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Internet Engineering Task Force                              Sally Floyd
INTERNET-DRAFT                                              Eddie Kohler
draft-ietf-dccp-ccid2-04.txt                                        ICIR
Expires: April 2004                                      27 October 2003


               Profile for DCCP Congestion Control ID 2:
                      TCP-like Congestion Control


Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of [RFC 2026].

    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
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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
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Copyright Notice

    Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

    This document contains the profile for Congestion Control Identifier
    2, TCP-like Congestion Control, in the Datagram Congestion Control
    Protocol (DCCP).  CCID 2 should be used by senders who would like to
    take advantage of the available bandwidth in an environment with
    rapidly changing conditions, and who are able to adapt to the abrupt
    changes in the congestion window typical of TCP's Additive Increase
    Multiplicative Decrease (AIMD) congestion control.




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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

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

    * Disallow direct tracking of TCP standards.

    Changes from draft-ietf-dccp-ccid2-02.txt:

    * Added to the section on application requirements.

    * Changed the default Ack Ratio to be two, as recommended for TCP.

    * Added a paragraph about packet sizes.

    Changes from draft-ietf-dccp-ccid2-01.txt:

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

    * Refer explicitly to SACK-based TCP, and flesh out Section 3
    ("Congestion Control on Data Packets").

    * When cwnd < ssthresh, increase cwnd by one per newly acknowledged
    packet up to some limit, in line with TCP Appropriate Byte Counting.

    * Refined definition of quiescence.

    Changes from draft-ietf-dccp-ccid2-00.txt:

    * Said that the Acknowledgement Number reports the largest sequence
    number, not the most recent packet, for consistency with draft-ietf-
    dccp-spec.

    * Added notes about ECN nonces for acknowledgements, and about
    dealing with piggybacked acknowledgements.
















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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   4
    2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
    3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1. Example Half-Connection. . . . . . . . . . . . . . . . .   5
       3.2. Updates. . . . . . . . . . . . . . . . . . . . . . . . .   6
    4. Connection Establishment. . . . . . . . . . . . . . . . . . .   7
    5. Congestion Control on Data Packets. . . . . . . . . . . . . .   7
       5.1. Response to Data Dropped . . . . . . . . . . . . . . . .   9
       5.2. Packet Size. . . . . . . . . . . . . . . . . . . . . . .   9
    6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .   9
       6.1. Congestion Control on Acknowledgements . . . . . . . . .  10
          6.1.1. Sending Acknowledgements. . . . . . . . . . . . . .  10
          6.1.2. Setting Ack Ratio . . . . . . . . . . . . . . . . .  11
          6.1.3. Derivation of Ack Ratio Decrease. . . . . . . . . .  12
       6.2. Quiescence . . . . . . . . . . . . . . . . . . . . . . .  12
       6.3. Acknowledgements of Acknowledgements . . . . . . . . . .  13
    7. Explicit Congestion Notification. . . . . . . . . . . . . . .  13
    8. Relevant Options and Features . . . . . . . . . . . . . . . .  14
    9. Security Considerations . . . . . . . . . . . . . . . . . . .  14
    10. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  14
    11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
    Normative References . . . . . . . . . . . . . . . . . . . . . .  14
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  15


























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1.  Introduction

    This document contains the profile for Congestion Control Identifier
    2, TCP-like Congestion Control, 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].)

    The TCP-like Congestion Control CCID sends data using a close
    variant of TCP's congestion control mechanisms, particularly SACK-
    based TCP's congestion control mechanisms [RFC 3517]. It is suitable
    for senders who can adapt to the abrupt changes in congestion window
    typical of AIMD (Additive Increase Multiplicative Decrease)
    congestion control in TCP, and particularly useful for senders who
    would like to take advantage of the available bandwidth in an
    environment with rapidly changing conditions.  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].

    For simplicity, we 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 piggybacked DCCP-DataAck packets.

3.  Usage

    TCP-like Congestion Control is intended to provide congestion
    control for applications that do not require fully reliable data
    transmission, or that desire to implement reliability on top of
    DCCP.  It is appropriate for flows that would like to receive as
    much bandwidth as possible over the long term, consistent with the
    use of end-to-end congestion control, and that are willing to
    undergo halving of the congestion window in response to a congestion
    event.

