Internet Engineering Task Force Eddie Kohler
INTERNET-DRAFT UCLA
draft-ietf-dccp-spec-08.txt Mark Handley	draft-ietf-dccp-spec-07.txt Mark Handley
Expires: 25 April 2005 UCL	Expires: April 2005 UCL
Sally Floyd
ICIR
25 October 2004


Datagram Congestion Control Protocol (DCCP)


Status of this Memo

This document is an Internet-Draft and is subject to all provisions	This document is an Internet-Draft.
of section 3 of RFC 3667. By submitting this Internet-Draft, each	By submitting this Internet-Draft, we certify that any applicable
author represents that any applicable patent or other IPR claims of	patent or other IPR claims of which we are aware have been
which he or she is aware have been or will be disclosed, and any of	disclosed, or will be disclosed, and any of which we become aware
which he or she become aware will be disclosed, in accordance with	will be disclosed, in accordance with RFC 3668 (BCP 79).
RFC 3668.	By submitting this Internet-Draft, we accept the provisions of
	Section 3 of RFC 3667 (BCP 78).
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
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This Internet-Draft will expire on 25 April 2005.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.






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Abstract

The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that provides bidirectional unicast connections of	protocol that implements bidirectional, unicast connections of
congestion-controlled unreliable datagrams. DCCP is suitable for	congestion-controlled, unreliable datagrams. It should be suitable
applications that transfer fairly large amounts of data, but can	for use by applications such as streaming media, Internet telephony,
benefit from control over the tradeoff between timeliness and	and on-line games.
reliability.











































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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

Changes since draft-ietf-dccp-spec-07.txt:

* Many changes, not listed here, for WGLC.

* The more stringent Sequence Number checks on DCCP-Sync and DCCP-
SyncAck packets become SHOULD, not MAY.

Changes since draft-ietf-dccp-spec-06.txt:

* Change extended sequence numbers. Now 48-bit sequence numbers are
MANDATORY, and all packet types except Data, Ack, and DataAck always
use 48-bit sequence numbers. This change improves DCCP's robustness
against blind attacks.

* Removed empty Change options.

* Allow preference list changes during feature negotiations (this
seems easier to implement than the alternative). This required a
new feature negotiation state, UNSTABLE.

* Add Minimum Checksum Coverage feature.

* Add Reset Congestion State option.

* Simplify the implementation of CCID-specific option processing: no
need to check whether the CCID feature is being negotiated.

* Many more minor changes.

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".




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* Rename "Reset Reason" to "Reset Code".

* Mobility ID becomes 128 bits long.

* Add probabilities to Mobility ID discussion.

* Add SyncAck.












































Kohler/Handley/Floyd [Page 4]

INTERNET-DRAFT Expires: 25 April 2005 October 2004	7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 46
	7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . . 46
	7.5. Validity and Synchronization . . . . . . . . . . . . . . 47
7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 48	7.5.1. Sequence-Validity Rules . . . . . . . . . . . . . . 47
7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 49	7.5.2. Handling Sequence-Invalid Packets . . . . . . . . . 49
7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 49	7.5.3. Sequence and Acknowledgement Number
7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 50	Windows. . . . . . . . . . . . . . . . . . . . . . . . . . 50
7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . . 51	7.5.4. Sequence Window Feature . . . . . . . . . . . . . . 51
7.5. Validity and Synchronization . . . . . . . . . . . . . . 51	7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 52
7.5.1. Sequence and Acknowledgement Number	7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . . 53
Windows. . . . . . . . . . . . . . . . . . . . . . . . . . 52	7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . . 54
7.5.2. Sequence Window Feature . . . . . . . . . . . . . . 53	7.6.1. Allow Short Sequence Numbers Feature. . . . . . . . 54
7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . . 53	7.6.2. When to Avoid Short Sequence Numbers. . . . . . . . 55
7.5.4. Handling Sequence-Invalid Packets . . . . . . . . . 55	7.7. NDP Count and Detecting Application Loss . . . . . . . . 55
7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 56	7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . . 56
7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . . 57	7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 57
7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . . 58	8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 57
7.6.1. Allow Short Sequence Numbers Feature. . . . . . . . 59	8.1. Connection Establishment . . . . . . . . . . . . . . . . 57
7.6.2. When to Avoid Short Sequence Numbers. . . . . . . . 59	8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 57
7.7. NDP Count and Detecting Application Loss . . . . . . . . 60	8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 58
7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . . 61	8.1.3. Server Response . . . . . . . . . . . . . . . . . . 59
7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 61	8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 60
8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 61	8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 61
8.1. Connection Establishment . . . . . . . . . . . . . . . . 62	8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 62
8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 62	8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 62
8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 63	8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 64
8.1.3. Server Response . . . . . . . . . . . . . . . . . . 64	8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 64
8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 65	8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 65
8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 66	9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 66	9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 69
8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 67	9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 70
8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 69	9.2.1. Minimum Checksum Coverage Feature . . . . . . . . . 71
8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 69	9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 71
8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 70	9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 72
9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 74	9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . . 73
9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 75	10. Congestion Control IDs . . . . . . . . . . . . . . . . . . . 73
9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 76	10.1. Unspecified Sender-Based Congestion Control . . . . . . 74
9.2.1. Minimum Checksum Coverage Feature . . . . . . . . . 77	10.2. TCP-like Congestion Control . . . . . . . . . . . . . . 75
9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 77	10.3. TFRC Congestion Control . . . . . . . . . . . . . . . . 76
9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 78	10.4. CCID-Specific Options, Features, and Reset
9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . . 78	Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 79	11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 78
10.1. TCP-like Congestion Control . . . . . . . . . . . . . . 80	11.1. Acks of Acks and Unidirectional Connections . . . . . . 78
10.2. TFRC Congestion Control . . . . . . . . . . . . . . . . 80	11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 80
10.3. CCID-Specific Options, Features, and Reset	11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 80
Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80	11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 82
10.4. CCID Profile Requirements . . . . . . . . . . . . . . . 83	11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 84
10.5. Congestion State. . . . . . . . . . . . . . . . . . . . 83	11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 85
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 84	11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 86
11.1. Acks of Acks and Unidirectional Connections . . . . . . 84	11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 86
11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 86	11.7. Reset Congestion State Option . . . . . . . . . . . . . 87
	11.8. Data Dropped Option . . . . . . . . . . . . . . . . . . 87
	11.8.1. Data Dropped and Normal Congestion
	Response . . . . . . . . . . . . . . . . . . . . . . . . . 90
Kohler/Handley/Floyd [Page 6]	11.8.2. Particular Drop Codes. . . . . . . . . . . . . . . 90

INTERNET-DRAFT Expires: 25 April 2005 October 2004	12. Explicit Congestion Notification . . . . . . . . . . . . . . 91
	12.1. ECN Capable Feature . . . . . . . . . . . . . . . . . . 92
	12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 92
11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 86	12.3. Other Aggression Penalties. . . . . . . . . . . . . . . 93
11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 88	13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 94
11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 90	13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 94
11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 92	13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 94
11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 92	13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 95
11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 93	14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 96
11.7. Data Dropped Option . . . . . . . . . . . . . . . . . . 93	15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 99
11.7.1. Data Dropped and Normal Congestion	16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 99
Response . . . . . . . . . . . . . . . . . . . . . . . . . 96	17. Relations to Other Specifications. . . . . . . . . . . . . . 101
11.7.2. Particular Drop Codes. . . . . . . . . . . . . . . 97	17.1. DCCP and RTP. . . . . . . . . . . . . . . . . . . . . . 101
12. Explicit Congestion Notification . . . . . . . . . . . . . . 98	17.2. Multiplexing Issues . . . . . . . . . . . . . . . . . . 102
12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . . 98	18. Security Considerations. . . . . . . . . . . . . . . . . . . 102
12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 99
12.3. Other Aggression Penalties. . . . . . . . . . . . . . . 100
13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 100
13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 101
13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 101
13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 102
14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 103
14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 104
14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 105
15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 106
16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 107
17. Relations to Other Specifications. . . . . . . . . . . . . . 108
17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 108
17.2. Congestion Manager and Multiplexing . . . . . . . . . . 109
18. Security Considerations. . . . . . . . . . . . . . . . . . . 110
18.1. Security Considerations for Partial
Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 110	Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 103
19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 111	19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 104
19.1. Packet Types. . . . . . . . . . . . . . . . . . . . . . 111	20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
19.2. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 111	A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 105
19.3. Option Types. . . . . . . . . . . . . . . . . . . . . . 112	A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 107
19.4. Feature Numbers . . . . . . . . . . . . . . . . . . . . 112	A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 107
19.5. Congestion Control Identifiers. . . . . . . . . . . . . 112	A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 108
19.6. Ack Vector States . . . . . . . . . . . . . . . . . . . 113	A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 109
19.7. Drop Codes. . . . . . . . . . . . . . . . . . . . . . . 113	A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 110
19.8. Service Codes . . . . . . . . . . . . . . . . . . . . . 113	A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 111
20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 113	B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 112
A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 114	B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 112
A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 116	Normative References . . . . . . . . . . . . . . . . . . . . . . 113
A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 116	Informative References . . . . . . . . . . . . . . . . . . . . . 114
A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 117	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 116
A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 118	Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 116
A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 119	Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 116
A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 120
B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 121
B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 121



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Normative References . . . . . . . . . . . . . . . . . . . . . . 122
Informative References . . . . . . . . . . . . . . . . . . . . . 123
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 125
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 125
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 125














































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List of Tables

Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . . 25
Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . . 33
Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . . 35
Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . . 39
Table 5: DCCP Congestion Control Identifiers . . . . . . . . . . 79
Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . . 88
Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . . 95










































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1. Introduction

The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that implements bidirectional, unicast connections of
congestion-controlled, unreliable datagrams. Specifically, DCCP
provides:

o Unreliable flows 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 servers to avoid holding state for
unacknowledged connection attempts and 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
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 Transmission Unit (PMTU) discovery, as per [RFC	o Path Maximum Transfer Unit (PMTU) discovery, as per [RFC 1191].
1191].	DCCP is intended for applications, such as streaming media and
	Internet telephony, where the costs of long delays can exceed the
o A choice of modular congestion control mechanisms. Two	costs of loss and out-of-order delivery. TCP is not well-suited for
mechanisms are currently specified, TCP-like Congestion Control	these applications, since its reliable in-order delivery, combined
[CCID 2 PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion	with congestion control, can cause arbitrarily long delays. UDP
Control [CCID 3 PROFILE], but DCCP is easily extensible to	avoids long delays, but UDP applications must implement congestion
further forms of unicast congestion control.	control on their own. DCCP provides built-in congestion control,
	including ECN support, for unreliable datagram flows. DCCP avoids
DCCP is intended for applications that can benefit from control over	the arbitrary delays associated with TCP. It also implements
the tradeoffs between delay and reliable in-order delivery. Such	reliable connection setup, teardown, and feature negotiation, and
applications include streaming media and Internet telephony. TCP is	provides a choice of congestion control mechanisms.
not well-suited for these applications, since reliable in-order
delivery and congestion control can cause arbitrarily long delays.
UDP avoids long delays, but UDP applications that implement
congestion control must do so on their own. DCCP provides built-in
congestion control, including ECN support, for unreliable datagram



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flows, avoiding the arbitrary delays associated with TCP. It also
implements reliable connection setup, teardown, and feature
negotiation.

