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

Internet Engineering Task Force                             Eddie Kohler
INTERNET-DRAFT                                                      UCLA
draft-ietf-dccp-spec-11.txt                                 Mark Handley
Expires: 10 September 2005                                           UCL
                                                             Sally Floyd
                                                                    ICIR
                                                           10 March 2005


              Datagram Congestion Control Protocol (DCCP)


Status of this Memo

    This document is an Internet-Draft and is subject to all provisions
    of section 3 of RFC 3667.  By submitting this Internet-Draft, each
    author represents that any applicable patent or other IPR claims of
    which he or she is aware have been or will be disclosed, and any of
    which he or she become aware will be disclosed, in accordance with
    RFC 3668.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-
    Drafts.

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

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

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

    This Internet-Draft will expire on 10 September 2005.

Copyright Notice

    Copyright (C) The Internet Society (2005). All Rights Reserved.






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Abstract

    The Datagram Congestion Control Protocol (DCCP) is a transport
    protocol that provides bidirectional unicast connections of
    congestion-controlled unreliable datagrams.  DCCP is suitable for
    applications that transfer fairly large amounts of data, but can
    benefit from control over the tradeoff between timeliness and
    reliability.











































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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes since draft-ietf-dccp-spec-08.txt:

    * Added minimum Sequence Window.

    * Init Cookie implementation sketch.

    * Include reasoning for ignoring options on DCCP-Data.

    * More Aggression Penalty explanation.

    * More explanation on Ack Vectors that report information on packets
    that haven't been sent.

    Changes since draft-ietf-dccp-spec-07.txt:

    * Many changes, not listed here, for WGLC.

    * The more stringent Sequence Number checks on DCCP-Sync and DCCP-
    SyncAck packets become SHOULD, not MAY.

    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.




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    * Add pseudocode for event processing.

    * Remove # NDP; replace with Ack Count.

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

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

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

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

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

    * Mobility ID becomes 128 bits long.

    * Add probabilities to Mobility ID discussion.

    * Add SyncAck.































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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  10
    2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . .  11
    3. Conventions and Terminology . . . . . . . . . . . . . . . . .  12
       3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . .  12
       3.2. Parts of a Connection. . . . . . . . . . . . . . . . . .  13
       3.3. Features . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . .  14
       3.5. Security Limitation. . . . . . . . . . . . . . . . . . .  14
       3.6. Robustness Principle . . . . . . . . . . . . . . . . . .  14
    4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  15
       4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . .  15
       4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . .  16
       4.3. States . . . . . . . . . . . . . . . . . . . . . . . . .  17
       4.4. Congestion Control . . . . . . . . . . . . . . . . . . .  19
       4.5. Features . . . . . . . . . . . . . . . . . . . . . . . .  20
       4.6. Differences From TCP . . . . . . . . . . . . . . . . . .  21
       4.7. Example Connection . . . . . . . . . . . . . . . . . . .  22
    5. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . .  23
       5.1. Generic Header . . . . . . . . . . . . . . . . . . . . .  24
       5.2. DCCP-Request Packets . . . . . . . . . . . . . . . . . .  27
       5.3. DCCP-Response Packets. . . . . . . . . . . . . . . . . .  28
       5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets. . . . . .  29
       5.5. DCCP-CloseReq and DCCP-Close Packets . . . . . . . . . .  30
       5.6. DCCP-Reset Packets . . . . . . . . . . . . . . . . . . .  31
       5.7. DCCP-Sync and DCCP-SyncAck Packets . . . . . . . . . . .  34
       5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . .  35
          5.8.1. Padding Option. . . . . . . . . . . . . . . . . . .  36
          5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . .  37
    6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . .  38
       6.1. Change Options . . . . . . . . . . . . . . . . . . . . .  38
       6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . .  39
       6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . .  39
          6.3.1. Server-Priority . . . . . . . . . . . . . . . . . .  39
          6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . .  40
       6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . .  40
       6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . .  41
       6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . .  42
          6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . .  43
          6.6.2. Processing Received Options . . . . . . . . . . . .  43
          6.6.3. Loss and Retransmission . . . . . . . . . . . . . .  45
          6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . .  46
          6.6.5. Preference Changes. . . . . . . . . . . . . . . . .  47
          6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . .  47
          6.6.7. Unknown Features. . . . . . . . . . . . . . . . . .  47
          6.6.8. Invalid Options . . . . . . . . . . . . . . . . . .  48
          6.6.9. Mandatory Feature Negotiation . . . . . . . . . . .  49



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    7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . .  49
       7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . .  50
       7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . .  50
       7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . .  51
       7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . .  52
       7.5. Validity and Synchronization . . . . . . . . . . . . . .  52
          7.5.1. Sequence and Acknowledgement Number
          Windows. . . . . . . . . . . . . . . . . . . . . . . . . .  53
          7.5.2. Sequence Window Feature . . . . . . . . . . . . . .  54
          7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . .  54
          7.5.4. Handling Sequence-Invalid Packets . . . . . . . . .  56
          7.5.5. Sequence Number Attacks . . . . . . . . . . . . . .  57
          7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . .  58
       7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . .  59
          7.6.1. Allow Short Sequence Numbers Feature. . . . . . . .  60
          7.6.2. When to Avoid Short Sequence Numbers. . . . . . . .  60
       7.7. NDP Count and Detecting Application Loss . . . . . . . .  61
          7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . .  62
          7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . .  62
    8. Event Processing. . . . . . . . . . . . . . . . . . . . . . .  62
       8.1. Connection Establishment . . . . . . . . . . . . . . . .  63
          8.1.1. Client Request. . . . . . . . . . . . . . . . . . .  63
          8.1.2. Service Codes . . . . . . . . . . . . . . . . . . .  64
          8.1.3. Server Response . . . . . . . . . . . . . . . . . .  65
          8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . .  66
          8.1.5. Handshake Completion. . . . . . . . . . . . . . . .  67
       8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . .  67
       8.3. Termination. . . . . . . . . . . . . . . . . . . . . . .  68
          8.3.1. Abnormal Termination. . . . . . . . . . . . . . . .  70
       8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . .  70
       8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . .  71
    9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . .  75
       9.1. Header Checksum Field. . . . . . . . . . . . . . . . . .  76
       9.2. Header Checksum Coverage Field . . . . . . . . . . . . .  77
          9.2.1. Minimum Checksum Coverage Feature . . . . . . . . .  78
       9.3. Data Checksum Option . . . . . . . . . . . . . . . . . .  78
          9.3.1. Check Data Checksum Feature . . . . . . . . . . . .  79
          9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . .  79
    10. Congestion Control . . . . . . . . . . . . . . . . . . . . .  80
       10.1. TCP-like Congestion Control . . . . . . . . . . . . . .  81
       10.2. TFRC Congestion Control . . . . . . . . . . . . . . . .  81
       10.3. CCID-Specific Options, Features, and Reset
       Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  81
       10.4. CCID Profile Requirements . . . . . . . . . . . . . . .  84
       10.5. Congestion State. . . . . . . . . . . . . . . . . . . .  84
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  85
       11.1. Acks of Acks and Unidirectional Connections . . . . . .  86
       11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . .  87



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       11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . .  87
       11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . .  89
          11.4.1. Ack Vector Consistency . . . . . . . . . . . . . .  91
          11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . .  93
       11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . .  94
       11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . .  94
       11.7. Data Dropped Option . . . . . . . . . . . . . . . . . .  95
          11.7.1. Data Dropped and Normal Congestion
          Response . . . . . . . . . . . . . . . . . . . . . . . . .  98
          11.7.2. Particular Drop Codes. . . . . . . . . . . . . . .  98
    12. Explicit Congestion Notification . . . . . . . . . . . . . .  99
       12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . . 100
       12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 100
       12.3. Aggression Penalties. . . . . . . . . . . . . . . . . . 101
    13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 102
       13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 102
       13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 103
       13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 104
    14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 105
       14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 105
       14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 107
    15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 108
    16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 108
    17. Relations to Other Specifications. . . . . . . . . . . . . . 110
       17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 110
       17.2. Congestion Manager and Multiplexing . . . . . . . . . . 111
    18. Security Considerations. . . . . . . . . . . . . . . . . . . 111
       18.1. Security Considerations for Partial
       Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 112
    19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 113
       19.1. Packet Types. . . . . . . . . . . . . . . . . . . . . . 113
       19.2. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 113
       19.3. Option Types. . . . . . . . . . . . . . . . . . . . . . 114
       19.4. Feature Numbers . . . . . . . . . . . . . . . . . . . . 114
       19.5. Congestion Control Identifiers. . . . . . . . . . . . . 114
       19.6. Ack Vector States . . . . . . . . . . . . . . . . . . . 115
       19.7. Drop Codes. . . . . . . . . . . . . . . . . . . . . . . 115
       19.8. Service Codes . . . . . . . . . . . . . . . . . . . . . 115
    20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
    A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 116
       A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 118
          A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 118
          A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 119
       A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 120
       A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 121
       A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 122
    B. Appendix: Partial Checksumming Design Motivation. . . . . . . 123
    Normative References . . . . . . . . . . . . . . . . . . . . . . 124



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    Informative References . . . . . . . . . . . . . . . . . . . . . 125
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 127
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 127
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 128















































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                               List of Tables

    Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . .  26
    Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . .  33
    Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . .  35
    Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . .  40
    Table 5: DCCP Congestion Control Identifiers . . . . . . . . . .  80
    Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . .  90
    Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . .  96










































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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) [RFC 3168] and the ECN Nonce [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 Transmission Unit (PMTU) discovery [RFC 1191].

    o  A choice of modular congestion control mechanisms.  Two
       mechanisms are currently specified, TCP-like Congestion Control
       [CCID 2 PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion
       Control [CCID 3 PROFILE], but DCCP is easily extensible to
       further forms of unicast congestion control.

