TOS (Type of Service)

The type of service (ToS) field is the second byte of the IPv4 header. It has had various purposes over the years, and has been defined in different ways by five RFCs.

Prior to the redefinition, the ToS field could specify a datagram’s priority and request a route for low-delay, 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 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, also used for the Traffic Class field in IPv6 packets, is an 8-bit differentiated services field (DS field) which consists of a 6-bit Differentiated Services Code Point (DSCP) field and a 2-bit Explicit Congestion Notification (ECN) field. While Differentiated Services is somewhat backwards compatible with ToS, ECN is not.

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