Jan.2001C.Watters1 World Wide Web Basics

Jan.2001C.Watters2 What is an internet anyway? 2 or more networks that can communicate

Jan.2001C.Watters3 Historical View: Internet Telnet computers –Stanford, UCLA, UC Santa Barbara, U Utah FTP computers on the internet million computers on the internet ?? Kazillions

Jan.2001C.Watters4 What is the World Wide Web ? Hypertext connectivity of “documents”

Jan.2001C.Watters5 Size of the Internet

Jan.2001C.Watters6

Jan.2001C.Watters7 The web The Web is protocol that uses the internet as the communication structure links documents stored in computers communicating with the internet main authority: W3 consortium

Jan.2001C.Watters8 Historical View: WWW Berners Lee - web doc proposal Berners Lee - text browser (physicists) public access to web docs at CERN web servers & Mosaic (graphics) – (500 servers by year end) more Internet than US post x million docs & y million servers

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Jan.2001C.Watters10 Basics Web server - machine that services internet request Web client - machine that initiates internet request Browser - software to interact with internet data at the client TCP/IP - internet data protocol FTP - internet file transfer protocol HTTP - hypertext transfer protocol HTML - hypertext markup language

Jan.2001C.Watters11 Client-Server Model

Jan.2001C.Watters12 Looking in the Cloud /opt/sbin/traceroute

Jan.2001C.Watters13 CA*net2 layer 2 links

Jan.2001C.Watters14 CA*net3 Physical Links

Jan.2001C.Watters15 1. Client-Server & Web Cloud model TCP/IP HTTP and MIME types FTP protocol stacks

Jan.2001C.Watters16 Servers and Clients Servers - computer systems at the end of a network that store files and provide other services Clients - computer systems that are end points for users of the data

Jan.2001C.Watters17 Network Architectures ISO’s OSI model 1970’s International Organization for Standards Open Systems Interconnection reference Model 7 layer architecture

Jan.2001C.Watters18 ISO - OSI Model Application layer presentation layer session layer transport layer network layer data link layer physical layer Ftp, telnet, etc data compression, format set up connections end-to-end trans of packets guide packets along links send packet between nodes deliver bits between nodes

Jan.2001C.Watters19 ISO OSI model

Jan.2001C.Watters20 INTERNET MODEL 4 layers Application layer –communication services (ftp, telnet, ) transport layer –transmission of messages end-to-end network layer –transmission of messages sequence of links link layer –transmission of packet across one link

Jan.2001C.Watters21 Internet layers

Jan.2001C.Watters22 Application Layer FTP HTTP SMTP telnet etc

Jan.2001C.Watters23 TCP/IP Suite of protocols made the standard for the Internet facilitates communication between heterogeneous and similar networks that are connected together reliable, connection oriented, byte stream protocol

Jan.2001C.Watters24 Transport layer: TCP and UDP TCP –transmission control protocol –full duplex byte stream –virtual path (connected) –error free –uses acknowledgements –16 bit address of ports UDP –user datagram protocol –connectionless –no acknowledgements –no flow control –no resending of erroroneous packets –some error detection –16 bit port addresses

Jan.2001C.Watters25 TCP/IP Transport Control Protocol Internet Protocol

Jan.2001C.Watters26 TCP and IP

Jan.2001C.Watters27 Network Layer: IP Delivers packets up to 64kbytes, 1 at a time Each packet has a header –sending host and intended host network addresses –32 bit addresses IP layer (like UDP) – unreliable –connectionless

Jan.2001C.Watters28 Link Layer: links Connect computer to Internet SLIP –serial line IP (asynchronous, 1 char at a time) –move IP packets to common link (phone line) PPP –point-to-point protocol –also synchronous transfer for packets

Jan.2001C.Watters29 Data encapsulation using TCP on Ethernet

Jan.2001C.Watters30 TCP/IP apps TCP/IP software usually includes: –remote terminal client using TELNET protocol for remote login –electronic mail client using SMTP protocol to transfer to remote system –file transfer client using FTP protocol to transfer files between 2 machines

Jan.2001C.Watters31 HTTP HyperText Transport Protocol Native protocol for WWW sits on top of internet’s TCP/IP protocol HTTP is a 4 step process per transaction uses a predefined set of document formats from MIME

Jan.2001C.Watters32 MIME MIME - multipurpose internet mail extensions –defines file formats (images, video, text, etc) –e.g. Content-type: text/html – Data type/subtype » text/html » text/plain » image/gif » video/mpeg » application/msword »etc!!!

