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The packet-switched networks were born following the philosophy best-effort, thus not guarantee delivery times or without differentiate traffic. To achieve this you must use other protocols:
-RSVP allows you to reserve some resources
-Differentiated services allow you to classify traffic (does not guarantee anything, but it ensures that some classes have priority over other classes).
-Integrated services allow you to guarantee resources (very difficult to implement).

Mechanisms to ensure QoS
-Scheduling policies in the tails of the router:
- Standard FIFO queue does not allow the QOS.
- Priority queue (without the right of first refusal) sends packets according to traffic class.
- Round robin cycles to shift all the queues.
- Weighted fair queuing allows to manage different weights for different queues, which however are cycled.

Waste-policies of the queues of the routers:
- Drop tail eliminates the package arrives.
- Elmina priority packet with lower priority.
- Random drops a package at random.
- RED eliminates packet with probability increased with increasing length of the tail.
-Mechanisms of policing

With the combination of the above techniques can not guarantee the QOS. For example if you use the token model combined with a system of priority queues packets higher priority will have a guaranteed crossing time of t = b * L / r, while the banda banda minimum guarantee is given by the network.

There are no problems of IP signaling connection routing because there is less and more is IP best-effort.
The problems arise when signaling over IP is to a certain QOS. For this reason I have introduced two approaches to the problem:
-Intserv plans to handle the traffic flow based on the membership of the packets.
DiffServ-plans to handle the traffic based on the type.

IntServ is a standard that specifies the descriptors of the flows.
-Filterspec, since the destination / source.
-Flowspec given by the characteristics of the traffic and the services that you want to guarantee.
To control the flow, the routers are equipped with a packet classifier, a scedulatore packets and a transmission buffer.In addition it is necessary to control input for incoming packets, so as to guarantee the reservation of resources.

RSVP provides a series of messages between routers in order to give the possibility to reserve resources for flows.The main problem is that using different priority flows is very complicated for high-traffic routers. Another problem is the low scalability of the protocol, because as the number of flows, it gets harder.
Most routers can handle RSVP messages but, in reality, only a few actually filter traffic through this protocol.RSVP provides:
RSVP access control and queuing techniques for flow within each of the routers that use it.
The receiver requests the network that is attributed to a stream, so that to be guaranteed a certain QoS. The resources are reserved according to the possibilities of the router and not on RSVPed.

DiffServ allows to classify traffic into different types independent of the data streams. In this way the work is simplified routers. All packets of a class are treated equally. At a time when a packet enters a network is classified and no further classificazion
The classification can be managed directly by the user, in case they do pay more services with the highest priority.
The IP ToS field indicates the class of the package.

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