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Versions: 00 01 02 03 04 05 06 RFC 5690

Internet Engineering Task Force                                 S. Floyd
INTERNET-DRAFT                                                      ICIR
Intended status: Experimental                                   A. Arcia
Expires: 14 January 2009                                          D. Ros
                                                           ENST Bretagne
                                                              J. Iyengar
                                                     Connecticut College
                                                            14 July 2008


            Adding Acknowledgement Congestion Control to TCP
                     draft-floyd-tcpm-ackcc-04.txt


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   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).





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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.
   This acknowledgement congestion control mechanism is being proposed
   as an experimental mechanism for TCP for evaluation by the network
   community.




































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                             Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   7
   2. Conventions and Terminology. . . . . . . . . . . . . . . . . .   8
   3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4. Acknowledgement Congestion Control . . . . . . . . . . . . . .   9
      4.1. Negotiating the Use of ACK Congestion Control . . . . . .   9
      4.2. The TCP ACK Ratio Option. . . . . . . . . . . . . . . . .  10
      4.3. The Receiver: Implementing the ACK Ratio. . . . . . . . .  10
      4.4. The Sender: Determining Lost or Marked ACK Pack-
      ets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
      4.5. The Sender: Adjusting the ACK Ratio . . . . . . . . . . .  12
      4.6. The Receiver: Sending ACKs for Out-of-Order Data
      Segments . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
      4.7. The Sender: Response to ACK Packets . . . . . . . . . . .  14
      4.8. Possible Addition: Receiver Bounds on the Ack
      Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5. Possible Complications . . . . . . . . . . . . . . . . . . . .  16
      5.1. Possible Complication: Delayed
      Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  16
      5.2. Possible Complication: Duplicate Acknowledge-
      ments. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
      5.3. Possible Complication: Two-Way Traffic. . . . . . . . . .  17
      5.4. Possible Complication: Reordering of ACK Pack-
      ets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
      5.5. Possible Complication: Abrupt Changes in the ACK
      Path.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
      5.6. Possible Complication: Corruption.. . . . . . . . . . . .  18
      5.7. Possible Complication: ACKs That Don't Contrib-
      ute to Congestion. . . . . . . . . . . . . . . . . . . . . . .  18
      5.8. Possible Complication: TCP Implementations that
      Skip ACK Packets . . . . . . . . . . . . . . . . . . . . . . .  21
      5.9. Possible Complication: Router or Middlebox-based
      ACK Mechanisms . . . . . . . . . . . . . . . . . . . . . . . .  22
      5.10. Other Issues . . . . . . . . . . . . . . . . . . . . . .  22
   6. Evaluating ACK Congestion Control. . . . . . . . . . . . . . .  22
      6.1. Contention in Wireless Links or in non-switched
      Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
      6.2. Keep-alive and Other Special ACK Packets. . . . . . . . .  23
   7. Measurements of ACK Traffic and Congestion . . . . . . . . . .  23
   8. Acknowledgement Congestion Control in DCCP's CCID 2. . . . . .  24
   9. Security Considerations. . . . . . . . . . . . . . . . . . . .  24
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  26
   12. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  26
   A. Appendix: Related Work . . . . . . . . . . . . . . . . . . . .  26
      A.1. ECN-only Mechanisms . . . . . . . . . . . . . . . . . . .  27



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      A.2. Receiver-only Mechanisms. . . . . . . . . . . . . . . . .  27
      A.3. Middlebox-based Mechanisms. . . . . . . . . . . . . . . .  28
   B. Appendix: Design Considerations. . . . . . . . . . . . . . . .  28
      B.1. The TCP ACK Ratio Option, or an AckNow bit in
      data packets?. . . . . . . . . . . . . . . . . . . . . . . . .  28
   Normative References. . . . . . . . . . . . . . . . . . . . . . .  29
   Informative References. . . . . . . . . . . . . . . . . . . . . .  29
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  32
   Intellectual Property . . . . . . . . . . . . . . . . . . . . . .  32










































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   TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

   Changes from draft-floyd-tcpm-ackcc-03.txt:

   * General editing.  Feedback from Alfred Hoenes.

   * General editing.  Feedback from Gorry Fairhurst.

   * Added a subsection on "Contention in Wireless Links
     or in non-switched Ethernet".

   * Modified Section 4.4 to say that the sender doesn't try to
     detect lost ACK packets during loss events even if
     the ACK Ratio is two.

   * Added a paragraph to Section 4.4 about "Detecting lost ACK
     packets after changes in the ACK Ratio".

   Changes from draft-floyd-tcpm-ackcc-02.txt:

   * Cited RFC 3449 (PILC), RFC 3135 (PILC), and RFC 2760 (TCPSAT).
     From December 2007 Working Group meeting.

   * Added a note about the problem of effective ACK Congestion
     Control for environments with systematic reordering in the
     data path.

