Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 alpha 4/15/85; site ucbvax.ARPA
Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!tcp-ip
From: tcp-ip@ucbvax.ARPA
Newsgroups: fa.tcp-ip
Subject: Re:  MILNET/ARPANET performance
Message-ID: <7594@ucbvax.ARPA>
Date: Thu, 30-May-85 01:31:34 EDT
Article-I.D.: ucbvax.7594
Posted: Thu May 30 01:31:34 1985
Date-Received: Thu, 30-May-85 20:45:20 EDT
Sender: daemon@ucbvax.ARPA
Organization: University of California at Berkeley
Lines: 41

From: Chris Torek <chris@gyre>

I might take this opportunity to note that many 4.2BSD sites are
retransmitting packets once every second, no matter what the actual
round trip ack time is; this doesn't help gateway load at all.

There is a bit of code in /sys/netinet/tcp_output.c that looks like this:

		if (SEQ_GT(tp->snd_nxt, tp->snd_max))
			tp->snd_max = tp->snd_nxt;
		
		/*
		 * Time this transmission if not a retransmittion and not
		 * currently timing anything.
		 */
		if (SEQ_GT(tp->snd_nxt, tp->snd_max) && tp->t_rtt == 0) {
			tp->t_rtt = 1;
			tp->t_rtseq = tp->snd_nxt - len;
		}

The second SEQ_GT is guaranteed to fail, thus nothing is ever timed; and
the retransmits happen at the maximum rate (1/second).

The code should be changed to:

		if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
			tp->snd_max = tp->snd_nxt;
			/*
			 * Time this transmission (it's not a retransmission)
			 * unless we're already timing something.
			 */
			 if (tp->t_rtt == 0) {
				tp->t_rtt = 1;
				tp->t_rtseq = tp->snd_nxt - len;
			}
		}

(Note, Berkeley has fixed this.)  I hope most 4.2 arpa sites are reading
this. . . .

Chris