Network Programming and Internet Applications

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• Chapter 3 4
• Network Programming and Internet Applications

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Topics

• Network communication
• client-server computing
• communication paradigm
• an example application program interface
• an intuitive look at the API
• definition of the API
• code for an echo application
• code for a chat application
• code for a web application

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Introduction

• An important idea
• A programmer can create Internet application
  software without understanding the underlying
  network technology or communication protocols.
• The key using libraries

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Network Communication

• Network data transfer without interpreting them
• Applications data processing and exchange
• doing it in pairs, both must know data format and
  meaning, e.g.,
• one side database services
• another side send requests

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Client-Server computing

• How the pairs know the location of each other?
• Server starts first and waits for the other
  application to contact it
• Client must know the location of the server and
  contact it.

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Specify the server

• The location of the server is given by a pair of
  identifiers
• (computer, application)
• The client can use alphabetic names as
  identifiers
• The network only knows binary numbers
• Network software does the translation

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Communication paradigm

• Sequence of operations
• The server application starts first, and waits
  for contact from a client
• The client contacts the server by specifying its
  location and requesting communication
• The client and server exchange messages
• After they finish sending data, the client and
  server each send an end-of-file to terminate
  communication

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An example Application Program Interface (API)

• API the set of operations available to an
  application programmer
• API specifies the arguments for each operation as
  well as the semantics

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The API for network programming
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An intuitive look at the API
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Definition of the API

• Data types used

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Functions defined by the API (1)

• connection await_contact(appnum a)
• connection make_contact(computer c, appnum a)
• appnum appname_to_appnum(char a)
• computer cname_to_comp(char c)

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Functions defined by the API (2)

• int send(connection con, char buffer, int
  length, int flags)
• con The connection identifier
• buffer Data buffer
• length Number of bytes to send.
• flag 0
• return length if successful negative
• otherwise

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Functions defined by the API (3)

• int recv(connection con, char buffer, int
  length, int flags)
• Block until data arrives.
• con The connection identifier
• buffer Data buffer
• length Buffer size.
• return of bytes in the buffer
• 0 if receives EOF
• negative otherwise

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Functions defined by the API (4)

• int recvln(connection con, char buffer, int
  length)
• int send_eof(connection con)

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Summary of API types
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Code for an echo application

• Server echoes back all the data it receives
• Client repeatedly does the following
• prompts the user for a line of input
• sends the line to the server
• displays whatever the server sends back

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Where the client and the server reside?
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How to run the programs?

• invoke the server on computer cms07201.uhd.edu
• gt echoserver 20000
• invoke the client
• gt echoclient cms07201.uhd.edu

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Example echo server code (1)

• / echoserver.c /
• include ltstdlib.hgt
• include ltstdio.hgt
• include ltcnaiapi.hgt
• define BUFFSIZE 256
• /------------------------------------------------
  ------------
• Program echoserver
• Purpose wait for a connection from an
  echoclient and echo data
• Usage echoserver ltappnumgt
• ------------------------------------------------
  ------------
• /

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Example echo server code (2)

• int
• main(int argc, char argv)
• connection conn
• int len
• char buffBUFFSIZE
• if (argc ! 2)
• (void) fprintf(stderr, "usage s ltappnumgt\n",
• argv0)
• exit(1)
• / wait for a connection from an echo client /
• conn await_contact((appnum) atoi(argv1))
• if (conn lt 0)
• exit(1)

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Example echo server code (3)

• / iterate, echoing all data received until end
  of file /
• while((len recv(conn, buff, BUFFSIZE, 0)) gt 0)
• (void) send(conn, buff, len, 0)
• send_eof(conn)
• return 0

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Example echo client code (1)

• / echoclient.c /
• include ltstdlib.hgt
• include ltstdio.hgt
• include ltcnaiapi.hgt
• define BUFFSIZE 256
• define INPUT_PROMPT "Input gt "
• define RECEIVED_PROMPT "Receivedgt "
• int readln(char , int)
• /------------------------------------------------
  ------------
• Program echoclient
• Purpose contact echoserver, send user input
  and print server response
• Usage echoclient ltcompnamegt appnum
• Note Appnum is optional. If not specified
  the standard echo appnum
• (7) is used.

