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

Internet Engineering Task Force                              Sally Floyd
INTERNET-DRAFT                                                      ICIR
draft-ietf-dccp-ccid2-10.txt                                Eddie Kohler
Expires: 10 September 2005                                          UCLA
                                                           10 March 2005


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


Status of this Memo

    This document is an Internet-Draft and is subject to all provisions
    of section 3 of RFC 3667. By submitting this Internet-Draft, each
    author represents that any applicable patent or other IPR claims of
    which he or she is aware have been or will be disclosed, and any of
    which he or she become aware will be disclosed, in accordance with
    RFC 3668.

    Internet-Drafts are working documents of the Internet Engineering
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    reference material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt.

    The list of Internet-Draft Shadow Directories can be accessed at
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    This Internet-Draft will expire on 10 September 2005.

Copyright Notice

    Copyright (C) The Internet Society (2005). All Rights Reserved.







Floyd/Kohler                                                    [Page 1]

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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.










































Floyd/Kohler                                                    [Page 2]

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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

    Changes from draft-ietf-dccp-ccid2-07.txt:

    * Restrict the use of byte-counting to be at most as aggressive
      as the current TCP (without byte-counting).

    Changes from draft-ietf-dccp-ccid2-06.txt:

    * Moved three citations to Informational.

    * Added that "The sender SHOULD not attempt Ack Ratio
      renegotiations more than once per round-trip time."

    * Specified that ssthresh is never less than two, instead of one.

    * Added references to RFC 2988 and RFC 2018.

    * Specify that the congestion window is only increased for packets
    that aren't ECN-marked.

    Changes from draft-ietf-dccp-ccid2-05.txt:

    * Changes to the discussion about how the sender infers that DCCP-
    Ack packets are lost.  The sender does not know for sure whether a
    missing sequence number is for a dropped ACK packet or a dropped
    data packet.  Our changes include a new appendix on "The Costs of
    Inferring Lost Ack Packets".

    * Minor editing for clarity, including some reordering of sections.

    * Added a section on response to idle and application-limited
    periods.

    * Clarifications on changing the Ack Ratio, based on feedback from
    Nils-Erik Mattsson.

    Changes from draft-ietf-dccp-ccid2-04.txt:

    * Minor editing, as follows:
      - Added a note that CCID2 implementations MAY check for apps that
    are
        gaming with regard to the packet size.
      - Deleted a statement that the maximum packet size is 1500 bytes.
      - Added that the receiver MAY know the round-trip time from its
    role as
      - Added a note that the initial cwnd is up to four packets.




Floyd/Kohler                                                    [Page 3]

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    * Added Intellectual Property Notice.

    Changes from draft-ietf-dccp-ccid2-03.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.
















Floyd/Kohler                                                    [Page 4]

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                             Table of Contents

    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   6
    2. Conventions and Notation. . . . . . . . . . . . . . . . . . .   6
    3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1. Relationship with TCP. . . . . . . . . . . . . . . . . .   7
       3.2. Example Half-Connection. . . . . . . . . . . . . . . . .   8
    4. Connection Establishment. . . . . . . . . . . . . . . . . . .   9
    5. Congestion Control on Data Packets. . . . . . . . . . . . . .   9
       5.1. Response to Idle and Application-limited
       Periods . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       5.2. Response to Data Dropped and Slow Receiver . . . . . . .  12
       5.3. Packet Size. . . . . . . . . . . . . . . . . . . . . . .  12
    6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  13
       6.1. Congestion Control on Acknowledgements . . . . . . . . .  13
          6.1.1. Detecting Lost and Marked
          Acknowledgements . . . . . . . . . . . . . . . . . . . . .  13
          6.1.2. Changing Ack Ratio. . . . . . . . . . . . . . . . .  14
       6.2. Acknowledgements of Acknowledgements . . . . . . . . . .  15
          6.2.1. Determining Quiescence. . . . . . . . . . . . . . .  15
    7. Explicit Congestion Notification. . . . . . . . . . . . . . .  16
    8. Options and Features. . . . . . . . . . . . . . . . . . . . .  16
    9. Security Considerations . . . . . . . . . . . . . . . . . . .  16
    10. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  16
       10.1. Reset Codes . . . . . . . . . . . . . . . . . . . . . .  17
       10.2. Option Types. . . . . . . . . . . . . . . . . . . . . .  17
       10.3. Feature Numbers . . . . . . . . . . . . . . . . . . . .  17
    11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
    A. Appendix: Derivation of Ack Ratio Decrease. . . . . . . . . .  18
    B. Appendix: Cost of Loss Inference Mistakes to Ack
    Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
    Normative References . . . . . . . . . . . . . . . . . . . . . .  20
    Informative References . . . . . . . . . . . . . . . . . . . . .  21
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  21
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  22
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  22