   * General editing.  Feedback from Alfred Hoenes.

   * Added more about keep-alive packets and window update packets.
     Feedback from Anantha Ramaiah.

   * Clarified that SACK "SHOULD" be used.  We don't know enough
     to say "MUST".

   Changes from draft-floyd-tcpm-ackcc-01.txt:

   * Added a section on "Keep-alive Packets".  Feedback from
     Anantha Ramaiah.

   * Added a section on "Possible Complication: TCP Implementations that
     Skip ACK Packets".  Motivated by reports at IETF that many
     high-bandwidth TCPs don't follow the MUST of sending an ACK for
     every other packet, if they don't have time.

   * Added that receivers might have buffer limitations that require
     that they ack at least every K packets, for some K.  Feedback
     from Sara Landstrom.



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   * Added to the discussion of "Possible Complication: Two-Way
     Traffic".  Feedback from Sara Landstrom.

   * Added a section on "Possible Complication: Router or
     Middlebox-based ACK Mechanisms".   Feedback from Sara Landstrom.

   * Added that SACK is required with ACK congestion control.
     Feedback from Sara Landstrom.

   * Added a discussion of "Reducing the TCP Acknowledgment Frequency"
     to the related work section.

   * Moved the Related Work section to the appendix.
     Feedback from Alfred Hoenes.

   * General editing from feedback from Alfred Hoenes.

   * Added an appendix on "Design Considerations", with a subsection
     on "The TCP ACK Ratio Option, or an AckNow bit in data packets?".

   Changes from draft-floyd-tcpm-ackcc-00.txt:

   * Added a discussion of environments where the reverse path
     is congested, but the TCP ACK traffic does not significantly
     contribute to that congestion.  In this case, the goal is
     to minimize the negative impact of AckCC on TCP performance.
     Feedback from Armando Caro.

   * In Section 5.7, added that when ABC is used with Aggregate
     Congestion Control, and rate-based pacing is also used, the sender
     MAY increase cwnd by more than 2 MSS.
     Feedback from Armando Caro.

   * Added a section about measurements of ACK traffic and congestion.
     Feedback from Armando Caro.

   * Added a section on the possibility of a TCP receiver-imposed
     lower bound on the ACK Ratio.  Suggested by Mark Allman.

   * Added to the discussion of the minimum ACK sending rate.
     Suggested by Mark Allman.

   * Added a note that if the TCP receiver doesn't sent an ACK for
     every duplicate data packet, the sender's Fast Recovery
     procedure will have to be modified to take this into
     account.  Feedback from Mark Allman.

   * Added a discussion of evaluating ACK Congestion Control.



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     From feedback from Mark Allman.

   * Some general editing in response to feedback from Mark Allman.

   END OF SECTION TO BE DELETED.

1.  Introduction

   This document adds an optional congestion control mechanism for
   acknowledgements (ACKs) to TCP.  This mechanism is based on the
   acknowledgement congestion control in DCCP's CCID 2 [RFC4340],
   [RFC4341], which is a successor to the TCP acknowledgement congestion
   control mechanism proposed by Balakrishnan et at. in [BPK97].

   In this document we use the terminology 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 uses an ACK Ratio R reported to it by the sender, 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.

   The goal of this document is to explore a similar congestion control
   mechanism for acknowledgement traffic for TCP.  The goal is for the
   TCP sender to monitor the packet drop rate for ACK packets, and to
   respond to a high ACK packet drop rate by instructing the receiver to
   reduce the sending rate for ACK packets.  The assumption is that in
   some environments with congestion on the reverse path, reducing the
   sending rate for ACK traffic traversing the congested path can help
   to reduce the congestion itself, in turn reducing the packet drop
   rates for the ACK traffic.  For those environments where the reverse
   path is congested but where TCP ACK traffic does not appreciably
   contribute to that aggregate congestion, the goal is for TCP's ACK
   congestion control to have a minimal negative effect on the
   performance of the TCP connection.

   Adding 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-



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   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 and Terminology

   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].   MSS
   refers to the Maximum Segment Size.

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.




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   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.  Acknowledgement Congestion Control

   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.  Unlike some of the related work
   cited in the appendix, 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 either marked or dropped ACK packets.

4.1.  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
   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 pure ACK packets as ECN-Capable.






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          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.

4.2.  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.

4.3.  The Receiver: 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.

   If the receiver has buffer limitations, the receiver might have to
   acknowledge K packets, for some K less than the current ACK Ratio R.
   In this case, the sender could observe from the acknowledgements that
   the receiver is acknowledging less than R packets.

   It is possible for there to be lost or marked ACK packets when there
   haven't yet been any lost or marked data packets.  Thus, the sender
   could increase the ACK Ratio R even during the initial slow-start.

   [RFC2581] recommends that the receiver SHOULD acknowledge out-of-



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   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.

