Per-Hop Behavior

Posted by AJ × 08:55

* The Per-Hop Behavior (PHB) is indicated by encoding a 6-bit value—called the

Differentiated Services Code Point (DSCP)—into the 8-bit Differentiated Services

(DS) field of the IP packet header.

* The DS field is the same as the TOS field, and ECN occupies the upper 2 bits.

* In theory, a network could have up to 64 (i.e. 26) different traffic classes using

different markings in the DSCP.

* This gives a network operator great flexibility in defining traffic classes.

* In practice, however, most networks use the following commonly-defined Per-Hop Behaviors:

* Default PHB—which is typically best-effort traffic

* Expedited Forwarding (EF) PHB—dedicated to low-loss, low-latency traffic

* Assured Forwarding (AF) PHB— which gives assurance of delivery under conditions

* Class Selector PHBs—which are defined to maintain backward compatibility with the IP Precedence field.

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