The version of draft-ietf-dccp-spec-13.txt used to generate this diff is slightly different from the published version, in that we altered its margins and whitespace formatting in order to obtain a closer correspondence with the published RFC.







Network Working Group E. Kohler	Internet Engineering Task Force Eddie Kohler
Request for Comments: 4340 UCLA	INTERNET-DRAFT UCLA
Category: Standards Track M. Handley	draft-ietf-dccp-spec-13.txt Mark Handley
UCL	Expires: 3 October 2006 UCL
S. Floyd	Sally Floyd
ICIR
March 2006	3 April 2006


Datagram Congestion Control Protocol (DCCP)

Status of This Memo

This document specifies an Internet standards track protocol for the	By submitting this Internet-Draft, each author represents that any
Internet community, and requests discussion and suggestions for	applicable patent or other IPR claims of which he or she is aware
improvements. Please refer to the current edition of the "Internet	have been or will be disclosed, and any of which he or she becomes
Official Protocol Standards" (STD 1) for the standardization state	aware will be disclosed, in accordance with Section 6 of BCP 79.
and status of this protocol. Distribution of this memo is unlimited.	Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that
Copyright Notice	other groups may also distribute working documents as Internet-
	Drafts.
Copyright (C) The Internet Society (2006).	Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any
Abstract	time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."
The Datagram Congestion Control Protocol (DCCP) is a transport	The list of current Internet-Drafts can be accessed at
protocol that provides bidirectional unicast connections of	http://www.ietf.org/ietf/1id-abstracts.txt.
congestion-controlled unreliable datagrams. DCCP is suitable for	The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.
	This Internet-Draft will expire on 3 October 2006.
applications that transfer fairly large amounts of data and that can	applications that transfer fairly large amounts of data, but can
benefit from control over the tradeoff between timeliness and
reliability.
\|\| TEXT DELETED \|\|	TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
	Changes since draft-ietf-dccp-spec-08.txt:
Table of Contents	* 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.
	* 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.

1. Introduction ....................................................5	1. Introduction ....................................................9
2. Design Rationale ................................................6	2. Design Rationale ...............................................10
3. Conventions and Terminology .....................................7	3. Conventions and Terminology ....................................11
3.1. Numbers and Fields .........................................7	3.1. Numbers and Fields ........................................11
3.2. Parts of a Connection ......................................8	3.2. Parts of a Connection .....................................12
3.3. Features ...................................................9	3.3. Features ..................................................12
3.4. Round-Trip Times ...........................................9	3.4. Round-Trip Times ..........................................13
3.5. Security Limitation ........................................9	3.5. Security Limitation .......................................13
3.6. Robustness Principle ......................................10	3.6. Robustness Principle ......................................13
4. Overview .......................................................10	4. Overview .......................................................13
4.1. Packet Types ..............................................10	4.1. Packet Types ..............................................14
4.2. Packet Sequencing .........................................11	4.2. Packet Sequencing .........................................15
4.3. States ....................................................12	4.3. States ....................................................16
4.4. Congestion Control Mechanisms .............................14	4.4. Congestion Control Mechanisms .............................17
	4.5. Connection Features .......................................18
	4.6. Differences From TCP ......................................19
	4.7. Example Connection ........................................20
Kohler, et al. Standards Track [Page 1]	5. Packet Formats .................................................22

