Specifying New Congestion Control Algorithms
draft-ietf-ccwg-rfc5033bis-04
| Document | Type | Active Internet-Draft (ccwg WG) | |
|---|---|---|---|
| Authors | Martin Duke , Gorry Fairhurst | ||
| Last updated | 2024-04-26 (Latest revision 2024-04-24) | ||
| Replaces | draft-scheffenegger-congress-rfc5033bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Best Current Practice | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Reese Enghardt | ||
| Shepherd write-up | Show Last changed 2024-04-26 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | ietf@tenghardt.net |
draft-ietf-ccwg-rfc5033bis-04
CCWG M. Duke, Ed.
Internet-Draft Google LLC
Obsoletes: 5033 (if approved) G. Fairhurst, Ed.
Intended status: Best Current Practice University of Aberdeen
Expires: 26 October 2024 24 April 2024
Specifying New Congestion Control Algorithms
draft-ietf-ccwg-rfc5033bis-04
Abstract
Introducing new or modified congestion control algorithms in the
global Internet has possible ramifications to both the flows using
the proposed congestion control algorithms and to flows using a
standardized congestion control algorithm. Therefore, the IETF must
proceed with caution when evaluating proposals for alternate
congestion control. The goal of this document is to provide guidance
for considering standardization of a proposed congestion control
algorithm at the IETF. It obsoletes RFC5033 to reflect changes in
the congestion control landscape.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-ccwg-rfc5033bis/.
Discussion of this document takes place on the Congestion Control
Working Group (ccwg) Working Group mailing list
(mailto:ccwg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ccwg/. Subscribe at
https://www.ietf.org/mailman/listinfo/ccwg/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-ccwg/rfc5033bis.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 26 October 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Guidelines for Authors about Document Status . . . . . . . . 6
3. Specifying Algorithms for Use in Controlled Environments . . 7
4. Evaluation Criteria . . . . . . . . . . . . . . . . . . . . . 8
4.1. Single Algorithm Behavior . . . . . . . . . . . . . . . . 8
4.1.1. Protection Against Congestion Collapse . . . . . . . 8
4.1.2. Protection Against Bufferbloat . . . . . . . . . . . 8
4.1.3. Protection Against High Packet Loss . . . . . . . . . 9
4.1.4. Fairness within the Proposed Congestion Control
Algorithm . . . . . . . . . . . . . . . . . . . . . . 9
4.1.5. Short Flows . . . . . . . . . . . . . . . . . . . . . 10
4.2. Mixed Algorithm Behavior . . . . . . . . . . . . . . . . 10
4.2.1. Existing General-Purpose Transports . . . . . . . . . 10
4.2.2. Real-Time Protocols . . . . . . . . . . . . . . . . . 11
4.2.3. Short and Long Flows . . . . . . . . . . . . . . . . 12
4.3. Other Criteria . . . . . . . . . . . . . . . . . . . . . 12
4.3.1. Differences with Congestion Control Principles . . . 12
4.3.2. Incremental Deployment . . . . . . . . . . . . . . . 12
5. General Use . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Paths with Tail-drop Queues . . . . . . . . . . . . . . . 13
5.2. Tunnel Behavior . . . . . . . . . . . . . . . . . . . . . 13
5.3. Wired Paths . . . . . . . . . . . . . . . . . . . . . . . 13
5.4. Wireless Paths . . . . . . . . . . . . . . . . . . . . . 14
6. Special Cases . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Active Queue Management (AQM) . . . . . . . . . . . . . . 14
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6.1.1. Operation with the Envelope set by Network Circuit
Breakers . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Paths with Varying Delay . . . . . . . . . . . . . . . . 15
6.3. Internet of Things . . . . . . . . . . . . . . . . . . . 15
6.4. Paths with High Delay . . . . . . . . . . . . . . . . . . 16
6.5. Misbehaving Nodes . . . . . . . . . . . . . . . . . . . . 16
6.6. Extreme Packet Reordering . . . . . . . . . . . . . . . . 16
6.7. Transient Events . . . . . . . . . . . . . . . . . . . . 17
6.7.1. Sudden changes in the Path . . . . . . . . . . . . . 17
6.8. Multipath Transport . . . . . . . . . . . . . . . . . . . 17
6.9. Data Centers . . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Evolution of RFC5033bis . . . . . . . . . . . . . . . . . . . . . 24
Since draft-ietf-ccwg-rfc5033bis-03 . . . . . . . . . . . . . . 24
Since draft-ietf-ccwg-rfc5033bis-02 . . . . . . . . . . . . . . 24
Since draft-ietf-ccwg-rfc5033bis-01 . . . . . . . . . . . . . . 24
Since draft-ietf-ccwg-rfc5033bis-00 . . . . . . . . . . . . . . 25
Since draft-scheffenegger-congress-rfc5033bis-00 . . . . . . . 25
Since RFC5033 . . . . . . . . . . . . . . . . . . . . . . . . . 25
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This document provides guidelines for the IETF to use when evaluating
a proposed congestion control algorithm that differs from the general
congestion control principles outlined in [RFC2914]. The guidance is
intended to be useful to authors proposing congestion control
algorithms and for the IETF community when evaluating whether a
proposal is appropriate for publication in the RFC series and for
deployment in the Internet.
