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From: forster@cisco.com (Jim Forster)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: minimum maximum transfer unit.
Message-ID: <34849@boulder.Colorado.EDU>
Date: 8 May 91 00:54:18 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 34

>> Some (many?) IP implementations compare the source and destination IP
>> addresses and use the interface MTU for "local" destinations (source and
>> destination addresses have matching network parts), and 576 for non-local
>> destinations because that is the smallest "required" MTU (see RFC 1122,
>> "Requirements for Internet Hosts -- Communication Layers", page 56).
>> This way, in theory, you "know" that no gateway along the way will have
>> to fragment your datagrams.  Do ciscos do a similar thing?

What've you've described used to be a semi-reasonable approach for a host
to take when deciding what size IP datagrams to send. A router don't have
any choice; it takes what the hosts send, and fragments if it must.

Note that in theory even packets destined for a different subnet should
be subject to the 576 rule, but in practice these days, serial lines
support the same size MTU as Ethernets.

Everything is different and harder when you move from just Ethernets & Serial
lines to Token Rings & FDDI.  Both TR's and FDDI have substantially larger
MTU's, and hosts would justifiably like to take advantage of this if
possible, but not cause fragmentation if an Ethernet must be used.  Thus
the relevance of:

	RFC-1191  Mogul, J.C.; Deering, S.E.  Path MTU discovery.  
	1990 November; 19 p.  (Format: TXT=47936 bytes)  (Obsoletes RFC 1063)

which we will support in 8.3.



  -- Jim


PS: I vaguely recall some BSD Unix flag "subnetsarelocal" which may
be relevant.  Does some know about this?