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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                                           Jitendra Padhye
draft-ietf-dccp-ccid3-00.txt                          Microsoft Research
                                                             Sally Floyd
                                                            Eddie Kohler
                                                                    ICIR
                                                         23 October 2002
                                                     Expires: April 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
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    Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
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    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.

















































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                           Table of Contents


     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   4
      1.1. Usage Scenario . . . . . . . . . . . . . . . . . . .   4
      1.2. Example Half-Connection. . . . . . . . . . . . . . .   4
     2. Connection Establishment. . . . . . . . . . . . . . . .   5
     3. Congestion Control on Data Packets. . . . . . . . . . .   5
     4. Acknowledgments . . . . . . . . . . . . . . . . . . . .   6
      4.1. Congestion Control on Acknowledgments. . . . . . . .   6
      4.2. Quiescence . . . . . . . . . . . . . . . . . . . . .   6
      4.3. Acknowledgments of Acknowledgments . . . . . . . . .   7
     5. Explicit Congestion Notification. . . . . . . . . . . .   7
     6. Relevant Options and Features . . . . . . . . . . . . .   7
      6.1. Window counter option. . . . . . . . . . . . . . . .   7
      6.2. Elapsed time option. . . . . . . . . . . . . . . . .   8
      6.3. Loss Event Rate Option . . . . . . . . . . . . . . .   8
      6.4. Receive Rate Option. . . . . . . . . . . . . . . . .   8
      6.5. ECN Nonce Option . . . . . . . . . . . . . . . . . .   8
     7. Application Requirements. . . . . . . . . . . . . . . .   9
     8. Design Considerations . . . . . . . . . . . . . . . . .  10
      8.1. Determining Loss Events. . . . . . . . . . . . . . .  10
      8.2. Sending Feedback Packets . . . . . . . . . . . . . .  11
     9. Thanks. . . . . . . . . . . . . . . . . . . . . . . . .  12
     10. References . . . . . . . . . . . . . . . . . . . . . .  12
     11. Authors' Addresses . . . . . . . . . . . . . . . . . .  14

























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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 acknowledgments sent from DCCP B to
    DCCP A. DCCP A is the sending DCCP, and DCCP B the acknowledging
    DCCP, for this half-connection.)

    TFRC is a receiver-based congestion control mechanism that provides
    a TCP-friendly send rate, while minimizing abrupt rate changes [1].

    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 at least 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 [2], 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, while the receiver calculates the value of p.

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, taken from the main DCCP draft, is of a half-
    connection using TFRC Congestion Control specified by CCID 3.  The
    "sender" is the HC-Sender, and the "receiver" is the HC-Receiver.

    (1) The sender sends DCCP-Data packets, where the number of packets
        sent is governed by an allowed transmit rate, as specified in
        [1]. Each DCCP-Data packet has a sequence number and a window



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        counter option.

        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.

    (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 [1]. 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 [1].

    (4) The sender estimates round-trip times and calculates a TimeOut
        value TO as specified in [1].

    (5) 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.  Connection Establishment

    The connection is initiated by the client using mechanisms described
    in the DCCP specification [3]. The client and the server MAY
    negotiate the use of the ACK Vector option.  The ACK vector option
    is described in [3].

3.  Congestion Control on Data Packets

    The sender sends DCCP-Data packets to the receiver at the rate
    specified by the TCP throughput equation [2].

    Each DCCP-Data packet has a sequence number, and an acknowledgment
    number that is the sequence number of the most recent acknowledgment
    packet received from the receiver. Each data packet contains the
    window counter option. The format of the window counter option is
    described below.





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    After each feedback packet is received from the receiver, the sender
    updates values of RTT, TO and the sending rate using procedures
    specified in [1].

    If no feedback packet is received from the receiver after an
    interval specified in [1], the sending rate is halved. However, the
    sending rate is never reduced below one packet per 64 seconds. See
    [1] for more details.


