Sign in Subscribe

PHB DiffServ Per Hop Behavior

Last updated on 

Per Hop Behavior (PHB) is a concept used in Quality of Service (QoS) implementations within computer networks. It refers to the behavior exhibited by network devices, specifically routers, when forwarding packets based on the Differentiated Services (DiffServ) architecture. DiffServ aims to provide traffic differentiation and prioritization in network environments, enabling more efficient and effective handling of diverse types of network traffic.

PHB is an essential component of the DiffServ model, defining the specific treatment that packets receive at each hop (router) along their path through the network. The DiffServ architecture divides network traffic into different classes or service levels based on their relative importance or requirements. Each class is associated with a specific PHB, which determines how packets of that class are treated and forwarded by routers.

The DiffServ model operates on the principle of marking packets with specific Differentiated Services Code Points (DSCPs) that indicate their class or priority level. These DSCPs are embedded in the IP header of the packet and are used by routers to classify and process packets accordingly. Routers then apply the appropriate PHB based on the DSCP value to determine the forwarding treatment for each packet.

There are several standardized PHBs defined by the Internet Engineering Task Force (IETF) that network administrators can implement in their networks to achieve different QoS objectives. These PHBs define specific behaviors for how packets are handled, including forwarding, dropping, scheduling, queuing, and marking.

The PHBs specified in the DiffServ architecture include:

  1. Default PHB (DSCP 000000): This PHB applies to packets that are not explicitly classified with a DSCP. Routers typically forward these packets in a best-effort manner, with no special treatment or guarantees.
  2. Expedited Forwarding (EF) PHB (DSCP 101110): This PHB provides a low-latency, low-jitter, and assured bandwidth service for real-time traffic, such as voice or video. Routers that support the EF PHB prioritize these packets and provide dedicated resources to ensure their timely delivery.
  3. Assured Forwarding (AF) PHB Group: The AF PHB group consists of four sub-classes, each with a different level of priority and forwarding treatment. These sub-classes are AF1 (DSCP 001010), AF2 (DSCP 010010), AF3 (DSCP 011010), and AF4 (DSCP 100010). The AF PHB group offers different levels of service guarantees and bandwidth allocations, allowing network administrators to prioritize and differentiate between various classes of traffic.
  4. Class Selector (CS) PHB Group: The CS PHB group provides backward compatibility with the older IP Precedence model. It consists of eight classes, each defined by a specific DSCP value ranging from CS0 (DSCP 000000) to CS7 (DSCP 111000). Each class can be mapped to a particular PHB, allowing for the differentiation of traffic based on its legacy IP Precedence value.

In addition to these standardized PHBs, network administrators can also define and implement their own custom PHBs to meet specific QoS requirements within their networks. Custom PHBs enable further flexibility and customization in tailoring the treatment of packets based on the unique needs of the network environment.

The implementation of PHBs in routers involves configuring various QoS mechanisms such as traffic classification, marking, policing, queuing, and scheduling. These mechanisms work in conjunction with the PHBs to ensure the proper treatment and prioritization of packets. For example, routers can use traffic classifiers to identify packets based on their DSCP values and then apply the corresponding PHB to determine the forwarding behavior.

To summarize, PHB is a fundamental concept in the DiffServ architecture that defines the behavior exhibited by routers when forwarding packets based on their assigned DSCP values. By implementing specific PHBs, network administrators can prioritize and differentiate traffic based on its importance, enabling efficient resource allocation and improved QoS in computer networks.