Internet DRAFT - draft-fairhurst-ccwg-cc
draft-fairhurst-ccwg-cc
Network Working Group G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Best Current Practice M. Welzl
Expires: 25 April 2024 University of Oslo
23 October 2023
Guidelines for Internet Congestion Control at Endpoints
draft-fairhurst-ccwg-cc-01
Abstract
When published as an RFC, this document provides guidance on the
design of methods to avoid congestion collapse and how an endpoint
needs to react to congestion. Based on these, and Internet
engineering experience, the document provides best current practice
for the design of new congestion control methods in Internet
protocols.
When published, the document will update or replace the Best Current
Practice in BCP 41, which currently includes "Congestion Control
Principles" provided in RFC2914.
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/.
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 25 April 2024.
Copyright Notice
Copyright (c) 2023 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/
license-info) in effect on the date of publication of this document.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Incipient and Persistent Congestion . . . . . . . . . . . 4
1.2. The Need to Mitigate the Effects of Incipient
Congestion . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. The Need to Avoid the Effects of Persistent Congestion . 6
1.4. New Congestion Control Methods . . . . . . . . . . . . . 6
1.5. Current Challenges . . . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Requirements from the RFC Series . . . . . . . . . . . . . . 8
3.1. The Need to React to Congestion . . . . . . . . . . . . . 8
3.2. Tolerance to a Diversity of Path Characteristics . . . . 9
3.3. Protection of Protocol Mechanisms . . . . . . . . . . . . 10
4. Principles of Congestion Control . . . . . . . . . . . . . . 10
4.1. Initialisation and Using Capacity . . . . . . . . . . . . 10
4.1.1. Starting to use Path Capacity . . . . . . . . . . . . 10
4.1.2. Using More Capacity . . . . . . . . . . . . . . . . . 11
4.2. Robustness: Timers and Retransmission . . . . . . . . . . 12
4.3. Detecting and Reacting to Incipient Congestion . . . . . 13
4.3.1. Congestion Control Initialization . . . . . . . . . . 14
4.3.2. Loss-Based Congestion Detection and Retransmission . 14
4.3.3. Responding to Incipient Congestion . . . . . . . . . 15
4.3.4. Utilising Additional Path Information . . . . . . . . 16
4.4. Avoiding Persistent Congestion . . . . . . . . . . . . . 16
4.4.1. Avoiding Congestion Collapse and Flow Starvation . . 17
4.5. Additional Considerations . . . . . . . . . . . . . . . . 17
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Normative References . . . . . . . . . . . . . . . . . . . . 18
9. Informative References . . . . . . . . . . . . . . . . . . . 19
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document has two purposes. It first identifices changes in
practice and network design that have occurred since the publications
of IETF BCPs on the topic of congestion control and identifies
current issues in congestion ocontrol. Second, it updates the
guidance on the use of Congestion Control (CC) mechanisms. It also
provides background information for the design of new mechanisms. A
related document provides guidance on the evaluation of these new
methods.
The IETF has specified a set of Internet transports (e.g., TCP
[RFC9293], UDP [RFC0768], UDP-Lite [RFC3828], SCTP [RFC4960], and
DCCP [RFC4340]) as well as protocols layered on top of these
transports (e.g., RTP [RFC3550], QUIC [RFC9000] [RFC9002], SCTP/UDP
[RFC6951], DCCP/UDP [RFC6773]) and transports that work directly over
the IP network layer. These transports are implemented in endpoints
(either Internet hosts or routers acting as endpoints), and can be
designed to detect and react to network congestion. TCP was the
first transport to provide this, although the specifications found in
RFC 793 [RFC793] predate the inclusion of CC and did not contain any
discussion of using or managing a congestion window (cwnd). RFC 9293
[RFC9293] has addressed this.
Section 3 of [RFC2914] states "The equitable sharing of bandwidth
among flows depends on the fact that all flows are running compatible
congestion control algorithms". Internet transports therefore need
to react to avoid congestion that could impact other flows sharing a
path. The Requirements for Internet Hosts [RFC1122] formally
mandates that endpoints perform CC. "Because congestion control is
critical to the stable operation of the Internet, applications and
other protocols that choose to use UDP as an Internet transport must
employ mechanisms to prevent congestion collapse and to establish
some degree of fairness with concurrent flows [RFC8085].
The popularity of the Internet has led to the deployment of many
implementations: Some use standard CC mechanisms, some have chosen to
adopt approaches that differ from present standards. Guidance is
needed to ensure safe evolution of the CC methods used by transport
protocols.
