Specifying New Congestion Control Algorithms
draft-ietf-ccwg-rfc5033bis-00
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Authors | Martin Duke , Gorry Fairhurst | ||
Last updated | 2023-09-25 | ||
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draft-ietf-ccwg-rfc5033bis-00
CCWG M. Duke, Ed. Internet-Draft Google LLC Obsoletes: 5033 (if approved) G. Fairhurst, Ed. Intended status: Best Current Practice University of Aberdeen Expires: 24 March 2024 21 September 2023 Specifying New Congestion Control Algorithms draft-ietf-ccwg-rfc5033bis-00 Abstract 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. 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 24 March 2024. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Duke & Fairhurst Expires 24 March 2024 [Page 1] Internet-Draft New CC Algorithms September 2023 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Document Status . . . . . . . . . . . . . . . . . . . . . . . 4 3. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Minimum Requirements . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12 Evolution of RFC5033bis . . . . . . . . . . . . . . . . . . . . . 13 Since draft-scheffenegger-congress-rfc5033bis-00 . . . . . . . 13 Since RFC5033 . . . . . . . . . . . . . . . . . . . . . . . . . 13 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 1. 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 [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. Since then, multiple congestion control algorithms were developed outside of the IETF, including at least two that saw large scale deployment: Cubic [HRX08] and BBR [BBR-draft]. Duke & Fairhurst Expires 24 March 2024 [Page 2] Internet-Draft New CC Algorithms September 2023 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. Duke & Fairhurst Expires 24 March 2024 [Page 3] Internet-Draft New CC Algorithms September 2023 2. Document Status 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 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 Duke & Fairhurst Expires 24 March 2024 [Page 4] Internet-Draft New CC Algorithms September 2023 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. 3. 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. (0) Differences with Congestion Control Principles [RFC2914] Proposed congestion control mechanisms should include a clear explanation of the deviations from [RFC2914]. (1) Impact on Standard TCP, SCTP [RFC2960], and DCCP [RFC4340]. Proposed congestion control mechanisms should be evaluated when competing with standard IETF congestion control [RFC2581], [RFC2960], [RFC4340]. 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 Duke & Fairhurst Expires 24 March 2024 [Page 5] Internet-Draft New CC Algorithms September 2023 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]. (2) Difficult Environments. The proposed algorithms should be assessed in difficult environments such as paths containing wireless links. Characteristics of wireless environments are discussed in [RFC3819] and in Section 16 of [Tools]. 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 [RFC2488], high delay bandwidth products [RFC3649], high packet corruption rates [RFC3155], packet reordering [RFC4653], and significantly slow links [RFC3150]. 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 [RFC4782] (Quick-Start). (3) 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) [FJ03] 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 Duke & Fairhurst Expires 24 March 2024 [Page 6] Internet-Draft New CC Algorithms September 2023 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 [RFC3649] (HighSpeed TCP) and Section 9.7 of [RFC4782] (Quick-Start). (4) Protection Against Congestion Collapse 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 "full backoff" is used, this bullet 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 bandwidth during backoff. (5) 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 [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 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 Duke & Fairhurst Expires 24 March 2024 [Page 7] Internet-Draft New CC Algorithms September 2023 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. (6) 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. (7) 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 [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. (8) 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 [Tools]. As an example from an Experimental RFC, response to transient events is discussed in Section 9.2 of [RFC4782] (Quick-Start). (9) 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 Duke & Fairhurst Expires 24 March 2024 [Page 8] Internet-Draft New CC Algorithms September 2023 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). 4. 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. 5. 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 [RFC2581]). Alternate congestion control schemes should be mindful of such pitfalls, as well, and should examine any potential security issues that may arise. Duke & Fairhurst Expires 24 March 2024 [Page 9] Internet-Draft New CC Algorithms September 2023 6. IANA Considerations This document has no IANA actions. 