Discussion Session 4 EE450 Computer Networks Topic Network Applications 1 Some network apps v v v v v v v e mail web instant messaging remote login P2P file sharing multi user network games streaming stored video YouTube v v v v v voice over IP real time video conferencing cloud computing v 2 Creating a network app write programs that run on different end systems communicate over network e g web server software communicates with browser software No need to write software for network core devices network core devices do not run user applications applications on end systems allows for rapid app development propagation application transport network data link physical application transport network data link physical application transport network data link physical 3 Processes communicating process program running within a host v within same host two processes communicate using inter process communication defined by OS v processes in different hosts communicate by exchanging messages client process process that initiates communication server process process that waits to be contacted v aside applications with P2P architectures have client processes server processes 4 Web and HTTP First a review v web page consists of objects v object can be HTML file JPEG image Java applet audio file v web page consists of base HTML file which includes several referenced objects v each object is addressable by a URL v example URL www someschool edu someDept pic gif host name path name 5 HTTP overview HTTP hypertext transfer protocol v v 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 PC running Explorer Server running Apache Web server Mac running Navigator 6 HTTP overview continued Uses TCP HTTP is stateless v v v v v 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 server maintains no information about past client requests aside protocols that maintain state are complex v past history state must be maintained v if server client crashes their views of state may be inconsistent must be reconciled 7 HTTP connections non persistent HTTP v at most one object sent over TCP connection persistent HTTP v multiple objects can be sent over single TCP connection between client server 8 Nonpersistent HTTP suppose user enters URL contains text www someSchool edu someDepartment home index references to 10 jpeg images 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 9 Nonpersistent HTTP cont 4 HTTP server closes TCP 5 HTTP client receives response connection message containing html file displays html Parsing html file finds 10 referenced jpeg objects time 6 Steps 1 5 repeated for each of 10 jpeg objects 10 Non Persistent HTTP Response time definition of RTT time for a small packet to travel from client to server and back response time v one RTT to initiate TCP connection v one RTT for HTTP request and first few bytes of HTTP response to return v file transmission time total 2RTT transmit time initiate TCP connection RTT request file RTT file received time time to transmit file time 11 Persistent HTTP non persistent HTTP issues v requires 2 RTTs per object v OS overhead for each TCP connection v browsers often open parallel TCP connections to fetch referenced objects persistent HTTP v server leaves connection open after sending response v subsequent HTTP messages between same client server sent over open connection v client sends requests as soon as it encounters a referenced object v as little as one RTT for all the referenced objects 12 HTTP request message v v two types of HTTP messages request response HTTP request message ASCII human readable format request line GET POST HEAD commands header lines carriage return line feed at start of line indicates end of header lines carriage return character line feed character GET index html HTTP 1 1 r n Host www net cs umass edu r n User Agent Firefox 3 6 10 r n Accept text html application xhtml xml r n Accept Language en us en q 0 5 r n Accept Encoding gzip deflate r n Accept Charset ISO 8859 1 utf 8 q 0 7 r n Keep Alive 115 r n Connection keep alive r n r n 13 HTTP response message status line protocol status code status phrase header lines data e g requested HTML file HTTP 1 1 200 OK r n Date Sun 26 Sep 2010 20 09 20 GMT r n Server Apache 2 0 52 CentOS r n Last Modified Tue 30 Oct 2007 17 00 02 GMT r n ETag 17dc6 a5c bf716880 r n Accept Ranges bytes r n Content Length 2652 r n Keep Alive timeout 10 max 100 r n Connection Keep Alive r n Content Type text html charset ISO 8859 1 r n r n data data data data data 14 HTTP response status codes v v status code appears in 1st line in server client response message some sample codes 200 OK request succeeded requested object later in this msg 301 Moved Permanently requested object moved new location specified later in this msg Location 400 Bad Request request msg not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported 15 Trying out HTTP client side for yourself 1 Telnet to your favorite Web server telnet cis poly edu 80 opens TCP connection to port 80 default HTTP server port at cis poly edu anything typed in sent to port 80 at cis poly edu 2 type in a GET HTTP request GET ross HTTP 1 1 Host cis poly edu by typing this in hit carriage return twice you send this minimal but complete GET request to HTTP server 3 look at response message sent by HTTP server or use Wireshark 16 FTP the file transfer protocol The image cannot be displayed Your computer may not have enough memory to open the image or the image may have been corrupted Restart your computer and then open the file again If the red x still appears you may have to delete the image and then insert it again user at host v v v v FTP FTP user client interface local file