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Internet Engineering Task Force                             Eddie Kohler
INTERNET-DRAFT                                                      UCLA
draft-ietf-dccp-spec-07.txt                                 Mark Handley
Expires: January 2005                                                UCL
                                                             Sally Floyd
                                                                    ICIR
                                                            18 July 2004


              Datagram Congestion Control Protocol (DCCP)


Status of this Memo

    This document is an Internet-Draft.

    By submitting this Internet-Draft, we certify that any applicable
    patent or other IPR claims of which we are aware have been
    disclosed, or will be disclosed, and any of which we become aware
    will be disclosed, in accordance with RFC 3668 (BCP 79).

    By submitting this Internet-Draft, we accept the provisions of
    Section 3 of RFC 3667 (BCP 78).

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
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    The list of current Internet-Drafts can be accessed at
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Copyright Notice

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





Kohler/Handley/Floyd                                            [Page 1]

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Abstract

    The Datagram Congestion Control Protocol (DCCP) is a transport
    protocol that implements bidirectional, unicast connections of
    congestion-controlled, unreliable datagrams.  It should be suitable
    for use by applications such as streaming media, Internet telephony,
    and on-line games.












































Kohler/Handley/Floyd                                            [Page 2]

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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes since draft-ietf-dccp-spec-06.txt:

    * Change extended sequence numbers.  Now 48-bit sequence numbers are
    MANDATORY, and all packet types except Data, Ack, and DataAck always
    use 48-bit sequence numbers.  This change improves DCCP's robustness
    against blind attacks.

    * Removed empty Change options.

    * Allow preference list changes during feature negotiations (this
    seems easier to implement than the alternative).  This required a
    new feature negotiation state, UNSTABLE.

    * Add Minimum Checksum Coverage feature.

    * Add Reset Congestion State option.

    * Simplify the implementation of CCID-specific option processing: no
    need to check whether the CCID feature is being negotiated.

    * Many more minor changes.

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

    * Organization overhaul.

    * Add pseudocode for event processing.

    * Remove # NDP; replace with Ack Count.

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

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

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

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

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

    * Mobility ID becomes 128 bits long.

    * Add probabilities to Mobility ID discussion.

    * Add SyncAck.



Kohler/Handley/Floyd                                            [Page 3]

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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. Security Limitation. . . . . . . . . . . . . . . . . . .  11
       3.6. Robustness Principle . . . . . . . . . . . . . . . . . .  11
    4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . .  11
       4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . .  13
       4.3. States . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.4. Congestion Control . . . . . . . . . . . . . . . . . . .  15
       4.5. Features . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.6. 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 . . . . . . . . . .  26
       5.6. DCCP-Reset Header. . . . . . . . . . . . . . . . . . . .  26
       5.7. DCCP-Sync and DCCP-SyncAck Headers . . . . . . . . . . .  29
       5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . .  30
          5.8.1. Padding Option. . . . . . . . . . . . . . . . . . .  31
          5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . .  31
    6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . .  32
       6.1. Change Options . . . . . . . . . . . . . . . . . . . . .  33
       6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . .  33
       6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . .  34
          6.3.1. Server-Priority . . . . . . . . . . . . . . . . . .  34
          6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . .  34
       6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . .  35
       6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . .  35
       6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . .  37
          6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . .  37
          6.6.2. Processing Received Options . . . . . . . . . . . .  38
          6.6.3. Loss and Retransmission . . . . . . . . . . . . . .  40
          6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . .  41
          6.6.5. Preference Changes. . . . . . . . . . . . . . . . .  42
          6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . .  42
          6.6.7. Unknown Features. . . . . . . . . . . . . . . . . .  42
          6.6.8. Invalid Options . . . . . . . . . . . . . . . . . .  43



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          6.6.9. Mandatory Feature Negotiation . . . . . . . . . . .  43
          6.6.10. Out-of-Band Agreement. . . . . . . . . . . . . . .  44
    7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . .  44
       7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . .  44
       7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . .  45
       7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . .  46
       7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . .  46
       7.5. Validity and Synchronization . . . . . . . . . . . . . .  47
          7.5.1. Sequence-Validity Rules . . . . . . . . . . . . . .  47
          7.5.2. Handling Sequence-Invalid Packets . . . . . . . . .  49
          7.5.3. Sequence and Acknowledgement Number
          Windows. . . . . . . . . . . . . . . . . . . . . . . . . .  50
          7.5.4. Sequence Window Feature . . . . . . . . . . . . . .  51
          7.5.5. Sequence Number Attacks . . . . . . . . . . . . . .  52
          7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . .  53
       7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . .  54
          7.6.1. Allow Short Sequence Numbers Feature. . . . . . . .  54
          7.6.2. When to Avoid Short Sequence Numbers. . . . . . . .  55
       7.7. NDP Count and Detecting Application Loss . . . . . . . .  55
          7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . .  56
          7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . .  57
    8. Event Processing. . . . . . . . . . . . . . . . . . . . . . .  57
       8.1. Connection Establishment . . . . . . . . . . . . . . . .  57
          8.1.1. Client Request. . . . . . . . . . . . . . . . . . .  57
          8.1.2. Service Codes . . . . . . . . . . . . . . . . . . .  58
          8.1.3. Server Response . . . . . . . . . . . . . . . . . .  59
          8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . .  60
          8.1.5. Handshake Completion. . . . . . . . . . . . . . . .  61
       8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . .  62
       8.3. Termination. . . . . . . . . . . . . . . . . . . . . . .  62
          8.3.1. Abnormal Termination. . . . . . . . . . . . . . . .  64
       8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . .  64
       8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . .  65
    9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . .  69
       9.1. Header Checksum Field. . . . . . . . . . . . . . . . . .  69
       9.2. Header Checksum Coverage Field . . . . . . . . . . . . .  70
          9.2.1. Minimum Checksum Coverage Feature . . . . . . . . .  71
       9.3. Data Checksum Option . . . . . . . . . . . . . . . . . .  71
          9.3.1. Check Data Checksum Feature . . . . . . . . . . . .  72
          9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . .  73
    10. Congestion Control IDs . . . . . . . . . . . . . . . . . . .  73
       10.1. Unspecified Sender-Based Congestion
       Control . . . . . . . . . . . . . . . . . . . . . . . . . . .  74
       10.2. TCP-like Congestion Control . . . . . . . . . . . . . .  75
       10.3. TFRC Congestion Control . . . . . . . . . . . . . . . .  76
       10.4. CCID-Specific Options, Features, and Reset
       Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  76
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  78



Kohler/Handley/Floyd                                            [Page 5]

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       11.1. Acks of Acks and Unidirectional
       Connections . . . . . . . . . . . . . . . . . . . . . . . . .  78
       11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . .  80
       11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . .  80
       11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . .  82
          11.4.1. Ack Vector Consistency . . . . . . . . . . . . . .  84
          11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . .  85
       11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . .  86
       11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . .  86
       11.7. Reset Congestion State Option . . . . . . . . . . . . .  87
       11.8. Data Dropped Option . . . . . . . . . . . . . . . . . .  87
          11.8.1. Data Dropped and Normal Congestion
          Response . . . . . . . . . . . . . . . . . . . . . . . . .  90
          11.8.2. Particular Drop Codes. . . . . . . . . . . . . . .  90
    12. Explicit Congestion Notification . . . . . . . . . . . . . .  91
       12.1. ECN Capable Feature . . . . . . . . . . . . . . . . . .  92
       12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . .  92
       12.3. Other Aggression Penalties. . . . . . . . . . . . . . .  93
    13. Timing Options . . . . . . . . . . . . . . . . . . . . . . .  94
       13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . .  94
       13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . .  94
       13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . .  95
    14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . .  96
    15. Forward Compatibility. . . . . . . . . . . . . . . . . . . .  99
    16. Middlebox Considerations . . . . . . . . . . . . . . . . . .  99
    17. Relations to Other Specifications. . . . . . . . . . . . . . 101
       17.1. DCCP and RTP. . . . . . . . . . . . . . . . . . . . . . 101
       17.2. Multiplexing Issues . . . . . . . . . . . . . . . . . . 102
    18. Security Considerations. . . . . . . . . . . . . . . . . . . 102
       18.1. Security Considerations for Partial Check-
       sums. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
    19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 104
    20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
    A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 105
       A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 107
          A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 107
          A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 108
       A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 109
       A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 110
       A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 111
    B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 112
       B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 112
    Normative References . . . . . . . . . . . . . . . . . . . . . . 113
    Informative References . . . . . . . . . . . . . . . . . . . . . 114
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 116
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 116
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 116




Kohler/Handley/Floyd                                            [Page 6]

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1.  Introduction

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

    o  Unreliable flows of datagrams, with acknowledgements.

    o  Reliable handshakes for connection setup and teardown.

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

    o  Mechanisms allowing servers to avoid holding state for
       unacknowledged connection attempts and already-finished
       connections.

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

    o  Acknowledgement mechanisms communicating packet loss and ECN
       information.  Acks are transmitted as reliably as the relevant
       congestion control mechanism requires, possibly completely
       reliably.

    o  Optional mechanisms that tell the sending application, with high
       reliability, which data packets reached the receiver, and whether
       those packets were ECN marked, corrupted, or dropped in the
       receive buffer.

    o  Path Maximum Transfer Unit (PMTU) discovery, as per [RFC 1191].

    DCCP is intended for applications, such as streaming media and
    Internet telephony, where the costs of long delays can exceed the
    costs of loss and out-of-order delivery.  TCP is not well-suited for
    these applications, since its reliable in-order delivery, combined
    with congestion control, can cause arbitrarily long delays.  UDP
    avoids long delays, but UDP applications must implement congestion
    control on their own.  DCCP provides built-in congestion control,
    including ECN support, for unreliable datagram flows.  DCCP avoids
    the arbitrary delays associated with TCP.  It also implements
    reliable connection setup, teardown, and feature negotiation, and
    provides a choice of congestion control mechanisms.







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2.  Design Rationale

    Most streaming UDP applications should have little reason not to
    switch to DCCP, once it is deployed.  To facilitate this, DCCP was
    designed to have as little overhead as possible, both in terms of
    the packet header size and in terms of the state and CPU overhead
    required at end hosts.  Only the minimal necessary functionality was
    included in DCCP, leaving other functionality, such as forward error
    correction (FEC), semi-reliability, and multiple streams, to be
    layered on top of DCCP as desired.  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, on-line games might want to
    make quick use of any available bandwidth, while streaming media
    might trade off this responsiveness for a steadier, less bursty
    rate.  (Sudden rate changes can cause unacceptable UI glitches, such
    as audible pauses or clicks in the playout stream.)  DCCP thus
    allows applications to choose 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 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 implements congestion control itself.

