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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4340

Internet Engineering Task Force                             Eddie Kohler
INTERNET-DRAFT                                                      UCLA
draft-ietf-dccp-spec-06.txt                                 Mark Handley
Expires: August 2004                                                 UCL
                                                             Sally Floyd
                                                                    ICIR
                                                        16 February 2004


              Datagram Congestion Control Protocol (DCCP)


Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of [RFC 2026].  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF), its
    areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time. It is inappropriate to use Internet-Drafts as reference
    material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html

Copyright Notice

    Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

    This document specifies the Datagram Congestion Control Protocol
    (DCCP), which implements a congestion-controlled, unreliable flow of
    unicast datagrams suitable for use by applications such as streaming
    media, Internet telephony, and on-line games.







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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes since draft-ietf-dccp-spec-05.txt:

    * Organization overhaul.

    * Add pseudocode for event processing.

    * Remove # NDP; replace with Ack Count.

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

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

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

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

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

    * Mobility ID becomes 128 bits long.

    * Add probabilities to Mobility ID discussion.

    * Add SyncAck.

























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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   7
    2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . .   8
    3. Conventions and Terminology . . . . . . . . . . . . . . . . .   9
       3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . .   9
       3.2. Parts of a Connection. . . . . . . . . . . . . . . . . .   9
       3.3. Features . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . .  10
       3.5. Robustness Principle . . . . . . . . . . . . . . . . . .  10
    4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . .  11
       4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . .  12
       4.3. States . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.4. Congestion Control . . . . . . . . . . . . . . . . . . .  15
       4.5. Features . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.6. Other Differences from TCP . . . . . . . . . . . . . . .  17
       4.7. Example Connection . . . . . . . . . . . . . . . . . . .  18
    5. Header Formats. . . . . . . . . . . . . . . . . . . . . . . .  19
       5.1. Generic Header . . . . . . . . . . . . . . . . . . . . .  20
       5.2. DCCP-Request Header. . . . . . . . . . . . . . . . . . .  23
       5.3. DCCP-Response Header . . . . . . . . . . . . . . . . . .  23
       5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Head-
       ers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
       5.5. DCCP-CloseReq and DCCP-Close Headers . . . . . . . . . .  25
       5.6. DCCP-Reset Header. . . . . . . . . . . . . . . . . . . .  26
       5.7. DCCP-Move Header . . . . . . . . . . . . . . . . . . . .  27
       5.8. DCCP-Sync and DCCP-SyncAck Headers . . . . . . . . . . .  28
       5.9. Options. . . . . . . . . . . . . . . . . . . . . . . . .  29
          5.9.1. Padding Option. . . . . . . . . . . . . . . . . . .  30
          5.9.2. Mandatory Option. . . . . . . . . . . . . . . . . .  30
    6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . .  31
       6.1. Change Options . . . . . . . . . . . . . . . . . . . . .  31
       6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . .  32
       6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . .  32
          6.3.1. Server-Priority . . . . . . . . . . . . . . . . . .  33
          6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . .  33
       6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . .  33
       6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . .  34
       6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . .  36
          6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . .  36
          6.6.2. Loss and Retransmission . . . . . . . . . . . . . .  37
          6.6.3. Reordering. . . . . . . . . . . . . . . . . . . . .  38
          6.6.4. Preference Changes. . . . . . . . . . . . . . . . .  39
          6.6.5. Simultaneous Negotiation. . . . . . . . . . . . . .  39
          6.6.6. Unknown Features. . . . . . . . . . . . . . . . . .  39
          6.6.7. Invalid Options . . . . . . . . . . . . . . . . . .  40
          6.6.8. Mandatory Feature Negotiation . . . . . . . . . . .  40



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          6.6.9. Out-of-Band Agreement . . . . . . . . . . . . . . .  41
          6.6.10. State Diagram. . . . . . . . . . . . . . . . . . .  41
    7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . .  42
       7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . .  42
       7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . .  43
       7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . .  44
       7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . .  44
       7.5. Validity and Synchronization . . . . . . . . . . . . . .  45
          7.5.1. Sequence-Validity Rules . . . . . . . . . . . . . .  45
          7.5.2. Handling Sequence-Invalid Packets . . . . . . . . .  47
          7.5.3. Sequence and Acknowledgement Number
          Windows. . . . . . . . . . . . . . . . . . . . . . . . . .  48
          7.5.4. Sequence Window Feature . . . . . . . . . . . . . .  49
          7.5.5. Sequence Number Attacks . . . . . . . . . . . . . .  49
          7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . .  50
       7.6. Extended Sequence Numbers. . . . . . . . . . . . . . . .  51
          7.6.1. When to Use Extended Sequence Numbers . . . . . . .  51
          7.6.2. Header Processing . . . . . . . . . . . . . . . . .  52
          7.6.3. Transitioning to Extended Sequence Num-
          bers . . . . . . . . . . . . . . . . . . . . . . . . . . .  53
          7.6.4. Sequence Transition Capable Feature . . . . . . . .  54
       7.7. NDP Count and Detecting Application Loss . . . . . . . .  55
          7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . .  56
          7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . .  56
    8. Event Processing. . . . . . . . . . . . . . . . . . . . . . .  56
       8.1. Connection Establishment . . . . . . . . . . . . . . . .  56
          8.1.1. Client Request. . . . . . . . . . . . . . . . . . .  57
          8.1.2. Service Codes . . . . . . . . . . . . . . . . . . .  57
          8.1.3. Server Response . . . . . . . . . . . . . . . . . .  59
          8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . .  60
          8.1.5. Handshake Completion. . . . . . . . . . . . . . . .  60
       8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . .  61
       8.3. Termination. . . . . . . . . . . . . . . . . . . . . . .  62
          8.3.1. Abnormal Termination. . . . . . . . . . . . . . . .  63
       8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . .  63
       8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . .  64
    9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . .  68
       9.1. Header Checksum Field. . . . . . . . . . . . . . . . . .  68
       9.2. Header Checksum Coverage Field . . . . . . . . . . . . .  69
       9.3. Data Checksum Option . . . . . . . . . . . . . . . . . .  70
          9.3.1. Check Data Checksum Feature . . . . . . . . . . . .  71
          9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . .  71
    10. Congestion Control IDs . . . . . . . . . . . . . . . . . . .  71
       10.1. Unspecified Sender-Based Congestion
       Control . . . . . . . . . . . . . . . . . . . . . . . . . . .  72
       10.2. TCP-like Congestion Control . . . . . . . . . . . . . .  74
       10.3. TFRC Congestion Control . . . . . . . . . . . . . . . .  74
       10.4. CCID-Specific Options, Features, and Reset



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       Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  74
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  76
       11.1. Acks of Acks and Unidirectional
       Connections . . . . . . . . . . . . . . . . . . . . . . . . .  77
       11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . .  78
       11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . .  79
       11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . .  79
          11.4.1. Ack Vector Consistency . . . . . . . . . . . . . .  81
          11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . .  83
       11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . .  83
       11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . .  84
       11.7. Data Dropped Option . . . . . . . . . . . . . . . . . .  84
          11.7.1. Data Dropped and Normal Congestion
          Response . . . . . . . . . . . . . . . . . . . . . . . . .  87
          11.7.2. Particular Drop Codes. . . . . . . . . . . . . . .  87
    12. Explicit Congestion Notification . . . . . . . . . . . . . .  88
       12.1. ECN Capable Feature . . . . . . . . . . . . . . . . . .  88
       12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . .  89
       12.3. Other Aggression Penalties. . . . . . . . . . . . . . .  90
    13. Timing Options . . . . . . . . . . . . . . . . . . . . . . .  90
       13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . .  90
       13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . .  91
       13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . .  92
    14. Multihoming and Mobility . . . . . . . . . . . . . . . . . .  92
       14.1. Mobility Capable Feature. . . . . . . . . . . . . . . .  93
       14.2. Mobility ID Feature . . . . . . . . . . . . . . . . . .  93
       14.3. Mobile Host Processing. . . . . . . . . . . . . . . . .  94
       14.4. Stationary Host Processing. . . . . . . . . . . . . . .  95
       14.5. Congestion Control State. . . . . . . . . . . . . . . .  96
       14.6. Security. . . . . . . . . . . . . . . . . . . . . . . .  96
    15. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . .  97
    16. Forward Compatibility. . . . . . . . . . . . . . . . . . . .  99
    17. Middlebox Considerations . . . . . . . . . . . . . . . . . . 100
    18. Relations to Other Specifications. . . . . . . . . . . . . . 101
       18.1. DCCP and RTP. . . . . . . . . . . . . . . . . . . . . . 101
       18.2. Multiplexing Issues . . . . . . . . . . . . . . . . . . 102
    19. Security Considerations. . . . . . . . . . . . . . . . . . . 103
       19.1. Security Considerations for Mobility. . . . . . . . . . 103
       19.2. Security Considerations for Partial Check-
       sums. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
    20. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 105
    21. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
    A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 106
       A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 108
          A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 108
          A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 109
       A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 110
       A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 110



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       A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 112
    B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 113
       B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 113
    Normative References . . . . . . . . . . . . . . . . . . . . . . 114
    Informative References . . . . . . . . . . . . . . . . . . . . . 115
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 116
    Intellectual Property Notice . . . . . . . . . . . . . . . . . . 117












































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1.  Introduction

    This document describes the Datagram Congestion Control Protocol
    (DCCP), a transport protocol that implements a congestion-
    controlled, bidirectional stream of unreliable datagrams.
    Specifically, DCCP provides:

    o An unreliable flow of datagrams, with acknowledgements.

    o Reliable handshakes for connection setup and teardown.

    o Reliable negotiation of options, including negotiation of a
      suitable congestion control mechanism.

    o Mechanisms allowing a server to avoid holding any state for
      unacknowledged connection attempts or already-finished
      connections.

    o Congestion control incorporating Explicit Congestion Notification
      (ECN) and the ECN Nonce, as per [RFC 3168] and [RFC 3540].

    o Acknowledgement mechanisms communicating packet loss and ECN mark
      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 Transfer Unit (PMTU) discovery, as per [RFC 1191].

    DCCP is intended for applications, such as streaming media and
    Internet telephony, where reliable in-order delivery, combined with
    congestion control, can result in some information arriving at the
    receiver after it is no longer of use.  So far, most such
    applications have either used TCP, with the attendant quality
    problems caused by late data delivery, or used UDP and implemented
    their own congestion control (or no congestion control at all).
    DCCP provides standard congestion control mechanisms for such
    applications.  It enables the use of ECN, along with conformant end-
    to-end congestion control, for applications that would otherwise be
    using UDP.  In addition, DCCP implements reliable connection setup,
    teardown, and feature negotiation.

    DCCP's target applications require the flow-based semantics of TCP,
    but do not want TCP's in-order delivery and reliability, or would



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    like different congestion control dynamics than TCP.

2.  Design Rationale

    DCCP was intended to be used by applications that currently use UDP
    without end-to-end congestion control.  Most streaming UDP
    applications should have little reason not to switch to DCCP, once
    it is deployed.  Thus, 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.  This desire for minimal overhead is also one of the
    reasons to avoid proposing an unreliable variant of the Stream
    Control Transmission Protocol (SCTP, [RFC 2960]).

    Different forms of conformant congestion control are appropriate for
    different applications.  For example, applications such as on-line
    games might want to make quick use of any available bandwidth.
    Other applications, such as streaming media, might trade off this
    responsiveness for a steadier, less bursty rate, since sudden rate
    changes cause unacceptable UI glitches (such as audible pauses or
    clicks in the playout stream).  Thus, DCCP allows applications to
    choose between several forms of congestion control.  One choice,
    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 be able to tolerate the abrupt changes
    in congestion window typical of TCP.  A second alternative, TCP-
    Friendly Rate Control (TFRC, [RFC 3448]), a form of equation-based
    congestion control, minimizes abrupt changes in the sending rate
    while maintaining longer-term fairness with TCP.

    DCCP also lets unreliable traffic safely use ECN.  A UDP kernel API
    might not allow applications to set UDP packets as ECN-capable,
    since the API could not guarantee the application would properly
    detect or respond to congestion.  DCCP kernel APIs will have no such
    issues, since DCCP itself implements congestion control.

    We chose not to require the use of the Congestion Manager [RFC
    3124], which allows multiple concurrent streams between the same
    sender and receiver to share congestion control.  The current
    Congestion Manager can only be used by applications that have their
    own end-to-end feedback about packet losses, but this is not the
    case for many of the applications currently using UDP.  In addition,
    the current Congestion Manager does not easily support multiple
    congestion control mechanisms, or lend itself to the use of forms of



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    TFRC where the state about past packet drops or marks is maintained
    at the receiver rather than at the sender.  DCCP should be able to
    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.

3.  Conventions and Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
    this document are to be interpreted as described in [RFC 2119].

3.1.  Numbers and Fields

    All multi-byte numerical quantities in DCCP, such as port numbers,
    Sequence Numbers, and arguments to options, are transmitted in
    network byte order (most significant byte first).

    We occasionally refer to the "left" and "right" sides of a bit
    field.  "Left" means towards the most significant bit, and "right"
    means towards the least significant bit.

    Reserved bitfields in DCCP packet headers MUST be ignored by
    receivers, and MUST be set to zero by senders, unless otherwise
    specified.

    Random numbers in DCCP are used for their security properties, and
    MUST be chosen according to the guidelines in [RFC 1750].

3.2.  Parts of a Connection

    Each DCCP connection runs between two endpoints, which we often name
    DCCP A and DCCP B.

    DCCP connections are actively initiated by one endpoint.  The active
    endpoint is called the client, and the passive endpoint is called
    the server.

    DCCP connections are bidirectional; data may pass from either
    endpoint to the other.  This means that data and acknowledgements
    may be flowing in both directions simultaneously.  Logically,
    however, a DCCP connection consists of two separate unidirectional
    connections, called half-connections.  Each half-connection consists
    of the data packets sent by one endpoint and the corresponding
    acknowledgements sent by the other endpoint.  We can illustrate this
    as follows:





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     +--------+  A-to-B half-connection:         +--------+
     |        |    -->    data packets    -->    |        |
     |        |    <--  acknowledgements  <--    |        |
     | DCCP A |                                  | DCCP B |
     |        |  B-to-A half-connection:         |        |
     |        |    <--    data packets    <--    |        |
     +--------+    -->  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 HC-Sender is the
    endpoint sending data, while the HC-Receiver is the endpoint sending
    acknowledgements.  For example, in the A-to-B half-connection,
    DCCP A is the HC-Sender and DCCP B is the HC-Receiver.

3.3.  Features

    A feature is a DCCP connection attribute, identified by a feature
    number and an endpoint, 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, whether mobility is allowed, and whether ECN is
    supported.  The endpoints can achieve agreement by out-of-band
    communication, or through the exchange of feature negotiation
    options in DCCP headers.

    The notation F/A represents the feature with feature number F
    located at DCCP endpoint A; the feature F/B has the same feature
    number, but is located at the other endpoint.  Both DCCP A and
    DCCP B know, and agree on, the values of both F/A and F/B, but F/A
    and F/B may have different values.

    DCCP A is called the feature location for all features F/A, and the
    feature remote for all features F/B.

3.4.  Round-Trip Times

    We sometimes refer to a round-trip time for setting timers, for
    example.  If no useful round-trip time estimate is available, a DCCP
    implementation SHOULD use 0.2 seconds instead.

