Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!PURDUE.EDU!narten
From: narten@PURDUE.EDU (Thomas Narten)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re-fragmenting IP Datagrams
Message-ID: <8902211357.AA01982@percival.cs.purdue.edu>
Date: 21 Feb 89 13:57:16 GMT
References: <4014@ingr.com>
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Organization: The Internet
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It is perfectly legal and sometimes necessary to further fragment
already fragmented datagrams.  One item to watch out for is being sure
that the MORE_FRAGMENTS bit is cleared only in the last fragment of
the original datagram.  That is, if a gateway fragments datagram A
into AF1 and AF2, AF1 will have the MORE_FRAGMENTS bit set, while AF2
will have it cleared.  If another gateway finds that it must fragment
AF1, all of AF1's fragments (including the last one) would have the
MORE_FRAGMENTS bit set.  Similar considerations apply to the
FRAGMENT_OFFSET field.

4.2 BSD got that case wrong the first time around, but it's been fixed
for a long time.

Thomas