Type=8 (DCCP-Move) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old Address Family | Old Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Old Address /
/ / [padding] /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options, including Identification / [padding] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Old Address Family: 16 bits
The Old Address Family field indicates the address family
formerly used for this connection, and takes values from the
Address Family Numbers registry administered by IANA. Particular
values include 1 for IPv4 and 2 for IPv6. An endpoint MUST
discard DCCP-Move packets with unrecognized Old Address Family
values.
Old Port: 16 bits
The former port number used by DCCP A's endpoint.
Old Address: at least 32 bits
The former address used by DCCP A's endpoint, padded on the
right to a multiple of 32 bits. The form and size of the address
are determined by the Old Address Family field. For instance, if
Old Address Family is 1, then Old Address contains an IPv4
address and takes 32 bits; if it is 2, then Old Address contains
an IPv6 address and takes 128 bits.
Options
Every DCCP-Move packet MUST include a valid Identification
option (see Section 6.4).
DCCP B SHOULD respond to the DCCP-Move with a DCCP-Reset (with
Reason set to "Invalid Move") if any of the following conditions
holds:
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(1) Neither the Old Address/Old Port combination nor the network
address/Source Port combination refers to a currently active
DCCP connection.
(2) The Identification option is not present or invalid.
(3) DCCP B does not support mobility, or its Mobility Capable
feature is off.
After receiving such an invalid DCCP-Move, DCCP B MAY ignore
subsequent DCCP-Move packets, valid or not, for a short period of
time, such as one round-trip time. This protects DCCP B against
denial-of-service attacks from floods of invalid DCCP-Moves.
DCCP B SHOULD respond to a valid DCCP-Move packet with a DCCP-Ack or
DCCP-DataAck packet acknowledging the move. If DCCP B accepts the
move, it MUST send this acknowledgement to the network
address/Source Port combination; if it rejects the move, which it
MAY do for any reason, it MUST send the acknowledgement to the Old
Address/Old Port combination.
If the acknowledgement is lost, DCCP A might resend the DCCP-Move
packet (using a new sequence number). DCCP B will detect this case
because the network address/Source Port combination corresponds to a
valid connection, for which the Sequence Number and Acknowledgement
Number fields are valid; the Identification option is valid for that
connection; and the Old Address/Old Port combination no longer
refers to a valid DCCP connection. It SHOULD respond by sending
another acknowledgement, as allowed by the congestion control
mechanism in use.
We note that DCCP mobility, as provided by DCCP-Move, may not be
useful in the context of IPv6, with its mandatory support for Mobile
IP.
6. Options and Features
All DCCP packets may contain options, which occupy space at the end
of the DCCP header and are a multiple of 8 bits in length. All
options are always included in the checksum. An option may begin on
any byte boundary.
The first byte of an option is the option type. Options with types 0
through 31 are single-byte options. Other options are followed by a
byte indicating the option's length. This length value includes the
two bytes of option-type and option-length as well as any option-
data bytes, and MUST therefore be greater than or equal to two.
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The following options are currently defined:
Option Section
Type Length Meaning Reference
---- ------ ------- ---------
0 1 Padding 6.1
2 1 Slow Receiver 8.6
32 3-4 Ignored 6.2
33 variable Change 6.3
34 variable Prefer 6.3
35 variable Confirm 6.3
36 variable Init Cookie 6.5
37 variable Ack Vector [Nonce 0] 8.5
38 variable Ack Vector [Nonce 1] 8.5
39 variable Data Dropped 8.7
40 6 Timestamp 6.6
41 6-10 Timestamp Echo 6.8
42 variable Identification 6.4.3
44 variable Challenge 6.4.4
45 4 Payload Checksum 8.8
46 4-6 Elapsed Time 6.7
128-255 variable CCID-specific options 7.4
6.1. Padding Option
The padding option, with type 0, is a single byte option used to pad
between or after options. It either ensures the payload begins on a
32-bit boundary (as required), or ensures alignment of following
options (not mandatory).
+--------+
|00000000|
+--------+
Type=0
6.2. Ignored Option
The Ignored option, with type 32, signals that a DCCP did not
understand some option. This can happen, for example, when a
conventional DCCP converses with an extended DCCP. Each Ignored
option has one or two bytes of data. The first byte contains the
offending option type; the second, if present, contains the first
byte of the offending option's data. If the offending option had no
data, the Ignored option MAY still supply two bytes of data, with
the second byte set to 0.
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Ignored options SHOULD be sent only on packets that contain
Acknowledgement Numbers (that is, DCCP-Reponse, DCCP-Ack, DCCP-
DataAck, DCCP-Close, DCCP-CloseReq, DCCP-Reset, and DCCP-Move), and
SHOULD concern options sent on the packet acknowledged by the
Acknowledgement Number.
+--------+--------+--------+
|00100000|00000011|Opt Type|
+--------+--------+--------+
Type=32 Length=3
+--------+--------+--------+--------+
|00100000|00000100|Opt Type|Opt Data|
+--------+--------+--------+--------+
Type=32 Length=4
6.3. Feature Negotiation
DCCP contains a mechanism for reliably negotiating features, notably
the congestion control mechanism in use on each half-connection. The
motivation is to implement reliable feature negotiation once, so
that different options need not reinvent that wheel.
Three options, Change, Prefer, and Confirm, implement feature
negotiation. Change is sent to a feature's location, asking it to
change the feature's value. The feature location may respond with
Prefer, which asks the other endpoint to Change again with different
values, or it may change the feature value and acknowledge the
request with Confirm.
Feature values MUST NOT change apart from feature negotiation, and
enforced retransmissions make feature negotiation reliable. This
ensures that both endpoints eventually agree on every feature's
value.
Some features are non-negotiable, meaning that the feature location
MUST set its value to whatever the other endpoint requests. For non-
negotiable features, the feature location MUST respond to Change
options with Confirm; Prefer is not useful. These features use the
feature framework simply to achieve reliability.
Negotiations for multiple features may take place simultaneously.
For instance, a packet may contain multiple Change options that
refer to different features.
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Feature negotiation generally takes place using packet types that
carry no user data, such as DCCP-Ack, particularly when the relevant
feature may affect how data will be treated.
6.3.1. Feature Numbers
The first data byte of every Change, Prefer, or Confirm option is a
feature number, defining the type of feature being negotiated. The
remainder of the data gives one or more values for the feature, and
is interpreted according to the feature. The current set of feature
numbers is as follows:
Section
Number Meaning Neg.? Reference
------ ------- ----- ---------
1 Congestion Control (CC) Y 7
2 ECN Capable Y 9.1
3 Ack Ratio N 8.3
4 Use Ack Vector Y 8.4
5 Mobility Capable Y 10.1
6 Loss Window N 6.9
7 Connection Nonce N 6.4.2
8 Identification Regime Y 6.4.1
128-255 CCID-Specific Features ? 7.4
The "Neg[otiable]?" column is "Y" for normal features and "N" for
non-negotiable features.
6.3.2. Change Option
DCCP A sends a Change option to DCCP B to ask it to change the value
of some feature located at DCCP B. DCCP B SHOULD respond to a Change
option for a known feature with either Prefer or Confirm. In
special circumstances, such as a Change option whose value is
inappropriate for the listed feature number or a negotiation that
seems to be going on forever, DCCP B MAY respond instead by ignoring
the Change (with or without sending an Ignored option), or by
resetting the connection with Reason set to "Fruitless Negotiation"
or "Feature Error". DCCP A SHOULD retransmit the Change option
until it receives some relevant response. DCCP A will always
generate a Change option in response to a Prefer option; it may also
generate a Change option due to some application event.
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+--------+--------+--------+--------+--------+--------
|00100001| Length |Feature#| Value or Values ...
+--------+--------+--------+--------+--------+--------
Type=33
6.3.3. Prefer Option
DCCP A sends a Prefer option to DCCP B to ask it to choose another
value for some feature located at DCCP B. DCCP B SHOULD respond to a
valid Prefer option with a Change; other possible responses include
ignoring the option, sending an Ignored option, or resetting the
connection, as described above. DCCP A SHOULD retransmit the Prefer
option until it receives some relevant response. DCCP A may generate
a Prefer option in response to some Change option, or in response to
some application event. Prefer options are not useful for non-
negotiable features.
+--------+--------+--------+--------+--------+--------
|00100010| Length |Feature#| Value or Values ...
+--------+--------+--------+--------+--------+--------
Type=34
6.3.4. Confirm Option
DCCP A sends a Confirm option to DCCP B to inform it that a Change
option for some feature located at DCCP A has been accepted.
Generally the Confirm option will include the feature's accepted
value. For some special features, such as Connection Nonce, a
Confirm option contains no data; these features are identified
explicitly. DCCP A MUST generate Confirm options only in response
to valid Change options. DCCP A SHOULD NOT retransmit Confirm
options: DCCP B will retransmit the relevant Changes as necessary.
The receipt of a valid Confirm option ends the negotiation over a
feature's value.
+--------+--------+--------+--------+--------+--------
|00100011| Length |Feature#| Value ...
+--------+--------+--------+--------+--------+--------
Type=35
6.3.5. Example Negotiations
This section demonstrates several negotiations of the congestion
control feature for the A-to-B half-connection. (This feature is
located at DCCP A.) In this sequence of packets, DCCP A is happy
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with DCCP B's suggestion of CC mechanism 2:
B > A Change(CC, 2)
A > B Confirm(CC, 2)
Here, A and B jointly settle on CC mechanism 5:
B > A Change(CC, 3, 4)
A > B Prefer(CC, 1, 2, 5)
B > A Change(CC, 5)
A > B Confirm(CC, 5)
In this sequence, A refuses to use CC mechanism 5. If this sequence
continued, one or the other endpoint would eventually abort the
connection via a DCCP-Reset packet with Reason set to "Fruitless
Negotiation":
B > A Change(CC, 3, 4, 5)
A > B Prefer(CC, 1, 2)
B > A Change(CC, 5)
A > B Prefer(CC, 1, 2)
Here, A elicits agreement from B that it is satisfied with
congestion control mechanism 2:
A > B Prefer(CC, 1, 2)
B > A Change(CC, 2)
A > B Confirm(CC, 2)
6.3.6. Unknown Features
If a DCCP receives a Change or Prefer option referring to a feature
number it does not understand, it MUST respond with an Ignored
option. This informs the remote DCCP that the local DCCP does not
implement the feature. No other action need be taken. (Ignored may
also indicate that the DCCP endpoint could not respond to a CCID-
specific feature request because the CCID was in flux; see Section
7.4.)
6.3.7. State Diagram
These state diagrams present the legal transitions in a DCCP feature
negotiation. They define DCCP's states and transitions with respect
to the negotiation of a single feature it understands. There are two
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diagrams, corresponding to the two endpoints: the feature location
DCCP A, and what we call the "feature requester", DCCP B.
Transitions between states are triggered by receiving a packet
("RECV") or by an application event ("APP"). Received packets are
further distinguished by any options relevant to the feature being
negotiated. "RECV -" means the packet contained no relevant option.
"RECV Chg" denotes a Change option, "RECV Pr" a Prefer option, and
"RECV Cfm" a Confirm option. The data contained in an option is
given in parentheses when necessary. The "SEND" action indicates
which option the DCCP will send next. Finally, the "SET-VALUE"
action causes the DCCP to change its value for the relevant feature.
"SEND" does not force DCCP to immediately generate a packet; rather,
it says which feature option must be sent on the next packet
generated. A DCCP MAY choose to generate a packet in response to
some "SEND" action. However, it MUST NOT generate a packet if doing
so would violate the congestion control mechanism in use.
