HP 7102dl - ProCurve Secure Router Configuration Manual page 358

Setting Up Quality of Service
Overview
8-8
The DSCP marks packets for a specific per-hop behavior (PHB). PHBs
describe forwarding behavior. That is, standards for PHBs determine such
issues as which packets should be forwarded first and which packets should
be dropped during network congestion. DiffServ defines four types of PHBs:
Default PHB—The Default PHB is for traffic with DSCP 0 (not set) or any
undefined DSCP. If a packet is configured for the Default PHB, the router
uses best-effort service to process and forward that packet.
Class-Selector PHB—The Class-Selector PHBs provide backward com-
patibility with IP precedence. In these PHBs, the last three bits in the
DSCP are always set to zero, so only the first three bits (those that match
the IP precedence bits) are significant for differentiating the eight classes.
Network devices must grant each class the type of service given to the
corresponding IP precedence value. Table 8-1 shows the DSCP for the
Class-Selector PHB.
Table 8-1. Class-selector PHBs
DiffServ Value DSCP
0 000000
8 001000
16 010000
24 011000
32 100000
40 101000
48 110000
56 111000
Assured Forwarding PHB—The Assured Forwarding PHBs allow you to
create four traffic classes (AF1, AF2, AF3, and AF4) and assign different
forwarding priorities to each. Values in the first three bits of the DSCP
determine the AF class. Several DSCPs match each of the AF PHBs. Within
each set of AF PHBs, different DSCPs can define different:
• buffer space
• bandwidth
• drop precedence
Table 8-2 shows the DSCP for Assured Forwarding PHBs.
First 3 bits IP Precedence
000 0
001 1
010 2
011 3
100 4
101 5
110 6
111 7