RFC4341: Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control

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


   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

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           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",
   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.

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   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

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   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

   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.

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   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

   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

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   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

   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

   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

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      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.

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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.

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   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

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

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   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.

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   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

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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

   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.

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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

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.

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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.

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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

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   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

   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

   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

   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.

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   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

   [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.

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   [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

   [RFC3692]      Narten, T., "Assigning Experimental and Testing
                  Numbers Considered Useful", BCP 82, RFC 3692, January

   [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                  Congestion Control Protocol (DCCP)", RFC 4340, March

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.

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Authors' Addresses

   Sally Floyd
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704

   EMail: floyd@icir.org

   Eddie Kohler
   4531C Boelter Hall
   UCLA Computer Science Department
   Los Angeles, CA 90095

   EMail: kohler@cs.ucla.edu

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Full Copyright Statement

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