The requester, DCCP B, has four states: Known, Unknown, Failed, and
Changing. Similarly, the feature location, DCCP A, has four states:
Known, Unknown, Failed, and Confirming. In both cases, Known denotes
a state where the DCCP knows the feature's current value, and
believes that the other DCCP agrees. Changing and Confirming denote
states where the DCCPs are in the process of negotiating a new value
for the feature. The Unknown state can occur only at connection
setup time. It denotes a state where the DCCP does not know any
value for the feature, and has not yet entered a negotiation to
determine its value. Finally, the Failed state represents a state
where the other DCCP does not implement the feature under
negotiation.
A DCCP may start in either the Unknown or Known state, depending on
the feature in question. In particular, some features have a well-
known value for new connections, in which case the DCCPs begin the
connection in the Known states.
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REQUESTER STATE DIAGRAM (DCCP B)
+-----------+
| Unknown |
+-----------+
+----------+ | +-----------+
| |RECV - |RECV -/Pr | APP | |RECV Pr/Cfm
V |SEND - |SEND Chg V |SEND Chg
+-----------+ | | +------------+ |
| |----+ +------------>| |-----+
| Known |------------------------------>| Changing |
| | RECV Pr | APP | |-----+
+-----------+ SEND Chg +------------+ |RECV -
^ | | ^ |SEND -/Chg
| | | | |
+------------------------------------------+ | +---------+
RECV Cfm(O) | +----------+
SEND - +--------->| Failed |
SET-VALUE O RECV Ign +----------+
SEND -
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FEATURE LOCATION STATE DIAGRAM (DCCP A)
(O represents any feature value acceptable to DCCP A; X is not acceptable.)
RECV Chg(O)
SEND Cfm(O) RECV - | APP
SET-VALUE O +-----------+ SEND Pr(O)
+--------------------| Unknown |------------+
| +-----------+ |
| +-------+ | | +-----------+
| | |RECV - |RECV Chg(X) | | |RECV Chg(X)
V V |SEND - |SEND Pr(O) V V |SEND Pr(O)
+-----------+ | | +------------+ | (need not be
| |----+ +------------>| |-----+ the same O)
| Known |------------------------------>| Confirming |
| |----+ RECV Chg | APP | |-----+
+-----------+ | SEND Pr(O) +------------+ |RECV -
^ ^ | | | ^ |SEND -/Pr(O)
| | |RECV Chg(O) | | | |
| | |SEND Cfm(O) | | +---------+
| | |SET-VALUE O | |
| +-------+ | | +----------+
+---------------------------------------------+ +-------->| Failed |
RECV Chg(O) RECV Ign +----------+
SEND Cfm(O) SEND -
SET-VALUE O
This specification allows several choices of action in certain
states. The implementation will generally use feature-specific
information to decide how to respond. For example, DCCP A in the
Known state may respond to a Change option with either Confirm or
Prefer. If DCCP A is willing to set the feature to the value
specified by Change, it will generally send Confirm; but if it would
like to negotiate further, it will send Prefer.
DCCP B retransmits Change options, and DCCP A retransmits Prefer
options, until receiving a relevant response. However, they need not
retransmit the option on every packet, as shown by the "RECV - /
SEND -" transitions in the Changing and Confirming states.
These state diagrams guarantee safety, but not liveness. Namely, no
unexpected or erroneous options will be sent, but option negotiation
might not terminate. For example, the following infinite negotiation
is legal according to this specification.
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A > B Prefer(1)
B > A Change(2)
A > B Prefer(1)
B > A Change(2)...
Implementations MAY choose to enforce a maximum length on any
negotiation---for example, by resetting the connection when any
negotiation lasts more than some maximum time. The DCCP-Reset Reason
"Fruitless Negotiation" SHOULD be used to signal that a connection
was aborted because of a negotiation that took too long.
In the Changing and Confirming states, the value of the
corresponding feature is in flux. DCCP MAY change its behavior in
these states---for example, by refusing to send data until
reentering a Known state.
6.4. Identification Options
The Identification options provide a way for DCCP endpoints to
confirm each others' identities, even after changes of address
(Section 10) or long bursts of loss that get the endpoints out of
sync (Section 5.2). Again, DCCP as specified here does not provide
cryptographic security guarantees, and attackers that can see every
packet are still capable of manipulating DCCP connections
inappropriately, but the Identification options make it more
difficult for some kinds of attacks to succeed.
The Identification option is used to prove an endpoint's identity,
while a Challenge option elicits an Identification from the other
endpoint. An Identification Regime determines how the
Identifications are calculated. In the default MD5 Regime, the
calculation involves an MD5 hash over packet data and two Connection
Nonces exchanged at the beginning of the connection.
6.4.1. Identification Regime Feature
Identification Regime has feature number 8. The ID Regime feature
located at DCCP B specifies the algorithm that DCCP A will use for
its Identification options. Each endpoint must keep track of both
its ID regime and, via the ID Regime feature, the regime used by the
other endpoint.
The value of ID Regime is a two-byte number, so a valid Confirm(ID
Regime) option takes exactly four bytes. Change or Prefer options
MAY list multiple ID Regimes in descending order of preference. ID
Regime defaults to 0, the MD5 Regime. Applications preferring
different security guarantees, particularly around mobility issues,
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may prefer to implement another identification algorithm and assign
it a different ID Regime value.
The ID Regime feature is negotiable, so an endpoint can request that
the other endpoint use a particular ID Regime, or one of a set of
Regimes, by sending a Prefer option. If the endpoints cannot agree
on mutually acceptable ID Regimes, the connection SHOULD be reset
due to "Fruitless Negotiation".
6.4.2. Connection Nonce Feature
Connection Nonce has feature number 7. The Connection Nonce feature
located at DCCP B is the value of DCCP A's connection nonce. Each
endpoint SHOULD keep track of both its nonce and, via the Connection
Nonce feature, the other endpoint's nonce. Connection Nonces are
used by Identification Regime 0.
The Connection Nonce feature takes arbitrary values of at least 4
bytes long. A Change(Connection Nonce) option therefore takes at
least 6 bytes. Confirm(Connection Nonce) options MUST NOT contain
the relevant value, so a Confirm(Connection Nonce) option takes
exactly 2 bytes.
Connection Nonce defaults to a random 8-byte string. To prevent
spoofing, this string MUST NOT have any predictable value. For
example, it MUST NOT be set deterministically to zero, and it MUST
change on every connection.
This feature is non-negotiable.
6.4.3. Identification Option
The Identification option serves as confirmation that a packet was
sent by an endpoint involved in the initiation of the DCCP
connection. It is permitted in any DCCP packet, but it might not be
useful until the endpoints have exchanged security information such
as connection nonces. The option takes the following form:
+--------+--------+--------+--------+--------+--------
|00101010| Length | Identification Data ...
+--------+--------+--------+--------+--------+--------
Type=42
The particular data included in an Identification option sent by
DCCP A depends on the ID Regime in force for the A-to-B sequence,
which is the value of the ID Regime feature located at DCCP B. The
remainder of this section describes ID Regime 0, the default MD5
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Regime.
The Identification data provided for the MD5 Regime consists of a
16-byte MD5 digest of: the second 32-bit word in the generic DCCP
header, including the Sequence Number; the value of the sender's
Connection Nonce; and the value of the other endpoint's Connection
Nonce, in that order. The total length of the option is therefore 18
bytes. Inclusion of the two Connection Nonces ensures that attackers
cannot fake an Identification Option, unless they snooped on the
beginning of the connection when nonces are exchanged. (No
mechanism protects against snoopers who know Connection Nonces,
since DCCP as specified here does not provide strong cryptographic
security guarantees; see Section 16.) Inclusion of the sequence
number protects against replay attacks within the connection.
To check an Identification option's value, the receiver simply
calculates the MD5 digest itself and compares that against the
option data. The MD5 calculation can be expensive, so an attacker
could conceivably disable a DCCP endpoint by sending it a flood of
invalid packets with bad Identification options. Rate limits
described in Sections 5.2 and 10 mitigate this issue. The receiver
MAY ignore an Identification option if it occurs on a packet that
would otherwise be considered valid.
Example C code for constructing the option's value follows:
unsigned char *packet_data;
int packet_length;
int id_option_offset; /* offset of option in packet_data */
const unsigned char *my_nonce, *other_nonce;
int my_nonce_length, other_nonce_length;
MD5_CTX md5_context;
MD5_Init(&md5_context);
MD5_Update(&md5_context, packet_data + 4, 4);
MD5_Update(&md5_context, my_nonce, my_nonce_length);
MD5_Update(&md5_context, other_nonce, other_nonce_length);
packet_data[id_option_offset] = 42; /* option value */
packet_data[id_option_offset+1] = 18; /* option length */
MD5_Final(packet_data + id_option_offset + 2, &md5_context);
6.4.4. Challenge Option
This option informs the receiving DCCP that one of its packets was
ignored, and that succeeding packets will be ignored until the
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endpoint sends a correct Identification option. The receiving DCCP
SHOULD include an Identification option on the next packet it sends.
The option takes the following form:
+--------+--------+--------+--------+--------+--------
|00101100| Length | Identification Data ...
+--------+--------+--------+--------+--------+--------
Type=44
The Identification Data on a packet sent by DCCP B is the same as
that for an Identification option sent by DCCP B. The receiver
SHOULD ignore a Challenge option, and the packet the Challenge
option contains, if the Identification Data is incorrect. The
purpose of this mechanism is to prevent denial-of-service attacks
where an attacker could cause the receiver to send many packets with
expensive-to-compute Identification options, since the receiver MAY
ignore Challenge options for some time after receiving an invalid
Challenge.
If, after several Challenge options, a DCCP is unable to elicit a
valid Identification from its partner, it MAY reset the connection
with Reason "Unanswered Challenge".
6.5. Init Cookie Option
This option is permitted in DCCP-Response, DCCP-Data, and DCCP-
DataAck messages. The option MAY be returned by the server in a
DCCP-Response. If so, then the client MUST echo the same Init
Cookie option in its ensuing DCCP-Data or DCCP-DataAck message. The
server SHOULD respond to an invalid Init Cookie option by resetting
the connection with Reason set to "Bad Init Cookie".
The purpose of this option is to allow a DCCP server to avoid having
to hold any state until the three-way connection setup handshake has
completed. The server wraps up the service name, server port, and
any options it cares about from both the DCCP-Request and DCCP-
Response in an opaque cookie. Typically the cookie will be
encrypted using a secret known only to the server and include a
cryptographic checksum or magic value so that correct decryption can
be verified. When the server receives the cookie back in the
response, it can decrypt the cookie and instantiate all the state it
avoided keeping.
The precise implementation of the Init Cookie does not need to be
specified here; since Init Cookies are opaque to the client, there
are no interoperability concerns.
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+--------+--------+--------+--------+--------+--------
|00100100| Length | Init Cookie Value ...
+--------+--------+--------+--------+--------+--------
Type=36
6.6. Timestamp Option
This option is permitted in any DCCP packet. The length of the
option is 6 bytes.
+--------+--------+--------+--------+--------+--------+
|00101000|00000110| Timestamp Value |
+--------+--------+--------+--------+--------+--------+
Type=40 Length=6
The four bytes of option data carry the timestamp of this packet in
some undetermined form. A DCCP receiving a Timestamp option SHOULD
respond with a Timestamp Echo option on the next packet it sends.
6.7. Elapsed Time Option
This option is permitted in any DCCP packet that contains an
Acknowledgement Number. It indicates how much time, in milliseconds,
has elapsed since the packet being acknowledged---the packet with
the given Acknowledgement Number---was received. The option may take
up 4 or 6 bytes, depending on how large Elapsed Time is.
