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ToS (Type of Service)

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The IP datagram header contains an 8-bit field called ToS (Type of Service). The field has been part of the IP header since the beginning, but it was rarely used until the recent introduction of Differentiated Services (Diff-Serv).

ToS was outlined in RFC 791 (Internet Protocol, September 1981). It was then clarified in RFC 1349 (Type of Service in the Internet Protocol Suite, July 1992). In the early days, some attempts were made to specify how it could be used to compute separate routes through networks depending on type of service specifications. The field was defined with two parts, a precedence value and the ToS bits. The precedence value occupies the leftmost 3 bits and was meant to privide a form of priority queuing. The ToS bits specified how the network should make trade-offs between throughput, delay, reliability, and cost.

The IETF Differentiated Services Working Group redefined the ToS field and renamed it the Differentiated Services field. Six bits of the DS field are used as a codepoints to select the PHB (per-hop behavior), which defines how packets are queued at network nodes. This discussion continues under the topic "Differentiated Services (Diff-Serv)."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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