In Chapter 11, we discussed several important concepts related to traffic classification, queueing algorithms, and congestion handling systems. Unfortunately, some of these concepts are unfamiliar to many network engineers, so this appendix includes some more detail and background.
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.
It is easy to get confused between the different uses of the term “TOS.” Sometimes it refers to the entire byte and sometimes to just the four bits that describe forwarding behavior. To help reduce the confusion, we call the four-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, which are strictly reserved for keepalive packets, routing protocols, and other important network traffic. The network must always give these packets higher priority than any application packets because no application will work if the network loses its topology information.