Internet DRAFT - draft-ietf-tcpm-ack-rate-request
draft-ietf-tcpm-ack-rate-request
TCPM Working Group C. Gomez
Internet-Draft UPC
Intended status: Experimental J. Crowcroft
Expires: 3 September 2024 University of Cambridge
March 2024
TCP ACK Rate Request Option
draft-ietf-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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This Internet-Draft will expire on 2 September 2024.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 4
3. TCP ACK Rate Request Functionality . . . . . . . . . . . . . 4
3.1. Sender behavior . . . . . . . . . . . . . . . . . . . . . 4
3.2. Receiver behavior . . . . . . . . . . . . . . . . . . . . 5
4. Option Format . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Issues of Stretch ACKs . . . . . . . . . . . . . . . . . . . 8
5.1. Sender burstiness . . . . . . . . . . . . . . . . . . . . 8
5.2. Slow cwnd opening . . . . . . . . . . . . . . . . . . . . 8
5.3. Lower frequency of RTT samples . . . . . . . . . . . . . 9
6. Changing the ACK rate during the lifetime of a TCP
connection . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Relation between the present document and RFC
5690 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
A.1. Motivation, goals and features . . . . . . . . . . . . . 14
A.2. New TCP option details . . . . . . . . . . . . . . . . . 15
Appendix B. Other documents that provide rules on sending
ACKs . . . . . . . . . . . . . . . . . . . . . . . . . . 15
B.1. Standards Track documents . . . . . . . . . . . . . . . . 15
B.2. Informational documents . . . . . . . . . . . . . . . . . 16
B.3. Experimental documents . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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
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transfers). The "SHOULD" requirement level for implementing Delayed
ACKs in RFC 1122 (subsequently reinforced in RFC 5681), 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
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 Round Trip Time (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
[RFC9006]. 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.
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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. Therefore, TARR allows to
override the Delayed ACKs mechanism [RFC1122] (while still complying
with the maximum delay to send an ACK of 500 ms). However, Standards
Track TCP specifications other than RFC 1122 and some Informational
specifications that recommend or mandate triggering ACKs in special
conditions prevail over TARR (see section 3.2 and Appendix B).
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. TCP ACK Rate Request Functionality
A TCP endpoint SHOULD announce 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.
In some cases (e.g. when SYN cookies are used [RFC4987]), the client
MAY announce that it supports the TARR option in packets subsequent
to the SYN packet. (Note that announcing TARR option support on the
ACK in the three-way handshake is not reliable.)
Upon reception of a segment announcing support of the TARR option, a
TARR-option-capable endpoint MUST announce support of the TARR option
as well by including it in the next segment to be sent.
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).
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A TCP sender MUST NOT communicate a value of R corresponding to an
amount of data bytes to be acknowledged at once by the receiver
greater than the last known rwin size or greater than the current
cwnd size.
Note that, at a given moment, the rwin size may have changed from the
last rwin size known by the sender. In such case: i) if rwin size
has increased, the sender will not request an R value corresponding
to an amount of data bytes to be acknowledged at once greater than
the current rwin size; ii) if rwin size has decreased, a request of
an R value corresponding to an amount of data bytes to be
acknowledged at once greater than the current rwin size will be
ignored by the receiver (see Section 3.2, second paragraph).
When a TCP sender needs a data segment to be acknowledged immediately
by a TARR-option-capable receiving TCP, without modifying the steady
state ACK rate being used by the receiver, the sender includes the
TARR option in the TCP header of the data segment, with a value of R
equal to 0. Requesting an immediate ACK from the receiver can help
reduce the time it takes to detect and/or recover from packet loss.
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 MUST ignore a TARR option
requesting a value of R corresponding to an amount of data bytes to
be acknowledged at once greater than its current rwin size.
Otherwise, a TARR-option-capable receiving TCP SHOULD modify its ACK
rate to one ACK every R received data segments from the sender. The
receivers's count of data segments received from the sender is reset
every time that an ACK is sent for any reason.
If the rwin size of a TARR-option-capable TCP decreases to a value
lower than the amount of data bytes to be acknowledged at once for
the latest R requested, the amount of data bytes acknowledged at once
by an ACK sent by the receiving TCP MUST NOT exceed its current rwin
size.
If a TARR-option-capable TCP receives a segment carrying the TARR
option with R=0, the receiving TCP SHOULD send an ACK immediately
while keeping its steady state ACK rate.
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Following the behavior specified in RFC 5681, in order to aid the
sender in segment loss detection and repair, a TARR-option-capable
receiver SHOULD send a duplicate ACK immediately when an out-of-order
segment arrives [RFC5681], regardless of the last ACK rate requested
by the sender. After sending a duplicate ACK, the receiver MAY send
the next non-duplicate ACK after R data segments received. In
addition, a TARR-option-capable receiver SHOULD send an immediate ACK
when the incoming segment fills in all or part of a gap in the
sequence space [RFC5681], regardless of the last ACK rate requested
by the sender.
