[e2e] ICMP & TCP segments with IP ID = 0?

Craig Partridge craig at aland.bbn.com
Wed May 16 09:24:11 PDT 2001

In message <20010516111231.A16714 at host-186.dyn.dev.zks.net>, Jerome Etienne wri

>what is so evil in setting the IPv4 id to 0, assuming the DF bit is set ?

I said it was "evil" because it is not in compliance with the specifications
(namely RFC791 and 1122).

Interestingly enough, while RFC 1122 certainly assumes that a new identifier is
normally required for each datagram, it doesn't say so.

But RFC 791 is quite clear:

    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.

So while 0 is a valid value, you can't repeat it for the lifetime of
a datagram or so (RFC 1122 permits reuse in the narrow case of retransmitting
exactly the same data).


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