| Internet-Draft | New CC Algorithms | October 2023 |
| Duke & Fairhurst | Expires 25 April 2024 | [Page] |
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.¶
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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 [RFC2914]. 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 [RFC5033] that was published in 2007 as a Best Current Practice to evaluate new congestion control algorithms as Experimental or Proposed Standard RFCs.¶
In 2007, TCP was the dominant consumer of this work, and proposals were typically discussed in research groups, for example the Internet Congestion Control Research Group (ICCRG).¶
Since RFC 5033 was published, many conditions have changed. The set of protocols using these algorithms has spread beyond TCP and SCTP to include DCCP, QUIC, and beyond. Some congestion control algorithm proponents now have the opportunity to test and deploy at scale without IETF review. There is more interest in specialized use cases such as data centers and real-time protocols. Finally, the community has gained much more experience with indications of congestion beyond packet loss.¶
Multiple congestion control algorithms have been developed outside of the IETF, including at least two that saw large scale deployment: Cubic [HRX08] and BBR [BBR-draft].¶
Cubic was documented in a research publication in 2007 [HRX08], 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 [RFC8312] and then as a proposed standard in 2023 [RFC9438].¶
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-draft]. BBR is widely used for Google services using either TCP or QUIC [RFC9000], and is also largely deployed outside of Google.¶
We cannot say now whether the original authors of [RFC5033] 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 [RFC2914] of preventing congestion collapse, considering fairness, and optimizing the 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 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.¶
Following the lead of HighSpeed TCP [RFC3649], 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, [RFC3649] 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 [RFC4782] 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 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 would not be 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].¶
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 ensure algorithm specifications are widely disseminated to facilitate further research. Such an internet-draft could also be considered as an Informational RFC, as a first step in the process towards standardization. Such a document would 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.¶
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.¶
When considering a new congestion control proposal, the community MUST consider the following criteria. These criteria will be evaluated in various domains (see Section 4 and Section 5).¶
The following criteria evaluate the proposed algorithm when one or more flows using that algorithm share a bottleneck link, with no other algorithms operating.¶
The alternate congestion control mechanism should either stop sending when the packet drop rate exceeds some threshold [RFC3714], or should include some notion of "full backoff". For "full backoff", at some point the algorithm would reduce the sending rate to one packet per round-trip time and then exponentially backoff the time between single packet transmissions if congestion persists. Exactly when either "full backoff" or a pause in sending comes into play will be algorithm-specific. However, as discussed in [RFC2914], this requirement is crucial to protect the network in times of extreme congestion.¶
If the result of full backoff is used, this test does not require that the full backoff mechanism must be identical to that of TCP [RFC2988]. As an example, this bullet does not preclude full backoff mechanisms that would give flows with different round- trip times comparable capacity during backoff.¶
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, but see [RFC8961] and [RFC8085] for requirements.¶
Bufferbloat [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 [RFC5681] 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 adds unwanted latency that impacts 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 [RFC2914]. The classic congestion control algorithm [RFC5681] and the widely deployed Cubic algorithm [RFC9438] do not address it, but newly designed congestion control algorithms have the opportunity to improve the state of the art.¶
When multiple competing flows all using the same alternate congestion control algorithm, the proposal should explore how the capacity is shared among the competing flows.¶
(TODO: Discuss short and long flows)¶
These criteria evaluate the interaction of the proposal with commonly deployed congestion controls.¶
Evaluate the impact on TCP [RFC9293], SCTP [RFC9260], DCCP [RFC4340], and QUIC [RFC9000].¶
Proposed congestion control mechanisms should be evaluated when competing with standard IETF congestion control [RFC5681], [RFC9260], [RFC4340], [RFC9002], [RFC9438]. 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. The community should also consider other non-standard congestion controls known to be widely deployed,¶
We note that this bullet is not a requirement for strict Reno- or Cubic- 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. When a new algorithm is deployed, the existing major deployments need to be considered to avoid severe performance degradation. We also note that this guideline does not constrain the interaction with non-best-effort traffic.¶
As an example from an Experimental RFC, fairness with standard TCP is discussed in Sections 4 and 6 of [RFC3649] (HighSpeed TCP) and using spare capacity is discussed in Sections 6, 11.1, and 12 of [RFC3649].¶
