Chapter 3 Network Programming and Applications

1. Chapter 3 Network Programming and Applications

2. Topics Covered • Client-Server Computing • Communication Paradigm • An Example Application Program Interface • Definition Of The API • An Echo Application • A Chat Application • A Web Application • Managing Multiple Connections With The Select Function

3. Network Communication • Applications us a network in pairs to exchange messages • Example • A distributed database service that allows remote users to access a central database • One application runs on a remote computer to send request • The other application on the computer that has the databases to send a response • Only the two applications understand the message format and meaning

4. Client-Server Computing (1) • One application starts first and waits for the other application to contact it • The second application must know the location where the first application is waiting • This is known as client-server computing • The program that waits for contact is called a server • The program that initiates contact is known as a client who must know where the server is running, and must specify the location

5. Client-Server Computing (2) How does a client specify the location of a server? • In the Internet, a location is given by a pair of identifiers • (computer, application) • computer identifies the computer on which the server is running • application identifies a particular application program on that computer • Humans enter the names (i.e. echo), and software translates each name to a corresponding binary value automatically because application software use binary value

6. Communication Paradigm • Two applications • establish communication, • exchange messages back and forth, • and then terminate • The steps (in details) are: • The server starts first, and waits for contact from a client • The client specifies the server's location and requests a connection be established • Once a connection is in place, the client and server use the connection to exchange messages • After they finish sending data, the client and server each send an end-of-file (EOF) and the connection is terminated

7. An Example Application Program Interface • Application Program Interface (API) is used to describe the set of operations available to a programmer • Seven functions that an application can call

8. An Intuitive Look At The API • A server begins by calling await_contact to wait for contact from a client • The client begins by calling make_contact to establish contact • Once the client has contacted the server, the two can exchange messages with send and recv • Order of send or receive must be known by server and client, if both sides try to receive without sending, they will block forever • An application calls send_eof to terminate the communication • On the other side, recv returns a value of zero to indicate the termination

9. Definition Of The API • Define 3 data types to keep our API independent of particular OS and NW software

10. Functions • Await_Contact Function: connection await_contact(appnum a) • A server calls function await_contact to wait for contact from a client • One argument specifies a number that identifies the server application and a client must specify the same number when contacting the server. • The server uses the return value (type connection ) to transfer data • Make_Contact Function: connection make_contact (computer c, appnum a) • client calls function make_contact to establish contact with a server • Two arguments to identify a computer of the server application number • The client uses the return value which is of type connection to transfer data

11. Functions • appnum appname_to_appnum(char *a) • Clients and servers both use it to translate from a human-readable service name for a service to an internal binary value • The call takes one argument and returns an equivalent binary value of type appnum • computer cname_to_comp (char *c) • Clients call it to convert from a human-readable computer name to the internal binary value • The call takes one argument and returns an equivalent binary value of type computer

12. Functions • int send (connection con, char *buffer, int length, int flags) • Both clients and servers use send to transfer data across the network • Four arguments specifies a connection previously established with await_contact or make_contact; the address of a buffer containing data to send; the length of the data in bytes (octets); the fourth argument is zero for normal transfer • Send returns the number of bytes transferred, or a negative value if an error occurred • int recv (connection con, char *buffer, int length, int flags) • Both clients and servers use recv to access data that arrives • Four arguments specifies: a connection with await_contact or make_contact; the address of a buffer into which the data to be placed; the size of the buffer in bytes; and the fourth is zero for normal transfer • recv returns the number of bytes that were placed in the buffer • zero to indicate that EOF has been reached or a negative value to indicate that an error occurred • int recvln (connection con, char *buffer, int length) • function recvln that repeatedly calls recv until an entire line of text has been received.

13. Functions • Both the client and server must use send_eof after sending data • to inform the other side that no further transmission will occur • On the other side, the recv function returns zero when it receives the EOF int send_eof(connection con) • Argument specifies a connection previously established with await_contact or make_contact • The function returns a negative value • to indicate that an error occurred, and a non-negative value otherwise

15. Code For An Echo Application (1) • The client repeatedly • prompts the user for a line of input, • sends the line to the server, • and then displays whatever the server sends back. • Like all the applications described in this chapter, • the echo application operates across a NW • That is, the client and server programs can run on separate computers

16. Code For An Echo Application (2) • Ex: suppose someone using computer lancelot.cs.purdue.edu chooses 20000 as the application number • The server is invoked by the command: echoserver 20000 • If some other application is using number 20000, • the server emits an appropriate error message and exits • the user must choose another number. • Once the server has been invoked, the client is invoked: echoclient lancelot.cs.purdue.edu 20000

17. Code For A Chat Application • A simplified version of chat that works between a single pair of users • One user begins by choosing an application number and running the server • Ex: suppose a user on excalibur.cs.purdue.edu runs the server: chatserver 25000 • A user on another computer can invoke the client: chatclient excalibur.cs.purdue.edu 25000 • To keep the code as small as possible • the scheme requires users to take turns entering text • Users alternate entering text until one of them sends an EOF

18. Code for Web Application

19. Browser Interface • The World Wide Web (WWW) is the large-scale, online repository of information • A browser is an interactive program used to access the Web • Web is a “distributed” “hypermedia” system that supports interactive access because a browser can display both text and graphics.

