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Versions: 00 01 02 03 04 05 06 RFC 5690

Internet Engineering Task Force                         S. Floyd, Editor
INTERNET-DRAFT                                                      ICIR
Intended status: Experimental                                   A. Arcia
Expires: 13 October 2007                                          D. Ros
                                                           ENST Bretagne
                                                              J. Iyengar
                                                     Connecticut College
                                                           13 April 2007


            Adding Acknowledgement Congestion Control to TCP
                     draft-floyd-tcpm-ackcc-00.txt


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Copyright Notice

   Copyright (C) The IETF Trust (2007).





Floyd                                                           [Page 1]


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Abstract

   This document adds an optional congestion control mechanism for
   acknowledgement traffic (ACKs) to TCP.  The document specifies an
   end-to-end acknowledgement congestion control mechanism for TCP that
   uses participation from both TCP hosts, the TCP data sender and the
   TCP data receiver.  The TCP data sender detects lost and ECN-marked
   ACK packets, and tells the TCP data receiver the ACK Ratio R to use
   to respond to the congestion on the reverse path from the data
   receiver to the data sender.  The TCP data receiver sends roughly one
   ACK packet for every R data packets received.  This mechanism is
   based on the acknowledgement congestion control in DCCP's CCID 2
   [RFC4340], [RFC4341].  This acknowledgement congestion control
   mechanism is being proposed as an experimental mechanism for TCP for
   evaluation by the network community.




































Floyd                                                           [Page 2]


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Table of Contents

   1. Introduction ....................................................4
   2. Conventions .....................................................4
   3. Overview ........................................................5
   4. Related Work ....................................................5
   5. Acknowledgement Congestion Control ..............................7
      5.1. Negotiating the Use of ACK Congestion Control ..............7
      5.2. The TCP ACK Ratio Option ...................................8
      5.3. Implementing the ACK Ratio .................................8
      5.4. Determining Lost or Marked ACK Packets .....................9
      5.5. Adjusting the ACK Ratio ...................................10
      5.6. Sending ACKs for Out-of-Order Data Segments ...............10
      5.7. The Sender's Response to ACK Packets ......................11
   6. Possible Complications .........................................12
      6.1. Possible Complications:  Delayed Acknowledgements .........12
      6.2. Possible Complications: Duplicate Acknowledgements. .......12
      6.3. Possible Complications:  Two-Way Traffic. .................13
      6.4. Possible Complications:  Reordering of ACK Packets. .......13
      6.5. Possible Complications: Abrupt changes in the ACK path. ...13
      6.6. Possible Complications: Corruption. .......................13
      6.7. Other Issues ..............................................13
   7. Acknowledgement Congestion Control in CCID 2 ...................14
   8. Security Considerations ........................................14
   9. IANA Considerations ............................................15
   10. Conclusions ...................................................15
   11. Acknowledgements ..............................................15
   Normative References ..............................................15
   Informative References ............................................16
   Full Copyright Statement ..........................................17
   Intellectual Property .............................................18




















Floyd                                                           [Page 3]


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1.  Introduction

   This documents adds an optional congestion control mechanism to TCP
   for acknowledgements (ACKs).  This mechanism is based on the
   acknowledgement congestion control in DCCP's CCID 2 [RFC4340],
   [RFC4341].

   In this document we use the termininology of senders and receivers,
   with the sender sending data traffic, and the receiver sending
   acknowledgement traffic in response.  In CCID 2's acknowledgement
   congestion control, specified in Section 6.1 of [RFC4341], the
   receiver maintains an ACK Ratio R, sending roughly one ACK packet for
   every R data packets received.  The CCID 2 sender keeps the
   acknowledgement rate roughly TCP friendly by monitoring the
   acknowledgement stream for lost and marked ACK packets and modifying
   the ACK Ratio accordingly.  For every RTT containing an ACK
   congestion event (that is, a lost or marked ACK packet), the sender
   halves the acknowledgement rate by doubling the ACK Ratio; for every
   RTT containing no ACK congestion event, the sender additively
   increases the acknowledgement rate through gradual decreases in the
   ACK Ratio.

   Adding a similar acknowledgement congestion control as an option in
   TCP requires the following:

   * An agreement from the TCP hosts on the use of ACK congestion
   control.  The TCP hosts use a new TCP option, the ACK-Congestion-
   Control-Permitted Option.

