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The Network Layer
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Network Performance Measures
• Two Performance Measures– Quantity of Service (Throughput)
• How much data travels across the net?• How long does it take to transfer long files?
– Quality of Service (Average packet delay)• How long does it take for a packet to arrive at its
destination?• How responsive is the system to user commands?• Can the network support real-time delivery such as
audio and video?
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Types of Routing AlgorithmsNonadaptive (static)
◦ Do not use measurements of current conditions◦ Static routes are downloaded at boot time
Adaptive Algorithms◦ Change routes dynamically
Gather information at runtime locally from adjacent routers from all other routers
Change routes Every delta T seconds When load changes When topology changes
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Network Layer Design Issues
• Store-and-forward packet switching• Services provided to transport layer• Implementation of connectionless service• Implementation of connection-oriented service• Comparison of virtual-circuit and datagram
networks
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Store-and-Forward Packet Switching
The environment of the network layer protocols.
ISP’s equipment
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Services Provided to the Transport Layer
1.Services independent of router technology.2.Transport layer shielded from number, type,
topology of routers.3.Network addresses available to transport layer
use uniform numbering plan– even across LANs and WANs
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Implementation of Connectionless Service
Routing within a datagram network
ISP’s equipment
A’s table (initially) A’s table (later) C’s Table E’s Table
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Implementation of Connection-Oriented Service
Routing within a virtual-circuit network
ISP’s equipment
A’s table C’s Table E’s Table
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Comparison of datagram and virtual-circuit networks
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Routing Algorithms
1.Distance Vector Routing2.Link State Routing
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The Concept of Distance Vector Routing
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Distance Vector Routing Table
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Routing Table Distribution
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Updating Routing Table for Router A
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Final Routing Tables
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Example 21.1
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Concept of Link State Routing
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Cost in Link State Routing
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Link State Packet
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Flooding of A’s LSP
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Link State Database
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Dijkstra algorithm
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Shortest Path Algorithm (1)
The first five steps used in computing the shortest path from A to D. The arrows indicate the working node
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Hierarchical Routing
Hierarchical routing.
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Broadcast Routing
Reverse path forwarding. (a) A network. (b) A sink tree. (c) The tree built by reverse path forwarding.
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Flooding
• Flooding is a simple routing algorithm in which every incoming packet is sent through every outgoing link except the one it arrived on.
• Algorithm– Each node acts as both a transmitter and a
receiver.– Each node tries to forward every message to
every one of its neighbors except the source node.
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Flooding
• Advantages:– If a packet can be delivered, it will (probably
multiple times).– Since flooding naturally utilizes every path
through the network, it will also use the shortest path.
– This algorithm is very simple to implement
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Flooding