TCP ACK Rate Request Option
draft-ietf-tcpm-ack-rate-request-04

TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: 3 September 2024                        University of Cambridge
                                                              March 2024

                      TCP ACK Rate Request Option
                  draft-ietf-tcpm-ack-rate-request-04

Abstract

   TCP Delayed Acknowledgments (ACKs) is a widely deployed mechanism
   that allows reducing protocol overhead in many scenarios.  However,
   Delayed ACKs may also contribute to suboptimal performance.  When a
   relatively large congestion window (cwnd) can be used, less frequent
   ACKs may be desirable.  On the other hand, in relatively small cwnd
   scenarios, eliciting an immediate ACK may avoid unnecessary delays
   that may be incurred by the Delayed ACKs mechanism.  This document
   specifies the TCP ACK Rate Request (TARR) option.  This option allows
   a sender to request the ACK rate to be used by a receiver, and it
   also allows to request immediate ACKs from a receiver.

Status of This Memo

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
   3.  TCP ACK Rate Request Functionality  . . . . . . . . . . . . .   4
     3.1.  Sender behavior . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Receiver behavior . . . . . . . . . . . . . . . . . . . .   5
   4.  Option Format . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Issues of Stretch ACKs  . . . . . . . . . . . . . . . . . . .   8
     5.1.  Sender burstiness . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Slow cwnd opening . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Lower frequency of RTT samples  . . . . . . . . . . . . .   9
   6.  Changing the ACK rate during the lifetime of a TCP
           connection  . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Relation between the present document and RFC
           5690  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     A.1.  Motivation, goals and features  . . . . . . . . . . . . .  14
     A.2.  New TCP option details  . . . . . . . . . . . . . . . . .  15
   Appendix B.  Other documents that provide rules on sending
           ACKs  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     B.1.  Standards Track documents . . . . . . . . . . . . . . . .  15
     B.2.  Informational documents . . . . . . . . . . . . . . . . .  16
     B.3.  Experimental documents  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Delayed Acknowledgments (ACKs) were specified for TCP with the aim to
   reduce protocol overhead [RFC1122].  With Delayed ACKs, a TCP delays
   sending an ACK by up to 500 ms (often 200 ms, with lower values in
   recent implementations such as ~50 ms also reported), and typically
   sends an ACK for at least every second segment received in a stream
   of full-sized segments.  This allows combining several segments into
   a single one (e.g. the application layer response to an application
   layer data message, and the corresponding ACK), and also saves up to
   one of every two ACKs, under many traffic patterns (e.g. bulk

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   transfers).  The "SHOULD" requirement level for implementing Delayed
   ACKs in RFC 1122 (subsequently reinforced in RFC 5681), along with
   its expected benefits, has led to a widespread deployment of this
   mechanism.

   However, there exist scenarios where Delayed ACKs contribute to
   suboptimal performance.  We next roughly classify such scenarios into
   two main categories, in terms of the congestion window (cwnd) size
   and the Maximum Segment Size (MSS) that would be used therein: i)
   "large" cwnd scenarios (i.e. cwnd >> MSS), and ii) "small" cwnd
   scenarios (e.g. cwnd up to ~MSS).

   In "large" cwnd scenarios, increasing the number of data segments
   after which a receiver transmits an ACK beyond the typical one (i.e.
   2 when Delayed ACKs are used) may provide significant benefits.  One
   example is mitigating performance limitations due to asymmetric path
   capacity (e.g. when the reverse path is significantly limited in
   comparison to the forward path) [RFC3449].  Another advantage is
   reducing the computational cost both at the sender and the receiver,
   and reducing network packet load, due to the lower number of ACKs
   involved.

   In many "small" cwnd scenarios, a sender may want to request the
   receiver to acknowledge a data segment immediately (i.e. without the
   additional delay incurred by the Delayed ACKs mechanism).  In high
   bit rate environments (e.g. data centers), a flow's fare share of the
   available Bandwidth Delay Product (BDP) may be in the order of one
   MSS, or even less.  For an accordingly set cwnd value (e.g. cwnd up
   to MSS), Delayed ACKs would incur a delay that is several orders of
   magnitude greater than the Round Trip Time (RTT), severely degrading
   performance.  Note that the Nagle algorithm may produce the same
   effect for some traffic patterns in the same type of environments
   [RFC8490].  In addition, when transactional data exchanges are
   performed over TCP, or when the cwnd size has been reduced, eliciting
   an immediate ACK from the receiver may avoid idle times and allow
   timely continuation of data transmission and/or cwnd growth,
   contributing to maintaining low latency.