    While CCID 3 is appropriate for flows that would prefer to minimize
    abrupt changes in the sending rate, CCID 2 is recommended for
    applications that simply need to transfer as much data as possible
    in as short a time.  For example, CCID 2 is recommended over CCID 3



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    for streaming media applications that buffer a considerable amount
    of data at the application receiver before playback time, insulating
    the application somewhat from abrupt changes in the sending rate.
    Such applications could easily choose DCCP's CCID 2 over TCP itself,
    possibly adding some form of selective reliability at the
    application layer.  CCID 2 is also recommended over CCID 3 for
    applications where the halving of the sending rate in response to
    congestion is not likely to interfere with application-level
    performance.

    An additional advantage of CCID 2 is that its TCP-like congestion
    control mechanisms are reasonably well-understood, with traffic
    dynamics quite similar to those of TCP.  While the network research
    community is still learning about the dynamics of TCP after 15 years
    of TCP congestion control as the dominant transport protocol in the
    Internet, some applications might prefer the more well-known
    dynamics of TCP-like congestion control over that of newer
    congestion control mechanisms that have not yet met the test of
    widespread deployment in the Internet.

3.1.  Example Half-Connection

    This example shows the typical progress of a half-connection using
    TCP-like Congestion Control specified by CCID 2, 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 a congestion window, cwnd, as in TCP.  Each
        DCCP-Data packet uses a sequence number.  The sender also sends
        an Ack Ratio feature option specifying the number of data
        packets to be covered by an Ack packet from the receiver.

        Assuming that the half-connection is ECN capable (the ECN
        Capable feature is turned on---the default), each DCCP-Data
        packet is sent as ECN-Capable with either the ECT(0) or the
        ECT(1) codepoint set, as described in [ECN NONCE].

    (2) The receiver sends a DCCP-Ack packet acknowledging the data
        packets for every Ack Ratio data packets transmitted by the
        sender.  Each DCCP-Ack packet uses a sequence number and
        contains an Ack Vector.  The sequence number acknowledged in
        DCCP-Ack packets is that of the received packet with the highest
        sequence number, rather than a TCP-like cumulative
        acknowledgement.





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        If the half-connection is ECN capable, the receiver returns the
        sum of received ECN Nonces via Ack Vector options, allowing the
        sender to probabilistically verify that the receiver is not
        misbehaving.  DCCP-Ack packets from the receiver are also sent
        as ECN-Capable, since the sender will control the
        acknowledgement rate in a roughly TCP-friendly way using the Ack
        Ratio feature.  There is little need for the receiver to verify
        the nonces on its Ack packets, since the sender cannot get
        significant benefit from misreporting the ack mark rate.

    (3) The sender continues sending DCCP-Data packets as controlled by
        the congestion window.  Upon receiving DCCP-Ack packets, the
        sender examines their Ack Vectors to learn about marked or
        dropped data packets, and adjusts its congestion window
        accordingly.  Because this is unreliable transfer, the sender
        does not retransmit dropped packets.

    (4) Because DCCP-Ack packets use sequence numbers, the sender has
        direct information about the fraction of lost or marked DCCP-Ack
        packets.  The sender responds to lost or marked DCCP-Ack packets
        by modifying the Ack Ratio sent to the receiver.

    (5) The sender acknowledges the receiver's acknowledgements at least
        once per congestion window.  If both half-connections are
        active, the sender's acknowledgement of the receiver's
        acknowledgements is included in the sender's acknowledgement of
        the receiver's data packets.  If the reverse-path half-
        connection is quiescent, the sender sends a DCCP-DataAck packet
        that includes an Acknowledgement Number in the header.

    (6) The sender estimates round-trip times, either through keeping
        track of acknowledgement round-trip times as TCP does or through
        explicit Timestamp options, and calculates a TimeOut (TO) value
        much as the RTO (Retransmit Timeout) is calculated in TCP.  The
        TO is used to determine when a new DCCP-Data packet can be
        transmitted when the sender has been limited by the congestion
        window and no feedback has been received from the receiver.

3.2.  Updates

    The congestion control mechanisms described here closely follow
    mechanisms standardized by the IETF for use in SACK-based TCP, and
    we rely partially on existing TCP documentation, such as [RFC 793],
    [RFC 3465], and [RFC 3517]. TCP congestion control continues to
    evolve, but conformant CCID 2 implementations SHOULD wait for
    explicit updates to CCID 2, rather than tracking TCP's evolution
    directly.  The differences between CCID 2 and straight TCP include:
    CCID 2 defines an additional mechanism not currently standardized



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    for use in TCP, namely congestion control on acknowledgements as
    achieved by the Ack Ratio.  DCCP is a datagram protocol, so several
    parameters whose units are bytes in TCP, such as the congestion
    window cwnd, have units of packets in DCCP.  Unreliability also
    leads to differences from TCP: DCCP never retransmits a packet, so
    congestion control mechanisms that distinguish retransmissions from
    new packets need rethinking in the DCCP context.

4.  Connection Establishment

    Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so
    the sender MUST send a "Change R(Use Ack Vector, 1)" option to the
    receiver as part of connection establishment.  The sender SHOULD NOT
    send data until it has received the corresponding "Confirm L(Use Ack
    Vector, 1)" from the receiver, except for possible data included on
    the initial DCCP-Request packet.

5.  Congestion Control on Data Packets

    CCID 2's congestion control mechanisms are based on those for SACK-
    based TCP [RFC 3517], since the Ack Vector provides strictly more
    information than that transmitted in SACK options.

    A CCID 2 data sender maintains three integer parameters.  All of
    their units are packets, not bytes; for example, CCID 2 expresses
    its window in terms of how many packets may be sent.

    (1) The congestion window "cwnd", which equals the maximum number of
        data-carrying packets allowed in the network at any time.
        ("Data-carrying packet" means any DCCP packet that contains user
        data: DCCP-Data, DCCP-DataAck, and occasionally DCCP-Request,
        DCCP-Response, and DCCP-Move.)

    (2) The slow-start threshold "ssthresh", which controls adjustments
        to cwnd.

        When halved, cwnd and ssthresh have their values rounded down,
        except that neither parameter is ever less than one.

    (3) The pipe value "pipe", which is the sender's estimate of the
        number of data-carrying packets outstanding in the network.

    These parameters are manipulated, and their initial values
    determined, according to SACK-based TCP's behavior.  The rest of
    this section provides more specific guidance.

    The sender MAY send a data-carrying packet only when pipe < cwnd.
    In particular, it MUST NOT send a data-carrying packet when pipe >=



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    cwnd.  Every data-carrying packet sent increases pipe by 1.

    The sender reduces pipe as it infers that data-carrying packets have
    left the network, either by being received or by being dropped.  In
    particular:

    (1) The sender reduces pipe by 1 for each packet newly-acknowledged
        as received (Ack Vector State 0 or State 1) by some DCCP-Ack.

    (2) The sender reduces pipe by 1 for each packet it can infer as
        lost due to the DCCP equivalent of TCP's "duplicate
        acknowledgements".  This depends on TCP's NUMDUPACK parameter,
        the number of duplicate acknowledgements TCP needs to infer a
        loss, which currently equals 3.  A packet P is inferred to be
        lost, rather than delayed, when at least NUMDUPACK packets after
        P have been acknowledged as received (Ack Vector State 0 or 1)
        by the receiver.  Note that these acknowledgements are not
        duplicates, and that the acknowledged packets might include
        DCCP-Ack packets.

    (3) Finally, the sender needs "retransmit" timeouts, handled like
        TCP's retransmission timeouts, in case an entire window of
        packets are lost.  The sender estimates the round-trip time at
        most once per window of data, and uses the TCP algorithms for
        maintaining the average round-trip time, mean deviation, and
        timeout value.  Because DCCP does not retransmit data, DCCP does
        not require TCP's recommended minimum timeout of one second.
        The exponential backoff of the timer is exactly as in TCP.

        When a "retransmit" timeout occurs, the sender sets pipe to 0.

    The sender MUST NOT decrement pipe more than once for any given
    packet.  True duplicate acknowledgements, for example, MUST not
    affect pipe.  Furthermore, the sender MUST NOT decrement pipe for
    non-data packets, such as DCCP-Acks, even though the Ack Vector will
    contain information about them.

