TCPM Working Group C. Gomez
Internet-Draft UPC
Intended status: Experimental J. Crowcroft
Expires: 28 September 2022 University of Cambridge
March 2022
TCP ACK Rate Request Option
draft-gomez-tcpm-ack-rate-request-04
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 request 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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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. Changing the ACK rate during the lifetime of a TCP
connection . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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).
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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
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
request 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 (with the appropriate Length value,
see Section 4) in packets that have the SYN bit set.
Upon reception of a SYN segment carrying the TARR option, a TARR-
option-capable endpoint MUST include the TARR option in the SYN-ACK
segment sent in response.
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 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 data segments received from
the sender. This request is performed by the sender by including the
TARR option in the TCP header of a segment. The TARR option carries
the R value requested by the sender (see section 4).
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 1.
A TCP segment carrying retransmitted data is not required to include
a TARR option.
3.2. Receiver behavior
A receiving TCP conforming to this specification MUST process a TARR
option present in a received segment.
A TARR-option-capable receiving TCP SHOULD modify its ACK rate to one
ACK every R received data segments from the sender. If a TARR-
option-capable TCP receives a segment carrying the TARR option with
R=1, the receiving TCP SHOULD send an ACK immediately.
If packet reordering occurs, a TARR-option-capable receiver should
send a duplicate ACK immediately when an out-of-order segment arrives
[RFC5681]. After sending a duplicate ACK, the receiver MAY send the
next non-duplicate ACK after R data segments received. Note also
that the receiver might be unable to send ACKs at the requested rate
(e.g., due to lack of resources); on the other hand, the receiver
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might opt not to fulfill a request for security reasons (e.g., to
avoid or mitigate an attack by which a large number of senders
request disabling delayed ACKs simultaneously and send a large number
of data segments to the receiver).
The request to modify the ACK rate of the receiver holds until the
next segment carrying a TARR option is received.
4. Option Format
The TARR option presents two different formats that can be identified
by the corresponding format length. For packets that have the SYN
bit set, the TARR option has the format shown in Fig. 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length | ExID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: TCP ACK Rate Request option format for packets that
have the SYN bit set.
Kind: The Kind field value is TBD.
Length: The Length field value is 4 bytes.
ExID: The experiment ID field size is 2 bytes, and its value is
0x00AC.
For packets that do not have the SYN bit set, the TARR option has the
format and content shown in Fig. 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length | ExID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R |V|
+-+-+-+-+-+-+-+-+
Figure 2: TCP ACK Rate Request option format.
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Kind: The Kind field value is TBD.
Length: The Length field value is 5 bytes.
ExID: The experiment ID field size is 2 bytes, and its value is
0x00AC.
R: The size of this field is 7 bits. The field carries the ACK rate
requested by the sender. The minimum value of R is 1.
Note: there are currently two options being considered regarding the
semantics of the R field:
OPTION 1: the R field corresponds to the binary encoding of the
requested ACK rate. The maximum value of R is 127. A receiver MUST
ignore an R field with all bits set to zero. (TO-DO: if OPTION 1 is
selected, see how to handle all bits of R being equal to zero.)
OPTION 2: the R field is composed of two subfields: the 5 leftmost
bits represent a mantissa (m) and the 2 rightmost bits represent an
exponent (e). The value of the requested ACK rate is obtained as R =
(m+1)*2^(2*e). The maximum value of R is 2048.
ReserVed (V): The size of this field is 1 bit. This bit is reserved
for future use.
5. Changing the ACK rate during the lifetime of a TCP connection
In some scenarios, setting the ACK rate once for the whole lifetime
of a TCP connection may be suitable. However, there are also cases
where it may be desirable to modify the ACK rate during the lifetime
of a connection.
The ACK rate to be used may depend on the cwnd value used by the
sender, which can change over the lifetime of a connection. cwnd will
start at a low value and grow rapidly during the slow-start phase,
then settle into a reasonably consistent range for the congestion-
avoidance phase - assuming the underlying bandwidth-delay product
(BDP) remains constant. Phenomena such as routing updates, link
capacity changes or path load changes may modify the underlying BDP
significantly; the cwnd should be expected to change accordingly,
prompting the need for ACK rate updates.
TARR can also be used to suppress Delayed ACKs in order to allow
measuring the RTT of each packet in specific intervals (e.g., during
flow start-up), and allow a different ACK rate afterwards.
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A Linux receiver has a heuristic to detect slow start and suppress
Delayed ACKs just for that period. However, some slow start variants
(e.g., HyStart, HyStart++, etc.) may alter the ending of slow start,
thus confusing the heuristics of the receiver. To avoid slow start
sender behavior ossification, an explicit signal such as TARR may be
useful.
Another reason to modify the ACK rate might be reducing the ACK load.
The sender may notice that the ACKs it receives cover more segments
than the ACK rate requested, indicating that ACK decimation is
occurring en route. The sender may then decide to reduce the ACK
frequency to reduce receiver workload and network load up to the ACK
decimation point.
Future TCP specifications may also permit Congestion Experienced (CE)
marks to appear on pure ACKs [I-D.ietf-tcpm-generalized-ecn]. This
might involve more frequent ACK rate updates (e.g., once an RTT), as
the sender probes around an operating point.
6. 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.
7. Security Considerations
The TARR option opens the door to new security threats. This section
discusses such new threats, and suggests mitigation techniques.
An attacker might be able to impersonate a legitimate sender, and
forge an apparently valid packet intended for the receiver. In such
case, the attacker may mount a variety of harmful actions. By using
TARR, the attacker may intentionally communicate a bad R value to the
latter with the aim to damage communication or device performance.
For example, in a small cwnd scenario, using a too high R value may
lead to exacerbated RTT increase and throughput decrease. In other
scenarios, a too low R value may contribute to depleting the energy
of a battery-operated receiver at a faster rate or may lead to
increased network packet load.
While Transport Layer Security (TLS) [RFC8446] is strongly
recommended for securing TCP-based communication, TLS does not
protect TCP headers, and thus cannot protect the TARR option fields
carried by a segment. One approach to address the problem is using
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network-layer protection, such as Internet Protocol Security (IPsec)
[RFC4301]. Another solution is using the TCP Authentication Option
(TCP-AO), which provides TCP segment integrity and protection against
replay attacks [RFC5925].
While it is relatively hard for an off-path attacker to attack an
unprotected TCP session, it is RECOMMENDED for a TARR receiver to use
the guidance and attack mitigation given in [RFC5961]. The TARR
option MUST be ignored on a packet that is deemed invalid.
A TARR receiver might opt not to fulfill a request to avoid or
mitigate an attack by which a large number of senders request
disabling delayed ACKs simultaneously and send a large number of data
segments to the receiver (see Section 3.2).
8. Acknowledgments
Bob Briscoe, Jonathan Morton, Richard Scheffenegger, Neal Cardwell,
Michael Tuexen, Yuchung Cheng, Matt Mathis, Jana Iyengar, Gorry
Fairhurst, Stuart Cheshire, Yoshifumi Nishida, Michael Scharf, Ian
Swett, and Martin Duke provided useful comments and input for this
document. Jana Iyengar suggested including a field to allow a sender
communicate its tolerance to reordering. Jonathan Morton and Bob
Briscoe provided the main input for Section 5.
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.
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,
<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>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
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[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>.
9.2. Informative References
[I-D.ietf-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
Work in Progress, Internet-Draft, draft-ietf-tcpm-
generalized-ecn-09, 31 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-tcpm-
generalized-ecn-09.txt>.
[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>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[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
08860 Castelldefels
Spain
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Email: carlesgo@entel.upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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