    DCCP is intended for applications such as streaming media that can
    benefit from control over the tradeoffs between delay and reliable
    in-order delivery.  TCP is not well-suited for these applications,
    since reliable in-order delivery and congestion control can cause
    arbitrarily long delays.  UDP avoids long delays, but UDP
    applications that implement congestion control must do so on their
    own.  DCCP provides built-in congestion control, including ECN
    support, for unreliable datagram flows, avoiding the arbitrary
    delays associated with TCP.  It also implements reliable connection



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    setup, teardown, and feature negotiation.

2.  Design Rationale

    One DCCP design goal was to give most streaming UDP applications
    little reason not to switch to DCCP, once it is deployed.  To
    facilitate this, DCCP was designed to have as little overhead as
    possible, both in terms of the packet header size and in terms of
    the state and CPU overhead required at end hosts.  Only the minimal
    necessary functionality was included in DCCP, leaving other
    functionality, such as forward error correction (FEC), semi-
    reliability, and multiple streams, to be layered on top of DCCP as
    desired.

    Different forms of conformant congestion control are appropriate for
    different applications.  For example, on-line games might want to
    make quick use of any available bandwidth, while streaming media
    might trade off this responsiveness for a steadier, less bursty
    rate.  (Sudden rate changes can cause unacceptable UI glitches, such
    as audible pauses or clicks in the playout stream.)  DCCP thus
    allows applications to choose from a set of congestion control
    mechanisms.  One alternative, TCP-like Congestion Control, halves
    the congestion window in response to a packet drop or mark, as in
    TCP.  Applications using this congestion control mechanism will
    respond quickly to changes in available bandwidth, but must tolerate
    the abrupt changes in congestion window typical of TCP.  A second
    alternative, TCP-Friendly Rate Control (TFRC) [RFC 3448], a form of
    equation-based congestion control, minimizes abrupt changes in the
    sending rate while maintaining longer-term fairness with TCP.  Other
    alternatives can be added as future congestion control mechanisms
    are standardized.

    DCCP also lets unreliable traffic safely use ECN.  A UDP kernel 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



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    make use of CM where desired by the application, but we do not see
    any benefit in making the deployment of DCCP contingent on the
    deployment of CM itself.

    We intend for DCCP's protocol mechanisms, which are described in
    this document, to suit any application desiring unicast congestion-
    controlled streams of unreliable datagrams.  The congestion control
    mechanisms currently approved for use with DCCP, which are described
    in separate Congestion Control ID Profiles [CCID 2 PROFILE, CCID 3
    PROFILE], may, however, cause problems for some applications,
    including high-bandwidth interactive video.  These applications
    should be able to use DCCP once suitable Congestion Control ID
    Profiles are standardized.

3.  Conventions and Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in 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
    SHOULD 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.  Implementation
    strategies for DCCP sequence numbers will resemble those for other
    circular arithmetic spaces, including TCP's sequence numbers [RFC
    793] and DNS's serial numbers [RFC 1982].  Note that the common
    technique for implementing circular comparison using two's-
    complement arithmetic, whereby A < B using circular arithmetic if
    and only if (A - B) < 0 using conventional two's-complement
    arithmetic, may be used for DCCP sequence numbers, provided they are
    stored in the most significant 48 bits of 64-bit integers.

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



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    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 hosts, which we often name
    DCCP A and DCCP B.  Each connection is actively initiated by one of
    the hosts, which we call the client; the other, initially passive
    host is called the server.  The term "DCCP endpoint" is used to
    refer to either of the two hosts explicitly named by the connection
    (the client and the server).  The term "DCCP processor" refers more
    generally to any host that might need to process a DCCP header; this
    includes the endpoints and any middleboxes on the path, such as
    firewalls and network address translators.

    DCCP connections are bidirectional: data may pass from either
    endpoint to the other.  This means that data and acknowledgements
    may 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:

     +--------+  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



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    in use on the two half-connections.  The endpoints achieve agreement
    through the exchange of feature negotiation options in DCCP headers.

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

3.4.  Round-Trip Times

    DCCP round-trip time measurements are performed by congestion
    control mechanisms; different mechanisms may measure round-trip time
    in different ways, or not measure it at all.  However, the main DCCP
    protocol does use round-trip times occasionally, such as in the
    initial values for certain timers.  Each DCCP implementation thus
    defines a default round-trip time for use when no estimate is
    available; this parameter should default to not less than
    0.2 seconds, a reasonably conservative round-trip time for Internet
    TCP connections.  Protocol behavior specified in terms of "round-
    trip time" values actually refers to "a current round-trip time
    estimate taken by some CCID, or, if no estimate is available, the
    default round-trip time parameter".

    The maximum segment lifetime, or MSL, is the maximum length of time
    a packet can survive in the network.  The DCCP MSL should equal that
    of TCP, which is normally two minutes.

3.5.  Security Limitation

    DCCP provides no protection against attackers who can snoop on a
    connection in progress, or who can guess valid sequence numbers in
    other ways.  Applications desiring stronger security should use
    IPsec [RFC 2401]; depending on the level of security required,
    application-level cryptography may also suffice.  These issues are
    discussed further in Sections 18 and 7.5.5.

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].







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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.

       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.



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    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 by the client or the server to close the connection;
        elicits a DCCP-Reset in response.

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

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

4.2.  Sequence Numbers

    Each DCCP packet carries a sequence number, so that losses can be
    detected and reported.  Unlike TCP sequence numbers, which are byte-
    based, DCCP sequence numbers increment by one per packet.  For
    example:










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

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

    Since DCCP provides unreliable semantics, there are no
    retransmissions, and 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.  They are listed in
    increasing order, so that "state >= CLOSEREQ" means the same as
    "state = CLOSEREQ or state = CLOSING or state = TIMEWAIT".  Section
    8 describes the states in more detail.

    CLOSED
        Represents nonexistent connections.

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

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

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

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




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

    CLOSEREQ
        A server socket enters this state, from SERVER-OPEN, to 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 server or client socket remains in this state for 2MSL (4
        minutes) after the connection has been torn down, to prevent
        mistakes due to the delivery of old packets.  Only one of the
        endpoints need enter TIMEWAIT state (the other can enter CLOSED
        state immediately), and a server can request its client to hold
        TIMEWAIT state using the DCCP-CloseReq packet type.

4.4.  Congestion Control

    DCCP connections are congestion controlled, but unlike in TCP, DCCP
    applications have a choice of congestion control mechanism.  In
    fact, the two half-connections can be governed by different
    mechanisms.  Mechanisms are denoted by one-byte congestion control
    identifiers, or CCIDs.  The endpoints negotiate their CCIDs during
    connection initiation.  Each CCID describes how the HC-Sender limits
    data packet rates, how the HC-Receiver sends congestion feedback via
    acknowledgements, and so forth.  CCIDs 2 and 3 are currently
    defined; CCIDs 0, 1, and 4-255 are reserved.  Other CCIDs may be
    defined in the future.

    CCID 2 provides TCP-like Congestion Control, which is similar to
    that of TCP.  The sender maintains a congestion window and sends
    packets until that window is full.  Packets are acknowledged by the
    receiver.  Dropped packets and ECN [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 2018].

    CCID 3 provides TFRC Congestion Control, an equation-based form of
    congestion control intended to respond to congestion more smoothly



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    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.

    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 use Change and Confirm options to negotiate and agree
    on feature values.  Feature negotiation will almost always happen on
    the connection initiation handshake, but it can begin at any time.

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

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

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

    Both exchanges negotiate the CCID/Server feature's value, which is
    the CCID in use on the server-to-client half-connection.  In the
    second exchange, the client requests that the server use either
    CCID 3 or CCID 4, with 3 preferred; the server chooses 4 and
    supplies its preference list, "4 2".

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







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


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

4.6.  Differences From TCP

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

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

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

    o  Denial-of-service (DoS) protection.  Several mechanisms help
       limit the amount of state 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 11.7) lets an endpoint declare that a packet was dropped
       because of corruption, because of receive buffer overflow, and so
       on.  This facilitates research into more appropriate rate-control
       responses for these non-network-congestion losses (although
       currently such losses will cause a congestion response).

    o  Acknowledgeability.  In TCP, a packet may be acknowledged only
       once the data is reliably queued for application delivery.  This
       does not make sense in DCCP, where an application might, for
       example, request a drop-from-front receive buffer.  A DCCP packet
       may be acknowledged as soon as its header has been successfully



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       processed.  Concretely, a packet becomes acknowledgeable at
       Step 8 of Section 8.5's packet processing pseudocode.
       Acknowledgeability does not guarantee data delivery, however: the
       Data Dropped option may later report that the packet's
       application data was discarded.

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

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

    o  No half-closed states.  DCCP has no states corresponding to TCP's
       FINWAIT and CLOSEWAIT, where one half-connection is explicitly
       closed while the other is still active.  The Data Dropped
       option's Drop Code 1, Application Not Listening (Section 11.7),
       can achieve a similar effect, however.

4.7.  Example Connection

    The progress of a typical DCCP connection is as follows.  (This
    description is informative, not normative.)

