[e2e] Re: Question on "identification" field of IP header

Felix Hernandez-Campos fhernand at cs.unc.edu
Fri Dec 13 12:42:08 PST 2002

Ramesh Shankar wrote:
> If the "Don't fragment bit" is set in the IP header, what purpose does 
> the "identification" field serve? Why can't I simply put 0 for this 
> field in such a case? I remember coming across some e-mail chain in one 
> of the mailing lists (TCP-IMPL, e2e, TSVWG) about this issue and the 
> interaction with NAT. But I am not sure what came out of that discussion.

You may want to have a look at Steve Bellovin's "A Technique for 
Counting NATed Hosts", presented at IMW 2002. The paper discusses how 
the IP header's ID field can be used to infer the number of hosts behind 
a NAT box.


Felix Hernandez-Campos

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