Floyd/Kohler                                                    [Page 5]

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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].  DCCP uses Congestion Control Identifiers,
    or CCIDs, to specify the congestion control mechanism in use on a
    half-connection.

    The TCP-like Congestion Control CCID sends data using a close
    variant of TCP's congestion control mechanisms, incorporating
    selective acknowledgements (SACK) [RFC 2018, RFC 3517].  CCID 2 is
    suitable for senders who can adapt to the abrupt changes in
    congestion window typical of TCP's Additive Increase Multiplicative
    Decrease (AIMD) congestion control, 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 and Notation

    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.

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

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

    The phrases "ECN-marked" and "marked" refer to packets marked ECN
    Congestion Experienced unless otherwise noted.

3.  Usage

    CCID 2, TCP-like Congestion Control, is appropriate for DCCP 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 can tolerate the large sending rate variations
    characteristic of AIMD congestion control, including halving of the
    congestion window in response to a congestion event.




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    Applications that simply need to transfer as much data as possible
    in as short a time as possible should use CCID 2.  This contrasts
    with CCID 3, TCP-Friendly Rate Control (TFRC) Congestion Control
    [CCID 3 PROFILE], which is appropriate for flows that would prefer
    to minimize abrupt changes in the sending rate.  For example, CCID 2
    is recommended over CCID 3 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 halving 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 its being 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,
    which haven't yet met the test of widespread Internet deployment.

3.1.  Relationship with TCP

    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 2581, RFC 3465, and RFC 3517.  TCP congestion control continues
    to evolve, but CCID 2 implementations SHOULD wait for explicit
    updates to CCID 2 rather than track TCP's evolution directly.
    Differences between CCID 2 and straight TCP congestion control
    include the following:

    o  CCID 2 applies congestion control to acknowledgements, a
       mechanism not currently standardized for use in TCP.

    o  DCCP is a datagram protocol, so several parameters whose units
       are specified in bytes in TCP, such as the congestion window
       cwnd, have units of packets in DCCP.

    o  As an unreliable protocol, DCCP never retransmits a packet, so
       congestion control mechanisms that distinguish retransmissions
       from new packets have been redesigned for the DCCP context.






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3.2.  Example Half-Connection

    This example shows the typical progress of a half-connection using
    CCID 2's TCP-like Congestion Control, not including connection
    initiation and termination.  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; Ack
        Ratio defaults to two.  The DCCP header's CCVal field is set to
        zero.

        Assuming that the half-connection is Explicit Congestion
        Notification (ECN) capable (the ECN Incapable feature is zero --
        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
        RFC 3540.

    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 a
        DCCP-Ack packet is that of the received packet with the highest
        sequence number, rather than a TCP-like cumulative
        acknowledgement.

        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 of its DCCP-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
        some information about lost or marked DCCP-Ack packets.  The



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        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.

4.  Connection Establishment

    Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so
    the sender MUST send a "Change R(Send 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(Send
    Ack Vector, 1)" from the receiver, except possibly for 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 all the
    information that might be transmitted in SACK options.

    A CCID 2 data sender maintains three integer parameters measured in
    packets.

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

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

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




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    These parameters are manipulated, and their initial values
    determined, according to SACK-based TCP's behavior, except that they
    are measured in packets, not bytes.  The rest of this section
    provides more specific guidance.

    The sender MAY send a data packet when pipe < cwnd, but MUST NOT
    send a data packet when pipe >= cwnd.  Every data packet sent
    increases pipe by 1.

