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EEC-484/584 Computer Networks

Learn about client-server model, sockets, addressing, protocols, inter-process communications, HTTP, and more in network applications.

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EEC-484/584 Computer Networks

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  1. EEC-484/584Computer Networks Lecture 4 Wenbing Zhao wenbingz@gmail.com (Part of the slides are based on Drs. Kurose & Ross’s slides for their Computer Networking book)

  2. Administrative: Lab1 (instruction posted online): Wednesday Lab report requirement: Typed hardcopy, must include questions/tasks, your answers, and snapshots to backup your answers Today’s topics Principles of networked applications Web and HTTP EEC-484/584: Computer Networks

  3. Application Layer Protocols Principles of networked applications Client server model Sockets Addressing Protocol What do we need from transport layer? 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  4. Creating a Network Application Write programs that run on different end systems and communicate over a network No need to write code for devices in subnet Subnet devices do not run user application code application 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 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  5. Inter-Process Communications Process: program running within a host Processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted More accurately, client and server should be regarded as the roles played by a process. A process can be both a client and a server 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  6. Sockets Process sends/receives messages to/from its socket For each point-to-point connection, there are two sockets, one on each side API (Application Programming Interface): (1) choice of transport protocol; (2) ability to fix a few parameters host or server host or server process process socket socket TCP with buffers, variables TCP with buffers, variables Controlled by app developer Internet Controlled by OS 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  7. Addressing To receive messages, a process must have an identifier Each host device has a unique 32-bit IP address Question: Does the IP address of the host on which the process runs suffice for identifying the process? 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  8. Addressing Identifier includes both IP address and port numbers (16-bit) associated with process on host Example port numbers: HTTP server: 80 SSH server: 22 To send HTTP request to academic.csuohio.edu Web server: IP address: 137.148.49.46 Port number: 80 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  9. Application Layer Protocol Defines Types of messages exchanged e.g., request, response Message syntax what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., KaZaA 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  10. What Transport Service Does an Application Need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” • Bandwidth • some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” • other apps (“elastic apps”) make use of whatever bandwidth they get 1/5/2020 EEC-484/584: Computer Networks Wenbing Zhao

  11. The World Wide Web Creation of Tim Berners-Lee, in 1989 CERN nuclear physics research Mosaic – first graphical interface, creation of Marc Andersson (and others), precursor to Netscape Uses a client-server architecture Web server Web browser Runs on HTTP over TCP EEC-484/584: Computer Networks

  12. Web and HTTP Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file,… A Web page consists of a base HTML-file which includes several referenced objects Each object is addressable by a URL The idea of having one page point to another is called hypertext Invented by Vannevar Bush, a MIT EE professor, in 1945 EEC-484/584: Computer Networks

  13. URL – Uniform Resource Locater Example URL: URL encodes three types of information What is the page called – local path name uniquely indicating the specific page Where is the page located – Host name of the server on which the page is located How can the page be accessed – protocol, e.g., http, ftp http://www.someschool.edu/someDept/pic.gif path name host name protocol name EEC-484/584: Computer Networks

  14. HTTP Overview HTTP: HyperText Transfer Protocol Web’s application layer protocol client/server model HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator EEC-484/584: Computer Networks

  15. HTTP Overview Client initiates TCP connection (creates socket) to server, port 80 Server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed EEC-484/584: Computer Networks

  16. HTTP Overview 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 EEC-484/584: Computer Networks

  17. 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 EEC-484/584: Computer Networks

  18. Nonpersistent HTTP Suppose user enters URL http://www.someSchool.edu/someDept/home.index 1a. HTTP client initiates TCP connection to HTTP server at www.someSchool.edu on port 80 (contains text, references to 10 jpeg images) 1b. HTTP server at host 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 socket. Message indicates that client wants object someDept/home.index 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time EEC-484/584: Computer Networks

