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Chapter 3
Basic QoS Concepts
E-NIT-CTC-20041213-0013 v0.5
17
Precedence values The table below gives the precedence values:
Second generation The Differentiated Service Code Point is a selector for router's per-hop behaviours.
The fields ECT and CE are spare bits in the IP header used by Explicit Congestion
Notification (RFC3168).
As can be seen, the DSCP field supersedes the old Precedence field. So the values of
DSCP provide limited backwards compatibility with Precedence.
This leads to notions of "class", each class being the group of DSCPs with the same
Precedence value. Values within a class would offer similar network services but
with slight differences (used to create different levels of service such as "gold",
"silver" and "bronze").
Precedence Purpose
0Routine
1Priority
2 Immediate
3Flash
4Flash Override
5CRITIC/ECP
6 Internetwork Control
7 Network Control
Note that IP Precedence is obsolete and is only implemented to provide
backwards compatibility.
01234567
Differentiated Service Code Point ECT CE