2. Design Rationale

One DCCP design goal was to give most streaming UDP applications	Most streaming UDP applications should have little reason not to
little reason not to switch to DCCP, once it is deployed. To	switch to DCCP, once it is deployed. To facilitate this, DCCP was
facilitate this, DCCP was designed to have as little overhead as	designed to have as little overhead as possible, both in terms of
possible, both in terms of the packet header size and in terms of	the packet header size and in terms of the state and CPU overhead
the state and CPU overhead required at end hosts. Only the minimal	required at end hosts. Only the minimal necessary functionality was
necessary functionality was included in DCCP, leaving other	included in DCCP, leaving other functionality, such as forward error
functionality, such as forward error correction (FEC), semi-	correction (FEC), semi-reliability, and multiple streams, to be
reliability, and multiple streams, to be layered on top of DCCP as	layered on top of DCCP as desired. This desire for minimal overhead
desired.	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, on-line games might want to
make quick use of any available bandwidth, while streaming media
might trade off this responsiveness for a steadier, less bursty
rate. (Sudden rate changes can cause unacceptable UI glitches, such
as audible pauses or clicks in the playout stream.) DCCP thus
allows applications to choose from a set of congestion control	allows applications to choose between several forms of congestion
mechanisms. One alternative, TCP-like Congestion Control, halves	control. One choice, TCP-like Congestion Control, halves the
the congestion window in response to a packet drop or mark, as in	congestion window in response to a packet drop or mark, as in TCP.
TCP. Applications using this congestion control mechanism will	Applications using this congestion control mechanism will respond
respond quickly to changes in available bandwidth, but must tolerate	quickly to changes in available bandwidth, but must tolerate the
the abrupt changes in congestion window typical of TCP. A second	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. Other	sending rate while maintaining longer-term fairness with TCP.
alternatives can be added as future congestion control mechanisms
are standardized.

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 implements congestion control itself.

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.

We intend for DCCP's protocol mechanisms, which are described in
this document, to suit any application desiring unicast congestion-
controlled streams of unreliable datagrams. The congestion control
mechanisms currently approved for use with DCCP, which are described
in separate Congestion Control ID Profiles [CCID 2 PROFILE] [CCID 3
PROFILE], may, however, cause problems for some applications,
including high-bandwidth interactive video. These applications
should be able to use DCCP once suitable Congestion Control ID
Profiles are standardized.

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.

Random numbers in DCCP are used for their security properties, and
SHOULD be chosen according to the guidelines in [RFC 1750].	MUST be chosen according to the guidelines in [RFC 1750].

All operations on DCCP sequence numbers, and comparisons such as
"greater" and "greatest", use circular arithmetic modulo 2**48.
This form of arithmetic preserves the relationships between sequence
numbers as they roll over from 2**48 - 1 to 0. We note that the	numbers as they roll over from 2**48 - 1 to 0.
two's-complement trick for implementing circular comparison --
namely, A < B in the circular comparison sense if and only if
(A - B) < 0 in the conventional arithmetic sense -- applies directly
to DCCP sequence numbers, as long as they are stored in the most
significant 48 bits of 64-bit integers.

Reserved bitfields in DCCP packet headers MUST be set to zero by
senders, and MUST be ignored by receivers, unless otherwise
specified. This is to allow for future protocol extensions. In



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particular, DCCP processors MUST NOT reset a DCCP connection simply
because a Reserved field has non-zero value [RFC 3360].

3.2. Parts of a Connection

Each DCCP connection runs between two hosts, which we often name	Each DCCP connection runs between two endpoints, which we often name
DCCP A and DCCP B. Each connection is actively initiated by one of	DCCP A and DCCP B.
the hosts, which we call the client; the other, initially passive	DCCP connections are actively initiated by one endpoint. The active
host is called the server. The term "DCCP endpoint" is used to	endpoint is called the client, and the passive endpoint is called
refer to either of the two hosts explicitly named by the connection	the server.
(the client and the server). The term "DCCP processor" refers more	DCCP connections are bidirectional; data may pass from either
generally to any host that might need to process a DCCP header; this
includes the endpoints and any middleboxes on the path, such as
firewalls and network address translators.

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 application data sent by one endpoint and the corresponding
acknowledgements sent by the other endpoint. We can illustrate this
as follows:

+--------+ A-to-B half-connection: +--------+
\| \| --> application data --> \| \|
\| \| <-- acknowledgements <-- \| \|
\| DCCP A \| \| DCCP B \|
\| \| B-to-A half-connection: \| \|
\| \| <-- application data <-- \| \|
+--------+ --> 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 terms "HC-Sender"
and "HC-Receiver" denote the endpoints sending application data and
acknowledgements, respectively. For example, DCCP A is the HC-
Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.

3.3. Features

A DCCP feature is a connection attribute 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. The endpoints achieve agreement	in use on the two half-connections. The endpoints can achieve
	agreement through the exchange of feature negotiation options in
	DCCP headers, or through out-of-band communication.

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through the exchange of feature negotiation options in DCCP headers.

DCCP features are identified by a feature number and an endpoint.
The notation "F/X" represents the feature with feature number F
located at DCCP endpoint X. Each valid feature number thus
corresponds to two features, which are negotiated separately and
need not have the same value. The two endpoints know, and agree on,
the value of every valid feature. DCCP A is the "feature location"
for all features F/A, and the "feature remote" for all features F/B.

3.4. Round-Trip Times

DCCP round-trip time measurements are performed by congestion	We sometimes refer to round-trip times -- for setting timers, for
control mechanisms; different mechanisms may measure round-trip time	example. If no useful round-trip time estimate is available, a DCCP
in different ways, or not measure it at all. However, the main DCCP	implementation SHOULD use 0.1 seconds instead.
protocol does use round-trip times occasionally, such as in the
initial values for certain timers. Each DCCP implementation thus
defines a default round-trip time for use when no estimate is
available; this parameter should default to not less than
0.2 seconds, a reasonable median round-trip time for Internet TCP
connections. Protocol behavior specified in terms of "round-trip
time" values actually refers to "a current round-trip time estimate
taken by some CCID, or, if no estimate is available, the default
round-trip time parameter".

The maximum segment lifetime, or MSL, is the maximum length of time
a packet can survive in the network. The DCCP MSL should equal that	a packet can survive in the network. The default MSL is two minutes
of TCP, which is normally two minutes.	for this specification.

3.5. Security Limitation

DCCP provides no protection against attackers who can snoop on a	DCCP is not robust against attackers who can snoop on a connection
connection in progress, or who can guess valid sequence numbers in	in progress, or who can guess valid sequence numbers in other ways.
other ways. Applications desiring stronger security should use	Applications desiring stronger security should use IPsec or
IPsec [RFC 2401]; depending on the level of security required,	application-level cryptography.
application-level cryptography may also suffice. These issues are
discussed further in Sections 18 and 7.5.5.

3.6. Robustness Principle

DCCP implementations will follow TCP's "general principle of
robustness": "be conservative in what you do, be liberal in what you
accept from others" [RFC 793].

4. Overview

DCCP's high-level connection dynamics echo those of TCP.
Connections progress through three phases: initiation, including a



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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, and thus 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 as TCP minus bytestream semantics and
reliability, or as UDP plus congestion control, handshakes, and
acknowledgements.

4.1. Packet Types

Ten packet types implement DCCP's 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, shown here. Note the three-way handshakes during
initiation and termination.

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

The two remaining packet types are used to resynchronize after
bursts of loss.

Every DCCP packet starts with a 12-byte generic header. Particular
packet types include additional fixed-size header data; for example,
DCCP-Acks include an Acknowledgement Number. DCCP options and any
application data follow the fixed-size header.

The packet types are as follows:





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DCCP-Request
Sent by the client to initiate a connection (the first part of
the three-way initiation handshake).

DCCP-Response
Sent by the server in response to a DCCP-Request (the second
part of the three-way initiation handshake).

DCCP-Data
Used to transmit application data.

DCCP-Ack
Used to transmit pure acknowledgements.

DCCP-DataAck
Used to transmit application data with piggybacked
acknowledgements.

DCCP-CloseReq
Sent by the server to request that the client close the
connection.

DCCP-Close
Used by the client or the server to close the connection;	Used to close the connection; elicits a DCCP-Reset in response.
elicits a DCCP-Reset in response.

DCCP-Reset
Used to terminate the connection, either normally or abnormally.

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. Unlike TCP sequence numbers, which are byte-	detected and reported. Unlike TCP's byte-based sequence numbers,
based, DCCP sequence numbers increment by one per packet. For	DCCP sequence numbers are packet-based: each packet sent increments
example:	the sequence number by one. For example:

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)




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Every DCCP packet increments the sequence number, whether or not it	Even DCCP-Ack pure acknowledgements increment the sequence number.
contains application data. DCCP-Ack pure acknowledgements increment	In the example, DCCP B's second packet uses sequence number 11,
the sequence number, for instance: DCCP B's second packet above uses	since sequence number 10 was used for an acknowledgement. This lets
sequence number 11, since sequence number 10 was used for an	endpoints detect lost acknowledgements. It also means that
acknowledgement. This lets endpoints detect all packet loss,	endpoints can get out of sync after long bursts of loss; the DCCP-
including acknowledgement loss. It also means that endpoints can	Sync and DCCP-SyncAck packet types are used to recover (Section
get out of sync after long bursts of loss; the DCCP-Sync and DCCP-	7.5).
SyncAck packet types are used to recover (Section 7.5).

Since DCCP provides unreliable semantics, there are no
retransmissions, and it doesn't make sense to have a TCP-style
cumulative acknowledgement field. DCCP's Acknowledgement Number
field equals the greatest sequence number received, rather than the
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.

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 nine possible states are as follows. They are listed in	The nine possible states are as follows. Section 8 describes them
increasing order, so that "state >= CLOSEREQ" means the same as	in more detail.
"state = CLOSEREQ or state = CLOSING or state = TIMEWAIT". Section
8 describes the states in more detail.




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CLOSED
Represents nonexistent connections.

LISTEN
Represents server sockets in the passive listening state.
LISTEN and CLOSED are not associated with any particular DCCP
connection.

REQUEST
A 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
A 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
application 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
and server sockets enter 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
Server and client sockets can both enter this state to close the
connection.

TIMEWAIT
A server or client socket remains in this state for 2MSL (4	A socket remains in this state for 2MSL (4 minutes) after the
minutes) after the connection has been torn down, to prevent	connection has been torn down, to prevent mistakes due to the
mistakes due to the delivery of old packets. Only one of the	delivery of old packets.
endpoints need enter TIMEWAIT state (the other can enter CLOSED
state immediately), and a server can request its client to hold
TIMEWAIT state using the DCCP-CloseReq packet type.





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4.4. Congestion Control

DCCP connections are congestion controlled, but unlike in TCP, DCCP
applications have a choice of congestion control mechanism. In
fact, the two half-connections can be governed by different
mechanisms. Mechanisms are denoted by one-byte congestion control
identifiers, or CCIDs. The endpoints negotiate their CCIDs during
connection initiation. Each CCID describes how the HC-Sender limits
data packet rates, how the HC-Receiver sends congestion feedback via
acknowledgements, and so forth. CCIDs 2 and 3 are currently
defined; CCIDs 0, 1, and 4-255 are reserved. Other CCIDs may be	defined; CCID 0 is reserved, and CCID 1 is used for special
defined in the future.	purposes.

