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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4342

Internet Engineering Task Force                              Sally Floyd
INTERNET-DRAFT                                                      ICIR
draft-ietf-dccp-ccid3-08.txt                                Eddie Kohler
Expires: 14 May 2005                                                UCLA
                                                         Jitendra Padhye
                                                      Microsoft Research
                                                        14 November 2004


               Profile for DCCP Congestion Control ID 3:
                        TFRC Congestion Control


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
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    Drafts.

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    reference material or to cite them other than as "work in progress."

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

    This Internet-Draft will expire on 14 May 2005.

Copyright Notice

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





Floyd/Kohler/Padhye                                             [Page 1]

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Abstract

    This document contains the profile for Congestion Control Identifier
    3, TCP-Friendly Rate Control (TFRC), in the Datagram Congestion
    Control Protocol (DCCP).  CCID 3 should be used by senders that want
    a TCP-friendly sending rate, possibly with Explicit Congestion
    Notification (ECN), while minimizing abrupt rate changes.












































Floyd/Kohler/Padhye                                             [Page 2]

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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes from draft-ietf-dccp-ccid3-07.txt:

    * Loss Intervals is mandatory.

    * Elapsed Time is mandatory, even if there's a Timestamp Echo.

    * Send Loss Event Rate defaults to zero.

    * Rewrite Section 5.

    * IANA Considerations.

    * Wording nits.

    Changes from draft-ietf-dccp-ccid3-06.txt:

    * Moved the sections on Possible Changes to the Initial Window and
    Other Possible Changes to TFRC to be the section on Possible Future
    Changes to CCID3 in the appendix.

    * Some rephrasing, as a result of Working Group Last Call.

    * Specified the value of the inverted loss event rate when the loss
    event rate is 0.  From a suggestion from David Vos.

    * Added that the optional procedure for estimated the RTT at the
    receiver does not work when the inter-packet sending times are
    greater than the RTT.  From a suggestion by Ladan Gharai.

    Changes from draft-ietf-dccp-ccid3-05.txt:

    * Added a section on Response to Idle and Application-limited
    Periods

    * Added a paragraph on the sending rate when no feedback is received
    from the receiver.

    * Expanded on the discussion of the packet size s used in the TCP
    throughput equation.

    * Some editing to improve the presentation.

    * Added to discussion of response to Data Dropped and Slow Receiver.

    * Deleted the optional algorithm given in Section 8.7.1 for
    receivers to estimate the RTT, and replaced it with one sentence.



Floyd/Kohler/Padhye                                             [Page 3]

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    * Added a section on Other Possible Changes to TFRC.

    Changes from draft-ietf-dccp-ccid3-04.txt:

    * Minor editing.

    * Said that implementations may check for apps that are manipulating
    the packet size inappropriately.

    * Deletes the maximum packet size of 1500 bytes.

    * Added discussion on using the CCVal counter for estimating the
    round-trip time.

    * Changed the option number for the Loss Intervals option.

    * Added the Intellectual Property Notice.

    Changes from draft-ietf-dccp-ccid3-03.txt:

    * Added more text to the section on Congestion Control on Data
    Packets to make it more readable, and to summarize the key
    mechanisms specified in the TFRC spec.

    * Said that it is OK to use an initial sending rate of 2-4 pkts/RTT,
    based on RFC 3390.  And that in the future an initial sending rate
    of up to 8 pkts/RTT might be specified, for very small packets.

    * Receive Rate is measured in bytes per second, as RFC 3448
    specifies.

    * New definition of Loss Intervals option, because old definition
    was 24-bit-sequence-number specific; and add an example.

    Changes from draft-ietf-dccp-ccid3-02.txt:

    * Added to the section on Application Requirements.

    * Added a section on Packet Sizes.

    Changes from draft-ietf-dccp-ccid3-01.txt:

    * Added "Security Considerations" and "IANA Considerations"
    sections.

    * Store Window Counter in the DCCP header's CCVal field, not a
    separate option.




Floyd/Kohler/Padhye                                             [Page 4]

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    * Add to the description of a loss interval in the Loss Intervals
    option: a loss interval includes at most one round-trip time's worth
    of possibly-marked packets, and at least one round-trip time's worth
    of packets in all.

    * Added a description of when the loss event rate calculated by the
    sender could differ from that calculated by the receiver.

    * Window counter fixups.

    * Add Use Loss Intervals and Use Loss Event Rate features, and
    explain their interaction.

    * Move Elapsed Time option to DCCP's main specification (and
    simultaneously change its units to tenths of milliseconds). Allow
    the use of either Elapsed Time or Timestamp Echo.

    * Clarify the definition of quiescence.

    * Change calculations for determining loss events to take window
    counter wrapping into account.

    Changes from draft-ietf-dccp-ccid3-00.txt:

    * Changed the guidelines to say that required acknowledgement
    packets should include one or more of the following:  The Loss Event
    Rate, Loss Intervals, or the Ack Vector.

    * Added a separate section on "The Use of Ack Vectors".  This
    section says that Ack-of-acks must be used when the Ack Vector is
    used.

    * Renamed the "ECN Nonce Option" to the "Loss Intervals" option, and
    extended this option to include up to eight loss intervals.  This is
    to enable more precise verification by the sender of the receiver's
    feedback.

    * Added a section about "When should Ack Vector or Loss Intervals be
    used?"  In progress.

    * Added a section about using the ECN Nonce to verify the receiver's
    feedback.

    * Said that the ECN-Nonce feedback must be returned in every
    required acknowledgement.

    * Added a sentence saying that the TFRC spec "separately specifies
    the minimum sending rate from rate reductions during an idle



Floyd/Kohler/Padhye                                             [Page 5]

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    period."


















































Floyd/Kohler/Padhye                                             [Page 6]

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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   9
    2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   9
    3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.1. Relationship with TFRC . . . . . . . . . . . . . . . . .  10
       3.2. Example Half-Connection. . . . . . . . . . . . . . . . .  10
    4. Connection Establishment. . . . . . . . . . . . . . . . . . .  11
    5. Congestion Control on Data Packets. . . . . . . . . . . . . .  11
       5.1. Response to Idle and Application-limited
       Periods . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2. Response to Data Dropped and Slow Receiver . . . . . . .  14
       5.3. Packet Sizes . . . . . . . . . . . . . . . . . . . . . .  15
    6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  15
       6.1. Loss Interval Definition . . . . . . . . . . . . . . . .  16
       6.2. Congestion Control on Acknowledgements . . . . . . . . .  17
       6.3. Acknowledgements of Acknowledgements . . . . . . . . . .  17
       6.4. Quiescence . . . . . . . . . . . . . . . . . . . . . . .  18
    7. Explicit Congestion Notification. . . . . . . . . . . . . . .  18
    8. Options and Features. . . . . . . . . . . . . . . . . . . . .  18
       8.1. Window Counter Value . . . . . . . . . . . . . . . . . .  19
       8.2. Elapsed Time Options . . . . . . . . . . . . . . . . . .  21
       8.3. Receive Rate Option. . . . . . . . . . . . . . . . . . .  21
       8.4. Send Loss Event Rate Feature . . . . . . . . . . . . . .  22
       8.5. Loss Event Rate Option . . . . . . . . . . . . . . . . .  22
       8.6. Loss Intervals Option. . . . . . . . . . . . . . . . . .  23
          8.6.1. Option Details. . . . . . . . . . . . . . . . . . .  23
          8.6.2. Example . . . . . . . . . . . . . . . . . . . . . .  24
    9. Verifying Congestion Control Compliance With ECN. . . . . . .  26
       9.1. Verifying the ECN Nonce Echo . . . . . . . . . . . . . .  26
       9.2. Verifying the Reported Loss Intervals and Loss
       Event Rate. . . . . . . . . . . . . . . . . . . . . . . . . .  27
    10. Implementation Issues. . . . . . . . . . . . . . . . . . . .  27
       10.1. Timestamp Usage . . . . . . . . . . . . . . . . . . . .  27
       10.2. Determining Loss Events at the Receiver . . . . . . . .  27
       10.3. Sending Feedback Packets. . . . . . . . . . . . . . . .  29
    11. Security Considerations. . . . . . . . . . . . . . . . . . .  31
    12. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  31
       12.1. Reset Codes . . . . . . . . . . . . . . . . . . . . . .  32
       12.2. Option Types. . . . . . . . . . . . . . . . . . . . . .  32
       12.3. Feature Numbers . . . . . . . . . . . . . . . . . . . .  32
    13. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
    A. Appendix: Possible Future Changes to CCID 3 . . . . . . . . .  33
    Normative References . . . . . . . . . . . . . . . . . . . . . .  34
    Informative References . . . . . . . . . . . . . . . . . . . . .  34
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  35
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  35
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  35