3.5.  Robustness Principle

    DCCP implementations should follow TCP's "general principle of
    robustness": be conservative in what you do, be liberal in what you



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    accept from others.

4.  Overview

    DCCP's high-level connection dynamics should seem familiar to anyone
    who knows TCP.  DCCP connections, like 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; congestion control uses this information to
    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 either as TCP minus bytestream semantics and
    reliability, or as UDP plus congestion control, handshakes, and
    acknowledgements.

4.1.  Packet Types

    DCCP uses eleven packet types to implement various protocol
    functions.  For example, every new connection attempt begins with a
    DCCP-Request packet sent by the client.  A DCCP-Request packet thus
    resembles a TCP SYN; but DCCP-Request is a packet type, not a flag,
    so there's no way to send an unexpected combination such as TCP's
    SYN+FIN+ACK+RST.

    Eight packet types occur during the progress of a typical
    connection---two only during the initiation phase, three during the
    data transfer phase, and three only during the termination phase:

       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

    Note the three-way handshakes during initiation and termination.
    The three remaining packet types are used for special purposes: when
    an endpoint moves, or to resynchronize after bursts of loss.



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    Every DCCP packet starts with a common, 12-byte generic header, but
    different packet types may include different amounts of additional
    data.  For example, the DCCP-Ack packet type includes an
    Acknowledgement Number.  Every packet type may also contain options,
    up to around 1000 bytes' worth.

    All of the packet types are described below.

    DCCP-Request
        Sent by the client to initiate a connection (the first part of
        the three-way handshake).

    DCCP-Response
        Sent by the server in response to a DCCP-Request (the second
        part of the three-way handshake).

    DCCP-Data
        Used to transmit data.

    DCCP-Ack
        Used for pure acknowledgements.

    DCCP-DataAck
        Used for piggybacked data-plus-acknowledgements.

    DCCP-CloseReq
        Sent by the server to request that the client close the
        connection.

    DCCP-Close
        Used to close the connection; elicits a DCCP-Reset in response.

    DCCP-Reset
        Used to terminate the connection, either normally or abnormally.

    DCCP-Move
        Supports multihoming and mobility.

    DCCP-Sync, DCCP-SyncAck
        Used to resynchronize sequence numbers after large bursts of
        loss.

4.2.  Sequence Numbers

    Each DCCP packet carries a sequence number, so that losses can be
    detected and reported.  But unlike TCP's byte-based sequence
    numbers, DCCP sequence numbers are attached to packets.  Each packet
    sent increments the sequence number by one.  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)

    Note that even DCCP-Ack pure acknowledgements increment the sequence
    number; after the DCCP-Ack with sequence number 10, the following
    DCCP-Data packet uses the next sequence number, 11.  This lets the
    endpoints tell when acknowledgements are lost in the network.  It
    also means that endpoints can get out of sync after a long burst of
    loss.  The DCCP-Sync and DCCP-SyncAck packet types let DCCP recover
    from large loss bursts; see Section 7.5.

    Also note that, since DCCP is an unreliable protocol, there are no
    retransmissions, and it doesn't make sense to have a cumulative
    acknowledgement field.  Acknowledgement Number (ackno) fields equal
    the largest sequence number received, rather than the TCP-style
    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 client and server's typical progress through states.

    The states are as follows; Section 8 describes them in more detail.

    CLOSED
        Represents a nonexistent connection.

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

    REQUEST
        The 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
        The 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 data in this state, but it MUST include an
        Acknowledgement Number on all of its packets.

    OPEN
        The central, data transfer portion of a DCCP connection.  Client



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        and server enter into this state from PARTOPEN and RESPOND,
        respectively.  Sometimes we speak of SERVER-OPEN and CLIENT-OPEN
        states, corresponding to the server's OPEN state and the
        client's OPEN state.

    CLOSEREQ
        A server socket enters this state, from SERVER-OPEN, to signal
        that the connection is over, but the client must hold TIMEWAIT
        state.

    CLOSING
        Either server or client can enter this state to close the
        connection.

    TIMEWAIT
        A socket remains in this state for 2MSL after the connection has
        been torn down, to prevent mistakes due to the delivery of old
        packets.  One MSL, or Maximum Segment Lifetime, is the maximum
        length of time a packet could survive in the network.

4.4.  Congestion Control

    DCCP connections are congestion controlled.  Unlike TCP, however,
    DCCP supports multiple congestion control mechanisms for
    applications to choose from.  In fact, the two half-connections can
    be governed by different mechanisms.  Each mechanism corresponds to
    a one-byte congestion control identifier, or CCID.  A CCID describes
    how the HC-Sender limits data packet rates; how it maintains
    necessary parameters, such as congestion windows; how the HC-
    Receiver sends congestion feedback via acknowledgements; and how it
    manages the acknowledgement rate.

    The endpoints negotiate their CCIDs during connection initiation.
    So far, CCIDs 2 and 3 have been defined for use with DCCP; CCID 0 is
    reserved, and CCID 1 is used for special purposes (see Section
    10.1).

    CCID 2 corresponds to TCP-like Congestion Control, which is similar
    to that of TCP.  The sender maintains a congestion window and sends
    packets until that window is full.  Packets are acknowledged by the
    receiver.  Dropped packets and ECN [RFC 3168] are 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 super
    selective-acknowledgement (SACK, [RFC 3517]).

    CCID 3 provides TFRC Congestion Control, an equation-based form of
    congestion control which is intended to provide a smoother response



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    to congestion than CCID 2.  The sender maintains a "transmit rate".
    The receiver sends acknowledgement packets containing information
    about the receiver's estimate of packet loss.  The sender uses this
    information to update its transmit rate.  Although CCID 3 behaves
    somewhat differently from TCP in its short term congestion response,
    it is designed to operate fairly with TCP over the long term.

    The behaviors of CCIDs 2 and 3 are fully defined in separate profile
    documents [CCID 2 PROFILE] [CCID 3 PROFILE].

4.5.  Features

    Agreement on DCCP feature values is achieved by explicit
    negotiation, using options in DCCP packet headers.  This generally
    happens at connection startup, but negotiation can begin at any
    time.  The relevant options are Change L, Confirm L, Change R, and
    Confirm R, with the "L" options sent by the feature location and the
    "R" options sent by the feature remote.

    A Change R message says to the peer, "change this feature value on
    your side".  The peer responds with a Confirm L, meaning "I've
    changed it".  The suggested option setting in Change R can sometimes
    contain multiple values, which are 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 *

    In the second exchange, the client requests that the server use
    either CCID 3 or CCID 4, with 3 preferred.  The server chooses 4,
    giving its preference list of "4 2".

    A party that wants to change a feature located at itself issues a
    "Change L" option, which elicits a "Confirm R" in reply.

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




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    In this example, the server requests CCID value 3 or 2 for the
    server's CCID, with 3 preferred, and the client agrees.

    Retransmissions make feature negotiation reliable. Section 6
    describes these options further.

4.6.  Other Differences from TCP

    Interesting differences between DCCP and TCP, apart from those
    discussed so far, include:

    o Copious space for options (up to 1020 bytes).

    o Different acknowledgement formats.  The CCID for a connection
      determines how much ack information needs to be transmitted. In
      CCID 2 (TCP-like), this is about one ack per 2 packets, and each
      ack must declare exactly which packets were received; in CCID 3
      (TFRC), it's about one ack per RTT, and acks must declare at
      minimum just the lengths of recent loss intervals.

    o Denial-of-service (DoS) protection.  Several DCCP mechanisms
      attempt to let servers limit the amount of state possibly-
      misbehaving clients can force them to maintain.  An Init Cookie
      option, analogous to TCP's SYN Cookies [SYNCOOKIES], avoids SYN-
      flood-like attacks.  Only one connection endpoint need hold
      TIMEWAIT state; the DCCP-CloseReq packet, which may only be sent
      by the server, passes that state to the client.  Various rate
      limits let servers avoid attacks that might force extensive
      computation or packet generation.

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

    o Acknowledgement readiness.  In TCP, a packet is acknowledged only
      when the data is queued for delivery to the application.  This
      does not make sense in DCCP, where an application might request a
      drop-from-front receive buffer, for example.  We acknowledge a
      packet when its options have been processed.  The Data Dropped
      option may later say that the packet's payload was discarded.

    o Integrated support for mobility and multihoming via the DCCP-Move
      packet type.





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    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.

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 -->
                                             <-- 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 some data on the DCCP-Request packet---an application-
        level request, say---which the server may ignore.

    2.  The server sends the client a DCCP-Response packet indicating
        that it is willing to communicate with the client.  The response
        indicates any features and options that the server agrees to,
        begins or continues other feature negotiations if desired, and
        optionally includes an Init Cookie that wraps up all this
        information and which must be returned by the client for the
        connection to complete.

    3.  The client sends the server a DCCP-Ack packet that acknowledges
        the DCCP-Response packet.  This acknowledges the server's
        initial sequence number and returns the Init Cookie if there was



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        one in the DCCP-Response.  It may also continue feature
        negotiation.  There might follow zero or more DCCP-Ack exchanges
        as required to finalize feature negotiation.  The client may
        piggyback an application-level request on its final ack,
        producing a DCCP-DataAck packet.

    4.  The server and client then exchange DCCP-Data packets, DCCP-Ack
        packets acknowledging that data, and, optionally, DCCP-DataAck
        packets containing piggybacked data and 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.

    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.  In DCCP, unlike TCP,
        Resets are part of normal connection termination; see Section
        5.6.

    8.  The client receives the DCCP-Reset packet and holds state for a
        reasonable interval of time 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 a
        reasonable interval of time to allow any remaining packets to
        clear the network.

5.  Header Formats

    The variable-length DCCP header appears first in every DCCP packet.
    A header can be from 12 to 1020 bytes long.  The initial 12 bytes of
    the header are the same regardless of packet type.  Following this
    comes optional additional fixed-length fields, depending on the
    packet type, and then a variable-length list of options.  Finally,
    some packet types include application data.





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


5.1.  Generic Header

    The DCCP generic header generally takes 12 bytes.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Type  |X| Res |              Sequence Number                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Actually, there are two types of generic header, depending on the
    value of X, the Extended Sequence Numbers bit.  If X is zero, the
    Sequence Number field takes 24 bits, as above.  If X is one, the
    Sequence Number field extends for an additional 24 bits, for a total
    of 48:

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


    Source and Destination Ports: 16 bits each
        These fields identify the connection, similar to the
        corresponding fields in TCP and UDP.  The Source Port represents
        the relevant port on the endpoint that sent this packet, the



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        Destination Port the relevant port on the other endpoint.
        Source Ports SHOULD be chosen randomly, to reduce the likelihood
        of attack.

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

    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.  CCVal MUST be set to zero unless the HC-
        Sender CCID specifies a different value.

    Checksum Coverage (CsCov): 4 bits
        Checksum Coverage specifies what parts of the packet are covered
        by the Checksum field.  This always includes the DCCP header and
        options, but if applications request it, some or all of the
        application data may be excluded.  This can improve performance
        on noisy links, assuming the application 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, some or all of the application data.  See
        Section 9.

    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-Move
          9    DCCP-Sync
         10    DCCP-SyncAck
        11-15  Reserved




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    Extended Sequence Numbers (X): 1 bit
        This bit is set to one to indicate the use of an extended
        generic header with 48-bit Sequence and Acknowledgement Numbers.
        Very-high-rate connections SHOULD set X to one, and use 48-bit
        sequence numbers, to gain increased protection against wrapped
        sequence numbers and attacks.  See Section 7.6.

    Reserved (Res): 3 bits
        The version of DCCP specified here MUST ignore this field on
        received packets, and MUST set it to all zeroes on generated
        packets.

    Sequence Number: 24 or 48 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.

    Sequence Number Transition (T): 1 bit [X=1 only]
        Set to one to indicate an ongoing transition from 24-bit to
        48-bit sequence numbers.  See Section 7.6.

    Many packet types also carry an Acknowledgement Number in the four
    or eight bytes immediately following the generic header.  When X=0,
    its format is:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    And when X=1:

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


    Acknowledgement Number: 24 or 48 bits
        The Acknowledgement Number field generally acknowledges the
        greatest valid sequence number received so far on this
        connection.  ("Greatest" is, of course, measured in circular
        sequence space.)  Acknowledgement numbers make no attempt to
        provide precise information about which packets have arrived;
        options such as the Ack Vector do this.





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    Reserved: 8 bits
        The version of DCCP specified here MUST ignore these fields on
        received packets, and MUST set them to all zeroes on generated
        packets.

5.2.  DCCP-Request Header

    A client initiates a DCCP connection by sending a DCCP-Request
    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 (12 or 16 bytes)             /
     /                   with Type=0 (DCCP-Request)                  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Service Code                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |


    Service Code: 32 bits
        Describes the service to which the client application wants to
        connect.  Examples might include RTSP and DOOM.  Service Codes
        are intended to make application protocols independent of well-
        known ports, and help middleboxes identify the protocol used on
        a given connection.  See Section 8.1.2.

5.3.  DCCP-Response Header

    The server responds to valid DCCP-Request packets with DCCP-Response
    packets.  This is the second phase of the three-way handshake.
















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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 (12 or 16 bytes)             /
     /                  with Type=1 (DCCP-Response)                  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Service Code                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |


    Acknowledgement Number: 24 or 48 bits
        The Acknowledgement Number field will generally equal the
        Sequence Number from the DCCP-Request.

    Service Code: 32 bits
        Echoes the Service Code on the DCCP-Request.

5.4.  DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers

    The central data transfer portion of every DCCP connection uses
    DCCP-Data, DCCP-Ack, and DCCP-DataAck packets.  DCCP-Data packets
    carry application data.

      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 (12 or 16 bytes)             /
     /                    with Type=2 (DCCP-Data)                    /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |

    DCCP-Ack packets dispense with the data, but contain an
    Acknowledgement Number.  They are used for pure 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 (12 or 16 bytes)             /
     /                    with Type=3 (DCCP-Ack)                     /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    DCCP-DataAck packets carry both application data and an
    Acknowledgement Number: acknowledgement information is piggybacked
    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 (12 or 16 bytes)             /
     /                  with Type=4 (DCCP-DataAck)                   /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |

    DCCP-Data and DCCP-DataAck packets may contain zero application data
    bytes if the application sends a zero-length datagram.  Also, a
    DCCP-Ack packet need not have a zero-length application data area.
    The receiver MUST ignore any "application data" in a DCCP-Ack
    packet.  The sender will not generally send such data, but it may
    occasionally do so---to perform PMTU discovery without risking loss
    of user data, for example.

    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 Headers

    DCCP-CloseReq and DCCP-Close packets begin the handshake that
    normally terminates a connection.  Either client or server may send



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    a DCCP-Close packet, which will elicit a DCCP-Reset packet (see the
    next section).  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.

      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 (12 or 16 bytes)             /
     /         with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close)         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    The receiver MUST ignore any "application data" in a DCCP-CloseReq
    or DCCP-Close packet.

5.6.  DCCP-Reset Header

    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.

      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 (12 or 16 bytes)             /
     /                   with Type=7 (DCCP-Reset)                    /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Reset Code   |    Data 1     |    Data 2     |    Data 3     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                          Error Text                           |
     |                              ...                              |


    Reset Code: 8 bits
        Represents the reason that the sender reset the DCCP connection.



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    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 Reason.

