path: root/doc/rfc/rfc4340.txt

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.

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.



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      +---------------------------------------+  -.
      |            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|                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   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.






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   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:







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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           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                        /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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.







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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /              Generic DCCP Header (16 or 12 bytes)             /
      /                    with Type=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



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   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).




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /            Generic DCCP Header with X=1 (16 bytes)            /
      /                   with Type=7 (DCCP-Reset)                    /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /          Acknowledgement Number Subheader (8 bytes)           /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  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.




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   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.






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   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).







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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /            Generic DCCP Header with X=1 (16 bytes)            /
      /          with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck)          /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /          Acknowledgement Number Subheader (8 bytes)           /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                      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



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   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



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   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.




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   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.








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              +--------+--------+--------+--------+--------
   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").





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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.
























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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.















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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.







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   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.







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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.




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   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



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   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



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   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.




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   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.














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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.





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   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




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   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



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   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.



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   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 sequence numbers.  A sent packet's
   Acknowledgement Number MUST equal the sending endpoint's GSR, the
   Greatest Sequence Number Received on an acknowledgeable packet, for
   all packet types except DCCP-Sync and DCCP-SyncAck.

   "Acknowledgeable" does not refer to data processing.  Even
   acknowledgeable packets may have their application data dropped, due
   to receive buffer overflow or corruption, for instance.  Data Dropped
   options report these data losses when necessary, letting congestion
   control mechanisms distinguish between network losses and endpoint
   losses.  This issue is discussed further in Sections 11.4 and 11.7.

   DCCP-Sync and DCCP-SyncAck packets' Acknowledgement Numbers differ as
   follows: The Acknowledgement Number on a DCCP-Sync packet corresponds
   to a received packet, but not necessarily to an acknowledgeable
   packet; in particular, it might correspond to an out-of-sync packet
   whose options were not processed.  The Acknowledgement Number on a
   DCCP-SyncAck packet always corresponds to an acknowledgeable DCCP-
   Sync packet; it might be less than GSR in the presence of reordering.

7.5.  Validity and Synchronization

   Any DCCP endpoint might receive packets that are not actually part of
   the current connection.  For instance, the network might deliver an
   old packet, an attacker might attempt to hijack a connection, or the
   other endpoint might crash, causing a half-open connection.

   DCCP, like TCP, uses sequence number checks to detect these cases.
   Packets whose Sequence and/or Acknowledgement Numbers are out of
   range are called sequence-invalid and are not processed normally.




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   Unlike TCP, DCCP requires a synchronization mechanism to recover from
   large bursts of loss.  One endpoint might send so many packets during
   a burst of loss that when one of its packets finally got through, the
   other endpoint would label its Sequence Number as invalid.  A
   handshake of DCCP-Sync and DCCP-SyncAck packets recovers from this
   case.

7.5.1.  Sequence and Acknowledgement Number Windows

   Each DCCP endpoint defines sequence validity windows that are subsets
   of the Sequence and Acknowledgement Number spaces.  These windows
   correspond to packets the endpoint expects to receive in the next few
   round-trip times.  The Sequence and Acknowledgement Number windows
   always contain GSR and GSS, respectively.  The window widths are
   controlled by Sequence Window features for the two half-connections.

   The Sequence Number validity window for packets from DCCP B is [SWL,
   SWH].  This window always contains GSR, the Greatest Sequence Number
   Received on a sequence-valid packet from DCCP B.  It is W packets
   wide, where W is the value of the Sequence Window/B feature.  One-
   fourth of the sequence window, rounded down, is less than or equal to
   GSR, and three-fourths is greater than GSR.  (This asymmetric
   placement assumes that bursts of loss are more common in the network
   than significant reorderings.)

     invalid  |       valid Sequence Numbers        |  invalid
   <---------*|*===========*=======================*|*--------->
         GSR -|GSR + 1 -   GSR                 GSR +|GSR + 1 +
    floor(W/4)|floor(W/4)                 ceil(3W/4)|ceil(3W/4)
               = SWL                           = SWH

   The Acknowledgement Number validity window for packets from DCCP B is
   [AWL, AWH].  The high end of the window, AWH, equals GSS, the
   Greatest Sequence Number Sent by DCCP A; the window is W' packets
   wide, where W' is the value of the Sequence Window/A feature.

     invalid  |    valid Acknowledgement Numbers    |  invalid
   <---------*|*===================================*|*--------->
      GSS - W'|GSS + 1 - W'                      GSS|GSS + 1
               = AWL                           = AWH

   SWL and AWL are initially adjusted so that they are not less than the
   initial Sequence Numbers received and sent, respectively:

         SWL := max(GSR + 1 - floor(W/4), ISR),
         AWL := max(GSS + 1 - W', ISS).





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   These adjustments MUST be applied only at the beginning of the
   connection.  (Long-lived connections may wrap sequence numbers so
   that they appear to be less than ISR or ISS; the adjustments MUST NOT
   be applied in that case.)

7.5.2.  Sequence Window Feature

   The Sequence Window/A feature determines the width of the Sequence
   Number validity window used by DCCP B and the width of the
   Acknowledgement Number validity window used by DCCP A.  DCCP A sends
   a "Change L(Sequence Window, W)" option to notify DCCP B that the
   Sequence Window/A value is W.

   Sequence Window has feature number 3 and is non-negotiable.  It takes
   48-bit (6-byte) integer values, like DCCP sequence numbers.  Change
   and Confirm options for Sequence Window are therefore 9 bytes long.
   New connections start with Sequence Window 100 for both endpoints.
   The minimum valid Sequence Window value is Wmin = 32.  The maximum
   valid Sequence Window value is Wmax = 2^46 - 1 = 70368744177663.
   Change options suggesting Sequence Window values out of this range
   are invalid and MUST be handled accordingly.

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

7.5.3.  Sequence-Validity Rules

   Sequence-validity depends on the received packet's type.  This table
   shows the sequence and acknowledgement number checks applied to each
   packet; a packet is sequence-valid if it passes both tests, and
   sequence-invalid if it does not.  Many of the checks refer to the
   sequence and acknowledgement number validity windows [SWL, SWH] and
   [AWL, AWH] defined in Section 7.5.1.






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                                             Acknowledgement Number
   Packet Type      Sequence Number Check    Check
   -----------      ---------------------    ----------------------
   DCCP-Request     SWL <= seqno <= SWH (*)  N/A
   DCCP-Response    SWL <= seqno <= SWH (*)  AWL <= ackno <= AWH
   DCCP-Data        SWL <= seqno <= SWH      N/A
   DCCP-Ack         SWL <= seqno <= SWH      AWL <= ackno <= AWH
   DCCP-DataAck     SWL <= seqno <= SWH      AWL <= ackno <= AWH
   DCCP-CloseReq    GSR <  seqno <= SWH      GAR <= ackno <= AWH
   DCCP-Close       GSR <  seqno <= SWH      GAR <= ackno <= AWH
   DCCP-Reset       GSR <  seqno <= SWH      GAR <= ackno <= AWH
   DCCP-Sync        SWL <= seqno             AWL <= ackno <= AWH
   DCCP-SyncAck     SWL <= seqno             AWL <= ackno <= AWH

   (*) Check not applied if connection is in LISTEN or REQUEST state.

   In general, packets are sequence-valid if their Sequence and
   Acknowledgement Numbers lie within the corresponding valid windows,
   [SWL, SWH] and [AWL, AWH].  The exceptions to this rule are as
   follows:

   o  Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a
      connection, they cannot have Sequence Numbers less than or equal
      to GSR, or Acknowledgement Numbers less than GAR.

   o  DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly
      checked.  These packet types exist specifically to get the
      endpoints back into sync; checking their Sequence Numbers would
      eliminate their usefulness.

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

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

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



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

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

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

7.5.4.  Handling Sequence-Invalid Packets

   Endpoints respond to received sequence-invalid packets as follows.

   o  Any sequence-invalid DCCP-Sync or DCCP-SyncAck packet MUST be
      ignored.

   o  A sequence-invalid DCCP-Reset packet MUST elicit a DCCP-Sync
      packet in response (subject to a possible rate limit).  This
      response packet MUST use a new Sequence Number, and thus will
      increase GSS; GSR will not change, however, since the received
      packet was sequence-invalid.  The response packet's
      Acknowledgement Number MUST equal GSR.

   o  Any other sequence-invalid packet MUST elicit a similar DCCP-Sync
      packet, except that the response packet's Acknowledgement Number
      MUST equal the sequence-invalid packet's Sequence Number.

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

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




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   DCCP endpoints MUST NOT process sequence-invalid packets except,
   perhaps, by generating a DCCP-Sync.  For instance, options MUST NOT
   be processed.  An endpoint MAY temporarily preserve sequence-invalid
   packets in case they become valid later, however; this can reduce the
   impact of bursts of loss by delivering more packets to the
   application.  In particular, an endpoint MAY preserve sequence-
   invalid packets for up to 2 round-trip times.  If, within that time,
   the relevant sequence windows change so that the packets become
   sequence-valid, the endpoint MAY process them again.

   Note that sequence-invalid DCCP-Reset packets cause DCCP-Syncs to be
   generated.  This is because endpoints in an unsynchronized state
   (CLOSED, REQUEST, and LISTEN) might not have enough information to
   generate a proper DCCP-Reset on the first try.  For example, if a
   peer endpoint is in CLOSED state and receives a DCCP-Data packet, it
   cannot guess the right Sequence Number to use on the DCCP-Reset it
   generates (since the DCCP-Data packet has no Acknowledgement Number).
   The DCCP-Sync generated in response to this bad reset serves as a
   challenge, and contains enough information for the peer to generate a
   proper DCCP-Reset.  However, the new DCCP-Reset may carry a different
   Reset Code than the original DCCP-Reset; probably the new Reset Code
   will be 3, "No Connection".  The endpoint SHOULD use information from
   the original DCCP-Reset when possible.

7.5.5.  Sequence Number Attacks

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

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

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

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




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

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

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

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

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

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

   Since DCCP-Data attacks with short sequence numbers are relatively
   easy for attackers to execute, DCCP has been engineered to prevent
   these attacks from escalating to connection resets or other serious



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   consequences.  In particular, any options whose processing might
   cause the connection to be reset are ignored when they appear on
   DCCP-Data packets.

7.5.6.  Sequence Number Handling Examples

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

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

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

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

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

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





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7.6.  Short Sequence Numbers

   DCCP sequence numbers are 48 bits long.  This large sequence space
   protects DCCP connections against some blind attacks, such as the
   injection of DCCP-Resets into the connection.  However, DCCP-Data,
   DCCP-Ack, and DCCP-DataAck packets, which make up the body of any
   DCCP connection, may reduce header space by transmitting only the
   lower 24 bits of the relevant Sequence and Acknowledgement Numbers.
   The receiving endpoint will extend these numbers to 48 bits using the
   following pseudocode:

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

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

7.6.1.  Allow Short Sequence Numbers Feature

   Endpoints can require that all packets use long sequence numbers by
   leaving the Allow Short Sequence Numbers feature value at its default
   of zero.  This can reduce the risk that data will be inappropriately
   injected into the connection.  DCCP A sends a "Change L(Allow Short
   Seqnos, 1)" option to indicate its desire to send packets with short
   sequence numbers.

   Allow Short Sequence Numbers has feature number 2 and is server-
   priority.  It takes one-byte Boolean values.  When Allow Short
   Seqnos/B is zero, DCCP B MUST NOT send packets with short sequence
   numbers and DCCP A MUST ignore any packets with short sequence



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   numbers that are received.  Values of two or more are reserved.  New
   connections start with Allow Short Sequence Numbers 0 for both
   endpoints.

7.6.2.  When to Avoid Short Sequence Numbers

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

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

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

7.7.  NDP Count and Detecting Application Loss

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

   +--------+--------+-------- ... --------+
   |00100101| Length |      NDP Count      |
   +--------+--------+-------- ... --------+
    Type=37  Len=3-8       (1-6 bytes)

   If a DCCP endpoint's Send NDP Count feature is one (see below), then
   that endpoint MUST send an NDP Count option on every packet whose



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   immediate predecessor was a non-data packet.  Non-data packets
   consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
   DCCP-Reset, DCCP-Sync, and DCCP-SyncAck.  The other packet types,
   namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are
   considered data packets, although not all DCCP-Request and DCCP-
   Response packets will actually carry application data.

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

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

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

7.7.1.  NDP Count Usage Notes

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

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

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

   Those packets would have NDP Counts as follows.

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

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

7.7.2.  Send NDP Count Feature

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




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   Send NDP Count has feature number 7 and is server-priority.  It takes
   one-byte Boolean values.  DCCP B MUST send NDP Count options as
   described above when Send NDP Count/B is one, although it MAY send
   NDP Count options even when Send NDP Count/B is zero.  Values of two
   or more are reserved.  New connections start with Send NDP Count 0
   for both endpoints.

8.  Event Processing

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

8.1.  Connection Establishment

   DCCP connections' initiation phase consists of a three-way handshake:
   an initial DCCP-Request packet sent by the client, a DCCP-Response
   sent by the server in reply, and finally an acknowledgement from the
   client, usually via a DCCP-Ack or DCCP-DataAck packet.  The client
   moves from the REQUEST state to PARTOPEN, and finally to OPEN; the
   server moves from LISTEN to RESPOND, and finally to OPEN.

     Client State                             Server State
        CLOSED                                   LISTEN
   1.   REQUEST   -->       Request        -->
   2.             <--       Response       <--   RESPOND
   3.   PARTOPEN  -->     Ack, DataAck     -->
   4.             <--  Data, Ack, DataAck  <--   OPEN
   5.   OPEN      <->  Data, Ack, DataAck  <->   OPEN

8.1.1.  Client Request

   When a client decides to initiate a connection, it enters the REQUEST
   state, chooses an initial sequence number (Section 7.2), and sends a
   DCCP-Request packet using that sequence number to the intended
   server.

   DCCP-Request packets will commonly carry feature negotiation options
   that open negotiations for various connection parameters, such as
   preferred congestion control IDs for each half-connection.  They may
   also carry application data, but the client should be aware that the
   server may not accept such data.

   A client in the REQUEST state SHOULD use an exponential-backoff timer
   to send new DCCP-Request packets if no response is received.  The
   first retransmission should occur after approximately one second,
   backing off to not less than one packet every 64 seconds; or the



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   endpoint can use whatever retransmission strategy is followed for
   retransmitting TCP SYNs.  Each new DCCP-Request MUST increment the
   Sequence Number by one and MUST contain the same Service Code and
   application data as the original DCCP-Request.

   A client MAY give up on its DCCP-Requests after some time (3 minutes,
   for example).  When it does, it SHOULD send a DCCP-Reset packet to
   the server with Reset Code 2, "Aborted", to clean up state in case
   one or more of the Requests actually arrived.  A client in REQUEST
   state has never received an initial sequence number from its peer, so
   the DCCP-Reset's Acknowledgement Number MUST be set to zero.

   The client leaves the REQUEST state for PARTOPEN when it receives a
   DCCP-Response from the server.

8.1.2.  Service Codes

   Each DCCP-Request contains a 32-bit Service Code, which identifies
   the application-level service to which the client application is
   trying to connect.  Service Codes should correspond to application
   services and protocols.  For example, there might be a Service Code
   for SIP control connections and one for RTP audio connections.
   Middleboxes, such as firewalls, can use the Service Code to identify
   the application running on a nonstandard port (assuming the DCCP
   header has not been encrypted).

   Endpoints MUST associate a Service Code with every DCCP socket, both
   actively and passively opened.  The application will generally supply
   this Service Code.  Each active socket MUST have exactly one Service
   Code.  Passive sockets MAY, at the implementation's discretion, be
   associated with more than one Service Code; this might let multiple
   applications, or multiple versions of the same application, listen on
   the same port, differentiated by Service Code.  If the DCCP-Request's
   Service Code doesn't equal any of the server's Service Codes for the
   given port, the server MUST reject the request by sending a DCCP-
   Reset packet with Reset Code 8, "Bad Service Code".  A middlebox MAY
   also send such a DCCP-Reset in response to packets whose Service Code
   is considered unsuitable.

   Service Codes are not intended to be DCCP-specific and are allocated
   by IANA.  Following the policies outlined in [RFC2434], most Service
   Codes are allocated First Come First Served, subject to the following
   guidelines.

   o  Service Codes are allocated one at a time, or in small blocks.  A
      short English description of the intended service is REQUIRED to
      obtain a Service Code assignment, but no specification, standards




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      track or otherwise, is necessary.  IANA maintains an association
      of Service Codes to the corresponding phrases.

   o  Users request specific Service Code values.  We suggest that users
      request Service Codes that can be represented using the "SC:"
      formatting convention described below.  Thus, the "Frobodyne Plotz
      Protocol" might correspond to Service Code 17178548426 or,
      equivalently, "SC:fdpz".  The canonical interpretation of a
      Service Code field is numeric.

   o  Service Codes whose bytes each have values in the set {32, 45-57,
      65-90} use a Specification Required allocation policy.  That is,
      these Service Codes are used for international standard or
      standards-track specifications, IETF or otherwise.  (This set
      consists of the ASCII digits, uppercase letters, and characters
      space, '-', '.', and '/'.)

   o  Service Codes whose high-order byte equals 63 (ASCII '?') are
      reserved for Private Use.

   o  Service Code 0 represents the absence of a meaningful Service Code
      and MUST NOT be allocated.

   o  The value 4294967295 is an invalid Service Code.  Servers MUST
      reject any DCCP-Request with this Service Code value by sending a
      DCCP-Reset packet with Reset Code 8, "Bad Service Code".

   This design for Service Code allocation is based on the allocation of
   4-byte identifiers for Macintosh resources, PNG chunks, and TrueType
   and OpenType tables.

   In text settings, we recommend that Service Codes be written in one
   of three forms, prefixed by the ASCII letters SC and either a colon
   ":" or equals sign "=".  These forms are interpreted as follows.

   SC:     Indicates a Service Code representable using a subset of the
           ASCII characters.  The colon is followed by one to four
           characters taken from the following set: letters, digits, and
           the characters in "-_+.*/?@" (not including quotes).
           Numerically, these characters have values in {42-43, 45-57,
           63-90, 95, 97-122}.  The Service Code is calculated by
           padding the string on the right with spaces (value 32) and
           intepreting the four-character result as a 32-bit big-endian
           number.

   SC=     Indicates a decimal Service Code.  The equals sign is
           followed by any number of decimal digits, which specify the
           Service Code.  Values above 4294967294 are illegal.



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   SC=x or SC=X
           Indicates a hexadecimal Service Code.  The "x" or "X" is
           followed by any number of hexadecimal digits (upper or lower
           case), which specify the Service Code.  Values above
           4294967294 are illegal.

   Thus, the Service Code 1717858426 might be represented in text as
   either SC:fdpz, SC=1717858426, or SC=x6664707A.

8.1.3.  Server Response

   In the second phase of the three-way handshake, the server moves from
   the LISTEN state to RESPOND and sends a DCCP-Response message to the
   client.  In this phase, a server will often specify the features it
   would like to use, either from among those the client requested or in
   addition to those.  Among these options is the congestion control
   mechanism the server expects to use.

