Internet Engineering Task Force Eddie Kohler
INTERNET-DRAFT UCLA
draft-ietf-dccp-spec-13.txt Mark Handley	draft-ietf-dccp-spec-12.txt Mark Handley
Expires: 1 June 2006 UCL	Expires: 28 May 2006 UCL
Sally Floyd
ICIR
1 December 2005	28 November 2005


Datagram Congestion Control Protocol (DCCP)


Status of this Memo

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
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This Internet-Draft will expire on 1 June 2006.	This Internet-Draft will expire on 28 May 2006.

Abstract

The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that provides bidirectional unicast connections of
congestion-controlled unreliable datagrams. DCCP is suitable for
applications that transfer fairly large amounts of data, but can
benefit from control over the tradeoff between timeliness and
reliability.



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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

Changes since draft-ietf-dccp-spec-08.txt:

* Added minimum Sequence Window.

* Init Cookie implementation sketch.

* Include reasoning for ignoring options on DCCP-Data.

* More Aggression Penalty explanation.

* More explanation on Ack Vectors that report information on packets
that haven't been sent.

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.




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* Add pseudocode for event processing.

* Remove # NDP; replace with Ack Count.

* Remove Identification, Challenge, ID Regime, and Connection Nonce.

* Data Checksum (formerly Payload Checksum) uses a 32-bit CRC.

* Switch location of non-negotiable features to clarify
presentation; now the feature location controls its value.

* Rename "value type" to "reconciliation rule".

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

* Mobility ID becomes 128 bits long.

* Add probabilities to Mobility ID discussion.

* Add SyncAck.































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Table of Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 9
2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . . 10
3. Conventions and Terminology . . . . . . . . . . . . . . . . . 11
3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . . 11
3.2. Parts of a Connection. . . . . . . . . . . . . . . . . . 12
3.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 12
3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . . 13
3.5. Security Limitation. . . . . . . . . . . . . . . . . . . 13
3.6. Robustness Principle . . . . . . . . . . . . . . . . . . 13
4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Packet Sequencing. . . . . . . . . . . . . . . . . . . . 15
4.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4. Congestion Control Mechanisms. . . . . . . . . . . . . . 18
4.5. Connection Features. . . . . . . . . . . . . . . . . . . 19
4.6. Differences From TCP . . . . . . . . . . . . . . . . . . 20
4.7. Example Connection . . . . . . . . . . . . . . . . . . . 21
5. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . . 22
5.1. Generic Header . . . . . . . . . . . . . . . . . . . . . 23
5.2. DCCP-Request Packets . . . . . . . . . . . . . . . . . . 26
5.3. DCCP-Response Packets. . . . . . . . . . . . . . . . . . 27
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets. . . . . . 28
5.5. DCCP-CloseReq and DCCP-Close Packets . . . . . . . . . . 29
5.6. DCCP-Reset Packets . . . . . . . . . . . . . . . . . . . 30
5.7. DCCP-Sync and DCCP-SyncAck Packets . . . . . . . . . . . 33
5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . . 34
5.8.1. Padding Option. . . . . . . . . . . . . . . . . . . 35
5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . . 36
6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . . 37
6.1. Change Options . . . . . . . . . . . . . . . . . . . . . 37
6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . . 38
6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . . 38
6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 38
6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . . 39
6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . . 39
6.5. Feature Negotiation Examples . . . . . . . . . . . . . . 40
6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 41
6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . . 42
6.6.2. Processing Received Options . . . . . . . . . . . . 42
6.6.3. Loss and Retransmission . . . . . . . . . . . . . . 44
6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . . 45
6.6.5. Preference Changes. . . . . . . . . . . . . . . . . 46
6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . . 46
6.6.7. Unknown Features. . . . . . . . . . . . . . . . . . 46
6.6.8. Invalid Options . . . . . . . . . . . . . . . . . . 47
6.6.9. Mandatory Feature Negotiation . . . . . . . . . . . 48



