A small area of the IP header which referes to what kind of priorities this packet should be given. In IPv4 you can say that this packet should:
- minimise delay: For ssh sessions for example
- maximise throughput: for ftp sessions for example (eg: an ftp over a highspeed satellite link would be great, but a ssh session would be horrible!)
- maximise reliability: for packets that should get there!
- minimise $cost$: for things like IRC and UseNet :)