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2.0.- Simplifying assumptions |
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| Extensions to LSAs include
only available bandwidth and delay. Path selection is limited
to considering only bandwidth requeriments. The path selection
algorithm selects paths capable of satisfying the bandwidth requeriment
of flows, while at the same time trying to minimize the number of hops
used. |
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Some times, bandwidth does not fully capture the complete range of
potential QoS requeriments. For example, a delay-sensitive
flow could be put on a path using a satellite link because it has
plenty of unused bandwidth. This is an undesirable choice. One approach
taken in this specifications is to assign delay-sensitive traffic
to a policy that would eliminate from the network all links with high
propagation delay, before invoking the path selection algorithm. |
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It is very important to develop a
solution that has the smallest possible computing overhead. One
possible approach is to allow the pre-computation of QoS routes. This
is the method selected for the QoS extensions to OSPF. It
should be noted also that although several alternative path selection
algorithms are possible, the same algorithm should be used consistently
within a given routing domain. This requeriment may be relaxed when explicit
routing is used, as the responsability for selecting a QoS path lies
with a single entity which ensures consistency even if each router uses a
different algorithm. Nevertheless, the use of a common path selection
algorithm within an AS is recommended, and even necessary, for proper
operation, |
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| A last aspect that will be treated
later, is how to control the overhead associated with the additional link
state updates caused by more frequent changes to link metrics.
The goal is to minimize the amount of additional update traffic without
adversely affecting the performance of path selection. |
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