IP Precedence, TOS, and DSCP Classifications

Every IP packet (including both IPv4 and IPv6) includes a TOS byte. This byte is broken up into fields that the network uses to help provide the appropriate QoS commitments. In the older TOS model defined in RFC 1349, the first three bits contain the IP Precedence value, and the next four bits contain the TOS value.

We note in passing that it is easy to get confused between the different uses of “TOS”. Sometimes it refers to the entire byte, and sometimes to just the 4 bits that describe forwarding behavior. To help reduce the confusion, we will call the 4-bit field the TOS field, and the entire byte the TOS byte.

Table B-1 shows the standard IP Precedence values. It is important to note that normal application traffic is not permitted to use IP Precedence values 6 or 7. These are strictly reserved for network purposes like keepalive packets and routing protocols. The network must always give these packets higher priority than any application packets. This is because no application will work if the network loses its topology information.

Table 2-1. Standard IP Precedence values

IP PrecedenceDecimal valueBit pattern
Routine0000
Priority1001
Immediate2010
Flash3011
Flash Override4100
Critical5101
Internetwork Control6110
Network Control7111

Table B-2 shows the standard IP TOS values, as defined in RFC 1349. The idea was that an application could use these bits to request the appropriate forwarding behavior. Because the values are specified in different ...

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