TCPM Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Experimental                               J. Crowcroft
Expires: May 7, 2020                             University of Cambridge
                                                        November 4, 2019

                              TCP ACK Pull


   Delayed Acknowledgments (ACKs) allow reducing protocol overhead in
   many scenarios.  However, in some cases, Delayed ACKs may
   significantly degrade network and device performance in terms of link
   utilization, latency, memory usage and/or energy consumption.  This
   document defines the TCP ACK Pull (AKP) mechanism, which allows a
   sender to request the ACK for a data segment to be sent without
   additional delay by the receiver.  AKP makes use of one of the
   reserved bits in the TCP header, which is defined in this
   specification as the AKP flag.

Status of This Memo

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   This Internet-Draft will expire on May 7, 2020.

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
   3.  ACK Pull Mechanism  . . . . . . . . . . . . . . . . . . . . .   4
   4.  The ACK Pull Flag . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Actions  . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Annex: Alternative approaches . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Delayed Acknowledgments (ACKs) were specified 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 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 it 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 traffic patterns and scenarios for which Delayed
   ACKs can actually be detrimental to performance.  When a segment
   carrying a message of a size up to one Maximum Segment Size (MSS) is
   transferred, if the message does not elicit an application-layer
   response, and a second data segment is not transferred earlier than
   the Delayed ACK timeout, the ACK is unnecessarily delayed, with a
   number of negative consequences.

   When the Nagle algorithm is used, in some cases the sender may be
   prevented from sending more data while awaiting the Delayed ACK.  In
   some high bit rate environment (e.g.  Gigabit Ethernet) use cases,
   such a delay may be very large, and link utilitzation may be

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   dramatically reduced, as the Delayed ACK timeout is several orders of
   magnitude greater than the Round Trip Time (RTT) [RFC8490].

   Delayed ACKs are also detrimental in Internet of Things (IoT)
   scenarios, where TCP is being increasingly used
   [I-D.ietf-lwig-tcp-constrained-node-networks].  Many IoT devices,
   such as sensors, transfer small messages (e.g. containing sensor
   readings) rather infrequently, therefore if the receiver uses Delayed
   ACKs, the ACK will often be unnecessarily delayed.  The sender cannot
   release the memory resources associated to a transferred data segment
   until the ACK is received and processed.  This may be a problem for
   many IoT devices, which are typically memory-constrained, and may
   even lead to subsequent packet drops if their scarce memory resources
   are blocked while awaiting an ACK.  Moreover, if the IoT device uses
   a radio interface for communication, in some scenarios Delayed ACKs
   will lead to increased energy consumption (e.g. with the radio
   interface of the device staying in receive mode while awaiting the
   ACK).  Since many IoT devices run on small batteries, the device
   lifetime may be significantly decreased.  Furthermore, the delay
   suffered by the ACK may interact negatively with layer two
   mechanisms, especially in wireless network technologies where devices
   remain in low-power states for long intervals [RFC 8352], potentially
   leading to a further exacerbated delay (by even one or more orders of

   One approach that cannot be recommended as a general solution to
   solve the described problems is disabling Delayed ACKs at the
   receiving TCP.  In fact, the latter may interact with a wide variety
   of devices and many of those may still benefit from the advantages of
   Delayed ACKs.  In addition, in some cases, a sender may offer a mixed
   traffic pattern comprising single data segments that will lead to
   unnecessarily delayed ACKs, with other data segments upon which
   Delayed ACKs will act as intended.  Therefore, the solution has to be
   provided at a per-segment granularity.

   Since the presented problem is about low performance in various
   scenarios, another requirement for the solution is to provide a
   sender with a mechanism to request an immediate ACK for a data
   segment without incurring overhead in terms of header size increase
   or additional packets sent.  For example, in IoT scenarios, every
   additional communicated byte consumes scarce resources (e.g. energy,
   bandwidth, computational resources); even further, each additional
   communicated packet may involve significant energy overhead.

   This document defines the TCP ACK Pull (AKP) mechanism and an AKP
   flag in the TCP header.  AKP allows a sender to request an ACK to be
   sent by a receiving TCP without additional delay upon reception of a
   data segment, by setting the AKP flag in that data segment.  The AKP

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   flag uses one of the reserved bits in the TCP header.  More
   specifically, the AKP flag uses bit 6 of byte 13 of the TCP header.

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.  ACK Pull Mechanism

   When a TCP sender needs a data segment to be acknowledged by the
   receiving TCP without additional delay, the sender sets the AKP flag
   of the data segment TCP header.  A receiving TCP conforming to this
   specification MUST process the AKP flag of a received segment.  If
   the AKP flag is set, the receiving TCP MUST send an ACK without
   additional delay, regardless of whether the receiving TCP uses the
   Delayed ACKs mechanism.

