IP: The Internet Protocol

Spring 2002Computer Networks

Applications

LAN---Bus Topology Each computer

is connected to a cable. Coordinates with others to send a message

BUS

Spring 2002Computer Networks

Applications

Issues in Sharing Communication Media Generally, most networks types involve

sharing: Bus, ring in LANs High speed backbone in WANs

Granting one party exclusive access may block all others for intolerable periods of time; Ex: to send a file of 5megabytes over a

network that may transfer 56,000 bits/s will require 12 minutes.

Spring 2002Computer Networks

Applications

Solution: Divide a message in small blocks, called

packets; Computers take turns in sending packets; insures fair, prompt access to the shared

resources; Ex. a packet of 8000 bits is transmitted in 0.143

s over a network that transfers 56,000 bits/s. easier to detect and recover from errors

Spring 2002Computer Networks

Applications

Packets Transmission All computers connected to a network have a unique

number, called address; Each type of network uses its own format for addresses:

from few digits to as much as 16 digits; At the beginning of a packet, there is a header which

contains the addresses of the sender and the destination; The hardware (NIC card) looks at every packet; if its for

the local machines it copies it;

A

B

C

DDc c D

Spring 2002Computer Networks

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Packets Transmission (cont.)

Packets are not all the same size any size can be sent up to a maximum Ex: a keystroke can be sent as one

individual packet. Sender and receiver have to agree

on how to specify the beginning and ending of each frame;

Ex: have dedicated characters for beginning and end;

Spring 2002Computer Networks

Applications

Packets Transmission (cont.)

Packet transmission is very fast: Typical LAN transfer ~ large 1000

packets/s Packet switching systems adapt

automatically as computers start/end sending data fair access to shared resources

Most networks, including Internet are a packet switching system

Spring 2002Computer Networks

Applications

Internet: a Collection of Disparate Networks Different goals:

Speed, cost, distance; Different standards for:

Expected carrier; Coding bits; Detecting and recovering from errors; Protocols for transmitting messages: bus,

token ring,… Packets sizes, and encoding for the start/end

of packets,… Types of computer addresses

Spring 2002Computer Networks

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Solution: Routers

RouterRouter

Router

High speed connection

Routers: computers design to interconnect different networks

Spring 2002Computer Networks

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Solution: Internet Protocol (IP)

IP hides the details of physical networks

IP specifies: Packet format; How routers should forward packets Define address format

Every computer connected to the Internet must run IP software

Spring 2002Computer Networks

Applications

The illusion of a single network The internet concept:

(a) the illusion of a single network that IP provides;

(b) the underlying physical structure.

Spring 2002Computer Networks

Applications

Datagrams A packet following IP specification is called

a datagram; The header of a datagram contains the

addresses of the sender and the destination;

But, each network type: defines its own packet format; Accepts and delivers only packets that adhere

to its own format. How can IP datagrams be sent across

networks that do not recognize IP format?

Spring 2002Computer Networks

Applications

Datagrams (cont.) A datagram travels across a given network

inside that network packet; When the packet arrives at the next router,

the router opens the packet and extracts the datagram;

If the datagram has to be sent to another network, the router:

Creates a new network packet; Encloses the datagram inside that packet; Sends it to the next router along the path;

If the datagram arrived at the destination: the receiver processes it.

Spring 2002Computer Networks

Applications

Datagram transmission

Spring 2002Computer Networks

Applications

Internet Addressing To make datagram routing and delivery

possible, each computer is assigned a unique address, called Internet address or IP address;

Each address is a 32-bit binary number; To make routing efficient, each address is

divided into two parts: a prefix and a suffix; Prefix: identifies the physical network to

which the computer is attached; Suffix: identifies each computer attached to

that network.

Spring 2002Computer Networks

Applications

Internet Addressing (cont.) To insure uniqueness:

two networks cannot be assigned the same address

network numbers are distributed by a centralized authority, called Internet Assigned Number Authority

two computers on the same network cannot be assigned the same number

suffixes can be assigned locally without global coordination

Spring 2002Computer Networks

Applications

Dotted decimal notation Convenient way to express IP

addresses Each 8-bit section represented as a

decimal number; Uses periods to separate the sections;

Spring 2002Computer Networks

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Forwarding a datagram Each router along the path, uses the

destination address to determine the next hop to which it has to be sent.

Each IP router keeps relevant information into a routing table;

Each entry specifies a destination and the next hop used to reach it;

Each destination is a network (an internet contains over 1000 times more hosts than networks)

Spring 2002Computer Networks

Applications

Routing table---an example

(a)An internet formed by 4 networks and 3 routers;(b)The conceptual routing table of router 2

Spring 2002Computer Networks

Applications

Address resolution IP addresses are abstractions provided by

software---differ usually from hardware addresses;

Mapping between a hardware address and an IP address is called address resolution;

Is used by routers/computers when need to send a packet on the same physical network.

Address resolution techniques: Table lookup: mappings are stored in memory, which the

software searches Message exchange: a computer sends a message that

requests an address binding, and another computer sends a reply that contains the requested info.

Spring 2002Computer Networks

Applications

Address resolution with table lookup

Spring 2002Computer Networks

Applications

An example trip through an internet

x

Y

R1

R2

R3