]> Google LLC

martin.h.duke@gmail.com

University of Aberdeen

gorry@erg.abdn.ac.uk

General CCWG Internet-Draft The IETF's standard congestion control schemes have been widely shown to be inadequate for various environments (e.g., high-speed networks, wireless technologies such as 3GPP and WiFi, long distance satellite links) and also in conflict with the needed, more isochronous, behaviors of VoIP, gaming, and videoconferencing traffic. Recent research has yielded many alternate congestion control schemes that significantly differ from the IETF's congestion control principles. Using these new congestion control schemes in the global Internet has possible ramifications to both the traffic using the new congestion control and to traffic using the currently standardized congestion control. Therefore, the IETF must proceed with caution when dealing with alternate congestion control proposals. The goal of this document is to provide guidance for considering alternate congestion control algorithms within the IETF.

Introduction This document provides guidelines for the IETF to use when evaluating suggested congestion control algorithms that significantly differ from the general congestion control principles outlined in . The guidance is intended to be useful to authors proposing alternate congestion control 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 updates the similarly titled that was published in 2007. Since then, multiple congestion control algorithms were developed outside of the IETF, including at least two that saw large scale deployment: Cubic and BBR . Cubic was documented in a research publication in 2007 , and 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, being published as an informational RFC in 2017 and then as a proposed standard in 2023 . BBR is developed as an internal research project by Google, with the first implementation contributed to Linux kernel 4.19 in 2016. It was described in an IRTF draft in 2018, and that draft is regularly updated to document the evolving versions of the algorithm . BBR is widely used for Google services using either TCP or QUIC , and is also largely deployed outside of Google. We cannot say now whether the original authors of expected that developers would be somehow waiting for IETF review before widely deploying a congestion control algorithm over the Internet, but the examples of Cubic and BBR teaches us that deployment of new algorithms is not in fact gated by publication of the algorithm as an RFC. Nevertheless, guidelines are important, if only to remind potential inventors and developers of the multiple facets of the congestion control problem. The guidelines in this document are intended to be consistent with the congestion control principles from of preventing congestion collapse, considering fairness, and optimizing the flow's own performance in terms of throughput, delay, and loss. 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 scheme. Rather, the document provides a set of criteria that should be considered and weighed by the developers of congestion control algorithms and by the IETF in the context of each proposal. The high-order criteria for any new proposal is that a serious scientific study of the pros and cons of the proposal needs to have been done before a proposal is considered for publication by the IETF or before it is deployed at large scale. After initial studies, we encourage authors to write a specification of their proposals for publication in the RFC series to allow others to concretely understand and investigate the wealth of proposals in this space.

Document Status Following the lead of HighSpeed TCP , alternate congestion control algorithms are expected to be published as "Experimental" RFCs until such time that the community better understands the solution space. Traditionally, the meaning of "Experimental" status has varied in its use and interpretation. As part of this document we define two classes of congestion control proposals that can be published with the "Experimental" status. The first class includes algorithms that are judged to be safe to deploy for best-effort traffic in the global Internet and further investigated in that environment. The second class includes algorithms that, while promising, are not deemed safe enough for widespread deployment as best-effort traffic on the Internet, but are being specified to facilitate investigations in simulation, testbeds, or controlled environments. The second class can also include algorithms where the IETF does not yet have sufficient understanding to decide if the algorithm is or is not safe for deployment on the Internet. Each alternate congestion control algorithm published is required to include a statement in the abstract indicating whether or not the proposal is considered safe for use on the Internet. Each alternate congestion control algorithm published is also required to include a statement in the abstract describing environments where the protocol is not recommended for deployment. There may be environments where the protocol is deemed safe for use, but still is not recommended for use because it does not perform well for the user. As examples of such statements, specifying HighSpeed TCP includes a statement in the abstract stating that the proposal is Experimental, but may be deployed in the current Internet. In contrast, the Quick-Start document includes a paragraph in the abstract stating the mechanism is only being proposed for controlled environments. The abstract specifies environments where the Quick-Start request could give false positives (and therefore would be unsafe to deploy). 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 would still not be recommended because it could cause unnecessary delays for the connections attempting to use Quick-Start. For authors of alternate congestion control schemes who are not ready to bring their congestion control mechanisms to the IETF for standardization (either as Experimental or as Proposed Standard), one possibility would be to submit an internet-draft that documents the alternate congestion control mechanism for the benefit of the IETF and IRTF communities. This is particularly encouraged in order to get algorithm specifications widely disseminated to facilitate further research. Such an internet-draft could be submitted to be considered as an Informational RFC, as a first step in the process towards standardization. Such a document would also be expected to carry an explicit warning against using the scheme in the global Internet. Note: we are not changing the RFC publication process for non-IETF produced documents (e.g., those from the IRTF or Independent Submissions via the RFC-Editor). However, we would hope the guidelines in this document inform the IESG as they consider whether to add a note to such documents.

