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Chapter 5 The Network Layer

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Ryerson University
Information Technology Management
ITM 301
Franklyn Prescod

Chapter 5 The Network LayerNetwork Layer is concerned with getting packets from the source all the way to destination The network layer is the lowest layer that deals with end to end transmissions51 Network Layer Design Issues 511 Store and Forward Packet Switching The major components of the network are the ISPs equipment routers connected by transmission linesStoreandForward Packet Switching 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 ISP The packet is stored there until it has fully arrived and the link has finished its processing by verifying the checksum Then it is forwarded to the next router along the path until it reaches the destination host where it is delivered 512 Services Provided to the Transport Layer The network layer provides services to the transport layer at the network layertransport layer interfaceThe services should be independent of the router technology the transport layer should be shielded from the number type and topology of the routers present and the network address made available to the transport layer should use a uniform numbering plan even across LANs and WANs513 Implementation of Connectionless Services Internet IF connectionless services are offered packets are injected into the network individually and routed independently of each other no setup is needed The packets are frequently called datagrams and the network is called a datagram network If a connectionoriented service is used a path from the source router all the way to the destination router must be established before any data packets can be sent This connection is called a VC virtual circuit and the network is called a virtual circuit networkEvery router has an internal table telling it where to send packets for each of the possible destinations Each table entry is a pair consisting of a destination and the outgoing line to use for that destination Only directly connected lines can be used The algorithm that manages the tables and makes the routing decisions is called the routing algorithmIP Internet Protocol which is the basis of the entire Internet is the dominant example of a connectionless network services Each packet carries a destination IP address that routers use to individually forward each packet The addresses are 32 bits in IPv4 packets and 128 bits in IPv6 packets514 Implementation of ConnectionOriented Services Telephone For connection oriented services we need a virtual circuit network 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 sources machine to the destination 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 When the connection is released the virtual circuit is also terminated With connectionoriented services each packet carries an identifier telling which virtual circuit it belongs toLabel Switching Router A assigns a different connection identifier to the outgoing traffic for the second connection Avoiding conflicts of this kind is why routers need the ability to replace connection identifiers in outgoing packets MultiProtocol Label Switching MPLS used within ISP networks in the Internet with IP packets wrapped in an MPLS header having a 20 bit connection identifier or label MPLS is often hidden from
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