This document obsoletes [RFC5033], which was published in 2007 as a
Best Current Practice to evaluate proposed congestion control
algorithms as Experimental or Proposed Standard RFCs.
The IETF's standard congestion control algorithms have performance
challenges in various environments (e.g., high-speed networks,
cellular and WiFi wireless technologies, and long distance satellite
links) and also for flOws carrying specific workloads (Voice over IP
(VoIP), gaming, and videoconferencing).
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When [RFC5033] was published in 2007, TCP [RFC9293] was the dominant
consumer of this work, and proposals were typically discussed in the
Internet Congestion Control Research Group (ICCRG). The Datagram
Congestion Control Protocol (DCCP) [RFC4340] was developed as a
method for defining new congestion control algorithms for datagram
traffic.
Since then, many conditions have changed. The set of protocols using
these algorithms has spread beyond TCP, Stream Control Transmission
Protocol (SCTP) [RFC9260], and DCCP, to include QUIC [RFC9000], RTP
Media Congestion Avoidance Techniques (RMCAT) (e.g., [RFC8836]) and
beyond.
Some proponents of alternative congestion control algorithms now have
the opportunity to test and deploy at scale without IETF review.
There is more interest in specialized use cases, such as data centers
(e.g., [RFC8257]), and in support for a variety of upper layer
protocols/applications, e.g., real-time protocols. Finally, the
community has gained much more experience with indications of
congestion beyond packet loss.
Multicast congestion control is a considerably less mature field of
study and are not in scope for this document. However, Section 4 of
[RFC8085] provides additional guidelines for multicast and broadcast
usage of UDP.
Congestion control algorithms have been developed outside of the
IETF, including at least two that saw large scale deployment: Cubic
[HRX08] and Bottleneck Bandwidth and Round-trip propagation time
(BBR) [BBR-draft].
Cubic was documented in a research publication in 2007 [HRX08], and
was then adopted as the default congestion control algorithm for the
TCP implementation in Linux. It was already used in a significant
fraction of TCP connections over the Internet before being documented
in an Informational Internet-Draft in 2015, published as an
Informational RFC in 2017 [RFC8312] and then as a Proposed Standard
in 2023 [RFC9438].
At the time of writing, BBR is being developed as an internal
research project by Google, with the first implementation contributed
to Linux kernel 4.19 in 2016. It was described in an IRTF Internet-
Draft in 2018, and that Internet- Draft is regularly updated to
document the evolving versions of the algorithm [BBR-draft]. BBR is
currently widely used for Google services using either TCP or QUIC,
and is also widely deployed outside of Google.
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We cannot say now whether the original authors of [RFC5033] expected
that developers would be somehow waiting for IETF review before
widely deploying a new congestion control algorithm over the
Internet, but the examples of Cubic and BBR teach us that deployment
of new algorithms is not in fact gated by the publication of the
algorithm as an RFC.
Nevertheless, specifying congestion control algorithms has a number
of advantages:
* A specification can help implementers, operators, and other
interested parties develop a shared understanding of how the
algorithm works and how it is expected to behave in various
scenarios and configurations.
* A specification can help potential contributors understand the
algorithm, which can make it easier for them to suggest
improvements and/or identify limitations. Furthermore, the
specification can help multiple contributors align on a consensus
change to the algorithm.
* A specification that is accessible to anyone can circumvent the
issue that some implementers may be unable to read open source
reference implementations due to the constraints of some open
source licenses.
Beyond helping develop specific algorithm proposals, guidelines can
also serve as a reminder to potential inventors and developers of the
multiple facets of the congestion control problem.
The evaluation guidelines in this document are intended to be
consistent with the congestion control principles from [RFC2914] of
preventing congestion collapse, considering fairness, and optimizing
a flow's own performance in terms of throughput, delay, and loss.
[RFC2914] also discusses the goal of avoiding a congestion control
"arms race" among competing transport protocols.
This document does not give hard-and-fast requirements for an
appropriate congestion control algorithm. Rather, the document
provides a set of criteria that should be considered and weighed by
the developers of alternative algorithms and by the IETF in the
context of each proposal.
The high-order criterion for advancing any proposal within the IETF
is a serious scientific study of the pros and cons that occurs when
the proposal is considered for publication by the IETF, or before it
is deployed at large scale.
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After initial studies, authors are encouraged to write a
specification of their proposal for publication in the RFC series.
This allows others to understand and investigate the wealth of
proposals in this space.
This document is intended to reduce the barriers to entry for new
congestion control work to the IETF. As such, proponents of new
congestion control algorithms ought not to interpret these criteria
as a checklist of requirements before approaching the IETF. Instead,
proponents are encouraged to think about these issues beforehand, and
have the willingness to do the work implied by the remainder of this
document.
2. Guidelines for Authors about Document Status
This document applies to proposed congestion control algorithms
proposals that seek Experimental or Standards Track status.
Evaluation of both cases involves the same questions, but with
different expectations for both the answers and the degree of
certainty of those answers.