CCID 2 provides 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] 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 selective
acknowledgement (SACK) [RFC 2018].	acknowledgement (SACK) [RFC 3517].

CCID 3 provides TFRC Congestion Control, an equation-based form of
congestion control intended to respond to congestion more smoothly
than CCID 2. The sender maintains a transmit rate, which it updates
using the receiver's estimate of the packet loss and mark rate.
CCID 3 behaves somewhat differently from TCP in the short term, it
is designed to operate fairly with TCP over the long term.

Section 10 describes DCCP's CCIDs in more detail. The behaviors of
CCIDs 2 and 3 are fully defined in separate profile documents [CCID
2 PROFILE] [CCID 3 PROFILE].

4.5. Features

DCCP endpoints use Change and Confirm options to negotiate and agree	DCCP endpoints generally use Change and Confirm options to negotiate
on feature values. Feature negotiation will almost always happen on	and agree on feature values, although agreement may also be achieved
the connection initiation handshake, but it can begin at any time.	using an out-of-band signalling channel. Feature negotiation will
	almost always happen on the connection initiation handshake, but it
There are four feature negotiation options in all: Change L,	can begin at any time.
Confirm L, Change R, and Confirm R. The "L" options are sent by the
feature location, and the "R" options are sent by the feature
remote. A Change R option says to the feature location, "change
this feature value as follows". The feature location responds with
Confirm L, meaning "I've changed it". Some features allow Change R
options to contain multiple values, sorted in preference order. For
example:




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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 *

Both exchanges negotiate the CCID/Server feature's value, which is	In the second exchange, the client requests that the server use
the CCID in use on the server-to-client half-connection. In the	either CCID 3 or CCID 4, with 3 preferred. The server chooses 4 and
second exchange, the client requests that the server use either
CCID 3 or CCID 4, with 3 preferred; the server chooses 4 and
supplies its preference list, "4 2".

The Change L and Confirm R options are used for feature negotiations
initiated by the feature location. In the following example, the
server requests that CCID/Server be set to 3 or 2, with 3 preferred,
and the client agrees.

Client Server
------ ------
<-- Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2) -->
* agreement that CCID/Server = 3 *


Section 6 describes the feature negotiation options further,
including the retransmission strategies that make negotiation
reliable.

4.6. Differences From TCP

Differences between DCCP and TCP apart from those discussed so far
include:

o Copious space for options (up to 1008 bytes or the PMTU).	o Copious space for options (up to 1008 bytes).
	o Different acknowledgement formats. The CCID for a connection
o Different acknowledgement formats. The CCID for a connection
determines how much acknowledgement information needs to be
transmitted. For example, in CCID 2 (TCP-like), this is about one	transmitted. In CCID 2 (TCP-like), this is about one ack per 2
ack per 2 packets, and each ack must declare exactly which	packets, and each ack must declare exactly which packets were
packets were received; in CCID 3 (TFRC), it's about one ack per	received; in CCID 3 (TFRC), it's about one ack per RTT, and acks
round-trip time, and acks must declare at minimum just the	must declare at minimum just the lengths of recent loss
lengths of recent loss intervals.	intervals.
	o Denial-of-service (DoS) protection. Several mechanisms help limit
	the amount of state possibly-misbehaving clients can force DCCP
	servers 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-
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o Denial-of-service (DoS) protection. Several mechanisms help
limit the amount of state possibly-misbehaving clients can force
DCCP servers 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	(Section 11.8) 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 such losses will cause a congestion response).

o Acknowledgeability. In TCP, a packet may be acknowledged only	o Acknowledgement readiness. In TCP, a packet is acknowledged only
once the data is reliably queued for application delivery. This	when the data is queued for delivery to the application. This
does not make sense in DCCP, where an application might, for	does not make sense in DCCP, where an application might request a
example, request a drop-from-front receive buffer. A DCCP packet	drop-from-front receive buffer, for example. DCCP acknowledges a
may be acknowledged as soon as its header has been successfully	packet when its options have been processed. The Data Dropped
processed. Concretely, a packet becomes acknowledgeable at	option may later report that the packet's payload was discarded.
Step 8 of Section 8.5's packet processing pseudocode.
Acknowledgeability does not guarantee data delivery, however: the
Data Dropped option may later report that the packet's
application data was discarded.

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. The Data Dropped	closed while the other is still active.
option's Drop Code 1, Application Not Listening (Section 11.7),
can achieve a similar effect, however.

4.7. Example Connection

The progress of a typical DCCP connection is as follows. (This
description is informative, not normative.)







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Client Server
------ ------
0. [CLOSED] [LISTEN]
1. DCCP-Request -->
2. <-- DCCP-Response
3. DCCP-Ack -->
\|\| TEXT DELETED \|\|	<-- 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 an application request on the DCCP-Request packet,
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. This
response indicates any features and options that the server
agrees to, begins other feature negotiations as desired, and
optionally includes an Init Cookie that wraps up all this
information and which must be returned by the client for the
connection to complete.

3. The client sends the server a DCCP-Ack packet that acknowledges
the DCCP-Response packet. This acknowledges the server's
initial sequence number and returns the Init Cookie if there was
one in the DCCP-Response. It may also continue feature
negotiation. 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 data with piggybacked 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. (However, the client may not send DCCP-Data	Acks exclusively.
packets before receiving at least one non-DCCP-Response packet
from the server.)

5. The server sends a DCCP-CloseReq packet requesting a close.





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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. DCCP-Resets are part
of normal connection termination; see Section 5.6.

8. The client receives the DCCP-Reset packet and holds state for	8. The client receives the DCCP-Reset packet and holds state for a
two maximum segment lifetimes, or 2MSL, to allow any remaining	reasonable interval of time to allow any remaining packets to
packets to clear the network.	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	7b. The client receives the DCCP-Reset packet and holds state for a
2MSL to allow any remaining packets to clear the network.	reasonable interval of time to allow any remaining packets to
	clear the network.
5. Packet Formats	5. Header Formats

The DCCP header can be from 12 to 1020 bytes long. The initial 12
bytes of the header have the same semantics for all currently-	bytes of the header have the same semantics for all packet types.
defined packet types. Following this comes any additional fixed-	Following this comes any additional fixed-length fields required by
length fields required by the packet type, and then a variable-	the packet type, and then a variable-length list of options. Some
length list of options. The application data area follows the	packet types allow application data to follow the header.
header. In some packet types, this area contains data for the
application; in other packet types, its contents are ignored.

+---------------------------------------+ -.
\| Generic Header \| \|	\| Generic Header \| \|
+---------------------------------------+ \|
\| Additional Fields (depending on type) \| +- DCCP Header
+---------------------------------------+ \|
\| Options (optional) \| \|	\| Options (optional) \| \|
+=======================================+ -'
\| Application Data Area \|	\| Application Data (optional) \|
+---------------------------------------+


5.1. Generic Header

The DCCP generic header takes different forms depending on the value
of X, the Extended Sequence Numbers bit. If X is one, the Sequence
Number field is 48 bits long and the generic header takes 16 bytes,
as follows.



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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Source Port \| Dest Port \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Data Offset \| CCVal \| CsCov \| Checksum \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| \| \|X\| \| .	\| \|X\| \| .
\| Res \| Type \|=\| Reserved \| Sequence Number (high bits) .	\| Res \|=\| Type \| Sequence Number (high bits) .
\| \| \|1\| \| .	\| \|1\| \| .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) \|	. Sequence Number (low bits) \| Reserved \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If X is zero, only the low 24 bits of the Sequence Number are
transmitted, and the generic header is 12 bytes long.

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 \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| \| \|X\| \|	\| \|X\| \| \|
\| Res \| Type \|=\| Sequence Number (low bits) \|	\| Res \|=\| Type \| Sequence Number (low bits) \|
\| \| \|0\| \|	\| \|0\| \| \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The generic header fields are defined as follows.

Source and Destination Ports: 16 bits each
These fields identify the connection, similar to the
corresponding fields in TCP and UDP. The Source Port represents
the relevant port on the endpoint that sent this packet, the
Destination Port the relevant port on the other endpoint. When	Destination Port the relevant port on the other endpoint.
initiating a connection, the client SHOULD choose its Source	Source Ports SHOULD be chosen randomly, to reduce the likelihood
Port randomly to reduce the likelihood of attack.	of attack.

DCCP APIs should treat port numbers similarly to TCP and UDP
port numbers. For example, machines that distinguish between
"privileged" and "unprivileged" ports for TCP and UDP should do
the same for DCCP.

Data Offset: 8 bits
The offset from the start of the packet's DCCP header to the	The offset from the start of the DCCP header to the beginning of
start of its application data area, in 32-bit words. The	the packet's application data, in 32-bit words.



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receiver MUST ignore packets whose Data Offset is smaller than
the minimum-sized header for the given Type, or larger than the
DCCP packet itself.

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. The sender MUST set CCVal to zero unless
its HC-Sender CCID specifies otherwise, and the receiver MUST
ignore the CCVal field unless its HC-Receiver CCID specifies
otherwise.

Checksum Coverage (CsCov): 4 bits
Checksum Coverage determines the parts of the packet that are
covered by the Checksum field. This always includes the DCCP
header and options, but some or all of the application data may
be excluded. This can improve performance on noisy links for
applications that 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, all, some, or none of the application data.	Checksum Coverage, some or all of the application data. See
See Section 9.	Section 9.

Reserved (Res): 3 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.

Type: 4 bits
The Type field specifies the type of the packet. The following
values are defined:


















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Type Meaning	Type Meaning
---- -------	---- -------
0 DCCP-Request	0 DCCP-Request
1 DCCP-Response	1 DCCP-Response
2 DCCP-Data	2 DCCP-Data
3 DCCP-Ack	3 DCCP-Ack
4 DCCP-DataAck	4 DCCP-DataAck
5 DCCP-CloseReq	5 DCCP-CloseReq
6 DCCP-Close	6 DCCP-Close
7 DCCP-Reset	7 DCCP-Reset
8 DCCP-Sync	8 DCCP-Sync
9 DCCP-SyncAck	9 DCCP-SyncAck
10-15 Reserved	10-15 Reserved

Table 1: DCCP Packet Types

Receivers MUST ignore any packets with reserved type. That is,
packets with reserved type MUST NOT be processed and they MUST
NOT be acknowledged as received.
\|\| TEXT DELETED \|\|	Reserved (Res): 3 bits
	Senders MUST set this field to all zeroes on generated packets,
Extended Sequence Numbers (X): 1 bit	and receivers MUST ignore its value.
Set to one to indicate the use of an extended generic header
with 48-bit Sequence and Acknowledgement Numbers. DCCP-Data,
DCCP-DataAck, and DCCP-Ack packets MAY set X to zero or one.
All DCCP-Request, DCCP-Response, DCCP-CloseReq, DCCP-Close,
DCCP-Reset, DCCP-Sync, and DCCP-SyncAck packets MUST set X to
one; endpoints MUST ignore any such packets with X set to zero.
High-rate connections SHOULD set X to one on all packets to gain
increased protection against wrapped sequence numbers and
attacks. See Section 7.6.