    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
    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.



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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.

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

    All operations on DCCP sequence numbers, and comparisons such as
    "greater" and "greatest", use circular arithmetic modulo 2**48.
    This form of arithmetic preserves the relationships between sequence
    numbers as they roll over from 2**48 - 1 to 0.

    Reserved bitfields in DCCP packet headers MUST be set to zero by
    senders, and MUST be ignored by receivers, unless otherwise
    specified.  This is to allow for future protocol extensions.  In
    particular, DCCP processors MUST NOT reset a DCCP connection simply
    because a Reserved field has non-zero value [RFC 3360].

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 application data sent by one endpoint and the corresponding
    acknowledgements sent by the other endpoint.  We can illustrate this
    as follows:




Kohler/Handley/Floyd                              Section 3.2.  [Page 9]

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     +--------+  A-to-B half-connection:         +--------+
     |        |    -->  application data  -->    |        |
     |        |    <--  acknowledgements  <--    |        |
     | DCCP A |                                  | DCCP B |
     |        |  B-to-A half-connection:         |        |
     |        |    <--  application data  <--    |        |
     +--------+    -->  acknowledgements  -->    +--------+

    Although they are logically distinct, in practice the half-
    connections overlap; a DCCP-DataAck packet, for example, contains
    application data relevant to one half-connection and acknowledgement
    information relevant to the other.

    In the context of a single half-connection, the terms "HC-Sender"
    and "HC-Receiver" denote the endpoints sending application data and
    acknowledgements, respectively.  For example, DCCP A is the HC-
    Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.

3.3.  Features

    A DCCP feature is a connection attribute on whose value the two
    endpoints agree.  Many properties of a DCCP connection are
    controlled by features, including the congestion control mechanisms
    in use on the two half-connections.  The endpoints can achieve
    agreement through the exchange of feature negotiation options in
    DCCP headers, or through out-of-band communication.

    DCCP features are identified by a feature number and an endpoint.
    The notation "F/X" represents the feature with feature number F
    located at DCCP endpoint X.  Each valid feature number thus
    corresponds to two features, which are negotiated separately and
    need not have the same value.  The two endpoints know, and agree on,
    the value of every valid feature.  DCCP A is the "feature location"
    for all features F/A, and the "feature remote" for all features F/B.

3.4.  Round-Trip Times

    We sometimes refer to round-trip times -- for setting timers, for
    example.  If no useful round-trip time estimate is available, a DCCP
    implementation SHOULD use 0.1 seconds instead.

    The maximum segment lifetime, or MSL, is the maximum length of time
    a packet can survive in the network.  The default MSL is two minutes
    for this specification.







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3.5.  Security Limitation

    DCCP is not robust against attackers who can snoop on a connection
    in progress, or who can guess valid sequence numbers in other ways.
    Applications desiring stronger security should use IPsec or
    application-level cryptography.

3.6.  Robustness Principle

    DCCP implementations will follow TCP's "general principle of
    robustness": "be conservative in what you do, be liberal in what you
    accept from others" [RFC 793].

4.  Overview

    DCCP's high-level connection dynamics echo those of TCP.
    Connections progress through three phases: initiation, including a
    three-way handshake; data transfer; and termination.  Data can flow
    both ways over the connection.  An acknowledgement framework lets
    senders discover how much data has been lost, and thus avoid
    unfairly congesting the network.  Of course, DCCP provides
    unreliable datagram semantics, not TCP's reliable bytestream
    semantics.  The application must package its data into explicit
    frames, and must retransmit its own data as necessary.  It may be
    useful to think of DCCP as TCP minus bytestream semantics and
    reliability, or as UDP plus congestion control, handshakes, and
    acknowledgements.

4.1.  Packet Types

    Ten packet types implement DCCP's protocol functions.  For example,
    every new connection attempt begins with a DCCP-Request packet sent
    by the client.  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, shown here.  Note the three-way handshakes during
    initiation and termination.












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       Client                                      Server
       ------                                      ------
                        (1) Initiation
       DCCP-Request -->
                                        <-- DCCP-Response
       DCCP-Ack -->
                        (2) Data transfer
       DCCP-Data, DCCP-Ack, DCCP-DataAck -->
                    <-- DCCP-Data, DCCP-Ack, DCCP-DataAck
                        (3) Termination
                                        <-- DCCP-CloseReq
       DCCP-Close -->
                                           <-- DCCP-Reset

    The two remaining packet types are used to resynchronize after
    bursts of loss.

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

    The packet types are as follows:

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

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

    DCCP-Data
        Used to transmit application data.

    DCCP-Ack
        Used to transmit pure acknowledgements.

    DCCP-DataAck
        Used to transmit application data with piggybacked
        acknowledgements.

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

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



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

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

4.2.  Sequence Numbers

    Each DCCP packet carries a sequence number, so that losses can be
    detected and reported.  Unlike TCP's byte-based sequence numbers,
    DCCP sequence numbers are packet-based: each packet sent increments
    the sequence number by one.  For example:

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

    Even DCCP-Ack pure acknowledgements increment the sequence number.
    In the example, DCCP B's second packet uses sequence number 11,
    since sequence number 10 was used for an acknowledgement.  This lets
    endpoints detect lost acknowledgements.  It also means that
    endpoints can get out of sync after long bursts of loss; the DCCP-
    Sync and DCCP-SyncAck packet types are used to recover (Section
    7.5).

    Since DCCP provides unreliable semantics, there are no
    retransmissions, and it doesn't make sense to have a TCP-style
    cumulative acknowledgement field.  DCCP's Acknowledgement Number
    field equals the greatest sequence number received, rather than the
    smallest sequence number not received.  Separate options indicate
    any intermediate sequence numbers that weren't received.

4.3.  States

    DCCP endpoints progress through different states during the course
    of a connection, corresponding roughly to the three phases of
    initiation, data transfer, and termination.  The figure below shows
    the typical progress through these states for a client and server.








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

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

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

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

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

    CLOSED
        Represents nonexistent connections.

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

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

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

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

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



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

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

    CLOSING
        Server and client sockets can both enter this state to close the
        connection.

    TIMEWAIT
        A socket remains in this state for 2MSL (4 minutes) after the
        connection has been torn down, to prevent mistakes due to the
        delivery of old packets.

4.4.  Congestion Control

    DCCP connections are congestion controlled, but unlike in TCP, DCCP
    applications have a choice of congestion control mechanism.  In
    fact, the two half-connections can be governed by different
    mechanisms.  Mechanisms are denoted by one-byte congestion control
    identifiers, or CCIDs.  The endpoints negotiate their CCIDs during
    connection initiation.  Each CCID describes how the HC-Sender limits
    data packet rates, how the HC-Receiver sends congestion feedback via
    acknowledgements, and so forth.  CCIDs 2 and 3 are currently
    defined; CCID 0 is reserved, and CCID 1 is used for special
    purposes.

    CCID 2 provides TCP-like Congestion Control, which is similar to
    that of TCP.  The sender maintains a congestion window and sends
    packets until that window is full.  Packets are acknowledged by the
    receiver.  Dropped packets and ECN [RFC 3168] indicate congestion;
    the response to congestion is to halve the congestion window.
    Acknowledgements in CCID 2 contain the sequence numbers of all
    received packets within some window, similar to a selective
    acknowledgement (SACK) [RFC 3517].

    CCID 3 provides TFRC Congestion Control, an equation-based form of
    congestion control intended to respond to congestion more smoothly
    than CCID 2.  The sender maintains a transmit rate, which it updates
    using the receiver's estimate of the packet loss and mark rate.
    CCID 3 behaves somewhat differently from TCP in the short term, it
    is designed to operate fairly with TCP over the long term.




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    Section 10 describes DCCP's CCIDs in more detail.  The behaviors of
    CCIDs 2 and 3 are fully defined in separate profile documents [CCID
    2 PROFILE] [CCID 3 PROFILE].

4.5.  Features

    DCCP endpoints generally use Change and Confirm options to negotiate
    and agree on feature values, although agreement may also be achieved
    using an out-of-band signalling channel.  Feature negotiation will
    almost always happen on the connection initiation handshake, but it
    can begin at any time.

    There are four feature negotiation options in all: Change L,
    Confirm L, Change R, and Confirm R.  The "L" options are sent by the
    feature location, and the "R" options are sent by the feature
    remote.  A Change R option says to the feature location, "change
    this feature value as follows".  The feature location responds with
    Confirm L, meaning "I've changed it".  Some features allow Change R
    options to contain multiple values, sorted in preference order.  For
    example:

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

       Change R(CCID, 3 4) -->
                                <-- Confirm L(CCID, 4, 4 2)
                  * agreement that CCID/Server = 4 *

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

    The Change L and Confirm R options are used for feature negotiations
    initiated by the feature location.  In the following example, the
    server requests that CCID/Server be set to 3 or 2, with 3 preferred,
    and the client agrees.

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






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

4.6.  Differences From TCP

    Differences between DCCP and TCP apart from those discussed so far
    include:

    o  Copious space for options (up to 1008 bytes).

    o  Different acknowledgement formats. The CCID for a connection
       determines how much acknowledgement 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 mechanisms help limit
       the amount of state possibly-misbehaving clients can force DCCP
       servers to maintain.  An Init Cookie option, analogous to TCP's
       SYN Cookies [SYNCOOKIES], avoids SYN-flood-like attacks.  Only
       one connection endpoint 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 11.8) lets an endpoint declare that a packet was dropped
       because of corruption, because of receive buffer overflow, and so
       on.  This facilitates research into more appropriate rate-control
       responses for these non-network-congestion losses (although
       currently such losses will cause a congestion response).

    o  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.  DCCP acknowledges a
       packet when its options have been processed.  The Data Dropped
       option may later report that the packet's payload was discarded.

    o  No receive window.  DCCP is a congestion control protocol, not a
       flow control protocol.

    o  No simultaneous open.  Every connection has one client and one
       server.



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    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 an application request on the DCCP-Request packet,
        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.  This
        response indicates any features and options that the server
        agrees to, begins other feature negotiations as 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
        one in the DCCP-Response.  It may also continue 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



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        packets containing data with piggybacked acknowledgements.  If
        the client has no data to send, then the server will send DCCP-
        Data and DCCP-DataAck packets, while the client will send DCCP-
        Acks exclusively.

    5.  The server sends a DCCP-CloseReq packet requesting a close.

    6.  The client sends a DCCP-Close packet acknowledging the close.

    7.  The server sends a DCCP-Reset packet with Reset Code 1,
        "Closed", and clears its connection state.  DCCP-Resets are part
        of normal connection termination; see Section 5.6.