  19. Nonpersistent HTTP 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 4. HTTP server closes TCP connection. time 6.Steps 1-5 repeated for each of 10 jpeg objects EEC-484/584: Computer Networks

  20. Non-Persistent HTTP: Response Time Definition of RTT: time to send a small packet to travel from client to server and back (Round Trip Time) initiate TCP connection RTT request file time to transmit file RTT file received time time EEC-484/584: Computer Networks

  21. Non-Persistent HTTP: Response Time Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time Total = 2RTT+transmission time EEC-484/584: Computer Networks

  22. Non-Persistent HTTP Issues Requires 2 RTTs per object OS overhead for each TCP connection To reduce response time, browsers often open parallel TCP connections to fetch referenced objects EEC-484/584: Computer Networks

  23. Persistent HTTP Server leaves connection open after sending response Subsequent HTTP messages between same client/server sent over open connection EEC-484/584: Computer Networks

  24. Persistent HTTP Persistent without pipelining: Client issues new request only when previous response has been received One RTT for each referenced object Persistent with pipelining: Default in HTTP/1.1 Multiple requests are sent over the same connection concurrently. That is, after the first request, the second request is sent before the reply for the first request is received As little as one RTT for all the referenced objects EEC-484/584: Computer Networks

  25. HTTP Request Message Two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message EEC-484/584: Computer Networks

  26. HTTP Request Message: General Format HTTP header is pure ASCII based. It is very different from lower layer protocols such as TCP, which is binary based EEC-484/584: Computer Networks

  27. Method Types HTTP/1.0 GET POST HEAD Asks server to include only the header part in response HTTP/1.1 GET, POST, HEAD PUT Uploads file in entity body to path specified in URL field DELETE Deletes file specified in the URL field EEC-484/584: Computer Networks

  28. HTTP Response Message status line (protocol status code status phrase) HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested HTML file EEC-484/584: Computer Networks

  29. HTTP Response Status Codes 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported Status code is in first line of the response message: EEC-484/584: Computer Networks

  30. Web Caching user sets browser: Web accesses via proxy server browser sends all HTTP requests to proxy server object in cache: returns cached object else cache requests object from origin server, then returns object to client Goal: satisfy client request without involving origin server origin server Proxy server HTTP request HTTP request client HTTP response HTTP response HTTP request HTTP response client origin server EEC-484/584: Computer Networks

  31. More about Web Caching Proxy server acts as both client and server Typically proxy server is installed by ISP (university, company, residential ISP) Why Web caching? Reduce response time for client request Reduce traffic on an institution’s access link Internet dense with caches: enables “poor” content providers to effectively deliver content EEC-484/584: Computer Networks

  32. Conditional GET: HTTP Build-in Support for Caching Goal: don’t send object if cache is up-to-date Proxy server: specify date of cached copy in HTTP request If-modified-since: <date> Origin server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP response HTTP/1.0 304 Not Modified Origin Server Proxy server HTTP request msg If-modified-since: <date> object not modified HTTP request msg If-modified-since: <date> object modified HTTP response HTTP/1.0 200 OK <data> EEC-484/584: Computer Networks

  33. Non-Caching Example Assumptions Average object size = 100,000 bits Avg. request rate from institution’s browsers to origin servers = 15/sec Delay from institutional router to any origin server and back to router = 2 sec origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN EEC-484/584: Computer Networks

  34. Non-Caching Example Consequences Utilization on LAN = 15% Utilization on access link = 100% Total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN EEC-484/584: Computer Networks

  35. Non-Caching Example Possible solution Increase bandwidth of access link to, say, 10 Mbps Consequences Utilization on LAN = 15% Utilization on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs Often a costly upgrade origin servers public Internet 10 Mbps access link institutional network 10 Mbps LAN EEC-484/584: Computer Networks

  36. Caching Example Install proxy server Suppose hit rate is 0.4 Consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server Utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) Total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN Institutional Proxy server EEC-484/584: Computer Networks

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