wdiff rfc9743v1.txt rfc9743.txt

Internet Engineering Task Force (IETF) M. Duke, Ed.
Request for Comments: 9743 Google LLC
BCP: 133 G. Fairhurst, Ed.
Obsoletes: 5033 University of Aberdeen
Category: Best Current Practice March 2025
ISSN: 2070-1721

Specifying New Congestion Control Algorithms

Abstract

This document replaces

RFC 5033, which 5033 discusses the principles and guidelines for standardizing
new congestion control algorithms. It This document obsoletes RFC 5033
to reflect changes in the congestion control landscape by providing a
framework for the development and assessment of congestion control
mechanisms, promoting stability across diverse network paths. This
document seeks to ensure that proposed congestion control algorithms
operate efficiently and without harm and efficiently alongside other algorithms when used in the global
Internet. It emphasizes the need for comprehensive testing and
validation to prevent adverse interactions with existing flows. This
document provides a framework for the development and assessment of
congestion control mechanisms, promoting stability across diverse
network environments. It obsoletes RFC 5033 to reflect changes in
the congestion control landscape.

Status of This Memo

This memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9743.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Specification of Requirements
3. Guidelines for Authors
3.1. Evaluation Guidelines
3.2. Document-Status Guidelines
4. Specifying Algorithms for Use in Controlled Environments
5. Evaluation Criteria
5.1. Single Algorithm Behavior
5.1.1. Protection Against Congestion Collapse
5.1.2. Protection Against Bufferbloat
5.1.3. Protection Against High Packet Loss
5.1.4. Fairness Within the Proposed Congestion Control
Algorithm
5.1.5. Short Flows
5.2. Mixed Algorithm Behavior
5.2.1. Existing General-Purpose Congestion Control
5.2.2. Real-Time Congestion Control
5.2.3. Short and Long Flows
5.3. Other Criteria
5.3.1. Differences with Congestion Control Principles
5.3.2. Incremental Deployment
6. General Use
6.1. Paths with Tail-Drop Queues
6.2. Tunnel Behavior
6.3. Wired Paths
6.4. Wireless Paths
7. Special Cases
7.1. Active Queue Management (AQM)
7.2. Operation with the Envelope Set by Network Circuit Breakers
7.3. Paths with Varying Delay
7.4. Internet of Things and Constrained Nodes
7.5. Paths with High Delay
7.6. Misbehaving Nodes
7.7. Extreme Packet Reordering
7.8. Transient Events
7.9. Sudden Changes in the Path
7.10. Multipath Transport
7.11. Data Centers
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Changes Since RFC 5033
Acknowledgments
Contributors
Authors' Addresses

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 for evaluating proposed congestion control
algorithms for publication in Experimental or Proposed Standard RFCs.

The IETF specifies standard Internet congestion control algorithms in
the RFC Series. These congestion control algorithms can suffer
performance challenges when used in differing environments (e.g.,
high-speed networks, cellular and Wi-Fi wireless technologies, and
long-distance satellite links), and also when flows carry specific
workloads (e.g., Voice over IP (VoIP), gaming, and
videoconferencing).

When [RFC5033] was published, TCP [RFC9293] was the primary focus of
IETF congestion control efforts, with proposals typically discussed
within the Internet Congestion Control Research Group (ICCRG).
Concurrently, the Datagram Congestion Control Protocol (DCCP)
[RFC4340] was developed to define new congestion control algorithms
for datagram traffic, while the Stream Control Transmission Protocol
(SCTP) [RFC9260] reused TCP congestion control algorithms.

Since then, several changes have occurred. The range of protocols
utilizing congestion control algorithms has expanded to include QUIC
[RFC9000] and RTP Media Congestion Avoidance Techniques (RMCAT)
(e.g., [RFC8836]). Additionally, some alternative congestion control
algorithms have been tested and deployed at scale without full IETF
review. There is increased interest in specialized use cases, such
as data centers (e.g., [RFC8257]), and in supporting a variety of
upper-layer protocols and applications, such as real-time protocols.
Moreover, the community has gained significant experience with
congestion indications beyond packet loss.