20. Document Representation • Web uses a standard representation known as the HyperText Markup Language (HTML) • HTML is classified as a “markup language” • because it only gives general guidelines for display • but does not include detailed formatting instructions • Markup language is important because it allows a browser to adapt the page to the underlying display: Thus, the same page can be formatted for • a large or small computer monitor • a high resolution or low resolution display • or a handheld device such as a PDA

21. HTML Format And Representation • Each HTML document is divided into • a head that contains information about the document • a body that contains the information to be displayed • HTML tags provide structure for the document as well as formatting

22. Other Markup Languages Other markup languages have been created for special purposes. • Voice Extensible Markup Language (VoiceXML) or VXML • specifies communication in human speech • Extensible Markup Language (XML) • Unlike HTML, XML does not specify layout • Instead, XML gives an internal representation that provides names for each field in a data item

24. www.someschool.edu/someDept/pic.gif path name host name 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:

25. 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 HTTP overview HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator

26. 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 HTTP is “stateless” server maintains no information about past client requests HTTP overview (continued) aside 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

27. 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

28. 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 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 someDepartment/home.index 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time

29. 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects Nonpersistent HTTP (cont.) 4. HTTP server closes TCP connection. time 6.Steps 1-5 repeated for each of 10 jpeg objects

30. initiate TCP connection RTT request file time to transmit file RTT file received time time Response time modeling Definition of RRT: time to send a small packet to travel from client to server and back. 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

31. Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate host resources for each TCP connection but browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server are sent over connection 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 Persistent HTTP

32. 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

33. HTTP request message: general format

34. 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 www.somesite.com/animalsearch?monkeys&banana

35. HTTP/1.0 GET requests a specified item from the server POST sends data to the server The server appends the data to a specified item HEAD requests status information about an item The server returns the status without returning a copy of the item itself HTTP/1.1 GET, POST, HEAD PUT sends data to the server The server uses the data to replace a specified item . It is seldom used; most Web pages that send data use POST instead DELETE deletes file specified in the URL field Method types

36. 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

37. 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 In first line in server->client response message. A few sample codes:

38. 1. Telnet to your favorite Web server: Trying out HTTP (client side) for yourself 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!

39. 35.18 Alternative Transfer Protocols • Several extensions and alternatives have been proposed to enhance or replace HTTP • HTTPS • protocol provides for secure HTTP transfers • (i.e., it uses encryption to guarantee confidentiality of the data being transferred in an HTTP session) • HTTP version 1.1 introduces the concept of persistent connections • in which a single TCP connection is used for multiple transfers • (i.e., multiple GET requests from the browser). • HTTP-NG (Next-Generation of HTTP) • that will use binary encoding for headers instead of the current ASCII representation

40. Many major Web sites use cookies Four components: 1) cookie header line in the HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host and managed by user’s browser 4) back-end database at Web site 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 User-server state: cookies

41. client server usual http request msg usual http response + Set-cookie: 1678 Cookie file Cookie file Cookie file amazon: 1678 ebay: 8734 ebay: 8734 amazon: 1678 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” (cont.) server creates ID 1678 for user entry in backend database access access one week later:

42. What cookies can bring: authorization shopping carts recommendations user session state (Web e-mail) Cookies (continued) aside Cookies and privacy: • cookies permit sites to learn a lot about you • you may supply name and e-mail to sites • search engines use redirection & cookies to learn yet more • advertising companies obtain info across sites

43. user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client Web caches (proxy server) 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

44. Cache acts as both client and server Typically cache 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 (but so does P2P file sharing) More about Web caching

45. 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 Consequences utilization on LAN = 15% utilization on access link = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds Caching example origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

46. 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 Caching example (cont) origin servers public Internet 10 Mbps access link institutional network 10 Mbps LAN institutional cache

47. Install cache suppose hit rate is .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 + milliseconds < 1.4 secs Caching example (cont) origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

48. Goal: don’t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> 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 server cache 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>

49. Code For A Web Application (1) • To run the server, a user chooses an application number and invokes the server • Ex: if a user on computer netbook.cs.purdue.edu chooses application number 27000, the server can be invoked with the command: webserver 27000 • The client specifies a computer, a path name, and an application number: webclient netbook.cs.purdue.edu /index.html 27000 • It is possible to use a conventional Web browser (such as, Internet Explorer or Netscape) to access the server http://netbook.cs.purdue.edu:27000/index.html

50. Code For A Web Application (2) • The client code is extremely simple: • after establishing communication with the Web server, it sends a request, which must have the form : GET / path HTTP/1.0 CRLF CRLF CRLF GET • where path denotes the name of an item such as index.html , • CRLF denotes the two characters carriage return and line feed. • After sending the request, the client receives and prints output from the server