Following RFC 5961, in order to protect from an attack whereby an
off-path attacker may inject a spoofed TCP segment hoping to cause
the connection to be torn down, if a TARR-option-capable TCP receiver
receives a segment with the RST bit set, and the sequence number does
not exactly match the next expected sequence value, yet is within the
current receive window, the receiver MUST send an ACK (called
"challenge ACK" [RFC5961]), regardless of the last requested ACK
rate.
A TARR-option-capable receiver in AccECN mode MUST comply with the
rules specified in draft-ietf-tcpm-accurate-ecn, section 3.2.2.5.1,
on when it emits an ACK, regardless of the last ACK rate requested.
That is, such a receiver SHOULD emit an ACK (Change-Triggered ACK)
whenever a data packet marked Congestion Experienced (CE) arrives
after the previous packet was not CE, and it MUST emit an ACK
(Increment-Triggered ACK) if 'n' CE marks have arrived since the
previous ACK [I-D.ietf-tcpm-accurate-ecn]. (If there is
unacknowledged data at the receiver, 'n' SHOULD be 2. If there is no
unacknowledged data at the receiver, 'n' SHOULD be 3 and MUST be no
less than 3. In either case, 'n' MUST be no greater than 7.)
When Data Center TCP (DCTCP) is used, if the receiver is TARR-option-
capable, it MUST comply with the rules specified in section 3.2 of
RFC 8257 that produce ACKs, regardless of the last ACK rate
requested. This is in order to allow the sender to be able to handle
congestion by determining the number of bytes sent that encountered
congestion [RFC8257].
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 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).
In any case, as specified in RFC 9293, the delay for an ACK MUST be
less than 0.5 seconds.
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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 announce TARR
option support by their senders, 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: Format used to announce support of the TARR option by
the sender.
Kind: The Kind field value is 254.
Length: The Length field value is 4 bytes.
ExID: The experiment ID field size is 2 bytes, and its value is
0x00AC.
When the sender requests an ACK rate of R, 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.
Kind: The Kind field value is 254.
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 7 bits. The field carries the binary
encoding of the ACK rate requested by the sender. The maximum value
of R is 127.
V (reserVed): The size of this field is 1 bit. This field is
reserved for future use.
5. Issues of Stretch ACKs
The TARR option can be used to increase the number of data segments
after which a receiver sends an ACK. ACKs that acknowledge more than
two previously unacknowledged data segments are known as "Stretch
ACKs" [StrACKs]. Stretch ACKs have been reported to produce a number
of undesirable effects [RFC2525], including increased TCP sender
burst size, increased time for TCP to open the cwnd, and reduced
frequency of RTT samples. Note that, as per this specification, R
values up to 127 are possible.
5.1. Sender burstiness
Increased TCP sender burstiness may contribute to router queue
overflow and packet loss. One technique that a sender MAY use to
mitigate the sender burstiness that stems from Stretch ACKs is TCP
Sender Pacing [RFC2760]. This technique allows to space the
transmission of data segments over a given time interval (e.g., the
RTT). TCP Sender Pacing requires an algorithm to determine the
appropriate data segment transmission rate, which needs to be
commensurate with the R value being used.
5.2. Slow cwnd opening
During slow start, cwnd increases by up to Sender Maximum Segment
Size (SMSS) upon receipt of an ACK covering new data [RFC5681].
However, Stretch ACKs (or even Delayed ACKs) reduce the amount of
ACKs received by the sender, thus reducing the rate of cwnd growth,
increasing transfer time and reducing throughput, when compared with
sending an ACK for each incoming data segment. Note that, while
Appropriate Byte Counting (ABC) [RFC3465] might be used to address
this problem, it remains an experimental mechanism, not fully
included in RFC 5681, which specifies standard TCP congestion
control.
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In order to avoid slow cwnd opening, a TCP sender SHOULD NOT use the
TARR option to produce Stretch ACKs during Slow Start. While there
exist TCP receiver implementations that send one ACK per received
data segment during Slow Start, a TCP sender MAY use the TARR option
with R=1 for data segments transmitted during Slow Start.
5.3. Lower frequency of RTT samples
When TARR produces Stretch ACKs, the number of RTT samples that the
sender can obtain decreases. This reduces the responsiveness of the
RTT estimate to path RTT changes. Therefore, time-based packet loss
detection may either be unnecessarily delayed or it may produce
spurious retransmissions. In order to limit this issue, when there
are segments in flight, a sender needs to trigger a sufficient number
of ACKs per round trip. (This number depends on the specific
scenario, with the best currently known value for such number being
roughly in the range of at least 1-4). This can be achieved by i)
sending a data segment with the TARR option with R=0 at the required
rate or ii) using a greater R value intended to produce the required
ACK rate.
6. 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. cwnd may also change due to
relatively sporadic phenomena, such as retransmission timer
expiration, regardless of the steady-state cwnd value for a given
path; in such cases, ACK rate updates may be needed as well. Note
that the sender may opt to request an ACK rate that it considers
appropriate at any moment.
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
[I-D.ietf-tcpm-hystartplusplus]. To avoid slow start sender behavior
ossification, an explicit signal such as TARR may be useful.
In some scenarios, the sender may notice that the ACKs it receives
cover more segments than the ACK rate requested. This may be due to
two reasons: i) ACK decimation is occurring en route; or ii) the
receiver uses Large Receive Offload (LRO). If the reason is known or
suspected by the sender, in the former, the sender may decide to
reduce the ACK frequency to reduce receiver workload and network load
up to the ACK decimation point. In the latter, the sender may want
to increase the ACK frequency to compensate for the impact of the LRO
engine on the ACK flow.
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.
7. 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.
8. 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.
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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
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).
9. 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. Jonathan Morton and Bob Briscoe provided the main input
for Section 6. Section 5.3 has been inspired by related guidance
(for QUIC) included in draft-ietf-quic-ack-frequency, and discussion
in the IETF QUIC working group.
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 through grant number 2017 SGR 376 and
grant number 2021 SGR 00330.
10. References
10.1. 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>.
[RFC2760] Allman, M., Ed., Dawkins, S., Glover, D., Griner, J.,
Tran, D., Henderson, T., Heidemann, J., Touch, J., Kruse,
H., Ostermann, S., Scott, K., and J. Semke, "Ongoing TCP
Research Related to Satellites", RFC 2760,
DOI 10.17487/RFC2760, February 2000,
<https://www.rfc-editor.org/info/rfc2760>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
[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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
10.2. Informative References
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[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
Accurate Explicit Congestion Notification (ECN) Feedback
in TCP", Work in Progress, Internet-Draft, draft-ietf-
tcpm-accurate-ecn-28, 17 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
accurate-ecn-28>.
[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-15, 2 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
generalized-ecn-15>.
[I-D.ietf-tcpm-hystartplusplus]
Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++:
Modified Slow Start for TCP", Work in Progress, Internet-
Draft, draft-ietf-tcpm-hystartplusplus-14, 27 February
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
tcpm-hystartplusplus-14>.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>.
[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>.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
DOI 10.17487/RFC5690, February 2010,
<https://www.rfc-editor.org/info/rfc5690>.
[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>.
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[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>.
[RFC9006] Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
Guidance in the Internet of Things (IoT)", RFC 9006,
DOI 10.17487/RFC9006, March 2021,
<https://www.rfc-editor.org/info/rfc9006>.
[StrACKs] Paxson, V., "Automated packet trace analysis of TCP
implementations", 1997.
Appendix A. Relation between the present document and RFC 5690
A previously published document, entitled "Adding Acknowledgment
Congestion Control to TCP" [RFC5690], includes functionality similar
to some aspects of the present document. However, the motivation,
main goals, and use cases of both documents are almost orthogonal.
In fact, some features of the present document were not considered in
[RFC5690]. This section compares the main features of RFC 5690 and
the present document.
A.1. Motivation, goals and features
RFC 5690 is an informational document that describes a possible
congestion control mechanism for TCP ACKs. The main goal is to
reduce ACK traffic when there is congestion on the reverse path in
order to reduce the congestion. The mechanism includes: i) a
component for the TCP sender to detect lost and ECN-marked pure ACKs,
ii) a mechanism for adjusting the ACK Ratio, iii) a method to
discover the support of the ACK congestion control mechanism by an
endpoint (by means of a new TCP option), and iv) a method for the TCP
sender to inform the TCP receiver of a new value for the ACK Ratio
(by means of a second new TCP option). As of the writing, and to the
best knowledge of the authors, RFC 5690 has not been implemented.
Option Kind values for the new TCP options described in RFC 5690 have
neither been allocated by IANA.
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The present document defines the TARR option. While it can be used
to reduce network load, its primary focus is rather on end-to-end
performance and end-system resource conservation. TARR serves two
purposes: i) allowing a sender to request a given ACK ratio from the
receiver, and ii) allowing a sender to request an immediate ACK,
without modifying the steady state ACK ratio. The latter is not
supported by RFC 5690. On the other hand, TARR might be used as a
component of other mechanisms (e.g. an ACK congestion control
mechanism). However, such mechanisms are out of the scope of the
present document.