   The server MAY respond to a DCCP-Request packet with a DCCP-Reset
   packet to refuse the connection.  Relevant Reset Codes for refusing a
   connection include 7, "Connection Refused", when the DCCP-Request's
   Destination Port did not correspond to a DCCP port open for
   listening; 8, "Bad Service Code", when the DCCP-Request's Service
   Code did not correspond to the service code registered with the
   Destination Port; and 9, "Too Busy", when the server is currently too
   busy to respond to requests.  The server SHOULD limit the rate at
   which it generates these resets; for example, to not more than 1024
   per second.

   The server SHOULD NOT retransmit DCCP-Response packets; the client
   will retransmit the DCCP-Request if necessary.  (Note that the
   "retransmitted" DCCP-Request will have, at least, a different
   sequence number from the "original" DCCP-Request.  The server can
   thus distinguish true retransmissions from network duplicates.)  The
   server will detect that the retransmitted DCCP-Request applies to an
   existing connection because of its Source and Destination Ports.
   Every valid DCCP-Request received while the server is in the RESPOND
   state MUST elicit a new DCCP-Response.  Each new DCCP-Response MUST
   increment the server's Sequence Number by one and MUST include the
   same application data, if any, as the original DCCP-Response.

   The server MUST NOT accept more than one piece of DCCP-Request
   application data per connection.  In particular, the DCCP-Response
   sent in reply to a retransmitted DCCP-Request with application data
   SHOULD contain a Data Dropped option, in which the retransmitted
   DCCP-Request data is reported with Drop Code 0, Protocol Constraints.
   The original DCCP-Request SHOULD also be reported in the Data Dropped
   option, either in a Normal Block (if the server accepted the data or



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   there was no data) or in a Drop Code 0 Drop Block (if the server
   refused the data the first time as well).

   The Data Dropped and Init Cookie options are particularly useful for
   DCCP-Response packets (Sections 11.7 and 8.1.4).

   The server leaves the RESPOND state for OPEN when it receives a valid
   DCCP-Ack from the client, completing the three-way handshake.  It MAY
   also leave the RESPOND state for CLOSED after a timeout of not less
   than 4MSL (8 minutes); when doing so, it SHOULD send a DCCP-Reset
   with Reset Code 2, "Aborted", to clean up state at the client.

8.1.4.  Init Cookie Option

   +--------+--------+--------+--------+--------+--------
   |00100100| Length |         Init Cookie Value   ...
   +--------+--------+--------+--------+--------+--------
    Type=36

   The Init Cookie option lets a DCCP server avoid having to hold any
   state until the three-way connection setup handshake has completed,
   in a similar fashion as for TCP SYN cookies [SYNCOOKIES].  The server
   wraps up the Service Code, server port, and any options it cares
   about from both the DCCP-Request and DCCP-Response in an opaque
   cookie.  Typically the cookie will be encrypted using a secret known
   only to the server and will include a cryptographic checksum or magic
   value so that correct decryption can be verified.  When the server
   receives the cookie back in the response, it can decrypt the cookie
   and instantiate all the state it avoided keeping.  In the meantime,
   it need not move from the LISTEN state.

   The Init Cookie option MUST NOT be sent on DCCP-Request or DCCP-Data
   packets.  Any Init Cookie options received on DCCP-Request or DCCP-
   Data packets, or after the connection has been established (when the
   connection's state is >= OPEN), MUST be ignored.  The server MAY
   include Init Cookie options in its DCCP-Response.  If so, then the
   client MUST echo the same Init Cookie options, in the same order, in
   each succeeding DCCP packet until one of those packets is
   acknowledged (showing that the three-way handshake has completed) or
   the connection is reset.  As a result, the client MUST NOT use DCCP-
   Data packets until the three-way handshake completes or the
   connection is reset.  The Init Cookie options on a client packet MUST
   equal those received on the DCCP-Request indicated by the client
   packet's Acknowledgement Number.  The server SHOULD design its Init
   Cookie format so that Init Cookies can be checked for tampering; it
   SHOULD respond to a tampered Init Cookie option by resetting the
   connection with Reset Code 10, "Bad Init Cookie".




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   Init Cookie's precise implementation need not be specified here;
   since Init Cookies are opaque to the client, there are no
   interoperability concerns.  An example cookie format might encrypt
   (using a secret key) the connection's initial sequence and
   acknowledgement numbers, ports, Service Code, any options included on
   the DCCP-Request packet and the corresponding DCCP-Response, a random
   salt, and a magic number.  On receiving a reflected Init Cookie, the
   server would decrypt the cookie, validate it by checking its magic
   number, sequence numbers, and ports, and, if valid, create a
   corresponding socket using the options.

   Each individual Init Cookie option can hold at most 253 bytes of
   data, but a server can send multiple Init Cookie options to gain more
   space.

8.1.5.  Handshake Completion

   When the client receives a DCCP-Response from the server, it moves
   from the REQUEST state to PARTOPEN and completes the three-way
   handshake by sending a DCCP-Ack packet to the server.  The client
   remains in PARTOPEN until it can be sure that the server has received
   some packet the client sent from PARTOPEN (either the initial DCCP-
   Ack or a later packet).  Clients in the PARTOPEN state that want to
   send data MUST do so using DCCP-DataAck packets, not DCCP-Data
   packets.  This is because DCCP-Data packets lack Acknowledgement
   Numbers, so the server can't tell from a DCCP-Data packet whether the
   client saw its DCCP-Response.  Furthermore, if the DCCP-Response
   included an Init Cookie, that Init Cookie MUST be included on every
   packet sent in PARTOPEN.

   The single DCCP-Ack sent when entering the PARTOPEN state might, of
   course, be dropped by the network.  The client SHOULD ensure that
   some packet gets through eventually.  The preferred mechanism would
   be a roughly 200-millisecond timer, set every time a packet is
   transmitted in PARTOPEN.  If this timer goes off and the client is
   still in PARTOPEN, the client generates another DCCP-Ack and backs
   off the timer.  If the client remains in PARTOPEN for more than 4MSL
   (8 minutes), it SHOULD reset the connection with Reset Code 2,
   "Aborted".

   The client leaves the PARTOPEN state for OPEN when it receives a
   valid packet other than DCCP-Response, DCCP-Reset, or DCCP-Sync from
   the server.

8.2.  Data Transfer

   In the central data transfer phase of the connection, both server and
   client are in the OPEN state.



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   DCCP A sends DCCP-Data and DCCP-DataAck packets to DCCP B due to
   application events on host A.  These packets are congestion-
   controlled by the CCID for the A-to-B half-connection.  In contrast,
   DCCP-Ack packets sent by DCCP A are controlled by the CCID for the
   B-to-A half-connection.  Generally, DCCP A will piggyback
   acknowledgement information on DCCP-Data packets when acceptable,
   creating DCCP-DataAck packets.  DCCP-Ack packets are used when there
   is no data to send from DCCP A to DCCP B, or when the congestion
   state of the A-to-B CCID will not allow data to be sent.

   DCCP-Sync and DCCP-SyncAck packets may also occur in the data
   transfer phase.  Some cases causing DCCP-Sync generation are
   discussed in Section 7.5.  One important distinction between DCCP-
   Sync packets and other packet types is that DCCP-Sync elicits an
   immediate acknowledgement.  On receiving a valid DCCP-Sync packet, a
   DCCP endpoint MUST immediately generate and send a DCCP-SyncAck
   response (subject to any implementation rate limits); the
   Acknowledgement Number on that DCCP-SyncAck MUST equal the Sequence
   Number of the DCCP-Sync.

   A particular DCCP implementation might decide to initiate feature
   negotiation only once the OPEN state was reached, in which case it
   might not allow data transfer until some time later.  Data received
   during that time SHOULD be rejected and reported using a Data Dropped
   Drop Block with Drop Code 0, Protocol Constraints (see Section 11.7).

8.3.  Termination

   DCCP connection termination uses a handshake consisting of an
   optional DCCP-CloseReq packet, a DCCP-Close packet, and a DCCP-Reset
   packet.  The server moves from the OPEN state, possibly through the
   CLOSEREQ state, to CLOSED; the client moves from OPEN through CLOSING
   to TIMEWAIT, and after 2MSL wait time (4 minutes) to CLOSED.

   The sequence DCCP-CloseReq, DCCP-Close, DCCP-Reset is used when the
   server decides to close the connection but doesn't want to hold
   TIMEWAIT state:

     Client State                             Server State
        OPEN                                     OPEN
   1.             <--       CloseReq       <--   CLOSEREQ
   2.   CLOSING   -->        Close         -->
   3.             <--        Reset         <--   CLOSED (LISTEN)
   4.   TIMEWAIT
   5.   CLOSED






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   A shorter sequence occurs when the client decides to close the
   connection.

     Client State                             Server State
        OPEN                                     OPEN
   1.   CLOSING   -->        Close         -->
   2.             <--        Reset         <--   CLOSED (LISTEN)
   3.   TIMEWAIT
   4.   CLOSED

   Finally, the server can decide to hold TIMEWAIT state:

     Client State                             Server State
        OPEN                                     OPEN
   1.             <--        Close         <--   CLOSING
   2.   CLOSED    -->        Reset         -->
   3.                                            TIMEWAIT
   4.                                            CLOSED (LISTEN)

   In all cases, the receiver of the DCCP-Reset packet holds TIMEWAIT
   state for the connection.  As in TCP, TIMEWAIT state, where an
   endpoint quietly preserves a socket for 2MSL (4 minutes) after its
   connection has closed, ensures that no connection duplicating the
   current connection's source and destination addresses and ports can
   start up while old packets might remain in the network.

   The termination handshake proceeds as follows.  The receiver of a
   valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet.
   The receiver of a valid DCCP-Close packet MUST respond with a DCCP-
   Reset packet with Reset Code 1, "Closed".  The receiver of a valid
   DCCP-Reset packet -- which is also the sender of the DCCP-Close
   packet (and possibly the receiver of the DCCP-CloseReq packet) --
   will hold TIMEWAIT state for the connection.

   A DCCP-Reset packet completes every DCCP connection, whether the
   termination is clean (due to application close; Reset Code 1,
   "Closed") or unclean.  Unlike TCP, which has two distinct termination
   mechanisms (FIN and RST), DCCP ends all connections in a uniform
   manner.  This is justified because some aspects of connection
   termination are the same independent of whether termination was
   clean.  For instance, the endpoint that receives a valid DCCP-Reset
   SHOULD hold TIMEWAIT state for the connection.  Processors that must
   distinguish between clean and unclean termination can examine the
   Reset Code.  DCCP implementations generally transition to the CLOSED
   state after sending a DCCP-Reset packet.

   Endpoints in the CLOSEREQ and CLOSING states MUST retransmit DCCP-
   CloseReq and DCCP-Close packets, respectively, until leaving those



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   states.  The retransmission timer should initially be set to go off
   in two round-trip times and should back off to not less than once
   every 64 seconds if no relevant response is received.

   Only the server can send a DCCP-CloseReq packet or enter the CLOSEREQ
   state.  A server receiving a sequence-valid DCCP-CloseReq packet MUST
   respond with a DCCP-Sync packet and otherwise ignore the DCCP-
   CloseReq.

   DCCP-Data, DCCP-DataAck, and DCCP-Ack packets received in CLOSEREQ or
   CLOSING states MAY be either processed or ignored.

8.3.1.  Abnormal Termination

   DCCP endpoints generate DCCP-Reset packets to terminate connections
   abnormally; a DCCP-Reset packet may be generated from any state.
   Resets sent in the CLOSED, LISTEN, and TIMEWAIT states use Reset Code
   3, "No Connection", unless otherwise specified.  Resets sent in the
   REQUEST or RESPOND states use Reset Code 4, "Packet Error", unless
   otherwise specified.

   DCCP endpoints in CLOSED, LISTEN, or TIMEWAIT state may need to
   generate a DCCP-Reset packet in response to a packet received from a
   peer.  Since these states have no associated sequence number
   variables, the Sequence and Acknowledgement Numbers on the DCCP-Reset
   packet R are taken from the received packet P, as follows.

   1. If P.ackno exists, then set R.seqno := P.ackno + 1.  Otherwise,
      set R.seqno := 0.

   2. Set R.ackno := P.seqno.

   3. If the packet used short sequence numbers (P.X == 0), then set the
      upper 24 bits of R.seqno and R.ackno to 0.

8.4.  DCCP State Diagram

   The most common state transitions discussed above can be summarized
   in the following state diagram.  The diagram is illustrative; the
   text in Section 8.5 and elsewhere should be considered definitive.
   For example, there are arcs (not shown) from every state except
   CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.









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   +---------------------------+    +---------------------------+
   |                           v    v                           |
   |                        +----------+                        |
   |          +-------------+  CLOSED  +------------+           |
   |          | passive     +----------+  active    |           |
   |          |  open                      open     |           |
   |          |                         snd Request |           |
   |          v                                     v           |
   |     +----------+                          +----------+     |
   |     |  LISTEN  |                          | REQUEST  |     |
   |     +----+-----+                          +----+-----+     |
   |          | rcv Request            rcv Response |           |
   |          | snd Response             snd Ack    |           |
   |          v                                     v           |
   |     +----------+                          +----------+     |
   |     | RESPOND  |                          | PARTOPEN |     |
   |     +----+-----+                          +----+-----+     |
   |          | rcv Ack/DataAck         rcv packet  |           |
   |          |                                     |           |
   |          |             +----------+            |           |
   |          +------------>|   OPEN   |<-----------+           |
   |                        +--+-+--+--+                        |
   |       server active close | |  |   active close            |
   |           snd CloseReq    | |  | or rcv CloseReq           |
   |                           | |  |    snd Close              |
   |                           | |  |                           |
   |     +----------+          | |  |          +----------+     |
   |     | CLOSEREQ |<---------+ |  +--------->| CLOSING  |     |
   |     +----+-----+            |             +----+-----+     |
   |          | rcv Close        |        rcv Reset |           |
   |          | snd Reset        |                  |           |
   |<---------+                  |                  v           |
   |                             |             +----+-----+     |
   |                   rcv Close |             | TIMEWAIT |     |
   |                   snd Reset |             +----+-----+     |
   +-----------------------------+                  |           |
                                                    +-----------+
                                                 2MSL timer expires

8.5.  Pseudocode

   This section presents an algorithm describing the processing steps a
   DCCP endpoint must go through when it receives a packet.  A DCCP
   implementation need not implement the algorithm as it is described
   here, but any implementation MUST generate observable effects exactly
   as indicated by this pseudocode, except where allowed otherwise by
   another part of this document.




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   The received packet is written as P, the socket as S.  Socket
   variables are:

   S.SWL - sequence number window low
   S.SWH - sequence number window high
   S.AWL - acknowledgement number window low
   S.AWH - acknowledgement number window high
   S.ISS - initial sequence number sent
   S.ISR - initial sequence number received
   S.OSR - first OPEN sequence number received
   S.GSS - greatest sequence number sent
   S.GSR - greatest valid sequence number received
   S.GAR - greatest valid acknowledgement number received on a
           non-Sync; initialized to S.ISS
   "Send packet" actions always use, and increment, S.GSS.

   Step 1: Check header basics
      /* This step checks for malformed packets.  Packets that fail
         these checks are ignored -- they do not receive Resets in
         response */
      If the packet is shorter than 12 bytes, drop packet and return
      If P.type is not understood, drop packet and return
      If P.Data Offset is smaller than the given packet type's
            fixed header length or larger than the packet's length,
            drop packet and return
      If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
            has short sequence numbers), drop packet and return
      If the header checksum is incorrect, drop packet and return
      If P.CsCov is too large for the packet size, drop packet and
            return

   Step 2: Check ports and process TIMEWAIT state
      /* Flow ID is <src addr, src port, dst addr, dst port> 4-tuple */
      Look up flow ID in table and get corresponding socket
      If no socket, or S.state == TIMEWAIT,
         /* The following Reset's Sequence and Acknowledgement Numbers
            are taken from the input packet; see Section 8.3.1. */
         Generate Reset(No Connection) unless P.type == Reset
         Drop packet and return












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   Step 3: Process LISTEN state
      If S.state == LISTEN,
         If P.type == Request or P contains a valid Init Cookie option,
            /* Must scan the packet's options to check for Init
               Cookies.  Only Init Cookies are processed here,
               however; other options are processed in Step 8.  This
               scan need only be performed if the endpoint uses Init
               Cookies */
            /* Generate a new socket and switch to that socket */
            Set S := new socket for this port pair
            S.state = RESPOND
            Choose S.ISS (initial seqno) or set from Init Cookies
            Initialize S.GAR := S.ISS
            Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookies
            Continue with S.state == RESPOND
            /* A Response packet will be generated in Step 11 */
         Otherwise,
            Generate Reset(No Connection) unless P.type == Reset
            Drop packet and return

   Step 4: Prepare sequence numbers in REQUEST
      If S.state == REQUEST,
         If (P.type == Response or P.type == Reset)
               and S.AWL <= P.ackno <= S.AWH,
            /* Set sequence number variables corresponding to the
               other endpoint, so P will pass the tests in Step 6 */
            Set S.GSR, S.ISR, S.SWL, S.SWH
            /* Response processing continues in Step 10; Reset
               processing continues in Step 9 */
         Otherwise,
            /* Only Response and Reset are valid in REQUEST state */
            Generate Reset(Packet Error)
            Drop packet and return

   Step 5: Prepare sequence numbers for Sync
      If P.type == Sync or P.type == SyncAck,
         If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,
            /* P is valid, so update sequence number variables
               accordingly.  After this update, P will pass the tests
               in Step 6.  A SyncAck is generated if necessary in
               Step 15 */
            Update S.GSR, S.SWL, S.SWH
         Otherwise,
            Drop packet and return







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   Step 6: Check sequence numbers
      If P.X == 0 and the relevant Allow Short Seqnos feature is 0,
         /* Packet has short seqnos, but short seqnos not allowed */
         Drop packet and return
      Otherwise, if P.X == 0,
         Extend P.seqno and P.ackno to 48 bits using the procedure
         in Section 7.6
      Let LSWL = S.SWL and LAWL = S.AWL
      If P.type == CloseReq or P.type == Close or P.type == Reset,
         LSWL := S.GSR + 1, LAWL := S.GAR
      If LSWL <= P.seqno <= S.SWH
            and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),
         Update S.GSR, S.SWL, S.SWH
         If P.type != Sync,
            Update S.GAR
      Otherwise,
         If P.type == Reset,
            Send Sync packet acknowledging S.GSR
         Otherwise,
            Send Sync packet acknowledging P.seqno
         Drop packet and return

   Step 7: Check for unexpected packet types
      If (S.is_server and P.type == CloseReq)
           or (S.is_server and P.type == Response)
           or (S.is_client and P.type == Request)
           or (S.state >= OPEN and P.type == Request
               and P.seqno >= S.OSR)
           or (S.state >= OPEN and P.type == Response
               and P.seqno >= S.OSR)
           or (S.state == RESPOND and P.type == Data),
         Send Sync packet acknowledging P.seqno
         Drop packet and return

   Step 8: Process options and mark acknowledgeable
      /* Option processing is not specifically described here.
         Certain options, such as Mandatory, may cause the connection
         to be reset, in which case Steps 9 and on are not executed */
      Mark packet as acknowledgeable (in Ack Vector terms, Received
           or Received ECN Marked)

   Step 9: Process Reset
      If P.type == Reset,
         Tear down connection
         S.state := TIMEWAIT
         Set TIMEWAIT timer
         Drop packet and return




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   Step 10: Process REQUEST state (second part)
      If S.state == REQUEST,
         /* If we get here, P is a valid Response from the server (see
            Step 4), and we should move to PARTOPEN state.  PARTOPEN
            means send an Ack, don't send Data packets, retransmit
            Acks periodically, and always include any Init Cookie from
            the Response */
         S.state := PARTOPEN
         Set PARTOPEN timer
         Continue with S.state == PARTOPEN
         /* Step 12 will send the Ack completing the three-way
            handshake */

   Step 11: Process RESPOND state
      If S.state == RESPOND,
         If P.type == Request,
            Send Response, possibly containing Init Cookie
            If Init Cookie was sent,
               Destroy S and return
               /* Step 3 will create another socket when the client
                  completes the three-way handshake */
         Otherwise,
            S.OSR := P.seqno
            S.state := OPEN

   Step 12: Process PARTOPEN state
      If S.state == PARTOPEN,
         If P.type == Response,
            Send Ack
         Otherwise, if P.type != Sync,
            S.OSR := P.seqno
            S.state := OPEN

   Step 13: Process CloseReq
      If P.type == CloseReq and S.state < CLOSEREQ,
         Generate Close
         S.state := CLOSING
         Set CLOSING timer

   Step 14: Process Close
      If P.type == Close,
         Generate Reset(Closed)
         Tear down connection
         Drop packet and return







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   Step 15: Process Sync
      If P.type == Sync,
         Generate SyncAck

   Step 16: Process data
      /* At this point any application data on P can be passed to the
         application, except that the application MUST NOT receive
         data from more than one Request or Response */

9.  Checksums

   DCCP uses a header checksum to protect its header against corruption.
   Generally, this checksum also covers any application data.  DCCP
   applications can, however, request that the header checksum cover
   only part of the application data, or perhaps no application data at
   all.  Link layers may then reduce their protection on unprotected
   parts of DCCP packets.  For some noisy links, and for applications
   that can tolerate corruption, this can greatly improve delivery rates
   and perceived performance.