   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 4.6
   specifies in detail the receiver's behavior for sending ACKs for out-
   of-order data packets.

4.4.  The Sender: 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.   Because SACK options in ACK packets can
   help the sender in detecting lost ACK packets, SACK SHOULD be used
   with ACK congestion control.

   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.

   Out-of-order data 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 should be able to detect lost DupACK
   packets by counting the number of DupACKs that arrive 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).  Simulations and/or experiments will be
   needed to determine whether, in practice, it works for the TCP sender
   to assess lost ACK packets during loss events, for an ACK Ratio of at
   most two.



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   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
   4.6.  For simplicity, 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 reverse-path traffic.

   Detecting lost ACK packets after changes in the ACK Ratio: In
   detecting lost ACK packets, the sender relies on its knowledge of the
   ACK Ratio used by the receiver.  But when the sender makes a change
   in the ACK Ratio, and then receives ACK packets, how does the sender
   know whether the receiver was using the new or the old ACK Ratio when
   it sent those ACK packets?  As specified in the next section, the
   sender can make only one of two possible changes to the ACK Ratio
   within one round-trip time.  The sender can decrease the ACK Ratio by
   one, from R to R-1, or the sender can double the ACK Ratio,
   increasing it from R to 2R.  But, in detecting lost ACK packets after
   an increase in the ACK Ratio, the sender needs to know whether the
   receiver was using the old ACK Ration R or the new ACK Ration 2R.

   The sender sends ACK Ratio Options only on data packets, and these
   data packets are acknowledged by the receiver.  One possibility would
   be for the sender to save the sequence number of the last data packet
   that contained an ACK Ratio Option, and to remember whether that ACK
   Ratio Option was for an increase or a decrease in the ACK Ratio.
   Then, if the sender receives an ACK packet acknowledging the saved
   sequence number, then the sender knows that the receiver has begun
   using the new ACK Ratio.

   It *might* be sufficient for the sender just to save the information
   of whether the last change in the ACK Ratio was an increase or a
   decrease, without saving the sequence number associated with the last
   ACK Ratio Option.  In this way, if the sender recently increased the
   ACK Ratio from R to 2R, the sender could be more cautious in
   detecting lost ACK packets.  However, we do not explore this approach
   in more detail in this document.

4.5.  The Sender: 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 the following three constraints.

   (1) The ACK Ratio is an integer.



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   (2) The minimum ACK sending rate: The ACK Ratio does not exceed
   max(2, cwnd/(K*MSS)), rounded up, for K=2.  This ensures that the TCP
   receiver sends at least two ACKs for a window of data (for a window
   of at least four full-sized segments).

   (3) If the congestion window is at least as large as four full-sized
   segments, then the ACK Ratio is at least two.  In other words, an ACK
   Ratio of one is only allowed when the congestion window is at most
   three 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.)  As
   stated in the previous section, when the ACK Ratio is greater than
   two the sender does not attempt to detect lost ACK packets during
   loss events for forward-path traffic.

   For a constant congestion window, these modifications to the ACK
   ratio give 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
   the ACK Ratio by one.

   The frequency of ACK Ratio negotiations: 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 the ACK Ratio
   to one for half-connections with persistent congestion windows of 1
   or 2 packets.

   The minimum ACK sending rate: From rule (2) above, the TCP receiver
   always sends at least K=2 ACKs for a 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.  One possibility would be to use a higher minimum ACK sending
   rate, adding a constant upper bound on the ACK Ratio.  That is, if
   the ACK Ratio also had an upper bound of J, independent of cwnd, then



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   the receiver would always send at least one ACK for every J data
   packets, regardless of the level of congestion on the reverse path.


4.6.  The Receiver: 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
   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.

   In general, 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
   return to sending an ACK for every R segments, instead of sending an
   ACK for every out-of-order segment in that reordering event.  [We
   note that the Fast Recovery procedure of the TCP sender might have to
   be modified to take this change into account.]  In addition, a
   receiver MUST NOT withhold an ACK for more than 500 ms.

   We note that in an environment with systematic reordering in the data
   path (e.g., every set of K data packets arrives in inverted order,
   for some value of K), the guideline above could result in the
   receiver sending an ACK for every data packet, regardless of the ACK
   Ratio.  In such an environment with persistent reordering, the
   receiver MAY decide not to send an immediate duplicate ACK for each
   of the first three out-of-order segments that arrive in a reordering
   event.  We leave the investigation of mechanisms for effective ACK
   Congestion Control in environments with systematic reordering for
   future work.

4.7.  The Sender: 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.



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

   Byte counting at the sender: 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 during congestion avoidance 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 this
   form of byte counting with Acknowledgement Congestion Control, so
   that the Acknowledgement Congestion Control doesn't slow down the
   window increases for the data traffic sent by the sender.  Because
   rate-based pacing should be used with Acknowledgement Congestion
   Control, as recommended earlier in this section, the TCP sender MAY
   increase the congestion window by more than two MSS for each ACK.