4.  Acknowledgments

    The receiver sends a DCCP-Ack 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 [1].

    The acknowledgment number in the DCCP-Ack packet acknowledges the
    most recent packet received from the sender. Each DCCP-Ack packet
    from the receiver includes the following options:

        1. An option specifying the amount of time elapsed between since
        the receiver received the packet whose sequence number appears
        in the acknowledgment field.

        2. An option specifying the loss event rate p calculated by the
        receiver as described in [1].

        3. An option specifying the rate at which the receiver received
        data since the last DCCP-Ack was sent.

    The format of these options is described below.


4.1.  Congestion Control on Acknowledgments

    The rate and timing for generating acknowledgments is determined by
    the TFRC algorithm [1]. 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 6.1
    of [3]): How does a CCID 3 receiver determine that the corresponding
    sender is not sending any data?





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    The receiver detects that the sender has gone quiescent after two
    round-trip times have passed without receiving any additional data.
    Since ACKs are not required to be reliable, the receiver needs to do
    nothing special in this case, unlike CCID 2 [5].

4.3.  Acknowledgments of Acknowledgments

    Acknowledgments in TFRC are entirely unreliable -- TFRC works even
    if every acknowledgment is dropped -- and it is never necessary for
    the sender to acknowledge an acknowledgment.

5.  Explicit Congestion Notification

    ECN [6] MAY be used with CCID 3.  If ECN is used, then the ECN Nonce
    will automatically be used for the data packets, following the
    specification for the ECN Nonce [4] for TCP.  For the data sub-flow,
    the sender sets either the ECT[0] or ECT[1] codepoint on DCCP-Data
    packets.

    If the ACK vector option is being used, the ECN-NONCE information is
    returned via the ACK vector.

    If the ACK vector option is not being used, the information about
    the ECN-NONCE is returned by the receiver using the ECN-NONCE option
    described below. In this case the receiver MUST return this option
    if it is reporting a lower packet loss rate than the one it reported
    in the previous acknowledgment.


6.  Relevant Options and Features


6.1.  Window counter option


    +--------+--------+--------+
    |10000000|00000011|WinCount|
    +--------+--------+--------+
     Type=128   Len=3

    This option is set by the data sender on all data packets. The
    option data gives the value of a counter which the sender sets to 0
    at the beginning of the transmission, and increases by 1 every
    quarter of round trip time as described in [1].







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6.2.  Elapsed time option


    +--------+--------+--------+--------+
    |11000001|00000100|   Elapsed Time  |
    +--------+--------+--------+--------+
     Type=193   Len=4

    This option is set by the data receiver on all acknowledgment
    packets.  The option value is the amount of time (in milliseconds)
    elapsed since the packet being acknowledged was received.


6.3.  Loss Event Rate Option


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

    This option is set by the data receiver on all acknowledgment
    packets.  The option value indicates the inverse of the loss event
    rate, rounded UP, as calculated by the receiver.


6.4.  Receive Rate Option


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

    This option is set by the data receiver on all acknowledgment
    packets.  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 acknowledgment,
    in bits per second.


6.5.  ECN Nonce Option









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    +--------+--------+----...----+----...----+--------+
    |11000011|00001001| Left Edge | Right Edge|X0000000|
    +--------+--------+----...----+----...----+--------+
     Type=195   Len=9    3 bytes     3 bytes

    If ECN is used without the ACK vector option, then the ECN Nonce
    option is set by the data receiver on any acknowledgment packet that
    reports a loss rate lower than the loss rate reported in the
    previous acknowledgment packet.  The first byte gives the option
    type and the second gives the option length.  The right edge (RE)
    and the left edge (LE) are sequence numbers of data packets, such
    that:

        - Let LastAck be the sequence number of the data packet
        acknowledged by the previous acknowledgment.

        - If (LastAck + 1) was a dropped or marked packet, then RE
        should be the highest non-dropped and non-marked packet before
        (LastAck + 1).

