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rfc:rfc4341

Network Working Group S. Floyd Request for Comments: 4341 ICIR Category: Standards Track E. Kohler

                                                                  UCLA
                                                            March 2006
      Profile for Datagram Congestion Control Protocol (DCCP)
        Congestion Control ID 2: TCP-like Congestion Control

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document contains the profile for Congestion Control Identifier
 2 (CCID 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.

Table of Contents

 1. Introduction ....................................................2
 2. Conventions and Notation ........................................2
 3. Usage ...........................................................3
    3.1. Relationship with TCP ......................................3
    3.2. Half-Connection Example ....................................4
 4. Connection Establishment ........................................5
 5. Congestion Control on Data Packets ..............................5
    5.1. Response to Idle and Application-Limited Periods ...........8
    5.2. Response to Data Dropped and Slow Receiver .................8
    5.3. Packet Size ................................................8
 6. Acknowledgements ................................................9
    6.1. Congestion Control on Acknowledgements .....................9
         6.1.1. Detecting Lost and Marked Acknowledgements .........10

Floyd & Kohler Standards Track [Page 1] RFC 4341 DCCP CCID2 March 2006

         6.1.2. Changing Ack Ratio .................................10
    6.2. Acknowledgements of Acknowledgements ......................11
         6.2.1. Determining Quiescence .............................12
 7. Explicit Congestion Notification ...............................12
 8. Options and Features ...........................................12
 9. Security Considerations ........................................13
 10. IANA Considerations ...........................................13
    10.1. Reset Codes ..............................................13
    10.2. Option Types .............................................13
    10.3. Feature Numbers ..........................................14
 11. Thanks ........................................................14
 A.  Appendix: Derivation of Ack Ratio Decrease ....................15
 B.  Appendix: Cost of Loss Inference Mistakes to Ack Ratio ........15
 Normative References ..............................................17
 Informative References ............................................18

1. Introduction

 This document contains the profile for Congestion Control Identifier
 2 (CCID 2), TCP-like Congestion Control, in the Datagram Congestion
 Control Protocol (DCCP) [RFC4340].  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 a variant of
 selective acknowledgements (SACK) [RFC2018, RFC3517].  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 [RFC2119].
 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 [RFC4340] for more discussion.

Floyd & Kohler Standards Track [Page 2] RFC 4341 DCCP CCID2 March 2006

 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.
 CCID 2 flows must also tolerate the large sending rate variations
 characteristic of AIMD congestion control, including halving of the
 congestion window in response to a congestion event.
 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) [RFC4342], 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 those 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 [RFC793],
 [RFC2581], [RFC3465], and [RFC3517].  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.

Floyd & Kohler Standards Track [Page 3] RFC 4341 DCCP CCID2 March 2006

 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.

3.2. Half-Connection Example

 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 [RFC3540].
 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; it is not 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.

Floyd & Kohler Standards Track [Page 4] RFC 4341 DCCP CCID2 March 2006

 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-connection is quiescent, the
    sender sends at least one DCCP-DataAck packet per congestion
    window.
 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
    determines 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 that it MAY send data on DCCP-
 Request packets.

5. Congestion Control on Data Packets

 CCID 2's congestion control mechanisms are based on those for SACK-
 based TCP [RFC3517], 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.

Floyd & Kohler Standards Track [Page 5] RFC 4341 DCCP CCID2 March 2006

 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.
 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 [RFC2988].  (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 to ensure that the
    average round-trip time is effectively derived from measurements

Floyd & Kohler Standards Track [Page 6] RFC 4341 DCCP CCID2 March 2006

    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.
 The sender also MUST NOT decrement pipe again upon receiving
 acknowledgement of a packet previously inferred as lost.
 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 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 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 [RFC3465] 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 [RFC3390]; 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.

Floyd & Kohler Standards Track [Page 7] RFC 4341 DCCP CCID2 March 2006

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, [RFC2581] 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.  [RFC2861], 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.  [RFC2861] 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

 DCCP's Data Dropped option lets a receiver declare that a packet was
 dropped at the end host before delivery to the application -- for
 instance, because of corruption or receive buffer overflow.  DCCP's
 Slow Receiver option lets a receiver declare that it is having
 trouble keeping up with the sender's packets, although nothing has
 yet been dropped.  CCID 2 senders respond to these options as
 described in [RFC4340], 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 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.

Floyd & Kohler Standards Track [Page 8] RFC 4341 DCCP CCID2 March 2006

 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
 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 that 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 [RFC4340], Section 11.4.2.  Any Data Dropped options
 SHOULD likewise cover the receiver's entire Acknowledgement Window.
 CCID 2 senders use DCCP's Ack Ratio feature to influence the rate at
 which receivers generate DCCP-Ack packets, 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 is 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.  [RFC4340], Section 11.3,
 describes the Ack Ratio in more detail, including its relationship to
 acknowledgement pacing and DCCP-DataAck packets.  This document's
 Section 6.1.1 describes how a CCID 2 sender detects lost or marked
 acknowledgements, and Section 6.1.2 describes how it changes 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

Floyd & Kohler Standards Track [Page 9] RFC 4341 DCCP CCID2 March 2006

 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 that 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 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
 independent of Ack Ratio 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 [RFC4340], Section 11.3.