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006	5.1. Generic Header ............................................22
	5.2. DCCP-Request Packets ......................................25
	5.3. DCCP-Response Packets .....................................26
4.5. Connection Features .......................................15	5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets .............27
4.6. Differences from TCP ......................................16	5.5. DCCP-CloseReq and DCCP-Close Packets ......................28
4.7. Example Connection ........................................17	5.6. DCCP-Reset Packets ........................................29
5. Packet Formats .................................................18	5.7. DCCP-Sync and DCCP-SyncAck Packets ........................32
5.1. Generic Header ............................................19	5.8. Options ...................................................33
5.2. DCCP-Request Packets ......................................22	5.8.1. Padding Option .....................................34
5.3. DCCP-Response Packets .....................................23	5.8.2. Mandatory Option ...................................35
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets .............23	6. Feature Negotiation ............................................35
5.5. DCCP-CloseReq and DCCP-Close Packets ......................25	6.1. Change Options ............................................36
5.6. DCCP-Reset Packets ........................................25	6.2. Confirm Options ...........................................36
5.7. DCCP-Sync and DCCP-SyncAck Packets ........................28	6.3. Reconciliation Rules ......................................37
5.8. Options ...................................................29	6.3.1. Server-Priority ....................................37
5.8.1. Padding Option .....................................31	6.3.2. Non-Negotiable .....................................37
5.8.2. Mandatory Option ...................................31	6.4. Feature Numbers ...........................................38
6. Feature Negotiation ............................................32	6.5. Feature Negotiation Examples ..............................39
6.1. Change Options ............................................32	6.6. Option Exchange ...........................................40
6.2. Confirm Options ...........................................33	6.6.1. Normal Exchange ....................................40
6.3. Reconciliation Rules ......................................33	6.6.2. Processing Received Options ........................41
6.3.1. Server-Priority ....................................33	6.6.3. Loss and Retransmission ............................43
6.3.2. Non-Negotiable .....................................34	6.6.4. Reordering .........................................44
6.4. Feature Numbers ...........................................34	6.6.5. Preference Changes .................................45
6.5. Sequence Number Handling Examples .........................35	6.6.6. Simultaneous Negotiation ...........................45
6.6. Option Exchange ...........................................36	6.6.7. Unknown Features ...................................45
6.6.1. Normal Exchange ....................................37	6.6.8. Invalid Options ....................................46
6.6.2. Processing Received Options ........................37	6.6.9. Mandatory Feature Negotiation ......................46
6.6.3. Loss and Retransmission ............................39	7. Sequence Numbers ...............................................47
6.6.4. Reordering .........................................40	7.1. Variables .................................................47
6.6.5. Preference Changes .................................41	7.2. Initial Sequence Numbers ..................................48
6.6.6. Simultaneous Negotiation ...........................41	7.3. Quiet Time ................................................49
6.6.7. Unknown Features ...................................41	7.4. Acknowledgement Numbers ...................................49
6.6.8. Invalid Options ....................................42	7.5. Validity and Synchronization ..............................50
6.6.9. Mandatory Feature Negotiation ......................43	7.5.1. Sequence and Acknowledgement Number Windows ........50
7. Sequence Numbers ...............................................43	7.5.2. Sequence Window Feature ............................51
7.1. Variables .................................................43	7.5.3. Sequence-Validity Rules ............................52
7.2. Initial Sequence Numbers ..................................44	7.5.4. Handling Sequence-Invalid Packets ..................53
7.3. Quiet Time ................................................45	7.5.5. Sequence Number Attacks ............................54
7.4. Acknowledgement Numbers ...................................46	7.5.6. Sequence Number Handling Examples ..................56
7.5. Validity and Synchronization ..............................46	7.6. Short Sequence Numbers ....................................57
7.5.1. Sequence and Acknowledgement Number Windows ........47	7.6.1. Allow Short Sequence Numbers Feature ...............57
7.5.2. Sequence Window Feature ............................48	7.6.2. When to Avoid Short Sequence Numbers ...............58
7.5.3. Sequence-Validity Rules ............................48	7.7. NDP Count and Detecting Application Loss ..................58
7.5.4. Handling Sequence-Invalid Packets ..................50	7.7.1. NDP Count Usage Notes ..............................59
7.5.5. Sequence Number Attacks ............................51	7.7.2. Send NDP Count Feature .............................60
7.5.6. Examples ...........................................53	8. Event Processing ...............................................60
7.6. Short Sequence Numbers ....................................54	8.1. Connection Establishment ..................................60
7.6.1. Allow Short Sequence Numbers Feature ...............54	8.1.1. Client Request .....................................60
7.6.2. When to Avoid Short Sequence Numbers ...............55	8.1.2. Service Codes ......................................61
7.7. NDP Count and Detecting Application Loss ..................55	8.1.3. Server Response ....................................63
	8.1.4. Init Cookie Option .................................64
	8.1.5. Handshake Completion ...............................65
	8.2. Data Transfer .............................................66
Kohler, et al. Standards Track [Page 2]	8.3. Termination ...............................................66