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

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

    2.  Dropped data packets.  The sender reduces pipe by 1 for each
        data packet it can infer as lost due to the DCCP equivalent of
        TCP's "duplicate acknowledgements".  This depends on the
        NUMDUPACK parameter, the number of duplicate acknowledgements
        needed to infer a loss.  The NUMDUPACK parameter is set to
        three, as is currently the case in TCP.  A packet P is inferred
        to be lost, rather than delayed, when at least NUMDUPACK packets
        transmitted after P have been acknowledged as received (Ack
        Vector State 0 or 1) by the receiver.  Note that the
        acknowledged packets following the hole may be DCCP-Acks or
        other non-data packets.

    3.  Transmit timeouts.  Finally, the sender needs transmit timeouts,
        handled like TCP's retransmission timeouts, in case an entire
        window of packets is 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 [RFC 2988].  (If more than one
        measurement per round-trip time was used for these calculations,
        then the weights of the averagers would have to be adjusted, so
        that the average round-trip time is effectively derived from
        measurements over multiple round-trip times.)  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 transmit timeout occurs, the
        sender sets pipe to zero.  The adjustments to cwnd and ssthresh
        are described below.

    The sender MUST NOT decrement pipe more than once per data packet.
    True duplicate acknowledgements, for example, MUST NOT affect pipe.
    Furthermore, the sender MUST NOT decrement pipe for non-data



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    packets, such as DCCP-Acks, even though the Ack Vector will contain
    information about them.

    Congestion events cause CCID 2 to reduce its congestion window.  A
    congestion event contains at least one lost or marked packet.  As in
    TCP, two losses or marks are considered to be part of a single
    congestion event when the second packet was sent before the loss or
    mark of the first packet was detected.  As an approximation, a
    sender can consider two losses or marks to be part of a single
    congestion event when the packets were sent within one RTT estimate
    of one another, using an RTT estimate current at the time the
    packets were sent.  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 halved, cwnd and ssthresh have
    their values rounded down, except that cwnd is never less than one
    and ssthresh is never less than two.

    When cwnd < ssthresh, meaning that the sender is in slow-start, the
    congestion window is increased by one packet for every two newly
    acknowledged data packets with Ack Vector State 0 (not ECN-marked),
    up to a maximum of Ack Ratio/2 packets per acknowledgement.  This is
    a modified form of Appropriate Byte Counting [RFC 3465] that is
    consistent with TCP's current standard (which does not include byte-
    counting), but allows CCID 2 to increase as aggressively as TCP when
    CCID-2's Ack Ratio is greater than the default value of two.  When
    cwnd >= ssthresh, the congestion window is increased by one packet
    for every window of data acknowledged without lost or marked
    packets.  The cwnd parameter is initialized to at most four packets
    for new connections, following the rules from RFC 3390; the ssthresh
    parameter is initialized to an arbitrarily high value.

    Senders MAY use a form of rate-based pacing when sending multiple
    data packets liberated by a single ack packet, rather than sending
    all liberated data packets in a single burst.

5.1.  Response to Idle and Application-limited Periods

    CCID 2 is designed to follow TCP's congestion control mechanisms to
    the extent possible, but TCP does not have complete standardization
    for its congestion control response to idle periods (when no data
    packets are sent) or to application-limited periods (when the
    sending rate is less than that allowed by cwnd).  This section is a
    brief guide to the standards for TCP in this area.





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    For idle periods, RFC 2581 recommends that the TCP sender SHOULD
    slow-start after an idle period, where an idle period is defined as
    a period exceeding the timeout interval.  RFC 2861, currently
    Experimental, suggests a slightly more moderate mechanism where the
    congestion window is halved for every round-trip time that the
    sender has remained idle.

    There are currently no standards governing TCP's use of the
    congestion window during an application-limited period.  In
    particular, it is possible for TCP's congestion window to grow quite
    large during a long uncongested period when the sender is
    application-limited, sending at a low rate.  RFC 2861 essentially
    suggests that TCP's congestion window not be increased during
    application-limited periods, when the congestion window is not being
    fully utilized.

5.2.  Response to Data Dropped and Slow Receiver

    As described in [DCCP], the Data Dropped option lets an endpoint
    declare that a packet was dropped at the end host before delivery to
    the application -- for instance, because of corruption or receive
    buffer overflow.  CCID 2 senders respond to these options 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.