    Error Text (application data area)
        If present, Error Text is a human-readable text string,
        preferably in English and encoded in Unicode UTF-8, that
        describes the error in more detail.  For example, a DCCP-Reset
        with Reset Code 12, "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.  The "Data" columns
    describe what the Data fields contain for a given Code.  N/A means
    the Data field MUST be set to 0 by the sender of the DCCP-Reset and
    ignored by its receiver.

     Reset                                                Section
     Code   Name                   Data 1 Data 2 Data 3  Reference
     -----  ----                   ------ ------ ------  ---------
       0    Unspecified             N/A    N/A    N/A
       1    Closed                  N/A    N/A    N/A      8.3
       2    Aborted                 N/A    N/A    N/A      8.1.1
       3    No Connection           N/A    N/A    N/A      8.3.1
       4    Packet Error           packet  N/A    N/A      8.3.1
                                    type
       5    Option Error           option  option data
                                   number   (if any)
       6    Mandatory Error        option  option data     5.9.2
                                   number   (if any)
       7    Extended Seqnos         N/A    N/A    N/A      7.6
       8    Connection Refused      N/A    N/A    N/A      8.1.3
       9    Bad Service Code        N/A    N/A    N/A      8.1.3
      10    Too Busy                N/A    N/A    N/A      8.1.3
      11    Bad Init Cookie         N/A    N/A    N/A      8.1.4
      12    Aggression Penalty      N/A    N/A    N/A      12.2
      13    Move Refused            N/A    N/A    N/A      14.4
     13-127 Reserved
    128-255 CCID-specific codes      ... variable ...      10.4


5.7.  DCCP-Move Header

    The DCCP-Move packet type is part of DCCP's support for multihoming
    and mobility, which is described further in Section 14. DCCP A sends
    a DCCP-Move packet to DCCP B after changing its address and/or port
    number.  The DCCP-Move packet requests that DCCP B start sending



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    packets to a new address and port number, which are read off the
    packet's network header and generic DCCP header.  The old address
    and port are defined through a Mobility ID, which provides some
    protection against hijacked connections.

      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 (12 or 16 bytes)             /
     /                    with Type=8 (DCCP-Move)                    /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Mobility ID (high bits)                    .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                   Mobility ID (bits 64-95)                    .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                   Mobility ID (bits 32-63)                    .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                    Mobility ID (low bits)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Mobility ID: 128 bits
        The value of the receiver's Mobility ID feature.  This value
        uniquely identifies the current connection among the set of
        connections terminating at the receiver (meaning, the stationary
        endpoint); it MUST have been set in an earlier exchange.  See
        Section 14.2.

    The receiver MUST ignore any "application data" in a DCCP-Move
    packet.

5.8.  DCCP-Sync and DCCP-SyncAck Headers

    DCCP-Sync packets help DCCP endpoints recover synchronization after
    bursts of loss, or recover from half-open connections.  Each valid
    DCCP-Sync received immediately elicits a DCCP-SyncAck.









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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 (12 or 16 bytes)             /
     /          with Type=9 (DCCP-Sync) or 10 (DCCP-SyncAck)         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |            Acknowledgement Number             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)when
    (.       Acknowledgement Number (low bits)       |   Reserved    |)X=1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    The Acknowledgement Number on DCCP-Sync and DCCP-SyncAck packets
    need not equal the generating endpoint's greatest valid sequence
    number received (GSR).  This differs from Acknowledgement Numbers on
    all other packet types.  If a DCCP-Sync was generated in response to
    a packet with invalid sequence numbers, then the DCCP-Sync's
    Acknowledgement Number will equal the invalid packet's sequence
    number.  The Acknowledgement Number on any DCCP-SyncAck packet MUST
    correspond to a received, valid DCCP-Sync's Sequence Number; in the
    presence of reordering, this might not equal GSR.

    The receiver MUST ignore any "application data" in a DCCP-Sync or
    DCCP-SyncAck packet.

5.9.  Options

    All DCCP packets may contain options, which occupy space at the end
    of the DCCP header.  Each option is a multiple of 8 bits in length.
    The combination of all options MUST add up to a multiple of 32 bits.
    Individual options are not padded to multiples of 32 bits, however;
    any option may begin on any byte boundary.  All options are always
    included in the checksum.

    The first byte of an option is the option type.  Options with types
    0 through 31 are single-byte options.  Other options are followed by
    a byte indicating the option's length.  This length value includes
    the two bytes of option-type and option-length as well as any
    option-data bytes, and must therefore be greater than or equal to
    two.

    Options are processed sequentially, starting at the first option in
    the packet header.

    The following options are currently defined:





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             Option                            Section
     Type    Length     Meaning               Reference
     ----    ------     -------               ---------
       0        1       Padding                 5.9.1
       1        1       Mandatory               5.9.2
       2        1       Slow Receiver           11.6
     3-31       1       Reserved
      32     variable   Change L                6.1
      33     variable   Confirm L               6.2
      34     variable   Change R                6.1
      35     variable   Confirm R               6.2
      36     variable   Init Cookie             8.1.4
      37       4-5      NDP Count               7.7
      38     variable   Ack Vector [Nonce 0]    11.4
      39     variable   Ack Vector [Nonce 1]    11.4
      40     variable   Data Dropped            11.7
      41        6       Timestamp               13.1
      42       6-10     Timestamp Echo          13.3
      43       4-6      Elapsed Time            13.2
      44        4       Data Checksum           9.3
     45-127  variable   Reserved
    128-255  variable   CCID-specific options   10.4

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

5.9.1.  Padding Option

    The Padding option, with type 0, is a single byte option used to pad
    between or after options.  It either ensures the application data
    begins on a 32-bit boundary (as required), or ensures alignment of
    following options (not mandatory).

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


5.9.2.  Mandatory Option

    The Mandatory option, with type 1, is a single byte option that
    indicates that the immediately following option is mandatory.  If
    the receiving DCCP does not understand that following option, it
    MUST reset the connection, generally using Reset Code 6, "Mandatory
    Failure".  For instance, say DCCP A receives a packet with two
    options: a Mandatory option, and immediately following, another
    option O.  Then DCCP A would reset the connection if it did not



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    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 DCCP A did not understand the enclosed feature number;
    if DCCP A understood O, but chose not to perform the action O
    implies; and so forth.  Section 6.6.8 describes the behavior of
    Mandatory feature negotiation options in more detail.

    +--------+
    |00000001|
    +--------+
      Type=1


6.  Feature Negotiation

    Four DCCP options, Change L, Confirm L, Change R, and Confirm R,
    implement in-band feature negotiation.  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 feature values are generally
    arranged in a linear preference list, where the first value is most
    preferred.

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

6.1.  Change Options

    Change L and Change R options initiate feature negotiation.  Either
    endpoint can start a negotiation for any feature; if DCCP A wants to
    start a negotiation for feature F/A, it will send a Change L option,
    while to start a negotiation for F/B, it will send a Change R
    option.  Change options are retransmitted until some response is
    received.  Normal Change options contain at least one Value, and
    thus have length 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

    The endpoint may check a feature's current value without attempting
    to change it by sending an empty Change option, containing just the
    feature number.  Such options have length 3.  The endpoints must
    agree on feature values anyway, so these options are useful in
    practice only in special situations, such as when a middlebox
    introduced in the middle of a connection wants to check a feature
    value.

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.  Normal Confirm options
    contain the selected Value, possibly followed by 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 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



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    the contents of Change options sent by the two endpoints.

    All current DCCP features use one of two reconciliation rules,
    server-priority ("SP") and non-negotiable ("NN").

6.3.1.  Server-Priority

    The feature value is a fixed-length byte string (length determined
    by the feature number).  Each Change option contains a preference
    list of values, 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.  Even responses
    to empty Change options contain the whole 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 6.6.8).

    DCCP endpoints need not calculate their value preference lists
    before feature negotiation begins.  Thus, a server might adjust its
    preference list based on the client's preference list, assuming the
    client opened the negotiation.  Once a negotiation for a feature has
    begun, however, the preference lists MUST remain stable until the
    negotiation has closed.

6.3.2.  Non-Negotiable

    The feature value is a byte string.  Each option contains exactly
    one feature value.  The feature location signals a value change by
    sending Change L options.  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.  Non-negotiable features aren't really negotiated; they use
    feature negotiation as a mechanism for achieving reliability.
    Change R and Confirm L options MUST NOT be sent for non-negotiable
    features.

6.4.  Feature Numbers

    This document defines the following feature numbers.







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                                           Rec'n Initial        Section
    Number   Meaning                       Rule   Value  Req'd Reference
    ------   -------                       -----  -----  ----- ---------
       0     Reserved
       1     Congestion Control ID (CCID)   SP      2      Y     10
       2     ECN Capable                    SP      1      Y     12.1
       3     Sequence Window                NN     100     Y     7.5.4
       4     Sequence Transition Capable    SP      0      N     7.6.4
       5     Mobility Capable               SP      0      N     14.1
       6     Mobility ID                    NN      0      N     14.2
       7     Ack Ratio                      NN      2      N     11.3
       8     Send Ack Vector                SP      0      N     11.5
       9     Send NDP Count                 SP      0      N     7.7.2
      10     Check Data Checksum            SP      0      N     9.3.1
     11-127  Reserved
    128-255  CCID-specific features          ?      ?      ?     10.4


    Rec'n Rule     The reconciliation rule used for the feature.  SP is
                   server-priority and NN is non-negotiable.

    Initial Value  The initial value for the feature.  Every feature has
                   a known initial value.

    Req'd          This column is "Y" iff every DCCP implementation MUST
                   understand the feature.  If it is "N", then the
                   feature behaves like an extension (see Section 16),
                   and it is safe to respond to Change options for the
                   feature with empty Confirm options.  Of course, a
                   CCID might require the feature; a DCCP that
                   implements CCID 2 MUST support Ack Ratio and Send Ack
                   Vector, for example.

6.5.  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 value 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 *






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     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)
                 (both endpoints in CHANGING)
    2a.                        <--  Confirm L(CCID, 3, 3 2 1)
     b. Confirm R(CCID, 3, 2 3 1)  -->
                 (both endpoints in STABLE)
                 * agreement that CCID/Server = 3 *

    Example Change and Confirm options follow, with their byte
    encodings.  Each option is sent by DCCP A.

    Change L(CCID, 2 3) = 32,5,1,2,3
        I want to change CCID/A's value (feature number 1, a server-
        priority feature); my preferred values are 2 and 3, in that
        preference order.

    Change L(Sequence Window, 1024) = 32,6,3,0,4,0
        Change Sequence Window/A's value (feature number 3, a non-
        negotiable feature) to the 3-byte string 0,4,0 (the value 1024).

    Empty Change L(CCID) = 32,3,1
        Tell me CCID/A's value using a Confirm R option.

    Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3
        I've changed CCID/A's value to 2; my preferred values are 2 and
        3, in that preference order.

    Empty Confirm L(126) = 33,3,126
        I don't implement feature number 126, or your proposed value for
        feature 126/A was invalid.

    Change R(CCID, 3 2) = 34,5,1,3,2
        Please change CCID/B's value; my preferred values are 3 and 2,
        in that preference order.



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    Empty Change R(CCID) = 34,3,1
        Tell me CCID/B's value using a Confirm L option.

    Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2
        I've changed CCID/B's value to 2; my preferred values were 3 and
        2, in that preference order.

    Confirm R(Sequence Window, 1024) = 35,6,3,0,4,0
        I've changed Sequence Window/B's value to the 3-byte string
        0,4,0 (the value 1024).

    Empty Confirm R(126) = 35,3,126
        I don't implement feature number 126, or your 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 some response is
        received.

    3.  Preference lists don't change during a negotiation.

    4.  Feature negotiation options are processed in strictly increasing
        order by Sequence Number.

    The rest of this section describes the consequences of these rules
    in more detail.

6.6.1.  Normal Exchange

    Change options are generated when a DCCP endpoint wants to change
    the value of some feature.  Generally, this will happen at the
    beginning of a connection, although it may happen at any time.  We
    say the endpoint "generates" or "sends" a Change L or Change R
    option; but, of course, the option must be attached to a packet.
    The endpoint may attach the option to a packet it would have
    generated anyway (such as a DCCP-Request), or it may create a new
    packet just to carry the options (often a DCCP-Sync).  If it does
    create a new packet, it MUST NOT create more than one such packet
    per round-trip time (or 0.2 seconds, if no RTT is available).

    On receiving a Change L or Change R option, a DCCP endpoint examines
    the included preference list, reconciles that with its own



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    preference list, calculates the new value, and sends back a
    Confirm R or Confirm L option, respectively, informing its partner
    of the new value.  The rule for reconciling the two preference lists
    is feature-specific; see Section 6.3. Every non-reordered Change
    option MUST result in a corresponding Confirm option.  Any packet
    including a Confirm option MUST carry an Acknowledgement Number;
    thus, Confirm options are not allowed on DCCP-Request and DCCP-Data
    packets.  Again, generated Confirm options may be attached to
    packets that would have been sent anyway (such as DCCP-Response or
    DCCP-SyncAck), or to new packets (usually DCCP-Ack).

    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.

    DCCP endpoints effectively exist in one of two states, STABLE and
    CHANGING, 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.

6.6.2.  Loss and Retransmission

    Packets containing Change and Confirm options might be lost or
    delayed by the network.  Therefore, Change options are retransmitted
    to achieve reliability.

    A CHANGING endpoint retransmits a Change option once it realizes
    that it has not heard back from the other endpoint.  Each
    retransmitted Change option MUST contain exactly the same payload as
    the original. The endpoint may piggyback its Change options on
    packets it would have sent anyway.  If it generates new packets for
    feature negotiation, it MUST use an exponential-backoff timer.  The
    timer's initial value is set to approximately one or two round-trip
    times (or 0.2-0.4 seconds, if no RTT is available), and it is pinned
    at roughly 32 RTTs.

    A CHANGING endpoint MUST continue retransmitting Change options
    until it gets some response.  Its only recourse is to reset the
    connection, which it SHOULD NOT do until at least 12 transmissions
    have failed.

    Change options SHOULD NOT be transmitted more frequently than once
    per RTT, or the reordering protection below would prevent any
    Confirm option from being accepted (since no Confirm would
    acknowledge the most recently transmitted Change).



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    Confirm options are never retransmitted, but the Confirm-sending
    endpoint MUST generate a new Confirm option for every non-reordered
    Change it receives.

6.6.3.  Reordering

    Reordering might cause packets containing Change and Confirm options
    to arrive in an unexpected order.  Endpoints MUST be robust to
    reordering, by ignoring feature negotiation options that do not
    arrive in strictly-increasing order by Sequence Number.

    The most straightforward way to implement this requirement is for an
    endpoint to associate two sequence number variables with every
    feature F/X, as follows.

    F/X.GSR   The Greatest Sequence Number Received from the other
              endpoint on a packet containing a Change or Confirm option
              for feature F/X.

    F/X.GSS   The Greatest Sequence Number Sent by this endpoint on a
              packet containing a Change option for feature F/X.

    Then DCCP A will check options relating to feature F/A as follows:

    1.  Ignore any received Change R(F) option whose packet's Sequence
        Number is not greater than F/A.GSR.

    2.  Ignore any received Confirm R(F) option whose packet's Sequence
        Number is not greater than F/A.GSR, or whose packet could not
        have acknowledged F/A.GSS.  Specifically, if the Acknowledgement
        Number is less than F/A.GSS, the endpoint MUST ignore the
        Confirm; and if the packet has an Ack Vector indicating that
        F/A.GSS was not received, the endpoint MAY ignore the Confirm.

