ipid-debuggery (was Re: [e2e] ICMP & TCP segments with IP ID = 0?
F.Potorti at cnuce.cnr.it
Fri May 18 01:33:47 PDT 2001
I am confused by your claim. Near the beginning of this thread,
someone quoted RFC 791 as saying exactly that incrementing the IP ID is
I've been too concise. Sorry about that.
Here is what rfc 791 says:
| The identification field is used to distinguish the fragments of one
| datagram from those of another.
This suggests that the identification field is not useful for datagrams
where DF is set.
| The originating protocol module of
| an internet datagram sets the identification field to a value that
| must be unique for that source-destination pair and protocol for the
| time the datagram will be active in the internet system.
Apparently we are talking about fragmentation.
Apart from that, I think that this statement has not a rigourous
meaning, because I think it is easy to get into a situation where it is
impossible to obtain a unique value, so it should be interpreted as a
guideline rather than a precription.
Also, it could be argued that, in order to increase probability of
uniqueness for fragmentable packets, DF packets should avoid consuming
sequence numbers in systems where a single counter is used at the IP
Anyway, I think that increasing when possible is a good thing. Only,
systems that don't are not violating any standard, in my opinion.
And because it increases the system's robustness against failures caused
by liberties (like turning off DF without resetting the ID)
that boxes in the middle of the network might take.
Francesco Potortì (researcher) Voice: +39 050 315 3058 (op.2111)
Area della ricerca CNR - CNUCE Fax: +39 050 313 8091
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