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006	8.3.1. Abnormal Termination ...............................68
	8.4. DCCP State Diagram ........................................68
	8.5. Pseudocode ................................................69
7.7.1. NDP Count Usage Notes ..............................56	9. Checksums ......................................................73
7.7.2. Send NDP Count Feature .............................56	9.1. Header Checksum Field .....................................74
8. Event Processing ...............................................57	9.2. Header Checksum Coverage Field ............................75
8.1. Connection Establishment ..................................57	9.2.1. Minimum Checksum Coverage Feature ..................76
8.1.1. Client Request .....................................57	9.3. Data Checksum Option ......................................76
8.1.2. Service Codes ......................................58	9.3.1. Check Data Checksum Feature ........................77
8.1.3. Server Response ....................................60	9.3.2. Checksum Usage Notes ...............................78
8.1.4. Init Cookie Option .................................61	10. Congestion Control ............................................78
8.1.5. Handshake Completion ...............................62	10.1. TCP-like Congestion Control ..............................79
8.2. Data Transfer .............................................62	10.2. TFRC Congestion Control ..................................79
8.3. Termination ...............................................63	10.3. CCID-Specific Options, Features, and Reset Codes .........79
8.3.1. Abnormal Termination ...............................65	10.4. CCID Profile Requirements ................................82
8.4. DCCP State Diagram ........................................65	10.5. Congestion State .........................................82
8.5. Pseudocode ................................................66	11. Acknowledgements ..............................................83
9. Checksums ......................................................70	11.1. Acks of Acks and Unidirectional Connections ..............84
9.1. Header Checksum Field .....................................71	11.2. Ack Piggybacking .........................................85
9.2. Header Checksum Coverage Field ............................72	11.3. Ack Ratio Feature ........................................85
9.2.1. Minimum Checksum Coverage Feature ..................73	11.4. Ack Vector Options .......................................87
9.3. Data Checksum Option ......................................73	11.4.1. Ack Vector Consistency ............................89
9.3.1. Check Data Checksum Feature ........................74	11.4.2. Ack Vector Coverage ...............................91
9.3.2. Checksum Usage Notes ...............................75	11.5. Send Ack Vector Feature ..................................91
10. Congestion Control ............................................75	11.6. Slow Receiver Option .....................................92
10.1. TCP-like Congestion Control ..............................76	11.7. Data Dropped Option ......................................93
10.2. TFRC Congestion Control ..................................76	11.7.1. Data Dropped and Normal Congestion Response .......95
10.3. CCID-Specific Options, Features, and Reset Codes .........77	11.7.2. Particular Drop Codes .............................96
10.4. CCID Profile Requirements ................................79	12. Explicit Congestion Notification ..............................97
10.5. Congestion State .........................................79	12.1. ECN Incapable Feature ....................................97
11. Acknowledgements ..............................................80	12.2. ECN Nonces ...............................................98
11.1. Acks of Acks and Unidirectional Connections ..............81	12.3. Aggression Penalties .....................................99
11.2. Ack Piggybacking .........................................82	13. Timing Options ...............................................100
11.3. Ack Ratio Feature ........................................82	13.1. Timestamp Option ........................................100
11.4. Ack Vector Options .......................................84	13.2. Elapsed Time Option .....................................100
11.5. Send Ack Vector Feature ..................................89	13.3. Timestamp Echo Option ...................................101
11.6. Slow Receiver Option .....................................89	14. Maximum Packet Size ..........................................102
11.7. Data Dropped Option ......................................90	14.1. Measuring PMTU ..........................................103
11.7.1. Data Dropped and Normal Congestion Response .......93	14.2. Sender Behavior .........................................104
11.7.2. Particular Drop Codes .............................93	15. Forward Compatibility ........................................105
12. Explicit Congestion Notification ..............................94	16. Middlebox Considerations .....................................106
12.1. ECN Incapable Feature ....................................95	17. Relations to Other Specifications ............................107
12.2. ECN Nonces ...............................................95	17.1. RTP .....................................................107
12.3. Aggression Penalties .....................................97	17.2. Congestion Manager and Multiplexing .....................109
13. Timing Options ................................................97	18. Security Considerations ......................................109
13.1. Timestamp Option .........................................97	18.1. Security Considerations for Partial Checksums ...........110
13.2. Elapsed Time Option ......................................98	19. IANA Considerations ..........................................110
13.3. Timestamp Echo Option ....................................99	19.1. Packet Types Registry ...................................111
14. Maximum Packet Size ..........................................100	19.2. Reset Codes Registry ....................................111
14.1. Measuring PMTU ..........................................100	19.3. Option Types Registry ...................................111
14.2. Sender Behavior .........................................102	19.4. Feature Numbers Registry ................................112
	19.5. Congestion Control Identifiers Registry .................112
	19.6. Ack Vector States Registry ..............................112
	19.7. Drop Codes Registry .....................................112
Kohler, et al. Standards Track [Page 3]	19.8. Service Codes Registry ..................................113

