Internet Traffic Quality of Service
The user priority and traffic class concepts enable MAC-level bridges and Layer 2 switches to implement a traffic-handling policy within a bridged collection of LANs that gives preference to certain types of traffic. These concepts are needed because these bridges and switches cannot see "above" the MAC layer and hence cannot recognize or utilize QoS indications in higher layers such as IP. However, it's often the case that traffic from a bridged set of LANs must cross WANs that make use of QoS functionality. An example of this is an ATM network, which provides for user-specified QoS. Another example is an IP-based intranet, which can provide IP-level QoS. Some means is needed for mapping between traffic classes and QoS for such configurations. This is an evolving area of technology and standardization, but a general picture can be provided.
In the case of IP-based internets, the IP type of service (ToS) field provides a way to label traffic with different QoS demands. The ToS field is preserved along the entire path from source to destinationpotentially through multiple routers. Fortunately, the mapping from traffic class to ToS is straightforward. The ToS field includes a 3-bit precedence subfield. A router connecting a LAN to an intranet can be configured to read the Layer 2 traffic class field and copy that into the ToS precedence field in one direction, and copy the 3-bit precedence field into the user priority field in the other direction.
In the case of an ATM connection, a bridge or Layer 2 switch might be connected to a LAN on one side and an ATM network on the other, using the ATM network to link to other remote LANs. For local LAN traffic arriving at the bridge, the bridge must match the user priority level with the appropriate ATM service class and other ATM parameters. For this purpose, the bridge can consult a mapping table whose settings have been predefined through the policy controls of network management software. An appropriate virtual connection is used to carry the traffic. If the traffic exits the ATM network at another LAN, the bridge on that end can map incoming traffic from each virtual connection into the appropriate traffic class and user priority.