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

[Greg Minshall]

> Why are they evil? ipid is only useful for defragmentation, and perhaps 
> to recover from bugs in IP checksum ...

i think this is one of those "be conservative in what you send" things.  the 
conservative thing to do, i would argue, is to obey the principle of least 
surprise and send unique IDs; this is what systems expect, orthogonal from DF 
or not DF.  (of course, the rest of the world should obey the second half: "be 
liberal in what you accept", and so deal with ID == 0.)

not everything is (or can be) written down.  but, as i found out to my 
incredible embarrassment with the telnet client in 4.3BSD [it *still* turns my 
face red when i think of it!], if you find yourself going out of your way to 
do something "out the ordinary" (out of the expected), it may well be the 
"wrong" thing to do.