There are several reasons to think that things have changed since the
original best current practice was published: At one time, it was
common that the serialisation delay of a packet at the bottleneck
formed a large proportion of the round trip time (RTT) of a path,
motivating a need for conservative loss recovery. This is not often
the case for today's higher capacity links. The increase in the link
speed often means that for many users, current traffic often does not
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normally experience persistent congestion, and under-load (inability
to achieve the bottleneck rate) is often as common as over-load
(exceeding the bottleneck rate) That is, a current challenge is that
conservative methods lead to under-utilisation of the path, and safe
scalable methods need to be found.
There also have been changes in the way that protocol mechanisms are
deployed in Internet endpoints:
On the one hand, techniques have evolved that allow incremental
deployment and testing of new methods which can enable the rapid
development of methods to detect and react to congestion. This
allows new mechanisms to be tested to ensure the majority users see
benefit in the networks they use. There has been considerable
progress in developing new loss recovery and congestion responses
that have been evaluated in this way.
On the other hand, the Internet continues to be heterogenous, some
endpoints experience very different network path characteristics and
some endpoints generate very different patterns of traffic. There is
still a need to avoid harm to other flows (stravation of capacity,
unecessary increase of latency, congestion collapse).
This document has a focus on unicast point-to-point transports, this
includes migration from using one path to another path. Some
recommendations [RFC5783] and requirements will apply to point-to-
multipoint transports (e.g., multicast), however this is beyond the
current document's scope. [RFC2914] provides additional guidance on
the use of multicast.
Finally, experience has shown that successful protocols developed in
a specific context, or for a particular application tend to also
become used in a wider range of contexts. Therefore, IETF
specifications ought to target deployment on the general Internet, or
be specified for use only within a controlled environment.
1.1. Incipient and Persistent Congestion
Internet paths experience congestion (loss or delay) when there is
excess load at a bottleneck that they traverse. This document
differentiates two levels of congestion:
* Incipient congestion: This is a consequential side effect of the
statistical multiplexing of packet flows. There will be times
when packets need to be buffered or dropped at the bottleneck(s)
on a path, irrespective of the long-term average load.
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* Persistent congestion: This occurs when the pattern of arriving
traffic results in over-consumption of a path's resources.
Typically this results in packet loss. The effects of persistent
congestion might impact the flow that induces the congestion, but
could adversely impact other flows, e.g., starving them of
resources or reducing the efficiency of the path (e.g., congestion
collapse).
Flows need to react when they encounter either form congestion to
reduce their contribution to the load. For persistent congestion,
the reaction needs to be sufficient to avoid excessive harm to other
flows.
1.2. The Need to Mitigate the Effects of Incipient Congestion
Incipient congestion results during normal operation of the Internet.
Buffering (which causes an increase in latency) or congestion loss
(discard of a packet) arises when the traffic arriving at a
bottleneck exceeds the resources available. A network device uses
will drop excess packets when its queue(s) becomes full. This can be
managed using Active Queue Management (AQM) [RFC7567], which can be
combined with Explicit Congestion Notification (ECN) signalling
[RFC3168] to mitigate incipient congestion [RFC8087].
Buffers can be divided into pools and traffic can be associated with
a specific pool (e.g., using local configuration, or coordinated
using the Differentiated Services [RFC2475] architecture). A
schedular can [RFC7806] isolate the queuing of packets for different
flows, or aggregates of flows, and reduce the impact of flow
multiplexing on other flows (e.g., flow scheduling [RFC7567]). This
could equally distribute resources between sharing flows, but this
equality is explicitly not a requirement [Flow-Rate-Fairness].
Even when a path is expected to support such methods, an endpoint
MUST NOT rely on the presence and correct configuration of these
methods, and therefore needs to employ CC methods that work end-to-
end, or employ in-network control, such as a circuit-breaker.
In some controlled environments, Internet transports can use
mechanisms to reserve capacity. Most Internet paths do not support
this. In the absence of such a reservation, endpoints are unable to
determine a safe rate at which to start a new transmission. The use
of an Internet path therefore requires end-to-end CC mechanisms to
detect and respond to congestion.
Section 3.3 of [RFC2914] notes that a flow can use CC to "optimize
its own performance regarding throughput, delay, and loss. In some
circumstances, for example in environments with high statistical
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multiplexing, the delay and loss rate experienced by a flow are
largely independent of its own sending rate." and continues: "in
environments with lower levels of statistical multiplexing or with
per-flow scheduling, the delay and loss rate experienced by a flow is
in part a function of the flow's own sending rate. Thus, a flow can
use end-to-end congestion control to limit the delay or loss
experienced by its own packets."
