Internet Engineering Task Force Eddie Kohler
INTERNET-DRAFT UCLA
draft-ietf-dccp-spec-06.txt Mark Handley
Expires: August 2004 UCL
Sally Floyd
ICIR
16 February 2004
Datagram Congestion Control Protocol (DCCP)
Status of this Memo
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
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Internet-Drafts are draft documents valid for a maximum of six
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies the Datagram Congestion Control Protocol
(DCCP), which implements a congestion-controlled, unreliable flow of
unicast datagrams suitable for use by applications such as streaming
media, Internet telephony, and on-line games.
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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes since draft-ietf-dccp-spec-05.txt:
* Organization overhaul.
* Add pseudocode for event processing.
* Remove # NDP; replace with Ack Count.
* Remove Identification, Challenge, ID Regime, and Connection Nonce.
* Data Checksum (formerly Payload Checksum) uses a 32-bit CRC.
* Switch location of non-negotiable features to clarify
presentation; now the feature location controls its value.
* Rename "value type" to "reconciliation rule".
* Rename "Reset Reason" to "Reset Code".
* Mobility ID becomes 128 bits long.
* Add probabilities to Mobility ID discussion.
* Add SyncAck.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . . 8
3. Conventions and Terminology . . . . . . . . . . . . . . . . . 9
3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . . 9
3.2. Parts of a Connection. . . . . . . . . . . . . . . . . . 9
3.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . . 10
3.5. Robustness Principle . . . . . . . . . . . . . . . . . . 10
4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 12
4.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4. Congestion Control . . . . . . . . . . . . . . . . . . . 15
4.5. Features . . . . . . . . . . . . . . . . . . . . . . . . 16
4.6. Other Differences from TCP . . . . . . . . . . . . . . . 17
4.7. Example Connection . . . . . . . . . . . . . . . . . . . 18
5. Header Formats. . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Generic Header . . . . . . . . . . . . . . . . . . . . . 20
5.2. DCCP-Request Header. . . . . . . . . . . . . . . . . . . 23
5.3. DCCP-Response Header . . . . . . . . . . . . . . . . . . 23
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Head-
ers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.5. DCCP-CloseReq and DCCP-Close Headers . . . . . . . . . . 25
5.6. DCCP-Reset Header. . . . . . . . . . . . . . . . . . . . 26
5.7. DCCP-Move Header . . . . . . . . . . . . . . . . . . . . 27
5.8. DCCP-Sync and DCCP-SyncAck Headers . . . . . . . . . . . 28
5.9. Options. . . . . . . . . . . . . . . . . . . . . . . . . 29
5.9.1. Padding Option. . . . . . . . . . . . . . . . . . . 30
5.9.2. Mandatory Option. . . . . . . . . . . . . . . . . . 30
6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . . 31
6.1. Change Options . . . . . . . . . . . . . . . . . . . . . 31
6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . . 32
6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . . 32
6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 33
6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . . 33
6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . . 33
6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 34
6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 36
6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . . 36
6.6.2. Loss and Retransmission . . . . . . . . . . . . . . 37
6.6.3. Reordering. . . . . . . . . . . . . . . . . . . . . 38
6.6.4. Preference Changes. . . . . . . . . . . . . . . . . 39
6.6.5. Simultaneous Negotiation. . . . . . . . . . . . . . 39
6.6.6. Unknown Features. . . . . . . . . . . . . . . . . . 39
6.6.7. Invalid Options . . . . . . . . . . . . . . . . . . 40
6.6.8. Mandatory Feature Negotiation . . . . . . . . . . . 40
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6.6.9. Out-of-Band Agreement . . . . . . . . . . . . . . . 41
6.6.10. State Diagram. . . . . . . . . . . . . . . . . . . 41
7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 42
7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 42
7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 43
7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 44
7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . . 44
7.5. Validity and Synchronization . . . . . . . . . . . . . . 45
7.5.1. Sequence-Validity Rules . . . . . . . . . . . . . . 45
7.5.2. Handling Sequence-Invalid Packets . . . . . . . . . 47
7.5.3. Sequence and Acknowledgement Number
Windows. . . . . . . . . . . . . . . . . . . . . . . . . . 48
7.5.4. Sequence Window Feature . . . . . . . . . . . . . . 49
7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 49
7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . . 50
7.6. Extended Sequence Numbers. . . . . . . . . . . . . . . . 51
7.6.1. When to Use Extended Sequence Numbers . . . . . . . 51
7.6.2. Header Processing . . . . . . . . . . . . . . . . . 52
7.6.3. Transitioning to Extended Sequence Num-
bers . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
7.6.4. Sequence Transition Capable Feature . . . . . . . . 54
7.7. NDP Count and Detecting Application Loss . . . . . . . . 55
7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . . 56
7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 56
8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 56
8.1. Connection Establishment . . . . . . . . . . . . . . . . 56
8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 57
8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 57
8.1.3. Server Response . . . . . . . . . . . . . . . . . . 59
8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 60
8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 60
8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 61
8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 62
8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 63
8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 63
8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 64
9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 68
9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 68
9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 69
9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 70
9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 71
9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . . 71
10. Congestion Control IDs . . . . . . . . . . . . . . . . . . . 71
10.1. Unspecified Sender-Based Congestion
Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
10.2. TCP-like Congestion Control . . . . . . . . . . . . . . 74
10.3. TFRC Congestion Control . . . . . . . . . . . . . . . . 74
10.4. CCID-Specific Options, Features, and Reset
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Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 76
11.1. Acks of Acks and Unidirectional
Connections . . . . . . . . . . . . . . . . . . . . . . . . . 77
11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 78
11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 79
11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 79
11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 81
11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 83
11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 83
11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 84
11.7. Data Dropped Option . . . . . . . . . . . . . . . . . . 84
11.7.1. Data Dropped and Normal Congestion
Response . . . . . . . . . . . . . . . . . . . . . . . . . 87
11.7.2. Particular Drop Codes. . . . . . . . . . . . . . . 87
12. Explicit Congestion Notification . . . . . . . . . . . . . . 88
12.1. ECN Capable Feature . . . . . . . . . . . . . . . . . . 88
12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 89
12.3. Other Aggression Penalties. . . . . . . . . . . . . . . 90
13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 90
13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 90
13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 91
13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 92
14. Multihoming and Mobility . . . . . . . . . . . . . . . . . . 92
14.1. Mobility Capable Feature. . . . . . . . . . . . . . . . 93
14.2. Mobility ID Feature . . . . . . . . . . . . . . . . . . 93
14.3. Mobile Host Processing. . . . . . . . . . . . . . . . . 94
14.4. Stationary Host Processing. . . . . . . . . . . . . . . 95
14.5. Congestion Control State. . . . . . . . . . . . . . . . 96
14.6. Security. . . . . . . . . . . . . . . . . . . . . . . . 96
15. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 97
16. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 99
17. Middlebox Considerations . . . . . . . . . . . . . . . . . . 100
18. Relations to Other Specifications. . . . . . . . . . . . . . 101
18.1. DCCP and RTP. . . . . . . . . . . . . . . . . . . . . . 101
18.2. Multiplexing Issues . . . . . . . . . . . . . . . . . . 102
19. Security Considerations. . . . . . . . . . . . . . . . . . . 103
19.1. Security Considerations for Mobility. . . . . . . . . . 103
19.2. Security Considerations for Partial Check-
sums. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
20. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 105
21. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 106
A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 108
A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 108
A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 109
A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 110
A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 110
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A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 112
B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 113
B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 113
Normative References . . . . . . . . . . . . . . . . . . . . . . 114
Informative References . . . . . . . . . . . . . . . . . . . . . 115
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 116
Intellectual Property Notice . . . . . . . . . . . . . . . . . . 117
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1. Introduction
This document describes the Datagram Congestion Control Protocol
(DCCP), a transport protocol that implements a congestion-
controlled, bidirectional stream of unreliable datagrams.
Specifically, DCCP provides:
o An unreliable flow of datagrams, with acknowledgements.
o Reliable handshakes 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 Congestion control incorporating Explicit Congestion Notification
(ECN) and the ECN Nonce, as per [RFC 3168] and [RFC 3540].
o Acknowledgement mechanisms communicating packet loss and ECN mark
information. Acks are transmitted as reliably as the relevant
congestion control mechanism requires, possibly completely
reliably.
o Optional mechanisms that tell the sending application, with high
reliability, which data packets reached the receiver, and whether
those packets were ECN marked, corrupted, or dropped in the
receive buffer.
o Path Maximum Transfer Unit (PMTU) discovery, as per [RFC 1191].
DCCP is intended for applications, such as streaming media and
Internet telephony, where reliable in-order delivery, combined with
congestion control, can result in some information arriving at the
receiver after it is no longer of use. So far, most such
applications have either used TCP, with the attendant quality
problems caused by late data delivery, or used UDP and implemented
their own congestion control (or no congestion control at all).
DCCP provides standard congestion control mechanisms for such
applications. It enables 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.
DCCP's target applications require the flow-based semantics of TCP,
but do not want TCP's in-order delivery and reliability, or would
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like different congestion control dynamics than TCP.
2. Design Rationale
DCCP was intended to be used by applications that currently use UDP
without end-to-end congestion control. Most streaming UDP
applications should have little reason not to switch to DCCP, once
it is deployed. Thus, DCCP was designed to have as little overhead
as possible, both in terms of the packet header size and in terms of
the state and CPU overhead required at end hosts. Only the minimal
necessary functionality was included in DCCP, leaving other
functionality, such as forward error correction (FEC), semi-
reliability, and multiple streams, to be layered on top of DCCP as
desired. This desire for minimal overhead is also one of the
reasons to avoid proposing an unreliable variant of the Stream
Control Transmission Protocol (SCTP, [RFC 2960]).
Different forms of conformant congestion control are appropriate for
different applications. For example, applications such as on-line
games might want to make quick use of any available bandwidth.
Other applications, such as streaming media, might trade off this
responsiveness for a steadier, less bursty rate, since sudden rate
changes cause unacceptable UI glitches (such as audible pauses or
clicks in the playout stream). Thus, DCCP allows applications to
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. Applications using
this congestion control mechanism will respond quickly to changes in
available bandwidth, but must be able to tolerate the abrupt changes
in congestion window typical of TCP. A second alternative, TCP-
Friendly Rate Control (TFRC, [RFC 3448]), a form of equation-based
congestion control, minimizes abrupt changes in the sending rate
while maintaining longer-term fairness with TCP.
DCCP also lets unreliable traffic safely use ECN. A UDP kernel API
might not allow applications to set UDP packets as ECN-capable,
since the API could not guarantee the application would properly
detect or respond to congestion. DCCP kernel APIs will have no such
issues, since DCCP itself implements congestion control.
We chose not to require the use of the Congestion Manager [RFC
3124], which allows multiple concurrent streams between the same
sender and receiver to share congestion control. The current
Congestion Manager can only be used by applications that have their
own end-to-end feedback about packet losses, but this is not the
case for many of the applications currently using UDP. In addition,
the current Congestion Manager does not easily support multiple
congestion control mechanisms, or lend itself to the use of forms of
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TFRC where the state about past packet drops or marks is maintained
at the receiver rather than at the sender. DCCP should be able to
make use of CM where desired by the application, but we do not see
any benefit in making the deployment of DCCP contingent on the
deployment of CM itself.
3. Conventions 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. Numbers and Fields
All multi-byte numerical quantities in DCCP, such as port numbers,
Sequence Numbers, and arguments to options, are transmitted in
network byte order (most significant byte first).
We occasionally refer to the "left" and "right" sides of a bit
field. "Left" means towards the most significant bit, and "right"
means towards the least significant bit.
Reserved bitfields in DCCP packet headers MUST be ignored by
receivers, and MUST be set to zero by senders, unless otherwise
specified.
Random numbers in DCCP are used for their security properties, and
MUST be chosen according to the guidelines in [RFC 1750].
3.2. Parts of a Connection
Each DCCP connection runs between two endpoints, which we often name
DCCP A and DCCP B.
DCCP connections are actively initiated by one endpoint. The active
endpoint is called the client, and the passive endpoint is called
the server.
DCCP connections are bidirectional; data may pass from either
endpoint to the other. This means that data and acknowledgements
may be flowing in both directions simultaneously. Logically,
however, a DCCP connection consists of two separate unidirectional
connections, called half-connections. Each half-connection consists
of the data packets sent by one endpoint and the corresponding
acknowledgements sent by the other endpoint. We can illustrate this
as follows:
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+--------+ A-to-B half-connection: +--------+
| | --> data packets --> | |
| | <-- acknowledgements <-- | |
| DCCP A | | DCCP B |
| | B-to-A half-connection: | |
| | <-- data packets <-- | |
+--------+ --> acknowledgements --> +--------+
Although they are logically distinct, in practice the half-
connections overlap; a DCCP-DataAck packet, for example, contains
application data relevant to one half-connection and acknowledgement
information relevant to the other.
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.
3.3. Features
A feature is a DCCP connection attribute, identified by a feature
number and an endpoint, on whose value the two endpoints agree.
Many properties of a DCCP connection are controlled by features,
including the congestion control mechanisms in use on the two half-
connections, whether mobility is allowed, and whether ECN is
supported. The endpoints can achieve agreement by out-of-band
communication, or through the exchange of feature negotiation
options in DCCP headers.
The notation F/A represents the feature with feature number F
located at DCCP endpoint A; the feature F/B has the same feature
number, but is located at the other endpoint. Both DCCP A and
DCCP B know, and agree on, the values of both F/A and F/B, but F/A
and F/B may have different values.
DCCP A is called the feature location for all features F/A, and the
feature remote for all features F/B.
3.4. Round-Trip Times
We sometimes refer to a round-trip time for setting timers, for
example. If no useful round-trip time estimate is available, a DCCP
implementation SHOULD use 0.2 seconds instead.
3.5. Robustness Principle
DCCP implementations should follow TCP's "general principle of
robustness": be conservative in what you do, be liberal in what you
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accept from others.
4. Overview
DCCP's high-level connection dynamics should seem familiar to anyone
who knows TCP. DCCP connections, like TCP connections, progress
through three phases: initiation (including a three-way handshake),
data transfer, and termination. Data can flow both ways over the
connection. An acknowledgement framework lets senders discover how
much data has been lost; congestion control uses this information to
avoid unfairly congesting the network. Of course, DCCP provides
unreliable datagram semantics, not TCP's reliable bytestream
semantics. The application must package its data into explicit
frames, and must retransmit its own data as necessary. It may be
useful to think of DCCP either as TCP minus bytestream semantics and
reliability, or as UDP plus congestion control, handshakes, and
acknowledgements.
4.1. Packet Types
DCCP uses eleven packet types to implement various protocol
functions. For example, every new connection attempt begins with a
DCCP-Request packet sent by the client. A DCCP-Request packet thus
resembles a TCP SYN; but DCCP-Request is a packet type, not a flag,
so there's no way to send an unexpected combination such as TCP's
SYN+FIN+ACK+RST.
Eight packet types occur during the progress of a typical
connection---two only during the initiation phase, three during the
data transfer phase, and three only during the termination phase:
Client Server
------ ------
(1) Initiation
DCCP-Request -->
<-- DCCP-Response
DCCP-Ack -->
(2) Data transfer
DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
(3) Termination
<-- DCCP-CloseReq
DCCP-Close -->
<-- DCCP-Reset
Note the three-way handshakes during initiation and termination.
The three remaining packet types are used for special purposes: when
an endpoint moves, or to resynchronize after bursts of loss.
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Every DCCP packet starts with a common, 12-byte generic header, but
different packet types may include different amounts of additional
data. For example, the DCCP-Ack packet type includes an
Acknowledgement Number. Every packet type may also contain options,
up to around 1000 bytes' worth.
All of the packet types are described below.
DCCP-Request
Sent by the client to initiate a connection (the first part of
the three-way handshake).
DCCP-Response
Sent by the server in response to a DCCP-Request (the second
part of the three-way handshake).
DCCP-Data
Used to transmit data.
DCCP-Ack
Used for pure acknowledgements.
DCCP-DataAck
Used for piggybacked data-plus-acknowledgements.
DCCP-CloseReq
Sent by the server to request that the client close the
connection.
DCCP-Close
Used to close the connection; elicits a DCCP-Reset in response.
DCCP-Reset
Used to terminate the connection, either normally or abnormally.
DCCP-Move
Supports multihoming and mobility.
DCCP-Sync, DCCP-SyncAck
Used to resynchronize sequence numbers after large bursts of
loss.
4.2. Sequence Numbers
Each DCCP packet carries a sequence number, so that losses can be
detected and reported. But unlike TCP's byte-based sequence
numbers, DCCP sequence numbers are attached to packets. Each packet
sent increments the sequence number by one. For example:
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DCCP A DCCP B
------ ------
DCCP-Data(seqno 1) -->
DCCP-Data(seqno 2) -->
<-- DCCP-Ack(seqno 10, ackno 2)
DCCP-DataAck(seqno 3, ackno 10) -->
<-- DCCP-Data(seqno 11)
Note that even DCCP-Ack pure acknowledgements increment the sequence
number; after the DCCP-Ack with sequence number 10, the following
DCCP-Data packet uses the next sequence number, 11. This lets the
endpoints tell when acknowledgements are lost in the network. It
also means that endpoints can get out of sync after a long burst of
loss. The DCCP-Sync and DCCP-SyncAck packet types let DCCP recover
from large loss bursts; see Section 7.5.
Also note that, since DCCP is an unreliable protocol, there are no
retransmissions, and it doesn't make sense to have a cumulative
acknowledgement field. Acknowledgement Number (ackno) fields equal
the largest sequence number received, rather than the TCP-style
smallest sequence number not received. Separate options indicate
any intermediate sequence numbers that weren't received.
4.3. States
DCCP endpoints progress through different states during the course
of a connection, corresponding roughly to the three phases of
initiation, data transfer, and termination. The figure below shows
the typical progress through these states for a client and server.
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Client Server
------ ------
(0) No connection
CLOSED LISTEN
(1) Initiation
REQUEST DCCP-Request -->
<-- DCCP-Response RESPOND
PARTOPEN DCCP-Ack or DCCP-DataAck -->
(2) Data transfer
OPEN <-- DCCP-Data, Ack, DataAck --> OPEN
(3) Termination
<-- DCCP-CloseReq CLOSEREQ
CLOSING DCCP-Close -->
<-- DCCP-Reset CLOSED
TIMEWAIT
CLOSED
The client and server's typical progress through states.
The states are as follows; Section 8 describes them in more detail.
CLOSED
Represents a nonexistent connection.
LISTEN
Represents a server socket in the passive listening state.
LISTEN and CLOSED are not associated with any particular DCCP
connection.
REQUEST
The client socket enters this state, from CLOSED, after sending
a DCCP-Request packet to try to initiate a connection.
RESPOND
A server socket enters this state, from LISTEN, after receiving
a DCCP-Request from a client.
PARTOPEN
The client socket enters this state, from REQUEST, after
receiving a DCCP-Response from the server. This state
represents the third phase of the three-way handshake. The
client may send data in this state, but it MUST include an
Acknowledgement Number on all of its packets.
OPEN
The central, data transfer portion of a DCCP connection. Client
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and server enter into this state from PARTOPEN and RESPOND,
respectively. Sometimes we speak of SERVER-OPEN and CLIENT-OPEN
states, corresponding to the server's OPEN state and the
client's OPEN state.
CLOSEREQ
A server socket enters this state, from SERVER-OPEN, to signal
that the connection is over, but the client must hold TIMEWAIT
state.
CLOSING
Either server or client can enter this state to close the
connection.
TIMEWAIT
A socket remains in this state for 2MSL after the connection has
been torn down, to prevent mistakes due to the delivery of old
packets. One MSL, or Maximum Segment Lifetime, is the maximum
length of time a packet could survive in the network.
4.4. Congestion Control
DCCP connections are congestion controlled. Unlike TCP, however,
DCCP supports multiple congestion control mechanisms for
applications to choose from. In fact, the two half-connections can
be governed by different mechanisms. Each mechanism corresponds to
a one-byte congestion control identifier, or CCID. A CCID 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.
The endpoints negotiate their CCIDs during connection initiation.
So far, CCIDs 2 and 3 have been defined for use with DCCP; CCID 0 is
reserved, and CCID 1 is used for special purposes (see Section
10.1).
