In message <878yp1wm1h.fsf@snark.piermont.com>, "Perry E. Metzger" writes:
>
>David Laight <david@l8s.co.uk> writes:
>> - sequences where values are guaranteed not to be reproduced
>> - random values
>
>One way to do this is encrypting a counter with a 32 bit block cipher,
>but until a few minutes ago I was unaware of any. (Now I've learned of
>one on the cryptography mailing list.)

Right, but some uses for such things have stronger non-repitition 
requirements.  For example, the TCP initial sequence number shouldn't 
repeat for 2*maximum segment lifetime.  The IPid field shouldn't repeat 
for somewhat longer than the fragment lifetime on the receiving system.


		--Steve Bellovin, http://www.research.att.com/~smb