- Stefan
> -----Original Message-----
> From: Neal Cardwell [SMTP:cardwell@cs.washington.edu]
> Sent: Friday, May 01, 1998 3:26 PM
> To: syn@cs
> Subject: fun with MSS
>
>
> I finally got around to installing linux on my pc and i've been
> playing
> around to see what the deal is with those 1500 byte packets from the
> MCI
> paper.
>
> Looks like Linux and most everything else seems to send and accept the
> MSS
> (maximum TCP segment size) option on the SYN segment. Here's a random
> sample from browsing from a Linux box:
>
> server presumed OS MSS MSS value advertised
> ------------ ----------- --- --------------------
> www.cnn.com Solaris yes 1460
> www.yahoo.com Solaris? yes 1460
> www.netscape Solaris? yes 1460
> www.hotbot.com Solaris yes 1460
> www.redhat.com Linux yes 1460
> www.freebsd.org FreeBSD yes 1460
> www.ibm.com AIX yes 536
> www.research.ibm. AIX yes 512
> www.hp.com HP/UX yes 1460
> www.digital.com Digital Unix yes 536
> www.altavista. Digital Unix yes 536
> www.sun.com Solaris yes 1460
> www.apple.com MacOS yes 1460
> www.microsoft.com NT yes 4312
> www.msn.com NT yes 4312
> www.sgi.com IRIX yes 4312
>
> From this i'm guessing that AIX and Digital UNIX don't do MTU
> discovery,
> but that the rest of the OSes do. 'Cause they'd have to be willing to
> do
> MTU discovery (accept ICMP fragmentation complaints and adjust their
> MSS
> downward) if they're trying packets bigger than 576 bytes, right?
>
> One interesting thing is the 4312 byte MSS advertised by MS and SGI.
> Anyone know what has an MSS that big?
>
> Tcpdump also reveals that NT4.0, Win95b, and recent MacOS clients
> advertise MSS 1460 and accept 1460 byte packets from a server as well.
>
> Win95 going over PPP also advertises MSS=1460. My MacOS 7.5 machine
> going
> over PPP uses MSS=536 but doesn't speak the MSS option.
>
> *If* all this is correct then most of the recent clients and servers
> should be sending 1500 byte packets, so it's just the older TCP/IP
> stacks
> (which is apparently the vast majority of hosts right now) that are
> sending smaller packets. Does that seem right?
>
> neal
>
>