   We note that for Appropriate Byte Counting (ABC) as specified in
   [RFC3465], during Slow-Start the sender is allowed to increase the
   congestion window by at most two MSS for each ACK.  It has not yet
   been determined whether, with Acknowledgement Congestion Control, the
   TCP sender could use ABC during Slow-Start.  If ABC is used with
   Acknowledgement Congestion Control, then when the TCP sender is in
   slow-start and the Ack Ratio is greater than two, the TCP sender MAY
   increase the congestion window by more that two MSS in response to a
   single ACK.  Section 4.2 of [LL07] explores some of the issues with
   the use of ABC for TCP connections with an Ack Ratio greater than
   two.

   Inferring lost data packets: 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



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   lost packet, 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.

4.8.  Possible Addition: Receiver Bounds on the Ack Ratio

   It has been suggested that in some environments, the TCP receiver
   might want to set lower bounds on the ACK Ratio.  For example, the
   TCP receiver might know from configuration or from past experience
   that the bandwidth on the return path is limited, and might want to
   set a lower bound (greater than two) on the ACK Ratio R.  If this is
   included, this would require a TCP Option from the TCP receiver to
   the TCP sender reporting the lower bound on the ACK Ratio.  Care
   would also be needed so that the lower bound on the ACK Ratio was
   only in effect when the TCP sender's congestion window was
   sufficiently high.


5.  Possible Complications

5.1.  Possible Complication: 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 acknowledgement.  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.

5.2.  Possible Complication: Duplicate Acknowledgements.

   As discussed in Section 4.3, 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



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   space" [RFC2581].  When ACK Congestion Control is used, the TCP
   receiver instead uses the guidelines from Section 4.6 to govern the
   sending of duplicate ACKs.  More work would be useful to evaluate the
   advantages and disadvantages of this approach in terms of the
   potential delay in triggering Fast Retransmit, and to explore
   alternate possibilities.

5.3.  Possible Complication: 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.  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.

   In a connection with two-way traffic, the TCP sender would not always
   be able to infer when a pure ACK packet had been lost.  For example,
   the receiver could send a pure ACK packet acknowledging packet K, and
   soon afterwards, the receiver could send a newly-generated data
   packet for the reverse-path flow also acknowledging packet K.  The
   pure ACK packet could be dropped in the network, and the sender would
   not be able to detect this drop.

   Fortunately, there are limitations to the potential problems caused
   by undetected ACK losses in two-way traffic.  The sender will only
   fail to detect the loss of a pure ACK packet if the ACK packet was
   followed by a data packet with the same acknowledgement number.  If
   the reverse-path traffic for the connection is dominated by data
   traffic, then the congestion control for the data traffic is more
   important than the congestion control for the pure ACK traffic.  If
   the reverse-path traffic is dominated by pure ACK traffic, then the
   sender would detect any losses of pure ACK packets followed by other
   pure ACK packets, and this would include most of the pure ACK packets
   for that connection.  Thus, the sender's failure to detect the loss
   of a pure ACK packet followed by a data packet with the same
   acknowledgement number would not disable acknowledgement congestion
   control for a TCP connection with two-way traffic.

5.4.  Possible Complication: 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].






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5.5.  Possible Complication: 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?

5.6.  Possible Complication: 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.  If there is ever some better
   mechanism for identifying and responding to corrupted TCP data
   packets, the same solution hopefully would apply to corrupted ACK
   packets as well.

   One problem with the interaction of packet corruption and congestion
   control, for both data and ACK packets, is that it is not always
   obvious when the packet corruption is related to congestion, and when
   the packet corruption is independent of the level of congestion on
   the corrupting link.  In environments where packet corruption exists
   and is independent of the level of congestion on the corrupting link,
   applying ACK congestion control would only make the connection more
   sensitive to ACK packet corruption, by reducing the number of ACKs
   that are sent.

5.7.  Possible Complication: ACKs That Don't Contribute to Congestion.

   It is possible for the ACK packets in a TCP connection to traverse a
   congested path where ACK packets are dropped, but where the ACK
   packets themselves don't significantly contribute to the congestion
   on the path.  In scenarios where ACK packets are dropped but where
   ACK traffic doesn't make a significant contribution of the congestion
   on the path, the use of ACK Congestion Control would not contribute
   to reducing the aggregate congestion on the path.  In this case, one
   goal is to minimize the negative impact of ACK Congestion Control on
   the overall performance of the TCP connection.













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       J TCP conns.            link L ->           J TCP conns.
         data ->      |---|                 |---|   <- ACKs
      <-------------> |   |                 |   | <------------->
                      |   | <-------------> |   |
      <-------------> |   |                 |   | <------------->
       K TCP conns.   |---|                 |---|  K TCP conns.
        ACKs ->               <- link L1            <- data

       A scenario with J forward and K reverse TCP connections.

