TCP window size issues

[Andre Oppermann]

Carl Mascott wrote:
> 
> Here's a case that your logic does not handle correctly.
> 
> 1. Kernel default buffer size = 32 KB
> 
> 2. Routing table buffer size = 48 KB
> 
> 3. Application sets buffer size to 32 KB
> 
> 4. tcp_mss() selects 48 KB buffer size, giving the routing
>    table precedence over the application.

Tough luck... can't have everything.

> I found it necessary to add a new flag with meaning
> "the application set my size" to struct sockbuf.

I think you are overdoing this thing a little bit. Sendpipe and
recvpipe can only be set by root with the route command. There
is nothing changing that automagically in the kernel.

Actually in my tcp_hostcache patch we still carry sendpipe and
recvpipe even though there ain't no way to change it anymore
(route has no longer access to it). It comes handy when we get
auto-sizing socket buffers... (hint hint!)

I don't think there are many people setting the pipesize at all.

> I'll e-mail you a copy of my document.  Note that I have
> backed out the patch from PR 11966.  You may not want to
> do this.  It depends upon the correct answer to Issue #3
> (shrinking the receive window) in my original posting.

Got it. Still have to read it.

> NOTE: AFAIK, 4.4BSD through FreeBSD 4.6-R seem to have done
> alright without the PR 11966 patch, but if someone knows
> different, please speak up.

-- 
Andre


> Andre Oppermann wrote:
> >
> > The patch in PR11966 has already been applied. It does not fix it.
> > I've fixed it by redoing this check in tcp_input.c::tcp_mss():
> >
> >  if ((bufsize = rt->rt_rmx.rmx_sendpipe) == 0)
> >       bufsize = so->sn_snd.sb_hiwat;
> >
> > to:
> >
> >  if ((so->so_snd.sb_hiwat == tcp_sendspace) && rt->rt_rmx.rmx_sendpipe)
> >       bufsize = rt->rt_rmx.rmx_sendpipe;
> >  else
> >       bufsize = so->so_snd.sb_hiwat;
> >
> >
> > The theory goes that with default buffers so_snd.sb_hiwat and
> > tcp_sendspace have the same size. If that is actually the case,
> > we use whatever we've got from the metrics. If not, the user
> > did some changes and we take that and ignore the metrics value.
> > This is not yet tested in reality... just theoretical ;-)
> >
> > Anyway, I'm interested in your solutions as well.
> >
> > Carl Mascott wrote:
> > >
> > > Actually I have just fixed it in my copy of 4.8-R.  I have a document
> > > that describes the problem and my solution.  I could send you that
> > > and/or a set of patches.  You might want to sketch out your own
> > > solution before you look at mine, though.  Also, I'm not done
> > > testing mine yet.
> > > > Carl Mascott wrote:
> > > > >
> > > > > 2. The 4.4BSD TCP implementation has never had the correct precedence
> > > > >    rules for setting window sizes.  App-set values should take
> > > > >    precedence over all others.  Routing table values should take
> > > > >    precedence over kernel default values.  The fix for PR 11966
> > > > >    alters the behavior but still doesn't implement these precedence
> > > > >    rules.
> > > >
> > > > I'll fix this in my tcp_hostcache patch currently under review by
> > > > Sam Leffler and Ruslan Ermilov (however that is for -CURRENT).