JUNOS Enterprise Routing

IP Differentiated Services

Over the years, there have been several false starts to a standardized IP CoS solution. This section summarizes the history of IP CoS, and it provides a primer on the current solution known as IP Differentiated Services (DiffServ).

The original use of IP networks was to support robust communications in the face of battlefield conditions, an application to which datagram (connectionless) operation is well suited. This discussion is tempered with the knowledge that the concept of integrating services over IP internetworks was not considered by the protocol’s architects, and wide-scale adoption of IP CoS has yet to occur. However, recent advancements in router platforms have enabled the high-bandwidth forwarding rates required to make IP-based convergence a commercial reality. With high-capacity forwarding in place, the final piece of the IP CoS puzzle is the intelligent handling of packets to effectively prioritize certain packets during times of reduced capacity or link congestion.

IP ToS

RFC 791 is the original RFC specification of the Internet Protocol (IP) and was published in 1981. The RFC defined an 8-bit field in the IP header as a Type of Service (ToS) field. Figure 9-8 shows the IP header and details the structure of the ToS field itself.

Figure 9-8. The IP ToS byte

The original IP ToS field is structured into a 3-bit precedence field, a 3-bit ToS indication ...