Multicast congestion control is a considerably less mature field of
study and is not in the scope of 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. These
include CUBIC [HRX08] and Bottleneck Bandwidth and Round-trip
propagation time (BBR) [BBR].

CUBIC was documented in a research publication in 2007 2008 [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, and published as an Informational RFC
in 2017 as [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 BBR was described in an Internet-Draft
in 2018, which 2018 and was first presented in the IRTF Internet Congestion
Control Research Group. It has since been regularly updated to
document the evolving versions of the algorithm [BBR]. BBR is
currently widely used for Google services using either TCP or QUIC
and is also widely deployed outside of Google.

We cannot say whether the original authors of [RFC5033] expected that
developers would be waiting for IETF review before widely deploying a
new congestion control algorithm over the Internet, but the examples
of CUBIC and BBR illustrate that deployment of new algorithms is not,
in fact, gated by the publication of the algorithm as an RFC.

Nevertheless, a specification for a congestion control algorithm
provides a number of advantages:

* It 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.

* It 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 It can help (by being accessible to anyone can anyone) to 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]
related to 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 occur when
the proposal is considered for publication by the IETF or before it
is deployed at a large scale.

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. Specification of Requirements

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. Guidelines for Authors

3.1. Evaluation Guidelines

This document does not provide specific evaluation methods, short of
Internet-scale deployment and measurement, to test the criteria
described below. There are multiple possible approaches to
evaluation. Each has a role, and the most appropriate approach
depends on the criteria being evaluated and the maturity of the
specification.

For many algorithms, an initial evaluation will consider individual
protocol mechanisms in a simulator to analyze their stability and
safety across a wide range of conditions, including overload. For
example, [RFC8869] describes evaluation test cases for interactive
real-time media over wireless networks. Such results could also be
published or discussed in IRTF research groups, such as ICCRG and
MAPRG.

Before a proposed congestion control algorithm is published as an
Experimental or Standards Track RFC, the community SHOULD gain
practical experience with implementations and experience using the
algorithm. Implementations by independent teams can help provide
assurance that a specification has avoided assumptions or ambiguity.
An independent evaluation by multiple teams helps provide assurance
that the design meets the evaluation criteria and can assess typical
interactions with other traffic. This evaluation could use an
emulated laboratory environment or a controlled experiment (within a
limited domain or at the Internet scale). Evidence of results is
normally considered by the When a working group in deciding is trying
to decide if a proposed specification is ready for publication and publication, it
will normally consider evidence of results. This ought to be
documented in any request for from the working group to publish the
specification.

Publication might occur

A congestion control algorithm without multiple implementations might
still be published as an RFC if a single implementation is widely
used, open source, and shown to have a positive impact on the
Internet, particularly if the target status is Experimental.

3.2. Document-Status Guidelines

The guidelines in this document apply to specifications of congestion
control algorithms that seek publication as an RFC via the IETF
Stream with an Experimental or Standards Track status. The
evaluation of either status involves the same questions, but with
different expectations for both the answers and the degree of
certainty of those answers.

Specifications on of congestion control algorithms without empirical
evidence of Internet-scale deployment MUST seek Experimental status,
unless they are not targeted for general use. Algorithms not
targeted at general use do not require Internet-scale data.

Specifications that seek to be published as Experimental IETF Stream
RFCs ought to explain the reason for the status and what further
information would be required to progress to a Standards Track RFC.
For example, Section 12 of [RFC6928] provides "Usage and Deployment
Recommendations" that describe the experiments expected by the TCPM
Working Group. Section 4 of [RFC4614] [RFC7414] provides other examples of
extensions that were considered experimental when the specification
was published (note that [RFC4614] has since been obsoleted by
[RFC7414]). published.

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.

