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.
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