Congestion control algorithms without empirical evidence of Internet-
scale deployment SHOULD seek Experimental status. Congestion control
algorithms with a record of measured Internet-scale deployment MAY
directly seek the Standards Track if there is solid data that
reflects that it is safe, and the design is stable, guided by the
considerations in Section 5. However, the existence of this data
does not waive the other considerations in this document.
Experimental specifications SHOULD NOT be deployed as a default.
They SHOULD only be deployed in situations where they are being
actively measured, and where it is possible to deactivate them if
there are signs of pathological behavior.
Each published congestion control algorithm is REQUIRED to include a
statement in the abstract indicating whether or not there is IETF
consensus that the proposed congestion control algorithm is
considered safe for use on the Internet. Each published algorithm is
also required to include a statement in the abstract describing
environments where the protocol is not recommended for deployment.
There can be environments where the congestion control algorithm is
deemed _safe_ for use, but it is still is _not recommended_ for use
because it does not perform well for the user.
As examples of such statements, [RFC3649] specifies HighSpeed TCP and
includes a statement in the abstract stating that the proposed
congestion control algorithm is Experimental, but may be deployed in
the Internet. In contrast, the Quick-Start document [RFC4782]
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includes a paragraph in the abstract stating that the mechanism is
only being proposed for use in controlled environments. The abstract
specifies environments where the Quick- Start request could give
false positives (and therefore would be unsafe for incremental
deployment where some routers forward, but do not process the
option). The abstract also specifies environments where packets
containing the Quick-Start request could be dropped in the network;
in such an environment, Quick-Start would not be unsafe to deploy,
but deployment is not recommended because it could lead to
unnecessary delays for the connections attempting to use Quick-Start.
The Quick-Start method is discussed as an example in [RFC9049].
Alhough out of the scope of this document, proponents of a new
algorithm could alternatively seek publication as an Informational or
Experimental RFC via the Internet Research Task Force (IRTF). In
general, these algorithms are expected to be less mature than ones
that follow the procedures in this document. Authors documenting
deployed congestion control algorithms that cannot be changed by IETF
or IRTF review are invited to publish as an Informational RFC via the
Independent Stream Editor (ISE).
3. Specifying Algorithms for Use in Controlled Environments
Algorithms can be designed for general Internet deployment or for use
in controlled environments [RFC8799]. An operator can ensure that
flows within a controlled environment are isolated from other
Internet flows, or they might allow these flows to share resources
with other Internet flows. Algorithms that rely on specific
functions or configurations in a network need to provide a reference
or specification for these functions (an RFC or another stable
specification). The IETF will need to assess whether the relevant
working group is able to review the proposed new algorithm and
whether there is sufficient experience to understand any dependent
functions.
A data center is an example of a controlled environment, which often
deploys fabrics with rich signalling from switches to endpoints.
In evaluating a new proposal for use in a controlled environment, the
IETF needs to understand the usage, e.g., how the usage is scoped to
the controlled environment, whether the algorithm will share
resources with Internet traffic, and consider what could happen if
used in a protocol that is bridged across an Internet path.
Algorithms that are designed to be confined to a controlled
environment and are not intended for use in the general Internet,
might instead seek real-world data for those environments. In such
cases, the evaluation criteria in the remainder of this document
might not apply.
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4. Evaluation Criteria
As noted above, authors are expected to do a well-rounded evaluation
of the pros and cons of congestion control algorithms that are
brought to the IETF. The following guidelines are designed to help
authors and the IETF community. Concerns that fall outside the scope
of these guidelines are certainly possible; these guidelines should
not be considered an all-encompassing check-list.
When considering a proposed congestion control algorithm, the
community MUST consider the following criteria. These criteria will
be evaluated in various domains (see Section 5 and Section 6).
4.1. Single Algorithm Behavior
The criteria in this section evaluate the congestion control
algorithm when one or more flows using that algorithm share a
bottleneck link (i.e., with no flows using a differing congestion
control algorithm).
4.1.1. Protection Against Congestion Collapse
A congestion control algorithm should either stop sending when the
packet drop rate exceeds some threshold [RFC3714], or should include
some notion of "full backoff". For "full backoff", at some point the
algorithm would reduce the sending rate to one packet per round-trip
time and then exponentially backoff the time between single packet
transmissions if the congestion persists. Exactly when either "full
backoff" or a pause in sending comes into play will be algorithm-
specific. However, as discussed in [RFC2914] and [RFC8961], this
requirement is crucial to protect the network in times of extreme
(persistent) congestion.
If full backoff is used, this test does not require that the
mechanism must be identical to that of TCP ([RFC2988], [RFC8961]).
For example, this does not preclude full backoff mechanisms that
would give flows with different round- trip times comparable capacity
during backoff.
4.1.2. Protection Against Bufferbloat
A congestion control algorithm should try to avoid maintaining
excessive queues in the network. Exactly how the algorithm achieves
this is algorithm-specific, but see [RFC8961] and [RFC8085] for
requirements.