Floyd/Kohler/Padhye                                             [Page 7]

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                               List of Tables

    Table 1: DCCP CCID 3 Options . . . . . . . . . . . . . . . . . .  19
    Table 2: DCCP CCID 3 Feature Numbers . . . . . . . . . . . . . .  19















































Floyd/Kohler/Padhye                                             [Page 8]

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1.  Introduction

    This document contains the profile for Congestion Control Identifier
    3, TCP-friendly rate control (TFRC), in the Datagram Congestion
    Control Protocol (DCCP) [DCCP].  DCCP uses Congestion Control
    Identifiers, or CCIDs, to specify the congestion control mechanism
    in use on a half-connection.

    TFRC is a receiver-based congestion control mechanism that provides
    a TCP-friendly sending rate, while minimizing the abrupt rate
    changes characteristic of TCP or of TCP-like congestion control [RFC
    3448].  The sender's allowed sending rate is set in response to the
    loss event rate, which is typically reported by the receiver to the
    sender.  See Section 3 for more on application requirements.

2.  Conventions

    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].

    All multi-byte numerical quantities in CCID 3, such as arguments to
    options, are transmitted in network byte order (most significant
    byte first).

    A DCCP half-connection consists of the application data sent by one
    endpoint and the corresponding acknowledgements sent by the other
    endpoint.  The terms "HC-Sender" and "HC-Receiver" denote the
    endpoints sending application data and acknowledgements,
    respectively.  Since CCIDs apply at the level of half-connections,
    we abbreviate HC-Sender to "sender" and HC-Receiver to "receiver" in
    this document.  See [DCCP] for more discussion.

    For simplicity, we say that senders send DCCP-Data packets and
    receivers send DCCP-Ack packets.  Both of these categories are meant
    to include DCCP-DataAck packets.

3.  Usage

    CCID 3's TFRC congestion control is appropriate for flows that would
    prefer to minimize abrupt changes in the sending rate, including
    streaming media applications with small or moderate buffering at the
    receive application before the playback time.  CCID 2, TCP-like
    congestion control [CCID 2 PROFILE], which halves the sending rate
    in response to a congestion event, cannot satisfy a preference for a
    relatively smooth sending rate.





Floyd/Kohler/Padhye                                 Section 3.  [Page 9]

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    As explained in [RFC 3448], the penalty of having smoother
    throughput than TCP while competing fairly for bandwidth is that the
    TFRC mechanism in CCID 3 responds slower than TCP or TCP-like
    mechanisms to changes in available bandwidth.  Thus, CCID 3 should
    only be used for applications with a requirement for smooth
    throughput, in particular avoiding TCP's halving of the sending rate
    in response to a single packet drop.  For applications that simply
    need to transfer as much data as possible in as short a time as
    possible, we recommend using TCP-like congestion control, such as
    CCID 2.

    As described in the TFRC specification [RFC 3448], CCID 3 should
    also not be used by applications that change their sending rate by
    varying the packet size, rather than varying the rate at which
    packets are sent.  A new CCID will be required for these
    applications.

3.1.  Relationship with TFRC

    The congestion control mechanisms described here follow the TFRC
    mechanism standardized by the IETF [RFC 3448].  Conformant CCID 3
    implementations MAY track updates to the TCP throughput equation
    directly, as updates are standardized in the IETF, rather than
    waiting for revisions of this document.  However, conformant
    implementations SHOULD wait for explicit updates to CCID 3 before
    implementing other changes to TFRC congestion control.

3.2.  Example Half-Connection

    This example shows the typical progress of a half-connection using
    CCID 3's TFRC Congestion Control, not including connection
    initiation and termination.  The example is informative, not
    normative.

    1.  The sender sends DCCP-Data packets, where the sending rate is
        governed by the allowed transmit rate as specified in [RFC
        3448].  Each DCCP-Data packet has a sequence number, and the
        DCCP header's CCVal field contains the Window Counter Value,
        used by the receiver in determining when multiple losses belong
        in a single loss event.

        If the connection isn't Explicit Congestion Notification (ECN)
        Incapable, then each DCCP-Data and DCCP-DataAck packet is sent
        as ECN-Capable, with either the ECT(0) or the ECT(1) codepoint
        set.  The use of the ECN Nonce with TFRC is described in Section
        9.





Floyd/Kohler/Padhye                              Section 3.2.  [Page 10]

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    2.  The receiver sends DCCP-Ack packets at least once per round-trip
        time acknowledging the data packets, unless the sender is
        sending at a rate of less than one packet per round-trip time,
        as indicated by the TFRC specification [RFC 3448].  Each DCCP-
        Ack packet uses a sequence number, identifies the most recent
        packet received from the sender, and includes feedback about the
        recent loss intervals experienced by the receiver.

    3.  The sender continues sending DCCP-Data packets as controlled by
        the allowed transmit rate.  Upon receiving DCCP-Ack packets, the
        sender updates its allowed transmit rate as specified in [RFC
        3448].  This update is based upon a loss event rate calculated
        by the sender, based on the receiver's loss intervals feedback.
        If it prefers, the sender can also use a loss event rate
        calculated and reported by the receiver.

    4.  The sender estimates round-trip times and calculates a
        nofeedback time, as specified in [RFC 3448].  If no feedback is
        received from the receiver in that time (at least four round-
        trip times), the sender halves its sending rate.

4.  Connection Establishment

    The connection is initiated by the client using mechanisms described
    in the DCCP specification [DCCP].  During or after CCID 3
    negotiation, the client and/or server may want to negotiate the
    values of the Send Ack Vector and Send Loss Event Rate features.

5.  Congestion Control on Data Packets

    CCID 3 uses the congestion control mechanisms of TFRC [RFC 3448].
    The following discussion summarizes information from RFC 3448; that
    RFC should be considered normative except where specifically
    indicated.

    Loss Event Rate

    The basic operation of CCID 3 centers around the calculation of a
    loss event rate: the number of loss events as a fraction of the
    number of packets transmitted, weighted over the last several loss
    intervals.  This loss event rate, a round-trip time estimate, and
    the average packet size are plugged into the TCP throughput
    equation, as specified in RFC 3448.  The result is a fair transmit
    rate, close to what a modern TCP would achieve in the same
    conditions.  CCID 3 senders are limited to this fair rate.

    The loss event rate itself is calculated in CCID 3 using recent loss
    interval lengths reported by the receiver.  Loss intervals are



Floyd/Kohler/Padhye                                Section 5.  [Page 11]

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    precisely defined in Section 6.1.  In summary, a loss interval is up
    to 1 RTT of possibly lost or ECN-marked packets, followed by an
    arbitrary number of non-dropped, non-marked packets.  Thus, long
    loss intervals represent low congestion rates.  The CCID 3 Loss
    Intervals option is used to report loss interval lengths; see
    Section 8.6.

