What is the class selector PHB used for in the differentiated services model?
A.
Best-effort service
B.
Low-delay service
C.
Bandwidth guarantee
D.
Backward compatibility
Explanation:
With the introduction of the DSCP markings, there were significantly more possible markings for packets (0-63 are the possible markings for packets). Because there were so many more possible markings, the IETF decided to standardize what some of the codepoints meant. In part, this is to provide backward compatibility to IP precedence and, in part, this is to facilitate certain types of behaviors that were seen as fundamental to the DiffServ architecture.
The following definition of a per-hop behavior is taken from Section 2.4 of RFC 2475:
A per-hop behavior (PHB) is a description of the externally observable forwarding behavior of a DS node applied to a particular DS behavior aggregate … In general, the observable behavior of a PHB may depend on certain constraints on the traffic characteristics of the associated behavior aggregate, or the characteristics of other behavior aggregates.
RFC 2597: The Assured Forwarding PHB
Other than those defined in RFC 2474, there are two main PHBs, RFC 2597 defines the first of these. It is called the assured forwarding (AF) PHB, and the concept behind the PHB is to provide a level of assurance as to a given packet’s probability of being forwarded during congestion.
RFC 2597 defines four classes, and each class is completely independent of the other classes. In addition, each class has three level of “drop precedence” to which packets of that class can be assigned.