1.3. The Need to Avoid the Effects of Persistent Congestion
Early RFCs recognised that a poorly designed transport can lead to
significant congestion, which could result in severe service
degradation or "Internet meltdown". One effect is called "Congestion
Collapse", where an increase in the network load results in a
decrease in the useful work done by the network. [RFC0896]
[RFC0970]. [RFC2914]. This was first observed in the mid 1980s At
that time, this was aggrevated by connections thjat did not use CC
and which unnecessarily retransmitted packets that were either in
transit or had already been received, resulting in a stable
persistent congestion [RFC0896].
[RFC2914] also notes that it is even more destructive when
applications increase their sending rate in response to an increase
in the packet loss rate (e.g., automatically using an increased level
of FEC (Forward Error Correction)).
The problems of congestion collapse have generally been corrected by
improvements to the loss recovery and congestion control mechanisms
in transport protocols [Jac88], designed to avoid starving other
flows of capacity (e.g., [RFC7567]). Section 3.1 describes
preventing congestion collapse. [RFC2309] adds that "all UDP-based
streaming applications should incorporate effective congestion
avoidance mechanisms." [RFC7567] and [RFC8085] both reaffirm the
continued need to provide methods to prevent starvation.
1.4. New Congestion Control Methods
CC is an evolving subject, responding to changes in protocol design,
operation of applications using the network and understanding of the
network operation under load. The IETF has provided guidance
[RFC5033] for considering and evaluating alternate CC algorithms.
The IRTF has described a set of metrics and related trade-off between
metrics to compare, contrast, and evaluate CC algorithms [RFC5166].
[RFC5783] provided a snapshot of CC research in 2008. [RFC6077]
discussed open issues in CC research in 2011.
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In contrast to considering the fairness in distributing capacity
between flows, a different approach is to analyse persistent
congestion effects to understand the harm to other flows (collateral
impact of loss, starvation, collapse, etc). Such an analysis of the
suitability of a new mechanism can evaluate how changes impact other
flows sharing a bottleneck, and consider the impact on the flows that
have outliers in performance (e.g., the last 5%, 1%) For example, the
performance often does not provide an indication that a new method
could starve other applications that share the bottleneck, or when
patterns of packets (e.g., bursts) are sent that disrupt the packet
timing needed by another application flow.
1.5. Current Challenges
Recommendations and requirements on CC control are distributed across
many documents in the RFC series. This section gathers and
consolidates these recommendations. These, and Internet engineering
experience are used to derive the best current practice in the design
of Internet CC methods.
Standardization of new CC algorithms can avoid an "arms race" among
competing protocols [RFC2914]. That is, avoid competition for
Internet resource in a way that significantly reduces the ability of
other flows to use the Internet.
The general recommendation in the UDP Guidelines [RFC8085] is that
applications SHOULD leverage existing CC techniques, such as those
defined for TCP [RFC9293], TCP-Friendly Rate Control (TFRC)
[RFC5348], SCTP [RFC4960], and other IETF-defined transports. This
is because there are many trade offs and details that can have a
serious impact on the performance of a CC mechanism and upon other
traffic that seeks to share a bottlneck.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The path between endpoints (sometimes called "Internet Hosts" for
IPv4 and called "source nodes" and "destination nodes" in IPv6)
consists of the endpoint protocol stack at the sender and the
receiver (which together implement the transport service), and a
succession of links and network devices (routers or middleboxes)
forming the network path. The set of network devices forming the
path is not usually fixed, and it should generally be assumed that
this set can change over arbitrary lengths of time.
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[RFC5783] defines CC as "the feedback-based adjustment of the rate at
which data is sent into the network. Congestion control is an
indispensable set of principles and mechanisms for maintaining the
stability of the Internet."
The document draws on language used in the specifications of TCP and
other IETF transports. For example, a protocol timer is generally
needed to detect persistent congestion, and this document uses the
term Retransmission Timeout (RTO) to refer to the operation of this
timer. Similarly, it refers to a congestion window (cwnd) as a
variable that controls the rate of transmission by the CC. Each new
transport needs to make its own design decisions about how to meet
the recommendations and requirements for CC. The use of these terms
does not imply that endpoints need to implement functions in the
current way used by TCP.
Other terminology is directly copied from the cited RFCs.
3. Requirements from the RFC Series
3.1. The Need to React to Congestion
This includes:
* Endpoints MUST perform congestion control [RFC1122] and SHOULD
leverage existing techniques [RFC8085].
* If an application or protocol chooses not to use a CC, it SHOULD
control the rate at which it sends datagrams to a destination
host, to fulfil the requirements of [RFC2914], as stated in
[RFC8085].
* Endpoints SHOULD control the aggregate traffic that is sent
[RFC8085]. An endpoint can become aware of congestion by various
means (including, delay variation, timeout, ECN, packet loss). A
signal that indicates congestion SHOULD result in a reaction to
reduce the sendding rate [RFC8087]).