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Example echo client code (2)

• int
• main(int argc, char argv)
• computer comp
• appnum app
• connection conn
• char buffBUFFSIZE
• int expect, received, len
• if (argc lt 2 argc gt 3)
• (void) fprintf(stderr,
• "usage s ltcompnamegt appnum\n", argv0)
• exit(1)
• / convert the arguments to binary format comp
  and appnum /
• comp cname_to_comp(argv1)
• if (comp -1)

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Example echo client code (3)

• if (argc 3)
• app (appnum) atoi(argv2)
• else
• if ((app appname_to_appnum("echo")) -1)
• exit(1)
• / form a connection with the echoserver /
• conn make_contact(comp, app)
• if (conn lt 0)
• exit(1)
• (void) printf(INPUT_PROMPT)
• (void) fflush(stdout)

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Example echo client code (4)

• / iterate read input from the user, send to
  the server, /
• / receive reply from the server, and
  display for user /
• while((len readln(buff, BUFFSIZE)) gt 0)
• / send the input to the echoserver /
• (void) send(conn, buff, len, 0)
• (void) printf(RECEIVED_PROMPT)
• (void) fflush(stdout)

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Example echo client code (5)

• / read and print same no. of bytes from echo
  server
• /
• expect len
• for (received 0 received lt expect)
• len recv(conn, buff, (expect - received) lt
  
• BUFFSIZE ? (expect - received)
• BUFFSIZE, 0)
• if (len lt 0)
• send_eof(conn)
• return 1
• (void) write(STDOUT_FILENO, buff, len)
• received len
• (void) printf("\n")
• (void) printf(INPUT_PROMPT)
• (void) fflush(stdout)

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Example echo client code (6)

• / iteration ends when EOF found on stdin /
• (void) send_eof(conn)
• (void) printf("\n")
• return 0

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Code for a chat application

• A simplified version of chat
• works between a single pair of users
• client and server take turns entering text
  client starts
• Both client and server enter a loop of reading
  and sending local users input and displaying the
  message from the other side
• Both client and server proceed until they receive
  end-of-file

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To run the aaplication

• Server
• chatserver 25000
• Client
• chatclient cms70201.uhd.edu 25000

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Example chat server code (1)

• / chatserver.c /
• include ltstdlib.hgt
• include ltstdio.hgt
• include ltcnaiapi.hgt
• define BUFFSIZE 256
• define INPUT_PROMPT "Input gt "
• define RECEIVED_PROMPT "Receivedgt "
• int recvln(connection, char , int)
• int readln(char , int)
• /------------------------------------------------
  ------------
• Program chatserver
• Purpose wait for a connection from a
  chatclient allow users to chat
• Usage chatserver ltappnumgt

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Example chat server code (2)

• int
• main(int argc, char argv)
• connection conn
• int len
• char buffBUFFSIZE
• if (argc ! 2)
• (void) fprintf(stderr, "usage s ltappnumgt\n",
• argv0)
• exit(1)
• (void) printf("Chat Server Waiting For
  Connection.\n")
• / wait for a connection from a chatclient /
• conn await_contact((appnum) atoi(argv1))
• if (conn lt 0)

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Example chat server code (3)

• / iterate, reading from the client and the
  local user /
• while((len recvln(conn, buff, BUFFSIZE)) gt 0)
  
• (void) printf(RECEIVED_PROMPT)
• (void) fflush(stdout)
• (void) write(STDOUT_FILENO, buff, len)
• / send a line to the chatclient /
• (void) printf(INPUT_PROMPT)
• (void) fflush(stdout)
• if ((len readln(buff, BUFFSIZE)) lt 1)
• break
• bufflen - 1 '\n'
• (void) send(conn, buff, len, 0)
• / iteration ends when EOF found on stdin or
  chat connection /

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Example chat client code (1)

• / chatclient.c /
• include ltstdlib.hgt
• include ltstdio.hgt
• include ltcnaiapi.hgt
• define BUFFSIZE 256
• define INPUT_PROMPT "Input gt "
• define RECEIVED_PROMPT "Receivedgt "
• int recvln(connection, char , int)
• int readln(char , int)
• /------------------------------------------------
  ------------
• Program chatclient
• Purpose contact a chatserver and allow users
  to chat
• Usage chatclient ltcompnamegt ltappnumgt

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Example chat client code (2)