    A similar procedure applies options relating to feature F/B, namely
    Change L(F) and Confirm L(F), except that F/B.GSR and F/B.GSS are
    checked.

    A less state-intensive way to implement this requirement would be to
    share the F.GSR and F.GSS variables among all features, rather than
    keeping one pair per feature.  Then the feature negotiation options
    on any received packet would be treated as a unit (either all
    accepted or all rejected).

    Checking Confirm options is easier if the endpoint only sends Change
    options on packet types that will be acknowledged immediately,
    namely DCCP-Request, DCCP-Response, and DCCP-Sync.  Then there is
    never any need to check Ack Vectors, although checking Ack Vectors



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    is NOT MANDATORY anyway.

6.6.4.  Preference Changes

    Endpoints MUST NOT change their preference lists in the middle of a
    negotiation.  This is because, if a preference list changed in the
    middle of a negotiation and the right packets were lost, the
    negotiation could terminate with the endpoints thinking the feature
    had different values.  In particular, an endpoint MUST NOT change
    its preference list while in the CHANGING state; this ensures that
    every Change option sent during that negotiation will contain the
    same data.

6.6.5.  Simultaneous Negotiation

    The two endpoints might simultaneously open negotiation for the same
    feature, after which an endpoint in the CHANGING state will receive
    a Change option for the same feature.  Such received Change options
    can act as responses to the original Change options.  The CHANGING
    endpoint MUST examine the received Change's preference list,
    reconcile that with its own preference list (as expressed in its
    generated Change options), and generate the corresponding Confirm
    option.  It can then transition to the STABLE state.

6.6.6.  Unknown Features

    An endpoint may receive a Change option referring to some feature
    number it does not understand.  This is particularly likely to
    happen when an extended DCCP converses with a non-extended DCCP.
    The receiving endpoint MUST respond to such Change options with
    corresponding 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
    preceded by a Mandatory option, the connection MUST be reset; see
    Section 6.6.8.

    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 16 suggests that the default
    value for any extension feature should correspond to "extension not
    available".

    An endpoint will also send an empty Confirm option when it
    understood the Change's feature number, but considered the Change's
    value invalid or inappropriate for the feature.  The next section
    describes this further.





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    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.  Endpoints MUST either
    reset the connection on receiving such options, or just ignore the
    options.

6.6.7.  Invalid Options

    A DCCP endpoint might receive a Change or Confirm option that lists
    one or more values that it does not understand.  Some, but not all,
    such options are invalid, depending on the relevant reconciliation
    rule (Section 6.3). For instance:

    o All features have length limitiations, and options with invalid
      lengths are invalid.  For example, the Mobility ID feature takes
      128-bit values, so valid "Confirm R(Mobility ID)" options have
      option length 19.

    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.

    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".

6.6.8.  Mandatory Feature Negotiation

    Change options may be preceded by Mandatory options (Section 5.9.2).
    Mandatory Change options are processed like normal Change options,
    except that various failure cases will cause the receiver to reset
    the connection with Reset Code 6, "Mandatory Failure", rather than
    send a Confirm option.  Specifically, the connection MUST be reset
    if:

    o The Change option's feature number was not understood;




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    o The Change option's value was invalid, and the receiver would
      normally have sent an empty Confirm option in response; or

    o For server-priority features, there was no shared entry in the two
      endpoints' preference lists.

    There's no reason to mark Confirm options as Mandatory in this
    version of DCCP, since Confirm options are sent only in response to
    Change options and therefore can't mention potentially-invalid
    values or unexpected feature numbers.

6.6.9.  Out-of-Band Agreement

    An endpoint MUST NOT unilaterally change the value of any DCCP
    feature.  However, endpoints MAY cooperatively change DCCP feature
    values without using in-band feature negotiation options---by using
    a separate signalling channel, for example.

6.6.10.  State Diagram

    This diagram illustrates feature-related state transitions, ignoring
    sequence number and option validity issues, for the endpoint that is
    the feature location.  For a feature remote state transition
    diagram, switch the "L"s and "R"s.

  rcv Confirm R        app/protocol evt : snd Change L
  : ignore      +--------------------------------------------+
       +----+   |                                            |
       |    v   |                    rcv Change R            v
    +------------+  rcv Confirm R    : calc new value, +------------+
    |            |  : accept value     snd Confirm L   |            |
    |   STABLE   |<------------------------------------|  CHANGING  |
    |            |         rcv empty Confirm R         |            |
    +------------+         : revert to old value       +------------+
        |    ^                                             |    ^
        +----+                                             +----+
  rcv Change R                                      timeout/rcv non-ack
  : calc new value, snd Confirm L                   : snd Change L

    This state diagram corresponds to the following procedure for
    reacting to received packets with feature negotiation options.  The
    procedure refers to "P.seqno", "P.ackno", "P.optiontype", and
    "P.optionlen", which are properties of the packet; "F.GSR" and
    "F.GSS", which are the variables mentioned in Section 6.6.3;
    "F.state", which is the feature's state (STABLE or CHANGING); and
    "F.value", which is the feature's value.





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    If F.state == STABLE:
       If P.optiontype == Change R && P.seqno > F.GSR:
          Calculate new value
          Send Confirm L on next packet
          F.GSR := P.seqno
       Otherwise:
          Ignore option

    If F.state == CHANGING:
       If P.optiontype == Confirm R && P.ackno >= F.GSS
             && P potentially acknowledges F.GSS:
          If P.optionlen == 3:
             /* empty Confirm R option */
             Retain old value
          Otherwise:
             Check new value
             F.value := new value
          F.state := STABLE
       Otherwise, if P.optiontype == Change R && P.seqno > F.GSR:
          Calculate new value
          Send Confirm L on next packet
          F.GSR := P.seqno
       Otherwise:
          Ignore option

7.  Sequence Numbers

    DCCP uses 24- or 48-bit sequence numbers to arrange packets into
    sequence, detect losses and network duplicates, and protect against
    attackers, half-open connections, and the delivery of very old
    packets.  Every packet carries a Sequence Number; most packet types
    carry an Acknowledgement Number as well.

    DCCP sequence numbers are per-packet.  Thus, each endpoint
    increments the DCCP Sequence Number field by one (modulo 2^24 or
    2^48) with every packet sent.  Even DCCP-Ack and DCCP-Sync packets,
    and other packets that don't carry user data, increment the Sequence
    Number.  Since DCCP is an unreliable protocol, there are no true
    retransmissions; but effective retransmissions, such as
    retransmissions of DCCP-Request packets, also increment the Sequence
    Number.  This lets DCCP implementations detect network duplication,
    retransmissions, and acknowledgement loss, and is a significant
    departure from TCP practice.

7.1.  Variables

    DCCP endpoints maintain a set of sequence number variables for each
    connection.



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    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.
            ("Greatest" is of course measured in circular sequence
            space.)

    GSR     The Greatest Sequence Number Received from the other
            endpoint on an acknowledgeable packet.  (Section 7.4 defines
            "acknowledgeable" packets.)

    GAR     The Greatest Acknowledgement Number Received from the other
            endpoint on an acknowledgeable packet.

    Some other variables are derived from these primitives.

    SWL and SWH
            (Sequence Number Window Low and High)  The extremes of the
            validity window for received packets' Sequence Numbers.

    AWL and AWH
            (Acknowledgement Number Window Low and High)  The extremes
            of the validity window for received packets' Acknowledgement
            Numbers.

7.2.  Initial Sequence Numbers

    The endpoints' initial sequence numbers are set by the first DCCP-
    Request and DCCP-Response packets sent.  Initial sequence numbers
    MUST be chosen to avoid two problems:

    o Delivery of old packets, where packets lingering in the network
      from an old connection are delivered to a new connection with the
      same addresses and port numbers.

    o Sequence number attacks, where an attacker can guess the sequence
      numbers that a future connection would use [M85].

    DCCP implementations may use TCP's strategies for avoiding these
    problems [RFC 793] [RFC 1948].

    To address the first problem, an implementation MUST ensure that the
    initial sequence number for a given <source address, source port,



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    destination address, destination port> 4-tuple doesn't overlap with
    recent sequence numbers on connections with the same 4-tuple
    ("recent" meaning sent within 2 maximum segment lifetimes).  If the
    implementation has state for a recent connection with the same
    4-tuple, it can simply pick a good initial sequence number;
    otherwise, it could tie initial sequence number selection to some
    clock, such as the 4-microsecond clock used by TCP [RFC 793].

    To address the second problem, an implementation MUST provide each
    4-tuple with an independent initial sequence number space; then an
    attacker can't learn anything about anyone else's initial sequence
    numbers.  RFC 1948 achieves this by adding a cryptographic hash, of
    the 4-tuple and a secret, to any initial sequence number.  For the
    secret, RFC 1948 recommends a combination of some truly-random data
    [RFC 1750], an administratively-installed passphrase, the endpoint's
    IP address, and the endpoint's boot time, but truly-random data is
    sufficient.  Care should be taken when changing the secret; such a
    change alters all initial sequence number spaces, which might make
    an initial sequence number for some 4-tuple equal a recently sent
    sequence number for the same 4-tuple.  To avoid this problem around
    such a change, the endpoint might remember dead connection state for
    each 4-tuple or stay quiet for 2 maximum segment lifetimes.

7.3.  Quiet Time

    DCCP endpoints, like TCP endpoints, must take care before initiating
    connections when they boot.  In particular, they MUST NOT send
    packets whose sequence numbers are close to the sequence numbers of
    packets lingering in the network from before the boot.  The simplest
    way to enforce this rule is for DCCP endpoints to avoid sending any
    packets until one maximum segment lifetime (2 minutes) after boot.
    Other enforcement mechanisms include remembering recent sequence
    numbers across boots, or reserving the upper 8 or so bits of initial
    sequence numbers for a persistent boot counter that decrements by
    two each boot (this would require the use of extended sequence
    numbers).

7.4.  Acknowledgement Numbers

    DCCP has no cumulative acknowledgement field; cumulative
    acknowledgements would be meaningless in an unreliable protocol.
    Therefore, the Acknowledgement Number field has a different meaning
    in DCCP than in TCP.

    A packet is classified as "acknowledgeable" if and only if its
    options were processed by the receiving DCCP.  This means, for
    example, that all acknowledgeable packets have valid header
    checksums and sequence numbers.  The Acknowledgement Number for most



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    packet types MUST equal GSR, the Greatest Sequence Number Received
    on an acknowledgeable packet.

    Note that "acknowledgeable" refers to option processing, not 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 are a special case to this rule.
    The Acknowledgement Number on a DCCP-Sync packet corresponds to a
    received packet, but not necessarily an acknowledgeable packet; in
    particular, it might correspond to an out-of-sync packet whose
    options were not processed.  The Acknowledgement Number on a DCCP-
    SyncAck packet always corresponds to an acknowledgeable DCCP-Sync
    packet; if there was reordering, that Acknowledgement Number might
    be less than GSR.

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.

    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 involving DCCP-Sync and DCCP-SyncAck packets
    recovers from this case.

7.5.1.  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 windows, [SWL, SWH] and [AWL,
    AWH], defined below in Section 7.5.3.





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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    SWL <= seqno <= SWH         AWL <= ackno <= AWH
    DCCP-Close       SWL <= seqno <= SWH         AWL <= ackno <= AWH
    DCCP-Reset       seqno == 0 or seqno > GSR   GAR <= ackno <= AWH
    DCCP-Move        seqno >= SWL                ISS <= ackno <= AWH
    DCCP-Sync        seqno >= SWL                AWL <= ackno <= AWH
    DCCP-SyncAck     seqno >= SWL                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 DCCP-Reset Sequence Numbers may be zero.  This is because during
      the cleanup of a half-open connection, an endpoint might generate
      a DCCP-Reset in response to a DCCP-Request or DCCP-Data packet
      with no Acknowledgement Number; the resetting endpoint would then
      use zero for the Reset's Sequence Number, since it has no valid
      Sequence Number available.

      DCCP-Reset Acknowledgement Numbers, and non-zero Sequence Numbers,
      are checked more stringently than those on other packet types,
      however.  This is because DCCP-Reset always ends a connection: no
      endpoint will send a non-Reset packet on a connection after it has
      sent a Reset.  Thus, a Reset packet whose Sequence Number is less
      than GSR, or whose Acknowledgement Number is less than GAR, must
      be sequence-invalid.

    o DCCP-Move Sequence and Acknowledgement Numbers are not strongly
      checked because moves might likely happen after long loss periods,
      and the mandatory Mobility ID provides good protection against
      unexpected packets.

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





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    These lenient checks all allow continued operation after unusual
    events, such as endpoint crashes and large bursts of loss.  There's
    no need for leniency when the endpoints are actively sending packets
    to one another.  Therefore, a DCCP endpoint SHOULD implement the
    following, tighter constraints for active connections.  An endpoint
    considers a connection active if it has received valid packets from
    the other endpoint within the last several round-trip times, or
    1 second, if the RTT is not known.

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

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

7.5.2.  Handling Sequence-Invalid Packets

    Sequence-invalid DCCP-Move, DCCP-Reset, DCCP-Sync, and DCCP-SyncAck
    packets MUST be ignored.

    When DCCP A receives any other sequence-invalid packet, it MUST
    reply with a DCCP-Sync packet.  This packet MUST acknowledge the
    packet's Sequence Number (not GSR!).  The DCCP-Sync MUST use a new
    Sequence Number, and thus will increase GSS; GSR will not change,
    however, since the received packet was sequence-invalid.  DCCP A
    MUST NOT otherwise process sequence-invalid packets.  For instance,
    it MUST NOT process their options.

    When the DCCP B endpoint receives the (sequence-valid) DCCP-Sync, it
    MUST update its GSR variable and reply with a DCCP-SyncAck packet
    acknowledging the DCCP-Sync (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.  Alternatively, if the
    connection was half-open (DCCP B is in CLOSED or REQUEST state),
    DCCP B will send a Reset.

    A DCCP endpoint MAY temporarily preserve sequence-invalid packets in
    case they become valid later.  This can reduce the impact of bursts
    of loss by delivering more packets to the application.  In
    particular, an endpoint MAY preserve a sequence-invalid packet for
    up to 2 round-trip times (or 1 second, if the RTT is unknown); if,
    within that time, the relevant sequence windows change so that the



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    packet becomes sequence-valid, the endpoint MAY process the packet
    again.

    To protect itself against denial-of-service attacks (where an
    attacker sends many sequence-invalid packets, trying to force the
    receiver to send many DCCP-Syncs), a DCCP implementation MAY rate-
    limit the DCCP-Syncs sent in response to sequence-invalid packets.

7.5.3.  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.

    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 placed at and before
    GSR, with three-fourths after GSR.  (This asymmetric placement
    assumes that bursts of loss are more common in the network than
    significant reordering.)

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

    The Acknowledgement Number validity window for packets from DCCP B
    is [AWL, AWH].  The high end of the window, AWH, always 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 don't go below the
    initial Sequence Numbers received and sent, respectively:
                 SWL := max(GSR + 1 - floor(W/4), ISR),
                 AWL := max(GSS - W' + 1, ISS).
    Of course, these adjustments MUST NOT be applied after the relevant



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    sequence numbers wrap.

