[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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-ccid2-06.txt                                Eddie Kohler
Expires: January 2005                                               UCLA
                                                            18 July 2004

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

Status of this Memo

    This document is an Internet-Draft.

    By submitting this Internet-Draft, we certify that any applicable
    patent or other IPR claims of which we are aware have been
    disclosed, or will be disclosed, and any of which we become aware
    will be disclosed, in accordance with RFC 3668 (BCP 79).

    By submitting this Internet-Draft, we accept the provisions of
    Section 3 of RFC 3667 (BCP 78).

    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-

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than a "work in progress."

    The list of current Internet-Drafts can be accessed at

    The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

    Copyright (C) The Internet Society (2004). All Rights Reserved.

Floyd/Kohler                                                    [Page 1]

INTERNET-DRAFT            Expires: January 2005                July 2004


    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]

INTERNET-DRAFT            Expires: January 2005                July 2004


    Changes from draft-ietf-dccp-ccid3-05.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

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

    Changes from draft-ietf-dccp-ccid3-04.txt:

    * Minor editing, as follows:
      - Added a note that CCID2 implementations MAY check for apps that
        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.

    * Added Intellectual Property Notice.

    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"

Floyd/Kohler                                                    [Page 3]

INTERNET-DRAFT            Expires: January 2005                July 2004

    * 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-

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

Floyd/Kohler                                                    [Page 4]

INTERNET-DRAFT            Expires: January 2005                July 2004

                             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. . . . . . . . . . . . . . . . .   7
    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. . . . . . . . . . . . . . . . . . . . . . .  12
       6.1. Congestion Control on Acknowledgements . . . . . . . . .  13
          6.1.1. Detecting Lost and Marked Acknowledge-
          ments. . . . . . . . . . . . . . . . . . . . . . . . . . .  13
          6.1.2. Changing Ack Ratio. . . . . . . . . . . . . . . . .  14
       6.2. Acknowledgements of Acknowledgements . . . . . . . . . .  15
          6.2.1. Determining Quiescence. . . . . . . . . . . . . . .  15
    7. Explicit Congestion Notification. . . . . . . . . . . . . . .  15
    8. Options and Features. . . . . . . . . . . . . . . . . . . . .  16
    9. Security Considerations . . . . . . . . . . . . . . . . . . .  16
    10. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  16
    11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
    A. Appendix: Derivation of Ack Ratio Decrease. . . . . . . . . .  17
    B. Appendix: Cost of Loss Inference Mistakes to Ack
    Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
    Normative References . . . . . . . . . . . . . . . . . . . . . .  19
    Informative References . . . . . . . . . . . . . . . . . . . . .  20
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  20
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  21
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  21

Floyd/Kohler                                                    [Page 5]

INTERNET-DRAFT            Expires: January 2005                July 2004

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

    The TCP-like Congestion Control CCID sends data using a close
    variant of TCP's congestion control mechanisms, incorporating
    selective acknowledgements (SACK) [RFC 3517].  CCID 2 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 and Notation

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    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.

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.

    CCID 2 is recommended for applications that simply need to transfer
    as much data as possible in as short a time as possible.  This
    contrasts with CCID 3, TCP-Friendly Rate Control (TFRC) Congestion

Floyd/Kohler                                        Section 3.  [Page 6]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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 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, 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 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.  The
    differences between CCID 2 and straight TCP include: CCID 2 applies
    congestion control to acknowledgements, a mechanism not currently
    standardized for use in TCP.  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.

3.2.  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.  The example is informative,
    not normative.

Floyd/Kohler                                      Section 3.2.  [Page 7]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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.

        Assuming that the half-connection is Explicit Congestion
        Notification (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 [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

        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 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
        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-

Floyd/Kohler                                      Section 3.2.  [Page 8]

INTERNET-DRAFT            Expires: January 2005                July 2004

        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 for possible data included
    on the initial DCCP-Request packet.

    CCID 2 requires only generic feedback, namely the Ack Vector.
    Therefore, CCID 2 MAY masquerade as CCID 1 as long as the receiver's
    Send Ack Vector feature is set to 1.

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

    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, DCCP-Response, and DCCP-

    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.

    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

Floyd/Kohler                                        Section 5.  [Page 9]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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

    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
        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.  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

    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
    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

Floyd/Kohler                                       Section 5.  [Page 10]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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 neither parameter is ever less than one.

