Web and HTtTP First some jargon Web page consists of objects Object can be HTML file,JPEG image,Java applet,audio file,. Web page consists of base HTML-file which includes several referenced ob.jects Each object is addressable by a URL ▣Example URL: www.someschool.edu/someDept/pic.gif host name path name 2:Application Layer 21
2: Application Layer 21 Web and HTTP First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file,. Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: www.someschool.edu/someDept/pic.gif host name path name
HTTP overview HTTP:hypertext transfer protocol Web's application layer PC running HTTP request protocol Explorer HTTP ▣client,/server model response *client:browser that requests,receives, "displays"Web objects HTTP request Server running server:Web server HTTP response Apache Web server sends objects in response to requests Mac running ▣HTTP1.0:RFC1945 Navigator HTTP 1.1:RFC 2068 2:Application Layer 22
2: Application Layer 22 HTTP overview HTTP: hypertext transfer protocol Web’s application layer protocol client/server model ❖ client: browser that requests, receives, “displays” Web objects ❖ server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 PC running Explorer Server running Apache Web server Mac running Navigator
HTTP overview (continued) Uses TCP: HTTP is "stateless" client initiates TCP ▣server maintains no connection(creates socket) information about to server,port 80 past client requests ▣server accepts TCP connection from client aside Protocols that maintain HTTP messages (application- "state"are complex! layer protocol messages) ▣ past history (state)must exchanged between browser be maintained (HTTP client)and Web server (HTTP server) if server/client crashes, their views of "state"may TCP connection closed be inconsistent,must be reconciled 2:Application Layer 23
2: Application Layer 23 HTTP overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is “stateless” server maintains no information about past client requests Protocols that maintain “state” are complex! past history (state) must be maintained if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside
HTTP connections Nonpersistent HTTP Persistent HTTP ▣At most one object is Multiple ob.jects can sent over a TCP be sent over single connection. TCP connection ▣HTTP/1.0uses between client and nonpersistent HTTP server. ▣HTTP/1.1uses persistent connections in default mode 2:Application Layer 24
2: Application Layer 24 HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode
Nonpersistent HTTP (contains text, Suppose user enters URL references to 10 www.someSchool.edu/someDepartment/home.index jpeg images) 1a.HTTP client initiates TCP connection to HTTP server 1b.HTTP server at host (process)at www.someSchool.edu on port 80 www.someSchool.edu waiting for TCP connection at port 80. "accepts"connection,notifying client 2.HTTP client sends HTTP request message(containing URL)into TCP connection~ 3.HTTP server receives request socket.Message indicates message,forms response that client wants object message containing requested someDepartment/home.index object,and sends message into its socket time 2:Application Layer 25
2: Application Layer 25 Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time (contains text, references to 10 jpeg images)