On Fri, 30 Nov 2001, Scott Barron wrote:

> > NetBSD doesn't appear to be handling the TCP MSS option properly.
> > According to RFC 1122, the MSS sent by a host is the maximum IP packet
> > size its willing to receive minus 40.  This correlates to the maximum
> > tcp payload when no tcp or ip options are present.  However, NetBSD
> > appears to use it as the maximum tcp payload even when options are
> > present.
>
> I am pretty much a novice when it comes to the networking code but the
> way I understand it is the MSS options are exchanged when a connection
> is created.  However the option length may not be constant during the
> lifetime of the connection (a host that sends SACK blocks, something I'm
> currently working on, is a good example of this).  So I see only two
> things, set it to the size that is sent, which is what I think it
> currently does (looking at tcp_input.c in 1.5.2) or set it to the size
> advertised minus MAX_TCPOPTLEN and take and go for the "worst case" so
> to speak.  Personally, I'm not sure which is a better idea and I don't
> know how other systems handle it (I suspect the same way).  Maybe
> somebody more experienced can chime in here (also please chime in if
> I've gotten anything incorrect).

Right.  That is exactly why the RFCs define the MSS to be the maximum IP
packet size minus 40.  That way, the extra IP or TCP options count against
the segment size to keep the total packet size bounded at a constant
(MSS+40).

> > I haven't had a chance to look into the source for either of theese
> > problems, but I hope to have time this weekend if someone doesn't beat me
> > to it.
>
> The sent MSS is recorded in tcp_dooptions() in tcp_input.c (grep for
> TCPOPT_MAXSEG, my file is too hacked up to give you a valid line
> number), to give you a starting point.

Cool, thanks.  I'll take a look this weekend and see if I can come up with
a fix.  All that should be required is to subtract the length of any TCP
or IP options when desciding how much payload to include in a TCP
packet...

Thanks,
	Rick