           Client                                  Server
           ------                                  ------
       0.  [CLOSED]                              [LISTEN]
       1.  DCCP-Request -->
       2.                               <-- DCCP-Response
       3.  DCCP-Ack -->
       4.  DCCP-Data, DCCP-Ack, DCCP-DataAck -->
                    <-- DCCP-Data, DCCP-Ack, DCCP-DataAck
       5.                               <-- DCCP-CloseReq
       6.  DCCP-Close -->
       7.                                  <-- DCCP-Reset
       8.  [TIMEWAIT]


    1.  The client sends the server a DCCP-Request packet specifying the
        client and server ports, the service being requested, and any
        features being negotiated, including the CCID that the client
        would like the server to use.  The client may optionally
        piggyback an application request on the DCCP-Request packet,
        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



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        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
        packets containing data with piggybacked acknowledgements.  If
        the client has no data to send, then the server will send DCCP-
        Data and DCCP-DataAck packets, while the client will send DCCP-
        Acks exclusively.  (However, the client may not send DCCP-Data
        packets before receiving at least one non-DCCP-Response packet
        from the server.)

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

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

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

    8.  The client receives the DCCP-Reset packet and holds state for
        two maximum segment lifetimes, or 2MSL, to allow any remaining
        packets to clear the network.

    An alternative connection closedown sequence is initiated by the
    client:

    5b. The client sends a DCCP-Close packet closing the connection.

    6b. The server sends a DCCP-Reset packet with Reset Code 1,
        "Closed", and clears its connection state.

    7b. The client receives the DCCP-Reset packet and holds state for
        2MSL to allow any remaining packets to clear the network.

5.  Packet Formats

    The DCCP header can be from 12 to 1020 bytes long.  The initial 12
    bytes of the header have the same semantics for all currently-
    defined packet types.  Following this comes any additional fixed-



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    length fields required by the packet type, and then a variable-
    length list of options.  The application data area follows the
    header.  In some packet types, this area contains data for the
    application; in other packet types, its contents are ignored.

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


5.1.  Generic Header

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

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

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












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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          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.  When
        initiating a connection, the client SHOULD choose its Source
        Port randomly to reduce the likelihood of attack.

        DCCP APIs should treat port numbers similarly to TCP and UDP
        port numbers.  For example, machines that distinguish between
        "privileged" and "unprivileged" ports for TCP and UDP should do
        the same for DCCP.

    Data Offset: 8 bits
        The offset from the start of the packet's DCCP header to the
        start of its application data area, in 32-bit words.  The
        receiver MUST ignore packets whose Data Offset is smaller than
        the minimum-sized header for the given Type, or larger than the
        DCCP packet itself.

    CCVal: 4 bits
        Used by the HC-Sender CCID.  For example, the A-to-B CCID's
        sender, which is active at DCCP A, MAY send 4 bits of
        information per packet to its receiver by encoding that
        information in CCVal.  The sender MUST set CCVal to zero unless
        its HC-Sender CCID specifies otherwise, and the receiver MUST
        ignore the CCVal field unless its HC-Receiver CCID specifies
        otherwise.

    Checksum Coverage (CsCov): 4 bits
        Checksum Coverage determines the parts of the packet that are
        covered by the Checksum field.  This always includes the DCCP
        header and options, but some or all of the application data may



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        be excluded.  This can improve performance on noisy links for
        applications that can tolerate corruption.  See Section 9.

    Checksum: 16 bits
        The Internet checksum of the packet's DCCP header (including
        options), a network-layer pseudoheader, and, depending on
        Checksum Coverage, all, some, or none of the application data.
        See Section 9.

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

    Type: 4 bits
        The Type field specifies the type of the packet.  The following
        values are defined:

                          Type   Meaning
                          ----   -------
                            0    DCCP-Request
                            1    DCCP-Response
                            2    DCCP-Data
                            3    DCCP-Ack
                            4    DCCP-DataAck
                            5    DCCP-CloseReq
                            6    DCCP-Close
                            7    DCCP-Reset
                            8    DCCP-Sync
                            9    DCCP-SyncAck
                          10-15  Reserved

                      Table 1: DCCP Packet Types

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

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




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    Sequence Number: 48 or 24 bits
        Identifies the packet uniquely in the sequence of all packets
        the source sent on this connection.  Sequence Number increases
        by one with every packet sent, including packets such as DCCP-
        Ack that carry no application data.  See Section 7.

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

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

    When X=0, only the low 24 bits of the Acknowledgement Number are
    transmitted, giving the Acknowledgement Number Subheader this
    format:

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


    Reserved: 16 or 8 bits
        Senders MUST set this field to all zeroes on generated packets,
        and receivers MUST ignore its value.

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

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

5.2.  DCCP-Request Packets

    A client initiates a DCCP connection by sending a DCCP-Request
    packet.  These packets MAY contain application data, and MUST use
    48-bit sequence numbers (X=1).




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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=0 (DCCP-Request)                  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Service Code                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                      Options and Padding                      /
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     /                       Application Data                        /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Service Code: 32 bits
        Describes the application-level service to which the client
        application wants to connect.  Service Codes are intended to
        provide information about which application protocol a
        connection intends to use, and thus aiding middleboxes and
        reducing reliance on globally well-known ports.  See Section
        8.1.2.

5.3.  DCCP-Response Packets

    The server responds to valid DCCP-Request packets with DCCP-Response
    packets.  This is the second phase of the three-way handshake.
    DCCP-Response packets MAY contain application data, and MUST use
    48-bit sequence numbers (X=1).

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


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



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        Number on a received DCCP-Request.

    Service Code: 32 bits
        MUST equal the Service Code on the corresponding DCCP-Request.

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

    The central data transfer portion of every DCCP connection uses
    DCCP-Data, DCCP-Ack, and DCCP-DataAck packets.  These packets MAY
    use 24-bit sequence numbers, depending on the value of the Allow
    Short Sequence Numbers feature (Section 7.6.1).  DCCP-Data packets
    carry application data without acknowledgements.

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

    DCCP-Ack packets dispense with the data, but contain an
    Acknowledgement Number.  They are used for pure acknowledgements.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /              Generic DCCP Header (16 or 12 bytes)             /
     /                    with Type=3 (DCCP-Ack)                     /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /        Acknowledgement Number Subheader (8 or 4 bytes)        /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                      Options and Padding                      /
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     /                Application Data Area (Ignored)                /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    DCCP-DataAck packets carry both application data and an
    Acknowledgement Number: acknowledgement information is piggybacked
    on a data packet.








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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /              Generic DCCP Header (16 or 12 bytes)             /
     /                  with Type=4 (DCCP-DataAck)                   /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /        Acknowledgement Number Subheader (8 or 4 bytes)        /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                      Options and Padding                      /
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     /                       Application Data                        /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    A DCCP-Data or DCCP-DataAck packet may have a zero-length
    application data area, which indicates that the application sent a
    zero-length datagram.  This differs from DCCP-Request and DCCP-
    Response packets, where an empty application data area indicates the
    absence of application data (not the presence of zero-length
    application data).  The API SHOULD report any received zero-length
    datagrams to the receiving application.

    A DCCP-Ack packet MAY have a non-zero-length application data area,
    which essentially pads the DCCP-Ack to a desired length.  Receivers
    MUST ignore the content of the application data area in DCCP-Ack
    packets.

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

5.5.  DCCP-CloseReq and DCCP-Close Packets

    DCCP-CloseReq and DCCP-Close packets begin the handshake that
    normally terminates a connection.  Either client or server may send
    a DCCP-Close packet, which will elicit a DCCP-Reset packet.  Only
    the server can send a DCCP-CloseReq packet, which indicates that the
    server wants to close the connection, but does not want to hold its
    TIMEWAIT state.  Both packet types MUST use 48-bit sequence numbers
    (X=1).












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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=5 (DCCP-CloseReq) or 6 (DCCP-Close)         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /          Acknowledgement Number Subheader (8 bytes)           /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                      Options and Padding                      /
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     /                Application Data Area (Ignored)                /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY
    have non-zero-length application data areas, whose contents
    receivers MUST ignore.

5.6.  DCCP-Reset Packets

    DCCP-Reset packets unconditionally shut down a connection.
    Connections normally terminate with a DCCP-Reset, but resets may be
    sent for other reasons, including bad port numbers, bad option
    behavior, incorrect ECN Nonce Echoes, and so forth.  DCCP-Resets
    MUST use 48-bit sequence numbers (X=1).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /            Generic DCCP Header with X=1 (16 bytes)            /
     /                   with Type=7 (DCCP-Reset)                    /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /          Acknowledgement Number Subheader (8 bytes)           /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Reset Code   |    Data 1     |    Data 2     |    Data 3     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                      Options and Padding                      /
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     /              Application Data Area (Error Text)               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

    Data 1, Data 2, and Data 3: 8 bits each
        The Data fields provide additional information about why the
        sender reset the DCCP connection.  The meanings of these fields
        depend on the value of Reset Code.



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    Application Data Area: Error Text
        If present, Error Text is a human-readable text string encoded
        in Unicode UTF-8, and preferably in English, that describes the
        error in more detail.  For example, a DCCP-Reset with Reset Code
        11, "Aggression Penalty", might contain Error Text such as
        "Aggression Penalty: Received 3 bad ECN Nonce Echoes, assuming
        misbehavior".

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

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

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

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

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

    4, "Packet Error"
        A valid packet arrived with unexpected type.  For example, a
        DCCP-Data packet with valid header checksum and sequence numbers
        arrived at a connection in the REQUEST state.  See Section
        8.3.1.  The Data 1 field equals the offending packet type as an
        eight-bit number; thus, an offending packet with Type 2 will
        result in a Data 1 value of 2.

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

    6, "Mandatory Error"
        The sending endpoint could not process an option O that was
        immediately preceded by Mandatory.  The Data fields report the
        option type and data of option O, using the format of Reset Code



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        5, "Option Error".  See Section 5.8.2.

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

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

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

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

    11, "Aggression Penalty"
        This endpoint has detected congestion control-related
        misbehavior on the part of the other endpoint.  See Section
        12.3.