    Other Congestion Control Mechanisms

    The sender starts in a slow-start phase, roughly doubling its
    allowed sending rate each round-trip time.  The slow-start phase is
    ended by the receiver's report of a packet drop or mark, after which
    the sender uses the loss event rate to calculate its allowed sending
    rate.

    RFC 3448 specifies an initial sending rate of one packet per RTT
    (Round-Trip Time) as follows: The sender initializes the allowed
    sending rate to one packet per second.  As soon as a feedback packet
    is received from the receiver, the sender has a measurement of the
    round-trip time, and then sets the initial allowed sending rate to
    one packet per RTT.  However, while the initial TCP window used to
    be one segment, RFC 2581 allows an initial TCP window of two
    segments, and RFC 3390 allows an initial TCP window of three or four
    segments (up to 4380 bytes).  RFC 3390 gives an upper bound on the
    initial window of
               min(4*MSS, max(2*MSS, 4380 bytes)).
    Translating this to the packet-based congestion control of CCID 3,
    the initial CCID 3 sending rate is allowed to be at least two
    packets per RTT, and at most four packets per RTT, depending on the
    packet size.  The initial rate is only allowed to be three or four
    packets per RTT when, in terms of segment size, that translates to
    at most 4380 bytes per RTT.

    The sender's measurement of the round-trip time uses the Elapsed
    Time and/or Timestamp Echo option contained in feedback packets, as
    described in Section 8.2. The Elapsed Time option is required, while
    the Timestamp Echo option is not required.  The sender maintains an
    average round-trip time heavily weighted on the most recent
    measurements.

    Each DCCP-Data packet contains a sequence number.  Each DCCP-Data
    packet also contains a Window Counter Value, as described in Section
    8.1 below.  The Window Counter Value is incremented by one every
    quarter round-trip time, and is used by the receiver in the
    calculation of loss intervals.  In particular, the Window Counter
    Value is used by the receiver as a coarse-grained timestamp to
    determine when a packet loss should be considered part of an
    existing loss interval, or must begin a new loss interval.



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    Because TFRC is rate-based instead of window-based, and because
    feedback packets can be dropped in the network, the sender needs
    some mechanism for reducing its sending rate in the absence of
    positive feedback from the receiver.  As described in Section 6, the
    receiver sends feedback packets roughly once per round-trip time.
    As specified in RFC 3448, the sender sets a nofeedback timer to at
    least four round-trip times, or to twice the interval between data
    packets, whichever is larger; if the sender hasn't received a
    feedback packet from the receiver when the nofeedback timer expires,
    then the sender halves its allowed sending rate.  The allowed
    sending rate is never reduced below one packet per 64 seconds.  Note
    that not all acknowledgements are considered feedback packets, since
    feedback packets must contain valid Loss Intervals, Elapsed Time,
    and Receive Rate options.

    If the sender never receives a feedback packet from the receiver,
    and as a consequence never gets to set the allowed sending rate to
    one packet per RTT, then the sending rate is left at its initial
    rate of one packet per second, with the nofeedback timer expiring
    after two seconds.  The allowed sending rate is halved each time the
    nofeedback timer expires.  Thus, if no feedback is received from the
    receiver, the allowed sending rate is never above one packet per
    second, and is quickly reduced below one packet per second.

    The feedback packets from the receiver contain a Receive Rate option
    specifying the rate at which data packets were received by the
    receiver since the last feedback packet.  The allowed sending rate
    can be at most twice the rate that the receiver received in the last
    round-trip time.  This may be less than the nominal fair rate if,
    for example, the application is sending less than its fair share.

5.1.  Response to Idle and Application-limited Periods

    One consequence of the nofeedback timer is that the sender reduces
    the allowed sending rate when the sender has been idle for a
    significant period of time.  As specified in RFC 3448, the allowed
    sending rate is never reduced to less than two packets per round-
    trip time as the result of an idle period.

    In CCID 3, we revise this specification from RFC 3448 to take into
    account the larger initial windows allowed by RFC 3390.  That is,
    the allowed sending rate is never reduced to less than the RFC 3390
    initial sending rate as the result of an idle period.  If the
    allowed sending rate is less than the initial sending rate upon
    entry to the idle period, then it will still be less than the
    initial sending rate when exiting the idle period.  However, the
    allowed sending rate should not be reduced to below the initial
    sending rate because of reductions of the allowed sending rate



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    during the idle period itself.

    The sender's allowed sending rate is limited to at most twice the
    receive rate reported by the receiver.  Thus, after an application-
    limited period, the sender can at most double its sending rate from
    one round-trip time to the next, until it reaches the allowed
    sending rate determined by the loss event rate.

5.2.  Response to Data Dropped and Slow Receiver

    A CCID 3 sender responds to packets acknowledged as Data Dropped as
    described in [DCCP], with the following further clarifications.

    o  Drop Code 2 ("receive buffer drop").  The allowed sending rate is
       reduced by one packet per RTT for each packet newly acknowledged
       as Drop Code 2, except that it is never reduced below one packet
       per round-trip time.

    o  Adjusting the receive rate X_recv.  A CCID 3 sender SHOULD also
       respond to non-congestion events, such as those implied by Data
       Dropped and Slow Receiver options, by adjusting X_recv, the
       receive rate reported by the receiver in Receive Rate options
       (see Section 8.3).  The CCID 3 sender's allowed sending rate is
       limited to at most twice the receive rate reported by the
       receiver, via the "min(..., 2*X_recv)" clause in RFC 3448's
       throughput calculations.  When the sender receives one or more
       Data Dropped and Slow Receiver options, the sender SHOULD adjust
       X_recv as follows:

       1.  Let X_inrecv equal the Receive Rate reported by the receiver
           in the most recent acknowledgement.

       2.  Let X_drop equal the upper bound on the sending rate implied
           by Data Dropped and Slow Receiver options.  If the sender
           receives a Slow Receiver option, which requests that the
           sender not increase its sending rate for roughly a round-trip
           time [DCCP], then X_drop should be set to X_inrecv.
           Similarly, if the sender receives a Data Dropped option
           indicating that three packets were dropped with Drop Code 2,
           then the upper bound on the sending rate will be decreased by
           three by the sender setting X_drop to X_inrecv - 3*s, where s
           is the packet size in bytes.

       3.  Set X_recv := min(X_inrecv, X_drop/2).

       As a result, the next round-trip time's sending rate will be
       limited to at most 2*(X_drop/2) = X_drop.  The effects of the
       Slow Receiver and Data Dropped options on X_recv will mostly



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       vanish by the round-trip time after that, which is appropriate
       for this non-congestion feedback.  This procedure MUST only be
       used for those Drop Codes not related to corruption (see [DCCP]).
       Currently, this is limited to Drop Codes 0, 1, and 2.

    o  Exiting slow-start.  The sender MUST also exit slow start
       whenever it receives a relevant Data Dropped or Slow Receiver
       option.

5.3.  Packet Sizes

    CCID 3 is intended for applications that use a fixed packet size,
    and that vary their sending rate in packets per second in response
    to congestion.   CCID 3 is not appropriate for applications that
    require a fixed interval of time between packets, and vary their
    packet size instead of their packet rate in response to congestion.
    However, some attention might be required for applications using
    CCID 3 that vary their packet size not in response to congestion,
    but in response to other application-level requirements.

    The packet size s is used in the TCP throughput equation.  A CCID 3
    implementation MAY calculate s as the segment size averaged over
    multiple round trip times -- for example, over the most recent four
    loss intervals, for loss intervals as defined in Section 6.1.
    Alternately, a CCID 3 implementation MAY use the Maximum Packet Size
    to derive s.  In this case, s is set to the Maximum Segment Size
    (MSS), the maximum size in bytes for the data segment, not including
    the default DCCP and IP packet headers.  In this case, each packet
    transmitted counts as one MSS, regardless of the actual segment
    size.  In this case, the TCP throughput equation can be interpreted
    as specifying the sending rate in packets per second.