Guidelines As noted above, authors are expected to do a well-rounded evaluation of the pros and cons of proposals brought to the IETF. The following are guidelines to help authors and the IETF community. Concerns that fall outside the scope of these guidelines are certainly possible; these guidelines should not be considered as an all-encompassing check-list.

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Differences with Congestion Control Principles

Proposed congestion control mechanisms should include a clear explanation of the deviations from .

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Impact on Standard TCP, SCTP , and DCCP .

Proposed congestion control mechanisms should be evaluated when competing with standard IETF congestion control , , . Alternate congestion controllers that have a significantly negative impact on traffic using standard congestion control may be suspect and this aspect should be part of the community's decision making with regards to the suitability of the alternate congestion control mechanism.

We note that this bullet is not a requirement for strict TCP- friendliness as a prerequisite for an alternate congestion control mechanism to advance to Experimental. As an example, HighSpeed TCP is a congestion control mechanism that is Experimental, but that is not TCP-friendly in all environments. We also note that this guideline does not constrain the fairness offered for non-best-effort traffic.

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

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Difficult Environments.

The proposed algorithms should be assessed in difficult environments such as paths containing wireless links. Characteristics of wireless environments are discussed in and in Section 16 of . Other difficult environments can include those with multipath routing within a connection. We note that there is still much to be desired in terms of the performance of TCP in some of these difficult environments. For congestion control mechanisms with explicit feedback from routers, difficult environments can include paths with non-IP queues at layer-two, IP tunnels, and the like. A minimum goal for experimental mechanisms proposed for widespread deployment in the Internet should be that they do not perform significantly worse than TCP in these environments.

While it is impossible to enumerate all the possible "difficult environments", we note that the IETF has previously grappled with paths with long delays , high delay bandwidth products , high packet corruption rates , packet reordering , and significantly slow links . Aspects of alternate congestion control that impact networks with these characteristics should be detailed.

As an example from an Experimental RFC, performance in difficult environments is discussed in Sections 6, 9.2, and 10.2 of (Quick-Start).

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Investigating a Range of Environments.

Similar to the last criteria, proposed alternate congestion controllers should be assessed in a range of environments. For instance, proposals should be investigated across a range of bandwidths, round-trip times, levels of traffic on the reverse path, and levels of statistical multiplexing at the congested link. Similarly, proposals should be investigated for robust performance with different queueing mechanisms in the routers, especially Random Early Detection (RED) and Drop-Tail. This evaluation is often not included in the internet-draft itself, but in related papers cited in the draft.

A particularly important aspect of evaluating a proposal for standardization is in understanding where the algorithm breaks down. Therefore, particular attention should be paid to characterizing the areas where the proposed mechanism does not perform well.

As an example from an Experimental RFC, performance in a range of environments is discussed in Section 12 of (HighSpeed TCP) and Section 9.7 of (Quick-Start).

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Protection Against Congestion Collapse

The alternate congestion control mechanism should either stop sending when the packet drop rate exceeds some threshold , or should include some notion of "full backoff". For "full backoff", at some point the algorithm would reduce the sending rate to one packet per round-trip time and then exponentially backoff the time between single packet transmissions if congestion persists. Exactly when either "full backoff" or a pause in sending comes into play will be algorithm-specific. However, as discussed in , this requirement is crucial to protect the network in times of extreme congestion.

