> However, what counts is "can be altered" - that happens 
  > only if things
  > don't break when it is altered.   If packets get rejected 
  > because it is
  > being authenticated, and the alteration is detected - 
  > that's breaking,
  > so anyone who tried to fiddle such a field would find 
  > themselves with
  > no customers.   

=> Exactly.

  > 
  > The reason that people got away with altering the IPv4 TOS field was
  > simply that no-one cared - it was used for nothing in 
  > practice.   Had
  > there been applications that actually used it, or routing 
  > schemes that
  > depended upon it, it wouldn't have been able to be altered 
  > without the
  > s**t hitting the fan.

=> True !

  > 
  > We don't need cryptographic type protection to avoid that kind of
  > manipulation - we just need to actually care what the value is.
  > 

=> ok but if 'care' does not translate into an implementation
that tells you when something breaks then we care
but we'll be frustrated with 'stupid computers
that don't work' without knowing why !
I'm obviously trying to think of an average user
here, (that excludes anyone on any IETF mailing list :))
who might notice a warning that data was tampered
with instead of just getting frustrated with bad 
service without knowing why. 

Hesham




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