Internet DRAFT - draft-gomez-tcpm-ack-pull
draft-gomez-tcpm-ack-pull
TCPM Working Group C. Gomez
Internet-Draft UPC
Intended status: Experimental J. Crowcroft
Expires: May 7, 2020 University of Cambridge
November 4, 2019
TCP ACK Pull
draft-gomez-tcpm-ack-pull-01
Abstract
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 defines the TCP ACK Pull (AKP) mechanism, which allows a
sender to request the ACK for a data segment to be sent without
additional delay by the receiver. AKP makes use of one of the
reserved bits in the TCP header, which is defined in this
specification as the AKP flag.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 4
3. ACK Pull Mechanism . . . . . . . . . . . . . . . . . . . . . 4
4. The ACK Pull Flag . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Actions . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. Annex: Alternative approaches . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Delayed Acknowledgments (ACKs) were specified 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 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.
When the Nagle algorithm is used, in some cases the sender may be
prevented from sending more data while awaiting the 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
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dramatically reduced, as the Delayed ACK timeout is several orders of
magnitude greater than the Round Trip Time (RTT) [RFC8490].
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 be significantly decreased. 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 [RFC 8352], potentially
leading to a further exacerbated delay (by even one or more orders of
magnitude).
One approach that cannot be recommended as a general solution to
solve the described problems is 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. In addition, in some cases, 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.
Since the presented problem is about low performance in various
scenarios, another requirement for the solution is to provide a
sender with a mechanism to request an immediate ACK for a data
segment without 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, computational resources); even further, each additional
communicated packet may involve significant energy overhead.
This document defines the TCP ACK Pull (AKP) mechanism and an AKP
flag in the TCP header. AKP allows a sender to request an ACK to be
sent by a receiving TCP without additional delay upon reception of a
data segment, by setting the AKP flag in that data segment. The AKP
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flag uses one of the reserved bits in the TCP header. More
specifically, the AKP flag uses bit 6 of byte 13 of the TCP header.
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. ACK Pull Mechanism
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. A receiving TCP conforming to this
specification MUST process the AKP flag of a received segment. If
the AKP flag is set, the receiving TCP MUST send an ACK without
additional delay, regardless of whether the receiving TCP uses the
Delayed ACKs mechanism.
4. The ACK Pull Flag
The AKP flag is defined as bit number 6 of the 13th byte of the TCP
header. Figure 1 illustrates bytes 13 and 14 of the TCP header,
including the AKP flag.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | A | R | C | E | U | A | P | R | S | F |
| Header Length |Reservd| K | v | W | C | R | C | S | S | Y | I |
| | | P | d | R | E | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 1: Definition of the AKP field within bytes 13 and 14 of the
TCP Header.
(Note: as of the writing, bit 7 in the above figure is reserved,
although this may change with the publication of
[I-D.ietf-tcpm-accurate-ecn].)
5. IANA Actions
This document assigns bit 6 of the TCP header flags to the AKP flag.
This flag will be defined as shown in Figure 2:
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+-----+-------------------+-----------+
| Bit | Name | Reference |
+-----+-------------------+-----------+
| 6 | AKP (ACK Pull) | RFC XXXX |
+-----+-------------------+-----------+
Figure 2
[TO BE REMOVED: IANA is requested to update the existing entry in the
Transmission Control Protocol (TCP) Header Flags registration
(https://www.iana.org/assignments/tcp-header-flags/tcp-header-
flags.xhtml#tcp-header-flags-1) for Bit 6 to 'AKP (ACK Pull)'.]
6. Security Considerations
TCP ACK Pull introduces a possible Denial of Service (DoS) attack on
a resource-constrained receiver. An attacker might send a large
number of messages to a victim node, requesting an immediate ACK in
response to each one of them. This attack is easily avoided by
ignoring the TCP ACK Pull flag.
7. Acknowledgments
Stuart Cheshire, Ted Lemon, Michael Scharf, and Christoph Paasch
participated in a discussion that was seminal to this document.
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Ciencia, Innovacion y Universidades) through the Jose
Castillejo grant CAS18/00170 and by European Regional Development
Fund (ERDF) and the Spanish Government through project
TEC2016-79988-P, AEI/FEDER, UE. His contribution to this work has
been carried out during his stay as a visiting scholar at the
Computer Laboratory of the University of Cambridge.
8. Annex: Alternative approaches
Several mechanisms that have been proposed in the past allow
increasing the amount of ACKs sent by the receiver. Some examples
are Acknowledgment Congestion Control (AckCC) [RFC5690] and Tail Loss
Probe (TLP) [I-D.ietf-tcpm-rack].
In AckCC, 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.
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TLP 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.
Another approach that allows eliciting an immediate ACK after sending
a data segment is sending a subsequent segment carrying e.g. an
already sent data byte. Another workaround, which is used in the
Contiki operating system (a popular operating system for constrained
devices in IoT scenarios) is to split the data to be sent into two
segments of smaller size. A standard compliant TCP receiver will
acknowledge the second MSS of data, which can improve throughput.
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'.
Note that all the approaches in this Annex involve increasing TCP
header size of some segments, or involve sending additional packets.
The main advantage of the AKP mechanism defined in this specification
is allowing a sender to request an immediate ACK while incurring no
overhead.
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>.
9.2. Informative References
[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-08 (work in progress),
June 2019.
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[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-09 (work in progress), July 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-06 (work in progress), November 2019.
[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>.
[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
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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