CCID 2 corresponds to TCP-like Congestion Control, which is similar
to that of TCP. The sender maintains a congestion window and sends
packets until that window is full. Packets are acknowledged by the
receiver. Dropped packets and ECN [RFC 3168] are indicate
congestion; the response to congestion is to halve the congestion
window. Acknowledgements in CCID 2 contain the sequence numbers of
all received packets within some window, similar to a super
selective-acknowledgement (SACK, [RFC 3517]).
CCID 3 provides TFRC Congestion Control, an equation-based form of
congestion control which is intended to provide a smoother response
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to congestion than CCID 2. The sender maintains a "transmit rate".
The receiver sends acknowledgement packets containing information
about the receiver's estimate of packet loss. The sender uses this
information to update its transmit rate. Although CCID 3 behaves
somewhat differently from TCP in its short term congestion response,
it is designed to operate fairly with TCP over the long term.
The behaviors of CCIDs 2 and 3 are fully defined in separate profile
documents [CCID 2 PROFILE] [CCID 3 PROFILE].
4.5. Features
Agreement on DCCP feature values is achieved by explicit
negotiation, using options in DCCP packet headers. This generally
happens at connection startup, but negotiation can begin at any
time. The relevant options are Change L, Confirm L, Change R, and
Confirm R, with the "L" options sent by the feature location and the
"R" options sent by the feature remote.
A Change R message says to the peer, "change this feature value on
your side". The peer responds with a Confirm L, meaning "I've
changed it". The suggested option setting in Change R can sometimes
contain multiple values, which are sorted in preference order. For
example:
Client Server
------ ------
Change R(CCID, 2) -->
<-- Confirm L(CCID, 2)
* agreement that CCID/Server = 2 *
Change R(CCID, 3 4) -->
<-- Confirm L(CCID, 4, 4 2)
* agreement that CCID/Server = 4 *
In the second exchange, the client requests that the server use
either CCID 3 or CCID 4, with 3 preferred. The server chooses 4,
giving its preference list of "4 2".
A party that wants to change a feature located at itself issues a
"Change L" option, which elicits a "Confirm R" in reply.
Client Server
------ ------
<-- Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2) -->
* agreement that CCID/Server = 3 *
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In this example, the server requests CCID value 3 or 2 for the
server's CCID, with 3 preferred, and the client agrees.
Retransmissions make feature negotiation reliable. Section 6
describes these options further.
4.6. Other Differences from TCP
Interesting differences between DCCP and TCP, apart from those
discussed so far, include:
o Copious space for options (up to 1020 bytes).
o Different acknowledgement formats. The CCID for a connection
determines how much ack information needs to be transmitted. In
CCID 2 (TCP-like), this is about one ack per 2 packets, and each
ack must declare exactly which packets were received; in CCID 3
(TFRC), it's about one ack per RTT, and acks must declare at
minimum just the lengths of recent loss intervals.
o Denial-of-service (DoS) protection. Several DCCP mechanisms
attempt to let servers limit the amount of state possibly-
misbehaving clients can force them to maintain. An Init Cookie
option, analogous to TCP's SYN Cookies [SYNCOOKIES], avoids SYN-
flood-like attacks. Only one connection endpoint need hold
TIMEWAIT state; the DCCP-CloseReq packet, which may only be sent
by the server, passes that state to the client. Various rate
limits let servers avoid attacks that might force extensive
computation or packet generation.
o Distinguishing different kinds of loss. A Data Dropped option
(Section 11.7) lets an endpoint declare that a packet was dropped
because of corruption, because of receive buffer overflow, and so
on. This facilitates research into more appropriate rate-control
responses for these non-network-congestion losses (although
currently all losses will cause a congestion response).
o Acknowledgement readiness. In TCP, a packet is acknowledged only
when the data is queued for delivery to the application. This
does not make sense in DCCP, where an application might request a
drop-from-front receive buffer, for example. We acknowledge a
packet when its options have been processed. The Data Dropped
option may later say that the packet's payload was discarded.
o Integrated support for mobility and multihoming via the DCCP-Move
packet type.
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o No receive window. DCCP is a congestion control protocol, not a
flow control protocol.
o No simultaneous open. Every connection has one client and one
server.
o No half-closed states. DCCP has no states corresponding to TCP's
FINWAIT and CLOSEWAIT, where one half-connection is explicitly
closed while the other is still active.
4.7. Example Connection
The progress of a typical DCCP connection is as follows. (This
description is informative, not normative.)
Client Server
------ ------
0. [CLOSED] [LISTEN]
1. DCCP-Request -->
2. <-- DCCP-Response
3. DCCP-Ack -->
<-- DCCP-Ack
4. DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
5. <-- DCCP-CloseReq
6. DCCP-Close -->
7. <-- DCCP-Reset
8. [TIMEWAIT]
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
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one in the DCCP-Response. It may also continue feature
negotiation. There might follow 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.
4. 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.
5. The server sends a DCCP-CloseReq packet requesting a close.
6. The client sends a DCCP-Close packet acknowledging the close.
7. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state. In DCCP, unlike TCP,
Resets are part of normal connection termination; see Section
5.6.
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.
An alternative connection closedown sequence is initiated by the
client:
5b. The client sends a DCCP-Close packet closing the connection.
6b. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state.
7b. 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.
5. Header Formats
The variable-length DCCP header appears first in every DCCP packet.
A header can be from 12 to 1020 bytes long. The initial 12 bytes of
the header are the same regardless of packet type. Following this
comes optional additional fixed-length fields, depending on the
packet type, and then a variable-length list of options. Finally,
some packet types include application data.
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+---------------------------------------+ -.
| Generic Header | |
+---------------------------------------+ |
| Additional Fields (depending on type) | +- DCCP Header
+---------------------------------------+ |
| Options (optional) | |
+=======================================+ -'
| Application Data (optional) |
+=======================================+
5.1. Generic Header
The DCCP generic header generally takes 12 bytes.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |X| Res | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Actually, there are two types of generic header, depending on the
value of X, the Extended Sequence Numbers bit. If X is zero, the
Sequence Number field takes 24 bits, as above. If X is one, the
Sequence Number field extends for an additional 24 bits, for a total
of 48:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |1| Res | Sequence Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) | Reserved |T|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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Destination Port the relevant port on the other endpoint.
Source Ports SHOULD be chosen randomly, to reduce the likelihood
of attack.
Data Offset: 8 bits
The offset from the start of the DCCP header to the beginning of
the packet's application data, in 32-bit words.
CCVal: 4 bits
Used by the HC-Sender CCID. For example, the A-to-B CCID's
sender, which is active at DCCP A, MAY send 4 bits of
information per packet to its receiver by encoding that
information in CCVal. CCVal MUST be set to zero unless the HC-
Sender CCID specifies a different value.
Checksum Coverage (CsCov): 4 bits
Checksum Coverage specifies what parts of the packet are covered
by the Checksum field. This always includes the DCCP header and
options, but if applications request it, some or all of the
application data may be excluded. This can improve performance
on noisy links, assuming the application can tolerate
corruption. See Section 9.
Checksum: 16 bits
The Internet checksum of the packet's DCCP header (including
options), a network-layer pseudoheader, and, depending on
Checksum Coverage, some or all of the application data. See
Section 9.
Type: 4 bits
The Type field specifies the type of the packet. The following
values are defined:
Type Meaning
---- -------
0 DCCP-Request
1 DCCP-Response
2 DCCP-Data
3 DCCP-Ack
4 DCCP-DataAck
5 DCCP-CloseReq
6 DCCP-Close
7 DCCP-Reset
8 DCCP-Move
9 DCCP-Sync
10 DCCP-SyncAck
11-15 Reserved
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Extended Sequence Numbers (X): 1 bit
This bit is set to one to indicate the use of an extended
generic header with 48-bit Sequence and Acknowledgement Numbers.
Very-high-rate connections SHOULD set X to one, and use 48-bit
sequence numbers, to gain increased protection against wrapped
sequence numbers and attacks. See Section 7.6.
Reserved (Res): 3 bits
The version of DCCP specified here MUST ignore this field on
received packets, and MUST set it to all zeroes on generated
packets.
Sequence Number: 24 or 48 bits
Identifies the packet uniquely in the sequence of all packets
the source sent on this connection. Sequence Number increases
by one with every packet sent, including packets such as DCCP-
Ack that carry no application data. See Section 7.
Sequence Number Transition (T): 1 bit [X=1 only]
Set to one to indicate an ongoing transition from 24-bit to
48-bit sequence numbers. See Section 7.6.
Many packet types also carry an Acknowledgement Number in the four
or eight bytes immediately following the generic header. When X=0,
its format is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
And when X=1:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Acknowledgement Number (low bits) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledgement Number: 24 or 48 bits
The Acknowledgement Number field generally acknowledges the
greatest valid sequence number received so far on this
connection. ("Greatest" is, of course, measured in circular
sequence space.) Acknowledgement numbers make no attempt to
provide precise information about which packets have arrived;
options such as the Ack Vector do this.
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Reserved: 8 bits
The version of DCCP specified here MUST ignore these fields on
received packets, and MUST set them to all zeroes on generated
packets.
5.2. DCCP-Request Header
A client initiates a DCCP connection by sending a DCCP-Request
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 or 16 bytes) /
/ with Type=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Application Data |
| ... |
Service Code: 32 bits
Describes the service to which the client application wants to
connect. Examples might include RTSP and DOOM. Service Codes
are intended to make application protocols independent of well-
known ports, and help middleboxes identify the protocol used on
a given connection. See Section 8.1.2.
5.3. DCCP-Response Header
The server responds to valid DCCP-Request packets with DCCP-Response
packets. This is the second phase of the three-way handshake.
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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 or 16 bytes) /
/ with Type=1 (DCCP-Response) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Application Data |
| ... |
Acknowledgement Number: 24 or 48 bits
The Acknowledgement Number field will generally equal the
Sequence Number from the DCCP-Request.
Service Code: 32 bits
Echoes the Service Code on the DCCP-Request.
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers
The central data transfer portion of every DCCP connection uses
DCCP-Data, DCCP-Ack, and DCCP-DataAck packets. DCCP-Data packets
carry application 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 or 16 bytes) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Application Data |
| ... |
DCCP-Ack packets dispense with the data, but contain an
Acknowledgement Number. They are used for pure acknowledgements.
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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 or 16 bytes) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-DataAck packets carry both application 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 or 16 bytes) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Application Data |
| ... |
DCCP-Data and DCCP-DataAck packets may contain zero application data
bytes if the application sends a zero-length datagram. Also, a
DCCP-Ack packet need not have a zero-length application data area.
The receiver MUST ignore any "application data" in a DCCP-Ack
packet. The sender will not generally send such data, but it may
occasionally do so---to perform PMTU discovery without risking loss
of user data, for example.
DCCP-Ack and DCCP-DataAck packets often include additional
acknowledgement options, such as Ack Vector, as required by the
congestion control mechanism in use.
5.5. DCCP-CloseReq and DCCP-Close Headers
DCCP-CloseReq and DCCP-Close packets begin the handshake that
normally terminates a connection. Either client or server may send
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a DCCP-Close packet, which will elicit a DCCP-Reset packet (see the
next section). Only the server can send a DCCP-CloseReq packet,
which indicates that the server wants to close the connection, but
does not want to hold its TIMEWAIT state.
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 or 16 bytes) /
/ with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The receiver MUST ignore any "application data" in a DCCP-CloseReq
or DCCP-Close packet.
5.6. DCCP-Reset Header
DCCP-Reset packets unconditionally shut down a connection.
Connections normally terminate 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.
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 or 16 bytes) /
/ with Type=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reset Code | Data 1 | Data 2 | Data 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Error Text |
| ... |
Reset Code: 8 bits
Represents the reason that the sender reset the DCCP connection.
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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.
Error Text (application data area)
If present, Error Text is a human-readable text string,
preferably in English and encoded in Unicode UTF-8, that
describes the error in more detail. For example, a DCCP-Reset
with Reset Code 12, "Aggression Penalty", might contain Error
Text such as "Aggression Penalty: Received 3 bad ECN Nonce
Echoes, assuming misbehavior".
The following Reset Codes are currently defined. The "Data" columns
describe what the Data fields contain for a given Code. N/A means
the Data field MUST be set to 0 by the sender of the DCCP-Reset and
ignored by its receiver.
Reset Section
Code Name Data 1 Data 2 Data 3 Reference
----- ---- ------ ------ ------ ---------
0 Unspecified N/A N/A N/A
1 Closed N/A N/A N/A 8.3
2 Aborted N/A N/A N/A 8.1.1
3 No Connection N/A N/A N/A 8.3.1
4 Packet Error packet N/A N/A 8.3.1
type
5 Option Error option option data
number (if any)
6 Mandatory Error option option data 5.9.2
number (if any)
7 Extended Seqnos N/A N/A N/A 7.6
8 Connection Refused N/A N/A N/A 8.1.3
9 Bad Service Code N/A N/A N/A 8.1.3
10 Too Busy N/A N/A N/A 8.1.3
11 Bad Init Cookie N/A N/A N/A 8.1.4
12 Aggression Penalty N/A N/A N/A 12.2
13 Move Refused N/A N/A N/A 14.4
13-127 Reserved
128-255 CCID-specific codes ... variable ... 10.4
5.7. DCCP-Move Header
The DCCP-Move packet type is part of DCCP's support for multihoming
and mobility, which is described further in Section 14. 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
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packets to a new address and port number, which are read off the
packet's network header and generic DCCP header. The old address
and port are defined through a Mobility ID, which provides some
protection against hijacked connections.
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 or 16 bytes) /
/ with Type=8 (DCCP-Move) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobility ID (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Mobility ID (bits 64-95) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Mobility ID (bits 32-63) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Mobility ID (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Mobility ID: 128 bits
The value of the receiver's Mobility ID feature. This value
uniquely identifies the current connection among the set of
connections terminating at the receiver (meaning, the stationary
endpoint); it MUST have been set in an earlier exchange. See
Section 14.2.
The receiver MUST ignore any "application data" in a DCCP-Move
packet.
5.8. DCCP-Sync and DCCP-SyncAck Headers
DCCP-Sync packets help DCCP endpoints recover synchronization after
bursts of loss, or recover from half-open connections. Each valid
DCCP-Sync received immediately elicits a DCCP-SyncAck.
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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 or 16 bytes) /
/ with Type=9 (DCCP-Sync) or 10 (DCCP-SyncAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number |
(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
(. Acknowledgement Number (low bits) | Reserved |)X=1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options / Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Acknowledgement Number on DCCP-Sync and DCCP-SyncAck packets
need not equal the generating endpoint's greatest valid sequence
number received (GSR). This differs from Acknowledgement Numbers on
all other packet types. If a DCCP-Sync was generated in response to
a packet with invalid sequence numbers, then the DCCP-Sync's
Acknowledgement Number will equal the invalid packet's sequence
number. The Acknowledgement Number on any DCCP-SyncAck packet MUST
correspond to a received, valid DCCP-Sync's Sequence Number; in the
presence of reordering, this might not equal GSR.
The receiver MUST ignore any "application data" in a DCCP-Sync or
DCCP-SyncAck packet.
5.9. Options
All DCCP packets may contain options, which occupy space at the end
of the DCCP header. Each option is a multiple of 8 bits in length.
The combination of all options MUST add up to a multiple of 32 bits.
Individual options are not padded to multiples of 32 bits, however;
any option may begin on any byte boundary. All options are always
included in the checksum.
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.
Options are processed sequentially, starting at the first option in
the packet header.
The following options are currently defined:
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Option Section
Type Length Meaning Reference
---- ------ ------- ---------
0 1 Padding 5.9.1
1 1 Mandatory 5.9.2
2 1 Slow Receiver 11.6
3-31 1 Reserved
32 variable Change L 6.1
33 variable Confirm L 6.2
34 variable Change R 6.1
35 variable Confirm R 6.2
36 variable Init Cookie 8.1.4
37 4-5 NDP Count 7.7
38 variable Ack Vector [Nonce 0] 11.4
39 variable Ack Vector [Nonce 1] 11.4
40 variable Data Dropped 11.7
41 6 Timestamp 13.1
42 6-10 Timestamp Echo 13.3
43 4-6 Elapsed Time 13.2
44 4 Data Checksum 9.3
45-127 variable Reserved
128-255 variable CCID-specific options 10.4
This section describes two generic options, Padding and Mandatory.
Other options are described later.
5.9.1. Padding Option
The Padding option, with type 0, is a single byte option used to pad
between or after options. It either ensures the application data
begins on a 32-bit boundary (as required), or ensures alignment of
following options (not mandatory).
+--------+
|00000000|
+--------+
Type=0
5.9.2. Mandatory Option
The Mandatory option, with type 1, is a single byte option that
indicates that the immediately following option is mandatory. If
the receiving DCCP does not understand that following option, it
MUST reset the connection, generally using Reset Code 6, "Mandatory
Failure". For instance, say DCCP A receives a packet with two
options: a Mandatory option, and immediately following, another
option O. Then DCCP A would reset the connection if it did not
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understand O's type; if it understood O's type, but not O's data; if
O's data was invalid for O's type; if O was a feature negotiation
option, and DCCP A did not understand the enclosed feature number;
if DCCP A understood O, but chose not to perform the action O
implies; and so forth. Section 6.6.8 describes the behavior of
Mandatory feature negotiation options in more detail.
+--------+
|00000001|
+--------+
Type=1
6. Feature Negotiation
Four DCCP options, Change L, Confirm L, Change R, and Confirm R,
implement in-band feature negotiation. Change options initiate a
negotiation; Confirm options complete that negotiation. The "L"
options are sent by the feature location, and the "R" options are
sent by the feature remote. Change options are retransmitted to
ensure reliability.
All these options have the same format. The first byte of option
data is the feature number, and the second and subsequent data bytes
hold one or more feature values. The feature values are generally
arranged in a linear preference list, where the first value is most
preferred.
+--------+--------+--------+--------+--------
| Type | Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Together, the feature number and the option type ("L" or "R")
uniquely identify the feature to which an option applies. The exact
format of the Value(s) area depends on the feature number.
6.1. Change Options
Change L and Change R options initiate feature negotiation. Either
endpoint can start a negotiation for any feature; if DCCP A wants to
start a negotiation for feature F/A, it will send a Change L option,
while to start a negotiation for F/B, it will send a Change R
option. Change options are retransmitted until some response is
received. Normal Change options contain at least one Value, and
thus have length at least 4.
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+--------+--------+--------+--------+--------
Change L: |00100000| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=32
+--------+--------+--------+--------+--------
Change R: |00100010| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=34
The endpoint may check a feature's current value without attempting
to change it by sending an empty Change option, containing just the
feature number. Such options have length 3. The endpoints must
agree on feature values anyway, so these options are useful in
practice only in special situations, such as when a middlebox
introduced in the middle of a connection wants to check a feature
value.
6.2. Confirm Options
Confirm L and Confirm R options complete feature negotiation, and
are sent in response to Change R and Change L options, respectively.
Confirm options MUST NOT be generated except in response to Change
options. Confirm options need not be retransmitted, since Change
options are retransmitted as necessary. Normal Confirm options
contain the selected Value, possibly followed by the sender's
preference list.
+--------+--------+--------+--------+--------
Confirm L: |00100001| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=33
+--------+--------+--------+--------+--------
Confirm R: |00100011| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=35
If an endpoint receives an invalid Change option -- with an unknown
feature number, or an invalid value -- it will respond with an empty
Confirm option containing no value. Such options have length 3.
6.3. Reconciliation Rules
Reconciliation rules determine how the two sets of preferences for a
given feature are resolved into a unique result. The reconciliation
rule depends only on the feature number. Each reconciliation rule
must have the property that the result is uniquely determined given
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the contents of Change options sent by the two endpoints.
All current DCCP features use one of two reconciliation rules,
server-priority ("SP") and non-negotiable ("NN").
6.3.1. Server-Priority
The feature value is a fixed-length byte string (length determined
by the feature number). Each Change option contains a preference
list of values, with the most preferred value coming first. Each
Confirm option contains the confirmed value, followed by the
confirmer's preference list. Thus, the feature's current value will
generally appear twice in Confirm options' data, once as the current
value and once in the confirmer's preference list. Even responses
to empty Change options contain the whole preference list.