+--------+--------+--------+--------+
|00101110|00000100| Elapsed Time |
+--------+--------+--------+--------+
Type=46 Len=4
+--------+--------+--------+--------+--------+--------+
|00101110|00000110| Elapsed Time |
+--------+--------+--------+--------+--------+--------+
Type=46 Len=6
The option data, Elapsed Time, represents the amount of time, in
tenths of milliseconds, elapsed since the packet being acknowledged
was received. If Elapsed Time is less than a minute, the first, more
parsimonious form of the option SHOULD be used. Elapsed Times of
more than 6.5535 seconds MUST be sent using the second form of the
option.
Elapsed Time is measured in tenths of milliseconds as a compromise
between two conflicting goals: first, to provide enough granularity
to reduce aliasing noise when measuring elapsed time over fast LANs;
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and second, to allow most reasonable elapsed times to fit into two
bytes of data.
6.8. Timestamp Echo Option
This option is permitted in any DCCP packet, as long as at least one
packet carrying the Timestamp option has been received. The length
of the option is between 6 and 10 bytes, depending on whether
Elapsed Time is included and how large it is.
+--------+--------+--------+--------+--------+--------+
|00101001|00000110| Timestamp Echo |
+--------+--------+--------+--------+--------+--------+
Type=41 Len=6
+--------+--------+------- ... -------+--------+--------+
|00101001|00001000| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+--------+--------+
Type=41 Len=8 (4 bytes)
+--------+--------+------- ... -------+------- ... -------+
|00101001|00001010| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+------- ... -------+
Type=41 Len=10 (4 bytes) (4 bytes)
The first four bytes of option data, Timestamp Echo, carry a
Timestamp Value taken from a preceding received Timestamp option.
Usually, this will be the last packet that was received---the packet
indicated by the Acknowledgement Number, if any---but it might be a
preceding packet.
The Elapsed Time field is similar to the value stored in the Elapsed
Time option. If present, it indicates the amount of time elapsed
since receiving the packet whose timestamp is being echoed. This
time MUST be in tenths of milliseconds. Elapsed Time is meant to
help the Timestamp sender separate the network round-trip time from
the Timestamp receiver's processing time. This may be particularly
important for CCIDs where acknowledgements are sent infrequently, so
that there might be considerable delay between receiving a Timestamp
option and sending the corresponding Timestamp Echo. A missing
Elapsed Time field is equivalent to an Elapsed Time of zero. The
smallest version of the option SHOULD be used that can hold the
relevant Elapsed Time value.
6.9. Loss Window Feature
Loss Window has feature number 6. The Loss Window feature located at
DCCP B is the width of the window DCCP B uses to determine whether
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packets from DCCP A are valid. Packets outside this window will be
dropped by DCCP B as old duplicates or spoofing attempts; see
Section 5.2 for more information. DCCP A sends a "Change(Loss
Window, W)" option to DCCP B to set DCCP B's Loss Window to W.
The Loss Window feature takes 3-byte integer values, like DCCP
sequence numbers. Change and Confirm options for Loss Window are
therefore 6 bytes long.
Loss Window defaults to 1000 for new connections. The Loss Window
value is the total width of the loss window. The receiver may
position the loss window asymmetrically around the greatest sequence
number seen---for example, by allocating 1/4 of the loss window
width for older sequence numbers and 3/4 of it for newer sequence
numbers.
This feature is non-negotiable.
7. Congestion Control IDs
Each congestion control mechanism supported by DCCP is assigned a
congestion control identifier, or CCID: a number from 0 to 255.
During connection setup, and optionally thereafter, the endpoints
negotiate their congestion control mechanisms by negotiating the
values for their Congestion Control features. Congestion Control has
feature number 1. The feature located at DCCP A is the CCID in use
for the A-to-B half-connection. DCCP B sends an "Change(CC, K)"
option to DCCP A to ask A to use CCID K for its data packets.
The data byte of Congestion Control feature negotiation options form
a list of acceptable CCIDs, sorted in descending order of priority.
For example, the option "Change(CC 1, 2, 3)" asks the sender to use
CCID 1, although CCIDs 2 and 3 are also acceptable. (This
corresponds to the bytes "33, 6, 1, 1, 2, 3": Change option (33),
option length (6), feature ID (1), CCIDs (1, 2, 3).) Similarly,
"Confirm(CC 1, 2, 3)" tells the receiver that the sender is using
CCID 1, but that CCIDs 2 or 3 might also be acceptable.
The CCIDs defined by this document are:
CCID Meaning
---- -------
0 Reserved
1 Unspecified Sender-Based Congestion Control
2 TCP-like Congestion Control
3 TFRC Congestion Control
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A new connection starts with CCID 2 for both DCCPs. If this is
unacceptable for either DCCP, that DCCP will start in the Unknown
state. A DCCP SHOULD NOT send data when its Congestion Control
feature is in the Unknown state.
All CCIDs standardized for use with DCCP will correspond to
congestion control mechanisms previously standardized by the IETF.
We expect that for quite some time, all such mechanisms will be TCP-
friendly, but TCP-friendliness is not an explicit DCCP requirement.
A DCCP implementation intended for general use---in a general-
purpose operating system kernel, for example---SHOULD implement at
least CCIDs 1 and 2. The intent is to make these CCIDs broadly
available for interoperability, although any given application might
disallow their use via the feature negotiation process.
7.1. Unspecified Sender-Based Congestion Control
CCID 1 denotes an unspecified sender-based congestion control
mechanism. Separate features negotiate the corresponding congestion
acknowledgement options---for example, Ack Vector. This provides a
limited, controlled form of interoperability for new IETF-approved
CCIDs.
Implementors MUST NOT use CCID 1 in production environments as a
proxy for congestion control mechanisms that have not entered the
IETF standards process. We intend that any production use of CCID 1
would have to be explicitly approved first by the IETF. Middleboxes
MAY choose to treat the use of CCID 1 as experimental or
unacceptable.
For example, say that CCID 98, a new sender-based congestion control
mechanism using Ack Vector for acknowledgements, has entered the
IETF standards process, and the IETF has approved the use of CCID 1
as a backup for CCID 98. Now, DCCP A, which understands and would
like to use CCID 98, is trying to communicate with DCCP B, which
doesn't yet know about CCID 98. DCCP A can simply negotiate use of
CCID 1 and, separately, negotiate Use Ack Vector. DCCP B will
provide the feedback DCCP A requires for CCID 98, namely Ack Vector,
without needing to understand the congestion control mechanism in
use.
CCID 1 has no sender implementation; it is exclusively meaningful at
the receiver to support forward compatibility. The sender always
uses a specific congestion control mechanism whose CCID is not 1.
However, the code implementing a CCID that requires only generic
feedback, such as Ack Vector, MAY add CCID 1 to the list of
acceptable CCIDs sent to the receiver (following the actual CCID),
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facilitating communication with receivers that do not understand the
actual CCID.
Any CCID feature negotiation in which the sender proposes the use of
CCID 1 without any other CCID is considered erroneous, and SHOULD
result in connection reset, with Reason set to "Fruitless
Negotiation".
DCCP implementations MAY provide APIs that allow applications to
suggest preferred CCIDs for sending and receiving data. Any such API
MUST NOT allow sending applications to suggest CCID 1; again, CCID 1
will be suggested when appropriate by the code implementing the
preferred CCID. In contrast, APIs SHOULD let applications allow or
prevent the use of CCID 1 for receiving.
7.2. TCP-like Congestion Control
CCID 2 denotes Additive Increase, Multiplicative Decrease (AIMD)
congestion control with behavior modelled directly on TCP, including
congestion window, slow start, timeouts, and so forth. CCID 2 is
further described in [CCID 2 PROFILE].
7.3. TFRC Congestion Control
CCID 3 denotes TCP-Friendly Rate Control, an equation-based rate-
controlled congestion control mechanism. CCID 3 is further described
in [CCID 3 PROFILE].
7.4. CCID-Specific Options and Features
Option and feature numbers 128 through 255 are available for CCID-
specific use. CCIDs may often need new option types---for
communicating acknowledgement or rate information, for example.
CCID-specific option types let them create options at will without
polluting the global option space. Option 128 might have different
meanings on a half-connection using CCID 4 and a half-connection
using CCID 8. CCID-specific options and features will never conflict
with global options introduced by later versions of this
specification.
Any packet may contain information meant for either half-connection,
so CCID-specific option and feature numbers explicitly signal the
half-connection to which they apply. Option numbers 128 through 191
are for options sent from the HC-Sender to the HC-Receiver; option
numbers 192 through 255 are for options sent from the HC-Receiver to
the HC-Sender. Similarly, feature numbers 128 through 191 are for
features located at the HC-Sender; feature numbers 192 through 255
are for features located at the HC-Receiver. (Change options for a
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feature are sent to the feature location; Prefer and Confirm options
are sent from the feature location. Thus, Change(128) options are
sent by the HC-Receiver by definition, while Change(192) options are
sent by the HC-Sender.)
For example, consider a DCCP connection where the A-to-B half-
connection uses CCID 4 and the B-to-A half-connection uses CCID 5.
Here is how a sampling of CCID-specific options and features are
assigned to half-connections:
Relevant Relevant
Packet Option Half-conn. CCID
------ ------ ---------- ----
A > B 128 A-to-B 4
A > B 192 B-to-A 5
A > B Change(128, ...) B-to-A 5
A > B Prefer(128, ...) A-to-B 4
A > B Confirm(128, ...) A-to-B 4
A > B Change(192, ...) A-to-B 4
A > B Prefer(192, ...) B-to-A 5
A > B Confirm(192, ...) B-to-A 5
CCID-specific options and features have no clear meaning when the
relevant CCID is in flux. A DCCP SHOULD respond to CCID-specific
options and features with Ignored options during those times.
8. Acknowledgements
Congestion control requires receivers to transmit information about
packet losses and ECN marks to senders. DCCP receivers MUST report
all congestion they see, as defined by the relevant CCID profile.
Each CCID says when acknowledgements should be sent, what options
they must use, how they should be congestion controlled, and so on.
Most acknowledgements use DCCP options. For example, on a half-
connection with CCID 2 (TCP-like), the receiver reports
acknowledgement information using the Ack Vector option. This
section describes common acknowledgement options and shows how acks
using those options will commonly work. Full descriptions of the
acknowledgement mechanisms used for each CCID are laid out in the
CCID profile specifications.
Acknowledgement options, such as Ack Vector, generally depend on the
DCCP Acknowledgement Number, and are thus only allowed on packet
types that carry that number (all packets except DCCP-Request and
DCCP-Data). However, detailed acknowledgement options are not
generally necessary on DCCP-Resets.
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8.1. Acks of Acks and Unidirectional Connections
DCCP was designed to work well for both bidirectional and
unidirectional flows of data, and for connections that transition
between these states. However, acknowledgements required for a
unidirectional connection are very different from those required for
a bidirectional connection. In particular, unidirectional
connections need to worry about acks of acks.
The ack-of-acks problem arises because some acknowledgement
mechanisms are reliable. For example, an HC-Receiver using CCID 2,
TCP-like Congestion Control, sends Ack Vectors containing completely
reliable acknowledgement information. The HC-Sender should
occasionally inform the HC-Receiver that it has received an ack. If
it did not, the HC-Receiver might resend complete Ack Vector
information, going back to the start of the connection, with every
DCCP-Ack packet! However, note that acks-of-acks need not be
reliable themselves: when an ack-of-acks is lost, the HC-Receiver
will simply maintain old acknowledgement-related state for a little
longer. Therefore, there is no need for acks-of-acks-of-acks.