* Although network devices can be configured to reduce the impact of
multiplexing on other flows, endpoints MUST NOT rely solely on the
presence and correct configuration, except in a controlled
environment.
* A transport that does not target Internet deployment needs to be
constrained to only operate in a controlled environment (e.g., see
Section 3.6 of [RFC8085]) and provide appropriate mechanisms to
prevent this traffic from accidentally leaving the controlled
environment [RFC8084].
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3.2. Tolerance to a Diversity of Path Characteristics
Internet transports need to use a CC method designed for Internet
paths.
* Path Capacity: The forward path can be congested in terms of the
number of packets it can support and/or the number of rate of
bytes it can transfer. The return path (towards the sender) can
also be congested. Methods need to operate over paths where
capacity in the forward and return directions are significantly
different.
* Path Loss: Paths can experience packet loss for various reasons
besides experiencing congestion (e.g., link corruption [RFC3819]),
but an endpoint cannot usually reliably disambiguate the cause of
loss. Whilst mechanisms below the transport layer can mitigate
this loss, the only way to surely confirm that a sending endpoint
has successfully communicated with a remote endpoint is to utilise
a timer (see Section 4.2) to detect a lack of response that could
result from a change in the path or the path characteristics. The
detection of congestion and the resulting reduction in rate MUST
NOT solely depend upon reception of a signal from the remote
endpoint, because congestion indications could themselves be lost
due to congestion.
* Path RTT: The RTT from an endpoint cannot be determined a-priori,
and must be measured dynamically (see Section 4.2).
* Path Change: An endpoint MUST assume that path characteristics can
change over time (i.e. path characteristics and sharing traffic
once discovered do not necessarily remain valid in the future).
* Network devices MAY provide mechanisms to mitigate the impact of
congestion by transport flows (e.g., priority forwarding of
control information, and starvation detection), and ought to
mitigate the impact of non-conformant and malicious flows
[RFC7567]). These mechanisms complement, but do not replace, the
endpoint congestion avoidance mechanisms.
* Security: Internet endpoints need to be protected from intentional
disruption of the service they provide, and from the exploitation
of methods to attack other endpoints or services (see
Section 3.3).
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3.3. Protection of Protocol Mechanisms
An endpoint needs to be protected from attacks on the traffic it
generates, or attacks that seek to increase the capacity that it
consumes (impacting other traffic that share a bottleneck).
The following guidance is provided on protection:
* Off-Path Attack: A design MUST protect from off-path attack to the
protocol [RFC8085] (i.e., where the attacker is unable to observe
packets). This can lead to a Denial of Service (DoS)
vulnerability for the flow being controlled and/or other flows
that share network resources along the path.
* On-Path Attack: A protocol can be designed to protect from on-path
attacks (i.e., where an attacker can observe the packets).
Protecting from on-path attacks can require more complexity and
typically utilises encryption and/or authentication mechanisms
(e.g., IPsec [RFC4301], QUIC [RFC9000]).
* Validation of Signals: To protect from malicious abuse, network
signals and control messages (e.g., ICMP [RFC0792]) MUST be
validated before they are used (see Section 3.3). Transports MUST
at least include protection from off-path attack using signals
[RFC8085] (e.g., validating the quoted information in an ICMP
message enables checksing that this corresponds to the flow,
before utilising the signalling it contains).
4. Principles of Congestion Control
This section summarises the principles for providing CC. It
describes principles associated with preventing persistent
congestion, reacting to incipient congestion and utilising additional
path information.
4.1. Initialisation and Using Capacity
4.1.1. Starting to use Path Capacity
A sender needs to regulate the maximum volume of data in flight over
the interval of the current RTT (the cwnd). It needs to react to
incipient congestion.
* Setting an initial cwnd: A TCP sender "SHOULD set the congestion
window to no more than the Restart Window (R)" before beginning
transmission, if the sender has not sent data in an interval that
exceeds the current retransmission timeout, i.e., when an
application becomes idle [RFC9293]. Congestion Window Validation
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(CWV) [RFC7661] describes how a TCP sender can tentatively
maintains a cwnd larger than the path has supported in the last
RTT when a flow is application-limited, provided that the endpoint
rapidly reduces the cwnd when congestion is detected.
* Using the cwnd: A sender that does not use capacity has no
understanding whether previously used capacity remains available,
or whether that capacity has disappeared (e.g., a change in the
path that causes a flow to experience a smaller bottleneck, or
when more traffic emerges that consumes previously available
capacity resulting in a new bottleneck). For this reason, a
transport that is limited by the volume of data available to send,
MUST NOT continue to grow its cwnd when the current cwnd is more
than twice the volume of data acknowledged in the last RTT. The
reduction needs to be commensurate with the increase that preceded
it. This factor of 2 decrease corresponds to an increase factor
of 2 in slow start.