To reconcile the preference lists, select the first entry in the
server's list that also occurs in the client's list. If there is no
shared entry, the feature's value MUST NOT change, and the Confirm
option will confirm the feature's previous value (unless the Change
option was Mandatory; see Section 6.6.8).
DCCP endpoints need not calculate their value preference lists
before feature negotiation begins. Thus, a server might adjust its
preference list based on the client's preference list, assuming the
client opened the negotiation. Once a negotiation for a feature has
begun, however, the preference lists MUST remain stable until the
negotiation has closed.
6.3.2. Non-Negotiable
The feature value is a byte string. Each option contains exactly
one feature value. The feature location signals a value change by
sending Change L options. The feature remote MUST accept any valid
value, responding with a Confirm R option containing the new value,
and it MUST send empty Confirm R options in response to invalid
values. Non-negotiable features aren't really negotiated; they use
feature negotiation as a mechanism for achieving reliability.
Change R and Confirm L options MUST NOT be sent for non-negotiable
features.
6.4. Feature Numbers
This document defines the following feature numbers.
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Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
0 Reserved
1 Congestion Control ID (CCID) SP 2 Y 10
2 ECN Capable SP 1 Y 12.1
3 Sequence Window NN 100 Y 7.5.4
4 Sequence Transition Capable SP 0 N 7.6.4
5 Mobility Capable SP 0 N 14.1
6 Mobility ID NN 0 N 14.2
7 Ack Ratio NN 2 N 11.3
8 Send Ack Vector SP 0 N 11.5
9 Send NDP Count SP 0 N 7.7.2
10 Check Data Checksum SP 0 N 9.3.1
11-127 Reserved
128-255 CCID-specific features ? ? ? 10.4
Rec'n Rule The reconciliation rule used for the feature. SP is
server-priority and NN is non-negotiable.
Initial Value The initial value for the feature. Every feature has
a known initial value.
Req'd This column is "Y" iff every DCCP implementation MUST
understand the feature. If it is "N", then the
feature behaves like an extension (see Section 16),
and it is safe to respond to Change options for the
feature with empty Confirm options. Of course, a
CCID might require the feature; a DCCP that
implements CCID 2 MUST support Ack Ratio and Send Ack
Vector, for example.
6.5. Examples
Here are three example feature negotiations for features located at
the server, the first two for the Congestion Control ID feature, the
last for the Ack Ratio:
Client Server
1. Change R(CCID, 2 3 1) -->
("2 3 1" is client's value preference list)
2. <-- Confirm L(CCID, 3, 3 2 1)
(3 is the negotiated value;
"3 2 1" is server's pref list)
* agreement that CCID/Server = 3 *
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1. XXX <-- Change L(CCID, 3 2 1)
2. Retransmission:
<-- Change L(CCID, 3 2 1)
3. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *
1. <-- Change L(Ack Ratio, 3)
2. Confirm R(Ack Ratio, 3) -->
* agreement that Ack Ratio/Server = 3 *
This example shows a simultaneous negotiation.
Client Server
1a. Change R(CCID, 2 3 1) -->
b. <-- Change L(CCID, 3 2 1)
(both endpoints in CHANGING)
2a. <-- Confirm L(CCID, 3, 3 2 1)
b. Confirm R(CCID, 3, 2 3 1) -->
(both endpoints in STABLE)
* agreement that CCID/Server = 3 *
Example Change and Confirm options follow, with their byte
encodings. Each option is sent by DCCP A.
Change L(CCID, 2 3) = 32,5,1,2,3
I want to change CCID/A's value (feature number 1, a server-
priority feature); my preferred values are 2 and 3, in that
preference order.
Change L(Sequence Window, 1024) = 32,6,3,0,4,0
Change Sequence Window/A's value (feature number 3, a non-
negotiable feature) to the 3-byte string 0,4,0 (the value 1024).
Empty Change L(CCID) = 32,3,1
Tell me CCID/A's value using a Confirm R option.
Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3
I've changed CCID/A's value to 2; my preferred values are 2 and
3, in that preference order.
Empty Confirm L(126) = 33,3,126
I don't implement feature number 126, or your proposed value for
feature 126/A was invalid.
Change R(CCID, 3 2) = 34,5,1,3,2
Please change CCID/B's value; my preferred values are 3 and 2,
in that preference order.
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Empty Change R(CCID) = 34,3,1
Tell me CCID/B's value using a Confirm L option.
Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2
I've changed CCID/B's value to 2; my preferred values were 3 and
2, in that preference order.
Confirm R(Sequence Window, 1024) = 35,6,3,0,4,0
I've changed Sequence Window/B's value to the 3-byte string
0,4,0 (the value 1024).
Empty Confirm R(126) = 35,3,126
I don't implement feature number 126, or your proposed value for
feature 126/B was invalid.
6.6. Option Exchange
A few basic rules govern feature negotiation option exchange.
1. Every non-reordered Change option gets a Confirm option in
response.
2. Change options are retransmitted until some response is
received.
3. Preference lists don't change during a negotiation.
4. Feature negotiation options are processed in strictly increasing
order by Sequence Number.
The rest of this section describes the consequences of these rules
in more detail.
6.6.1. Normal Exchange
Change options are generated when a DCCP endpoint wants to change
the value of some feature. Generally, this will happen at the
beginning of a connection, although it may happen at any time. We
say the endpoint "generates" or "sends" a Change L or Change R
option; but, of course, the option must be attached to a packet.
The endpoint may attach the option to a packet it would have
generated anyway (such as a DCCP-Request), or it may create a new
packet just to carry the options (often a DCCP-Sync). If it does
create a new packet, it MUST NOT create more than one such packet
per round-trip time (or 0.2 seconds, if no RTT is available).
On receiving a Change L or Change R option, a DCCP endpoint examines
the included preference list, reconciles that with its own
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preference list, calculates the new value, and sends back a
Confirm R or Confirm L option, respectively, informing its partner
of the new value. The rule for reconciling the two preference lists
is feature-specific; see Section 6.3. Every non-reordered Change
option MUST result in a corresponding Confirm option. Any packet
including a Confirm option MUST carry an Acknowledgement Number;
thus, Confirm options are not allowed on DCCP-Request and DCCP-Data
packets. Again, generated Confirm options may be attached to
packets that would have been sent anyway (such as DCCP-Response or
DCCP-SyncAck), or to new packets (usually DCCP-Ack).
The Change-sending endpoint MUST wait to receive a corresponding
Confirm option before changing its stored feature value. The
Confirm-sending endpoint changes its stored feature value as soon as
it sends the Confirm.
DCCP endpoints effectively exist in one of two states, STABLE and
CHANGING, relative to each feature. STABLE is the normal state,
where the endpoint knows the feature's value and thinks the other
endpoint agrees. An endpoint enters the CHANGING state when it
first sends a Change for the feature, and returns to STABLE once it
receives a corresponding Confirm.
6.6.2. Loss and Retransmission
Packets containing Change and Confirm options might be lost or
delayed by the network. Therefore, Change options are retransmitted
to achieve reliability.
A CHANGING endpoint retransmits a Change option once it realizes
that it has not heard back from the other endpoint. Each
retransmitted Change option MUST contain exactly the same payload as
the original. The endpoint may piggyback its Change options on
packets it would have sent anyway. If it generates new packets for
feature negotiation, it MUST use an exponential-backoff timer. The
timer's initial value is set to approximately one or two round-trip
times (or 0.2-0.4 seconds, if no RTT is available), and it is pinned
at roughly 32 RTTs.
A CHANGING endpoint MUST continue retransmitting Change options
until it gets some response. Its only recourse is to reset the
connection, which it SHOULD NOT do until at least 12 transmissions
have failed.
Change options SHOULD NOT be transmitted more frequently than once
per RTT, or the reordering protection below would prevent any
Confirm option from being accepted (since no Confirm would
acknowledge the most recently transmitted Change).
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Confirm options are never retransmitted, but the Confirm-sending
endpoint MUST generate a new Confirm option for every non-reordered
Change it receives.
6.6.3. Reordering
Reordering might cause packets containing Change and Confirm options
to arrive in an unexpected order. Endpoints MUST be robust to
reordering, by ignoring feature negotiation options that do not
arrive in strictly-increasing order by Sequence Number.
The most straightforward way to implement this requirement is for an
endpoint to associate two sequence number variables with every
feature F/X, as follows.
F/X.GSR The Greatest Sequence Number Received from the other
endpoint on a packet containing a Change or Confirm option
for feature F/X.
F/X.GSS The Greatest Sequence Number Sent by this endpoint on a
packet containing a Change option for feature F/X.
Then DCCP A will check options relating to feature F/A as follows:
1. Ignore any received Change R(F) option whose packet's Sequence
Number is not greater than F/A.GSR.
2. Ignore any received Confirm R(F) option whose packet's Sequence
Number is not greater than F/A.GSR, or whose packet could not
have acknowledged F/A.GSS. Specifically, if the Acknowledgement
Number is less than F/A.GSS, the endpoint MUST ignore the
Confirm; and if the packet has an Ack Vector indicating that
F/A.GSS was not received, the endpoint MAY ignore the Confirm.
A similar procedure applies options relating to feature F/B, namely
Change L(F) and Confirm L(F), except that F/B.GSR and F/B.GSS are
checked.
A less state-intensive way to implement this requirement would be to
share the F.GSR and F.GSS variables among all features, rather than
keeping one pair per feature. Then the feature negotiation options
on any received packet would be treated as a unit (either all
accepted or all rejected).
Checking Confirm options is easier if the endpoint only sends Change
options on packet types that will be acknowledged immediately,
namely DCCP-Request, DCCP-Response, and DCCP-Sync. Then there is
never any need to check Ack Vectors, although checking Ack Vectors
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is NOT MANDATORY anyway.
6.6.4. Preference Changes
Endpoints MUST NOT change their preference lists in the middle of a
negotiation. This is because, if a preference list changed in the
middle of a negotiation and the right packets were lost, the
negotiation could terminate with the endpoints thinking the feature
had different values. In particular, an endpoint MUST NOT change
its preference list while in the CHANGING state; this ensures that
every Change option sent during that negotiation will contain the
same data.
6.6.5. Simultaneous Negotiation
The two endpoints might simultaneously open negotiation for the same
feature, after which an endpoint in the CHANGING state will receive
a Change option for the same feature. Such received Change options
can act as responses to the original Change options. The CHANGING
endpoint MUST examine the received Change's preference list,
reconcile that with its own preference list (as expressed in its
generated Change options), and generate the corresponding Confirm
option. It can then transition to the STABLE state.
6.6.6. Unknown Features
An endpoint may receive a Change option referring to some feature
number it does not understand. This is particularly likely to
happen when an extended DCCP converses with a non-extended DCCP.
The receiving endpoint MUST respond to such Change options with
corresponding empty Confirm options (that is, Confirm options
containing no data), which inform the CHANGING endpoint that the
feature was not understood. However, if the Change option was
preceded by a Mandatory option, the connection MUST be reset; see
Section 6.6.8.
On receiving an empty Confirm option for some feature, the CHANGING
endpoint MUST transition back to the STABLE state, leaving the
feature's value unchanged. Section 16 suggests that the default
value for any extension feature should correspond to "extension not
available".
An endpoint will also send an empty Confirm option when it
understood the Change's feature number, but considered the Change's
value invalid or inappropriate for the feature. The next section
describes this further.
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Some features are required to be understood by all DCCPs (see
Section 6.4); the CHANGING endpoint SHOULD reset the connection
(with Reset Code 5, "Option Error") if it receives an empty Confirm
option for such a feature.
Since Confirm options are generated only in response to Change
options, an endpoint should never receive a Confirm option referring
to a feature number it does not understand. Endpoints MUST either
reset the connection on receiving such options, or just ignore the
options.
6.6.7. Invalid Options
A DCCP endpoint might receive a Change or Confirm option that lists
one or more values that it does not understand. Some, but not all,
such options are invalid, depending on the relevant reconciliation
rule (Section 6.3). For instance:
o All features have length limitiations, and options with invalid
lengths are invalid. For example, the Mobility ID feature takes
128-bit values, so valid "Confirm R(Mobility ID)" options have
option length 19.
o Some non-negotiable features have value limitations. The Ack
Ratio feature takes two-byte, non-zero integer values, so a
"Change L(Ack Ratio, 0)" option is never valid. Note that server-
priority features do not have value limitations, since unknown
values are handled as a matter of course.
o Any Confirm option that selects the wrong value, based on the two
preference lists and the relevant reconciliation rule, is invalid.
An endpoint receiving an invalid Change option MUST respond with the
corresponding empty Confirm option. An endpoint receiving an
invalid Confirm option MUST reset the connection, with Reset Code 5,
"Option Error".
6.6.8. Mandatory Feature Negotiation
Change options may be preceded by Mandatory options (Section 5.9.2).
Mandatory Change options are processed like normal Change options,
except that various failure cases will cause the receiver to reset
the connection with Reset Code 6, "Mandatory Failure", rather than
send a Confirm option. Specifically, the connection MUST be reset
if:
o The Change option's feature number was not understood;
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o The Change option's value was invalid, and the receiver would
normally have sent an empty Confirm option in response; or
o For server-priority features, there was no shared entry in the two
endpoints' preference lists.
There's no reason to mark Confirm options as Mandatory in this
version of DCCP, since Confirm options are sent only in response to
Change options and therefore can't mention potentially-invalid
values or unexpected feature numbers.
6.6.9. Out-of-Band Agreement
An endpoint MUST NOT unilaterally change the value of any DCCP
feature. However, endpoints MAY cooperatively change DCCP feature
values without using in-band feature negotiation options---by using
a separate signalling channel, for example.
6.6.10. State Diagram
This diagram illustrates feature-related state transitions, ignoring
sequence number and option validity issues, for the endpoint that is
the feature location. For a feature remote state transition
diagram, switch the "L"s and "R"s.
rcv Confirm R app/protocol evt : snd Change L
: ignore +--------------------------------------------+
+----+ | |
| v | rcv Change R v
+------------+ rcv Confirm R : calc new value, +------------+
| | : accept value snd Confirm L | |
| STABLE |<------------------------------------| CHANGING |
| | rcv empty Confirm R | |
+------------+ : revert to old value +------------+
| ^ | ^
+----+ +----+
rcv Change R timeout/rcv non-ack
: calc new value, snd Confirm L : snd Change L
This state diagram corresponds to the following procedure for
reacting to received packets with feature negotiation options. The
procedure refers to "P.seqno", "P.ackno", "P.optiontype", and
"P.optionlen", which are properties of the packet; "F.GSR" and
"F.GSS", which are the variables mentioned in Section 6.6.3;
"F.state", which is the feature's state (STABLE or CHANGING); and
"F.value", which is the feature's value.
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If F.state == STABLE:
If P.optiontype == Change R && P.seqno > F.GSR:
Calculate new value
Send Confirm L on next packet
F.GSR := P.seqno
Otherwise:
Ignore option
If F.state == CHANGING:
If P.optiontype == Confirm R && P.ackno >= F.GSS
&& P potentially acknowledges F.GSS:
If P.optionlen == 3:
/* empty Confirm R option */
Retain old value
Otherwise:
Check new value
F.value := new value
F.state := STABLE
Otherwise, if P.optiontype == Change R && P.seqno > F.GSR:
Calculate new value
Send Confirm L on next packet
F.GSR := P.seqno
Otherwise:
Ignore option
7. Sequence Numbers
DCCP uses 24- or 48-bit sequence numbers to arrange packets into
sequence, detect losses and network duplicates, and protect against
attackers, half-open connections, and the delivery of very old
packets. Every packet carries a Sequence Number; most packet types
carry an Acknowledgement Number as well.
DCCP sequence numbers are per-packet. Thus, each endpoint
increments the DCCP Sequence Number field by one (modulo 2^24 or
2^48) with every packet sent. Even DCCP-Ack and DCCP-Sync packets,
and other packets that don't carry user data, increment the Sequence
Number. Since DCCP is an unreliable protocol, there are no true
retransmissions; but effective retransmissions, such as
retransmissions of DCCP-Request packets, also increment the Sequence
Number. This lets DCCP implementations detect network duplication,
retransmissions, and acknowledgement loss, and is a significant
departure from TCP practice.
7.1. Variables
DCCP endpoints maintain a set of sequence number variables for each
connection.
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ISS The Initial Sequence Number Sent by this endpoint. This
equals the Sequence Number of the first DCCP-Request or
DCCP-Response sent.
ISR The Initial Sequence Number Received from the other
endpoint. This equals the Sequence Number of the first
DCCP-Request or DCCP-Response received.
GSS The Greatest Sequence Number Sent by this endpoint.
("Greatest" is of course measured in circular sequence
space.)
GSR The Greatest Sequence Number Received from the other
endpoint on an acknowledgeable packet. (Section 7.4 defines
"acknowledgeable" packets.)
GAR The Greatest Acknowledgement Number Received from the other
endpoint on an acknowledgeable packet.
Some other variables are derived from these primitives.
SWL and SWH
(Sequence Number Window Low and High) The extremes of the
validity window for received packets' Sequence Numbers.
AWL and AWH
(Acknowledgement Number Window Low and High) The extremes
of the validity window for received packets' Acknowledgement
Numbers.
7.2. Initial Sequence Numbers
The endpoints' initial sequence numbers are set by the first DCCP-
Request and DCCP-Response packets sent. Initial sequence numbers
MUST be chosen to avoid two problems:
o Delivery of old packets, where packets lingering in the network
from an old connection are delivered to a new connection with the
same addresses and port numbers.
o Sequence number attacks, where an attacker can guess the sequence
numbers that a future connection would use [M85].
DCCP implementations may use TCP's strategies for avoiding these
problems [RFC 793] [RFC 1948].
To address the first problem, an implementation MUST ensure that the
initial sequence number for a given <source address, source port,
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destination address, destination port> 4-tuple doesn't overlap with
recent sequence numbers on connections with the same 4-tuple
("recent" meaning sent within 2 maximum segment lifetimes). If the
implementation has state for a recent connection with the same
4-tuple, it can simply pick a good initial sequence number;
otherwise, it could tie initial sequence number selection to some
clock, such as the 4-microsecond clock used by TCP [RFC 793].
To address the second problem, an implementation MUST provide each
4-tuple with an independent initial sequence number space; then an
attacker can't learn anything about anyone else's initial sequence
numbers. RFC 1948 achieves this by adding a cryptographic hash, of
the 4-tuple and a secret, to any initial sequence number. For the
secret, RFC 1948 recommends a combination of some truly-random data
[RFC 1750], an administratively-installed passphrase, the endpoint's
IP address, and the endpoint's boot time, but truly-random data is
sufficient. Care should be taken when changing the secret; such a
change alters all initial sequence number spaces, which might make
an initial sequence number for some 4-tuple equal a recently sent
sequence number for the same 4-tuple. To avoid this problem around
such a change, the endpoint might remember dead connection state for
each 4-tuple or stay quiet for 2 maximum segment lifetimes.
7.3. Quiet Time
DCCP endpoints, like TCP endpoints, must take care before initiating
connections when they boot. In particular, they MUST NOT send
packets whose sequence numbers are close to the sequence numbers of
packets lingering in the network from before the boot. The simplest
way to enforce this rule is for DCCP endpoints to avoid sending any
packets until one maximum segment lifetime (2 minutes) after boot.
Other enforcement mechanisms include remembering recent sequence
numbers across boots, or reserving the upper 8 or so bits of initial
sequence numbers for a persistent boot counter that decrements by
two each boot (this would require the use of extended sequence
numbers).
7.4. Acknowledgement Numbers
DCCP has no cumulative acknowledgement field; cumulative
acknowledgements would be meaningless in an unreliable protocol.
Therefore, the Acknowledgement Number field has a different meaning
in DCCP than in TCP.
A packet is classified as "acknowledgeable" if and only if its
options were processed by the receiving DCCP. This means, for
example, that all acknowledgeable packets have valid header
checksums and sequence numbers. The Acknowledgement Number for most
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packet types MUST equal GSR, the Greatest Sequence Number Received
on an acknowledgeable packet.
Note that "acknowledgeable" refers to option processing, not data
processing. Even acknowledgeable packets may have their application
data dropped, due to receive buffer overflow or corruption, for
instance. Data Dropped options report these data losses when
necessary, letting congestion control mechanisms distinguish between
network losses and endpoint losses. This issue is discussed further
in Sections 11.4 and 11.7.
