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since the SLA also contains feedback parameters to the customer about the actually
achieved service levels. In traditional SLAs the customer can perform only a limited
number of actions.
[SP01] explains SLAs and the process in detail. To sum-marize the above, SLA
defines the agreed upon quality lev-els as demanded by different users and applications
and as agreed to be provided by network service provider SLAs also exist between
network providers. Curently, the major items specified by SLA include availability,
transmission delay and packet loss probability. They are mean values of one or more
months, and they show average performance of the provider's entire network. Currently,
service provider doesnot guarantee detailed service levels and QoS to individual
customers, but in the future, it needs to be done because of the increasing business
opportunities on the internet and the need to support real time transactions and services.
D. QoS control Mechanism of Network Elements
This section explains the mechanism to allow QoS control at each network element.
1) Packet Classifer: This is used to classify packets based on some predefined rules.
As determined by the IETF DiffServ Working Group, packet classifier can be of 2 types:
Multi-field (MF) classfier and Behaviour Aggregate classfier (BA). MF classifier is used
at the edges of the network and BA is used at the core network to ensure scalability.
_ Multi-Field Classi®er: This classifies based on the com-bination of one or more header
(like source/destinationaddress, DS field, protocol ID) information and input
interface
information.
_ Behaviour Aggregate Clasi®er: It classifies packet based only on their DS value.
2) Traffic Conditioner: Traffic Conditioner measures the input traffic and assures
that the packet behaviour follows the predefined profiles. The traffic conditioner consists
of a combination of Meter, Marker, Shaper and Dropper. In the DiffServ network,
packets are conditioned at the edges only.
3) Scheduler: The scheduler controls the packet trans-mission sequence from
queues of individual classes to pro-vide traffic of each class with the QoS level
appropriate to PHB of the respective class. The scheduling algorithms are grouped into
two major categories: Priority Based Scheduling and Bandwidth-Sharing Scheduling. The
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