Packet Switching

Packet is a unit of data that can be transferred at one time.

Packet includes a header and the data.

Packets are of variable sizes with a maximum size limit.

All data is transferred across the Internet in packets.

Packet Switching (contd.)

Packet switching is the process of routing communications traffic in the form of data packets from a sender to the (correct) receiver.

LANs and WANs use packet switching.Packet switching avoids delays; short

messages need not wait for long transfers to complete.

Packet Switching (contd.)

Packet switching permits multiple pairs of computers to communicate simultaneously in a shared network.

Computers take turns sending packets.

Interface hardware handles sharing automatically.

Packet Switching (contd.)

Each device connected to a network contains a small computer that handles the communication.

Network software on the sender side divides data into packets.

Network software on the receiver side reassembles the packets.

Internet - A Network of Networks

Interconnection of multiple packet switched networks.

Multiple packet switching technologies - different speed, distances, & cost

characteristicsInternet provides

a mechanism to interconnect arbitrary networks software to transfer data across the connections

Routers

Computers with special purpose software that interconnects networks.

Forward packets from one computer to another.

Routing is the process of selecting a network over which to send a packet.

Router can interconnect diverse technologies, for example, a LAN to a WAN.

Backbone Network & Sites

A major WAN to which other networks attach is called a backbone network. Example: vBNS - very high-speed Backbone

Network System (by MCI in 1995).Sites reached by the backbone are

called backbone sites.At each backbone site, a router

connects a LAN to the backbone.

Wide Area Backbone

Wide Area Backbone

LAN at site 1 LAN at site 2 LAN at site 3

LAN at site 4 LAN at site 5 LAN at site 6 LAN at site 7

Router

net at 6net at 7

net at 2 net at 3

Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

IP - Internet Protocol

A Communication Protocol is an agreement that specifies a common language two computers use to exchange messages.

A protocol specifies exact format & meaning of each message conditions under which a message can be

sent how a computer should respond to a message

IP - Internet Protocol (contd.)

IP specifies how a packet must be formed, and how a router must forward it to its destination.

IP software is needed on a computer connected to the Internet.

IP software is memory resident.Internet packets are called IP

datagrams.

IP - Internet Protocol (contd.)

IP transforms a collection of networks and routers into a seamless communication system.

IP makes the complex internal physical structure of the Internet transparent to the end user.

Internal Structure

f

e

d

c

b

a

Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

Network a Router connecting networks a and b

Computer attached to network d

IP Datagrams & Network Packets

IP datagram defines a standard format for all Internet packets.

IP datagram travels inside a network packet.

IP datagram is data within a network.Router creates a new network packet for

transmission across another network.Datagram is processed by software on the

destination computer.

Internet or IP Address

IP address is a unique number assigned to a computer.

Computer stores an IP address in 4 bytes.IP address is displayed as 4 decimal

numbers separated by periods,139.78.100.1

IP addresses are not random; prefix is the same for all computers on the same network.

Routing Example

d e

a

f

b c

C3

C2

C1Computer C3

Computer C1

Router 1

Router 2

Router 3

Router 5

Router 4

Computer C2

Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

TCP - Transmission Control Protocol

In addition to IP software, most of the computers that connect to the Internet also run TCP software.

TCP/IP - an entire set of Internet communication protocols.

TCP enables two computer programs to communicate across the Internet - connect, exchange data, and disconnect.

TCP (contd.)

TCP makes the Internet reliable. Checks for lost datagrams that were

discarded by routers. Puts incoming datagrams in the order they

were sent. Checks for duplicate datagrams.

TCP handles the problem of lost datagrams by using timers and acknowledgements.

TCP (contd.)

TCP software on the receiver side sends an acknowledgement back to the sender.

TCP on the sender side starts a timer when it sends data. If an ack. arrives before the timer expires then the TCP cancels the timer, else it retransmits another copy.

TCP’s timer mechanism adapts to “distance” of destination and Internet traffic delays.

Names for Computers

Users prefer alphabetic names to IP addresses.

Each computer on the Internet must have a unique name.

Names with many parts Full Name = local name + organization

suffixE.g. www.okstate.edu = www + okstate.edu

Full Name = computer name + dept. name + organization suffixE.g. cimctr.inden.okstate.edu

Names for Computers (contd.)

Domain Name System (DNS) is a software that translates a domain name to an IP address.

DNS uses the client/server approach; application program contacts the domain name server to get the IP address.

Domain name servers store names of computers at only one company or enterprise.

Names for Computers (contd.)

Computer name lookup is automatic. Application asks a local domain name

server and if needed this server then contacts a remote domain name server.

There is no correspondence between the parts of a domain name and the parts of its IP address.

Reasons for Internet’s success

IP provides flexibility Accommodates many types of hardware - WAN

or LAN, high-speed or slow, wired or wireless, etc.

TCP/IP standards specify how to send IP datagrams on each type of network.

TCP provides reliability Automatically adapts to Internet conditions Makes reliable communication possible even

during periods of congestion.