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

5.3.  Packet Size

    CCID 2 is optimized for applications that generally 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.  CCID 2 maintains a congestion window in packets, and
    does not increase the congestion window in response to a decrease in
    the packet size.  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 implementations MAY check for applications that appear to be
    manipulating the packet size inappropriately.  For example, an
    application might send small packets for a while, building up a fast



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    rate, then switch to large packets to take advantage of the fast
    rate.  (Preliminary simulations indicate that applications may not
    be able to increase their overall transfer rates this way, so it is
    not clear this manipulation will occur in practice [V03].)

6.  Acknowledgements

    CCID 2 acknowledgements are generally paced by the sender's data
    packets.  Each required acknowledgement MUST contain Ack Vector
    options that declare exactly which packets arrived, and whether
    those packets were ECN-marked.  Acknowledgement data in the Ack
    Vector options SHOULD generally cover the receiver's entire
    Acknowledgement Window; see [DCCP] (Section 11.4.2).

    CCID 2 senders use DCCP's Ack Ratio feature to influence the rate at
    which DCCP-Ack packets are generated, thus controlling reverse-path
    congestion.  This differs from TCP, which presently has no
    congestion control for pure acknowledgement traffic.  CCID 2's
    reverse-path congestion control does not try to be TCP-friendly; it
    just tries to avoid congestion collapse, and to be somewhat better
    than TCP in the presence of a high packet loss or mark rate on the
    reverse path.  The default Ack Ratio is two, and CCID 2 with this
    Ack Ratio behaves like TCP with delayed acks.  [DCCP] (Section 11.3)
    describes the Ack Ratio in more detail, including its relationship
    to acknowledgement pacing and DCCP-DataAck packets.  Section 6.1.1
    below describes the sender's detection of lost or marked
    acknowledgements, and Section 6.1.2 gives the sender's rules for
    changing the Ack Ratio.

6.1.  Congestion Control on Acknowledgements

    When Ack Ratio is R, the receiver sends one DCCP-Ack packet per R
    data packets, more or less.  Since the sender sends cwnd data
    packets per round-trip time, the acknowledgement rate equals cwnd/R
    DCCP-Acks per round-trip time.  The sender keeps the acknowledgement
    rate roughly TCP-friendly by monitoring the acknowledgement stream
    for lost and marked DCCP-Ack packets, and modifying R accordingly.
    For every RTT containing a DCCP-Ack congestion event (that is, a
    lost or marked DCCP-Ack), the sender halves the acknowledgement rate
    by doubling Ack Ratio; for every RTT containing no DCCP-Ack
    congestion event, it additively increases the acknowledgement rate
    through gradual decreases in Ack Ratio.

6.1.1.  Detecting Lost and Marked Acknowledgements

    All packets from the receiver contain sequence numbers, so the
    sender can detect both losses and marks on the receiver's packets.
    The sender infers receiver packet loss in the same way as it infers



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    losses of its data packets: a packet from the receiver is considered
    lost after at least NUMDUPACK packets with greater sequence numbers
    have been received.

    DCCP-Ack packets are generally small, so they might impose less load
    on congested network links than DCCP-Data and DCCP-DataAck packets.
    For this reason, Ack Ratio depends on losses and marks on the
    receiver's non-data packets, not on aggregate losses and marks on
    all of the receiver's packets.  The non-data packet category
    consists of those packet types that cannot carry application data:
    DCCP-Ack, DCCP-Close, DCCP-CloseReq, DCCP-Reset, DCCP-Sync, and
    DCCP-SyncAck.  The sender can easily distinguish non-data marks from
    other marks.  This is harder for losses, though, since the sender
    can't always know whether a lost packet carried data.  Unless it has
    better information, the sender SHOULD assume, for the purpose of Ack
    Ratio calculation, that every lost packet was a non-data packet.
    Better information is available via DCCP's NDP Count option, if
    necessary.  (Appendix B discusses the costs of mistaking data packet
    loss for non-data packet loss.)

    A receiver that implements its own acknowledgement congestion
    control SHOULD NOT reduce its DCCP-Ack acknowledgement rate due to
    losses or marks on its data packets.

6.1.2.  Changing Ack Ratio

    Ack Ratio always meets three constraints: (1) Ack Ratio is an
    integer.  (2) Ack Ratio does not exceed cwnd/2, rounded up, except
    that Ack Ratio 2 is always acceptable.  (3) Ack Ratio is two or more
    for a congestion window of four or more packets.

