Bug 243590
| Summary: | TCP ECN not adhering extremely strictly to RFC3168 can cause massive TCP perf issues | ||
|---|---|---|---|
| Product: | Base System | Reporter: | Richard Scheffenegger <rscheff> |
| Component: | kern | Assignee: | Richard Scheffenegger <rscheff> |
| Status: | Closed FIXED | ||
| Severity: | Affects Some People | CC: | rscheff, transport, tuexen |
| Priority: | --- | Flags: | rscheff:
maintainer-feedback-
rscheff: mfc-stable12? rscheff: mfc-stable11+ rscheff: mfc-stable10- rscheff: exp-run- |
| Version: | 11.2-STABLE | ||
| Hardware: | Any | ||
| OS: | Any | ||
A commit references this bug: Author: rscheff Date: Sun May 24 17:51:14 UTC 2020 New revision: 361436 URL: https://svnweb.freebsd.org/changeset/base/361436 Log: MFC r361347: With RFC3168 ECN, CWR SHOULD only be sent with new data Overly conservative data receivers may ignore the CWR flag on other packets, and keep ECE latched. This can result in continous reduction of the congestion window, and very poor performance when ECN is enabled. This does NOT contain the merge of the change to RACK since at this time that code does not exist in stable/11, and there is no plan to merge RACK to stable/11. PR: 243590 Reviewed by: rgrimes (mentor), rrs Approved by: rgrimes (mentor), tuexen (mentor) MFC after: 3 days Sponsored by: NetApp, Inc. Differential Revision: https://reviews.freebsd.org/D23364 Changes: _U stable/11/ stable/11/sys/netinet/tcp_input.c stable/11/sys/netinet/tcp_output.c rS361347 fixes this issue in HEAD, scheduled for MFC to stable/12 soonish, after rS355273 has been MFC'd A commit references this bug: Author: rscheff Date: Wed May 27 22:34:47 UTC 2020 New revision: 361565 URL: https://svnweb.freebsd.org/changeset/base/361565 Log: MFS r361436: MFC r361347: With RFC3168 ECN, CWR SHOULD only be sent with new data. Overly conservative data receivers may ignore the CWR flag on other packets, and keep ECE latched. This can result in continuous reduction of the congestion window, and very poor performance when ECN is enabled. This does NOT contain the merge of the change to RACK since at this time that code does not exist in stable/11, and there is no plan to merge RACK to stable/11. PR: 243590 Reviewed by: rgrimes (mentor), rrs Approved by: re(gjb) Sponsored by: NetApp, Inc. Differential Revision: https://reviews.freebsd.org/D23364 Changes: _U releng/11.4/ releng/11.4/sys/netinet/tcp_input.c releng/11.4/sys/netinet/tcp_output.c A commit references this bug: Author: rscheff Date: Wed Jun 24 16:17:59 UTC 2020 New revision: 362586 URL: https://svnweb.freebsd.org/changeset/base/362586 Log: MFC r361347: With RFC3168 ECN, CWR SHOULD only be sent with new data Overly conservative data receivers may ignore the CWR flag on other packets, and keep ECE latched. This can result in continous reduction of the congestion window, and very poor performance when ECN is enabled. PR: 243590 Reviewed by: rgrimes (mentor), rrs Approved by: rgrimes (mentor, blanket) MFC after: 3 weeks Sponsored by: NetApp, Inc. Differential Revision: https://reviews.freebsd.org/D23364 Changes: _U stable/12/ stable/12/sys/netinet/tcp_input.c stable/12/sys/netinet/tcp_output.c stable/12/sys/netinet/tcp_stacks/rack.c |
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), while 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. 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.