The Internet and the World Wide Web

ICS 61– Winter, 2015

Why computers?

Arithmetic Replicate human thinking Store and retrieve information Control other hardware Communicate Entertain Educate

J. C. R. Licklider

Man-Computer Symbiosis, 1960:

The hope is that, in not too many years, human brains and computing machines will be coupled together very tightly, and that the resulting partnership will think as no human brain has ever thought and process data in a way not approached by the information-handling machines we know today.

The “Galactic Network”

Licklider’s 1962 concept of a global computer network.

Also in 1962, Licklider became the first head of computer research at DARPA.

2 Types of networks

Circuit switching Path decided on before data transmission starts Dedicated circuit (e.g. actual copper wire in early

telephony) Packet switching

Break message into small chunks (“packets”) Each chunk has a destination address inside Each chunk may take a different path from source

to destination

Leonard Kleinrock

Proposed packet switching in 1960.

His computer at UCLA became the first node on the Internet in September, 1969. SRI UCSB Utah

Then called “Arpanet.”

What do the packets look like?

Current system, called TCP/IP, was adopted by the Internet in 1983.

IP: Internet Protocol TCP: Transmission Control Protocol

The IP Address

32 bits, written as four eight-bit (0-255) numbers

128.195.4.228 http://whatismyipaddress.com/

Originally, the first 8 bits specified the network, and the remaining 24 bits designated the computer on that network

New version of IP (v6) has 64 bit addresses http://www.nytimes.com/2011/02/15/technology/15internet.html?

scp=1&sq=internet%20addresses&st=cse

Why Protocols are necessary

Big-endian v. little-endian Little-endian: later digits more significant

Intel x86 architecture Irvine CA USA

Big-endian: earlier digits more significant Motorola processors thirty two

Why Protocols are necessary

Big-endian v. little-endian Little-endian: later digits more significant

Intel x86 architecture Irvine CA USA

Big-endian: earlier digits more significant Motorola processors thirty two

Blefuscu v. Lilliput in Gulliver’s Travels, 1726

The IP Packet

IP Version number Total length of packet Source IP Address Destination IP Addr. Time To Live (TTL) Checksum Data being transmitted (000110100100101…)

No guarantee packets arrive, or arrive in order.

TCP

Provides a “virtual circuit” between two computers. Defines a port HTTP: 80, IRC 194)

Guarantees reliable and in-order delivery of packets. TCP packets have sequence numbers. TCP programs at each end acknowledge packets

and resend if necessary. TCP practices congestion avoidance and

provides error checking.

TCP/IP packet

UDP

User Datagram Protocol “Lightweight” when compared to TCP,

doesn’t provide reliability or ordering. Same port facilities, provides error checking. Better for many time-sensitive purposes. Used for games, VoIP, IPTV.

The Layers of the Network

Application Layer (FTP, HTTP, IMAP) Transport Layer (TCP, UDP) Network Layer (IP) Data Link Layer (802.11, Ethernet, WiMax) Physical Layer (Modems, Coax, Ethernet)

The World Wide Web

Tim Berners-Lee, 1989 - 1991 “Any network-accessible information could be

referred to by a single ‘Universal Document Identifier.’”

More conventions, languages, and protocols: URL, HTML, HTTP

Mosaic (from NCSA), 1993-1995 Client-server architecture

Uniform Resource Locator

More URL

scheme://[user:password@]domain:port/path?query_string#fragment_id scheme: how to connect, e.g. https domain: where to connect port: optional (if omitted, determined by scheme) path, etc: what to ask for

query string: info for program, e.g. q=The+Stanley+Parable

fragment: position in document encodings: %2D = - %24 = $ %20, + = space

Peer to Peer Architecture

Each computer is connected every other computer.

Client-Server Architecture

Client:•Rendering•Sound•Some rules•Some physics

Server:•Game State•All rules•All physics•All AI