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Bufferbloat [Bufferbloat] refers to the building of excessive queues
in the network. Many network routers are configured with very large
buffers. The standards-track Reno [RFC5681] and Cubic [RFC9438]
congestion control algorithms send at progressively higher rates
until a First-In First-Out (FIFO) buffer completely fills, and packet
losses then occur. Every connection passing through that bottleneck
experiences increased latency due to the high buffer occupancy. This
adds unwanted latency that negatively impacts highly interactive
applications such as videoconferencing or games, but it also affects
routine web browsing and video playing.
This problem has been widely discussed since 2011 [Bufferbloat], but
was not discussed in the Congestion Control Principles published in
September 2002 [RFC2914]. The Reno and Cubic congestion control
algorithms do not address it, but a new congestion control algorithm
has the opportunity to improve the state of the art.
4.1.3. Protection Against High Packet Loss
A congestion control algorithm should try to avoid causing
excessively high rates of packet loss. To accomplish this, it should
avoid excessive increases in sending rate, and reduce its sending
rate if experiencing high packet loss.
The first version of the BBR algorithm [BBRv1-draft] failed this
requirement. Experimental evaluation [BBRv1-Evaluation] showed that
it caused a sustained rate of packet loss when multiple BBRv1 flows
shared a bottleneck and the buffer size was less than roughly one and
a half BDP. This was unsatisfactory, and indeed further versions of
BBR [BBR-draft] fixed it.
This requirement does not imply that the algorithm should react to
packet losses in exactly the same way as current standards-track
congestion control algorithms (e.g., [RFC5681]).
4.1.4. Fairness within the Proposed Congestion Control Algorithm
When multiple competing flows all use the same proposed congestion
control algorithm, the proposal should explore how the capacity is
shared among the competing flows. Capacity fairness can be important
when a small number of similar flows compete to fill a bottleneck.
However, it can also not be useful, for example, when comparing flows
that seek to send at different rates, or if some of the flows do not
last sufficiently long to approach asymptotic behavior.
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4.1.5. Short Flows
A great deal of congestion control analysis concerns the steady-state
behavior of long flows. However, many Internet flows are relatively
short-lived. If flows never experience a packet loss, many short-
lived flows today remain in the "slow start" mode of operation
[RFC5681] that commonly features exponential congestion window
growth.
A proposed congestion control algorithm MUST consider how new and
short-lived flows affect long-lived flows, and vice versa.
4.2. Mixed Algorithm Behavior
Mixed algorithm behavior criteria evaluate the interaction of the
proposed congestion control algorithm with commonly deployed
congestion control algorithms.
In contexts where differing congestion control algorithms are used,
it is important to understand whether the proposed congestion control
algorithm could result in more harm than previous standards-track
algorithms (e.g., [RFC5681], [RFC9002], [RFC9438]) to flows sharing a
common bottleneck. The measure of harm is not restricted to unequal
capacity, but ought also to consider metrics such as the introduced
latency, or an increase in packet loss. An evaluation MUST assess
the potential to cause starvation, including assurance that a loss of
all feedback (e.g., detected by expiry of a retransmission time out)
results in backoff.
4.2.1. Existing General-Purpose Transports
A proposed congestion control algorithm SHOULD be evaluated when
competing using standard IETF congestion controls, e.g. [RFC5681],
[RFC9002], [RFC9438]. A proposed congestion control algorithm that
has a significantly negative impact on flows using standard
congestion control might be suspect, and this aspect should be part
of the community's decision making with regards to the suitability of
the proposed congestion control algorithm. The community should also
consider other non-standard congestion control algorithms that are
known to be widely deployed.
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Note that this guideline is not a requirement for strict Reno- or
Cubic- friendliness as a prerequisite for a proposed congestion
control mechanism to advance to Experimental or Standards Track
status. As an example, HighSpeed TCP is a congestion control
mechanism specified as Experimental, that is not TCP- friendly in all
environments. When a new congestion control algorithm is deployed,
the existing major algorithm deployments need to be considered to
avoid severe performance degradation. Note that this guideline does
not constrain the interaction with non-best-effort flows.
As an example from an Experimental RFC, fairness with standard TCP is
discussed in Sections 4 and 6 of [RFC3649] (HighSpeed TCP) and using
spare capacity is discussed in Sections 6, 11.1, and 12 of [RFC3649].
4.2.2. Real-Time Protocols
General-purpose protocols need to coexist in the Internet with real-
time congestion control algorithms, which, in general, have finite
throughput requirements (i.e., do not seek to utilize all available
capacity) and more strict latency bounds.
[RFC8868] provides suggestions for real-time congestion control
design and [RFC8867] suggests test cases. [RFC9392] describes some
considerations for the RTP Control Protocol (RTCP). In particular,
feedback for real-time flows can be less frequent than the
acknowledgements provided by reliable transports. This document does
not change the informational status of those RFCs.
A proposed congestion control algorithm SHOULD consider coexistence
with widely deployed real-time congestion control algorithms.
Regrettably, at the time of writing (2024), many algorithms with
detailed public specifications are not widely deployed, while many
widely deployed real-time congestion control algorithms have
incomplete public specifications. It is hoped that this situation
will change.
