Before prioritizing traffic, you must identify it. Classification examines packets and assigns them to service classes.
Classification methods:
Port numbers. Port is HTTP. Port is SIP.
IP addresses. Traffic from specific servers gets special treatment.
Protocol. ICMP, TCP, UDP have different needs.
Deep packet inspection. Examine application layer for dynamic ports.
Existing markings. Trust DSCP or CoS already set.
Classify traffic close to the source. Edge devices know application context. Core routers should trust edge markings.