Datagram and Virtual Network in Computer Network, Slides of Computer Networks

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Program Name :B.Tech

Semester :ECE 6th Sem

Course Name: Computer Network

Course Code:EC 602

Facilitator Name: Megha Dewan

Module:- 5

th

The Network Layer

Description of the network layer

a) The network layer is concerned with getting p-ackets from the source all the way to the desti-nation by using Logical Address(IP). To achieve its goals, the network layer must know about the topology of the communica- tion subnet and choose appropriate paths th- rough it. It must also take care to choose routes to avoid overloading some of the commun- ication lines and routers while leaving others idle.

Chapter 5 The Network Layer

**1. Network Layer Design Issues

2. Routing Algorithms**

Network Layer functionalities: Devices which works on Network Layer mainly focus on routing. Routing may include variety of tasks aimed to achieve a single goal. These can be:

• (^) Addressing Devices and Networks.
• (^) Populating Routing tables (or static routes).
• (^) Queuing incoming and outgoing data and then forwarding them according to Quality of Service constraints set for those packets.
• (^) Internetworking between two different subnets.
• (^) Delivering packets to destination with best efforts.
• (^) Provides connection oriented and connection less mechanism.

5.1 Network Layer Design Issues a) Store-and-Forward Packet Switching b) Services Provided to the Transport Layer c) Implementation of Connectionless Service d) Implementation of Connection-Oriented Service e) Comparison of Virtual-Circuit and Datagram Subnets

a) A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-to-point link to the carrier. The packet is stored there until it has fully arrived so the che- cksum can be verified. Then it is forwarded to the next router along the path until it reaches the destination host, where it is delivered. This mechanism is store-and-forward packet switch- ing, as we have seen in previous chapters.

5.1 Network Layer Design Issues a) Store-and-Forward Packet Switching b) Services Provided to the Transport Layer c) Implementation of Connectionless Service d) Implementation of Connection-Oriented Service e) Comparison of Virtual-Circuit and Datagram Subnets

One camp’s view

The routers' job is moving packets around and no- thing else. In their view , the subnet is inherently un- reliable, no matter how it is designed. Therefore, the hosts should accept the fact that the network is unre- liable and do error control (i.e., error detection and correction) and flow control themselves. So the network service should be connectionless. The Internet offers connectionless network-layer service

The other camp’s view

The subnet should provide a reliable, connection- oriented service. In this view, quality of service is the dominant factor, and without connections in the sub- net, quality of service is very difficult to achieve, esp- ecially for real-time traffic such as voice and video. ATM networks offer connection-oriented network-layer service.

a) Two different organizations are possible, depending on the type of service offered. b) If connectionless service is offered, packets are injected into the subnet individually and routed independently of each other. No advance setup is needed. In this context, the packets are frequently called datagrams and the subnet is called a datagram subnet. c) If connection-oriented service is used, a path from the source router to the destination router must be established before any data packets can be sent. This connection is called a VC (virtual circuit) and the subnet is called a virtual-circuit subnet.

Implementation of Connectionless

Service

Routing within a diagram subnet. 14 The question is: a packet with a destination D arrives at router A. then which router will router A send this packet to?

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a) For connection-oriented service, we need a virtual- circuit subnet. b) The idea behind virtual circuits is to avoid having to choose a new route for every packet sent. Instead,when a connection is established, a route from the source machine to the destination machine is chosen as part of the connection setup and stored in tables inside the routers. That route is used for all traffic flowing over the connection, exactly the same way that the teleph- one system works. When the connection is released, the virtual circuit is also terminated. c) With connection-oriented service, each packet carries an identifier telling which virtual circuit it belongs to.

Implementation of Connection-Oriented Service Routing within a virtual-circuit subnet.

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