Type of serviceOriginal intention is prioritization. ENCinvolves both IP (IP header ECN bit marking) and TCP (TCP header C,E bit marking ECN Flag )TCP Congestion control Type of serviceThe 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.[1]https://en.wikipedia.org/wiki/Type_of_service#DSCP_and_ECN