Date: Fri, 24 Jun 2005 14:07:26 +0900 (JST) From: Noritoshi Demizu <demizu@dd.iij4u.or.jp> To: freebsd-net@freebsd.org Subject: A burst from NewReno when a partial ACK is received Message-ID: <20050624.140726.43393126.Noritoshi@Demizu.ORG>
next in thread | raw e-mail | index | archive | help
I'm using FreeBSD current for my experiences. I observed bursts sent by NewReno when a partial ACK is received. I have two packet traces of such bursts. One of such bursts is analyzed at http://www.demizu.org/~noritosi/memo/2005/0623/ . I think tcp_newreno_partial_ack() in tcp_input.c rev 1.275 has a bug in calculating a new value of snd_cwnd at the tail of the function. Currently, snd_cwnd is re-calculated as following: L.3113: tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg); Since snd_cwnd is u_long, if snd_cwnd < SEG.ACK - SND.UNA - MSS, snd_cwnd becomes awfully huge and a burst of data can be sent. To fix this problem, I'd like to suggest the patch below. Thanks. Regards, Noritoshi Demizu Index: tcp_input.c =================================================================== RCS file: /home/cvsup/FreeBSD/ncvs/src/sys/netinet/tcp_input.c,v retrieving revision 1.275 diff -u -r1.275 tcp_input.c --- tcp_input.c 1 Jun 2005 12:03:18 -0000 1.275 +++ tcp_input.c 23 Jun 2005 06:20:28 -0000 @@ -3110,7 +3112,11 @@ * Partial window deflation. Relies on fact that tp->snd_una * not updated yet. */ - tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg); + if (tp->snd_cwnd > th->th_ack - tp->snd_una) + tp->snd_cwnd -= th->th_ack - tp->snd_una; + else + tp->snd_cwnd = 0; + tp->snd_cwnd += tp->t_maxseg; } /*
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20050624.140726.43393126.Noritoshi>