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7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 48
7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 49
7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 49
7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 50
7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . . 51
7.5. Validity and Synchronization . . . . . . . . . . . . . . 51
7.5.1. Sequence and Acknowledgement Number
Windows. . . . . . . . . . . . . . . . . . . . . . . . . . 52
7.5.2. Sequence Window Feature . . . . . . . . . . . . . . 53
7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . . 53
7.5.4. Handling Sequence-Invalid Packets . . . . . . . . . 55
7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 56
7.5.6. Sequence Number Handling Examples . . . . . . . . . 58
7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . . 58
7.6.1. Allow Short Sequence Numbers Feature. . . . . . . . 59
7.6.2. When to Avoid Short Sequence Numbers. . . . . . . . 60
7.7. NDP Count and Detecting Application Loss . . . . . . . . 60
7.7.1. NDP Count Usage Notes . . . . . . . . . . . . . . . 61
7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 61
8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 62
8.1. Connection Establishment . . . . . . . . . . . . . . . . 62
8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 62
8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 63
8.1.3. Server Response . . . . . . . . . . . . . . . . . . 65
8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 66
8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 67
8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 67
8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 68
8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 70
8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 70
8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 71
9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 75
9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 76
9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 77
9.2.1. Minimum Checksum Coverage Feature . . . . . . . . . 78
9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 78
9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 79
9.3.2. Checksum Usage Notes. . . . . . . . . . . . . . . . 79
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 80
10.1. TCP-like Congestion Control . . . . . . . . . . . . . . 81
10.2. TFRC Congestion Control . . . . . . . . . . . . . . . . 81
10.3. CCID-Specific Options, Features, and Reset
Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
10.4. CCID Profile Requirements . . . . . . . . . . . . . . . 84
10.5. Congestion State. . . . . . . . . . . . . . . . . . . . 84
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 85
11.1. Acks of Acks and Unidirectional Connections . . . . . . 86
11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 87



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11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 87
11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 89
11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 91
11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 93
11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 94
11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 94
11.7. Data Dropped Option . . . . . . . . . . . . . . . . . . 95
11.7.1. Data Dropped and Normal Congestion
Response . . . . . . . . . . . . . . . . . . . . . . . . . 98
11.7.2. Particular Drop Codes. . . . . . . . . . . . . . . 98
12. Explicit Congestion Notification . . . . . . . . . . . . . . 99
12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . . 100
12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 100
12.3. Aggression Penalties. . . . . . . . . . . . . . . . . . 101
13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 102
13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 102
13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 103
13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 104
14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 105
14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 105
14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 107
15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 108
16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 108
17. Relations to Other Specifications. . . . . . . . . . . . . . 110
17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 110
17.2. Congestion Manager and Multiplexing . . . . . . . . . . 111
18. Security Considerations. . . . . . . . . . . . . . . . . . . 111
18.1. Security Considerations for Partial
Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 112
19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 113
19.1. Packet Types Registry . . . . . . . . . . . . . . . . . 113
19.2. Reset Codes Registry. . . . . . . . . . . . . . . . . . 113	19.2. Reset Codes Registry. . . . . . . . . . . . . . . . . . 114
19.3. Option Types Registry . . . . . . . . . . . . . . . . . 114
19.4. Feature Numbers Registry. . . . . . . . . . . . . . . . 114
19.5. Congestion Control Identifiers Registry . . . . . . . . 114	19.5. Congestion Control Identifiers Registry . . . . . . . . 115
19.6. Ack Vector States Registry. . . . . . . . . . . . . . . 115
19.7. Drop Codes Registry . . . . . . . . . . . . . . . . . . 115
19.8. Service Codes Registry. . . . . . . . . . . . . . . . . 115
19.9. Port Numbers Registry . . . . . . . . . . . . . . . . . 116
20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 117	20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 118
A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 120
A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 120	A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 121
A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 121	A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 122
A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 122
A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 123
A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 124
B. Appendix: Partial Checksumming Design Motivation. . . . . . . 125



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Normative References . . . . . . . . . . . . . . . . . . . . . . 126	Normative References . . . . . . . . . . . . . . . . . . . . . . 127
Informative References . . . . . . . . . . . . . . . . . . . . . 127
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 129
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 129	Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 130
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 130














































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List of Tables

Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . . 25
Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . . 32
Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . . 34
Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . . 39
Table 5: DCCP Congestion Control Identifiers . . . . . . . . . . 80
Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . . 90
Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . . 96










































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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) [RFC 3168] and the ECN Nonce [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 [RFC 1191].

o A choice of modular congestion control mechanisms. Two
mechanisms are currently specified, TCP-like Congestion Control
[CCID 2 PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion
Control [CCID 3 PROFILE], but DCCP is easily extensible to
further forms of unicast congestion control.

DCCP is intended for applications such as streaming media that can
benefit from control over the tradeoffs between delay and reliable
in-order delivery. TCP is 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 flows, avoiding the arbitrary
delays associated with TCP. It also implements reliable connection



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setup, teardown, and feature negotiation.

2. Design Rationale

One DCCP design goal was to give most streaming UDP applications
little reason not to switch to DCCP, once it is deployed. To
facilitate this, DCCP was designed to have as little overhead as
possible, both in terms of the packet header size and in terms of
the state and CPU overhead required at end hosts. Only the minimal
necessary functionality was included in DCCP, leaving other
functionality, such as forward error correction (FEC), semi-
reliability, and multiple streams, to be layered on top of DCCP as
desired.