    8.  The client receives the DCCP-Reset packet and holds state for 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 DCCP header can be from 12 to 1020 bytes long.  The initial 12
    bytes of the header have the same semantics for all packet types.
    Following this comes any additional fixed-length fields required by
    the packet type, and then a variable-length list of options.  Some
    packet types allow application data to follow the header.

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





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5.1.  Generic Header

    The DCCP generic header takes different forms depending on the value
    of X, the Extended Sequence Numbers bit.  If X is one, the Sequence
    Number field is 48 bits long and the generic header takes 16 bytes,
    as follows.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Source Port          |           Dest Port           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Data Offset  | CCVal | CsCov |           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     |X|       |                                               .
     | Res |=| Type  |          Sequence Number (high bits)          .
     |     |1|       |                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .          Sequence Number (low bits)           |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    If X is zero, only the low 24 bits of the Sequence Number are
    transmitted, and the generic header is 12 bytes long.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Source Port          |           Dest Port           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Data Offset  | CCVal | CsCov |           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     |X|       |                                               |
     | Res |=| Type  |          Sequence Number (low bits)           |
     |     |0|       |                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    The generic header fields are defined as follows.

    Source and Destination Ports: 16 bits each
        These fields identify the connection, similar to the
        corresponding fields in TCP and UDP.  The Source Port represents
        the relevant port on the endpoint that sent this packet, the
        Destination Port the relevant port on the other endpoint.
        Source Ports SHOULD be chosen randomly, to reduce the likelihood
        of attack.





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    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.  The sender MUST set CCVal to zero unless
        its HC-Sender CCID specifies otherwise, and the receiver MUST
        ignore the CCVal field unless its HC-Receiver CCID specifies
        otherwise.

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

    Checksum: 16 bits
        The Internet checksum of the packet's DCCP header (including
        options), a network-layer pseudoheader, and, depending on
        Checksum Coverage, 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-Sync
          9    DCCP-SyncAck
        10-15  Reserved

        Receivers MUST ignore any packets with reserved type.  That is,
        packets with reserved type MUST NOT be processed and they MUST
        NOT be acknowledged as received.




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    Reserved (Res): 3 bits
        Senders MUST set this field to all zeroes on generated packets,
        and receivers MUST ignore its value.

    Extended Sequence Numbers (X): 1 bit
        Set to one to indicate the use of an extended generic header
        with 48-bit Sequence and Acknowledgement Numbers.  DCCP-Data,
        DCCP-DataAck, and DCCP-Ack packets MAY set X to zero or one.
        All DCCP-Request, DCCP-Response, DCCP-CloseReq, DCCP-Close,
        DCCP-Reset, DCCP-Sync, and DCCP-SyncAck packets MUST set X to
        one; endpoints MUST ignore any such packets with X set to zero.
        High-rate connections SHOULD set X to one on all packets to gain
        increased protection against wrapped sequence numbers and
        attacks.  See Section 7.6.

    Sequence Number: 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.

    All currently defined packet types except DCCP-Request and DCCP-Data
    carry an Acknowledgement Number in the four or eight bytes
    immediately following the generic header.  When X=1, its format is:

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

    When X=0, only the low 24 bits of the Acknowledgement Number are
    transmitted.

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


    Acknowledgement Number: 24 or 48 bits
        Generally contains GSR, the Greatest Sequence Number Received on
        any acknowledgeable packet so far.  A packet is acknowledgeable
        if and only if its header options were processed by the
        receiver.  Section 7.4 describes this further.  Options such as
        Ack Vector (Section 11.4) combine with the Acknowledgement
        Number to provide precise information about which packets have
        arrived.




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        Acknowledgement Numbers on DCCP-Sync and DCCP-SyncAck packets
        need not equal GSR; see Section 5.7.

    Reserved: 8 bits
        Senders MUST set this field to all zeroes on generated packets,
        and receivers MUST ignore its value.

5.2.  DCCP-Request Header

    A client initiates a DCCP connection by sending a DCCP-Request
    packet.  These packets MAY contain 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 with X=1 (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.
    DCCP-Response packets MAY contain application data.













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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /            Generic DCCP Header with X=1 (16 bytes)            /
     /                  with Type=1 (DCCP-Response)                  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |       Acknowledgement Number (high bits)      .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .       Acknowledgement Number (low bits)       |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Service Code                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |


    Acknowledgement Number: 48 bits
        Contains GSR.  Since DCCP-Responses are only sent during
        connection initiation, this will always equal the Sequence
        Number on a received DCCP-Request.

    Service Code: 32 bits
        Echoes the Service Code on a received 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             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)
    (.       Acknowledgement Number (low bits)       |   Reserved    |)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    (The parenthesized fields appear only when the header's Extended
    Sequence Numbers field is 1.)  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             |
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+)
    (.       Acknowledgement Number (low bits)       |   Reserved    |)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Application Data                       |
     |                              ...                              |

    A DCCP-Data or DCCP-DataAck packet may have a zero-length
    application data area, which indicates that the application sent a
    zero-length datagram.  This differs from DCCP-Request and DCCP-
    Response packets, where an empty application data area indicates the
    absence of application data (as opposed to the presence of zero-
    length application data).

    Receivers MUST ignore the application data area in DCCP-Ack packets.
    DCCP-Ack senders will generally leave this area empty.

    DCCP-Ack and DCCP-DataAck packets often include additional
    acknowledgement options, such as Ack Vector, as required by the
    congestion control mechanism in use.





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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
    a DCCP-Close packet, which will elicit a DCCP-Reset packet.  Only
    the server can send a DCCP-CloseReq packet, which indicates that the
    server wants to close the connection, but does not want to hold its
    TIMEWAIT state.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /            Generic DCCP Header with X=1 (16 bytes)            /
     /         with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close)         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |       Acknowledgement Number (high bits)      .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .       Acknowledgement Number (low bits)       |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Receivers MUST ignore the application data area in DCCP-CloseReq and
    DCCP-Close packets.

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.




















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

    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 11, "Aggression Penalty", might contain Error
        Text such as "Aggression Penalty: Received 3 bad ECN Nonce
        Echoes, assuming misbehavior".

    The following Reset Codes are currently defined.  Unless otherwise
    specified, the Data 1, 2, and 3 fields MUST be set to 0 by the
    sender of the DCCP-Reset and ignored by its receiver.  Section
    references describe concrete situations that will cause each Reset
    Code to be generated; they are not meant to be exhaustive.

    0, "Unspecified"
        Indicates the absence of a meaningful Reset Code.  Use of Reset
        Code 0 is NOT RECOMMENDED: the sender should choose a Reset Code
        that more clearly defines why the connection is being reset.

    1, "Closed"
        Normal connection close.  See Section 8.3.




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    2, "Aborted"
        The sending endpoint gave up on the connection because of lack
        of progress.  See Sections 8.1.1 and 8.1.5.

    3, "No Connection"
        No connection exists.  See Section 8.3.1.

    4, "Packet Error"
        An unexpected packet type arrived; for example, a DCCP-Data
        packet arrived at a connection in the REQUEST state.  See
        Section 8.3.1. The Data 1 field equals the offending packet
        type.

    5, "Option Error"
        An option was erroneous, and the error was serious enough to
        warrant resetting the connection.  See Sections 6.6.7, 6.6.8,
        and 11.4.  The Data 1 field equals the offending option type;
        Data 2 and Data 3 equal the first two bytes of option data (or
        zero if the option had less than two bytes of data).

    6, "Mandatory Error"
        The sending endpoint could not process an option marked
        Mandatory.  The Data fields report the option type and data of
        the unprocessed option (not the Mandatory option), using the
        format of Reset Code 5, "Option Error".  See Section 5.8.2.

    7, "Connection Refused"
        The Destination Port didn't correspond to a port open for
        listening.  Sent only in response to DCCP-Requests.  See Section
        8.1.3.

    8, "Bad Service Code"
        The Service Code didn't equal the service code attached to the
        Destination Port.  Sent only in response to DCCP-Requests.  See
        Section 8.1.3.

    9, "Too Busy"
        The server is too busy to accept new connections.  Sent only in
        response to DCCP-Requests.  See Section 8.1.3.

    10, "Bad Init Cookie"
        The Init Cookie echoed by the client was incorrect or missing.
        See Section 8.1.4.

    11, "Aggression Penalty"
        This endpoint has detected congestion control-related
        misbehavior on the part of the other endpoint.  See Sections
        12.2 and 13.2.



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    12-127, Reserved
        Receivers should treat these codes like Reset Code 0,
        "Unspecified".

    128-255, CCID-specific codes
        Semantics depend on the connection's CCIDs.  See Section 10.4.
        Receivers should treat unknown CCID-specific Reset Codes like
        Reset Code 0, "Unspecified".

    The following table summarizes this information.

     Reset
     Code   Name                    Data 1     Data 2 & 3
     -----  ----                    ------     ----------
       0    Unspecified               0            0
       1    Closed                    0            0
       2    Aborted                   0            0
       3    No Connection             0            0
       4    Packet Error           pkt type        0
       5    Option Error           option #   option data
       6    Mandatory Error        option #   option data
       7    Connection Refused        0            0
       8    Bad Service Code          0            0
       9    Too Busy                  0            0
      10    Bad Init Cookie           0            0
      11    Aggression Penalty        0            0
     12-127 Reserved
    128-255 CCID-specific codes


5.7.  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
    received DCCP-Sync 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 with X=1 (16 bytes)            /
     /          with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck)          /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |       Acknowledgement Number (high bits)      .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .       Acknowledgement Number (low bits)       |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Options                   /    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    The Acknowledgement Number field has special semantics for DCCP-Sync
    and DCCP-SyncAck packets.  First, the packet corresponding to a
    DCCP-Sync's Acknowledgement Number need not have been
    acknowledgeable.  Thus, receivers MUST NOT assume that a packet was
    processed simply because it appears in the Acknowledgement Number
    field of a DCCP-Sync packet.  This differs from all other packet
    types, where the Acknowledgement Number by definition corresponds to
    an acknowledgeable packet.  Second, the Acknowledgement Number on
    any DCCP-SyncAck packet MUST correspond to the Sequence Number on an
    acknowledgeable DCCP-Sync packet.  In the presence of reordering,
    this might not equal GSR.

    Receivers MUST ignore the application data area in DCCP-Sync and
    DCCP-SyncAck packets.  Endpoints may find it useful to pad DCCP-Sync
    packets with "application data" when performing PMTU discovery; see
    Section 14.

5.8.  Options

    Any DCCP packet may contain options, which occupy space at the end
    of the DCCP header.  Each option is a multiple of 8 bits in length.
    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.  Any options present are
    included in the header checksum.