   To explore the relative contribution of ACK traffic on congestion, it
   is useful to consider a simple scenario with a congested
   unidirectional link L carrying data traffic from J TCP connections
   (the forward TCP connections) and ACK traffic from K TCP connections
   (the reverse TCP connections).  We assume that all TCP connections
   have the same round-trip time R and the same data packet size S of
   1500 bytes.  We further assume that all of the forward TCP
   connections have the same data packet drop rate p and the same
   congestion window W, and that all of the reverse TCP connections have
   the same congestion window W1 and the same ACK packet drop rate p1.
   The J TCP connections each use a bandwidth on link L of 1500*W/R
   bytes per second, and the K TCP connections, without ACK Congestion
   Control, each use an bandwidth on link L of 40*(W1/2)/R bytes per
   second.  This gives a ratio of 75*(J/K)*(W/W1) for TCP data bandwidth
   to TCP ACK bandwidth on link L.  The ratio J/K is the ratio between
   the number of forward and reverse TCP connections on link L, and
   could have a wide range of values (e.g., large for an access link
   from a web server, and small for an access link to a web server).
   For this scenario, the ratio W/W1 is largely a function of the
   different levels of congestion on the forward and reverse paths.

   To explore the possibilities, we will consider some of the range of
   congestion control mechanisms for the congested link.  First, we
   consider scenarios where the limitation on the congested path is in
   the link bandwidth in bytes per second.

   Cases (1), (2), (3), (5), and (7) below represent the best scenarios
   for ACK Congestion Control, where the fraction of packet drops for
   TCP ACK packets roughly matches the TCP ACK packets' contribution to
   congestion.  [In several of these cases this is at best a rough match
   because the data packets are a factor in the bandwidth and in the
   queue limitations, while the TCP ACK packets are only a factor in the
   queue limitations.]  Cases (4) and (8) below represent problematic
   scenarios where the fraction of packet drops for TCP ACK packets is
   much higher than the TCP ACK packets' contribution to congestion.
   Case (6) below represents scenarios where ACK Congestion Control
   would not be effective because it would not be invoked.  In the
   scenarios in case (6), the fraction of packet drops for TCP ACK



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   packets would be much smaller than the TCP ACK packets' contribution
   to congestion.

   (1) The Drop-Tail queue for link L is measured in packets.  In this
   case, the congested queue can accommodate N packets, regardless of
   packet size, there is a limitation of both bandwidth in bytes per
   second and also in queue space in packets, and large data packets and
   small TCP ACK packets should see similar packet drop rates.  Although
   TCP ACK packets most likely aren't a major factor in the bandwidth
   limitation, they can be a significant contribution to the limitation
   of queue space.  So, while the drop rate for ACK packets could be
   high in times of congestion, the ACK packets are contributing to that
   congestion somewhat by using scarce buffer space.

   (2) The Drop-Tail queue is measured in bytes.  In this case, the
   congested queue can accommodate M bytes of packets, and TCP ACK
   packets don't make a significant contribution to either the bandwidth
   limitation or to the limitation in queue space.  It is also the case
   that in this scenario, even if there is heavy congestion, the drop
   rate for TCP ACK packets should be small (because small ACK packets
   can often find space on the congested queue when large data packets
   can't find space).  In this case, ACK Congestion Control should not
   present any problems; the TCP ACK packets aren't contributing
   significantly to congestion, and aren't experiencing significant
   packet drop rates.

   (3) The RED queue is in packet mode, and is measured in packets.
   This is similar to case (1) above.  Because the queue is measured in
   packets, small TCP ACK packets contribute to the limitation in queue
   space, but not to the limitation in link bandwidth.  Because the
   queue is in packet mode, large data packets and small TCP ACK packets
   should see similar packet drop rates.

   (4) The RED queue is in packet mode, but is measured in bytes.
   Because the queue is measured in bytes, small TCP ACK packets don't
   contribute significantly to either the limitation in queue space or
   to the limitation in link bandwidth.  Because the queue is in packet
   mode, large data packets and small TCP ACK packets should see similar
   packet drop rates.  If it existed, this case would be problematic,
   because the TCP ACK packets would not be contributing significantly
   to the congestion, but they would see a similar drop rate as the
   large data packets that are contributing to congestion.

   (5) The RED queue is in byte mode, and is measured in bytes.  This is
   similar to case (2) above.  Because the queue is measured in bytes,
   small TCP ACK packets don't contribute significantly to either the
   limitation in queue space or to the limitation in link bandwidth.  At
   the same time, because the queue is in byte mode, small TCP ACK



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   packets see much smaller packet drop rates that those of large data
   packets.