   Further "small" cwnd scenarios can be found in Internet of Things
   (IoT) environments.  Many IoT devices exhibit significant memory
   constraints, such as only enough RAM for a send buffer size of 1 MSS
   [RFC9006].  In that case, if the data segment does not elicit an
   application-layer response, the Delayed ACKs mechanism unnecessarily
   contributes a delay equal to the Delayed ACK timer to ACK
   transmission.  The sender cannot transmit a new data segment until
   the ACK corresponding to the previous data segment is received and
   processed.

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   With the aim to provide a tool for performance improvement in both
   "large" and "small" cwnd scenarios, this document specifies the TCP
   ACK Rate request (TARR) option.  This option allows a sender to
   request the ACK rate to be used by a receiver, and it also allows to
   request immediate ACKs from a receiver.  Therefore, TARR allows to
   override the Delayed ACKs mechanism [RFC1122] (while still complying
   with the maximum delay to send an ACK of 500 ms).  However, Standards
   Track TCP specifications other than RFC 1122 and some Informational
   specifications that recommend or mandate triggering ACKs in special
   conditions prevail over TARR (see section 3.2 and Appendix B).

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  TCP ACK Rate Request Functionality

   A TCP endpoint SHOULD announce that it supports the TARR option by
   including the TARR option format (with the appropriate Length value,
   see Section 4) in packets that have the SYN bit set.

   In some cases (e.g. when SYN cookies are used [RFC4987]), the client
   MAY announce that it supports the TARR option in packets subsequent
   to the SYN packet.  (Note that announcing TARR option support on the
   ACK in the three-way handshake is not reliable.)

   Upon reception of a segment announcing support of the TARR option, a
   TARR-option-capable endpoint MUST announce support of the TARR option
   as well by including it in the next segment to be sent.

   The next two subsections define the sender and receiver behaviors for
   devices that support the TARR option, respectively.

3.1.  Sender behavior

   A TCP sender MUST NOT include the TARR option in TCP segments to be
   sent if the TCP receiver does not support the TARR option.

   A TCP sender MAY request a TARR-option-capable receiver to modify the
   ACK rate of the latter to one ACK every R data segments received from
   the sender.  This request is performed by the sender by including the
   TARR option in the TCP header of a segment.  The TARR option carries
   the R value requested by the sender (see section 4).

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   A TCP sender MUST NOT communicate a value of R corresponding to an
   amount of data bytes to be acknowledged at once by the receiver
   greater than the last known rwin size or greater than the current
   cwnd size.

   Note that, at a given moment, the rwin size may have changed from the
   last rwin size known by the sender.  In such case: i) if rwin size
   has increased, the sender will not request an R value corresponding
   to an amount of data bytes to be acknowledged at once greater than
   the current rwin size; ii) if rwin size has decreased, a request of
   an R value corresponding to an amount of data bytes to be
   acknowledged at once greater than the current rwin size will be
   ignored by the receiver (see Section 3.2, second paragraph).

   When a TCP sender needs a data segment to be acknowledged immediately
   by a TARR-option-capable receiving TCP, without modifying the steady
   state ACK rate being used by the receiver, the sender includes the
   TARR option in the TCP header of the data segment, with a value of R
   equal to 0.  Requesting an immediate ACK from the receiver can help
   reduce the time it takes to detect and/or recover from packet loss.

   A TCP segment carrying retransmitted data is not required to include
   a TARR option.

3.2.  Receiver behavior

   A receiving TCP conforming to this specification MUST process a TARR
   option present in a received segment.

   A TARR-option-capable receiving TCP MUST ignore a TARR option
   requesting a value of R corresponding to an amount of data bytes to
   be acknowledged at once greater than its current rwin size.
   Otherwise, a TARR-option-capable receiving TCP SHOULD modify its ACK
   rate to one ACK every R received data segments from the sender.  The
   receivers's count of data segments received from the sender is reset
   every time that an ACK is sent for any reason.