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

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



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    The following options are currently defined:

             Option                            Section
     Type    Length     Meaning               Reference
     ----    ------     -------               ---------
       0        1       Padding                 5.8.1
       1        1       Mandatory               5.8.2
       2        1       Slow Receiver           11.6
       3        1       Reset Congestion State  11.7
     4-31       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            11.8
      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.

5.8.1.  Padding Option

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

    Padding 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).

5.8.2.  Mandatory Option

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




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    Mandatory is a single byte option that marks the immediately
    following option as mandatory.  Say that the immediately following
    option is OP.  Then the Mandatory option has no effect if the
    receiving DCCP endpoint understands and processes OP.  If the
    endpoint does not understand or process OP, however, then it MUST
    reset the connection using Reset Code 6, "Mandatory Failure".  For
    instance, the endpoint would reset the connection if it did not
    understand OP's type; if it understood OP's type, but not OP's data;
    if OP's data was invalid for OP's type; if OP was a feature
    negotiation option, and the endpoint did not understand the enclosed
    feature number; if the endpoint understood OP, but chose not to
    perform the action OP implies; and so forth.

    The connection is in error and should be reset with Reset Code 5,
    "Option Error" if option OP is absent (Mandatory was the last byte
    of the option list), or if option OP equals Mandatory.  However, the
    combination "Mandatory Padding" is valid, and MUST behave like two
    bytes of Padding.

    Section 6.6.9 describes the behavior of Mandatory feature
    negotiation options in more detail.

6.  Feature Negotiation

    Four DCCP options, Change L, Confirm L, Change R, and Confirm R,
    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.







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6.1.  Change Options

    Change L and Change R options initiate feature negotiation.  Which
    option to use depends on where the negotiated feature is located.
    To start a negotiation for feature F/A, DCCP A must send a Change L
    option; to start a negotiation for F/B, it must send a Change R
    option.  Change options are retransmitted until some response is
    received.  Change options contain at least one Value, and thus have
    length at least 4.

               +--------+--------+--------+--------+--------
    Change L:  |00100000| Length |Feature#| Value(s) ...
               +--------+--------+--------+--------+--------
                Type=32

               +--------+--------+--------+--------+--------
    Change R:  |00100010| Length |Feature#| Value(s) ...
               +--------+--------+--------+--------+--------
                Type=34


6.2.  Confirm Options

    Confirm L and Confirm R options complete feature negotiation, and
    are sent in response to Change R and Change L options, respectively.
    Confirm options MUST NOT be generated except in response to Change
    options.  Any packet including a Confirm option MUST carry an
    Acknowledgement Number; thus, Confirm options are not allowed on
    DCCP-Request and DCCP-Data packets.  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.





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6.3.  Reconciliation Rules

    Reconciliation rules determine how the two sets of preferences for a
    given feature are resolved into a unique result.  The reconciliation
    rule depends only on the feature number.  Each reconciliation rule
    must have the property that the result is uniquely determined given
    the contents of Change options sent by the two endpoints.

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

6.3.1.  Server-Priority

    The feature value is a fixed-length byte string (length determined
    by the feature number).  Each Change option contains a 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.

    To reconcile the preference lists, select the first entry in the
    server's list that also occurs in the client's list.  If there is no
    shared entry, the feature's value MUST NOT change, and the Confirm
    option will confirm the feature's previous value (unless the Change
    option was Mandatory; see Section 6.6.9).

    A single feature negotiation may, because of loss or delay, contain
    retransmitted Change options and multiple Confirm options.  Each of
    the retransmitted Change options MUST contain the same payload; see
    Section 6.6.3.  For server-priority features, this means that an
    endpoint sending Change options MUST NOT change its preference list
    during a negotiation.  However, the other endpoint MAY change its
    preference list at will, assuming it hasn't recently sent a Change
    option for the same feature.  Reordering protection (Section 6.6.4)
    ensures that agreement is reached.

6.3.2.  Non-Negotiable

    The feature value is a byte string.  Each option contains exactly
    one feature value.  The feature location signals a new value by
    sending a Change L option.  The feature remote MUST accept any valid
    value, responding with a Confirm R option containing the new value,
    and it MUST send empty Confirm R options in response to invalid
    values (unless the Change L option was Mandatory; see Section
    6.6.9).  Change R and Confirm L options MUST NOT be sent for non-
    negotiable features.  Non-negotiable features use the feature
    negotiation mechanism to achieve reliability.



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6.4.  Feature Numbers

    This document defines the following feature numbers.

                                           Rec'n Initial        Section
    Number   Meaning                       Rule   Value  Req'd Reference
    ------   -------                       -----  -----  ----- ---------
       0     Reserved
       1     Congestion Control ID (CCID)   SP      2      Y     10
       2     Allow Short Seqnos             SP      1      Y     7.6.1
       3     Sequence Window                NN     100     Y     7.5.4
       4     ECN Capable                    SP      1      Y     12.1
       5     Ack Ratio                      NN      2      N     11.3
       6     Send Ack Vector                SP      0      N     11.5
       7     Send NDP Count                 SP      0      N     7.7.2
       8     Minimum Checksum Coverage      SP      0      N     9.2.1
       9     Check Data Checksum            SP      0      N     9.3.1
     10-127  Reserved
    128-255  CCID-specific features                              10.4


    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 15),
                   and it is safe to respond to Change options for the
                   feature with empty Confirm options.  Of course, a
                   CCID might require the feature; a DCCP that
                   implements CCID 2 MUST support Ack Ratio and Send Ack
                   Vector, for example.

6.5.  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.











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                Client                     Server
                ------                     ------
     1. Change R(CCID, 2 3 1)  -->
        ("2 3 1" is client's preference list)
     2.                        <--  Confirm L(CCID, 3, 3 2 1)
                              (3 is the negotiated value;
                              "3 2 1" is server's pref list)
                 * agreement that CCID/Server = 3 *


     1.                   XXX  <--  Change L(CCID, 3 2 1)
     2.                             Retransmission:
                               <--  Change L(CCID, 3 2 1)
     3. Confirm R(CCID, 3, 2 3 1)  -->
                 * agreement that CCID/Server = 3 *


     1.                        <--  Change L(Ack Ratio, 3)
     2. Confirm R(Ack Ratio, 3)  -->
              * agreement that Ack Ratio/Server = 3 *

    This example shows a simultaneous negotiation.

                Client                     Server
                ------                     ------
    1a. Change R(CCID, 2 3 1)  -->
     b.                        <--  Change L(CCID, 3 2 1)
    2a.                        <--  Confirm L(CCID, 3, 3 2 1)
     b. Confirm R(CCID, 3, 2 3 1)  -->
                 * agreement that CCID/Server = 3 *

    Here are the byte encodings of several Change and Confirm options.
    Each option is sent by DCCP A.

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

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

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




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    Empty Confirm L(126) = 33,3,126
        DCCP A doesn't implement feature number 126, or DCCP B's
        proposed value for feature 126/A was invalid.

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

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

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

    Empty Confirm R(126) = 35,3,126
        DCCP A doesn't implement feature number 126, or DCCP B's
        proposed value for feature 126/B was invalid.

6.6.  Option Exchange

    A few basic rules govern feature negotiation option exchange.

    1.  Every non-reordered Change option gets a Confirm option in
        response.

    2.  Change options are retransmitted until a response for the latest
        Change is received.

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

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

6.6.1.  Normal Exchange

    Change options are generated when a DCCP endpoint wants to change
    the value of some feature.  Generally, this will happen at the
    beginning of a connection, although it may happen at any time.  We
    say the endpoint "generates" or "sends" a Change L or Change R
    option, but of course the option must be attached to a packet.  The
    endpoint may attach the option to a packet it would have generated
    anyway (such as a DCCP-Request).  Alternatively, it may create a
    "feature negotiation packet", often a DCCP-Ack or DCCP-Sync, just to
    carry the option.  Feature negotiation packets MUST be rate-limited
    by the relevant congestion control mechanisms.  In addition, an



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    endpoint SHOULD generate at most one feature negotiation packet per
    round-trip time (0.1 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
    preference list, calculates the new value, and sends back a
    Confirm R or Confirm L option, respectively, informing its peer of
    the new value.  Every non-reordered Change option MUST result in a
    corresponding Confirm option, and any packet including a Confirm
    option MUST carry an Acknowledgement Number.  Generated Confirm
    options may be attached to packets that would have been sent anyway
    (such as DCCP-Response or DCCP-SyncAck), or to new feature
    negotiation packets, as described above.

    The Change-sending endpoint MUST wait to receive a corresponding
    Confirm option before changing its stored feature value.  The
    Confirm-sending endpoint changes its stored feature value as soon as
    it sends the Confirm.

    Endpoints MUST NOT send packets that contain more than one feature
    negotiation option referring to the same feature.  Note, however,
    that a packet is allowed to contain one L option and one R option
    with the same feature number F, since the two options actually refer
    to different features (F/A and F/B).

6.6.2.  Processing Received Options

    DCCP endpoints exist in one of three states relative to each
    feature.  STABLE is the normal state, where the endpoint knows the
    feature's value and thinks the other endpoint agrees.  An endpoint
    enters the CHANGING state when it first sends a Change for the
    feature, and returns to STABLE once it receives a corresponding
    Confirm.  The final state, UNSTABLE, indicates that an endpoint in
    CHANGING state changed its preference list, but has not yet
    transmitted a Change option with the new preference list.

    Feature-related state transitions at the feature location are
    implemented as shown in the diagram below.  For feature-related
    state transitions at the feature remote, switch the "L"s and "R"s.
    The diagram ignores sequence number and option validity issues;
    these are handled explicitly in the pseudocode that follows the
    diagram.









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

    Endpoints SHOULD use the following pseudocode, which corresponds to
    the state diagram, to react to each feature negotiation option on
    each valid packet received.  The pseudocode refers to "P.seqno" and
    "P.ackno", which are properties of the packet; "O.type", and
    "O.len", which are properties of the option; "FGSR" and "FGSS",
    which are properties of the connection, and handle reordering as
    described in Section 6.6.4; "F.state", which is the feature's state
    (STABLE, CHANGING, or UNSTABLE); and "F.value", which is the
    feature's value.

