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Configuring Differentiated Services
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308620-14.00 Rev 00
Marking Packets for Specific Services
A packet’s assigned level of service determines whether it receives preferential
treatment as it travels through the network. The DS field has a specific value that
determines the service.
If the router communicates with a bandwidth broker, the process of installing
filters and marking packets is as follows:
1.
The edge router contacts the bandwidth broker with a configuration request.
This happens automatically as part of the router’s normal differentiated
services activity for each differentiated services interface.
2.
The bandwidth broker responds by downloading a set of trigger and flow
filters from its database to the edge router.
3.
When a packet arrives at a routers differentiated services interface, the router
checks its trigger filter and flow filters, if any are installed, to see if the packet
matches either filter’s criteria. If the packet matches the trigger filter’s criteria,
the router sends a message to the bandwidth broker. (For the purposes of this
explanation, we will assume that the packet did not match any previously
installed flow filter.)
4.
After the bandwidth broker receives a message from the router, it responds,
possibly with its own request to install additional filters.
The router may or may not accept the additional filters because it may have a
limit on the number of filters it can handle, or the filter is misconfigured with
an action that is unknown to the router. If the bandwidth broker does install
additional filters, they are added to the filter table of the differentiated services
interface.
5.
Packets that arrive on that interface that match the flow filter criteria are
marked by the differentiated services application with a DS-field pattern
specified by the filter. The value in the DS field determines whether the packet
is sent to a high-priority data queue for preferential treatment. This is how
each packet gets marked or “differentiated” for a specific type of service.
6.
After a packet is marked, it is sent to the core network device where,
depending on the marking, the packet receives preferential treatment as it is
routed through the network and on to the destination host.
Figure 1-3
shows the Nortel Networks differentiated services architecture and
how the router marks a packet.
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