Draft email to freebsd-net

[James Healy]

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We've recently been doing some TCP congestion control research, and have
 written a small logging module for 6.2 that outputs the cwnd of a tcp
flow to a log file. The logging module is called SIFTR and is available
on our website: http://caia.swin.edu.au/urp/newtcp/tools.html.

In analysing the output, we noticed that despite the rfc3390 sysctl
variable being enabled, there were times when flows started with a cwnd
equal to 1 MSS instead of 4380 as suggested by rfc3390.

We realised the cwnd was constrained to 1 MSS when it's starting value
was influenced by the hostcache, so we created the attached patch to
view relevant variable values in the tcp_mss function in tcp_input.c.

The following are the logged values for 2 sequential ssh sessions to the
same host. The first time cwnd is set according to rfc3390, and the
second time based on the hostcache.
- -------
one:
Jul 16 13:31:20 jhealy kernel: setting snd_cwnd according to rfc3390
Jul 16 13:31:20 jhealy kernel: tp->snd_cwnd now set to: 4380

two:
Jul 16 13:31:31 jhealy kernel: setting snd_cwnd from hostcache
Jul 16 13:31:31 jhealy kernel: tp->snd_cwnd: 1073725440
Jul 16 13:31:31 jhealy kernel: mss: 1448
Jul 16 13:31:31 jhealy kernel: metrics.rmx_cwnd: 14480
Jul 16 13:31:31 jhealy kernel: tp->snd_wnd: 0
Jul 16 13:31:31 jhealy kernel: so->so_snd.sb_hiwat: 33304
Jul 16 13:31:31 jhealy kernel: tp->snd_cwnd now set to: 1448
- ------
The formula used to calculate the cwnd when a relevant entry exists in
the hostcache is on line 3054 of tcp_input.c:

tp->snd_cwnd = max(mss, min(metrics.rmx_cwnd / 2, min(tp->snd_wnd,
               so->so_snd.sb_hiwat)));

Using the values that we logged earlier, this breaks down as:

= max(1448, min(7240, min(0, 33304)))
= max(1448, min(7240, 0))
= max(1448, 0)
= 1448

Given that the snd_wnd value during the connection initiation seems to
always be 0, the cwnd is always going to be set to 1 MSS.

This behaviour seems a little odd to us - can anyone shed some light on
it? Our assumption is that the use of the hostcache is designed to
increase performance where appropriate by seeding the initial cwnd based
on past experience.

For this section of code to return a cwnd that is successfully
influenced by the hostcache, it would seem that the use of tp->snd_wnd
should be avoided when the connection is still being initialised:

tp->snd_cwnd = max(mss, min(metrics.rmx_cwnd / 2, so->so_snd.sb_hiwat));

James Healy & Lawrence Stewart
Centre for Advanced Internet Architectures
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