   Checksum coverage may eventually impact congestion control mechanisms
   as well.  A packet with corrupt application data and complete
   checksum coverage is treated as lost.  This incurs a heavy-duty loss
   response from the sender's congestion control mechanism, which can
   unfairly penalize connections on links with high background
   corruption.  The combination of reduced checksum coverage and Data
   Checksum options may let endpoints report packets as corrupt rather
   than dropped, using Data Dropped options and Drop Code 3 (see Section
   11.7).  This may eventually benefit applications.  However, further
   research is required to determine an appropriate response to
   corruption, which can sometimes correlate with congestion.  Corrupt
   packets currently incur a loss response.

   The Data Checksum option, which contains a strong CRC, lets endpoints
   detect application data corruption.  An API can then be used to avoid
   delivering corrupt data to the application, even if links deliver
   corrupt data to the endpoint due to reduced checksum coverage.
   However, the use of reduced checksum coverage for applications that
   demand correct data is currently considered experimental.  This is
   because the combined loss-plus-corruption rate for packets with
   reduced checksum coverage may be significantly higher than that for
   packets with full checksum coverage, although the loss rate will
   generally be lower.  Actual behavior will depend on link design;
   further research and experience is required.

   Reduced checksum coverage introduces some security considerations;
   see Section 18.1.  See Appendix B for further motivation and




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   discussion.  DCCP's implementation of reduced checksum coverage was
   inspired by UDP-Lite [RFC3828].

9.1.  Header Checksum Field

   DCCP uses the TCP/IP checksum algorithm.  The Checksum field in the
   DCCP generic header (see Section 5.1) equals the 16-bit one's
   complement of the one's complement sum of all 16-bit words in the
   DCCP header, DCCP options, a pseudoheader taken from the network-
   layer header, and, depending on the value of the Checksum Coverage
   field, some or all of the application data.  When calculating the
   checksum, the Checksum field itself is treated as 0.  If a packet
   contains an odd number of header and payload bytes to be checksummed,
   8 zero bits are added on the right to form a 16-bit word for checksum
   purposes.  The pad byte is not transmitted as part of the packet.

   The pseudoheader is calculated as for TCP.  For IPv4, it is 96 bits
   long and consists of the IPv4 source and destination addresses, the
   IP protocol number for DCCP (padded on the left with 8 zero bits),
   and the DCCP length as a 16-bit quantity (the length of the DCCP
   header with options, plus the length of any data); see [RFC793],
   Section 3.1.  For IPv6, it is 320 bits long, and consists of the IPv6
   source and destination addresses, the DCCP length as a 32-bit
   quantity, and the IP protocol number for DCCP (padded on the left
   with 24 zero bits); see [RFC2460], Section 8.1.

   Packets with invalid header checksums MUST be ignored.  In
   particular, their options MUST NOT be processed.

9.2.  Header Checksum Coverage Field

   The Checksum Coverage field in the DCCP generic header (see Section
   5.1) specifies what parts of the packet are covered by the Checksum
   field, as follows:

   CsCov = 0      The Checksum field covers the DCCP header, DCCP
                  options, network-layer pseudoheader, and all
                  application data in the packet, possibly padded on the
                  right with zeros to an even number of bytes.

   CsCov = 1-15   The Checksum field covers the DCCP header, DCCP
                  options, network-layer pseudoheader, and the initial
                  (CsCov-1)*4 bytes of the packet's application data.

   Thus, if CsCov is 1, none of the application data is protected by the
   header checksum.  The value (CsCov-1)*4 MUST be less than or equal to
   the length of the application data.  Packets with invalid CsCov
   values MUST be ignored; in particular, their options MUST NOT be



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   processed.  The meanings of values other than 0 and 1 should be
   considered experimental.

   Values other than 0 specify that corruption is acceptable in some or
   all of the DCCP packet's application data.  In fact, DCCP cannot even
   detect corruption in areas not covered by the header checksum, unless
   the Data Checksum option is used.  Applications should not make any
   assumptions about the correctness of received data not covered by the
   checksum and should, if necessary, introduce their own validity
   checks.

   A DCCP application interface should let sending applications suggest
   a value for CsCov for sent packets, defaulting to 0 (full coverage).
   The Minimum Checksum Coverage feature, described below, lets an
   endpoint refuse delivery of application data on packets with partial
   checksum coverage; by default, only fully covered application data is
   accepted.  Lower layers that support partial error detection MAY use
   the Checksum Coverage field as a hint of where errors do not need to
   be detected.  Lower layers MUST use a strong error detection
   mechanism to detect at least errors that occur in the sensitive part
   of the packet, and to discard damaged packets.  The sensitive part
   consists of the bytes between the first byte of the IP header and the
   last byte identified by Checksum Coverage.

   For more details on application and lower-layer interface issues
   relating to partial checksumming, see [RFC3828].

9.2.1.  Minimum Checksum Coverage Feature

   The Minimum Checksum Coverage feature lets a DCCP endpoint determine
   whether its peer is willing to accept packets with reduced Checksum
   Coverage.  For example, DCCP A sends a "Change R(Minimum Checksum
   Coverage, 1)" option to DCCP B to check whether B is willing to
   accept packets with Checksum Coverage set to 1.

   Minimum Checksum Coverage has feature number 8 and is server-
   priority.  It takes one-byte integer values between 0 and 15; values
   of 16 or more are reserved.  Minimum Checksum Coverage/B reflects
   values of Checksum Coverage that DCCP B finds unacceptable.  Say that
   the value of Minimum Checksum Coverage/B is MinCsCov.  Then:

   o  If MinCsCov = 0, then DCCP B only finds packets with CsCov = 0
      acceptable.

   o  If MinCsCov > 0, then DCCP B additionally finds packets with
      CsCov >= MinCsCov acceptable.





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   DCCP B MAY refuse to process application data from packets with
   unacceptable Checksum Coverage.  Such packets SHOULD be reported
   using Data Dropped options (Section 11.7) with Drop Code 0, Protocol
   Constraints.  New connections start with Minimum Checksum Coverage 0
   for both endpoints.

9.3.  Data Checksum Option

   The Data Checksum option holds a 32-bit CRC-32c cyclic redundancy-
   check code of a DCCP packet's application data.

   +--------+--------+--------+--------+--------+--------+
   |00101100|00000110|              CRC-32c              |
   +--------+--------+--------+--------+--------+--------+
    Type=44  Length=6

   The sending DCCP computes the CRC of the bytes comprising the
   application data area and stores it in the option data.  The CRC-32c
   algorithm used for Data Checksum is the same as that used for SCTP
   [RFC3309]; note that the CRC-32c of zero bytes of data equals zero.
   The DCCP header checksum will cover the Data Checksum option, so the
   data checksum must be computed before the header checksum.

   A DCCP endpoint receiving a packet with a Data Checksum option either
   MUST or MAY check the Data Checksum; the choice depends on the value
   of the Check Data Checksum feature described below.  If it checks the
   checksum, it computes the received application data's CRC-32c using
   the same algorithm as the sender and compares the result with the
   Data Checksum value.  If the CRCs differ, the endpoint reacts in one
   of two ways:

   o  The receiving application may have requested delivery of known-
      corrupt data via some optional API.  In this case, the packet's
      data MUST be delivered to the application, with a note that it is
      known to be corrupt.  Furthermore, the receiving endpoint MUST
      report the packet as delivered corrupt using a Data Dropped option
      (Drop Code 7, Delivered Corrupt).

   o  Otherwise, the receiving endpoint MUST drop the application data
      and report that data as dropped due to corruption using a Data
      Dropped option (Drop Code 3, Corrupt).

   In either case, the packet is considered acknowledgeable (since its
   header was processed) and will therefore be acknowledged using the
   equivalent of Ack Vector's Received or Received ECN Marked states.

   Although Data Checksum is intended for packets containing application
   data, it may be included on other packets, such as DCCP-Ack, DCCP-



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   Sync, and DCCP-SyncAck.  The receiver SHOULD calculate the
   application data area's CRC-32c on such packets, just as it does for
   DCCP-Data and similar packets.  If the CRCs differ, the packets
   similarly MUST be reported using Data Dropped options (Drop Code 3),
   although their application data areas would not be delivered to the
   application in any case.

9.3.1.  Check Data Checksum Feature

   The Check Data Checksum feature lets a DCCP endpoint determine
   whether its peer will definitely check Data Checksum options.  DCCP A
   sends a Mandatory "Change R(Check Data Checksum, 1)" option to DCCP B
   to require it to check Data Checksum options (the connection will be
   reset if it cannot).

   Check Data Checksum has feature number 9 and is server-priority.  It
   takes one-byte Boolean values.  DCCP B MUST check any received Data
   Checksum options when Check Data Checksum/B is one, although it MAY
   check them even when Check Data Checksum/B is zero.  Values of two or
   more are reserved.  New connections start with Check Data Checksum 0
   for both endpoints.

9.3.2.  Checksum Usage Notes

   Internet links must normally apply strong integrity checks to the
   packets they transmit [RFC3828, RFC3819].  This is the default case
   when the DCCP header's Checksum Coverage value equals zero (full
   coverage).  However, the DCCP Checksum Coverage value might not be
   zero.  By setting partial Checksum Coverage, the application
   indicates that it can tolerate corruption in the unprotected part of
   the application data.  Recognizing this, link layers may reduce error
   detection and/or correction strength when transmitting this
   unprotected part.  This, in turn, can significantly increase the
   likelihood of the endpoint's receiving corrupt data; Data Checksum
   lets the receiver detect that corruption with very high probability.

10.  Congestion Control

   Each congestion control mechanism supported by DCCP is assigned a
   congestion control identifier, or CCID: a number from 0 to 255.
   During connection setup, and optionally thereafter, the endpoints
   negotiate their congestion control mechanisms by negotiating the
   values for their Congestion Control ID features.  Congestion Control
   ID has feature number 1.  The CCID/A value equals the CCID in use for
   the A-to-B half-connection.  DCCP B sends a "Change R(CCID, K)"
   option to ask DCCP A to use CCID K for its data packets.





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   CCID is a server-priority feature, so CCID negotiation options can
   list multiple acceptable CCIDs, sorted in descending order of
   priority.  For example, the option "Change R(CCID, 2 3 4)" asks the
   receiver to use CCID 2 for its packets, although CCIDs 3 and 4 are
   also acceptable.  (This corresponds to the bytes "35, 6, 1, 2, 3, 4":
   Change R option (35), option length (6), feature ID (1), CCIDs (2, 3,
   4).)  Similarly, "Confirm L(CCID, 2, 2 3 4)" tells the receiver that
   the sender is using CCID 2 for its packets, but that CCIDs 3 and 4
   might also be acceptable.

   Currently allocated CCIDs are as follows:

           CCID   Meaning                      Reference
           ----   -------                      ---------
            0-1   Reserved
             2    TCP-like Congestion Control  [RFC4341]
             3    TCP-Friendly Rate Control    [RFC4342]
           4-255  Reserved

           Table 5: DCCP Congestion Control Identifiers

   New connections start with CCID 2 for both endpoints.  If this is
   unacceptable for a DCCP endpoint, that endpoint MUST send Mandatory
   Change(CCID) options on its first packets.

   All CCIDs standardized for use with DCCP will correspond to
   congestion control mechanisms previously standardized by the IETF.
   We expect that for quite some time, all such mechanisms will be TCP
   friendly, but TCP-friendliness is not an explicit DCCP requirement.

   A DCCP implementation intended for general use, such as an
   implementation in a general-purpose operating system kernel, SHOULD
   implement at least CCID 2.  The intent is to make CCID 2 broadly
   available for interoperability, although particular applications
   might disallow its use.

10.1.  TCP-like Congestion Control

   CCID 2, TCP-like Congestion Control, denotes Additive Increase,
   Multiplicative Decrease (AIMD) congestion control with behavior
   modelled directly on TCP, including congestion window, slow start,
   timeouts, and so forth [RFC2581].  CCID 2 achieves maximum bandwidth
   over the long term, consistent with the use of end-to-end congestion
   control, but halves its congestion window in response to each
   congestion event.  This leads to the abrupt rate changes typical of
   TCP.  Applications should use CCID 2 if they prefer maximum bandwidth
   utilization to steadiness of rate.  This is often the case for
   applications that are not playing their data directly to the user.



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   For example, a hypothetical application that transferred files over
   DCCP, using application-level retransmissions for lost packets, would
   prefer CCID 2 to CCID 3.  On-line games may also prefer CCID 2.

   CCID 2 is further described in [RFC4341].

10.2.  TFRC Congestion Control

   CCID 3 denotes TCP-Friendly Rate Control (TFRC), an equation-based
   rate-controlled congestion control mechanism.  TFRC is designed to be
   reasonably fair when competing for bandwidth with TCP-like flows,
   where a flow is "reasonably fair" if its sending rate is generally
   within a factor of two of the sending rate of a TCP flow under the
   same conditions.  However, TFRC has a much lower variation of
   throughput over time compared with TCP, which makes CCID 3 more
   suitable than CCID 2 for applications such as streaming media where a
   relatively smooth sending rate is important.

   CCID 3 is further described in [RFC4342].  The TFRC congestion
   control algorithms were initially described in [RFC3448].

10.3.  CCID-Specific Options, Features, and Reset Codes

   Half of the option types, feature numbers, and Reset Codes are
   reserved for CCID-specific use.  CCIDs may often need new options,
   for communicating acknowledgement or rate information, for example;
   reserved option spaces let CCIDs create options at will without
   polluting the global option space.  Option 128 might have different
   meanings on a half-connection using CCID 4 and a half-connection
   using CCID 8.  CCID-specific options and features will never conflict
   with global options and features introduced by later versions of this
   specification.

   Any packet may contain information meant for either half-connection,
   so CCID-specific option types, feature numbers, and Reset Codes
   explicitly signal the half-connection to which they apply.

   o  Option numbers 128 through 191 are for options sent from the
      HC-Sender to the HC-Receiver; option numbers 192 through 255 are
      for options sent from the HC-Receiver to the HC-Sender.

   o  Reset Codes 128 through 191 indicate that the HC-Sender reset the
      connection (most likely because of some problem with
      acknowledgements sent by the HC-Receiver).  Reset Codes 192
      through 255 indicate that the HC-Receiver reset the connection
      (most likely because of some problem with data packets sent by the
      HC-Sender).




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   o  Finally, feature numbers 128 through 191 are used for features
      located at the HC-Sender; feature numbers 192 through 255 are for
      features located at the HC-Receiver.  Since Change L and Confirm L
      options for a feature are sent by the feature location, we know
      that any Change L(128) option was sent by the HC-Sender, while any
      Change L(192) option was sent by the HC-Receiver.  Similarly,
      Change R(128) options are sent by the HC-Receiver, while Change
      R(192) options are sent by the HC-Sender.

   For example, consider a DCCP connection where the A-to-B half-
   connection uses CCID 4 and the B-to-A half-connection uses CCID 5.
   Here is how a sampling of CCID-specific options are assigned to
   half-connections.

                                   Relevant    Relevant
        Packet  Option             Half-conn.  CCID
        ------  ------             ----------  ----
        A > B   128                  A-to-B     4
        A > B   192                  B-to-A     5
        A > B   Change L(128, ...)   A-to-B     4
        A > B   Change R(192, ...)   A-to-B     4
        A > B   Confirm L(128, ...)  A-to-B     4
        A > B   Confirm R(192, ...)  A-to-B     4
        A > B   Change R(128, ...)   B-to-A     5
        A > B   Change L(192, ...)   B-to-A     5
        A > B   Confirm R(128, ...)  B-to-A     5
        A > B   Confirm L(192, ...)  B-to-A     5
        B > A   128                  B-to-A     5
        B > A   192                  A-to-B     4
        B > A   Change L(128, ...)   B-to-A     5
        B > A   Change R(192, ...)   B-to-A     5
        B > A   Confirm L(128, ...)  B-to-A     5
        B > A   Confirm R(192, ...)  B-to-A     5
        B > A   Change R(128, ...)   A-to-B     4
        B > A   Change L(192, ...)   A-to-B     4
        B > A   Confirm R(128, ...)  A-to-B     4
        B > A   Confirm L(192, ...)  A-to-B     4

   Using CCID-specific options and feature options during a negotiation
   for the corresponding CCID feature is NOT RECOMMENDED, since it is
   difficult to predict which CCID will be in force when the option is
   processed.  For example, if a DCCP-Request contains the option
   sequence "Change L(CCID, 3), 128", the CCID-specific option "128" may
   be processed either by CCID 3 (if the server supports CCID 3) or by
   the default CCID 2 (if it does not).  However, it is safe to include
   CCID-specific options following certain Mandatory Change(CCID)





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   options.  For example, if a DCCP-Request contains the option sequence
   "Mandatory, Change L(CCID, 3), 128", then either the "128" option
   will be processed by CCID 3 or the connection will be reset.

   Servers that do not implement the default CCID 2 might nevertheless
   receive CCID 2-specific options on a DCCP-Request packet.  (Such a
   server MUST send Mandatory Change(CCID) options on its DCCP-Response,
   so CCID-specific options on any other packet won't refer to CCID 2.)
   The server MUST treat such options as non-understood.  Thus, it will
   reset the connection on encountering a Mandatory CCID-specific option
   or feature negotiation request, send an empty Confirm for a non-
   Mandatory Change option for a CCID-specific feature, and ignore other
   CCID-specific options.