Jan.2001C.Watters33 HTTP Connection 1. Client –makes an HTTP request for a web page –makes a TCP/IP connection 2. Server accepts request –sends page as HTTP 3. Client downloads page 4. Server breaks the connection

Jan.2001C.Watters34 HTTP is Stateless!!!! Each operation or transaction makes a new connection each operation is unaware of any other connection each click is a new connection So how do they do those shopping carts??

Jan.2001C.Watters35 What does it look like? Header + object file Header –plain text –info about the object (MIME etc) –methods allowed –etc –browser sends a header to server each time you ask for information –server sends a header and possibly content

Jan.2001C.Watters36 HTTP Transaction Example GET /catalog/ip/ip.htm HTTP 1.0 Accept: text/plain Accept: text/html Referer: User-Agent: Mozilla/2.0

Jan.2001C.Watters37 HTTP REQUEST PROTOCOL Request = Simple | Full Simple = GET CRLF Full = Method URI ProtVersion CRLF [ *] [CRLF ] Method = GET | POST | HEAD | …. = : CRLF = MIME conforming message

Jan.2001C.Watters38 HTTP Header fields General-header fields –used for both requests and responses Request-header fields –used for responses –extra client information for use by server –optional

Jan.2001C.Watters39 General-header fields Date: mon,11, Jan :14:32 GMT MIME-version: 1.0 Pragma: no cache –directives

Jan.2001C.Watters40 Request-header fields acceptable MIME types for response –Accept:text/html – Accept:*.* 401 response from client –Authorization: Basic abcdef (uuencoded username and password) From:client- -addr

Jan.2001C.Watters41 More Request-header fields If-Modified-Since:date –conditional get source of current requested Url –Referer:URL robot/browser identification –User-Agent:Mozilla/2.0

Jan.2001C.Watters42 Looking at the HTTP Header Values In Perl –$ENV{“From”} In Netscape –

Jan.2001C.Watters43 HTTP Methods Client requests either –simple request –full request Request-line= method Request-URI HTTP-version CRLF GET /catalog/ip.html HTTP/1.0

Jan.2001C.Watters44 Simple requests Only for HTTP 0.9 only uses Get method causes the server to locate and transfer the object specified client responsible for handling the object GET CRLF

Jan.2001C.Watters45 Full Request Uses HTTP version and more methods method tells server what to do to the resource requested Methods –GET –POST –HEAD

Jan.2001C.Watters46 GET Method Request server to retrieve object specified conditional GET –request message includes –If-Modified-Since in header

Jan.2001C.Watters47 HEAD Method Like GET but does not return the object returns a header about the resource requested (metainformation) good way to test link validity

Jan.2001C.Watters48 POST Method Include an object in the request server should use that object in processing the request must include a Content-Length in header

Jan.2001C.Watters49 HTTP Response Message HTTP protocol version 3 digit status code reason phrase CRLF optional header fields CRLF

Jan.2001C.Watters50 HTTP Response Header Fields Additional information about the server such as: –LOCATION: exact URI address –SERVER: server software (CERN/3.0) –WWW-AUTHENTICATE: status 401 responses (unauthorized request) server challenges client client may use to send authorization info to server

Jan.2001C.Watters51 Understanding STATUS Codes 1xx - not yet in use 2xx - action successful 3xx - further action needed 4xx - client request error 5xx - server error

Jan.2001C.Watters52 HTTP Transaction 1. Client and server establish a connection 2. Client makes a request 3. Server makes a response 4. Server terminates connection

Jan.2001C.Watters53 Step 1 establish connection –TCP/IP connection set up –uses a port number as application reference –usually port 80 –ports 1024 are open) Step 2 client request –Http message sent with a request line –request-line = method URL HTTP version

Jan.2001C.Watters54 Web Port Assignments 21FTP 23 Telnet 25 smtp (mail) 70 gopher 79 finger 80 HTTP

Jan.2001C.Watters55 Step 3 Server response –server sends Http message and optionally requested data –resp-message = HTTP version statuscode reason-phrase [optional stuff] Step 4 connection terminated –usually the server –sometimes the client “stops” it –anything else, whoever notices terminates