To the extent that behavior of widely deployed algorithms is
understood, proponents of a proposed congestion control algorithm can
analyze and simulate its interaction with those algorithms. To the
extent they are not, experiments can be conducted where possible.
Real-time flows can be directed into distinct queues via
Differentiated Services Code Points (DSCP) or other mechanisms, which
can substantially reduce the interplay with other traffic. However,
a proposal targeting general Internet use can not assume this is
always the case.
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Section 6.1.1 describes the impact of network transport circuit
breaker algorithms. [RFC8083] also defines a minimal set of RTP
circuit breakers that operate across a path. This identifies
conditions under which a sender needs to stop transmitting media data
to protect the network from excessive congestion. It is expected
that, in the absence of long-lived excessive congestion, RTP
applications running on best-effort IP networks will be able to
operate without triggering these circuit breakers.
4.2.3. Short and Long Flows
The effect on short-lived and long-lived flows using other common
congestion control algorithms MUST be evaluated, as in Section 4.1.5.
4.3. Other Criteria
4.3.1. Differences with Congestion Control Principles
A proposed congestion control algorithm SHOULD include a clear
explanation of any deviations from [RFC2914] and [RFC7141].
4.3.2. Incremental Deployment
The proposed congestion control algorithm ought to discuss whether it
allows for incremental deployment in the targeted environment. For a
mechanism targeted for deployment in the current Internet, it would
be helpful for the proposal to discuss what is known (if anything)
about the correct operation of the mechanisms with some of the
equipment in the current Internet, e.g., routers, transparent
proxies, WAN optimizers, intrusion detection systems, home routers,
and the like.
Similarly, if the proposed congestion control algorithm is intended
only for specific environments (and not the global Internet), the
proposal should consider how this intention is to be realised. The
community will have to address the question of whether the scope can
be enforced by stating the restrictions, or whether additional
protocol mechanisms are required to enforce this scoping. The answer
will necessarily depend on the proposed change.
As an example from an Experimental RFC, deployment issues are
discussed in Sections 10.3 and 10.4 of [RFC4782] (Quick-Start).
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5. General Use
The criteria in Section 4 will be evaluated in the following
scenarios. Unless a proposed congestion control specification
explicitly forbids use on the public Internet, the community MUST
find that it meets the criteria in these scenarios for the proposed
congestion control algorithm to progress.
The evaluation in each scenario SHOULD occur over a representative
range of bandwidths, delays, and queue depths. Of course, the set of
parameters representative of the public Internet will change over
time.
These criteria are intended to capture a statistically dominant set
of Internet conditions. In the case that a proposed congestion
control algorithm has been tested at Internet scale, the results from
that deployment are often useful for answering these questions.
5.1. Paths with Tail-drop Queues
The performance of a congestion control algorithm is affected by the
queue discipline applied at the bottleneck link. The drop-tail queue
discipline (using a FIFO buffer) MUST be evaluated. See Section 6.1
for evaluation of other queue disciplines.
5.2. Tunnel Behavior
When a proposed congestion control algorithm relies on explicit
signals from the path, the proposal MUST consider the effect of
traffic passing through a tunnel, where routers may not be aware of
the flow.
5.3. Wired Paths
Wired networks are usually characterized by extremely low rates of
packet loss except for those due to queue drops. They tend to have
stable aggregate bandwidth, usually higher than other types of links,
and low non-queueing delay. Because the properties are relatively
simple, wired links are typically used as a "baseline" case even if
they are not always the bottleneck link in the modern Internet.
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5.4. Wireless Paths
While the early Internet was dominated by wired links, the properties
of wireless links have become extremely important to Internet
performance. In particular, a proposed congestion control algorithm
should be evaluated in situations where some packet losses are due to
radio effects, rather than router queue drops; the link capacity
varies over time due to changing link conditions; and media access
delays and link-layer retransmission lead to increased jitter in
round-trip times. See [RFC3819] and Section 16 of [Tools] for
further discussion of wireless properties.
6. Special Cases
The criteria in Section 4 will be evaluated in the following
scenarios, unless the proposed congestion control algorithm
specifically excludes its use in a scenario. For these specific use-
cases, the community MAY allow a proposal to progress even if the
criteria indicate an unsatisfactory result for these scenarios.
In general, measurements from Internet-scale deployments might not
expose the properties of operation in each of these scenarios,
because they are not as ubiquitous as the General Use scenarios.
6.1. Active Queue Management (AQM)
The proposed congestion control algorithm SHOULD be evaluated under a
variety of bottleneck queue disciplines. The effect of an AQM
discipline can be hard to detect by Internet evaluation. At a
minimum, a proposal should reason about an algorithm's response to
various AQM disciplines. Simulation or empirical results are, of
course, valuable.
Among the AQM techniques that might have an impact on a proposed
congestion control algorithm are FQ-CoDel [RFC8290]; Proportional
Integral Controller Enhanced (PIE) [RFC8033]; and Low Latency, Low
Loss, and Scalable Throughput (L4S) [RFC9332].