10.4.  CCID Profile Requirements

   Each CCID Profile document MUST address at least the following
   requirements:

   o  The profile MUST include the name and number of the CCID being
      described.

   o  The profile MUST describe the conditions in which it is likely to
      be useful.  Often the best way to do this is by comparison to
      existing CCIDs.

   o  The profile MUST list and describe any CCID-specific options,
      features, and Reset Codes and SHOULD list those general options
      and features described in this document that are especially
      relevant to the CCID.

   o  Any newly defined acknowledgement mechanism MUST include a way to
      transmit ECN Nonce Echoes back to the sender.

   o  The profile MUST describe the format of data packets, including
      any options that should be included and the setting of the CCval
      header field.

   o  The profile MUST describe the format of acknowledgement packets,
      including any options that should be included.

   o  The profile MUST define how data packets are congestion
      controlled.  This includes responses to congestion events, to idle
      and application-limited periods, and to the DCCP Data Dropped and
      Slow Receiver options.  CCIDs that implement per-packet congestion
      control SHOULD discuss how packet size is factored in to
      congestion control decisions.




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   o  The profile MUST specify when acknowledgement packets are
      generated and how they are congestion controlled.

   o  The profile MUST define when a sender using the CCID is considered
      quiescent.

   o  The profile MUST say whether its CCID's acknowledgements ever need
      to be acknowledged and, if so, how often.

10.5.  Congestion State

   Most congestion control algorithms depend on past history to
   determine the current allowed sending rate.  In CCID 2, this
   congestion state includes a congestion window and a measurement of
   the number of packets outstanding in the network; in CCID 3, it
   includes the lengths of recent loss intervals.  Both CCIDs use an
   estimate of the round-trip time.  Congestion state depends on the
   network path and is invalidated by path changes.  Therefore, DCCP
   senders and receivers SHOULD reset their congestion state --
   essentially restarting congestion control from "slow start" or
   equivalent -- on significant changes in the end-to-end path.  For
   example, an endpoint that sends or receives a Mobile IPv6 Binding
   Update message [RFC3775] SHOULD reset its congestion state for any
   corresponding DCCP connections.

   A DCCP implementation MAY also reset its congestion state when a CCID
   changes (that is, when a negotiation for the CCID feature completes
   successfully and the new feature value differs from the old value).
   Thus, a connection in a heavily congested environment might evade
   end-to-end congestion control by frequently renegotiating a CCID,
   just as it could evade end-to-end congestion control by opening new
   connections for the same session.  This behavior is prohibited.  To
   prevent it, DCCP implementations MAY limit the rate at which CCID can
   be changed -- for instance, by refusing to change a CCID feature
   value more than once per minute.

11.  Acknowledgements

   Congestion control requires that receivers transmit information about
   packet losses and ECN marks to senders.  DCCP receivers MUST report
   all congestion they see, as defined by the relevant CCID profile.
   Each CCID says when acknowledgements should be sent, what options
   they must use, and so on.  DCCP acknowledgements are congestion
   controlled, although it is not required that the acknowledgement
   stream be more than very roughly TCP friendly; each CCID defines how
   acknowledgements are congestion controlled.





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   Most acknowledgements use DCCP options.  For example, on a half-
   connection with CCID 2 (TCP-like), the receiver reports
   acknowledgement information using the Ack Vector option.  This
   section describes common acknowledgement options and shows how acks
   using those options will commonly work.  Full descriptions of the ack
   mechanisms used for each CCID are laid out in the CCID profile
   specifications.

   Acknowledgement options, such as Ack Vector, depend on the DCCP
   Acknowledgement Number and are thus only allowed on packet types that
   carry that number.  Acknowledgement options received on other packet
   types, namely DCCP-Request and DCCP-Data, MUST be ignored.  Detailed
   acknowledgement options are not necessarily required on every packet
   that carries an Acknowledgement Number, however.

11.1.  Acks of Acks and Unidirectional Connections

   DCCP was designed to work well for both bidirectional and
   unidirectional flows of data, and for connections that transition
   between these states.  However, acknowledgements required for a
   unidirectional connection are very different from those required for
   a bidirectional connection.  In particular, unidirectional
   connections need to worry about acks of acks.

   The ack-of-acks problem arises because some acknowledgement
   mechanisms are reliable.  For example, an HC-Receiver using CCID 2,
   TCP-like Congestion Control, sends Ack Vectors containing completely
   reliable acknowledgement information.  The HC-Sender should
   occasionally inform the HC-Receiver that it has received an ack.  If
   it did not, the HC-Receiver might resend complete Ack Vector
   information, going back to the start of the connection, with every
   DCCP-Ack packet!  However, note that acks-of-acks need not be
   reliable themselves: when an ack-of-acks is lost, the HC-Receiver
   will simply maintain, and periodically retransmit, old
   acknowledgement-related state for a little longer.  Therefore, there
   is no need for acks-of-acks-of-acks.

   When communication is bidirectional, any required acks-of-acks are
   automatically contained in normal acknowledgements for data packets.
   On a unidirectional connection, however, the receiver DCCP sends no
   data, so the sender would not normally send acknowledgements.
   Therefore, the CCID in force on that half-connection must explicitly
   say whether, when, and how the HC-Sender should generate acks-of-
   acks.

   For example, consider a bidirectional connection where both half-
   connections use the same CCID (either 2 or 3), and where DCCP B goes
   "quiescent".  This means that the connection becomes unidirectional:



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   DCCP B stops sending data and sends only DCCP-Ack packets to DCCP A.
   In CCID 2, TCP-like Congestion Control, DCCP B uses Ack Vector to
   reliably communicate which packets it has received.  As described
   above, DCCP A must occasionally acknowledge a pure acknowledgement
   from DCCP B so that B can free old Ack Vector state.  For instance, A
   might send a DCCP-DataAck packet instead of DCCP-Data every now and
   then.  In CCID 3, however, acknowledgement state is generally
   bounded, so A does not need to acknowledge B's acknowledgements.

   When communication is unidirectional, a single CCID -- in the
   example, the A-to-B CCID -- controls both DCCPs' acknowledgements, in
   terms of their content, their frequency, and so forth.  For
   bidirectional connections, the A-to-B CCID governs DCCP B's
   acknowledgements (including its acks of DCCP A's acks) and the B-to-A
   CCID governs DCCP A's acknowledgements.

   DCCP A switches its ack pattern from bidirectional to unidirectional
   when it notices that DCCP B has gone quiescent.  It switches from
   unidirectional to bidirectional when it must acknowledge even a
   single DCCP-Data or DCCP-DataAck packet from DCCP B.

   Each CCID defines how to detect quiescence on that CCID, and how that
   CCID handles acks-of-acks on unidirectional connections.  The B-to-A
   CCID defines when DCCP B has gone quiescent.  Usually, this happens
   when a period has passed without B sending any data packets; in CCID
   2, for example, this period is the maximum of 0.2 seconds and two
   round-trip times.  The A-to-B CCID defines how DCCP A handles
   acks-of-acks once DCCP B has gone quiescent.

11.2.  Ack Piggybacking

   Acknowledgements of A-to-B data MAY be piggybacked on data sent by
   DCCP B, as long as that does not delay the acknowledgement longer
   than the A-to-B CCID would find acceptable.  However, data
   acknowledgements often require more than 4 bytes to express.  A large
   set of acknowledgements prepended to a large data packet might exceed
   the allowed maximum packet size.  In this case, DCCP B SHOULD send
   separate DCCP-Data and DCCP-Ack packets, or wait, but not too long,
   for a smaller datagram.

   Piggybacking is particularly common at DCCP A when the B-to-A
   half-connection is quiescent -- that is, when DCCP A is just
   acknowledging DCCP B's acknowledgements.  There are three reasons to
   acknowledge DCCP B's acknowledgements: to allow DCCP B to free up
   information about previously acknowledged data packets from A; to
   shrink the size of future acknowledgements; and to manipulate the
   rate at which future acknowledgements are sent.  Since these are




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   secondary concerns, DCCP A can generally afford to wait indefinitely
   for a data packet to piggyback its acknowledgement onto; if DCCP B
   wants to elicit an acknowledgement, it can send a DCCP-Sync.

   Any restrictions on ack piggybacking are described in the relevant
   CCID's profile.

11.3.  Ack Ratio Feature

   The Ack Ratio feature lets HC-Senders influence the rate at which
   HC-Receivers generate DCCP-Ack packets, thus controlling reverse-path
   congestion.  This differs from TCP, which presently has no congestion
   control for pure acknowledgement traffic.  Ack Ratio reverse-path
   congestion control does not try to be TCP friendly.  It just tries to
   avoid congestion collapse, and to be somewhat better than TCP in the
   presence of a high packet loss or mark rate on the reverse path.

   Ack Ratio applies to CCIDs whose HC-Receivers clock acknowledgements
   off the receipt of data packets.  The value of Ack Ratio/A equals the
   rough ratio of data packets sent by DCCP A to DCCP-Ack packets sent
   by DCCP B.  Higher Ack Ratios correspond to lower DCCP-Ack rates; the
   sender raises Ack Ratio when the reverse path is congested and lowers
   Ack Ratio when it is not.  Each CCID profile defines how it controls
   congestion on the acknowledgement path, and, particularly, whether
   Ack Ratio is used.  CCID 2, for example, uses Ack Ratio for
   acknowledgement congestion control, but CCID 3 does not.  However,
   each Ack Ratio feature has a value whether or not that value is used
   by the relevant CCID.

   Ack Ratio has feature number 5 and is non-negotiable.  It takes two-
   byte integer values.  An Ack Ratio/A value of four means that DCCP B
   will send at least one acknowledgement packet for every four data
   packets sent by DCCP A.  DCCP A sends a "Change L(Ack Ratio)" option
   to notify DCCP B of its ack ratio.  An Ack Ratio value of zero
   indicates that the relevant half-connection does not use an Ack Ratio
   to control its acknowledgement rate.  New connections start with Ack
   Ratio 2 for both endpoints; this Ack Ratio results in acknowledgement
   behavior analogous to TCP's delayed acks.

   Ack Ratio should be treated as a guideline rather than a strict
   requirement.  We intend Ack Ratio-controlled acknowledgement behavior
   to resemble TCP's acknowledgement behavior when there is no reverse-
   path congestion, and to be somewhat more conservative when there is
   reverse-path congestion.  Following this intent is more important
   than implementing Ack Ratio precisely.  In particular:






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   o  Receivers MAY piggyback acknowledgement information on data
      packets, creating DCCP-DataAck packets.  The Ack Ratio does not
      apply to piggybacked acknowledgements.  However, if the data
      packets are too big to carry acknowledgement information, or if
      the data sending rate is lower than Ack Ratio would suggest, then
      DCCP B SHOULD send enough pure DCCP-Ack packets to maintain the
      rate of one acknowledgement per Ack Ratio received data packets.

   o  Receivers MAY rate-pace their acknowledgements rather than send
      acknowledgements immediately upon the receipt of data packets.
      Receivers that rate-pace acknowledgements SHOULD pick a rate that
      approximates the effect of Ack Ratio and SHOULD include Elapsed
      Time options (Section 13.2) to help the sender calculate round-
      trip times.

   o  Receivers SHOULD implement delayed acknowledgement timers like
      TCP's, whereby any packet's acknowledgement is delayed by at most
      T seconds.  This delay lets the receiver collect additional
      packets to acknowledge and thus reduce the per-packet overhead of
      acknowledgements; but if T seconds have passed by and the ack is
      still around, it is sent out right away.  The default value of T
      should be 0.2 seconds, as is common in TCP implementations.  This
      may lead to sending more acknowledgement packets than Ack Ratio
      would suggest.

   o  Receivers SHOULD send acknowledgements immediately on receiving
      packets marked ECN Congestion Experienced or packets whose out-
      of-order sequence numbers potentially indicate loss.  However,
      there is no need to send such immediate acknowledgements for
      marked packets more than once per round-trip time.

   o  Receivers MAY ignore Ack Ratio if they perform their own
      congestion control on acknowledgements.  For example, a receiver
      that knows the loss and mark rate for its DCCP-Ack packets might
      maintain a TCP-friendly acknowledgement rate on its own.  Such a
      receiver MUST either ensure that it always obtains sufficient
      acknowledgement loss and mark information or fall back to Ack
      Ratio when sufficient information is not available, as might
      happen during periods when the receiver is quiescent.

11.4.  Ack Vector Options

   The Ack Vector gives a run-length encoded history of data packets
   received at the client.  Each byte of the vector gives the state of
   that data packet in the loss history, and the number of preceding
   packets with the same state.  The option's data looks like this:





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   +--------+--------+--------+--------+--------+--------
   |0010011?| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL|  ...
   +--------+--------+--------+--------+--------+--------
   Type=38/39         \___________ Vector ___________...

   The two Ack Vector options (option types 38 and 39) differ only in
   the values they imply for ECN Nonce Echo.  Section 12.2 describes
   this further.

   The vector itself consists of a series of bytes, each of whose
   encoding is:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |Sta| Run Length|
   +-+-+-+-+-+-+-+-+

   Sta[te] occupies the most significant two bits of each byte and can
   have one of four values, as follows:

                    State  Meaning
                    -----  -------
                      0    Received
                      1    Received ECN Marked
                      2    Reserved
                      3    Not Yet Received

                  Table 6: DCCP Ack Vector States

   The term "ECN marked" refers to packets with ECN code point 11, CE
   (Congestion Experienced); packets received with this ECN code point
   MUST be reported using State 1, Received ECN Marked.  Packets
   received with ECN code points 00, 01, or 10 (Non-ECT, ECT(0), or
   ECT(1), respectively) MUST be reported using State 0, Received.

   Run Length, the least significant six bits of each byte, specifies
   how many consecutive packets have the given State.  Run Length zero
   says the corresponding State applies to one packet only; Run Length
   63 says it applies to 64 consecutive packets.  Run lengths of 65 or
   more must be encoded in multiple bytes.

   The first byte in the first Ack Vector option refers to the packet
   indicated in the Acknowledgement Number; subsequent bytes refer to
   older packets.  Ack Vector MUST NOT be sent on DCCP-Data and DCCP-
   Request packets, which lack an Acknowledgement Number, and any Ack
   Vector options encountered on such packets MUST be ignored.





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   An Ack Vector containing the decimal values 0,192,3,64,5 and for
   which the Acknowledgement Number is decimal 100 indicates that:

      Packet 100 was received (Acknowledgement Number 100, State 0, Run
      Length 0);

      Packet 99 was lost (State 3, Run Length 0);

      Packets 98, 97, 96 and 95 were received (State 0, Run Length 3);

      Packet 94 was ECN marked (State 1, Run Length 0); and

      Packets 93, 92, 91, 90, 89, and 88 were received (State 0, Run
      Length 5).

   A single Ack Vector option can acknowledge up to 16192 data packets.
   Should more packets need to be acknowledged than can fit in 253 bytes
   of Ack Vector, then multiple Ack Vector options can be sent; the
   second Ack Vector begins where the first left off, and so forth.

   Ack Vector states are subject to two general constraints.  (These
   principles SHOULD also be followed for other acknowledgement
   mechanisms; referring to Ack Vector states simplifies their
   explanation.)

   1. Packets reported as State 0 or State 1 MUST be acknowledgeable:
      their options have been processed by the receiving DCCP stack.
      Any data on the packet need not have been delivered to the
      receiving application; in fact, the data may have been dropped.

   2. Packets reported as State 3 MUST NOT be acknowledgeable.  Feature
      negotiations and options on such packets MUST NOT have been
      processed, and the Acknowledgement Number MUST NOT correspond to
      such a packet.

   Packets dropped in the application's receive buffer MUST be reported
   as Received or Received ECN Marked (States 0 and 1), depending on
   their ECN state; such packets' ECN Nonces MUST be included in the
   Nonce Echo.  The Data Dropped option informs the sender that some
   packets reported as received actually had their application data
   dropped.

   One or more Ack Vector options that, together, report the status of a
   packet with a sequence number less than ISN, the initial sequence
   number, SHOULD be considered invalid.  The receiving DCCP SHOULD
   either ignore the options or reset the connection with Reset Code 5,
   "Option Error".  No Ack Vector option can refer to a packet that has
   not yet been sent, as the Acknowledgement Number checks in Section



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   7.5.3 ensure, but because of attack, implementation bug, or
   misbehavior, an Ack Vector option can claim that a packet was
   received before it is actually delivered.  Section 12.2 describes how
   this is detected and how senders should react.  Packets that haven't
   been included in any Ack Vector option SHOULD be treated as "not yet
   received" (State 3) by the sender.

   Appendix A provides a non-normative description of the details of
   DCCP acknowledgement handling in the context of an abstract Ack
   Vector implementation.

11.4.1.  Ack Vector Consistency

   A DCCP sender will commonly receive multiple acknowledgements for
   some of its data packets.  For instance, an HC-Sender might receive
   two DCCP-Acks with Ack Vectors, both of which contained information
   about sequence number 24.  (Information about a sequence number is
   generally repeated in every ack until the HC-Sender acknowledges an
   ack.  In this case, perhaps the HC-Receiver is sending acks faster
   than the HC-Sender is acknowledging them.)  In a perfect world, the
   two Ack Vectors would always be consistent.  However, there are many
   reasons why they might not be.  For example:

   o  The HC-Receiver received packet 24 between sending its acks, so
      the first ack said 24 was not received (State 3) and the second
      said it was received or ECN marked (State 0 or 1).

   o  The HC-Receiver received packet 24 between sending its acks, and
      the network reordered the acks.  In this case, the packet will
      appear to transition from State 0 or 1 to State 3.

   o  The network duplicated packet 24, and one of the duplicates was
      ECN marked.  This might show up as a transition between States 0
      and 1.

   To cope with these situations, HC-Sender DCCP implementations SHOULD
   combine multiple received Ack Vector states according to this table:

                               Received State
                                 0   1   3
                               +---+---+---+
                             0 | 0 |0/1| 0 |
                       Old     +---+---+---+
                             1 | 1 | 1 | 1 |
                      State    +---+---+---+
                             3 | 0 | 1 | 3 |
                               +---+---+---+




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   To read the table, choose the row corresponding to the packet's old
   state and the column corresponding to the packet's state in the newly
   received Ack Vector; then read the packet's new state off the table.
   For an old state of 0 (received non-marked) and received state of 1
   (received ECN marked), the packet's new state may be set to either 0
   or 1.  The HC-Sender implementation will be indifferent to ack
   reordering if it chooses new state 1 for that cell.

   The HC-Receiver should collect information about received packets
   according to the following table:

                              Received Packet
                                 0   1   3
                               +---+---+---+
                             0 | 0 |0/1| 0 |
                     Stored    +---+---+---+
                             1 |0/1| 1 | 1 |
                      State    +---+---+---+
                             3 | 0 | 1 | 3 |
                               +---+---+---+

   This table equals the sender's table except that, when the stored
   state is 1 and the received state is 0, the receiver is allowed to
   switch its stored state to 0.

   An HC-Sender MAY choose to throw away old information gleaned from
   the HC-Receiver's Ack Vectors, in which case it MUST ignore newly
   received acknowledgements from the HC-Receiver for those old packets.
   It is often kinder to save recent Ack Vector information for a while
   so that the HC-Sender can undo its reaction to presumed congestion
   when a "lost" packet unexpectedly shows up (the transition from State
   3 to State 0).