    First, check for unknown features (Section 6.6.7);
       If F is unknown:
          If option was Mandatory:   /* Section 6.6.9 */
             Reset connection and return
          Otherwise, if O.type == Change R:
             Send Empty Confirm L on a future packet
          Return

    Second, check for reordering (Section 6.6.4);
       If F.state == UNSTABLE or P.seqno <= FGSR
               or (O.type == Confirm R and P.ackno < FGSS)
          Ignore option and return

    Third, process Change R options;
       If O.type == Change R:
          If option's value is valid:   /* Section 6.6.8 */
             Calculate new value
             Send Confirm L on a future packet



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             Set F.state := STABLE
          Otherwise, if option was Mandatory:
             Reset connection and return
          Otherwise:
             Send Empty Confirm L on a future packet
             /* Remain in existing state.  If that's CHANGING, this
                endpoint will retransmit its Change L option later. */

    Fourth, process Confirm R options (but only in CHANGING state).
       If F.state == CHANGING and O.type == Confirm R:
          If O.len > 3:   /* nonempty */
             If option's value is valid:
                Set F.value := new value
             Otherwise:
                Reset connection and return
          Set F.state := STABLE

6.6.3.  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 transmits another Change option once it realizes
    that it has not heard back from the other endpoint.  The new Change
    option need not contain the same payload as the original; reordering
    protection will ensure that agreement is reached based on the most
    recently transmitted option.  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 is initially set to approximately one or two
    round-trip times (or 0.1-0.2 seconds, if no RTT is available), and
    pinned at roughly 32 RTTs.

    A CHANGING endpoint MUST continue retransmitting Change options
    until it gets some response or the connection terminates.

    Endpoints SHOULD NOT send Change options for a given feature more
    frequently than once per RTT.  Otherwise, the reordering protection
    algorithms described in the next subsection may delay agreement,
    since no received Confirm option would acknowledge the most recently
    transmitted Change.

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





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6.6.4.  Reordering

    Reordering might cause packets containing Change and Confirm options
    to arrive in an unexpected order.  Endpoints MUST ignore feature
    negotiation options that do not arrive in strictly-increasing order
    by Sequence Number.  The rest of this section presents two
    algorithms that fulfill this requirement.

    The first algorithm introduces two sequence number variables that
    each endpoint maintains for the connection.

    FGSR      Feature Greatest Sequence Number Received: The greatest
              sequence number received, considering only valid packets
              that contained one or more feature negotiation options
              (Change and/or Confirm).  This value is initialized to
              ISR - 1.

    FGSS      Feature Greatest Sequence Number Sent: The greatest
              sequence number sent, considering only packets that
              contained one or more non-retransmitted Change options.
              (Retransmitted Change options MUST have exactly the same
              contents as previously transmitted options, so limited
              reordering can safely be tolerated.)  This value is
              initialized to ISS.

    Each endpoint checks two conditions on sequence numbers to decide
    whether to process received feature negotiation options.

    1.  If a packet's Sequence Number is less than or equal to FGSR,
        then its Change options MUST be ignored.

    2.  If a packet's Sequence Number is less than or equal to FGSR, OR
        it has no Acknowledgement Number, OR its Acknowledgement Number
        is less than FGSS, then its Confirm options MUST be ignored.

    Alternatively, an endpoint MAY maintain separate FGSR and FGSS
    values for every feature.  FGSR(F/X) would equal the greatest
    sequence number received, considering only packets that contained
    Change or Confirm options applying to feature F/X; FGSS(F/X) would
    be defined similarly.  This algorithm requires more state, but is
    slightly more forgiving to multiple overlapped feature negotiations.
    Either algorithm MAY be used; the first algorithm, with connection-
    wide FGSR and FGSS variables, is RECOMMENDED.

    One consequence of these rules is that a CHANGING endpoint will
    ignore any Confirm option that does not acknowledge the latest
    Change option sent.  This ensures that agreement, once achieved,
    used the most recent available information about the endpoints'



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    preferences.

6.6.5.  Preference Changes

    Endpoints are allowed to change their preference lists at any time.
    However, an endpoint that changes its preference list while in the
    CHANGING state MUST transition to the UNSTABLE state.  It will
    transition back to CHANGING once it has transmitted a Change option
    with the new preference list.  This ensures that agreement is based
    on active preference lists.  Without the UNSTABLE state,
    simultaneous negotiation -- where the endpoints began independent
    negotiations for the same feature at the same time -- might lead to
    the negotiation terminating with the endpoints thinking the feature
    had different values.

6.6.6.  Simultaneous Negotiation

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

6.6.7.  Unknown Features

    Endpoints may receive Change options referring to feature numbers
    they do not understand -- for instance, when an extended DCCP
    converses with a non-extended DCCP.  Endpoints MUST respond to
    unknown Change options with Empty Confirm options (that is, Confirm
    options containing no data), which inform the CHANGING endpoint that
    the feature was not understood.  However, if the Change option was
    preceded by a Mandatory option, the connection MUST be reset; see
    Section 6.6.9.

    On receiving an empty Confirm option for some feature, the CHANGING
    endpoint MUST transition back to the STABLE state, leaving the
    feature's value unchanged.  Section 15 suggests that the default
    value for any extension feature should correspond to "extension not
    available".

    Some features are required to be understood by all DCCPs (see
    Section 6.4).  The CHANGING endpoint SHOULD reset the connection
    (with Reset Code 5, "Option Error") if it receives an empty Confirm
    option for such a feature.




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    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 ignore
    such options.

6.6.8.  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 Ack Ratio feature takes
       16-bit values, so valid "Confirm R(Ack Ratio)" options have
       option length 5.

    o  Some non-negotiable features have value limitations.  The Ack
       Ratio feature takes two-byte, non-zero integer values, so a
       "Change L(Ack Ratio, 0)" option is never valid.  Note that
       server-priority features do not have value limitations, since
       unknown values are handled as a matter of course.

    o  Any Confirm option that selects the wrong value, based on the two
       preference lists and the relevant reconciliation rule, is
       invalid.

    o  However, unexpected Confirm options -- that refer to unknown
       feature numbers, or that don't appear to be part of a current
       negotiation -- are considered valid, although they are ignored by
       the receiver.

    An endpoint receiving an invalid Change option MUST respond with the
    corresponding empty Confirm option.  An endpoint receiving an
    invalid Confirm option MUST reset the connection, with Reset Code 5,
    "Option Error".

6.6.9.  Mandatory Feature Negotiation

    Change options may be preceded by Mandatory options (Section 5.8.2).
    Mandatory Change options are processed like normal Change options,
    except that the following failure cases will cause the receiver to
    reset the connection with Reset Code 6, "Mandatory Failure", rather
    than send a Confirm option.  The connection MUST be reset if:

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





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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.

6.6.10.  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.  For
    example, features MAY be changed via negotation over a separate
    signaling channel, for example.

7.  Sequence Numbers

    DCCP uses 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 packet-based.  That is, the packets
    generated by each endpoint have Sequence Numbers that increase by
    one, modulo 2^48, for every packet.  Even DCCP-Ack and DCCP-Sync
    packets, and other packets that don't carry user data, increment the
    Sequence Number.  Since DCCP is an unreliable protocol, there are no
    true retransmissions; but effective retransmissions, such as
    retransmissions of DCCP-Request packets, also increment the Sequence
    Number.  This lets DCCP implementations detect network duplication,
    retransmissions, and acknowledgement loss, and is a significant
    departure from TCP practice.

7.1.  Variables

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

    ISS     The Initial Sequence Number Sent by this endpoint.  This
            equals the Sequence Number of the first DCCP-Request or
            DCCP-Response sent.





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    ISR     The Initial Sequence Number Received from the other
            endpoint.  This equals the Sequence Number of the first
            DCCP-Request or DCCP-Response received.

    GSS     The Greatest Sequence Number Sent by this endpoint.  Here,
            and elsewhere, "greatest" is measured in circular sequence
            space.

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

    GAR     The Greatest Acknowledgement Number Received from the other
            endpoint on an acknowledgeable packet that was not a DCCP-
            Sync.

    Some other variables are derived from these primitives.

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

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

7.2.  Initial Sequence Numbers

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

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

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

    These problems are the same as problems faced by TCP, and DCCP
    implementations SHOULD use TCP's strategies to avoid them [RFC 793]
    [RFC 1948].  The rest of this section explains these strategies in
    more detail.

    To address the first problem, an implementation MUST ensure that the
    initial sequence number for a given <source address, source port,
    destination address, destination port> 4-tuple doesn't overlap with



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    recent sequence numbers on previous connections with the same
    4-tuple.  ("Recent" means sent within 2 maximum segment lifetimes,
    or 4 minutes.)  The implementation MUST additionally ensure that the
    lower 24 bits of the initial sequence number don't overlap with the
    lower 24 bits of recent sequence numbers (unless the implementation
    plans to avoid short sequence numbers; see Section 7.6).  An
    implementation that has state for a recent connection with the same
    4-tuple can pick a good initial sequence number explicitly.
    Otherwise, it could tie initial sequence number selection to some
    clock, such as the 4-microsecond clock used by TCP [RFC 793].  Two
    separate clocks may be required, one for the upper 24 bits and one
    for the lower 24 bits.

    To address the second problem, an implementation MUST provide each
    4-tuple with an independent initial sequence number space.  Then
    opening a connection doesn't provide any information about initial
    sequence numbers on other connections to the same host.  RFC 1948
    achieves this by adding a cryptographic hash of the 4-tuple and a
    secret to each 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, the endpoint
    might remember dead connection state for each 4-tuple or stay quiet
    for 2 maximum segment lifetimes around such a change.

7.3.  Quiet Time

    DCCP endpoints, like TCP endpoints, must take care before initiating
    connections when they boot.  In particular, they MUST NOT send
    packets whose sequence numbers are close to the sequence numbers of
    packets lingering in the network from before the boot.  The simplest
    way to enforce this rule is for DCCP endpoints to avoid sending any
    packets until one maximum segment lifetime (2 minutes) after boot.
    Other enforcement mechanisms include remembering recent sequence
    numbers across boots, and reserving the upper 8 or so bits of
    initial sequence numbers for a persistent counter that decrements by
    two each boot.  (The latter mechanism would require disallowing
    packets with short sequence numbers; see Section 7.6.1.)

7.4.  Acknowledgement Numbers

    Cumulative acknowledgements are meaningless in an unreliable
    protocol.  Therefore, DCCP's Acknowledgement Number field has a
    different meaning than TCP's.



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    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 MUST
    equal GSR, the Greatest Sequence Number Received on an
    acknowledgeable packet, for all packet types except DCCP-Sync and
    DCCP-SyncAck.

    "Acknowledgeable" does not refer to data processing.  Even
    acknowledgeable packets may have their application data dropped, due
    to receive buffer overflow or corruption, for instance.  Data
    Dropped options report these data losses when necessary, letting
    congestion control mechanisms distinguish between network losses and
    endpoint losses.  This issue is discussed further in Sections 11.4
    and 11.8.

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

7.5.  Validity and Synchronization

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

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

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

7.5.1.  Sequence-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



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    sequence-invalid if it does not.  Many of the checks refer to the
    sequence and acknowledgement number windows [SWL, SWH] and [AWL,
    AWH], which are defined in Section 7.5.3.

