The Network Layer

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?

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

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

Store-and-Forward Packet Switching

The environment of the network layer protocols.

ISP’s equipment

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

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

Implementation of Connection-Oriented Service

Routing within a virtual-circuit network

ISP’s equipment

A’s table C’s Table E’s Table

Comparison of datagram and virtual-circuit networks

Routing Algorithms

1.Distance Vector Routing2.Link State Routing

The Concept of Distance Vector Routing

Distance Vector Routing Table

Routing Table Distribution

Updating Routing Table for Router A

Final Routing Tables

Example 21.1

Concept of Link State Routing

Cost in Link State Routing

Link State Packet

Flooding of A’s LSP

Link State Database

Dijkstra algorithm

Shortest Path Algorithm (1)

The first five steps used in computing the shortest path from A to D. The arrows indicate the working node

Hierarchical Routing

Hierarchical routing.

Broadcast Routing

Reverse path forwarding. (a) A network. (b) A sink tree. (c) The tree built by reverse path forwarding.

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.

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

Flooding