Which of these is a valid differentiated services PHB?

A.
Guaranteed PHB

B.
Class-Selector PHB

C.
Reserved Forwarding PHB

D.
Discard Eligible PHB

E.
Priority PHB

Explanation:
Differentiated Services Definition
Differentiated Services is a multiple service model that can satisfy differing QoS requirements.
With Differentiated Services, the network tries to deliver a particular kind of service based on the
QoS specified by each packet. This specification can occur in different ways, for example, using
the 6-bit differentiated services code point (DSCP) setting in IP packets or source and destination
addresses. The network uses the QoS specification to classify, mark, shape, and police traffic and
to perform intelligent queuing.
Differentiated Services is used for several mission-critical applications and for providing end-toend QoS.
Typically, Differentiated Services is appropriate for aggregate flows because it performs a
relatively coarse level of traffic classification.
DS Field Definition
A replacement header field, called the DS field, is defined by Differentiated Services. The DS field
supersedes the existing definitions of the IP version 4 (IPv4) type of service (ToS) octet (RFC 791)
and the IPv6 traffic class octet. Six bits of the DS field are used as the DSCP to select the Per-Hop
Behavior (PHB) at each interface. A currently unused 2-bit (CU) field is reserved for explicit
congestion notification (ECN). The value of the CU bits is ignored by DS-compliant interfaces
when determining the PHB to apply to a received packet.
Per-Hop Behaviors
RFC 2475 defines PHB as the externally observable forwarding behavior applied at a DiffServcompliant node to a DiffServ Behavior Aggregate (BA).
With the ability of the system to mark packets according to DSCP setting, collections of packets
with the same DSCP setting that are sent in a particular direction can be grouped into a BA.
Packets from multiple sources or applications can belong to the same BA.
In other words, a PHB refers to the packet scheduling, queueing, policing, or shaping behavior of a
node on any given packet belonging to a BA, as configured by a service level agreement (SLA) or
a policy map.
The following sections describe the four available standard PHBs:
Default PHB
Class-Selector PHB (as defined in RFC 2474)
Assured Forwarding PHB (as defined in RFC 2597)
Expedited Forwarding PHB (as defined in RFC 2598)

For more information about default PHB, see RFC 2474, Definition of the Differentiated Services
Field (DS
Field) in the IPv4 and IPv6 Headers.
Class-Selector PHB
To preserve backward-compatibility with any IP precedence scheme currently in use on the
network, DiffServ has defined a DSCP value in the form xxx000, where x is either 0 or 1. These
DSCP values are called Class-Selector Code Points. (The DSCP value for a packet with default
PHB 000000 is also called the Class-Selector Code Point.)
The PHB associated with a Class-Selector Code Point is a Class-Selector PHB. These ClassSelector PHBs retain most of the forwarding behavior as nodes that implement IP Precedencebased classification and forwarding.
For example, packets with a DSCP value of 11000 (the equivalent of the IP Precedence-based
value of 110) have preferential forwarding treatment (for scheduling, queueing, and so on), as
compared to packets with a DSCP value of 100000 (the equivalent of the IP Precedence-based
value of 100). These Class-Selector PHBs ensure that DS-compliant nodes can coexist with IP
Precedence-based nodes.
For more information about Class-Selector PHB, see RFC 2474, Definition of the Differentiated
Services Field (DS Field) in the IPv4 and IPv6 Headers.

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