What exactly is the Internet?

A networking infrastructure that allows hardware devices to physically connect to one another.

Ø Servers, Desktop PCs, Mobile devices etc.

Ø Based on common standards – notably the TCP/IP protocol suite.

Protocols specify how the transactions are carried out.

Ø How communication is initiated and maintained.

Ø Type, size and structure of the data sent/received.

Ø Enable the services running over the Internet (“the web” is just one!)

Software clients (and servers) allow users and devices to access and use Internet services.

Ø Using centralized servers or distributed peer-to-peer systems.

Clients, Servers and Peers

Local software clients access centralized remote servers

Ø Email via an Outlook client accessing an ISP’s mail server.

Ø Web pages via a Firefox web client accessing an Apache web server.

Peer-to-peer technology can directly connect multiple, decentralized computers – each acting as both client and server.

Ø Responsibly implemented P2P can be very efficient for data-rich services.

Ø Distributed download, media on demand, VOIP/video calling etc.

Protocols to build services

Clients and servers typically use a combination (stack) of protocols to provide specific services.

Ø Web access

Ø Email

Ø File transfer

Ø Peer-to-peer data exchange

Ø Chat/instant messaging

Ø Shell/console access to computers

Most common protocols in use today are those associated with web and email services

Ø HTTP/HTTPS, SMTP/IMAP/POP etc.

Common Internet Protocols

Protocols

Notes

HTTP

Hypertext Transfer Protocol

Core protocol for web transactions

SSL

Secure Sockets Layer

Data encryption

HTTPS

Secure HTTP

A combination of HTTP and SSL

SMTP

POP

IMAP

Simple Mail Transfer Protocol

Post Office Protocol

Internet Message Access Protocol

Used in combination to provide most email services

FTP

File Transfer Protocol

Used to exchange files between computers

SSH and SFTP

Secure Shell and Secure/SSH FTP

Encrypted access to remote computers plus extension for encrypted file transfer

BitTorrent

Protocol for peer-to-peer data access and exchange

TCP/IP: The heart of the Internet

TCP (Transmission Control Protocol)

Ø Handles direct connection on computer between client/server software and network interface(s)

Ø Breaks outgoing messages from application into packets to send via IP

Ø Assembles incoming packets into messages to pass back to application

Packets have two parts:

Ø Header (addressing/sequencing information and metadata)

Ø Body (actual data payload)

IP (Internet Protocol)

Ø Handles delivery of packets to/from any device with an IP address

Ø A unique numeric identifier for a device on the internet e.g. (128.240.233.249)

IP: Routing packets

IP routes packets using address info in header to direct them to their destination

IP routing is connection-less

Ø Does not require a fixed connection between endpoints

Ø Enables data to travel via multiple routes to reach endpoint

Ø One “wire” can handle multiple connections/services

Ø Packets can be re-routed around points of failure

But risks include…

Ø Corruption of data/loss of packets/duplicate packets arriving

Ø Packets delivered out of sequence (sent A -> B, received B -> A).

As a result IP only promises best-effort-delivery

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