If "full backoff" is used, this bullet does not require that the full backoff mechanism must be identical to that of TCP . As an example, this bullet does not preclude full backoff mechanisms that would give flows with different round- trip times comparable bandwidth during backoff.

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Protection Against Bufferbloat

The alternate congestion control mechanism should reduce its sending rate if the round trip time (RTT) significantly increases. Exactly how the algorithm reduces its sending rate is algorithm specific.

Bufferbloat refers to the building of long queues in the network. Many network routers are configured with very large buffers. If congestion starts happening, classic TCP congestion control algorithms will continue sending at a high rate until the buffer fills up completely and packet losses occur. Every connection going through that bottleneck will experience high latency. This is particularly bad for highly interactive applications like games, but it also affects routine web browsing and video playing.

This problem became apparent in the last decade and was not discussed in the Congestion Control Principles published in September 2002 . The classic congestion control algorithm and the widely deployed Cubic algorithm do not address it, but newly designed congestion control algorithms have the opportunity to improve the state of the art.

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Fairness within the Alternate Congestion Control Algorithm.

In environments with multiple competing flows all using the same alternate congestion control algorithm, the proposal should explore how bandwidth is shared among the competing flows.

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Performance with Misbehaving Nodes and Outside Attackers.

The proposal should explore how the alternate congestion control mechanism performs with misbehaving senders, receivers, or routers. In addition, the proposal should explore how the alternate congestion control mechanism performs with outside attackers. This can be particularly important for congestion control mechanisms 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 (Quick-Start). 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.

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Responses to Sudden or Transient Events.

The proposal should consider how the alternate congestion control mechanism would perform in the presence of transient events such as sudden congestion, a routing change, or a mobility event. Routing changes, link disconnections, intermittent link connectivity, and mobility are discussed in more detail in Section 17 of .

As an example from an Experimental RFC, response to transient events is discussed in Section 9.2 of (Quick-Start).

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Incremental Deployment.

The proposal should discuss whether the alternate congestion control mechanism allows for incremental deployment in the targeted environment. For a mechanism targeted for deployment in the current Internet, it would be helpful for the proposal to discuss what is known (if anything) about the correct operation of the mechanism with some of the equipment installed in the current Internet, e.g., routers, transparent proxies, WAN optimizers, intrusion detection systems, home routers, and the like.

As a similar concern, if the alternate congestion control mechanism is intended only for specific environments (and not the global Internet), the proposal should consider how this intention is to be carried out. The community will have to address the question of whether the scope can be enforced by simply stating the restrictions or whether additional protocol mechanisms are required to enforce the scoping. The answer will necessarily depend on the change being proposed.

As an example from an Experimental RFC, deployment issues are discussed in Sections 10.3 and 10.4 of (Quick-Start).

Minimum Requirements This section suggests minimum requirements for a document to be approved as Experimental with approval for widespread deployment in the global Internet. The minimum requirements for approval for widespread deployment in the global Internet include the following guidelines on: (1) assessing the impact on standard congestion control, (3) investigation of the proposed mechanism in a range of environments, (4) protection against congestion collapse, and (8) discussing whether the mechanism allows for incremental deployment. For other guidelines, i.e., (2), (5), (6), and (7), the author must perform the suggested evaluations and provide recommended analysis. Evidence that the proposed mechanism has significantly more problems than those of TCP should be a cause for concern in approval for widespread deployment in the global Internet.

Security Considerations This document does not represent a change to any aspect of the TCP/IP protocol suite and therefore does not directly impact Internet security. The implementation of various facets of the Internet's current congestion control algorithms do have security implications (e.g., as outlined in ). Alternate congestion control schemes should be mindful of such pitfalls, as well, and should examine any potential security issues that may arise.

IANA Considerations This document has no IANA actions.