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
mechanisms. One alternative, TCP-like Congestion Control, halves
the congestion window in response to a packet drop or mark, as in
TCP. Applications using this congestion control mechanism will
respond quickly to changes in available bandwidth, but must tolerate
the abrupt changes in congestion window typical of TCP. A second
alternative, TCP-Friendly Rate Control (TFRC) [RFC 3448], a form of
equation-based congestion control, minimizes abrupt changes in the
sending rate while maintaining longer-term fairness with TCP. Other
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
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



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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.

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. Implementation
strategies for DCCP sequence numbers will resemble those for other
circular arithmetic spaces, including TCP's sequence numbers [RFC
793] and DNS's serial numbers [RFC 1982]. Note that the common
technique for implementing circular comparison using two's-
complement arithmetic, whereby A < B using circular arithmetic if
and only if (A - B) < 0 using conventional two's-complement
arithmetic, may be used for DCCP sequence numbers, provided 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



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specified. This is to allow for future protocol extensions. In
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
DCCP A and DCCP B. Each connection is actively initiated by one of
the hosts, which we call the client; the other, initially passive
host is called the server. The term "DCCP endpoint" is used to
refer to either of the two hosts explicitly named by the connection
(the client and the server). The term "DCCP processor" refers more
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



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in use on the two half-connections. The endpoints achieve agreement
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
control mechanisms; different mechanisms may measure round-trip time
in different ways, or not measure it at all. However, the main DCCP
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 reasonably conservative 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
of TCP, which is normally two minutes.

3.5. Security Limitation

DCCP provides no protection against attackers who can snoop on a
connection in progress, or who can guess valid sequence numbers in
other ways. Applications desiring stronger security should use
IPsec [RFC 2401]; depending on the level of security required,
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].







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4. Overview

DCCP's high-level connection dynamics echo those of TCP.
Connections progress through three phases: initiation, including a
three-way handshake; data transfer; and termination. Data can flow
both ways over the connection. An acknowledgement framework lets
senders discover how much data has been lost, 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.



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The packet types are as follows:

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;
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. Packet Sequencing

Each DCCP packet carries a sequence number, so that losses can be
detected and reported. Unlike TCP sequence numbers, which are byte-
based, DCCP sequence numbers increment by one per packet. For
example:










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DCCP A DCCP B
------ ------
DCCP-Data(seqno 1) -->
DCCP-Data(seqno 2) -->
<-- DCCP-Ack(seqno 10, ackno 2)
DCCP-DataAck(seqno 3, ackno 10) -->
<-- DCCP-Data(seqno 11)

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





















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Client Server
------ ------
(0) No connection
CLOSED LISTEN

(1) Initiation
REQUEST DCCP-Request -->
<-- DCCP-Response RESPOND
PARTOPEN DCCP-Ack or DCCP-DataAck -->

(2) Data transfer
OPEN <-- DCCP-Data, Ack, DataAck --> OPEN

(3) Termination
<-- DCCP-CloseReq CLOSEREQ
CLOSING DCCP-Close -->
<-- DCCP-Reset CLOSED
TIMEWAIT
CLOSED

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

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.




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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
minutes) after the connection has been torn down, to prevent
mistakes due to the delivery of old packets. Only one of the
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.

4.4. Congestion Control Mechanisms

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 in the future.

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].

CCID 3 provides TFRC Congestion Control, an equation-based form of
congestion control intended to respond to congestion more smoothly



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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. Connection Features

DCCP endpoints use Change and Confirm options to negotiate and agree
on feature values. Feature negotiation will almost always happen on
the connection initiation handshake, but it can begin at any time.

There are four feature negotiation options in all: Change L,
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:

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
the CCID in use on the server-to-client half-connection. In the
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.







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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 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 ack per 2 packets, and each ack must declare exactly which
packets were received; in CCID 3 (TFRC), it's about one ack per
round-trip time, and acks must declare at minimum just the
lengths of recent loss 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-
CloseReq packet, which may only be sent by the server, passes
that state to the client. Various rate limits let servers avoid
attacks that might force extensive computation or packet
generation.

o Distinguishing different kinds of loss. A Data Dropped option
(Section 11.7) lets an endpoint declare that a packet was dropped
because of corruption, because of receive buffer overflow, and so
on. This facilitates research into more appropriate rate-control
responses for these non-network-congestion losses (although
currently such losses will cause a congestion response).

o Acknowledgeability. In TCP, a packet may be acknowledged only
once the data is reliably queued for application delivery. This
does not make sense in DCCP, where an application might, for
example, request a drop-from-front receive buffer. A DCCP packet
may be acknowledged as soon as its header has been successfully



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processed. Concretely, a packet becomes acknowledgeable at
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
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.)