When communication is bidirectional, any required acks-of-acks are
automatically contained in normal acknowledgements for data packets.
On a unidirectional connection, however, the receiver DCCP sends no
data, so the sender would not normally send acknowledgements.
Therefore, the CCID in force on that half-connection must explicitly
say whether, when, and how the HC-Sender should generate acks-of-
acks.
For example, consider a bidirectional connection where both half-
connections use the same CCID (either 2 or 3), and where DCCP B goes
"quiescent". This means that the connection becomes unidirectional:
DCCP B stops sending data, and sends only sends DCCP-Ack packets to
DCCP A. For CCID 2, TCP-like Congestion Control, DCCP B uses Ack
Vector to reliably communicate which packets it has received. As
described above, DCCP A must occasionally acknowledge a pure
acknowledgement from DCCP B, so that DCCP B can free old Ack Vector
state. For instance, DCCP A might send a DCCP-DataAck packet every
now and then, instead of DCCP-Data. In contrast, for CCID 3, TFRC
Congestion Control, DCCP B's acknowledgements generally need not be
reliable, since they contain cumulative loss rates; TFRC works even
if every DCCP-Ack is lost. Therefore, DCCP A need never acknowledge
an acknowledgement.
When communication is unidirectional, a single CCID---in the
example, the A-to-B CCID---controls both DCCPs' acknowledgements, in
terms of their content, their frequency, and so forth. For
bidirectional connections, the A-to-B CCID governs DCCP B's
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acknowledgements (including its acks of DCCP A's acks), while the B-
to-A CCID governs DCCP A's acknowledgements.
DCCP A switches its ack pattern from bidirectional to unidirectional
when it notices that DCCP B has gone quiescent. It switches from
unidirectional to bidirectional when it must acknowledge even a
single DCCP-Data or DCCP-DataAck packet from DCCP B. (This includes
the case where a single DCCP-Data or DCCP-DataAck packet was lost in
transit, which is detectable using the # NDP field in the DCCP
packet header.)
Each CCID defines how to detect quiescence on that CCID, and how
that CCID handles acks-of-acks on unidirectional connections. The B-
to-A CCID defines when DCCP B has gone quiescent. Usually, this
happens when a period has passed without B sending any data packets.
For CCID 2, this period is roughly two round-trip times. The A-to-B
CCID defines how DCCP A handles acks-of-acks once DCCP B has gone
quiescent.
8.2. Ack Piggybacking
Acknowledgements of A-to-B data MAY be piggybacked on data sent by
DCCP B, as long as that does not delay the acknowledgement longer
than the A-to-B CCID would find acceptable. However, data
acknowledgements often require more than 4 bytes to express. A large
set of acknowledgements prepended to a large data packet might
exceed the path's MTU. In this case, DCCP B SHOULD send separate
DCCP-Data and DCCP-Ack packets, or wait, but not too long, for a
smaller datagram.
Piggybacking is particularly common at DCCP A when the B-to-A half-
connection is quiescent---that is, when DCCP A is just acknowledging
DCCP B's acknowledgements, as described above. There are three
reasons to acknowledge DCCP B's acknowledgements: to allow DCCP B to
free up information about previously acknowledged data packets from
A; to shrink the size of future acknowledgements; and to manipulate
the rate future acknowledgements are sent. Since these are secondary
concerns, DCCP A can generally afford to wait indefinitely for a
data packet to piggyback its acknowledgement onto.
Any restrictions on ack piggybacking are described in the relevant
CCID's profile.
8.3. Ack Ratio Feature
Ack Ratio provides a common mechanism by which CCIDs that clock
acknowledgements off of data packets can perform rudimentary
congestion control on the acknowledgement stream. CCID 2, TCP-like
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Congestion Control, uses Ack Ratio to limit the rate of its
acknowledgement stream, for example. Some CCIDs ignore Ack Ratio,
performing congestion control on acknowledgements in some other way.
Ack Ratio has feature number 3. The Ack Ratio feature located at
DCCP B equals the ratio of data packets sent by DCCP A to
acknowledgement packets sent back by DCCP B. For example, if it is
set to four, then DCCP B will send at least one acknowledgement
packet for every four data packets DCCP A sends. DCCP A sends a
"Change(Ack Ratio)" option to DCCP B to change DCCP B's ack ratio.
An Ack Ratio option contains two bytes of data: a sixteen-bit
integer representing the ratio. A new connection starts with Ack
Ratio 2 for both DCCPs.
This feature is non-negotiable.
8.4. Use Ack Vector Feature
The Use Ack Vector feature lets DCCPs negotiate whether they should
use Ack Vector options to report congestion. Ack Vector provides
detailed loss information, and lets senders report back to their
applications whether particular packets were dropped. Use Ack Vector
is mandatory for some CCIDs, and optional for others.
Use Ack Vector has feature number 4. The Use Ack Vector feature
located at DCCP B specifies whether DCCP B MUST use Ack Vector
options on its acknowledgements to DCCP A, although DCCP B MAY send
Ack Vector options even when Use Ack Vector is false. DCCP A sends a
"Change(Use Ack Vector, 1)" option to DCCP B to ask B to send Ack
Vector options as part of its acknowledgement traffic.
Use Ack Vector feature values are a single byte long. The receiver
MUST send Ack Vector options if this byte is nonzero. A new
connection starts with Use Ack Vector 0 for both DCCPs.
8.5. Ack Vector Options
The Ack Vector gives a run-length encoded history of data packets
received at the client. Each byte of the vector gives the state of
that data packet in the loss history, and the number of preceding
packets with the same state. The option's data looks like this:
+--------+--------+--------+--------+--------+--------
|001001??| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL| ...
+--------+--------+--------+--------+--------+--------
Type=37/38 \___________ Vector ___________...
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The two Ack Vector options (option types 37 and 38) differ only in
the values they imply for ECN Nonce Echo. Section 9.2 describes this
further.
The vector itself consists of a series of bytes, each of whose
encoding is:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|St | Run Length|
+-+-+-+-+-+-+-+-+
St[ate]: 2 bits
Run Length: 6 bits
State occupies the most significant two bits of each byte, and can
have one of four values:
0 Packet received (and not ECN marked).
1 Packet received ECN marked.
2 Reserved.
3 Packet not yet received.
The first byte in the first Ack Vector option refers to the packet
indicated in the Acknowledgement Number; subsequent bytes refer to
older packets. (Ack Vector MUST NOT be sent on DCCP-Data and DCCP-
Request packets, which lack an Acknowledgement Number.) If an Ack
Vector contains the decimal values 0,192,3,64,5 and the
Acknowledgement Number is decimal 100, then:
Packet 100 was received (Acknowledgement Number 100, State 0,
Run Length 0).
Packet 99 was lost (State 3, Run Length 0).
Packets 98, 97, 96 and 95 were received (State 0, Run Length 3).
Packet 94 was ECN marked (State 1, Run Length 0).
Packets 93, 92, 91, 90, 89, and 88 were received (State 0, Run
Length 5).
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Run lengths of more than 64 must be encoded in multiple bytes. A
single Ack Vector option can acknowledge up to 16192 data packets.
Should more packets need to be acknowledged than can fit in 253
bytes of Ack Vector, then multiple Ack Vector options can be sent.
The second Ack Vector option will begin where the first Ack Vector
option left off, and so forth.
Ack Vector states are subject to two general constraints. (These
principles SHOULD also be followed for other acknowledgement
mechanisms; referring to Ack Vector states simplifies their
explanation.)
(1) Packets reported as State 0 or State 1 MUST have been processed
by the receiving DCCP stack. In particular, their options must
have been processed. Any data on the packet need not have been
delivered to the receiving application; in fact, the data may
have been dropped.
(2) Packets reported as State 3 MUST NOT have been received by DCCP.
Feature negotiations and options on such packets MUST NOT have
been processed, and the Acknowledgement Number MUST NOT
correspond to such a packet.
Packets dropped in the application's receive buffer SHOULD be
reported as Received or Received ECN Marked (States 0 and 1),
depending on their ECN state; such packets' ECN Nonces MUST be
included in the Nonce Echo. The Data Dropped option informs the
sender that some packets reported as received actually had their
payloads dropped.
One or more Ack Vector options that, together, report the status of
more packets than have actually been sent SHOULD be considered
invalid. The receiving DCCP SHOULD either ignore the options or
reset the connection with Reason set to "Option Error". Packets
whose status has not reported by any Ack Vector option SHOULD be
treated as "not yet received" (State 3) by the sender.
8.5.1. Ack Vector Consistency
A DCCP sender will commonly receive multiple acknowledgements for
some of its data packets. For instance, an HC-Sender might receive
two DCCP-Acks with Ack Vectors, both of which contained information
about sequence number 24. (Because of cumulative acking,
information about a sequence number is repeated in every ack until
the HC-Sender acknowledges an ack. Perhaps the HC-Receiver is
sending acks faster than the HC-Sender is acknowledging them.) In a
perfect world, the two Ack Vectors would always be consistent.
However, there are many reasons why they might not be:
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o The HC-Receiver received packet 24 between sending its acks, so
the first ack said 24 was not received (State 3) and the second
said it was received or ECN marked (State 0 or 1).
o The HC-Receiver received packet 24 between sending its acks, and
the network reordered the acks. In this case, the packet will
appear to transition from State 0 or 1 to State 3.
o The network duplicated packet 24, and one of the duplicates was
ECN marked. This might show up as a transition between States 0
and 1.
To cope with these situations, HC-Sender DCCP implementations SHOULD
combine multiple received Ack Vector states according to this table:
Received State
0 1 3
+---+---+---+
0 | 0 | 1 | 0 |
Old +---+---+---+
1 | 1 | 1 | 1 |
State +---+---+---+
3 | 0 | 1 | 3 |
+---+---+---+
To read the table, choose the row corresponding to the packet's old
state and the column corresponding to the packet's state in the
newly received Ack Vector, then read the packet's new state off the
table. The table is symmetric about the main diagonal, so it is
indifferent to ack reordering.
This table defines how the HC-Sender should react to received Ack
Vector states. This is equivalent to how the HC-Receiver should
collect information about received packets, with two symmetric
exceptions: when one State is 0 (received non-marked) and the other
is 1 (received ECN marked). According to the table, the HC-Sender
should react to this combination of Ack Vector information as if
only State 1 had been reported. But what state should the HC-
Receiver report in Ack Vector if two duplicates are received for a
packet, and only one is ECN marked? We explicitly allow the HC-
Receiver to report the combination as State 0 (received non-marked)
or State 1. After all, one duplicate was non-marked, and depending
on how much state the HC-Receiver keeps about packets it receives,
it might be impossible to change a packet from State 0 to State 1
and preserve correct ECN Nonce Echo information.
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A HC-Sender MAY choose to throw away old information gleaned from
the HC-Receiver's Ack Vectors, in which case it MUST ignore newly
received acknowledgements from the HC-Receiver for those old
packets. It is often kinder to save recent Ack Vector information
for a while, so that the HC-Sender can undo its reaction to presumed
congestion when a "lost" packet unexpectedly shows up (the
transition from State 3 to State 0).
8.5.2. Ack Vector Coverage
We can divide the packets that have been sent from an HC-Sender to
an HC-Receiver into four roughly contiguous groups. From oldest to
youngest, these are:
(1) Packets already acknowledged by the HC-Receiver, where the HC-
Receiver knows that the HC-Sender has definitely received the
acknowledgements.
(2) Packets already acknowledged by the HC-Receiver, where the HC-
Receiver cannot be sure that the HC-Sender has received the
acknowledgements.
