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TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: May 4, 2021                             University of Cambridge
                                                        October 31, 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 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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   This Internet-Draft will expire on May 4, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   publication of this document.  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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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

   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
   indicate 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",
   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 in packets that have the SYN bit
   set.  In such packets, the values carried by the TARR option format
   other than Kind, Length and ExID MUST be ignored by the receiving

   TBD1: perhaps two formats (with one codepoint each), a shorter one
   just to advertise that TARR is supported, and a larger one which is
   the current TARR option format defined in section 4?

   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 data 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 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 carries the R value requested by the sender (see section
   4).  For the described purpose, the value of R MUST NOT be zero.  The
   TARR option also carries the N field, which MUST be ignored when R is
   not set to zero.

   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 zero.  When R is set to zero, the N field of the same option
   indicates the number of subsequent data segments for which the sender
   also requests immediate ACKs.

   A TCP sender MAY indicate that it has a reordering tolerance of R
   packets by setting the Ignore Order field of the TARR option to True
   (see Section 4).

3.2.  Receiver behavior

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

   When the TARR option of a received segment carries an R value
   different from zero, a TARR-option-capable receiving TCP MUST modify
   its ACK rate to one ACK every R full-sized received data segments

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   from the sender, as long as packet reordering does not occur.  When R
   is different from zero, the receiving TCP MUST ignore the N field of
   the TARR option.

   A TARR-option-capable TCP that receives a TARR option with the Ignore
   Order field set to True (see Section 4), MUST NOT send an ACK after
   each reordered data segment.  Instead, it MUST continue to send one
   ACK every R received data segments.  Otherwise (i.e., Ignore Order =
   False), such a receiver will need to send an ACK after each reordered
   data segment received.

   If a TARR-option-capable TCP receives a segment carrying the TARR
   option with R=0, the receiving TCP MUST send an ACK immediately, and
   it MUST also send an ACK immediately after each one of the N next
   consecutive segments to be received.  N refers to the corresponding
   field in the TARR option of the received segment (see Section 4).

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    | IgnOrd |    N   |

               Figure 1: TCP ACK Rate Request option format.

   Kind: The Kind field value is TBD.

   Length: The Length field value is 7 bytes.

   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 one or more ACKs 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.

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   Ignore Order: The size of this field is 1 byte.  This field MUST have
   the value 0x01 ("True") or 0x00 ("False").  When this field is set to
   True, the receiver MUST NOT send an ACK after each reordered data
   segment.  Instead, it MUST continue to send one ACK every R received
   data segments.

   TBD2: perhaps 7 bits for R and 1 bit for Ignore Order?

   N: The size of this field is 1 byte.  When R=0, the N field indicates
   the number of subsequent consecutive data segments to be sent for
   which immediate ACKs are requested by the sender.

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, Gorry
   Fairhurst, and Stuart Cheshire provided useful comments and input for
   this document.  Jana Iyengar suggested including a field to allow a
   sender communicate its tolerance to reordering.  Gorry Fairhurst
   suggested adding a mechanism to request a number of consecutive
   immediate ACKs.

   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,

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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,

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

8.2.  Informative References

   [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>.

   [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

   Email: carlesgo@entel.upc.edu

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

   Email: jon.crowcroft@cl.cam.ac.uk

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