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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 4341

Internet Engineering Task Force
INTERNET-DRAFT                                               Sally Floyd
draft-ietf-dccp-ccid2-02.txt                                Eddie Kohler
                                                                    ICIR
                                                             10 May 2003
                                                  Expires: November 2003


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



Status of this Document

    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
    months and may be updated, replaced, or obsoleted by other documents
    at any time. It is inappropriate to use Internet-Drafts as reference
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    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 2, TCP-like Congestion Control, in the Datagram
     Congestion Control Protocol (DCCP) [DCCP]. DCCP implements a
     congestion-controlled, unreliable flow of datagrams suitable
     for use by applications such as streaming media. The TCP-like
     Congestion Control CCID is used by senders who are able to
     adapt to the abrupt changes in the congestion window typical
     of TCP's AIMD (Additive Increase Multiplicative Decrease)
     congestion control.  TCP-like Congestion Control is
     particularly useful for senders who would like to take



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     advantage of the available bandwidth in an environment with
     rapidly changing conditions.

     TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

     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
      1.1. Usage Scenario . . . . . . . . . . . . . . . . . . .   5
      1.2. Example Half-Connection. . . . . . . . . . . . . . .   5
     2. Connection Establishment. . . . . . . . . . . . . . . .   6
     3. Congestion Control on Data Packets. . . . . . . . . . .   6
     4. Acknowledgements. . . . . . . . . . . . . . . . . . . .   8
      4.1. Congestion Control on Acknowledgements . . . . . . .   8
       4.1.1. Derivation of Ack Ratio Decrease. . . . . . . . .  10
      4.2. Quiescence . . . . . . . . . . . . . . . . . . . . .  10
      4.3. Acknowledgements of Acknowledgements . . . . . . . .  11
     5. Explicit Congestion Notification. . . . . . . . . . . .  11
     6. Relevant Options and Features . . . . . . . . . . . . .  12
     7. Application Requirements. . . . . . . . . . . . . . . .  12
     8. Thanks. . . . . . . . . . . . . . . . . . . . . . . . .  12
     9. Normative References. . . . . . . . . . . . . . . . . .  12
     10. Informative References . . . . . . . . . . . . . . . .  12
     11. Security Considerations. . . . . . . . . . . . . . . .  13
     12. IANA Considerations. . . . . . . . . . . . . . . . . .  13
     13. Authors' Addresses . . . . . . . . . . . . . . . . . .  13





























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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.

    The congestion control mechanisms described here closely follow
    mechanisms standardized by the IETF for use in SACK-based TCP. We do
    not define these mechanisms anew; instead, we rely on existing TCP
    documentation, such as [RFC 793], [RFC 3465], and [RFC 3517]. This
    is both to avoid respecifying TCP, and to allow our specification to
    track TCP as it evolves. Conformant CCID 2 implementations MAY track
    TCP's evolution directly, as updates are standardized in the IETF,
    rather than waiting for revisions of this document. CCID 2 does
    define an additional mechanism not currently standardized for use in
    TCP, namely congestion control on acknowledgements as achieved by
    the Ack Ratio. Also, 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.

    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.

    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

    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.

1.2.  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.

        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, but there is no need to verify the nonces.

    (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



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        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 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.

2.  Connection Establishment

    Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so
    the sender MUST send a "Change(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(Use Ack
    Vector, 1)" from the receiver.

3.  Congestion Control on Data Packets

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

    In particular, a CCID 2 data sender maintains three integer
    parameters, whose units are in packets:

    (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.



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        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 >=
    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 "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.

    (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.  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.





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    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
    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.

4.  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
    Unacknowledged Window, as described in [DCCP].

    The sender specifies the Ack Ratio to be used by the receiver.  In
    the absence of congestion on the reverse path, the Ack Ratio is set
    to two if the congestion window is three or more packets, and is set
    to one otherwise.  The receiver sends a DCCP-Ack packet for every
    Ack Ratio packets sent by the sender.

4.1.  Congestion Control on Acknowledgements

    In CCID 2, the acknowledgement subflow is loosely congestion-
    controlled by the Ack Ratio specified by the sender.  The receiver
    sends (cwnd / Ack Ratio) acknowledgement packets for each congestion
    window of data packets.  We note that CCID 2 differs from TCP, which
    presently has no congestion control for pure acknowledgement
    traffic.  For congestion control for the pure ack stream, DCCP 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.



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    If DCCP B, the HC-Receiver, is actively sending data---it is not
    quiescent---then required acknowledgements may be piggybacked on
    DCCP B's data packets. In this situation, DCCP B MAY send more
    piggybacked acknowledgements than the Ack Ratio would allow; but it
    MUST send at least as many acknowledgements as the Ack Ratio
    requires. Conceivably, the CCID in use for the B-to-A half-
    connection might limit DCCP B's sending rate to less than the
    acknowledgement rate required for the A-to-B half-connection.  DCCP
    B MUST follow both constraints. In practice, this means that DCCP B
    will not piggyback data on every acknowledgement.

    There are three constraints on the Ack Ratio.  First, it is always
    an integer.  Second, it is never greater than half the congestion
    window (with fractions rounded up).  Third, it is at least two 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.  We note
    that, because the sending rate for the 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.

4.1.1.  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).


4.2.  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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4.3.  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 encapsulating a datagram in
    a DCCP-DataAck packet. No acknowledgement options are necessary,
    just the relevant Acknowledgement Number in the DCCP-DataAck header.
    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.

5.  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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6.  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.

7.  Application Requirements

    There are no specific application requirements for TCP-like
    Congestion Control.

8.  Thanks

    We thank Mark Handley and Jitendra Padhye for their help in defining
    CCID 2.

9.  Normative References

    [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 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.

    [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.

10.  Informative References







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11.  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.

12.  IANA Considerations

    There are no new IANA considerations created in this document.

13.  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.




























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