    Congestion events, namely one or more packets lost or marked from a
    window of data, cause CCID 2 to reduce its congestion window.  For
    each congestion event, either indicated explicitly as an Ack Vector
    State 1 (ECN-marked) acknowledgement or inferred via "duplicate
    acknowledgements", cwnd is halved, then ssthresh is set to the new
    cwnd.  Cwnd is never reduced below one packet.  After a timeout, the
    slow-start threshold is set to cwnd/2, then cwnd is set to one
    packet.

    When cwnd < ssthresh, meaning that the sender is in slow-start, the
    congestion window is increased by one packet for every newly



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    acknowledged (with Ack Vector State 0 or 1) data-carrying packet, up
    to a maximum of Ack Ratio packets per acknowledgement.  This differs
    from TCP's historical behavior, which (in DCCP terms) would increase
    cwnd by one per DCCP-Ack received, not by one per packet newly
    acknowledged by some DCCP-Ack; but it is in line with TCP's behavior
    with appropriate byte counting [RFC 3465]. When cwnd >= ssthresh,
    the congestion window is increased by one packet for every window of
    data acknowledged without lost or marked packets.

5.1.  Response to Data Dropped

    CCID 2 senders respond to packets acknowledged as Data Dropped as
    described in [DCCP], with the following further clarifications.

    o Drop Code 2 ("receive buffer drop").  The congestion window "cwnd"
      is reduced by one for each packet newly acknowledged as Drop Code
      2, except that it is never reduced below one.

5.2.  Packet Size

    CCID 2 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 2 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 2 that vary their packet size not in response to congestion,
    but in response to other application-level requirements.

    CCID 2 enforces a maximum packet size of 1500 bytes on applications.
    Thus, in CCID 2, DCCP's CCMPS parameter equals 1500.  This reduces
    the possible damage from applications that would try to evade
    congestion control by increasing the data sent per packet during a
    congestion event.

6.  Acknowledgements

    This section describes how the receiver reports acknowledgement
    information back to the sender.  DCCP-Ack packets from the receiver
    MUST include Ack Vector options, as well as an Acknowledgement
    Number acknowledging the packet with the largest valid sequence
    number received from the sender.  Acknowledgement data in the Ack
    Vector options SHOULD generally cover the receiver's entire
    Acknowledgement Window, as described in [DCCP].







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6.1.  Congestion Control on Acknowledgements

    The acknowledgement subflow is loosely congestion-controlled by an
    Ack Ratio specified by the sender.  The receiver sends approximately
    (cwnd / Ack Ratio) acknowledgement packets for each congestion
    window of data packets.  When the acknowledgement stream is
    congested, the sender will increase the receiver's Ack Ratio,
    limiting its acknowledgement rate.  This differs from TCP, which
    presently has no congestion control for pure acknowledgement
    traffic.

    In the absence of congestion on the ack stream, CCID 2
    acknowledgements will be sent in roughly the same way as TCP
    acknowledgements.  For instance, the Ack Ratio will be set to 2,
    leading to behavior like TCP's delayed acks.  When the ack stream is
    congested, CCID 2 does not try to be TCP-friendly, but just tries to
    avoid congestion collapse, and to be somewhat better than TCP in
    explicitly reducing the ack sending rate in the presence of a high
    packet loss or marking rate on the return path.

6.1.1.  Sending Acknowledgements

    A CCID 2 receiver SHOULD send one acknowledgement for every Ack
    Ratio data packets it receives.

    This is only a rough guideline, however.  We intend CCID 2's
    acknowledgement behavior to resemble TCP's when there is no ack-
    stream congestion, and to be somewhat more conservative when there
    is ack-stream congestion; following this intent is more important
    than implementing Ack Ratio precisely.  Suggested variations from
    strict Ack Ratio compliance include:

    (1) If the HC-Receiver, DCCP B, is not quiescent---it is actively
        sending data---then its acknowledgements may be piggybacked on
        its data packets.  It is acceptable in this case to send more
        piggybacked acknowledgements than the Ack Ratio would suggest.
        If the data packets are too big to carry acknowledgement
        information, or the data sending rate is too low, then DCCP B
        SHOULD send some pure acknowledgements as well as piggybacked
        data-plus-acknowledgement packets, to maintain the rate of one
        acknowledgement per Ack Ratio received data packets.

