Per hop behaviour

In computer networking, per-hop behaviour (PHB) is a term used in differentiated services (DiffServ) or multiprotocol label switching (MPLS). It defines the policy and priority applied to a packet when traversing a hop (such as a router) in a DiffServ network.

Rule that governs how packets are handled within a diffserv network is called the Per-Hop Behavior (PHB). PHBs are defined to support the general properties controlled by IP precedence. DSCP Contains 6-bits, PHBs are created (one for each combination of the top 3 bits) of the form bbb000 to match the precedence behaviors and leaves the other DSCP values open where each b may take the value zero or 1.