(3) Packets not yet acknowledged by the HC-Receiver.
(4) Packets not yet received by the HC-Receiver.
The union of groups 2 and 3 is called the Unacknowledged Window.
Generally, every Ack Vector generated by the HC-Receiver will cover
the whole Unacknowledged Window: Ack Vector acknowledgements are
cumulative. (This simplifies Ack Vector maintenance at the HC-
Receiver; see Section 8.9, below.) As packets are received, this
window both grows on the right and shrinks on the left. It grows
because there are more packets, and shrinks because the data
packets' Acknowledgement Numbers will acknowledge previous
acknowledgements, moving packets from group 2 into group 1.
8.6. Slow Receiver Option
An HC-Receiver sends the Slow Receiver option to its sender to
indicate that it is having trouble keeping up with the sender's
data. The HC-Sender SHOULD NOT increase its sending rate for
approximately one round-trip time after seeing a packet with a Slow
Receiver option. However, the Slow Receiver option does not indicate
congestion, and the HC-Sender need not reduce its sending rate. (If
necessary, the receiver can force the sender to slow down by
dropping packets or reporting false ECN marks.) APIs SHOULD let
receiver applications set Slow Receiver, and sending applications
determine whether or not their receivers are Slow.
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The Slow Receiver option takes just one byte:
+--------+
|00000010|
+--------+
Type=2
Slow Receiver does not specify why the receiver is having trouble
keeping up with the sender. Possible reasons include lack of buffer
space, CPU overload, and application quotas. A sending application
might react to Slow Receiver by reducing its sending rate or by
switching to a lossier compression algorithm. However, a smart
sender might actually *increase* its sending rate in response to
Slow Receiver, by switching to a less-compressed sending format. (A
highly-compressed data format might overwhelm a slow CPU more
seriously than the higher memory requirements of a less-compressed
data format.) This tension between transfer size (less compression
means more congestion) and processing speed (less compression means
less processing) cannot be resolved in general.
Slow Receiver implements a portion of TCP's receive window
functionality. We believe receiver operating systems and
applications will find it much easier to send Slow Receiver when
appropriate than they currently find it to correctly set a TCP
receive window.
8.7. Data Dropped Option
The Data Dropped option indicates that some packets reported as
received actually had their data dropped before it reached the
application. The sender's congestion control mechanism MAY react to
data-dropped packets; such responses MAY be less severe than
responses triggered by a lost or marked packet. (For instance, a
windowed mechanism might subtract a constant value from its
congestion window, rather than cut it in half.) When ECN-marked
packets are included in Data Dropped, the sender's congestion
control mechanism MUST react to the ECN marks as usual.
The option's data looks like this:
+--------+--------+--------+--------+--------+--------
|00100111| Length | Block | Block | Block | ...
+--------+--------+--------+--------+--------+--------
Type=39 \___________ Vector ___________ ...
The vector itself consists of a series of bytes, called Blocks,
each of whose encoding corresponds to one of these choices:
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| Run Length | or |1|Dr St|Run Len|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Normal Block Drop Block
The first byte in the first Data Dropped option refers to the packet
indicated in the Acknowledgement Number; subsequent bytes refer to
older packets. (Data Dropped MUST NOT be sent on DCCP-Data or DCCP-
Request packets, which lack an Acknowledgement Number.) Normal
Blocks, which have high bit 0, indicate that any received packets in
the Run Length had their data delivered to the application. Drop
Blocks, which have high bit 1, indicate that received packets in the
Run Len[gth] were not delivered as usual. The 3-bit Dr[op] St[ate]
field says what happened; generally, no data from that packet
reached the application. Packets reported as "not yet received" MUST
be included in Normal Blocks; packets not covered by any Data
Dropped option are treated as if they were in a Normal Block.
Defined Drop States for Drop Blocks are:
0 Packet data dropped due to protocol constraints. For
example, the data was included on a DCCP-Request packet, and
the receiving application does not allow that piggybacking;
or the data was sent during an important feature
negotiation.
1 Packet data dropped in the receive buffer.
2 Packet data dropped due to corruption.
3 Packet data corrupted, but delivered to the application
anyway.
4 Packet data dropped because the application is no longer
listening.
5-7 Reserved.
For example, if a Data Dropped option contains the decimal values
0,144,3,146, the Acknowledgement Number is 100, and an Ack Vector
reported all packets as received, then:
Packet 100 was received (Acknowledgement Number 100, Normal
Block, Run Length 0).
Packet 99 was dropped in the receive buffer (Drop Block, Drop
State 1, Run Length 0).
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Packets 98, 97, 96, and 95 were received (Normal Block, Run
Length 3).
Packets 95, 94, and 93 were dropped in the receive buffer (Drop
Block, Drop State 1, Run Length 2).
Run lengths of more than 128 (for Normal Blocks) or 16 (for Drop
Blocks) must be encoded in multiple Blocks. A single Data Dropped
option can acknowledge up to 32384 Normal Block data packets,
although the receiver SHOULD NOT send a Data Dropped option when all
relevant packets fit into Normal Blocks. Should more packets need to
be acknowledged than can fit in 253 bytes of Data Dropped, then
multiple Data Dropped options can be sent. The second option will
begin where the first option left off, and so forth.
One or more Data Dropped options that, together, report the status
of more packets than have been sent, or that change the status of a
packet, or that disagree with Ack Vector or equivalent options (by
reporting a "not yet received" packet as "dropped in the receive
buffer", for example), SHOULD be considered invalid. The receiving
DCCP SHOULD respond to invalid Data Dropped options by ignoring them
or by resetting the connection with Reason set to "Option Error".
Drop State 4 ("application no longer listening") means the
application running at the endpoint that sent the option is no
longer listening for data. For example, a server might close its
receiving half-connection to new data after receiving a complete
request from the client. This would limit the amount of state the
server would expend on incoming data, and thus reduce the potential
damage from certain denial-of-service attacks. A Data Dropped option
containing Drop State 4 SHOULD be sent whenever received data is
ignored due to a non-listening application. Once a DCCP reports Drop
State 4 for a packet, it SHOULD report Drop State 4 for every
succeeding data packet on that half-connection; once a DCCP receives
a Drop State 4 report, it SHOULD expect that no more data will ever
be delivered to the other endpoint's application. A DCCP receiving
Drop State 4 MAY report this event to the application. (Previous
versions of this specification used a "Buffer Closed" option instead
of Drop State 4.)
8.8. Payload Checksum Option
The Payload Checksum option holds the 16 bit one's complement of the
one's complement sum of all 16 bit words in the DCCP payload (the
data contained in a DCCP-Request, DCCP-Response, DCCP-Data, DCCP-
DataAck, or DCCP-Move packet). When combined with a Checksum Length
of less than 15, this lets DCCP distinguish between corruption in a
packet's payload and corruption in its header. Corrupted-header
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packets MUST be treated as dropped by the network, while corrupted-
payload packets MAY be treated differently; for example, the
sender's response to corruption might be less stringent than its
response to congestion. A low Checksum Length lets DCCP process
packets with valid headers, even if the payload is corrupt, avoiding
the congestion response to corruption. The Payload Checksum option
then lets DCCP detect payload corruption, and therefore avoid
delivering bad data to the application.
The option's data looks like this:
+--------+--------+--------+--------+
|00101101|00000100| Checksum |
+--------+--------+--------+--------+
Type=45 Length=4
The receiving DCCP MUST check the Payload Checksum's value against
the actual payload checksum. If the values differ, the packet's data
SHOULD be dropped, and reported as dropped due to corruption (Drop
State 2) using a Data Dropped option (Section 8.7). Optionally, DCCP
MAY provide an API through which the receiving application could
request delivery of known-corrupt data. When that API is active, the
packet's data SHOULD be delivered, but reported as delivered corrupt
(Drop State 3) using a Data Dropped option. In either case, the
packet will be reported as Received or Received ECN Marked by Ack
Vector or equivalent options.
See Section 18.1 for a discussion of the issues related to the use
of this option.
8.9. Ack Vector Implementation Notes
This section discusses particulars of DCCP acknowledgement handling,
in the context of an abstract implementation for Ack Vector. It is
informative rather than normative.
The first part of our implementation runs at the HC-Receiver, and
therefore acknowledges data packets. It generates Ack Vector
options. The implementation has the following characteristics:
o At most one byte of state per acknowledged packet.
o O(1) time to update that state when a new packet arrives (normal
case).
o Cumulative acknowledgements.
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o Quick removal of old state.
The basic data structure is a circular buffer containing information
about acknowledged packets. Each byte in this buffer contains a
state and run length; the state can be 0 (packet received), 1
(packet ECN marked), or 3 (packet not yet received). The live
portion of the buffer is marked off by head and tail pointers, each
marked with the HC-Sender sequence number to which it corresponds.
The buffer also stores a single-bit ECN Nonce Echo, which equals the
one-bit sum of the ECN Nonces received on state-0 packets. The
buffer grows from right to left. For example:
+-------------------------------------------------------------------+
|S,L|S,L|S,L|S,L| | | | | |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|
+-------------------------------------------------------------------+
^ ^
Tail, seqno = T Head, seqno = H ECN Nonce Echo = E
<=== Head and Tail move this way <===
Each `S,L' represents a State/Run length byte. We will draw these
buffers showing only their live portion; for example, here is
another representation for the buffer above:
+-----------------------------------------------+
H |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L| T ENE[E]
+-----------------------------------------------+
This smaller Example Buffer contains actual data.
+---------------------------+
10 |0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 ENE[1] [Example Buffer]
+---------------------------+
In concrete terms, its meaning is as follows:
Packet 10 was received. (The head of the buffer has sequence
number 10, state 0, and run length 0.)
Packets 9, 8, and 7 have not yet been received. (The three bytes
preceding the head each have state 3 and run length 0.)
Packets 6, 5, 4, 3, and 2 were received.
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Packet 1 was ECN marked.
Packet 0 was received.
The one-bit sum of the ECN Nonces on packets 10, 6, 5, 4, 3, 2,
and 0 equals 1.
8.9.1. New Packets
When a packet arrives whose sequence number is larger than any in
the buffer, the HC-Receiver simply moves the Head pointer to the
left, increases the head sequence number, and stores a byte
representing the packet into the buffer. For example, if HC-Sender
packet 11 arrived ECN marked, the Example Buffer above would enter
this new state (the change is marked with stars):
+***----------------------------+
11 |1,0|0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 ENE[1]
+***----------------------------+
If the packet's state equals the state at the head of the buffer,
the HC-Receiver may choose to increment its run length (up to the
maximum). For example, if HC-Sender packet 11 arrived without ECN
marking and with ECN Nonce 0, the Example Buffer might enter this
state instead:
+--*------------------------+
11 |0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0 ENE[1]
+--*------------------------+
Of course, the new packet's sequence number might not equal the
expected sequence number. In this case, the HC-Receiver should enter
the intervening packets as State 3. If several packets are missing,
the HC-Receiver may prefer to enter multiple bytes with run length
0, rather than a single byte with a larger run length; this
simplifies table updates when one of the missing packets arrives.
For example, if HC-Sender packet 12 arrived with ECN Nonce 1, the
Example Buffer would enter this state:
+*******----------------------------+ *
12 |0,0|3,0|0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0 ENE[0]
+*******----------------------------+ *
When a new packet's sequence number is less than the head sequence
number, the HC-Receiver should scan the table for the byte
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corresponding to that sequence number. (Slightly more complex
indexing structures could reduce the complexity of this scan.)