    The sender changes Ack Ratio within those constraints as follows.
    For each congestion window of data with lost or marked DCCP-Ack
    packets, Ack Ratio is doubled; and for each cwnd/(R^2 - R)
    consecutive congestion windows of data with no lost or marked DCCP-
    Ack packets, Ack Ratio is decreased by 1.  (See Appendix A for the
    derivation.)  Changes in Ack Ratio are signalled through feature
    negotiation; see [DCCP] (Section 11.3).

    For a constant congestion window, this gives an Ack sending rate
    that is roughly TCP-friendly.  Of course, cwnd usually varies over
    time; the dynamics will be rather complex, but roughly TCP-friendly.
    We recommend that the sender use the most recent value of cwnd when
    determining whether to decrease Ack Ratio by 1.

    The sender need not keep 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.  The



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    sender SHOULD NOT attempt to renegotiate the Ack Ratio more than
    once per round-trip time.  Additionally, it MAY enforce a minimum
    Ack Ratio of two, or it MAY set Ack Ratio to one for half-
    connections with persistent congestion windows of 1 or 2 packets.

    Putting it all together, the receiver always sends at least one
    acknowledgement per window of data when cwnd = 1, and at least two
    acknowledgements 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 an
    exponentially-backed-off timeout, 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
    reverse path as well.

6.2.  Acknowledgements of Acknowledgements

    An active sender DCCP A MUST occasionally acknowledge its peer DCCP
    B's acknowledgements, so that DCCP B can free up Ack Vector state.
    When both half-connections are active, A's acknowledgements of B's
    acknowledgements are automatically contained in A's acknowledgements
    of B's data. If the B-to-A half-connection is quiescent, however,
    DCCP A must occasionally send acknowledgements proactively, such as
    by sending a DCCP-DataAck packet that includes an Acknowledgement
    Number in the header.

    An active sender SHOULD acknowledge the receiver's acknowledgements
    at least once 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.

6.2.1.  Determining Quiescence

    This section describes how a CCID 2 receiver determines that the
    corresponding sender is not sending any data, and therefore has gone
    quiescent.  See [DCCP] (Section 11.1) for general information on
    quiescence.

    Let T equal the greater of 0.2 seconds and two round-trip times.
    (The receiver may know the round-trip time in its role as the sender
    for the other half-connection.  If it does not, it should use a
    default RTT of 0.2 seconds, as described in [DCCP] (Section 3.4).)
    Once the sender acknowledges the receiver's Ack Vectors, and the
    sender has not sent additional data for at least T seconds, the
    receiver can infer that the sender is quiescent.  More precisely,



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    the receiver infers that the sender has gone quiescent when at least
    T seconds have passed without receiving any data from the sender,
    and the sender has acknowledged receiver Ack Vectors covering all
    data packets received at the receiver.

7.  Explicit Congestion Notification

    CCID 2 supports Explicit Congestion Notification (ECN) [RFC 3168].
    The sender will use the ECN Nonce for data packets, and the receiver
    will echo those nonces in its Ack Vectors, as specified in [DCCP]
    (Section 12.2).  Information about marked packets is also 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 RFC 3540, and returns it as part of its Ack Vector options.
    The sender SHOULD check these ECN Nonce Echoes against the expected
    values, thus protecting against the accidental or malicious
    concealment of marked packets.

    Because CCID 2 acknowledgements are congestion-controlled, ECN may
    also be used for its acknowledgements.  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 much motivation for
    lying about the mark rate on acknowledgements.

8.  Options and Features

    DCCP's Ack Vector option, and its ECN Capable, Ack Ratio, and Send
    Ack Vector 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

    This specification defines the value 2 in the DCCP CCID namespace
    managed by IANA.  This assignment is also mentioned in [DCCP].



Floyd/Kohler                                      Section 10.  [Page 16]

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    CCID 2 also introduces three sets of numbers whose values should be
    allocated by IANA, namely CCID 2-specific Reset Codes, option types,
    and feature numbers.  These ranges will prevent any future
    CCID 2-specific allocations from polluting DCCP's corresponding
    global namespaces; see [DCCP] (Section 10.3).  However, this
    document makes no particular allocations from any range, except for
    experimental and testing use [RFC 3692].  We refer to the Standards
    Action policy outlined in RFC 2434.

