QoS Per-Hop Behavior (PHB) Tutorial

Quality of Service (QoS) is the set of tools a network uses to prioritize some traffic over other traffic. Rather than treating every packet equally, QoS lets a network device decide which packets deserve better treatment and act on that decision as the packet passes through. The concrete action a device takes on a packet — holding it, dropping it, or changing something in its header — is called a per-hop behavior, or PHB, because it is applied independently at each hop the packet crosses along a path of devices sharing a consistent QoS policy.

The DSCP Field

Per-hop behavior relies on a marking carried inside the packet itself: the Differentiated Services Code Point (DSCP), which occupies part of the old Type of Service field in the IP header. By writing a particular codepoint into this field, a device can tag a packet with a certain priority, and every subsequent device along the path can read that marking and decide how to treat the packet without having to re-examine it from scratch.

The Building Blocks of PHB

Implementing QoS is really a combination of several distinct mechanisms, each handling one part of the job.