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

Document Type Active Internet-Draft (individual)
Authors Carles Gomez , Jon Crowcroft
Last updated 2022-03-30
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draft-gomez-tcpm-ack-rate-request-04
TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: 28 September 2022                       University of Cambridge
                                                              March 2022

                      TCP ACK Rate Request Option
                  draft-gomez-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

   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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   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 September 2022.

Copyright Notice

   Copyright (c) 2022 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.

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   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 . . . . . . . . . . . . . .   3
   3.  TCP ACK Rate Request Functionality  . . . . . . . . . . . . .   4
     3.1.  Sender behavior . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Receiver behavior . . . . . . . . . . . . . . . . . . . .   4
   4.  Option Format . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Changing the ACK rate during the lifetime of a TCP
           connection  . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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, 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).

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   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 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.
   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.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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3.  TCP ACK Rate Request Functionality

   A TCP endpoint announces 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.

   Upon reception of a SYN segment carrying the TARR option, a TARR-
   option-capable endpoint MUST include the TARR option in the SYN-ACK
   segment sent in response.

   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).

   When a TCP sender needs a data segment to be acknowledged immediately
   by a TARR-option-capable receiving TCP, the sender includes the TARR
   option in the TCP header of the data segment, with a value of R equal
   to 1.

   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 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=1, the receiving TCP SHOULD send an ACK immediately.

   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

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   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 have the SYN
   bit set, 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: TCP ACK Rate Request option format for packets that
                           have the SYN bit set.

   Kind: The Kind field value is TBD.

   Length: The Length field value is 4 bytes.

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

   For packets that do not have the SYN bit set, 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.

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   Kind: The Kind field value is TBD.

   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 ACK rate
   requested by the sender.  The minimum value of R is 1.

   Note: there are currently two options being considered regarding the
   semantics of the R field:

   OPTION 1: the R field corresponds to the binary encoding of the
   requested ACK rate.  The maximum value of R is 127.  A receiver MUST
   ignore an R field with all bits set to zero.  (TO-DO: if OPTION 1 is
   selected, see how to handle all bits of R being equal to zero.)

   OPTION 2: the R field is composed of two subfields: the 5 leftmost
   bits represent a mantissa (m) and the 2 rightmost bits represent an
   exponent (e).  The value of the requested ACK rate is obtained as R =
   (m+1)*2^(2*e).  The maximum value of R is 2048.

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

5.  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.

   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.  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.

6.  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.

7.  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

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   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).

8.  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.  Jana Iyengar suggested including a field to allow a sender
   communicate its tolerance to reordering.  Jonathan Morton and Bob
   Briscoe provided the main input for Section 5.

   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.

9.  References

9.1.  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>.

   [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>.

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   [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>.

9.2.  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-09, 31 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-tcpm-
              generalized-ecn-09.txt>.

   [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>.

   [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>.

Authors' Addresses

   Carles Gomez
   UPC
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain

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   Email: carlesgo@entel.upc.edu

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

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