Priority (System Administration Guide, Volume 3)

System Administration Guide, Volume 3

Priority

The 4-bit priority field in the IPv6 header enables a source to identify the desired delivery priority of its packets, relative to other packets from the same source. The priority values are divided into the following two ranges:

Values 0 through 7 specify the priority of traffic for which the source provides congestion control, that is, traffic that backs off in response to congestion, such as TCP traffic.
Values 8 through 15 specify the priority of traffic that does not back off in response to congestion, for example, real-time packets being sent at a constant rate.

For congestion-controlled traffic, the priority values are recommended for particular application categories. The following table shows the priority values.

Table 14-9 Priority Values


Priority	Meaning
0	Uncharacterized traffic
1	"Filler" traffic (for example, netnews)
2	Unattended data transfer (for example, email)
3	(Reserved)
4	Attended bulk transfer (for example, FTP, HTTP).
5	(Reserved)
6	Interactive traffic (for example, telnet, X)
7	Internet control traffic (for example, routing protocols, SNMP)

For non-congestion-controlled traffic, you should use the lowest priority value (8) for those packets that the sender is most willing to have discarded under conditions of congestion (for example, high-fidelity video traffic). You should use the highest value (15) for those packets that the sender is least willing to have discarded (for example, low-fidelity audio traffic). There is no relative ordering implied between the congestion-controlled priorities and the non-congestion-controlled priorities.