A proposed congestion control algorithm that sets one of the two
Explicit Congestion Transport (ECT) codepoints in the IP header can
gain the benefits of receiving Explicit Congestion Notification (ECN)
Congestion Experienced (CE) signals from an on-path AQM [RFC8087].
Use of ECN [RFC3168], [RFC9332] requires the congestion control
algorithm to react when it receives a packet with an ECN-CE marking.
This reaction needs to be evaluated to confirm that the algorithm
conforms with the requirements of the ECT codepoint that was used.
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Note that evaluation of AQM techniques -- as opposed to their impact
on a specific proposed congestion control algorithm -- is out of
scope of this document. [RFC7567] describes design considerations
for AQMs.
6.1.1. Operation with the Envelope set by Network Circuit Breakers
Some equipment in the network uses an automatic mechanism to
continuously monitor the use of resources by a flow or aggregate set
of flows [RFC8084]. Such a network transport circuit breaker can
automatically detect excessive congestion, and when detected, it can
terminate (or significantly reduce the rate of) the flow(s). A well-
designed congestion control algorithm ought to react before the flow
uses excessive resources, and therefore will operate within the
envelope set by network transport circuit breaker algorithms.
6.2. Paths with Varying Delay
An Internet Path can include simple links, where the minimum delay is
the propagation delay, and any additional delay can be attributed to
link buffering. This cannot be assumed. An Internet Path can also
include complex subnetworks where the minimum delay changes over
various time scales, resulting in a non- stationary minimum delay.
This occurs when a subnet changes the forwarding path to optimise
capacity, resilience, etc. It could also arise when a subnet uses a
capacity management method where the available resource is
periodically distributed among the active nodes. A node might then
have to buffer data until an assigned transmission opportunity or
until the physical path changes (e.g., when the length of a wireless
path changes, or the physical layer changes its mode of operation).
Variation also arises when traffic with a higher priority DSCP pre-
empts transmission of traffic with a lower class. In these cases,
the delay varies as a function of external factors, and attempting to
infer congestion from an increase in the delay results in reduced
throughput. This variation in the delay over short timescales
(jitter) might not be distinguishable from jitter that results from
other effects.
A proposed congestion control algorithm SHOULD be evaluated to ensure
its operation is robust when there is a significant change in the
minimum delay.
6.3. Internet of Things
The "Internet of Things" (IoT) is a broad concept, but when
evaluating a proposed congestion control algorithm, it is often
associated with unique characteristics.
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IoT nodes might be more constrained in power, CPU, or other
parameters than conventional Internet hosts. This might place limits
on the complexity of any given algorithm. These power and radio
constraints might make the volume of control packets in a given
algorithm a key evaluation metric.
Furthermore, many IoT applications do not a have a human in the loop,
and therefore can have weaker latency constraints because they do not
relate to a user experience. They mightt still need to share the
path with other flows with different constraints.
Extremely low-power links can lead to very low throughput and a low
bandwidth- delay product, well below the standard operating range of
most Internet flows.
6.4. Paths with High Delay
A proposed congestion control algorithm ought not to presume that all
general Internet paths have a low delay. Some paths include links
that contibute much more delay than for a typical Internet path.
Satellite links often have delays longer than typical for wired paths
[RFC2488] and high delay bandwidth products [RFC3649].
They can also have variable delay as described in Section 6.2.
6.5. Misbehaving Nodes
A proposed congestion control algorithm should explore how the
algorithm performs with non-compliant senders, receivers, or routers.
In addition, the proposal should explore how a proposed congestion
control algorithm performs with outside attackers. This can be
particularly important for proposed congestion control algorithms
that involve explicit feedback from routers along the path.
As an example from an Experimental RFC, performance with misbehaving
nodes and outside attackers is discussed in Sections 9.4, 9.5, and
9.6 of [RFC4782]. This includes discussion of misbehaving senders
and receivers; collusion between misbehaving routers; misbehaving
middleboxes; and the potential use of Quick- Start to attack routers
or to tie up available Quick-Start bandwidth.
6.6. Extreme Packet Reordering
A proposed congestion control algorithm ought not to presume that all
general Internet paths reliably deliver packets in order. [RFC4653]
discusses the effect of extreme packet reordering.
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6.7. Transient Events
A proposed congestion control algorithm SHOULD consider how the
proposed congestion control algorithm would perform in the presence
of transient events such as sudden onset of congestion, a routing
change, or a mobility event. Routing changes, link disconnections,
intermittent link connectivity, and mobility are discussed in more
detail in Section 16 of [Tools].
As an example from an Experimental RFC, response to transient events
is discussed in Section 9.2 of [RFC4782].
6.7.1. Sudden changes in the Path
An IETF transport is not tied to a specific Internet path or type of
path. The set of routers that form a path can and do change with
time. This will cause the properties of the path to change with
respect to time. A proposed congestion control algorithm MUST
evaluate the impact of changes in the path, and be robust to changes
in path characteristics on the interval of common Internet re-routing
intervals.
6.8. Multipath Transport
Multipath transport protocols permit more than one path to be
differentiated and used by a single connection at the sender. A
multipath sender can schedule which packets travel on which of its
active paths. This enables a tradeoff in timeliness and reliability.