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

    128-255, CCID-specific codes
        Semantics depend on the connection's CCIDs.  See Section 10.3.
        Receivers should treat unknown CCID-specific Reset Codes like
        Reset Code 0, "Unspecified".

    The following table summarizes this information.
















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

                        Table 2: DCCP Reset Codes

    Options on DCCP-Reset packets are processed before the connection is
    shut down.  This means that certain combinations of options,
    particularly involving Mandatory, may cause an endpoint to respond
    to a valid DCCP-Reset with another DCCP-Reset.  This cannot lead to
    a reset storm; since the first endpoint has already reset the
    connection, the second DCCP-Reset will be ignored.

5.7.  DCCP-Sync and DCCP-SyncAck Packets

    DCCP-Sync packets help DCCP endpoints recover synchronization after
    bursts of loss, or recover from half-open connections.  Each valid
    received DCCP-Sync immediately elicits a DCCP-SyncAck.  Both packet
    types MUST use 48-bit sequence numbers (X=1).

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

    The Acknowledgement Number field has special semantics for DCCP-Sync



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

    As with DCCP-Ack packets, DCCP-Sync and DCCP-SyncAck packets MAY
    have non-zero-length application data areas, whose contents
    receivers MUST ignore.  Padded DCCP-Sync packets may be useful when
    performing Path MTU discovery; see Section 14.

5.8.  Options

    Any DCCP packet may contain options, which occupy space at the end
    of the DCCP header.  Each option is a multiple of 8 bits in length.
    Individual options are not padded to multiples of 32 bits, and any
    option may begin on any byte boundary.  However, the combination of
    all options MUST add up to a multiple of 32 bits; Padding options
    MUST be added as necessary to fill out option space to a word
    boundary.  Any options present are included in the header checksum.

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

    Options are processed sequentially, starting at the first option in
    the packet header.  Options with unknown types MUST be ignored.
    Also, options with nonsensical lengths (length byte less than two or
    more than the remaining space in the options portion of the header)
    MUST be ignored, and any option space following an option with
    nonsensical length MUST likewise be ignored.

    The following options are currently defined:










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               Option                           DCCP-  Section
       Type    Length     Meaning               Data?  Reference
       ----    ------     -------               -----  ---------
         0        1       Padding                 Y      5.8.1
         1        1       Mandatory               N      5.8.2
         2        1       Slow Receiver           Y      11.6
       3-31       1       Reserved
        32     variable   Change L                N      6.1
        33     variable   Confirm L               N      6.2
        34     variable   Change R                N      6.1
        35     variable   Confirm R               N      6.2
        36     variable   Init Cookie             N      8.1.4
        37       3-5      NDP Count               Y      7.7
        38     variable   Ack Vector [Nonce 0]    N      11.4
        39     variable   Ack Vector [Nonce 1]    N      11.4
        40     variable   Data Dropped            N      11.7
        41        6       Timestamp               Y      13.1
        42      6/8/10    Timestamp Echo          Y      13.3
        43       4/6      Elapsed Time            N      13.2
        44        6       Data Checksum           Y      9.3
       45-127  variable   Reserved
      128-255  variable   CCID-specific options   -      10.3

                        Table 3: DCCP Options

    Not all options are suitable for all packet types.  For example,
    since the Ack Vector option is interpreted relative to the
    Acknowledgement Number, it isn't suitable on DCCP-Request and DCCP-
    Data packets, which have no Acknowledgement Number.  If an option
    occurs on an unexpected packet type, it MUST generally be ignored;
    any such restrictions are mentioned in each option's description.
    The table summarizes the most common restriction: when the DCCP-
    Data? column value is N, the corresponding option MUST be ignored
    when received on a DCCP-Data packet.  (Section 7.5.5 describes why
    such options are ignored as opposed to, say, causing a reset.)

    Options with invalid values MUST be ignored unless otherwise
    specified.  For example, any Data Checksum option with option length
    4 MUST be ignored, since all valid Data Checksum options have option
    length 6.

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

5.8.1.  Padding Option






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

    Padding is a single-byte "no-operation" option used to pad between
    or after options.  If the length of a packet's other options is not
    a multiple of 32 bits, then Padding options are REQUIRED to pad out
    the options area to the length implied by Data Offset.  Padding may
    also be used between options -- for example, to align the beginning
    of a subsequent option on a 32-bit boundary.  There is no guarantee
    that senders will use this option, so receivers must be prepared to
    process options even if they do not begin on a word boundary.

5.8.2.  Mandatory Option

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

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

    Mandatory options MUST NOT be sent on DCCP-Data packets, and any
    Mandatory options received on DCCP-Data packets MUST be ignored.

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

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






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6.  Feature Negotiation

    Four DCCP options, Change L, Confirm L, Change R, and Confirm R, are
    used to negotiate feature values.  Change options initiate a
    negotiation; Confirm options complete that negotiation.  The "L"
    options are sent by the feature location, and the "R" options are
    sent by the feature remote.  Change options are retransmitted to
    ensure reliability.

    All these options have the same format.  The first byte of option
    data is the feature number, and the second and subsequent data bytes
    hold one or more feature values.  The exact format of the feature
    value area depends on the feature type; see Section 6.3.

    +--------+--------+--------+--------+--------
    |  Type  | Length |Feature#| Value(s) ...
    +--------+--------+--------+--------+--------

    Together, the feature number and the option type ("L" or "R")
    uniquely identify the feature to which an option applies.  The exact
    format of the Value(s) area depends on the feature number.

    Feature negotiation options MUST NOT be sent on DCCP-Data packets,
    and any feature negotiation options received on DCCP-Data packets
    MUST be ignored.

6.1.  Change Options

    Change L and Change R options initiate feature negotiation.  The
    option to use depends on the relevant feature's location: To start a
    negotiation for feature F/A, DCCP A will send a Change L option; to
    start a negotiation for F/B, it will send a Change R option.  Change
    options are retransmitted until some response is received.  They
    contain at least one Value, and thus have length at least 4.

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

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







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6.2.  Confirm Options

    Confirm L and Confirm R options complete feature negotiation, and
    are sent in response to Change R and Change L options, respectively.
    Confirm options MUST NOT be generated except in response to Change
    options.  Confirm options need not be retransmitted, since Change
    options are retransmitted as necessary.  The first byte of the
    Confirm option contains the feature number from the corresponding
    Change.  Following this is the selected Value, and then possibly the
    sender's preference list.

               +--------+--------+--------+--------+--------
    Confirm L: |00100001| Length |Feature#| Value(s) ...
               +--------+--------+--------+--------+--------
                Type=33

               +--------+--------+--------+--------+--------
    Confirm R: |00100011| Length |Feature#| Value(s) ...
               +--------+--------+--------+--------+--------
                Type=35

    If an endpoint receives an invalid Change option -- with an unknown
    feature number, or an invalid value -- it will respond with an empty
    Confirm option containing the problematic feature number, but no
    value.  Such options have length 3.

6.3.  Reconciliation Rules

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

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

6.3.1.  Server-Priority

    The feature value is a fixed-length byte string (length determined
    by the feature number).  Each Change option contains a list of
    values ordered by preference, with the most preferred value coming
    first.  Each Confirm option contains the confirmed value, followed
    by the confirmer's preference list.  Thus, the feature's current
    value will generally appear twice in Confirm options' data, once as
    the current value and once in the confirmer's preference list.





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

6.3.2.  Non-Negotiable

    The feature value is a byte string.  Each option contains exactly
    one feature value.  The feature location signals a new value by
    sending a Change L option.  The feature remote MUST accept any valid
    value, responding with a Confirm R option containing the new value,
    and it MUST send empty Confirm R options in response to invalid
    values (unless the Change L option was Mandatory; see Section
    6.6.9).  Change R and Confirm L options MUST NOT be sent for non-
    negotiable features; see Section 6.6.8.  Non-negotiable features use
    the feature negotiation mechanism to achieve reliability.

6.4.  Feature Numbers

    This document defines the following feature numbers.

                                           Rec'n Initial        Section
    Number   Meaning                       Rule   Value  Req'd Reference
    ------   -------                       -----  -----  ----- ---------
       0     Reserved
       1     Congestion Control ID (CCID)   SP      2      Y     10
       2     Allow Short Seqnos             SP      1      Y     7.6.1
       3     Sequence Window                NN     100     Y     7.5.2
       4     ECN Incapable                  SP      0      N     12.1
       5     Ack Ratio                      NN      2      N     11.3
       6     Send Ack Vector                SP      0      N     11.5
       7     Send NDP Count                 SP      0      N     7.7.2
       8     Minimum Checksum Coverage      SP      0      N     9.2.1
       9     Check Data Checksum            SP      0      N     9.3.1
     10-127  Reserved
    128-255  CCID-specific features                              10.3

                       Table 4: DCCP Feature Numbers


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

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





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    Req'd          This column is "Y" if and only if every DCCP
                   implementation MUST understand the feature.  If it is
                   "N", then the feature behaves like an extension (see
                   Section 15), and it is safe to respond to Change
                   options for the feature with empty Confirm options.
                   Of course, a CCID might require the feature; a DCCP
                   that implements CCID 2 MUST support Ack Ratio and
                   Send Ack Vector, for example.

6.5.  Examples
    Here are three example feature negotiations for features located at
    the server, the first two for the Congestion Control ID feature, the
    last for the Ack Ratio.

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


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


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

    This example shows a simultaneous negotiation.

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

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




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

    Change L(Sequence Window, 1024) = 32,9,3,0,0,0,0,4,0
        DCCP B should change Sequence Window/A's value (feature number
        3, a non-negotiable feature) to the 6-byte string 0,0,0,0,4,0
        (the value 1024).