Assume that the sequence number was previously lost (State 3), and
that it was stored in a byte with run length 0. Then the HC-Receiver
can simply change the byte's state. For example, if HC-Sender packet
8 was received with ECN Nonce 0, the Example Buffer would enter this
state:
+--------*------------------+
10 |0,0|3,0|0,0|3,0|0,4|1,0|0,0| 0 ENE[1]
+--------*------------------+
If the packet is not marked as lost, or if its sequence number is
not contained in the table, the packet is probably a duplicate, and
should be ignored. (The new packet's ECN marking state might differ
from the state in the buffer; Section 8.5.1 describes what to do
then.) If the packet's corresponding buffer byte has a non-zero run
length, then the buffer might need be reshuffled to make space for
one or two new bytes.
Of course, the circular buffer may overflow, either when the HC-
Sender is sending data at a very high rate, when the HC-Receiver's
acknowledgements are not reaching the HC-Sender, or when the HC-
Sender is forgetting to acknowledge those acks (so the HC-Receiver
is unable to clean up old state). In this case, the HC-Receiver
should either compress the buffer, transfer its state to a larger
buffer, or drop all received packets, without processing them
whatsoever, until its buffer shrinks again.
8.9.2. Sending Acknowledgements
Whenever the HC-Receiver needs to generate an acknowledgement, the
buffer's contents can simply be copied into one or more Ack Vector
options. Copied Ack Vectors might not be maximally compressed; for
example, the Example Buffer above contains three adjacent 3,0 bytes
that could be combined into a single 3,2 byte. The HC-Receiver
might, therefore, choose to compress the buffer in place before
sending the option, or to compress the buffer while copying it;
either operation is simple.
Every acknowledgement sent by the HC-Receiver SHOULD include the
entire state of the buffer. That is, acknowledgements are
cumulative.
The HC-Receiver should store information about each acknowledgement
it sends in another buffer. Specifically, for every acknowledgement
it sends, the HC-Receiver should store:
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o The HC-Receiver sequence number it used for the ack packet.
o The HC-Sender sequence number it acknowledged (that is, the
packet's Acknowledgement Number). Since acknowledgements are
cumulative, this single number completely specifies the set of HC-
Sender packets acknowledged by this ack packet.
8.9.3. Clearing State
Some of the HC-Sender's packets will include acknowledgement
numbers, which ack the HC-Receiver's acknowledgements. When such an
ack is received, the HC-Receiver simply finds the HC-Sender sequence
number corresponding to that acked HC-Receiver packet, and moves the
buffer's Tail pointer up to that sequence number. (It may choose to
keep some older information, in case a lost packet shows up late.)
For example, say that the HC-Receiver storing the Example Buffer had
sent two acknowledgements already:
(1) HC-Receiver Ack 59 acknowledged HC-Sender Seq 3 with ECN Nonce
Echo 1.
(2) HC-Receiver Ack 60 acknowledged HC-Sender Seq 10 with ECN Nonce
Echo 0.
Say the HC-Receiver then received a DCCP-DataAck packet from the HC-
Sender with Acknowledgement Number 59. This informs the HC-Receiver
that the HC-Sender received, and processed, all the information in
HC-Receiver packet 59. This packet acknowledged HC-Sender packet 3,
so the HC-Sender has now received HC-Receiver's acknowledgements for
packets 0, 1, 2, and 3. The Example Buffer should enter this state:
+------------------*+ * *
10 |0,0|3,0|3,0|3,0|0,2| 4 ENE[0]
+------------------*+ * *
The tail byte's run length was adjusted, since packet 3 was in the
middle of that byte. The new ECN Nonce Echo field equals the
exclusive-or of the old field, and the ECN Nonce Echo reported with
the relevant acknowledgement. The HC-Receiver can also throw away
stored information about HC-Receiver Ack 59.
A careful implementation might try to ensure reasonable robustness
to reordering. Suppose that the Example Buffer is as before, but
that packet 9 now arrives, out of sequence. The buffer would enter
this state:
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+----*----------------------+
10 |0,0|0,0|3,0|3,0|0,4|1,0|0,0| 0 ENE[1]
+----*----------------------+
The danger is that the HC-Sender might acknowledge the P2's previous
acknowledgement (with sequence number 60), which says that Packet 9
was not received, before the HC-Receiver has a chance to send a new
acknowledgement saying that Packet 9 actually was received.
Therefore, when packet 9 arrived, the HC-Receiver might modify its
acknowledgement record to:
(1) HC-Receiver Ack 59 acknowledged HC-Sender Seq 3 with ECN Nonce
Echo 1.
(2) HC-Receiver Ack 60 also acknowledged HC-Sender Seq 3 with ECN
Nonce Echo 1.
That is, Ack 60 is now treated like a duplicate of Ack 59. This
would prevent the Tail pointer from moving past packet 9 until the
HC-Receiver knows that the HC-Sender has seen an Ack Vector
indicating that packet's arrival.
8.9.4. Processing Acknowledgements
When the HC-Sender receives an acknowledgement, it generally cares
about the number of packets that were dropped and/or ECN marked. It
simply reads this off the Ack Vector. Additionally, it may check the
ECN Nonce for correctness. (As described in Section 8.5.1, it may
want to keep more detailed information about acknowledged packets in
case packets change states between acknowledgements, or in case the
application queries whether a packet arrived.)
The HC-Sender must also acknowledge the HC-Receiver's
acknowledgements so that the HC-Receiver can free old Ack Vector
state. (Since Ack Vector acknowledgements are reliable, the HC-
Receiver must maintain and resend Ack Vector information until it is
sure that the HC-Sender has received that information.) A simple
algorithm suffices: since Ack Vector acknowledgements are
cumulative, a single acknowledgement number tells HC-Receiver how
much ack information has arrived. Assuming that the HC-Receiver
sends no data, the HC-Sender can simply ensure that at least once a
round-trip time, it sends a DCCP-DataAck packet acknowledging the
latest DCCP-Ack packet it has received. Of course, the HC-Sender
only needs to acknowledge the HC-Receiver's acknowledgements if the
HC-Sender is also sending data. If the HC-Sender is not sending
data, then the HC-Receiver's Ack Vector state is stable, and there
is no need to shrink it. The HC-Sender must watch for drops and ECN
marks on received DCCP-Ack packets so that it can adjust the HC-
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Receiver's ack-sending rate---for example, with Ack Ratio---in
response to congestion.
If the other half-connection is not quiescent---that is, the HC-
Receiver is sending data to the HC-Sender, possibly using another
CCID---then the acknowledgements on that half-connection are
sufficient for the HC-Receiver to free its state.
9. Explicit Congestion Notification
The DCCP protocol is fully ECN-aware. Each CCID specifies how its
endpoints respond to ECN marks. Furthermore, DCCP, unlike TCP,
allows senders to control the rate at which acknowledgements are
generated (with options like Ack Ratio); this means that
acknowledgements are generally congestion-controlled, and may have
ECN-Capable Transport set.
A CCID profile describes how that CCID interacts with ECN, both for
data traffic and pure-acknowledgement traffic. A sender SHOULD set
ECN-Capable Transport on its packets whenever the receiver has its
ECN Capable feature turned on and the relevant CCID allows it,
unless the sending application indicates that ECN should not be
used.
The rest of this section describes the ECN Capable feature and the
interaction of the ECN Nonce with acknowledgement options such as
Ack Vector.
9.1. ECN Capable Feature
The ECN Capable feature lets a DCCP inform its partner that it
cannot read ECN bits from received IP headers, so the partner must
not set ECN-Capable Transport on its packets.
ECN Capable has feature number 2. The ECN Capable feature located at
DCCP A indicates whether or not A can successfully read ECN bits
from received frames' IP headers. (This is independent of whether it
can set ECN bits on sent frames.) DCCP A sends a "Prefer(ECN
Capable, 0)" option to DCCP B to inform B that A cannot read ECN
bits.
An ECN Capable feature contains a single byte of data. ECN
capability is on if and only if this byte is nonzero.
A new connection starts with ECN Capable 1 (that is, ECN capable)
for both DCCPs. If a DCCP is not ECN capable, it MUST send
"Prefer(ECN Capable, 0)" options to the other endpoint until
acknowledged (by "Change(ECN Capable, 0)") or the connection closes.
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Furthermore, it MUST NOT accept any data until the other endpoint
sends "Change(ECN Capable, 0)". It SHOULD send Data Dropped options
on its acknowledgements, with Drop State 0 ("protocol constraints"),
if the other endpoint does send data inappropriately.
9.2. ECN Nonces
Congestion avoidance will not occur, and the receiver will sometimes
get its data faster, when the sender is not told about any
congestion events. Thus, the receiver has some incentive to falsify
acknowledgement information, reporting that marked or dropped
packets were actually received unmarked. This problem is more
serious with DCCP than with TCP, since TCP provides reliable
transport: it is more difficult with TCP to lie about lost packets
without breaking the application.
ECN Nonces are a general mechanism to prevent ECN cheating (or loss
cheating). Two values for the two-bit ECN header field indicate ECN-
Capable Transport, 01 and 10. The second code point, 10, is the ECN
Nonce. In general, a protocol sender chooses between these code
points randomly on its output packets, remembering the sequence it
chose. The protocol receiver reports, on every acknowledgement, the
number of ECN Nonces it has received thus far. This is called the
ECN Nonce Echo. Since ECN marking and packet dropping both destroy
the ECN Nonce, a receiver that lies about an ECN mark or packet drop
has a 50% chance of guessing right and avoiding discipline. The
sender may react punitively to an ECN Nonce mismatch, possibly up to
dropping the connection. The ECN Nonce Echo field need not be an
integer; one bit is enough to catch 50% of infractions.
In DCCP, the ECN Nonce Echo field is encoded in acknowledgement
options. For example, the Ack Vector option comes in two forms, Ack
Vector [Nonce 0] (option 37) and Ack Vector [Nonce 1] (option 38),
corresponding to the two values for a one-bit ECN Nonce Echo. The
Nonce Echo for a given Ack Vector equals the one-bit sum (exclusive-
or, or parity) of ECN nonces for packets reported by that Ack Vector
as received and not ECN marked. Thus, only packets marked as State
0 matter for this calculation (that is, valid received packets that
were not ECN marked). Every Ack Vector option is detailed enough
for the sender to determine what the Nonce Echo should have been. It
can check this calculation against the actual Nonce Echo, and
complain if there is a mismatch.
(The Ack Vector could conceivably report every packet's ECN Nonce
state, but this would severely limit Ack Vector's compressibility
without providing much extra protection.)
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Consider a half-connection from DCCP A to DCCP B. DCCP A SHOULD set
ECN Nonces on its packets, and remember which packets had nonces,
whenever DCCP B reports that it is ECN Capable. An ECN-capable
endpoint MUST calculate and use the correct value for ECN Nonce Echo
when sending acknowledgement options. An ECN-incapable endpoint,
however, SHOULD treat the ECN Nonce Echo as always zero. When a
sender detects an ECN Nonce Echo mismatch, it SHOULD behave as if
the receiver had reported one or more packets as ECN-marked (instead
of unmarked). It MAY take more punitive action, such as resetting
the connection. The Reason for such DCCP-Reset packets SHOULD be set
to "Aggression Penalty".
An ECN-incapable DCCP SHOULD ignore received ECN nonces and generate
ECN nonces of zero. For instance, out of the two Ack Vector options,
an ECN-incapable DCCP SHOULD generate Ack Vector [Nonce 0] (option
37) exclusively. (Again, the ECN Capable feature MUST be set to zero
in this case.)
