Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!batcomputer!itsgw!steinmetz!uunet!ingr!ken
From: ken@ingr.com (Ken Cox)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re-fragmenting IP Datagrams
Message-ID: <4015@ingr.com>
Date: 21 Feb 89 13:44:56 GMT
References: <4014@ingr.com>
Distribution: usa
Organization: Intergraph Corp. Huntsville, Al
Lines: 19

in article <4014@ingr.com>, sale@ed.uucp (Ed Sale) says:
> 
> I have just spent the large part of the day trying to determine if it is
> legal for an IP gateway to re-fragment an already fragmented datagram
> when it attempts to route the datagram from a network with a large MTU
> (Max Transmission Unit) to one with a smaller MTU.  I have perused all
> *current* RFC's containing the word "fragment" and Douglas Comer's book
> to no avail.
> 

RFC 791 addresses this problem, but you have to read carefully to
find it.  To summarize, you can fragment as long as the don't fragment
bit is not set.  The receiving end does not know or care how many times
the original datagram has been fragmented.

If you look at the fragmentation example in the RFC, it will clearly
outline the algorithm for fragmentation (or re-fragmentation.)

--Ken Cox