Crete

The layer of network is the third layer of OSI model.

Definition OSI

The network layer builds a transportation route from beginning to end (termination) starting from ways of communication with its direct neighbors. Its principal functional contributions are thus: ; the Routing: determination of a way allowing to connect the 2 distant machines; ; and the relayage: retransmission of a PDU whose destination is not local to bring it closer to its final destination.

This layer is thus the only one with being directly concerned by topology of the network. It is also the last layer supported by all the machines network for the data transmission user: the roadbases are realized only in the machines of end.

The PDU of this layer is often called “package”. The function of “relayage” (term OSI) is sometimes called “routing”.

Remarks

This layer is most characteristic of an architecture network. This is why architecture often takes the name of the principal protocol of level network.

To determine a way is a complex task normally carried out in the large ones networks by dedicated protocols whose role is to discover the topology of network and to draw the best road from it. The protocols of routing are different by the selection criteria of the roads and the precision of topology discovered. In outside case of the small networks, the routing is hierarchical: precision of knowledge of the environment of a router decrease with the distance.

If the routers do not have a roadbase than the network layer from the point of view of the machines users of the network, they can support protocols of level transport and with above for the management of the network (supervision and execution of the protocols of routing for example). Although the data calculated by the protocols of routing are used by the network layer, they are not protocols of level network because they do not serve to transport the data of the machines users as the network. Moreover, in world IP, the protocols of nonlocal routing (i.e except RIP) are transported by TCP.

Some protocols of layer 3

• Internet Protocol (IP)

• Internet Control Message Protocol (ICMP): protocol of support of IP, in particular for the notification of the errors
• IPX
• Connectionless Network Protocol (CLNP): a protocol OSI whose service is inspired by that of IP.
• Wireless Distribution System (WDS)

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