The goal of this document is to explain the need for congestion control in the Internet, and to discuss what constitutes correct congestion control. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The IETF's standard congestion control schemes have been widely shown to be inadequate for various environments (e.g., high-speed networks). Recent research has yielded many alternate congestion control schemes that significantly differ from the IETF's congestion control principles. Using these new congestion control schemes in the global Internet has possible ramifications to both the traffic using the new congestion control and to traffic using the currently standardized congestion control. Therefore, the IETF must proceed with caution when dealing with alternate congestion control proposals. The goal of this document is to provide guidance for considering alternate congestion control algorithms within the IETF. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the Stream Control Transmission Protocol (SCTP). [STANDARDS-TRACK] The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. [STANDARDS-TRACK] This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods. [STANDARDS-TRACK] This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods. This document obsoletes RFC 2581. [STANDARDS-TRACK] Google Google Google Google Google This document specifies the BBR congestion control algorithm. BBR ("Bottleneck Bandwidth and Round-trip propagation time") uses recent measurements of a transport connection's delivery rate, round-trip time, and packet loss rate to build an explicit model of the network path. BBR then uses this model to control both how fast it sends data and the maximum volume of data it allows in flight in the network at any time. Relative to loss-based congestion control algorithms such as Reno [RFC5681] or CUBIC [RFC8312], BBR offers substantially higher throughput for bottlenecks with shallow buffers or random losses, and substantially lower queueing delays for bottlenecks with deep buffers (avoiding "bufferbloat"). BBR can be implemented in any transport protocol that supports packet-delivery acknowledgment. Thus far, open source implementations are available for TCP [RFC793] and QUIC [RFC9000]. This document specifies version 2 of the BBR algorithm, also sometimes referred to as BBRv2 or bbr2. CUBIC is an extension to the current TCP standards. It differs from the current TCP standards only in the congestion control algorithm on the sender side. In particular, it uses a cubic function instead of a linear window increase function of the current TCP standards to improve scalability and stability under fast and long-distance networks. CUBIC and its predecessor algorithm have been adopted as defaults by Linux and have been used for many years. This document provides a specification of CUBIC to enable third-party implementations and to solicit community feedback through experimentation on the performance of CUBIC. CUBIC is a standard TCP congestion control algorithm that uses a cubic function instead of a linear congestion window increase function to improve scalability and stability over fast and long-distance networks. CUBIC has been adopted as the default TCP congestion control algorithm by the Linux, Windows, and Apple stacks. This document updates the specification of CUBIC to include algorithmic improvements based on these implementations and recent academic work. Based on the extensive deployment experience with CUBIC, this document also moves the specification to the Standards Track and obsoletes RFC 8312. This document also updates RFC 5681, to allow for CUBIC's occasionally more aggressive sending behavior. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. The proposals in this document are experimental. While they may be deployed in the current Internet, they do not represent a consensus that this is the best method for high-speed congestion control. In particular, we note that alternative experimental proposals are likely to be forthcoming, and it is not well understood how the proposals in this document will interact with such alternative proposals. This document proposes HighSpeed TCP, a modification to TCP's congestion control mechanism for use with TCP connections with large congestion windows. The congestion control mechanisms of the current Standard TCP constrains the congestion windows that can be achieved by TCP in realistic environments. For example, for a Standard TCP connection with 1500-byte packets and a 100 ms round-trip time, achieving a steady-state throughput of 10 Gbps would require an average congestion window of 83,333 segments, and a packet drop rate of at most one congestion event every 5,000,000,000 packets (or equivalently, at most one congestion event every 1 2/3 hours). This is widely acknowledged as an unrealistic constraint. To address his limitation of TCP, this document proposes HighSpeed TCP, and solicits experimentation and feedback from the wider community. This document specifies an optional Quick-Start mechanism for transport protocols, in cooperation with routers, to determine an allowed sending rate at the start and, at times, in the middle of a data transfer (e.g., after an idle period). While Quick-Start is designed to be used by a range of transport protocols, in this document we only specify its use with TCP. Quick-Start is designed to allow connections to use higher sending rates when there is significant unused bandwidth along the path, and the sender and all of the routers along the path approve the Quick-Start Request. This document describes many paths where Quick-Start Requests would not be approved. These paths include all paths containing routers, IP tunnels, MPLS paths, and the like that do not support Quick- Start. These paths also include paths with routers or middleboxes that drop packets containing IP options. Quick-Start Requests could be difficult to approve