7.5.4.  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 3- or 6-byte integer values, like DCCP sequence numbers.
    Change and Confirm options for Sequence Window are therefore either
    6 or 9 bytes long.  New connections start with Sequence Window 100
    for both endpoints.

    A proper Sequence Window/A value should reflect how many packets
    DCCP A expects to be in flight.  Only DCCP A can anticipate this
    number.  Too-small values increase the risk of the endpoints getting
    out sync after bursts of loss; too-large values increase the risk of
    connection hijacking.  (The next section quantifies this risk.)  One
    good guideline is for each endpoint to set Sequence Window to a
    small multiple of the maximum number of packets it expects to send
    in a round-trip time.  This value may not be available at connection
    initiation, when the round-trip time is unknown, but the endpoint
    can always send updates as the connection progresses.

7.5.5.  Sequence Number Attacks

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

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

    o Send DCCP-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.

    o Send DCCP-Reset packets with Sequence Number zero and random
      Acknowledgement Numbers.  If one of these packets hits the valid



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      acknowledgement number window, the connection will be shut down.

    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.

    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 and DCCP-Reset packets to
    succeed, assuming the victim implemented the more stringent checks
    for active connections recommended in Section 7.5.1.

    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 one of these attacks will succeed
    with probability P = WN/2^L.  For N = 1000, W = 100, and L = 24,
    this probability is about 0.006.  (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---will
    succeed with probability 0.002 for N = 1000, W = 8760 [a common
    default], and L = 32.)  Connections with sequence windows much
    larger than 100 SHOULD use extended sequence numbers to reduce the
    probability of attack success.

7.5.6.  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.

                    Recovery from Burst of Loss
    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
    attack.  The attacker cannot guess sequence numbers.  (DCCP is not



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    robust to attackers who can guess sequence numbers.)

                        Recovery from 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.

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


7.6.  Extended Sequence Numbers

    Extended 48-bit sequence numbers increase the rate DCCP connections
    can achieve without wrapping sequence numbers, and provide
    additional protection against the sequence number attacks described
    above.  Very-high-rate DCCP connections, and connections with large
    sequence windows, SHOULD therefore use extended sequence numbers
    rather than the default 24-bit sequence numbers.

7.6.1.  When to Use Extended Sequence Numbers

    The sequence-validity mechanism protects against the network
    delivering old data, but it 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.  We can easily
    calculate the maximum connection rate that can be safely achieved
    given this constraint.  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.




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    For the default MSL of 2 minutes, 1500-byte DCCP packets, and 24-bit
    sequence numbers, the safe rate is therefore approximately 800 Mb/s.
    Of course, 2 minutes is a very large MSL for any networks that could
    sustain that rate with such small packets.  Nevertheless, 48-bit
    sequence numbers allow much higher rates, up to 14 petabits a second
    for 1500-byte packets and the default MSL.

    The probability of sequence number attack success P = WN/2^L,
    discussed in Section 7.5.5, may also be relevant when deciding
    whether to use extended sequence numbers.  A fast connection will
    generally have a relatively high W (sequence window size),
    increasing the attack success probability for fixed N (number of
    attack packets); if the probability gets uncomfortably high with L =
    24, the connection should use 48-bit sequence numbers instead.

7.6.2.  Header Processing

    Extended sequence numbers are activated when the header's X bit is
    set to one (see Section 5.1). This extends the Sequence Number and
    Acknowledgement Number fields by an additional 24 bits, for a total
    of 48 bits.  The 48-bit numbers are stored in network order, with
    most significant bit first.  All packet types except for DCCP-Data
    and DCCP-Request will follow this generic header with an extended
    48-bit Acknowledgement Number.

    Once an endpoint has transitioned to 48-bit sequence numbers (X=1),
    it MUST send all succeeding packets with 48-bit sequence numbers.
    Furthermore, once an endpoint has received a sequence-valid packet
    with 48-bit sequence numbers, it MUST either send all succeeding
    packets with 48-bit sequence numbers, or reset the connection with
    Reset Code 7, "Extended Sequence Numbers".  (But note that an
    endpoint may send extended DCCP-Sync packets before transitioning to
    extended sequence numbers.)

    Clients SHOULD decide whether to use extended sequence numbers
    before sending their DCCP-Requests.  However, the Transition bit (T)
    and Sequence Transition Capable feature support transitioning to
    extended sequence numbers during an active connection, in case this
    proves necessary; see below.  A client that sends an extended DCCP-
    Request might receive a DCCP-Reset in response with Reset Code 7,
    "Extended Sequence Numbers"; the client SHOULD respond by sending
    another Request using 24-bit sequence numbers.

    Extended sequence numbers are treated simply as longer sequence
    numbers.  For instance, the sequence-validity mechanisms work the
    same way whether or not sequence numbers are extended.  Care is
    required when comparing a 24-bit sequence number with an 48-bit
    sequence number, however; see the next section.



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7.6.3.  Transitioning to Extended Sequence Numbers

    The Transition bit (T) following the extended Sequence Number field
    makes it possible to transition to 48-bit sequence numbers in the
    middle of a connection.  T is set to one only during such a
    transition.  When DCCP A switches to 48-bit sequence numbers, it
    MUST set the T bit to one on all of its packets for some period.
    This period SHOULD last on the order of a few round trip times, or
    until DCCP A receives an acknowledgement from DCCP B proving that
    one of its 48-bit-sequence-number packets has been received,
    whichever comes later.

    Each DCCP MUST choose its first 48-bit sequence number to have its
    lower 24 bits equal the 24-bit sequence number it expected to send
    (GSS+1).  The upper 24 bits may be chosen arbitrarily.  This applies
    to Acknowledgement Numbers as well as Sequence Numbers; if DCCP A
    sends an extended packet containing an Acknowledgement Number before
    DCCP B sends it a 48-bit Sequence Number, DCCP A can choose any
    value for the upper 24 bits of the Acknowledgement Number, but the
    lower 24 bits MUST equal the expected 24-bit Acknowledgement Number
    (GSR).  Furthermore, DCCP A MUST leave GSR as a 24-bit number until
    receiving an extended packet from DCCP B.

    Switching to 48-bit sequence numbers in the middle of a connection
    complicates sequence number comparison.  Endpoints must compare
    48-bit sequence numbers with 24-bit sequence numbers, and compare
    48-bit sequence numbers that might have different, arbitrary values
    in the upper 24 bits, while remaining robust to reordering and to
    old or malicious packets.  The following procedure describes how
    sequence numbers should be compared during and immediately after a
    transition.

    Let P be the packet sequence number received from DCCP B, and E be
    the sequence number DCCP A expects.  During sequence-validity
    computations, for example, P might be the packet's Acknowledgement
    Number and E might be AWL, the left edge of the appropriate
    acknowledgement number window.  Then DCCP A should perform the
    comparison as follows.

    o If P and E are both 24 bits, compare them modulo 2^24.

    o If P and E are both 48 bits, you generally compare them modulo
      2^48, except that during a transition, the two values might have
      arbitrary values in the upper 24 bits.

      - If the packet's Transition bit is set, and the last packet sent
        by DCCP A had its Transition bit set, then compare P and E
        modulo 2^24.



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      - Otherwise, compare them modulo 2^48.

    o If P is 48 bits but E is 24, the remote DCCP may want to
      transition to extended sequence numbers.

      - If the packet's Transition bit is set, compare P with E modulo
        2^24.  If the packet proves sequence-valid, then it is OK;
        transition to extended sequence numbers, and set E according to
        the full 48 bits of P.

      - Otherwise, the packet is sequence-invalid.

      Either way, if the packet proves to be sequence-invalid, send an
      extended DCCP-Sync if required (with T set to one), but do not yet
      transition to extended sequence numbers.

    o If P is 24 bits but E is 48, there may have been benign packet
      reordering.  The correct action depends on whether the last
      sequence-valid packet received from DCCP B had the Transition bit
      set.

      - If Transition was set, extend P to a 48-bit value P'.  First,
        let EH equal the upper 24 bits of E, and EL equal the lower 24
        bits of E.  Then:

          If  EL > P,  set  P' = (EH << 24) | P.
          Otherwise,   set  P' = (((EH - 1) mod 2^24) << 24) | P.

        The "EL > P" test uses arithmetic comparison, NOT circular
        comparison.  Compare P' with E modulo 2^48.

      - Otherwise, the packet is sequence-invalid.

      Either way, if the packet proves to be sequence-invalid, send an
      extended DCCP-Sync if required, with T set to one.

    DCCP implementations can, of course, avoid most of this complexity
    by disallowing transitions to extended sequence numbers (and by
    resetting the connection when the other endpoint attempts such a
    transition).  Connections that use 48-bit sequence numbers
    throughout, starting with the DCCP-Request, MUST have T set to zero
    on all their packets.

7.6.4.  Sequence Transition Capable Feature

    The Sequence Transition Capable feature expresses whether DCCP
    endpoints are capable of transitioning to extended sequence numbers
    in the course of an active connection.  DCCP A sends a



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    "Change R(Sequence Transition Capable, 1)" option to DCCP B to
    discover whether B can transition to extended sequence numbers.

    Sequence Transition Capable has feature number 4, and is server-
    priority.  It takes one-byte Boolean values.  DCCP B MUST allow
    transitions to extended sequence numbers when Sequence Transition
    Capable/B is one.  It MUST NOT reset the connection with Reset Code
    7, "Extended Sequence Numbers", under those circumstances.  However,
    DCCP B MAY allow such transitions even when Sequence Transition
    Capable/B is zero.  Values of two or more are reserved.  New
    connections start with Sequence Transition Capable 0 (that is, not
    capable) for both endpoints.

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 determine, for every burst of loss, whether
    or not application data was lost.

    +--------+--------+-------- ... --------+
    |00100101| Length |      NDP Count      |
    +--------+--------+-------- ... --------+
     Type=37  Len=3-5

    If a DCCP endpoint's Send NDP Count feature is one (see below), then
    that endpoint MUST send an NDP Count option on every packet whose
    immediate predecessor was a non-data packet.  Non-data packets
    consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
    DCCP-Reset, DCCP-Move, DCCP-Sync, and DCCP-SyncAck.  All other
    packet types 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 three bytes of data.  The
    smallest option format that can hold the NDP Count SHOULD be used.







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7.7.1.  Usage Notes

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

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

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

    Send NDP Count has feature number 9, and is server-priority.  It
    takes one-byte Boolean values.  DCCP B MUST send NDP Count options
    on its non-data packets (and some of its data packets) 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-



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    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 send new DCCP-Request packets
    after some timeout if no response is received.  The retransmission
    strategy SHOULD be similar to that for retransmitting TCP SYNs; for
    instance, a first timeout on the order of a second, with an
    exponential backoff timer.  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 after some number of DCCP-Requests.  If so, 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.

    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 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 HTTP



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    connections, one for FTP control connections, and one for FTP data
    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, while passive sockets MAY have more than one; this
    might let multiple applications listen on the same port,
    differentiated by Service Code.  If the DCCP-Request's Service Code
    doesn't match 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 9, "Bad Service Code".  A middlebox MAY also send
    such a DCCP-Reset in response to packets whose Service Code is
    considered unsuitable.

    Service Codes should be allocated by IANA.  We intend for Service
    Code allocation to be allocated to anyone who asks, first-come
    first-serve, subject to the following guidelines.

    o Service Codes should be 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-track or otherwise, is necessary.  IANA
      should maintain an association of Service Codes to the
      corresponding phrases.

    o Users may request specific Service Code values, which should be
      assigned first-come first-serve.  We suggest that users request
      Service Codes that can be interpreted as meaningful four-byte
      ASCII strings.  Thus, the "Frobodyne Plotz Protocol" might
      correspond to "fdpz", or the number 1717858426.  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} should be reserved 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 '?') should
      never be allocated.  These Service Codes are reserved for private
      use.

    o Service Code 0 should never be allocated.  It represents the
      absence of a meaningful Service Code.




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    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.

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 receiver MAY respond to a DCCP-Request packet with a DCCP-Reset
    packet to refuse the connection.  Relevant Reset Codes for refusing
    a connection include 8, "Connection Refused", when the DCCP-
    Request's Destination Port did not correspond to a DCCP port open
    for listening; 9, "Bad Service Code", when the DCCP-Request's
    Service Code did not correspond to the service code registered with
    the Destination Port; and 10, "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.

    The receiver SHOULD NOT retransmit DCCP-Response packets; the sender
    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 receiver can
    thus distinguish true retransmissions from network duplicates.)  The
    responder 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 responder's Sequence Number by one, and MUST include
    the same application data, if any, as the original DCCP-Response.

    The responder MUST accept at most one piece of DCCP-Request data per
    connection.  In particular, the DCCP-Response sent in reply to a
    retransmitted DCCP-Request with data SHOULD contain a Data Dropped
    option, in which the retransmitted DCCP-Request is reported as "data
    dropped due to protocol constraints" (Drop Code 0). The original
    DCCP-Request SHOULD also be reported in the Data Dropped option,
    either in a Normal Block (if the responder accepted the data, or
    there was no data), or in a Drop Code 0 Drop Block (if the responder
    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).




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    The server leaves the RESPOND state for OPEN when it receives a
    valid DCCP-Ack from the client, completing the three-way handshake.

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.
    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 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.

    This option is permitted in DCCP-Response, DCCP-Data, DCCP-Ack,
    DCCP-DataAck, DCCP-Sync, and DCCP-SyncAck packets.  The server MAY
    include an Init Cookie option in its DCCP-Response.  If so, then the
    client MUST echo the same Init Cookie option in each succeeding DCCP
    packet until one of those packets is acknowledged, meaning the
    three-way handshake has completed, or the connection is reset.  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 11, "Bad
    Init Cookie".

    The precise implementation of the Init Cookie does not need to be
    specified here; since Init Cookies are opaque to the client, there
    are no interoperability concerns.

    Init Cookies are limited to at most 253 bytes in length.

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 three-way
    handshake by sending a DCCP-Ack packet to the server.  The PARTOPEN
    state represents that the client isn't sure whether the server has
    received any of its DCCP-Acks.  The client MUST NOT send DCCP-Data
    packets while it remains in PARTOPEN.  This is because DCCP-Data
    packets lack Acknowledgement Numbers, so the server can't tell from



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    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 delayed-ack-like 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, it SHOULD reset the connection with Reset Code 2,
    "Aborted".

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

    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.

    The DCCP-Move, DCCP-Sync, and DCCP-SyncAck packets will also occur
    in the data transfer phase.  DCCP-Move handling is discussed in
    Section 14, and 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 in
    response; and 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.




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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, 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
    4.   TIMEWAIT
    5.   CLOSED

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

      Client State                             Server State
         OPEN                                     OPEN
    1.   CLOSING   -->        Close         -->
    2.             <--        Reset         <--   CLOSED
    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


    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.





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    The termination handshake proceeds as follows.  The receiver of a
    valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet;
    that receiving endpoint will expect to hold TIMEWAIT state after
    later receiving a DCCP-Reset.  The receiver of a valid DCCP-Close
    packet MUST respond with a DCCP-Reset packet, with Reset Code 1,
    "Closed"; the endpoint that originally sent the DCCP-Close will hold
    TIMEWAIT state.  The endpoint that receives a valid DCCP-Reset
    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 responses to
    connection termination close are the same no matter 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-Reset packets MUST NOT
    be generated in response to received DCCP-Reset packets.  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
    states.  The retransmission timer should initially be set to go off
    in two RTTs, or 0.4 seconds if the RTT is not known, and should back
    off to not less than once every 64 RTTs if no relevant response is
    received.