* Collateral Damage: Even in the absence of congestion, statistical
multiplexing can result in transient effects for flows sharing
common resources. A sender SHOULD avoid persistently inducing
excessive congestion to other flows (collateral damage that could
result in flow starvation). For example, avoid a sudden surge in
sending rate that lasts for more than one RTT.
* Burst Mitigation: While an endpoint ought to limit its sending
rate at the granularity of the current RTT, this can be
insufficient to satisfy the need to mitigate collateral damage.
Endpoints SHOULD provide mechanisms to regulate the bursts of
transmission that the application/protocol sends (section 3.1.6 of
[RFC8085]). ACK-Clocking [RFC9293] can help mitigate bursts when
they receive continuous feedback of reception (such as TCP).
Sender pacing can also mitigate this [RFC8085], (described in
Section 4.6 of [RFC3449]), and has been recommended for TCP in
conditions where ACK-Clocking is not effective, (e.g., [RFC3742],
[RFC7661]). SCTP [RFC4960] defines a maximum burst length
(Max.Burst) with a recommended value of 4 segments to limit the
SCTP burst size. QUIC recommends that a sender paces all in-
flight packets based on input from the CC [RFC9002].
4.1.2. Using More Capacity
When the CC is increasing the cwnd, it transmits faster than the last
confirmed safe rate. Such an increase needs to be regarded as
tentative and a sender needs to reduce its rate below the last
confirmed safe rate when congestion is detected.
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* Increasing the cwnd: In the absence of congestion, an endpoint MAY
increase the sending rate or cwnd. This limit should only be
increased when there is additional data available to send (i.e.,
the sender will utilise the additional capacity in the next RTT).
* A sender MUST NOT increase the sending rate for a time longer than
one RTT period after congestion is first detected. This helps
manage incipient congestion.
* Avoiding Overshoot: Overshoot of the cwnd beyond the point of
congestion can significantly impact other flows sharing resources
along a path, and can impact the performance of the flow itself.
As endpoints experience more paths with a large Bandwidth Delay
Product (BDP) and a wider range of potential path RTTs,
variability or changes in the path can significantly impact the
appropriate dynamics for increasing the cwnd (see also burst
mitigation, Section 4.1.1). Methods such as HyStart are designed
to avoid overshoot [RFC9406].
* Response to Detected Congestion: The sending rate MUST be below
the previously confirmed safe rate for multiple RTT periods after
a congestion event. In TCP Reno [RFC9293], this is performed by
using a conservative (linear) increase from a slow start threshold
that is re-initialised each time congestion is experienced.
4.2. Robustness: Timers and Retransmission
An endpoint can utilise timers to implement transport mechanisms,
e.g., to recover from loss, to trigger pre-emptive retransmission and
other protocol functions. An endpoint that does utilise timers needs
to follow the rules in section 3.3 of [RFC8085].
Principles include:
* Initial RTO Interval: When a flow sends the first packet(s), it
has no way to know the RTT of the path. An initial timer value is
needed to detect any lack of responsiveness from the remote
endpoint. In TCP, this is the starting value of the RTO. A safe
initial value is important for overall Internet stability
[RFC6298] [RFC8085]. In the absence of any knowledge about the
latency of a path (including the initial value), senders SHOULD
conservatively set the RTO to no less than 1 second. (Although
Linux TCP has deployed a smaller initial RTO value, the appendix
of [RFC6298] confirms that values shorter than 1 second can be
problematic.)
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* Initial RTO Expiry: If the RTO timer expires while awaiting
completion of a connection setup, or handshake (e.g., the ACK of a
SYN segment in the three-way handshake in TCP), and the
implementation is using an RTO of less than 3 seconds, the local
endpoint can resend the connection setup. The RTO MUST then be
re-initialized to increase it to 3 seconds once data transmission
begins (i.e., after the handshake completes) [RFC6298] [RFC8085].
This conservative increase is necessary to avoid congestion
collapse when many flows retransmit across a shared bottleneck
with restricted capacity.
* Initial Measured RTO: Once an RTT measurement is available (e.g.,
through reception of an acknowledgement), the timeout value must
be adjusted. This adjustment MUST take into account the RTT
variance. For the first sample, this variance cannot be
determined, and a local endpoint MUST therefore initialise the
variance to RTT/2 (see equation 2.2 of [RFC6298] and related text
for UDP in section 3.1.1 of [RFC8085]).