    CCID 3 implementations MAY check for applications that appear to be
    manipulating the packet size inappropriately.  For example, an
    application might send small packets for a while, building up a fast
    rate, then switch to large packets to take advantage of the fast
    rate.  (Preliminary simulations indicate that applications may not
    be able to increase their overall transfer rates this way, so it is
    not clear this manipulation will occur in practice [V03].)

6.  Acknowledgements

    The receiver sends an acknowledgement to the sender roughly once per
    round-trip time, if the sender is sending packets that frequently.
    This rate is determined by the TFRC protocol, specified in [RFC
    3448].





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    As specified in [DCCP], the acknowledgement number acknowledges the
    greatest valid sequence number received so far on this connection.
    ("Greatest" is, of course, measured in circular sequence space.)
    Each acknowledgement required by TFRC also includes at least the
    following options:

    1.  An Elapsed Time and/or Timestamp Echo option specifying the
        amount of time elapsed since the receiver received the packet
        whose sequence number appears in the Acknowledgement Number
        field.  These options are described in Sections 13.2 and 13.1 of
        [DCCP].

    2.  A Receive Rate option (Section 8.3) specifying the rate at which
        the receiver received data since the last DCCP-Ack was sent.

    3.  A Loss Intervals option (Section 8.6) specifying the beginning
        and end of the most recent loss intervals experienced by the
        receiver.  (The definition of a loss interval is provided
        below.)  From Loss Intervals, the sender can easily calculate
        the loss event rate p using the procedure described in [RFC
        3448].

    Acknowledgements not containing at least these three options are not
    considered feedback packets.

    The receiver MAY also include other options concerning the loss
    event rate, including Loss Event Rate, which gives the loss event
    rate calculated by the receiver (Section 8.5), and DCCP's generic
    Ack Vector option, which reports which specific packets were lost or
    marked (Section 11.4 of [DCCP]).  Ack Vector is not required by
    CCID 3's congestion control mechanisms: the Loss Intervals option
    provides all the information needed to manage the transmit rate and
    probabilistically verify receiver reports.  However, Ack Vector may
    be useful for applications that need to determine exactly which
    packets were lost.

    If the HC-Receiver is also sending data packets to the HC-Sender,
    then it MAY piggyback acknowledgement information on those data
    packets more frequently than TFRC's specified acknowledgement rate
    allows.

6.1.  Loss Interval Definition

    As described in [RFC 3448] (Section 5.2), a loss interval begins
    with a lost or ECN-marked packet; continues with at most one round
    trip time's worth of packets that may or may not be lost or marked;
    and completes with an arbitrarily-long series of non-dropped, non-
    marked packets.  Call these the lossy part and the lossless part of



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    the loss interval.  For example, here is a single loss interval,
    assuming that sequence numbers increase as you move right:

               Lossy Part
                <= 1 RTT   __________ Lossless Part __________
              /          \/                                   \
              *----*--*--*-------------------------------------
              ^    ^  ^  ^
             losses or marks


    Note that a loss interval's lossless part might be empty, as in the
    first interval below:

             Lossy Part   Lossy Part
              <= 1 RTT     <= 1 RTT   _____ Lossless Part _____
            /          \/           \/                         \
            *----*--*--***--------*-*---------------------------
            ^    ^  ^  ^^^        ^ ^
            \_ Int. 1 _/\_____________ Interval 2 _____________/


    [RFC 3448] specifies that the length of the lossy part must be <=
    1 RTT.  CCID 3 uses the Window Counter, not receive times, to
    determine whether multiple packets occurred in the same RTT, and
    thus belong to the same loss event; see Section 10.2.

    A missing packet doesn't begin a new loss interval until NDUPACK
    packets have been seen after the "hole", where NDUPACK = 3 (see
    Section 5.1 of [RFC 3448]).  Thus, up to NDUPACK of the most recent
    sequence numbers (including the sequence numbers of any holes) might
    temporarily not be part of any loss interval, while the
    implementation waits to see whether a hole will be filled.

6.2.  Congestion Control on Acknowledgements

    The rate and timing for generating acknowledgements is determined by
    the TFRC algorithm [RFC 3448].  The sending rate for
    acknowledgements is relatively low -- roughly once per round-trip
    time -- so there is no need for explicit congestion control on
    acknowledgements.

6.3.  Acknowledgements of Acknowledgements

    TFRC acknowledgements don't generally need to be reliable, so the
    sender generally need not acknowledge the receiver's
    acknowledgements.  When Ack Vector is used, however, the sender,
    DCCP A, MUST occasionally acknowledge the receiver's



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    acknowledgements so that the receiver can free up Ack Vector state.
    When both half-connections are active, the necessary
    acknowledgements will be contained in A's acknowledgements to B's
    data.  If the B-to-A half-connection goes quiescent, however, DCCP A
    must send an acknowledgement proactively.

    Thus, when Ack Vector is used, an active sender MUST acknowledge the
    receiver's acknowledgements approximately once per round-trip time,
    within a factor of two or three, probably by sending a DCCP-DataAck
    packet.  No acknowledgement options are necessary, just the
    Acknowledgement Number in the DCCP-DataAck header.

    The sender MAY choose to acknowledge the receiver's acknowledgements
    even if they do not contain Ack Vectors.  For instance, regular
    acknowledgements can shrink the size of the Loss Intervals option.
    Unlike the Ack Vector, however, the Loss Intervals option is bounded
    in size (and receiver state), so acks-of-acks are not required.

6.4.  Quiescence

    This section refers to quiescence in the DCCP sense (see Section
    11.1 of [DCCP]): How does a CCID 3 receiver determine that the
    corresponding sender is not sending any data?

    Let T equal the greater of 0.2 seconds and two round-trip times.  (A
    CCID 3 receiver has a rough measure of the round-trip time, so that
    it can pace its acknowledgements.)  The receiver detects that the
    sender has gone quiescent after T seconds have passed without
    receiving any additional data from the sender.

7.  Explicit Congestion Notification

    CCID 3 supports Explicit Congestion Notification (ECN) [RFC 3168].
    The sender will use the ECN Nonce for its data packets, as specified
    in Section 12.2 of [DCCP].  Information about the ECN Nonce is
    returned by the receiver using the Loss Intervals option.
    Additionally, any Ack Vector options will include the ECN Nonce Sum.
    The sender can maintain a table with the ECN nonce sum for each
    packet, and use this information to probabilistically verify the ECN
    nonce sums returned in Loss Intervals or Ack Vector options.
    Section 9 describes this further.

8.  Options and Features

    CCID 3 can make use of DCCP's Ack Vector, Timestamp, Timestamp Echo,
    and Elapsed Time options, and its Send Ack Vector and ECN Incapable
    features.  In addition, the following CCID-specific options are
    defined for use with CCID 3.



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                   Option                        DCCP-   Section
          Type     Length     Meaning            Data?  Reference
          -----    ------     -------            -----  ---------
         128-191              Reserved
           192        6       Loss Event Rate      N      8.5
           193     variable   Loss Intervals       N      8.6
           194        6       Receive Rate         N      8.3
         195-255              Reserved

                       Table 1: DCCP CCID 3 Options

    The DCCP-Data? column indicates that all currently defined
    CCID 3-specific options MUST be ignored when they occur on DCCP-Data
    packets.

    The following CCID-specific feature is also defined.

                                        Rec'n Initial        Section
      Number   Meaning                  Rule   Value  Req'd Reference
      ------   -------                  -----  -----  ----- ---------
      128-191  Reserved
        192    Send Loss Event Rate      SP      0      N      8.4
      193-255  Reserved

                   Table 2: DCCP CCID 3 Feature Numbers

    The columns are defined in Table 4 of [DCCP].  Rec'n Rule defines
    the feature's reconciliation rule, where "SP" means server-priority.
    Req'd specifies whether every CCID 3 implementation MUST understand
    a feature; in this case, Send Loss Event Rate is optional, in that
    it behaves like an extension (see Section 15 of [DCCP]).