Client Server
------ ------
0. [CLOSED] [LISTEN]
1. DCCP-Request -->
2. <-- DCCP-Response
3. 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



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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
packets before receiving at least one non-DCCP-Response packet
from the server.)

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

6. The client sends a DCCP-Close packet acknowledging the close.

7. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state. DCCP-Resets are part
of normal connection termination; see Section 5.6.

8. The client receives the DCCP-Reset packet and holds state for
two maximum segment lifetimes, or 2MSL, to allow any remaining
packets to clear the network.

An alternative connection closedown sequence is initiated by the
client:

5b. The client sends a DCCP-Close packet closing the connection.

6b. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state.

7b. The client receives the DCCP-Reset packet and holds state for
2MSL to allow any remaining packets to clear the network.

5. Packet 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-
defined packet types. Following this comes any additional fixed-



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length fields required by the packet type, and then a variable-
length list of options. The application data area follows the
header. In some packet types, this area contains data for the
application; in other packet types, its contents are ignored.

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


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.

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\| \| .
\| Res \| Type \|=\| Reserved \| Sequence Number (high bits) .
\| \| \|1\| \| .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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












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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\| \|
\| Res \| Type \|=\| Sequence Number (low bits) \|
\| \| \|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
initiating a connection, the client SHOULD choose its Source
Port randomly to reduce the likelihood 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. See Section 19.9 for more discussion.

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



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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.
See 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:

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

Extended Sequence Numbers (X): 1 bit
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.




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Sequence Number: 48 or 24 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
immediately following the generic header. When X=1, its format is:

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

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

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


Reserved: 16 or 8 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
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.

5.2. DCCP-Request Packets

A client initiates a DCCP connection by sending a DCCP-Request
packet. These packets MAY contain application data, and 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=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Service Code: 32 bits
Describes the application-level service to which the client
application wants to connect. Service Codes are intended to
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
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) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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



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Number on a received DCCP-Request.

Service Code: 32 bits
MUST equal the Service Code on the corresponding DCCP-Request.

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
use 24-bit sequence numbers, depending on the value of the Allow
Short Sequence Numbers feature (Section 7.6.1). DCCP-Data packets
carry application data without 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) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ 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) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DCCP-DataAck packets carry both application data and an
Acknowledgement Number: acknowledgement information is piggybacked
on a data packet.








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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) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ 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
absence of application data (not the presence of zero-length
application data). The API SHOULD report any received zero-length
datagrams to the receiving application.

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

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
(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=5 (DCCP-CloseReq) or 6 (DCCP-Close) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY
have non-zero-length application data areas, whose contents
receivers MUST ignore.

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
MUST use 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=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reset Code \| Data 1 \| Data 2 \| Data 3 \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (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.



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Application Data Area: Error Text
If present, Error Text is a human-readable text string encoded
in Unicode UTF-8, and preferably in English, that describes the
error in more detail. For example, a DCCP-Reset with Reset Code
11, "Aggression Penalty", might contain Error Text such as
"Aggression Penalty: Received 3 bad ECN Nonce Echoes, assuming
misbehavior".

The following Reset Codes are currently defined. 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
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
DCCP-Data packet with valid header checksum and sequence numbers
arrived at a connection in the REQUEST state. See Section
8.3.1. The Data 1 field equals the offending packet type as an
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
immediately preceded by Mandatory. The Data fields report the
option type and data of option O, using the format of Reset Code



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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 Section
12.3.

12-127, Reserved
Receivers should treat these codes as they do Reset Code 0,	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.
Receivers should treat unknown CCID-specific Reset Codes as they	Receivers should treat unknown CCID-specific Reset Codes like
do Reset Code 0, "Unspecified".	Reset Code 0, "Unspecified".

The following table summarizes this information.
















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

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
types MUST use 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=8 (DCCP-Sync) or 9 (DCCP-SyncAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Acknowledgement Number field has special semantics for DCCP-Sync



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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
have non-zero-length application data areas, whose contents
receivers MUST ignore. Padded DCCP-Sync packets may be useful when
performing Path MTU discovery; see 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
option may begin on any byte boundary. However, the combination of
all options MUST add up to a multiple of 32 bits; Padding options
MUST be added as necessary to fill out option space to a word
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.

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

The following options are currently defined:










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

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. (Section 7.5.5 describes why
such options are ignored as opposed to, say, causing a reset.)

Options with invalid values MUST be ignored unless otherwise
specified. For example, any Data Checksum option with option length
4 MUST be ignored, since all valid Data Checksum options have option
length 6.