A.2. New TCP option details
As part of the ACK congestion control mechanism, RFC 5690 proposes
the use of two new TCP options: one intented to announce support of
TCP ACK Congestion Control, and another one which is used by the
sender to communicate the ACK ratio to the receiver. The former can
only be sent on packets that have the SYN bit set. In the latter, a
one-byte field is used to carry the ACK ratio, but the encoding to be
used for this field is not defined.
The present document uses a single TCP experimental option Kind value
(following RFC 6994) for both announcing support of the TARR option,
and for communicating the requested ACK ratio. In the present
document, announcing support of the TARR option may be done in
packets that do not have the SYN bit set, with the aim to alleviate
the need for TCP option space in SYN packets. In contrast with RFC
5690, the encoding to be used for the ACK ratio field is specified
(see Section 4).
Appendix B. Other documents that provide rules on sending ACKs
B.1. Standards Track documents
RFC 1122, Section 4.2.2.21 allows (via a normative "MAY") a receiver
to send an ACK in response to an out-of-order data segment, i.e., a
segment that is in the window, but not at the left window edge
(MAY-13 in RFC 9293). RFC 2018 states that, in such circumstance,
the receiver "SHOULD send an ACK", including a SACK option, "for
every valid segment that arrives containing new data".
Following similar principles, RFC 5681 states that, in order to aid
the sender in segment loss detection and repair, a receiver SHOULD
send a duplicate ACK immediately when an out-of-order segment
arrives. In addition, a receiver SHOULD send an immediate ACK when
the incoming segment fills in all or part of a gap in the sequence
space.
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RFC 5961 updates RFC 793 to state that a TCP that receives a segment
with the RST bit "SHOULD implement" a set of instructions which
include: "If the RST bit is set and the sequence number does not
exactly match the next expected sequence value, yet is within the
current receive window, TCP MUST send an acknowledgment (challenge
ACK)".
draft-ietf-tcpm-accurate-ecn, section 3.2.2.5.1, provides "The
following rules define when the receiver of a packet in AccECN mode
emits an ACK:
Change-Triggered ACKs: An AccECN Data Receiver SHOULD emit an ACK
whenever a data packet marked CE arrives after the previous packet
was not CE.
Even though this rule is stated as a "SHOULD", it is important for a
transition to trigger an ACK if at all possible, The only valid
exception to this rule is given below these bullets.
For the avoidance of doubt, this rule is deliberately worded to apply
solely when _data_ packets arrive, but the comparison with the
previous packet includes any packet, not just data packets.
Increment-Triggered ACKs: An AccECN receiver of a packet MUST emit an
ACK if 'n' CE marks have arrived since the previous ACK. If there is
unacknowledged data at the receiver, 'n' SHOULD be 2. If there is no
unacknowledged data at the receiver, 'n' SHOULD be 3 and MUST be no
less than 3. In either case, 'n' MUST be no greater than 7."
B.2. Informational documents
RFC 5690 describes a possible congestion control mechanism for TCP
ACKs (see Appendix A).
RFC 8257 describes Data Center TCP (DCTCP). In order to handle
congestion, this scheme requires that the sender must be able to
determine the number of bytes sent that encountered congestion. To
this end, "DCTCP introduces a new Boolean TCP state variable, DCTCP
Congestion Encountered (DCTCP.CE)". And follows: "When receiving
packets, the CE codepoint MUST be processed as follows:
1. If the CE codepoint is set and DCTCP.CE is false, set DCTCP.CE to
true and send an immediate ACK.
2. If the CE codepoint is not set and DCTCP.CE is true, set DCTCP.CE
to false and send an immediate ACK.
3. Otherwise, ignore the CE codepoint.
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Since the immediate ACK reflects the new DCTCP.CE state, it may
acknowledge any previously unacknowledged packets in the old state.
This can lead to an incorrect rate computation at the sender per
Section 3.3. To avoid this, an implementation MAY choose to send two
ACKs: one for previously unacknowledged packets and another
acknowledging the most recently received packet."
B.3. Experimental documents
RFC 4782 describes the experimental Quick-Start mechanism for
transport protocols, and specifies its use with TCP. Quick-Start may
produce a sudden increase of pure ACKs transmitted on the reverse
path. The document proposes that "In the absence of congestion
control for acknowledgement traffic, the TCP receiver could limit its
sending rate for ACK packets sent in response to Quick-Start data
packets". The document proposes that the receiver can acknowledge
the first Quick-Start data packet, and every succeeding K data
packets, and gives a formula to determine K.
Note that TARR can be used to allow a Quick-Start sender to request
the ACK rate to be used by the receiver.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carles.gomez@upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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