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. 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. The Type of Service field in the IP header was originally defined in RFC 791, and has been interpreted for IP Precedence and ToS ever since. The definition was largely derived from a US DoD Specification JANAP-128, which defines message precedence. It defined a mechanism for assigning a precedence to each IP packet, as well as a mechanism to request specific treatment such as high throughput, high reliability or low latency, etc. In the RFC 1349 update, the Monetary Cost bit is introduced (this bit was previously marked 'Reserved for Future Use'). Section 2.4 of RFC 1583 (OSPFv2) introduces a ToS-aware routing method. In practice, only the IP Precedence part of the field was ever used outside US DoD networks: the higher the value of the IP Precedence field, the higher the priority of the IP packet. Some US DoD networks did use the delay bit for route selection between oceanic cable paths and Satellite Communication (SATCOM) paths when both paths existed. IPv6 has never had an IPv4-like 'traditional' ToS field, partially because the authors were aware of DiffServ efforts at its drafting (RFC 2460 Section 7). In RFC 2474 the definition of this entire field was changed. It is now called the 'DS' (Differentiated Services, 'DiffServ') field and the upper 6 bits contain a value called the 'DSCP' (Differentiated Services Code Point). The upper 3 bits of DS maintains compatibility with IP Precedence. Since RFC 3168, the remaining two bits (the two least significant bits) are used for Explicit Congestion Notification. Prior to its deprecation, the Type of Service field was defined as follows from RFC 791: