Differentiated Services
A replacement header field, called the DS field, is defined to supersede the existing definitions of the IPv4 TOS octet (RFC-791) and the IPv6 Traffic Class octet (RFC-1883)
Six bits of the Differentiated Services (DS) field are used as a codepoint (DSCP) to select the PHB (Per-Hop Behavior) a packet experiences at each node
A two-bit currently unused (CU) field is reserved, the value of the CU bits are ignored by differentiated services-compliant nodes
The Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers is defined in RFC-2474 (December, 1998)
The Architecture for Differentiated Services is defined in RFC-2475 (December, 1998)
The DS field structure is presented on the next page