Sequence Number: 48 or 24 bits	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.

All currently defined packet types except DCCP-Request and DCCP-Data
carry an Acknowledgement Number Subheader in the four or eight bytes	carry an Acknowledgement Number in the four or eight bytes
immediately following the generic header. When X=1, its format is:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reserved \| Acknowledgement Number .	\| Reserved \| Acknowledgement Number (high bits) .
\| \| (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Acknowledgement Number (low bits) \|	. Acknowledgement Number (low bits) \| Reserved \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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When X=0, only the low 24 bits of the Acknowledgement Number are
transmitted, giving the Acknowledgement Number Subheader this	transmitted.
format:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reserved \| Acknowledgement Number (low bits) \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Reserved: 16 or 8 bits	Acknowledgement Number: 24 or 48 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.

Acknowledgement Number: 48 or 24 bits
Generally contains GSR, the Greatest Sequence Number Received on
any acknowledgeable packet so far. A packet is acknowledgeable
if and only if its header was successfully processed by the	if and only if its header options were processed by the
receiver; Section 7.4 describes this further. Options such as	receiver. Section 7.4 describes this further. Options such as
Ack Vector (Section 11.4) combine with the Acknowledgement
Number to provide precise information about which packets have
arrived.

Acknowledgement Numbers on DCCP-Sync and DCCP-SyncAck packets
need not equal GSR. See Section 5.7.	need not equal GSR; see Section 5.7.
	Reserved: 8 bits
5.2. DCCP-Request Packets	Senders MUST set this field to all zeroes on generated packets,
	and receivers MUST ignore its value.
A client initiates a DCCP connection by sending a DCCP-Request	5.2. DCCP-Request Header
packet. These packets MAY contain application data, and MUST use	packet. These packets MAY contain application data.
48-bit sequence numbers (X=1).

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 with X=1 (16 bytes) /
/ with Type=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /	\| Application Data \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	\| ... \|


Service Code: 32 bits
Describes the application-level service to which the client	Describes the service to which the client application wants to
application wants to connect. Service Codes are intended 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.
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provide information about which application protocol a
connection intends to use, and thus aiding middleboxes and
reducing reliance on globally well-known ports. See Section
8.1.2.

5.3. DCCP-Response Packets

The server responds to valid DCCP-Request packets with DCCP-Response
packets. This is the second phase of the three-way handshake.
DCCP-Response packets MAY contain application data, and MUST use	DCCP-Response packets MAY contain application data.
48-bit sequence numbers (X=1).

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 with X=1 (16 bytes) /
/ with Type=1 (DCCP-Response) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /	\| Reserved \| Acknowledgement Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|	. Acknowledgement Number (low bits) \| Reserved \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /	\| Application Data \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	\| ... \|


Acknowledgement Number: 48 bits
Contains GSR. Since DCCP-Responses are only sent during
connection initiation, this will always equal the Sequence
Number on a received DCCP-Request.

Service Code: 32 bits
MUST equal the Service Code on the corresponding DCCP-Request.	Echoes the Service Code on a received DCCP-Request.
	5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets

The central data transfer portion of every DCCP connection uses
DCCP-Data, DCCP-Ack, and DCCP-DataAck packets. These packets MAY	DCCP-Data, DCCP-Ack, and DCCP-DataAck packets. DCCP-Data packets
use 24-bit sequence numbers, depending on the value of the Allow	carry application data.
Short Sequence Numbers feature (Section 7.6.1). DCCP-Data packets
carry application data without 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 (16 or 12 bytes) /	/ Generic DCCP Header (12 or 16 bytes) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /	\| Application Data \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	\| ... \|

DCCP-Ack packets dispense with the data, but contain an
Acknowledgement Number. They are used for pure acknowledgements.

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 (16 or 12 bytes) /	/ Generic DCCP Header (12 or 16 bytes) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /	\| Reserved \| Acknowledgement Number \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)
/ Options and Padding /	(. Acknowledgement Number (low bits) \| Reserved \|)
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Application Data Area (Ignored) /	\| Options / Padding \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	(The parenthesized fields appear only when the header's Extended
DCCP-DataAck packets carry both application data and an	Sequence Numbers field is 1.) DCCP-DataAck packets carry both
Acknowledgement Number: acknowledgement information is piggybacked	application data and an Acknowledgement Number: acknowledgement
on a data packet.	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 (16 or 12 bytes) /	/ Generic DCCP Header (12 or 16 bytes) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /	\| Reserved \| Acknowledgement Number \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	(+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)
	(. Acknowledgement Number (low bits) \| Reserved \|)
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /	\| Application Data \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	\| ... \|

A DCCP-Data or DCCP-DataAck packet may have a zero-length
application data area, which indicates that the application sent a
zero-length datagram. This differs from DCCP-Request and DCCP-
Response packets, where an empty application data area indicates the



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absence of application data (not the presence of zero-length	absence of application data (as opposed to the presence of zero-
application data). The API SHOULD report any received zero-length	length application data).
datagrams to the receiving application.	Receivers MUST ignore the application data area in DCCP-Ack packets.
	DCCP-Ack senders will generally leave this area empty.
A DCCP-Ack packet MAY have a non-zero-length application data area,
which essentially pads the DCCP-Ack to a desired length. Receivers
MUST ignore the content of the application data area in DCCP-Ack
packets.

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 Packets	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
a DCCP-Close packet, which will elicit a DCCP-Reset packet. 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. Both packet types MUST use 48-bit sequence numbers	TIMEWAIT state.
(X=1).

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 with X=1 (16 bytes) /
/ with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /	\| Reserved \| Acknowledgement Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /	. Acknowledgement Number (low bits) \| Reserved \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Application Data Area (Ignored) /	\| Options / Padding \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY	Receivers MUST ignore the application data area in DCCP-CloseReq and
have non-zero-length application data areas, whose contents	DCCP-Close packets.
receivers MUST ignore.	5.6. DCCP-Reset Header

5.6. DCCP-Reset Packets

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. DCCP-Resets	behavior, incorrect ECN Nonce Echoes, and so forth.
MUST use 48-bit sequence numbers (X=1).




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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 with X=1 (16 bytes) /
/ with Type=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /	\| Reserved \| Acknowledgement Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reset Code \| Data 1 \| Data 2 \| Data 3 \|	. Acknowledgement Number (low bits) \| Reserved \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Error Text) /	\| Error Text \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	\| ... \|


Reset Code: 8 bits
Represents the reason that the sender reset the DCCP connection.

Data 1, Data 2, and Data 3: 8 bits each
The Data fields provide additional information about why the
sender reset the DCCP connection. The meanings of these fields
depend on the value of Reset Code.	depend on the value of Reason.
	Error Text (application data area)
Application Data Area: Error Text	If present, Error Text is a human-readable text string,
If present, Error Text is a human-readable text string encoded	preferably in English and encoded in Unicode UTF-8, that
in Unicode UTF-8, and preferably in English, that describes the	describes the error in more detail. For example, a DCCP-Reset
error in more detail. For example, a DCCP-Reset with Reset Code	with Reset Code 11, "Aggression Penalty", might contain Error
11, "Aggression Penalty", might contain Error Text such as	Text such as "Aggression Penalty: Received 3 bad ECN Nonce
"Aggression Penalty: Received 3 bad ECN Nonce Echoes, assuming	Echoes, assuming misbehavior".
misbehavior".

The following Reset Codes are currently defined. Unless otherwise
specified, the Data 1, 2, and 3 fields MUST be set to 0 by the
sender of the DCCP-Reset and ignored by its receiver. Section
references describe concrete situations that will cause each Reset
Code to be generated; they are not meant to be exhaustive.

0, "Unspecified"
Indicates the absence of a meaningful Reset Code. Use of Reset
Code 0 is NOT RECOMMENDED: the sender should choose a Reset Code
that more clearly defines why the connection is being reset.

1, "Closed"
Normal connection close. See Section 8.3.

2, "Aborted"
The sending endpoint gave up on the connection because of lack



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of progress. See Sections 8.1.1 and 8.1.5.

3, "No Connection"
No connection exists. See Section 8.3.1.

4, "Packet Error"
A valid packet arrived with unexpected type. For example, a	An unexpected packet type arrived; for example, a DCCP-Data
DCCP-Data packet with valid header checksum and sequence numbers	packet arrived at a connection in the REQUEST state. See
arrived at a connection in the REQUEST state. See Section	Section 8.3.1. The Data 1 field equals the offending packet
8.3.1. The Data 1 field equals the offending packet type as an	type.
eight-bit number; thus, an offending packet with Type 2 will
result in a Data 1 value of 2.

5, "Option Error"
An option was erroneous, and the error was serious enough to
warrant resetting the connection. See Sections 6.6.7, 6.6.8,
and 11.4. The Data 1 field equals the offending option type;
Data 2 and Data 3 equal the first two bytes of option data (or
zero if the option had less than two bytes of data).

6, "Mandatory Error"
The sending endpoint could not process an option O that was	The sending endpoint could not process an option marked
immediately preceded by Mandatory. The Data fields report the	Mandatory. The Data fields report the option type and data of
option type and data of option O, using the format of Reset Code	the unprocessed option (not the Mandatory option), using the
5, "Option Error". See Section 5.8.2.	format of Reset Code 5, "Option Error". See Section 5.8.2.

7, "Connection Refused"
The Destination Port didn't correspond to a port open for
listening. Sent only in response to DCCP-Requests. See Section
8.1.3.

8, "Bad Service Code"
The Service Code didn't equal the service code attached to the
Destination Port. Sent only in response to DCCP-Requests. See
Section 8.1.3.

9, "Too Busy"
The server is too busy to accept new connections. Sent only in
response to DCCP-Requests. See Section 8.1.3.

10, "Bad Init Cookie"
The Init Cookie echoed by the client was incorrect or missing.
See Section 8.1.4.

11, "Aggression Penalty"
This endpoint has detected congestion control-related
misbehavior on the part of the other endpoint. See Sections
12.2 and 13.2.



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12-127, Reserved
Receivers should treat these codes like Reset Code 0,
"Unspecified".

128-255, CCID-specific codes
Semantics depend on the connection's CCIDs. See Section 10.3.	Semantics depend on the connection's CCIDs. See Section 10.4.
Receivers should treat unknown CCID-specific Reset Codes like
Reset Code 0, "Unspecified".

The following table summarizes this information.

Reset	Reset
Code Name Data 1 Data 2 & 3	Code Name Data 1 Data 2 & 3
----- ---- ------ ----------	----- ---- ------ ----------
0 Unspecified 0 0	0 Unspecified 0 0
1 Closed 0 0	1 Closed 0 0
2 Aborted 0 0	2 Aborted 0 0
3 No Connection 0 0	3 No Connection 0 0
4 Packet Error pkt type 0	4 Packet Error pkt type 0
5 Option Error option # option data	5 Option Error option # option data
6 Mandatory Error option # option data	6 Mandatory Error option # option data
7 Connection Refused 0 0	7 Connection Refused 0 0
8 Bad Service Code 0 0	8 Bad Service Code 0 0
9 Too Busy 0 0	9 Too Busy 0 0
10 Bad Init Cookie 0 0	10 Bad Init Cookie 0 0
11 Aggression Penalty 0 0	11 Aggression Penalty 0 0
12-127 Reserved	12-127 Reserved
128-255 CCID-specific codes	128-255 CCID-specific codes
	5.7. DCCP-Sync and DCCP-SyncAck Headers
Table 2: DCCP Reset Codes

Options on DCCP-Reset packets are processed before the connection is
shut down. This means that certain combinations of options,
particularly involving Mandatory, may cause an endpoint to respond
to a valid DCCP-Reset with another DCCP-Reset. This cannot lead to
a reset storm; since the first endpoint has already reset the
connection, the second DCCP-Reset will be ignored.