DCCP-Sync and DCCP-SyncAck packets are a special case to this rule.
The Acknowledgement Number on a DCCP-Sync packet corresponds to a
received packet, but not necessarily an acknowledgeable packet; in
particular, it might correspond to an out-of-sync packet whose
options were not processed. The Acknowledgement Number on a DCCP-
SyncAck packet always corresponds to an acknowledgeable DCCP-Sync
packet; if there was reordering, that Acknowledgement Number might
be less than GSR.
7.5. Validity and Synchronization
Any DCCP endpoint might receive packets that are not actually part
of the current connection. For instance, the network might deliver
an old packet, an attacker might attempt to hijack a connection, or
the other endpoint might crash, causing a half-open connection.
DCCP, like TCP, uses sequence number checks to detect these cases
Packets whose Sequence and/or Acknowledgement Numbers are out of
range are called sequence-invalid, and are not processed normally.
Unlike TCP, DCCP requires a synchronization mechanism to recover
from large bursts of loss. One endpoint might send so many packets
during a burst of loss that when one of its packets finally got
through, the other endpoint would label its Sequence Number as
invalid. A handshake involving DCCP-Sync and DCCP-SyncAck packets
recovers from this case.
7.5.1. Sequence-Validity Rules
Sequence-validity depends on the received packet's type. This table
shows the sequence and acknowledgement number checks applied to each
packet; a packet is sequence-valid if it passes both tests, and
sequence-invalid if it does not. Many of the checks refer to the
sequence and acknowledgement number windows, [SWL, SWH] and [AWL,
AWH], defined below in Section 7.5.3.
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Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Request SWL <= seqno <= SWH (*) N/A
DCCP-Response SWL <= seqno <= SWH (*) AWL <= ackno <= AWH
DCCP-Data SWL <= seqno <= SWH N/A
DCCP-Ack SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-DataAck SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-CloseReq SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-Close SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-Reset seqno == 0 or seqno > GSR GAR <= ackno <= AWH
DCCP-Move seqno >= SWL ISS <= ackno <= AWH
DCCP-Sync seqno >= SWL AWL <= ackno <= AWH
DCCP-SyncAck seqno >= SWL AWL <= ackno <= AWH
(*) Check not applied if connection is in LISTEN or REQUEST state.
In general, packets are sequence-valid if their Sequence and
Acknowledgement Numbers lie within the corresponding valid windows,
[SWL, SWH] and [AWL, AWH]. The exceptions to this rule are as
follows:
o DCCP-Reset Sequence Numbers may be zero. This is because during
the cleanup of a half-open connection, an endpoint might generate
a DCCP-Reset in response to a DCCP-Request or DCCP-Data packet
with no Acknowledgement Number; the resetting endpoint would then
use zero for the Reset's Sequence Number, since it has no valid
Sequence Number available.
DCCP-Reset Acknowledgement Numbers, and non-zero Sequence Numbers,
are checked more stringently than those on other packet types,
however. This is because DCCP-Reset always ends a connection: no
endpoint will send a non-Reset packet on a connection after it has
sent a Reset. Thus, a Reset packet whose Sequence Number is less
than GSR, or whose Acknowledgement Number is less than GAR, must
be sequence-invalid.
o DCCP-Move Sequence and Acknowledgement Numbers are not strongly
checked because moves might likely happen after long loss periods,
and the mandatory Mobility ID provides good protection against
unexpected packets.
o DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly
checked. These packet types exist specifically to get the
endpoints back into sync after bursts of loss; checking their
Sequence Numbers would eliminate their usefulness.
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These lenient checks all allow continued operation after unusual
events, such as endpoint crashes and large bursts of loss. There's
no need for leniency when the endpoints are actively sending packets
to one another. Therefore, a DCCP endpoint SHOULD implement the
following, tighter constraints for active connections. An endpoint
considers a connection active if it has received valid packets from
the other endpoint within the last several round-trip times, or
1 second, if the RTT is not known.
Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Reset GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Move SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-Sync SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno <= SWH AWL <= ackno <= AWH
Note that sequence-validity is only one of the validity checks
applied to received packets.
7.5.2. Handling Sequence-Invalid Packets
Sequence-invalid DCCP-Move, DCCP-Reset, DCCP-Sync, and DCCP-SyncAck
packets MUST be ignored.
When DCCP A receives any other sequence-invalid packet, it MUST
reply with a DCCP-Sync packet. This packet MUST acknowledge the
packet's Sequence Number (not GSR!). The DCCP-Sync MUST use a new
Sequence Number, and thus will increase GSS; GSR will not change,
however, since the received packet was sequence-invalid. DCCP A
MUST NOT otherwise process sequence-invalid packets. For instance,
it MUST NOT process their options.
When the DCCP B endpoint receives the (sequence-valid) DCCP-Sync, it
MUST update its GSR variable and reply with a DCCP-SyncAck packet
acknowledging the DCCP-Sync (not necessarily GSR!). Upon receiving
this DCCP-SyncAck, which will be sequence-valid since it
acknowledges the DCCP-Sync, DCCP A will update its GSR variable, and
the endpoints will be back in sync. Alternatively, if the
connection was half-open (DCCP B is in CLOSED or REQUEST state),
DCCP B will send a Reset.
A DCCP endpoint MAY temporarily preserve sequence-invalid packets in
case they become valid later. This can reduce the impact of bursts
of loss by delivering more packets to the application. In
particular, an endpoint MAY preserve a sequence-invalid packet for
up to 2 round-trip times (or 1 second, if the RTT is unknown); if,
within that time, the relevant sequence windows change so that the
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packet becomes sequence-valid, the endpoint MAY process the packet
again.
To protect itself against denial-of-service attacks (where an
attacker sends many sequence-invalid packets, trying to force the
receiver to send many DCCP-Syncs), a DCCP implementation MAY rate-
limit the DCCP-Syncs sent in response to sequence-invalid packets.
7.5.3. Sequence and Acknowledgement Number Windows
Each DCCP endpoint defines sequence validity windows that are
subsets of the Sequence and Acknowledgement Number spaces. These
windows correspond to packets the endpoint expects to receive in the
next few round-trip times. The Sequence and Acknowledgement Number
windows always contain GSR and GSS, respectively; the window widths
are controlled by Sequence Window features.
The Sequence Number validity window for packets from DCCP B is [SWL,
SWH]. This window always contains GSR, the Greatest Sequence Number
Received on a sequence-valid packet from DCCP B. It is W packets
wide, where W is the value of the Sequence Window/B feature. One-
fourth of the sequence window, rounded down, is placed at and before
GSR, with three-fourths after GSR. (This asymmetric placement
assumes that bursts of loss are more common in the network than
significant reordering.)
invalid | valid Sequence Numbers | invalid
<---------*|*===========*=======================*|*--------->
GSR -|GSR + 1 - GSR GSR +|GSR + 1 +
floor(W/4)|floor(W/4) ceil(3W/4)|ceil(3W/4)
= SWL = SWH
The Acknowledgement Number validity window for packets from DCCP B
is [AWL, AWH]. The high end of the window, AWH, always equals GSS,
the Greatest Sequence Number Sent by DCCP A; the window is W'
packets wide, where W' is the value of the Sequence Window/A
feature.
invalid | valid Acknowledgement Numbers | invalid
<---------*|*===================================*|*--------->
GSS - W'|GSS + 1 - W' GSS|GSS + 1
= AWL = AWH
SWL and AWL are initially adjusted so that they don't go below the
initial Sequence Numbers received and sent, respectively:
SWL := max(GSR + 1 - floor(W/4), ISR),
AWL := max(GSS - W' + 1, ISS).
Of course, these adjustments MUST NOT be applied after the relevant
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sequence numbers wrap.
7.5.4. Sequence Window Feature
The Sequence Window/A feature determines the width of the Sequence
Number validity window used by DCCP B, and the width of the
Acknowledgement Number validity window used by DCCP A. DCCP A sends
a "Change L(Sequence Window, W)" option to notify DCCP B that the
Sequence Window/A value is W.
Sequence Window has feature number 3, and is non-negotiable. It
takes 3- or 6-byte integer values, like DCCP sequence numbers.
Change and Confirm options for Sequence Window are therefore either
6 or 9 bytes long. New connections start with Sequence Window 100
for both endpoints.
A proper Sequence Window/A value should reflect how many packets
DCCP A expects to be in flight. Only DCCP A can anticipate this
number. 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. (The next section quantifies this risk.) One
good guideline is for each endpoint to set Sequence Window to a
small multiple of the maximum number of packets it expects to send
in a round-trip time. This value may not be available at connection
initiation, when the round-trip time is unknown, but the endpoint
can always send updates as the connection progresses.
7.5.5. Sequence Number Attacks
Sequence and Acknowledgement Numbers form DCCP's main line of
defense against attackers. An attacker that cannot guess sequence
numbers cannot easily manipulate or hijack a DCCP connection, and
requirements like careful initial sequence number choice eliminate
the most serious attacks.
An attacker might still send many packets with randomly chosen
Sequence and Acknowledgement Numbers, however. If one of those
probes ends up sequence-valid, it may shut down the connection or
otherwise cause problems. The easiest such attacks to execute are:
o Send DCCP-Sync packets with random Sequence and Acknowledgement
Numbers. If one of these packets hits the valid acknowledgement
number window, the receiver will shift its sequence number window
accordingly, getting out of sync with the correct
endpoint---perhaps permanently.
o Send DCCP-Reset packets with Sequence Number zero and random
Acknowledgement Numbers. If one of these packets hits the valid
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acknowledgement number window, the connection will be shut down.
o Send DCCP-Data packets with random Sequence Numbers. If one of
these packets hits the valid sequence number window, the attack
packet's application data may be inserted into the data stream.
The attacker has to guess both Source and Destination Ports for any
of these attacks to succeed. Additionally, the connection would
have to be inactive for the DCCP-Sync and DCCP-Reset packets to
succeed, assuming the victim implemented the more stringent checks
for active connections recommended in Section 7.5.1.
To quantify the probability of success, let N be the number of
attack packets the attacker is willing to send, W be the relevant
sequence window width, and L be the length of sequence numbers (24
or 48). The attacker's best strategy is to space the attack packets
evenly over sequence space. Then one of these attacks will succeed
with probability P = WN/2^L. For N = 1000, W = 100, and L = 24,
this probability is about 0.006. (For reference, the easiest TCP
attack---sending a SYN with a random sequence number, which will
cause a connection reset if it falls within the window---will
succeed with probability 0.002 for N = 1000, W = 8760 [a common
default], and L = 32.) Connections with sequence windows much
larger than 100 SHOULD use extended sequence numbers to reduce the
probability of attack success.
7.5.6. Examples
In the following example, DCCP A and DCCP B recover from a large
burst of loss that runs DCCP A's sequence numbers out of DCCP B's
appropriate sequence number window.
Recovery from Burst of Loss
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
--> DCCP-Data(seq 2) XXX
...
--> DCCP-Data(seq 100) XXX
--> DCCP-Data(seq 101) --> ???
seqno out of range;
send Sync
OK <-- DCCP-Sync(seq 11, ack 101) <--
(GSS=11,GSR=1)
--> DCCP-SyncAck(seq 102, ack 11) --> OK
(GSS=102,GSR=11) (GSS=11,GSR=102)
In the next example, a DCCP connection recovers from a simple
attack. The attacker cannot guess sequence numbers. (DCCP is not
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robust to attackers who can guess sequence numbers.)
Recovery from Attack
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
*ATTACKER* --> DCCP-Data(seq 10^6) --> ???
seqno out of range;
send Sync
??? <-- DCCP-Sync(seq 11, ack 10^6) <--
ackno out of range; ignore
(GSS=1,GSR=10) (GSS=11,GSR=1)
The final example demonstrates recovery from a half-open connection.
Recovery from a Half-Open Connection
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
(Crash)
CLOSED OPEN
REQUEST --> DCCP-Request(seq 400) --> ???
!! <-- DCCP-Sync(seq 11, ack 400) <-- OPEN
REQUEST --> DCCP-Reset(seq 401, ack 11) --> (Abort)
REQUEST CLOSED
REQUEST --> DCCP-Request(seq 402) --> ...
7.6. Extended Sequence Numbers
Extended 48-bit sequence numbers increase the rate DCCP connections
can achieve without wrapping sequence numbers, and provide
additional protection against the sequence number attacks described
above. Very-high-rate DCCP connections, and connections with large
sequence windows, SHOULD therefore use extended sequence numbers
rather than the default 24-bit sequence numbers.
7.6.1. When to Use Extended Sequence Numbers
The sequence-validity mechanism protects against the network
delivering old data, but it assumes that the network does not
deliver extremely old data. In particular, it assumes that the
network must have dropped any packet by the time the connection
wraps around and uses its sequence number again. We can easily
calculate the maximum connection rate that can be safely achieved
given this constraint. Let MSL equal the maximum segment lifetime,
P equal the average DCCP packet size in bits, and L equal the length
of sequence numbers (24 or 48 bits). Then the maximum safe rate, in
bits per second, is R = P*(2^L)/2MSL.
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For the default MSL of 2 minutes, 1500-byte DCCP packets, and 24-bit
sequence numbers, the safe rate is therefore approximately 800 Mb/s.
Of course, 2 minutes is a very large MSL for any networks that could
sustain that rate with such small packets. Nevertheless, 48-bit
sequence numbers allow much higher rates, up to 14 petabits a second
for 1500-byte packets and the default MSL.
The probability of sequence number attack success P = WN/2^L,
discussed in Section 7.5.5, may also be relevant when deciding
whether to use extended sequence numbers. A fast connection will
generally have a relatively high W (sequence window size),
increasing the attack success probability for fixed N (number of
attack packets); if the probability gets uncomfortably high with L =
24, the connection should use 48-bit sequence numbers instead.
7.6.2. Header Processing
Extended sequence numbers are activated when the header's X bit is
set to one (see Section 5.1). This extends the Sequence Number and
Acknowledgement Number fields by an additional 24 bits, for a total
of 48 bits. The 48-bit numbers are stored in network order, with
most significant bit first. All packet types except for DCCP-Data
and DCCP-Request will follow this generic header with an extended
48-bit Acknowledgement Number.
Once an endpoint has transitioned to 48-bit sequence numbers (X=1),
it MUST send all succeeding packets with 48-bit sequence numbers.
Furthermore, once an endpoint has received a sequence-valid packet
with 48-bit sequence numbers, it MUST either send all succeeding
packets with 48-bit sequence numbers, or reset the connection with
Reset Code 7, "Extended Sequence Numbers". (But note that an
endpoint may send extended DCCP-Sync packets before transitioning to
extended sequence numbers.)
Clients SHOULD decide whether to use extended sequence numbers
before sending their DCCP-Requests. However, the Transition bit (T)
and Sequence Transition Capable feature support transitioning to
extended sequence numbers during an active connection, in case this
proves necessary; see below. A client that sends an extended DCCP-
Request might receive a DCCP-Reset in response with Reset Code 7,
"Extended Sequence Numbers"; the client SHOULD respond by sending
another Request using 24-bit sequence numbers.
Extended sequence numbers are treated simply as longer sequence
numbers. For instance, the sequence-validity mechanisms work the
same way whether or not sequence numbers are extended. Care is
required when comparing a 24-bit sequence number with an 48-bit
sequence number, however; see the next section.
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7.6.3. Transitioning to Extended Sequence Numbers
The Transition bit (T) following the extended Sequence Number field
makes it possible to transition to 48-bit sequence numbers in the
middle of a connection. T is set to one only during such a
transition. When DCCP A switches to 48-bit sequence numbers, it
MUST set the T bit to one on all of its packets for some period.
This period SHOULD last on the order of a few round trip times, or
until DCCP A receives an acknowledgement from DCCP B proving that
one of its 48-bit-sequence-number packets has been received,
whichever comes later.
Each DCCP MUST choose its first 48-bit sequence number to have its
lower 24 bits equal the 24-bit sequence number it expected to send
(GSS+1). The upper 24 bits may be chosen arbitrarily. This applies
to Acknowledgement Numbers as well as Sequence Numbers; if DCCP A
sends an extended packet containing an Acknowledgement Number before
DCCP B sends it a 48-bit Sequence Number, DCCP A can choose any
value for the upper 24 bits of the Acknowledgement Number, but the
lower 24 bits MUST equal the expected 24-bit Acknowledgement Number
(GSR). Furthermore, DCCP A MUST leave GSR as a 24-bit number until
receiving an extended packet from DCCP B.
Switching to 48-bit sequence numbers in the middle of a connection
complicates sequence number comparison. Endpoints must compare
48-bit sequence numbers with 24-bit sequence numbers, and compare
48-bit sequence numbers that might have different, arbitrary values
in the upper 24 bits, while remaining robust to reordering and to
old or malicious packets. The following procedure describes how
sequence numbers should be compared during and immediately after a
transition.
Let P be the packet sequence number received from DCCP B, and E be
the sequence number DCCP A expects. During sequence-validity
computations, for example, P might be the packet's Acknowledgement
Number and E might be AWL, the left edge of the appropriate
acknowledgement number window. Then DCCP A should perform the
comparison as follows.
o If P and E are both 24 bits, compare them modulo 2^24.
o If P and E are both 48 bits, you generally compare them modulo
2^48, except that during a transition, the two values might have
arbitrary values in the upper 24 bits.
- If the packet's Transition bit is set, and the last packet sent
by DCCP A had its Transition bit set, then compare P and E
modulo 2^24.
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- Otherwise, compare them modulo 2^48.
o If P is 48 bits but E is 24, the remote DCCP may want to
transition to extended sequence numbers.
- If the packet's Transition bit is set, compare P with E modulo
2^24. If the packet proves sequence-valid, then it is OK;
transition to extended sequence numbers, and set E according to
the full 48 bits of P.
- Otherwise, the packet is sequence-invalid.
Either way, if the packet proves to be sequence-invalid, send an
extended DCCP-Sync if required (with T set to one), but do not yet
transition to extended sequence numbers.
o If P is 24 bits but E is 48, there may have been benign packet
reordering. The correct action depends on whether the last
sequence-valid packet received from DCCP B had the Transition bit
set.
- If Transition was set, extend P to a 48-bit value P'. First,
let EH equal the upper 24 bits of E, and EL equal the lower 24
bits of E. Then:
If EL > P, set P' = (EH << 24) | P.
Otherwise, set P' = (((EH - 1) mod 2^24) << 24) | P.
The "EL > P" test uses arithmetic comparison, NOT circular
comparison. Compare P' with E modulo 2^48.
- Otherwise, the packet is sequence-invalid.
Either way, if the packet proves to be sequence-invalid, send an
extended DCCP-Sync if required, with T set to one.
DCCP implementations can, of course, avoid most of this complexity
by disallowing transitions to extended sequence numbers (and by
resetting the connection when the other endpoint attempts such a
transition). Connections that use 48-bit sequence numbers
throughout, starting with the DCCP-Request, MUST have T set to zero
on all their packets.
7.6.4. Sequence Transition Capable Feature
The Sequence Transition Capable feature expresses whether DCCP
endpoints are capable of transitioning to extended sequence numbers
in the course of an active connection. DCCP A sends a
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"Change R(Sequence Transition Capable, 1)" option to DCCP B to
discover whether B can transition to extended sequence numbers.
Sequence Transition Capable has feature number 4, and is server-
priority. It takes one-byte Boolean values. DCCP B MUST allow
transitions to extended sequence numbers when Sequence Transition
Capable/B is one. It MUST NOT reset the connection with Reset Code
7, "Extended Sequence Numbers", under those circumstances. However,
DCCP B MAY allow such transitions even when Sequence Transition
Capable/B is zero. Values of two or more are reserved. New
connections start with Sequence Transition Capable 0 (that is, not
capable) for both endpoints.
7.7. NDP Count and Detecting Application Loss
DCCP's sequence numbers increment by one on every packet, including
non-data packets (packets that don't carry application data). This
makes DCCP sequence numbers suitable for detecting any network loss,
but not for detecting the loss of application data. The NDP Count
option reports the length of each burst of non-data packets. This
lets the receiving DCCP determine, for every burst of loss, whether
or not application data was lost.
