Type of Service

To give priority for traffic at Layer 3, type of service (ToS), or now known as the DS byte, needs to be manipulated. The ToS field is composed of 1 byte in the IP packet. IP precedence is the three most significant bits. As a result, a total of 8 different types of priorities are found in IP precedence. Types 6 and 7 are used by control traffic.

The Differentiated Services Code Point (DSCP) uses the first 6 bits of ToS byte rather than the original 3 as mentioned, which can provide up to 64 different types of priorities. DSCP is backward compatible to IP precedence as the mapping shows in Example 8-1.

Example 8-1. ToS to DSCP Map

IP Precedence 0 =0 DSCP

IP Precedence 1 =8 DSCP

IP Precedence 2 =16 DSCP

IP Precedence 3 =24 DSCP

IP Precedence 4 =32 DSCP

IP Precedence 5 =40 DSCP

IP Precedence 6 =48 DSCP

IP Precedence 7 =56 DSCP