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

Internet Engineering Task Force
INTERNET-DRAFT                                               Sally Floyd
draft-ietf-dccp-ccid3-03.txt                                Eddie Kohler
                                                                    ICIR
                                                         Jitendra Padhye
                                                      Microsoft Research
                                                            30 June 2003
                                                  Expires: December 2003


               Profile for DCCP Congestion Control ID 3:
                        TFRC Congestion Control



Status of this Document

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    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

                                Abstract


     This document contains the profile for Congestion Control
     Identifier 3, TCP-friendly rate control (TFRC), in the
     Datagram Congestion Control Protocol (DCCP).  DCCP implements
     a congestion-controlled unreliable datagram flow suitable for
     use by applications such as streaming media. The TFRC CCID is
     used by applications that want a TCP-friendly send rate,



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     possibly with Explicit Congestion Notification (ECN), while
     minimizing abrupt rate changes.

     TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

     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.

     * 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.




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     * 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 period."






























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                           Table of Contents


     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   5
      1.1. Usage Scenario . . . . . . . . . . . . . . . . . . .   6
      1.2. Example Half-Connection. . . . . . . . . . . . . . .   6
     2. Connection Establishment. . . . . . . . . . . . . . . .   7
     3. Congestion Control on Data Packets. . . . . . . . . . .   7
     4. Acknowledgements. . . . . . . . . . . . . . . . . . . .   7
      4.1. Congestion Control on Acknowledgements . . . . . . .   8
      4.2. Quiescence . . . . . . . . . . . . . . . . . . . . .   8
      4.3. Acknowledgements of Acknowledgements . . . . . . . .   8
     5. Explicit Congestion Notification. . . . . . . . . . . .   9
     6. Relevant Options and Features . . . . . . . . . . . . .   9
      6.1. Window Counter Value . . . . . . . . . . . . . . . .  10
      6.2. Elapsed Time Options . . . . . . . . . . . . . . . .  11
      6.3. Receive Rate Option. . . . . . . . . . . . . . . . .  11
      6.4. Use Loss Event Rate Feature. . . . . . . . . . . . .  11
      6.5. Loss Event Rate Option . . . . . . . . . . . . . . .  12
      6.6. Use Loss Intervals Feature . . . . . . . . . . . . .  12
      6.7. Loss Intervals Option. . . . . . . . . . . . . . . .  12
     7. Verifying Congestion Control Compliance With
     ECN. . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
      7.1. Verifying the ECN Nonce Echo . . . . . . . . . . . .  14
      7.2. Verifying the Reported Loss Event Rate . . . . . . .  15
     8. Application Requirements. . . . . . . . . . . . . . . .  16
     9. Design Considerations . . . . . . . . . . . . . . . . .  16
      9.1. Determining Loss Events at the Receiver. . . . . . .  16
      9.2. Sending Feedback Packets . . . . . . . . . . . . . .  18
      9.3. When Should Ack Vector And Loss Intervals Be
      Used? . . . . . . . . . . . . . . . . . . . . . . . . . .  19
      9.4. Packet Sizes . . . . . . . . . . . . . . . . . . . .  20
     10. Thanks . . . . . . . . . . . . . . . . . . . . . . . .  20
     11. Normative References . . . . . . . . . . . . . . . . .  20
     12. Informative References . . . . . . . . . . . . . . . .  21
     13. Security Considerations. . . . . . . . . . . . . . . .  21
     14. IANA Considerations. . . . . . . . . . . . . . . . . .  21
     15. Authors' Addresses . . . . . . . . . . . . . . . . . .  21













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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 uses Congestion Control Identifiers,
    or CCIDs, to specify the congestion control mechanism in use on a
    half-connection. (A half-connection might consist of data packets
    sent from DCCP A to DCCP B, plus acknowledgements sent from DCCP B
    to DCCP A. DCCP A is the HC-Sender, and DCCP B the HC-Receiver, for
    this half-connection. In this document, we abbreviate HC-Sender and
    HC-Receiver as "sender" and "receiver", respectively. These terms
    are defined more fully in [DCCP].)

