2.4.1 The DS Field Definition

The DS field is used to replace the ToS octet in IPv4 and  IPv6 Traffic class octet. Now six bits out of the 8 bitsof the DS field are used as a code point i.e. the DSCP is used to select the PHB a packet experiences at each node. The remaining two bits called Currently Unused (CU) bits are reserved for future use.

The DS field structure is given below:

DSCP: Differentiated Services CodePoint.
    CUCurrently Unused.

Within the six available bits, the following mappings are defined:

It is imperative that the above mentioned mappings and that too EF and AF are to be comprehensively explained. The code points for the different PHB's are defined in the next page.