• int
• main(int argc, char argv)
• computer comp
• connection conn
• char buffBUFFSIZE
• int len
• if (argc ! 3)
• (void) fprintf(stderr,
• "usage s ltcompnamegt ltappnumgt\n",
• argv0)
• exit(1)
• / convert the compname to binary form comp /
• comp cname_to_comp(argv1)
• if (comp -1)

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Example chat client code (3)

• / make a connection to the chatserver /
• conn make_contact(comp, (appnum)
  atoi(argv2))
• if (conn lt 0)
• exit(1)
• (void) printf("Chat Connection Established.\n")
• (void) printf(INPUT_PROMPT)
• (void) fflush(stdout)

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Example chat client code (4)

• / iterate, reading from local user and then
  from chatserver /
• while((len readln(buff, BUFFSIZE)) gt 0)
• bufflen - 1 '\n'
• (void) send(conn, buff, len, 0)
• / receive and print a line from the chatserver
  /
• if ((len recvln(conn, buff, BUFFSIZE)) lt 1)
• break
• (void) printf(RECEIVED_PROMPT)
• (void) fflush(stdout)
• (void) write(STDOUT_FILENO, buff, len)
• (void) printf(INPUT_PROMPT)
• (void) fflush(stdout)
• / iteration ends when stdin or the connection
  indicates EOF /

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Code for a web application

• Simplifications
• the server only supplies three web pages
• none of the pages contains anything except text
• pages are hard-wired into the code
• the client does not format the web pages

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Use standard protocol

• The web server can communicate with a
  conventional web browser, e.g., enter the
  following in IE
• http//cms70201.uhd.edu27000/index.html
• The web client can inter-operate with a
  commercial web server, e.g.,
• webclient www.uhd.edu /

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To run the applications

• To run the server
• webserver 27000
• To run the client
• webclient cms70201.uhd.edu /index.html 27000

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Example web client code (1)

• / webclient.c /
• include ltstdlib.hgt
• include ltstdio.hgt
• include ltcnaiapi.hgt
• define BUFFSIZE 256
• /------------------------------------------------
  ------------
• Program webclient
• Purpose fetch page from webserver and dump to
  stdout with headers
• Usage webclient ltcompnamegt ltpathgt appnum
• Note Appnum is optional. If not specified
  the standard www appnum (80) is used.
• ------------------------------------------------
  ------------
• /

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Example web client code (2)

• int
• main(int argc, char argv)
• computer comp
• appnum app
• connection conn
• char buffBUFFSIZE
• int len
• if (argc lt 3 argc gt 4)
• (void) fprintf(stderr, "sss", "usage ",
  argv0,
• " ltcompanegt ltpathgt appnum\n")
• exit(1)
• / convert arguments to binary computer and
  appnum /
• comp cname_to_comp(argv1)

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Example web client code (3)

• if (argc 4)
• app (appnum) atoi(argv3)
• else
• if ((app appname_to_appnum("www")) -1)
• exit(1)
• / contact the web server /
• conn make_contact(comp, app)
• if (conn lt 0)
• exit(1)
• / send an HTTP/1.0 request to the webserver /
• len sprintf(buff, "GET s HTTP/1.0\r\n\r\n",
  argv2)
• (void) send(conn, buff, len, 0)

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Example web client code (4)

• / dump all data received from the server to
  stdout /
• while((len recv(conn, buff, BUFFSIZE, 0)) gt 0)
• (void) write(STDOUT_FILENO, buff, len)
• return 0

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Example web server code (1)

• / webserver.c /
• include ltstdio.hgt
• include ltstdlib.hgt
• include lttime.hgt
• include ltcnaiapi.hgt
• if defined(LINUX) defined(SOLARIS)
• include ltsys/time.hgt
• endif
• define BUFFSIZE 256
• define SERVER_NAME "CNAI Demo Web Server"
• define ERROR_400 "ltheadgtlt/headgtltbodygtlthtmlgtlth1gtEr
  ror 400lt/h1gtltpgtThe server couldn't understand
  your request.lt/htmlgtlt/bodygt\n"
• define ERROR_404 "ltheadgtlt/headgtltbodygtlthtmlgtlth1gtEr
  ror 404lt/h1gtltpgtDo\
• cument not found.lt/htmlgtlt/bodygt\n"

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Example web server code (2)

