QoS markings in the IP packet have evolved over time. The IP header has always contained a byte known as the type of service (ToS) byte. The 8 bits within this byte have evolved and have been redefined over time. It's a little confusing to start with, because the ToS byte has contained multiple things, some of which were called ToS bits.
Figure 6-1 shows the first four bytes of the IP header as defined in the original IP header (RFC 791, circa 1981) and as redefined in RC 1349 (circa 1992), RFC 2474 (circa 1998), and RFC 3168 (circa 2001).
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