ipid-debuggery (was Re: [e2e] ICMP & TCP segments with IP ID = 0?

Kastenholz, Frank FKastenholz at unispherenetworks.com
Wed May 16 14:32:14 PDT 2001

>Contrary to a recent statement, I doubt that the IP ID has been used
>to find more than real problems in routing, bridging and so forth than
>could be counted on the fingers of one hand.  Real packets almost
>always have plenty of serial numbers, time stamps, TTL's, and other
>stigmata at least as unique as the IP ID.

Actually, the Frag-ID field is incredibly useful in debugging
in that it's really the only place one can put numbers of one's
own choosing in the IP header that a router does not care about.
So, if one were, say, developing an ASIC to forward IP packets and
that hypothetical ASIC saw only the IP header, and one needed to 
follow some hypothetical packets through this hypothetical ASIC, then
one could insert a monotonically increasing number into the fragment-
ID and quite easily differentiate packet 1 from packet 2 from ...
Hypothetically speaking, of course ;-)

Of course, that's not "real networks", so we now return to our 
regularly scheduled pin-heads-dancing-on-angels discussion :-)

Frank Kastenholz

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