4.  The ACK Pull Flag

   The AKP flag is defined as bit number 6 of the 13th byte of the TCP
   header.  Figure 1 illustrates bytes 13 and 14 of the TCP header,
   including the AKP flag.

         0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
       |               |       | A | R | C | E | U | A | P | R | S | F |
       | Header Length |Reservd| K | v | W | C | R | C | S | S | Y | I |
       |               |       | P | d | R | E | G | K | H | T | N | N |

    Figure 1: Definition of the AKP field within bytes 13 and 14 of the
                                TCP Header.

   (Note: as of the writing, bit 7 in the above figure is reserved,
   although this may change with the publication of

5.  IANA Actions

   This document assigns bit 6 of the TCP header flags to the AKP flag.
   This flag will be defined as shown in Figure 2:

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                    | Bit | Name              | Reference |
                    |  6  | AKP (ACK Pull)    | RFC XXXX  |

                                 Figure 2

   [TO BE REMOVED: IANA is requested to update the existing entry in the
   Transmission Control Protocol (TCP) Header Flags registration
   flags.xhtml#tcp-header-flags-1) for Bit 6 to 'AKP (ACK Pull)'.]

6.  Security Considerations

   TCP ACK Pull introduces a possible Denial of Service (DoS) attack on
   a resource-constrained receiver.  An attacker might send a large
   number of messages to a victim node, requesting an immediate ACK in
   response to each one of them.  This attack is easily avoided by
   ignoring the TCP ACK Pull flag.

7.  Acknowledgments

   Stuart Cheshire, Ted Lemon, Michael Scharf, and Christoph Paasch
   participated in a discussion that was seminal to this document.

   Carles Gomez has been funded in part by the Spanish Government
   (Ministerio de Ciencia, Innovacion y Universidades) through the Jose
   Castillejo grant CAS18/00170 and by European Regional Development
   Fund (ERDF) and the Spanish Government through project
   TEC2016-79988-P, AEI/FEDER, UE.  His contribution to this work has
   been carried out during his stay as a visiting scholar at the
   Computer Laboratory of the University of Cambridge.

8.  Annex: Alternative approaches

   Several mechanisms that have been proposed in the past allow
   increasing the amount of ACKs sent by the receiver.  Some examples
   are Acknowledgment Congestion Control (AckCC) [RFC5690] and Tail Loss
   Probe (TLP) [I-D.ietf-tcpm-rack].

   In AckCC, the sender tells the receiver the ACK Ratio R to use, where
   the receiver sends one ACK per R data packets received.  AckCC
   defines a 2-byte "TCP ACK Congestion Control Permitted Option" for
   negotiating use of AckCC, whereas it defines a 3-byte "ACK Ratio TCP
   option" to communicate the ACK Ratio value from the sender to the

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   TLP is intended to avoid RTO-expiration-based retransmission when
   tail loss occurs by inducing additional ACKs at the receiver.  This
   is achieved by sending a probe segment after a probe time-out (PTO)
   when data have been sent but not confirmed.

   Another approach that allows eliciting an immediate ACK after sending
   a data segment is sending a subsequent segment carrying e.g. an
   already sent data byte.  Another workaround, which is used in the
   Contiki operating system (a popular operating system for constrained
   devices in IoT scenarios) is to split the data to be sent into two
   segments of smaller size.  A standard compliant TCP receiver will
   acknowledge the second MSS of data, which can improve throughput.
   However, this 'split hack' may not always work since a TCP receiver
   is required to acknowledge every second full-sized segment, but not
   two consecutive small segments.  Furthermore, the overhead of sending
   two IP packets instead of one is another downside of the 'split

   Note that all the approaches in this Annex involve increasing TCP
   header size of some segments, or involve sending additional packets.
   The main advantage of the AKP mechanism defined in this specification
   is allowing a sender to request an immediate ACK while incurring no

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,

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

9.2.  Informative References

              Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
              Guidance in the Internet of Things (IoT)", draft-ietf-
              lwig-tcp-constrained-node-networks-08 (work in progress),
              June 2019.

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              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-09 (work in progress), July 2019.

              Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-06 (work in progress), November 2019.

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              DOI 10.17487/RFC5690, February 2010,

   [RFC8352]  Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, Ed.,
              "Energy-Efficient Features of Internet of Things
              Protocols", RFC 8352, DOI 10.17487/RFC8352, April 2018,

   [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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