TCP ACK Rate Request Option
draft-ietf-tcpm-ack-rate-request-02

TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: 8 January 2024                          University of Cambridge
                                                               July 2023

                      TCP ACK Rate Request Option
                  draft-ietf-tcpm-ack-rate-request-02

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 2 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 1]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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 . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Issues of Stretch ACKs  . . . . . . . . . . . . . . . . . . .   6
     5.1.  Sender burstiness . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Slow cwnd opening . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Lower frequency of RTT samples  . . . . . . . . . . . . .   7
   6.  Changing the ACK rate during the lifetime of a TCP
           connection  . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Relation between the present document and RFC
           5690  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  Motivation, goals and features  . . . . . . . . . . . . .  12
     A.2.  New TCP option details  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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
   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.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 2]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   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.

   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.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 3]
Internet-Draft         TCP ACK Rate Request Option             July 2023

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).

   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 rwin size or greater than cwnd size.

   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.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 4]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   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 value of R
   corresponding to an amount of data bytes to be acknowledged at once
   greater than the last rwin size it has announced.  Otherwise, a TARR-
   option-capable receiving TCP SHOULD modify its ACK rate to one ACK
   every R received data segments from the sender.  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.

   If packet reordering occurs, a TARR-option-capable receiver should
   send a duplicate ACK immediately when an out-of-order segment arrives
   [RFC5681].  After sending a duplicate ACK, the receiver MAY send the
   next non-duplicate ACK after R data segments received.  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).

   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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Gomez & Crowcroft        Expires 8 January 2024                 [Page 5]
Internet-Draft         TCP ACK Rate Request Option             July 2023

      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.

   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

Gomez & Crowcroft        Expires 8 January 2024                 [Page 6]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   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.

   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, a sender
   needs to trigger a sufficient number of ACKs per RTT (such 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.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 7]
Internet-Draft         TCP ACK Rate Request Option             July 2023

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.

   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.

   Another reason to modify the ACK rate might be reducing the ACK load.
   The sender may notice that the ACKs it receives cover more segments
   than the ACK rate requested, indicating that ACK decimation is
   occurring en route.  The sender may then decide to reduce the ACK
   frequency to reduce receiver workload and network load up to the ACK
   decimation point.

   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.

Gomez & Crowcroft        Expires 8 January 2024                 [Page 8]
Internet-Draft         TCP ACK Rate Request Option             July 2023

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.

   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).

Gomez & Crowcroft        Expires 8 January 2024                 [Page 9]
Internet-Draft         TCP ACK Rate Request Option             July 2023

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.

   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 2017 through grant SGR 376.

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>.

   [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>.

Gomez & Crowcroft        Expires 8 January 2024                [Page 10]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   [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>.

10.2.  Informative References

   [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-11, 21 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
              generalized-ecn-11>.

   [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>.

Gomez & Crowcroft        Expires 8 January 2024                [Page 11]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   [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>.

   [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.

Gomez & Crowcroft        Expires 8 January 2024                [Page 12]
Internet-Draft         TCP ACK Rate Request Option             July 2023

   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).

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

Gomez & Crowcroft        Expires 8 January 2024                [Page 13]