   (6) The RED queue is in byte mode, but is measured in packets.
   Because the queue is measured in packets, small TCP ACK packets
   contribute to the limitation in queue space, but not to the
   limitation in link bandwidth.  Because the queue is in byte mode,
   small TCP ACK packets see much smaller packet drop rates that those
   of large data packets.  If this case existed, TCP ACK packets would
   contribute somewhat to congestion, but would see a much smaller
   packet drop rate than that of large data packets.

   Next, we consider scenarios where the limitation on the congested
   link is in CPU cycles at the router in packets per second, not in
   bandwidth in bytes per second.

   (7) The CPU load imposed by TCP ACK packets is similar to the load
   imposed by other packets (e.g., TCP data packets).  ACK Congestion
   Control would be useful in this scenario, particularly if TCP ACK
   packets saw the same packet drop rates as TCP data packets.

   (8) The CPU load imposed by TCP ACK packets is much less than the
   load imposed by other packets (e.g., TCP data packets).  If TCP ACK
   packets saw a smaller packet drop rate than TCP data packets, then
   the TCP ACK packet drop rate would roughly match the TCP ACK packets'
   contribution to congestion, and this would be good.  If TCP ACK
   packets saw the same packet drop rate as TCP data packets, this case
   would be problematic, because the TCP ACK packets would not be
   contributing significantly to the congestion, but they would see a
   similar drop rate as the large data packets that are contributing to
   congestion.

5.8.  Possible Complication: TCP Implementations that Skip ACK Packets

   It has been reported in IETF meetings that current TCP
   implementations do not always acknowledge at least every other data
   packet, as required by the TCP specifications.  In particular, it has
   been reported that if a TCP receiver receives many data packets in a
   burst, before it is able to send an acknowledgement, then it might
   send a single acknowledgement for the burst of packets.  We note that
   such a behavior would cause complications for a TCP connection that
   used ACK congestion control, as the sender would not be able to
   determine when an ACK packet had been dropped in the network, and
   when the packet had been skipped by the receiver because it was
   processing a burst of data packet arrivals.

   One possibility for addressing this problem would be for TCP
   receivers using ACK congestion control to be required to send an



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   acknowledgement for each R packets, for ACK Ratio R.  In this case,
   if the receiver received a large burst of data packets back-to-back,
   the receiver would be required to send a responding burst of ACK
   packets, one for each set of R data packets.

   A second possibility for addressing this problem would be to define a
   TCP option or flag that the TCP receiver could use when sending an
   ACK packet to inform the sender that the TCP receiver `skipped' some
   ACK packets, so that the sender should not infer ACK loss if some
   previous ACK packets seem to be missing.

   Future work will explore the costs and benefits of these two
   approaches.

5.9.  Possible Complication: Router or Middlebox-based ACK Mechanisms

   One possible complication would be the interaction of ACK Congestion
   Control with router-based or middlebox-based ACK mechanisms such as
   ACK filtering along the reverse path [BPK97] [WWCM99] [BA03] [KLS07].
   We are not aware of the deployment of ACK filtering in the Internet,
   but any testing of ACK congestion control would have to look for
   interactions with any middlebox-based mechanisms regarding ACK
   packets.  In particular, we would consider interactions of ACK
   congestion control with the possible deployment of ACK filtering on
   satellite links, cable modems, or the like.

5.10.  Other Issues

   Are there any problems caused by the combination of two-way traffic
   and reordering?  Or other issues that have not yet been addressed?


6.  Evaluating ACK Congestion Control

   Evaluating ACK Congestion Control will have two components: (1)
   evaluating the effects of ACK Congestion Control on an individual TCP
   connection; and (2) evaluating the effects of ACK Congestion Control
   on aggregate traffic (including the effects of ACK Congestion Control
   on the aggregate congestion of the path).

   The first part, evaluating ACK Congestion Control on the performance
   of an individual TCP connection, will have to examine those scenarios
   where ACK Congestion Control might help the performance of a TCP
   connection, and those scenarios where the use of ACK Congestion
   Control might cause problems.

   The second part, evaluating the effects of ACK Congestion Control on
   aggregate traffic, should consider scenarios where the use of ACK



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   Congestion Control helps all of the connections sharing a path by
   reducing the aggregate congestion on the path. This part should also
   see if there are scenarios where ACK Congestion Control causes
   problems by increasing the burstiness of aggregate traffic, or by
   otherwise changing traffic dynamics.

6.1.  Contention in Wireless Links or in non-switched Ethernet

   One possible benefit of ACK congestion control is that it could
   reduce contention in wireless links, shared Ethernet, or other
   environments with contention between forward-path and reverse-path
   traffic [AJ03] [KIA07].  At the same time, contention on the shared
   medium won't necessarily result in dropped ACK packets, and therefore
   wouldn't necessarily be detected by ACK congestion control.

