Roxen Community: RFC 4340 Datagram Congestion Control Protocol (DCCP) (Standards Track)

[Text version]

Network Working Group
Request for Comments: 4340
Category: Standards Track E. Kohler
UCLA
M. Handley
UCL
S. Floyd
ICIR
March 2006
Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (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 and that can benefit from control over the tradeoff between timeliness and reliability.

Table of Contents
1. Introduction ....................................................5 2. Design Rationale ................................................6 3. Conventions and Terminology .....................................7 3.1. Numbers and Fields .........................................7 3.2. Parts of a Connection ......................................8 3.3. Features ...................................................9 3.4. Round-Trip Times ...........................................9 3.5. Security Limitation ........................................9 3.6. Robustness Principle ......................................10 4. Overview .......................................................10 4.1. Packet Types ..............................................10 4.2. Packet Sequencing .........................................11 4.3. States ....................................................12 4.4. Congestion Control Mechanisms .............................14
4.5. Feature Negotiation Options ...............................15 4.6. Differences from TCP ......................................16 4.7. Example Connection ........................................17 5. Packet Formats .................................................18 5.1. Generic Header ............................................19 5.2. DCCP-Request Packets ......................................22 5.3. DCCP-Response Packets .....................................23 5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets .............23 5.5. DCCP-CloseReq and DCCP-Close Packets ......................25 5.6. DCCP-Reset Packets ........................................25 5.7. DCCP-Sync and DCCP-SyncAck Packets ........................28 5.8. Options ...................................................29 5.8.1. Padding Option .....................................31 5.8.2. Mandatory Option ...................................31 6. Feature Negotiation ............................................32 6.1. Change Options ............................................32 6.2. Confirm Options ...........................................33 6.3. Reconciliation Rules ......................................33 6.3.1. Server-Priority ....................................34 6.3.2. Non-Negotiable .....................................34 6.4. Feature Numbers ...........................................35 6.5. Feature Negotiation Examples ..............................36 6.6. Option Exchange ...........................................37 6.6.1. Normal Exchange ....................................38 6.6.2. Processing Received Options ........................38 6.6.3. Loss and Retransmission ............................40 6.6.4. Reordering .........................................41 6.6.5. Preference Changes .................................42 6.6.6. Simultaneous Negotiation ...........................42 6.6.7. Unknown Features ...................................43 6.6.8. Invalid Options ....................................43 6.6.9. Mandatory Feature Negotiation ......................44 7. Sequence Numbers ...............................................44 7.1. Variables .................................................45 7.2. Initial Sequence Numbers ..................................45 7.3. Quiet Time ................................................46 7.4. Acknowledgement Numbers ...................................47 7.5. Validity and Synchronization ..............................47 7.5.1. Sequence and Acknowledgement Number Windows ........48 7.5.2. Sequence Window Feature ............................49 7.5.3. Sequence-Validity Rules ............................49 7.5.4. Handling Sequence-Invalid Packets ..................51 7.5.5. Sequence Number Attacks ............................52 7.5.6. Sequence Number Handling Examples ..................54 7.6. Short Sequence Numbers ....................................55 7.6.1. Allow Short Sequence Numbers Feature ...............55 7.6.2. When to Avoid Short Sequence Numbers ...............56 7.7. NDP Count and Detecting Application Loss ..................56
7.7.1. NDP Count Usage Notes ..............................57 7.7.2. Send NDP Count Feature .............................57 8. Event Processing ...............................................58 8.1. Connection Establishment ..................................58 8.1.1. Client Request .....................................58 8.1.2. Service Codes ......................................59 8.1.3. Server Response ....................................61 8.1.4. Init Cookie Option .................................62 8.1.5. Handshake Completion ...............................63 8.2. Data Transfer .............................................63 8.3. Termination ...............................................64 8.3.1. Abnormal Termination ...............................66 8.4. DCCP State Diagram ........................................66 8.5. Pseudocode ................................................67 9. Checksums ......................................................72 9.1. Header Checksum Field .....................................73 9.2. Header Checksum Coverage Field ............................73 9.2.1. Minimum Checksum Coverage Feature ..................74 9.3. Data Checksum Option ......................................75 9.3.1. Check Data Checksum Feature ........................76 9.3.2. Checksum Usage Notes ...............................76 10. Congestion Control ............................................76 10.1. TCP-like Congestion Control ..............................77 10.2. TFRC Congestion Control ..................................78 10.3. CCID-Specific Options, Features, and Reset Codes .........78 10.4. CCID Profile Requirements ................................80 10.5. Congestion State .........................................81 11. Acknowledgements ..............................................81 11.1. Acks of Acks and Unidirectional Connections ..............82 11.2. Ack Piggybacking .........................................83 11.3. Ack Ratio Feature ........................................84 11.4. Ack Vector Options .......................................85 11.4.1. Ack Vector Consistency ............................88 11.4.2. Ack Vector Coverage ...............................89 11.5. Send Ack Vector Feature ..................................90 11.6. Slow Receiver Option .....................................90 11.7. Data Dropped Option ......................................91 11.7.1. Data Dropped and Normal Congestion Response .......94 11.7.2. Particular Drop Codes .............................95 12. Explicit Congestion Notification ..............................96 12.1. ECN Incapable Feature ....................................96 12.2. ECN Nonces ...............................................97 12.3. Aggression Penalties .....................................98 13. Timing Options ................................................99 13.1. Timestamp Option .........................................99 13.2. Elapsed Time Option ......................................99 13.3. Timestamp Echo Option ...................................100 14. Maximum Packet Size ..........................................101
14.1. Measuring PMTU ..........................................102 14.2. Sender Behavior .........................................103 15. Forward Compatibility ........................................104 16. Middlebox Considerations .....................................105 17. Relations to Other Specifications ............................106 17.1. RTP .....................................................106 17.2. Congestion Manager and Multiplexing .....................108 18. Security Considerations ......................................108 18.1. Security Considerations for Partial Checksums ...........109 19. IANA Considerations ..........................................110 19.1. Packet Types Registry ...................................110 19.2. Reset Codes Registry ....................................110 19.3. Option Types Registry ...................................110 19.4. Feature Numbers Registry ................................111 19.5. Congestion Control Identifiers Registry .................111 19.6. Ack Vector States Registry ..............................111 19.7. Drop Codes Registry .....................................112 19.8. Service Codes Registry ..................................112 19.9. Port Numbers Registry ...................................112 20. Thanks .......................................................114 A. Appendix: Ack Vector Implementation Notes ....................116 A.1. Packet Arrival ..........................................118 A.1.1. New Packets ......................................118 A.1.2. Old Packets ......................................119 A.2. Sending Acknowledgements ................................120 A.3. Clearing State ..........................................120 A.4. Processing Acknowledgements .............................122 B. Appendix: Partial Checksumming Design Motivation .............123 Normative References .............................................124 Informative References ...........................................125
List of Tables
Table 1: DCCP Packet Types .......................................21 Table 2: DCCP Reset Codes ........................................28 Table 3: DCCP Options ............................................30 Table 4: DCCP Feature Numbers.....................................35 Table 5: DCCP Congestion Control Identifiers .....................77 Table 6: DCCP Ack Vector States ..................................86 Table 7: DCCP Drop Codes .........................................92
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:

o Unreliable flows of datagrams.
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].
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 A choice of modular congestion control mechanisms. Two mechanisms are currently specified: TCP-like Congestion Control [RFC4341] and TCP-Friendly Rate Control (TFRC) [RFC4342]. 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 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) [RFC3448], 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 Application Programming Interface (API) might not allow applications to set UDP packets as ECN capable, since the API could not guarantee that 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 [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 mechanisms where the state about past packet drops or marks is maintained at the receiver rather than 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 control mechanisms currently approved for use with DCCP, which are described in separate Congestion Control ID Profiles [RFC4341, RFC4342], may 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 [RFC2119].

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 [RFC4086].
All operations on DCCP sequence numbers use circular arithmetic modulo 2^48, as do comparisons such as "greater" and "greatest". 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] and DNS's serial numbers [RFC1982]. It may make sense to store DCCP sequence numbers in the most significant 48 bits of 64-bit integers and set the least significant 16 bits to zero, since this supports a common technique that implements circular comparison A < B by testing whether (A - B) < 0 using conventional two's-complement arithmetic. Reserved bitfields in DCCP packet headers MUST be set to zero by senders and MUST be ignored by receivers, unless otherwise specified. This allows for future protocol extensions. In particular, DCCP processors MUST NOT reset a DCCP connection simply because a Reserved field has non-zero value [RFC3360].

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 connection consists of two separate unidirectional connections, called half-connections. Each half-connection consists of the application data sent by one endpoint and the corresponding acknowledgements sent by the other endpoint. We can illustrate this as follows:
+--------+ A-to-B half-connection: +--------+ | | --> application data --> | | | | <-- acknowledgements <-- | | | DCCP A | | DCCP B | | | B-to-A half-connection: | | | | <-- application data <-- | | +--------+ --> acknowledgements --> +--------+
Although they are logically distinct, in practice the half- connections overlap; a DCCP-DataAck packet, for example, contains application data relevant to one half-connection and acknowledgement information relevant to the other.
In the context of a single half-connection, the terms "HC-Sender" and "HC-Receiver" denote the endpoints sending application data and acknowledgements, respectively. For example, DCCP A is the HC-Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.

3.3. Features

A DCCP feature is a connection attribute on whose value the two endpoints agree. Many properties of a DCCP connection are controlled by features, including the congestion control mechanisms in use on the two half-connections. The endpoints achieve agreement 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 [RFC2401]; depending on the level of security required, application- level cryptography may also suffice. These issues are discussed further in Sections 7.5.5 and 18.

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].

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. In this way a DCCP-Request packet resembles a TCP SYN, but since DCCP-Request is a packet type there is no way to send an unexpected flag 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 fixed-size 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 acknowledgement information.
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:
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 having a TCP-style cumulative acknowledgement field doesn't make sense. DCCP's Acknowledgement Number field equals the greatest sequence number received, rather than the smallest sequence number not received. Separate options indicate any intermediate sequence numbers that weren't received.