    Only the server can send a DCCP-CloseReq packet or enter the
    CLOSEREQ state.

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.
    However, a DCCP endpoint in the CLOSED or LISTEN state may not have
    a proper sequence number available to send a Reset.  In these cases,
    it MUST set the Reset's Sequence Number to zero.  Resets sent in the
    CLOSED, LISTEN, and TIMEWAIT states often use Reset Code 3, "No
    Connection".  Resets sent in the REQUEST or RESPOND states often use
    Reset Code 4, "Packet Error".

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



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    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.

    +---------------------------+    +---------------------------+
    |                           v    v                           |
    |                        +----------+                        |
    |          +-------------+  CLOSED  +------------+           |
    |          |             +----------+  active    |           |
    |          | passive                    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        |                  |           |
    |          | snd Reset        |        rcv Reset |           |
    |<---------+                  |                  v           |
    |                   rcv Close |             +----+-----+     |
    |                   snd Reset |             | TIMEWAIT |     |
    |                             |             +----+-----+     |
    +-----------------------------+                  |           |
                                                     +-----------+
                                                  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



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    implementation need not implement the algorithm as it is described
    here, but any implementation MUST generate observable effects
    (meaning packets) exactly as indicated by this pseudocode, except
    where allowed otherwise by another part of this document.

    The received packet is written as P, the socket as S.  Socket variables:
    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 acknowledgement number received; initialized to S.ISS
    "Send packet" actions always use, and increment, S.GSS.

    First, check the header basics;
       If the header checksum is incorrect, drop packet and return.
       If the packet type is not understood, drop packet and return.
       If Data Offset is too small for packet type, or too large for packet,
       drop packet and return.

    Second, process DCCP-Move;
       If P.type == Move,
          Look up the Mobility ID in table; get socket.
          If socket exists && P.seqno >= S.SWL && P.ackno <= S.AWH
                && P.ackno >= S.ISS && S.state >= PARTOPEN && S.state < TIMEWAIT,
             Process options
             Set socket to point at new address/ports
             Add reference to new address/ports
             Set timer to remove old address/ports after 2MSL
             Choose new Mobility ID, add to table
             Send DCCP-Sync[Change L[Mobility ID, new ID]]
             Update S.GSR, S.SWL, S.SWH
             Drop packet and return
          Otherwise,
             Drop packet and return

    Third, check ports and process TIMEWAIT state;
       Look up flow ID; get socket.
       If no socket, or S.state == TIMEWAIT,
          Generate Reset(No Connection) unless P.type == Reset
          Drop packet and return

    Fourth, process LISTEN state;
       If S.state == LISTEN,



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          If P.type == Request,
             /* Init Cookie processing would go here */
             Set S := new socket for this port pair
             S.state = RESPOND
             Choose S.ISS (initial seqno)
             Set S.ISR, S.GSR, S.SWL, S.SWH from packet
             Continue (with S.state == RESPOND)
          Otherwise,
             Generate Reset(No Connection) unless P.type == Reset
             Drop packet and return

    Fifth, process Reset;
       If P.type == Reset,
          If S.GAR <= P.ackno <= S.AWH
                && (P.seqno == 0 || P.seqno > S.GSR || S.state == REQUEST),
             Tear down connection
             S.state := TIMEWAIT
             Set TIMEWAIT timer
             Drop packet and return
          Otherwise (sequence numbers out of whack),
             Drop packet and return

    Sixth, process REQUEST state;
       If S.state == REQUEST,
          If P.type == Response && S.AWL <= P.ackno <= S.AWH,
             Set S.GSR, S.ISR, S.SWL, S.SWH
          Otherwise,
             Generate Reset(Packet Error)
             Drop packet and return

    Seventh, process Sync sequence numbers;
       If P.type == Sync || P.type == SyncAck,
          If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,
             Update S.GSR, S.SWL, S.SWH
          Otherwise,
             Drop packet and return

    Eighth, check sequence numbers;
       If S.SWL <= P.seqno <= S.SWH
             && (P.ackno does not exist || S.AWL <= P.ackno <= S.AWH),
          Update S.GSR, S.GAR, S.SWL, S.SWH
       Otherwise,
          Send Sync packet acknowledging P.seqno
          Drop packet and return

    Ninth, check packet type;
       If (S.is_server && P.type == CloseReq)
            || (S.is_server && P.type == Response)



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            || (S.is_client && P.type == Request)
            || (S.state >= OPEN && P.type == Request && P.seqno >= S.OSR)
            || (S.state >= OPEN && P.type == Response && P.seqno >= S.OSR)
            || (S.state == RESPOND && P.type == Data),
          Send Sync packet acknowledging P.seqno
          Drop packet and return

    Tenth, process options;
       /* may involve resetting connection, etc. */
       Mark packet as "received" for acknowledgement purposes
       On processing Confirm R(Mobility ID),
          Check that the confirmed Mobility ID is correct
          If a DCCP-Move was recently processed,
             Remove any old Mobility ID from table

    Eleventh, process RESPOND state;
       If S.state == RESPOND,
          If P.type == Request,
             Send Response
          Otherwise,
             S.OSR := P.seqno
             S.state := OPEN

    Twelfth, process REQUEST state;
       If S.state == REQUEST,
          S.state := PARTOPEN
          /* Do not send Data packets in PARTOPEN; furthermore, include Init
             Cookie on every packet */
          Set PARTOPEN timer

    Thirteenth, process PARTOPEN state;
       If S.state == PARTOPEN,
          If P.type == Response,
             Send Ack
          Otherwise,
             S.OSR := P.seqno
             S.state := OPEN

    Fourteenth, process CloseReq;
       If P.type == CloseReq && S.state < CLOSEREQ,
          Generate Close
          S.state := CLOSING
          Set CLOSING timer

    Fifteenth, process Close;
       If P.type == Close,
          Generate Reset(Closed)
          Tear down connection



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          Drop packet and return

    Sixteenth, process Sync;
       If P.type == Sync,
          Generate SyncAck

    Seventeenth, process data.
       Do not deliver data from more than one Request or Response

9.  Checksums

    DCCP uses a header checksum to protect its header against
    corruption.  Generally, this checksum covers any application data as
    well.  However, DCCP applications can 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 applications that can tolerate corruption, this can
    greatly improve delivery rates and perceived performance.

    If checksum coverage is complete, packets with corrupt application
    data must be treated as network losses, thus incurring a loss
    response from the sender's congestion control mechanism.  Such a
    heavy-duty response may unfairly penalize connections on links with
    high background corruption.  It is to the application's benefit to
    report corruption losses differently from network losses.
    Therefore, even applications that demand correct data can make use
    of reduced checksum coverage, by including a Data Checksum option.
    Data Checksum holds a strong checksum of the application data.  The
    combination of reduced checksum coverage and Data Checksum can
    detect application data corruption, but report it as corruption, not
    congestion, via Data Dropped options (see Section 11.7).

    Reduced checksum coverage introduces some security considerations;
    see Section 19.2. See Appendix B.1 for further motivation and
    discussion.  DCCP's implementation of reduced checksum coverage was
    inspired by UDP-Lite [UDP-LITE].

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 text bytes to be checksummed, 8



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    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 Section 3.1
    of [RFC 793]. 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 Section 8.1 of [RFC 2460].

    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 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).



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    It should also let receiving applications refuse delivery of packets
    with checksum coverage less than a value provided by the
    application; by default, only packets with fully-covered application
    data should be accepted.  (Note that, for short packets, application
    data might be fully covered by a nonzero Checksum Coverage value.)
    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 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 [UDP-LITE].

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

    Data Checksum is intended for packets containing application data,
    such as DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck,
    but it may be included on any packet.  The sending DCCP computes the
    CRC of the bytes comprising the application data and stores it in
    the option data.  The CRC-32c algorithm used for Data Checksum is
    the same as that used for SCTP [RFC 3309]; 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.

    The receiving DCCP SHOULD compute the received application data's
    CRC-32c using the same algorithm as the sender, and compare the
    result and the Data Checksum value.  If the values differ, the
    packet's application data MUST be dropped, and reported using a Data
    Dropped option as dropped due to corruption (Drop Code 3). However,
    DCCP MAY provide an API through which the receiving application
    could request delivery of known-corrupt data.  When that API is
    active, the packet's data SHOULD be delivered, but reported as
    delivered corrupt (Drop Code 7) using a Data Dropped option.  In
    either case, the packet will be reported as Received or Received ECN
    Marked by Ack Vector or similar options.




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9.3.1.  Check Data Checksum Feature

    The Check Data Checksum feature lets a sending DCCP determine
    whether or not its partner can check Data Checksum options.  DCCP A
    sends a Mandatory "Change R(Check Data Checksum, 1)" option to
    DCCP B to require B to check Data Checksum options (the connection
    will be reset if DCCP B cannot).

    Check Data Checksum has feature number 10, 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.  Usage Notes

    Internet links must normally apply strong integrity checks to the
    packets they transmit [UDP-LITE] [LINK BCP]. Data Checksum is
    redundant for DCCP packets whose integrity is checked by every link
    they traverse.  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 the strength of their error
    detection and/or correction when transmitting this unprotected part,
    which can significantly increase the probability of the endpoint
    receiving corrupt data.  Data Checksum lets the receiver detect any
    ensuing corruption.

10.  Congestion Control IDs

    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.

    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, 1 2 3)" asks the
    receiver to use CCID 1 for its packets, although CCIDs 2 and 3 are
    also acceptable.  (This corresponds to the bytes "35, 6, 1, 1, 2,
    3": Change R option (35), option length (6), feature ID (1), CCIDs



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    (1, 2, 3).)  Similarly, "Confirm L(CCID, 1, 1 2 3)" tells the
    receiver that the sender is using CCID 1 for its packets, but that
    CCIDs 2 or 3 might also be acceptable.

    The CCIDs defined by this document are:

         CCID   Meaning
         ----   -------
           0    Reserved
           1    Unspecified Sender-Based Congestion Control
           2    TCP-like Congestion Control
           3    TFRC Congestion Control

    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 CCIDs 1 and 2.  The intent is to make these CCIDs
    broadly available for interoperability, although particular
    applications might disallow their use.

10.1.  Unspecified Sender-Based Congestion Control

    CCID 1 denotes an unspecified sender-based congestion control
    mechanism.  This provides a limited, controlled form of
    interoperability for new IETF-approved CCIDs: with CCID 1, an HC-
    Sender can use a new sender-based congestion control mechanism whose
    details the HC-Receiver does not understand.

    Some congestion control mechanisms require only generic behavior
    from the receiver.  For example, CCID 2, TCP-like Congestion
    Control, requires that the receiver (1) send Ack Vectors and (2)
    respond to Ack Ratio.  Both of these requirements use generic
    mechanisms described in this document.  Thus, a CCID 2 HC-Receiver
    doesn't really need to understand the details of CCID 2.

    CCID 1 uses this insight to support forward compatibility for
    sender-based congestion control mechanisms.  An HC-Sender proposes
    CCID 1 as a proxy for a sender-based mechanism whose details the HC-
    Receiver doesn't need to understand.  The HC-Receiver can then agree
    to CCID 1, and provide generic acknowledgement feedback as requested



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    by other features (such as Send Ack Vector).  Individual CCID
    profile documents say whether or not they can masquerade as CCID 1.

    For example, say that CCID 98, a new sender-based congestion control
    mechanism using Ack Vector for acknowledgements, has entered the
    IETF standards process, and the IETF has approved the use of CCID 1
    as a proxy for CCID 98.  Now, say DCCP A would like to use CCID 98
    for its data packets.  It should therefore send a "Change L(CCID, 98
    1)" option to open a CCID negotiation.  98 comes first, since that
    is the preferred CCID; 1 comes next, as a potential proxy for 98.
    If DCCP B understands CCID 98, it will respond with "Confirm R(CCID,
    98, ...)" and all is well.  But if it does not understand CCID 98,
    it may respond with "Confirm R(CCID, 1, ...)", still allowing DCCP A
    to use CCID 98.  DCCP A will separately negotiate Send Ack Vector,
    and thus DCCP B will provide the feedback DCCP A requires, namely
    Ack Vector, without needing to understand the operation of CCID 98.

    Implementors MUST NOT use CCID 1 in production environments as a
    proxy for congestion control mechanisms that have not entered the
    IETF standards process.  We intend that any production use of CCID 1
    would have to be explicitly approved first by the IETF.  Middleboxes
    MAY choose to treat the use of CCID 1 as experimental or
    unacceptable.

    Since CCID 1 should be used only as a proxy for other, defined
    CCIDs, an HC-Sender MUST NOT report a preference list consisting
    only of CCID 1, and the option "Change L(CCID, 1)" is illegal.
    Receiving such an option SHOULD result in connection reset with
    Reset Code 5, "Option Error".  An HC-Receiver MAY suggest CCID 1
    exclusively: the option "Change R(CCID, 1)" is not illegal.

    If CCID 1 is the result of a CCID feature negotiation, the HC-Sender
    determines which CCID to actually use by picking the earliest CCID
    in its preference list that can masquerade as CCID 1.  The HC-Sender
    MUST pick a CCID that appeared explicitly in its preference list.

    Many DCCP APIs will allow applications to suggest preferred CCIDs
    for sending and receiving data.  Such APIs might let applications
    allow or prevent the use of CCID 1 for receiving, but they should
    not let applications suggest the use of CCID 1 for sending.  The
    code implementing a particular CCID should add CCID 1 to the HC-
    Sender's CCID preference list when appropriate, unless the
    application disagrees.  The default for both sender and receiver
    should be to allow CCID 1 when possible.

    CCID 1 places no restrictions on how often the HC-Receiver may send
    DCCP-Ack packets.  A careful implementation SHOULD implement a
    liberal rate limit on DCCP-Acks to prevent ack storms.



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10.2.  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.  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.
    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 [CCID 2 PROFILE].

10.3.  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 telephony or streaming
    media where a relatively smooth sending rate is of importance.

    CCID 3 is further described in [CCID 3 PROFILE]. The TFRC congestion
    control algorithms were initially described in [RFC 3448].

10.4.  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



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    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).

    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 and features are
    assigned to half-connections:

























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                                    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

    CCID-specific options and features have no clear meaning when a
    nontrivial negotiation for the relevant CCID is in progress.  This
    can happen when a CCID-specific option follows a Change(CCID)
    option.  Say the Change option lists CCID X first.  Then the
    negotiation is nontrivial if and only if its result is not X.  CCID-
    specific options and features MUST be ignored during a nontrivial
    CCID negotiation, except that Mandatory CCID-specific options and
    features MUST induce a DCCP-Reset with Reset Code 6, "Mandatory
    Error".

11.  Acknowledgements

    Congestion control requires receivers to 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, how they should be congestion controlled, and so on.

    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



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    ack mechanisms used for each CCID are laid out in the CCID profile
    specifications.