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

5.8.1. Padding Option






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

Padding is a single-byte "no-operation" option used to pad between
or after options. If the length of a packet's other options is not
a multiple of 32 bits, then Padding options are REQUIRED to pad out
the options area to the length implied by Data Offset. Padding may
also be used between options -- for example, to align the beginning
of a subsequent option on a 32-bit 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
receiving DCCP endpoint understands and processes O. If the
endpoint does not understand or process O, 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
O's data was invalid for O's type; if O was a feature negotiation
option, and the endpoint did not understand the enclosed feature
number; if the endpoint understood O, but chose not to perform the
action O 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
the option list), or if option O 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
used to negotiate feature values. 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
value area depends on the feature type; see Section 6.3.

+--------+--------+--------+--------+--------
\| 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
option to use depends on the relevant feature's location: To start a
negotiation for feature F/A, DCCP A will send a Change L option; to
start a negotiation for F/B, it will send a Change R option. Change
options are retransmitted until some response is received. They
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 are retransmitted as necessary. The first byte of the
Confirm option contains the feature number from the corresponding
Change. Following this is the selected Value, and then possibly 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
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
values ordered by preference, with the most preferred value coming
first. Each Confirm option contains the confirmed value, followed
by the confirmer's preference list. Thus, the feature's current
value will generally appear twice in Confirm options' data, once as
the current value and once in the confirmer's preference list.





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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).

6.3.2. Non-Negotiable

The feature value is a byte string. Each option contains exactly
one feature value. The feature location signals a new value by
sending a Change L option. The feature remote MUST accept any valid
value, responding with a Confirm R option containing the new value,
and it MUST send empty Confirm R options in response to invalid
values (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
the feature 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
4 ECN Incapable SP 0 N 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

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

6.5. Feature Negotiation 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) = 32,9,3,0,0,0,0,4,0
DCCP B should change Sequence Window/A's value (feature number
3, a non-negotiable feature) to the 6-byte string 0,0,0,0,4,0
(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) = 35,9,3,0,0,0,0,4,0
DCCP A has changed Sequence Window/B's value to the 6-byte
string 0,0,0,0,4,0 (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
negotiation packet", often a DCCP-Ack or DCCP-Sync, just to carry
the option. Feature negotiation packets are controlled by the
relevant congestion control mechanism. For example, DCCP A may send
a DCCP-Ack or DCCP-Sync for feature negotiation only if the B-to-A
CCID would allow sending a DCCP-Ack. In addition, an endpoint
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-
reordered Change option MUST result in a corresponding Confirm
option, and any packet including a Confirm option MUST carry an
Acknowledgement Number. (Section 6.6.4 describes how Change
reordering is detected and handled.) Generated Confirm options may
be attached to packets that would have been sent anyway (such as
DCCP-Response or DCCP-SyncAck), or to new feature negotiation
packets, as 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
long as no two options refer 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
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



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feature's value and thinks the other endpoint agrees. An endpoint
enters the CHANGING state when it first sends a Change for the
feature, and returns to STABLE once it receives a corresponding
Confirm. 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
according to this diagram. The diagram ignores sequence number and
option validity issues; these are handled explicitly in the
pseudocode that follows.

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

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



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Return

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),
Ignore option and return

Third, process Change R options;
If O.type == Change R,
If the 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,
Reset connection and return
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 O.len > 3, /* nonempty */
If the option's value is valid,
Set F.value := new value
Otherwise,
Reset connection and return
Set F.state := STABLE

Versions of this diagram and pseudocode are also used by 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
transmitted to achieve reliability. We refer to this as
"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.




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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
retransmit Change options. (Endpoints that generate packets
specifically for feature negotiation MUST use such a timer.) The
timer interval is initially set to not less than one round-trip
time, and should back off to not less than 64 seconds. The backoff
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.

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
MUST ignore any such options they receive.

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 limitations, 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.

7. Sequence Numbers

DCCP uses sequence numbers to arrange packets into sequence, detect
losses and network duplicates, and protect against attackers, half-
open connections, and the delivery of very old packets. Every
packet carries a Sequence Number; most packet types carry an
Acknowledgement Number as well.

DCCP sequence numbers are 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.)

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
its header was succesfully processed by the receiving DCCP. In
terms of the pseudocode in Section 8.5, a received packet becomes
acknowledgeable when the receiving endpoint reaches Step 8. This
means, for example, that all acknowledgeable packets have valid
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.

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

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.
Change and Confirm options for Sequence Window are therefore 9 bytes
long. New connections start with Sequence Window 100 for both
endpoints. The minimum valid Sequence Window value is Wmin = 32.
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 Sequence
Window values out of this range are invalid and MUST be handled
accordingly.