5.7. DCCP-Sync and DCCP-SyncAck Packets

DCCP-Sync packets help DCCP endpoints recover synchronization after
bursts of loss, or recover from half-open connections. Each valid
received DCCP-Sync immediately elicits a DCCP-SyncAck. Both packet	received DCCP-Sync immediately elicits a DCCP-SyncAck.
types MUST use 48-bit sequence numbers (X=1).







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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 with X=1 (16 bytes) /
/ with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /	\| Reserved \| Acknowledgement Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	. Acknowledgement Number (low bits) \| Reserved \|
/ Options and Padding /	\| Options / Padding \|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Acknowledgement Number field has special semantics for DCCP-Sync
and DCCP-SyncAck packets. First, the packet corresponding to a
DCCP-Sync's Acknowledgement Number need not have been
acknowledgeable. Thus, receivers MUST NOT assume that a packet was
processed simply because it appears in the Acknowledgement Number
field of a DCCP-Sync packet. This differs from all other packet
types, where the Acknowledgement Number by definition corresponds to
an acknowledgeable packet. Second, the Acknowledgement Number on
any DCCP-SyncAck packet MUST correspond to the Sequence Number on an
acknowledgeable DCCP-Sync packet. In the presence of reordering,
this might not equal GSR.

As with DCCP-Ack packets, DCCP-Sync and DCCP-SyncAck packets MAY	Receivers MUST ignore the application data area in DCCP-Sync and
have non-zero-length application data areas, whose contents	DCCP-SyncAck packets. Endpoints may find it useful to pad DCCP-Sync
receivers MUST ignore. Padded DCCP-Sync packets may be useful when	packets with "application data" when performing PMTU discovery; see
performing Path MTU discovery; see Section 14.	Section 14.

5.8. Options

Any DCCP packet may contain options, which occupy space at the end
of the DCCP header. Each option is a multiple of 8 bits in length.
Individual options are not padded to multiples of 32 bits, and any	The combination of all options MUST add up to a multiple of 32 bits.
option may begin on any byte boundary. However, the combination of	Individual options are not padded to multiples of 32 bits, however;
all options MUST add up to a multiple of 32 bits; Padding options	any option may begin on any byte boundary. Any options present are
MUST be added as necessary to fill out option space to a word	included in the header checksum.
boundary. Any options present are included in the header 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.





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Options are processed sequentially, starting at the first option in
the packet header. Options with unknown types, and options with	the packet header.
invalid lengths (length byte less than two or more than the
remaining space in the options portion of the header), MUST be
ignored.

The following options are currently defined:

Option DCCP- Section	Option Section
Type Length Meaning Data? Reference	Type Length Meaning Reference
---- ------ ------- ----- ---------	---- ------ ------- ---------
0 1 Padding Y 5.8.1	0 1 Padding 5.8.1
1 1 Mandatory N 5.8.2	1 1 Mandatory 5.8.2
2 1 Slow Receiver Y 11.6	2 1 Slow Receiver 11.6
3-31 1 Reserved	3 1 Reset Congestion State 11.7
32 variable Change L N 6.1	4-31 1 Reserved
33 variable Confirm L N 6.2	32 variable Change L 6.1
34 variable Change R N 6.1	33 variable Confirm L 6.2
35 variable Confirm R N 6.2	34 variable Change R 6.1
36 variable Init Cookie N 8.1.4	35 variable Confirm R 6.2
37 3-5 NDP Count Y 7.7	36 variable Init Cookie 8.1.4
38 variable Ack Vector [Nonce 0] N 11.4	37 4-5 NDP Count 7.7
39 variable Ack Vector [Nonce 1] N 11.4	38 variable Ack Vector [Nonce 0] 11.4
40 variable Data Dropped N 11.7	39 variable Ack Vector [Nonce 1] 11.4
41 6 Timestamp Y 13.1	40 variable Data Dropped 11.8
42 6/8/10 Timestamp Echo Y 13.3	41 6 Timestamp 13.1
43 4/6 Elapsed Time N 13.2	42 6-10 Timestamp Echo 13.3
44 6 Data Checksum Y 9.3	43 4-6 Elapsed Time 13.2
45-127 variable Reserved	44 4 Data Checksum 9.3
128-255 variable CCID-specific options - 10.3	45-127 variable Reserved
	128-255 variable CCID-specific options 10.4
Table 3: DCCP Options

Not all options are suitable for all packet types. For example,
since the Ack Vector option is interpreted relative to the
Acknowledgement Number, it isn't suitable on DCCP-Request and DCCP-
Data packets, which have no Acknowledgement Number. If an option
occurs on an unexpected packet type, it MUST generally be ignored;
any such restrictions are mentioned in each option's description.
The table summarizes the most common restriction: when the DCCP-
Data? column value is N, the corresponding option MUST be ignored
when received on a DCCP-Data packet.

This section describes two generic options, Padding and Mandatory.
Other options are described later.






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5.8.1. Padding Option

+--------+
\|00000000\|
+--------+
Type=0

Padding is a single-byte "no-operation" option used to pad between	Padding is a single byte option used to pad between or after
or after options. If the length of a packet's other options is not	options. It either ensures the application data begins on a 32-bit
a multiple of 4, then Padding options are REQUIRED to pad out the	boundary (as required), or ensures alignment of following options
options area to the length implied by Data Offset. Padding may also	(not mandatory).
be used between options -- for example, to align the beginning of a
subsequent option on a word boundary. There is no guarantee that
senders will use this option, so receivers must be prepared to
process options even if they do not begin on a word boundary.

5.8.2. Mandatory Option

+--------+
\|00000001\|
+--------+
Type=1

Mandatory is a single byte option that marks the immediately
following option as mandatory. Say that the immediately following
option is O. Then the Mandatory option has no effect if the	option is OP. Then the Mandatory option has no effect if the
receiving DCCP endpoint understands and processes O. If the	receiving DCCP endpoint understands and processes OP. If the
endpoint does not understand or process O, however, then it MUST	endpoint does not understand or process OP, however, then it MUST
reset the connection using Reset Code 6, "Mandatory Failure". For
instance, the endpoint would reset the connection if it did not
understand O's type; if it understood O's type, but not O's data; if	understand OP's type; if it understood OP's type, but not OP's data;
O's data was invalid for O's type; if O was a feature negotiation	if OP's data was invalid for OP's type; if OP was a feature
option, and the endpoint did not understand the enclosed feature	negotiation option, and the endpoint did not understand the enclosed
number; if the endpoint understood O, but chose not to perform the	feature number; if the endpoint understood OP, but chose not to
action O implies; and so forth.	perform the action OP implies; and so forth.

Mandatory options MUST NOT be sent on DCCP-Data packets, and any
Mandatory options received on DCCP-Data packets MUST be ignored.

The connection is in error and should be reset with Reset Code 5,
"Option Error" if option O is absent (Mandatory was the last byte of	"Option Error" if option OP is absent (Mandatory was the last byte
the option list), or if option O equals Mandatory. However, the	of the option list), or if option OP equals Mandatory. However, the
combination "Mandatory Padding" is valid, and MUST behave like two
bytes of Padding.

Section 6.6.9 describes the behavior of Mandatory feature
negotiation options in more detail.




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6. Feature Negotiation

Four DCCP options, Change L, Confirm L, Change R, and Confirm R, are	Four DCCP options, Change L, Confirm L, Change R, and Confirm R,
used to negotiate feature values. Change options initiate a	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 exact format of the feature	hold one or more feature values. The feature values are generally
value area depends on the feature type; see Section 6.3.	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.

Feature negotiation options MUST NOT be sent on DCCP-Data packets,
and any feature negotiation options received on DCCP-Data packets
MUST be ignored.

6.1. Change Options

Change L and Change R options initiate feature negotiation. The	Change L and Change R options initiate feature negotiation. Which
option to use depends on the relevant feature's location: To start a	option to use depends on where the negotiated feature is located.
negotiation for feature F/A, DCCP A will send a Change L option; to	To start a negotiation for feature F/A, DCCP A must send a Change L
start a negotiation for F/B, it will send a Change R option. Change	option; to start a negotiation for F/B, it must send a Change R
options are retransmitted until some response is received. They	option. Change options are retransmitted until some response is
contain at least one Value, and thus have length at least 4.	received. Change options contain at least one Value, and thus have
	length at least 4.
+--------+--------+--------+--------+--------
Change L: \|00100000\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=32

+--------+--------+--------+--------+--------
Change R: \|00100010\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=34







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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. Any packet including a Confirm option MUST carry an
options are retransmitted as necessary. The first byte of the	Acknowledgement Number; thus, Confirm options are not allowed on
Confirm option contains the feature number from the corresponding	DCCP-Request and DCCP-Data packets. Confirm options need not be
Change. Following this is the selected Value, and then possibly the	retransmitted, since Change options are retransmitted as necessary.
sender's preference list.	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 the problematic feature number, but no	Confirm option containing no value. Such options have length 3.
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
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 list of	by the feature number). Each Change option contains a preference
values ordered by preference, with the most preferred value coming	list of values, with the most preferred value coming first. Each
first. Each Confirm option contains the confirmed value, followed	Confirm option contains the confirmed value, followed by the
by the confirmer's preference list. Thus, the feature's current	confirmer's preference list. Thus, the feature's current value will
value will generally appear twice in Confirm options' data, once as	generally appear twice in Confirm options' data, once as the current
the current value and once in the confirmer's preference list.	value and once in the confirmer's preference list.





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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 6.6.9).
\|\| TEXT DELETED \|\|	A single feature negotiation may, because of loss or delay, contain
	retransmitted Change options and multiple Confirm options. Each of
6.3.2. Non-Negotiable	the retransmitted Change options MUST contain the same payload; see
	Section 6.6.3. For server-priority features, this means that an
The feature value is a byte string. Each option contains exactly	endpoint sending Change options MUST NOT change its preference list
one feature value. The feature location signals a new value by	during a negotiation. However, the other endpoint MAY change its
sending a Change L option. The feature remote MUST accept any valid	preference list at will, assuming it hasn't recently sent a Change
value, responding with a Confirm R option containing the new value,	option for the same feature. Reordering protection (Section 6.6.4)
and it MUST send empty Confirm R options in response to invalid	ensures that agreement is reached.
values (unless the Change L option was Mandatory; see Section
6.6.9). Change R and Confirm L options MUST NOT be sent for non-
negotiable features; see Section 6.6.8. Non-negotiable features use	negotiable features. Non-negotiable features use the feature
the feature negotiation mechanism to achieve reliability.	negotiation mechanism to achieve reliability.

6.4. Feature Numbers

This document defines the following feature numbers.

Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
0 Reserved
1 Congestion Control ID (CCID) SP 2 Y 10
2 Allow Short Seqnos SP 1 Y 7.6.1
3 Sequence Window NN 100 Y 7.5.2	3 Sequence Window NN 100 Y 7.5.4
4 ECN Incapable SP 0 N 12.1	4 ECN Capable SP 1 Y 12.1
5 Ack Ratio NN 2 N 11.3
6 Send Ack Vector SP 0 N 11.5
7 Send NDP Count SP 0 N 7.7.2
8 Minimum Checksum Coverage SP 0 N 9.2.1
9 Check Data Checksum SP 0 N 9.3.1
10-127 Reserved
128-255 CCID-specific features 10.3	128-255 CCID-specific features 10.4

Table 4: DCCP Feature Numbers


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.





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Req'd This column is "Y" if and only if every DCCP	Req'd This column is "Y" iff every DCCP implementation MUST
implementation MUST understand the feature. If it is	understand the feature. If it is "N", then the
"N", then the feature behaves like an extension (see	feature behaves like an extension (see Section 15),
Section 15), and it is safe to respond to Change	and it is safe to respond to Change options for the
options for the feature with empty Confirm options.	feature with empty Confirm options. Of course, a
Of course, a CCID might require the feature; a DCCP	CCID might require the feature; a DCCP that
that implements CCID 2 MUST support Ack Ratio and	implements CCID 2 MUST support Ack Ratio and Send Ack
Send Ack Vector, for example.	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 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 *


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)
2a. <-- Confirm L(CCID, 3, 3 2 1)
b. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *

Here are the byte encodings of several Change and Confirm options.
Each option is sent by DCCP A.




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Change L(CCID, 2 3) = 32,5,1,2,3
DCCP B should change CCID/A's value (feature number 1, a server-
priority feature); DCCP A's preferred values are 2 and 3, in
that preference order.

Change L(Sequence Window, 1024) = 32,6,3,0,4,0
DCCP B should change Sequence Window/A's value (feature number
3, a non-negotiable feature) to the 3-byte string 0,4,0 (the
value 1024).

Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3
DCCP A has changed CCID/A's value to 2; its preferred values are
2 and 3, in that preference order.

Empty Confirm L(126) = 33,3,126
DCCP A doesn't implement feature number 126, or DCCP B's
proposed value for feature 126/A was invalid.

Change R(CCID, 3 2) = 34,5,1,3,2
DCCP B should change CCID/B's value; DCCP A's preferred values
are 3 and 2, in that preference order.

Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2
DCCP A has changed CCID/B's value to 2; its preferred values
were 3 and 2, in that preference order.

Confirm R(Sequence Window, 1024) = 35,6,3,0,4,0
DCCP A has changed Sequence Window/B's value to the 3-byte
string 0,4,0 (the value 1024).

Empty Confirm R(126) = 35,3,126
DCCP A doesn't implement feature number 126, or DCCP B's
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 a response for the latest
Change is received.

3. Feature negotiation options are processed in strictly increasing
order by Sequence Number.





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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 "feature	anyway (such as a DCCP-Request). Alternatively, it may create a
negotiation packet", often a DCCP-Ack or DCCP-Sync, just to carry	"feature negotiation packet", often a DCCP-Ack or DCCP-Sync, just to
the option. Feature negotiation packets are controlled by the	carry the option. Feature negotiation packets MUST be rate-limited
relevant congestion control mechanism. For example, DCCP A may send	by the relevant congestion control mechanisms. In addition, an
a DCCP-Ack or DCCP-Sync for feature negotiation only if the B-to-A	endpoint SHOULD generate at most one feature negotiation packet per
CCID would allow sending a DCCP-Ack. In addition, an endpoint	round-trip time (0.1 seconds, if no RTT is available).
SHOULD generate at most one feature negotiation packet per round-
trip time.

On receiving a Change L or Change R option, a DCCP endpoint examines
the included preference list, reconciles that with its own
preference list, calculates the new value, and sends back a
Confirm R or Confirm L option, respectively, informing its peer of
the new value or that the feature was not understood. Every non-	the new value. Every non-reordered Change option MUST result in a
reordered Change option MUST result in a corresponding Confirm	corresponding Confirm option, and any packet including a Confirm
option, and any packet including a Confirm option MUST carry an	option MUST carry an Acknowledgement Number. Generated Confirm
Acknowledgement Number. Generated Confirm options may be attached	options may be attached to packets that would have been sent anyway
to packets that would have been sent anyway (such as DCCP-Response	(such as DCCP-Response or DCCP-SyncAck), or to new feature
or DCCP-SyncAck), or to new feature negotiation packets, as	negotiation packets, as described above.
described above.

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.

A packet MAY contain more than one feature negotiation option, as	Endpoints MUST NOT send packets that contain more than one feature
long as no two options refer to the same feature. Note, however,	negotiation option referring to the same feature. Note, however,
that a packet is allowed to contain one L option and one R option
with the same feature number, since the two options actually refer	with the same feature number F, since the two options actually refer
to different features (F/A and F/B).

6.6.2. Processing Received Options

DCCP endpoints exist in one of three states 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



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enters the CHANGING state when it first sends a Change for the
feature, and returns to STABLE once it receives a corresponding
Confirm. The final state, UNSTABLE, indicates that an endpoint in
CHANGING state changed its preference list, but has not yet
transmitted a Change option with the new preference list.

Feature state transitions at a feature location are implemented	Feature-related state transitions at the feature location are
according to this diagram. The diagram ignores sequence number and	implemented as shown in the diagram below. For feature-related
option validity issues; these are handled explicitly in the	state transitions at the feature remote, switch the "L"s and "R"s.
pseudocode that follows.	The diagram ignores sequence number and option validity issues;
	these are handled explicitly in the pseudocode that follows the
timeout/	diagram.
rcv Confirm R app/protocol evt : snd Change L rcv non-ack
: ignore +---------------------------------------+ : snd Change L
+----+ \| \| +----+
\| v \| rcv Change R v \| v
+------------+ rcv Confirm R : calc new value, +------------+
\| \| : accept value snd Confirm L \| \|
\| STABLE \|<-----------------------------------\| CHANGING \|
\| \| rcv empty Confirm R \| \|
+------------+ : revert to old value +------------+
\| ^ \| ^
+----+ pref list \| \| snd
rcv Change R changes \| \| Change L
: calc new value, snd Confirm L v \|
+------------+
+---\| \|
rcv Confirm/Change R \| \| UNSTABLE \|
: ignore +-->\| \|
+------------+

Feature locations SHOULD use the following pseudocode, which	Endpoints SHOULD use the following pseudocode, which corresponds to
corresponds to the state diagram, to react to each feature	the state diagram, to react to each feature negotiation option on
negotiation option on each valid packet received. The pseudocode	each valid packet received. The pseudocode refers to "P.seqno" and
refers to "P.seqno" and "P.ackno", which are properties of the	"P.ackno", which are properties of the packet; "O.type", and
packet; "O.type", and "O.len", which are properties of the option;	"O.len", which are properties of the option; "FGSR" and "FGSS",
"FGSR" and "FGSS", which are properties of the connection, and	which are properties of the connection, and handle reordering as
handle reordering as described in Section 6.6.4; "F.state", which is	described in Section 6.6.4; "F.state", which is the feature's state
the feature's state (STABLE, CHANGING, or UNSTABLE); and "F.value",	(STABLE, CHANGING, or UNSTABLE); and "F.value", which is the
which is the feature's value.	feature's value.

First, check for unknown features (Section 6.6.7);
If F is unknown,	If F is unknown:
If the option was Mandatory, /* Section 6.6.9 */	If option was Mandatory: /* Section 6.6.9 */
Reset connection and return
Otherwise, if O.type == Change R,	Otherwise, if O.type == Change R:
Send Empty Confirm L on a future packet
Return



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Second, check for reordering (Section 6.6.4);
If F.state == UNSTABLE or P.seqno <= FGSR
or (O.type == Confirm R and P.ackno < FGSS),	or (O.type == Confirm R and P.ackno < FGSS)
Ignore option and return

Third, process Change R options;
If O.type == Change R,	If O.type == Change R:
If the option's value is valid, /* Section 6.6.8 */	If option's value is valid: /* Section 6.6.8 */
Calculate new value
Send Confirm L on a future packet
Set F.state := STABLE
Otherwise, if the option was Mandatory,	Otherwise, if option was Mandatory:
Reset connection and return
Otherwise,	Otherwise:
Send Empty Confirm L on a future packet
/* Remain in existing state. If that's CHANGING, this
endpoint will retransmit its Change L option later. */

Fourth, process Confirm R options (but only in CHANGING state).
If F.state == CHANGING and O.type == Confirm R,	If F.state == CHANGING and O.type == Confirm R:
If O.len > 3, /* nonempty */	If O.len > 3: /* nonempty */
If the option's value is valid,	If option's value is valid:
Set F.value := new value
Otherwise,	Otherwise:
Reset connection and return
Set F.state := STABLE

Of course, every DCCP endpoint is both a feature location and a
feature remote. A similar diagram and pseudocode applies to feature
remotes; simply switch the "L"s and "R"s, so that the relevant
options are Change R and Confirm L.

6.6.3. Loss and Retransmission

Packets containing Change and Confirm options might be lost or
delayed by the network. Therefore, Change options are repeatedly	delayed by the network. Therefore, Change options are retransmitted
transmitted to achieve reliability. We refer to this as	to achieve reliability.
"retransmission", although of course there are no packet-level
retransmissions in DCCP: a Change option that is sent again will be
sent on a new packet with a new sequence number.

A CHANGING endpoint transmits another Change option once it realizes
that it has not heard back from the other endpoint. The new Change
option need not contain the same payload as the original; reordering
protection will ensure that agreement is reached based on the most
recently transmitted option.	recently transmitted option. 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 is initially set to approximately one or two
	round-trip times (or 0.1-0.2 seconds, if no RTT is available), and
	pinned at roughly 32 RTTs.
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A CHANGING endpoint MUST continue retransmitting Change options
until it gets some response or the connection terminates.

Endpoints SHOULD use an exponential-backoff timer to decide when to	Endpoints SHOULD NOT send Change options for a given feature more
retransmit Change options. (Endpoints that generate packets	frequently than once per RTT. Otherwise, the reordering protection
specifically for feature negotiation MUST use such a timer.) The	algorithms described in the next subsection may delay agreement,
timer interval is initially set to not less than one round-trip	since no received Confirm option would acknowledge the most recently
time, and should back off to not less than 64 seconds. The backoff	transmitted Change.
protects against delayed agreement due to the reordering protection
algorithms described in the next section. Again, endpoints may
piggyback Change options on packets they would have sent anyway, or
create new packets to carry the options; any such new packets are
controlled by the relevant congestion-control mechanism.

Confirm options are never retransmitted, but the Confirm-sending
endpoint MUST generate a Confirm option after every non-reordered
Change.

6.6.4. Reordering

Reordering might cause packets containing Change and Confirm options
to arrive in an unexpected order. Endpoints MUST ignore feature
negotiation options that do not arrive in strictly-increasing order
by Sequence Number. The rest of this section presents two
algorithms that fulfill this requirement.

The first algorithm introduces two sequence number variables that
each endpoint maintains for the connection.

