Session Date/Time: 24 Jul 2024 20:00

ccwg

Summary

This session of the Congestion Control Working Group (CCWG) focused on discussing potential new work items for the group, now that RFC 5033bis is nearing completion. Several proposals were presented, covering rate-limited sending, BBRv3 updates, BBR modifications for real-time communication, HPCC++, and a new slow start algorithm. The session also included a rechartering discussion, primarily concerning the addition of introductory text and focus topics to the charter.

Key Discussion Points

Rate-Limited Sending: Michael Rosa presented a draft addressing an issue where senders, when application or receiver-limited, could theoretically grow their congestion window indefinitely. The proposed solution involves limiting congestion window growth based on the maximum flight size. There was discussion regarding the time period over which max flight size should be considered.
BBRv3: Neil presented an update on BBRv3, now a CCWG draft. Changes include moving to GitHub, incorporating the delivery rate draft, bug fixes, parameter tunings, and editorial improvements. The discussion centered on fairness concerns, especially with coexistence with Reno and Cubic, and the need to balance fairness with aggressive bandwidth probing. It was suggested that BBR3 should be an experimental standard. There was a general concern about BBR being "bug compatible with Reno", and a debate about how aggressive a new congestion control should be.
BBR for Real-time Communication: Christian Huitema presented work on adapting BBR for real-time communication, specifically for video streaming over QUIC. Issues were identified with BBR's start-up phase, its behavior on Wi-Fi networks with frequent suspensions, and the bandwidth-limited nature of video codecs. He proposed modifications to address these problems, including a mechanism for detecting loss of feedback. A cross layer communication between WiFi and the IP/Transport layer was discussed as valuable.
HPCC++: Rui provided an update on HPCC++, a precise congestion control mechanism leveraging in-band telemetry in data center networks. Discussion focused on queue length measurements and support for multi-queue environments. There was conversation about clarifying the use case and potential audience for this technology.
New Slow Start Algorithm (Search): Mark Claypool presented "Search," a new slow start algorithm aimed at addressing the limitations of High Start, especially on wireless networks. Search uses delivery rate and predicted sending rate to determine when to exit slow start. Discussion centered on dealing with application rate limits and minimizing state.
Rechartering: The group discussed a proposal to add introductory text to the charter explaining congestion control. Also discussed were the addition of focus topics. A key point of discussion revolved around how the group should approach the adoption of new congestion control algorithms and how much testing and evaluation should be performed before adoption.

Decisions and Action Items

Rate-Limited Sending: The group expressed interest in addressing the rate-limited sending issue using the provided draft as a starting point.
- Action Item: Send comments to the list and/or file GitHub issues.
BBRv3: The group expressed general interest in working on BBR as a congestion control algorithm and using the current draft as a starting point. IETF will gain change control of the BBRv3 document.
- Action Item: Read and provide comments on the BBRv3 draft.
BBR for Real-time Communication: The group seemed to agree this should be folded into existing BBR work.
HPCC++: It was expressed that group would like to discuss CCWG topic sometime in the future.
- Action Item: Rui to clarify the use case and intended audience for HPCC++ for the group.
New Slow Start Algorithm (Search): The group expressed interest in dedicating working group time to improving the slow start algorithm and initial rate estimates.
Rechartering: Add introductory text to the CCWG charter around congestion control.
- Action Item: Contribute to the open pull request on GitHub to wordsmith the introductory text for the charter.
General Adoption Process:
- Start by picking one congestion control algorithm and adopting it to build the tools and processes needed for future evaluations.
Action Item: All should review the 5033BIS test before adding new features.

Next Steps

Authors of the drafts should address the feedback received during the meeting and revise their documents accordingly.
Members are encouraged to review the drafts and provide comments on the mailing list or via GitHub.
The CCWG chairs will work with authors and the community to prioritize the work items and determine the next steps for each proposal.
The CCWG chairs will work with the ICCRG chairs to coordinate work between the two groups.
The CCWG chairs will send out a link to the PR for charter introductory text.