8.1.  Window Counter Value

    The data sender stores a 4-bit window counter value in the DCCP
    generic header's CCVal field on every data packet it sends.  This
    value is set to 0 at the beginning of the transmission, and
    generally increased by 1 every quarter of a round-trip time, as
    described in [RFC 3448].  For reference, the DCCP generic header is
    as follows (diagram repeated from [DCCP], which also shows the
    generic header with a 24-bit Sequence Number field).










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

    The CCVal field has enough space to express 4 round-trip times at
    quarter-RTT granularity.  The sender MUST avoid wrapping CCVal on
    adjacent packets, as might happen, for example, if two data-carrying
    packets were sent 4 round-trip times apart with no packets
    intervening.  Therefore, the sender SHOULD use the following
    algorithm for setting CCVal.  The algorithm uses three variables:
    "last_WC" holds the last window counter value sent, "last_WC_time"
    is the time at which the first packet with window counter value
    "last_WC" was sent, and "RTT" is the current round-trip time
    estimate.  last_WC is initialized to zero, and last_WC_time to the
    time of the first packet sent.  Then, before sending a new packet,
    proceed like this:

      Let quarter_RTTs = floor((current_time - last_WC_time) / (RTT/4)).
      If quarter_RTTs > 0, then:
          Set last_WC := (last_WC + min(quarter_RTTs, 5)) mod 16, and
          Set last_WC_time := current_time.
      Set the packet header's CCVal field to last_WC.

    When this algorithm is used, adjacent data-carrying packets' CCVal
    counters never differ by more than five, modulo 16.

    The window counter value may also change as feedback packets arrive.
    In particular, after receiving an acknowledgement for a packet sent
    with window counter WC, the sender SHOULD increase its window
    counter, if necessary, so that subsequent packets have window
    counter value at least (WC + 4) mod 16.

    The CCVal counters are used by the receiver to determine whether
    multiple losses belong to a single loss event, to determine the
    interval to use for calculating the receive rate, and to determine
    when to send feedback packets.  None of these procedures require the
    receiver to maintain an explicit estimate of the round-trip time.
    However, implementors who wish to keep such an RTT estimate may do
    so using CCVal.  Let T(I) be the arrival time of the earliest valid
    received packet with CCVal = I.  (Of course, when the window counter



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    value wraps around to the same value mod 16, we must recalculate
    T(I).)  Let D = 2, 3, or 4, and say that T(K) and T(K+D) both exist
    (packets were received with window counters K and K+D).  Then the
    value (T(K+D) - T(K)) * 4/D MAY serve as an estimate of the round-
    trip time.  Values of D = 4 SHOULD be preferred for RTT estimation.
    Concretely, say that the following packets arrived:

    Time:       T1  T2  T3 T4  T5           T6  T7   T8  T9
           ------*---*---*-*----*------------*---*----*--*---->
    CCVal:      K-1 K-1  K K   K+1          K+3 K+4  K+3 K+4

    Then T7 - T3, the difference between the receive times of the first
    packet received with window counter K+4 and the first packet
    received with window counter K, is a reasonable round-trip time
    estimate.  Because of the necessary constraint that measurements can
    only come from packet pairs whose CCVals differ by at most 4, this
    procedure does not work when the inter-packet sending times are
    significantly greater than the RTT, resulting in packet pairs whose
    CCVals differ by 5.  Explicit RTT measurement techniques, such as
    Timestamp and Timestamp Echo, should be used in that case.

8.2.  Elapsed Time Options

    The data receiver MUST include an elapsed time value on every
    required acknowledgement.  This helps the sender distinguish between
    network round-trip time, which it must include in its rate
    equations, and delay at the receiver due to TFRC's infrequent
    acknowledgement rate, which it need not include.  The elapsed time
    value is included in one, or possibly two, ways:

    1.  If at least one recent data packet (i.e., a packet received
        after the previous DCCP-Ack was sent) included a Timestamp
        option, then the receiver SHOULD include the corresponding
        Timestamp Echo option, with Elapsed Time value.

    2.  In any case, the receiver MUST include an Elapsed Time option.

    All these option types are defined in the main DCCP specification
    [DCCP].

8.3.  Receive Rate Option

    +--------+--------+--------+--------+--------+--------+
    |11000010|00000110|            Receive Rate           |
    +--------+--------+--------+--------+--------+--------+
     Type=194   Len=6

    This option MUST be sent by the data receiver on all required



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    acknowledgements.  Its four data bytes indicate the rate at which
    the receiver has received data since it last sent an
    acknowledgement, in bytes per second.  The Receive Rate is
    calculated as the number of bytes received in the most recent t
    seconds, divided by t, where t is the larger of the following:  the
    time since the last Receive Rate Option was sent, and the estimated
    round-trip time.  The receiver can use the Window Counter Value in
    received data packets to determine if an interval of t seconds
    corresponds to at least a round-trip time.

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

8.4.  Send Loss Event Rate Feature

    The Send Loss Event Rate feature lets CCID 3 endpoints negotiate
    whether the receiver MUST provide Loss Event Rate options on its
    acknowledgements.  DCCP A sends a "Change R(Send Loss Event Rate,
    1)" option to ask DCCP B to send Loss Event Rate options as part of
    its acknowledgement traffic.

    Send Loss Event Rate has feature number 192, and is server-priority.
    It takes one-byte Boolean values.  DCCP B MUST send Loss Event Rate
    options on its acknowledgements when Set Loss Event Rate/B is one,
    although it MAY send Loss Event Rate options even when Send Loss
    Event Rate/B is zero.  Values of two or more are reserved.  A CCID 3
    half-connection starts with Send Loss Event Rate equal to zero.

8.5.  Loss Event Rate Option

    +--------+--------+--------+--------+--------+--------+
    |11000000|00000110|          Loss Event Rate          |
    +--------+--------+--------+--------+--------+--------+
     Type=192   Len=6

    The option value indicates the inverse of the loss event rate,
    rounded UP, as calculated by the receiver.  Its units are packets
    per loss interval.  Thus, if the Loss Event Rate option value is
    100, then the loss event rate is 0.01 loss events per packet (and
    the average loss interval contains 100 packets).  When each loss
    event has exactly one packet loss, the loss event rate is the same
    as the packet drop rate.

    See [RFC 3448] for a normative calculation of loss event rate.
    Before any losses have occurred, when the loss event rate is zero,
    the Loss Event Rate option value is set to
    "11111111111111111111111111111111" in binary (or equivalently, to
    2^32 - 1).



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

8.6.  Loss Intervals Option

                                 ___ Loss Interval ___
                                /                     \
    +--------+--------+--------+----...----+----...----+--------+---
    |11000001| Length |  Skip  | Lossless  |E|  Loss   | More Loss
    |        |        | Length |  Length   | | Length  | Intervals...
    +--------+--------+--------+----...----+----...----+--------+---
     Type=193                     3 bytes     3 bytes

    This option MUST be sent by the data receiver on all required
    acknowledgements.  The option reports up to 42 loss intervals seen
    by the receiver (although TFRC currently uses at most the latest 9
    of these).  This lets the sender calculate a loss event rate and
    probabilistically verify the receiver's ECN Nonce Echo.

    As specified in [RFC 3448], the length of the loss interval is the
    number of packets transmitted in the lossy and lossless parts of the
    loss interval.  The receiver computes the weighted average of the
    last eight loss interval lengths, to get the average loss interval
    in packets.  The Loss Event Rate, discussed in option 192, is the
    inverse of the average loss interval, in units of loss events per
    packet.  Thus, if the average loss interval is 100 packets, this
    gives a loss event rate of 0.01 loss events per packet.