7. References 7.1. Normative References [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion Control", RFC 2581, DOI 10.17487/RFC2581, April 1999, <https://www.rfc-editor.org/rfc/rfc2581>. [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000, <https://www.rfc-editor.org/rfc/rfc2914>. [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, DOI 10.17487/RFC2960, October 2000, <https://www.rfc-editor.org/rfc/rfc2960>. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006, <https://www.rfc-editor.org/rfc/rfc4340>. [RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion Control Algorithms", BCP 133, RFC 5033, DOI 10.17487/RFC5033, August 2007, <https://www.rfc-editor.org/rfc/rfc5033>. [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, <https://www.rfc-editor.org/rfc/rfc5681>. 7.2. Informative References [BBR-draft] Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V. Jacobson, "BBR Congestion Control", Work in Progress, Internet-Draft, draft-cardwell-iccrg-bbr-congestion- control-02, 7 March 2022, <https://datatracker.ietf.org/doc/html/draft-cardwell- iccrg-bbr-congestion-control-02>. Duke & Fairhurst Expires 24 March 2024 [Page 10] Internet-Draft New CC Algorithms September 2023 [Bufferbloat] Gettys, J., "The Blind Men and the Elephant", IETF Blog , 10 February 2018, <https://www.ietf.org/blog/blind-men-and-elephant/>. [FJ03] Floyd, S. and V. Jacobson, "Random Early Detection Gateways for Congestion Avoidance", IEEE/ACM Transactions on Networking, V.1 N.4 , August 1993. [HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: a new TCP-friendly high-speed TCP variant", ACM SIGOPS Operating Systems Review, vol. 42, no. 5, pp. 64-74 , July 2008, <https://doi.org/10.1145/1400097.1400105>. [RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP Over Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488, DOI 10.17487/RFC2488, January 1999, <https://www.rfc-editor.org/rfc/rfc2488>. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, DOI 10.17487/RFC2988, November 2000, <https://www.rfc-editor.org/rfc/rfc2988>. [RFC3150] Dawkins, S., Montenegro, G., Kojo, M., and V. Magret, "End-to-end Performance Implications of Slow Links", BCP 48, RFC 3150, DOI 10.17487/RFC3150, July 2001, <https://www.rfc-editor.org/rfc/rfc3150>. [RFC3155] Dawkins, S., Montenegro, G., Kojo, M., Magret, V., and N. Vaidya, "End-to-end Performance Implications of Links with Errors", BCP 50, RFC 3155, DOI 10.17487/RFC3155, August 2001, <https://www.rfc-editor.org/rfc/rfc3155>. [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", RFC 3649, DOI 10.17487/RFC3649, December 2003, <https://www.rfc-editor.org/rfc/rfc3649>. [RFC3714] Floyd, S., Ed. and J. Kempf, Ed., "IAB Concerns Regarding Congestion Control for Voice Traffic in the Internet", RFC 3714, DOI 10.17487/RFC3714, March 2004, <https://www.rfc-editor.org/rfc/rfc3714>. [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004, <https://www.rfc-editor.org/rfc/rfc3819>. Duke & Fairhurst Expires 24 March 2024 [Page 11] Internet-Draft New CC Algorithms September 2023 [RFC4653] Bhandarkar, S., Reddy, A. L. N., Allman, M., and E. Blanton, "Improving the Robustness of TCP to Non- Congestion Events", RFC 4653, DOI 10.17487/RFC4653, August 2006, <https://www.rfc-editor.org/rfc/rfc4653>. [RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick- Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782, January 2007, <https://www.rfc-editor.org/rfc/rfc4782>. [RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion Control Mechanisms", RFC 5166, DOI 10.17487/RFC5166, March 2008, <https://www.rfc-editor.org/rfc/rfc5166>. [RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", RFC 8312, DOI 10.17487/RFC8312, February 2018, <https://www.rfc-editor.org/rfc/rfc8312>. [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/rfc/rfc9000>. [RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed., "CUBIC for Fast and Long-Distance Networks", RFC 9438, DOI 10.17487/RFC9438, August 2023, <https://www.rfc-editor.org/rfc/rfc9438>. [Tools] Floyd, S. and E. Kohler, "Tools for the Evaluation of Simulation and Testbed Scenarios", Work in Progress , July 2007, <https://datatracker.ietf.org/doc/draft-irtf-tmrg-tools>. Acknowledgments Sally Floyd and Mark Allman were the authors of this document's predecessor, RFC5033, which served the community well for over a decade. Thanks to Richard Scheffenegger for helping to get this revision process started. 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]. Duke & Fairhurst Expires 24 March 2024 [Page 12] Internet-Draft New CC Algorithms September 2023 These individuals suggested improvements to this document: * Dave Taht 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 Christian Huitema Private Octopus, Inc. Email: huitema@huitema.net Authors' Addresses Martin Duke (editor) Google LLC Email: martin.h.duke@gmail.com Godred Fairhurst (editor) University of Aberdeen Email: gorry@erg.abdn.ac.uk Duke & Fairhurst Expires 24 March 2024 [Page 13]