11.4.2.  Ack Vector Coverage

   We can divide the packets that have been sent from an HC-Sender to an
   HC-Receiver into four roughly contiguous groups.  From oldest to
   youngest, these are:

   1. Packets already acknowledged by the HC-Receiver, where the
      HC-Receiver knows that the HC-Sender has definitely received the
      acknowledgements;

   2. Packets already acknowledged by the HC-Receiver, where the
      HC-Receiver cannot be sure that the HC-Sender has received the
      acknowledgements;

   3. Packets not yet acknowledged by the HC-Receiver; and



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   4. Packets not yet received by the HC-Receiver.

   The union of groups 2 and 3 is called the Acknowledgement Window.
   Generally, every Ack Vector generated by the HC-Receiver will cover
   the whole Acknowledgement Window: Ack Vector acknowledgements are
   cumulative.  (This simplifies Ack Vector maintenance at the
   HC-Receiver; see Appendix A, below.)  As packets are received, this
   window both grows on the right and shrinks on the left.  It grows
   because there are more packets, and shrinks because the HC-Sender's
   Acknowledgement Numbers will acknowledge previous acknowledgements,
   moving packets from group 2 into group 1.

11.5.  Send Ack Vector Feature

   The Send Ack Vector feature lets DCCPs negotiate whether they should
   use Ack Vector options to report congestion.  Ack Vector provides
   detailed loss information and lets senders report back to their
   applications whether particular packets were dropped.  Send Ack
   Vector is mandatory for some CCIDs and optional for others.

   Send Ack Vector has feature number 6 and is server-priority.  It
   takes one-byte Boolean values.  DCCP A MUST send Ack Vector options
   on its acknowledgements when Send Ack Vector/A has value one,
   although it MAY send Ack Vector options even when Send Ack Vector/A
   is zero.  Values of two or more are reserved.  New connections start
   with Send Ack Vector 0 for both endpoints.  DCCP B sends a "Change
   R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack Vector
   options as part of its acknowledgement traffic.

11.6.  Slow Receiver Option

   An HC-Receiver sends the Slow Receiver option to its sender to
   indicate that it is having trouble keeping up with the sender's data.
   The HC-Sender SHOULD NOT increase its sending rate for approximately
   one round-trip time after seeing a packet with a Slow Receiver
   option.  After one round-trip time, the effect of Slow Receiver
   disappears, allowing the HC-Sender to increase its rate.  Therefore,
   the HC-Receiver SHOULD continue to send Slow Receiver options if it
   needs to prevent the HC-Sender from going faster in the long term.
   The Slow Receiver option does not indicate congestion, and the HC-
   Sender need not reduce its sending rate.  (If necessary, the receiver
   can force the sender to slow down by dropping packets, with or
   without Data Dropped, or by reporting false ECN marks.)  APIs should
   let receiver applications set Slow Receiver and sending applications
   determine whether their receivers are Slow.






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   Slow Receiver is a one-byte option.

   +--------+
   |00000010|
   +--------+
    Type=2

   Slow Receiver does not specify why the receiver is having trouble
   keeping up with the sender.  Possible reasons include lack of buffer
   space, CPU overload, and application quotas.  A sending application
   might react to Slow Receiver by reducing its application-level
   sending rate, for example.

   The sending application should not react to Slow Receiver by sending
   more data, however.  Although the optimal response to a CPU-bound
   receiver might be to reduce compression and send more data (a
   highly-compressed data format might overwhelm a slow CPU more
   seriously than would the higher memory requirements of a less-
   compressed data format), this kind of format change should be
   requested at the application level, not via the Slow Receiver option.

   Slow Receiver implements a portion of TCP's receive window
   functionality.

11.7.  Data Dropped Option

   The Data Dropped option indicates that the application data on one or
   more received packets did not actually reach the application.  Data
   Dropped additionally reports why the data was dropped: perhaps the
   data was corrupt, or perhaps the receiver cannot keep up with the
   sender's current rate and the data was dropped in some receive
   buffer.  Using Data Dropped, DCCP endpoints can discriminate between
   different kinds of loss; this differs from TCP, in which all loss is
   reported the same way.

   Unless it is explicitly specified otherwise, DCCP congestion control
   mechanisms MUST react as if each Data Dropped packet was marked as
   ECN Congestion Experienced by the network.  We intend for Data
   Dropped to enable research into richer congestion responses to
   corrupt and other endpoint-dropped packets, but DCCP CCIDs MUST react
   conservatively to Data Dropped until this behavior is standardized.
   Section 11.7.2, below, describes congestion responses for all current
   Drop Codes.

   If a received packet's application data is dropped for one of the
   reasons listed below, this SHOULD be reported using a Data Dropped
   option.  Alternatively, the receiver MAY choose to report as




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   "received" only those packets whose data were not dropped, subject to
   the constraint that packets not reported as received MUST NOT have
   had their options processed.

   The option's data looks like this:

   +--------+--------+--------+--------+--------+--------
   |00101000| Length | Block  | Block  | Block  |  ...
   +--------+--------+--------+--------+--------+--------
    Type=40          \___________ Vector ___________ ...

   The Vector consists of a series of bytes, called Blocks, each of
   whose encoding corresponds to one of two choices:

    0 1 2 3 4 5 6 7                  0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+                +-+-+-+-+-+-+-+-+
   |0| Run Length  |       or       |1|DrpCd|Run Len|
   +-+-+-+-+-+-+-+-+                +-+-+-+-+-+-+-+-+
     Normal Block                      Drop Block

   The first byte in the first Data Dropped option refers to the packet
   indicated by the Acknowledgement Number; subsequent bytes refer to
   older packets.  Data Dropped MUST NOT be sent on DCCP-Data or DCCP-
   Request packets, which lack an Acknowledgement Number, and any Data
   Dropped options received on such packets MUST be ignored.

   Normal Blocks, which have high bit 0, indicate that any received
   packets in the Run Length had their data delivered to the
   application.  Drop Blocks, which have high bit 1, indicate that
   received packets in the Run Len[gth] were not delivered as usual.
   The 3-bit Drop Code [DrpCd] field says what happened; generally, no
   data from that packet reached the application.  Packets reported as
   "not yet received" MUST be included in Normal Blocks; packets not
   covered by any Data Dropped option are treated as if they were in a
   Normal Block.  Defined Drop Codes for Drop Blocks are as follows.

                  Drop Code  Meaning
                  ---------  -------
                      0      Protocol Constraints
                      1      Application Not Listening
                      2      Receive Buffer
                      3      Corrupt
                     4-6     Reserved
                      7      Delivered Corrupt

                   Table 7: DCCP Drop Codes





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   In more detail:

      0   The packet data was dropped due to protocol constraints.  For
          example, the data was included on a DCCP-Request packet, but
          the receiving application does not allow such piggybacking; or
          the data was included on a packet with inappropriately low
          Checksum Coverage.

      1   The packet data was dropped because the application is no
          longer listening.  See Section 11.7.2.

      2   The packet data was dropped in a receive buffer, probably
          because of receive buffer overflow.  See Section 11.7.2.

      3   The packet data was dropped due to corruption.  See Section
          9.3.

      7   The packet data was corrupted but was delivered to the
          application anyway.  See Section 9.3.

   For example, assume that a packet arrives with Acknowledgement Number
   100, an Ack Vector reporting all packets as received, and a Data
   Dropped option containing the decimal values 0,160,3,162.  Then:

      Packet 100 was received (Acknowledgement Number 100, Normal Block,
      Run Length 0).

      Packet 99 was dropped in a receive buffer (Drop Block, Drop Code
      2, Run Length 0).

      Packets 98, 97, 96, and 95 were received (Normal Block, Run Length
      3).

      Packets 95, 94, and 93 were dropped in the receive buffer (Drop
      Block, Drop Code 2, Run Length 2).

   Run lengths of more than 128 (for Normal Blocks) or 16 (for Drop
   Blocks) must be encoded in multiple Blocks.  A single Data Dropped
   option can acknowledge up to 32384 Normal Block data packets,
   although the receiver SHOULD NOT send a Data Dropped option when all
   relevant packets fit into Normal Blocks.  Should more packets need to
   be acknowledged than can fit in 253 bytes of Data Dropped, then
   multiple Data Dropped options can be sent.  The second option will
   begin where the first left off, and so forth.

   One or more Data Dropped options that, together, report the status of
   more packets than have been sent, or that change the status of a
   packet, or that disagree with Ack Vector or equivalent options (by



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   reporting a "not yet received" packet as "dropped in the receive
   buffer", for example) SHOULD be considered invalid.  The receiving
   DCCP SHOULD either ignore such options, or respond by resetting the
   connection with Reset Code 5, "Option Error".

   A DCCP application interface should let receiving applications
   specify the Drop Codes corresponding to received packets.  For
   example, this would let applications calculate their own checksums
   but still report "dropped due to corruption" packets via the Data
   Dropped option.  The interface SHOULD NOT let applications reduce the
   "seriousness" of a packet's Drop Code; for example, the application
   should not be able to upgrade a packet from delivered corrupt (Drop
   Code 7) to delivered normally (no Drop Code).

   Data Dropped information is transmitted reliably.  That is, endpoints
   SHOULD continue to transmit Data Dropped options until receiving an
   acknowledgement indicating that the relevant options have been
   processed.  In Ack Vector terms, each acknowledgement should contain
   Data Dropped options that cover the whole Acknowledgement Window
   (Section 11.4.2), although when every packet in that window would be
   placed in a Normal Block, no actual option is required.

11.7.1.  Data Dropped and Normal Congestion Response

   When deciding on a response to a particular acknowledgement or set of
   acknowledgements containing Data Dropped options, a congestion
   control mechanism MUST consider dropped packets, ECN Congestion
   Experienced marks (including marked packets that are included in Data
   Dropped), and packets singled out in Data Dropped.  For window-based
   mechanisms, the valid response space is defined as follows.

   Assume an old window of W.  Independently calculate a new window
   W_new1 that assumes no packets were Data Dropped (so W_new1 contains
   only the normal congestion response), and a new window W_new2 that
   assumes no packets were lost or marked (so W_new2 contains only the
   Data Dropped response).  We are assuming that Data Dropped
   recommended a reduction in congestion window, so W_new2 < W.

   Then the actual new window W_new MUST NOT be larger than the minimum
   of W_new1 and W_new2; and the sender MAY combine the two responses,
   by setting

         W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).

   The details of how this is accomplished are specified in CCID profile
   documents.  Non-window-based congestion control mechanisms MUST
   behave analogously; again, CCID profiles define how.




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11.7.2.  Particular Drop Codes

   Drop Code 0, Protocol Constraints, does not indicate any kind of
   congestion, so the sender's CCID SHOULD react to packets with Drop
   Code 0 as if they were received (with or without ECN Congestion
   Experienced marks, as appropriate).  However, the sending endpoint
   SHOULD NOT send data until it believes the protocol constraint no
   longer applies.

   Drop Code 1, Application Not Listening, means the application running
   at the endpoint that sent the option is no longer listening for data.
   For example, a server might close its receiving half-connection to
   new data after receiving a complete request from the client.  This
   would limit the amount of state available at the server for incoming
   data and thus reduce the potential damage from certain denial-of-
   service attacks.  A Data Dropped option containing Drop Code 1 SHOULD
   be sent whenever received data is ignored due to a non-listening
   application.  Once an endpoint reports Drop Code 1 for a packet, it
   SHOULD report Drop Code 1 for every succeeding data packet on that
   half-connection; once an endpoint receives a Drop State 1 report, it
   SHOULD expect that no more data will ever be delivered to the other
   endpoint's application, so it SHOULD NOT send more data.

   Drop Code 2, Receive Buffer, indicates congestion inside the
   receiving host.  For instance, if a drop-from-tail kernel socket
   buffer is too full to accept a packet's application data, that packet
   should be reported as Drop Code 2.  For a drop-from-head or more
   complex socket buffer, the dropped packet should be reported as Drop
   Code 2.  DCCP implementations may also provide an API by which
   applications can mark received packets as Drop Code 2, indicating
   that the application ran out of space in its user-level receive
   buffer.  (However, it is not generally useful to report packets as
   dropped due to Drop Code 2 after more than a couple of round-trip
   times have passed.  The HC-Sender may have forgotten its
   acknowledgement state for the packet by that time, so the Data
   Dropped report will have no effect.)  Every packet newly acknowledged
   as Drop Code 2 SHOULD reduce the sender's instantaneous rate by one
   packet per round-trip time, unless the sender is already sending one
   packet per RTT or less.  Each CCID profile defines the CCID-specific
   mechanism by which this is accomplished.

   Currently, the other Drop Codes (namely Drop Code 3, Corrupt; Drop
   Code 7, Delivered Corrupt; and reserved Drop Codes 4-6) MUST cause
   the relevant CCID to behave as if the relevant packets were ECN
   marked (ECN Congestion Experienced).






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12.  Explicit Congestion Notification

   The DCCP protocol is fully ECN-aware [RFC3168].  Each CCID specifies
   how its endpoints respond to ECN marks.  Furthermore, DCCP, unlike
   TCP, allows senders to control the rate at which acknowledgements are
   generated (with options like Ack Ratio); since acknowledgements are
   congestion controlled, they also qualify as ECN-Capable Transport.

   Each CCID profile describes how that CCID interacts with ECN, both
   for data traffic and pure-acknowledgement traffic.  A sender SHOULD
   set ECN-Capable Transport on its packets' IP headers unless the
   receiver's ECN Incapable feature is on or the relevant CCID disallows
   it.

   The rest of this section describes the ECN Incapable feature and the
   interaction of the ECN Nonce with acknowledgement options such as Ack
   Vector.

12.1.  ECN Incapable Feature

   DCCP endpoints are ECN-aware by default, but the ECN Incapable
   feature lets an endpoint reject the use of Explicit Congestion
   Notification.  The use of this feature is NOT RECOMMENDED.  ECN
   incapability both avoids ECN's possible benefits and prevents senders
   from using the ECN Nonce to check for receiver misbehavior.  A DCCP
   stack MAY therefore leave the ECN Incapable feature unimplemented,
   acting as if all connections were ECN capable.  Note that the
   inappropriate firewall interactions that dogged TCP's implementation
   of ECN [RFC3360] involve TCP header bits, not the IP header's ECN
   bits; we know of no middlebox that would block ECN-capable DCCP
   packets but allow ECN-incapable DCCP packets.

   ECN Incapable has feature number 4 and is server-priority.  It takes
   one-byte Boolean values.  DCCP A MUST be able to read ECN bits from
   received frames' IP headers when ECN Incapable/A is zero.  (This is
   independent of whether it can set ECN bits on sent frames.)  DCCP A
   thus sends a "Change L(ECN Inapable, 1)" option to DCCP B to inform
   it that A cannot read ECN bits.  If the ECN Incapable/A feature is
   one, then all of DCCP B's packets MUST be sent as ECN incapable.  New
   connections start with ECN Incapable 0 (that is, ECN capable) for
   both endpoints.  Values of two or more are reserved.

   If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN
   Incapable, 1)" options to the other endpoint until acknowledged (by
   "Confirm R(ECN Incapable, 1)") or the connection closes.
   Furthermore, it MUST NOT accept any data until the other endpoint





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   sends "Confirm R(ECN Incapable, 1)".  It SHOULD send Data Dropped
   options on its acknowledgements, with Drop Code 0 ("protocol
   constraints"), if the other endpoint does send data inappropriately.

12.2.  ECN Nonces

   Congestion avoidance will not occur, and the receiver will sometimes
   get its data faster, if the sender isn't told about congestion
   events.  Thus, the receiver has some incentive to falsify
   acknowledgement information, reporting that marked or dropped packets
   were actually received unmarked.  This problem is more serious with
   DCCP than with TCP, since TCP provides reliable transport: it is more
   difficult with TCP to lie about lost packets without breaking the
   application.

   ECN Nonces are a general mechanism to prevent ECN cheating (or loss
   cheating).  Two values for the two-bit ECN header field indicate
   ECN-Capable Transport, 01 and 10.  The second code point, 10, is the
   ECN Nonce.  In general, a protocol sender chooses between these code
   points randomly on its output packets, remembering the sequence it
   chose.  On every acknowledgement, the protocol receiver reports the
   number of ECN Nonces it has received thus far.  This is called the
   ECN Nonce Echo.  Since ECN marking and packet dropping both destroy
   the ECN Nonce, a receiver that lies about an ECN mark or packet drop
   has a 50% chance of guessing right and avoiding discipline.  The
   sender may react punitively to an ECN Nonce mismatch, possibly up to
   dropping the connection.  The ECN Nonce Echo field need not be an
   integer; one bit is enough to catch 50% of infractions, and the
   probability of success drops exponentially as more packets are sent
   [RFC3540].

   In DCCP, the ECN Nonce Echo field is encoded in acknowledgement
   options.  For example, the Ack Vector option comes in two forms, Ack
   Vector [Nonce 0] (option 38) and Ack Vector [Nonce 1] (option 39),
   corresponding to the two values for a one-bit ECN Nonce Echo.  The
   Nonce Echo for a given Ack Vector equals the one-bit sum (exclusive-
   or, or parity) of ECN nonces for packets reported by that Ack Vector
   as received and not ECN marked.  Thus, only packets marked as State 0
   matter for this calculation (that is, valid received packets that
   were not ECN marked).  Every Ack Vector option is detailed enough for
   the sender to determine what the Nonce Echo should have been.  It can
   check this calculation against the actual Nonce Echo and complain if
   there is a mismatch.  (The Ack Vector could conceivably report every
   packet's ECN Nonce state, but this would severely limit its
   compressibility without providing much extra protection.)

   Each DCCP sender SHOULD set ECN Nonces on its packets and remember
   which packets had nonces.  When a sender detects an ECN Nonce Echo



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   mismatch, it behaves as described in the next section.  Each DCCP
   receiver MUST calculate and use the correct value for ECN Nonce Echo
   when sending acknowledgement options.

   ECN incapability, as indicated by the ECN Incapable feature, is
   handled as follows: an endpoint sending packets to an ECN-incapable
   receiver MUST send its packets as ECN incapable, and an ECN-
   incapable receiver MUST use the value zero for all ECN Nonce Echoes.

12.3.  Aggression Penalties

   DCCP endpoints have several mechanisms for detecting congestion-
   related misbehavior.  For example:

   o  A sender can detect an ECN Nonce Echo mismatch, indicating
      possible receiver misbehavior.

   o  A receiver can detect whether the sender is responding to
      congestion feedback or Slow Receiver.

   o  An endpoint may be able to detect that its peer is reporting
      inappropriately small Elapsed Time values (Section 13.2).

   An endpoint that detects possible congestion-related misbehavior
   SHOULD try to verify that its peer is truly misbehaving.  For
   example, a sending endpoint might send a packet whose ECN header
   field is set to Congestion Experienced, 11; a receiver that doesn't
   report a corresponding mark is most likely misbehaving.

   Upon detecting possible misbehavior, a sender SHOULD respond as if
   the receiver had reported one or more recent packets as ECN-marked
   (instead of unmarked), while a receiver SHOULD report one or more
   recent non-marked packets as ECN-marked.  Alternately, a sender might
   act as if the receiver had sent a Slow Receiver option, and a
   receiver might send Slow Receiver options.  Other reactions that
   serve to slow the transfer rate are also acceptable.  An entity that
   detects particularly egregious and ongoing misbehavior MAY also reset
   the connection with Reset Code 11, "Aggression Penalty".