9.3. Other Aggression Penalties
The ECN Nonce provides one way for a DCCP sender to discover that a
receiver is misbehaving. There may be other mechanisms, and a
receiver or middlebox may also discover that a sender is
misbehaving---sending more data than it should. In any of these
cases, the entity that discovers the misbehavior MAY react by
resetting the connection, with Reason set to "Aggression Penalty". A
receiver that detects marginal (meaning possibly spurious) sender
misbehavior MAY instead react with a Slow Receiver option, or by
reporting some packets as ECN marked that were not, in fact, marked.
10. Multihoming and Mobility
DCCP provides primitive support for multihoming and mobility via a
mechanism for transferring a connection endpoint from one address to
another. The moving endpoint must negotiate mobility support
beforehand, and both endpoints must share their Connection Nonces.
When the moving endpoint gets a new address, it sends a DCCP-Move
packet from that address to the stationary endpoint. The stationary
endpoint then changes its connection state to use the new address.
DCCP's support for mobility is intended to solve only the simplest
multihoming and mobility problems. For instance, DCCP has no support
for simultaneous moves. Applications requiring more complex mobility
semantics, or more stringent security guarantees, should use an
existing solution like Mobile IP or [SB00].
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10.1. Mobility Capable Feature
A DCCP uses the Mobility Capable feature to inform its partner that
it would like to be able to change its address and/or port during
the course of the connection.
Mobility Capable has feature number 5. The Mobility Capable feature
located at DCCP A indicates whether or not A will accept a DCCP-Move
packet sent by B. DCCP B sends a "Change(Mobility Capable, 1)"
option to DCCP A to inform it that B might like to move later.
A Mobility Capable feature contains a single byte of data. Mobility
is allowed if and only if this byte is nonzero. A DCCP MUST reject a
DCCP-Move packet referring to a connection when Mobility Capable is
0; however, it MAY reject a valid DCCP-Move packet even when
Mobility Capable is 1.
A new connection starts with Mobility Capable 0 (that is, mobility
is not allowed) for both DCCPs.
10.2. Security
The DCCP mobility mechanism, like DCCP in general, does not provide
cryptographic security guarantees. Nevertheless, mobile hosts must
use valid sequence numbers and include valid Identifications in
their DCCP-Move packets, providing protection against some classes
of attackers. Specifically, an attacker cannot move a DCCP
connection to a new address unless they know valid sequence numbers
and how to generate valid Identifications. Even with the default MD5
Identification Regime, this means that an attacker must have snooped
on every packet in the connection to get a reasonable probability of
success, assuming that initial sequence numbers and Connection
Nonces are chosen well (that is, randomly). Section 16 further
describes DCCP security considerations.
10.3. Congestion Control State
Once an endpoint has transitioned to a new address, the connection
is effectively a new connection in terms of its congestion control
state: the accumulated information about congestion between the old
endpoints no longer applies. Both DCCPs MUST initialize their
congestion control state (windows, rates, and so forth) to that of a
new connection---that is, they must "slow start"---unless they have
high-quality information about actual network conditions between the
two new endpoints. Normally, the only way to get this information
would be by instrumenting a DCCP connection between the new
addresses.
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Similarly, the endpoints' configured MTUs (see 11) SHOULD be
reinitialized, and PMTU discovery performed again, following an
address change.
10.4. Loss During Transition
Several loss and delay events may affect the transition of a DCCP
connection from one address to another. The DCCP-Move packet itself
might be lost; the acknowledgement to that packet might be lost,
leaving the mobile endpoint unsure of whether the transition has
completed; and data from the old endpoint might continue to arrive
at the receiver even after the transition.
To protect against lost DCCP-Move packets, the mobile host SHOULD
retransmit a DCCP-Move packet if it does not receive an
acknowledgement within a reasonable time period. Section 5.9
describes the mechanism used to protect against duplicate DCCP-Move
packets.
A receiver MAY drop all data received from the old address/port pair
once a DCCP-Move has successfully completed. Alternately, it MAY
accept one Loss Window's worth of this data. Congestion and loss
events on this data SHOULD NOT affect the new connection's
congestion control state. The receiver MUST NOT accept data with the
old address/port pair past one Loss Window, and SHOULD send DCCP-
Resets in response to those packets.
During some transition period, acknowledgements from the receiver to
the mobile host will contain information about packets sent both
from the old address/port pair, and from the new address/port pair.
The mobile DCCP MUST NOT let loss events on packets from the old
address/port pair affect the new congestion control state.
11. Path MTU Discovery
A DCCP implementation SHOULD be capable of performing Path MTU
(PMTU) discovery, as described in [RFC 1191]. The API to DCCP SHOULD
allow this mechanism to be disabled in cases where IP fragmentation
is preferred. The rest of this section assumes PMTU discovery has
not been disabled.
A DCCP implementation MUST maintain its idea of the current PMTU for
each active DCCP session. The PMTU SHOULD be initialized from the
interface MTU that will be used to send packets.
To perform PMTU discovery, the DCCP sender sets the IP Don't
Fragment (DF) bit. However, it is undersirable for MTU discovery to
occur on the initial connection setup handshake, as the connection
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setup process may not be representative of packet sizes used during
the connection, and performing MTU discovery on the initial
handshake might unnecessarily delay connection establishment. Thus,
DF SHOULD NOT be set on DCCP-Request and DCCP-Response packets. In
addition DF SHOULD NOT be set on DCCP-Reset packets, although
typically these would be small enough to not be a problem. On all
other DCCP packets, DF SHOULD be set.
Any API to DCCP MUST allow the application to discover DCCP's
current PMTU. DCCP applications SHOULD use the API to discover the
PMTU, and SHOULD NOT send datagrams that are greater than the PMTU;
the only exception to this is if the application disables PMTU
discovery. If the application tries to send a packet bigger than the
PMTU, the DCCP implementation MUST drop the packet and return an
appropriate error.
As specified in [RFC 1191], when a router receives a packet with DF
set that is larger than the PMTU, it sends an ICMP Destination
Unreachable message to the source of the datagram with the Code
indicating "fragmentation needed and DF set" (also known as a
"Datagram Too Big" message). When a DCCP implementation receives a
Datagram Too Big message, it decreases its PMTU to the Next-Hop MTU
value given in the ICMP message. If the MTU given in the message is
zero, the sender chooses a value for PMTU using the algorithm
described in Section 7 of [RFC 1191]. If the MTU given in the
message is greater than the current PMTU, the Datagram Too Big
message is ignored, as described in [RFC 1191]. (We are aware that
this may cause problems for DCCP endpoints behind certain
firewalls.)
If the DCCP implementation has decreased the PMTU, and the sending
application attempts to send a packet larger than the new MTU, the
API MUST cause the send to fail returning an appropriate error to
the application, and the application SHOULD then use the API to
query the new value of the PMTU. When this occurs, it is possible
that the kernel has some packets buffered for transmission that are
smaller than the old PMTU, but larger than the new PMTU. The kernel
MAY send these packets with the DF bit cleared, or it MAY discard
these packets; it MUST NOT transmit these datagrams with the DF bit
set.
DCCP currently provides no way to increase the PMTU once it has
decreased.
A DCCP sender MAY optionally treat the reception of an ICMP Datagram
Too Big message as an indication that the packet being reported was
not lost due congestion, and so for the purposes of congestion
control it MAY ignore the DCCP receiver's indication that this
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packet did not arrive. However, if this is done, then the DCCP
sender MUST check the ECN bits of the IP header echoed in the ICMP
message, and only perform this optimization if these ECN bits
indicate that the packet did not experience congestion prior to
reaching the router whose MTU it exceeded.
12. Middlebox Considerations
This section describes properties of DCCP that firewalls, network
address translators, and other middleboxes must consider, including
parts of the packet that middleboxes must not change.
The Service Name field in DCCP-Request packets provide information
that may be useful for stateful middleboxes. With Service Name, a
middlebox can tell what protocol a connection will use, without
relying on port numbers. Middleboxes MAY disallow attempted
connections with zero Service Names by sending a DCCP-Reset.
Middleboxes SHOULD NOT modify the Service Name.
The Source and Destination Port fields are in the same packet
locations as the corresponding fields in TCP and UDP, which may
simplify some middlebox implementations.
Middleboxes MUST NOT modify DCCP packets' Sequence Number,
Acknowledgement Number, and # NDP fields in order to add or remove
packets from a packet stream. Any such modification would affect the
endpoints' accounting of which packets have been lost, destroy the
Identification mechanism, and confuse the congestion control
mechanisms in use. Note that there is less need to modify DCCP's
per-packet sequence numbers than TCP's per-byte sequence numbers;
for example, a middlebox can change the contents of a packet without
changing its sequence number. (In TCP, sequence number modification
is required to support legacy protocols like FTP that carry
variable-length addresses in the data stream. If such an application
were deployed over DCCP, middleboxes would simply grow or shrink the
relevant packets as necessary, without changing their sequence
numbers.)
The exception to this rule is that middleboxes MAY reset connections
in progress. Clearly this requires inserting a packet into one or
both packet streams, as well as dropping all later packets on the
connection.
This does not explicitly prevent one sequence number modification
occasionally seen with TCP, namely proxies with "connection
splicing" [SHHP00]. Such proxies intercept TCP connection attempts
from a client, but may later "splice" data from an external server
connection into that client connection via sequence number
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manipulations. Packets are not added to or removed from the spliced-
in stream, reducing the sequence number issues somewhat.
Nevertheless, DCCP, with its extensive end-to-end feature
negotiation, is inherently unfriendly to the idea of connection
splicing: the proxy would have to ensure that the server chose the
same feature values that the proxy had previously negotiated with
the client. Furthermore, Identification options would require
special handling; and there may be other issues. We suggest that
DCCP splicing, if implemented, should take place at the application
level.
A middlebox that wants to trivially support the MD5 Identification
Regime (Section 6.4.3) MUST NOT alter packets' Sequence Number,
Type, and CCval fields, or the Connection Nonce feature values,
which are included in the MD5 hash sent with Identification and
Challenge options.
The contents of this section SHOULD NOT be interpreted as a
wholesale endorsement of stateful middleboxes.
13. Abstract API
API issues for DCCP are discussed in another Internet-Draft, in
progress.
14. Multiplexing Issues
In contrast to TCP, DCCP does not offer reliable ordered delivery.
As a consequence, with DCCP there are no inherent performance
penalties in layering functionality above DCCP to multiplex several
sub-flows into a single DCCP connection.
However, this approach of multiplexing sub-flows above DCCP will not
work in circumstances such as RTP where the RTP subflows require
separate port numbers. In this case, if it is desired to share
congestion control state among multiple DCCP flows that share the
same source and destination addresses, the possibilities are to add
DCCP-specific mechanisms to enable this, or to use a generic
multiplexing facility like the Congestion Manager [RFC 3124]
residing below the transport layer. For some DCCP flows, the
ability to specify the congestion control mechanism might be
critical, and for these flows the Congestion Manager will only be a
viable tool if it allows DCCP to specify the congestion control
mechanism used by the Congestion Manager for that flow. Thus, to
allow the sharing of congestion control state among multiple DCCP
flows, the alternatives seem to be to add DCCP-specific
functionality to the Congestion Manager, or to add a similar layer
below DCCP that is specific to DCCP. We defer issues of DCCP
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operating over a revised version of the Congestion Manager, or over
a DCCP-specific module for the sharing of congestion control state,
to later work.
15. DCCP and RTP
The real-time transport protocol, RTP [RFC 1889], is currently used
(over UDP) by many of DCCP's target applications (for instance,
streaming media). This section therefore discusses the relationship
between DCCP and RTP, and in particular, the question of whether any
changes in RTP are necessary or desirable when it is layered over
DCCP instead of UDP. The main issue here is header size: a DCCP
header is at least 4 bytes larger than a UDP header.