A proper Sequence Window/A value must reflect the number of packets
DCCP A expects to be in flight. Only DCCP A can anticipate this
number. Values that are too small increase the risk of the
endpoints getting out sync after bursts of loss, and values that are
much too small can prevent productive communication whether or not
there is loss. On the other hand, too-large values increase the
risk of connection hijacking; Section 7.5.5 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. Endpoints SHOULD send Change L(Sequence Window)
options as necessary as the connection progresses. Also, an
endpoint MUST NOT persistently send more than its Sequence Window
number of packets per round-trip time; that is, DCCP A MUST NOT
persistently send more than Sequence Window/A packets per RTT.

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
[AWL, AWH] defined in Section 7.5.1.






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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-SyncAck SWL <= seqno 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
eliminate their usefulness.

The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow
continued operation after unusual events, such as endpoint crashes
and large bursts of loss, but there's no need for leniency in the
absence of unusual events -- that is, during ongoing successful
communication. Therefore, DCCP implementations SHOULD use the
following, more stringent checks for active connections, where a
connection is considered active if it has received valid packets
from the other endpoint 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

Endpoints MUST ignore sequence-invalid DCCP-Sync and DCCP-SyncAck
packets, and MUST respond to other sequence-invalid packets with
(possibly rate-limited) DCCP-Sync packets. Each such DCCP-Sync
packet MUST use a new Sequence Number, and thus will increase GSS;
GSR will not change, however, since the received packet was
sequence-invalid. Each such DCCP-Sync packet's Acknowledgement
Number MUST equal GSR when the received sequence-invalid packet has
type DCCP-Reset, and the received packet's Sequence Number
otherwise.

On receiving a sequence-valid DCCP-Sync packet, the peer endpoint
(say, DCCP B) MUST update its GSR variable and reply with a DCCP-
SyncAck packet. The DCCP-SyncAck packet's Acknowledgement Number
will equal the DCCP-Sync's Sequence Number, not necessarily GSR.
Upon receiving this DCCP-SyncAck, which will be sequence-valid since
it acknowledges the DCCP-Sync, DCCP A will update its GSR variable,
and the endpoints will be back in sync. As an exception, if the
peer endpoint is in the REQUEST state, it MUST respond with a DCCP-
Reset instead of a DCCP-SyncAck. This serves to clean up DCCP A's
half-open connection.

To protect against denial-of-service attacks, DCCP implementations
SHOULD impose a rate limit on DCCP-Syncs sent in response to
sequence-invalid packets, such as not more than eight DCCP-Syncs per
second.

DCCP endpoints MUST NOT process sequence-invalid packets except,
perhaps, by generating a DCCP-Sync. For instance, options MUST NOT
but processed. An endpoint MAY temporarily preserve sequence-
invalid packets in case they become valid later, however; this can
reduce the impact of bursts of loss by delivering more packets to
the application. In particular, an endpoint MAY preserve sequence-



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invalid packets for up to 2 round-trip times. If, within that time,
the relevant sequence windows change so that the packets become
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.

7.5.5. Sequence Number Attacks

Sequence and Acknowledgement Numbers form DCCP's main line of
defense against attackers. An attacker that cannot guess sequence
numbers cannot easily manipulate or hijack a DCCP connection, and
requirements like careful initial sequence number choice eliminate
the most serious attacks.

An attacker might still send many packets with randomly chosen
Sequence and Acknowledgement Numbers, however. If one of those
probes ends up sequence-valid, it may shut down the connection or
otherwise cause problems. The easiest such attacks to execute are:

o Send DCCP-Data packets with random Sequence Numbers. If one of
these packets hits the valid sequence number window, the attack
packet's application data may be inserted into the data stream.

o Send DCCP-Sync packets with random Sequence and Acknowledgement
Numbers. If one of these packets hits the valid acknowledgement
number window, the receiver will shift its sequence number window
accordingly, getting out of sync with the correct endpoint --
perhaps permanently.

The attacker has to guess both Source and Destination Ports for any
of these attacks to succeed. Additionally, the connection would
have to be inactive for the DCCP-Sync attack to succeed, assuming
the victim implemented the more stringent checks for active
connections recommended in Section 7.5.3.





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To quantify the probability of success, let N be the number of
attack packets the attacker is willing to send, W be the relevant
sequence window width, and L be the length of sequence numbers (24
or 48). The attacker's best strategy is to space the attack packets
evenly over sequence space. Then the probability of hitting one
sequence number window is P = WN/2^L.

The success probability for a DCCP-Data attack using short sequence
numbers thus equals P = WN/2^24. For W = 100, then, the attacker
must send more than 83,000 packets to achieve a 50% chance of
success. For reference, the easiest TCP attack -- sending a SYN
with a random sequence number, which will cause a connection reset
if it falls within the window -- has W = 8760 (a common default) and
L = 32, and requires more than 245,000 packets to achieve a 50%
chance of success.