Floyd & Kohler Standards Track [Page 10] RFC 4341 DCCP CCID2 March 2006

 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
 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, all the ack
 packets are dropped, and 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.

Floyd & Kohler Standards Track [Page 11] RFC 4341 DCCP CCID2 March 2006

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 [RFC4340], 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 [RFC4340], 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, the
 receiver infers that the sender has gone quiescent when at least T
 seconds have passed without receiving any data from the sender, and
 when 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) [RFC3168].
 The sender will use the ECN Nonce for data packets, and the receiver
 will echo those nonces in its Ack Vectors, as specified in [RFC4340],
 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 [RFC3540] 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.

Floyd & Kohler Standards Track [Page 12] RFC 4341 DCCP CCID2 March 2006

9. Security Considerations

 Security considerations for DCCP have been discussed in [RFC4340],
 and security considerations for TCP have been discussed in [RFC2581].
 [RFC2581] discusses ways in which 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 [RFC4340].
 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 [RFC4340], Section 10.3.  However, this document
 makes no particular allocations from any range, except for
 experimental and testing use [RFC3692].  We refer to the Standards
 Action policy outlined in [RFC2434].

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.

Floyd & Kohler Standards Track [Page 13] RFC 4341 DCCP CCID2 March 2006

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

Floyd & Kohler Standards Track [Page 14] RFC 4341 DCCP CCID2 March 2006

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.  When this congestion avoidance
 behavior is applied 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 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 to a too-slow acknowledgement rate.  All
 acknowledgement information will still get through -- DCCP

Floyd & Kohler Standards Track [Page 15] RFC 4341 DCCP CCID2 March 2006

 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:
 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 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.

Floyd & Kohler Standards Track [Page 16] RFC 4341 DCCP CCID2 March 2006

 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

 [RFC793]       Postel, J., "Transmission Control Protocol", STD 7,
                RFC 793, September 1981.
 [RFC2018]      Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
                "TCP Selective Acknowledgement Options", RFC 2018,
                October 1996.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                2434, October 1998.
 [RFC2581]      Allman, M., Paxson, V., and W. Stevens, "TCP
                Congestion Control", RFC 2581, April 1999.
 [RFC2988]      Paxson, V. and M. Allman, "Computing TCP's
                Retransmission Timer", RFC 2988, November 2000.

Floyd & Kohler Standards Track [Page 17] RFC 4341 DCCP CCID2 March 2006

 [RFC3168]      Ramakrishnan, K., Floyd, S., and D. Black, "The
                Addition of Explicit Congestion Notification (ECN) to
                IP", RFC 3168, September 2001.
 [RFC3390]      Allman, M., Floyd, S., and C. Partridge, "Increasing
                TCP's Initial Window", RFC 3390, October 2002.
 [RFC3517]      Blanton, E., Allman, M., Fall, K., and L. Wang, "A
                Conservative Selective Acknowledgment (SACK)-based
                Loss Recovery Algorithm for TCP", RFC 3517, April
                2003.
 [RFC3692]      Narten, T., "Assigning Experimental and Testing
                Numbers Considered Useful", BCP 82, RFC 3692, January
                2004.
 [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                Congestion Control Protocol (DCCP)", RFC 4340, March
                2006.

Informative References

 [RFC2861]      Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
                Window Validation", RFC 2861, June 2000.
 [RFC3465]      Allman, M., "TCP Congestion Control with Appropriate
                Byte Counting (ABC)", RFC 3465, February 2003.
 [RFC3540]      Spring, N., Wetherall, D., and D. Ely, "Robust
                Explicit Congestion Notification (ECN) Signaling with
                Nonces", RFC 3540, June 2003.
 [RFC4342]      Floyd, S., Kohler, E., and J. Padhye, "Profile for
                Datagram Congestion Control Protocol (DCCP) Congestion
                Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
                4342, March 2006.
 [V03]          Arun Venkataramani, August 2003.  Citation for
                acknowledgement purposes only.

Floyd & Kohler Standards Track [Page 18] RFC 4341 DCCP CCID2 March 2006

Authors' Addresses

 Sally Floyd
 ICSI Center for Internet Research
 1947 Center Street, Suite 600
 Berkeley, CA 94704
 USA
 EMail: floyd@icir.org
 Eddie Kohler
 4531C Boelter Hall
 UCLA Computer Science Department
 Los Angeles, CA 90095
 USA
 EMail: kohler@cs.ucla.edu

Floyd & Kohler Standards Track [Page 19] RFC 4341 DCCP CCID2 March 2006

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Floyd & Kohler Standards Track [Page 20]

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