Essential function of network:

• The essential function of network is to transfer information among the users that are attached to the network.

• Transfer of information may be single block of information or sequence of blocks as shown in below figure.

• In case of single block of information, we are interested in having the block delivered correctly to destination and also interested in delay experienced in traversing the network.

• In case of sequence of blocks, we are interested not only in receiving the blocks correctly and in right sequence.

Network service can be Connection-oriented service or connectionless service

Connectionless service:

• Connectionless service is simple with two basic interactions (1) a request to network layer that it send a packet (2) an indication from the network layer that a packet has arrived

• It puts total responsibility of error control, sequencing and flow control on the end system transport layer

Connection-oriented service

l The Transport layer can not request transmission of information until a connection is established between end systems

l Network layer must be informed about the new flow

l Network layer maintains state information about the flows it is handling

l During connection set up, parameters related to usage and quality of services may be negotiated and network resources may be allocated

l Connection release procedure may be required to terminate the connection

It is also possible for a network layer to provide a choice of services to the user of network like:

l best-effort connectionless services

l Low delay connectionless services

l Connection oriented reliable stream services

l Connection oriented transfer of packets with guaranteed delay and bandwidth

End To End argument for system design

§ The end to end argument in system design state that an end-to-end function is best implemented at higher level than at a lower level.

§ The reason is that the correct end-to-end implementation requires all intermediate low-level components to operate correctly.

§ The higher-level components at the ends are in better position to determine that a function has been carried out correctly and in better position to take corrective action if they have not.

Ø Keeping the core of the network simple and adding the necessary complexity at the edge enhances the scalability of the network to larger size and scope

Internal network operation

The fig above shows the relation between the service offered by the network and the internal network operation

• The internal operation of the network is connectionless if packets are transferred within the network as datagrams

• Each packets are routed independently

• Packets follow different paths from end to end and arrive out of order

• The internal operation of the network is connection-oriented if packets follow a virtual circuit along a path that has been established from source to destination.

• Virtual circuit setup is done once, then packets are simply forwarded

• If resources are reserved then bandwidth, delay and loss guarantees are provided.

Network layer essentials

The Functions that need to be carried out at every node in the Network Layer are:-

l Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements

l Forwarding: transfer of packets from NE inputs to outputs

l Priority & Scheduling: determining order of packet transmission in each NE

Optional: congestion control, segmentation & reassembly, security