   * A mechanism for the TCP sender to detect lost and ECN-marked pure
   acknowledgement packets.

   * A mechanism for adjusting the ACK Ratio.  The TCP sender adjusts
   the ACK Ratio as specified in Section 6.1.2 of [RFC4341].

   * A method for the TCP sender to inform the TCP receiver of a new
   value for the ACK Ratio.  The TCP sender uses a new TCP option, the
   ACK Ratio Option.


2.  Conventions

   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].






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3.  Overview

   This section gives a non-normative overview of acknowledgement
   congestion control for TCP.

   [Graphics will be added.]

   During connection initiation, TCP host B sends an ACK-Congestion-
   Control-Permitted option on its SYN or SYN/ACK packet.  This allows
   TCP host A (now called the sender) to send instructions to TCP host B
   (now called the receiver) about the Ack Ratio to use in responding to
   data packets.

   Also during connection initiation, TCP host A sends an ACK-
   Congestion-Control-Permitted option on its SYN or SYN/ACK packet.  In
   combination with TCP host B's sending of an ACK-Congestion-Control-
   Permitted option, this allows TCP host B to send its ACK packets as
   ECN-Capable.

   The TCP receiver starts with an ACK Ratio of two, generally sending
   one ACK packet for every two data packets received.

   The TCP sender detects lost or ECN-marked ACK packets from the TCP
   receiver, and at some point sends an ACK Ratio option of three to the
   receiver.  The TCP receiver changes to an ACK Ratio of three,
   generally sending one ACK packet for every three data packets.  The
   TCP sender uses Appropriate Byte Counting and rate-based pacing in
   responding to these ACK packets.

   The TCP sender detects fewer lost ACK packets, and at some point
   sends an ACK Ratio option of two to the TCP receiver.  The TCP
   receiver changes back to an ACK Ratio of two, generally sending one
   ACK packet for every two data packets.

4.  Related Work

   The goal of the mechanism proposed in this document is to control
   pure ACK traffic on the path from the TCP data receiver to the TCP
   data sender.  Note that the approach outlined here is an end-to-end
   one (as is the approach followed by DCCP's CCID 2 [RFC4341]), but it
   may also take advantage of explicit congestion information from the
   network conveyed by ECN [RFC3168], if available.  The ECN
   specification [RFC3168, section 6.1.4] prohibits a TCP receiver from
   setting the ECT(0) or ECT(1) codepoints in IP packets carrying pure
   ACKs, but *only* as long as the receiver does *not* implement any
   form of ACK congestion control.

   There exist several papers dealing with controlling congestion in the



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   reverse path of a TCP connection, especially in the context of
   networks with bandwidth asymmetry.  Some of these proposals require
   explicit support from routers or middleboxes, whereas others are
   "pure" end-to-end schemes.

   Balakrishnan et al. ([BPK97]) describe the use of ECN to detect
   congestion in the return path, in order to reduce the sending rate of
   ACKs.  The use of a RED queue in the reverse path allows for marking
   of ACK packets.  The sender echoes back ECN congestion marks to the
   receiver.  The receiver keeps an ACK ratio d (called the "delayed-ACK
   factor"), specifying the number of data segments that have to be
   received before the receiver sends a new ACK.  The ACK ratio d is
   managed using multiplicative-increase, additive-decrease; upon
   reception of a congestion mark, the receiver doubles the value of d
   (hence dividing the ACK sending rate by two).  The ACK ratio
   decreases linearly for each RTT in which no ECN-marked ACKs are
   received.  Multiple congestion marks received in an RTT are treated
   as a single congestion event, i.e., d can be doubled at most once per
   RTT.  The TCP timestamp option is used to keep track of the RTT
   values.

   In [TJW00], Tam Ming-Chit et al. propose a receiver-based method for
   calculating an "appropriate" number of ACKs per congestion window
   (cwnd) of data, in order to alleviate congestion on the reverse path.
   The sender's cwnd is estimated at the receiver by counting the number
   of received packets per RTT (which also has to be estimated by the
   receiver).  From this estimate, a simple algorithm is used to compute
   the number of ACKs to be sent per cwnd.  The algorithm enforces a
   lower bound on the number of ACKs per cwnd, aiming at minimizing the
   probability of timeout at the sender due to ACK loss.  Similarly, the
   ACK ratio is upper-bounded so as to avoid excessive ACK delay.