Specifications on of congestion control algorithms with a record of
measured Internet-scale deployment MAY directly seek Standards Track
status if there is solid data that reflects that the algorithm is
safe and the design is stable, guided by the considerations in
Section 6. However, the existence of this data does not waive the
other considerations in this document.

Each specification submitted for publication as an RFC 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 such specification 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 not recommended for use because
it does not perform well for the user.

Examples of such statements exist in [RFC3649], which 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] 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].

Strictly speaking, documents for publication as Informational RFCs
from the IETF Stream need not meet all of the criteria in this
document, as they do not carry a formal recommendation from the IETF
community. Instead, the community judges the publication of these
Informational RFCs based on the value of their addition to the
information captured by the RFC Series.

Although it is out of scope for this document, proponents of a new
algorithm could alternatively seek publication of their specification
as an Informational or Experimental RFC via the Internet Research
Task Force (IRTF) Stream. In general, these algorithms are expected
to be less mature than ones that follow the procedures in this
document for publication via the IETF Stream. Authors documenting
deployed congestion control algorithms that cannot be changed by IETF
or IRTF review are invited to seek publication of their specification
as an Informational RFC via the Independent Submission Stream.

4. Specifying Algorithms for Use in Controlled Environments

Algorithms can be designed for general Internet deployment or for use
in controlled environments [RFC8799]. Within a controlled
environment, an operator can ensure that flows are isolated from
other Internet flows or they might allow these flows to share
resources with other Internet flows. A data center is an example of
a controlled environment that often deploys fabrics with rich
signaling from switches to endpoints.

Algorithms that rely on specific functions or configurations in the
network need to provide a reference or specification for these
functions (such as an RFC or another stable specification). For
publication of a specification of one of these algorithms to proceed,
the IETF will need to consider whether a working group exists that
can properly assess the network-layer aspects and their interaction
with the congestion control.

In evaluating a new proposal for use in a controlled environment, the
IETF community needs to understand the usage, e.g., usage (e.g., how the usage is
scoped to the controlled environment, 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 real-
world data for those environments. In such cases, the evaluation
criteria in the remainder of this document might not apply.

5. Evaluation Criteria

As previously noted, authors of a specification on a congestion
control algorithm are expected to conduct a comprehensive evaluation
of the advantages and disadvantages of any congestion control
algorithms presented to the IETF community. The following guidelines
are intended to assist authors and the community in this endeavor.
While these guidelines provide a helpful framework, they should not
be regarded as an exhaustive checklist as concerns beyond the scope
of these guidelines may also arise.

When considering a proposed congestion control algorithm, the
community MUST consider the criteria in the following sections.
These criteria will be evaluated in various domains (see Sections 6
and 7).

Some of the sections below will list criteria that SHOULD be met. It
could happen that these criteria are not, in fact, met by the
proposal. In such cases, the community MUST document whether not
meeting the criteria is acceptable, for example, if there are
practical limitations on carrying out an evaluation of the criteria.

The requirement that the community consider a criterion does not
imply that the result needs to be described in an RFC: there is no
formal requirement to document the results, although normal IETF
policies for archiving proceedings will provide a record.

This document, except where otherwise noted, does not provide
normative guidance on the acceptable thresholds for any of these
criteria. Instead, the community will use these evaluations as an
input when considering whether to progress the proposed algorithm. algorithm
specification in the publication process.

5.1. Single Algorithm Behavior

The criteria in the following subsections 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).

5.1.1. Protection Against Congestion Collapse

A congestion control algorithm should either stop sending when the
packet drop rate exceeds some threshold [RFC3714] or 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; then, it would exponentially back off 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 (and persistent) congestion.

If full backoff is used, this test does not require that the
mechanism be identical to that of TCP (see [RFC6298] and [RFC8961]).
For example, this does not preclude full backoff mechanisms that
would give flows with different round-trip times comparable capacity
during backoff.

