Date: Sat, 25 Jan 2020 16:19:02 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 243590] TCP ECN not adhering extremely strictly to RFC3168 can cause massive TCP perf issues Message-ID: <bug-243590-227@https.bugs.freebsd.org/bugzilla/>
next in thread | raw e-mail | index | archive | help
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D243590 Bug ID: 243590 Summary: TCP ECN not adhering extremely strictly to RFC3168 can cause massive TCP perf issues Product: Base System Version: 11.2-STABLE Hardware: Any OS: Any Status: New Severity: Affects Some People Priority: --- Component: kern Assignee: bugs@FreeBSD.org Reporter: rscheff@gmx.at The TCP/IP ECN signaling loop looks like this -CE-> (latches ECE) <---- ACK, ECE ACK, CWR ----> (unlatches ECE) However, RFC3168 has a SHOULD for sending the CWR with a *new* data packet. New data packets usually are also marked as ECN-capable transport (ECT), wh= ile control packets, pure ACKs and retransmissions are sent without IP ECT codepoint. Some overly restrictive clients (Linux) explicitly only accept and process CWR, if they are received on (new) data segments, but ignore them completely when set in other packets. While such an overly restrictive behavior is also problematic, BSD *should* adhere to the guidance of RFC3168 and sent out CWR flags only when (new) data packets are sent out (sec 6.1.2) rather than simply the very next segment (sec. 6.1): RFC3168, section 6.1: * The sender sets the CWR flag in the TCP header of the next packet sent to the receiver to acknowledge its receipt of and reaction to the ECN-Echo flag. RFC3168, section 6.1.2: When an ECN-Capable TCP sender reduces its congestion window for any reason (because of a retransmit timeout, a Fast Retransmit, or in response to an ECN Notification), the TCP sender sets the CWR flag in the TCP header of the first new data packet sent after the window reduction.=20 The interaction can lead to a race to the bottom, where cwnd is continually decreased and kept at a very small value. This results in extremely poor performance for ECN-enabled TCP sessions. --=20 You are receiving this mail because: You are the assignee for the bug.=
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?bug-243590-227>