TCP ACK Rate Request Option
draft-gomez-tcpm-ack-rate-request-00
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| Authors | Carles Gomez , Jon Crowcroft | ||
| Last updated | 2020-07-13 | ||
| Replaced by | draft-ietf-tcpm-ack-rate-request | ||
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draft-gomez-tcpm-ack-rate-request-00
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
draft-gomez-tcpm-ack-rate-request-00
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 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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 14, 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
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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 . . . . . . . . . . . . . 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
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. 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
0x00AC.
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
TBD
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,
<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>.
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[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>.
8.2. Informative References
[I-D.gomez-tcpm-delack-suppr-reqs]
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, <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,
<https://www.rfc-editor.org/info/rfc8490>.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
Castelldefels 08860
Spain
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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