4.3. States

DCCP endpoints progress through different states during the course of a connection, corresponding roughly to the three phases of initiation, data transfer, and termination. The figure below shows the typical progress through these states for a client and server.
Client Server ------ ------ (0) No connection CLOSED LISTEN
(1) Initiation REQUEST DCCP-Request --> <-- DCCP-Response RESPOND PARTOPEN DCCP-Ack or DCCP-DataAck -->
(2) Data transfer OPEN <-- DCCP-Data, Ack, DataAck --> OPEN
(3) Termination <-- DCCP-CloseReq CLOSEREQ CLOSING DCCP-Close --> <-- DCCP-Reset CLOSED TIMEWAIT CLOSED
The nine possible states are as follows. They are listed in 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.
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 order the client to close the connection and to 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 has to 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 [RFC3168] 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) [RFC2018].
CCID 3 provides TCP-Friendly Rate Control (TFRC), an equation-based form of congestion control intended to respond to congestion more smoothly than CCID 2. The sender maintains a transmit rate, which it updates using the receiver's estimate of the packet loss and mark

rate. CCID 3 behaves somewhat differently than TCP in the short term, but 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 [RFC4341, RFC4342].

4.5. Feature Negotiation Options

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.
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

DCCP's differences from TCP apart from those discussed so far include the following:

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 is 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 that 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 has to 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 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. The server may ignore this application request.
2. The server sends the client a DCCP-Response packet indicating that it is willing to communicate with the client. This response indicates any features and options that the server agrees to, begins other feature negotiations as desired, and optionally includes Init Cookies that wrap up all this information and that 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 any Init Cookies in the DCCP-Response. It may also continue feature negotiation. The client may piggyback an application-level request on this 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 part of the header has the same semantics for all currently defined packet types. Following this comes any additional fixed-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.
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, and the Destination Port represents 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.
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 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.
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).
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, 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 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. This piggybacks acknowledgement information on a data packet.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / Generic DCCP Header (16 or 12 bytes) / / 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).
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.
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 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, "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 do Reset Code 0, "Unspecified".
The following table summarizes this information.
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 and 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 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; it must therefore be greater than or equal to two.
Options MUST be processed sequentially, starting with 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. Unless otherwise specified, multiple occurrences of the same option MUST be processed independently; for some options, this will mean in practice that the last valid occurrence of an option takes precedence.

The following options are currently defined:
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 3-8 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
+--------+ |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; or if the endpoint understood O, but chose not to perform the action O implies. This list is not exhaustive and, in particular, individual option specifications may describe additional situations in which the endpoint should reset the connection and situations in which it should not.
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.

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 a length of at least 4.
+--------+--------+--------+--------+-------- Change L: |00100000| Length |Feature#| Value(s) ... +--------+--------+--------+--------+-------- Type=32
+--------+--------+--------+--------+-------- Change R: |00100010| Length |Feature#| Value(s) ... +--------+--------+--------+--------+-------- Type=34
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.

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 0 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 means server-priority, NN means non-negotiable.
Initial Value The initial value for the feature. Every feature has a known initial value.
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.
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.
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, possibly including two options that refer to the same feature; as usual, the options are processed sequentially.

6.6.2. Processing Received Options

DCCP endpoints exist in one of three states relative to each feature. STABLE is the normal state, where the endpoint knows the feature's value and thinks the other endpoint agrees. An endpoint 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 non-Data 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
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.
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. (Packets generated 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 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 new Change options. A Change option is new if and only if it was generated during a transition from the STABLE or UNSTABLE state to the CHANGING state; Change options generated within the CHANGING state are retransmissions and MUST have exactly the same contents as previously transmitted options, allowing tolerance for reordering. FGSS 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.
2. If a packet's Sequence Number is less than or equal to FGSR, if it has no Acknowledgement Number, OR if 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's 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 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 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 for a known feature 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.
However, unexpected Confirm options -- that refer to unknown feature numbers, or that don't appear to be part of a current negotiation -- are not invalid, although they are ignored by the receiver.
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.
Other failure cases do not cause connection reset; in particular, reordering protection may cause a Mandatory Change option to be ignored without resetting the connection.
Confirm options behave identically and have the same reset conditions whether or not they are Mandatory.

7. Sequence Numbers

DCCP uses sequence numbers to arrange packets into sequence, to detect losses and network duplicates, and to 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, Sequence Numbers generated by each endpoint increase by one, modulo 2^48, per 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; it is a significant departure from TCP practice.

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; and
o sequence number attacks, where an attacker can guess the sequence numbers that a future connection would use [M85].
These problems are the same as those faced by TCP, and DCCP implementations SHOULD use TCP's strategies to avoid them [RFC793, RFC1948]. 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 [RFC793]. 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. [RFC1948] achieves this by adding a cryptographic hash of the 4-tuple and a secret to each initial sequence number. For the secret, [RFC1948] recommends a combination of some truly random data [RFC4086], 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 the secret is changed; 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 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 from TCP's.

A received packet is classified as acknowledgeable if and only if its header was successfully 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