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From: mogul@wrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: problem with ftp
Message-ID: <1991Feb13.015310.27301@pa.dec.com>
Date: 13 Feb 91 01:53:10 GMT
References: <9102111926.AA09697@jessica.stanford.edu> <19910211212243.8.BARMAR@OCCAM.THINK.COM>
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In article <19910211212243.8.BARMAR@OCCAM.THINK.COM> barmar@THINK.COM (Barry Margolin) writes:
>    From: "Philip Almquist" <almquist@jessica.stanford.edu>
>
>    IP networks are required to support 68 octet packets without network
>    layer (IP) fragmentation (RFC 791 page 25).  The Internet folklore that
>    says that this number is 576 is erroneous.
>
>Thanks for the correction.  However, it's reasonably safe to assume that
>most media have an MTU of at least 512.  I can't think of any that
>don't, and I would be surprised if anyone invented a new medium with a
>smaller MTU (the current direction is generally towards larger packet
>sizes).

RFC1191 ("Path MTU Discovery Protocol") lists all the MTUs I could
track down when I was writing it.  Several networks have MTU == 508,
and in RFC1144, Van Jacobson points out that on low-speed links
(like telephone lines) the use of too large an MTU may increase the
delays encountered to the point where they become unpleasant.  To quote:

   From the discussion in sec. 2, it seems desirable to limit the maximum
   packet size (MTU) on any line where there might be interactive traffic
   and multiple active connections (to maintain good interactive response
   between the different connections competing for the line).  The obvious
   question is `how much does this hurt throughput?'  It doesn't.

However, when Chris Kent invented the old "use 576 if non-local" rule,
he was attempting to avoid fragmentation-induced communication failure
without unduly reducing throughput.  576 is a compromise; it is essentially
an arbitrary number that was chosen because it seemed less arbitrary than
any other choice.  Until you can rely on PMTU Discovery, the 576-rule
works pretty well, but it does not guarantee non-fragmentation.

-Jeff