+--------+--------+-------- ... --------+
|00100101| Length | NDP Count |
+--------+--------+-------- ... --------+
Type=37 Len=3-5
If a DCCP endpoint's Send NDP Count feature is one (see below), then
that endpoint MUST send an NDP Count option on every packet whose
immediate predecessor was a non-data packet. Non-data packets
consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
DCCP-Reset, DCCP-Move, DCCP-Sync, and DCCP-SyncAck. All other
packet types are considered data packets, although not all DCCP-
Request and DCCP-Response packets will actually carry application
data.
The value stored in NDP Count equals the number of consecutive non-
data packets in the run immediately previous to the current packet.
Packets with no NDP Count option are considered to have NDP Count
zero.
The NDP Count option can carry one to three bytes of data. The
smallest option format that can hold the NDP Count SHOULD be used.
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7.7.1. Usage Notes
Say that K consecutive sequence numbers are missing in some burst of
loss, and the Send NDP Count feature is on. Then some application
data was lost within those sequence numbers unless the packet
following the hole contains an NDP Count option whose value is
greater than or equal to K.
For example, say that the following sequence of non-data packets
(Nx) and data packets (Dx) were sent.
N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
Those packets would have NDP Counts as follows.
N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
- 1 2 - 1 - - 1 - - - - 1 2
NDP Count is not useful for applications that include their own
sequence numbers with their packet headers.
7.7.2. Send NDP Count Feature
The Send NDP Count feature lets DCCPs negotiate whether they should
send NDP Count options on their packets. DCCP A sends a
"Change R(Send NDP Count, 1)" option to ask DCCP B to send NDP Count
options.
Send NDP Count has feature number 9, and is server-priority. It
takes one-byte Boolean values. DCCP B MUST send NDP Count options
on its non-data packets (and some of its data packets) when Send NDP
Count/B is one, although it MAY send NDP Count options even when
Send NDP Count/B is zero. Values of two or more are reserved. New
connections start with Send NDP Count 0 for both endpoints.
8. Event Processing
This section describes how DCCP connections move between states, and
which packets are sent when. Note that feature negotiation takes
place in parallel with the connection-wide state transitions
described here.
8.1. Connection Establishment
DCCP connections' initiation phase consists of a three-way
handshake: an initial DCCP-Request packet sent by the client, a
DCCP-Response sent by the server in reply, and finally an
acknowledgement from the client, usually via a DCCP-Ack or DCCP-
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DataAck packet. The client moves from the REQUEST state to
PARTOPEN, and finally to OPEN; the server moves from LISTEN to
RESPOND, and finally to OPEN.
Client State Server State
CLOSED LISTEN
1. REQUEST --> Request -->
2. <-- Response <-- RESPOND
3. PARTOPEN --> Ack, DataAck -->
4. <-- Data, Ack, DataAck <-- OPEN
5. OPEN <-> Data, Ack, DataAck <-> OPEN
8.1.1. Client Request
When a client decides to initiate a connection, it enters the
REQUEST state, chooses an initial sequence number (Section 7.2), and
sends a DCCP-Request packet using that sequence number to the
intended server.
DCCP-Request packets will commonly carry feature negotiation options
that open negotiations for various connection parameters, such as
preferred congestion control IDs for each half-connection. They may
also carry application data, but the client should be aware that the
server may not accept such data.
A client in the REQUEST state SHOULD send new DCCP-Request packets
after some timeout if no response is received. The retransmission
strategy SHOULD be similar to that for retransmitting TCP SYNs; for
instance, a first timeout on the order of a second, with an
exponential backoff timer. Each new DCCP-Request MUST increment the
Sequence Number by one, and MUST contain the same Service Code and
application data as the original DCCP-Request.
A client MAY give up after some number of DCCP-Requests. If so, it
SHOULD send a DCCP-Reset packet to the server with Reset Code 2,
"Aborted", to clean up state in case one or more of the Requests
actually arrived.
The client leaves the REQUEST state for PARTOPEN when it receives a
DCCP-Response from the server.
8.1.2. Service Codes
Each DCCP-Request contains a 32-bit Service Code, which identifies
the service to which the client application is trying to connect.
Service Codes should correspond to application services and
protocols. For example, there might be a Service Code for HTTP
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connections, one for FTP control connections, and one for FTP data
connections. Middleboxes, such as firewalls, can use the Service
Code to identify the application running on a nonstandard port
(assuming the DCCP header has not been encrypted).
Endpoints MUST associate a Service Code with every DCCP socket, both
actively and passively opened. The application will generally
supply this Service Code. Each active socket MUST have exactly one
Service Code, while passive sockets MAY have more than one; this
might let multiple applications listen on the same port,
differentiated by Service Code. If the DCCP-Request's Service Code
doesn't match any of the server's Service Codes for the given port,
the server MUST reject the request by sending a DCCP-Reset packet
with Reset Code 9, "Bad Service Code". A middlebox MAY also send
such a DCCP-Reset in response to packets whose Service Code is
considered unsuitable.
Service Codes should be allocated by IANA. We intend for Service
Code allocation to be allocated to anyone who asks, first-come
first-serve, subject to the following guidelines.
o Service Codes should be allocated one at a time, or in small
blocks. A short English description of the intended service is
required to obtain a Service Code assignment, but no
specification, standards-track or otherwise, is necessary. IANA
should maintain an association of Service Codes to the
corresponding phrases.
o Users may request specific Service Code values, which should be
assigned first-come first-serve. We suggest that users request
Service Codes that can be interpreted as meaningful four-byte
ASCII strings. Thus, the "Frobodyne Plotz Protocol" might
correspond to "fdpz", or the number 1717858426. The canonical
interpretation of a Service Code field is numeric.
o Service Codes whose bytes each have values in the set {32, 45-57,
65-90} should be reserved for international standard or standards-
track specifications, IETF or otherwise. (This set consists of
the ASCII digits, uppercase letters, and characters space, '-',
'.', and '/'.)
o Service Codes whose high-order byte equals 63 (ASCII '?') should
never be allocated. These Service Codes are reserved for private
use.
o Service Code 0 should never be allocated. It represents the
absence of a meaningful Service Code.
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This design for Service Code allocation is based on the allocation
of 4-byte identifiers for Macintosh resources, PNG chunks, and
TrueType and OpenType tables.
8.1.3. Server Response
In the second phase of the three-way handshake, the server moves
from the LISTEN state to RESPOND, and sends a DCCP-Response message
to the client. In this phase, a server will often specify the
features 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.
The receiver MAY respond to a DCCP-Request packet with a DCCP-Reset
packet to refuse the connection. Relevant Reset Codes for refusing
a connection include 8, "Connection Refused", when the DCCP-
Request's Destination Port did not correspond to a DCCP port open
for listening; 9, "Bad Service Code", when the DCCP-Request's
Service Code did not correspond to the service code registered with
the Destination Port; and 10, "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. (Note that the
"retransmitted" DCCP-Request will have, at least, a different
sequence number from the "original" DCCP-Request; the receiver can
thus distinguish true retransmissions from network duplicates.) 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 while the server is in the RESPOND
state MUST elicit a new DCCP-Response. Each new DCCP-Response MUST
increment the responder's Sequence Number by one, and MUST include
the same application data, if any, as the original DCCP-Response.
The responder MUST accept at most one piece of DCCP-Request data per
connection. In particular, the DCCP-Response sent in reply to a
retransmitted DCCP-Request with data SHOULD contain a Data Dropped
option, in which the retransmitted DCCP-Request is reported as "data
dropped due to protocol constraints" (Drop Code 0). The original
DCCP-Request SHOULD also be reported in the Data Dropped option,
either in a Normal Block (if the responder accepted the data, or
there was no data), or in a Drop Code 0 Drop Block (if the responder
refused the data the first time as well).
The Data Dropped and Init Cookie options are particularly useful for
DCCP-Response packets (Sections 11.7 and 8.1.4).
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The server leaves the RESPOND state for OPEN when it receives a
valid DCCP-Ack from the client, completing the three-way handshake.
8.1.4. Init Cookie Option
+--------+--------+--------+--------+--------+--------
|00100100| Length | Init Cookie Value ...
+--------+--------+--------+--------+--------+--------
Type=36
The Init Cookie option lets a DCCP server avoid having to hold any
state until the three-way connection setup handshake has completed.
The server wraps up the service code, 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. In the
meantime, it need not move from the LISTEN state.
This option is permitted in DCCP-Response, DCCP-Data, DCCP-Ack,
DCCP-DataAck, DCCP-Sync, and DCCP-SyncAck packets. The server MAY
include an Init Cookie option in its DCCP-Response. If so, then the
client MUST echo the same Init Cookie option in each succeeding DCCP
packet until one of those packets is acknowledged, meaning the
three-way handshake has completed, or the connection is reset. The
server SHOULD design its Init Cookie format so that Init Cookies can
be checked for tampering; it SHOULD respond to a tampered Init
Cookie option by resetting the connection with Reset Code 11, "Bad
Init Cookie".
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.
Init Cookies are limited to at most 253 bytes in length.
8.1.5. Handshake Completion
When the client receives a DCCP-Response from the server, it moves
from the REQUEST state to PARTOPEN, and completes three-way
handshake by sending a DCCP-Ack packet to the server. The PARTOPEN
state represents that the client isn't sure whether the server has
received any of its DCCP-Acks. The client MUST NOT send DCCP-Data
packets while it remains in PARTOPEN. This is because DCCP-Data
packets lack Acknowledgement Numbers, so the server can't tell from
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a DCCP-Data packet whether the client saw its DCCP-Response.
Furthermore, if the DCCP-Response included an Init Cookie, that Init
Cookie MUST be included on every packet sent in PARTOPEN.
The single DCCP-Ack sent when entering the PARTOPEN state might, of
course, be dropped by the network. The client SHOULD ensure that
some packet gets through eventually. The preferred mechanism would
be a delayed-ack-like 200-millisecond timer, set every time a packet
is transmitted in PARTOPEN. If this timer goes off and the client
is still in PARTOPEN, the client generates another DCCP-Ack and
backs off the timer. If the client remains in PARTOPEN for more
than 4MSL, it SHOULD reset the connection with Reset Code 2,
"Aborted".
The client leaves the PARTOPEN state for OPEN when it receives a
packet other than DCCP-Response or DCCP-Reset from the server.
8.2. Data Transfer
In the central, data transfer phase of the connection, both server
and client are in the OPEN state.
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 DCCP-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 congestion
state of the A-to-B CCID will not allow data to be sent.
The DCCP-Move, DCCP-Sync, and DCCP-SyncAck packets will also occur
in the data transfer phase. DCCP-Move handling is discussed in
Section 14, and some cases causing DCCP-Sync generation are
discussed in Section 7.5. One important distinction between DCCP-
Sync packets and other packet types is that DCCP-Sync elicits an
immediate acknowledgement. On receiving a valid DCCP-Sync packet, a
DCCP endpoint MUST immediately generate and send a DCCP-SyncAck in
response; and the Acknowledgement Number on that DCCP-SyncAck MUST
equal the Sequence Number of the DCCP-Sync.
A particular DCCP implementation might decide to initiate feature
negotiation only once the OPEN state was reached, in which case it
might not allow data transfer until some time later. Data received
during that time SHOULD be rejected and reported using a Data
Dropped Drop Block with Drop Code 0.
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8.3. Termination
DCCP connection termination uses a handshake consisting of an
optional DCCP-CloseReq packet, a DCCP-Close packet, and a DCCP-Reset
packet. The server moves from the OPEN state, possibly through the
CLOSEREQ state, to CLOSED; the client moves from OPEN through
CLOSING to TIMEWAIT, and after 2MSL wait time, to CLOSED.
The sequence DCCP-CloseReq, DCCP-Close, DCCP-Reset is used when the
server decides to close the connection, but doesn't want to hold
TIMEWAIT state:
Client State Server State
OPEN OPEN
1. <-- CloseReq <-- CLOSEREQ
2. CLOSING --> Close -->
3. <-- Reset <-- CLOSED
4. TIMEWAIT
5. CLOSED
A shorter sequence occurs when the client decides to close the
connection.
Client State Server State
OPEN OPEN
1. CLOSING --> Close -->
2. <-- Reset <-- CLOSED
3. TIMEWAIT
4. CLOSED
Finally, the server can decide to hold TIMEWAIT state:
Client State Server State
OPEN OPEN
1. <-- Close <-- CLOSING
2. CLOSED --> Reset -->
3. TIMEWAIT
4. CLOSED
In all cases, the receiver of the DCCP-Reset packet holds TIMEWAIT
state for the connection. As in TCP, TIMEWAIT state, where an
endpoint quietly preserves a socket for 2MSL (4 minutes) after its
connection has closed, ensures that no connection duplicating the
current connection's source and destination addresses and ports can
start up while old packets might remain in the network.
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The termination handshake proceeds as follows. The receiver of a
valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet;
that receiving endpoint will expect to hold TIMEWAIT state after
later receiving a DCCP-Reset. The receiver of a valid DCCP-Close
packet MUST respond with a DCCP-Reset packet, with Reset Code 1,
"Closed"; the endpoint that originally sent the DCCP-Close will hold
TIMEWAIT state. The endpoint that receives a valid DCCP-Reset
packet will hold TIMEWAIT state for the connection.
A DCCP-Reset packet completes every DCCP connection, whether the
termination is clean (due to application close; Reset Code 1,
"Closed") or unclean. Unlike TCP, which has two distinct
termination mechanisms (FIN and RST), DCCP ends all connections in a
uniform manner. This is justified because some responses to
connection termination close are the same no matter whether
termination was clean. For instance, the endpoint that receives a
valid DCCP-Reset should hold TIMEWAIT state for the connection.
Processors that must distinguish between clean and unclean
termination can examine the Reset Code. DCCP-Reset packets MUST NOT
be generated in response to received DCCP-Reset packets. DCCP
implementations generally transition to the CLOSED state after
sending a DCCP-Reset packet.
Endpoints in the CLOSEREQ and CLOSING states MUST retransmit DCCP-
CloseReq and DCCP-Close packets, respectively, until leaving those
states. The retransmission timer should initially be set to go off
in two RTTs, or 0.4 seconds if the RTT is not known, and should back
off to not less than once every 64 RTTs if no relevant response is
received.
Only the server can send a DCCP-CloseReq packet or enter the
CLOSEREQ state.
8.3.1. Abnormal Termination
DCCP endpoints generate DCCP-Reset packets to terminate connections
abnormally; a DCCP-Reset packet may be generated from any state.
However, a DCCP endpoint in the CLOSED or LISTEN state may not have
a proper sequence number available to send a Reset. In these cases,
it MUST set the Reset's Sequence Number to zero. Resets sent in the
CLOSED, LISTEN, and TIMEWAIT states often use Reset Code 3, "No
Connection". Resets sent in the REQUEST or RESPOND states often use
Reset Code 4, "Packet Error".
8.4. DCCP State Diagram
The most common state transitions discussed above can be summarized
in the following state diagram. The diagram is illustrative; the
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text in Section 8.5 and elsewhere should be considered definitive.
For example, there are arcs (not shown) from every state except
CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.
+---------------------------+ +---------------------------+
| v v |
| +----------+ |
| +-------------+ CLOSED +------------+ |
| | +----------+ active | |
| | passive open | |
| | open snd Request | |
| v v |
| +----------+ +----------+ |
| | LISTEN | | REQUEST | |
| +----+-----+ +----+-----+ |
| | rcv Request rcv Response | |
| | snd Response snd Ack | |
| v v |
| +----------+ +----------+ |
| | RESPOND | | PARTOPEN | |
| +----+-----+ +----+-----+ |
| | rcv Ack/DataAck rcv packet | |
| | | |
| | +----------+ | |
| +------------>| OPEN |<-----------+ |
| +--+-+--+--+ |
| server active close | | | active close |
| snd CloseReq | | | or rcv CloseReq |
| | | | snd Close |
| | | | |
| +----------+ | | | +----------+ |
| | CLOSEREQ |<---------+ | +--------->| CLOSING | |
| +----+-----+ | +----+-----+ |
| | rcv Close | | |
| | snd Reset | rcv Reset | |
|<---------+ | v |
| rcv Close | +----+-----+ |
| snd Reset | | TIMEWAIT | |
| | +----+-----+ |
+-----------------------------+ | |
+-----------+
2MSL timer expires
8.5. Pseudocode
This section presents an algorithm describing the processing steps a
DCCP endpoint must go through when it receives a packet. A DCCP
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implementation need not implement the algorithm as it is described
here, but any implementation MUST generate observable effects
(meaning packets) exactly as indicated by this pseudocode, except
where allowed otherwise by another part of this document.
The received packet is written as P, the socket as S. Socket variables:
S.SWL - sequence number window low
S.SWH - sequence number window high
S.AWL - acknowledgement number window low
S.AWH - acknowledgement number window high
S.ISS - initial sequence number sent
S.ISR - initial sequence number received
S.OSR - first OPEN sequence number received
S.GSS - greatest sequence number sent
S.GSR - greatest valid sequence number received
S.GAR - greatest acknowledgement number received; initialized to S.ISS
"Send packet" actions always use, and increment, S.GSS.
First, check the header basics;
If the header checksum is incorrect, drop packet and return.
If the packet type is not understood, drop packet and return.
If Data Offset is too small for packet type, or too large for packet,
drop packet and return.
Second, process DCCP-Move;
If P.type == Move,
Look up the Mobility ID in table; get socket.
If socket exists && P.seqno >= S.SWL && P.ackno <= S.AWH
&& P.ackno >= S.ISS && S.state >= PARTOPEN && S.state < TIMEWAIT,
Process options
Set socket to point at new address/ports
Add reference to new address/ports
Set timer to remove old address/ports after 2MSL
Choose new Mobility ID, add to table
Send DCCP-Sync[Change L[Mobility ID, new ID]]
Update S.GSR, S.SWL, S.SWH
Drop packet and return
Otherwise,
Drop packet and return
Third, check ports and process TIMEWAIT state;
Look up flow ID; get socket.
If no socket, or S.state == TIMEWAIT,
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return
Fourth, process LISTEN state;
If S.state == LISTEN,
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If P.type == Request,
/* Init Cookie processing would go here */
Set S := new socket for this port pair
S.state = RESPOND
Choose S.ISS (initial seqno)
Set S.ISR, S.GSR, S.SWL, S.SWH from packet
Continue (with S.state == RESPOND)
Otherwise,
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return
Fifth, process Reset;
If P.type == Reset,
If S.GAR <= P.ackno <= S.AWH
&& (P.seqno == 0 || P.seqno > S.GSR || S.state == REQUEST),
Tear down connection
S.state := TIMEWAIT
Set TIMEWAIT timer
Drop packet and return
Otherwise (sequence numbers out of whack),
Drop packet and return
Sixth, process REQUEST state;
If S.state == REQUEST,
If P.type == Response && S.AWL <= P.ackno <= S.AWH,
Set S.GSR, S.ISR, S.SWL, S.SWH
Otherwise,
Generate Reset(Packet Error)
Drop packet and return
Seventh, process Sync sequence numbers;
If P.type == Sync || P.type == SyncAck,
If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,
Update S.GSR, S.SWL, S.SWH
Otherwise,
Drop packet and return
Eighth, check sequence numbers;
If S.SWL <= P.seqno <= S.SWH
&& (P.ackno does not exist || S.AWL <= P.ackno <= S.AWH),
Update S.GSR, S.GAR, S.SWL, S.SWH
Otherwise,
Send Sync packet acknowledging P.seqno
Drop packet and return
Ninth, check packet type;
If (S.is_server && P.type == CloseReq)
|| (S.is_server && P.type == Response)
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|| (S.is_client && P.type == Request)
|| (S.state >= OPEN && P.type == Request && P.seqno >= S.OSR)
|| (S.state >= OPEN && P.type == Response && P.seqno >= S.OSR)
|| (S.state == RESPOND && P.type == Data),
Send Sync packet acknowledging P.seqno
Drop packet and return
Tenth, process options;
/* may involve resetting connection, etc. */
Mark packet as "received" for acknowledgement purposes
On processing Confirm R(Mobility ID),
Check that the confirmed Mobility ID is correct
If a DCCP-Move was recently processed,
Remove any old Mobility ID from table
Eleventh, process RESPOND state;
If S.state == RESPOND,
If P.type == Request,
Send Response
Otherwise,
S.OSR := P.seqno
S.state := OPEN
Twelfth, process REQUEST state;
If S.state == REQUEST,
S.state := PARTOPEN
/* Do not send Data packets in PARTOPEN; furthermore, include Init
Cookie on every packet */
Set PARTOPEN timer
Thirteenth, process PARTOPEN state;
If S.state == PARTOPEN,
If P.type == Response,
Send Ack
Otherwise,
S.OSR := P.seqno
S.state := OPEN
Fourteenth, process CloseReq;
If P.type == CloseReq && S.state < CLOSEREQ,
Generate Close
S.state := CLOSING
Set CLOSING timer
Fifteenth, process Close;
If P.type == Close,
Generate Reset(Closed)
Tear down connection
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Drop packet and return
Sixteenth, process Sync;
If P.type == Sync,
Generate SyncAck
Seventeenth, process data.