over paths that include multi-access layer- two networks. This document also describes environments where the Quick-Start process could fail with false positives, with the sender incorrectly assuming that the Quick-Start Request had been approved by all of the routers along the path. As a result of these concerns, and as a result of the difficulties and seeming absence of motivation for routers, such as core routers to deploy Quick-Start, Quick-Start is being proposed as a mechanism that could be of use in controlled environments, and not as a mechanism that would be intended or appropriate for ubiquitous deployment in the global Internet. This memo defines an Experimental Protocol for the Internet community. This document provides advice to the designers of digital communication equipment, link-layer protocols, and packet-switched local networks (collectively referred to as subnetworks), who wish to support the Internet protocols but may be unfamiliar with the Internet architecture and the implications of their design choices on the performance and efficiency of the Internet. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Transmission Control Protocol (TCP) provides reliable delivery of data across any network path, including network paths containing satellite channels. While TCP works over satellite channels there are several IETF standardized mechanisms that enable TCP to more effectively utilize the available capacity of the network path. This document outlines some of these TCP mitigations. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document discusses the specific TCP mechanisms that are problematic in environments with high uncorrected error rates, and discusses what can be done to mitigate the problems without introducing intermediate devices into the connection. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies Non-Congestion Robustness (NCR) for TCP. In the absence of explicit congestion notification from the network, TCP uses loss as an indication of congestion. One of the ways TCP detects loss is using the arrival of three duplicate acknowledgments. However, this heuristic is not always correct, notably in the case when network paths reorder segments (for whatever reason), resulting in degraded performance. TCP-NCR is designed to mitigate this degraded performance by increasing the number of duplicate acknowledgments required to trigger loss recovery, based on the current state of the connection, in an effort to better disambiguate true segment loss from segment reordering. This document specifies the changes to TCP, as well as the costs and benefits of these modifications. This memo defines an Experimental Protocol for the Internet community. This document makes performance-related recommendations for users of network paths that traverse "very low bit-rate" links. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document discusses IAB concerns about effective end-to-end congestion control for best-effort voice traffic in the Internet. These concerns have to do with fairness, user quality, and with the dangers of congestion collapse. The concerns are particularly relevant in light of the absence of a widespread Quality of Service (QoS) deployment in the Internet, and the likelihood that this situation will not change much in the near term. This document is not making any recommendations about deployment paths for Voice over Internet Protocol (VoIP) in terms of QoS support, and is not claiming that best-effort service can be relied upon to give acceptable performance for VoIP. We are merely observing that voice traffic is occasionally deployed as best-effort traffic over some links in the Internet, that we expect this occasional deployment to continue, and that we have concerns about the lack of effective end-to-end congestion control for this best-effort voice traffic. This memo provides information for the Internet community. This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. [STANDARDS-TRACK] This document discusses the metrics to be considered in an evaluation of new or modified congestion control mechanisms for the Internet. These include metrics for the evaluation of new transport protocols, of proposed modifications to TCP, of application-level congestion control, and of Active Queue Management (AQM) mechanisms in the router. This document is the first in a series of documents aimed at improving the models that we use in the evaluation of transport protocols. This document is a product of the Transport Modeling Research Group (TMRG), and has received detailed feedback from many members of the Research Group (RG). As the document tries to make clear, there is not necessarily a consensus within the research community (or the IETF community, the vendor community, the operations community, or any other community) about the metrics that congestion control mechanisms should be designed to optimize, in terms of trade-offs between throughput and delay, fairness between competing flows, and the like. However, we believe that there is a clear consensus that congestion control mechanisms should be evaluated in terms of trade-offs between a range of metrics, rather than in terms of optimizing for a single metric. This memo provides information for the Internet community.

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. 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 . These individuals suggested improvements to this document:

Evolution of RFC5033bis

Since draft-scheffenegger-congress-rfc5033bis-00 Renamed file to reflect WG adpotion Updated authorship and acknowledgements. Include updated text suggested by Dave Taht Added criterion for bufferbloat Mentioned Cubic and BBR as motivation Include section to track updates between revisions Update references

Since RFC5033 converted to Markdown and xml2rfc v3 various formatting changes

Contributors Private Octopus, Inc.

huitema@huitema.net