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1.4.- Some IETF Projects
related to TE |
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Integrated Services (IS) |
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The IS model requires resources, such as bandwidth and
buffers, to be reserved a priori for a given traffic flow to ensure that
QoS requested by the flow can be satisfied. Two services have been
defined under this model: Guaranteed Service [RFC 2212] and
Controlled-Load Service [RFC 2211]. |
| Guaranteed Service can be
used for applications requiring bounded packet delivery time, this means,
those for which data that is delivered after a pre-defined amount of time
has elapsed is usually considered worthless. The service was intended to
provide a firm quantitative bound on the end-to-end packet delay for
a flow, by controlling the queuing delay on network elements along the data
flow path. |
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Controlled-Load Service can be used for adaptive applications that
can tolerate some delay but are sensitive to traffic overload conditions.
These applications function satisfactorily when the network is lightly
loaded but their performance degrade significantly when the network is
heavily loaded. Then, the service has been designed to provide aproximately
the same quality as best-effort service in a lightly loaded network,
regardless of actual network conditions. |
| The IS model requires
additional components beyond those used in the best-effort model such
as packet classifiers, packet schedulers, admission control
and explicit signaling (using RSVP, see below). Because
individual state is needed for each flow it manages, the main issue against
it has been scalability, especially in large public IP networks which
may potentially have millions of active micro-flows in transit concurrently. |
| RSVP |
| The Resource Reservation
Protocol (RSVP) performs the signaling function in the
Integrated Service model. RSVP is a soft-state signaling
protocol. It supports receiver initiated establishment of resource
reservation for both multicast and unicast flows. |
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Under RSVP, the sender or
source node sends a PATH Message to the receiver with the same source
and destination addresses as the traffic which the sender generate. Every
intermediate router along the path forwards the PATH Message to the
next hop determined by the routing protocol. Upon receiving a PATH
Message, the receiver responds with a RESV Message which includes
a flow descriptor used to request resource reservations. The RSVP Message
travels to the sender node in the opposite direction along the path the
PATH Message traversed. Every intermediate router along the path can
reject or accept the reservation request. If the request is rejected, the
rejecting router will send an error message to the receiver and the
signaling process will terminate. If the request is accepted, link bandwidth
and buffer space are allocated for the flow and the related flow information
is installaed in the router. |
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| One of the problems with RSVP
is scalability. Because reservation is required for each microflow, the
amount of state maintained by the network increases linearly with the number
of microflows. However, RSVP has been modified and extended to
mitigate the scaling problems. For example, it has been extended to reserve
resources for aggregation of flows, to set up MPLS explicit label
switched paths, and to perform other signaling functions within the
Internet. |
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