   If the rwin size of a TARR-option-capable TCP decreases to a value
   lower than the amount of data bytes to be acknowledged at once for
   the latest R requested, the amount of data bytes acknowledged at once
   by an ACK sent by the receiving TCP MUST NOT exceed its current rwin
   size.

   If a TARR-option-capable TCP receives a segment carrying the TARR
   option with R=0, the receiving TCP SHOULD send an ACK immediately
   while keeping its steady state ACK rate.

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   Following the behavior specified in RFC 5681, in order to aid the
   sender in segment loss detection and repair, a TARR-option-capable
   receiver SHOULD send a duplicate ACK immediately when an out-of-order
   segment arrives [RFC5681], regardless of the last ACK rate requested
   by the sender.  After sending a duplicate ACK, the receiver MAY send
   the next non-duplicate ACK after R data segments received.  In
   addition, a TARR-option-capable receiver SHOULD send an immediate ACK
   when the incoming segment fills in all or part of a gap in the
   sequence space [RFC5681], regardless of the last ACK rate requested
   by the sender.

   Following RFC 5961, in order to protect from an attack whereby an
   off-path attacker may inject a spoofed TCP segment hoping to cause
   the connection to be torn down, if a TARR-option-capable TCP receiver
   receives a segment with the RST bit set, and the sequence number does
   not exactly match the next expected sequence value, yet is within the
   current receive window, the receiver MUST send an ACK (called
   "challenge ACK" [RFC5961]), regardless of the last requested ACK
   rate.

   A TARR-option-capable receiver in AccECN mode MUST comply with the
   rules specified in draft-ietf-tcpm-accurate-ecn, section 3.2.2.5.1,
   on when it emits an ACK, regardless of the last ACK rate requested.
   That is, such a receiver SHOULD emit an ACK (Change-Triggered ACK)
   whenever a data packet marked Congestion Experienced (CE) arrives
   after the previous packet was not CE, and it MUST emit an ACK
   (Increment-Triggered ACK) if 'n' CE marks have arrived since the
   previous ACK [I-D.ietf-tcpm-accurate-ecn].  (If there is
   unacknowledged data at the receiver, 'n' SHOULD be 2.  If there is no
   unacknowledged data at the receiver, 'n' SHOULD be 3 and MUST be no
   less than 3.  In either case, 'n' MUST be no greater than 7.)

   When Data Center TCP (DCTCP) is used, if the receiver is TARR-option-
   capable, it MUST comply with the rules specified in section 3.2 of
   RFC 8257 that produce ACKs, regardless of the last ACK rate
   requested.  This is in order to allow the sender to be able to handle
   congestion by determining the number of bytes sent that encountered
   congestion [RFC8257].

   Note also that the receiver might be unable to send ACKs at the
   requested rate (e.g., due to lack of resources).  On the other hand,
   the receiver might opt not to fulfill a request for security reasons
   (e.g., to avoid or mitigate an attack by which a large number of
   senders request disabling delayed ACKs simultaneously and send a
   large number of data segments to the receiver).

   In any case, as specified in RFC 9293, the delay for an ACK MUST be
   less than 0.5 seconds.

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   The request to modify the ACK rate of the receiver holds until the
   next segment carrying a TARR option is received.

4.  Option Format

   The TARR option presents two different formats that can be identified
   by the corresponding format length.  For packets that announce TARR
   option support by their senders, the TARR option has the format shown
   in Fig. 1.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Kind      |     Length    |              ExID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: Format used to announce support of the TARR option by
                                the sender.

   Kind: The Kind field value is 254.

   Length: The Length field value is 4 bytes.

   ExID: The experiment ID field size is 2 bytes, and its value is
   0x00AC.

   When the sender requests an ACK rate of R, the TARR option has the
   format and content shown in Fig. 2.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Kind      |     Length    |              ExID             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      R      |V|
     +-+-+-+-+-+-+-+-+

               Figure 2: TCP ACK Rate Request option format.

   Kind: The Kind field value is 254.

   Length: The Length field value is 5 bytes.

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   ExID: The experiment ID field size is 2 bytes, and its value is
   0x00AC.

   R: The size of this field is 7 bits.  The field carries the binary
   encoding of the ACK rate requested by the sender.  The maximum value
   of R is 127.

   V (reserVed): The size of this field is 1 bit.  This field is
   reserved for future use.

