Pacing in Transport Protocols
draft-welzl-iccrg-pacing-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Michael Welzl , Wesley Eddy , Vidhi Goel , Michael Tüxen | ||
| Last updated | 2024-10-21 | ||
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draft-welzl-iccrg-pacing-01
Internet Congestion Control M. Welzl
Internet-Draft University of Oslo
Intended status: Informational W. Eddy
Expires: 24 April 2025 MTI Systems
V. Goel
Apple Inc.
M. Tüxen
Münster University of Applied Sciences
21 October 2024
Pacing in Transport Protocols
draft-welzl-iccrg-pacing-01
Abstract
Applications or congestion control mechanisms can produce bursty
traffic which can cause unnecessary queuing and packet loss. To
reduce the burstiness of traffic, the concept of evenly spacing out
the traffic from a data sender over a round-trip time known as
"pacing" has been used in many transport protocol implementations.
This document gives an overview of pacing and how some known pacing
implementations work.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://mwelzl.github.io/draft-iccrg-pacing/draft-welzl-iccrg-
pacing.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-welzl-iccrg-pacing/.
Discussion of this document takes place on the Internet Congestion
Control Research Group mailing list (mailto:iccrg@irtf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/iccrg.
Subscribe at https://www.ietf.org/mailman/listinfo/iccrg/.
Source for this draft and an issue tracker can be found at
https://github.com/mwelzl/draft-iccrg-pacing.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Pacing: general considerations and consequences . . . . . . . 3
3.1. More likely to saturate a bottleneck . . . . . . . . . . 4
3.1.1. Backing off after the increase . . . . . . . . . . . 5
3.1.2. Able to work with smaller queues . . . . . . . . . . 5
3.2. Getting good RTT estimates . . . . . . . . . . . . . . . 5
4. Implementation examples . . . . . . . . . . . . . . . . . . . 6
4.1. Linux TCP . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Apple OSes . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. QUIC BBR implementations . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Applications commonly generate either bulk data (e.g. files) or
bursts of data (e.g. segments of media) that transport protocols
deliver into the network based on congestion control algorithms.
RFCs describing congestion control generally refer to a congestion
window (cwnd) state variable as an upper limit for either the number
of unacknowledged packets or bytes that a sender is allowed to emit.
This limits the sender's transmission rate at the granularity of a
round-trip time (RTT). If the sender transmits the entire cwnd sized
data in an instant, this can result in unnecessarily high queuing and
eventually packet losses at the bottleneck. Such consequences are
detrimental to users' applications in terms of both responsiveness
and goodput. To solve this problem, the concept of pacing was
introduced. Pacing allows to send the same cwnd sized data but
spread it across a round-trip time more evenly.
Congestion control specifications always allow to send less than the
cwnd, or temporarily emit packets at a lower rate. Accordingly, it
is in line with these specifications to pace packets. Pacing is
known to have advantages -- if some packets arrive at a bottleneck as
a burst (all packets being back-to-back), loss can be more likely to
happen than in a case where there are time gaps between packets
(e.g., when they are spread out over the RTT). It also means that
pacing is less likely to cause any sudden, ephemeral increases in
queuing delay. Since keeping the queues short reduces packet losses,
pacing can also yield higher goodput by reducing the time lost in
loss recovery.
Because of its known advantages, pacing has become common in
implementations of congestion controlled transports. It is also an
integral element of the "BBR" congestion control mechanism
[I-D.cardwell-iccrg-bbr-congestion-control].
2. Conventions and Definitions
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. Pacing: general considerations and consequences
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3.1. More likely to saturate a bottleneck
We can distinguish between two reasons for packet losses that are due
to congestion at a bottleneck with a DropTail (FIFO) queue:
1. A flight of N packets arrives. The amount of data in this flight
exceeds the amount of data that can be transmitted by the
bottleneck during the flight's arrival plus the queue length,
i.e. some data do not fit into the queue.
2. The bottleneck is fully saturated. The queue is full, and
packets drain from it more slowly than new packets arrive.
The second type of loss matches the typical expectation of a
congestion control algorithm: the cwnd value when loss happens is
indicative of the bottleneck being fully saturated. When the first
type of loss happens, however, a sender's cwnd can be much smaller
than the Bandwidth*Delay Product (BDP) of the path (the amount of
data that can be in flight, ignoring the queue). In the absence of
other traffic, the probability for the first type of loss to happen
depends on the queue length and the ratio between the departure and
the arrival rate during the flight's arrival. By introducing time
gaps between the packets of a burst, this ratio is increased, i.e.
the difference between the departure and the arrival rate becomes
smaller, and the second type of loss is more likely.
