Date: Mon, 16 Nov 2009 07:15:40 GMT From: Richard Scheffenegger <rs@netapp.com> To: freebsd-gnats-submit@FreeBSD.org Subject: kern/140597: Lost Retransmission Detection Message-ID: <200911160715.nAG7FeZ9031471@www.freebsd.org> Resent-Message-ID: <200911160720.nAG7K1bV086516@freefall.freebsd.org>
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>Number: 140597 >Category: kern >Synopsis: Lost Retransmission Detection >Confidential: no >Severity: serious >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Mon Nov 16 07:20:00 UTC 2009 >Closed-Date: >Last-Modified: >Originator: Richard Scheffenegger >Release: 8.0-RC2 >Organization: NetApp >Environment: FreeBSD rsFreeBSD.vie.demo 8.0-RC2 FreeBSD 8.0-RC2 #16: Sat Nov 14 22:25:28 CET 2009 root@rsFreeBSD.vie.demo:/usr/obj/usr/src/sys/GENERIC i386 >Description: SACK TCP in FreeBSD does not do Lost Retransmission Detection. Unlike linux, that functionality is not implemented, leading to a RTO when a single segment is lost twice. This reduces Goodput considerably during phases of higher congestion. See http://ubinet.yonsei.ac.kr/v2/publication/hpmn_papaers/ij/29.pdf And http://projects.itri.aist.go.jp/gnet/pfldnet08kodama.ppt >How-To-Repeat: Drop the same segment of a TCP Stram twice, when cwnd is open enough (3+ segments). >Fix: Simple fix is rather easy (doesn't work perfectly under all circumstances, but doesn't need new state variable and runs in constant time): When sending the last segment in a sackhole, store snd.max in rxmit. During SACK Ack processing, check the value of rxmit from the lowest hole against snd.fack; if snd.fack <= rxmit of that hole, set rxmit to start, and sackhint.nexthole == sackhole[0]. Care must be taken during hole-splitting (if rxmit > end of lower hole, set rxmit to snd.max), and all checks of sackhole[].rxmit need to deal with the signal rxmit > end. See http://www.ietf.org/mail-archive/web/tcpm/current/msg05003.html ff. >Release-Note: >Audit-Trail: >Unformatted:
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