10.1.  Reset Codes

    Each entry in the DCCP CCID 2 Reset Code registry contains a
    CCID 2-specific Reset Code, which is a number in the range 128-255;
    a short description of the Reset Code; and a reference to the RFC
    defining the Reset Code.  Reset Codes 184-190 and 248-254 are
    permanently reserved for experimental and testing use.  The
    remaining Reset Codes -- 128-183, 191-247, and 255 -- are currently
    reserved, and should be allocated with the Standards Action policy,
    which requires IESG review and approval and standards-track IETF RFC
    publication.

10.2.  Option Types

    Each entry in the DCCP CCID 2 option type registry contains a
    CCID 2-specific option type, which is a number in the range 128-255;
    the name of the option; and a reference to the RFC defining the
    option type.  Option types 184-190 and 248-254 are permanently
    reserved for experimental and testing use.  The remaining option
    types -- 128-183, 191-247, and 255 -- are currently reserved, and
    should be allocated with the Standards Action policy, which requires
    IESG review and approval and standards-track IETF RFC publication.

10.3.  Feature Numbers

    Each entry in the DCCP CCID 2 feature number registry contains a
    CCID 2-specific feature number, which is a number in the range
    128-255; the name of the feature; and a reference to the RFC
    defining the feature number.  Feature numbers 184-190 and 248-254
    are permanently reserved for experimental and testing use.  The
    remaining feature numbers -- 128-183, 191-247, and 255 -- are
    currently reserved, and should be allocated with the Standards
    Action policy, which requires IESG review and approval and
    standards-track IETF RFC publication.

11.  Thanks

    We thank Mark Handley and Jitendra Padhye for their help in defining
    CCID 2.  We also thank Mark Allman, Aaron Falk, Nils-Erik Mattsson,



Floyd/Kohler                                      Section 11.  [Page 17]

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    Greg Minshall, Arun Venkataramani, Magnus Westerlund, and members of
    the DCCP Working Group for feedback on this document.

A.  Appendix: Derivation of Ack Ratio Decrease

    This section justifies the algorithm for increasing and decreasing
    the Ack Ratio given in Section 6.1.2.

    The congestion avoidance phase of TCP halves the cwnd for every
    window with congestion.  Similarly, CCID 2 doubles Ack Ratio for
    every window with congestion on the return path, roughly halving the
    DCCP-Ack sending rate.

    The congestion avoidance phase of TCP increases cwnd by one MSS for
    every congestion-free window.  Applying this congestion avoidance
    behavior to acknowledgement 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 Ack Ratio, since it is an integer.  Instead, we
    must decrease 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, rough TCP-friendliness for the ack traffic can be
    accomplished by setting K to 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.
        Since we can only reduce R by increments of one, we find K
        so that, after K congestion-free windows,
        W/R + K would equal W/(R-1).

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


B.  Appendix: Cost of Loss Inference Mistakes to Ack Ratio

    As discussed in Section 6.1.1, the sender often cannot determine
    whether lost packets carried data.  This hinders its ability to
    separate non-data loss events from other loss events.  In the



Floyd/Kohler                                       Section B.  [Page 18]

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    absence of better information, the sender assumes, for the purpose
    of Ack Ratio calculation, that all lost packets were non-data
    packets.  This may overestimate the non-data loss event rate, which
    can lead to a too-high Ack Ratio, and thus a too-slow
    acknowledgement rate.  All acknowledgement information will still
    get through -- DCCP acknowledgements are reliable -- but
    acknowledgement information will arrive in a burstier fashion.
    Absent some form of rate-based pacing, this could lead to increased
    burstiness for the sender's data traffic.

