EEC-484/584 Computer Networks

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

2. Outline • 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? EEC-484/584: Computer Networks

4. 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 Creating a Network Application EEC-484/584: Computer Networks

5. 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 Inter-Process Communications EEC-484/584: Computer Networks

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

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

8. 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 Addressing EEC-484/584: Computer Networks

9. 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 Application Layer Protocol Defines EEC-484/584: Computer Networks

10. 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” What Transport Service Does an Application Need? 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 EEC-484/584: Computer Networks

11. Transport Service Requirements of Common Applications Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games instant messaging Bandwidth elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss EEC-484/584: Computer Networks

12. Large Scale Internet Application Architecture http://img.microsoft.com/resources/documentation/wss/2/all/adminguide/en-us/stsb02b.gif

13. 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

14. Web and HTTP • 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 • 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

15. http://www.someschool.edu/someDept/pic.gif path name host name protocol name 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 EEC-484/584: Computer Networks

16. Some Common URLs EEC-484/584: Computer Networks

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

18. HTTP Overview • Uses TCP: • 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

19. 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

20. 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 HTTP Connections EEC-484/584: Computer Networks

21. 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 Nonpersistent HTTP (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

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

23. initiate TCP connection RTT request file time to transmit file RTT file received time time 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) EEC-484/584: Computer Networks

24. 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+transmit time EEC-484/584: Computer Networks

25. Non-Persistent HTTP Issues • Requires 2 RTTs per object • OS overhead for each TCP connection • Browsers often open parallel TCP connections to fetch referenced objects (to reduce response time) EEC-484/584: Computer Networks

26. 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

27. 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 • Client sends requests as soon as it encounters a referenced object • As little as one RTT for all the referenced objects EEC-484/584: Computer Networks

28. 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 HTTP Request Message • Two types of HTTP messages: request, response • HTTP request message: • ASCII (human-readable format) EEC-484/584: Computer Networks

29. HTTP Request Message: General Format EEC-484/584: Computer Networks

30. Post method: Web page often includes form input Input is uploaded to server in entity body URL method: Uses GET method Input is uploaded in URL field of request line: Uploading Form Input http://www.somesite.com/animalsearch?monkeys&banana EEC-484/584: Computer Networks

31. 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 Method types EEC-484/584: Computer Networks

32. 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

33. 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 HTTP Response Status Codes Status code is in first line of the response message: EEC-484/584: Computer Networks

34. 1. Telnet to your favorite Web server: Trying out HTTP 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 telnet cis.poly.edu 80 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET /~ross/ HTTP/1.1 Host: cis.poly.edu 3. Look at response message sent by HTTP server! EEC-484/584: Computer Networks

35. User-Server State: Cookies Many major Web sites use cookies Four components: 1) cookie header line in HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site EEC-484/584: Computer Networks

36. User-Server State: Cookies Example: • Susan access Internet always from same PC • She visits a specific e-commerce site for first time • When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID EEC-484/584: Computer Networks

37. client server usual http request msg usual http response + Set-cookie: 1678 Cookie file Cookie file Cookie file amazon: 1678 ebay: 8734 amazon: 1678 ebay: 8734 ebay: 8734 cookie- specific action usual http request msg cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http response msg cookie- spectific action Cookies: Keeping “State” server creates ID 1678 for user entry in backend database access access one week later: EEC-484/584: Computer Networks

38. Cookies What cookies can bring: • authorization • shopping carts • recommendations • user session state (Web e-mail) How to keep “state”: • Protocol endpoints: maintain state at sender/receiver over multiple transactions • cookies: http messages carry state EEC-484/584: Computer Networks

39. Performance Enhancement • Caching • Save pages that have been requested in case they are used again • Client-side technique • Server replication • Replicate server’s contents at multiple locations • Sometimes called mirroring • Content delivery networks • Deliver contents for their providers to end users efficiently for a fee EEC-484/584: Computer Networks

40. 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 Web Caching 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

41. 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 More about Web Caching EEC-484/584: Computer Networks

42. 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 Non-Caching Example origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN EEC-484/584: Computer Networks

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

44. 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 Non-Caching Example origin servers public Internet 10 Mbps access link institutional network 10 Mbps LAN EEC-484/584: Computer Networks

45. 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 Caching Example origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN Institutional Proxy server EEC-484/584: Computer Networks

46. 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 Conditional GET 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