Type of service
（重定向自IP precedence）
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. The modern redefinition of the ToS field is a six-bit Differentiated Services Code Point (DSCP) field and a two-bit Explicit Congestion Notification (ECN) field. While Differentiated Services is somewhat backwards compatible with ToS, ECN is not.