   However, ECN Nonce mismatches and other warning signs can result from
   innocent causes, such as implementation bugs or attack.  In
   particular, a successful DCCP-Data attack (Section 7.5.5) can cause
   the receiver to report an incorrect ECN Nonce Echo.  Therefore,
   connection reset and other heavyweight mechanisms SHOULD be used only
   as last resorts, after multiple round-trip times of verified
   aggression.





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13.  Timing Options

   The Timestamp, Timestamp Echo, and Elapsed Time options help DCCP
   endpoints explicitly measure round-trip times.

13.1.  Timestamp Option

   This option is permitted in any DCCP packet.  The length of the
   option is 6 bytes.

   +--------+--------+--------+--------+--------+--------+
   |00101001|00000110|          Timestamp Value          |
   +--------+--------+--------+--------+--------+--------+
    Type=41  Length=6

   The four bytes of option data carry the timestamp of this packet.
   The timestamp is a 32-bit integer that increases monotonically with
   time, at a rate of 1 unit per 10 microseconds.  At this rate,
   Timestamp Value will wrap approximately every 11.9 hours.  Endpoints
   need not measure time at this fine granularity; for example, an
   endpoint that preferred to measure time at millisecond granularity
   might send Timestamp Values that were all multiples of 100.  The
   precise time corresponding to Timestamp Value zero is not specified:
   Timestamp Values are only meaningful relative to other Timestamp
   Values sent on the same connection.  A DCCP receiving a Timestamp
   option SHOULD respond with a Timestamp Echo option on the next packet
   it sends.

13.2.  Elapsed Time Option

   This option is permitted in any DCCP packet that contains an
   Acknowledgement Number; such options received on other packet types
   MUST be ignored.  It indicates how much time has elapsed since the
   packet being acknowledged -- the packet with the given
   Acknowledgement Number -- was received.  The option may take 4 or 6
   bytes, depending on the size of the Elapsed Time value.  Elapsed Time
   helps correct round-trip time estimates when the gap between
   receiving a packet and acknowledging that packet may be long -- in
   CCID 3, for example, where acknowledgements are sent infrequently.












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   +--------+--------+--------+--------+
   |00101011|00000100|   Elapsed Time  |
   +--------+--------+--------+--------+
    Type=43    Len=4

   +--------+--------+--------+--------+--------+--------+
   |00101011|00000110|            Elapsed Time           |
   +--------+--------+--------+--------+--------+--------+
    Type=43    Len=6

   The option data, Elapsed Time, represents an estimated lower bound on
   the amount of time elapsed since the packet being acknowledged was
   received, with units of hundredths of milliseconds.  If Elapsed Time
   is less than a half-second, the first, smaller form of the option
   SHOULD be used.  Elapsed Times of more than 0.65535 seconds MUST be
   sent using the second form of the option.  The special Elapsed Time
   value 4294967295, which corresponds to approximately 11.9 hours, is
   used to represent any Elapsed Time greater than 42949.67294 seconds.
   DCCP endpoints MUST NOT report Elapsed Times that are significantly
   larger than the true elapsed times.  A connection MAY be reset with
   Reset Code 11, "Aggression Penalty", if one endpoint determines that
   the other is reporting a much-too-large Elapsed Time.

   Elapsed Time is measured in hundredths of milliseconds as a
   compromise between two conflicting goals.  First, it provides enough
   granularity to reduce rounding error when measuring elapsed time over
   fast LANs; second, it allows many reasonable elapsed times to fit
   into two bytes of data.

13.3.  Timestamp Echo Option

   This option is permitted in any DCCP packet, as long as at least one
   packet carrying the Timestamp option has been received.  Generally, a
   DCCP endpoint should send one Timestamp Echo option for each
   Timestamp option it receives, and it should send that option as soon
   as is convenient.  The length of the option is between 6 and 10
   bytes, depending on whether Elapsed Time is included and how large it
   is.













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   +--------+--------+--------+--------+--------+--------+
   |00101010|00000110|           Timestamp Echo          |
   +--------+--------+--------+--------+--------+--------+
    Type=42    Len=6

   +--------+--------+------- ... -------+--------+--------+
   |00101010|00001000|  Timestamp Echo   |   Elapsed Time  |
   +--------+--------+------- ... -------+--------+--------+
    Type=42    Len=8       (4 bytes)

   +--------+--------+------- ... -------+------- ... -------+
   |00101010|00001010|  Timestamp Echo   |    Elapsed Time   |
   +--------+--------+------- ... -------+------- ... -------+
    Type=42   Len=10       (4 bytes)           (4 bytes)

   The first four bytes of option data, Timestamp Echo, carry a
   Timestamp Value taken from a preceding received Timestamp option.
   Usually, this will be the last packet that was received -- the packet
   indicated by the Acknowledgement Number, if any -- but it might be a
   preceding packet.  Each Timestamp received will generally result in
   exactly one Timestamp Echo transmitted.  If an endpoint has received
   multiple Timestamp options since the last time it sent a packet, then
   it MAY ignore all Timestamp options but the one included on the
   packet with the greatest sequence number.  Alternatively, it MAY
   include multiple Timestamp Echo options in its response, each
   corresponding to a different Timestamp option.

   The Elapsed Time value, similar to that in the Elapsed Time option,
   indicates the amount of time elapsed since receiving the packet whose
   timestamp is being echoed.  This time MUST have units of hundredths
   of milliseconds.  Elapsed Time is meant to help the Timestamp sender
   separate the network round-trip time from the Timestamp receiver's
   processing time.  This may be particularly important for CCIDs where
   acknowledgements are sent infrequently, so that there might be
   considerable delay between receiving a Timestamp option and sending
   the corresponding Timestamp Echo.  A missing Elapsed Time field is
   equivalent to an Elapsed Time of zero.  The smallest version of the
   option SHOULD be used that can hold the relevant Elapsed Time value.

14.  Maximum Packet Size

   A DCCP implementation MUST maintain the maximum packet size (MPS)
   allowed for each active DCCP session.  The MPS is influenced by the
   maximum packet size allowed by the current congestion control
   mechanism (CCMPS), the maximum packet size supported by the path's
   links (PMTU, the Path Maximum Transmission Unit) [RFC1191], and the
   lengths of the IP and DCCP headers.




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   A DCCP application interface SHOULD let the application discover
   DCCP's current MPS.  Generally, the DCCP implementation will refuse
   to send any packet bigger than the MPS, returning an appropriate
   error to the application.  A DCCP interface MAY allow applications to
   request fragmentation for packets larger than PMTU, but not larger
   than CCMPS.  (Packets larger than CCMPS MUST be rejected in any
   case.)  Fragmentation SHOULD NOT be the default, since it decreases
   robustness: an entire packet is discarded if even one of its
   fragments is lost.  Applications can usually get better error
   tolerance by producing packets smaller than the PMTU.

   The MPS reported to the application SHOULD be influenced by the size
   expected to be required for DCCP headers and options.  If the
   application provides data that, when combined with the options the
   DCCP implementation would like to include, would exceed the MPS, the
   implementation should either send the options on a separate packet
   (such as a DCCP-Ack) or lower the MPS, drop the data, and return an
   appropriate error to the application.

14.1.  Measuring PMTU

   Each DCCP endpoint MUST keep track of the current PMTU for each
   connection, except that this is not required for IPv4 connections
   whose applications have requested fragmentation.  The PMTU SHOULD be
   initialized from the interface MTU that will be used to send packets.
   The MPS will be initialized with the minimum of the PMTU and the
   CCMPS, if any.

   Classical PMTU discovery uses unfragmentable packets.  In IPv4, these
   packets have the IP Don't Fragment (DF) bit set; in IPv6, all packets
   are unfragmentable once emitted by an end host.  As specified in
   [RFC1191], when a router receives a packet with DF set that is larger
   than the next link's MTU, it sends an ICMP Destination Unreachable
   message back to the source whose Code indicates that an
   unfragmentable packet was too large to forward (a "Datagram Too Big"
   message).  When a DCCP implementation receives a Datagram Too Big
   message, it decreases its PMTU to the Next-Hop MTU value given in the
   ICMP message.  If the MTU given in the message is zero, the sender
   chooses a value for PMTU using the algorithm described in [RFC1191],
   Section 7.  If the MTU given in the message is greater than the
   current PMTU, the Datagram Too Big message is ignored, as described
   in [RFC1191].  (We are aware that this may cause problems for DCCP
   endpoints behind certain firewalls.)

   A DCCP implementation may allow the application occasionally to
   request that PMTU discovery be performed again.  This will reset the
   PMTU to the outgoing interface's MTU.  Such requests SHOULD be rate
   limited, to one per two seconds, for example.



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   A DCCP sender MAY treat the reception of an ICMP Datagram Too Big
   message as an indication that the packet being reported was not lost
   due to congestion, and so for the purposes of congestion control it
   MAY ignore the DCCP receiver's indication that this packet did not
   arrive.  However, if this is done, then the DCCP sender MUST check
   the ECN bits of the IP header echoed in the ICMP message and only
   perform this optimization if these ECN bits indicate that the packet
   did not experience congestion prior to reaching the router whose link
   MTU it exceeded.

   A DCCP implementation SHOULD ensure, as far as possible, that ICMP
   Datagram Too Big messages were actually generated by routers, so that
   attackers cannot drive the PMTU down to a falsely small value.  The
   simplest way to do this is to verify that the Sequence Number on the
   ICMP error's encapsulated header corresponds to a Sequence Number
   that the implementation recently sent.  (According to current
   specifications, routers should return the full DCCP header and
   payload up to a maximum of 576 bytes [RFC1812] or the minimum IPv6
   MTU [RFC2463], although they are not required to return more than 64
   bits [RFC792].  Any amount greater than 128 bits will include the
   Sequence Number.)  ICMP Datagram Too Big messages with incorrect or
   missing Sequence Numbers may be ignored, or the DCCP implementation
   may lower the PMTU only temporarily in response.  If more than three
   odd Datagram Too Big messages are received and the other DCCP
   endpoint reports more than three lost packets, however, the DCCP
   implementation SHOULD assume the presence of a confused router and
   either obey the ICMP messages' PMTU or (on IPv4 networks) switch to
   allowing fragmentation.

   DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP
   endpoint begins by sending small packets with DF set and then
   gradually increases the packet size until a packet is lost.  This
   mechanism does not require any ICMP error processing.  DCCP-Sync
   packets are the best choice for upward probing, since DCCP-Sync
   probes do not risk application data loss.  The DCCP implementation
   inserts arbitrary data into the DCCP-Sync application area, padding
   the packet to the right length.  Since every valid DCCP-Sync
   generates an immediate DCCP-SyncAck in response, the endpoint will
   have a pretty good idea of when a probe is lost.

14.2.  Sender Behavior

   A DCCP sender SHOULD send every packet as unfragmentable, as
   described above, with the following exceptions.

   o  On IPv4 connections whose applications have requested
      fragmentation, the sender SHOULD send packets with the DF bit not
      set.



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   o  On IPv6 connections whose applications have requested
      fragmentation, the sender SHOULD use fragmentation extension
      headers to fragment packets larger than PMTU into suitably-sized
      chunks.  (Those chunks are, of course, unfragmentable.)

   o  It is undesirable for PMTU discovery to occur on the initial
      connection setup handshake, as the connection setup process may
      not be representative of packet sizes used during the connection,
      and performing MTU discovery on the initial handshake might
      unnecessarily delay connection establishment.  Thus, DCCP-Request
      and DCCP-Response packets SHOULD be sent as fragmentable.  In
      addition, DCCP-Reset packets SHOULD be sent as fragmentable,
      although typically these would be small enough to not be a
      problem.  For IPv4 connections, these packets SHOULD be sent with
      the DF bit not set; for IPv6 connections, they SHOULD be
      preemptively fragmented to a size not larger than the relevant
      interface MTU.

   If the DCCP implementation has decreased the PMTU, the sending
   application has not requested fragmentation, and the sending
   application attempts to send a packet larger than the new MPS, the
   API MUST refuse to send the packet and return an appropriate error to
   the application.  The application should then use the API to query
   the new value of MPS.  The kernel might have some packets buffered
   for transmission that are smaller than the old MPS but larger than
   the new MPS.  It MAY send these packets as fragmentable, or it MAY
   discard these packets; it MUST NOT send them as unfragmentable.

15.  Forward Compatibility

   Future versions of DCCP may add new options and features.  A few
   simple guidelines will let extended DCCPs interoperate with normal
   DCCPs.

   o  DCCP processors MUST NOT act punitively towards options and
      features they do not understand.  For example, DCCP processors
      MUST NOT reset the connection if some field marked Reserved in
      this specification is non-zero; if some unknown option is present;
      or if some feature negotiation option mentions an unknown feature.
      Instead, DCCP processors MUST ignore these events.  The Mandatory
      option is the single exception: if Mandatory precedes some unknown
      option or feature, the connection MUST be reset.

   o  DCCP processors MUST anticipate the possibility of unknown feature
      values, which might occur as part of a negotiation for a known
      feature.  For server-priority features, unknown values are handled
      as a matter of course: since the non-extended DCCP's priority list
      will not contain unknown values, the result of the negotiation



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      cannot be an unknown value.  A DCCP MUST respond with an empty
      Confirm option if it is assigned an unacceptable value for some
      non-negotiable feature.

   o  Each DCCP extension SHOULD be controlled by some feature.  The
      default value of this feature SHOULD correspond to "extension not
      available".  If an extended DCCP wants to use the extension, it
      SHOULD attempt to change the feature's value using a Change L or
      Change R option.  Any non-extended DCCP will ignore the option,
      thus leaving the feature value at its default, "extension not
      available".

   Section 19 lists DCCP assigned numbers reserved for experimental and
   testing purposes.

16.  Middlebox Considerations

   This section describes properties of DCCP that firewalls, network
   address translators, and other middleboxes should consider, including
   parts of the packet that middleboxes should not change.  The intent
   is to draw attention to aspects of DCCP that may be useful, or
   dangerous, for middleboxes, or that differ significantly from TCP.

   The Service Code field in DCCP-Request packets provides information
   that may be useful for stateful middleboxes.  With Service Code, a
   middlebox can tell what protocol a connection will use without
   relying on port numbers.  Middleboxes can disallow connections that
   attempt to access unexpected services by sending a DCCP-Reset with
   Reset Code 8, "Bad Service Code".  Middleboxes should not modify the
   Service Code unless they are really changing the service a connection
   is accessing.

   The Source and Destination Port fields are in the same packet
   locations as the corresponding fields in TCP and UDP, which may
   simplify some middlebox implementations.

   The forward compatibility considerations in Section 15 apply to
   middleboxes as well.  In particular, middleboxes generally shouldn't
   act punitively towards options and features they do not understand.

   Modifying DCCP Sequence Numbers and Acknowledgement Numbers is more
   tedious and dangerous than modifying TCP sequence numbers.  A
   middlebox that added packets to or removed packets from a DCCP
   connection would have to modify acknowledgement options, such as Ack
   Vector, and CCID-specific options, such as TFRC's Loss Intervals, at
   minimum.  On ECN-capable connections, the middlebox would have to
   keep track of ECN Nonce information for packets it introduced or
   removed, so that the relevant acknowledgement options continued to



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   have correct ECN Nonce Echoes, or risk the connection being reset for
   "Aggression Penalty".  We therefore recommend that middleboxes not
   modify packet streams by adding or removing packets.

   Note that there is less need to modify DCCP's per-packet sequence
   numbers than to modify TCP's per-byte sequence numbers; for example,
   a middlebox can change the contents of a packet without changing its
   sequence number.  (In TCP, sequence number modification is required
   to support protocols like FTP that carry variable-length addresses in
   the data stream.  If such an application were deployed over DCCP,
   middleboxes would simply grow or shrink the relevant packets as
   necessary without changing their sequence numbers.  This might
   involve fragmenting the packet.)

   Middleboxes may, of course, reset connections in progress.  Clearly,
   this requires inserting a packet into one or both packet streams, but
   the difficult issues do not arise.

   DCCP is somewhat unfriendly to "connection splicing" [SHHP00], in
   which clients' connection attempts are intercepted, but possibly
   later "spliced in" to external server connections via sequence number
   manipulations.  A connection splicer at minimum would have to ensure
   that the spliced connections agreed on all relevant feature values,
   which might take some renegotiation.

   The contents of this section should not be interpreted as a wholesale
   endorsement of stateful middleboxes.

17.  Relations to Other Specifications

17.1.  RTP

   The Real-Time Transport Protocol, RTP [RFC3550], is currently used
   over UDP by many of DCCP's target applications (for instance,
   streaming media).  Therefore, it is important to examine the
   relationship between DCCP and RTP and, in particular, the question of
   whether any changes in RTP are necessary or desirable when it is
   layered over DCCP instead of UDP.

   There are two potential sources of overhead in the RTP-over-DCCP
   combination: duplicated acknowledgement information and duplicated
   sequence numbers.  Together, these sources of overhead add slightly
   more than 4 bytes per packet relative to RTP-over-UDP, and
   eliminating the redundancy would not reduce the overhead.

   First, consider acknowledgements.  Both RTP and DCCP report feedback
   about loss rates to data senders, via RTP Control Protocol Sender and
   Receiver Reports (RTCP SR/RR packets) and via DCCP acknowledgement



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   options.  These feedback mechanisms are potentially redundant.
   However, RTCP SR/RR packets contain information not present in DCCP
   acknowledgements, such as "interarrival jitter", and DCCP's
   acknowledgements contain information not transmitted by RTCP, such as
   the ECN Nonce Echo.  Neither feedback mechanism makes the other
   redundant.

   Sending both types of feedback need not be particularly costly
   either.  RTCP reports may be sent relatively infrequently: once every
   5 seconds on average, for low-bandwidth flows.  In DCCP, some
   feedback mechanisms are expensive -- Ack Vector, for example, is
   frequent and verbose -- but others are relatively cheap: CCID 3
   (TFRC) acknowledgements take between 16 and 32 bytes of options sent
   once per round-trip time.  (Reporting less frequently than once per
   RTT would make congestion control less responsive to loss.)  We
   therefore conclude that acknowledgement overhead in RTP-over-DCCP
   need not be significantly higher than for RTP-over-UDP, at least for
   CCID 3.

   One clear redundancy can be addressed at the application level.  The
   verbose packet-by-packet loss reports sent in RTCP Extended Reports
   Loss RLE Blocks [RFC3611] can be derived from DCCP's Ack Vector
   options.  (The converse is not true, since Loss RLE Blocks contain no
   ECN information.)  Since DCCP implementations should provide an API
   for application access to Ack Vector information, RTP-over-DCCP
   applications might request either DCCP Ack Vectors or RTCP Extended
   Report Loss RLE Blocks, but not both.

   Now consider sequence number redundancy on data packets.  The
   embedded RTP header contains a 16-bit RTP sequence number.  Most data
   packets will use the DCCP-Data type; DCCP-DataAck and DCCP-Ack
   packets need not usually be sent.  The DCCP-Data header is 12 bytes
   long without options, including a 24-bit sequence number.  This is 4
   bytes more than a UDP header.  Any options required on data packets
   would add further overhead, although many CCIDs (for instance, CCID
   3, TFRC) don't require options on most data packets.

