Layer 2 Integration

Windows 2000 QoS supports mapping of RSVP signals to layer 2 signals using IEEE 802.1p priority markings to enable the prioritization of traffic across layer 2 devices, such as switches, on a network segment. IEEE 802 refers to the layer 2 technology, including the Data Link layer and the Media Access Control (MAC) layer. The IEEE 802.1p standard defines how layer 2 devices handle traffic marked with 802.1p priority. The QoS Packet Scheduler performs 802.1p marking for any application that requests QoS using GQoS or the Traffic Control API.

On Ethernet, 802.1p priority is carried in Virtual Local Area Network (VLAN) tags defined in IEEE 802.1q/p (802.1p). A field in the 802.1q tag carries one of eight priority values (3 bits in length), recognizable by layer 2 devices on a network segment. This marking determines the service level the packet receives when crossing an 802.1p-enabled network segment. Figure 9.5 shows the location of the 802.1p priority bits within the 802.1q tag.

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Figure 9.5 802.1p Tag

The 802.1p tag is placed inside the Ethernet header, between the MAC header and the data payload. A mapping from the service-type used by RSVP is made to one of these 802.1p priority values. A default mapping is defined on the hosts, however, sophisticated switches might direct hosts or routers to use mappings other than the default. The default markings for 802.1p priority service levels listed in Table 9.4 are hard-coded into the Windows 2000 QoS Packet Scheduler and can only be modified via the host registry.

Table 9.4 Windows 2000 Default 802.1p Priority Levels

The QoS Packet Scheduler must be installed on any host that performs 802.1p marking. If the layer 2 devices between the end nodes are not 802.1p-capable or enabled, layer 2 prioritization cannot be guaranteed across that segment.

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