By backoff if you are referring to sender side mistakenly counting the window updates for a "fastretransmit" on a count of 3 dup acks, isn't the following in tcp_input.c taking care of it ? [Only when twin == tp->snd_wnd it is counted as a dup ack.] 1836 if (SEQ_LEQ(th->th_ack, tp->snd_una)) { 1837 if (tlen == 0 && tiwin == tp->snd_wnd) { 1838 V_tcpstat.tcps_rcvdupack++; 1839 /* 1840 * If we have outstanding data (other than 1841 * a window probe), this is a completely 1842 * duplicate ack (ie, window info didn't 1843 * change), the ack is the biggest we've 1844 * seen and we've seen exactly our rexmt 1845 * threshhold of them, assume a packet 1846 * has been dropped and retransmit it. 1847 * Kludge snd_nxt & the congestion 1848 * window so we send only this one 1849 * packet. 1850 * How many acks do you see for every 2 full-sized segments received ? 2 or 1 ? At least one ack needs to go out for every 2 incoming segments as per the rfc. Associating window updates with delayed ack was not clear to me. If I see the tcpdump output I might understand better.
Venkat ________________________________ From: Rui Paulo <[EMAIL PROTECTED]> To: David Malone <[EMAIL PROTECTED]> Cc: freebsd-net@freebsd.org; Kevin Oberman <[EMAIL PROTECTED]> Sent: Friday, November 28, 2008 9:36:45 AM Subject: Re: FreeBSD Window updates On 27 Nov 2008, at 14:03, David Malone wrote: > I was looking at some tcpdumps from a FreeBSD box receiving a TCP > stream with someone yesterday and noticed that it seemed to be > generating quite a lot of dupliacte acks. Looking more carefully, > we noticed that the duplicates were actually window updates. The > code for sending window updates can be found in: > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/netinet/tcp_output.c?annot > ate=1.157 > > around lines 541-565. It seems that we generate a window update if > the available buffer space changes by more than two packets, or if > it changes by more than half the buffer size. This is probably a > little aggressive, and in some cases seems to result in bursts of > window updates that look like they should be batched. I wonder if > it would make sense to not do a window update if a delayed ack is > scheduled? It might even be possible to do them as a delayed ack > without causing problems. Yes, this makes sense. Probably this is a bug since 4.4BSD-Lite. > I note that there is at least one PR mentioning we generate many > window updates: > > http://www.freebsd.org/cgi/query-pr.cgi?pr=116335 > > I also wonder if it might cause non-FreeBSD senders to back off, > we are careful not to retransmit if we get window updates (see > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/netinet/tcp_input.c?annota > te=1.392 > > about line 1836, but other OSes might not be and that might degrade > our performance when receiving from a non-FreeBSD sender. So, from what I understand, we do back off and that implies we are losing performance in the FreeBSD to FreeBSD case, right? -- Rui Paulo _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "[EMAIL PROTECTED]" _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "[EMAIL PROTECTED]"