   ACK filtering (AF) [BPK97] from Balakrishnan et al. is a router-based
   technique that tries to reduce the number of ACKs sent over the
   congested return link.  With AF, an arriving ACK may replace
   preceding, older ACKs at the bottleneck queue.  An aggressive
   replacement policy might guarantee that at most one ACK per
   connection is waiting in the queue, alleviating congestion.  However,
   as in other proposals, care must be taken to avoid sender timeouts in
   case the (too few) ACKs resulting from the filtering get lost.  The
   idea of filtering ACKs has been extended in [YMH03] to deal with SACK
   information.

   Blandford et al. [BGG+07] propose an end-to-end, receiver-oriented
   scheme called "smartacking".  The algorithm is based upon the
   receiver monitoring the inter-segment arrival time for data packets
   and adapting the ACK sending rate in response.  When the bottleneck
   link is underutilized, ACKs are sent frequently (up to one ACK per



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   received segment) to promote fast growth of the congestion window.
   On the other hand, when the bottleneck is close to full utilization,
   the algorithm tries to reduce control traffic overhead and slow
   congestion window growth by generating ACKs at the minimum rate
   needed to keep the data pipe full.

   Reducing the number of ACKs (or, equivalently, increasing the amount
   of bytes acknowledged by each ACK) can increase the burstiness of the
   TCP sender.  Hence, any mechanism as those cited above should be
   coupled with some burst mitigation technique [AB05].  Such a
   technique may consist in either limiting the size of the bursts or
   pacing the sending of data segments [ASA00], or a combination of both
   like the Sender Adaptation proposal in [BPK97].

   Aweya et al. [AOM02] present a middlebox-based approach for
   mitigating data packet bursts and for controlling the uplink ACK
   congestion.  The main idea is to perform pacing on ACK segments on an
   edge device close to the sender, so as to control the ACK arrival
   rate at the sender.

   Unlike some of the related work cited above, in this document we are
   proposing an end-to-end ACK congestion control mechanism that
   controls congestion on the reverse path (the path followed by the ACK
   traffic) by detecting and responding to marked or dropped ACK
   packets.

5.  Acknowledgement Congestion Control

5.1.  Negotiating the Use of ACK Congestion Control

   The TCP end-points negotiate the use of ACK Congestion Control
   (ACKCC) with a TCP option, the ACK-Congestion-Control-Permitted
   Option.  The option number will be allocated by IANA.

   The ACK-Congestion-Control-Permitted option can only be sent on
   packets that have the SYN bit set.  If TCP end-point A receives an
   ACK-Congestion-Control-Permitted option from TCP end-point B, then
   the TCP end-points MAY use ACK Congestion Control on the pure
   acknowledgements sent from B to A.  This means that TCP end-point A
   MAY send ACK Ratio values to TCP end-point B, for TCP end-point B to
   use on pure acknowledgement packets.

   Similarly, if TCP end-point B receives an ACK-Congestion-Control-
   Permitted option from TCP end-point A, then the TCP end-points MAY
   use ACK Congestion Control on the pure acknowledgements sent from A
   to B.

   If TCP end-point B receives an ACK-Congestion-Control-Permitted



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   option from TCP end-point A and also sent an ACK-Congestion-Control-
   Permitted option to TCP end-point A, then TCP end-point B can send
   its ACK packets as ECN-Capable.


          TCP ACK-Congestion-Control-Permitted Option:

          Kind: N

          +-----------+-----------+
          |  Kind=N   |  Length=2 |
          +-----------+-----------+

   When ACK Congestion Control is used, the default initial ACK Ratio is
   two, with the receiver acknowledging at least every other data
   packet.

5.2.  The TCP ACK Ratio Option

   The sender uses a ACK Ratio TCP Option to communicate the ACK Ratio
   value from the sender to the receiver.


          TCP ACK Ratio Option:

          Kind: N+1

          +-----------+-----------+-----------+
          |  Kind=N+1 |  Length=3 | ACK Ratio |
          +-----------+-----------+-----------+

   The ACK Ratio Option is only sent on data packets.  Because TCP uses
   reliable delivery for data packets, the TCP sender can tell if the
   TCP receiver has received an ACK Ratio Option.

5.3.  Implementing the ACK Ratio

   With an ACK Ratio of R, the receiver should send one pure ACK for
   every R newly received data packets unless the delayed ACK timer
   expires first.  A receiver could simply maintain a counter that
   increments up to R for each new data packet received, and then reset
   the counter to zero when an ACK is sent, either pure or piggybacked.