        - If (LastAck + 1) was not a dropped or marked packet, the RE
        should be the greatest sequence number such that all data
        packets between (LastAck + 1) and  RE, inclusive, were received
        and not ECN-marked.  Clearly (RE >= LastAck + 1).

        - LE should be the smallest sequence number such that all data
        packets between LE and RE, inclusive, were received and not ECN-
        marked.  Clearly (LE <= RE).

    The first bit of the final byte is the Nonce Echo.  It equals the
    base-2 modulus of the number of received ECN Nonce packets between
    LE and RE, both included.

    Note that the interval [LE, RE] would be the largest non-loss
    interval containing the first packet received since the last report,
    or, if that was a dropped packet, containing the run before this
    drop.  That is, [LE, RE] would continue to grow during non-drop and
    non-mark periods.  Thus, for every loss event, the receiver reports
    the Nonce Echo for the consecutive sequence of packets received
    before the beginning of that loss event.


7.  Application Requirements

    As described in the TFRC specifications [1], this CCID should 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.




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    As it is presently specified, this CCID should only be used by
    senders that are willing to trust the receiver to report the correct
    loss event rate.  If ECN is used, the ECN Nonce Option allows the
    sender to probabilistically verify the loss rate reported by the
    receiver. However, we have not specified such a verification
    procedure in this document.


8.  Design Considerations

    The data packets do not carry timestamps. The sender can store the
    times at which recent packets were sent. When an acknowledgement
    arrives, the acknowledgement number and the elapsed time option
    provide sufficient information to compute the round trip time.


8.1.  Determining Loss Events

    The window counter option is used by the receiver to determine if
    multiple lost packets belong to the same loss event. The sender
    increases the window counter by 1 every quarter round trip time. To
    determine whether two lost packets, with sequence numbers X and Y (Y
    > X), belong to different loss events, the receiver proceeds as
    follows:

        - Let X_prev be the highest sequence number which was received
        with X_prev < X.

        - Let Y_prev be the highest sequence number which was received
        with Y_prev < Y.

        - Let CX_prev and CY_prev be the window counters associated with
        packets X_prev and Y_prev respectively. Clearly, CY_prev >=
        CX_prev.

        - Packets X and Y belong to different loss events if (CY_prev -
        CX_prev) > 4

    The use of the window counter option 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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    As an alternative to the window counter option, 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
    round-trip time estimate than does the window counter option.
    However, the window counter option 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 option 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.


8.2.  Sending Feedback Packets

    The window counter option 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 highest 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 last_counter + 4, then the receiver sends a new feedback
    packet.

    The TFRC protocol [1] 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 option is used, the
    receiver can use information from the window counter option 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



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    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.

    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.  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).


9.  Thanks

    We thank Mark Handley for his help in defining CCID 3.  We thank
    Sara Karlberg and Yufei Wang for feedback on an earlier version of
    this document.

10.  References


    [1] M. Handley, J. Padhye, and S. Floyd.  TCP Friendly Rate Control
        (TFRC): Protocol Specification, draft-ietf-tsvwg-tfrc-04.txt,
        work in progress, April 2002.


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




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    [3] E. Kohler, M. Handley, S. Floyd, and J. Padhye.  Datagram
        Congestion Control Protocol, draft-kohler-dcp-02.txt, work in
        progress, March 2002.


    [4] Neil Spring, David Wetherall, and David Ely.  Robust ECN
        Signaling with Nonces, draft-ietf-tsvwg-tcp-nonce-03.txt, work
        in progress, April 2002.


    [5] S. Floyd, E. Kohler. Profile for DCCP Congestion Control ID 2:
        TCP-like Congestion Control, draft-floyd-dcp-ccid2-03.txt, work
        in progress, May 2002.


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

































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11.  Authors' Addresses

    Jitendra Padhye <padhye@microsoft.com>

    Microsoft Research
    One Microsoft Way
    Redmond, WA 98052 USA

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

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





































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