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

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

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

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

    Confirm R(Sequence Window, 1024) = 35,9,3,0,0,0,0,4,0
        DCCP A has changed Sequence Window/B's value to the 6-byte
        string 0,0,0,0,4,0 (the value 1024).

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

6.6.  Option Exchange

    A few basic rules govern feature negotiation option exchange.

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

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

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





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

6.6.1.  Normal Exchange

    Change options are generated when a DCCP endpoint wants to change
    the value of some feature.  Generally, this will happen at the
    beginning of a connection, although it may happen at any time.  We
    say the endpoint "generates" or "sends" a Change L or Change R
    option, but of course the option must be attached to a packet.  The
    endpoint may attach the option to a packet it would have generated
    anyway (such as a DCCP-Request), or it may create a "feature
    negotiation packet", often a DCCP-Ack or DCCP-Sync, just to carry
    the option.  Feature negotiation packets are controlled by the
    relevant congestion control mechanism.  For example, DCCP A may send
    a DCCP-Ack or DCCP-Sync for feature negotiation only if the B-to-A
    CCID would allow sending a DCCP-Ack.  In addition, an endpoint
    SHOULD generate at most one feature negotiation packet per round-
    trip time.

    On receiving a Change L or Change R option, a DCCP endpoint examines
    the included preference list, reconciles that with its own
    preference list, calculates the new value, and sends back a
    Confirm R or Confirm L option, respectively, informing its peer of
    the new value or that the feature was not understood.  Every non-
    reordered Change option MUST result in a corresponding Confirm
    option, and any packet including a Confirm option MUST carry an
    Acknowledgement Number.  (Section 6.6.4 describes how Change
    reordering is detected and handled.)  Generated Confirm options may
    be attached to packets that would have been sent anyway (such as
    DCCP-Response or DCCP-SyncAck), or to new feature negotiation
    packets, as described above.

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

    A packet MAY contain more than one feature negotiation option, as
    long as no two options refer 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, 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



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    feature's value and thinks the other endpoint agrees.  An endpoint
    enters the CHANGING state when it first sends a Change for the
    feature, and returns to STABLE once it receives a corresponding
    Confirm.  The final state, UNSTABLE, indicates that an endpoint in
    CHANGING state changed its preference list, but has not yet
    transmitted a Change option with the new preference list.

    Feature state transitions at a feature location are implemented
    according to this diagram.  The diagram ignores sequence number and
    option validity issues; these are handled explicitly in the
    pseudocode that follows.

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

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

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



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          Return

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

    Third, process Change R options;
       If O.type == Change R,
          If the option's value is valid,   /* Section 6.6.8 */
             Calculate new value
             Send Confirm L on a future packet
             Set F.state := STABLE
          Otherwise, if the option was Mandatory,
             Reset connection and return
          Otherwise,
             Send Empty Confirm L on a future packet
             /* Remain in existing state.  If that's CHANGING, this
                endpoint will retransmit its Change L option later. */

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

    Versions of this diagram and pseudocode are also used by feature
    remotes; simply switch the "L"s and "R"s, so that the relevant
    options are Change R and Confirm L.

6.6.3.  Loss and Retransmission

    Packets containing Change and Confirm options might be lost or
    delayed by the network.  Therefore, Change options are repeatedly
    transmitted to achieve reliability.  We refer to this as
    "retransmission", although of course there are no packet-level
    retransmissions in DCCP: a Change option that is sent again will be
    sent on a new packet with a new sequence number.

    A CHANGING endpoint transmits another Change option once it realizes
    that it has not heard back from the other endpoint.  The new Change
    option need not contain the same payload as the original; reordering
    protection will ensure that agreement is reached based on the most
    recently transmitted option.




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

    Endpoints SHOULD use an exponential-backoff timer to decide when to
    retransmit Change options.  (Endpoints that generate packets
    specifically for feature negotiation MUST use such a timer.)  The
    timer interval is initially set to not less than one round-trip
    time, and should back off to not less than 64 seconds.  The backoff
    protects against delayed agreement due to the reordering protection
    algorithms described in the next section.  Again, endpoints may
    piggyback Change options on packets they would have sent anyway, or
    create new packets to carry the options; any such new packets are
    controlled by the relevant congestion-control mechanism.

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

6.6.4.  Reordering

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

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

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

    FGSS      Feature Greatest Sequence Number Sent: The greatest
              sequence number sent, considering only packets that
              contained one or more 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.



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    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'
    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



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    unknown Change options with Empty Confirm options (that is, Confirm
    options containing no data), which inform the CHANGING endpoint that
    the feature was not understood.  However, if the Change option was
    Mandatory, the connection MUST be reset; see Section 6.6.9.

    On receiving an empty Confirm option for some feature, the CHANGING
    endpoint MUST transition back to the STABLE state, leaving the
    feature's value unchanged.  Section 15 suggests that the default
    value for any extension feature 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.

    Since Confirm options are generated only in response to Change
    options, an endpoint should never receive a Confirm option referring
    to a feature number it does not understand.  Nevertheless, endpoints
    MUST ignore any such options they receive.

6.6.8.  Invalid Options

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

    o  All features have length limitations, and options with invalid
       lengths are invalid.  For example, the Ack Ratio feature takes
       16-bit values, so valid "Confirm R(Ack Ratio)" options have
       option length 5.

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

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

    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.




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

6.6.9.  Mandatory Feature Negotiation

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

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

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

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

    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.

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.







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7.1.  Variables

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

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

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

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

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

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

    Some other variables are derived from these primitives.

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

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

7.2.  Initial Sequence Numbers

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

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

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



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    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
    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



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    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.

    A received packet is classified as acknowledgeable if and only if
    its header was succesfully processed by the receiving DCCP.  In
    terms of the pseudocode in Section 8.5, a received packet becomes
    acknowledgeable when the receiving endpoint reaches Step 8.  This
    means, for example, that all acknowledgeable packets have valid
    header checksums and sequence numbers.  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 11.7.

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

7.5.  Validity and Synchronization

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

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





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

7.5.1.  Sequence and Acknowledgement Number Windows

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

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

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

    The Acknowledgement Number validity window for packets from DCCP B
    is [AWL, AWH].  The high end of the window, AWH, 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



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    that they appear to be less than ISR or ISS; the adjustments MUST
    NOT be applied in that case.)

7.5.2.  Sequence Window Feature

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

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

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

7.5.3.  Sequence-Validity Rules

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






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

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

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

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

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

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

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

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



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    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.

7.5.4.  Handling Sequence-Invalid Packets

    Endpoints MUST ignore sequence-invalid DCCP-Sync and DCCP-SyncAck
    packets, and MUST respond to other sequence-invalid packets with
    (possibly rate-limited) DCCP-Sync packets.  Each DCCP-Sync packet
    MUST acknowledge the corresponding 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.

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

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

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



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    sequence-valid, the endpoint MAY process them again.

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

7.5.5.  Sequence Number Attacks

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

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

    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 7.5.3.

    To quantify the probability of success, let N be the number of
    attack packets the attacker is willing to send, W be the relevant
    sequence window width, and L be the length of sequence numbers (24



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    or 48).  The attacker's best strategy is to space the attack packets
    evenly over sequence space.  Then the probability of hitting one
    sequence number window is P = WN/2^L.

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

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

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

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

    Since DCCP-Data attacks with short sequence numbers are relatively
    easy for attackers to execute, DCCP has been engineered to prevent
    these attacks from escalating to connection resets or other serious
    consequences.  In particular, any options whose processing might
    cause the connection to be reset are ignored when they appear on
    DCCP-Data packets.

7.5.6.  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.



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

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

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

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

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


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:



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    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,    /* conventional, non-circular
                                     comparison */
          Return (((REF_hi + 1) mod 2^24) << 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.  (The
    circular comparison "REF_low (<) S" returns true if and only if
    (S - REF_low), calculated using two's-complement arithmetic and then
    represented as an unsigned number, is less than or equal to 2^23
    (mod 2^24).)  When this happens, the high-order bits are
    incremented.

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.

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

    The sequence-validity mechanism assumes that the network does not
    deliver extremely old data.  In particular, it assumes that the



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    network must have dropped any packet by the time the connection
    wraps around and uses its sequence number again.  This constraint
    limits the maximum connection rate that can be safely achieved.  Let
    MSL equal the maximum segment lifetime, P equal the average DCCP
    packet size in bits, and L equal the length of sequence numbers (24
    or 48 bits).  Then the maximum safe rate, in bits per second, is R =
    P*(2^L)/2MSL.

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

7.7.  NDP Count and Detecting Application Loss

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

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





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

7.7.1.  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.

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.





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8.1.  Connection Establishment

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

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


8.1.1.  Client Request

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

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

    A client in the REQUEST state SHOULD use an exponential-backoff
    timer to send new DCCP-Request packets if no response is received.
    The first retransmission should occur after approximately one
    second, backing off to not less than one packet every 64 seconds; or
    the endpoint can use whatever retransmission strategy is followed
    for retransmitting TCP SYNs.  Each new DCCP-Request MUST increment
    the Sequence Number by one, and MUST contain the same Service Code
    and application data as the original DCCP-Request.

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



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

8.1.2.  Service Codes

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

    Endpoints MUST associate a Service Code with every DCCP socket, both
    actively and passively opened.  The application will generally
    supply this Service Code.  Each active socket MUST have exactly one
    Service Code.  Passive sockets MAY, at the implementation's
    discretion, be associated with more than one Service Code; this
    might let multiple applications, or multiple versions of the same
    application, listen on the same port, differentiated by Service
    Code.  If the DCCP-Request's Service Code doesn't 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.

    Service Codes are not intended to be DCCP-specific, and 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



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       consists of the ASCII digits, uppercase letters, and characters
       space, '-', '.', and '/'.)