   [RFC2581] recommends that the receiver SHOULD acknowledge out-of-
   order data packets immediately, sending an immediate duplicate ACK
   when it receives a data segment above a gap in the sequence space,
   and sending an immediate ACK when it receives a data segment that
   fills in all or part of a gap in the sequence space.



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   When ACK Congestion Control is being used and the ACK Ratio is at
   most two, the TCP receiver MUST acknowledge each out-of-order data
   packet immediately.  For an ACK Ratio greater than two, Section 5.6
   specifies in detail the receiver's behavior for sending ACKs for out-
   of-order data packets.


5.4.  Determining Lost or Marked ACK Packets

   The TCP data sender uses its knowledge of the ACK Ratio in use by the
   receiver to infer when an ACK packet has been lost.

   Because the TCP sender knows the ACK Ratio R in use by the receiver,
   the TCP sender knows that in the absence of dropped or reordered
   acknowledgement packets, each new acknowledgement received will
   acknowledge at most R additional data packets.  Thus, if the sender
   receives an acknowledgement acknowledging more than R data packets,
   and does not receive a subsequent acknowledgement acknowledging a
   strict subset (with a smaller cumulative acknowledgement, or with the
   same cumulative acknowledgement but a strict subset of data
   acknowledged in SACK blocks), then the sender can infer that an ACK
   packet has been dropped.

   Similarly, the TCP sender knows that in the absence of dropped or
   delayed data packets from the sender, and in the absence of delayed
   acknowledgements due to a timer expiring at the receiver, each new
   pure acknowledgement received will acknowledge at least R additional
   data packets.  In terms of ACK congestion control, the TCP sender
   does not have to take any actions when it receives an acknowledgement
   acknowledging less than R additional packets.

   If the ACK Ratio is at most two, then the TCP receiver sends a dupACK
   for every out-of-order data packet.  In this case, the TCP sender can
   detect lost DupACK packets by counting the number of DupACKs that
   arrived between the beginning of the loss event and the arrival of
   the first full or partial ACK, and comparing this number with the
   number of DupACKs that should have arrived (based on the number of
   packets being ACKed by the full or partial ACK).

   If the ACK Ratio is greater than two, the TCP receiver does not send
   a dupACK for every out-of-order data packet, as specified in Section
   5.6.  For simplicity, if the ACK Ratio is greater than two, the TCP
   sender does not attempt to detect lost ACK packets during loss events
   involving forward-path data traffic.  That is, as soon as the sender
   infers a packet loss for a forward-path data packet, it stops
   detection of ACK loss on the reverse path. The sender waits until a
   new cumulative acknowledgement is received that covers the
   retransmitted data, and then restarts detection of ACK loss for



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   reverse-path traffic.

5.5.  Adjusting the ACK Ratio

   The TCP sender will adjust the ACK Ratio as specified in Section
   6.1.2 of [RFC4341], as follows.

   The ACK Ratio always meets three constraints: (1) the ACK Ratio is an
   integer.  (2) the ACK Ratio does not exceed cwnd/(2*MSS), rounded up,
   except that ACK Ratio 2 is always acceptable.  (3) the ACK Ratio is
   two or more for a congestion window of four or more full-sized
   segments.

   The sender changes the ACK Ratio within those constraints as follows.
   For each congestion window of data with lost or marked ACK packets,
   the ACK Ratio R is doubled; and for each cwnd/(MSS*(R^2 - R))
   consecutive congestion windows of data with no lost or marked ACK
   packets, the ACK Ratio is decreased by 1.  (See Appendix A of RFC
   4341 for the derivation.  Note that Appendix A of RFC 4341 assumes a
   congestion window W in packets, while we use cwnd in bytes.)

   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 one.

   The sender need not keep the 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 change 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.

   With ACK congestion control, 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. 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.

5.6.  Sending ACKs for Out-of-Order Data Segments

   RFC 2581 says that "a TCP receiver SHOULD send an immediate duplicate
   ACK when an out-of-order segment arrives."  After three duplicate



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   ACKs are received, the TCP sender infers a packet loss and implements
   Fast Retransmit and Fast Recovery, retransmitting the missing packet.
   When the ACK Ratio is at most two, the TCP receiver SHOULD still send
   an immediate duplicate ACK when an out-of-order segment arrives.