   The DCCP sequence number cannot be inferred from the RTP sequence
   number since it increments on non-data packets as well as data
   packets.  The RTP sequence number cannot be inferred from the DCCP
   sequence number either [RFC3550].  Furthermore, removing RTP's
   sequence number would not save any header space because of alignment
   issues.  We therefore recommend that RTP transmitted over DCCP use
   the same headers currently defined.  The 4 byte header cost is a
   reasonable tradeoff for DCCP's congestion control features and access
   to ECN.  Truly bandwidth-starved endpoints should use some header
   compression scheme.




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17.2.  Congestion Manager and Multiplexing

   Since DCCP doesn't provide reliable, ordered delivery, multiple
   application sub-flows may be multiplexed over a single DCCP
   connection with no inherent performance penalty.  Thus, there is no
   need for DCCP to provide built-in support for multiple sub-flows.
   This differs from SCTP [RFC2960].

   Some applications might want to share congestion control state among
   multiple DCCP flows that share the same source and destination
   addresses.  This functionality could be provided by the Congestion
   Manager [RFC3124], a generic multiplexing facility.  However, the CM
   would not fully support DCCP without change; it does not gracefully
   handle multiple congestion control mechanisms, for example.

18.  Security Considerations

   DCCP does not provide cryptographic security guarantees.
   Applications desiring cryptographic security services (integrity,
   authentication, confidentiality, access control, and anti-replay
   protection) should use IPsec or end-to-end security of some kind;
   Secure RTP is one candidate protocol [RFC3711].

   Nevertheless, DCCP is intended to protect against some classes of
   attackers: Attackers cannot hijack a DCCP connection (close the
   connection unexpectedly, or cause attacker data to be accepted by an
   endpoint as if it came from the sender) unless they can guess valid
   sequence numbers.  Thus, as long as endpoints choose initial sequence
   numbers well, a DCCP attacker must snoop on data packets to get any
   reasonable probability of success.  Sequence number validity checks
   provide this guarantee.  Section 7.5.5 describes sequence number
   security further.  This security property only holds assuming that
   DCCP's random numbers are chosen according to the guidelines in
   [RFC4086].

   DCCP also provides mechanisms to limit the potential impact of some
   denial-of-service attacks.  These mechanisms include Init Cookie
   (Section 8.1.4), the DCCP-CloseReq packet (Section 5.5), the
   Application Not Listening Drop Code (Section 11.7.2), limitations on
   the processing of options that might cause connection reset (Section
   7.5.5), limitations on the processing of some ICMP messages (Section
   14.1), and various rate limits, which let servers avoid extensive
   computation or packet generation (Sections 7.5.3, 8.1.3, and others).

   DCCP provides no protection against attackers that can snoop on data
   packets.





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18.1.  Security Considerations for Partial Checksums

   The partial checksum facility has a separate security impact,
   particularly in its interaction with authentication and encryption
   mechanisms.  The impact is the same in DCCP as in the UDP-Lite
   protocol, and what follows was adapted from the corresponding text in
   the UDP-Lite specification [RFC3828].

   When a DCCP packet's Checksum Coverage field is not zero, the
   uncovered portion of a packet may change in transit.  This is
   contrary to the idea behind most authentication mechanisms:
   authentication succeeds if the packet has not changed in transit.
   Unless authentication mechanisms that operate only on the sensitive
   part of packets are developed and used, authentication will always
   fail for partially-checksummed DCCP packets whose uncovered part has
   been damaged.

   The IPsec integrity check (Encapsulation Security Protocol, ESP, or
   Authentication Header, AH) is applied (at least) to the entire IP
   packet payload.  Corruption of any bit within that area will then
   result in the IP receiver's discarding a DCCP packet, even if the
   corruption happened in an uncovered part of the DCCP application
   data.

   When IPsec is used with ESP payload encryption, a link can not
   determine the specific transport protocol of a packet being forwarded
   by inspecting the IP packet payload.  In this case, the link MUST
   provide a standard integrity check covering the entire IP packet and
   payload.  DCCP partial checksums provide no benefit in this case.

   Encryption (e.g., at the transport or application levels) may be
   used.  Note that omitting an integrity check can, under certain
   circumstances, compromise confidentiality [B98].

   If a few bits of an encrypted packet are damaged, the decryption
   transform will typically spread errors so that the packet becomes too
   damaged to be of use.  Many encryption transforms today exhibit this
   behavior.  There exist encryption transforms, stream ciphers, that do
   not cause error propagation.  Proper use of stream ciphers can be
   quite difficult, especially when authentication checking is omitted
   [BB01].  In particular, an attacker can cause predictable changes to
   the ultimate plaintext, even without being able to decrypt the
   ciphertext.








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19.  IANA Considerations

   IANA has assigned IP Protocol Number 33 to DCCP.

   DCCP introduces eight sets of numbers whose values should be
   allocated by IANA.  We refer to allocation policies, such as
   Standards Action, outlined in [RFC2434], and most registries reserve
   some values for experimental and testing use [RFC3692].  In addition,
   DCCP requires that the IANA Port Numbers registry be opened for DCCP
   port registrations; Section 19.9 describes how.  The IANA should feel
   free to contact the DCCP Expert Reviewer with questions on any
   registry, regardless of the registry policy, for clarification or if
   there is a problem with a request.

19.1.  Packet Types Registry

   Each entry in the DCCP Packet Types registry contains a packet type,
   which is a number in the range 0-15; a packet type name, such as
   DCCP-Request; and a reference to the RFC defining the packet type.
   The registry is initially populated using the values in Table 1
   (Section 5.1).  This document allocates packet types 0-9, and packet
   type 14 is permanently reserved for experimental and testing use.
   Packet types 10-13 and 15 are currently reserved and should be
   allocated with the Standards Action policy, which requires IESG
   review and approval and standards-track IETF RFC publication.

19.2.  Reset Codes Registry

   Each entry in the DCCP Reset Codes registry contains a Reset Code,
   which is a number in the range 0-255; a short description of the
   Reset Code, such as "No Connection"; and a reference to the RFC
   defining the Reset Code.  The registry is initially populated using
   the values in Table 2 (Section 5.6).  This document allocates Reset
   Codes 0-11, and Reset Codes 120-126 are permanently reserved for
   experimental and testing use.  Reset Codes 12-119 and 127 are
   currently reserved and should be allocated with the IETF Consensus
   policy, requiring an IETF RFC publication (standards track or not)
   with IESG review and approval.  Reset Codes 128-255 are permanently
   reserved for CCID-specific registries; each CCID Profile document
   describes how the corresponding registry is managed.

19.3.  Option Types Registry

   Each entry in the DCCP option types registry contains an option type,
   which is a number in the range 0-255; the name of the option, such as
   "Slow Receiver"; and a reference to the RFC defining the option type.
   The registry is initially populated using the values in Table 3
   (Section 5.8).  This document allocates option types 0-2 and 32-44,



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   and option types 31 and 120-126 are permanently reserved for
   experimental and testing use.  Option types 3-30, 45-119, and 127 are
   currently reserved and should be allocated with the IETF Consensus
   policy, requiring an IETF RFC publication (standards track or not)
   with IESG review and approval.  Option types 128-255 are permanently
   reserved for CCID-specific registries; each CCID Profile document
   describes how the corresponding registry is managed.

19.4.  Feature Numbers Registry

   Each entry in the DCCP feature numbers registry contains a feature
   number, which is a number in the range 0-255; the name of the
   feature, such as "ECN Incapable"; and a reference to the RFC defining
   the feature number.  The registry is initially populated using the
   values in Table 4 (Section 6).  This document allocates feature
   numbers 0-9, and feature numbers 120-126 are permanently reserved for
   experimental and testing use.  Feature numbers 10-119 and 127 are
   currently reserved and should be allocated with the IETF Consensus
   policy, requiring an IETF RFC publication (standards track or not)
   with IESG review and approval.  Feature numbers 128-255 are
   permanently reserved for CCID-specific registries; each CCID Profile
   document describes how the corresponding registry is managed.

19.5.  Congestion Control Identifiers Registry

   Each entry in the DCCP Congestion Control Identifiers (CCIDs)
   registry contains a CCID, which is a number in the range 0-255; the
   name of the CCID, such as "TCP-like Congestion Control"; and a
   reference to the RFC defining the CCID.  The registry is initially
   populated using the values in Table 5 (Section 10).  CCIDs 2 and 3
   are allocated by concurrently published profiles, and CCIDs 248-254
   are permanently reserved for experimental and testing use.  CCIDs 0,
   1, 4-247, and 255 are currently reserved and should be allocated with
   the IETF Consensus policy, requiring an IETF RFC publication
   (standards track or not) with IESG review and approval.

19.6.  Ack Vector States Registry

   Each entry in the DCCP Ack Vector States registry contains an Ack
   Vector State, which is a number in the range 0-3; the name of the
   State, such as "Received ECN Marked"; and a reference to the RFC
   defining the State.  The registry is initially populated using the
   values in Table 6 (Section 11.4).  This document allocates States 0,
   1, and 3.  State 2 is currently reserved and should be allocated with
   the Standards Action policy, which requires IESG review and approval
   and standards-track IETF RFC publication.





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19.7.  Drop Codes Registry

   Each entry in the DCCP Drop Codes registry contains a Data Dropped
   Drop Code, which is a number in the range 0-7; the name of the Drop
   Code, such as "Application Not Listening"; and a reference to the RFC
   defining the Drop Code.  The registry is initially populated using
   the values in Table 7 (Section 11.7).  This document allocates Drop
   Codes 0-3 and 7.  Drop Codes 4-6 are currently reserved, and should
   be allocated with the Standards Action policy, which requires IESG
   review and approval and standards-track IETF RFC publication.

19.8.  Service Codes Registry

   Each entry in the Service Codes registry contains a Service Code,
   which is a number in the range 0-4294967294; a short English
   description of the intended service; and an optional reference to an
   RFC or other publicly available specification defining the Service
   Code.  The registry should list the Service Code's numeric value as a
   decimal number.  When the Service Code may be represented in "SC:"
   format according to the rules in Section 8.1.2, the registry should
   also show the corresponding ASCII interpretation of the Service Code
   minus the "SC:" prefix.  Thus, the number 1717858426 would
   additionally appear as "fdpz".  Service Codes are not DCCP-specific.
   Service Code 0 is permanently reserved (it represents the absence of
   a meaningful Service Code), and Service Codes 1056964608-1073741823
   (high byte ASCII "?") are reserved for Private Use.  Note that
   4294967295 is not a valid Service Code.  Most of the remaining
   Service Codes are allocated First Come First Served, with no RFC
   publication required; exceptions are listed in Section 8.1.2.  This
   document allocates a single Service Code, 1145656131 ("DISC").  This
   corresponds to the discard service, which discards all data sent to
   the service and sends no data in reply.

19.9.  Port Numbers Registry

   DCCP services may use contact port numbers to provide service to
   unknown callers, as in TCP and UDP.  IANA is therefore requested to
   open the existing Port Numbers registry for DCCP using the following
   rules, which we intend to mesh well with existing Port Numbers
   registration procedures.

   Port numbers are divided into three ranges.  The Well Known Ports are
   those from 0 through 1023, the Registered Ports are those from 1024
   through 49151, and the Dynamic and/or Private Ports are those from
   49152 through 65535.  Well Known and Registered Ports are intended
   for use by server applications that desire a default contact point on
   a system.  On most systems, Well Known Ports can only be used by
   system (or root) processes or by programs executed by privileged



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   users, while Registered Ports can be used by ordinary user processes
   or programs executed by ordinary users.  Dynamic and/or Private Ports
   are intended for temporary use, including client-side ports, out-of-
   band negotiated ports, and application testing prior to registration
   of a dedicated port; they MUST NOT be registered.

   The Port Numbers registry should accept registrations for DCCP ports
   in the Well Known Ports and Registered Ports ranges.  Well Known and
   Registered Ports SHOULD NOT be used without registration.  Although
   in some cases -- such as porting an application from UDP to DCCP --
   it may seem natural to use a DCCP port before registration completes,
   we emphasize that IANA will not guarantee registration of particular
   Well Known and Registered Ports.  Registrations should be requested
   as early as possible.

   Each port registration SHALL include the following information:

   o  A short port name, consisting entirely of letters (A-Z and a-z),
      digits (0-9), and punctuation characters from "-_+./*" (not
      including the quotes).

   o  The port number that is requested to be registered.

   o  A short English phrase describing the port's purpose.  This MUST
      include one or more space-separated textual Service Code
      descriptors naming the port's corresponding Service Codes (see
      Section 8.1.2).

   o  Name and contact information for the person or entity performing
      the registration, and possibly a reference to a document defining
      the port's use.  Registrations coming from IETF working groups
      need only name the working group, but indicating a contact person
      is recommended.

   Registrants are encouraged to follow these guidelines when submitting
   a registration.

   o  A port name SHOULD NOT be registered for more than one DCCP port
      number.

   o  A port name registered for UDP MAY be registered for DCCP as well.
      Any such registration SHOULD use the same port number as the
      existing UDP registration.

   o  Concrete intent to use a port SHOULD precede port registration.
      For example, existing UDP ports SHOULD NOT be registered in
      advance of any intent to use those ports for DCCP.




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   o  A port name generally associated with TCP and/or SCTP SHOULD NOT
      be registered for DCCP, since that port name implies reliable
      transport.  For example, we discourage registration of any "http"
      port for DCCP.  However, if such a registration makes sense (that
      is, if there is concrete intent to use such a port), the DCCP
      registration SHOULD use the same port number as the existing
      registration.

   o  Multiple DCCP registrations for the same port number are allowed
      as long as the registrations' Service Codes do not overlap.

   This document registers the following port.  (This should be
   considered a model registration.)

   discard    9/dccp    Discard SC:DISC
   # IETF dccp WG, Eddie Kohler <kohler@cs.ucla.edu>, [RFC4340]

   The discard service, which accepts DCCP connections on port 9,
   discards all incoming application data and sends no data in response.
   Thus, DCCP's discard port is analogous to TCP's discard port, and
   might be used to check the health of a DCCP stack.

20.  Thanks

   Thanks to Jitendra Padhye for his help with early versions of this
   specification.

   Thanks to Junwen Lai and Arun Venkataramani, who, as interns at ICIR,
   built a prototype DCCP implementation.  In particular, Junwen Lai
   recommended that the old feature negotiation mechanism be scrapped
   and co-designed the current mechanism.  Arun Venkataramani's feedback
   improved Appendix A.

   We thank the staff and interns of ICIR and, formerly, ACIRI, the
   members of the End-to-End Research Group, and the members of the
   Transport Area Working Group for their feedback on DCCP.  We
   especially thank the DCCP expert reviewers Greg Minshall, Eric
   Rescorla, and Magnus Westerlund for detailed written comments and
   problem spotting, and Rob Austein and Steve Bellovin for verbal
   comments and written notes.  We also especially thank Aaron Falk, the
   working group chair during the development of this specification.

   We also thank those who provided comments and suggestions via the
   DCCP BOF, Working Group, and mailing lists, including Damon Lanphear,
   Patrick McManus, Colin Perkins, Sara Karlberg, Kevin Lai, Bernard
   Aboba, Youngsoo Choi, Pengfei Di, Dan Duchamp, Lars Eggert, Gorry
   Fairhurst, Derek Fawcus, David Timothy Fleeman, John Loughney,
   Ghyslain Pelletier, Hagen Paul Pfeifer, Tom Phelan, Stanislav



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   Shalunov, Somsak Vanit-Anunchai, David Vos, Yufei Wang, and Michael
   Welzl.  In particular, Colin Perkins provided extensive, detailed
   feedback, Michael Welzl suggested the Data Checksum option, Gorry
   Fairhurst provided extensive feedback on various checksum issues, and
   Somsak Vanit-Anunchai, Jonathan Billington, and Tul Kongprakaiwoot's
   Colored Petri Net model [VBK05] discovered several problems with
   message exchange.












































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A.  Appendix: Ack Vector Implementation Notes

   This appendix discusses particulars of DCCP acknowledgement handling
   in the context of an abstract implementation for Ack Vector.  It is
   informative and not normative.

   The first part of our implementation runs at the HC-Receiver, and
   therefore acknowledges data packets.  It generates Ack Vector
   options.  The implementation has the following characteristics:

   o  At most one byte of state per acknowledged packet.

   o  O(1) time to update that state when a new packet arrives (normal
      case).

   o  Cumulative acknowledgements.

   o  Quick removal of old state.

   The basic data structure is a circular buffer containing information
   about acknowledged packets.  Each byte in this buffer contains a
   state and run length; the state can be 0 (packet received), 1 (packet
   ECN marked), or 3 (packet not yet received).  The buffer grows from
   right to left.  The implementation maintains five variables, aside
   from the buffer contents:

   o  "buf_head" and "buf_tail", which mark the live portion of the
      buffer.

   o  "buf_ackno", the Acknowledgement Number of the most recent packet
      acknowledged in the buffer.  This corresponds to the "head"
      pointer.

   o  "buf_nonce", the one-bit sum (exclusive-or, or parity) of the ECN
      Nonces received on all packets acknowledged by the buffer with
      State 0.

   We draw acknowledgement buffers like this:

      +---------------------------------------------------------------+
      |S,L|S,L|S,L|S,L|   |   |   |   |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|
      +---------------------------------------------------------------+
                    ^                   ^
                 buf_tail     buf_head, buf_ackno = A     buf_nonce = E

                <=== buf_head and buf_tail move this way <===





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   Each "S,L" represents a State/Run length byte.  We will draw these
   buffers showing only their live portion and will add an annotation
   showing the Acknowledgement Number for the last live byte in the
   buffer.  For example:

        +-----------------------------------------------+
      A |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L| T    BN[E]
        +-----------------------------------------------+

   Here, buf_nonce equals E and buf_ackno equals A.

   We will use this buffer as a running example.

         +---------------------------+
      10 |0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0    BN[1]   [Example Buffer]
         +---------------------------+

   In concrete terms, its meaning is as follows:

      Packet 10 was received.  (The head of the buffer has sequence
      number 10, state 0, and run length 0.)

      Packets 9, 8, and 7 have not yet been received.  (The three bytes
      preceding the head each have state 3 and run length 0.)

      Packets 6, 5, 4, 3, and 2 were received.

      Packet 1 was ECN marked.

      Packet 0 was received.

      The one-bit sum of the ECN Nonces on packets 10, 6, 5, 4, 3, 2,
      and 0 equals 1.

   Additionally, the HC-Receiver must keep some information about the
   Ack Vectors it has recently sent.  For each packet sent carrying an
   Ack Vector, it remembers four variables:

   o  "ack_seqno", the Sequence Number used for the packet.  This is an
      HC-Receiver sequence number.

   o  "ack_ptr", the value of buf_head at the time of acknowledgement.

   o  "ack_runlen", the run length stored in the byte of buffer data at
      buf_head at the time of acknowledgement.






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   o  "ack_ackno", the Acknowledgement Number used for the packet.  This
      is an HC-Sender sequence number.  Since acknowledgements are
      cumulative, this single number completely specifies all necessary
      information about the packets acknowledged by this Ack Vector.

   o  "ack_nonce", the one-bit sum of the ECN Nonces for all State 0
      packets in the buffer from buf_head to ack_ackno, inclusive.
      Initially, this equals the Nonce Echo of the acknowledgement's Ack
      Vector (or, if the ack packet contained more than one Ack Vector,
      the exclusive-or of all the acknowledgement's Ack Vectors).  It
      changes as information about old acknowledgements is removed (so
      ack_ptr and buf_head diverge) and as old packets arrive (so they
      change from State 3 or State 1 to State 0).