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

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

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

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

    The server MUST NOT accept more than one piece of DCCP-Request
    application data per connection.  In particular, the DCCP-Response
    sent in reply to a retransmitted DCCP-Request with application data



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    SHOULD contain a Data Dropped option, in which the retransmitted
    DCCP-Request data is reported with Drop Code 0, Protocol
    Constraints.  The original DCCP-Request SHOULD also be reported in
    the Data Dropped option, either in a Normal Block (if the server
    accepted the data, or there was no data), or in a Drop Code 0 Drop
    Block (if the server refused the data the first time as well).

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

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

8.1.4.  Init Cookie Option

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


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

    The Init Cookie option MUST NOT be sent on DCCP-Request or DCCP-Data
    packets, and any such options received on DCCP-Request or DCCP-Data
    packets MUST be ignored.  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.  (As a result, the client
    MUST NOT use DCCP-Data packets until the three-way handshake
    completes 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



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    resetting the connection with Reset Code 10, "Bad Init Cookie".

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

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

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

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

8.2.  Data Transfer

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



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

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

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

8.3.  Termination

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

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

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




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

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

    Finally, the server can decide to hold TIMEWAIT state:

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


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

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

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



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    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 round-trip times, and should back off to not less than once
    every 64 seconds if no relevant response is received.

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

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

8.3.1.  Abnormal Termination

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

    DCCP endpoints in CLOSED 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.
    For example, there are arcs (not shown) from every state except
    CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.







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


8.5.  Pseudocode

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



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    The received packet is written as P, the socket as S.
    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.

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

    Step 2: Check ports and process TIMEWAIT state
       Look up flow ID in table and get corresponding socket
       If no socket, or S.state == TIMEWAIT,
          Generate Reset(No Connection) unless P.type == Reset
          Drop packet and return

    Step 3: Process LISTEN state
       If S.state == LISTEN,
          If P.type == Request or P contains a valid Init Cookie option,
             /* Must scan the packet's options to check for an Init
                Cookie.  Only the Init Cookie is processed here,
                however; other options are processed in Step 8.  This
                scan need only be performed if the endpoint uses Init
                Cookies */
             /* Generate a new socket and switch to that socket */
             Set S := new socket for this port pair
             S.state = RESPOND
             Choose S.ISS (initial seqno) or set from Init Cookie



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             Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookie
             Continue with S.state == RESPOND
             /* A Response packet will be generated in Step 11 */
          Otherwise,
             Generate Reset(No Connection) unless P.type == Reset
             Drop packet and return

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

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

    Step 6: Check sequence numbers
       Let LSWL = S.SWL and LAWL = S.AWL
       If P.type == CloseReq or P.type == Close or P.type == Reset,
          LSWL := S.GSR + 1, LAWL := S.GAR
       If LSWL <= P.seqno <= S.SWH
             and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),
          Update S.GSR, S.SWL, S.SWH
          If P.type != Sync,
             Update S.GAR
       Otherwise,
          Send Sync packet acknowledging P.seqno
          Drop packet and return

    Step 7: Check for unexpected packet types
       If (S.is_server and P.type == CloseReq)
            or (S.is_server and P.type == Response)



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

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

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

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

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




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

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

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

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

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

9.  Checksums

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

    Checksum coverage may eventually impact congestion control
    mechanisms as well.  A packet with corrupt application data and
    complete checksum coverage is treated as lost.  This incurs a heavy-
    duty loss response from the sender's congestion control mechanism,
    which can unfairly penalize connections on links with high
    background corruption.  The combination of reduced checksum coverage
    and Data Checksum options may let endpoints report packets as
    corrupt rather than dropped, using Data Dropped options and Drop



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    Code 3 (see Section 11.7).  This may eventually benefit
    applications.  However, further research is required to determine an
    appropriate response to corruption, which can sometimes correlate
    with congestion.  Corrupt packets currently incur a loss response.

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

    Reduced checksum coverage introduces some security considerations;
    see Section 18.1.  See Appendix B for further motivation and
    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 payload bytes to be
    checksummed, 8 zero bits are added on the right to form a 16 bit
    word for checksum purposes.  The pad byte is not transmitted as part
    of the packet.

    The pseudoheader is calculated as for TCP.  For IPv4, it is 96 bits
    long, and consists of the IPv4 source and destination addresses, the
    IP protocol number for DCCP (padded on the left with 8 zero bits),
    and the DCCP length as a 16-bit quantity (the length of the DCCP
    header with options, plus the length of any data); see RFC 793
    (Section 3.1).  For IPv6, it is 320 bits long, and consists of the
    IPv6 source and destination addresses, the DCCP length as a 32-bit
    quantity, and the IP protocol number for DCCP (padded on the left
    with 24 zero bits); see RFC 2460 (Section 8.1).

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



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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
    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].






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9.2.1.  Minimum Checksum Coverage Feature

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

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

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

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

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

9.3.  Data Checksum Option

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

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

    The sending DCCP computes the CRC of the bytes comprising the
    application data area and stores it in the option data.  The CRC-32c
    algorithm used for Data Checksum is the same as that used for SCTP
    [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 its willingness to check
    Data Checksums using the Check Data Checksum feature, described



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    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, Delivered Corrupt).

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

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

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

9.3.1.  Check Data Checksum Feature

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

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

9.3.2.  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



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    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

    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, 2 3 4)" asks the
    receiver to use CCID 2 for its packets, although CCIDs 3 and 4 are
    also acceptable.  (This corresponds to the bytes "35, 6, 1, 2, 3,
    4": Change R option (35), option length (6), feature ID (1), CCIDs
    (2, 3, 4).)  Similarly, "Confirm L(CCID, 1, 2 3 4)" tells the
    receiver that the sender is using CCID 2 for its packets, but that
    CCIDs 3 and 4 might also be acceptable.

    Currently allocated CCIDs are as follows.

              CCID   Meaning                      Reference
              ----   -------                      ---------
               0-1   Reserved
                2    TCP-like Congestion Control  [RFC TBA]
                3    TFRC Congestion Control      [RFC TBA]
              4-255  Reserved

              Table 5: DCCP Congestion Control Identifiers

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

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




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

10.1.  TCP-like Congestion Control

    CCID 2, TCP-like Congestion Control, denotes Additive Increase,
    Multiplicative Decrease (AIMD) congestion control with behavior
    modelled directly on TCP, including congestion window, slow start,
    timeouts, and so forth [RFC 2581].  CCID 2 achieves maximum
    bandwidth over the long term, consistent with the use of end-to-end
    congestion control, but halves its congestion window in response to
    each congestion event.  This leads to the abrupt rate changes
    typical of TCP.  Applications should use CCID 2 if they prefer
    maximum bandwidth utilization to steadiness of rate.  This is often
    the case for applications that are not playing their data directly
    to the user.  For example, a hypothetical application that
    transferred files over DCCP, using application-level retransmissions
    for lost packets, would prefer CCID 2 to CCID 3.  On-line games may
    also prefer CCID 2.

    CCID 2 is further described in [CCID 2 PROFILE].

10.2.  TFRC Congestion Control

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

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

    Half of the option types, feature numbers, and Reset Codes are
    reserved for CCID-specific use.  CCIDs may often need new options,
    for communicating acknowledgement or rate information, for example;
    reserved option spaces let CCIDs create options at will without
    polluting the global option space.  Option 128 might have different
    meanings on a half-connection using CCID 4 and a half-connection



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    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
       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.



















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

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

    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.

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






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10.4.  CCID Profile Requirements

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

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

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

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

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

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

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

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

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

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

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

10.5.  Congestion State

    Most congestion control algorithms depend on past history to
    determine the current allowed sending rate.  In CCID 2, this
    congestion state includes a congestion window and a measurement of



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    the number of packets outstanding in the network; in CCID 3, it
    includes the lengths of recent loss intervals; and both CCIDs use an
    estimate of the round-trip time.  Congestion state depends on the
    network path, and is invalidated by path changes.  Therefore, DCCP
    senders and receivers SHOULD reset their congestion state --
    essentially restarting congestion control from "slow start" or
    equivalent -- on significant changes in end-to-end path.  For
    example, an endpoint that sends or receives a Mobile IPv6 Binding
    Update message [RFC 3775] SHOULD reset its congestion state for any
    corresponding DCCP connections.

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

11.  Acknowledgements

    Congestion control requires 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, and so on.  DCCP acknowledgements are congestion
    controlled, although it is not required that the acknowledgement
    stream be more than very roughly TCP-friendly; each CCID defines how
    acknowledgements are congestion controlled.

    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.




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11.1.  Acks of Acks and Unidirectional Connections

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

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

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

    For example, consider a bidirectional connection where both half-
    connections use the same CCID (either 2 or 3), and where DCCP B goes
    "quiescent".  This means that the connection becomes unidirectional:
    DCCP B stops sending data, and sends only sends DCCP-Ack packets to
    DCCP A.  For example, in CCID 2, TCP-like Congestion Control, DCCP B
    uses Ack Vector to reliably communicate which packets it has
    received.  As described above, DCCP A must occasionally acknowledge
    a pure acknowledgement from DCCP B, so that B can free old Ack
    Vector state.  For instance, A might send a DCCP-DataAck packet
    every now and then, instead of DCCP-Data.  In contrast, in 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



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    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;
    in CCID 2, for example, this period is the maximum of 0.2 seconds
    and two round-trip times.  The A-to-B CCID defines how DCCP A
    handles acks-of-acks once DCCP B has gone quiescent.

