TCPM Working Group C. Gomez
Internet-Draft UPC
Intended status: Informational J. Crowcroft
Expires: September 27, 2020 University of Cambridge
March 26, 2020
Sender Control of Delayed Acknowledgments in TCP: Problem Statement,
Requirements and Analysis of Potential Solutions
draft-gomez-tcpm-delack-suppr-reqs-01
Abstract
TCP 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 presents the problem statement regarding sender control of
Delayed ACKs in TCP. The document discusses the scenarios and use
cases in which sender control of Delayed ACKs offers advantages.
Then, requirements for a potential solution are derived. Finally, a
number of potential solutions are discussed, based on the
requirements, and also considering pros and cons in each case.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Problem statement: issues due to Delayed ACKs . . . . . . . . 3
3.1. Slow start . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. High bit rate environments and short data segments . . . 4
3.3. IoT scenarios . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Beyond classic ACK transmission behavior . . . . . . . . 4
4. Requirements for sender control of Delayed ACKs . . . . . . . 5
4.1. Sender-triggered mechanism . . . . . . . . . . . . . . . 5
4.2. Per-segment granularity . . . . . . . . . . . . . . . . . 5
4.3. Header/Message overhead . . . . . . . . . . . . . . . . . 6
4.4. Support for enabling generic ACK ratios . . . . . . . . . 6
4.5. Middlebox traversal . . . . . . . . . . . . . . . . . . . 6
4.6. Safe return to normal Delayed ACKs operation . . . . . . 6
4.7. Impact on existing TCP functionality . . . . . . . . . . 7
4.8. Impact on future TCP development . . . . . . . . . . . . 7
4.9. Avoidance of 'hacks' . . . . . . . . . . . . . . . . . . 7
4.10. Who is in control? . . . . . . . . . . . . . . . . . . . 7
5. Potential solutions for sender control of Delayed ACKs . . . 7
5.1. AckCC . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. TLP . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. TCP ACK Pull (AKP) flag . . . . . . . . . . . . . . . . . 8
5.4. A new 'ACK Pull' TCP option . . . . . . . . . . . . . . . 9
5.5. Reuse of existing TCP header fields . . . . . . . . . . . 9
5.6. 'Hacks' . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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
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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 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. Furthermore, there may be reasons
to allow a sender communicate the ACK ratio to be used in a TCP
connection, and thus dynamically override (or restore) use of Delayed
ACKs at the receiver.
This document presents the problem statement regarding sender control
of Delayed ACKs. The document discusses the scenarios and use cases
in which sender control of Delayed ACKs offers advantages. Then,
requirements for a potential solution are derived. Finally, a number
of potential solutions are discussed, based on the requirements, and
also considering pros and cons in each case.
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. Problem statement: issues due to Delayed ACKs
This section provides scenarios and use cases where performance
issues arise due to Delayed ACKs.
3.1. Slow start
During slow start, the congestion window (cwnd) increases by up to
Sender Maximum Segment Size (SMSS) upon receipt of an ACK covering
new data [RFC5681]. However, use of Delayed ACKs reduces 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
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to address this problem, it remains as an experimental mechanism, not
fully included in RFC 5681, which specifies standard TCP congestion
control. Furthermore, Delayed ACKs preclude using sender behaviors
intended to quickly and non-intrusively probe for available capacity
during slow start. One example of such behaviors is chirping, where
a sender follows a predetermined pattern to send data segments (e.g.
decreasing intersegment time gaps) and measures the gaps between ACKs
[I-D.kuehlewind-tcpm-accurate-ecn] (see Appendix B.4 of revision -03
of the referenced document).
3.2. High bit rate environments and short data segments
When the Nagle algorithm is used, in some cases the sender may be
prevented from sending more data while awaiting a 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
dramatically reduced, since the Delayed ACK timeout may be several
orders of magnitude greater than the Round Trip Time (RTT) [RFC8490].
3.3. IoT scenarios
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 significantly decrease. 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 [RFC8352], potentially leading to
a further exacerbated delay (by even one or more orders of
magnitude).
3.4. Beyond classic ACK transmission behavior
In some scenarios, it may be desirable to enable ACK transmission
behaviors beyond the classic ones.
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For example, it may be beneficial to apply congestion control to ACKs
[RFC5690]. Reducing the amount of ACKs on a congested reverse path
may allow alleviating congestion on that path, with minimal impact on
a relatively independent forward path.
On the other hand, in some scenarios, the rate at which ACKs arrive
at the sender limits the achievable performance of data transfer.
This happens due to forward and reverse path asymmetric capacity
(with the latter being significantly limited, e.g. in terms of
bandwidth) [RFC3449]. In some environments, the issue is mitigated
by using middleboxes that perform ACK thinning, i.e., deleting a
subset of the ACKs. Examples of technologies where deployments have
been reported to do ACK thinning include satellite links, DOCSIS
cable networks, mobile cellular networks, among others.