    The Loss Intervals option serves several purposes.

    o  The sender uses the Loss Intervals option to calculate the Loss
       Event Rate.

    o  Loss Intervals information is easily checked for consistency
       against previous Loss Intervals options, and against any Loss
       Event Rate calculated by the receiver.

    o  The sender can probabilistically verify the ECN Nonce Echo for
       each Loss Interval, reducing the likelihood of misbehavior.

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

8.6.1.  Option Details

    The Loss Intervals option contains information about between one and
    42 consecutive loss intervals, always including the most recent loss



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    interval.  Intervals are listed in reverse chronological order.  The
    option MUST contain information about at least the most recent
    NINTERVAL + 1 = 9 loss intervals unless (1) there have not yet been
    NINTERVAL + 1 loss intervals, or (2) the receiver knows, because of
    the sender's acknowledgements, that some previously-transmitted loss
    interval information has been received.  In this second case, the
    receiver need not send loss intervals that the sender already knows
    about, except that it MUST transmit at least one loss interval
    regardless.  The NINTERVAL parameter is equal to "n" as defined in
    Section 5.4 of [RFC 3448].

    Loss interval sequence numbers are delta-encoded starting from the
    Acknowledgement Number.  Therefore, Loss Intervals options MUST NOT
    be sent on packets without an Acknowledgement Number.

    The first byte of option data is Skip Length, which indicates the
    number of packets up to and including the Acknowledgement Number
    that are not part of any Loss Interval.  As discussed above, Skip
    Length must be less than or equal to NDUPACK = 3.

    Loss Interval structures follow Skip Length.  Each Loss Interval
    consists of a Lossless Length, a Loss Length, and an ECN Nonce Echo
    (E).

    Lossless Length, a 24-bit number, specifies the number of packets in
    the loss interval's lossless part.

    Loss Length, a 23-bit number, specifies the number of packets in the
    loss interval's lossy part.

    The ECN Nonce Echo, stored in the high-order bit of the 3-byte field
    containing Loss Length, equals the one-bit sum (exclusive-or, or
    parity) of nonces received over the loss interval's lossless part
    (which is Lossless Length packets long).  If Lossless Length is 0,
    or if the receiver is ECN-incapable, the ECN Nonce Echo MUST be
    reported as 0.

8.6.2.  Example

    Consider the following sequence of packets, where "-" represents a
    safely delivered packet and "*" represents a lost or marked packet.

    Sequence
     Numbers: 0         10        20        30        40  44
              |         |         |         |         |   |
              --*-*-----*--------***-*--------*----------*-

    Assuming that packet 43 was lost, not marked, this sequence might be



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    divided into loss intervals as follows:

              0         10        20        30        40  44
              |         |         |         |         |   |
              --*-*-----*--------***-*--------*----------*-
              \/\______/\_______/\___________/\_________/
              L0   L1      L2         L3           L4

    A Loss Intervals option sent to acknowledge this set of loss
    intervals, on a packet with Acknowledgement Number 44, might contain
    the bytes 193,33,2, 0,0,10, 128,0,1, 0,0,8, 0,0,5, 0,0,8, 0,0,1,
    0,0,5, 128,0,3, 0,0,2, 128,0,0.  This option is interpreted as
    follows.

    193 The Loss Intervals option number.

    33  The length of the option, including option type and length
        bytes.  This option contains information about (33 - 3)/6 = 5
        loss intervals.

    2   The Skip Length is 2 packets.  Thus, the most recent loss
        interval, L4, ends immediately before sequence number 44 - 2 + 1
        = 43.

    0,0,10, 128,0,1
        These bytes define L4.  L4 consists of a 10-packet lossless part
        (0,0,10), preceded by a 1-packet lossy part.  Continuing to
        subtract, the lossless part begins with sequence number 43 - 10
        = 33, and the lossy part begins with sequence number 33 - 1 =
        32.  The ECN Nonce Echo for the lossless part, namely packets 33
        through 42, inclusive, equals 1.

    0,0,8, 0,0,5
        This defines L3, whose lossless part begins with sequence number
        32 - 8 = 24; whose lossy part begins with sequence number 24 - 5
        = 19; and whose ECN Nonce Echo (for packets [24,31]) equals 0.

    0,0,8, 0,0,1
        L2's lossless part begins with sequence number 11, its lossy
        part begins with sequence number 10, and its ECN Nonce Echo (for
        packets [11,18]) equals 0.

    0,0,5, 128,0,3
        L1's lossless part begins with sequence number 5, its lossy part
        begins with sequence number 2, and its ECN Nonce Echo (for
        packets [5,9]) equals 1.





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    0,0,2, 128,0,0
        L0's lossless part begins with sequence number 0, it has no
        lossy part, and its ECN Nonce Echo (for packets [0,1]) equals 1.

9.  Verifying Congestion Control Compliance With ECN

    The sender can use Loss Intervals options' ECN Nonce Echoes (and
    possibly any Ack Vectors' ECN Nonce Echoes) to probabilistically
    verify that the receiver is correctly reporting all dropped or
    marked packets.  Even if ECN is not used (the ECN Incapable feature
    is set to one), the sender could still check on the receiver by
    occasionally not sending a packet, or sending a packet out-of-order,
    to catch the receiver in an error in Loss Intervals or Ack Vector
    information.  This is not as robust or as non-intrusive as the
    verification provided by the ECN Nonce, however.

9.1.  Verifying the ECN Nonce Echo

    To verify the ECN Nonce Echo included with a Loss Intervals option,
    the sender maintains a table with the ECN nonce sum for each packet.
    As defined in [RFC 3540], the nonce sum for sequence number S is the
    one-bit sum (exclusive-or, or parity) of nonces over the sequence
    number range [I,S], where I is the initial sequence number.  Let
    NonceSum(S) represent this nonce sum for sequence number S, and let
    NonceSum(I - 1) equal 0.  Then the Nonce Echo for a loss interval
    [Left Edge, Left Edge + Offset) should equal the following one-bit
    sum:

       NonceSum(Left Edge - 1) + NonceSum(Left Edge + Offset - 1).

    Since an ECN Nonce Echo is returned for the lossless part of each
    Loss Interval, a misbehaving receiver -- meaning a receiver that
    reports a lost or marked packet as "received non-marked", to avoid
    rate reductions -- has only a 50% chance of guessing the correct
    Nonce Echo for each loss interval.

    To verify the ECN Nonce Echo included with an Ack Vector option, the
    sender maintains a table with the ECN nonce value sent for each
    packet.  The Ack Vector option explicitly says which packets were
    received non-marked; the sender just adds up the nonces for those
    packets using a one-bit sum, and compares the result to the Nonce
    Echo encoded in the Ack Vector's option type.  Again, a misbehaving
    receiver has only a 50% chance of guessing an Ack Vector's correct
    Nonce Echo.  Appendix A of [DCCP] describes this further.
    Alternatively, an Ack Vector's ECN Nonce Echo may also be calculated
    from a table of ECN nonce sums, rather than ECN nonces.  If the Ack
    Vector contains many long runs of non-marked, non-dropped packets,
    the nonce sum-based calculation will probably be faster than a



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    straightforward nonce-based calculation.

9.2.  Verifying the Reported Loss Intervals and Loss Event Rate

    Besides probabilistically verifying the ECN Nonce Echoes reported by
    the receiver, the sender may also verify the loss intervals and any
    loss event rate reported by the receiver, if it so desires.
    Specifically, the Loss Intervals option explicitly reports the size
    of each loss interval as seen by the receiver; the sender can verify
    that the receiver is not falsely combining two loss events into one
    reported Loss Interval by using saved Window Counter information.
    The sender can also compare any Loss Event Rate option to the loss
    event rate it calculates using the Loss Intervals option.