5.  Issues of Stretch ACKs

   The TARR option can be used to increase the number of data segments
   after which a receiver sends an ACK.  ACKs that acknowledge more than
   two previously unacknowledged data segments are known as "Stretch
   ACKs" [StrACKs].  Stretch ACKs have been reported to produce a number
   of undesirable effects [RFC2525], including increased TCP sender
   burst size, increased time for TCP to open the cwnd, and reduced
   frequency of RTT samples.  Note that, as per this specification, R
   values up to 127 are possible.

5.1.  Sender burstiness

   Increased TCP sender burstiness may contribute to router queue
   overflow and packet loss.  One technique that a sender MAY use to
   mitigate the sender burstiness that stems from Stretch ACKs is TCP
   Sender Pacing [RFC2760].  This technique allows to space the
   transmission of data segments over a given time interval (e.g., the
   RTT).  TCP Sender Pacing requires an algorithm to determine the
   appropriate data segment transmission rate, which needs to be
   commensurate with the R value being used.

5.2.  Slow cwnd opening

   During slow start, cwnd increases by up to Sender Maximum Segment
   Size (SMSS) upon receipt of an ACK covering new data [RFC5681].
   However, Stretch ACKs (or even Delayed ACKs) reduce the amount of
   ACKs received by the sender, thus reducing the rate of cwnd growth,
   increasing transfer time and reducing throughput, when compared with
   sending an ACK for each incoming data segment.  Note that, while
   Appropriate Byte Counting (ABC) [RFC3465] might be used to address
   this problem, it remains an experimental mechanism, not fully
   included in RFC 5681, which specifies standard TCP congestion
   control.

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   In order to avoid slow cwnd opening, a TCP sender SHOULD NOT use the
   TARR option to produce Stretch ACKs during Slow Start.  While there
   exist TCP receiver implementations that send one ACK per received
   data segment during Slow Start, a TCP sender MAY use the TARR option
   with R=1 for data segments transmitted during Slow Start.

5.3.  Lower frequency of RTT samples

   When TARR produces Stretch ACKs, the number of RTT samples that the
   sender can obtain decreases.  This reduces the responsiveness of the
   RTT estimate to path RTT changes.  Therefore, time-based packet loss
   detection may either be unnecessarily delayed or it may produce
   spurious retransmissions.  In order to limit this issue, when there
   are segments in flight, a sender needs to trigger a sufficient number
   of ACKs per round trip.  (This number depends on the specific
   scenario, with the best currently known value for such number being
   roughly in the range of at least 1-4).  This can be achieved by i)
   sending a data segment with the TARR option with R=0 at the required
   rate or ii) using a greater R value intended to produce the required
   ACK rate.

6.  Changing the ACK rate during the lifetime of a TCP connection

   In some scenarios, setting the ACK rate once for the whole lifetime
   of a TCP connection may be suitable.  However, there are also cases
   where it may be desirable to modify the ACK rate during the lifetime
   of a connection.

   The ACK rate to be used may depend on the cwnd value used by the
   sender, which can change over the lifetime of a connection. cwnd will
   start at a low value and grow rapidly during the slow-start phase,
   then settle into a reasonably consistent range for the congestion-
   avoidance phase - assuming the underlying bandwidth-delay product
   (BDP) remains constant.  Phenomena such as routing updates, link
   capacity changes or path load changes may modify the underlying BDP
   significantly.  The cwnd should be expected to change accordingly,
   prompting the need for ACK rate updates. cwnd may also change due to
   relatively sporadic phenomena, such as retransmission timer
   expiration, regardless of the steady-state cwnd value for a given
   path; in such cases, ACK rate updates may be needed as well.  Note
   that the sender may opt to request an ACK rate that it considers
   appropriate at any moment.

   TARR can also be used to suppress Delayed ACKs in order to allow
   measuring the RTT of each packet in specific intervals (e.g., during
   flow start-up), and allow a different ACK rate afterwards.

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   A Linux receiver has a heuristic to detect slow start and suppress
   Delayed ACKs just for that period.  However, some slow start variants
   (e.g., HyStart, HyStart++, etc.) may alter the ending of slow start,
   thus confusing the heuristics of the receiver
   [I-D.ietf-tcpm-hystartplusplus].  To avoid slow start sender behavior
   ossification, an explicit signal such as TARR may be useful.