A fast connection's W will generally be high, increasing the attack
success probability for fixed N. If this probability gets
uncomfortably high with L = 24, the endpoint SHOULD prevent the use
of short sequence numbers by manipulating the Allow Short Sequence
Numbers feature (see Section 7.6.1). The probability limit depends
on the application, however. Some applications, such as those
already designed to handle corruption, are quite resilient to data
injection attacks.

The DCCP-Sync attack has L = 48, since DCCP-Sync packets use long
sequence numbers exclusively; in addition, the success probability
is halved, since only half the Sequence Number space is valid.
Attacks have a correspondingly smaller probability of success. For
a large W of 2000 packets, then, the attacker must send more than
10^11 packets to achieve a 50% chance of success.

Attacks involving DCCP-Ack, DCCP-DataAck, DCCP-CloseReq, DCCP-Close,
and DCCP-Reset packets are more difficult, since Sequence and
Acknowledgement Numbers must both be guessed. The probability of
attack success for these packet types equals P = WXN/2^(2L), where W
is the Sequence Number window, X is the Acknowledgement Number
window, and N and L are as before.

Since DCCP-Data attacks with short sequence numbers are relatively
easy for attackers to execute, DCCP has been engineered to prevent
these attacks from escalating to connection resets or other serious
consequences. In particular, any options whose processing might
cause the connection to be reset are ignored when they appear on
DCCP-Data packets.






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7.5.6. Sequence Number Handling Examples

In the following example, DCCP A and DCCP B recover from a large
burst of loss that runs DCCP A's sequence numbers out of DCCP B's
appropriate sequence number window.

DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
--> DCCP-Data(seq 2) XXX
...
--> DCCP-Data(seq 100) XXX
--> DCCP-Data(seq 101) --> ???
seqno out of range;
send Sync
OK <-- DCCP-Sync(seq 11, ack 101) <--
(GSS=11,GSR=1)
--> DCCP-SyncAck(seq 102, ack 11) --> OK
(GSS=102,GSR=11) (GSS=11,GSR=102)

In the next example, a DCCP connection recovers from a simple blind
attack.

DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
ATTACKER --> DCCP-Data(seq 10^6) --> ???
seqno out of range;
send Sync
??? <-- DCCP-Sync(seq 11, ack 10^6) <--
ackno out of range; ignore
(GSS=1,GSR=10) (GSS=11,GSR=1)

The final example demonstrates recovery from a half-open connection.

DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
(Crash)
CLOSED OPEN
REQUEST --> DCCP-Request(seq 400) --> ???
!! <-- DCCP-Sync(seq 11, ack 400) <-- OPEN
REQUEST --> DCCP-Reset(seq 401, ack 11) --> (Abort)
REQUEST CLOSED
REQUEST --> DCCP-Request(seq 402) --> ...


7.6. Short Sequence Numbers

DCCP sequence numbers are 48 bits long. This large sequence space
protects DCCP connections against some blind attacks, such as the



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injection of DCCP-Resets into the connection. However, DCCP-Data,
DCCP-Ack, and DCCP-DataAck packets, which make up the body of any
DCCP connection, may reduce header space by transmitting only the
lower 24 bits of the relevant Sequence and Acknowledgement Numbers.
The receiving endpoint will extend these numbers to 48 bits using
the following pseudocode:

procedure Extend_Sequence_Number(S, REF)
/* S is a 24-bit sequence number from the packet header.
REF is the relevant 48-bit reference sequence number:
GSS if S is an Acknowledgement Number, and GSR if S is a
Sequence Number. */
Set REF_low := low 24 bits of REF
Set REF_hi := high 24 bits of REF
If REF_low (<) S /* circular comparison mod 2^24 */
and S \|<\| REF_low, /* conventional, non-circular
comparison */
Return (((REF_hi + 1) mod 2^24) << 24) \| S
Otherwise, if S (<) REF_low and REF_low \|<\| S,
Return (((REF_hi - 1) mod 2^24) << 24) \| S
Otherwise,
Return (REF_hi << 24) \| S

The two different kinds of comparison in the if statements detect
when the low-order bits of the sequence space have wrapped. (The
circular comparison "REF_low (<) S" returns true if and only if
(S - REF_low), calculated using two's-complement arithmetic and then
represented as an unsigned number, is less than or equal to 2^23
(mod 2^24).) When this happens, the high-order bits are incremented
or decremented, as appropriate.

7.6.1. Allow Short Sequence Numbers Feature

Endpoints can require that all packets use long sequence numbers by
setting the Allow Short Sequence Numbers feature to false. This can
reduce the risk that data will be inappropriately injected into the
connection. DCCP A sends a "Change R(Allow Short Seqnos, 0)" option
to ask DCCP B to send only long sequence numbers.

