Mapping of User Priority to Traffic Class
IEEE 802.1D provides guidance on the mapping of user priorities into traffic classes. Figure 3 shows the recommended mapping. There are two comments that we can make immediately:
The mapping is based on the user priority associated with the frame, which, as was mentioned earlier, has end-to-end significance. However, the 802.3 and 802.11 frame formats don't include a priority field, which means that this end-to-end information could be lost. To address this issue, the bridge is able to reference the priority field contained in a tag header defined in IEEE 802.1Q, which deals with virtual LANs. The 802.1Q specification defines a tag header of 32 bits that is inserted after the frame header's source and destination address fields. This tag header includes a 3-bit priority field. Thus, if 802.1Q is in use by Ethernet and wireless LAN sources, a user priority can be defined that stays with the frame from source to destination.
Outbound ports associated with MAC methods that support only a single access priority, such as 802.3 and 802.11, can support multiple traffic classes. Recall that the traffic class deals with queuing delay, while the access priority deals with access delay.
Figure 3 Recommended user priority to traffic class mapping.
To understand the reason for the mappings recommended in Figure 3, we need to consider the types of traffic that are associated with each traffic class. IEEE 802.1D provides a list of traffic types, each of which can benefit from simple segregation from the others. In descending importance, these types are as follows:
Network control (7): Both time-critical and safety-critical, consisting of traffic needed to maintain and support the network infrastructure, such as routing protocol frames.
Voice (6): Time-critical, characterized by less than 10 ms delay, such as interactive voice.
Video (5): Time-critical, characterized by less than 100 ms delay, such as interactive video.
Controlled load (4): Not time-critical but loss-sensitive, such as streaming multimedia and business-critical traffic. A typical use is for business applications subject to some form of reservation or admission control, such as capacity reservation per flow.
Excellent effort (3): Also not time-critical but loss-sensitive, but of lower priority than controlled load. This is a best-effort type of service that an information services organization would deliver to its most important customers.
Best effort (2): Not time-critical or loss-sensitive. This is LAN traffic handled in the traditional fashion.
Background (0): Not time-critical or loss-sensitive, and of lower priority than best effort. This type includes bulk transfers and other activities that are permitted on the network but that should not impact the use of the network by other users and applications.
The numbers in parentheses in the preceding list are the traffic class values corresponding to each traffic type if there are eight queues and hence eight traffic classes available at a given output port.
Only seven traffic types are defined in IEEE 802.1D. The standard leaves as spare an eighth type, which could be used for traffic of more importance than background but less importance than best effort.
We can now address the issue of the mapping between user priority and traffic class value. If eight traffic class values are available (eight queues at this output port), the obvious mapping would be equality; that is, a user priority of K would map into traffic class K for 0 £ K < 7. This obvious mapping is not desirable because of the treatment of default priorities. For 802.3 and 802.11, which don't use priorities, the default user priority is 0. For other MAC types, such as 802.5, if the user doesn't specify a priority, the MAC level assigns a default value of 0. The 802.1D standard points out that using a different default value would result in some confusion and probably a lack of interoperability. However, the logical default traffic type is best effort. The solution proposed by 802.1D is to map a user priority of 0 to traffic class value 2. When eight traffic class values are available, user priority values 1 and 2 map to traffic class values 0 (background) and 1 (spare value), respectively.
This solution is reflected in Figure 4, which shows the mapping of user priority to traffic class when eight traffic classes are available. The figure also shows the mapping when there are fewer traffic classes. To understand the entries in this table, we need to consider the way in which 802.1D recommends grouping traffic types when fewer than eight queues are configured at a given output port. Figure 4 shows this grouping. The first row in the figure shows that if there is only one queue, all traffic classes are carried on that queue. This is obvious. If there are two queues (second row), 802.1D recommends assigning network control, voice, video, and controlled load to the higher-priority queue, and excellent effort, best effort, and background to the lower-priority queue. The reasoning supplied by the standard is this: To support a variety of services in the presence of bursty best-effort traffic, it's necessary to segregate time-critical traffic from other traffic. In addition, further traffic that is to receive superior service and that is operating under admission control also needs to be separated from the uncontrolled traffic. The allocation of traffic types to queues for the remaining rows of the figure can be explained similarly.
Figure 4 Suggested traffic types.