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The type of service (ToS) field in the IPv4 header has had various purposes over the years, and has been defined in different ways by five RFCs.〔RFC 791, RFC 1122, RFC 1349, RFC 2474, and RFC 3168. For a full history of the ToS field, see section 22 of RFC 3168.〕 The modern redefinition of the ToS field is a six-bit Differentiated Services Code Point (DSCP) field〔RFC 3260 Section 4〕 and a two-bit Explicit Congestion Notification (ECN) field.〔RFC 3168 Section 5〕 While Differentiated Services is somewhat backwards compatible with ToS, ECN is not. 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,〔http://www.lartc.org/howto/lartc.qdisc.classless.html Linux Advanced Routing & Traffic Control〕 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. * Interpretation history for IP Precedence, TOS & DSCP :1) The Type of Service field in the IP header was originally defined in RFC 791. The definition was largely derived from a US DoD Specification JANAP-128, which defines message precedence. :2) 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... :3) In practice, only the IP Precedence part of the field was ever used outside US DoD networks. 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. :4) At its simplest, the higher the value of the IP Precedence field, the higher the priority of the IP packet. :5) In the update version: RFC 1349 the Monetary Cost bit is introduced (this bit was previously marked "Reserved for Future Use"). :6) In the update version: RFC 2474 the definition of this entire field was changed. :7) It is now called the "DS" (Differentiated Services) field and the upper 6 bits contain a value called the DSCP" (Differentiated Services Code Point). :8) Since RFC 3168, the remaining two bits (the two least siginficant bits) are used for Explicit Congestion Notification ==Implementation== Prior to its deprecation, the Type of Service field was defined as follows. Precedence was a 3 bit field which treats high priority packets as more important than other packets.If a router is congested and needs to discard some packets, it will discard packets having lowest priority first. Although precedence field was part of version 4, it was never used. The last bit of Type of Service (bit 7) was defined as "Must Be Zero". Because the last three bits went through many definitions prior to RFC 2474 (see below), documentation and implementations may be confusing and contradictory. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Type of service」の詳細全文を読む スポンサード リンク
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