    (2) The receiver SHOULD implement an algorithm like TCP's delayed
        acknowledgements, whereby every data packet is acknowledged
        within at most T seconds of its receipt, regardless of Ack
        Ratio.  The delayed-ack timeout T SHOULD be set as for TCP---to
        200 milliseconds, for example.




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6.1.2.  Setting Ack Ratio

    There are three guidelines for setting Ack Ratio.  First, it is
    always an integer.  Second, it should not exceed half the congestion
    window, rounded up (except that Ack Ratio 2 is always acceptable).
    Third, it should be two or larger for a congestion window of four or
    more packets.

    DCCP-Ack packets from the receiver contain sequence numbers, so the
    sender can infer when DCCP-Ack packets are lost.  The sender
    considers a DCCP-Ack packet lost if at least NUMDUPACK packets with
    higher sequence numbers have been received from the receiver.
    (Again, NUMDUPACK equals 3.)  If DCCP-Ack packets from the receiver
    are marked in the network, the sender sees these marks directly.

    DCCP responds to congestion events on the return path by modifying
    the Ack Ratio, loosely emulating TCP.  For each congestion window of
    data with lost or marked DCCP-Ack packets, the Ack Ratio is doubled,
    subject to the constraints noted above.  Similarly, if the Ack Ratio
    is R, then for each (cwnd/(R^2 - R)) congestion windows of data with
    no lost or marked DCCP-Ack packets, the Ack Ratio is decreased by 1,
    again subject to the constraints on the Ack Ratio.  See the section
    below for the derivation.  For a constant congestion window, this
    gives an Ack sending rate that is roughly TCP-friendly.

    The sender need not keep the receiver's Ack Ratio completely up to
    date.  For instance, it MAY rate-limit Ack Ratio renegotiations to
    once every four or five round-trip times, or to once every second or
    two.  Additionally, it MAY bound Ack Ratio below by two, or it MAY
    set Ack Ratio to one for half-connections with persistent congestion
    windows of 1 or 2 packets.

    Since the sending rate for acknowledgement packets changes as a
    function of both the Ack Ratio and the congestion window, the
    dynamics will be rather complex, and this Ack congestion control
    mechanism is intended only to be very roughly TCP-friendly.

    As a result of the constraints given earlier in this section, the
    receiver always sends at least one ack packet for a congestion
    window of one packet, and the receiver always sends at least two ack
    packets per window of data otherwise.  Thus, the receiver could be
    sending two ack packets per window of data even in the face of very
    heavy congestion on the reverse path.  We would note, however, that
    if congestion is sufficiently heavy that all of the ack packets are
    dropped, then the sender falls back on a timeout, and the
    exponential backoff of the timer, as in TCP.  Thus, if congestion is
    sufficiently heavy on the reverse path, then the sender reduces its
    sending rate on the forward path, which reduces the rate on the



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    reverse path as well.

6.1.3.  Derivation of Ack Ratio Decrease

    The congestion avoidance phase of TCP increases cwnd by one MSS for
    every congestion-free window.  Applying this congestion avoidance
    behavior to the ack traffic, this would correspond to increasing the
    number of DCCP-Ack packets per window by one after every congestion-
    free window of DCCP-Ack packets.  We cannot achieve this exactly
    using the Ack Ratio, since the Ack Ratio is an integer.  Instead, we
    must decrease the Ack Ratio by one after K windows have been sent
    without a congestion event on the reverse path, where K is chosen so
    that the long-term number of DCCP-Ack packets per congestion window
    is roughly TCP-friendly, following AIMD congestion control.

    In CCID 2, K = (cwnd/(R^2 - R)), where R is the current Ack Ratio.
    This result was calculated as follows:

                R = Ack Ratio = # data packets / ack packets, and
                W = Congestion Window = # data packets / window, so
              W/R = # ack packets / window.

        Requirement: Increase W/R by 1 per congestion-free window.
        But can only reduce R by increments of one.

        Therefore, find K so that, after K congestion-free windows,
        the adjusted W/R would equal W/(R-1).

        (W/R) + K = W/(R-1), so
                K = W/(R-1) - W/R = W/(R^2 - R).


