11.3. Type of Service
At the network layer, the IP datagram header includes an 8-bit TOS field that may be used to indicate preference for data delivery class of service. As shown in Figure 11-2, the TOS field is divided into a 3-bit precedence field, a 4-bit field for setting TOS bits, and a single bit that is always zero (must be zero, or MBZ for acronym afficionados). The three precedence bits plus the zeroed bit leaves 4 bits for TOS designation.
Figure 11-2. IP header TOS field.
Although the 4-bit TOS field could provide 16 different types of service, RFC 1349 defines only five classes of service:
1000—Minimize delay
0100—Maximize throughput ...
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