Virtual Circuit-II

For the sake of discussion, we follow the path of one single packages from Ilir’s computer for tek Fatjon’s computer. We will also assume that the package before it reaches at its destination, pass through routers R1 R3, R5, R6, and R8. Two computers include five layers of TCP / IP while routers include three stack layers of TCP / IP protocols.

The network layer on the source computer provides four services:

1.packaging,

2.finding the account address k of the next step,

3.finding the physical address (MAC) of the step ofnext, and

4.the fragmentation of Datagram if it is necessary.

Since the network layer finally provides communication(end to end), both PCs wanting to communicate with each other the other needs a universal identification system, which is recognized as a network layer address or logical address.

-This type of identification is given in the network layer through one uniform and global addressing mechanism.

-The Internet uses an address space. Any subject in need to use the internet must be equipped with a unique address from this group.

•Iliri and Fatjoni need two network layer addresses to communicate. Task the first of the network layer is to encapsulate the data coming from upper layers in a Datagram

-This is done by adding a header to the data it contains

1.logical address source and packet destination,

2.information about fragmentation,

3.Our protocol ID that requested the service,

4.data length, and

5.maybe some other options as well. Network layer

6.it also includes a checksum that is calculated only above

Datagram header.

•Note that the top layer protocol only provides the addresslogical destination

-while the source logical address comes from the network layer itself (each host hasneed to know his own logical address).The prepared datagram contains the source and destination addresses of

package. To him, as we have seen before, he may have to passthrough many networks to reach its final destination.

-If the destination computer is not connected to the same network as source, Datagrams must be delivered to the next router

– Source and destination addresses on Datagram say nothing for the logical address of the next step.

– The network layer on the source computer should be consulted with one routing table to find the address logic step (jump) of next.The network layer does not deliver Datagram itself to the step another, but

– it is the task of the datalink layer to do the distribution.

– The datalink layer needs the next step MAC address to make the distribution

•To find the next step MAC address, the network layer may use another table to link the address logical next step with MAC address.

-This task is assigned to another auxiliary protocol called ARP Adress Resolution Protocol) Protocol i

address decomposition. The network layer on the destination computer is simple

•It is not necessary for forwarding. However, the destination computer must collect the pieces before passing the data to its above layers

•After evaluating each Datagram, the data is extracted from each fragment and preserved When all fragments t have come, of the data is reassembled and delivered to the top layer.

• The network layer also defines a timer) reunion. If the set time is up, all data fragments are destroyed and one is sent error message which says all datagrams should resend. These actually represent the services that normally discussed for the network layer, but they either apply partially to the coating network or not implemented at all.

•Some services are provided by some protocols ancillary or by protocols added to

Internet later.

•Wrong control means,

-to include a mechanism for detecting corruption, loss,or duplication of datagrams Error checking also includes.

-a mechanism for correcting errors.

•The online network layer does not provide a real mechanism error control. At the superficial level, it seems like there is no need for error checking in the network layer because each Datagram passes through several networks before reaching its destination final.

•Datalink layer that controls the behavior of these networks (LAN or WAN) uses error control. In other words, if applied a step-by-step error check step by step in layer e datalink, why do we need error checking even in the layer of network?

-Although step-by-step error checking can protect one Datagram to some extent, it does not offer complete protection.

Another reasoning for not giving the possibility of control of the error in this layer may be related to fragmentation.

• Since the data can be fragmented into several routers then even a portion of the network layer may change due to of fragmentation,

• if we use the wrong check, it should be checked on any router

– This makes the situation very difficult and makes error checking in this layer not very efficient.

•Network layer designers, however, have added a field checksum on Datagram

-to control any corruption in his header,

-but not all of Datagram.

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