Re: precedence


John Limpert (rochester!cci632!rlgvax!dolqci!irs3!wb6rqn!n3dmc!johnl@rutgers.edu)
24 Mar 88 08:26:39 EST (Thu)


>I note in the RFC's that TCP and IP have three bits reserved
>for precedence, but UDP does not. I have a number of questions
>about this. The obvious place to start: is precedence really implemented?

I don't know about the arpanet, but many other DOD voice/data networks
implement precedence.

>I can see many obvious fairness problems. One user could
>effectively shut out other users until their TCP connections
>start timing out. If they are allowed to, every one will
>want to send high precedence packets. How are these problems
>resolved?

Precedence is supposed to be unfair :-). The idea is to reduce delays
for high priority traffic, seizing resources from lower priority traffic
if necessary. You need to have prior authorization for the use of the
higher precedence levels. You shouldn't be able to use precedence
levels higher than ROUTINE without authority and need. The requirement
for authorization can be enforced in a switch. The switch can check a
table of maximum user precedence levels. Just because you mark something
with 'FLASH OVERRIDE' doesn't mean the switch will believe you. In a circuit
switched environment, the switch will preempt (drop) lower priority
connections if it runs out of resources. I would expect packet switches
to sort queues by precedence.

By the way, this is one use of the 4 extra/missing DTMF (touch tone TM)
buttons on your telephone. Some phones have 16 buttons, with 4 buttons
assigned to precedence.

John Limpert John Limpert johnl@n3dmc.UUCP
                bellcore!wb6rqn!n3dmc!johnl



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