TCP ACK Rate Request Option

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Authors Carles Gomez  , Jon Crowcroft 
Last updated 2020-07-13
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TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: January 14, 2021                        University of Cambridge
                                                           July 13, 2020

                      TCP ACK Rate Request Option


   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 indicate the ACK rate to be used by a receiver, and it
   also allows to request an immediate ACK from a receiver.

Status of This Memo

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   This Internet-Draft will expire on January 14, 2021.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  TCP ACK Rate Request Functionality  . . . . . . . . . . . . .   3
     3.1.  Sender behavior . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Receiver behavior . . . . . . . . . . . . . . . . . . . .   4
   4.  Option Format . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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).

   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

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   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

   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.  Furthermore, in IoT scenarios, where small messages may be
   sent infrequently, Delayed ACKs may also deplete the scarce resources
   of IoT devices unnecessarily [I-D.gomez-tcpm-delack-suppr-reqs].

   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
   indicate the ACK rate to be used by a receiver, and it also allows to
   request an immediate ACK from a receiver.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  TCP ACK Rate Request Functionality

   This section defines the sender and receiver behaviors for devices
   that support the TARR option.

3.1.  Sender behavior

   A TCP sender MAY request a receiver to modify the ACK rate of the
   latter to one ACK every R full-sized data segments received from the
   sender.  This request is performed by the sender by including the
   TARR option in the TCP header of a data segment.  The TARR option

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   carries the R value requested by the sender (see section 4).  For the
   described purpose, the value of R MUST NOT be zero.

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

   TO-DO: option negotiation.

3.2.  Receiver behavior

   A receiving TCP conforming to this specification MUST process the
   TARR option of a received data segment.

   When the TARR option of a received segment carries an R value
   different from zero, the receiving TCP MUST modify its ACK rate to
   one ACK every R full-sized received data segments from the sender,
   and MUST keep that ACK rate unless a new TARR carrying a different R
   value is received from the same sender.

   If the R value of the TARR option of a received segment is set to
   zero, the receiving TCP MUST send an ACK immediately, even if the
   receiving TCP implements the Delayed ACKs mechanism.  This kind of
   request from the sender MUST NOT be understood by the receiver as a
   request to modify its ACK rate.

   TO-DO: option negotiation.

4.  Option Format

   The TARR option has the format and content shown in Fig. 1.

         0          1          2          3
         01234567 89012345 67890123 45678901
        |  Kind  | Length |       ExID      |
        |   R    |

               Figure 1: TCP ACK Rate Request option format.

   Kind: The Kind field value is TBD.

   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

   R: The size of this field is 1 byte.  If all bits of this field are
   set to 0, the field indicates a request by the sender for the
   receiver to trigger an ACK immediately.  Otherwise, the field carries
   the binary encoding of the number of full-sized segments received
   after which the receiver is requested by the sender to send an ACK.

5.  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.

6.  Security Considerations


7.  Acknowledgments

   Bob Briscoe, Jonathan Morton, Richard Scheffenegger, Neal Cardwell,
   Michael Tuexen, Yuchung Cheng, Matt Mathis, Jana Iyengar, and Gorry
   Fairhurst provided useful comments and input for this document.

   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.

8.  References

8.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,

8.2.  Informative References

              Gomez, C. and J. Crowcroft, "Sender Control of Delayed
              Acknowledgments in TCP: Problem Statement, Requirements
              and Analysis of Potential Solutions", draft-gomez-tcpm-
              delack-suppr-reqs-01 (work in progress), March 2020.

   [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, <>.

   [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,

Authors' Addresses

   Carles Gomez
   C/Esteve Terradas, 7
   Castelldefels  08860


   Jon Crowcroft
   University of Cambridge
   JJ Thomson Avenue
   Cambridge, CB3 0FD
   United Kingdom


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