* Updating the Path RTT: Once an endpoint has started communicating
with its peer, the RTT MUST be adjusted by measuring the actual
path RTT. This adjustment MUST include adapting to the measured
RTT variance (see equation 2.3 of [RFC6298]). An RTO interval
SHOULD be set based on recent RTT observations (including the RTT
variance) (e.g., Section 3.1.1 of [RFC8085]).
* Persistent Lack of Feedback: Persistent lack of feedback (e.g.,
detected by an RTO expiry, or other means) MUST be treated as
persistent congestion. A failure to receive any specific response
could be a result of a RTT change, change of path, excessive loss,
or even congestion collapse. If there is no response within the
RTO interval, TCP collapses the cwnd to one segment [RFC9293].
Other transports MUST similarly respond when they fail to receive
confirmation of feedback. An endpoint MUST exponentially backoff
the RTO interval [RFC8085] each time persistent congestion is
detected [RFC1122], until the path characteristics can again be
confirmed [RFC6298] [RFC8085].
* Maximum RTO: A maximum value MAY be placed on the RTO interval.
This maximum RTO interval MUST NOT be less than 60 seconds
[RFC6298].
* [[Author Note: Re-check RTO-Consider. ]]
4.3. Detecting and Reacting to Incipient Congestion
This section describes the principles related to mitigation of
incipient congestion (see Section 1.2).
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4.3.1. Congestion Control Initialization
When a connection to a new destination is first established, the
endpoints have little information about the characteristics of the
network path they will use. The safety and responsiveness of new CC
proposals needs to be evaluated [RFC5166].
* Flow Start: A new flow between two endpoints needs to initialise a
CC for the path. The TCP slow-start algorithm is an accepted
standard for flow startup [RFC9293]. This uses the notion of an
Initial coingestion Window (IW) [RFC3390], updated by [RFC6928]).
The IW is not the smallest burst size, nor the smallest cwnd. It
t is a safe starting point for a path that is not suffering
persistent congestion, and is applicable until feedback about the
path is received.
* Utilised Capacity: A CC MAY assume that the recently used capacity
between a pair of endpoints is an indication of future capacity
that might be available in the near future between the same
endpoints (Section 4.3.4). The CC MUST reduce its rate if this is
not subsequently confirmed to be true. [[Author note: we likely
need to bound this reaction in time or size]].
4.3.2. Loss-Based Congestion Detection and Retransmission
This section describes mechanisms to detect loss and provide
retransmission, and to protect the network in the absence of timely
feedback.
* Congestion Detection: Loss is typically detected when a sender
cannot confirm delivery within an expected period (e.g., by
observing the time-ordering of the reception of ACKs, as in TCP
DupACK) or by utilising a timer to detect loss (e.g., a
transmission timer with a period less than the RTO, [RFC8085]
[RFC8985]) or a combination of the two. A transport is usually
unable to reliably detect whether a loss is a result of
congestion. For this reason, loss needs to be treated as
incipient congestion, at least until the cause of loss can be
reliably determined.
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* Retransmission: When loss is detected, the sender can choose to
retransmit the lost data, ignore the loss, or send other data
(e.g., [RFC8085] [RFC9002]), depending on the reliability provided
by the transport service. All transmissions consume network
capacity, therefore retransmissions MUST NOT increase the network
load in response to congestion loss (which worsens that
congestion) [RFC8085]. Any method that sends additional data
following loss is therefore responsible for CC of the
retransmissions (and any other packets sent, including FEC
information) as well as the original traffic.
4.3.3. Responding to Incipient Congestion
In determining an appropriate congestion response to incipient
congestion, designs could consider the size of the packets that
experience congestion [RFC4828].
* Congestion Response: An endpoint MUST promptly reduce the sending
rate when there is an indication of congestion (e.g., loss)
[RFC2914]. TCP Reno established a method that relies on
multiplicative-decrease to halve the sending rate while congestion
is detected. This response to congestion indications is
sufficient for safe Internet operation, but other decrease factors
have also been published in the RFC Series [RFC9438].
* ECN Detection: ECN can help determine an appropriate cwnd to
enable early indication of incipient congestion when it is
supported by routers on the path [RFC7567]. An early detection of
incipient congestion allows a different reaction to an explicit
congestion signal compared to the reaction to a detected packet
loss [RFC8311] [RFC8087]. Congestion control design should
provide the necessary mechanisms to support ECN [RFC3168]
[RFC6679], as described in section 3.1.7 of [RFC8085].
* Response to ECN Congestion Marking: Simple feedback of received
Congestion Experienced (CE) marks [RFC3168] relies only on an
indication that congestion has been experienced within the last
RTT. This response is appropriate when a flow uses ECT(0)
[RFC3168]. ABE modified this reaction to ECN [RFC8511]. Extended
RTP feedback and accurate TCP receiver feedback more detail about
the CE-marking [I-D.ietf-tcpm-accurate-ecn], supporting a finer
granularity of congestion response. The L4S architecture
[RFC9330] allows routers to use a different marking system that
can provide early reaction to incipient congestion [RFC9332] and
defines a reaction for this feedback when packets are marked with
ECT(1).
