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RFC 4340

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Proposed STD
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Datagram Congestion Control Protocol (DCCP)

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


              Datagram Congestion Control Protocol (DCCP)

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 (C) 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

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      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

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           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

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      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

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1.  Introduction

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

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   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

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   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.

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   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.

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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
   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.

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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].

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.

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   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:

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

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

   Since DCCP provides unreliable semantics, there are no
   retransmissions, and 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.

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

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

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

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

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

   CLOSED
      Represents nonexistent connections.

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

   REQUEST
      A client socket enters this state, from CLOSED, after sending a
      DCCP-Request packet to try to initiate a connection.

   RESPOND
      A server socket enters this state, from LISTEN, after receiving a
      DCCP-Request from a client.

   PARTOPEN
      A client socket enters this state, from REQUEST, after receiving a
      DCCP-Response from the server.  This state represents the third
      phase of the three-way handshake.  The client may send application
      data in this state, but it MUST include an Acknowledgement Number
      on all of its packets.

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   OPEN
      The central data transfer portion of a DCCP connection.  Client
      and server sockets enter this state from PARTOPEN and RESPOND,
      respectively.  Sometimes we speak of SERVER-OPEN and CLIENT-OPEN
      states, corresponding to the server's OPEN state and the client's
      OPEN state.

   CLOSEREQ
      A server socket enters this state, from SERVER-OPEN, to 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

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   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.

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

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

4.6.  Differences from TCP

   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

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      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.

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   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.



(page 18 continued on part 2)

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