    Acknowledgement options, such as Ack Vector, generally depend on the
    DCCP Acknowledgement Number, and are thus only allowed on packet
    types that carry that number (all packets except DCCP-Request and
    DCCP-Data).  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:
    DCCP B stops sending data, and sends only sends DCCP-Ack packets to
    DCCP A.  For 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



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    state.  For instance, A might send a DCCP-DataAck packet every now
    and then, instead of DCCP-Data.  In contrast, for CCID 3, TFRC
    Congestion Control, DCCP B's acknowledgements generally need not be
    reliable, since they contain cumulative loss rates; TFRC works even
    if every DCCP-Ack is lost.  Therefore, DCCP A need never acknowledge
    an acknowledgement.

    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), while 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;
    for CCID 2, 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, as described above.  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
    secondary concerns, DCCP A can generally afford to wait indefinitely
    for a data packet to piggyback its acknowledgement onto.




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

11.3.  Ack Ratio Feature

    Ack Ratio provides a common mechanism by which CCIDs that clock
    acknowledgements off data packets can perform rudimentary congestion
    control on the acknowledgement stream.  CCID 2, TCP-like Congestion
    Control, uses Ack Ratio to limit the rate of its acknowledgement
    stream, for example.  Some CCIDs ignore Ack Ratio, performing
    congestion control on acknowledgements in some other way.

    Ack Ratio has feature number 7, and is non-negotiable.  It takes
    two-byte integer values.  The Ack Ratio/A feature is the rough ratio
    of data packets sent by DCCP A to acknowledgement packets sent back
    by DCCP B.  For example, if Ack Ratio/A is four, then 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.  New connections start with Ack
    Ratio 2 for both endpoints.

    Implementations should treat Ack Ratio as a loose guideline.  For
    instance, a DCCP endpoint might implement a delayed acknowledgement
    timer like TCP's, whereby each packet is acknowledged within at most
    T seconds of its receipt.  (In TCP, T is commonly set to 200
    milliseconds.)  This is explicitly allowed even though it might lead
    to sending more acknowledgement packets than Ack Ratio would
    suggest.  Particular algorithms for setting and using Ack Ratio are
    discussed in the relevant CCID drafts.

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:

    +--------+--------+--------+--------+--------+--------
    |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:



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     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:

        0   Packet received (and not ECN marked).

        1   Packet received ECN marked.

        2   Reserved.

        3   Packet not yet 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.)  If an Ack
    Vector contains the decimal values 0,192,3,64,5 and the
    Acknowledgement Number is decimal 100, then:

        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).

        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



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    mechanisms; referring to Ack Vector states simplifies their
    explanation.)

    1.  Packets reported as State 0 or State 1 MUST have been processed
        by the receiving DCCP stack.  In particular, their options must
        have been processed.  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 have been received by DCCP.
        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 SHOULD 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
    more packets than have actually been sent SHOULD be considered
    invalid.  The receiving DCCP SHOULD either ignore the options or
    reset the connection with Reset Code 5, "Option Error".  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:

    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).




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    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 |
                                +---+---+---+

    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,
    which it will eventually report to the HC-Sender on one or more
    acknowledgements, 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.





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    A 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.

    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 Section 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 data
    packets' 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 8, 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



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    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.  However, 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 reporting
    false ECN marks.)  APIs should let receiver applications set Slow
    Receiver, and sending applications determine whether or not their
    receivers are Slow.

    The Slow Receiver option takes just one byte:

    +--------+
    |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 sending rate or by
    switching to a lossier compression algorithm.

    The sending application should not react to Slow Receiver by sending
    more data, however.  The optimal response to a CPU-bound receiver
    might be to increase the sending rate, by switching to a less-
    compressed sending format, since a highly-compressed data format
    might overwhelm a slow CPU more seriously than the higher memory
    requirements of a less-compressed data format.  The Slow Receiver
    option is not appropriate for this case; a CPU-bound receiver should
    not ask for Slow Receiver options to be sent.

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

11.7.  Data Dropped Option

    The Data Dropped option indicates that some packets reported as
    received actually had their data dropped before it reached the



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    application.  The sender's congestion control mechanism may respond
    to data-dropped packets less severely than to lost or marked
    packets.  For instance, a windowed mechanism might subtract a
    constant value from its congestion window, rather than cut it in
    half.

    Data Dropped lets a sender differentiate between different kinds of
    loss (network and endpoint), but it does not allow total freedom in
    how to react.  The congestion control response to a Data Dropped
    packet must be approved by the IETF.  Each congestion control
    mechanism MUST react to a Data Dropped packet as if the packet were
    ECN marked, unless explicitly specified otherwise.

    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
    "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 itself consists of a series of bytes, called Blocks, each
    of whose encoding corresponds to one of these 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 in 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.)  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



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    Block.  Defined Drop Codes for Drop Blocks are:

        0   Packet data dropped due to protocol constraints.  For
            example, the data was included on a DCCP-Request packet, and
            the receiving application does not allow that piggybacking;
            or the data was sent during an important feature
            negotiation.

        1   Packet data dropped because the application is no longer
            listening.

        2   Packet data dropped in the receive buffer.

        3   Packet data dropped due to corruption.

        4-6 Reserved.

        7   Packet data corrupted, but delivered to the application
            anyway.

    For example, if a Data Dropped option contains the decimal values
    0,160,3,162, the Acknowledgement Number is 100, and an Ack Vector
    reported all packets as received, then:

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

        Packet 99 was dropped in the 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 respond to invalid Data Dropped options by ignoring
    them, or 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).

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 packets, a congestion
    control mechanism MUST consider dropped packets and ECN marks
    (including ECN-marked packets that are included in Data Dropped), as
    well as the Data Dropped packets.  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).

    Non-window-based congestion control mechanisms MUST behave
    analogously.

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 non-marked packets
    with Drop Code 0 as if they were received.  However, the sending
    DCCP SHOULD NOT send more data until it believes the relevant
    protocol constraint has passed.





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    Drop Code 1 ("application no longer 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 the
    server would expend on 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 a DCCP reports
    Drop Code 1 for a packet, it SHOULD report Drop Code 1 for every
    succeeding data packet on that half-connection; once a DCCP 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.  A DCCP receiving Drop Code 1 MAY report this event
    to the application.  (Previous versions of this specification used a
    "Buffer Closed" option instead of Drop Code 1.)

    Drop Code 2 ("receive buffer drop") indicates congestion inside the
    receiving host.  Every packet newly acknowledged as Drop Code 2
    SHOULD reduce the sender's instantaneous rate by one packet per
    round trip time, using whatever mechanism is appropriate for the
    relevant CCID.  Further details may be available in CCID documents.

12.  Explicit Congestion Notification

    The DCCP protocol is fully ECN-aware [RFC 3168]. 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); this means that
    acknowledgements are generally congestion-controlled, and may have
    ECN-Capable Transport set.

    A 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 whenever the receiver has its
    ECN Capable feature turned on and the relevant CCID allows it,
    unless the sending application indicates that ECN should not be
    used.

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

12.1.  ECN Capable Feature

    The ECN Capable feature lets a DCCP inform its partner that it
    cannot read ECN bits from received IP headers, so the partner must
    not set ECN-Capable Transport on its packets.



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    ECN Capable has feature number 2, 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 Capable/A is one.  (This is
    independent of whether it can set ECN bits on sent frames.)  DCCP A
    thus sends a "Change L(ECN Capable, 0)" option to DCCP B to inform
    it that A cannot read ECN bits.  New connections start with ECN
    Capable 1 (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
    Capable, 0)" options to the other endpoint until acknowledged (by
    "Confirm R(ECN Capable, 0)") or the connection closes.  Furthermore,
    it MUST NOT accept any data until the other endpoint sends
    "Confirm R(ECN Capable, 0)".  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.  The protocol receiver reports, on every acknowledgement, 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.

    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



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    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
    Ack Vector's compressibility without providing much extra
    protection.)

    Given an A-to-B half-connection, DCCP A SHOULD set ECN Nonces on its
    packets, and remember which packets had nonces, whenever DCCP B
    reports that it is ECN Capable.  An ECN-capable endpoint MUST
    calculate and use the correct value for ECN Nonce Echo when sending
    acknowledgement options.  An ECN-incapable endpoint, however, SHOULD
    treat the ECN Nonce Echo as always zero.  When a sender detects an
    ECN Nonce Echo mismatch, it SHOULD behave as if the receiver had
    reported one or more packets as ECN-marked (instead of unmarked).
    It MAY take more punitive action, such as resetting the connection
    with Reset Code 12, "Aggression Penalty".

    An ECN-incapable DCCP SHOULD ignore received ECN nonces and generate
    ECN nonces of zero.  For instance, out of the two Ack Vector
    options, an ECN-incapable DCCP SHOULD generate Ack Vector [Nonce 0]
    (option 38) exclusively.  (Again, the ECN Capable feature MUST be
    set to zero in this case.)

12.3.  Other Aggression Penalties

    The ECN Nonce provides one way for a DCCP sender to discover that a
    receiver is misbehaving.  There may be other mechanisms, and a
    receiver or middlebox may also discover that a sender is
    misbehaving---sending more data than it should.  In any of these
    cases, the entity that discovers the misbehavior MAY react by
    resetting the connection with Reset Code 12, "Aggression Penalty".
    A receiver that detects marginal (meaning possibly spurious) sender
    misbehavior MAY instead react with a Slow Receiver option, or by
    reporting some packets as ECN marked that were not, in fact, marked.

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.



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

    The four bytes of option data carry the timestamp of this packet in
    some undetermined form.  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.  It indicates how much time, in tenths of
    milliseconds, 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.

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

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

    The option data, Elapsed Time, represents an estimated upper bound
    on the amount of time elapsed since the packet being acknowledged
    was received, with units of tenths of milliseconds.  If Elapsed Time
    is less than a second, the first, smaller form of the option SHOULD
    be used.  Elapsed Times of more than 6.5535 seconds MUST be sent
    using the second form of the option.  DCCP endpoints MUST NOT report
    Elapsed Times that are significantly larger than the true elapsed
    times.  A connection MAY be reset with Reset Code 12, "Aggression
    Penalty", if one endpoint determines that the other is reporting a
    much-too-large Elapsed Time.

    Elapsed Time is measured in tenths 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 most reasonable elapsed times to
    fit into two bytes of data.




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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.

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

    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 be in tenths 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.  Multihoming and Mobility

    DCCP provides primitive support for multihoming and mobility via a
    mechanism for transferring a connection endpoint from one address to
    another.  The moving endpoint must negotiate mobility support



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    beforehand.  When the moving endpoint gets a new address, it sends a
    DCCP-Move packet from that address to the stationary endpoint.  The
    stationary endpoint then changes its connection state to use the new
    address.

    DCCP's support for mobility is intended to solve only the simplest
    multihoming and mobility problems; for instance, there's no support
    for simultaneous moves.  Applications requiring more complex
    mobility semantics, or more stringent security guarantees, should
    use an existing solution like Mobile IP or [SB00]. DCCP mobility may
    not be useful in the context of IPv6, with its mandatory support for
    Mobile IP.

14.1.  Mobility Capable Feature

    A DCCP uses the Mobility Capable feature to inform its partner that
    it would like to be able to change its address and/or port during
    the course of the connection.  DCCP B sends a "Change R(Mobility
    Capable, 1)" option to DCCP A to inform it that B might like to move
    later.

    Mobility Capable has feature number 5, and is server-priority.  It
    takes one-byte Boolean values.  DCCP A agrees in principle to accept
    DCCP-Move packets from DCCP B when Mobility Capable/A is one.
    DCCP A MUST reject any DCCP-Move packet for a connection whose
    Mobility Capable/A feature is zero, although it MAY reject a valid
    DCCP-Move packet even when Mobility Capable/A is one.  Values of two
    or more are reserved.  New connections start with Mobility Capable 0
    (that is, mobility is not allowed) for both endpoints.

14.2.  Mobility ID Feature

    A DCCP uses the Mobility ID feature to inform its partner of a
    128-bit number that will act as identification, should the partner
    change its address and/or port during the course of the connection.
    DCCP A sends a "Change L(Mobility ID, N)" option to notify DCCP B of
    the ID it has chosen for B's use.

    Mobility ID has feature number 6, and is non-negotiable.  Its values
    are sixteen-byte integers.  The Mobility ID/A feature equals the
    identifier that DCCP B should use on DCCP-Move packets sent to A.
    DCCP A chooses Mobility ID/A to uniquely identify the connection
    among all connections that terminate at A.  For security, DCCP A
    MUST choose Mobility ID/A randomly.  Furthermore, it MUST reassign
    Mobility ID/A after each successful move by DCCP B, and it MAY
    reassign Mobility ID/A more frequently.  New connections start with
    Mobility ID 0 for both endpoints.  However, Mobility IDs of zero
    MUST NOT be accepted on DCCP-Move packets; an endpoint cannot



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    successfully move until the relevant Mobility ID has been set to a
    nonzero value.

14.3.  Mobile Host Processing

    When DCCP A changes its address and/or port, it MUST signal this by
    sending DCCP B a DCCP-Move packet.  The Mobility ID in the DCCP-Move
    packet uniquely identifies the connection; DCCP B will read the new
    address and port off the DCCP-Move's network and DCCP headers.
    Eventually, DCCP A will receive a DCCP-Sync sent to its new address
    that negotiates a new Mobility ID/B feature.  This confirms the
    move.  DCCP A SHOULD retransmit the DCCP-Move packet until it
    receives a DCCP-Sync confirmation.  The retransmission strategy
    SHOULD be similar to that for retransmitting DCCP-Requests (Section
    8.1.1); for instance, a first timeout on the order of a second, with
    an exponential backoff timer.

    DCCP A MUST reset its congestion control state after sending a DCCP-
    Move, since nothing is known about conditions on the new path.
    Essentially, DCCP A must "slow start" up to its new fair rate, as
    appropriate for its congestion control mechanism.  Section 14.5
    discusses this further.

    DCCP A SHOULD NOT send non-DCCP-Move packets to DCCP B until the
    move is confirmed.  If it did so, and the DCCP-Move packet was lost
    or reordered, then DCCP B would react by sending DCCP-Resets with
    Reset Code 3, "No Connection".  DCCP A might implement special
    handling for such resets to avoid any post-move quiet period, but
    this is NOT RECOMMENDED.

    DCCP B MAY refuse to accept a move, perhaps because of address
    policy.  In this case, DCCP A will receive a DCCP-Reset with Reset
    Code 13, "Move Refused", rather than a confirming DCCP-Sync.  DCCP A
    MAY react by tearing down the connection, or by trying another DCCP-
    Move---for instance, back to the old address, if possible.

    DCCP endpoints SHOULD NOT use an old address-port pair after sending
    a DCCP-Move.  If it becomes necessary to switch back to the old
    address-port pair, the endpoint MUST do so explicitly using another
    DCCP-Move.

    DCCP-Move packets SHOULD NOT be sent until the connection is
    established; it is illegal to send a DCCP-Move in REQUEST or RESPOND
    state.  If an endpoint moves during connection establishment, it
    SHOULD abandon the old connection and initiate a new one.  No
    connection exists to move until the three-way handshake has
    completed.




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14.4.  Stationary Host Processing

    The stationary endpoint, DCCP B, uses DCCP-Move packets' destination
    address, destination port, and Mobility ID fields to look up the
    relevant connection.  This differs from all other packet types,
    which use the source address/source port/destination
    address/destination port 4-tuple.