5.1.2. Protection Against Bufferbloat

A congestion control algorithm should ought to try to avoid maintaining
excessive queues in the network. Exactly how the algorithm achieves
this is algorithm specific; see [RFC8961] and [RFC8085] for
requirements.

"Bufferbloat" refers to the building of excessive queues in the
network [BUFFERBLOAT]. Many network routers are configured with very
large buffers. The Standards Track RFCs [RFC5681] and [RFC9438]
describing
describe the Reno and CUBIC congestion control algorithms
(respectively)
(respectively), which send at progressively higher rates until a
First In, First Out (FIFO) buffer completely fills; then packet
losses 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 this problem, but a new congestion control
algorithm has the opportunity to improve the state of the art.

5.1.3. Protection Against High Packet Loss

A congestion control algorithm should ought to 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] 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 times the Bandwidth Delay Product (BDP). This was
unsatisfactory, and, indeed, further versions provided a fix for this
aspect of BBR [BBR].

This requirement does not imply that the algorithm should react to
packet losses in exactly the same way as congestion control
algorithms described in current Standards Track RFCs (e.g.,
[RFC5681]).

5.1.4. Fairness Within the Proposed Congestion Control Algorithm

When multiple competing flows all use the same proposed congestion
control algorithm, the specification evaluation 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.

5.1.5. 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. Many short-lived flows today remain in the "slow start"
mode of operation [RFC5681] that commonly features exponential
congestion window growth because the flow never experiences
congestion (e.g., packet loss).

A proposed proposal for a congestion control algorithm MUST consider how new
and short-lived flows affect long-lived flows, and vice versa.

5.2. Mixed Algorithm Behavior

The mixed algorithm behavior criteria evaluate the interaction of the
proposed congestion control algorithms being specified 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 algorithms published in
previous Standards Track RFCs (e.g., [RFC5681], [RFC9002], and
[RFC9438]) to flows sharing a common bottleneck. The measure of harm
is not restricted to unequal capacity, but also ought 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.

5.2.1. Existing General-Purpose Congestion Control

A proposed congestion control algorithm MUST be evaluated when
competing against standard IETF congestion controls (e.g., [RFC5681],
[RFC9002], and [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.

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 that is specified in an Experimental RFC and 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 flows that are
not best effort.

As an example from an Experimental RFC, fairness with standard TCP is
discussed in Sections 4 and 6 of [RFC3649], and using spare capacity
is discussed in Sections 6, 11.1, and 12 of [RFC3649].

5.2.2. Real-Time Congestion Control

General-purpose algorithms need to coexist in the Internet with real-
time congestion control algorithms, which in general have finite
throughput requirements (i.e., they do not seek to utilize all
available capacity) and more strict latency bounds. See [RFC8836]
for a description of the characteristics of this use case and the
resulting requirements.

[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,
real-time flows can use less frequent feedback (acknowledgment) than
that provided by reliable transports. This document does not change
the Informational status of those RFCs.

A proposed proposal for a 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 a proposal's interaction with those algorithms.
To the extent that they are not, experiments can be conducted where
possible.

Real-time flows can be directed into distinct queues via
Differentiated Services Code Points (DSCPs) or other mechanisms,
which can substantially reduce the interplay with other traffic.
However, a proposal targeting general Internet use cannot assume this
is always the case.

Section 7.2 describes the impact of network transport circuit breaker
algorithms. [RFC8083] also defines a minimal set of RTP circuit
breakers that operate end-to-end 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.

5.2.3. 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 5.1.5.

5.3. Other Criteria

5.3.1. Differences with Congestion Control Principles

A proposed proposal for a congestion control algorithm MUST clearly explain
any deviations from [RFC2914] and [RFC7141].

5.3.2. Incremental Deployment

A congestion control algorithm proposal MUST discuss whether it
allows for incremental deployment in the targeted environment. For a
mechanism targeted for deployment in the current Internet, the
proposal SHOULD 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 realized. The
IETF 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 of Quick-
Start are discussed in Sections 10.3 and 10.4 of [RFC4782].