• define HOME_PAGE "ltheadgtlt/headgtltbodygtlthtmlgtlth1gtWe
  lcome to the CNAI Demo Serverlt/h1gtltpgtWhy not
  visit ltulgtltligtlta href\"http//netbook.cs.purdue.
  edu\"gtNetbook Home Pagelt/agtltligtlta
  href\"http//www.comerbooks.com\"gtComer Books
  Home Pageltagtlt/ulgtlt/htmlgtlt/bodygt\n"
• define TIME_PAGE "ltheadgtlt/headgtltbodygtlthtmlgtlth1gtTh
  e current date is slt/h1gtlt/htmlgtlt/bodygt\n"
• int recvln(connection, char , int)
• void send_head(connection, int, int)
• /------------------------------------------------
  ------------
• Program webserver
• Purpose serve hard-coded webpages to web
  clients
• Usage webserver ltappnumgt
• ------------------------------------------------
  ------------
• /

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Example web server code (3)

• int
• main(int argc, char argv)
• connection conn
• int n
• char buffBUFFSIZE, cmd16, path64,
  vers16
• char timestr
• if defined(LINUX) defined(SOLARIS)
• struct timeval tv
• elif defined(WIN32)
• time_t tv
• endif
• if (argc ! 2)
• (void) fprintf(stderr, "usage s ltappnumgt\n",
  argv0)
• exit(1)

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Example web server code (4)

• while(1)
• / wait for contact from a client on specified
  appnum
• /
• conn await_contact((appnum) atoi(argv1))
• if (conn lt 0)
• exit(1)
• / read and parse the request line /
• n recvln(conn, buff, BUFFSIZE)
• sscanf(buff, "s s s", cmd, path, vers)
• / skip all headers - read until we get \r\n
  alone /
• while((n recvln(conn, buff, BUFFSIZE)) gt 0)
• if (n 2 buff0 '\r' buff1
• '\n')

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Example web server code (5)

• / check for unexpected end of file /
• if (n lt 1)
• (void) send_eof(conn)
• continue
• / check for a request that we cannot
  understand /
• if (strcmp(cmd, "GET") (strcmp(vers,
  "HTTP/1.0")
• strcmp(vers, "HTTP/1.1")))
• send_head(conn, 400, strlen(ERROR_400))
• (void) send(conn, ERROR_400,
• strlen(ERROR_400),0)
• (void) send_eof(conn)
• continue

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Example web server code (6)

• / send the requested web page or a "not found"
  error
• /
• if (strcmp(path, "/") 0)
• send_head(conn, 200, strlen(HOME_PAGE))
• (void) send(conn, HOME_PAGE,
• strlen(HOME_PAGE),0)
• else if (strcmp(path, "/time") 0)
• if defined(LINUX) defined(SOLARIS)
• gettimeofday(tv, NULL)
• timestr ctime(tv.tv_sec)
• elif defined(WIN32)
• time(tv)
• timestr ctime(tv)
• endif
• (void) sprintf(buff, TIME_PAGE, timestr)
• send_head(conn, 200, strlen(buff))
• (void) send(conn, buff, strlen(buff), 0)
• else / not found /

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Example web server code (7)

• /------------------------------------------------
  ------------
• send_head - send an HTTP 1.0 header with given
  status and content-len
• ------------------------------------------------
  ------------
• /
• void
• send_head(connection conn, int stat, int len)
• char statstr, buffBUFFSIZE
• / convert the status code to a string /
• switch(stat)
• case 200
• statstr "OK"
• break
• case 400
• statstr "Bad Request"
• break
• case 404

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Example web server code (8)

• default
• statstr "Unknown"
• break
• /
• send an HTTP/1.0 response with Server,
  Content-Length,
• and Content-Type headers.
• /
• (void) sprintf(buff, "HTTP/1.0 d s\r\n", stat,
  statstr)
• (void) send(conn, buff, strlen(buff), 0)
• (void) sprintf(buff, "Server s\r\n",
  SERVER_NAME)
• (void) send(conn, buff, strlen(buff), 0)
• (void) sprintf(buff, "Content-Length d\r\n",
  len)
• (void) send(conn, buff, strlen(buff), 0)

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Example web server code (9)

• (void) sprintf(buff, "\r\n")
• (void) send(conn, buff, strlen(buff), 0)

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The format of HTTP header

• HTTP/1.0 status status_string CRLF
• Server CNAI Demo Server CRLF
• Content-Length datasize CRLF
• Content-Type text/html CRLF
• CRLF

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The end of Chapter 3 4