    We note that in some cases the loss event rate calculated by the
    sender could differ from an explicit Loss Event Rate option sent by
    the receiver.  In particular, when a number of successive packets
    are dropped, the receiver does not know the sending times for these
    packets, and interprets these losses as a single loss event.  In
    contrast, if the sender has saved the sending times or the window
    counter information for these packets, then the sender can determine
    if these losses constitute a single loss event, or several
    successive loss events.   Thus, with its knowledge of the sending
    times of dropped packets, the sender is able to make a more accurate
    calculation of the loss event rate.  These kinds of differences
    SHOULD NOT be misinterpreted as attempted receiver misbehavior.

10.  Implementation Issues

10.1.  Timestamp Usage

    CCID 3 data packets need not carry Timestamp options.  The sender
    can store the times at which recent packets were sent; the
    Acknowledgement Number and Elapsed Time option contained on each
    required acknowledgement then provide sufficient information to
    compute the round trip time.  Alternatively, the sender MAY include
    Timestamp options on a limited subset of its data packets.  The
    receiver will respond with Timestamp Echo options including Elapsed
    Times, allowing the sender to calculate round-trip times without
    storing timestamps at all.

10.2.  Determining Loss Events at the Receiver

    The window counter is used by the receiver to determine if multiple
    lost packets belong to the same loss event.  The sender increases
    the window counter by one every quarter round-trip time.  This
    section describes in detail the procedure for using the window
    counter to determine when two lost packets belong to the same loss



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    event.

    RFC 3448 specifies that each data packet contains a timestamp, and
    gives as an alternative implementation a "timestamp" that is
    incremented every quarter of an RTT, as is the window counter in
    CCID 3.  However, the section in [RFC 3448] on "Translation from
    Loss History to Loss Events" is written in terms of timestamps, not
    in terms of window counters.  In this section, we give an procedure
    for the translation from loss history to loss events that is
    explicitly in terms of window counters.

    To determine whether two lost packets with sequence numbers X and Y
    belong to different loss events, the receiver proceeds as follows.
    Assume Y > X in circular sequence space.

    o  Let X_prev be the greatest valid sequence number received with
       X_prev < X.

    o  Let Y_prev be the greatest valid sequence number received with
       Y_prev < Y.

    o  Given a sequence number N, let C(N) be the window counter value
       associated with that packet.

    o  Packets X and Y belong to different loss events if there exists a
       packet with sequence number S so that X_prev < S <= Y_prev, and
       the distance from C(X_prev) to C(S) is greater than 4.  (The
       distance is the number D so that C(X_prev) + D = C(S) (mod
       WCTRMAX), where WCTRMAX is the maximum value for the window
       counter -- in our case, 16.)

       That is, the receiver only considers losses X and Y as separate
       loss events if there exists some packet S received between X and
       Y, with the distance from C(X_prev) to C(S) greater than 4.  This
       complex calculation is necessary to handle the case where window
       counter space wrapped completely between X and Y.  Generally, the
       receiver can simply check whether the distance from C(X_prev) to
       C(Y_prev) is greater than 4;  if so, then X and Y belong to
       separate loss events.

    Window counters can help the receiver to disambiguate multiple
    losses after a sudden decrease in the actual round-trip time.  When
    the sender receives an acknowledgement acknowledging a data packet
    with window counter i, the sender increases its window counter, if
    necessary, so that subsequent data packets are sent with window
    counter values of at least i+4.  This can help minimize errors on
    the part of the receiver of incorrectly interpreting multiple loss
    events as a single loss event.



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    We note that if all of the packets between X and Y are lost in the
    network, then X_prev and Y_prev are both set to X-1, and the series
    of consecutive losses is treated by the receiver as a single loss
    event.  However, the sender will receive no DCCP-Ack packets during
    a period of consecutive losses, and the sender will reduce its
    sending rate accordingly.

    As an alternative to the window counter, the sender could have sent
    its estimate of the round-trip time to the receiver directly in a
    round-trip time option; the receiver would use the sender's round-
    trip time estimate to infer when multiple lost or marked packets
    belong in the same loss event.  In some respects, a round-trip time
    option would give a more precise encoding of the sender's round-trip
    time estimate than does the window counter.  However, the window
    counter conveys information about the relative *sending* times for
    packets, while the receiver could only use the round-trip time
    option to distinguish between the relative *receive* times (in the
    absence of timestamps).  That is, the window counter will give more
    robust performance when there is a large variation in delay for
    packets sent within a window of data.  Slightly more speculatively,
    a round-trip time option might possibly be used more easily by
    middleboxes attempting to verify that a flow was using conformant
    end-to-end congestion control.

10.3.  Sending Feedback Packets

    In CCID 3, the window counter is used by the receiver to decide when
    to send feedback packets.  [RFC 3448] specifies that the TFRC
    receiver sends a feedback packet when the new loss event rate p is
    less that the old value.  This rule is followed by CCID 3.

    In addition, [RFC 3448] specifies that the receiver uses a feedback
    timer to decide when to send additional feedback packets.  If the
    feedback timer expires, and data packets have been received since
    the previous feedback was sent, then the receiver sends a feedback
    packet.  When the feedback timer expires, the receiver resets the
    timer to expire after R_m seconds, where R_m is the most recent
    estimate of the round-trip time received by the receiver from the
    sender.  This section describes how CCID 3 uses the window counter
    instead of the feedback timer to determine when to send additional
    feedback packets.

    Whenever the receiver sends a feedback message, the receiver sets a
    local variable last_counter to the greatest received value of the
    window counter since the last feedback message was sent, if any data
    packets have been received since the last feedback message was sent.
    If the receiver receives a data packet with a window counter value
    greater than or equal to last_counter + 4, then the receiver sends a



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    new feedback packet.  ("Greater" and "greatest" are measured in
    circular window counter space.)

    This procedure ensures that when the sender is sending less than one
    packet per round-trip time, then the receiver sends a feedback
    packet after each data packet.  Similarly, this procedure ensures
    that when the sender is sending several packets per round-trip time,
    then the receiver will send a feedback packet each time that a data
    packet arrives with a window counter more than four greater than the
    window counter when the last feedback packet was sent.  Thus, the
    feedback timer is not necessary when the window counter is used.

    However, the feedback timer still could be useful in some rare cases
    to prevent the sender from unnecessarily halving its sending rate.
    In particular, one could construct scenarios where the use of the
    feedback timer at the receiver would prevent the unnecessary
    expiration of the nofeedback timer at the sender.  Consider the case
    below, in which a feedback packet is sent when a data packet arrives
    with a window counter of K.

     Window
     Counters: K   K+1 K+2 K+3 K+4 K+5 K+6  ...  K+15 K+16 K+17 ...
               |   |   |   |   |   |   |         |    |    |
     Data      |   |   |   |   |   |   |         |    |    |
     Packets   |   |   |   |   |   |   |         |    |    |
     Received:   - -  ---  -                ...   - - -- -  -- --  -
                 |                |               |    |    |        |
                 |                |               |    |    |        |
     Events:     1:               2:              3:   4:   5:       6:
                "A"                              "B"  Timer "B"
                sent                             sent       received

          1:  Feedback message A is sent.
          2:  A feedback message would have been sent if feedback timers
              had been used.
          3:  Feedback message B is sent.
          4:  Sender's nofeedback timer expires.
          5:  Feedback message B is received at the sender.
          6:  Sender's nofeedback timer would have expired if feedback
              timers had been used, and the feedback message at 2 had
              been sent.

    The receiver receives data after the feedback packet has been sent,
    but has received no data packets with a window counter between K+4
    and K+14.  A data packet with a window counter of K+4 or larger
    would have triggered sending a new feedback packet, but no feedback
    packet is sent until time 3.




