Date: Tue, 17 Apr 2001 18:04:38 +0300 (EET DST) From: Andrei Gurtov <gurtov@cs.Helsinki.FI> To: <freebsd-net@freebsd.org> Cc: <Reiner.Ludwig@Ericsson.com> Subject: initial congestion window Message-ID: <Pine.LNX.4.30.0104171618460.5636-100000@saviletto.cs.Helsinki.FI>
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Hi folks, At the last IETF meeting there were some debates around FreeBSD using a 16-KB initial congestion window in TCP when destination IP address is from the local subnet. Does anybody remember when it was introduced into the code and what kind of ideas were behind? Some reasons were given why it may not be a good idea: -the benefit of not having slow start on LANs is very small, i.e. some milliseconds -it is not a conformant TCP feature, i.e. not allowed by TCP Congestion Control (RFC2581) and is explicitly given in Known TCP Implementation Problems (RFC2525) "2.1 No initial slow start" and "2.3 Uninitialized CWND" -people may have the same subnet mask also over a slow PPP link. In this case the effect of the huge initial window is quite bad, see for example http://www.cs.Helsinki.FI/u/gurtov/papers/effect_of_delays_on_tcp_performance.pdf -in case of congestion on Ethernet, packets queues build up at the network interfaces in hosts and agressive TCP start-up behaviour can further increase congestion losses What are your thoughts on this? Andrei tcp_output.c: int ss_fltsz = 1; SYSCTL_INT(_net_inet_tcp, OID_AUTO, slowstart_flightsize, CTLFLAG_RW, &ss_fltsz, 1, "Slow start flight size"); int ss_fltsz_local = TCP_MAXWIN; /* something large */ SYSCTL_INT(_net_inet_tcp, OID_AUTO, local_slowstart_flightsize, CTLFLAG_RW, &ss_fltsz_local, 1, "Slow start flight size for local networks"); [...] if ( in_localaddr(tp->t_inpcb->inp_faddr) ) tp->snd_cwnd = tp->t_maxseg * ss_fltsz_local; else tp->snd_cwnd = tp->t_maxseg * ss_fltsz; To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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