11.2.  Ack Piggybacking

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

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



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    reverse-path congestion control does not try to be TCP-friendly.  It
    just tries to avoid congestion collapse, and to be somewhat better
    than TCP in the presence of a high packet loss or mark rate on the
    reverse path.

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

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

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

    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.



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

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

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

11.4.  Ack Vector Options

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

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

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

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








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

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

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

                     Table 6: DCCP Ack Vector States

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

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

    The first byte in the first Ack Vector option refers to the packet
    indicated in the Acknowledgement Number; subsequent bytes refer to
    older packets.  (Ack Vector MUST NOT be sent on DCCP-Data and DCCP-
    Request packets, which lack an Acknowledgement Number.)  An Ack
    Vector containing the decimal values 0,192,3,64,5 and the
    Acknowledgement Number is decimal 100 indicates that:

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

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

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

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

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




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

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

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

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

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

    One or more Ack Vector options that, together, report the status of
    a packet with sequence number less than ISN, the initial sequence
    number, SHOULD be considered invalid.  The receiving DCCP SHOULD
    either ignore the options or reset the connection with Reset Code 5,
    "Option Error".  No Ack Vector option can refer to a packet that has
    not yet been sent, as the Acknowledgement Number checks in Section
    7.5.3 ensure, but because of attack, implementation bug, or
    misbehavior, an Ack Vector option can claim that a packet was
    received before it is actually delivered; Section 12.2 describes how
    this is detected and how senders should react.  Packets that haven't
    been included in any Ack Vector option SHOULD be treated as "not yet
    received" (State 3) by the sender.

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

11.4.1.  Ack Vector Consistency

    A DCCP sender will commonly receive multiple acknowledgements for
    some of its data packets.  For instance, an HC-Sender might receive



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    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
    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:







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                               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.

    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



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    acknowledgements, moving packets from group 2 into group 1.

11.5.  Send Ack Vector Feature

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

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

11.6.  Slow Receiver Option

    An HC-Receiver sends the Slow Receiver option to its sender to
    indicate that it is having trouble keeping up with the sender's
    data.  The HC-Sender SHOULD NOT increase its sending rate for
    approximately one round-trip time after seeing a packet with a Slow
    Receiver option.  After one round-trip time, the effect of Slow
    Receiver disappears and the HC-Sender may again increase its rate,
    so the HC-Receiver SHOULD continue to send Slow Receiver options if
    it needs to prevent the HC-Sender from going faster in the long
    term.  The Slow Receiver option does not indicate congestion, and
    the HC-Sender need not reduce its sending rate.  (If necessary, the
    receiver can force the sender to slow down by dropping packets, with
    or without Data Dropped, or 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.

    +--------+
    |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, for
    example.



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    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.  This kind of format
    change should be requested at the application level, not via the
    Slow Receiver option.

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

11.7.  Data Dropped Option

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

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

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



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

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

                        Table 7: DCCP Drop Codes

    In more detail:

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

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

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

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



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

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

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

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

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

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

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

    One or more Data Dropped options that, together, report the status
    of more packets than have been sent, or that change the status of a
    packet, or that disagree with Ack Vector or equivalent options (by
    reporting a "not yet received" packet as "dropped in the receive
    buffer", for example), SHOULD be considered invalid.  The receiving
    DCCP SHOULD either ignore such options, or respond by resetting the
    connection with Reset Code 5, "Option Error".

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

    Data Dropped information is transmitted reliably.  That is,
    endpoints SHOULD continue to transmit Data Dropped options until
    receiving an acknowledgement indicating that the relevant options



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    have been processed.  In Ack Vector terms, each acknowledgement
    should contain Data Dropped options that cover the whole
    Acknowledgement Window (Section 11.4.2), although when every packet
    in that window would be placed in a Normal Block no actual option is
    required.

11.7.1.  Data Dropped and Normal Congestion Response

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

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

    Then the actual new window W_new MUST NOT be larger than the minimum
    of W_new1 and W_new2; and the sender MAY combine the two responses,
    by setting
          W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).

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

11.7.2.  Particular Drop Codes

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

    Drop Code 1, Application Not Listening, means the application
    running at the endpoint that sent the option is no longer listening
    for data.  For example, a server might close its receiving half-
    connection to new data after receiving a complete request from the
    client.  This would limit the amount of state available at the
    server for incoming data, and thus reduce the potential damage from
    certain denial-of-service attacks.  A Data Dropped option containing



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    Drop Code 1 SHOULD be sent whenever received data is ignored due to
    a non-listening application.  Once an endpoint reports Drop Code 1
    for a packet, it SHOULD report Drop Code 1 for every succeeding data
    packet on that half-connection; once an endpoint receives a Drop
    State 1 report, it SHOULD expect that no more data will ever be
    delivered to the other endpoint's application, so it SHOULD NOT send
    more data.

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

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

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); since
    acknowledgements are congestion-controlled, they also qualify as
    ECN-Capable Transport.

    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' IP headers, unless the
    receiver's ECN Incapable feature is on or the relevant CCID
    disallows it.

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



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    Ack Vector.

12.1.  ECN Incapable Feature

    DCCP endpoints are ECN-aware by default, but the ECN Incapable
    feature lets an endpoint reject the use of Explicit Congestion
    Notification.  The use of this feature is NOT RECOMMENDED.  ECN
    incapability both avoids ECN's possible benefits and prevents
    senders from using the ECN Nonce to check for receiver misbehavior.
    A DCCP stack MAY therefore leave the ECN Incapable feature
    unimplemented, acting as if all connections were ECN capable.  It is
    worth noting that the inappropriate firewall interactions that
    dogged TCP's implementation of ECN [RFC 3360] involve TCP header
    bits, not the IP header's ECN bits; we know of no middlebox that
    would block ECN-capable DCCP packets, but allow ECN-incapable DCCP
    packets.

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

    If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN
    Incapable, 1)" options to the other endpoint until acknowledged (by
    "Confirm R(ECN Incapable, 1)") or the connection closes.
    Furthermore, it MUST NOT accept any data until the other endpoint
    sends "Confirm R(ECN Incapable, 1)".  It SHOULD send Data Dropped
    options on its acknowledgements, with Drop Code 0 ("protocol
    constraints"), if the other endpoint does send data inappropriately.

12.2.  ECN Nonces

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

    ECN Nonces are a general mechanism to prevent ECN cheating (or loss
    cheating).  Two values for the two-bit ECN header field indicate



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    ECN-Capable Transport, 01 and 10.  The second code point, 10, is the
    ECN Nonce.  In general, a protocol sender chooses between these code
    points randomly on its output packets, remembering the sequence it
    chose.  The protocol receiver reports, on every acknowledgement, the
    number of ECN Nonces it has received thus far.  This is called the
    ECN Nonce Echo.  Since ECN marking and packet dropping both destroy
    the ECN Nonce, a receiver that lies about an ECN mark or packet drop
    has a 50% chance of guessing right and avoiding discipline.  The
    sender may react punitively to an ECN Nonce mismatch, possibly up to
    dropping the connection.  The ECN Nonce Echo field need not be an
    integer; one bit is enough to catch 50% of infractions, and the
    probability of success drops exponentially as more packets are sent
    [RFC 3540].

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

    Each DCCP sender SHOULD set ECN Nonces on its packets, and remember
    which packets had nonces.  When a sender detects an ECN Nonce Echo
    mismatch, it behaves as described in the next section.  Each DCCP
    receiver MUST calculate and use the correct value for ECN Nonce Echo
    when sending acknowledgement options.

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

12.3.  Aggression Penalties

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

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




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

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

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

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

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

13.  Timing Options

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

13.1.  Timestamp Option

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

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

    The four bytes of option data carry the timestamp of this packet.
    The timestamp is a 32-bit integer that increases monotonically with
    time, at a rate of 1 unit per 10 microseconds.  At this rate,



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    Timestamp Value will wrap approximately every 11.9 hours.  Endpoints
    need not measure time at this fine granularity; for example, an
    endpoint that preferred to measure time at millisecond granularity
    might send Timestamp Values that were all multiples of 100.  The
    precise time corresponding to Timestamp Value zero is not specified:
    Timestamp Values are only meaningful relative to other Timestamp
    Values sent on the same connection.  A DCCP receiving a Timestamp
    option SHOULD respond with a Timestamp Echo option on the next
    packet it sends.

13.2.  Elapsed Time Option

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

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

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

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




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

13.3.  Timestamp Echo Option

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

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

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

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

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

    The Elapsed Time value, similar to that in the Elapsed Time option,
    indicates the amount of time elapsed since receiving the packet
    whose timestamp is being echoed.  This time MUST be in hundredths of
    milliseconds.  Elapsed Time is meant to help the Timestamp sender
    separate the network round-trip time from the Timestamp receiver's



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    processing time.  This may be particularly important for CCIDs where
    acknowledgements are sent infrequently, so that there might be
    considerable delay between receiving a Timestamp option and sending
    the corresponding Timestamp Echo.  A missing Elapsed Time field is
    equivalent to an Elapsed Time of zero.  The smallest version of the
    option SHOULD be used that can hold the relevant Elapsed Time value.

14.  Maximum Packet Size

    A DCCP implementation MUST maintain the maximum packet size (MPS)
    allowed for each active DCCP session.  The MPS is influenced by the
    maximum packet size allowed by the current congestion control
    mechanism (CCMPS), the maximum packet size supported by the path's
    links (PMTU, the Path Maximum Transmission Unit) [RFC 1191], and the
    lengths of the IP and DCCP headers.