There are two potential sources of overhead in the RTP-over-DCCP
combination, duplicated acknowledgement information and duplicated
sequence numbers. We argue that together, these sources of overhead
add just 4 bytes per packet relative to RTP-over-UDP, and that
eliminating the redundancy would not reduce the overhead. However,
particular CCIDs might make productive use of the space occupied by
RTP's sequence number.
First, consider acknowledgements. The information on packet loss
that RTP communicates via RTCP SR/RR packets is communicated by DCCP
via acknowledgement options. Much of the information in an RTCP
receiver report could be divined from DCCP acknowledgements,
depending on the CCID in use. Acknowledgement options, such as Ack
Vector, can be frequent and verbose, whereas RTCP reports are sent
only rarely, with a minimum interval of 5 seconds between reports
[RFC 1889].
However, not all CCIDs require such verbose acknowledgements. CCID 3
(TFRC) reports acknowledgements at a low rate---between 16 and 32
bytes of options (depending on ECN usage), sent once per round trip
time. This is not an undue burden. Furthermore, the options are
necessary to implement responsive congestion control, and we cannot
report less frequently, although we might design alternative
acknowledgement options that take fewer bytes. DCCP gives the
application the trade off between small packet overhead and the
precise feedback provided by Ack Vector.
While RTP receiver reports might be considered "redundant" in the
presence of DCCP's more precise acknowledgements, they are sent so
infrequently that it is not worth optimizing them away. Also, note
that in the common case of a one-way data stream, acknowledgement
packets contain no data, so acknowledgement header size (as distinct
from congestion on the acknowledgement path) is not an issue.
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We now consider sequence number redundancy on data packets. The
embedded RTP header contains a 16-bit RTP sequence number. Most data
packets will use the DCCP-Data type; DCCP-DataAck and DCCP-Ack
packets need not usually be sent. The DCCP-Data header is 12 bytes
long without options, including a 24-bit sequence number. This is 4
bytes more than a UDP header. Any options required on data packets
would add further overhead, although many CCIDs (for instance, CCID
3 [TFRC]) don't require options on most data packets.
The DCCP sequence number cannot be inferred from the RTP sequence
number since it increments on non-data packets as well as data
packets. The RTP sequence number could be inferred from the DCCP
sequence number, though; it might equal the DCCP sequence number
minus the total number of non-data packets seen so far in the
connection (as tracked by DCCP's # NDP header field).
Removing RTP's sequence number would not save any header space
because of alignment issues. However, particular DCCP CCIDs might
make use of the 16 bits occupied by the RTP sequence number.
Therefore, particular DCCP CCIDs MAY provide optional CCID-specific
features that store DCCP quantities in place of the embedded RTP
sequence number. A conforming DCCP would write in the calculated RTP
sequence number before passing the packet to RTP. (The DCCP checksum
would use the DCCP quantity, not the RTP sequence number.)
Given RTP-over-DCCP's small overhead, however, implementors
demanding tiny headers will probably prefer more comprehensive
header compression to this ad-hoc compression technique.
16. Security Considerations
DCCP does not provide cryptographic security guarantees.
Applications desiring hard security should use IPsec or end-to-end
security of some kind.
Nevertheless, DCCP is intended to protect against some classes of
attackers. Attackers cannot hijack a DCCP connection (close the
connection unexpectedly, or cause attacker data to be accepted by an
endpoint as if it came from the sender) unless they can guess valid
sequence numbers. Thus, as long as endpoints choose initial sequence
numbers well, a DCCP attacker must snoop on data packets to get any
reasonable probability of success. The sequence number validity
(Section 5.2), Identification (Section 6.4.3), and mobility (Section
10) mechanisms provide this guarantee.
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17. IANA Considerations
DCCP introduces six sets of numbers whose values should be allocated
by IANA.
o 32-bit Service Names (Section 5.4; not exclusive to DCCP).
o 8-bit DCCP-Reset Reasons (Section 5.8).
o 8-bit DCCP Option Types (Section 6). The CCID-specific options 128
through 255 need not be allocated by IANA.
o 8-bit DCCP Feature Numbers (Section 6.3). The CCID-specific
features 128 through 255 need not be allocated by IANA.
o 8-bit DCCP Congestion Control Identifiers (CCIDs) (Section 7).
o 16-bit Identification Regimes, for use with DCCP Identification
and Challenge options (Section 6.4).
In addition, DCCP requires a Protocol Number to be added to the
registry of Assigned Internet Protocol Numbers. Experimental
implementors should use Protocol Number 33 for DCCP, but this number
may change in future.
18. Design Motivation
In the section we attempt to capture some of the rationale behind
specific details of DCCP design.
18.1. CSlen and Partial Checksumming
A great deal of discussion has taken place regarding the utility of
allowing a DCCP sender to restrict the checksum so that it does not
cover the complete packet.
Many of the applications that we envisage using DCCP are resilient
to some degree of data loss, or they would typically have chosen a
reliable transport. Some of these applications may also be
resilient to data corruption---some audio payloads, for example.
These resilient applications might prefer to receive corrupted data
than to have DCCP drop a corrupted packet. This is particularly
because of congestion control: DCCP cannot tell the difference
between packets dropped due to corruption and packets dropped due to
congestion, and so it must reduce the transmission rate accordingly.
This response may cause the connection to receive less bandwidth
than it is due; corruption in some networking technologies is
independent of, or at least not always correlated to, congestion.
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Therefore, corrupted packets do not need to cause as strong a
reduction in transmission rate as the congestion response would
dictate (so long as the DCCP header and options are not corrupt).
Thus DCCP allows the checksum to cover all of the packet, just the
DCCP header, or both the DCCP header and some number of bytes from
the payload. If the application cannot tolerate any payload
corruption, then the checksum SHOULD cover the whole packet. If the
application would prefer to tolerate some corruption rather than
have the packet dropped, then it can set the checksum to cover only
part of the packet (but always the DCCP header). In addition, if
the application wishes to decouple checksumming of the DCCP header
from checksumming of the payload, it may do so by including the
Payload Checksum option. This would allow payload corruption to
cause DCCP to discard a corrupted payload, but still not mistake the
corruption for network congestion.
Thus, from the application point of view, partial checksums seem to
be a desirable feature. However, the usefulness of partial
checksums depends on partially corrupted packets being delivered to
the receiver. If the link-layer CRC always discards corrupted
packets, then this will not happen, and so the usefulness of partial
checksums would be restricted to corruption that occurred in routers
and other places not covered by link CRCs. There does not appear to
be consensus on how likely it is that future network links that
suffer significant corruption will not cover the entire packet with
a single strong CRC. DCCP makes it possible to tailor such links to
the application, but it is difficult to predict if this will be
compelling for future link technologies.
In addition, partial checksums do not co-exist well with IP-level
authentication mechanisms such as IPsec AH, which cover the entire
packet with a cryptographic hash. Thus, if cryptographic
authentication mechanisms are required to co-exist with partial
checksums, the authentication must be carried in the DCCP payload.
A possible mode of usage would appear to be similar to that of
Secure RTP. However, such "application-level" authentication does
not protect the DCCP option negotiation and state machine from
forged packets. An alternative would be to use IPsec ESP, and use
encryption to protect the DCCP headers against attack, while using
the DCCP header validity checks to authenticate that the header is
from someone who possessed the correct key. However, while this is
resistant to replay (due to the DCCP sequence number), it is not by
itself resistant to some forms of man-in-the-middle attacks because
the payload is not tightly coupled to the packet header. Thus an
application-level authentication probably needs to be coupled with
IPsec ESP or a similar mechanism to provide a reasonably complete
security solution. The overhead of such a solution might be
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unacceptable for some applications that would otherwise wish to use
partial checksums.
On balance, the authors believe that DCCP partial checksums have the
potential to enable some future uses that would otherwise be
difficult. As the cost and complexity of supporting them is small,
it seems worth including them at this time. It remains to be seen
whether they are useful in practice.
19. Thanks
There is a wealth of work in this area, including the Congestion
Manager. We thank the staff and interns of ICIR and, formerly,
ACIRI, the members of the End-to-End Research Group, and the members
of the Transport Area Working Group for their feedback on DCCP. We
also thank those who provided comments and suggestions via the DCCP
BOF, Working Group, and mailing lists, including Damon Lanphear,
Patrick McManus, Sara Karlberg, Kevin Lai, Youngsoo Choi, Dan
Duchamp, Derek Fawcus, David Timothy Fleeman, John Loughney,
Ghyslain Pelletier, Tom Phelan, Stanislav Shalunov, Yufei Wang, and
Michael Welzl.
20. Normative References
[RFC 793] J. Postel, editor. Transmission Control Protocol. RFC 793.
[RFC 1191] J. C. Mogul and S. E. Deering. Path MTU Discovery. RFC
1191.
[RFC 2026] S. Bradner. The Internet Standards Process---Revision 3.
RFC 2026.
[RFC 2119] S. Bradner. Key Words For Use in RFCs to Indicate
Requirement Levels. RFC 2119.
[RFC 2460] S. Deering and R. Hinden. Internet Protocol, Version 6
(IPv6) Specification. RFC 2460.
[RFC 3168] K.K. Ramakrishnan, S. Floyd, and D. Black. The Addition
of Explicit Congestion Notification (ECN) to IP. RFC 3168.
September 2001.
21. Informative References
[CCID 2 PROFILE] S. Floyd and E. Kohler. Profile for DCCP Congestion
Control ID 2: TCP-like Congestion Control. draft-ietf-dccp-
ccid2-01.txt, work in progress, March 2003.
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[CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye. Profile for
DCCP Congestion Control ID 3: TFRC Congestion Control. draft-
ietf-dccp-ccid3-01.txt, work in progress, March 2003.
[ECN NONCE] David Wetherall, David Ely, and Neil Spring. Robust ECN
Signaling with Nonces. draft-ietf-tsvwg-tcp-nonce-04.txt, work
in progress, October 2002.
[RFC 1889] Audio-Video Transport Working Group, H. Schulzrinne, S.
Casner, R. Frederick, and V. Jacobson. RTP: A Transport
Protocol for Real-Time Applications. RFC 1889.
[RFC 1948] S. Bellovin. Defending Against Sequence Number Attacks.
RFC 1948.
[RFC 2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.
Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V.
Paxson. Stream Control Transmission Protocol. RFC 2960.
[RFC 3124] H. Balakrishnan and S. Seshan. The Congestion Manager.
RFC 3124.
[SB00] Alex C. Snoeren and Hari Balakrishnan. An End-to-End Approach
to Host Mobility. Proc. 6th Annual ACM/IEEE International
Conference on Mobile Computing and Networking (MOBICOM '00),
August 2000.
[SHHP00] Oliver Spatscheck, Jorgen S. Hansen, John H. Hartman, and
Larry L. Peterson. Optimizing TCP Forwarder Performance.
IEEE/ACM Transactions on Networking 8(2):146-157, April 2000.
[UDP-LITE] L-A. Larzon, M. Degermark, S. Pink, L-E. Jonsson
(editor), and G. Fairhurst (editor). The UDP-Lite Protocol.
draft-ietf-tsvwg-udp-lite-01.txt, work in progress, December
2002.
22. Authors' Addresses
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Eddie Kohler <kohler@icir.org>
Mark Handley <mjh@icir.org>
Sally Floyd <floyd@icir.org>
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704 USA
Jitendra Padhye <padhye@microsoft.com>
Microsoft Research
One Microsoft Way
Redmond, WA 98052 USA
Kohler/Handley/Floyd/Padhye Section 22. [Page 78]
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