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006	19.9. Port Numbers Registry ...................................113
	20. Thanks .......................................................115
	A. Appendix: Ack Vector Implementation Notes .....................115
15. Forward Compatibility ........................................103	A.1. Packet Arrival ...........................................118
16. Middlebox Considerations .....................................103	A.1.1. New Packets .......................................118
17. Relations to Other Specifications ............................105	A.1.2. Old Packets .......................................119
17.1. RTP .....................................................105	A.2. Sending Acknowledgements .................................119
17.2. Congestion Manager and Multiplexing .....................106	A.3. Clearing State ...........................................120
18. Security Considerations ......................................107	A.4. Processing Acknowledgements ..............................121
18.1. Security Considerations for Partial Checksums ...........107	B. Appendix: Partial Checksumming Design Motivation ..............122
19. IANA Considerations ..........................................108	Normative References .............................................123
19.1. Packet Types Registry ...................................108	Informative References ...........................................124
19.2. Reset Codes Registry ....................................109	Authors' Addresses ...............................................126
19.3. Option Types Registry ...................................109	Full Copyright Statement .........................................127
19.4. Feature Numbers Registry ................................109	Intellectual Property ............................................127
19.5. Congestion Control Identifiers Registry .................110
19.6. Ack Vector States Registry ..............................110
19.7. Drop Codes Registry .....................................110
19.8. Service Codes Registry ..................................110
19.9. Port Numbers Registry ...................................111
20. Thanks .......................................................113
A. Appendix: Ack Vector Implementation Notes ....................114
A.1. Packet Arrival ..........................................116
A.1.1. New Packets ......................................116
A.1.2. Old Packets ......................................117
A.2. Sending Acknowledgements ................................118
A.3. Clearing State ..........................................118
A.4. Processing Acknowledgements .............................120
B. Appendix: Partial Checksumming Design Motivation .............120
Normative References .............................................122
Informative References ...........................................123

List of Tables

Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . . 21	Table 1: DCCP Packet Types ........................................24
Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . . 28	Table 2: DCCP Reset Codes .........................................31
Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . . 30	Table 3: DCCP Options .............................................33
Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . . 34	Table 4: DCCP Feature Numbers .....................................38
Table 5: DCCP Congestion Control Identifiers . . . . . . . . . . 75	Table 5: DCCP Congestion Control Identifiers ......................78
Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . . 85	Table 6: DCCP Ack Vector States ...................................88
Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . . 91	Table 7: DCCP Drop Codes ..........................................94













Kohler, et al. Standards Track [Page 4]

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006


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 the following:	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) [RFC3168] and the ECN Nonce [RFC3540].	(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 [RFC1191].	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 [RFC4341] and	are currently specified, TCP-like Congestion Control [CCID 2
TFRC (TCP-Friendly Rate Control) Congestion Control [RFC4342].	PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion Control
DCCP is easily extensible to further forms of unicast congestion	[CCID 3 PROFILE], but DCCP is easily extensible to further forms
control.	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,	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





Kohler, et al. Standards Track [Page 5]

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006


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

2. Design Rationale

One DCCP design goal was to give most streaming UDP applications
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	(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	changes in available bandwidth, but must tolerate the abrupt changes
in congestion window typical of TCP. A second alternative, TCP-
Friendly Rate Control (TFRC) [RFC3448], a form of equation-based	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	might not allow applications to set UDP packets as ECN-capable, since
the API could not guarantee that the application would properly	the API could not guarantee the application would properly detect or
detect or respond to congestion. DCCP kernel APIs will have no such	respond to congestion. DCCP kernel APIs will have no such issues,
issues, since DCCP implements congestion control itself.	since DCCP implements congestion control itself.
	We chose not to require the use of the Congestion Manager [RFC 3124],
We chose not to require the use of the Congestion Manager [RFC3124],
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	control mechanisms, or lend itself to the use of forms of TFRC where