    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 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.  The cwnd
    parameter is initialized to at most four for new connections [RFC
    3390]; the ssthresh parameter is initialized to an arbitrarily high

    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.

    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.

Floyd/Kohler                                     Section 5.1.  [Page 11]

INTERNET-DRAFT            Expires: January 2005                July 2004

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 packets acknowledged as
    Data Dropped as described in [DCCP], with the following further

    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 intended 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.  That is, CCID 2 is optimized for a sender that
    generally sends fixed-sized packets; the congestion window is in
    packets, and CCID 2 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
    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.

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 were lost or marked.
    Acknowledgement data in the Ack Vector options SHOULD generally
    cover the receiver's entire Acknowledgement Window (Section 11.4.2
    of [DCCP]).

Floyd/Kohler                                       Section 6.  [Page 12]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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.  Section 11.3 of
    [DCCP] describes the Ack Ratio in more detail, including its
    relationship to acknowledgement pacing and DCCP-DataAck packets.

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-Ack packets per round-trip time.  The sender modifies R so as
    to keep the acknowledgement rate roughly TCP-friendly, by monitoring
    the acknowledgement stream for lost and marked DCCP-Ack packets.
    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
    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

Floyd/Kohler                                   Section 6.1.1.  [Page 13]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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 Section 11.3 of [DCCP].

    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.
    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.

    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.

Floyd/Kohler                                   Section 6.1.2.  [Page 14]

INTERNET-DRAFT            Expires: January 2005                July 2004

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 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.
    (The receiver may know the round-trip time in its role as the sender
    for the other half-connection; or if it does not, it should use an
    estimated RTT of 0.1 seconds.)  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, 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

    Explicit Congestion Notification (ECN) [RFC 3168] 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 [RFC 3540].  The sender sets either the ECT(0) or
    ECT(1) codepoint on 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 [RFC 3540], and returns it as part of its Ack Vector options.
    The sender SHOULD check these ECN Nonce Echoes against the expected

Floyd/Kohler                                       Section 7.  [Page 15]

INTERNET-DRAFT            Expires: January 2005                July 2004

    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 and Elapsed Time options, 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].

    CCID 2 also introduces the following three sets of numbers whose
    values should be allocated by IANA.  Following the policies outlined
    in [RFC 2434], these sets of numbers are allocated through an IETF
    Consensus action, with the specified exceptions for experimental and
    testing use [RFC 3692].

    o  CCID 2-specific option numbers 128-183, 191-247, and 255 are
       allocated through an IETF Consensus action.  Option numbers
       184-190 and 248-254 are reserved for experimental and testing

    o  CCID 2-specific feature numbers 128-183, 191-247, and 255 are
       allocated through an IETF Consensus action.  Feature numbers
       184-190 and 248-254 are reserved for experimental and testing

Floyd/Kohler                                      Section 10.  [Page 16]

INTERNET-DRAFT            Expires: January 2005                July 2004

    o  CCID 2-specific Reset Codes 128-183, 191-247, and 255 are
       allocated through an IETF Consensus action.  Reset Codes 184-190
       and 248-254 are reserved for experimental and testing use.

11.  Thanks

    We thank Mark Handley and Jitendra Padhye for their help in defining
    CCID 2.  We also thank Nils-Erik Mattsson, Greg Minshall, Arun
    Venkataramani, and Magnus Westerlund 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:

Floyd/Kohler                                       Section A.  [Page 17]

INTERNET-DRAFT            Expires: January 2005                July 2004

           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
    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 burstiness problem will not arise.
    In particular, call the receiver DCCP B and the sender DCCP A.

    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

Floyd/Kohler                                       Section B.  [Page 18]

INTERNET-DRAFT            Expires: January 2005                July 2004

       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

    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
    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)),
           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-07.txt, work in progress,
        July 2004.

Floyd/Kohler                                                   [Page 19]

INTERNET-DRAFT            Expires: January 2005                July 2004

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

    [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.

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

    [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 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.

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

    [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-ccid3-06.txt, work in progress, July 2004.

Authors' Addresses

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

Floyd/Kohler                                                   [Page 20]

INTERNET-DRAFT            Expires: January 2005                July 2004

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

Full Copyright Statement

    Copyright (C) The Internet Society 2004.  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    This document and the information contained herein are provided on

Intellectual Property

    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.

    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at ietf-

Floyd/Kohler                                                   [Page 21]

Html markup produced by rfcmarkup 1.111, available from https://tools.ietf.org/tools/rfcmarkup/