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.
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