    TFRC is a receiver-based congestion control mechanism that provides
    a TCP-friendly send rate, while minimizing abrupt rate changes [RFC
    3448].

    The basic TFRC protocol is as follows. The sender sends a stream of
    data packets to the receiver at some rate. The receiver sends a
    feedback packet to the sender roughly once every round-trip time.
    Based on the information contained in the feedback packets, the
    sender adjusts its sending rate in accordance with the TCP
    throughput equation [PFTK98], to maintain TCP-friendliness. If no
    feedback is received from the receiver in several round-trip times
    (four, in the current TFRC specification), the sender halves its
    sending rate.

    The values of the round-trip time RTT, the loss event rate p and the
    base timeout value TO are needed by the sender to calculate the send
    rate using the TCP throughput equation. The sender calculates the
    values of RTT and TO, and the receiver calculates the value of p.
    (If it prefers, the sender can also calculate p based on loss
    intervals provided by the receiver.)

    The congestion control mechanisms described here follow the TFRC
    mechanism standardized by the IETF. Conformant CCID 3
    implementations MAY track TFRC's evolution directly, as updates are
    standardized in the IETF, rather than waiting for revisions of this
    document.

    For simplicity, we occasionally refer to DCCP-Data packets sent by
    the sender and DCCP-Ack packets sent by the receiver. Both of these
    categories are meant to include DCCP-DataAck packets.

    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].




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1.1.  Usage Scenario

    DCCP with TFRC congestion control is intended to provide congestion
    control for the flow of data packets from the server to the client
    for applications that do not require fully reliable data
    transmission, or that desire to implement reliability on top of
    DCCP.  TFRC congestion control is appropriate for flows that would
    prefer to minimize abrupt changes in the sending rate.



1.2.  Example Half-Connection

    This example shows the typical progress of a half-connection using
    TFRC Congestion Control specified by CCID 3, not including
    connection initiation and termination.  Again, the "sender" is the
    HC-Sender, and the "receiver" is the HC-Receiver.  (The example is
    informative, not normative.)

    (1) The sender sends DCCP-Data packets, where the number of packets
        sent is governed by an 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.

        One or more of these data packets are DCCP-DataAck packets
        acknowledging the data packet from the receiver, but for
        simplicity we will not discuss the half-connection of data from
        the receiver to the sender in this example.

        If the use of ECN has been negotiated, 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 below.

    (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 RTT, as indicated
        by the TFRC specification [RFC 3448]. Each DCCP-Ack packet uses
        a sequence number and identifies the most recent packet received
        from the sender.  Each DCCP-Ack packet includes feedback about
        the loss event rate calculated by the receiver, as specified
        below.

    (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].




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    (4) The sender estimates round-trip times and calculates a TimeOut
        value TO as specified in [RFC 3448].

2.  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 Use Ack Vector, Use Loss Intervals, and Use Loss Event
    Rate features.

3.  Congestion Control on Data Packets

    The sender sends DCCP-Data packets to the receiver at the rate
    specified by the TCP throughput equation [PFTK98].

    Each DCCP-Data packet has a sequence number and, in the DCCP
    header's CCval field, a window counter value. The window counter is
    described below.

    After each feedback packet is received from the receiver, the sender
    updates values of RTT, TO and the sending rate using procedures
    specified in [RFC 3448].

    If no feedback packet is received from the receiver after an
    interval specified in [RFC 3448], the sending rate is halved.
    However, the sending rate is never reduced below one packet per 64
    seconds. See [RFC 3448] for more details.  [RFC 3448] separately
    specifies the minimum sending rate from rate reductions during an
    idle period.

4.  Acknowledgements

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

    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.



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

    (3) One or more options concerning the loss event rate p experienced
        by the receiver, as described in [RFC 3448]. Relevant options
        include Loss Event Rate, which simply gives the loss event rate
        calculated by the receiver; Loss Intervals, which specifies the
        beginning and end of each loss interval, from which the sender
        can easily calculate and/or verify the loss event rate; and Ack
        Vector, which says exactly which packets were lost or marked,
        again allowing the sender to calculate and/or verify the loss
        event rate.