FGSR Feature Greatest Sequence Number Received: The greatest
sequence number received, considering only valid packets
that contained one or more feature negotiation options
(Change and/or Confirm). This value is initialized to
ISR - 1.

FGSS Feature Greatest Sequence Number Sent: The greatest
sequence number sent, considering only packets that
contained one or more non-retransmitted Change options.
(Retransmitted Change options MUST have exactly the same
contents as previously transmitted options, so limited
reordering can safely be tolerated.) This value is
initialized to ISS.

Each endpoint checks two conditions on sequence numbers to decide
whether to process received feature negotiation options.

1. If a packet's Sequence Number is less than or equal to FGSR,
then its Change options MUST be ignored.



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2. If a packet's Sequence Number is less than or equal to FGSR, OR
it has no Acknowledgement Number, OR its Acknowledgement Number
is less than FGSS, then its Confirm options MUST be ignored.

Alternatively, an endpoint MAY maintain separate FGSR and FGSS
values for every feature. FGSR(F/X) would equal the greatest
sequence number received, considering only packets that contained
Change or Confirm options applying to feature F/X; FGSS(F/X) would
be defined similarly. This algorithm requires more state, but is
slightly more forgiving to multiple overlapped feature negotiations.
Either algorithm MAY be used; the first algorithm, with connection-
wide FGSR and FGSS variables, is RECOMMENDED.

One consequence of these rules is that a CHANGING endpoint will
ignore any Confirm option that does not acknowledge the latest
Change option sent. This ensures that agreement, once achieved,
used the most recent available information about the endpoints'
preferences.

6.6.5. Preference Changes

Endpoints are allowed to change their preference lists at any time.
However, an endpoint that changes its preference list while in the
CHANGING state MUST transition to the UNSTABLE state. It will
transition back to CHANGING once it has transmitted a Change option
with the new preference list. This ensures that agreement is based
on active preference lists. Without the UNSTABLE state,
simultaneous negotiation -- where the endpoints began independent
negotiations for the same feature at the same time -- might lead to
the negotiation terminating with the endpoints thinking the feature
had different values.

6.6.6. 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.

6.6.7. Unknown Features

Endpoints may receive Change options referring to feature numbers
they do not understand -- for instance, when an extended DCCP
converses with a non-extended DCCP. Endpoints MUST respond to



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unknown Change options with 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
Mandatory, the connection MUST be reset; see Section 6.6.9.	preceded by a Mandatory option, the connection MUST be reset; see
	Section 6.6.9.
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 15 suggests that the default
value for any extension feature should correspond to "extension not
available".

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. Nevertheless, endpoints	to a feature number it does not understand. Endpoints MUST ignore
MUST ignore any such options they receive.	such options.

6.6.8. 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 Ack Ratio feature takes
16-bit values, so valid "Confirm R(Ack Ratio)" options have
option length 5.

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.

o However, unexpected Confirm options -- that refer to unknown
feature numbers, or that don't appear to be part of a current
negotiation -- are considered valid, although they are ignored by
the receiver.




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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".

6.6.9. Mandatory Feature Negotiation

Change options may be preceded by Mandatory options (Section 5.8.2).
Mandatory Change options are processed like normal Change options,
except that the following failure cases will cause the receiver to
reset the connection with Reset Code 6, "Mandatory Failure", rather
than send a Confirm option. The connection MUST be reset if:

o The Change option's feature number was not understood;

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.
\|\| TEXT DELETED \|\|	6.6.10. Out-of-Band Agreement
	An endpoint MUST NOT unilaterally change the value of any DCCP
7. Sequence Numbers	feature. However, endpoints MAY cooperatively change DCCP feature
	values without using in-band feature negotiation options. For
DCCP uses sequence numbers to arrange packets into sequence, detect	example, features MAY be changed via negotation over a separate
losses and network duplicates, and protect against attackers, half-	signaling channel, for example.
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 packet-based. That is, the packets
generated by each endpoint have Sequence Numbers that increase by
one, modulo 2^48, for every packet. 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.







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7.1. Variables

DCCP endpoints maintain a set of sequence number variables for each
connection.

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. Here,
and elsewhere, "greatest" is measured in circular sequence
space.

GSR The Greatest Sequence Number Received from the other
endpoint on an acknowledgeable packet. (Section 7.4 defines
this term.)	"acknowledgeable" packets.)

GAR The Greatest Acknowledgement Number Received from the other
endpoint on an acknowledgeable packet that was not a DCCP-
Sync.

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].



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These problems are the same as problems faced by TCP, and DCCP
implementations SHOULD use TCP's strategies to avoid them [RFC 793]
[RFC 1948]. The rest of this section explains these strategies in
more detail.

To address the first problem, an implementation MUST ensure that the
initial sequence number for a given <source address, source port,
destination address, destination port> 4-tuple doesn't overlap with
recent sequence numbers on previous connections with the same
4-tuple. ("Recent" means sent within 2 maximum segment lifetimes,
or 4 minutes.) The implementation MUST additionally ensure that the
lower 24 bits of the initial sequence number don't overlap with the
lower 24 bits of recent sequence numbers (unless the implementation
plans to avoid short sequence numbers; see Section 7.6). An
implementation that has state for a recent connection with the same
4-tuple can pick a good initial sequence number explicitly.
Otherwise, it could tie initial sequence number selection to some
clock, such as the 4-microsecond clock used by TCP [RFC 793]. Two
separate clocks may be required, one for the upper 24 bits and one
for the lower 24 bits.

To address the second problem, an implementation MUST provide each
4-tuple with an independent initial sequence number space. Then
opening a connection doesn't provide any information about initial
sequence numbers on other connections to the same host. RFC 1948
achieves this by adding a cryptographic hash of the 4-tuple and a
secret to each 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, the endpoint
might remember dead connection state for each 4-tuple or stay quiet
for 2 maximum segment lifetimes around such a change.

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, and reserving the upper 8 or so bits of
initial sequence numbers for a persistent counter that decrements by



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two each boot. (The latter mechanism would require disallowing
packets with short sequence numbers; see Section 7.6.1.)

7.4. Acknowledgement Numbers

Cumulative acknowledgements are meaningless in an unreliable
protocol. Therefore, DCCP's Acknowledgement Number field has a
different meaning than TCP's.

A received packet is classified as acknowledgeable if and only if	A packet is classified as "acknowledgeable" if and only if its
its header was succesfully processed by the receiving DCCP. In	options were processed by the receiving DCCP. This means, for
terms of the pseudocode in Section 8.5, a received packet becomes	example, that all acknowledgeable packets have valid header
acknowledgeable when the receiving endpoint reaches Step 8. This	checksums and sequence numbers. The Acknowledgement Number MUST
means, for example, that all acknowledgeable packets have valid	equal GSR, the Greatest Sequence Number Received on an
header checksums and sequence numbers. The Acknowledgement Number
MUST equal GSR, the Greatest Sequence Number Received on an
acknowledgeable packet, for all packet types except DCCP-Sync and
DCCP-SyncAck.

"Acknowledgeable" does not refer to 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.	and 11.8.

DCCP-Sync and DCCP-SyncAck packets' Acknowledgement Numbers differ
as follows: 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; it might be less than GSR in
the presence of reordering.

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.





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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 of DCCP-Sync and DCCP-SyncAck packets recovers
from this case.

7.5.1. Sequence and Acknowledgement Number Windows	7.5.1. Sequence-Validity Rules

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 for the two half-
connections.

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 less than or equal
to GSR, and three-fourths is greater than 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, 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 are not less than
the initial Sequence Numbers received and sent, respectively:
SWL := max(GSR + 1 - floor(W/4), ISR),
AWL := max(GSS - W' + 1, ISS).
These adjustments MUST be applied only at the beginning of the
connection. (Long-lived connections may wrap sequence numbers so



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that they appear to be less than ISR or ISS; the adjustments MUST
NOT be applied in that case.)

7.5.2. 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 48-bit (6-byte) integer values, like DCCP sequence numbers,
but 1- to 5-byte values are also allowed in options -- they are
padded on the left with zero bytes as necessary to total 48 bits.
Change and Confirm options for Sequence Window are therefore between
4 and 9 bytes long. New connections start with Sequence Window 100
for both endpoints. The maximum valid Sequence Window value is
Wmax = 2^46 - 1 = 70368744177663; circular sequence number
comparisons would stop working absent this constraint. Change
options suggesting larger Sequence Window values are invalid and
MUST be handled accordingly.

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 about
five times 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.

7.5.3. 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 validity windows [SWL, SWH] and	sequence and acknowledgement number windows [SWL, SWH] and [AWL,
[AWL, AWH], which are defined in Section 7.5.1.	AWH], which are defined 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 GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Close GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Reset GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Sync SWL <= seqno AWL <= ackno <= AWH	DCCP-Sync seqno >= SWL AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno 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 Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a
connection, they cannot have Sequence Numbers less than or equal
to GSR, or Acknowledgement Numbers less than GAR.

o DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly
checked. These packet types exist specifically to get the
endpoints back into sync; checking their Sequence Numbers would	endpoints back into sync after bursts of loss; checking their
eliminate their usefulness.	Sequence Numbers would eliminate their usefulness.
	The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow
Although the lenient checks on DCCP-Sync and DCCP-SyncAck packets	continued operation after unusual events, such as endpoint crashes
allow continued operation after unusual events, such as endpoint	and large bursts of loss. There's no need for leniency when the
crashes and large bursts of loss, there's no need for leniency when	endpoints are actively sending packets to one another. Therefore,
the endpoints are actively sending packets to one another.	DCCP implementations SHOULD use the following, more stringent checks
Therefore, DCCP implementations SHOULD use the following, more	for active connections. A connection is considered active if it has
stringent checks for active connections. A connection is considered	received valid packets from the other endpoint within the last
active if it has received valid packets from the other endpoint	several round-trip times, or 0.5 seconds, if the RTT is not known.
within the last five round-trip times.

Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Sync SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno <= SWH AWL <= ackno <= AWH

Finally, an endpoint MAY apply the following more stringent checks
to DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets, further
lowering the probability of successful blind attacks using those



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packet types. Since these checks can cause extra synchronization
overhead and delay connection closing when packets are lost, they
should be considered experimental.

Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-CloseReq seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Close seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Reset seqno == GSR + 1 GAR <= ackno <= AWH

Note that sequence-validity is only one of the validity checks
applied to received packets.