6. General Use

The criteria in Section 5 will be evaluated in the scenarios
described in the following subsections. Unless a proposed congestion
control algorithm specification of the IETF Stream explicitly forbids
use on the public Internet, there MUST be IETF consensus that it
meets the criteria in these scenarios for the proposed congestion
control algorithm to progress.

The evaluation of 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.

6.1. 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 7.1
for evaluation of other queue disciplines.

6.2. 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.

Designers of tunnels and similar encapsulations might need to
consider nested congestion control interactions, for example, when
the Explicit Congestion Notification (ECN) is used by both an IP and
lower-layer technology [ECN-ENCAPS].

6.3. 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 capacity, 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.

6.4. Wireless Paths

While the early Internet was dominated by wired links, the properties
of wireless links have become 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.

7. Special Cases

The criteria in Section 5 will be evaluated in the scenarios
described in the following subsections, unless the proposed
congestion control algorithm specifically excludes its use in a
scenario. For these specific use cases, the IETF 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.

7.1. 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.

Some of the AQM techniques that might have an impact on a proposed
congestion control algorithm include:

* Flow Queue CoDel (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 ECN-
Capable Transport (ECT) codepoints in the IP header can gain the
benefits of receiving Explicit Congestion Notification -
Congestion Notification-Congestion
Experienced (ECN-CE) signals from an on-path AQM [RFC8087]. Use of
ECN (see [RFC3168] and [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.

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.

7.2. 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; 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; therefore, it will operate within the
envelope set by network transport circuit breaker algorithms.

7.3. 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 minimum delay that is not
stationary.

Varying delay occurs when a subnet changes the forwarding path to
optimize 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 when the physical layer changes its mode of
operation). Variation also arises when traffic with a higher
priority DSCP preempts 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.

7.4. Internet of Things and Constrained Nodes

The "Internet of Things" (IoT) is a broad concept, but when
evaluating a proposed congestion control algorithm, it is often
associated with unique characteristics. For example, 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.

Extremely low-power links can lead to very low throughput and a low
bandwidth-delay product, which is well below the standard operating
range of most Internet flows.

Furthermore, many IoT applications do not a have a human in the loop;
therefore, they might have weaker latency constraints because they do
not relate to a user experience. Congestion control algorithms still
may
might need to share the path with other flows with different
constraints.

7.5. Paths with High Delay

Authors of a proposed congestion control algorithm ought not to
presume that all general Internet paths have a low delay. Some paths
include links that contribute much more delay than for a typical
Internet path. Satellite links often have delays longer than is
typical for wired paths [RFC2488] and high-delay-bandwidth products
[RFC3649].

Paths can also present a variable delay as described in Section 7.3.

7.6. Misbehaving Nodes

A proposed proposal for a 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.

7.7. Extreme Packet Reordering

Authors of 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.

7.8. Transient Events

A proposed proposal for a congestion control algorithm SHOULD consider how it
would perform in the presence of transient events such as a 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, a response to transient
events is discussed in Section 9.2 of [RFC4782].

7.9. 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 proposal for a 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 rerouting
intervals.

7.10. 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 trade-off in timeliness and
reliability. There are various ways that multipath techniques can be
used.

One example use is to provide failover 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 failover MUST evaluate the
harm to performance resulting from a change in the path and show that
this does not result in flow starvation. Synchronization of failover
(e.g., where multiple flows change their path on similar time frames)
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. The Internet provides no guarantee that
different paths (e.g., using different endpoint addresses) are
disjoint. This introduces 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. A proposal SHOULD consider the potential for
harm to other flows. Synchronization of congestion control
mechanisms (e.g., where multiple flows change their behavior on
similar time frames) 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 Multipath TCP (MPTCP) [RFC8684].

7.11. 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 signaling 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 4). A
proposed congestion control algorithm 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.