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* [RFC8085] provides guidelines for a sender that does not, or is
unable to, adapt the cwnd.
4.3.4. Utilising Additional Path Information
Path information can be cached. In TCP, this was previously called
TCP Control Block (TCB) sharing, and is now called TCP Control Block
Interdependence, [RFC9040]. A CC can also utilise signals from the
network to help determine how to regulate the traffic it sends.
* Utilising Cached Path Information: A transport connection between
a pair of endpoints can share CC parameters with other connections
that share the same path. A CC that recently used a specific path
could allow another flow to take-over the previously consumed
capacity. Information used to accelerate the growth of the cwnd
MUST be viewed as tentative until it is confirmed that the flow
was able to utilise the capacity (i.e., the new flow needs to
either "use or loose" the capacity). A sender MUST reduce its
rate if the capacity is not confirmed within the current RTO
interval.
* [RFC8085] adds "An application that forks multiple worker
processes or otherwise uses multiple sockets to generate UDP
datagrams SHOULD perform congestion control over the aggregate
traffic."
* Utilising Network Signals: A mechanism that utilises signals
originating in the network (e.g., RSVP, NSIS, Quick-Start, ECN),
MUST assume that the set of network devices on the path can
change. This motivates use of soft-state for protocols [RFC9049]
(e.g., ECN) and includes context-sensitive treatment of "soft"
signals provided to the endpoint [RFC5164]. Endpoints MUST assume
the set of routers and links forming the path can change and that
network devices can be reconfigured or reset. A changing set of
on-path devices can also affect which types of packets traverse a
path (e.g. whether IP options are supported, or a specific
treatment applies.)
4.4. Avoiding Persistent Congestion
All endpoints are required to implement mechanisms that avoid
persistent congestion and can demonstrate that they do not induce
starvation and congestion collapse (see Section 1.3).
Principles include:
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* Persistent congestion can result in congestion collapse, which
MUST be aggressively avoided [RFC2914]. Endpoints that experience
persistent congestion and have already reduced their cwnd to the
loss window (e.g., one packet) MUST further reduce the rate if the
RTO timer continues to expire. For example, TFRC [RFC5348]
continues to reduce its sending rate under persistent congestion
to one packet per RTT, and then exponentially backs-off the time
between single packet transmissions if a congestion event
continues to persist [RFC2914]. QUIC [RFC9002] does not directly
specify a period, but does specify a probe to detect tail loss.
The Tail Loss Probe (TLP) mechanism [RFC8985] determines that
persisent congestion is experienced after a loss for a duration of
2 TLP probes plus the RTO.
4.4.1. Avoiding Congestion Collapse and Flow Starvation
Principles include:
* Transports MUST avoid inducing flow starving flows that share
resources along the path.
* Endpoints MUST treat a loss of all feedback (e.g., RTO expiry) as
an indication of persistent congestion.
* When an endpoint detects persistent congestion, it MUST reduce the
maximum rate/cwnd.
4.5. Additional Considerations
Many designs place the responsibility of rate-adaption for CC at the
sender (source) endpoint, utilising feedback information provided by
the remote endpoint (receiver). CC can also be implemented by
determining an appropriate rate limit at a receiver and using this
limit to control the maximum transport rate (e.g., using methods such
as [RFC5348] and [RFC4828]).
Applications at an endpoint can send more than one flow. "The
specific issue of a browser opening multiple connections to the same
destination has been addressed by [RFC2616]. Section 8.1.4 states
that "Clients that use persistent connections SHOULD limit the number
of simultaneous connections that they maintain to a given server. A
single-user client SHOULD NOT maintain more than 2 connections with
any server or proxy." [RFC9040].
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5. Acknowledgements
This document owes much to the insight offered by Sally Floyd, both
at the time of writing of RFC2914 and her help and review in the many
years that followed this.
Nicholas Kuhn helped develop the first draft of these guidelines.
Tom Jones and Ana Custura reviewed the first version of this draft.
Many discussions with Michael Welzl and others have provided
immeasurable help to get this far. The University of Aberdeen
received funding to support this work from the European Space Agency.
6. IANA Considerations
This memo includes no request to IANA.
RFC Editor Note: If there are no requirements for IANA, the section
will be removed during conversion into an RFC by the RFC Editor.