                                              Acknowledgement Number
    Packet Type      Sequence Number Check    Check
    -----------      ---------------------    ----------------------
    DCCP-Request     SWL <= seqno <= SWH (*)  N/A
    DCCP-Response    SWL <= seqno <= SWH (*)  AWL <= ackno <= AWH
    DCCP-Data        SWL <= seqno <= SWH      N/A
    DCCP-Ack         SWL <= seqno <= SWH      AWL <= ackno <= AWH
    DCCP-DataAck     SWL <= seqno <= SWH      AWL <= ackno <= AWH
    DCCP-CloseReq    GSR <  seqno <= SWH      GAR <= ackno <= AWH
    DCCP-Close       GSR <  seqno <= SWH      GAR <= ackno <= AWH
    DCCP-Reset       GSR <  seqno <= SWH      GAR <= ackno <= AWH
    DCCP-Sync        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  Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a
       connection, they cannot have Sequence Numbers less than or equal
       to GSR, or Acknowledgement Numbers less than GAR.

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

    The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow
    continued operation after unusual events, such as endpoint crashes
    and large bursts of loss.  There's no need for leniency when the
    endpoints are actively sending packets to one another.  Therefore,
    DCCP implementations SHOULD use the following, more stringent checks
    for active connections.  A connection is considered active if it has
    received valid packets from the other endpoint within the last
    several round-trip times, or 0.5 seconds, if the RTT is not known.









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

    Finally, an endpoint MAY apply the following more stringent checks
    to DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets, further
    lowering the probability of successful blind attacks using those
    packet types.  Since these checks can cause extra synchronization
    overhead and delay connection closing when packets are lost, they
    should be considered experimental.

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

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

7.5.2.  Handling Sequence-Invalid Packets

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

    On receiving any other sequence-invalid packet, an endpoint (say,
    DCCP A) MUST reply with a DCCP-Sync packet.  This packet MUST
    acknowledge the sequence-invalid 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.

    On receiving a sequence-valid DCCP-Sync, the peer endpoint (DCCP B)
    MUST either respond with a DCCP-Reset packet, or update its GSR
    variable and reply with a DCCP-SyncAck packet.  The DCCP-SyncAck
    packet's Acknowledgement Number will equal the DCCP-Sync's Sequence
    Number, 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.

    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 sequence-invalid packets for up



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    to 2 round-trip times (or 0.2 seconds, if the RTT is unknown); if,
    within that time, the relevant sequence windows change so that the
    packets becomes sequence-valid, the endpoint MAY process the packets
    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 SHOULD
    rate-limit the DCCP-Syncs sent in response to sequence-invalid
    packets.

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

7.5.3.  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 for the two half-
    connections.

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








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

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

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

    SWL and AWL are initially adjusted so that they are not less than
    the initial Sequence Numbers received and sent, respectively:
                 SWL := max(GSR + 1 - floor(W/4), ISR),
                 AWL := max(GSS - W' + 1, ISS).
    These adjustments MUST be applied only at the beginning of the
    connection.  (Long-lived connections may wrap sequence numbers so
    that they appear to be less than ISR or ISS; the adjustments MUST
    NOT be applied in that case.)

7.5.4.  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 about
    five times 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



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

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

    The attacker has to guess both Source and Destination Ports for any
    of these attacks to succeed.  Additionally, the connection would
    have to be inactive for the DCCP-Sync attack to succeed, assuming
    the victim implemented the more stringent checks for active
    connections recommended in Section 7.5.1.

    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 the probability of hitting one
    sequence number window is P = WN/2^L.

    For N = 1000, W = 100, and L = 24, P is about 0.006.  This is the
    probability of a successful DCCP-Data attack using short sequence
    numbers.  (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.)  A
    connection can reduce this probability by requiring long sequence
    numbers; see Section 7.6.1.





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    The DCCP-Sync attack has L = 48, since DCCP-Sync packets use long
    sequence numbers exclusively, and attacks correspondingly have a
    smaller probability of success.  For N = 10,000, W = 2000, and L =
    48, a DCCP-Sync attack will succeed with probability 7*10^-8.
    Attacks involving DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets
    are more difficult still, since 48-bit Sequence and Acknowledgement
    Numbers must both be guessed.

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.

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

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

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

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











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    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.  Short Sequence Numbers

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

    procedure Extend_Sequence_Number(S, REF)
       /* S is a 24-bit sequence number from the packet header.
          REF is the relevant 48-bit reference sequence number:
          GSS if S is an Acknowledgement Number, and GSR if S is a
          Sequence Number. */
       set REF_low := low 24 bits of REF
       set REF_hi := high 24 bits of REF
       if REF_low (<) S           /* CIRCULAR comparison mod 2^24 */
             && S |<| REF_low:    /* NON-CIRCULAR comparison */
          return ((REF_hi + 1) << 24) | S
       otherwise:
          return (REF_hi << 24) | S

    The two different kinds of comparison in the if statement detect
    when the low-order bits of the sequence space have wrapped.  When
    this happens, the high-order bits are incremented.

7.6.1.  Allow Short Sequence Numbers Feature

    Endpoints can require that all packets use long sequence numbers by
    setting the Allow Short Sequence Numbers feature to false.  This can
    reduce the risk that data will be inappropriately injected into the
    connection.  DCCP A sends a "Change R(Allow Short Seqnos, 0)" option
    to ask DCCP B to send only long sequence numbers.





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    Allow Short Sequence Numbers has feature number 2, and is server-
    priority.  It takes one-byte Boolean values.  DCCP B MUST NOT send
    packets with short sequence numbers when Allow Short Seqnos/B is
    zero.  Values of two or more are reserved.  New connections start
    with Allow Short Sequence Numbers 1 for both endpoints.

7.6.2.  When to Avoid Short Sequence Numbers

    Short sequence numbers increase the risks of certain kinds of
    attacks, including blind data injection, and reduce the rate DCCP
    connections can safely achieve.  Very-high-rate DCCP connections,
    and connections with large sequence windows (Section 7.5.4), SHOULD
    NOT use short sequence numbers on their data packets.

    The rate limitation imposed by short sequence numbers is easy to
    calculate.  The sequence-validity mechanism assumes that the network
    does not deliver extremely old data.  In particular, it assumes that
    the network must have dropped any packet by the time the connection
    wraps around and uses its sequence number again.  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.

    For the default MSL of 2 minutes, 1500-byte DCCP packets, and short
    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, long
    sequence numbers allow much higher rates, up to 14 petabits a second
    for 1500-byte packets and the default MSL.

    The probability of data injection attack success P = WN/2^L,
    discussed in Section 7.5.5, may also be relevant when deciding
    whether to use short 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 avoid short sequence numbers entirely.

7.7.  NDP Count and Detecting Application Loss

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



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    included application data.

    +--------+--------+-------- ... --------+
    |00100101| Length |      NDP Count      |
    +--------+--------+-------- ... --------+
     Type=37  Len=3-5       (1-3 bytes)

    If a DCCP endpoint's Send NDP Count feature is one (see below), then
    that endpoint MUST send an NDP Count option on every packet whose
    immediate predecessor was a non-data packet.  Non-data packets
    consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
    DCCP-Reset, DCCP-Sync, and DCCP-SyncAck.  The other packet types,
    namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are
    considered data packets, although not all DCCP-Request and DCCP-
    Response packets will actually carry application data.

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

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

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 an endpoint sent the following sequence of
    non-data packets (Nx) and data packets (Dx).

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

    Those packets would have NDP Counts as follows.

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

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







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7.7.2.  Send NDP Count Feature

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

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

8.  Event Processing

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

8.1.  Connection Establishment

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

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


8.1.1.  Client Request

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





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    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.  A client in REQUEST state has never received an
    initial sequence number from its peer, so the DCCP-Reset's
    Acknowledgement Number should be set to zero.

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

8.1.2.  Service Codes

    Each DCCP-Request contains a 32-bit Service Code, which identifies
    the 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
    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 8, "Bad Service Code".  A middlebox MAY also send
    such a DCCP-Reset in response to packets whose Service Code is
    considered unsuitable.





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    Service Codes are allocated by IANA.  Following the policies
    outlined in [RFC 2434], most Service Codes are allocated First Come
    First Served, subject to the following guidelines.

    o  Service Codes are allocated one at a time, or in small blocks.  A
       short English description of the intended service is required to
       obtain a Service Code assignment, but no specification,
       standards-track or otherwise, is necessary.  IANA maintains an
       association of Service Codes to the corresponding phrases.

    o  Users request specific Service Code values.  We suggest that
       users request Service Codes that can be 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} use a Specification Required allocation policy.  That is,
       these Service Codes are used for international standard or
       standards-track specifications, IETF or otherwise.  (This set
       consists of the ASCII digits, uppercase letters, and characters
       space, '-', '.', and '/'.)

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

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

    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 7, "Connection Refused", when the DCCP-
    Request's Destination Port did not correspond to a DCCP port open
    for listening; 8, "Bad Service Code", when the DCCP-Request's
    Service Code did not correspond to the service code registered with



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    the Destination Port; and 9, "Too Busy", when the server is
    currently too busy to respond to requests.  The server SHOULD limit
    the rate at which it generates these resets.

    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 11.8 and 8.1.4).

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

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



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    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 10, "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 client
    remains in the PARTOPEN state until it can be sure that the server
    has received this DCCP-Ack, or another packet sent later.  Clients
    in the PARTOPEN state that want to send data MUST do so using DCCP-
    DataAck packets, not DCCP-Data packets.  This is because DCCP-Data
    packets lack Acknowledgement Numbers, so the server can't tell from
    a DCCP-Data packet whether the client saw its DCCP-Response.
    Furthermore, if the DCCP-Response included an Init Cookie, that Init
    Cookie MUST be included on every packet sent in PARTOPEN.

    The single DCCP-Ack sent when entering the PARTOPEN state might, of
    course, be dropped by the network.  The client SHOULD ensure that
    some packet gets through eventually.  The preferred mechanism would
    be a 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 (8 minutes), 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.





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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.

    DCCP-Sync and DCCP-SyncAck packets may also occur in the data
    transfer phase.  Some cases causing DCCP-Sync generation are
    discussed in Section 7.5.  One important distinction between DCCP-
    Sync packets and other packet types is that DCCP-Sync elicits an
    immediate acknowledgement.  On receiving a valid DCCP-Sync packet, a
    DCCP endpoint MUST immediately generate and send a DCCP-SyncAck
    response; 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.

8.3.  Termination

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

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









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

    Finally, the server can decide to hold TIMEWAIT state:

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


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

    The termination handshake proceeds as follows.  The receiver of a
    valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet;
    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



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    uniform manner.  This is justified because some responses to
    connection termination 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 0.2 seconds if the RTT is not known, and should back
    off to not less than once every 64 seconds if no relevant response
    is received.