For example, consider a network path with a bottleneck capacity of 50
Mbit/s, a queue length of 15000 bytes (or 10 packets of size 1500
bytes) and an RTT of 30 ms. Assume that all packets emitted by the
sender have a size of 1500 bytes. Then, the BDP equals 125 packets.
The bottleneck of this network path is fully saturated when a (BDP +
queue length) amount of bytes are in flight: 135 packets.
In this network, the first type of loss can happen as follows: say,
N=40 packets arrive at this bottleneck at a rate of 100 Mbit/s. In
an otherwise empty network and assuming an initial window of 10
packets and no delayed ACKs, this occurs in the third round of slow
start without pacing, provided that the capacities of all links
before the bottleneck are at least 100 Mbit/s. In this case, an
overshoot will occur: packets are forwarded with half their arrival
rate, i.e. less than 20 packets can be forwarded during the burst's
arrival. The remaining 20 (or more) packets cannot fit into the
10-packet queue. A cwnd of 40 packets is much smaller than the (BDP
+ queue) limit of 135 packets, and the bottleneck is not fully
saturated.
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Let us now assume that the flight of 40 packets is instead paced,
such that the arrival rate only mildly exceeds the departure rate --
e.g., they arrive at a rate of 60 Mbit/s. When the last packet of
this flight arrives at the bottleneck, the bottleneck should already
have forwarded 5/6 * 39 = 32.5 packets. Since only complete packets
can be sent, the bottleneck has really forwarded 32 packets, and the
remaining 40-32 = 8 packets fit in the queue. No loss occurs.
This example explains how pacing can enable a rate increase to last
longer than without pacing. This makes it more likely that a
bottleneck is saturated, such that cwnd reflects the BDP plus the
queue length (loss type 2).
3.1.1. Backing off after the increase
The two loss types explained in Section 3.1 require a different back-
off factor to allow the queue to drain and congestion to dissipate.
Specifically, in the single-sender single-bottleneck example above,
when a slow start overshoot occurs as loss type 2, a back-off
function such as: ssthresh = cwnd * beta with beta >= 0.5 is
guaranteed to cause a second loss after the end of loss recovery.
This is because, when cwnd exceeds a fully saturated bottleneck
(i.e., cwnd > BDP + queue length), cwnd will have grown further by
another (BDP + queue length) by the time the sender learns about the
loss. In this case, beta = 0.5 will cause ssthresh to exceed (BDP +
queue length) again.
Since pacing makes loss type 2 more likely, beta < 0.5 may be a
better choice after slow start overshoot when pacing is used.
3.1.2. Able to work with smaller queues
The probability of loss type 1 in Section 3.1 is indirectly
proportional to the queue length. Pacing therefore enables a rate
increase to continue with a smaller queue at the bottleneck than in
the case without pacing.
3.2. Getting good RTT estimates
Since pacing algorithms generally attempt to spread out packets
evenly across an RTT, it is important to have a good RTT estimate.
Especially in the beginning of a transfer, when sending the initial
window, the only RTT estimate available may be from the connection
establishment handshake. Being based on only one sample, this is a
very unreliable estimate, and using it to pace the initial window can
cause unnecessary delay. This may be the reason why the Linux TCP
implementation does not pace the first 10 packets (see Section 4.1).
As a possible improvement, the initial RTT estimate could also be
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based on a previous connection (temporal sharing) or on another
ongoing connection (ensemble sharing) [RFC9040].
4. Implementation examples
4.1. Linux TCP
The following description is based on Linux kernel version 6.8.9.
There are two ways to enable pacing in Linux: 1) via a socket option,
2) by configuring the FQ queue discipline. We describe case 1.
Independent of the value of the Initial Window (IW), the first 10
(hardcoded) packets are not paced. Later, 10 packets will generally
be sent without pacing every 2^32 packets.
Every time an ACK arrives, a pacing rate is calculated, as: factor *
MSS * cwnd / SRTT, where "factor" is a configurable value that, by
default, is 2 in slow start and 1.2 in congestion avoidance. MSS is
the sender maximum segment size [RFC5681], and SRTT is the smoothed
round-trip time [RFC6298] [TODO check: Linux calculates SRTT
different from the standard, though RFC 6298 relaxes the rules, so
maybe it's ok?] The sender transmits data in line with the
calculated pacing rate; this is approximated by calculating the rate
per millisecond, and generally sending the resulting amount of data
per millisecond as a small burst, every millisecond. As an
exception, the per-millisecond amount of data can be a little larger
when the peer is very close, depending on a configurable value (per
default, when the minimum RTT is less than 3 milliseconds).