    The format of these options is described below (except Ack Vector,
    Timestamp Echo, and Elapsed Time, which are described in [DCCP]).

    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.

4.1.  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, and there is no explicit congestion control on
    the acknowledgements.

4.2.  Quiescence

    This section refers to quiescence in the DCCP sense (see section 8.1
    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.
    The receiver detects that the sender has gone quiescent after T
    seconds have passed without receiving any additional data from the
    sender.

4.3.  Acknowledgements of Acknowledgements

    TFRC acknowledgements are not generally required 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 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-



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    connection goes quiescent, however, DCCP A must do it proactively.

    When Ack Vector is used, therefore, an active sender MUST
    occasionally acknowledge the receiver's acknowledgements, probably
    by encapsulating a datagram in a DCCP-DataAck packet. No
    acknowledgement options are necessary, just the relevant
    Acknowledgement Number in the DCCP-DataAck header. Such
    acknowledgements should be sent approximately once per round-trip
    time, within a factor of two or three.

    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.

5.  Explicit Congestion Notification

    ECN [RFC 3168] MAY be used with CCID 3.  If ECN is enabled, then the
    ECN Nonce will automatically be used following the specification for
    the ECN Nonce for TCP [ECN NONCE]. For the data sub-flow, the sender
    sets either the ECT[0] or ECT[1] codepoint on DCCP-Data packets.

    If ECN is used, then the receiver MUST use at least one of Ack
    Vector and Loss Intervals to return ECN Nonce information to the
    sender.

    If the Ack Vector option is being used, the ECN nonce sum is
    returned in DCCP-Ack packets, as described in [CCID 2 PROFILE]. The
    sender can maintain a table with the ECN nonce sum for each packet,
    and use this information to probabilistically verify the ECN nonce
    sum returned in each DCCP-Ack packet.

    If the Ack Vector option is not being used, the information about
    the ECN Nonce is returned by the receiver using the Loss Intervals
    option described below. The receiver MUST include this option on
    every required acknowledgement.

6.  Relevant Options and Features

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

    The use of Ack Vector, Loss Intervals, and Loss Event Rate are
    controlled by separate features, but only some combinations of these
    features make sense. In particular, if ECN Capable is true, then



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    every required acknowledgement MUST include at least one of Ack
    Vector and Loss Intervals; otherwise, every required acknowledgement
    MUST include at least one of Ack Vector, Loss Intervals, and Loss
    Event Rate. This may impel the receiver to send certain options even
    when their corresponding Use features are false.  A sender that
    receives several invalid acknowledgements---that include only Loss
    Event Rate on an ECN-capable connection, for example---MAY respond
    by resetting the connection with Reason set to "Option Error".

6.1.  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]):

     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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Type  | CCval |              Sequence Number                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Data Offset  | # NDP | Cslen |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    The CCval field has enough space to express 4 round-trip times at
    quarter-RTT granularity. The sender SHOULD try to 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. For example, the sender MAY 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.




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    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.

6.2.  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. The elapsed time value MUST be included in one
    of 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) Otherwise, the receiver MUST include an Elapsed Time option.

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

6.3.  Receive Rate Option


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

    This option MUST be sent by the data receiver on all required
    acknowledgements.  The first byte gives the option type and the
    second gives the option length.  The last four bytes indicate the
    rate at which the receiver has received data since it last sent an
    acknowledgement, in bits per second.

6.4.  Use Loss Event Rate Feature

    The Use Loss Event Rate feature lets CCID 3 endpoints negotiate
    whether the receiver MUST provide Loss Event Rate options on its
    acknowledgements.

    Use Loss Event Rate has feature number 192. The Use Loss Event Rate
    feature located at DCCP B specifies whether DCCP B MUST send Loss
    Event Rate options on its acknowledgements, although DCCP B MAY send



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    Loss Event Rate options even if Use Loss Event Rate is false. DCCP A
    sends a "Change(Use Loss Event Rate, 1)" option to ask DCCP B to
    send Loss Event Rate options as part of its acknowledgement traffic.