8. Security Considerations

This document does not represent a change to any aspect of the TCP/IP
protocol suite; therefore, it 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]).

Proposed

A proposal for a congestion control algorithms algorithm MUST examine any
potential security or privacy issues that may arise from their
design.

9. IANA Considerations

This document has no IANA actions.

10. References

10.1. Normative References

[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>.

[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/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/info/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/info/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/info/rfc8083>.

[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/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/info/rfc8085>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8961] Allman, M., "Requirements for Time-Based Loss Detection",
BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/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/info/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/info/rfc9438>.

10.2. Informative References

[BBR] 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] 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]
Hock, M., Bless, R., and M. Zitterbart, "Experimental
evaluation of BBR congestion control", 2017 IEEE 25th
International Conference on Network Protocols (ICNP),
DOI 10.1109/ICNP.2017.8117540, 2017,
<https://ieeexplore.ieee.org/document/8117540>.

[BUFFERBLOAT]
Nichols, K. and J. Gettys, "Bufferbloat: Dark Buffers in
the Internet", ACM Queue Volume 9, Issue 11,
DOI 10.1145/2063166.2071893, November 2011, <https://queue.acm.org/detail.cfm?id=2071893>.
<https://dl.acm.org/doi/10.1145/2063166.2071893>.

[ECN-ENCAPS]
Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
Congestion Notification to Protocols that Encapsulate IP",
BCP 89, RFC 9599, DOI 10.17487/RFC9599, August 2024,
<https://www.rfc-editor.org/info/rfc9599>.

[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,
DOI 10.1145/1400097.1400105, 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/info/rfc2488>.

[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/info/rfc3168>.

[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<https://www.rfc-editor.org/info/rfc3649>.

[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/info/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/info/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/info/rfc4340>.

[RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap
for Transmission Control Protocol (TCP) Specification
Documents", RFC 4614, DOI 10.17487/RFC4614, September
2006, <https://www.rfc-editor.org/info/rfc4614>.

[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/info/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/info/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/info/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/info/rfc5166>.

[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/info/rfc6298>.

[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/info/rfc6356>.

[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.

[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.

[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/info/rfc7567>.

[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/info/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/info/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/info/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/info/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/info/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/info/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/info/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/info/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/info/rfc8867>.

[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/info/rfc8868>.

[RFC8869] Sarker, Z., Zhu, X., and J. Fu, "Evaluation Test Cases for
Interactive Real-Time Media over Wireless Networks",
RFC 8869, DOI 10.17487/RFC8869, January 2021,
<https://www.rfc-editor.org/info/rfc8869>.

[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/info/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/info/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/info/rfc9260>.

[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/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/info/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/info/rfc9392>.

[TOOLS] Floyd, S. and E. Kohler, "Tools for the Evaluation of
Simulation and Testbed Scenarios", Work in Progress,
Internet-Draft, draft-irtf-tmrg-tools-05, 23 February
2008, <https://datatracker.ietf.org/doc/html/draft-irtf-
tmrg-tools-05>.

Appendix A. Changes Since RFC 5033

* Examined BCP 14 keywords and consistency with other RFCs

* Rewrote the "Document Status" section

* Added QUIC and other more recent congestion control standards

* Aligned motivation for this work with the CCWG charter

* Refined discussion of Quick-Start

* Added criterion for bufferbloat

* Added text on constrained environments/limited domains and circuit
breakers and aligned with other RFCs

* Added discussion of real-time protocols, short flows, AQM
response, and multipath transports

* Listed properties of wired and wireless networks

* Added sections addressing IoT and Multicast (noting this is out of
scope)

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.

The editors would like to thank Mohamed Boucadair, Neal Cardwell,
Reese Enghardt, Jonathan Lennox, Matt Mathis, Zahed Sarker, Juergen
Schoenwaelder, Dave Taht, Sean Turner, 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].

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
Email: gorry@erg.abdn.ac.uk