    DCCP B MUST ignore DCCP-Moves whose Mobility ID is zero, or whose
    Mobility ID does not correspond to any active connection.  It also
    MUST ignore DCCP-Moves sent to sockets in CLOSED, LISTEN, REQUEST,
    RESPOND, or TIMEWAIT state, and it MUST ignore DCCP-Moves with
    invalid Sequence or Acknowledgement Numbers (see Section 7.5).
    DCCP B MUST NOT respond to invalid DCCP-Moves with DCCP-Reset or
    DCCP-Sync packets, since any active response would leak information
    about the connection to a possibly malicious host.  After receiving
    an invalid DCCP-Move, DCCP B MAY ignore subsequent DCCP-Move
    packets, valid or not, for a short period of time, such as one
    second or one round-trip time.  This protects DCCP B against denial-
    of-service attacks from floods of invalid DCCP-Moves.

    On receiving a valid DCCP-Move, DCCP B decides whether to accept or
    refuse the move request.  To accept the request, it performs several
    actions:

    o It changes the connection to use the new address and port.

    o It sets a timer to remove the old address and port after 2MSL.
      This delay allows the receipt of any delayed packets from the old
      address and port, and essentially represents TIMEWAIT state for
      the old connection.

    o It chooses a new Mobility ID for the connection, which temporarily
      coexists with the old Mobility ID.

    o It generates and sends a confirmation DCCP-Sync packet, which
      includes a "Change L(Mobility ID)" option for the new Mobility ID.

    If the DCCP-Sync is lost, then DCCP A will send another DCCP-Move
    packet with the old Mobility ID.  DCCP B MUST send another DCCP-Sync
    packet in this situation, but SHOULD NOT choose yet another new
    Mobility ID.

    The move's three-way handshake completes once DCCP B receives a
    DCCP-SyncAck from DCCP A that confirms the new Mobility ID option.
    At that point, DCCP B MUST remove the old Mobility ID.





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    DCCP B MAY refuse a valid DCCP-Move request for any reason; for
    instance, the new address space might be considered unsuitable.  To
    refuse a valid DCCP-Move, DCCP B sends a DCCP-Reset packet to the
    new address and port pair with Reset Code 13, "Move Refused".  It
    need take no other action; for example, it MAY tear down the
    connection, or not.  If DCCP B plans to refuse every DCCP-Move
    request, it MUST negotiate a zero value for the Mobility Capable/A
    feature.

    DCCP B MUST ignore any data following the header in a DCCP-Move
    packet.

14.5.  Congestion Control State

    Once an endpoint has transitioned to a new address, the connection
    is effectively a new connection in terms of its congestion control
    state: the accumulated information about congestion between the old
    endpoints no longer applies.  Both DCCPs MUST initialize their
    congestion control state (windows, rates, and so forth) to that of a
    new connection.  That is, they must "slow start".

    Similarly, the endpoints' PMTUs SHOULD be reinitialized, and PMTU
    discovery performed again, following an address change.  See Section
    15.

    During the transition period between addresses, the endpoints might
    receive congestion feedback from both before the move and after the
    move.  Congestion and loss events on packets sent before the move
    SHOULD NOT affect the new connection's congestion control state.

14.6.  Security

    The DCCP mobility mechanism, like DCCP in general, does not provide
    cryptographic security guarantees.  Nevertheless, mobile hosts must
    use valid Mobility IDs, providing protection against some classes of
    attackers: An attacker cannot move a DCCP connection to a new
    address unless it knows a valid Mobility ID.  This generally means
    that an attacker must have snooped on every packet in the connection
    to get a reasonable probability of success, assuming that the
    Mobility ID was chosen well (that is, randomly).

    An attacker could choose a server running many mobility-capable
    connections, and simply guess random Mobility IDs until one hit.
    Let N equal the number of mobility-capable connections at the
    server, X equal the number of attack attempts, and D equal the
    number of possible Mobility IDs, namely 2^128.  Then the probability
    of at least one attack succeeding is




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                 (D - N) choose X           (D-N)! (D-X)!
     P  =  1  -  ----------------  =  1  -  ------------- .
                    D choose X               D! (D-N-X)!

    For N = 10^6 and X = 10^9, the attack success probability is less
    than 10^-23.

    Section 19 further describes DCCP security considerations.

15.  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 Transfer Unit) [RFC 1191], and the
    lengths of the IP and DCCP headers.

    A DCCP application interface should let the application discover
    DCCP's current MPS.  DCCP applications should use the API to
    discover the 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 that packets
    larger than PMTU be fragmented on IPv4 networks.  This only matters
    when CCMPS > PMTU; packets larger than CCMPS MUST be rejected
    regardless.  Fragmentation should not be the default.  The rest of
    this section assumes the application has not requested
    fragmentation.

    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.

    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.

    To perform classical PMTU discovery, the DCCP sender sets the IP
    Don't Fragment (DF) bit.  However, it is undesirable for MTU
    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



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    initial handshake might unnecessarily delay connection
    establishment.  Thus, DF SHOULD NOT be set on DCCP-Request and DCCP-
    Response packets. In addition DF SHOULD NOT be set on DCCP-Reset
    packets, although typically these would be small enough to not be a
    problem.  On all other DCCP packets, DF SHOULD be set.

    As specified in [RFC 1191], 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 to the source of the datagram with
    the Code indicating "fragmentation needed and DF set" (also known as
    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 Section 7 of [RFC 1191]. If the MTU given in
    the message is greater than the current PMTU, the Datagram Too Big
    message is ignored, as described in [RFC 1191]. (We are aware that
    this may cause problems for DCCP endpoints behind certain
    firewalls.)

    If the DCCP implementation has decreased the PMTU, 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 with the DF bit
    cleared, or it MAY discard these packets; it MUST NOT transmit these
    datagrams with the DF bit set.

    A DCCP implementation may allow the application to occasionally
    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.  A successful DCCP-
    Move will also reset the PMTU.

    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 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



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    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.  (Routers are not
    required to return more than 64 bits of the DCCP header [RFC 792],
    but most modern routers will return far more, including 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 commensurate loss, 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, 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; and 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.

16.  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
      cannot be an unknown value.  A DCCP SHOULD reset the connection if
      it is assigned an unacceptable value for some non-negotiable



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      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 20 lists DCCP assigned numbers reserved for experimental and
    testing purposes.

17.  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 provide 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 attempted
    connections accessing unexpected services by sending a DCCP-Reset
    with Reset Code 9, "Bad Service Code".  Middleboxes probably
    shouldn't 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.

    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
    have correct ECN Nonce Echoes, or risk the connection being reset
    for "Aggression Penalty".  Furthermore, if a middlebox completely
    changed sequence numbers, the DCCP-Move mobility mechanism might
    stop working.  We therefore recommend that middleboxes not modify
    packet streams by adding or removing packets.



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    Note that there is less need to modify DCCP's per-packet sequence
    numbers than 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.

18.  Relations to Other Specifications

18.1.  DCCP and RTP

    The Real-Time Transport Protocol, RTP [RFC 3550], 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 that
    eliminating the redundancy would not reduce the overhead.

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



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    such as the ECN Nonce Echo.  Neither feedback mechanism makes the
    other redundant.

    Sending both types of feedback isn't particularly costly either.
    RTCP reports are sent relatively infrequently: once every 5 seconds,
    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 is not 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
    (RTCP XR) Loss RLE Blocks 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; for instance, RTP sequence numbers might be
    sent out of order.  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 header compression.

18.2.  Multiplexing Issues

    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



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    need for DCCP to provide built-in, SCTP-style support for multiple
    sub-flows.

    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 [RFC 3124], 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.

19.  Security Considerations

    DCCP does not provide cryptographic security guarantees.
    Applications desiring hard security should use IPsec or end-to-end
    security of some kind.

    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 [RFC 1750].

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

19.1.  Security Considerations for Mobility

    Mobility slightly changes DCCP's security properties by introducing
    a new mechanism by which an attacker can hijack a connection.  This
    mechanism, DCCP-Move, has the unfortunate property that, given a
    successful attack, the victim could not realize that the connection
    has been stolen---its connection would simply be reset unexpectedly.

    Nevertheless, a DCCP attacker still must snoop on data packets to
    get any reasonable probability of success, since it must guess a
    valid Mobility ID.  Section 14.6 quantifies the probability of
    successful attack; with DCCP's 128-bit Mobility IDs, that
    probability is quite low.





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19.2.  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 [UDP-LITE].

    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 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 [BEL98].

    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,
    which 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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20.  IANA Considerations

    DCCP introduces several sets of numbers whose values should be
    allocated by IANA.  The following sets of numbers should require an
    IETF standards-track specification as a prerequisite for new
    registrations.

    o DCCP Packet Types 9 through 15 (Section 5.1).

    o 8-bit DCCP-Reset Codes (Section 5.6).

    o 8-bit DCCP Option Types (Section 5.9). The CCID-specific options
      128 through 255 need not be allocated by IANA, although particular
      CCIDs may request that IANA allocate their CCID-specific options.

    o 8-bit DCCP Feature Numbers (Section 6). The CCID-specific features
      128 through 255 need not be allocated by IANA, although particular
      CCIDs may request that IANA allocate their CCID-specific features.

    o 8-bit DCCP Congestion Control Identifiers (CCIDs) (Section 10).

    o Ack Vector States (Section 11.4). Only State 2 remains
      unallocated.

    o Data Dropped Drop Codes 4 through 6 (Section 11.7).

    IANA should also provide a registry for 32-bit Service Codes.
    Registering a Service Code should not require a standards-track
    specification.  Our liberal proposed registration rules for Service
    Codes are presented in detail in Section 8.1.2.

    Finally, DCCP requires a Protocol Number to be added to the registry
    of Assigned Internet Protocol Numbers.  Protocol Number 33 has
    informally been made available for experimental DCCP use, but this
    number may change in future.

    The following DCCP assigned numbers should be reserved specifically
    for experimental and testing use [RFC 3692]: packet type 15, option
    number 31, option numbers 120 through 126, feature numbers 120
    through 126, Reset Codes 248 through 254, and CCID 254.

21.  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



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    Lai recommended that the old feature negotiation mechanism be
    scrapped and helped design the current mechanism, and 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 thank those who provided comments and suggestions via the
    DCCP BOF, Working Group, and mailing lists, including Damon
    Lanphear, Patrick McManus, Sara Karlberg, Kevin Lai, Youngsoo Choi,
    Dan Duchamp, Gorry Fairhurst, Derek Fawcus, David Timothy Fleeman,
    John Loughney, Ghyslain Pelletier, Tom Phelan, Stanislav Shalunov,
    Yufei Wang, and Michael Welzl.  In particular, Michael Welzl
    suggested the Data Checksum option.

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 rather than 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.



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    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 <===

    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.  This smaller
    Example Buffer contains actual data.

             +---------------------------+
          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.




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        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_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



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    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]
              ** +--*------------------------+

    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, either 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
    whatsoever, until its buffer shrinks again.

A.1.2.  Old Packets

    When a packet with Sequence Number S arrives, and S <= buf_ackno,
    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



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    buffer byte has a non-zero run length, then the buffer might need 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.

    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_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



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    with the appropriate ack_seqno, then:

    o Sets buf_tail to R.ack_ptr + 1.

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

    o 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_ackno = 3, ack_nonce = 1.

    2.  ack_seqno = 60, 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
    set 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 P2'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



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    acknowledgement saying that Packet 9 actually was received.
    Therefore, when packet 9 arrived, the HC-Receiver might modify its
    acknowledgement record to:

    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---for example, with Ack Ratio---in response to
    congestion.

    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.





Kohler/Handley/Floyd                            Section A.4.  [Page 112]

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B.  Appendix: Design Motivation

    This section attempts to capture some of the rationale behind
    specific details of DCCP design.

B.1.  CsCov and Partial Checksumming

    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.

    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 prefer to receive corrupted data
    than to have DCCP drop a corrupted packet.  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 (so 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, but still not mistake 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



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    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.

    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 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.  However, 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

    [RFC 793] J. Postel, editor.  Transmission Control Protocol.
        RFC 793.

    [RFC 1191] J. C. Mogul and S. E. Deering.  Path MTU Discovery.
        RFC 1191.

    [RFC 1750] D. Eastlake, S. Crocker, and J. Schiller.  Randomness
        Recommendations for Security.  RFC 1750.

    [RFC 2026] S. Bradner.  The Internet Standards Process---Revision 3.
        RFC 2026.

    [RFC 2119] S. Bradner.  Key Words For Use in RFCs to Indicate
        Requirement Levels.  RFC 2119.



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

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

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

    [RFC 3692] T. Narten.  Assigning Experimental and Testing Numbers
        Considered Useful.  RFC 3692.

    [UDP-LITE] L-A. Larzon, M. Degermark, S. Pink, L-E. Jonsson
        (editor), and G. Fairhurst (editor).  The UDP-Lite Protocol.
        draft-ietf-tsvwg-udp-lite-02.txt, work in progress, August 2003.

Informative References

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

    [BEL98] S.M. Bellovin.  Cryptography and the Internet.  Proc. CRYPTO
        '98 (LNCS 1462), pp46-55, August, 1988.

    [CCID 2 PROFILE] S. Floyd and E. Kohler.  Profile for DCCP
        Congestion Control ID 2: TCP-like Congestion Control.  draft-
        ietf-dccp-ccid2-05.txt, work in progress, February 2004.

    [CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye.  Profile for
        DCCP Congestion Control ID 3: TFRC Congestion Control.  draft-
        ietf-dccp-ccid3-05.txt, work in progress, February 2004.

    [LINK BCP] Phil Karn, editor.  Advice for Internet Subnetwork
        Designers.  draft-ietf-pilc-link-design-13.txt, work in
        progress, February 2003.

    [M85] Robert T. Morris.  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] Matt Mathis, John Heffner, and Kevin Lahey.  Path MTU
        Discovery.  draft-ietf-pmtud-method-00.txt, work in progress,
        October 2003.

    [RFC 792] J. Postel, editor.  Internet Control Message Protocol.
        RFC 792.




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    [RFC 1948] S. Bellovin.  Defending Against Sequence Number Attacks.
        RFC 1948.

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

    [RFC 3124] H. Balakrishnan and S. Seshan.  The Congestion Manager.
        RFC 3124.

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

    [RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang. A
        Conservative Selective Acknowledgment (SACK)-based Loss Recovery
        Algorithm for TCP. RFC 3517.

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

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

    [SB00] Alex C. Snoeren and Hari Balakrishnan.  An End-to-End
        Approach to Host Mobility.  Proc. 6th Annual ACM/IEEE
        International Conference on Mobile Computing and Networking
        (MOBICOM '00), August 2000.

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

    [SYNCOOKIES] Daniel J. Bernstein.  SYN Cookies.
        http://cr.yp.to/syncookies.html, as of July 2003.

Authors' Addresses















Kohler/Handley/Floyd                                          [Page 116]

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    Eddie Kohler <kohler@cs.ucla.edu>
    4531C Boelter Hall
    UCLA Computer Science Department
    Los Angeles, CA 90095
    USA

    Mark Handley <M.Handley@cs.ucl.ac.uk>
    Department of Computer Science
    University College London
    Gower Street
    London WC1E 6BT
    UK

    Sally Floyd <floyd@icir.org>
    ICSI Center for Internet Research
    1947 Center Street, Suite 600
    Berkeley, CA 94704
    USA


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    This document and translations of it may be copied and furnished to
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Kohler/Handley/Floyd                                          [Page 117]

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    and distributed, in whole or in part, without restriction of any
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Kohler/Handley/Floyd                                          [Page 118]


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