   When the ACK Ratio is greater than two, the TCP receiver still SHOULD
   send an immediate duplicate ACK for each of the first three out-of-
   order segments that arrive in a reordering event.  (We define a
   reordering event at the receiver as beginning when an out-of-order
   segment arrives, and ending when the receiver holds no more out-of-
   order segments.)  However, when the ACK Ratio is greater than two,
   after the first three duplicate ACKs have been sent, the TCP receiver
   should perform ACK congestion control on the remaining ACKs to be
   sent during the current reordering event.  That is, after the first
   three duplicate ACKs have been sent, the TCP receiver SHOULD send an
   ACK for every R out-of-order segments, instead of sending an ACK for
   every out-of-order segment.  In addition, a receiver MUST NOT
   withhold an ACK for more than 500 ms.

5.7.  The Sender's Response to ACK Packets

   The use of a large ACK Ratio can generate line rate data bursts at a
   TCP sender.  When the ACK Ratio is greater than two, the TCP sender
   SHOULD use some form of burst mitigation, or rate-based pacing for
   sending data packets in response to a single acknowledgement.  The
   use of rate-based pacing will be limited by the timer granularity at
   the TCP sender.

   We note that the interaction of ACK congestion control and burst
   mitigation schemes needs further study.

   In addition to the impact of a large ACK Ratio on the burstiness of
   the TCP sender's sending rate, a large ACK Ratio can also affect the
   data sending rate by slowing down the increase of the congestion
   window cwnd.  As specified in RFC 2581, in slow-start the TCP sender
   increases cwnd by one full-sized segment for each new ACK received
   (in this context, a "new ACK" is an ACK that acknowledges new data).
   RFC 2581 also specifies that in congestion avoidance, the TCP sender
   increases cwnd by roughly 1/cwnd full-sized segments for each ACK
   received, resulting in an increase in cwnd of roughly one full-sized
   segment per round-trip time.  In this case, the use of a large ACK
   Ratio would slow down the increase of the sender's congestion window.

   RFC 2581 notes that it is also acceptable to count the number of
   bytes acknowledged by new ACKs, and to increase cwnd based on the
   number of bytes acknowledged, rather than on the number of new ACKs
   received.  Thus, the sender SHOULD use Appropriate Byte Counting
   [RFC3465] with Acknowledgement Congestion Control, so that the



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   Acknowledgement Congestion Control doesn't slow down the window
   increases for the data traffic sent by the sender.

   As cited earlier, RFC 2581 infers that a packet has been lost after
   it receives three duplicate acknowledgements.  Because ACK Congestion
   Control is only used when there is congestion on the reverse path,
   after a packet loss one or more of the three duplicate ACKs sent by
   the receiver could be lost on the reverse path, and the receiver
   might wait until it has received R more out-of-order segments before
   sending the next duplicate ACK. All this could slow down Fast
   Recovery and Fast Retransmit quite a bit.  To reduce the potential
   delay in detecting a lost packet, we add that when SACK is used, a
   TCP sender SHOULD use the information in the SACK option to detect
   when the receiver has received at least three out-of-order data
   packets, and to initiate Fast Retransmit and Fast Recovery in this
   case, even if the TCP sender has not yet received three dup ACKs.

6.  Possible Complications

6.1.  Possible Complications:  Delayed Acknowledgements

   The receiver could send a delayed acknowledgement acknowledging a
   single packet, even when the ACK Ratio is two or more.

   This should not cause false positives (when the TCP sender infers a
   loss when no loss happened).  The TCP sender only infers that a pure
   ACK packet has been lost when no data packet has been lost, and an
   ACK packet arrives acknowledging more than R new packets.

   Delayed acknowledgements could, however, cause false negatives, with
   the TCP sender unable to detect the loss of an ack packet sent as a
   delayed acknowedgement.  False negatives seem acceptable; this would
   result in approximate ACK congestion control, which would be better
   than no ACK congestion control at all.  In particular, when this form
   of false negative occurs, it is because the receiver is sending
   acknowledgements at such a low rate that it is sending delayed
   acknowledgements, rather than acknowledging at least R data packets
   with each acknowledgement.