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

8.3.1.  Abnormal Termination

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

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

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

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

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

8.4.  DCCP State Diagram

    The most common state transitions discussed above can be summarized
    in the following state diagram.  The diagram is illustrative; the
    text in Section 8.5 and elsewhere should be considered definitive.



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


8.5.  Pseudocode

    This section presents an algorithm describing the processing steps a
    DCCP endpoint must go through when it receives a packet.  A DCCP
    implementation need not implement the algorithm as it is described



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    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.
    Packet variables P.seqno and P.ackno are 48-bit sequence numbers.
    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 valid acknowledgement number received on a
            non-Sync; 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 P.Data Offset is too small for packet type, or too large for
             packet, drop packet and return
       If P.CsCov is too large for the packet size, drop packet and
             return
       If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
             has short sequence numbers), drop packet and return

    Second, 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

    Third, process LISTEN state;
       If S.state == LISTEN,
          If P.type == Request or P contains a valid Init Cookie,
             Set S := new socket for this port pair
             S.state = RESPOND
             Choose S.ISS (initial seqno) or set from Init Cookie
             Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookie
             Continue (with S.state == RESPOND)
          Otherwise,
             Generate Reset(No Connection) unless P.type == Reset
             Drop packet and return




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    Fourth, process Reset;
       If P.type == Reset,
          If S.GSR < P.seqno <= S.SWH
                and S.GAR <= P.ackno <= S.AWH,
             Tear down connection
             S.state := TIMEWAIT
             Set TIMEWAIT timer
             Drop packet and return
          Otherwise,
             Send Sync packet acknowledging P.seqno
             Drop packet and return

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

    Sixth, process Sync sequence numbers;
       If P.type == Sync or 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

    Seventh, check sequence numbers;
       Let LSWL = S.SWL and LAWL = S.AWL
       If P.type == CloseReq or P.type == Close,
          LSWL := S.GSR + 1, LAWL := S.GAR
       If LSWL <= P.seqno <= S.SWH
             and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),
          Update S.GSR, S.SWL, S.SWH
          If P.type != Sync,
             Update S.GAR
       Otherwise,
          Send Sync packet acknowledging P.seqno
          Drop packet and return

    Eighth, check packet type;
       If (S.is_server and P.type == CloseReq)
            or (S.is_server and P.type == Response)
            or (S.is_client and P.type == Request)
            or (S.state >= OPEN and P.type == Request
                and P.seqno >= S.OSR)
            or (S.state >= OPEN and P.type == Response
                and P.seqno >= S.OSR)



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            or (S.state == RESPOND and P.type == Data),
          Send Sync packet acknowledging P.seqno
          Drop packet and return

    Ninth, process options;
       /* May involve resetting connection, etc. */
       Mark packet as "received" for acknowledgement purposes

    Tenth, process RESPOND state;
       If S.state == RESPOND,
          If P.type == Request,
             Send Response, possibly containing Init Cookie
             If Init Cookie was sent,
                Destroy S and return
                /* Step Three will create another socket when the client
                   responds. */
          Otherwise,
             S.OSR := P.seqno
             S.state := OPEN

    Eleventh, process REQUEST state;
       If S.state == REQUEST,
          S.state := PARTOPEN
          /* PARTOPEN means don't send Data packets, retransmit
             Acks periodically, and include any Init Cookie on
             every packet sent */
          Set PARTOPEN timer

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

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

    Fourteenth, process Close;
       If P.type == Close,
          Generate Reset(Closed)
          Tear down connection
          Drop packet and return




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

    Sixteenth, 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 also covers any application
    data.  DCCP applications can, however, request that the header
    checksum cover only part of the application data, or perhaps no
    application data at all.  Link layers may then reduce their
    protection on unprotected parts of DCCP packets.  For some noisy
    links, and 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 drop
    corrupt application data, but report such drops as corruption, not
    congestion, via Data Dropped options (see Section 11.8).

    Reduced checksum coverage introduces some security considerations;
    see Section 18.1.  See Appendix B.1 for further motivation and
    discussion.  DCCP's implementation of reduced checksum coverage was
    inspired by UDP-Lite [RFC 3828].

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
    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.



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    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).
    The Minimum Checksum Coverage feature, described below, lets an
    endpoint refuse delivery of application data on packets with partial
    checksum coverage; by default, only fully-covered application data



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    is accepted.  Lower layers that support partial error detection MAY
    use the Checksum Coverage field as a hint of where errors do not
    need to be detected.  Lower layers MUST use a strong error detection
    mechanism to detect at least errors that occur in the sensitive part
    of the packet, and 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 [RFC 3828].

9.2.1.  Minimum Checksum Coverage Feature

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

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

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

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

    DCCP B MAY refuse to process application data from packets with
    unacceptable Checksum Coverage.  Such packets SHOULD be reported
    using Data Dropped options (Section 11.8) with Drop Code 0,
    "Protocol Constraints".  New connections start with Minimum Checksum
    Coverage 0 for both endpoints.

9.3.  Data Checksum Option

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

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

    Data Checksum is intended for packets containing application data,



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    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 area 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.

    A DCCP endpoint receiving a packet with a Data Checksum option
    SHOULD compute the received application data's CRC-32c, using the
    same algorithm as the sender, and compare the result with the Data
    Checksum value.  (The endpoint can indicate whether it will is
    willing to check Data Checksums using the Check Data Checksum
    feature, described below.)  If the CRCs differ, the endpoint reacts
    in one of two ways.

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

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

    In either case, the packet will be reported as Received or Received
    ECN Marked by Ack Vector or similar options.

9.3.1.  Check Data Checksum Feature

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






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9.3.2.  Usage Notes

    Internet links must normally apply strong integrity checks to the
    packets they transmit [RFC 3828] [RFC 3819].  This is the default
    case when the DCCP header's Checksum Coverage value equals zero
    (full coverage).  However, the DCCP Checksum Coverage value might
    not be zero.  By setting partial Checksum Coverage, the application
    indicates that it can tolerate corruption in the unprotected part of
    the application data.  Recognizing this, link layers may reduce
    error detection and/or correction strength when transmitting this
    unprotected part.  This, in turn, can significantly increase the
    likelihood of the endpoint receiving corrupt data; Data Checksum
    lets the receiver detect that corruption with very high probability.

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
    (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.

    Currently allocated CCIDs are as follows.

         CCID   Meaning
         ----   -------
           0    Reserved
           1    Unspecified Sender-Based Congestion Control
           2    TCP-like Congestion Control
           3    TFRC Congestion Control
         4-255  Reserved

    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.



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    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
    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.




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    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.

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.



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    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
    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



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       Confirm L options for a feature are sent by the feature location,
       we know that any Change L(128) option was sent by the HC-Sender,
       while any Change L(192) option was sent by the HC-Receiver.
       Similarly, Change R(128) options are sent by the HC-Receiver,
       while Change R(192) options are sent by the HC-Sender.

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

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

         B > A   128                  B-to-A     5
         B > A   192                  A-to-B     4
         B > A   Change L(128, ...)   B-to-A     5
         B > A   Change R(192, ...)   B-to-A     5
         B > A   Confirm L(128, ...)  B-to-A     5
         B > A   Confirm R(192, ...)  B-to-A     5
         B > A   Change R(128, ...)   A-to-B     4
         B > A   Change L(192, ...)   A-to-B     4
         B > A   Confirm R(128, ...)  A-to-B     4
         B > A   Confirm L(192, ...)  A-to-B     4

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




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

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
    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



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    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
    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.



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11.2.  Ack Piggybacking

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

    Piggybacking is particularly common at DCCP A when the B-to-A half-
    connection is quiescent -- that is, when DCCP A is just
    acknowledging DCCP B's acknowledgements.  There are three reasons to
    acknowledge DCCP B's acknowledgements: to allow DCCP B to free up
    information about previously acknowledged data packets from A; to
    shrink the size of future acknowledgements; and to manipulate the
    rate at which future acknowledgements are sent.  Since these are
    secondary concerns, DCCP A can generally afford to wait indefinitely
    for a data packet to piggyback its acknowledgement onto; if DCCP B
    wants to elicit an acknowledgement, it can send a DCCP-Sync.

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

11.3.  Ack Ratio Feature

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

    Ack Ratio applies to CCIDs whose HC-Receivers clock acknowledgements
    off the receipt of data packets.  The value of Ack Ratio/A equals
    the rough ratio of data packets sent by DCCP A to DCCP-Ack packets
    sent by DCCP B.  Higher Ack Ratios correspond to lower DCCP-Ack
    rates; the sender raises Ack Ratio when the reverse path is
    congested and lowers Ack Ratio when it is not.  CCID 2, TCP-like
    Congestion Control, use Ack Ratio for acknowledgement congestion
    control.  Other CCIDs can ignore Ack Ratio if they perform
    congestion control on acknowledgements in some other way.

    Ack Ratio has feature number 5, and is non-negotiable.  It takes
    two-byte integer values.  If Ack Ratio/A is four, then DCCP B will



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    send at least one acknowledgement packet for every four data packets
    sent by DCCP A.  DCCP A sends a "Change L(Ack Ratio)" option to
    notify DCCP B of its ack ratio.  An Ack Ratio value of zero
    indicates that the relevant half-connection does not use an Ack
    Ratio to control its acknowledgement rate.  New connections start
    with Ack Ratio 2 for both endpoints; this Ack Ratio results in
    acknowledgement behavior analogous to TCP's delayed acks.

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

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

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

    o  Receivers SHOULD implement delayed acknowledgement timers like
       TCP's, whereby each packet is acknowledged within at most T
       seconds of its receipt.  The default value of T should be
       0.2 seconds, as is common in TCP implementations.  This may lead
       to sending more acknowledgement packets than Ack Ratio would
       suggest.

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

    o  Receivers MAY ignore Ack Ratio if they perform their own
       congestion control on acknowledgements.  For example, a receiver
       that knows the loss and mark rate for its DCCP-Ack packets might
       maintain a TCP-friendly acknowledgement rate on its own.  Such a
       receiver MUST ensure that it always obtains sufficient



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       acknowledgement loss and mark information or fall back to Ack
       Ratio when sufficient information is not available, as might
       happen during periods when the receiver is quiescent.

11.4.  Ack Vector Options

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

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

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

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

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

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

        0   Packet received (and not ECN Congestion Experienced).

        1   Packet received with ECN Congestion Experienced ("ECN
            marked" for short).

        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



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    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
    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



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    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.  For example:

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

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

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

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

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

    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



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    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.

    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.





