THE NETWORK LAYER

Services provided to the transport layer:

1. The services are independent of subnet technology.
2. The transport layer is shielded from the number, type and topology of subnets present.

3. Virtual Circuit   à  A connection of the subnet. Data gram  à  Independent packets of a connection-less organization.

   The network addresses made available to  transport layer uses a uniform numbering plan .

Internetworking Styles:

·         Concatenated Virtual Circuit Subnets                        (Connection-Oriented)

·         Datagram Model                                             (Connection-Less)

Concatenated Virtual Circuit Subnets:

            A connection to a host in a distant network is set up in a way similar to the way connections are normally established. The subnet sees that the destination is remote and builds a virtual circuit to the router nearest the destination network. Then it constructs a virtual circuit from that router to an external gateway, which records the existence of virtual circuit in its tables and proceeds to build another virtual circuit to a router in next subnet. This process continues until the destination host has been reached.

Once data packets begin following along the path, each gateway relays incoming packets, converting between packet formats and virtual circuit numbers as needed. Clearly, all data packets must traverse the same sequence of gateways, and thus arrive in order.

            The essential feature of this approach is that a sequence of virtual circuits is set up from the source through one or more gateways to the destination. Each gateway maintains tables telling which virtual circuits pass through it, where they are to be routed, and what the new virtual circuit number is.

Datagram Model:

In this model, the only service the n/w layer offers to the transport layer is the ability to inject datagrams into the subnet and hope for the best. Datagrams from one host to another host travel through different routes through the inter network. A routing decision is made separately for each packet, possibly depending on the traffic at the moment the packet is sent. This strategy can use multiple routes and thus achieve a higher band width than the concatenated virtual circuit model.

   Differences between a Virtual Circuit and a Datagram:

s.no	Issue	Data gram subnet	Virtual circuit subnet
1. 2. 3. 4. 5.	Circuit setup Addressing State information Routing Congestion control	Not required Each packet contains the full source and destination address Subnet does not hold state information Each packet is routed independently Difficult	Required Each packet contains a short virtual circuit number Each virtual circuit  requires subnet table space Route chosen when virtual circuit is setup &all packets follow this route Easy, if enough buffers can be allocated in advance for each virtual circuit