Do not deliver data from more than one Request or Response
9. Checksums
DCCP uses a header checksum to protect its header against
corruption. Generally, this checksum covers any application data as
well. However, DCCP applications can request that the header
checksum cover only part of the application data, or perhaps no
application data at all. Link layers may then reduce their
protection on unprotected parts of DCCP packets. For some noisy
links, and applications that can tolerate corruption, this can
greatly improve delivery rates and perceived performance.
If checksum coverage is complete, packets with corrupt application
data must be treated as network losses, thus incurring a loss
response from the sender's congestion control mechanism. Such a
heavy-duty response may unfairly penalize connections on links with
high background corruption. It is to the application's benefit to
report corruption losses differently from network losses.
Therefore, even applications that demand correct data can make use
of reduced checksum coverage, by including a Data Checksum option.
Data Checksum holds a strong checksum of the application data. The
combination of reduced checksum coverage and Data Checksum can
detect application data corruption, but report it as corruption, not
congestion, via Data Dropped options (see Section 11.7).
Reduced checksum coverage introduces some security considerations;
see Section 19.2. See Appendix B.1 for further motivation and
discussion. DCCP's implementation of reduced checksum coverage was
inspired by UDP-Lite [UDP-LITE].
9.1. Header Checksum Field
DCCP uses the TCP/IP checksum algorithm. The Checksum field in the
DCCP generic header (see Section 5.1) 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 Coverage
field, some or all of the application data. When calculating the
checksum, the Checksum field itself is treated as 0. If a packet
contains an odd number of header and text bytes to be checksummed, 8
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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 header checksums MUST be ignored. In
particular, their options MUST NOT be processed.
9.2. Header Checksum Coverage Field
The Checksum Coverage field in the DCCP generic header (see Section
5.1) specifies what parts of the packet are covered by the Checksum
field, as follows:
CsCov = 0 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and all
application data in the packet, possibly padded on
the right with zeros to an even number of bytes.
CsCov = 1-15 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and the initial
(CsCov-1)*4 bytes of the packet's application data.
Thus, if CsCov is 1, none of the application data is protected by
the header checksum. The value (CsCov-1)*4 MUST be less than or
equal to the length of the application data. Packets with invalid
CsCov values MUST be ignored; in particular, their options MUST NOT
be processed. The meanings of values other than 0 and 1 should be
considered experimental.
Values other than 0 specify that corruption is acceptable in some or
all of the DCCP packet's application data. In fact, DCCP cannot
even detect corruption in areas not covered by the header checksum,
unless the Data Checksum option is used. Applications should not
make any assumptions about the correctness of received data not
covered by the checksum, and should if necessary introduce their own
validity checks.
A DCCP application interface should let sending applications suggest
a value for CsCov for sent packets, defaulting to 0 (full coverage).
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It should also let receiving applications refuse delivery of packets
with checksum coverage less than a value provided by the
application; by default, only packets with fully-covered application
data should be accepted. (Note that, for short packets, application
data might be fully covered by a nonzero Checksum Coverage value.)
Lower layers that support partial error detection MAY use the
Checksum Coverage field as a hint of where errors do not need to be
detected. Lower layers MUST use a strong error detection mechanism
to detect at least errors that occur in the sensitive part of the
packet, and discard damaged packets. The sensitive part consists of
the bytes between the first byte of the IP header and the last byte
identified by Checksum Coverage.
For more details on application and lower-layer interface issues
relating to partial checksumming, see [UDP-LITE].
9.3. Data Checksum Option
The Data Checksum option holds a 32-bit CRC-32c cyclic redundancy-
check code of a DCCP packet's application data.
+--------+--------+--------+--------+--------+--------+
|00101100|00000110| CRC-32c |
+--------+--------+--------+--------+--------+--------+
Type=44 Length=6
Data Checksum is intended for packets containing application data,
such as DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck,
but it may be included on any packet. The sending DCCP computes the
CRC of the bytes comprising the application data and stores it in
the option data. The CRC-32c algorithm used for Data Checksum is
the same as that used for SCTP [RFC 3309]; note that the CRC-32c of
zero bytes of data equals zero. The DCCP header checksum will cover
the Data Checksum option, so the data checksum must be computed
before the header checksum.
The receiving DCCP SHOULD compute the received application data's
CRC-32c using the same algorithm as the sender, and compare the
result and the Data Checksum value. If the values differ, the
packet's application data MUST be dropped, and reported using a Data
Dropped option as dropped due to corruption (Drop Code 3). However,
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 Code 7) using a Data Dropped option. In
either case, the packet will be reported as Received or Received ECN
Marked by Ack Vector or similar options.
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9.3.1. Check Data Checksum Feature
The Check Data Checksum feature lets a sending DCCP determine
whether or not its partner can check Data Checksum options. DCCP A
sends a Mandatory "Change R(Check Data Checksum, 1)" option to
DCCP B to require B to check Data Checksum options (the connection
will be reset if DCCP B cannot).
Check Data Checksum has feature number 10, and is server-priority.
It takes one-byte Boolean values. DCCP B MUST check any received
Data Checksum options when Check Data Checksum/B is one, although it
MAY check them even when Check Data Checksum/B is zero. Values of
two or more are reserved. New connections start with Check Data
Checksum 0 for both endpoints.
9.3.2. Usage Notes
Internet links must normally apply strong integrity checks to the
packets they transmit [UDP-LITE] [LINK BCP]. Data Checksum is
redundant for DCCP packets whose integrity is checked by every link
they traverse. This is the default case when the DCCP header's
Checksum Coverage value equals zero (full coverage). However, the
DCCP Checksum Coverage value might not be zero. By setting partial
Checksum Coverage, the application indicates that it can tolerate
corruption in the unprotected part of the application data.
Recognizing this, link layers may reduce the strength of their error
detection and/or correction when transmitting this unprotected part,
which can significantly increase the probability of the endpoint
receiving corrupt data. Data Checksum lets the receiver detect any
ensuing corruption.
10. 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 ID features. Congestion Control
ID has feature number 1. The CCID/A value equals the CCID in use
for the A-to-B half-connection. DCCP B sends a "Change R(CCID, K)"
option to ask DCCP A to use CCID K for its data packets.
CCID is a server-priority feature, so CCID negotiation options can
list multiple acceptable CCIDs, sorted in descending order of
priority. For example, the option "Change R(CCID, 1 2 3)" asks the
receiver to use CCID 1 for its packets, although CCIDs 2 and 3 are
also acceptable. (This corresponds to the bytes "35, 6, 1, 1, 2,
3": Change R option (35), option length (6), feature ID (1), CCIDs
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(1, 2, 3).) Similarly, "Confirm L(CCID, 1, 1 2 3)" tells the
receiver that the sender is using CCID 1 for its packets, 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
New connections start with CCID 2 for both endpoints. If this is
unacceptable for a DCCP endpoint, that endpoint MUST send Mandatory
Change(CCID) options on its first packets.
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, such as an
implementation in a general-purpose operating system kernel, SHOULD
implement at least CCIDs 1 and 2. The intent is to make these CCIDs
broadly available for interoperability, although particular
applications might disallow their use.
10.1. Unspecified Sender-Based Congestion Control
CCID 1 denotes an unspecified sender-based congestion control
mechanism. This provides a limited, controlled form of
interoperability for new IETF-approved CCIDs: with CCID 1, an HC-
Sender can use a new sender-based congestion control mechanism whose
details the HC-Receiver does not understand.
Some congestion control mechanisms require only generic behavior
from the receiver. For example, CCID 2, TCP-like Congestion
Control, requires that the receiver (1) send Ack Vectors and (2)
respond to Ack Ratio. Both of these requirements use generic
mechanisms described in this document. Thus, a CCID 2 HC-Receiver
doesn't really need to understand the details of CCID 2.
CCID 1 uses this insight to support forward compatibility for
sender-based congestion control mechanisms. An HC-Sender proposes
CCID 1 as a proxy for a sender-based mechanism whose details the HC-
Receiver doesn't need to understand. The HC-Receiver can then agree
to CCID 1, and provide generic acknowledgement feedback as requested
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by other features (such as Send Ack Vector). Individual CCID
profile documents say whether or not they can masquerade as CCID 1.
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 proxy for CCID 98. Now, say DCCP A would like to use CCID 98
for its data packets. It should therefore send a "Change L(CCID, 98
1)" option to open a CCID negotiation. 98 comes first, since that
is the preferred CCID; 1 comes next, as a potential proxy for 98.
If DCCP B understands CCID 98, it will respond with "Confirm R(CCID,
98, ...)" and all is well. But if it does not understand CCID 98,
it may respond with "Confirm R(CCID, 1, ...)", still allowing DCCP A
to use CCID 98. DCCP A will separately negotiate Send Ack Vector,
and thus DCCP B will provide the feedback DCCP A requires, namely
Ack Vector, without needing to understand the operation of CCID 98.
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.
Since CCID 1 should be used only as a proxy for other, defined
CCIDs, an HC-Sender MUST NOT report a preference list consisting
only of CCID 1, and the option "Change L(CCID, 1)" is illegal.
Receiving such an option SHOULD result in connection reset with
Reset Code 5, "Option Error". An HC-Receiver MAY suggest CCID 1
exclusively: the option "Change R(CCID, 1)" is not illegal.
If CCID 1 is the result of a CCID feature negotiation, the HC-Sender
determines which CCID to actually use by picking the earliest CCID
in its preference list that can masquerade as CCID 1. The HC-Sender
MUST pick a CCID that appeared explicitly in its preference list.
Many DCCP APIs will allow applications to suggest preferred CCIDs
for sending and receiving data. Such APIs might let applications
allow or prevent the use of CCID 1 for receiving, but they should
not let applications suggest the use of CCID 1 for sending. The
code implementing a particular CCID should add CCID 1 to the HC-
Sender's CCID preference list when appropriate, unless the
application disagrees. The default for both sender and receiver
should be to allow CCID 1 when possible.
CCID 1 places no restrictions on how often the HC-Receiver may send
DCCP-Ack packets. A careful implementation SHOULD implement a
liberal rate limit on DCCP-Acks to prevent ack storms.
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10.2. TCP-like Congestion Control
CCID 2, TCP-like Congestion Control, 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 achieves maximum bandwidth over the
long term, consistent with the use of end-to-end congestion control,
but halves its congestion window in response to each congestion
event. This leads to the abrupt rate changes typical of TCP.
Applications should use CCID 2 if they prefer maximum bandwidth
utilization to steadiness of rate. This is often the case for
applications that are not playing their data directly to the user.
For example, a hypothetical application that transferred files over
DCCP, using application-level retransmissions for lost packets,
would prefer CCID 2 to CCID 3. On-line games may also prefer CCID
2.
CCID 2 is further described in [CCID 2 PROFILE].
10.3. TFRC Congestion Control
CCID 3 denotes TCP-Friendly Rate Control (TFRC), an equation-based
rate-controlled congestion control mechanism. TFRC is designed to
be reasonably fair when competing for bandwidth with TCP-like flows,
where a flow is "reasonably fair" if its sending rate is generally
within a factor of two of the sending rate of a TCP flow under the
same conditions. However, TFRC has a much lower variation of
throughput over time compared with TCP, which makes CCID 3 more
suitable than CCID 2 for applications such as telephony or streaming
media where a relatively smooth sending rate is of importance.
CCID 3 is further described in [CCID 3 PROFILE]. The TFRC congestion
control algorithms were initially described in [RFC 3448].
10.4. CCID-Specific Options, Features, and Reset Codes
Half of the option types, feature numbers, and Reset Codes are
reserved for CCID-specific use. CCIDs may often need new options,
for communicating acknowledgement or rate information, for example;
reserved option spaces let CCIDs 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 and features introduced by later
versions of this specification.
Any packet may contain information meant for either half-connection,
so CCID-specific option types, feature numbers, and Reset Codes
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explicitly signal the half-connection to which they apply.
o 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.
o Reset Codes 128 through 191 indicate that the HC-Sender reset the
connection (most likely because of some problem with
acknowledgements sent by the HC-Receiver); Reset Codes 192 through
255 indicate that the HC-Receiver reset the connection (most
likely because of some problem with data packets sent by the HC-
Sender).
o Finally, feature numbers 128 through 191 are used for features
located at the HC-Sender; feature numbers 192 through 255 are for
features located at the HC-Receiver. Since Change L and Confirm L
options for a feature are sent by the feature location, we know
that any Change L(128) option was sent by the HC-Sender, while any
Change L(192) option was sent by the HC-Receiver. Similarly,
Change R(128) options are sent by the HC-Receiver, while
Change R(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:
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Relevant Relevant
Packet Option Half-conn. CCID
------ ------ ---------- ----
A > B 128 A-to-B 4
A > B 192 B-to-A 5
A > B Change L(128, ...) A-to-B 4
A > B Change R(192, ...) A-to-B 4
A > B Confirm L(128, ...) A-to-B 4
A > B Confirm R(192, ...) A-to-B 4
A > B Change R(128, ...) B-to-A 5
A > B Change L(192, ...) B-to-A 5
A > B Confirm R(128, ...) B-to-A 5
A > B Confirm L(192, ...) B-to-A 5
B > A 128 B-to-A 5
B > A 192 A-to-B 4
B > A Change L(128, ...) B-to-A 5
B > A Change R(192, ...) B-to-A 5
B > A Confirm L(128, ...) B-to-A 5
B > A Confirm R(192, ...) B-to-A 5
B > A Change R(128, ...) A-to-B 4
B > A Change L(192, ...) A-to-B 4
B > A Confirm R(128, ...) A-to-B 4
B > A Confirm L(192, ...) A-to-B 4
CCID-specific options and features have no clear meaning when a
nontrivial negotiation for the relevant CCID is in progress. This
can happen when a CCID-specific option follows a Change(CCID)
option. Say the Change option lists CCID X first. Then the
negotiation is nontrivial if and only if its result is not X. CCID-
specific options and features MUST be ignored during a nontrivial
CCID negotiation, except that Mandatory CCID-specific options and
features MUST induce a DCCP-Reset with Reset Code 6, "Mandatory
Error".
11. 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
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ack 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). Detailed acknowledgement options are not necessarily
required on every packet that carries an Acknowledgement Number,
however.
11.1. 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, and periodically retransmit, 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 B can free old Ack Vector
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state. For instance, 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
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.
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 the maximum of 0.2 seconds and two round-
trip times. The A-to-B CCID defines how DCCP A handles acks-of-acks
once DCCP B has gone quiescent.
11.2. 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 allowed maximum packet size. 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 at which 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.
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Any restrictions on ack piggybacking are described in the relevant
CCID's profile.
11.3. Ack Ratio Feature
Ack Ratio provides a common mechanism by which CCIDs that clock
acknowledgements off data packets can perform rudimentary congestion
control on the acknowledgement stream. CCID 2, TCP-like 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 7, and is non-negotiable. It takes
two-byte integer values. The Ack Ratio/A feature is the rough ratio
of data packets sent by DCCP A to acknowledgement packets sent back
by DCCP B. For example, if Ack Ratio/A is four, then DCCP B will
send at least one acknowledgement packet for every four data packets
sent by DCCP A. DCCP A sends a "Change L(Ack Ratio)" option to
notify DCCP B of its ack ratio. New connections start with Ack
Ratio 2 for both endpoints.
Implementations should treat Ack Ratio as a loose guideline. For
instance, a DCCP endpoint might implement a delayed acknowledgement
timer like TCP's, whereby each packet is acknowledged within at most
T seconds of its receipt. (In TCP, T is commonly set to 200
milliseconds.) This is explicitly allowed even though it might lead
to sending more acknowledgement packets than Ack Ratio would
suggest. Particular algorithms for setting and using Ack Ratio are
discussed in the relevant CCID drafts.
11.4. 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:
+--------+--------+--------+--------+--------+--------
|0010011?| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL| ...
+--------+--------+--------+--------+--------+--------
Type=38/39 \___________ Vector ___________...
The two Ack Vector options (option types 38 and 39) differ only in
the values they imply for ECN Nonce Echo. Section 12.2 describes
this further.
The vector itself consists of a series of bytes, each of whose
encoding is:
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Sta| Run Length|
+-+-+-+-+-+-+-+-+
Sta[te] 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.
Run Length, the least significant six bits of each byte, specifies
how many consecutive packets have the given State. Run Length zero
says the corresponding State applies to one packet only; Run Length
63 says it applies to 64 consecutive packets. Run lengths of 65 or
more must be encoded in multiple bytes.
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).
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 begins where the first left off, and so forth.
Ack Vector states are subject to two general constraints. (These
principles SHOULD also be followed for other acknowledgement
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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
application data 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 Reset Code 5, "Option Error". Packets
that haven't been included in any Ack Vector option SHOULD be
treated as "not yet received" (State 3) by the sender.
Appendix A provides a non-normative description of the details of
DCCP acknowledgement handling, in the context of an abstract Ack
Vector implementation.
11.4.1. 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. (Information about a sequence number is
generally repeated in every ack until the HC-Sender acknowledges an
ack. In this case, 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:
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).
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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 |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. For an old state of 0 (received non-marked) and received
state of 1 (received ECN marked), the packet's new state may be set
to either 0 or 1. The HC-Sender implementation will be indifferent
to ack reordering if it chooses new state 1 for that cell.
The HC-Receiver should collect information about received packets,
which it will eventually report to the HC-Sender on one or more
acknowledgements, according to the following table:
Received Packet
0 1 3
+---+---+---+
0 | 0 |0/1| 0 |
Stored +---+---+---+
1 |0/1| 1 | 1 |
State +---+---+---+
3 | 0 | 1 | 3 |
+---+---+---+
This table equals the sender's table, except that when the stored
state is 1 and the received state is 0, the receiver is allowed to
switch its stored state to 0.
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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).
11.4.2. 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 Acknowledgement Window.
Generally, every Ack Vector generated by the HC-Receiver will cover
the whole Acknowledgement Window: Ack Vector acknowledgements are
cumulative. (This simplifies Ack Vector maintenance at the HC-
Receiver; see Section A, 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.
11.5. Send Ack Vector Feature
The Send 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. Send Ack
Vector is mandatory for some CCIDs, and optional for others.
Send Ack Vector has feature number 8, and is server-priority. It
takes one-byte Boolean values. DCCP A MUST send Ack Vector options
on its acknowledgements when Send Ack Vector/A has value one,
although it MAY send Ack Vector options even when Send Ack Vector/A
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is zero. Values of two or more are reserved. New connections start
with Send Ack Vector 0 for both endpoints. DCCP B sends a
"Change R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack
Vector options as part of its acknowledgement traffic.
11.6. 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, with or without Data Dropped, or reporting
false ECN marks.) APIs should let receiver applications set Slow
Receiver, and sending applications determine whether or not their
receivers are Slow.
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.
The sending application should not react to Slow Receiver by sending
more data, however. The optimal response to a CPU-bound receiver
might be to increase the sending rate, by switching to a less-
compressed sending format, since a highly-compressed data format
might overwhelm a slow CPU more seriously than the higher memory
requirements of a less-compressed data format. The Slow Receiver
option is not appropriate for this case; a CPU-bound receiver should
not ask for Slow Receiver options to be sent.
Slow Receiver implements a portion of TCP's receive window
functionality.
11.7. Data Dropped Option
The Data Dropped option indicates that some packets reported as
received actually had their data dropped before it reached the
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application. The sender's congestion control mechanism may respond
to data-dropped packets less severely than to lost or marked
packets. For instance, a windowed mechanism might subtract a
constant value from its congestion window, rather than cut it in
half.