6.2.  Possible Complications: Duplicate Acknowledgements.

   As discussed in Section 5.3, RFC 2581 states that "a TCP receiver
   SHOULD send an immediate duplicate ACK when an out-of-order segment
   arrives," and that "a TCP receiver SHOULD send an immediate ACK when
   the incoming segment fills in all or part of a gap in the sequence
   space" [RFC2581].  When ACK Congestion Control is used, the TCP
   receiver instead uses the guidelines from Section 5.6 to govern the
   sending of duplicate ACKs.  More work would be useful to evaluate the



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   advantages and disadvantages of this approach in terms of the
   potential delay in triggering Fast Retransmit, and to explore
   alternate possibilities.

6.3.  Possible Complications:  Two-Way Traffic.

   In a TCP connection with two-way traffic, the receiver could send
   some pure ACK packets, and some acknowledgements piggy-backed on data
   packets.  In this case, how well can the TCP sender infer when pure
   ACK packets have been lost?  The receiver would still follow the rule
   of only sending a pure ACK packet when there is a need for a delayed
   ack, or there are R new data packets to acknowledge.

6.4.  Possible Complications:  Reordering of ACK Packets.

   It is possible for ACK packets to be reordered on the reverse path.
   The TCP sender could either use a parallel mechanism to the dupACK
   threshold to infer when an ACK packet has been lost, as with TCP, or,
   more robustly, the TCP sender could wait an entire round-trip time
   before inferring that an ACK packet has been lost [RFC4653].

6.5.  Possible Complications: Abrupt changes in the ACK path.

   What happens when there are abrupt changes in the reverse path, such
   as from vertical handovers?  Can there be any problems that would be
   worse than those experienced by a TCP connection that is not using
   ACK congestion control?

6.6.  Possible Complications: Corruption.

   As with data packets, it is possible for ACK packets to be dropped in
   the network due to corruption rather than congestion.  The current
   assumption of ACK congestion control is that all losses should be
   taken as indications of congestion.  When there is some better answer
   for corrupted TCP data packets, the same solution hopefully would
   apply to corrupted ACK packets as well.

6.7.  Other Issues

   Are there any problems caused by the combination of two-way traffic
   and reordering?

   How well would ACK congestion control work without SACK information?
   Or would SACK be required with ACK congestion control?







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7.  Acknowledgement Congestion Control in CCID 2

   Rate-based pacing: For CCID 2, RFC 4341 says that "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."  However, rate-based pacing is not
   required in CCID 2.

   Increasing the congestion window: For CCID 2, RFC 4341 says that
   "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."


8.  Security Considerations

   [To be finished later.]

   What are the sender's incentives to cheat on ACK congestion control?
   What are the receiver's incentives to cheat?  What are the avenues
   open for cheating?

   As long as ACK congestion control is optional, neither host can be
   forced to use ACK congestion control if it doesn't want to.  So ACK
   congestion control will only be used if the sender or receiver have
   some chance of receiving some benefit.

   As long as ACK congestion control is optional for TCP, there is
   little incentive for the TCP end nodes to cheat on non-ECN-based ACK
   congestion control.  There is nothing now that requires TCP hosts to
   use congestion control in response to dropped ACK packets.

   What avenues for cheating are opened by the use of ECN-Capable ACK
   packets?  If the end nodes can use ECN to have ACK packets marked
   rather than dropped, and if the end nodes can then avoid the use of
   ACK congestion control that goes along with the use of ECN on ACK
   packets, then the end nodes could have an incentive to cheat.
   Senders could cheat by not instructing the receiver to use a higher
   ACK Ratio; the receiver would have a hard time detecting this
   cheating.  Receivers could cheat by not using the ACK Ratio they were



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   instructed to use, but senders could easily detect this cheating.
   However, receivers could also cheat by not using ACK congestion
   control and still sending ACK packets as ECN-capable, so ACK
   congestion control is not a necessary component for receivers to
   cheat about sending ECN-capable ACK packets.  One question would be
   whether there is any way for receivers to cheat about sending ECN-
   Capable ACK packets and not using appropriate ACK congestion control
   without this cheating being easily detected by the sender.

   What about the ability of routers or middleboxes to detect TCP
   receivers that cheat by inappropriately sending ACK packets as ECN-
   capable?  The router will only know if the receiver is authorized to
   send ACK packets as ECN-Capable if it monitored both the SYN and
   SYN/ACK packets (and was able to read the TCP options in the packet
   headers).  If ACK congestion control has been negotiated, the router
   will only know if ACK congestion control is being used correctly by
   the receiver if it can monitor the ACK Ratio options sent from the
   sender to the receiver.  If ACK congestion control is being used, the
   router will not necessarily be able to tell if ACK congestion control
   is being used correctly by the sender, because drops of ACK packets
   might be occurring after the ACK packets have left the router.
   However, if the router sees the ACK Ratio options sent from the
   sender, the router will be able to tell if the sender is correctly
   accounting for those ACK packets that are dropped or ECN-marked on
   the path from the receiver to the router.