If the pacing rate is smaller than 2 packets per millisecond, these
bursts will become 2 packets in size, and they will not be sent every
millisecond but with a varying time delay (depending on the pacing
rate). If the pacing rate is larger than 64 Kbyte per millisecond,
these bursts will be 64 Kbyte in size, and they will not be sent
every millisecond but with a varying time delay (depending on the
pacing rate). Bursts can always be smaller than described above, or
be "nothing", if a limiting factor such as the receiver window (rwnd)
[RFC5681] or the current cwnd disallows transmission. If the
previous packet was not sent when expected by the pacing logic, but
more than half of a pacing gap ago (e.g., due to a cwnd limitation),
the pacing gap is halved.
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*TEMPORARY NOTE - TO BE REMOVED:* This description is based on the
longer Linux pacing analysis text that is currently available at:
https://docs.google.com/document/
d/1h5hN9isFjT76YjaCphHZdW9LCRYqV4y3GKwRKxgqEO0/edit?usp=sharing
(https://docs.google.com/document/
d/1h5hN9isFjT76YjaCphHZdW9LCRYqV4y3GKwRKxgqEO0/edit?usp=sharing) -
comments or corrections are very welcome!
4.2. Apple OSes
Pacing was added to Apple OS as a private API in iOS 17 and macOS 14.
In its current form, an application or transport protocol computes
and sets the desired transmit timestamp on a per packet basis and
sends it to the pacing module in AQM. The packets are queued in the
AQM until the current time becomes greater than or equal to
corresponding packet's transmit timestamp. There is an upper limit
of 3 seconds for how long the AQM will hold a queued packet before
sending it.
The above simplicity in the kernel allows upper layer protocols or
applications to set a transmit timestamp in a manner that is suitable
for them. For example, a stream based protocol like TCP might pace
packets differently than a video conferencing app.
4.3. FreeBSD
FreeBSD has the infrastructure to support multiple TCP stacks. Each
TCP stack has a tcp_output() function, which handles most of the
sending of TCP segments. The default stack does not support pacing
and its tcp_output() may be called whenever
1. a TCP segment is received or
2. a TCP timer (like the retransmission or delayed ACK timer) runs
off or
3. the application provides new data to be sent
and sends as many TCP segments as is allowed by the congestion and
flow control resulting in burst of TCP segments. However, this also
allows to make use of TCP Segment Offload (TSO), which reduces the
CPU load.
The RACK [RACK] and BBR stacks both support pacing by leveraging the
TCP High Precision Timer System (HPTS) [HPTS], which is a kernel
loadable module available in FreeBSD 14 and higher. The tcp_output()
function of a TCP stack which supports pacing will not send as much
as is allowed by congestion and flow control, but may only send a
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micro burst and schedule itself for being called after the inter-
burst send time using the HPTS. The RACK stack supports an
application setting a pacing rate and a maximum burst size using TCP
socket options. The RACK stack then uses these values to compute the
actual micro burst size and the inter-burst send time.
The HPTS is optimized for handling a large number of TCP connections
and the tcp_output() function of the RACK stack is also optimized for
being called more often than the tcp_output() function of the default
stack. This allows to use TSO in combination with TCP pacing.
This subsystem underpins recently published research by Netflix and
Stanford into application-informed pacing at scale [Sammy].
4.4. QUIC BBR implementations
Pacing capability is expected in QUIC senders. While standard QUIC
congestion control [RFC9002] is based on TCP Reno, which is not
defined to include pacing (but also does not prohibit it), QUIC
congestion control requires either pacing or some other burst
limitation (Section 7.7 of [RFC9002]). BBR congestion control
implementations are common in QUIC stacks, and pacing is integral to
BBR, so this document focuses on it.
Pacing in QUIC stacks commonly involves:
1. Access to lower-level (e.g. OS and hardware) capabilities needed
for effective pacing.
2. Managing additional timers related to pacing, along with those
already needed for retransmission, and other events.
3. Details of the actual pacing algorithm (e.g. granularity of
bursts allowed, etc.).
Examples of different approaches to dealing with these challenges in
ways that work on multiple operating systems and hardware platforms
can be found in open source QUIC stacks, such as Google's QUIC
implementation and Meta's "mvfst". These provide examples for some
of the concepts discussed below.
Unlike TCP implementations that typically run within the operating
system kernel, QUIC implementations more typically run in user space
and are thus faced with more challenges regarding timing and coupling
with the underlying protocol stack and hardware needed to achieve
pacing. For instance, if an application trying to do pacing is
running on a highly loaded system, it may often "wake up late" and
miss the times that it intends to pace packets.
