27Feb2020 [FreeBSD WIKI]

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Attendees: jtl, tuexen, thj, Richard

===== Discussion =====
  * Whitespace commits are in head. tuexen will MFC all relevant changes to stable/12.
  * Cleanup (combine sack_newdata and snd_recover and reclaim one 32-bit integer): [[https://reviews.freebsd.org/D18811|D18811]]. Michael committed.
  * Improve [[https://tools.ietf.org/html/rfc6675|RFC 6675]] support: [[https://reviews.freebsd.org/D18624|D18624]], [[https://reviews.freebsd.org/D18985|D18985]]. Reviews needed.
  * Add [[https://tools.ietf.org/html/rfc6937|RFC 6937]] support: [[https://reviews.freebsd.org/D18892|D18892]]. **This depends on the RFC 6675 support.** Richard plans to test more before proposing we commit this. (The testing is also on hold until RFC 6675 support is committed.) Also, needs reviews.
  * No update on blackhole detection.
    * IP MTU discovery is enabled by default. Discovery works by setting DF bit and lowering MTU when we get back ICMP fragmentation needed responses.
    * Blackhole discovery is disabled by default. There is no probe back, so you get stuck at a lower rate permanently.
    * jtl@ to provide notes on problems he thought he saw.
    * Consensus is to make blackhole discovery "good enough".
  * Fix RTO timer during SACK retransmissions [[https://reviews.freebsd.org/D23160|D23160]]: Idea is correct; questions about implementation. JTL and Michael will try to look.
    * Richard is now having trouble reproducing.
    * Richard judges this as perhaps aa less-urgent problem.
  * ENOBUF handling in base stack seems suboptimal. RACK/BBR uses a different approach.
  * Richard reports on a timer problem with Cubic. It doesn't reset the timer when exiting slow start after going idle. Draft patch is [[https://reviews.freebsd.org/D23655|D23655]], but needs further review.
    * Later, we also need to review Cubic and bring it up to the latest RFC.
    * jtl added some more people to review.
  * ECN:
    * tuexen notices (at least) three problems in SYN handling:
      * When acting as server and getting IPv6 SYN, the TOS field is not set correctly.
      * When retransmitting SYN/ACKs, we are not honoring the max retries sysctl is not set correctly.
      * If client resends SYN, we don't update our syncache entry.
    * tuexen will submit fixes. 
    * Richard reports an interoperability problem between Linux and %%FreeBSD%% when using TCP with ECN. A patch is available at [[https://reviews.freebsd.org/D23364|D23364]]. Michael will review it.
      * Extensive discussion on the interaction of reusing snd_recover to track recovery windows for both ECN and CC algorithms. Richard argues from standards that we should have a unified view of the recovery window. tuexen and jtl think that from a code perspective it would be cleaner to have separate tracking to avoid unintended interactions. Richard points out that checking so many references to snd_recover might be difficult. jtl points out that there really aren't that many references. Richard thinks that there are sufficient barriers to prevent us from getting snd_recover use confused.
    * rrs@ found some other ECN bugs, and will commit fixes.
    * Richard reports Linux and %%FreeBSD%% with Cubic and ECN don't play well together due to a (different) bug on each side. Bugs open in each bug tracker.

===== Next Meeting =====
Next meeting is 12 Mar 2020 @ 1400 UTC.