Data Dropped lets a sender differentiate between different kinds of
loss (network and endpoint), but it does not allow total freedom in
how to react. The congestion control response to a Data Dropped
packet must be approved by the IETF. Each congestion control
mechanism MUST react to a Data Dropped packet as if the packet were
ECN marked, unless explicitly specified otherwise.
If a received packet's application data is dropped for one of the
reasons listed below, this SHOULD be reported using a Data Dropped
option. Alternatively, the receiver MAY choose to report as
"received" only those packets whose data were not dropped, subject
to the constraint that packets not reported as received MUST NOT
have had their options processed.
The option's data looks like this:
+--------+--------+--------+--------+--------+--------
|00101000| Length | Block | Block | Block | ...
+--------+--------+--------+--------+--------+--------
Type=40 \___________ Vector ___________ ...
The vector itself consists of a series of bytes, called Blocks, each
of whose encoding corresponds to one of these choices:
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| Run Length | or |1|DrpCd|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 Drop Code
[DrpCd] 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
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Block. Defined Drop Codes 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 because the application is no longer
listening.
2 Packet data dropped in the receive buffer.
3 Packet data dropped due to corruption.
4-6 Reserved.
7 Packet data corrupted, but delivered to the application
anyway.
For example, if a Data Dropped option contains the decimal values
0,160,3,162, 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
Code 2, Run Length 0).
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 Code 2, 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 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
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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 Reset Code 5, "Option
Error".
A DCCP application interface should let receiving applications
specify the Drop Codes corresponding to received packets. For
example, this would let applications calculate their own checksums,
but still report "dropped due to corruption" packets via the Data
Dropped option. The interface should not let applications reduce
the "seriousness" of a packet's Drop Code; for example, the
application should not be able to upgrade a packet from delivered
corrupt (Drop Code 7) to delivered normally (no Drop Code).
11.7.1. Data Dropped and Normal Congestion Response
When deciding on a response to a particular acknowledgement or set
of acknowledgements containing Data Dropped packets, a congestion
control mechanism MUST consider dropped packets and ECN marks
(including ECN-marked packets that are included in Data Dropped), as
well as the Data Dropped packets. For window-based mechanisms, the
valid response space is defined as follows.
Assume an old window of W. Independently calculate a new window
W_new1 that assumes no packets were Data Dropped (so W_new1 contains
only the normal congestion response), and a new window W_new2 that
assumes no packets were lost or marked (so W_new2 contains only the
Data Dropped response). We are assuming that Data Dropped
recommended a reduction in congestion window, so W_new2 < W.
Then the actual new window W_new MUST NOT be larger than the minimum
of W_new1 and W_new2; and the sender MAY combine the two responses,
by setting
W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).
Non-window-based congestion control mechanisms MUST behave
analogously.
11.7.2. Particular Drop Codes
Drop Code 0 ("protocol constraints") does not indicate any kind of
congestion, so the sender's CCID SHOULD react to non-marked packets
with Drop Code 0 as if they were received. However, the sending
DCCP SHOULD NOT send more data until it believes the relevant
protocol constraint has passed.
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Drop Code 1 ("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 Code 1 SHOULD be sent whenever received data
is ignored due to a non-listening application. Once a DCCP reports
Drop Code 1 for a packet, it SHOULD report Drop Code 1 for every
succeeding data packet on that half-connection; once a DCCP receives
a Drop State 1 report, it SHOULD expect that no more data will ever
be delivered to the other endpoint's application, so it SHOULD NOT
send more data. A DCCP receiving Drop Code 1 MAY report this event
to the application. (Previous versions of this specification used a
"Buffer Closed" option instead of Drop Code 1.)
Drop Code 2 ("receive buffer drop") indicates congestion inside the
receiving host. Every packet newly acknowledged as Drop Code 2
SHOULD reduce the sender's instantaneous rate by one packet per
round trip time, using whatever mechanism is appropriate for the
relevant CCID. Further details may be available in CCID documents.
12. Explicit Congestion Notification
The DCCP protocol is fully ECN-aware [RFC 3168]. 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.
12.1. 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.
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ECN Capable has feature number 2, and is server-priority. It takes
one-byte Boolean values. DCCP A MUST be able to read ECN bits from
received frames' IP headers when ECN Capable/A is one. (This is
independent of whether it can set ECN bits on sent frames.) DCCP A
thus sends a "Change L(ECN Capable, 0)" option to DCCP B to inform
it that A cannot read ECN bits. New connections start with ECN
Capable 1 (that is, ECN capable) for both endpoints. Values of two
or more are reserved.
If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN
Capable, 0)" options to the other endpoint until acknowledged (by
"Confirm R(ECN Capable, 0)") or the connection closes. Furthermore,
it MUST NOT accept any data until the other endpoint sends
"Confirm R(ECN Capable, 0)". It SHOULD send Data Dropped options on
its acknowledgements, with Drop Code 0 ("protocol constraints"), if
the other endpoint does send data inappropriately.
12.2. ECN Nonces
Congestion avoidance will not occur, and the receiver will sometimes
get its data faster, if the sender isn't told about 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 38) and Ack Vector [Nonce 1] (option 39),
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
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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.)
Given an A-to-B half-connection, 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
with Reset Code 12, "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 38) exclusively. (Again, the ECN Capable feature MUST be
set to zero in this case.)
12.3. 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 Reset Code 12, "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.
13. Timing Options
The Timestamp, Timestamp Echo, and Elapsed Time options help DCCP
endpoints explicitly measure round-trip times.
13.1. Timestamp Option
This option is permitted in any DCCP packet. The length of the
option is 6 bytes.
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+--------+--------+--------+--------+--------+--------+
|00101001|00000110| Timestamp Value |
+--------+--------+--------+--------+--------+--------+
Type=41 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.
13.2. Elapsed Time Option
This option is permitted in any DCCP packet that contains an
Acknowledgement Number. It indicates how much time, in tenths of
milliseconds, has elapsed since the packet being acknowledged---the
packet with the given Acknowledgement Number---was received. The
option may take 4 or 6 bytes, depending on the size of the Elapsed
Time value. Elapsed Time helps correct round-trip time estimates
when the gap between receiving a packet and acknowledging that
packet may be long---in CCID 3, for example, where acknowledgements
are sent infrequently.
+--------+--------+--------+--------+
|00101011|00000100| Elapsed Time |
+--------+--------+--------+--------+
Type=43 Len=4
+--------+--------+--------+--------+--------+--------+
|00101011|00000110| Elapsed Time |
+--------+--------+--------+--------+--------+--------+
Type=43 Len=6
The option data, Elapsed Time, represents an estimated upper bound
on the amount of time elapsed since the packet being acknowledged
was received, with units of tenths of milliseconds. If Elapsed Time
is less than a second, the first, smaller 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. DCCP endpoints MUST NOT report
Elapsed Times that are significantly larger than the true elapsed
times. A connection MAY be reset with Reset Code 12, "Aggression
Penalty", if one endpoint determines that the other is reporting a
much-too-large Elapsed Time.
Elapsed Time is measured in tenths of milliseconds as a compromise
between two conflicting goals. First, it provides enough
granularity to reduce rounding error when measuring elapsed time
over fast LANs; second, it allows most reasonable elapsed times to
fit into two bytes of data.
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13.3. 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. Generally,
a DCCP endpoint should send one Timestamp Echo option for each
Timestamp option it receives; and it should send that option as soon
as is convenient. The length of the option is between 6 and 10
bytes, depending on whether Elapsed Time is included and how large
it is.
+--------+--------+--------+--------+--------+--------+
|00101010|00000110| Timestamp Echo |
+--------+--------+--------+--------+--------+--------+
Type=42 Len=6
+--------+--------+------- ... -------+--------+--------+
|00101010|00001000| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+--------+--------+
Type=42 Len=8 (4 bytes)
+--------+--------+------- ... -------+------- ... -------+
|00101010|00001010| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+------- ... -------+
Type=42 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 value, similar to that in the Elapsed Time option,
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.
14. 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
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beforehand. 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, there's 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]. DCCP mobility may
not be useful in the context of IPv6, with its mandatory support for
Mobile IP.
14.1. 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. DCCP B sends a "Change R(Mobility
Capable, 1)" option to DCCP A to inform it that B might like to move
later.
Mobility Capable has feature number 5, and is server-priority. It
takes one-byte Boolean values. DCCP A agrees in principle to accept
DCCP-Move packets from DCCP B when Mobility Capable/A is one.
DCCP A MUST reject any DCCP-Move packet for a connection whose
Mobility Capable/A feature is zero, although it MAY reject a valid
DCCP-Move packet even when Mobility Capable/A is one. Values of two
or more are reserved. New connections start with Mobility Capable 0
(that is, mobility is not allowed) for both endpoints.
14.2. Mobility ID Feature
A DCCP uses the Mobility ID feature to inform its partner of a
128-bit number that will act as identification, should the partner
change its address and/or port during the course of the connection.
DCCP A sends a "Change L(Mobility ID, N)" option to notify DCCP B of
the ID it has chosen for B's use.
Mobility ID has feature number 6, and is non-negotiable. Its values
are sixteen-byte integers. The Mobility ID/A feature equals the
identifier that DCCP B should use on DCCP-Move packets sent to A.
DCCP A chooses Mobility ID/A to uniquely identify the connection
among all connections that terminate at A. For security, DCCP A
MUST choose Mobility ID/A randomly. Furthermore, it MUST reassign
Mobility ID/A after each successful move by DCCP B, and it MAY
reassign Mobility ID/A more frequently. New connections start with
Mobility ID 0 for both endpoints. However, Mobility IDs of zero
MUST NOT be accepted on DCCP-Move packets; an endpoint cannot
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successfully move until the relevant Mobility ID has been set to a
nonzero value.
14.3. Mobile Host Processing
When DCCP A changes its address and/or port, it MUST signal this by
sending DCCP B a DCCP-Move packet. The Mobility ID in the DCCP-Move
packet uniquely identifies the connection; DCCP B will read the new
address and port off the DCCP-Move's network and DCCP headers.
Eventually, DCCP A will receive a DCCP-Sync sent to its new address
that negotiates a new Mobility ID/B feature. This confirms the
move. DCCP A SHOULD retransmit the DCCP-Move packet until it
receives a DCCP-Sync confirmation. The retransmission strategy
SHOULD be similar to that for retransmitting DCCP-Requests (Section
8.1.1); for instance, a first timeout on the order of a second, with
an exponential backoff timer.
DCCP A MUST reset its congestion control state after sending a DCCP-
Move, since nothing is known about conditions on the new path.
Essentially, DCCP A must "slow start" up to its new fair rate, as
appropriate for its congestion control mechanism. Section 14.5
discusses this further.
DCCP A SHOULD NOT send non-DCCP-Move packets to DCCP B until the
move is confirmed. If it did so, and the DCCP-Move packet was lost
or reordered, then DCCP B would react by sending DCCP-Resets with
Reset Code 3, "No Connection". DCCP A might implement special
handling for such resets to avoid any post-move quiet period, but
this is NOT RECOMMENDED.
DCCP B MAY refuse to accept a move, perhaps because of address
policy. In this case, DCCP A will receive a DCCP-Reset with Reset
Code 13, "Move Refused", rather than a confirming DCCP-Sync. DCCP A
MAY react by tearing down the connection, or by trying another DCCP-
Move---for instance, back to the old address, if possible.
DCCP endpoints SHOULD NOT use an old address-port pair after sending
a DCCP-Move. If it becomes necessary to switch back to the old
address-port pair, the endpoint MUST do so explicitly using another
DCCP-Move.
DCCP-Move packets SHOULD NOT be sent until the connection is
established; it is illegal to send a DCCP-Move in REQUEST or RESPOND
state. If an endpoint moves during connection establishment, it
SHOULD abandon the old connection and initiate a new one. No
connection exists to move until the three-way handshake has
completed.
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14.4. Stationary Host Processing
The stationary endpoint, DCCP B, uses DCCP-Move packets' destination
address, destination port, and Mobility ID fields to look up the
relevant connection. This differs from all other packet types,
which use the source address/source port/destination
address/destination port 4-tuple.
DCCP B MUST ignore DCCP-Moves whose Mobility ID is zero, or whose
Mobility ID does not correspond to any active connection. It also
MUST ignore DCCP-Moves sent to sockets in CLOSED, LISTEN, REQUEST,
RESPOND, or TIMEWAIT state, and it MUST ignore DCCP-Moves with
invalid Sequence or Acknowledgement Numbers (see Section 7.5).
DCCP B MUST NOT respond to invalid DCCP-Moves with DCCP-Reset or
DCCP-Sync packets, since any active response would leak information
about the connection to a possibly malicious host. After receiving
an invalid DCCP-Move, DCCP B MAY ignore subsequent DCCP-Move
packets, valid or not, for a short period of time, such as one
second or one round-trip time. This protects DCCP B against denial-
of-service attacks from floods of invalid DCCP-Moves.
On receiving a valid DCCP-Move, DCCP B decides whether to accept or
refuse the move request. To accept the request, it performs several
actions:
o It changes the connection to use the new address and port.
o It sets a timer to remove the old address and port after 2MSL.
This delay allows the receipt of any delayed packets from the old
address and port, and essentially represents TIMEWAIT state for
the old connection.
o It chooses a new Mobility ID for the connection, which temporarily
coexists with the old Mobility ID.
o It generates and sends a confirmation DCCP-Sync packet, which
includes a "Change L(Mobility ID)" option for the new Mobility ID.
If the DCCP-Sync is lost, then DCCP A will send another DCCP-Move
packet with the old Mobility ID. DCCP B MUST send another DCCP-Sync
packet in this situation, but SHOULD NOT choose yet another new
Mobility ID.
The move's three-way handshake completes once DCCP B receives a
DCCP-SyncAck from DCCP A that confirms the new Mobility ID option.
At that point, DCCP B MUST remove the old Mobility ID.
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DCCP B MAY refuse a valid DCCP-Move request for any reason; for
instance, the new address space might be considered unsuitable. To
refuse a valid DCCP-Move, DCCP B sends a DCCP-Reset packet to the
new address and port pair with Reset Code 13, "Move Refused". It
need take no other action; for example, it MAY tear down the
connection, or not. If DCCP B plans to refuse every DCCP-Move
request, it MUST negotiate a zero value for the Mobility Capable/A
feature.
DCCP B MUST ignore any data following the header in a DCCP-Move
packet.
14.5. 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".
Similarly, the endpoints' PMTUs SHOULD be reinitialized, and PMTU
discovery performed again, following an address change. See Section
15.
During the transition period between addresses, the endpoints might
receive congestion feedback from both before the move and after the
move. Congestion and loss events on packets sent before the move
SHOULD NOT affect the new connection's congestion control state.
14.6. Security
The DCCP mobility mechanism, like DCCP in general, does not provide
cryptographic security guarantees. Nevertheless, mobile hosts must
use valid Mobility IDs, providing protection against some classes of
attackers: An attacker cannot move a DCCP connection to a new
address unless it knows a valid Mobility ID. This generally means
that an attacker must have snooped on every packet in the connection
to get a reasonable probability of success, assuming that the
Mobility ID was chosen well (that is, randomly).
An attacker could choose a server running many mobility-capable
connections, and simply guess random Mobility IDs until one hit.
Let N equal the number of mobility-capable connections at the
server, X equal the number of attack attempts, and D equal the
number of possible Mobility IDs, namely 2^128. Then the probability
of at least one attack succeeding is
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(D - N) choose X (D-N)! (D-X)!
P = 1 - ---------------- = 1 - ------------- .
D choose X D! (D-N-X)!
For N = 10^6 and X = 10^9, the attack success probability is less
than 10^-23.
Section 19 further describes DCCP security considerations.
15. Maximum Packet Size
A DCCP implementation MUST maintain the maximum packet size (MPS)
allowed for each active DCCP session. The MPS is influenced by the
maximum packet size allowed by the current congestion control
mechanism (CCMPS), the maximum packet size supported by the path's
links (PMTU, the Path Maximum Transfer Unit) [RFC 1191], and the
lengths of the IP and DCCP headers.
A DCCP application interface should let the application discover
DCCP's current MPS. DCCP applications should use the API to
discover the MPS. Generally, the DCCP implementation will refuse to
send any packet bigger than the MPS, returning an appropriate error
to the application.
A DCCP interface may allow applications to request that packets
larger than PMTU be fragmented on IPv4 networks. This only matters
when CCMPS > PMTU; packets larger than CCMPS MUST be rejected
regardless. Fragmentation should not be the default. The rest of
this section assumes the application has not requested
fragmentation.
The MPS reported to the application SHOULD be influenced by the size
expected to be required for DCCP headers and options. If the
application provides data that, when combined with the options the
DCCP implementation would like to include, would exceed the MPS, the
implementation should either send the options on a separate packet
(such as a DCCP-Ack) or lower the MPS, drop the data, and return an
appropriate error to the application.
The PMTU SHOULD be initialized from the interface MTU that will be
used to send packets. The MPS will be initialized with the minimum
of the PMTU and the CCMPS, if any.
To perform classical PMTU discovery, the DCCP sender sets the IP
Don't Fragment (DF) bit. However, it is undesirable for MTU
discovery to occur on the initial connection setup handshake, as the
connection setup process may not be representative of packet sizes
used during the connection, and performing MTU discovery on the
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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.
As specified in [RFC 1191], when a router receives a packet with DF
set that is larger than the next link's MTU, 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 MPS, the
API must refuse to send the packet and return an appropriate error
to the application. The application should then use the API to
query the new value of MPS. The kernel might have some packets
buffered for transmission that are smaller than the old MPS, but
larger than the new MPS. It 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.
A DCCP implementation may allow the application to occasionally
request that PMTU discovery be performed again. This will reset the
PMTU to the outgoing interface's MTU. Such requests SHOULD be rate
limited, to one per two seconds, for example. A successful DCCP-
Move will also reset the PMTU.
A DCCP sender MAY 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 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
link MTU it exceeded.
A DCCP implementation SHOULD ensure, as far as possible, that ICMP
Datagram Too Big messages were actually generated by routers, so
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that attackers cannot drive the PMTU down to a falsely small value.
The simplest way to do this is to verify that the Sequence Number on
the ICMP error's encapsulated header corresponds to a Sequence
Number that the implementation recently sent. (Routers are not
required to return more than 64 bits of the DCCP header [RFC 792],
but most modern routers will return far more, including the Sequence
Number.) ICMP Datagram Too Big messages with incorrect or missing
Sequence Numbers may be ignored, or the DCCP implementation may
lower the PMTU only temporarily in response. If more than three odd
Datagram Too Big messages are received and the other DCCP endpoint
reports commensurate loss, however, the DCCP implementation SHOULD
assume the presence of a confused router, and either obey the ICMP
messages' PMTU or (on IPv4 networks) switch to allowing
fragmentation.
DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP
endpoint begins by sending small packets with DF set, then gradually
increases the packet size until a packet is lost. This mechanism
does not require any ICMP error processing. DCCP-Sync packets are
the best choice for upward probing, since DCCP-Sync probes do not
risk application data loss. The DCCP implementation inserts
arbitrary data into the DCCP-Sync application area, padding the
packet to the right length; and since every valid DCCP-Sync
generates an immediate DCCP-SyncAck in response, the endpoint will
have a pretty good idea of when a probe is lost.
16. Forward Compatibility
Future versions of DCCP may add new options and features. A few
simple guidelines will let extended DCCPs interoperate with normal
DCCPs.
o DCCP processors MUST NOT act punitively towards options and
features they do not understand. For example, DCCP processors
MUST NOT reset the connection if some field marked Reserved in
this specification is non-zero; if some unknown option is present;
or if some feature negotiation option mentions an unknown feature.
Instead, DCCP processors MUST ignore these events. The Mandatory
option is the single exception: if Mandatory precedes some unknown
option or feature, the connection MUST be reset.
o DCCP processors MUST anticipate the possibility of unknown feature
values, which might occur as part of a negotiation for a known
feature. For server-priority features, unknown values are handled
as a matter of course: since the non-extended DCCP's priority list
will not contain unknown values, the result of the negotiation
cannot be an unknown value. A DCCP SHOULD reset the connection if
it is assigned an unacceptable value for some non-negotiable
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feature.
o Each DCCP extension SHOULD be controlled by some feature. The
default value of this feature should correspond to "extension not
available". If an extended DCCP wants to use the extension, it
SHOULD attempt to change the feature's value using a Change L or
Change R option. Any non-extended DCCP will ignore the option,
thus leaving the feature value at its default, "extension not
available".