6.2.  Keep-alive and Other Special ACK Packets

   Some TCP hosts send keep-alive packets when no data or ACK packets
   have been received over a long period of time [KEEP-ALIVE].  This
   keep-alive mechanism is not addressed in TCP specifications.
   However, such keep-alive packets, if used, should not interact with
   ACK congestion control one way or another.  For ACK congestion
   control, the ACK Ratio is set small enough to allow the receiver to
   send at least two ACKs for a window of data, In addition, the
   receiver uses a delayed ACK timer with the Ack Ratio, always sending
   an acknowledgement if the delayed ACK timer expires.  Thus, ACK
   congestion control will never cause the receiver to delay
   indefinitely in sending an acknowledgement for a received data
   packet.

   Some TCP implementations send pure ACK packets as window probes, to
   solicit an ACK packet from the other end with current window
   information.  Such ACK packets will generally be orthogonal to the
   ACK Congestion Control specified in this draft.

   TCP receivers also can send pure ACK packets as window update packets
   announcing a new value for the receive window, even when the
   acknowledgement number and SACK options in the ACK packet are not
   new.  The receiver MAY send window update packets even if the ACK
   Congestion Control mechanism would say that it is not time yet to
   send a pure ACK.  The sender will not necessarily be able to detect
   the loss of a window update ACK packet.

7.  Measurements of ACK Traffic and Congestion

   There are a number of studies about the traffic composition on
   various links in the Internet, reporting the fraction of bandwidth
   used by TCP data and by TCP ACK traffic.  [Pointers to be added.]



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   Are there any studies that show the relative drop rates for TCP data
   and ACK traffic, for particular links or for particular TCP
   connections?

   Are there any studies of congested links that show the fraction of
   traffic on the congested link, or in the congested queue, that
   consist of TCP ACK packets?

8.  Acknowledgement Congestion Control in DCCP's CCID 2

   In the transport protocol DCCP [RFC4340], the congestion control
   mechanism for the CCID 2 profile is based on that of TCP.  This
   section briefly discusses some of the issues that have been addressed
   in the acknowledgement congestion control already standardized 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."


9.  Security Considerations

   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



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   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
   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 the router can see traffic on both
   the forward and reverse paths, and 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.


10.  IANA Considerations

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







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11.  Conclusions

   This document specifies a congestion control mechanism for
   acknowledgement traffic (ACKs) for TCP, and discusses the possible
   complications.  We are deferring a recommendation on the use of this
   mechanism for TCP until it has been evaluated more fully.

12.  Acknowledgements

   Many thanks for feedback from Mark Allman, Armando Caro, Alfred
   Hoenes, Sara Landstrom, Anantha Ramaiah, and Michael Welzl, and for
   contributed text from Michael Welzl.

A.  Appendix: Related Work

   There exist several papers dealing with controlling congestion in the
   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.

   RFC 3449 [RFC3449] discusses TCP performance problems that arise in
   TCP connections over asymmetric paths.  Section 3 of RFC 3449
   describes in detail how congestion on the ACK path can affect overall
   TCP performance.  RFC 3449 also outlines a number of proposed
   mitigations, including ACK Congestion Control.  The experimental ACK
   Congestion Control mechanism discussed in the RFC relies on ECN, with
   the TCP sender detecting congestion on the ACK path from ECN-marked
   packets.  The RFC also discusses two receiver-based mechanisms,
   Window Prediction Mechanism (WPM) [CLP98] and Acknowledgement based
   on Cwnd Estimation (ACE) [MJW00], for using a dynamic ACK Ratio.
   RFC 3449 also considers link and network layer techniques that
   address congestion on the upstream path.  These include header
   compression, and bandwidth management techniques for the upstream
   link including ACK filtering and ACK reconstruction.

   RFC 3135 [RFC3135] on "Performance Enhancing Proxies Intended to
   Mitigate Link-Related Degradations" surveys a range of Performance
   Enhancing Proxies used to improve TCP behavior, including proxies for
   ACK filtering and reconstruction.  RFC 2760 [RFC2760] on "Ongoing TCP
   Research Related to Satellites" discusses both ACK Congestion Control
   and ACK Filtering and Reconstruction, with detailed descriptions of
   the mechanisms proposed by Balakrishnan et al. in [BPK97].

   Landstrom et al. in [LL07] explore a mechanism where the the receiver
   sends only four acknowledgements per window of data, along with the
   sender using a form of Appropriate Byte Counting.  In addition, the
   receiver reverts to a lower acknowledgement frequency after a



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   timeout, to avoid unnecessary Retransmit Timeouts.  One conclusion of
   the paper is that pacing at the sender introduces an additional
   delay, and might not be necessary.  A key result of the paper is that
   with the use of some form of byte counting at the sender, it is
   possible for TCP to use a lower acknowledgement frequency than that
   of delayed acknowledgements.