7.5.4. Handling Sequence-Invalid Packets	7.5.2. Handling Sequence-Invalid Packets
	Sequence-invalid DCCP-Sync and DCCP-SyncAck packets MUST be ignored.
Endpoints MUST ignore sequence-invalid DCCP-Sync and DCCP-SyncAck	On receiving any other sequence-invalid packet, an endpoint (say,
packets, and MUST respond to other sequence-invalid packets with	DCCP A) MUST reply with a DCCP-Sync packet. This packet MUST
(possibly rate-limited) DCCP-Sync packets. Each DCCP-Sync packet	acknowledge the sequence-invalid packet's Sequence Number, not GSR.
MUST acknowledge the corresponding sequence-invalid packet's	The DCCP-Sync MUST use a new Sequence Number, and thus will increase
Sequence Number, not GSR. The DCCP-Sync MUST use a new Sequence	GSS; GSR will not change, however, since the received packet was
Number, and thus will increase GSS; GSR will not change, however,	sequence-invalid. DCCP A MUST NOT otherwise process sequence-
since the received packet was sequence-invalid.	invalid packets. For instance, it MUST NOT process their options.
	On receiving a sequence-valid DCCP-Sync, the peer endpoint (DCCP B)
On receiving a sequence-valid DCCP-Sync packet, the peer endpoint	MUST either respond with a DCCP-Reset packet, or update its GSR
(say, DCCP B) MUST update its GSR variable and reply with a DCCP-	variable and reply with a DCCP-SyncAck packet. The DCCP-SyncAck
SyncAck packet. The DCCP-SyncAck packet's Acknowledgement Number	packet's Acknowledgement Number will equal the DCCP-Sync's Sequence
will equal the DCCP-Sync's Sequence Number, not necessarily GSR.	Number, not necessarily GSR. Upon receiving this DCCP-SyncAck,
Upon receiving this DCCP-SyncAck, which will be sequence-valid since	which will be sequence-valid since it acknowledges the DCCP-Sync,
it acknowledges the DCCP-Sync, DCCP A will update its GSR variable,	DCCP A will update its GSR variable, and the endpoints will be back
and the endpoints will be back in sync. As an exception, if the	in sync.
peer endpoint is in the REQUEST state, it MUST respond with a DCCP-	A DCCP endpoint MAY temporarily preserve sequence-invalid packets in
Reset instead of a DCCP-SyncAck. This serves to clean up DCCP A's	case they become valid later. This can reduce the impact of bursts
half-open connection.	of loss by delivering more packets to the application. In
	particular, an endpoint MAY preserve sequence-invalid packets for up
To protect against denial-of-service attacks, DCCP implementations	to 2 round-trip times (or 0.2 seconds, if the RTT is unknown); if,
SHOULD impose a rate limit on DCCP-Syncs sent in response to	within that time, the relevant sequence windows change so that the
sequence-invalid packets, such as not more than eight DCCP-Syncs per	packets becomes sequence-valid, the endpoint MAY process the packets
second.	again.
	To protect itself against denial-of-service attacks (where an
DCCP endpoints MUST NOT process sequence-invalid packets except,	attacker sends many sequence-invalid packets, trying to force the
perhaps, by generating a DCCP-Sync. For instance, options MUST NOT	receiver to send many DCCP-Syncs), a DCCP implementation SHOULD
but processed. An endpoint MAY temporarily preserve sequence-	rate-limit the DCCP-Syncs sent in response to sequence-invalid
invalid packets in case they become valid later, however; this can	packets.
reduce the impact of bursts of loss by delivering more packets to
the application. In particular, an endpoint MAY preserve sequence-
invalid packets for up to 2 round-trip times. If, within that time,
the relevant sequence windows change so that the packets become



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sequence-valid, the endpoint MAY process them again.

Note that sequence-invalid DCCP-Reset packets cause DCCP-Syncs to be
generated. This is because endpoints in an unsynchronized state
(CLOSED, REQUEST, and LISTEN) might not have enough information to
generate a proper DCCP-Reset on the first try. For example, if a
peer endpoint is in CLOSED state and receives a DCCP-Data packet, it
cannot guess the right Sequence Number to use on the DCCP-Reset it
generates (since the DCCP-Data packet has no Acknowledgement
Number). The DCCP-Sync generated in response to this bad reset
serves as a challenge, and contains enough information for the peer
to generate a proper DCCP-Reset. However, the new DCCP-Reset may
carry a different Reset Code than the original DCCP-Reset; probably
the new Reset Code will be 3, "No Connection". The endpoint SHOULD
use information from the original DCCP-Reset when possible.
\|\| TEXT DELETED \|\|	7.5.3. Sequence and Acknowledgement Number Windows
	Each DCCP endpoint defines sequence validity windows that are
7.5.5. Sequence Number Attacks	subsets of the Sequence and Acknowledgement Number spaces. These
	windows correspond to packets the endpoint expects to receive in the
Sequence and Acknowledgement Numbers form DCCP's main line of	next few round-trip times. The Sequence and Acknowledgement Number
defense against attackers. An attacker that cannot guess sequence	windows always contain GSR and GSS, respectively. The window widths
numbers cannot easily manipulate or hijack a DCCP connection, and	are controlled by Sequence Window features for the two half-
requirements like careful initial sequence number choice eliminate	connections.
the most serious attacks.	The Sequence Number validity window for packets from DCCP B is [SWL,
	SWH]. This window always contains GSR, the Greatest Sequence Number
An attacker might still send many packets with randomly chosen	Received on a sequence-valid packet from DCCP B. It is W packets
Sequence and Acknowledgement Numbers, however. If one of those	wide, where W is the value of the Sequence Window/B feature. One-
probes ends up sequence-valid, it may shut down the connection or	fourth of the sequence window, rounded down, is less than or equal
otherwise cause problems. The easiest such attacks to execute are:	to GSR, and three-fourths is greater than GSR. (This asymmetric
	placement assumes that bursts of loss are more common in the network
o Send DCCP-Data packets with random Sequence Numbers. If one of	than significant reordering.)
these packets hits the valid sequence number window, the attack	invalid \| valid Sequence Numbers \| invalid
packet's application data may be inserted into the data stream.	<---------\|==================================\|*--------->
	GSR -\|GSR + 1 - GSR GSR +\|GSR + 1 +
o Send DCCP-Sync packets with random Sequence and Acknowledgement	floor(W/4)\|floor(W/4) ceil(3W/4)\|ceil(3W/4)
Numbers. If one of these packets hits the valid acknowledgement	= SWL = SWH
number window, the receiver will shift its sequence number window	The Acknowledgement Number validity window for packets from DCCP B
accordingly, getting out of sync with the correct endpoint --	is [AWL, AWH]. The high end of the window, AWH, equals GSS, the
perhaps permanently.	Greatest Sequence Number Sent by DCCP A; the window is W' packets
	wide, where W' is the value of the Sequence Window/A feature.
The attacker has to guess both Source and Destination Ports for any	invalid \| valid Acknowledgement Numbers \| invalid
of these attacks to succeed. Additionally, the connection would	<---------\|===================================\|--------->
have to be inactive for the DCCP-Sync attack to succeed, assuming	GSS - W'\|GSS + 1 - W' GSS\|GSS + 1
the victim implemented the more stringent checks for active	= AWL = AWH
	SWL and AWL are initially adjusted so that they are not less than
	the initial Sequence Numbers received and sent, respectively:
	SWL := max(GSR + 1 - floor(W/4), ISR),
	AWL := max(GSS - W' + 1, ISS).
	These adjustments MUST be applied only at the beginning of the
	connection. (Long-lived connections may wrap sequence numbers so
	that they appear to be less than ISR or ISS; the adjustments MUST
	NOT be applied in that case.)
	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 </

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Table of Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 10	1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . . 11	2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . . 8
3. Conventions and Terminology . . . . . . . . . . . . . . . . . 12	3. Conventions and Terminology . . . . . . . . . . . . . . . . . 9
3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . . 12	3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . . 9
3.2. Parts of a Connection. . . . . . . . . . . . . . . . . . 13	3.2. Parts of a Connection. . . . . . . . . . . . . . . . . . 9
3.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 13	3.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . . 14	3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . . 10
3.5. Security Limitation. . . . . . . . . . . . . . . . . . . 14	3.5. Security Limitation. . . . . . . . . . . . . . . . . . . 11
3.6. Robustness Principle . . . . . . . . . . . . . . . . . . 14	3.6. Robustness Principle . . . . . . . . . . . . . . . . . . 11
4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 14	4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . . 15	4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 16	4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 13
4.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 17	4.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4. Congestion Control . . . . . . . . . . . . . . . . . . . 19	4.4. Congestion Control . . . . . . . . . . . . . . . . . . . 15
4.5. Features . . . . . . . . . . . . . . . . . . . . . . . . 19	4.5. Features . . . . . . . . . . . . . . . . . . . . . . . . 16
4.6. Differences From TCP . . . . . . . . . . . . . . . . . . 20	4.6. Differences From TCP . . . . . . . . . . . . . . . . . . 17
4.7. Example Connection . . . . . . . . . . . . . . . . . . . 21	4.7. Example Connection . . . . . . . . . . . . . . . . . . . 18
5. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . . 23	5. Header Formats. . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Generic Header . . . . . . . . . . . . . . . . . . . . . 23	5.1. Generic Header . . . . . . . . . . . . . . . . . . . . . 20
5.2. DCCP-Request Packets . . . . . . . . . . . . . . . . . . 27	5.2. DCCP-Request Header. . . . . . . . . . . . . . . . . . . 23
5.3. DCCP-Response Packets. . . . . . . . . . . . . . . . . . 28	5.3. DCCP-Response Header . . . . . . . . . . . . . . . . . . 23
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets. . . . . . 28	5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers. . . . . . 24
5.5. DCCP-CloseReq and DCCP-Close Packets . . . . . . . . . . 30	5.5. DCCP-CloseReq and DCCP-Close Headers . . . . . . . . . . 26
5.6. DCCP-Reset Packets . . . . . . . . . . . . . . . . . . . 30	5.6. DCCP-Reset Header. . . . . . . . . . . . . . . . . . . . 26
5.7. DCCP-Sync and DCCP-SyncAck Packets . . . . . . . . . . . 33	5.7. DCCP-Sync and DCCP-SyncAck Headers . . . . . . . . . . . 29
5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . . 34	5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . . 30
5.8.1. Padding Option. . . . . . . . . . . . . . . . . . . 36	5.8.1. Padding Option. . . . . . . . . . . . . . . . . . . 31
5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . . 36	5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . . 31
6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . . 37	6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . . 32
6.1. Change Options . . . . . . . . . . . . . . . . . . . . . 37	6.1. Change Options . . . . . . . . . . . . . . . . . . . . . 33
6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . . 38	6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . . 33
6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . . 38	6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . . 34
6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 38	6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 34
6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . . 39	6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . . 34
6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . . 39	6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . . 35
6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 40	6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 35
6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 41	6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 37
6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . . 42	6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . . 37
6.6.2. Processing Received Options . . . . . . . . . . . . 42	6.6.2. Processing Received Options . . . . . . . . . . . . 38
6.6.3. Loss and Retransmission . . . . . . . . . . . . . . 44	6.6.3. Loss and Retransmission . . . . . . . . . . . . . . 40
6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . . 45	6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . . 41
6.6.5. Preference Changes. . . . . . . . . . . . . . . . . 46	6.6.5. Preference Changes. . . . . . . . . . . . . . . . . 42
6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . . 46	6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . . 42
6.6.7. Unknown Features. . . . . . . . . . . . . . . . . . 46	6.6.7. Unknown Features. . . . . . . . . . . . . . . . . . 42
6.6.8. Invalid Options . . . . . . . . . . . . . . . . . . 47	6.6.8. Invalid Options . . . . . . . . . . . . . . . . . . 43
6.6.9. Mandatory Feature Negotiation . . . . . . . . . . . 48	6.6.9. Mandatory Feature Negotiation . . . . . . . . . . . 43
	6.6.10. Out-of-Band Agreement. . . . . . . . . . . . . . . 44
	7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 44
	7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 44
Kohler/Handley/Floyd [Page 5]	7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 45