johnl@n3dmc.UUCP (John Limpert) (03/24/88)
>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 johnl@n3dmc.UUCP bellcore!wb6rqn!n3dmc!johnl