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Web Basics and Protocols CS 502 – 20020129 Carl Lagoze Acks to McCracken Syracuse Univ. Cornell CS502 In the beginning…. Cornell CS502 In the beginning… Cornell CS502 ARPANET • DoD funded through leadership of Licklider • Inspired by move from batch to timesharing • Allowed remote login Cornell CS502 Packet Switching • Invented in early 1960’s by Baran, Davies, Kleinrock • digital, redundant, efficient, upgradeable (software) • 1969 ARPANET first network implementation Cornell CS502 Packet Switching • Network messages broken up into packets • Each pocket has a destination address • Pass and forward model – router gets packet, examine, decides where to send next • Message reassembled on other end Cornell CS502 Layered Protocol Model Cornell CS502 TCP/IP Protocol Suite • IP – packet delivery • TCP – virtual circuits, packet reassembly • ARP/RARP – address resolution Cornell CS502 Protocol Layers Cornell CS502 Internet Issues (Internet 2) • Demands of multimedia applications • Virtual circuit reservations – bandwidth and quality of service guarantees • Real time streaming protocols • State saving Cornell CS502 Internet Governance • Internet Society (ISOC) – Evolution, social & political issues • Internet Architecture Board (IAB) – Oversees standards process • Internet Engineering Task Force (IETF) – standards development • Internet Assigned Names Authority (IANA) – protocol # assignment • Internet Corporation for Assigned Names and Numbers (ICANN) – IP and DNS addresses • World Wide Web Consortium (W3C) – web standards and evolution Cornell CS502 Internet Documents • STD’s – Official IETF Internet standards – http://www.rfc-editor.org/rfcxx00.html • RFC’s – “Requests for Comments” to IETF community for information, standardization – http://www.ietf.org/rfc.html • Internet Drafts – IETF working documents – http://www.ietf.org/ID.html • W3C Reports (recommendations, drafts, notes) – http://www.w3.org/TR/ Cornell CS502 Well-Known Protocols • Telnet – external terminal interface, RFC 854 (1983) • FTP – file transfer, RFC 959 (1985) • SMTP – mail transport, RFC 821 (1982) • HTTP – distributed, collaborative hypermedia systems, RFC 1945 (1.0 1996), RFC 2616 (1.1 1999) Cornell CS502 Daemons and Ports Socket (Virtual Circuit) Cornell CS502 23 telnetd 80 httpd 21 ftpd Basic Socket ServerProgramming Cornell CS502 HTTP • HTTP is… – Designed for document transfer – Generic • not tied to web browsers exclusively • can serve any data type – Stateless • no persistent client/server connection Cornell CS502 HTTP Session • An HTTP session consists of a client request followed by a server response • Requests and responses: – are sent in plain text – conform to the HTTP syntax – consist of start line, headers, blank line, and message body Cornell CS502 HTTP Request • Start line – Consists of method, URL, version GET index.html HTTP/1.1 – Valid methods include: • GET, POST, HEAD, PUT, DELETE • Headers – HTTP/1.1 requires a Host: header • Body content Cornell CS502 HTTP Request Methods • Methods include – – – – GET: retrieve information identified by the URL HEAD: same as get but don't get message body (content) POST: accept the request content and send it to the URL PUT: store the request content as the given URL Cornell CS502 HTTP Response • Start line – consists of HTTP version, status code, and description HTTP/1.1 200 OK HTTP/1.1 404 Not Found • Headers Content-type: text/html • Content Cornell CS502 HTTP Response Codes • Respose coded by first digit – – – – – 1xx: informational, request received 2xx: success, request accepted 3xx: redirection 4xx: client error 5xx: server error Cornell CS502 HTTP Content Body • Header fields can affect content interpretation – required header field: Content-type – others: Content-Encoding, Content-Length, Expires, LastModified – added by web server - we will configure some of these later Cornell CS502 Serving a Page • User of client machine types in a URL client (Netscape) http://www.smallco.com/index.html Cornell CS502 server (Apache) Serving a Page • Server name is translated to an IP address via DNS client (Netscape) http://www.smallco.com/index.html 192.22.107.5 Cornell CS502 server (Apache) Serving a Page • Client connects to server using IP address and port number client (Netscape) http://www.smallco.com/index.html 192.22.107.5 Cornell CS502 192.22.107.5 port 80 server (Apache) Serving a Page • Client determines path and file to request client (Netscape) http://www.smallco.com/index.html Cornell CS502 server (Apache) Serving a Page • Client sends HTTP request to server client (Netscape) http://www.smallco.com/index.html Cornell CS502 GET index.html HTTP/1.1 server (Apache) Serving a Page • Server determines which file to send client (Netscape) http://www.smallco.com/index.html Cornell CS502 server (Apache) "index.html" is really /etc/httpd/htdocs/index.html Serving a Page • Server sends response code and the document client (Netscape) HTTP/1.1 200 OK Content-type: text/html [contents of index.html] http://www.smallco.com/index.html Cornell CS502 server (Apache) Serving a Page • Connection is broken client (Netscape) Cornell CS502 server (Apache)