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    The TFRC protocol specifies that after a feedback packet is
    received, the sender sets a nofeedback timer to at least four times
    the round-trip time estimate.  If the sender doesn't receive any
    feedback packets before the nofeedback timer expires, then the
    sender halves its sending rate.  In the figure, the sender receives
    feedback message A (time 1), then sets the nofeedback timer to
    expire roughly four round-trip times later (time 4).  The sender
    starts sending again just before the nofeedback timer expires, but
    doesn't receive the resulting feedback message until after its
    expiration, resulting in an unnecessary halving of the sending rate.
    If the connection had used feedback timers, the receiver would have
    sent a feedback message when the feedback timer expired at time 2,
    and the halving of the sending rate would have been avoided.

    For implementors who wish to implement a feedback timer for the data
    receiver, we suggest estimating the round-trip time from the most
    recent data packet as described in Section 8.1.  We note that this
    procedure does not work when the inter-packet sending times are
    greater than the RTT.

11.  Security Considerations

    Security considerations for DCCP have been discussed in [DCCP], and
    security considerations for TFRC have been discussed in [RFC 3448].
    The security considerations for TFRC include the need to protect
    against spoofed feedback, and the need for protection mechanisms to
    protect the congestion control mechanisms against incorrect
    information from the receiver.

    In this document we have extensively discussed the mechanisms the
    sender can use to verify the information sent by the receiver.  As
    the document described, ECN may be used with CCID 3.  When ECN is
    used, the receiver must use either Ack Vector or Loss Intervals to
    return ECN Nonce information to the sender.  When ECN is not used,
    then, as Section 9 shows, the sender could still use various
    techniques that might catch the receiver in an error in reporting
    congestion, but this is not as robust or as non-intrusive as the
    verification provided by the ECN Nonce.

12.  IANA Considerations

    This specification defines the value 3 in the DCCP CCID namespace
    managed by IANA.  This assignment is also mentioned in [DCCP].
    CCID 3 also introduces three 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].




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12.1.  Reset Codes

    Each entry in the DCCP CCID 3 Reset Code registry contains a
    CCID 3-specific Reset Code, which is a number in the range 128-255;
    a short description of the Reset Code; and a reference to the RFC
    defining the Reset Code.  Reset Codes 184-190 and 248-254 are
    permanently reserved for experimental and testing use.  The
    remaining Reset Codes -- 128-183, 191-247, and 255 -- are currently
    reserved, and should be allocated with the IETF Consensus policy,
    which requires RFC publication (not necessarily standards-track).

12.2.  Option Types

    Each entry in the DCCP CCID 3 option type registry contains a
    CCID 3-specific option type, which is a number in the range 128-255;
    the name of the option, such as "Loss Intervals"; and a reference to
    the RFC defining the option type.  The registry is initially
    populated using the values in Table 1 (Section 8).  This document
    allocates option types 192-194, and option types 184-190 and 248-254
    are permanently reserved for experimental and testing use.  The
    remaining option types -- 128-183, 191, 195-247, and 255 -- are
    currently reserved, and should be allocated with the IETF Consensus
    policy, which requires RFC publication (not necessarily standards-
    track).

12.3.  Feature Numbers

    Each entry in the DCCP CCID 3 feature number registry contains a
    CCID 3-specific feature number, which is a number in the range
    128-255; the name of the feature, such as "Send Loss Event Rate";
    and a reference to the RFC defining the feature number.  The
    registry is initially populated using the values in Table 2 (Section
    8).  This document allocates feature number 192, and feature numbers
    184-190 and 248-254 are permanently reserved for experimental and
    testing use.  The remaining feature numbers -- 128-183, 191,
    193-247, and 255 -- are currently reserved, and should be allocated
    with the IETF Consensus policy, which requires RFC publication (not
    necessarily standards-track).

13.  Thanks

    We thank Mark Handley for his help in defining CCID 3.  We also
    thank Mark Allman, Aaron Falk, Ladan Gharai, Sara Karlberg, Greg
    Minshall, Arun Venkataramani, David Vos, Yufei Wang, Magnus
    Westerlund, and members of the DCCP Working Group for feedback on
    versions of this document.





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A.  Appendix: Possible Future Changes to CCID 3

    There are a number of cases where the behavior of TFRC as specified
    in [RFC 3448] does not match the desires of possible users of DCCP.
    These include the following:

    1.  The initial sending rate of at most four packets per RTT, as
        specified in [RFC 3390].

    2.  The receiver's sending of an acknowledgement for every data
        packet received, when the receiver receives less than one packet
        per round-trip time.

    3.  The sender's limitation of at most doubling the sending rate
        from one round-trip time to the next (or more specifically, of
        limiting the sending rate to at most twice the reported receive
        rate over the previous round-trip time).

    4.  The limitation of halving the allowed sending rate after an idle
        period of four round-trip times (possibly down to the initial
        sending rate of two to four packets per round-trip time).

    5.  Another change that is needed is to modify the response function
        used in [RFC 3448] to match more closely the behavior of TCP in
        environments with high packet drop rates [RFC 3714].

    One suggestion for higher initial sending rates is that of an
    initial sending rate of up to eight small packets per RTT, when the
    total packet size, including headers, is at most 4380 bytes.
    Because the packets would be rate-paced out over a round-trip time,
    instead of sent back-to-back as they would be in TCP, an initial
    sending rate of eight small packets per RTT with TFRC-based
    congestion control would be considerably milder than the impact of
    an initial window of eight small packets sent back-to-back in TCP.
    As Section 5.1 describes, the initial sending rate also serves as a
    lower bound for reductions of the allowed sending rate during an
    idle period.

    We note that with CCID 3, the sender is in slow-start in the
    beginning, and responds promptly to the report of a packet loss or
    mark.  However, in the absence of feedback from the receiver, the
    sender can maintain its old sending rate for up to four round-trip
    times.  One possibility would be that for an initial window of eight
    small packets, the initial nofeedback timer would be set to two
    round-trip times instead of four, so that the sending rate would be
    reduced after two round-trips without feedback.





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    Research and engineering will be needed to investigate the pros and
    cons of modifying these limitations in order to allow larger initial
    sending rates, to send fewer acknowledgements when the data sending
    rate is low, to allow more abrupt changes in the sending rate, or to
    allow a higher sending rate after an idle period.

Normative References

    [DCCP] E. Kohler, M. Handley, and S. Floyd.  Datagram Congestion
        Control Protocol, draft-ietf-dccp-spec-09.txt, work in progress,
        November 2004.

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

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

    [RFC 3390] M. Allman, S. Floyd, and C. Partridge.  Increasing TCP's
        Initial Window.  RFC 3390.

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

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

Informative References

    [CCID 2 PROFILE] S. Floyd and E. Kohler. Profile for DCCP Congestion
        Control ID 2: TCP-like Congestion Control, draft-ietf-dccp-
        ccid2-08.txt, work in progress, November 2004.

    [MAF04] A. Medina, M. Allman, and S. Floyd. Measuring Interactions
        Between Transport Protocols and Middleboxes. ACM SIGCOMM/USENIX
        Internet Measurement Conference, Sicily, Italy, October 2004.
        URL "http://www.icir.org/tbit/".

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



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    [RFC 3714] S. Floyd and J. Kempf, Editors.  IAB Concerns Regarding
        Congestion Control for Voice Traffic in the Internet.  RFC 3714.

    [V03] Arun Venkataramani, August 2003.  Citation for acknowledgement
        purposes only.

Authors' Addresses

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

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

    Jitendra Padhye <padhye@microsoft.com>
    Microsoft Research
    One Microsoft Way
    Redmond, WA 98052
    USA

Full Copyright Statement

    Copyright (C) The Internet Society 2004.  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
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    This document and the information contained herein are provided on
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Floyd/Kohler/Padhye                                            [Page 35]

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