4. Requirements for sender control of Delayed ACKs
This section provides the requirements for a potential solution to
enable sender control of Delayed ACKs.
4.1. Sender-triggered mechanism
An assumption is that the sender knows when Delayed ACKs operation
should be overriden. For example, the sender may know in advance the
pattern of the traffic it will generate, or it may know whether an
application-layer response will be sent by the receiving endpoint
upon reception of a given message. Therefore, control of Delayed
ACKs has to be sender-triggered.
4.2. Per-segment granularity
One approach that cannot be recommended as a general solution for
controlling Delayed ACKs is (permanently) 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.
Another approach would be determining suppression of Delayed-ACKs per
connection. However, within the same connection, 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.
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4.3. Header/Message overhead
Since the presented problem is about low performance in various
scenarios, another requirement for the solution is to minimize
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
and computational resources).
Another benefit of keeping a low header/message overhead is
alleviating the processing workload of a receiving TCP.
4.4. Support for enabling generic ACK ratios
For many of the scenarios and use cases described in Section 2, an
ACK ratio of 1 (i.e. a receiver sending one ACK per incoming data
segment) would solve the mentioned issues. However, the ability of
enforcing a generic ACK ratio (including values different from 1 and
2) allows to enable a wider range of ACK behaviors, which may support
congestion control for ACKs, sender behaviors not based on ACK-
clocking, etc.
The desired generic ACK ratio is intended to be in force for the
current data segment, and for subsequent data segments (at least,
during a time interval of a duration that may depend on several
factors).
The mechanism used to indicate the desired receiver Delayed ACK
behavior might exploit soft state (i.e. using explicit information
carried by data segments) or connection state (which needs to be
stored by the receiver).
4.5. Middlebox traversal
Deployment of new functionality for TCP faces the risk of packets
being discarded by existing middleboxes upon detection of unexpected
or discouraged formats, header field values or even traffic patterns.
A solution for sender control of Delayed ACKs should offer relatively
good middlebox traversal (to the extent possible).
4.6. Safe return to normal Delayed ACKs operation
A solution for sender control of Delayed ACKs must ensure that normal
Delayed ACKs operation is in force by default, and also once
temporary action on Delayed ACKs needs to end.
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4.7. Impact on existing TCP functionality
A solution for sender control of Delayed ACKs should not reduce the
space of existing TCP functionality.
4.8. Impact on future TCP development
A solution for sender control of Delayed ACKs should not pose
significant risk of preventing future TCP development. If an
available resource (e.g. a reserved bit of the TCP header, a new TCP
option, etc.) is used by a solution, careful analysis must be carried
out regarding the risks and benefits of using such resource.
4.9. Avoidance of 'hacks'
Sender control of Delayed ACKs might be achieved by using
workarounds, such as implementation techniques that may produce the
desired effect. However, such approaches may be suboptimal regarding
implementation cleanliness, and may entail other performance issues
(see section 5.6).
4.10. Who is in control?
The receiver might not always be able to honour the ACK behavior
desired by the sender. Therefore, the semantics of sender control of
Delayed ACKs have to be of a hint, not a command.
If the receiver is actually not able to apply the ACK behavior
desired by the sender, then the former has a range of options with
regard to communicating so to the latter, from remaining silent, to
providing explicit feedback to the sender. Each option has
associated trade-offs. For example, remaining silent might degrade
performance if a sender relies on a receiver that uses the ACK
behavior intended by the sender.
5. Potential solutions for sender control of Delayed ACKs
This section enumerates and discusses potential solutions that might
be considered to enable sender control of Delayed ACKs. The list of
solutions is not necessarily comprehensive. This section intends to
illustrate the trade-offs that arise when considering potential
solutions for sender control of Delayed ACKs. (Note: the analysis
needs to be completed for many of the solutions below.)
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5.1. AckCC
In Acknowledgment Congestion Control (AckCC) [RFC5690], 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 receiver.
Middlebox traversal of a new TCP option is often regarded as 'bad'
(to be confirmed).
5.2. TLP
Tail Loss Probe (TLP) [I-D.ietf-tcpm-rack] 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. Of course, this means sending a whole new packet to
trigger ACKs, which adds significant overhead.
This approach might offer good middlebox traversal (to be confirmed).
5.3. TCP ACK Pull (AKP) flag
One solution that has been proposed for sender control of Delayed
ACKs is called 'TCP ACK Pull' [I-D.gomez-tcpm-ack-pull].
TCP ACK Pull defines the AKP flag as bit number 6 of the 13th byte of
the TCP header. 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. Upon
reception of a segment with the AKP flag set, a conforming receiving
TCP behaves accordingly by sending the corresponding ACK without
additional delay.
This solution would entail zero header or message overhead. However,
it would consume a TCP header bit, leaving only two available TCP
header reserved bits. A question is thus whether one TCP header bit
should be dedicated to this purpose or not.
