nternet. Diffserv is much newer. The values for diffserv are called differential service code point (DSCP) values. The TOS field is illustrated in Figure 31-5. DiffServ IP precedence TOS field Figure 31-5. The IP TOS field The field is eight bits long, with the first three bits used for IP precedence. The IP precedence values are called service mappings, and are defined in RFC 795 as follows: Network Working Group J. Postel Request for Comments: 795 September 1981 SERVICE MAPPINGS This memo describes the relationship between the Internet Protocol (IP) [1] Type of Service and the service parameters of specific networks. The IP Type of Service has the following fields: Bits 0-2: Precedence. 3: 0 = Normal Delay, 1 = Low Delay. Bits 4: 0 = Normal Throughput, 1 = High Throughput. 5: 0 = Normal Relibility, 1 = High Relibility. 6-7: Reserved for Future Use. PRECEDENCE 111 - Network Control 110 - Internetwork Control 101 - CRITIC/ECP 100 - Flash Override 011 Flash 010 Immediate 001 Priority 000 Routine The early Internet (this RFC was written in 1981) was pretty simple by today's stand- ards, and three bits of IP precedence were fine. But eventually, the issue of scalability QoS Mechanics became apparent as users began to want more than eight levels of distinction in their priorities. So, along came RFC 2474, which redefined the TOS field: 3. Differentiated Services Field Definition A replacement header field, called the DS field, is defined, which is intended to supersede the existing definitions of the IPv4 TOS octet [RFC791] and the IPv6 Traffic Class octet [IPv6]. Six bits of the DS field are used as a codepoint (DSCP) to select the PHB a packet experiences at each node. A two-bit currently unused (CU) field is reserved and its definition and interpretation are outside the scope of this document. The value of the CU bits are ignored by differentiated services-compliant nodes when determining the per-hop behavior to apply to a received packet. The DS field structure is presented below: DSCP: differentiated services co