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Date:      Sat, 18 Feb 2017 11:33:58 +1100
From:      Lawrence Stewart <lstewart@freebsd.org>
To:        Steven Hartland <steven.hartland@multiplay.co.uk>, freebsd-transport@freebsd.org
Subject:   Re: TCP Receive buffer scaling without Timestamps
Message-ID:  <be22c783-26d8-321d-d7b1-1edeb27d11e8@freebsd.org>
In-Reply-To: <b9906198-a026-1403-73e6-279066b4e197@multiplay.co.uk>
References:  <b9906198-a026-1403-73e6-279066b4e197@multiplay.co.uk>
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On 18/02/2017 09:37, Steven Hartland wrote:
[snip]
> 
> So the questions:
> 
> 1. Has anyone looked at this issue before or is working on it?
> 2. Do people think we could add support for RTT estimates and enable
>    receive scaling without major tcp stack changes?

We already have a non-TS-enabled RTT estimator in place c.f. the
relevant code in tcp_input.c:

if ((to.to_flags & TOF_TS) != 0 &&
    to.to_tsecr) {
	uint32_t t;

	t = tcp_ts_getticks() - to.to_tsecr;
	if (!tp->t_rttlow || tp->t_rttlow > t)
		tp->t_rttlow = t;
	tcp_xmit_timer(tp,
	    TCP_TS_TO_TICKS(t) + 1);
} else if (tp->t_rtttime &&
    SEQ_GT(th->th_ack, tp->t_rtseq)) {
	if (!tp->t_rttlow ||
	    tp->t_rttlow > ticks - tp->t_rtttime)
		tp->t_rttlow = ticks - tp->t_rtttime;
	tcp_xmit_timer(tp,
			ticks - tp->t_rtttime);
}


The autoscaling implementation just needs to have its insistence on
using timestamps stuffed into a cannon and shot in the direction of the
sun. This is the relevant block of rcvbuf code from tcp_input.c:


if (V_tcp_do_autorcvbuf &&
    (to.to_flags & TOF_TS) &&
    to.to_tsecr &&
    (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
	if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
	    to.to_tsecr - tp->rfbuf_ts < hz) {
		if (tp->rfbuf_cnt >
		    (so->so_rcv.sb_hiwat / 8 * 7) &&
		    so->so_rcv.sb_hiwat <
		    V_tcp_autorcvbuf_max) {
			newsize =
			    min(so->so_rcv.sb_hiwat +
			    V_tcp_autorcvbuf_inc,
			    V_tcp_autorcvbuf_max);
		}
		/* Start over with next RTT. */
		tp->rfbuf_ts = 0;
		tp->rfbuf_cnt = 0;
	} else
		tp->rfbuf_cnt += tlen;  /* add up */
}

It's pretty trivial to fix by anyone so inclined.

Cheers,
Lawrence

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