    Use Loss Event Rate feature values are a single byte long. The
    receiver MUST send Loss Event Rate options if this byte is nonzero.
    A CCID 3 half-connection starts with Use Loss Event Rate unknown.

6.5.  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.

6.6.  Use Loss Intervals Feature

    The Use Loss Intervals feature lets CCID 3 endpoints negotiate
    whether the receiver MUST provide Loss Intervals options on its
    acknowledgements.

    Use Loss Intervals has feature number 195. The Use Loss Intervals
    feature located at DCCP B specifies whether DCCP B MUST send Loss
    Intervals options on its acknowledgements, although DCCP B MAY send
    Loss Intervals options even if Use Loss Intervals is false. DCCP A
    sends a "Change(Use Loss Intervals, 1)" option to ask DCCP B to send
    Loss Intervals options as part of its acknowledgement traffic.

    Use Loss Intervals feature values are a single byte long. The
    receiver MUST send Loss Intervals options if this byte is nonzero. A
    CCID 3 half-connection starts with Use Loss Intervals unknown.

6.7.  Loss Intervals Option


                        ___ Loss Interval ___
                       /                     \
    +--------+--------+----...----+----...----+--------+--------+--------
    |11000011| Length | Left Edge |E|  Offset | Up to 7 Loss Intervals ...
    +--------+--------+----...----+----...----+--------+--------+--------
     Type=195            3 bytes     3 bytes

    This option MAY be set by the data receiver on acknowledgements. (If



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    ECN is enabled and Ack Vector is off, or if the Use Loss Intervals
    feature is true, it MUST be sent with every required
    acknowledgement.)  The option reports up to 8 loss intervals seen by
    the receiver.  As described in [RFC 3448], 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.  In addition, as specified in [RFC 3448], a loss
    interval continues for at least one round trip time; a lost or
    marked packet starts a new loss interval only if it was sent at
    least one round trip time after the start of the previous loss
    interval.  The Loss Event Rate, reported by option 192, is the
    weighted average of the last 8 loss interval lengths, inverted.

    The Loss Intervals option contains information about between one and
    eight consecutive loss intervals, always including the most recent
    loss interval.  Intervals are listed in reverse chronological order.
    The option MUST contain information about the most recent 8 loss
    intervals unless (1) there have not yet been 8 loss intervals, in
    which case the receiver SHOULD send information about all the loss
    intervals it has experienced; or (2) the receiver knows, because of
    acknowledgements from the sender, that information about older loss
    intervals has been received by the sender, in which case the
    receiver MUST send at least information about the loss intervals the
    sender has not acknowledged. In any case, the Loss Intervals option
    MUST contain the most recent loss interval.

    Each Loss Interval structure consists of a Left Edge, an Offset, and
    an ECN Nonce Echo (E). Left Edge, a 24-bit DCCP sequence number,
    specifies the first sequence number in the interval's loss- and
    mark-free tail. Offset, a 23-bit number, specifies the number of
    packets in that loss- and mark-free tail. The ECN Nonce Echo, stored
    in the high-order bit of the 3-byte field containing Offset, equals
    the one-bit sum (exclusive-or, or parity) of nonces received over
    the range of packets [Left Edge, Left Edge + Offset).  If Offset is
    0, or if the receiver is ECN-incapable, the ECN Nonce Echo SHOULD be
    reported as 0.

    Note that each Loss Interval structure explicitly specifies when the
    loss interval in question ends (that is, at Left Edge + Offset), but
    not when it began. That quantity equals the Left Edge + Offset of
    the chronologically preceding loss interval. Furthermore, the most
    recent Loss Interval's Left Edge + Offset need not equal the
    Acknowledgement Number. As Section 5.1 of [RFC 3448] says, a lost
    packet doesn't begin a new loss interval until 3 packets have been
    seen after the "hole". Acknowledgements sent in the meantime will
    acknowledge some sequence number larger than the "hole", but the
    most recent Loss Interval's Left Edge + Offset will equal the



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    sequence number of the "hole".