A.1.  ECN-only Mechanisms

   Balakrishnan et al. in [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.

A.2.  Receiver-only Mechanisms

   In [MJW00], 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.

   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
   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.



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   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 a burst mitigation technique, Rate-Based Pacing, that
   paces the sending of data segments [AB05] [ASA00] [BPK97].

A.3.  Middlebox-based Mechanisms

   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.

   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.

B.  Appendix: Design Considerations

B.1.  The TCP ACK Ratio Option, or an AckNow bit in data packets?

   In the ACK congestion control mechanism specified in this document,
   the sender uses the TCP ACK Ratio Option to tell the receiver the ACK
   Ratio to use.  An alternate approach to the TCP ACK Ratio Option
   could be for the sender to use an AckNow bit in the TCP header of
   data packets, telling the receiver to acknowledge this data packet.
   In the discussion below, we call these two approaches the TCP ACK
   Ratio Option approach and the AckNow approach.

   An advantage of an AckNow approach is that it would require less
   state from the receiver;  the receiver would not need to maintain a
   variable for the current Ack Ratio, and would not need to keep track
   of the number of data packets unacked to date.

   However, a disadvantage of the AckNow approach is that the sender
   does not know when packets will be reordered, delayed, or dropped on
   the path to the receiver.  In particular, the sender does not have
   control over whether a data packet with the AckNow bit set is
   reordered, delayed, or dropped in the network.  For this reason, we
   have chosen the approach of the sender determining the Ack Ratio that



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   should be used, and sending the Ack Ratio to the receiver, rather
   than the sender telling the receiver exactly which data packets to
   acknowledge.

   An additional disadvantage of the AckNow approach is that it would
   add complications and add difficulties for the default cases of the
   receiver using an Ack Ratio of one or two, as is done in the absence
   of ACK congestion control.

   For these reasons, we have specified that the sender determines the
   ACK Ratio to use and tells the receiver, rather than the receiver
   setting an AckNow bit in the TCP Header of selected data packets.

Normative References

   [RFC2119] S. Bradner, "Key Words For Use in RFCs to Indicate
      Requirement Levels", RFC 2119, March 1997.

   [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 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

   [RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J., Henderson,
      T., Heidemann, J., Kruse, H., Ostermann, S., Scott, K., Semke, J.,
      Touch, J. and D. Tran, "Ongoing TCP Research Related to
      Satellites", RFC 2760, February 2000.

   [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
      Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-
      Related Degradations", RFC 3135, June 2001.

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

   [RFC3449] H. Balakrishnan, V. N. Padmanabhan, G. Fairhurst, and M.
      Sooriyabandara, "TCP Performance Implications of Network Path



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      Asymmetry", RFC 3449, December 2002.

   [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", INFOCOM (3), pp. 1157-1165, 2000.

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

   [AJ03] Altman, E., and Jimenez, T.  Novel Delayed ACK Techniques for
      Improving TCP Performance in Multihop Wireless Networks, Proc. of
      the Personal Wireless Communications, 2003.

   [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.

   [BA03] C. Barakat and E. Altman, On ACK Filtering on a Slow Reverse
      Channel, International Journal of Satellite Communications and
      Networking, V.21 N.3, 2003.

   [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.

   [CLP98] Calveras, A., Linares, J., and J. Paradells, "Window
      Prediction Mechanism for Improving TCP in Wireless Asymmetric
      Links". Proc. IEEE Global Communications Conference (GLOBECOM),
      Sydney Australia, November 1998, pp. 533-538.

   [KIA07] Keceli, F., I. Inan, and E. Ayanoglu, TCP ACK Congestion
      Control and Filtering for Fairness Provision in the Uplink of IEEE
      802.11 Infrastructure Basic Service Set, Proc. IEEE ICC 2007, June
      2007.

   [KEEP-ALIVE] Fabio Busatto, "TCP Keepalive HOWTO", May 2007, URL
      "http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/index.html".

   [KLS07] H. Kim, H. Lee, and S. Shin, On the Cross-Layer Impact of TCP
      ACK Thinning on IEEE 802.11 Wireless MAC Dynamics, IEICE



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      Transactions on Communications, 2007

   [LL07] Landstrom, S., and Larzon, L.-A., "Reducing the TCP
      Acknowledgement Frequency", CCR, July 2007.

   [MJW00] Ming-Chit, I.T., Jinsong, D., and W. Wang,"Improving TCP
      Performance Over Asymmetric Networks", ACM SIGCOMM, ACM Computer
      Communications Review (CCR), Vol.30, No.3, 2000.

   [WWCM99] H. Wu, J. Wu, S. Cheng, and J. Ma, ACK Filtering on
      Bandwidth Asymmetry Networks, IEEE Communications, 1999.

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

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

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

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

   Email: iyengar <at> conncoll <dot> edu








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   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


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