(TODO: Clarify that real time congestion controls are included, with allowances for the poor documentation / open source availability of these)¶
(TODO: Discuss short and long flows)¶
Proposed congestion control mechanisms SHOULD include a clear explanation of the deviations from [RFC2914].¶
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 [RFC4782] (Quick-Start).¶
The criteria in Section 3 will be evaluated in the following scenarios. Unless a proposal is explicitly forbidden on the public internet, the community MUST find that it meets the criteria in these scenarios for the proposal to progress.¶
The evaluation in each scenario should occur over a representative range of bandwidths, delays, and queue depths. Of course, the set of parameters representative of the public internet will change over time.¶
These criteria are meant to capture a statistically dominant set of internet conditions. In the case that the algorithm has been tested at internet scale, the results from that deployment are often useful for answering these questions.¶
(TODO: Describe properties of wired networks.)¶
Proposals should be investigated for robust performance with different queueing mechanisms in the routers, especially Random Early Detection (RED) [FJ03] and Drop-Tail. This evaluation is often not included in the internet-draft itself, but in related papers cited in the draft.¶
While the early Internet was dominated by wired links, the properties of wireless links have become extremely important to Internet performance. In particular, congestion controllers 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.¶
The criteria in Section 3 will be evaluated in the following scenarios, unless the proposal specifically excludes its use in a scenario. The community MAY allow a proposal to progress even if the criteria indicate an unsatisfactory result for these scenarios.¶
In general, measurements from internet-scale deployments will not expose the properties of operation in these scenarios, as they are statistically small.¶
(TODO: Write this section)¶
Satellite links often have delays longer than typical for wired paths [RFC2488] and high delay bandwidth products [RFC3649].¶
The proposal should explore how the alternate congestion control mechanism performs with non-compliant 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 [RFC4782] (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.¶
When the proposal relies on explicit signals from the path, the effect of traffic passing through the tunnel -- where routers may not be aware of the underlying flow -- MUST be considered.¶
[RFC4653] discusses the effect of extreme packet reordering.¶
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 [Tools].¶
As an example from an Experimental RFC, response to transient events is discussed in Section 9.2 of [RFC4782] (Quick-Start).¶
An IETF transport is not tied to a specific Internet path. The set of routers forming a path can and do change with time, this will also cause the properties of the path to change with respect to time. New CCs MUST evaluate the impact of changes in the path, and be robust to changes in path characteristics on the interval of common Internet re-routing intervals.¶
Event when the routers constituting a path does not change, the properties of that path can vary, with similar impacts on congestion control¶
Multipath transport protocols permit more than one path to be differentiated and used by a single connection at the sender. A multipath sender can schedule which packets travel on which of its active paths. This enables a tradeoff in timeliness and reliability.¶
One use is to provide fail-over from one path to another when the original path is no longer viable or to switch the traffic from one path to another when this is expected to improve performance (latency, throughput, reliability, cost). Designs need to independently track the congestion state of each path, and need to demonstrate independent congestion control for each path being used. New multipath CCs that implement path fail-over MUST evaluate the harm resulting from a change in the path, and show that this does not result in flow starvation. Synchronisation of failover (e.g., where multiple flows change their path on similar timeframes) can also contribute to harm and/or reduce fairness, these effects also ought to be evaluated.¶
A concurrent multipath transport protocol simultaneously schedules flows to aggregate the capacity across multiple paths. The Internet provides no guarantee that different paths (e.g., using different endpoint addresses) are disjoint. This has additional implications: New CCs 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), and SHOULD consider the fairness with other flows. Synchronisation of CC mechanisms (e.g., where multiple flows change their behaviour on similar timeframes) can also contribute to harm and/or reduce fairness, these effects also ought to be evaluated. At the time of writing, there are no IETF standards for concurrent multipath congestion control in the general Internet.¶
This document does not represent a change to any aspect of the TCP/IP protocol suite and therefore does not directly impact Internet security. The implementation of various facets of the Internet's current congestion control algorithms do have security implications (e.g., as outlined in [RFC5681]). Alternate congestion control schemes should be mindful of such pitfalls, as well, and should examine any potential security issues that may arise.¶
This document has no IANA actions.¶
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 [RFC5166].¶
These individuals suggested improvements to this document:¶
Dave Taht¶