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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
draft-gomez-tcpm-ack-rate-request-01
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 immediate ACKs from a receiver.
Status of This Memo
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This Internet-Draft will expire on May 4, 2021.
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document authors. All rights reserved.
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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
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
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",
"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 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
TCP.
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
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 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
TBD
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,
<https://www.rfc-editor.org/info/rfc1122>.
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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,
<https://www.rfc-editor.org/info/rfc2119>.
[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
[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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