COMPUTER NETWORKS Chapter 05 The Network Layer Part 1

Chapter 5 THE NETWORK LAYER Provide facilities for getting data from a source to a destination May require making many hops at intermediate routers along the way

5.1 Network Layer Design Issues 5.1.1 Store-and-Forward Packet Switching Ø A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-topoint link to the carrier. The packet is stored there until it has fully arrived so the checksum can be verified. Then it is forwarded to the next router along the path until it reaches the destination host, where it is

delivered.

Computer Networks - 05 -1 The environment of the network layer protocols

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5.1.2 Services provided to the Transport Layer

Service goals:

Ø Services should be independent of technology Ø Transport layer is shielded from number, type, and topology of the subnet

• Ø Uniform numbering plan across LANs and WANs

• Issues about connection-oriented and connectionless services, the bottom line is about where to put the complexity.

Ø In connection-oriented service, it is in the network layer (subnet)

Ø In connectionless service, it is in the transport layer(hosts).

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The subnet is inherently unreliable

The Internet community argues that the routers’ job is moving packets around and nothing else. The subnet is inherently unreliable, no matter how it is designed. Therefore, the hosts should accept the fact that the network is unreliable and do error control, i.e., error detection and correction and flow control themselves.

Ø Network service should be connectionless, with primitives SEND PACKET and RECEIVE PACKET and little else. No packet ordering and flow control should be done, the hosts are going to do that anyway; each packet must carry the full destination address, for it is carried independently of its predecessors. Computer Networks - 05 -1

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Subnet should provide a reliable, connection-oriented service? The telephone companies argue that the subnet should provide a reliable, connection-oriented service. In this view, quality of service is the dominant factor, and without connections in the subnet, quality of service is very difficult to achieve, especially for real-time traffic such as voice and video.

• These two camps are best exemplified by the Internet and ATM.

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Internal organization of the network layer

Virtual circuit

Ø A route from source to destination is determined during connection setup, which is used for all traffic flowing over the connection. When the connection is released, the virtual circuit is terminated.

Example: Setup procedure of virtual circuit Datagrams

Ø No routes are worked out in advance, even if the service is connection-oriented; each packet sent is routed independently of its predecessors; successive packets may follow different routes; more robust and more easily adapt to failures and congestion.

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5.1.3 Implementation of Connectionless Service

Routing within a datagram subnet.

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5.1.4 Implementation of Connection-oriented Service

Routing within a virtual-circuit subnet. Computer Networks - 05 -1

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5.1.5 Comparison of virtual circuit and datagram

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