   In some scenarios, the sender may notice that the ACKs it receives
   cover more segments than the ACK rate requested.  This may be due to
   two reasons: i) ACK decimation is occurring en route; or ii) the
   receiver uses Large Receive Offload (LRO).  If the reason is known or
   suspected by the sender, in the former, the sender may decide to
   reduce the ACK frequency to reduce receiver workload and network load
   up to the ACK decimation point.  In the latter, the sender may want
   to increase the ACK frequency to compensate for the impact of the LRO
   engine on the ACK flow.

   Future TCP specifications may also permit Congestion Experienced (CE)
   marks to appear on pure ACKs [I-D.ietf-tcpm-generalized-ecn].  This
   might involve more frequent ACK rate updates (e.g., once an RTT), as
   the sender probes around an operating point.

7.  IANA Considerations

   This document specifies a new TCP option (TCP ACK Rate Request) that
   uses the shared experimental options format [RFC6994], with ExID in
   network-standard byte order.

   The authors plan to request the allocation of ExID value 0x00AC for
   the TCP option specified in this document.

8.  Security Considerations

   The TARR option opens the door to new security threats.  This section
   discusses such new threats, and suggests mitigation techniques.

   An attacker might be able to impersonate a legitimate sender, and
   forge an apparently valid packet intended for the receiver.  In such
   case, the attacker may mount a variety of harmful actions.  By using
   TARR, the attacker may intentionally communicate a bad R value to the
   latter with the aim to damage communication or device performance.
   For example, in a small cwnd scenario, using a too high R value may
   lead to exacerbated RTT increase and throughput decrease.  In other
   scenarios, a too low R value may contribute to depleting the energy
   of a battery-operated receiver at a faster rate or may lead to
   increased network packet load.

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   While Transport Layer Security (TLS) [RFC8446] is strongly
   recommended for securing TCP-based communication, TLS does not
   protect TCP headers, and thus cannot protect the TARR option fields
   carried by a segment.  One approach to address the problem is using
   network-layer protection, such as Internet Protocol Security (IPsec)
   [RFC4301].  Another solution is using the TCP Authentication Option
   (TCP-AO), which provides TCP segment integrity and protection against
   replay attacks [RFC5925].

   While it is relatively hard for an off-path attacker to attack an
   unprotected TCP session, it is RECOMMENDED for a TARR receiver to use
   the guidance and attack mitigation given in [RFC5961].  The TARR
   option MUST be ignored on a packet that is deemed invalid.

   A TARR receiver might opt not to fulfill a request to avoid or
   mitigate an attack by which a large number of senders request
   disabling delayed ACKs simultaneously and send a large number of data
   segments to the receiver (see Section 3.2).

9.  Acknowledgments

   Bob Briscoe, Jonathan Morton, Richard Scheffenegger, Neal Cardwell,
   Michael Tuexen, Yuchung Cheng, Matt Mathis, Jana Iyengar, Gorry
   Fairhurst, Stuart Cheshire, Yoshifumi Nishida, Michael Scharf, Ian
   Swett, and Martin Duke provided useful comments and input for this
   document.  Jonathan Morton and Bob Briscoe provided the main input
   for Section 6.  Section 5.3 has been inspired by related guidance
   (for QUIC) included in draft-ietf-quic-ack-frequency, and discussion
   in the IETF QUIC working group.

   Carles Gomez has been funded in part by the Spanish Government
   through project PID2019-106808RA-I00, and by Secretaria
   d'Universitats i Recerca del Departament d'Empresa i Coneixement de
   la Generalitat de Catalunya through grant number 2017 SGR 376 and
   grant number 2021 SGR 00330.

10.  References

10.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2760]  Allman, M., Ed., Dawkins, S., Glover, D., Griner, J.,
              Tran, D., Henderson, T., Heidemann, J., Touch, J., Kruse,
              H., Ostermann, S., Scott, K., and J. Semke, "Ongoing TCP
              Research Related to Satellites", RFC 2760,
              DOI 10.17487/RFC2760, February 2000,
              <https://www.rfc-editor.org/info/rfc2760>.

   [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
              Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
              2003, <https://www.rfc-editor.org/info/rfc3465>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <https://www.rfc-editor.org/info/rfc4987>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,
              <https://www.rfc-editor.org/info/rfc5961>.