A.1.  Packet Arrival

   This section describes how the HC-Receiver updates its
   acknowledgement buffer as packets arrive from the HC-Sender.

A.1.1.  New Packets

   When a packet with Sequence Number greater than buf_ackno arrives,
   the HC-Receiver updates buf_head (by moving it to the left
   appropriately), buf_ackno (which is set to the new packet's Sequence
   Number), and possibly buf_nonce (if the packet arrived unmarked with
   ECN Nonce 1), in addition to the buffer itself.  For example, if
   HC-Sender packet 11 arrived ECN marked, the Example Buffer above
   would enter this new state (changes are marked with stars):

         ** +***----------------------------+
         11 |1,0|0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0    BN[1]
         ** +***----------------------------+

   If the packet's state equals the state at the head of the buffer, the
   HC-Receiver may choose to increment its run length (up to the
   maximum).  For example, if HC-Sender packet 11 arrived without ECN
   marking and with ECN Nonce 0, the Example Buffer might enter this
   state instead:

             ** +--*------------------------+
             11 |0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0    BN[1]
             ** +--*------------------------+









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   Of course, the new packet's sequence number might not equal the
   expected sequence number.  In this case, the HC-Receiver will enter
   the intervening packets as State 3.  If several packets are missing,
   the HC-Receiver may prefer to enter multiple bytes with run length 0,
   rather than a single byte with a larger run length; this simplifies
   table updates if one of the missing packets arrives.  For example, if
   HC-Sender packet 12 arrived with ECN Nonce 1, the Example Buffer
   would enter this state:

      ** +*******----------------------------+         *
      12 |0,0|3,0|0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0    BN[0]
      ** +*******----------------------------+         *

   Of course, the circular buffer may overflow when the HC-Sender is
   sending data at a very high rate, when the HC-Receiver's
   acknowledgements are not reaching the HC-Sender, or when the
   HC-Sender is forgetting to acknowledge those acks (so the HC-Receiver
   is unable to clean up old state).  In this case, the HC-Receiver
   should either compress the buffer (by increasing run lengths when
   possible), transfer its state to a larger buffer, or, as a last
   resort, drop all received packets, without processing them at all,
   until its buffer shrinks again.

A.1.2.  Old Packets

   When a packet with Sequence Number S <= buf_ackno arrives, the
   HC-Receiver will scan the table for the byte corresponding to S.
   (Indexing structures could reduce the complexity of this scan.)  If S
   was previously lost (State 3), and it was stored in a byte with run
   length 0, the HC-Receiver can simply change the byte's state.  For
   example, if HC-Sender packet 8 was received with ECN Nonce 0, the
   Example Buffer would enter this state:

               +--------*------------------+
            10 |0,0|3,0|0,0|3,0|0,4|1,0|0,0| 0    BN[1]
               +--------*------------------+

   If S was not marked as lost, or if it was not contained in the table,
   the packet is probably a duplicate and should be ignored.  (The new
   packet's ECN marking state might differ from the state in the buffer;
   Section 11.4.1 describes what is allowed then.)  If S's buffer byte
   has a non-zero run length, then the buffer might need to be
   reshuffled to make space for one or two new bytes.

   The ack_nonce fields may also need manipulation when old packets
   arrive.  In particular, when S transitions from State 3 or State 1 to
   State 0, and S had ECN Nonce 1, then the implementation should flip
   the value of ack_nonce for every acknowledgement with ack_ackno >= S.



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   It is impossible with this data structure to shift packets from State
   0 to State 1, since the buffer doesn't store individual packets' ECN
   Nonces.

A.2.  Sending Acknowledgements

   Whenever the HC-Receiver needs to generate an acknowledgement, the
   buffer's contents can simply be copied into one or more Ack Vector
   options.  Copied Ack Vectors might not be maximally compressed; for
   example, the Example Buffer above contains three adjacent 3,0 bytes
   that could be combined into a single 3,2 byte.  The HC-Receiver
   might, therefore, choose to compress the buffer in place before
   sending the option, or to compress the buffer while copying it;
   either operation is simple.

   Every acknowledgement sent by the HC-Receiver SHOULD include the
   entire state of the buffer.  That is, acknowledgements are
   cumulative.

   If the acknowledgement fits in one Ack Vector, that Ack Vector's
   Nonce Echo simply equals buf_nonce.  For multiple Ack Vectors, more
   care is required.  The Ack Vectors should be split at points
   corresponding to previous acknowledgements, since the stored
   ack_nonce fields provide enough information to calculate correct
   Nonce Echoes.  The implementation should therefore acknowledge data
   at least once per 253 bytes of buffer state.  (Otherwise, there'd be
   no way to calculate a Nonce Echo.)

   For each acknowledgement it sends, the HC-Receiver will add an
   acknowledgement record.  ack_seqno will equal the HC-Receiver
   sequence number it used for the ack packet; ack_ptr will equal
   buf_head; ack_runlen will equal the run length stored in the buffer's
   buf_head byte; ack_ackno will equal buf_ackno; and ack_nonce will
   equal buf_nonce.

A.3.  Clearing State

   Some of the HC-Sender's packets will include acknowledgement numbers,
   which ack the HC-Receiver's acknowledgements.  When such an ack is
   received, the HC-Receiver finds the acknowledgement record R with the
   appropriate ack_seqno and then does the following:

   o  If the run length in the buffer's R.ack_ptr byte is greater than
      R.ack_runlen, then it decrements that run length by
      R.ack_runlen + 1 and sets buf_tail to R.ack_ptr.  Otherwise, it
      sets buf_tail to R.ack_ptr + 1.





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   o  If R.ack_nonce is 1, it flips buf_nonce, and the value of
      ack_nonce for every later ack record.

   o  It throws away R and every preceding ack record.

   (The HC-Receiver may choose to keep some older information, in case a
   lost packet shows up late.)  For example, say that the HC-Receiver
   storing the Example Buffer had sent two acknowledgements already:

   1. ack_seqno = 59, ack_runlen = 1, ack_ackno = 3, ack_nonce = 1.

   2. ack_seqno = 60, ack_runlen = 0, ack_ackno = 10, ack_nonce = 0.

   Say the HC-Receiver then received a DCCP-DataAck packet with
   Acknowledgement Number 59 from the HC-Sender.  This informs the
   HC-Receiver that the HC-Sender received, and processed, all the
   information in HC-Receiver packet 59.  This packet acknowledged
   HC-Sender packet 3, so the HC-Sender has now received HC-Receiver's
   acknowledgements for packets 0, 1, 2, and 3.  The Example Buffer
   should enter this state:

               +------------------*+ *       *
            10 |0,0|3,0|3,0|3,0|0,2| 4    BN[0]
               +------------------*+ *       *

   The tail byte's run length was adjusted, since packet 3 was in the
   middle of that byte.  Since R.ack_nonce was 1, the buf_nonce field
   was flipped, as were the ack_nonce fields for later acknowledgements
   (here, the HC-Receiver Ack 60 record, not shown, has its ack_nonce
   flipped to 1).  The HC-Receiver can also throw away stored
   information about HC-Receiver Ack 59 and any earlier
   acknowledgements.

   A careful implementation might try to ensure reasonable robustness to
   reordering.  Suppose that the Example Buffer is as before, but that
   packet 9 now arrives, out of sequence.  The buffer would enter this
   state:

                +----*----------------------+
             10 |0,0|0,0|3,0|3,0|0,4|1,0|0,0| 0     BN[1]
                +----*----------------------+

   The danger is that the HC-Sender might acknowledge the HC-Receiver's
   previous acknowledgement (with sequence number 60), which says that
   Packet 9 was not received, before the HC-Receiver has a chance to
   send a new acknowledgement saying that Packet 9 actually was
   received.  Therefore, when packet 9 arrived, the HC-Receiver might
   modify its acknowledgement record as follows:



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   1. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.

   2. ack_seqno = 60, ack_ackno = 3, ack_nonce = 1.

   That is, Ack 60 is now treated like a duplicate of Ack 59.  This
   would prevent the Tail pointer from moving past packet 9 until the
   HC-Receiver knows that the HC-Sender has seen an Ack Vector
   indicating that packet's arrival.

A.4.  Processing Acknowledgements

   When the HC-Sender receives an acknowledgement, it generally cares
   about the number of packets that were dropped and/or ECN marked.  It
   simply reads this off the Ack Vector.  Additionally, it should check
   the ECN Nonce for correctness.  (As described in Section 11.4.1, it
   may want to keep more detailed information about acknowledged packets
   in case packets change states between acknowledgements, or in case
   the application queries whether a packet arrived.)

   The HC-Sender must also acknowledge the HC-Receiver's
   acknowledgements so that the HC-Receiver can free old Ack Vector
   state.  (Since Ack Vector acknowledgements are reliable, the
   HC-Receiver must maintain and resend Ack Vector information until it
   is sure that the HC-Sender has received that information.)  A simple
   algorithm suffices: since Ack Vector acknowledgements are cumulative,
   a single acknowledgement number tells HC-Receiver how much ack
   information has arrived.  Assuming that the HC-Receiver sends no
   data, the HC-Sender can ensure that at least once a round-trip time,
   it sends a DCCP-DataAck packet acknowledging the latest DCCP-Ack
   packet it has received.  Of course, the HC-Sender only needs to
   acknowledge the HC-Receiver's acknowledgements if the HC-Sender is
   also sending data.  If the HC-Sender is not sending data, then the
   HC-Receiver's Ack Vector state is stable, and there is no need to
   shrink it.  The HC-Sender must watch for drops and ECN marks on
   received DCCP-Ack packets so that it can adjust the HC-Receiver's
   ack-sending rate in response to congestion, for example, with Ack
   Ratio.

   If the other half-connection is not quiescent -- that is, the
   HC-Receiver is sending data to the HC-Sender, possibly using another
   CCID -- then the acknowledgements on that half-connection are
   sufficient for the HC-Receiver to free its state.









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B.  Appendix: Partial Checksumming Design Motivation

   A great deal of discussion has taken place regarding the utility of
   allowing a DCCP sender to restrict the checksum so that it does not
   cover the complete packet.  This section attempts to capture some of
   the rationale behind specific details of DCCP design.

   Many of the applications that we envisage using DCCP are resilient to
   some degree of data loss, or they would typically have chosen a
   reliable transport.  Some of these applications may also be resilient
   to data corruption -- some audio payloads, for example.  These
   resilient applications might rather receive corrupted data than have
   DCCP drop corrupted packets.  This is particularly because of
   congestion control: DCCP cannot tell the difference between packets
   dropped due to corruption and packets dropped due to congestion, and
   so it must reduce the transmission rate accordingly.  This response
   may cause the connection to receive less bandwidth than it is due;
   corruption in some networking technologies is independent of, or at
   least not always correlated to, congestion.  Therefore, corrupted
   packets do not need to cause as strong a reduction in transmission
   rate as the congestion response would dictate (as long as the DCCP
   header and options are not corrupt).

   Thus DCCP allows the checksum to cover all of the packet, just the
   DCCP header, or both the DCCP header and some number of bytes from
   the application data.  If the application cannot tolerate any data
   corruption, then the checksum must cover the whole packet.  If the
   application would prefer to tolerate some corruption rather than have
   the packet dropped, then it can set the checksum to cover only part
   of the packet (but always the DCCP header).  In addition, if the
   application wishes to decouple checksumming of the DCCP header from
   checksumming of the application data, it may do so by including the
   Data Checksum option.  This would allow DCCP to discard corrupted
   application data without mistaking the corruption for network
   congestion.

   Thus, from the application point of view, partial checksums seem to
   be a desirable feature.  However, the usefulness of partial checksums
   depends on partially corrupted packets being delivered to the
   receiver.  If the link-layer CRC always discards corrupted packets,
   then this will not happen, and so the usefulness of partial checksums
   would be restricted to corruption that occurred in routers and other
   places not covered by link CRCs.  There does not appear to be
   consensus on how likely it is that future network links that suffer
   significant corruption will not cover the entire packet with a single
   strong CRC.  DCCP makes it possible to tailor such links to the
   application, but it is difficult to predict if this will be
   compelling for future link technologies.



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   In addition, partial checksums do not co-exist well with IP-level
   authentication mechanisms such as IPsec AH, which cover the entire
   packet with a cryptographic hash.  Thus, if cryptographic
   authentication mechanisms are required to co-exist with partial
   checksums, the authentication must be carried in the application
   data.  A possible mode of usage would appear to be similar to that of
   Secure RTP.  However, such "application-level" authentication does
   not protect the DCCP option negotiation and state machine from forged
   packets.  An alternative would be to use IPsec ESP, and to use
   encryption to protect the DCCP headers against attack, while using
   the DCCP header validity checks to authenticate that the header is
   from someone who possessed the correct key.  While this is resistant
   to replay (due to the DCCP sequence number), it is not by itself
   resistant to some forms of man-in-the-middle attacks because the
   application data is not tightly coupled to the packet header.  Thus,
   an application-level authentication probably needs to be coupled with
   IPsec ESP or a similar mechanism to provide a reasonably complete
   security solution.  The overhead of such a solution might be
   unacceptable for some applications that would otherwise wish to use
   partial checksums.

   On balance, the authors believe that DCCP partial checksums have the
   potential to enable some future uses that would otherwise be
   difficult.  As the cost and complexity of supporting them is small,
   it seems worth including them at this time.  It remains to be seen
   whether they are useful in practice.

Normative References

   [RFC793]       Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

   [RFC1191]      Mogul, J. and S. Deering, "Path MTU discovery", RFC
                  1191, November 1990.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26, RFC
                  2434, October 1998.

   [RFC2460]      Deering, S. and R. Hinden, "Internet Protocol, Version
                  6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC3168]      Ramakrishnan, K., Floyd, S., and D. Black, "The
                  Addition of Explicit Congestion Notification (ECN) to
                  IP", RFC 3168, September 2001.



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   [RFC3309]      Stone, J., Stewart, R., and D. Otis, "Stream Control
                  Transmission Protocol (SCTP) Checksum Change", RFC
                  3309, September 2002.

   [RFC3692]      Narten, T., "Assigning Experimental and Testing
                  Numbers Considered Useful", BCP 82, RFC 3692, January
                  2004.

   [RFC3775]      Johnson, D., Perkins, C., and J. Arkko, "Mobility
                  Support in IPv6", RFC 3775, June 2004.

   [RFC3828]      Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.,
                  and G. Fairhurst, "The Lightweight User Datagram
                  Protocol (UDP-Lite)", RFC 3828, July 2004.

Informative References

   [B98]          Bellovin, S.M., "Cryptography and the Internet",
                  CRYPTO '98 (LNCS 1462), pp 46-55, August 1988.

   [BB01]         Bellovin, S.M. and M. Blaze, "Cryptographic Modes of
                  Operation for the Internet", 2nd NIST Workshop on
                  Modes of Operation, August 2001.

   [M85]          Morris, R.T., "A Weakness in the 4.2BSD Unix TCP/IP
                  Software", Computer Science Technical Report 117, AT&T
                  Bell Laboratories, Murray Hill, NJ, February 1985.

   [PMTUD]        Mathis, M. and J. Heffner, "Path MTU Discovery", Work
                  in Progress, March 2006.

   [RFC792]       Postel, J., "Internet Control Message Protocol", STD
                  5, RFC 792, September 1981.

   [RFC1812]      Baker, F., "Requirements for IP Version 4 Routers",
                  RFC 1812, June 1995.

   [RFC1948]      Bellovin, S., "Defending Against Sequence Number
                  Attacks", RFC 1948, May 1996.

   [RFC1982]      Elz, R. and R. Bush, "Serial Number Arithmetic", RFC
                  1982, August 1996.

   [RFC2018]      Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
                  "TCP Selective Acknowledgement Options", RFC 2018,
                  October 1996.





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   [RFC2401]      Kent, S. and R. Atkinson, "Security Architecture for
                  the Internet Protocol", RFC 2401, November 1998.

   [RFC2463]      Conta, A. and S. Deering, "Internet Control Message
                  Protocol (ICMPv6) for the Internet Protocol Version 6
                  (IPv6) Specification", RFC 2463, December 1998.

   [RFC2581]      Allman, M., Paxson, V., and W. Stevens, "TCP
                  Congestion Control", RFC 2581, April 1999.

   [RFC2960]      Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                  Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                  Zhang, L., and V. Paxson, "Stream Control Transmission
                  Protocol", RFC 2960, October 2000.

   [RFC3124]      Balakrishnan, H. and S. Seshan, "The Congestion
                  Manager", RFC 3124, June 2001.

   [RFC3360]      Floyd, S., "Inappropriate TCP Resets Considered
                  Harmful", BCP 60, RFC 3360, August 2002.

   [RFC3448]      Handley, M., Floyd, S., Padhye, J., and J. Widmer,
                  "TCP Friendly Rate Control (TFRC): Protocol
                  Specification", RFC 3448, January 2003.

   [RFC3540]      Spring, N., Wetherall, D., and D. Ely, "Robust
                  Explicit Congestion Notification (ECN) Signaling with
                  Nonces", RFC 3540, June 2003.

   [RFC3550]      Schulzrinne, H., Casner, S., Frederick, R., and V.
                  Jacobson, "RTP: A Transport Protocol for Real-Time
                  Applications", STD 64, RFC 3550, July 2003.

   [RFC3611]      Friedman, T., Caceres, R., and A. Clark, "RTP Control
                  Protocol Extended Reports (RTCP XR)", RFC 3611,
                  November 2003.

   [RFC3711]      Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                  K. Norrman, "The Secure Real-time Transport Protocol
                  (SRTP)", RFC 3711, March 2004.

   [RFC3819]      Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
                  Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
                  and L. Wood, "Advice for Internet Subnetwork
                  Designers", BCP 89, RFC 3819, July 2004.






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   [RFC4086]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                  "Randomness Requirements for Security", BCP 106, RFC
                  4086, June 2005.

   [RFC4341]      Floyd, S. and E. Kohler, "Profile for Datagram
                  Congestion Control Protocol (DCCP) Congestion Control
                  ID 2: TCP-like Congestion Control", RFC 4341, March
                  2006.

   [RFC4342]      Floyd, S., Kohler, E., and J. Padhye, "Profile for
                  Datagram Congestion Control Protocol (DCCP) Congestion
                  Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
                  4342, March 2006.

   [SHHP00]       Spatscheck, O., Hansen, J.S., Hartman, J.H., and L.L.
                  Peterson, "Optimizing TCP Forwarder Performance",
                  IEEE/ACM Transactions on Networking 8(2):146-157,
                  April 2000.

   [SYNCOOKIES]   Bernstein, D.J., "SYN Cookies",
                  http://cr.yp.to/syncookies.html, as of March 2006.

   [VBK05]        Vanit-Anunchai, S., Billington, J., and T.
                  Kongprakaiwoot, "Discovering Chatter and
                  Incompleteness in the Datagram Congestion Control
                  Protocol", FORTE 2005, pp 143-158, October 2005.

























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Authors' Addresses

   Eddie Kohler
   4531C Boelter Hall
   UCLA Computer Science Department
   Los Angeles, CA 90095
   USA

   EMail: kohler@cs.ucla.edu


   Mark Handley
   Department of Computer Science
   University College London
   Gower Street
   London WC1E 6BT
   UK

   EMail: M.Handley@cs.ucl.ac.uk


   Sally Floyd
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704
   USA

   EMail: floyd@icir.org























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Full Copyright Statement

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