Middlebox traversal characteristics of bit 6 of the TCP header need
to be assessed.
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5.4. A new 'ACK Pull' TCP option
Another approach relies on defining a new option-kind-only TCP option
with the same semantics as the AKP flag, which might be called 'ACK
Pull Option' or 'AKP Option'.
This solution would consume an available TCP Option Kind number.
However, most of the 256 numbers in the TCP Option Kind number space
are currently available. Therefore, consuming one such number does
not appear to significantly limit future TCP development.
The header overhead of the AKP Option is one byte.
Middlebox traversal of a new TCP option is often regarded as 'bad'
(to be confirmed).
5.5. Reuse of existing TCP header fields
Another approach that might be used to enable sender control of
Delayed ACKs is based on reusing existing TCP header fields. For
example, use of the Urgent pointer has been suggested (e.g. by
reserving 3 of its 16 bits to encode an ACK ratio exponent that may
be communicated by the sender to the receiver), when URG=0. A
problem with this approach is that the semantics of the reused TCP
header field may become overloaded. Therefore, in some cases either
the original intended use of the reused TCP header field may become
limited, or if it prevails, then sender control of Delayed ACKs might
not always be available for use.
Middlebox traversal characteristics of this approach might be
relatively good (to be confirmed).
5.6. 'Hacks'
One approach that allows eliciting an immediate ACK after sending a
data segment is sending a subsequent segment carrying a previously
acknowledged data byte. However, in addition to the inefficiency of
sending a byte that has previously been sent, this approach may
require the transmission of a new packet (even carrying a single byte
of data payload) just for that purpose, which represents significant
overhead. Furthermore, sending a previously sent byte is not a clean
solution from an implementation perspective.
Another workaround intended to trigger an immediate ACK from the
receiving TCP, which is used in the Contiki operating system (a
popular operating system for constrained devices in IoT scenarios) is
splitting the data to be sent into two segments of smaller size. A
standard compliant TCP receiver will acknowledge the second MSS of
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data. 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 hack'.
6. Summary
The next table summarizes whether the different solutions presented
in Section 4 are able to satisfy the requirements stated in
Section 4.
+-------+-------+-------+-------+-------+------+------+
| Per- | Over- |Generic|Middle-|Impact |Impact|Hack |
|segment| head |ACK rat|box tr.|current|future|Avoid.|
+------------+-------+-------+-------+-------+-------+------+------+
| ACKcc | Yes | Low | Yes | Bad? | No | Low | Yes |
+------------+-------+-------+-------+-------+-------+------+------+
| TLP | No | High | No | Good | No | No | Yes |
+------------+-------+-------+-------+-------+-------+------+------+
| AKP flag | Yes | No | Yes | ? | No |Med/Hi| Yes |
+------------+-------+-------+-------+-------+-------+------+------+
| AKP option | Yes | Low | Yes | Bad? | No | Low | Yes |
+------------+-------+-------+-------+-------+-------+------+------+
|Reuse fields| Yes | No | Yes | Good? | Yes | ? | Yes |
+------------+-------+-------+-------+-------+-------+------+------+
| Hacks | ? |Med/Hig| No | Good? | No | No | No |
+------------+-------+-------+-------+-------+-------+------+------+
Note: all considered potential solutions satisfy the following requirements:
i) sender control of Delayed ACKs, and ii) safe return to normal
Delayed ACKs operation. A receiver may be unable to always honour
the ACK behavior desired by the sender regardless of the specific
potential solution considered.
Figure 1: Summary of potential solutions for sender control of
Delayed ACKs.
7. Security Considerations
TBD
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8. Acknowledgments
Bob Briscoe, Jonathan Morton, Richard Scheffenegger, Michael Tuexen
and Jana Iyengar provided useful input for this document.
Stuart Cheshire, Ted Lemon, Michael Scharf, and Christoph Paasch
participated in a discussion that was seminal to the TCP ACK Pull
proposal, which eventually led to this document.
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Ciencia, Innovacion y Universidades) through
Secretaria d'Universitats i Recerca del Departament d'Empresa i
Coneixement de la Generalitat de Catalunya 2017 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>.
[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>.
[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>.
[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>.
[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>.
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9.2. Informative References
[I-D.gomez-tcpm-ack-pull]
Gomez, C. and J. Crowcroft, "TCP ACK Pull", draft-gomez-
tcpm-ack-pull-01 (work in progress), November 2019.
[I-D.ietf-lwig-tcp-constrained-node-networks]
Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
Guidance in the Internet of Things (IoT)", draft-ietf-
lwig-tcp-constrained-node-networks-09 (work in progress),
November 2019.
[I-D.ietf-tcpm-rack]
Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP",
draft-ietf-tcpm-rack-08 (work in progress), March 2020.
[I-D.kuehlewind-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-kuehlewind-tcpm-
accurate-ecn-05 (work in progress), October 2015.
[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,
<https://www.rfc-editor.org/info/rfc8352>.
[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
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Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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