7. Security Considerations
This document introduces no new security considerations. Each RFC
listed in this document discusses the security considerations of the
specification it contains. The security considerations for the use
of transports are provided in the references section of the cited
RFCs. Security guidance for applications using UDP is provided in
the UDP Usage Guidelines [RFC8085].
Section 3.3 describes general requirements relating to the design of
safe protocols and their protection from on and off path attack.
Section 4.3.4 follows current best practice to validate ICMP messages
prior to use.
8. Normative References
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[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>.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, DOI 10.17487/RFC3390, October
2002, <https://www.rfc-editor.org/info/rfc3390>.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
<https://www.rfc-editor.org/info/rfc5348>.
[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>.
[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>.
[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>.
9. Informative References
[Flow-Rate-Fairness]
Briscoe, Bob., "Flow Rate Fairness: Dismantling a
Religion, ACM Computer Communication Review 37(2):63-74",
April 2007.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
Accurate Explicit Congestion Notification (ECN) Feedback
in TCP", Work in Progress, Internet-Draft, draft-ietf-
tcpm-accurate-ecn-26, 24 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
accurate-ecn-26>.
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[Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329 , August
1988, <ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC0896] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC 896, DOI 10.17487/RFC0896, January 1984,
<https://www.rfc-editor.org/info/rfc896>.
[RFC0970] Nagle, J., "On Packet Switches With Infinite Storage",
RFC 970, DOI 10.17487/RFC0970, December 1985,
<https://www.rfc-editor.org/info/rfc970>.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
<https://www.rfc-editor.org/info/rfc2309>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>.
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[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
2004, <https://www.rfc-editor.org/info/rfc3742>.
[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>.
[RFC3828] Larzon, L., Degermark, M., Pink, S., Jonsson, L., Ed., and
G. Fairhurst, Ed., "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July 2004,
<https://www.rfc-editor.org/info/rfc3828>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[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>.
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
(TFRC): The Small-Packet (SP) Variant", RFC 4828,
DOI 10.17487/RFC4828, April 2007,
<https://www.rfc-editor.org/info/rfc4828>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[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>.
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[RFC5164] Melia, T., Ed., "Mobility Services Transport: Problem
Statement", RFC 5164, DOI 10.17487/RFC5164, March 2008,
<https://www.rfc-editor.org/info/rfc5164>.
[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>.
[RFC5783] Welzl, M. and W. Eddy, "Congestion Control in the RFC
Series", RFC 5783, DOI 10.17487/RFC5783, February 2010,
<https://www.rfc-editor.org/info/rfc5783>.
[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
Briscoe, "Open Research Issues in Internet Congestion
Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
<https://www.rfc-editor.org/info/rfc6077>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/info/rfc6363>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>.
[RFC6773] Phelan, T., Fairhurst, G., and C. Perkins, "DCCP-UDP: A
Datagram Congestion Control Protocol UDP Encapsulation for
NAT Traversal", RFC 6773, DOI 10.17487/RFC6773, November
2012, <https://www.rfc-editor.org/info/rfc6773>.
[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>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
TCP to Support Rate-Limited Traffic", RFC 7661,
DOI 10.17487/RFC7661, October 2015,
<https://www.rfc-editor.org/info/rfc7661>.
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[RFC7806] Baker, F. and R. Pan, "On Queuing, Marking, and Dropping",
RFC 7806, DOI 10.17487/RFC7806, April 2016,
<https://www.rfc-editor.org/info/rfc7806>.
[RFC793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.
[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>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
[RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC8511, December 2018,
<https://www.rfc-editor.org/info/rfc8511>.
[RFC8985] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
DOI 10.17487/RFC8985, February 2021,
<https://www.rfc-editor.org/info/rfc8985>.
[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>.
[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>.
[RFC9040] Touch, J., Welzl, M., and S. Islam, "TCP Control Block
Interdependence", RFC 9040, DOI 10.17487/RFC9040, July
2021, <https://www.rfc-editor.org/info/rfc9040>.
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[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>.
[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>.
[RFC9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC9330, January 2023,
<https://www.rfc-editor.org/info/rfc9330>.
[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>.
[RFC9406] Balasubramanian, P., Huang, Y., and M. Olson, "HyStart++:
Modified Slow Start for TCP", RFC 9406,
DOI 10.17487/RFC9406, May 2023,
<https://www.rfc-editor.org/info/rfc9406>.
[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>.
Appendix A. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Previous versions of the document were presented and discsussed in
tsvwg, and eveolved through several versions. This version is a
refocus towards the newly formed CC Working Group where it is offered
as a candidate for progression.
Individual draft -00:
* First draft contributed to CC WG targeting publication as BCP.
* Reduced overlap
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Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
Michael Welzl
University of Oslo
Oslo
Norway
Email: michawe@ifi.uio.no
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