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

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

14.1.  Measuring PMTU

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

    Classical PMTU discovery uses unfragmentable packets.  In IPv4,
    these packets have the IP Don't Fragment (DF) bit set; in IPv6, all
    packets are unfragmentable once emitted by an end host.  As
    specified in RFC 1191, when a router receives a packet with DF set



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    that is larger than the next link's MTU, it sends an ICMP
    Destination Unreachable message back to the source whose Code
    indicates that an unfragmentable packet was too large to forward (a
    "Datagram Too Big" message).  When a DCCP implementation receives a
    Datagram Too Big message, it decreases its PMTU to the Next-Hop MTU
    value given in the ICMP message.  If the MTU given in the message is
    zero, the sender chooses a value for PMTU using the algorithm
    described in RFC 1191 (Section 7).  If the MTU given in the message
    is greater than the current PMTU, the Datagram Too Big message is
    ignored, as described in RFC 1191.  (We are aware that this may
    cause problems for DCCP endpoints behind certain firewalls.)

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

    A DCCP implementation SHOULD ensure, as far as possible, that ICMP
    Datagram Too Big messages were actually generated by routers, so
    that attackers cannot drive the PMTU down to a falsely small value.
    The simplest way to do this is to verify that the Sequence Number on
    the ICMP error's encapsulated header corresponds to a Sequence
    Number that the implementation recently sent.  (According to current
    specifications, routers should return the full DCCP header and
    payload up to a maximum of 576 bytes [RFC 1812] or the minimum IPv6
    MTU [RFC 2463], although they are not required to return more than
    64 bits [RFC 792].  Any amount greater than 128 bits will include
    the Sequence Number.)  ICMP Datagram Too Big messages with incorrect
    or missing Sequence Numbers may be ignored, or the DCCP
    implementation may lower the PMTU only temporarily in response.  If
    more than three odd Datagram Too Big messages are received and the
    other DCCP endpoint reports more than three lost packets, however,
    the DCCP implementation SHOULD assume the presence of a confused
    router, and either obey the ICMP messages' PMTU or (on IPv4
    networks) switch to allowing fragmentation.

    DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP
    endpoint begins by sending small packets with DF set, then gradually



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    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.

14.2.  Sender Behavior

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

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

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

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

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





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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 provides 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 connections that
    attempt to access unexpected services by sending a DCCP-Reset with
    Reset Code 8, "Bad Service Code".  Middleboxes should not modify the
    Service Code unless they are really changing the service a
    connection is accessing.

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

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

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





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

17.  Relations to Other Specifications

17.1.  RTP

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

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



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    applications might request either DCCP Ack Vectors or RTCP Extended
    Report Loss RLE Blocks, but not both.

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

    The DCCP sequence number cannot be inferred from the RTP sequence
    number since it increments on non-data packets as well as data
    packets.  The RTP sequence number cannot be inferred from the DCCP
    sequence number either [RFC 3550].  Furthermore, removing RTP's
    sequence number would not save any header space because of alignment
    issues.  We therefore recommend that RTP transmitted over DCCP use
    the same headers currently defined.  The 4 byte header cost is a
    reasonable tradeoff for DCCP's congestion control features and
    access to ECN.  Truly bandwidth-starved endpoints should use some
    future header compression scheme.

17.2.  Congestion Manager and Multiplexing

    Since DCCP doesn't provide reliable, ordered delivery, multiple
    application sub-flows may be multiplexed over a single DCCP
    connection with no inherent performance penalty.  Thus, there is no
    need for DCCP to provide built-in, 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 cryptographic security services (integrity,
    authentication, confidentiality, access control, and anti-replay
    protection) should use IPsec or end-to-end security of some kind;
    Secure RTP is one candidate protocol [RFC 3711].





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    Nevertheless, DCCP is intended to protect against some classes of
    attackers: Attackers cannot hijack a DCCP connection (close the
    connection unexpectedly, or cause attacker data to be accepted by an
    endpoint as if it came from the sender) unless they can guess valid
    sequence numbers.  Thus, as long as endpoints choose initial
    sequence numbers well, a DCCP attacker must snoop on data packets to
    get any reasonable probability of success.  Sequence number validity
    checks provide this guarantee.  Section 7.5.5 describes sequence
    number security further.  This security property only holds assuming
    that DCCP's random numbers are chosen according to the guidelines in
    RFC 1750.

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

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

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.



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

    Encryption (e.g., at the transport or application levels) may be
    used.  Note that omitting an integrity check can, under certain
    circumstances, compromise confidentiality [BEL98].

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

19.  IANA Considerations

    DCCP introduces eight sets of numbers whose values should be
    allocated by IANA.  We refer to allocation policies, such as
    Standards Action, outlined in RFC 2434, and most registries reserve
    some values for experimental and testing use [RFC 3692].  In
    addition, DCCP requires a Protocol Number to be added to the
    registry of Assigned Internet Protocol Numbers.  IANA is requested
    to assign IP Protocol Number 33 to DCCP; this number has already
    been informally made available for experimental DCCP use.

19.1.  Packet Types

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

19.2.  Reset Codes

    Each entry in the DCCP Reset Code registry contains a Reset Code,
    which is a number in the range 0-255; a short description of the



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

19.3.  Option Types

    Each entry in the DCCP option type registry contains an option type,
    which is a number in the range 0-255; the name of the option, such
    as "Slow Receiver"; and a reference to the RFC defining the option
    type.  The registry is initially populated using the values in Table
    3 (Section 5.8).  This document allocates option types 0-2 and
    32-44, and option types 31 and 120-126 are permanently reserved for
    experimental and testing use.  Option types 3-30, 45-119, and 127
    are currently reserved, and should be allocated with the IETF
    Consensus policy, requiring an IETF RFC publication (standards-track
    or not) with IESG review and approval.  Option types 128-255 are
    permanently reserved for CCID-specific registries; each CCID Profile
    document describes how the corresponding registry is managed.

19.4.  Feature Numbers

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

19.5.  Congestion Control Identifiers

    Each entry in the DCCP Congestion Control Identifier (CCID) registry
    contains a CCID, which is a number in the range 0-255; the name of
    the CCID, such as "TCP-like Congestion Control"; and a reference to
    the RFC defining the CCID.  The registry is initially populated



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    using the values in Table 5 (Section 10).  CCIDs 2 and 3 are
    allocated by concurrently published profiles, and CCIDs 248-254 are
    permanently reserved for experimental and testing use.  CCIDs 0, 1,
    4-247, and 255 are currently reserved, and should be allocated with
    the IETF Consensus policy, requiring an IETF RFC publication
    (standards-track or not) with IESG review and approval.

19.6.  Ack Vector States

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

19.7.  Drop Codes

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

19.8.  Service Codes

    Each entry in the Service Code registry contains a Service Code,
    which is a number in the range 0-4294967295; a short English
    description of the intended service; and an optional reference to an
    RFC or other publicly available specification defining the Service
    Code.  The registry should list the Service Code's numeric value as
    a decimal number, but when each byte of the four-byte Service Code
    is in the range 32-127, the registry should also show a four-
    character ASCII interpretation of the Service Code.  Thus, the
    number 1717858426 would additionally appear as "fdpz".  Service
    Codes are not DCCP-specific.  This document does not allocate any
    Service Codes, but Service Code 0 is permanently reserved (it
    represents the absence of a meaningful Service Code), and Service
    Codes 1056964608-1073741823 (high byte ASCII "?") are reserved for
    Private Use.  Most of the remaining Service Codes are allocated
    First Come First Served, with no RFC publication required.
    Exceptions are listed in Section 8.1.2.




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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 co-designed the current mechanism.  Arun
    Venkataramani's feedback improved Appendix A.

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

    We also thank those who provided comments and suggestions via the
    DCCP BOF, Working Group, and mailing lists, including Damon
    Lanphear, Patrick McManus, Colin Perkins, Sara Karlberg, Kevin Lai,
    Bernard Aboba, Youngsoo Choi, Pengfei Di, Dan Duchamp, Gorry
    Fairhurst, Derek Fawcus, David Timothy Fleeman, John Loughney,
    Ghyslain Pelletier, Tom Phelan, Stanislav Shalunov, David Vos, Yufei
    Wang, and Michael Welzl.  In particular, Colin Perkins provided
    extensive, detailed feedback, 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.





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    o  Quick removal of old state.

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

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

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

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

    We draw acknowledgement buffers like this:

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

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

    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: Partial Checksumming Design Motivation

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

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

    Thus DCCP allows the checksum to cover all of the packet, just the
    DCCP header, or both the DCCP header and some number of bytes from
    the application data.  If the application cannot tolerate any data
    corruption, then the checksum must cover the whole packet.  If the
    application would prefer to tolerate some corruption rather than
    have the packet dropped, then it can set the checksum to cover only
    part of the packet (but always the DCCP header).  In addition, if
    the application wishes to decouple checksumming of the DCCP header
    from checksumming of the application data, it may do so by including
    the Data Checksum option.  This would allow DCCP to discard
    corrupted application data, but still not mistake the corruption for
    network congestion.





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    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.

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




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

    [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-ccid2-10.txt, work in progress, March 2005.

    [CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye.  Profile for
        DCCP Congestion Control ID 3: TFRC Congestion Control.  draft-
        ietf-dccp-ccid3-11.txt, work in progress, March 2005.

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

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



Kohler/Handley/Floyd                                          [Page 125]

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        February 2004.

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

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

    [RFC 1812] F. Baker, editor.  Requirements for IP Version 4 Routers.
        RFC 1812.

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

    [RFC 1982] R. Elz and R. Bush.  Serial Number Arithmetic.  RFC 1982.

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

    [RFC 2401] S. Kent and R. Atkinson.  Security Architecture for the
        Internet Protocol.  RFC 2401.

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

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

    [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 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.  STD 64.
        RFC 3550.



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

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

    [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.

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

Full Copyright Statement

    Copyright (C) The Internet Society 2005.  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE



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    INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
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