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When a large amount of data needs to be sent, pacing naively could
result in an excessive number of timers to be managed and adjusted
along with all of the other timers that the QUIC stack and rest of
the application require. The Hashed Hierarchical Timing Wheel [VL87]
provides one approach for such cases, but implementations may also
simply schedule the next send event based on the current pacing rate,
and then schedule subsequent events as needed, rather than adjusting
timers for them. In any case, typically a pacing algorithm should
allow for some amount of burstiness, in order to efficiently use the
hardware as well as to be responsive for bursty (but low overall
rate) applications, and to avoid excessive timer management.
Pacing can be done based on different approaches such as a token-
based or tokenless algorithm. For instance, a tokenless algorithm
(e.g. as used in mvfst) might compute a regular interval time and
batch size (number of packets) to be released every interval and
achieve the pacing rate. This allows specific future transmissions
to be scheduled. In contrast, a token-based algorithm accumulates
tokens to permit transmission based on the pacing rate, using a
"leaky bucket" to control bursts. In this case the size of bursts
may be more granular, depending on how much time has elapsed between
evaluations.
The additional notion of "burst tokens" (or other burst allowance)
may be present in order to rapidly transmit data if coming out of a
quiescent period (e.g. when a flow has been application-limited
without data to send, e.g. as used in Google's implementation). A
number of burst tokens, representing packets that can be sent
unpaced, is initialized to some value (e.g. 10) when a flow starts or
becomes quiescent. If burst tokens are available, outgoing packets
are sent immediately, without pacing, up to the limit permitted by
the congestion window, and the burst tokens are depleted by each
packet sent. The number of burst tokens is reduced to zero on
congestion events. When coming out of quiescence, it is set to the
minimum of the initial burst size, or the amount of packets that the
congestion window (in bytes) represents.
There may be additional "lumpy tokens" that further allow unpaced
packets after the burst tokens have been consumed, and the congestion
window does not limit sending. The amount of lumpy tokens that might
be present is determined using heuristics, generally limiting to a
small number of packets (e.g. 1 or 2).
5. Security Considerations
While congestion control designs, including aspects such as pacing,
could result in unwanted competing traffic, they do not directly
result in new security considerations.
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Transport protocols that provide authentication (including those
using encryption), or are carried over protocols that provide
authentication, can protect their congestion control algorithm from
network attack. This is orthogonal to the congestion control rules.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V.
Jacobson, "BBR Congestion Control", Work in Progress,
Internet-Draft, draft-cardwell-iccrg-bbr-congestion-
control-02, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-cardwell-
iccrg-bbr-congestion-control-02>.
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
7.2. Informative References
[HPTS] Stewart, R. and M. Tüxen, "Pacing in the FreeBSD TCP
Stack", Upcoming in FreeBSD Journal (September/October
2024) , October 2024.
[HPTSCode] "tcp_hpts.c", October 2024, <https://github.com/freebsd/
freebsd-src/blob/main/sys/netinet/tcp_hpts.c#L31-L99>.
[RACK] Stewart, R. and M. Tüxen, "RACK and Alternate TCP Stacks
for FreeBSD", FreeBSD Journal (January/February 2024) ,
February 2024, <https://freebsdfoundation.org/our-
work/journal/browser-based-edition/networking-10th-
anniversary/rack-and-alternate-tcp-stacks-for-freebsd/>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/rfc/rfc5681>.
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[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/rfc/rfc6298>.
[RFC9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.
[RFC9040] Touch, J., Welzl, M., and S. Islam, "TCP Control Block
Interdependence", RFC 9040, DOI 10.17487/RFC9040, July
2021, <https://www.rfc-editor.org/rfc/rfc9040>.
[Sammy] Spang, B., Kunamalla, S., Teixeira, R., Huang, T.,
Armitage, G., Johari, R., and N. McKeown, "Sammy:
Smoothing Video Traffic to be a Friendly Internet
Neighbor", ACM SIGCOMM '23: Proceedings of the ACM SIGCOMM
2023 Conference , 1 September 2023,
<https://doi.org/10.1145/3603269.3604839>.
[VL87] Varghese, G. and T. Tauck, "Hashed and hierarchical timing
wheels: data structures for the efficient implementation
of a timer facility", DOI 10.1145/37499.37504, 1 November
1987, <https://doi.org/10.1145/37499.37504>.
Appendix A. Change Log
* -00 was the first individual submission for feedback by ICCRG.
* -01 adds Michael Tuexen as new co-author, and adds the following
new descriptions:
- a first version of text for Apple OSes
- a first version of text for FreeBSD
Authors' Addresses
Michael Welzl
University of Oslo
PO Box 1080 Blindern
0316 Oslo
Norway
Email: michawe@ifi.uio.no
URI: http://welzl.at/
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Wesley Eddy
MTI Systems
25111 Country Club Blvd, Suite 295
North Olmsted, OH 44070,
United States of America
Email: wes@mti-systems.com
Vidhi Goel
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: vidhi_goel@apple.com
Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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