There are various ways that multipath techniques can be used.
One example use is to provide fail-over from one path to another when
the original path is no longer viable, or provides inferior
performance. Designs need to independently track the congestion
state of each path, and demonstrate independent congestion control
for each path being used. Authors of a proposed multipath congestion
control algorithm that implements path fail-over MUST evaluate the
harm resulting from a change in the path, and show that this does not
result in flow starvation. Synchronisation of failover (e.g., where
multiple flows change their path on similar timeframes) can also
contribute to harm and/or reduce fairness. These effects also ought
to be evaluated.
Another example use is concurrent multipath, where the transport
protocol simultaneously schedules a flow to aggregate the capacity
across multiple paths.
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The Internet provides no guarantee that different paths (e.g., using
different endpoint addresses) are disjoint. This has additional
implications. A congestion control algorithm proposal MUST evaluate
the potential harm to other flows when the multiple paths share a
common congested bottleneck or share resources that are coupled
between different paths, such as an overall capacity limit). It
SHOULD consider the potential for harm to other flows.
Synchronisation of congestion control mechanisms (e.g., where
multiple flows change their behaviour on similar timeframes) can also
contribute to harm and/or reduce fairness. These effects also ought
to be evaluated.
At the time of writing (2024), there are currently no Standards Track
RFCs for concurrent multipath, but there is an Experimental RFC
[RFC6356] that specifies a concurrent multipath congestion control
algorithm for MPTCP [RFC8684].
6.9. Data Centers
Data centers are characterized by very low latencies (< 2 ms). Many
workloads involve bursty traffic where many nodes complete a task at
the same time. As a controlled environment, data centers often
deploy fabrics that employ rich signalling from switches to
endpoints. Furthermore, the operator can often limit the number of
operating congestion control algorithms.
For these reasons, data center congestion controls are often distinct
from those running elsewhere on the Internet (see Section 3. A
proposed congestion control need not coexist well with all other
algorithms if it is intended for data centers, but the proposal
SHOULD indicate which are expected to safely coexist with it.
7. Security Considerations
This document does not represent a change to any aspect of the TCP/IP
protocol suite and therefore does not directly impact Internet
security. The implementation of various facets of the Internet's
current congestion control algorithms do have security implications
(e.g., as outlined in [RFC5681]).
The IETF process that results in publication needs to ensure that
these security implications are considered. Proposed congestion
control algorithms therefore ought to be mindful of pitfalls, and
should examine any potential security issues that may arise.
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8. IANA Considerations
This document has no IANA actions.
9. References
9.1. Normative References
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/rfc/rfc2914>.
[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>.
[RFC7141] Briscoe, B. and J. Manner, "Byte and Packet Congestion
Notification", BCP 41, RFC 7141, DOI 10.17487/RFC7141,
February 2014, <https://www.rfc-editor.org/rfc/rfc7141>.
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
<https://www.rfc-editor.org/rfc/rfc8083>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/rfc/rfc8084>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.
[RFC8961] Allman, M., "Requirements for Time-Based Loss Detection",
BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/rfc/rfc8961>.
[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>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/rfc/rfc9438>.
9.2. Informative References
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[BBR-draft]
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>.
[BBRv1-draft]
Cardwell, N., Cheng, Y., Yeganeh, S. H., and V. Jacobson,
"BBR Congestion Control", Work in Progress, Internet-
Draft, draft-cardwell-iccrg-bbr-congestion-control-00, 3
July 2017, <https://datatracker.ietf.org/doc/html/draft-
cardwell-iccrg-bbr-congestion-control-00>.
[BBRv1-Evaluation]
Zitterbart, M., "Experimental evaluation of BBR congestion
control", 2017 IEEE 25th International Conference on
Network Protocols (ICNP) , 2017,
<https://ieeexplore.ieee.org/document/8117540>.
[Bufferbloat]
Kathleen Nichols, "Bufferbloat: Dark Buffers in the
Internet", ACM Queue Volume 9, Issue 11 , 2011,
<https://queue.acm.org/detail.cfm?id=2071893>.
[HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: a new TCP-friendly
high-speed TCP variant", ACM SIGOPS Operating Systems
Review, vol. 42, no. 5, pp. 64-74 , July 2008,
<https://doi.org/10.1145/1400097.1400105>.
[RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms",
BCP 28, RFC 2488, DOI 10.17487/RFC2488, January 1999,
<https://www.rfc-editor.org/rfc/rfc2488>.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, DOI 10.17487/RFC2988, November 2000,
<https://www.rfc-editor.org/rfc/rfc2988>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/rfc/rfc3168>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<https://www.rfc-editor.org/rfc/rfc3649>.
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[RFC3714] Floyd, S., Ed. and J. Kempf, Ed., "IAB Concerns Regarding
Congestion Control for Voice Traffic in the Internet",
RFC 3714, DOI 10.17487/RFC3714, March 2004,
<https://www.rfc-editor.org/rfc/rfc3714>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004,
<https://www.rfc-editor.org/rfc/rfc3819>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/rfc/rfc4340>.
[RFC4653] Bhandarkar, S., Reddy, A. L. N., Allman, M., and E.