6.2.  Quiescence

    This section refers to quiescence in the DCCP sense (see section 8.1
    of [DCCP]): How does a CCID 2 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.
    Then the receiver detects that the sender has gone quiescent when at
    least T seconds have passed without receiving any additional data
    from the sender, and the sender has acknowledged receiver Ack
    Vectors that covered all data packets sent.  That is, once the
    sender acknowledges the receiver's Ack Vectors and the sender has
    not sent additional data for at least T, the receiver can determine
    that the sender is quiescent.





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6.3.  Acknowledgements of Acknowledgements

    The sender, DCCP A, must occasionally acknowledge the receiver's
    acknowledgements, so that the receiver can free up Ack Vector state.
    The sender can also send acknowledgements to make changes to the Ack
    Ratio.  We assume that DCCP A simply sends Change(Ack Ratio) options
    whenever required.  To let the receiver free Ack Vector state, DCCP
    A must occasionally acknowledge that it has received one of DCCP B's
    acknowledgements.  When both half-connections are active, this
    information is automatically contained in A's acknowledgements to
    B's data.  If the B-to-A half-connection goes quiescent, however,
    DCCP A must do it proactively.

    In particular, an active sender MUST occasionally acknowledge the
    receiver's acknowledgements, probably by sending a DCCP-DataAck
    packet (rather than DCCP-Data).  No acknowledgement options are
    necessary; the Acknowledgement Number suffices.  The sender SHOULD
    acknowledge approximately one of the receiver's acknowledgements per
    congestion window.  Of course, the sender's application might fall
    silent.  This is no problem; when neither side is sending data, a
    sender can wait arbitrarily long before sending an ack.

7.  Explicit Congestion Notification

    ECN may be used with CCID 2.  If ECN is used, then the ECN Nonce
    will automatically be used for the data packets, following the
    specification for the ECN Nonce in TCP in [ECN NONCE]. For the data
    subflow, the sender sets either the ECT(0) or ECT(1) codepoint on
    DCCP-Data packets.  Information about marked packets is returned in
    the Ack Vector.  Because the information in the Ack Vector is
    reliably transferred, DCCP does not need the TCP flags of ECN-Echo
    and Congestion Window Reduced.

    For unmarked data packets, the receiver computes the ECN Nonce Echo
    as in [ECN NONCE], and returns the ECN Nonce Echo in DCCP-Ack
    packets.  The sender uses the ECN Nonce to protect against the
    accidental or malicious concealment of marked packets.

    Because the ack subflow is congestion-controlled, ECN can also be
    used for DCCP-Ack packets.  In this case we do not make use of the
    ECN Nonce, because it would not be easy to provide protection
    against the concealment of marked ack packets by the sender, and
    because the sender does not have as much motivation for lying about
    the mark rate on acknowledgements.







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8.  Relevant Options and Features

    DCCP's Ack Vector option and its Ack Ratio, Use Ack Vector, and ECN
    Capable features are relevant for CCID 2.

9.  Security Considerations

    Security considerations for DCCP have been discussed in [DCCP], and
    security considerations for TCP have been discussed in [RFC 2581].

    [RFC 2581] discusses ways that an attacker could impair the
    performance of a TCP connection by dropping packets, or by forging
    extra duplicate acknowledgements or acknowledgements for new data.
    We are not aware of any new security considerations created by this
    document in its use of TCP-like congestion control.

10.  IANA Considerations

    There are no new IANA considerations created in this document.

11.  Thanks

    We thank Mark Handley and Jitendra Padhye for their help in defining
    CCID 2.  We also thank Greg Minshall and Arun Venkataramani for
    feedback on this document.

Normative References

    [DCCP] E. Kohler, M. Handley, S. Floyd, and J. Padhye.  Datagram
        Congestion Control Protocol, draft-ietf-dccp-spec-05.txt, work
        in progress, October 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 793] J. Postel, editor. Transmission Control Protocol. RFC 793.

    [RFC 2026] S. Bradner. The Internet Standards Process -- Revision 3.
        RFC 2026.

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

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





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    [RFC 3465] M. Allman. TCP Congestion Control with Appropriate Byte
        Counting (ABC). RFC 3465.

    [RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang. A
        Conservative Selective Acknowledgment (SACK)-based Loss Recovery
        Algorithm for TCP. RFC 3517.

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.


Full Copyright Statement

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    kind, provided that the above copyright notice and this paragraph
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    The limited permissions granted above are perpetual and will not be
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