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,
              <https://www.rfc-editor.org/info/rfc6994>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

10.2.  Informative References

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   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
              Accurate Explicit Congestion Notification (ECN) Feedback
              in TCP", Work in Progress, Internet-Draft, draft-ietf-
              tcpm-accurate-ecn-28, 17 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
              accurate-ecn-28>.

   [I-D.ietf-tcpm-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
              Congestion Notification (ECN) to TCP Control Packets",
              Work in Progress, Internet-Draft, draft-ietf-tcpm-
              generalized-ecn-15, 2 December 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
              generalized-ecn-15>.

   [I-D.ietf-tcpm-hystartplusplus]
              Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++:
              Modified Slow Start for TCP", Work in Progress, Internet-
              Draft, draft-ietf-tcpm-hystartplusplus-14, 27 February
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              tcpm-hystartplusplus-14>.

   [RFC2525]  Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
              J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
              TCP Implementation Problems", RFC 2525,
              DOI 10.17487/RFC2525, March 1999,
              <https://www.rfc-editor.org/info/rfc2525>.

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <https://www.rfc-editor.org/info/rfc3449>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              DOI 10.17487/RFC5690, February 2010,
              <https://www.rfc-editor.org/info/rfc5690>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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   [RFC8490]  Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              RFC 8490, DOI 10.17487/RFC8490, March 2019,
              <https://www.rfc-editor.org/info/rfc8490>.

   [RFC9006]  Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
              Guidance in the Internet of Things (IoT)", RFC 9006,
              DOI 10.17487/RFC9006, March 2021,
              <https://www.rfc-editor.org/info/rfc9006>.

   [StrACKs]  Paxson, V., "Automated packet trace analysis of TCP
              implementations", 1997.

Appendix A.  Relation between the present document and RFC 5690

   A previously published document, entitled "Adding Acknowledgment
   Congestion Control to TCP" [RFC5690], includes functionality similar
   to some aspects of the present document.  However, the motivation,
   main goals, and use cases of both documents are almost orthogonal.
   In fact, some features of the present document were not considered in
   [RFC5690].  This section compares the main features of RFC 5690 and
   the present document.

A.1.  Motivation, goals and features

   RFC 5690 is an informational document that describes a possible
   congestion control mechanism for TCP ACKs.  The main goal is to
   reduce ACK traffic when there is congestion on the reverse path in
   order to reduce the congestion.  The mechanism includes: i) a
   component for the TCP sender to detect lost and ECN-marked pure ACKs,
   ii) a mechanism for adjusting the ACK Ratio, iii) a method to
   discover the support of the ACK congestion control mechanism by an
   endpoint (by means of a new TCP option), and iv) a method for the TCP
   sender to inform the TCP receiver of a new value for the ACK Ratio
   (by means of a second new TCP option).  As of the writing, and to the
   best knowledge of the authors, RFC 5690 has not been implemented.
   Option Kind values for the new TCP options described in RFC 5690 have
   neither been allocated by IANA.

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   The present document defines the TARR option.  While it can be used
   to reduce network load, its primary focus is rather on end-to-end
   performance and end-system resource conservation.  TARR serves two
   purposes: i) allowing a sender to request a given ACK ratio from the
   receiver, and ii) allowing a sender to request an immediate ACK,
   without modifying the steady state ACK ratio.  The latter is not
   supported by RFC 5690.  On the other hand, TARR might be used as a
   component of other mechanisms (e.g. an ACK congestion control
   mechanism).  However, such mechanisms are out of the scope of the
   present document.

A.2.  New TCP option details

   As part of the ACK congestion control mechanism, RFC 5690 proposes
   the use of two new TCP options: one intented to announce support of
   TCP ACK Congestion Control, and another one which is used by the
   sender to communicate the ACK ratio to the receiver.  The former can
   only be sent on packets that have the SYN bit set.  In the latter, a
   one-byte field is used to carry the ACK ratio, but the encoding to be
   used for this field is not defined.

   The present document uses a single TCP experimental option Kind value
   (following RFC 6994) for both announcing support of the TARR option,
   and for communicating the requested ACK ratio.  In the present
   document, announcing support of the TARR option may be done in
   packets that do not have the SYN bit set, with the aim to alleviate
   the need for TCP option space in SYN packets.  In contrast with RFC
   5690, the encoding to be used for the ACK ratio field is specified
   (see Section 4).