Blanton, "Improving the Robustness of TCP to Non-
Congestion Events", RFC 4653, DOI 10.17487/RFC4653, August
2006, <https://www.rfc-editor.org/rfc/rfc4653>.
[RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782,
January 2007, <https://www.rfc-editor.org/rfc/rfc4782>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/rfc/rfc5033>.
[RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion
Control Mechanisms", RFC 5166, DOI 10.17487/RFC5166, March
2008, <https://www.rfc-editor.org/rfc/rfc5166>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/rfc/rfc6356>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/rfc/rfc7567>.
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[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/rfc/rfc8033>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/rfc/rfc8087>.
[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/rfc/rfc8257>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/rfc/rfc8290>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/rfc/rfc8312>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/rfc/rfc8684>.
[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
<https://www.rfc-editor.org/rfc/rfc8799>.
[RFC8836] Jesup, R. and Z. Sarker, Ed., "Congestion Control
Requirements for Interactive Real-Time Media", RFC 8836,
DOI 10.17487/RFC8836, January 2021,
<https://www.rfc-editor.org/rfc/rfc8836>.
[RFC8867] Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating Congestion Control for Interactive
Real-Time Media", RFC 8867, DOI 10.17487/RFC8867, January
2021, <https://www.rfc-editor.org/rfc/rfc8867>.
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[RFC8868] Singh, V., Ott, J., and S. Holmer, "Evaluating Congestion
Control for Interactive Real-Time Media", RFC 8868,
DOI 10.17487/RFC8868, January 2021,
<https://www.rfc-editor.org/rfc/rfc8868>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC9049] Dawkins, S., Ed., "Path Aware Networking: Obstacles to
Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
DOI 10.17487/RFC9049, June 2021,
<https://www.rfc-editor.org/rfc/rfc9049>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/rfc/rfc9260>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/rfc/rfc9293>.
[RFC9332] De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
Queue Coupled Active Queue Management (AQM) for Low
Latency, Low Loss, and Scalable Throughput (L4S)",
RFC 9332, DOI 10.17487/RFC9332, January 2023,
<https://www.rfc-editor.org/rfc/rfc9332>.
[RFC9392] Perkins, C., "Sending RTP Control Protocol (RTCP) Feedback
for Congestion Control in Interactive Multimedia
Conferences", RFC 9392, DOI 10.17487/RFC9392, April 2023,
<https://www.rfc-editor.org/rfc/rfc9392>.
[Tools] Floyd, S. and E. Kohler, "Tools for the Evaluation of
Simulation and Testbed Scenarios", Work in Progress , July
2007,
<https://datatracker.ietf.org/doc/draft-irtf-tmrg-tools>.
Acknowledgments
Sally Floyd and Mark Allman were the authors of this document's
predecessor, [RFC5033], which served the community well for over a
decade.
Thanks to Richard Scheffenegger for helping to get this revision
process started.
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The editors would like to thank Mohamed Boucadair, Neal Cardwell,
Reese Enghardt, Jonathan Lennox, Matt Mathis, Dave Taht, Michael
Welzl, Magnus Westerlund, and Greg White for suggesting improvements
to this document.
Discussions with Lars Eggert and Aaron Falk seeded the original
RFC5033. Bob Briscoe, Gorry Fairhurst, Doug Leith, Jitendra Padhye,
Colin Perkins, Pekka Savola, members of TSVWG, and participants at
the TCP Workshop at Microsoft Research all provided feedback and
contributions to that document. It also drew from [RFC5166].
Evolution of RFC5033bis
Since draft-ietf-ccwg-rfc5033bis-03
* Harmonised the "proposed congestion control algorithm"
* addressed issues.
Since draft-ietf-ccwg-rfc5033bis-02
* Added discussion of real-time protocols
* Added discussion of short flows
* Listed properties of wired networks
* Added IoT section
* Added discussion of AQM response
* Rewrote the "Document Status" section
* Adding improved first sentence of abstract and intro.
* Added section on Multicast, noting this is out of scope
* Editorial changes
Since draft-ietf-ccwg-rfc5033bis-01
* Added discussion of multipath transports
* Totally reorganized central sections of the draft
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Since draft-ietf-ccwg-rfc5033bis-00
* Added QUIC, other congestion control standards
* Added wireless environments
* Aligned motivation for this work with the CCWG charter
* Refined discussion of QuickStart
Since draft-scheffenegger-congress-rfc5033bis-00
* Renamed file to reflect WG adpotion
* Updated authorship and acknowledgements.
* Include updated text suggested by Dave Taht
* Added criterion for bufferbloat
* Mentioned Cubic and BBR as motivation
* Include section to track updates between revisions
* Update references
Since RFC5033
* converted to Markdown and xml2rfc v3
* various formatting changes
Contributors
Christian Huitema
Private Octopus, Inc.
Email: huitema@huitema.net
Authors' Addresses
Martin Duke (editor)
Google LLC
Email: martin.h.duke@gmail.com
Godred Fairhurst (editor)
University of Aberdeen
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Email: gorry@erg.abdn.ac.uk
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