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To: michael butler <imb@scgt.oz.au>
Cc: current@freebsd.org
Subject: Re: apparent tcp double-send ? 
In-Reply-To: Your message of "Thu, 02 Nov 1995 07:23:40 +1100."
             <199511012023.HAA19813@asstdc.scgt.oz.au> 
Date: Thu, 02 Nov 1995 10:10:44 +0100
Message-Id: <17470.815303444@curie.cs.utwente.nl>
From: Andras Olah <olah@cs.utwente.nl>
Sender: owner-current@freebsd.org
Precedence: bulk

On Thu, 02 Nov 1995 07:23:40 +1100, michael butler wrote:
> I noticed this whilst doing a little snooping ..
> 
> ppp1.asstdc.com.au.1091 > asstdc.scgt.oz.au.http: S 951910400:951910400(0) win 4096 <mss 536> (ttl 59, id 6771)
> ppp1.asstdc.com.au.1091 > asstdc.scgt.oz.au.http: S 951910400:951910400(0) win 4096 <mss 536> (ttl 58, id 6771)
> asstdc.scgt.oz.au.http > ppp1.asstdc.com.au.1091: S 4154539922:4154539922(0) ack 951910401 win 16384 <mss 1460> (ttl 64, id 12805)
> asstdc.scgt.oz.au.http > ppp1.asstdc.com.au.1091: S 4154539922:4154539922(0) ack 951910401 win 16384 <mss 1460> (ttl 63, id 12805)
[ ... ] 
> ppp1 is connected via another -current machine which uses the same address
> for ethernet and ppp but no proxy arp. Strange MSS .. anyway, ppp1 is using
> Trumpet 2.0b .. is this BSD, Trumpet or TcpDump ? Suggestions ?

The duplicates are caused by some routing anomaly.  Notice that the
TTL of the duplicate is always one less than that of the original. 

As for the MSS, they're OK.  Please notice that the behavior of
FreeBSD changed by 2.0.5R, because the original BSD treatment of MSS
was not correct.  The MSS you send should depend only on the
capabilities of your i/f, but not on the route.  It's the
responsibility of the other end, which recieves your MSS, to
calculate its optimum outgoing segment size based on your MSS option
and on the route's MTU.

Andras