Type of service

The type of service (ToS) field is the second byte of the IPv4header. It has had various purposes over the years, and has been defined in different ways by five RFCs.

Before the redefinition, the ToS field could specify a datagram's priority and request a route for low-latency, high-throughput, or highly-reliable service. Based on these ToS values, a packet would be placed in a prioritized outgoing queue or take a route with appropriate latency, throughput, or reliability. In practice, the ToS field never saw widespread use outside of the US Department of Defense networks. However, a great deal of experimental, research, and deployment work has focused on how to make use of these eight bits, resulting in the current DS field definition.

The modern redefinition of the ToS field (as well as the Traffic Class field in IPv6 packets) splits this byte into a 6-bit Differentiated Services (DS) field and a 2-bit Explicit Congestion Notification (ECN) field. While Differentiated Services is somewhat backwards compatible with ToS, ECN is not.