15-1
15 Appendix B Introduction to Packet Precedences
IP Precedence and DSCP Values
Figure 15-1 ToS and DS fields
As shown in
Figure 15-1, the ToS field of the IP header contains eight bits, and the first three bits (0 to
2) represent IP precedence from 0 to 7. According to RFC 2474, the ToS field of the IP header is
redefined as the differentiated services (DS) field, where a DSCP value is represented by the first six
bits (0 to 5) and is in the range 0 to 63. The remaining two bits (6 and 7) are reserved.
Table 15-1 Description on IP precedence
IP precedence (decimal) IP precedence (binary) Description
0 000 Routine
1 001 priority
2 010 immediate
3 011 flash
4 100 flash-override
5 101 critical
6 110 internet
7 111 network
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