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.