Allow Short Sequence Numbers has feature number 2, and is server-
priority. It takes one-byte Boolean values. DCCP B MUST NOT send
packets with short sequence numbers when Allow Short Seqnos/B is
zero. Values of two or more are reserved. New connections start
with Allow Short Sequence Numbers 1 for both endpoints.







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7.6.2. When to Avoid Short Sequence Numbers

Short sequence numbers reduce the rate DCCP connections can safely
achieve, and increase the risks of certain kinds of attacks,
including blind data injection. Very-high-rate DCCP connections,
and connections with large sequence windows (Section 7.5.2), SHOULD
NOT use short sequence numbers on their data packets. The attack
risk issues have been discussed in Section 7.5.5; we discuss the
rate limitation issue here.

The sequence-validity mechanism assumes that the network does not
deliver extremely old data. In particular, it assumes that the
network must have dropped any packet by the time the connection
wraps around and uses its sequence number again. This constraint
limits the maximum connection rate that can be safely achieved. Let
MSL equal the maximum segment lifetime, P equal the average DCCP
packet size in bits, and L equal the length of sequence numbers (24
or 48 bits). Then the maximum safe rate, in bits per second, is R =
P*(2^L)/2MSL.

For the default MSL of 2 minutes, 1500-byte DCCP packets, and short
sequence numbers, the safe rate is therefore approximately 800 Mb/s.
Although 2 minutes is a very large MSL for any networks that could
sustain that rate with such small packets, long sequence numbers
allow much higher rates under the same constraints: up to
14 petabits a second for 1500-byte packets and the default MSL.

7.7. NDP Count and Detecting Application Loss

DCCP's sequence numbers increment by one on every packet, including
non-data packets (packets that don't carry application data). This
makes DCCP sequence numbers suitable for detecting any network loss,
but not for detecting the loss of application data. The NDP Count
option reports the length of each burst of non-data packets. This
lets the receiving DCCP reliably determine when a burst of loss
included application data.

+--------+--------+-------- ... --------+
\|00100101\| Length \| NDP Count \|
+--------+--------+-------- ... --------+
Type=37 Len=3-5 (1-3 bytes)

If a DCCP endpoint's Send NDP Count feature is one (see below), then
that endpoint MUST send an NDP Count option on every packet whose
immediate predecessor was a non-data packet. Non-data packets
consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
DCCP-Reset, DCCP-Sync, and DCCP-SyncAck. The other packet types,
namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are



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considered data packets, although not all DCCP-Request and DCCP-
Response packets will actually carry application data.

The value stored in NDP Count equals the number of consecutive non-
data packets in the run immediately previous to the current packet.
Packets with no NDP Count option are considered to have NDP Count
zero.

The NDP Count option can carry one to three bytes of data. The
smallest option format that can hold the NDP Count SHOULD be used.

With NDP Count, the receiver can reliably tell only whether a burst
of loss contained at least one data packet. For example, the
receiver cannot always tell whether a burst of loss contained a non-
data packet.

7.7.1. NDP Count Usage Notes

Say that K consecutive sequence numbers are missing in some burst of
loss, and the Send NDP Count feature is on. Then some application
data was lost within those sequence numbers unless the packet
following the hole contains an NDP Count option whose value is
greater than or equal to K.

For example, say that an endpoint sent the following sequence of
non-data packets (Nx) and data packets (Dx).

N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13

Those packets would have NDP Counts as follows.

N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
- 1 2 - 1 - - 1 - - - - 1 2

NDP Count is not useful for applications that include their own
sequence numbers with their packet headers.

7.7.2. Send NDP Count Feature

The Send NDP Count feature lets DCCP endpoints negotiate whether
they should send NDP Count options on their packets. DCCP A sends a
"Change R(Send NDP Count, 1)" option to ask DCCP B to send NDP Count
options.

Send NDP Count has feature number 7, and is server-priority. It
takes one-byte Boolean values. DCCP B MUST send NDP Count options
as described above when Send NDP Count/B is one, although it MAY
send NDP Count options even when Send NDP Count/B is zero. Values



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of two or more are reserved. New connections start with Send NDP
Count 0 for both endpoints.

8. Event Processing

This section describes how DCCP connections move between states, and
which packets are sent when. Note that feature negotiation takes
place in parallel with the connection-wide state transitions
described here.

8.1. Connection Establishment

DCCP connections' initiation phase consists of a three-way
handshake: an initial DCCP-Request packet sent by the client, a
DCCP-Response sent by the server in reply, and finally an
acknowledgement from the client, usually via a DCCP-Ack or DCCP-