    There are several cases when the problem of an overly-high Ack
    Ratio, and the resulting increased burstiness of the data traffic,
    will not arise.  In particular, call the receiver DCCP B and the
    sender DCCP A.  Then:

    o  The problem won't arise unless DCCP B is sending a significant
       amount of data itself.  When the B-to-A half-connection is
       quiescent or low-rate, most packets sent by DCCP B will, in fact,
       be pure acknowledgements, and DCCP A's estimate of the DCCP-Ack
       loss rate will be reasonably accurate.

    o  The problem won't arise if DCCP B habitually piggybacks
       acknowledgement information on its data packets.  The piggybacked
       acknowledgements are not limited by Ack Ratio, so they can arrive
       frequently enough to prevent burstiness.

    o  The problem won't arise if DCCP A's sending rate is low, since
       burstiness isn't a problem at low rates.

    o  The problem won't arise if DCCP B's sending rate is high relative
       to DCCP A's sending rate, since the B-to-A loss rate must be low
       to support DCCP B's sending rate.  This bounds the Ack Ratio to
       reasonable values even when DCCP A labels every loss as a DCCP-
       Ack loss.

    o  The problem won't arise if DCCP B sends NDP Count options when
       appropriate (the Send NDP Count/B feature is true).  Then the
       sender can use the receiver's NDP Count options to detect, in
       most cases, whether lost packets were data packets or DCCP-Acks.

    o  Finally, the problem won't arise if DCCP A rate-paces its data
       packets.

    This leaves the case when DCCP B is sending roughly the same amount
    of data packets and non-data packets, without NDP Count options, and
    with all acknowledgement information in DCCP-Ack packets.  We now
    quantify the potential cost, in terms of a too-large Ack Ratio, due
    to the sender's misclassifying data packet losses as DCCP-Ack



Floyd/Kohler                                       Section B.  [Page 19]

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    losses.  For simplicity, we assume an environment of large-scale
    statistical multiplexing, where the packet drop rate is independent
    of the sending rate of any individual connection.

    Assume that when DCCP A correctly counts non-data losses, Ack Ratio
    is set so that B-to-A data and acknowledgement traffic both have a
    sending rate of D packets per second.  Then when DCCP A incorrectly
    counts data losses as non-data losses, the sending rate for the B-
    to-A data traffic is still D pps, but the reduced sending rate for
    the B-to-A acknowledgement traffic is f*D pps, with f < 1.  Let the
    packet loss rate be p.  The sender incorrectly estimates the non-
    data loss rate as (pD+pfD)/fD, or, equivalently, as p(1 + 1/f).
    Because the congestion control mechanism for acknowledgement traffic
    is roughly TCP-friendly, and therefore the non-data sending rate and
    the data sending rate both grow as 1/sqrt(x) for x the packet drop
    rate, we have
           fD/D = sqrt(p)/sqrt(p(1 + 1/f)),
    so
           f^2 = 1/(1 + 1/f).
    Solving, we get f = 0.62.  If the sender incorrectly counts lost
    data packets as non-data in this scenario, the acknowledgement rate
    is decreased by a factor of 0.62.  This would result in a moderate
    increase in burstiness for the A-to-B data traffic, which could be
    mitigated by sending NDP Count options or piggybacked
    acknowledgements, or by rate-pacing out the data.

Normative References

    [DCCP] E. Kohler, M. Handley, and S. Floyd.  Datagram Congestion
        Control Protocol, draft-ietf-dccp-spec-11.txt, work in progress,
        March 2005.

    [RFC 793] J. Postel, editor.  Transmission Control Protocol.
        RFC 793.

    [RFC 2018] M. Mathis, J. Mahdavi, A. Floyd, and A. Romanow. TCP
        Selective Acknowledgement Options, RFC 2018, October 1996.

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

    [RFC 2434] T. Narten and H. Alvestrand.  Guidelines for Writing an
        IANA Considerations Section in RFCs.  RFC 2434.

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





Floyd/Kohler                                                   [Page 20]

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    [RFC 2988] V. Paxson and M. Allman, Computing TCP's Retransmission
        Timer, RFC 2988, November 2000.

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

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

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

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

Informative References

    [CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye.  Profile for
        DCCP Congestion Control ID 3: TFRC Congestion Control.  draft-
        ietf-dccp-ccid3-11.txt, work in progress, March 2005.

    [RFC 2861] M. Handley, J. Padhye, and S. Floyd.  TCP Congestion
        Window Validation.  RFC 2861.

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

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

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

Authors' Addresses

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

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




Floyd/Kohler                                                   [Page 21]

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Full Copyright Statement

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Floyd/Kohler                                                   [Page 22]


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