Section 20 lists DCCP assigned numbers reserved for experimental and
testing purposes.
17. Middlebox Considerations
This section describes properties of DCCP that firewalls, network
address translators, and other middleboxes should consider,
including parts of the packet that middleboxes should not change.
The intent is to draw attention to aspects of DCCP that may be
useful, or dangerous, for middleboxes, or that differ significantly
from TCP.
The Service Code field in DCCP-Request packets provide information
that may be useful for stateful middleboxes. With Service Code, a
middlebox can tell what protocol a connection will use without
relying on port numbers. Middleboxes can disallow attempted
connections accessing unexpected services by sending a DCCP-Reset
with Reset Code 9, "Bad Service Code". Middleboxes probably
shouldn't modify the Service Code, unless they are really changing
the service a connection is accessing.
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.
Modifying DCCP Sequence Numbers and Acknowledgement Numbers is more
tedious and dangerous than modifying TCP sequence numbers. A
middlebox that added packets to, or removed packets from, a DCCP
connection would have to modify acknowledgement options, such as Ack
Vector, and CCID-specific options, such as TFRC's Loss Intervals, at
minimum. On ECN-capable connections, the middlebox would have to
keep track of ECN Nonce information for packets it introduced or
removed, so that the relevant acknowledgement options continued to
have correct ECN Nonce Echoes, or risk the connection being reset
for "Aggression Penalty". Furthermore, if a middlebox completely
changed sequence numbers, the DCCP-Move mobility mechanism might
stop working. We therefore recommend that middleboxes not modify
packet streams by adding or removing packets.
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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 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. This might
involve fragmenting the packet.)
Middleboxes may, of course, reset connections in progress. Clearly
this requires inserting a packet into one or both packet streams,
but the difficult issues do not arise.
DCCP is somewhat unfriendly to "connection splicing" [SHHP00], in
which clients' connection attempts are intercepted, but possibly
later "spliced in" to external server connections via sequence
number manipulations. A connection splicer at minimum would have to
ensure that the spliced connections agreed on all relevant feature
values, which might take some renegotiation.
The contents of this section should not be interpreted as a
wholesale endorsement of stateful middleboxes.
18. Relations to Other Specifications
18.1. DCCP and RTP
The Real-Time Transport Protocol, RTP [RFC 3550], is currently used
over UDP by many of DCCP's target applications (for instance,
streaming media). Therefore, it is important to examine 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.
There are two potential sources of overhead in the RTP-over-DCCP
combination, duplicated acknowledgement information and duplicated
sequence numbers. Together, these sources of overhead add slightly
more than 4 bytes per packet relative to RTP-over-UDP, and that
eliminating the redundancy would not reduce the overhead.
First, consider acknowledgements. Both RTP and DCCP report feedback
about loss rates to data senders, via Real-Time Control Protocol
Sender and Receiver Reports (RTCP SR/RR packets) and via DCCP
acknowledgement options. These feedback mechanisms are potentially
redundant. However, RTCP SR/RR packets contain information not
present in DCCP acknowledgements, such as "interarrival jitter", and
DCCP's acknowledgements contain information not transmitted by RTCP,
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such as the ECN Nonce Echo. Neither feedback mechanism makes the
other redundant.
Sending both types of feedback isn't particularly costly either.
RTCP reports are sent relatively infrequently: once every 5 seconds,
for low-bandwidth flows. In DCCP, some feedback mechanisms are
expensive---Ack Vector, for example, is frequent and verbose---but
others are relatively cheap: CCID 3 (TFRC) acknowledgements take
between 16 and 32 bytes of options sent once per round trip time.
(Reporting less frequently than once per RTT would make congestion
control less responsive to loss.) We therefore conclude that
acknowledgement overhead in RTP-over-DCCP is not significantly
higher than for RTP-over-UDP, at least for CCID 3.
One clear redundancy can be addressed at the application level. The
verbose packet-by-packet loss reports sent in RTCP Extended Reports
(RTCP XR) Loss RLE Blocks can be derived from DCCP's Ack Vector
options. (The converse is not true, since Loss RLE Blocks contain
no ECN information.) Since DCCP implementations should provide an
API for application access to Ack Vector information, RTP-over-DCCP
applications might request either DCCP Ack Vectors or RTCP Extended
Report Loss RLE Blocks, but not both.
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 cannot be inferred from the DCCP
sequence number either; for instance, RTP sequence numbers might be
sent out of order. Furthermore, removing RTP's sequence number
would not save any header space because of alignment issues. We
therefore recommend that RTP transmitted over DCCP use the same
headers currently defined. The 4 byte header cost is a reasonable
tradeoff for DCCP's congestion control features and access to ECN.
Truly bandwidth-starved endpoints should use header compression.
18.2. Multiplexing Issues
Since DCCP doesn't provide reliable, ordered delivery, multiple
application sub-flows may be multiplexed over a single DCCP
connection with no inherent performance penalty. Thus, there is no
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need for DCCP to provide built-in, SCTP-style support for multiple
sub-flows.
Some applications might want to share congestion control state among
multiple DCCP flows that share the same source and destination
addresses. This functionality could be provided by the Congestion
Manager [RFC 3124], a generic multiplexing facility. However, the
CM would not fully support DCCP without change; it does not
gracefully handle multiple congestion control mechanisms, for
example.
19. 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. Sequence number validity
checks provide this guarantee. Section 7.5.5 describes sequence
number security further.
This security property only holds assuming that DCCP's random
numbers are chosen according to the guidelines in [RFC 1750].
DCCP provides no protection against attackers that can snoop on data
packets.
19.1. Security Considerations for Mobility
Mobility slightly changes DCCP's security properties by introducing
a new mechanism by which an attacker can hijack a connection. This
mechanism, DCCP-Move, has the unfortunate property that, given a
successful attack, the victim could not realize that the connection
has been stolen---its connection would simply be reset unexpectedly.
Nevertheless, a DCCP attacker still must snoop on data packets to
get any reasonable probability of success, since it must guess a
valid Mobility ID. Section 14.6 quantifies the probability of
successful attack; with DCCP's 128-bit Mobility IDs, that
probability is quite low.
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19.2. Security Considerations for Partial Checksums
The partial checksum facility has a separate security impact,
particularly in its interaction with authentication and encryption
mechanisms. The impact is the same in DCCP as in the UDP-Lite
protocol, and what follows was adapted from the corresponding text
in the UDP-Lite specification [UDP-LITE].
When a DCCP packet's Checksum Coverage field is not zero, the
uncovered portion of a packet may change in transit. This is
contrary to the idea behind most authentication mechanisms:
authentication succeeds if the packet has not changed in transit.
Unless authentication mechanisms that operate only on the sensitive
part of packets are developed and used, authentication will always
fail for partially-checksummed DCCP packets whose uncovered part has
been damaged.
The IPsec integrity check (Encapsulation Security Protocol, ESP, or
Authentication Header, AH) is applied (at least) to the entire IP
packet payload. Corruption of any bit within that area will then
result in the IP receiver discarding a DCCP packet, even if the
corruption happened in an uncovered part of the DCCP application
data.
When IPsec is used with ESP payload encryption, a link can not
determine the specific transport protocol of a packet being
forwarded by inspecting the IP packet payload. In this case, the
link MUST provide a standard integrity check covering the entire IP
packet and payload. DCCP partial checksums provide no benefit in
this case.
Encryption (e.g., at the transport or application levels) may be
used. Note that omitting an integrity check can, under certain
circumstances, compromise confidentiality [BEL98].
If a few bits of an encrypted packet are damaged, the decryption
transform will typically spread errors so that the packet becomes
too damaged to be of use. Many encryption transforms today exhibit
this behavior. There exist encryption transforms, stream ciphers,
which do not cause error propagation. Proper use of stream ciphers
can be quite difficult, especially when authentication-checking is
omitted [BB01]. In particular, an attacker can cause predictable
changes to the ultimate plaintext, even without being able to
decrypt the ciphertext.
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20. IANA Considerations
DCCP introduces several sets of numbers whose values should be
allocated by IANA. The following sets of numbers should require an
IETF standards-track specification as a prerequisite for new
registrations.
o DCCP Packet Types 9 through 15 (Section 5.1).
o 8-bit DCCP-Reset Codes (Section 5.6).
o 8-bit DCCP Option Types (Section 5.9). The CCID-specific options
128 through 255 need not be allocated by IANA, although particular
CCIDs may request that IANA allocate their CCID-specific options.
o 8-bit DCCP Feature Numbers (Section 6). The CCID-specific features
128 through 255 need not be allocated by IANA, although particular
CCIDs may request that IANA allocate their CCID-specific features.
o 8-bit DCCP Congestion Control Identifiers (CCIDs) (Section 10).
o Ack Vector States (Section 11.4). Only State 2 remains
unallocated.
o Data Dropped Drop Codes 4 through 6 (Section 11.7).
IANA should also provide a registry for 32-bit Service Codes.
Registering a Service Code should not require a standards-track
specification. Our liberal proposed registration rules for Service
Codes are presented in detail in Section 8.1.2.
Finally, DCCP requires a Protocol Number to be added to the registry
of Assigned Internet Protocol Numbers. Protocol Number 33 has
informally been made available for experimental DCCP use, but this
number may change in future.
The following DCCP assigned numbers should be reserved specifically
for experimental and testing use [RFC 3692]: packet type 15, option
number 31, option numbers 120 through 126, feature numbers 120
through 126, Reset Codes 248 through 254, and CCID 254.
21. Thanks
Thanks to Jitendra Padhye for his help with early versions of this
specification.
Thanks to Junwen Lai and Arun Venkataramani, who, as interns at
ICIR, built a prototype DCCP implementation. In particular, Junwen
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Lai recommended that the old feature negotiation mechanism be
scrapped and helped design the current mechanism, and Arun
Venkataramani's feedback improved Appendix A.
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
especially thank the DCCP expert reviewers: Greg Minshall, Eric
Rescorla, and Magnus Westerlund for detailed written comments and
problem spotting, and Rob Austein and Steve Bellovin for verbal
comments and written notes.
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, Gorry Fairhurst, Derek Fawcus, David Timothy Fleeman,
John Loughney, Ghyslain Pelletier, Tom Phelan, Stanislav Shalunov,
Yufei Wang, and Michael Welzl. In particular, Michael Welzl
suggested the Data Checksum option.
A. Appendix: Ack Vector Implementation Notes
This appendix 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.
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 buffer
grows from right to left. The implementation maintains five
variables, aside from the buffer contents:
o "buf_head" and "buf_tail", which mark the live portion of the
buffer.
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o "buf_ackno", the Acknowledgement Number of the most recent packet
acknowledged in the buffer. This corresponds to the "head"
pointer.
o "buf_nonce", the one-bit sum (exclusive-or, or parity) of the ECN
Nonces received on all packets acknowledged by the buffer with
State 0.
We draw acknowledgement buffers like this:
+-------------------------------------------------------------------+
|S,L|S,L|S,L|S,L| | | | | |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|
+-------------------------------------------------------------------+
^ ^
buf_tail buf_head, buf_ackno = A buf_nonce = E
<=== buf_head and buf_tail move this way <===
Each `S,L' represents a State/Run length byte. We will draw these
buffers showing only their live portion, and will add an annotation
showing the Acknowledgement Number for the last live byte in the
buffer. For example:
+-----------------------------------------------+
A |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 BN[E]
+-----------------------------------------------+
Here, buf_nonce equals E and buf_ackno equals A. This smaller
Example Buffer contains actual data.
+---------------------------+
10 |0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[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.
Packet 1 was ECN marked.
Packet 0 was received.
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The one-bit sum of the ECN Nonces on packets 10, 6, 5, 4, 3, 2,
and 0 equals 1.
Additionally, the HC-Receiver must keep some information about the
Ack Vectors it has recently sent. For each packet sent carrying an
Ack Vector, it remembers four variables:
o "ack_seqno", the Sequence Number used for the packet. This is an
HC-Receiver sequence number.
o "ack_ptr", the value of buf_head at the time of acknowledgement.
o "ack_ackno", the Acknowledgement Number used for the packet. This
is an HC-Sender sequence number. Since acknowledgements are
cumulative, this single number completely specifies all necessary
information about the packets acknowledged by this Ack Vector.
o "ack_nonce", the one-bit sum of the ECN Nonces for all State 0
packets in the buffer from buf_head to ack_ackno, inclusive.
Initially, this equals the Nonce Echo of the acknowledgement's Ack
Vector (or, if the ack packet contained more than one Ack Vector,
the exclusive-or of all the acknowledgement's Ack Vectors). It
changes as information about old acknowledgements is removed (so
ack_ptr and buf_head diverge), and as old packets arrive (so they
change from State 3 or State 1 to State 0).
A.1. Packet Arrival
This section describes how the HC-Receiver updates its
acknowledgement buffer as packets arrive from the HC-Sender.
A.1.1. New Packets
When a packet with Sequence Number greater than buf_ackno arrives,
the HC-Receiver updates buf_head (by moving it to the left
appropriately), buf_ackno (which is set to the new packet's Sequence
Number), and possibly buf_nonce (if the packet arrived unmarked with
ECN Nonce 1), in addition to the buffer itself. For example, if HC-
Sender packet 11 arrived ECN marked, the Example Buffer above would
enter this new state (changes are marked with stars):
** +***----------------------------+
11 |1,0|0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[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
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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 BN[1]
** +--*------------------------+
Of course, the new packet's sequence number might not equal the
expected sequence number. In this case, the HC-Receiver will 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 if 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 BN[0]
** +*******----------------------------+ *
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 (by increasing run lengths when
possible), transfer its state to a larger buffer, or, as a last
resort, drop all received packets, without processing them
whatsoever, until its buffer shrinks again.
A.1.2. Old Packets
When a packet with Sequence Number S arrives, and S <= buf_ackno,
the HC-Receiver will scan the table for the byte corresponding to S.
(Indexing structures could reduce the complexity of this scan.) If
S was previously lost (State 3), and it was stored in a byte with
run length 0, 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 BN[1]
+--------*------------------+
If S was not marked as lost, or if it was 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 11.4.1 describes what is allowed then.) If S's
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buffer byte has a non-zero run length, then the buffer might need be
reshuffled to make space for one or two new bytes.
The ack_nonce fields may also need manipulation when old packets
arrive. In particular, when S transitions from State 3 or State 1
to State 0, and S had ECN Nonce 1, then the implementation should
flip the value of ack_nonce for every acknowledgement with ack_ackno
>= S.
It is impossible with this data structure to shift packets from
State 0 to State 1, since the buffer doesn't store individual
packets' ECN Nonces.
A.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.
If the acknowledgement fits in one Ack Vector, that Ack Vector's
Nonce Echo simply equals buf_nonce. For multiple Ack Vectors, more
care is required. The Ack Vectors should be split at points
corresponding to previous acknowledgements, since the stored
ack_nonce fields provide enough information to calculate correct
Nonce Echoes. The implementation should therefore acknowledge data
at least once per 253 bytes of buffer state. (Otherwise, there'd be
no way to calculate a Nonce Echo.)
For each acknowledgement it sends, the HC-Receiver will add an
acknowledgement record. ack_seqno will equal the HC-Receiver
sequence number it used for the ack packet; ack_ptr will equal
buf_head; ack_ackno will equal buf_ackno; and ack_nonce will equal
buf_nonce.
A.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 finds the acknowledgement record R
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with the appropriate ack_seqno, then:
o Sets buf_tail to R.ack_ptr + 1.
o If R.ack_nonce is 1, it flips buf_nonce, and the value of
ack_nonce for every later ack record.
o Throws away R and every preceding ack record.
(The HC-Receiver 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. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.
2. ack_seqno = 60, ack_ackno = 10, ack_nonce = 0.
Say the HC-Receiver then received a DCCP-DataAck packet with
Acknowledgement Number 59 from the HC-Sender. 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 BN[0]
+------------------*+ * *
The tail byte's run length was adjusted, since packet 3 was in the
middle of that byte. Since R.ack_nonce was 1, the buf_nonce field
was flipped, as were the ack_nonce fields for later acknowledgements
(here, the HC-Receiver Ack 60 record, not shown, has its ack_nonce
set to 1). The HC-Receiver can also throw away stored information
about HC-Receiver Ack 59 and any earlier acknowledgements.
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:
+----*----------------------+
10 |0,0|0,0|3,0|3,0|0,4|1,0|0,0| 0 BN[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
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acknowledgement saying that Packet 9 actually was received.
Therefore, when packet 9 arrived, the HC-Receiver might modify its
acknowledgement record to:
1. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.
2. ack_seqno = 60, ack_ackno = 3, ack_nonce = 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.
A.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 should check
the ECN Nonce for correctness. (As described in Section 11.4.1, 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 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-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.
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B. Appendix: Design Motivation
This section attempts to capture some of the rationale behind
specific details of DCCP design.
B.1. CsCov 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.
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 application data. If the application cannot tolerate any data
corruption, then the checksum must 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 application data, it may do so by including
the Data Checksum option. This would allow DCCP to discard
corrupted application data, 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
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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 application
data. 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 application data 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 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.
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 1750] D. Eastlake, S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750.
[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.
Kohler/Handley/Floyd [Page 114]
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[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.
[RFC 3309] J. Stone, R. Stewart, and D. Otis. Stream Control
Transmission Protocol (SCTP) Checksum Change. RFC 3309.
[RFC 3692] T. Narten. Assigning Experimental and Testing Numbers
Considered Useful. RFC 3692.
[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-02.txt, work in progress, August 2003.
Informative References
[BB01] S.M. Bellovin and M. Blaze. Cryptographic Modes of Operation
for the Internet. 2nd NIST Workshop on Modes of Operation,
August 2001.
[BEL98] S.M. Bellovin. Cryptography and the Internet. Proc. CRYPTO
'98 (LNCS 1462), pp46-55, August, 1988.
[CCID 2 PROFILE] S. Floyd and E. Kohler. Profile for DCCP
Congestion Control ID 2: TCP-like Congestion Control. draft-
ietf-dccp-ccid2-05.txt, work in progress, February 2004.
[CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye. Profile for
DCCP Congestion Control ID 3: TFRC Congestion Control. draft-
ietf-dccp-ccid3-05.txt, work in progress, February 2004.
[LINK BCP] Phil Karn, editor. Advice for Internet Subnetwork
Designers. draft-ietf-pilc-link-design-13.txt, work in
progress, February 2003.
[M85] Robert T. Morris. A Weakness in the 4.2BSD Unix TCP/IP
Software. Computer Science Technical Report 117, AT&T Bell
Laboratories, Murray Hill, NJ, February 1985.
[PMTUD] Matt Mathis, John Heffner, and Kevin Lahey. Path MTU
Discovery. draft-ietf-pmtud-method-00.txt, work in progress,
October 2003.
[RFC 792] J. Postel, editor. Internet Control Message Protocol.
RFC 792.
Kohler/Handley/Floyd [Page 115]
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[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.
[RFC 3448] M. Handley, S. Floyd, J. Padhye, and J. Widmer. TCP
Friendly Rate Control (TFRC): Protocol Specification. RFC 3448.
[RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang. A
Conservative Selective Acknowledgment (SACK)-based Loss Recovery
Algorithm for TCP. RFC 3517.
[RFC 3540] N. Spring, D. Wetherall, and D. Ely. Robust Explicit
Congestion Notification (ECN) Signaling with Nonces. RFC 3540.
[RFC 3550] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson.
RTP: A Transport Protocol for Real-Time Applications. RFC 3550.
[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.
[SYNCOOKIES] Daniel J. Bernstein. SYN Cookies.
http://cr.yp.to/syncookies.html, as of July 2003.
Authors' Addresses
Kohler/Handley/Floyd [Page 116]
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Eddie Kohler <kohler@cs.ucla.edu>
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA
Mark Handley <M.Handley@cs.ucl.ac.uk>
Department of Computer Science
University College London
Gower Street
London WC1E 6BT
UK
Sally Floyd <floyd@icir.org>
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA
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and distributed, in whole or in part, without restriction of any
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Kohler/Handley/Floyd [Page 118]