9.  IANA Considerations

   IANA will allocate the option numbers for the two TCP options, the
   ACK-Congestion-Control-Permitted Option, and the ACK Ratio Option.

10.  Conclusions

11.  Acknowledgements

   Many thanks for feedback and contributed text from Michael Welzl.

Normative References

   [RFC2119]      S. Bradner, Key Words For Use in RFCs to Indicate
                  Requirement Levels, RFC 2119.

   [RFC2581]      Allman, M., V. Paxson, and W. Stevens, "TCP Congestion
                  Control", RFC 2581, April 1999.

   [RFC3465]      Allman, M., TCP Congestion Control with Appropriate
                  Byte Counting (ABC), RFC 3465, Experimental, February



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                  2003.

   [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                  Congestion Control Protocol (DCCP)", RFC 4340, March
                  2006.

   [RFC4341]      Floyd, S., and E. Kohler, Profile for Datagram
                  Congestion Control Protocol (DCCP) Congestion Control
                  ID 2: TCP-like Congestion Control, RFC 4341, March
                  2006.

Informative References

   [RFC3168]      K. Ramakrishnan, S. Floyd and D. Black. The Addition
                  of Explicit Congestion Notification (ECN) to IP. RFC
                  3168, September 2001.

   [RFC4653]      S. Bhandarkar, A. L. N. Reddy, M. Allman and E.
                  Blanton, Improving the Robustness of TCP to Non-
                  Congestion Events, RFC 4653, August 2006.

   [ASA00]        A. Aggarwal, S. Savage, and T. Anderson. Understanding
                  the Performance of TCP Pacing. In INFOCOM (3), pages
                  11571165, 2000.

   [AB05]         M. Allman and E. Blanton. Notes on Burst Mitigation
                  for Transport Protocols. SIGCOMM Comput. Commun. Rev.,
                  35(2):5360, 2005.

   [AOM02]        J. Aweya, M. Ouellette, and D. Y.  Montuno. A Self-
                  regulating TCP Acknowledgement (ack) Pacing Scheme.
                  Int. J. Netw. Manag., 12(3):145163, 2002.

   [BPK97]        Balakrishnan, H., V. Padmanabhan, and Katz, R., The
                  Effects of Asymmetry on TCP Performance, Third
                  ACM/IEEE Mobicom Conference, September 1997.

   [BGG+07]       D.K. Blandford, S.A. Goldman, S. Gorinsky, Y. Zhou,
                  and D.R. Dooly.  Smartacking: Improving TCP
                  Performance from the Receiving End. Journal of
                  Internet Engineering, 1(1), 2007.

   [TJW00]        I. Tam Ming-Chit, D. Jinsong and W. Wang. Improving
                  TCP Performance Over Asymmetric Networks. ACM SIGCOMM
                  Computer Communication Review, 30(3), July 2000.

   [YMH03]        L. Yu, Y. Minhua, and Z. Huimin. The Improvement of
                  TCP Performance in Bandwidth Asymmetric Network.  IEEE



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                  PIMRC, 1:482-486, September 2003.


Authors' Addresses

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

   EMail: floyd <at> icir <dot> org

   Andres Arcia
   Networking, Security & Multimedia (RSM) Dpt.
   GET / ENST Bretagne
   Rue de la Chataigneraie, CS 17607
   35576 Cesson Sevigne Cedex
   France

   Email: AE <dot> ARCIA <at> enst-bretagne <dot> fr

   Janardhan R. Iyengar
   Connecticut College
   270 Mohegan Avenue
   New London, CT 06320
   USA

   Email: iyengar <at> conncoll <dot> edu


   David Ros
   Networking, Security & Multimedia (RSM) Dpt.
   GET / ENST Bretagne
   Rue de la Chataigneraie, CS 17607
   35576 Cesson Sevigne Cedex
   France

   Email: David <dot> Ros <at> enst-bretagne <dot> fr


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   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.



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INTERNET-DRAFT        TCPM - ACK CONGESTION CONTROL           April 2007


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Floyd                                                          [Page 18]


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