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

    The union of groups 2 and 3 is called the Acknowledgement Window.
    Generally, every Ack Vector generated by the HC-Receiver will cover
    the whole Acknowledgement Window: Ack Vector acknowledgements are
    cumulative.  (This simplifies Ack Vector maintenance at the HC-
    Receiver; see Appendix A, below.)  As packets are received, this
    window both grows on the right and shrinks on the left.  It grows
    because there are more packets, and shrinks because the 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 6, and is server-priority.  It
    takes one-byte Boolean values.  DCCP A MUST send Ack Vector options
    on its acknowledgements when Send Ack Vector/A has value one,
    although it MAY send Ack Vector options even when Send Ack Vector/A
    is zero.  Values of two or more are reserved.  New connections start
    with Send Ack Vector 0 for both endpoints.  DCCP B sends a
    "Change R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack
    Vector options as part of its acknowledgement traffic.

11.6.  Slow Receiver Option

    An HC-Receiver sends the Slow Receiver option to its sender to
    indicate that it is having trouble keeping up with the sender's
    data.  The HC-Sender SHOULD NOT increase its sending rate for
    approximately one round-trip time after seeing a packet with a Slow
    Receiver option.  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.

    Slow Receiver is a one-byte option.







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    +--------+
    |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.  Reset Congestion State Option

    An HC-Receiver sends the Reset Congestion State option to its sender
    to force the sender to reset its congestion state -- that is, to
    "slow start", as if the connection were beginning again.  Reset
    Congestion State is a one-byte option.

    +--------+
    |00000011|
    +--------+
     Type=3

    The Reset Congestion State option is reserved for the very few cases
    when an endpoint knows that the congestion properties of a path have
    changed.  Currently, this reduces to mobility: a DCCP endpoint on a
    mobile host MUST send Reset Congestion State to its peer after the
    mobile host changes address or path.  DCCP endpoints MUST NOT use
    Reset Congestion State for other purposes.

11.8.  Data Dropped Option

    The Data Dropped option indicates that the application data on one
    or more received packets did not actually reach the application.
    Data Dropped additionally reports why the data was dropped: perhaps
    the data was corrupt, or perhaps the receiver cannot keep up with



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    the sender's current rate and the data was dropped in some receive
    buffer.  Using Data Dropped, DCCP endpoints can discriminate between
    different kinds of loss; this differs from TCP, in which all loss is
    reported the same way.

    Unless explicitly specified otherwise, DCCP congestion control
    mechanisms MUST react as if each Data Dropped packet was marked as
    ECN Congestion Experienced by the network.  We intend for Data
    Dropped to enable research into richer congestion responses to
    corrupt and other endpoint-dropped packets, but DCCP CCIDs MUST
    react conservatively to Data Dropped until this research is done.
    Section 11.8.2, below, describes congestion responses for all
    current Drop Codes.

    If a received packet's application data is dropped for one of the
    reasons listed below, this SHOULD be reported using a Data Dropped
    option.  Alternatively, the receiver MAY choose to report as
    "received" only those packets whose data were not dropped, subject
    to the constraint that packets not reported as received MUST NOT
    have had their options processed.

    The option's data looks like this:

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

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

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

    The first byte in the first Data Dropped option refers to the packet
    indicated 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



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    any Data Dropped option are treated as if they were in a Normal
    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, but
            the receiving application does not allow such piggybacking;
            or the data was included on a packet with inappropriately
            low Checksum Coverage.

        1   Packet data dropped because the application is no longer
            listening.  See Section 11.8.2.

        2   Packet data dropped in a receive buffer.  See Section
            11.8.2.

        3   Packet data dropped due to corruption.  See Section 9.3.

        4-6 Reserved.

        7   Packet data corrupted, but delivered to the application
            anyway.  See Section 9.3.

    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 a receive buffer (Drop Block, Drop Code
        2, Run Length 0).

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

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

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





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    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
    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).

11.8.1.  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.

11.8.2.  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
    endpoint SHOULD NOT send data until it believes the protocol



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    constraint isn't relevant any longer.

    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
    available at the server for incoming data, and thus reduce the
    potential damage from certain denial-of-service attacks.  A Data
    Dropped option containing Drop Code 1 SHOULD be sent whenever
    received data is ignored due to a non-listening application.  Once
    an endpoint reports Drop Code 1 for a packet, it SHOULD report Drop
    Code 1 for every succeeding data packet on that half-connection;
    once an endpoint receives a Drop State 1 report, it SHOULD expect
    that no more data will ever be delivered to the other endpoint's
    application, so it SHOULD NOT send more data.

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

    The other Drop Codes, namely Drop Code 3 ("corrupt"), Drop Code 7
    ("delivered corrupt"), and reserved Drop Codes 4-6, MUST currently
    be treated like ECN Congestion Experienced marks.

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.





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    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 peer that it cannot
    read ECN bits from received IP headers, so the peer must not set
    ECN-Capable Transport on its packets.

    ECN Capable has feature number 4, and is server-priority.  It takes
    one-byte Boolean values.  DCCP A MUST be able to read ECN bits from
    received frames' IP headers when ECN 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



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    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
    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 11, "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



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    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 11, "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.

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













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

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

    The option data, Elapsed Time, represents an estimated 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 11, "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.

13.3.  Timestamp Echo Option

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















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

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

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

    The first four bytes of option data, Timestamp Echo, carry a
    Timestamp Value taken from a preceding received Timestamp option.
    Usually, this will be the last packet that was received -- the
    packet indicated by the Acknowledgement Number, if any -- but it
    might be a preceding packet.

    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.  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.




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    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
    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



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    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 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
    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.



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15.  Forward Compatibility

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

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

    o  DCCP processors MUST anticipate the possibility of unknown
       feature values, which might occur as part of a negotiation for a
       known feature.  For server-priority features, unknown values are
       handled as a matter of course: since the non-extended DCCP's
       priority list will not contain unknown values, the result of the
       negotiation cannot be an unknown value.  A DCCP SHOULD respond
       with an empty Confirm option if it is assigned an unacceptable
       value for some non-negotiable feature.

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

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

16.  Middlebox Considerations

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

    The Service Code field in DCCP-Request packets provide information
    that may be useful for stateful middleboxes.  With Service Code, a
    middlebox can tell what protocol a connection will use without



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    relying on port numbers.  Middleboxes can disallow attempted
    connections accessing unexpected services by sending a DCCP-Reset
    with Reset Code 8, "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".  We therefore recommend that middleboxes
    not modify packet streams by adding or removing packets.

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






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17.  Relations to Other Specifications

17.1.  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,
    such as the ECN Nonce Echo.  Neither feedback mechanism makes the
    other redundant.

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

    One clear redundancy can be addressed at the application level.  The
    verbose packet-by-packet loss reports sent in RTCP Extended Reports
    Loss RLE Blocks [RFC 3611] 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.




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    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.

17.2.  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
    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.

18.  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



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    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.

18.1.  Security Considerations for Partial Checksums

    The partial checksum facility has a separate security impact,
    particularly in its interaction with authentication and encryption
    mechanisms.  The impact is the same in DCCP as in the UDP-Lite
    protocol, and what follows was adapted from the corresponding text
    in the UDP-Lite specification [RFC 3828].

    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



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    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.

19.  IANA Considerations

    DCCP introduces several sets of numbers whose values should be
    allocated by IANA.  Following the policies outlined in [RFC 2434],
    the following sets of numbers are allocated through an IETF
    Consensus action, with the specified exceptions for CCID-specific
    ranges and experimental and testing use [RFC 3692].

    o  Packet types 10-13 and 15 (Section 5.1).  Packet type 14 is
       reserved for experimental and testing use.

    o  Reset Codes 12-119 and 127 (Section 5.6).  Reset Codes 120-126
       are reserved for experimental and testing use, and Reset Codes
       128-255 are allocated in CCID-specific registries.

    o  Option types 4-30, 45-119, and 127 (Section 5.8).  Option types
       31 and 120-126 are reserved for experimental and testing use, and
       option types 128-255 are allocated in CCID-specific registries.

    o  Feature numbers 10-119 and 127 (Section 6).  Feature numbers
       120-126 are reserved for experimental and testing use, and
       feature numbers 128-255 are allocated in CCID-specific
       registries.

    o  Congestion Control Identifiers (CCIDs) 4-247 and 255 (Section
       10).  CCIDs 248-254 are reserved for experimental and testing
       use.

    o  Ack Vector State 2 (Section 11.4).

    o  Data Dropped Drop Codes 4-6 (Section 11.8).

    DCCP also introduces an IANA registry for 32-bit Service Codes.
    Most Codes are allocated First Come First Served; the exceptions,
    and more specific rules for registration, are presented in Section
    8.1.2.

    DCCP also 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.



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20.  Thanks

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

    Thanks to Junwen Lai and Arun Venkataramani, who, as interns at
    ICIR, built a prototype DCCP implementation.  In particular, Junwen
    Lai recommended that the old feature negotiation mechanism be
    scrapped and 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, and Gorry Fairhurst provided
    extensive feedback on various checksum issues.

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.





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

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

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

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

    We draw acknowledgement buffers like this:

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

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

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

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

    Here, buf_nonce equals E and buf_ackno equals A.

    We will use this buffer as a running example.

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

    In concrete terms, its meaning is as follows:





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

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

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

        Packet 1 was ECN marked.

        Packet 0 was received.

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

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

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

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

    o  "ack_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



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    Number), and possibly buf_nonce (if the packet arrived unmarked with
    ECN Nonce 1), in addition to the buffer itself.  For example, if HC-
    Sender packet 11 arrived ECN marked, the Example Buffer above would
    enter this new state (changes are marked with stars):

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

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

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

    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



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    run length 0, the HC-Receiver can simply change the byte's state.
    For example, if HC-Sender packet 8 was received with ECN Nonce 0,
    the Example Buffer would enter this state:

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

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



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    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
    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
    flipped to 1).  The HC-Receiver can also throw away stored
    information about HC-Receiver Ack 59 and any earlier
    acknowledgements.




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

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

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



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    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.

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



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    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
    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.




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

    [RFC 2434] T. Narten and H. Alvestrand.  Guidelines for Writing an
        IANA Considerations Section in RFCs.  RFC 2434.

    [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.

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

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.

    [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.



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    [PMTUD] Matt Mathis, John Heffner, and Kevin Lahey.  Path MTU
        Discovery.  draft-ietf-pmtud-method-01.txt, work in progress,
        February 2004.

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

    [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 3360] S. Floyd.  Inappropriate TCP Resets Considered Harmful.
        RFC 3360.

    [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.

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

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

    [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.





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Authors' Addresses

    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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    attempt made to obtain a general license or permission for the use
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