Appendix B.  Other documents that provide rules on sending ACKs

B.1.  Standards Track documents

   RFC 1122, Section 4.2.2.21 allows (via a normative "MAY") a receiver
   to send an ACK in response to an out-of-order data segment, i.e., a
   segment that is in the window, but not at the left window edge
   (MAY-13 in RFC 9293).  RFC 2018 states that, in such circumstance,
   the receiver "SHOULD send an ACK", including a SACK option, "for
   every valid segment that arrives containing new data".

   Following similar principles, RFC 5681 states that, in order to aid
   the sender in segment loss detection and repair, a receiver SHOULD
   send a duplicate ACK immediately when an out-of-order segment
   arrives.  In addition, a receiver SHOULD send an immediate ACK when
   the incoming segment fills in all or part of a gap in the sequence
   space.

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   RFC 5961 updates RFC 793 to state that a TCP that receives a segment
   with the RST bit "SHOULD implement" a set of instructions which
   include: "If the RST bit is set and the sequence number does not
   exactly match the next expected sequence value, yet is within the
   current receive window, TCP MUST send an acknowledgment (challenge
   ACK)".

   draft-ietf-tcpm-accurate-ecn, section 3.2.2.5.1, provides "The
   following rules define when the receiver of a packet in AccECN mode
   emits an ACK:

   Change-Triggered ACKs: An AccECN Data Receiver SHOULD emit an ACK
   whenever a data packet marked CE arrives after the previous packet
   was not CE.

   Even though this rule is stated as a "SHOULD", it is important for a
   transition to trigger an ACK if at all possible, The only valid
   exception to this rule is given below these bullets.

   For the avoidance of doubt, this rule is deliberately worded to apply
   solely when _data_ packets arrive, but the comparison with the
   previous packet includes any packet, not just data packets.

   Increment-Triggered ACKs: An AccECN receiver of a packet MUST emit an
   ACK if 'n' CE marks have arrived since the previous ACK.  If there is
   unacknowledged data at the receiver, 'n' SHOULD be 2.  If there is no
   unacknowledged data at the receiver, 'n' SHOULD be 3 and MUST be no
   less than 3.  In either case, 'n' MUST be no greater than 7."

B.2.  Informational documents

   RFC 5690 describes a possible congestion control mechanism for TCP
   ACKs (see Appendix A).

   RFC 8257 describes Data Center TCP (DCTCP).  In order to handle
   congestion, this scheme requires that the sender must be able to
   determine the number of bytes sent that encountered congestion.  To
   this end, "DCTCP introduces a new Boolean TCP state variable, DCTCP
   Congestion Encountered (DCTCP.CE)".  And follows: "When receiving
   packets, the CE codepoint MUST be processed as follows:

   1.  If the CE codepoint is set and DCTCP.CE is false, set DCTCP.CE to
   true and send an immediate ACK.

   2.  If the CE codepoint is not set and DCTCP.CE is true, set DCTCP.CE
   to false and send an immediate ACK.

   3.  Otherwise, ignore the CE codepoint.

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   Since the immediate ACK reflects the new DCTCP.CE state, it may
   acknowledge any previously unacknowledged packets in the old state.
   This can lead to an incorrect rate computation at the sender per
   Section 3.3.  To avoid this, an implementation MAY choose to send two
   ACKs: one for previously unacknowledged packets and another
   acknowledging the most recently received packet."

B.3.  Experimental documents

   RFC 4782 describes the experimental Quick-Start mechanism for
   transport protocols, and specifies its use with TCP.  Quick-Start may
   produce a sudden increase of pure ACKs transmitted on the reverse
   path.  The document proposes that "In the absence of congestion
   control for acknowledgement traffic, the TCP receiver could limit its
   sending rate for ACK packets sent in response to Quick-Start data
   packets".  The document proposes that the receiver can acknowledge
   the first Quick-Start data packet, and every succeeding K data
   packets, and gives a formula to determine K.

   Note that TARR can be used to allow a Quick-Start sender to request
   the ACK rate to be used by the receiver.

Authors' Addresses

   Carles Gomez
   UPC
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain
   Email: carles.gomez@upc.edu

   Jon Crowcroft
   University of Cambridge
   JJ Thomson Avenue
   Cambridge
   United Kingdom
   Email: jon.crowcroft@cl.cam.ac.uk

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