    The Loss Intervals option serves several purposes.

    o The sender can use the Loss Intervals to easily calculate the Loss
      Event Rate, perhaps using a later version of the TFRC algorithm
      than that deployed at the receiver.

    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.

7.  Verifying Congestion Control Compliance With ECN

    If ECN is used, the sender can use Ack Vector or the Loss Intervals
    option to probabilistically verify that the receiver is not lying in
    reporting packets received undropped and unmarked.  The sender could
    then use the information in acknowledgement packets to roughly
    verify the Loss Event Rate reported by the receiver, if it so
    desired.

    We note that if ECN is not used, 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 Ack Vector or
    Loss Intervals information.  Similarly, the sender would still use
    the Ack Vector or Loss Intervals information to verify the loss
    event rate reported by the receiver.  However, this is not as robust
    or as non-intrusive as the verification provided by the ECN Nonce.

7.1.  Verifying the ECN Nonce Echo

    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 (exclusive-or, or parity), and compares
    the result to the Nonce Echo encoded in the Ack Vector's option
    type.

    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 [ECN NONCE], the nonce sum for sequence
    number S is the one-bit sum 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 -



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    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).


    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
    straightforward nonce-based calculation.

    In either of these cases, 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.

7.2.  Verifying the Reported Loss Event Rate

    Once the sender has probabilistically verified the ECN Nonce Echoes
    reported by the receiver, the sender can calculate for itself the
    number of packets in each loss interval, to roughly verify the loss
    event rate reported by the receiver, if it so desires.  We note that
    DCCP's Loss Event Rate Option reports the average loss interval
    size, which is the inverse of the loss event rate.

    If the Ack Vector is used, the sender can identify the packet that
    begins each new loss interval from the Ack Vector in each DCCP-Ack
    packet.  If the sender saves information about the window counter
    for each data packet, then the sender also can tell when two lost or
    marked packets would have been interpreted by the receiver as
    separate loss events.

    The Loss Intervals option explicitly reports the size of each loss
    interval, as seen by the receiver. The sender can, using saved
    information about window counters, verify that the receiver is not
    falsely combining two loss events into one reported loss interval.

    Once the sender has reconstructed or verified Loss Intervals, it can
    easily calculate the expected loss event rate, and compare against
    the receiver's reported loss event rate.

    We note that in some cases the loss event rate calculated by the
    sender could differ from that calculated 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



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    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.

8.  Application Requirements

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

    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 when the application has 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.

    As described in the TFRC specifications [RFC 3448], this CCID 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.

9.  Design Considerations

    CCID 3 data packets need not carry Timestamp options. The sender can
    store the times at which recent packets were sent. Then the
    Acknowledgement Number and Elapsed Time option contained on each
    required acknowledgement 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.

9.1.  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



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    window counter by 1 every quarter round trip time. To determine
    whether two lost packets, with sequence numbers X and Y (Y > X in
    circular sequence space), belong to different loss events, the
    receiver proceeds as follows:

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

    o Let Y_prev be the greatest sequence number which was 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.)

      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.

    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.

    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, and the receiver should 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 gives a more precise encoding of the sender's



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    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 in some cases when there is a large
    variation in delay for packets sent within a window of data.  As a
    slightly more speculative consideration, the round-trip time option
    could possibly be used more easily by middleboxes attempting to
    verify that a flow was using conformant end-to-end congestion
    control.

9.2.  Sending Feedback Packets

    The window counter is also used by the receiver to decide when to
    send feedback packets.  Feedback packets should normally be sent at
    least once per round-trip time, if the sender is sending at least
    one data packet per round-trip time.  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 new feedback packet.
    ("Greater" and "greatest" are measured in circular window counter
    space.)

    The TFRC protocol [RFC 3448] specifies that the receiver uses a
    feedback timer to decide when to send feedback packets.  In the TFRC
    protocol, 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.  However, when the window counter is used, the receiver can
    use its information in deciding when to send feedback packets.

    When the sender is sending less than one packet per round-trip time,
    then the receiver sends a feedback packet after each data packet,
    and the feedback timer is not required.  Similarly, 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, and again the feedback
    counter is not required.  Similarly, the receiver always sends a
    feedback packet after the detection of a loss event.  Thus, the
    feedback timer is not absolutely necessary when the window counter
    is used.