Kohler, et al. Standards Track [Page 6]

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006


the state about past packet drops or marks is maintained at the
receiver rather than at the sender. DCCP should be able to make use
of CM where desired by the application, but we do not see any benefit
in making the deployment of DCCP contingent on the deployment of CM
itself.

We intend for DCCP's protocol mechanisms, which are described in this
document, to suit any application desiring unicast congestion-
controlled streams of unreliable datagrams. However, the congestion	controlled streams of unreliable datagrams. The congestion control
control mechanisms currently approved for use with DCCP, which are	mechanisms currently approved for use with DCCP, which are described
described in separate Congestion Control ID Profiles [RFC4341,	in separate Congestion Control ID Profiles [CCID 2 PROFILE, CCID 3
RFC4342], may cause problems for some applications, including high-	PROFILE], may, however, cause problems for some applications,
bandwidth interactive video. These applications should be able to	including high-bandwidth interactive video. These applications
use DCCP once suitable Congestion Control ID Profiles are	should be able to use DCCP once suitable Congestion Control ID
standardized.	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 [RFC2119].	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	Random numbers in DCCP are used for their security properties, and
SHOULD be chosen according to the guidelines in RFC 4086.	SHOULD be chosen according to the guidelines in RFC 1750.
	All operations on DCCP sequence numbers, and comparisons such as
All operations on DCCP sequence numbers and comparisons such as	"greater" and "greatest", use circular arithmetic modulo 2**48. This
"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 [RFC793]	circular arithmetic spaces, including TCP's sequence numbers [RFC
and DNS's serial numbers [RFC1982]. Note that the common technique	793] and DNS's serial numbers [RFC 1982]. Note that the common
for implementing circular comparison using twos-complement	technique for implementing circular comparison using two's-complement
arithmetic, whereby A < B using circular arithmetic if and only if (A	arithmetic, whereby A < B using circular arithmetic if and only if
- B) < 0 using conventional twos-complement arithmetic, may be used	(A - B) < 0 using conventional two's-complement arithmetic, may be
for DCCP sequence numbers, provided that they are stored in the most	used for DCCP sequence numbers, provided they are stored in the most
significant 48 bits of 64-bit integers.




Kohler, et al. Standards Track [Page 7]

RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006


Reserved bitfields in DCCP packet headers MUST be set to zero by
senders and MUST be ignored by receivers, unless otherwise specified.	senders, and MUST be ignored by receivers, unless otherwise
This allows for future protocol extensions. In particular, DCCP	specified. This is to allow for future protocol extensions. In
processors MUST NOT reset a DCCP connection simply because a Reserved	particular, DCCP processors MUST NOT reset a DCCP connection simply
field has non-zero value [RFC3360].	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
flow in both directions simultaneously. Logically, however, a DCCP	be flowing in both directions simultaneously. Logically, however, a
connection consists of two separate unidirectional connections,	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-	acknowledgements, respectively. For example, DCCP A is the HC-Sender
Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.	and DCCP B is the HC-Receiver in the A-to-B half-connection.







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3.3. Features

A DCCP feature is a connection attribute on whose value the two
endpoints agree. Many properties of a DCCP connection are controlled
by features, including the congestion control mechanisms in use on
the two half-connections. The endpoints achieve agreement 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	available; this parameter should default to not less than
seconds, a reasonably conservative round-trip time for Internet TCP	0.2 seconds, a reasonably conservative round-trip time for Internet
connections. Protocol behavior specified in terms of "round-trip	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
[RFC2401]; depending on the level of security required, application-	[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.








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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" [RFC793].	accept from others" [RFC 793].

4. Overview

DCCP's high-level connection dynamics echo those of TCP. Connections
progress through three phases: initiation, including a 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	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	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 is no way to send	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.	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.



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

The packet types are as follows:

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	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