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    However, the feedback timer still could be useful in some rare cases
    to prevent the sender from unnecessarily halving its sending rate.
    Consider the case when the receiver receives data soon after the
    most recent feedback packet has been sent, but has received no data
    packets with a window counter sufficiently large to trigger sending
    a new feedback packet.  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.  One could
    construct scenarios where the use of a feedback timer at the
    receiver would prevent the unnecessary expiration of the nofeedback
    timer at the sender.

    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 follows: Let K be the window counter from the
    most recent data packet, and let T_k be the time that that packet
    was received, as in the table below.  Let J be the highest window
    counter received that was less than K-4, and let T_j be the most
    recent time that such a packet was received.  Then the round-trip
    time can be very roughly estimated as 4*(T_k-T_j)/(K-J).
      Time  |           Event                 |   Window Counter
     -----------------------------------------------------------
       T_j  |  packet received with WC < K-4  |   J   (J<K-4)
       T_k  |  most recent packet received    |   K

9.3.  When Should Ack Vector And Loss Intervals Be Used?

    If the use of ECN has not been negotiated, then the receiver is not
    required to use either Ack Vector or Loss Intervals.  Essentially,
    in this case the sender is completely relying on the Loss Event Rate
    reported by the receiver.  If the Ack Vector or Loss Intervals is
    used, however, then the sender could test that the receiver is
    correctly reporting dropped and marked packets by conducting a test
    and skipping a packet in its transmissions.

    In the common case, it is assumed that the use of ECN will be
    negotiated with CCID 3.  However, it is possible that either the
    sender or the receiver will want to negotiate the use of CCID 3
    without ECN, e.g., if there happens to be a known broken middlebox
    along the path that blocks the use of ECN in the IP packet header.

    If ECN is used, then the receiver is required to use at least one of
    Ack Vector and Loss Intervals to return ECN Nonce information to the
    sender.  The Ack Vector returns more information about which packets
    were lost or marked during a loss event.  The sender uses more
    computation and state for verifying receiver feedback with the Ack



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    Vector than with Loss Intervals, because then it must reconstruct
    loss intervals from the Ack Vector.  The Ack Vector also requires
    that the sender occasionally acknowledge the receiver's
    acknowledgements; this is optional with Loss Intervals.

9.4.  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.

10.  Thanks

    We thank Mark Handley for his help in defining CCID 3.  We thank
    Sara Karlberg, Arun Venkataramani, and Yufei Wang for feedback on
    earlier versions of this document.

11.  Normative References

    [CCID 2 PROFILE] S. Floyd and E. Kohler. Profile for DCCP Congestion
        Control ID 2: TCP-like Congestion Control, draft-ietf-dccp-
        ccid2-01.txt, work in progress, March 2003.

    [DCCP] E. Kohler, M. Handley, S. Floyd, and J. Padhye.  Datagram
        Congestion Control Protocol, draft-ietf-dccp-spec-01.txt, work
        in progress, March 2003.

    [ECN NONCE] Neil Spring, David Wetherall, and David Ely.  Robust ECN
        Signaling with Nonces, draft-ietf-tsvwg-tcp-nonce-04.txt, work
        in progress, October 2002.

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

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

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





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12.  Informative References

    [PFTK98] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose.  Modeling
        TCP Throughput: A Simple Model and its Empirical Validation.
        Proc ACM SIGCOMM 1998.

13.  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.

14.  IANA Considerations

    This section will contain the namespaces that have been created in
    this specification, and the values assigned in existing namespaces
    managed by IANA.

    This will include the following: The Receive Rate, Loss Event Rate,
    and Loss Intervals Options; the Use Loss Event Rate and Use Loss
    Intervals features.

15.  Authors' Addresses

    Sally Floyd <floyd@icir.org>
    Eddie Kohler <kohler@icir.org>

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

    Jitendra Padhye <padhye@microsoft.com>

    Microsoft Research
    One Microsoft Way
    Redmond, WA 98052 USA









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