Computer Networks

1
Computer Networks

• Project 1
• Client Server Programming

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TABLE OF CONTENTS

• Executive Summary
• How It Works
• The Big Picture
• Client and Server Operation
• User - Client Interface
• Client - Server Connection
• Server - Client Connection
• Client - Server Interface

Interacting With Oracle oracle.h Client -
Oracle Interaction Server - Oracle
Interaction Tools Available To You General
Advice The Steps To Follow Good
Citizenship Plagiarism How You Will Be
Evaluated
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EXECUTIVE SUMMARY

• This assignment serves as an introduction to
  client-server programming using the TCP/IP
  protocols. This is valuable in that you will
  gain some actual experience in system programming
  using standard communications protocols. You
  will write code that allows several processes to
  communicate with each other, and demonstrate that
  this communication actually occurs.

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HOW IT WORKS

• In this assignment, you will write both a client
  and a server. Your client and server will
  communicate using TCP, and your server may
  implement any service you choose. The ground
  rules are simple
• A) The server reads and writes data to and
  from the TCP connection.
• B) The server may prompt the client for input, or
  simply print a random message.
• C) Once you have debugged the server, it executes
  in background (even after you log out) waiting
  for service requests from a client.
• To access a service, the client opens a TCP
  connection to the server, sends and receives
  data, and then closes the connection.
  Conceptually, the client acts as a pipe between
  your terminal and the server, copying data sent
  by the server to standard output and sending data
  read from standard input to the server. The
  client terminates the connection with the server
  when it receives an end-of-file from either the
  server or standard input.

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HOW IT WORKS

• The above scenario omits a key aspect of
  client-server programming
• How does the client find out where the server is?
  What transport-level address (Internet address
  and port number) should the client connect to?
• One solution is to use a name server that
  dynamically maps service names into their
  transport-level addresses. We will supply you
  with a such a server, called oracle, allowing you
  to register the service you provide and advertise
  it to other students in the class.
• The oracle is like the white pages in your phone
  book. A server registers the name and transport
  address of its service in the phone book, and
  clients use the phone book to map service names
  to transport addresses.

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HOW IT WORKS

• When your server starts, the operating system
  could assign it an unused port number (e.g., XXX)
  on which that server can wait for incoming
  connection requests. (Alternatively, and the way
  well do it, is to use fixed ports. You will
  each have several ports that you own for the
  semester and that you can use for your servers.)
  
• The server then advertises the availability of
  its service by sending a short message to the
  oracle containing the name of the service (e.g.,
  "daytime") together with the transport address
  (host number and port XXX). The oracle server
  records the name-to-address mapping in its local
  database.
• When a client wishes to connect to a server, it
  first sends a message containing the desired
  service name (e.g., "daytime") to the oracle, and
  the oracle returns a message with the appropriate
  transport address (e.g., XXX). The client then
  opens a TCP connection to that service. Exact
  details for communicating with the oracle are
  described below.

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THE BIG PICTURE

• An Overview of How it works

SERVER
CLIENT
OPERATING SYSTEM
Getservbyname ?
? Getservbyname
? Oracle port returned.
Oracle port returned ?
ORACLE
UDP
UDP
Register the server port number using the OM
protocol
Ask the oracle for the number of the server port.
? Client uses the server port to request result
from server.
Server returns result to client ?
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CLIENT AND SERVER OPERATION
User - Client Interface

• The client accepts the following set of commands
  from standard input
• list Send a message to the oracle requesting a
  listing of all the services currently available.
  
• connect service uid Open a connection to
  the server providing service service. Service is
  the user-friendly service name registered with
  the oracle . An optional argument uid is used to
  distinguish between services provided by
  different users having the same name. That is,
  multiple users may register services having the
  same name.
• quit terminate the client program.

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CLIENT AND SERVER OPERATION
Client - Server Connection

• When the client wishes to connect to a server, it
  performs the following steps
• Contact the oracle to find the transport-level
  address (host name and port number) of the
  service you are trying to locate (details below).
• Open a TCP connection to the server supplying the
  desired service.
• Copy standard input to the server, and copy all
  data sent by the server to standard output.
• After receiving an end-of-file from either
  standard input or the TCP connection, close the
  TCP connection. Note if your client receives
  an end-of-file from standard input, it should
  terminate the connection to the server, but
  should continue reading additional commands from
  standard input because the user may want to
  connect to another service (see clearerr(3)).

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CLIENT AND SERVER OPERATION
Server - Client Connection

• The server performs the following steps when
  making a service available
• Create a TCP socket (similar to a UNIX file
  descriptor -- see socket(2)).
• Bind the socket to a sockaddr_in structure with
  family AF_INET, port number X, and address
  INADDR_ANY. This directs the kernel to accept TCP
  connection requests from any machine (INADDR_ANY)
  in the Internet, and specifying a port number of
  X directs the kernel to allocate the port number
  you request. (see bind(2) ). (The port numbers X
  available to you are in the range xxx - yyy. We
  will assign these in class/lab.)
• Extract the port number allocated in the previous
  step (see getsockname(2) ), and fetch the
  Internet address of the host on which your server
  resides (see gethostname(2) and gethostbyname(3).
  Fill in the appropriate fields in the om
  structure (described below) and register the
  service with oracle.

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CLIENT AND SERVER OPERATION
Server - Client Connection

• Server
• The server performs the following steps when
  making a service available
• Specify the backlog of incoming connection
  requests you are willing to tolerate (see
  listen(2)).
• Finally, wait for a connection request and
  service it. When you have serviced the request,
  repeat the process by waiting for the next
  connection attempt (see accept(2), and close(2)
  ).

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CLIENT AND SERVER OPERATION
Client - Server Interface

• One of the requirements not mentioned previously
  is interoperability. This means for this project
  that I expect that your client will operate with
  my servers and my client will operate with your
  servers.
• These extended rules are as follows
• 1. You will provide at least two services.
  These can be either servers that require no input
  from the client or ones that do require input.
• 2. You must provide each of these services in a
  separate program. Each server program, upon
  initialization, will register its service with
  the Oracle.

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CLIENT AND SERVER OPERATION
Client - Server Interface
A Server Requiring NO Client Interaction

• CLIENT
• - - - -
• a service is selected by the human user
  ( connect daytime )
• connect()
• recv()
• close()

SERVER - - - - accept()
send( Result of service - daytime)
close()
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CLIENT AND SERVER OPERATION
Client - Server Interface
A Server Requiring Client Interaction

• CLIENT
• - - - -
• a service is selected by the human user
  ( connect checksum )
• connect()
• recv()
• if ( "Help" seen )
• read from standard in
• while ( "quit" not seen )
• send()
• recv()
• print out what server sent.
• read from standard in

SERVER - - - - accept()
send( HELP A useful help message)
while( recv( request from client ) )
do useful work with client
request send( result back to
client )
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CLIENT AND SERVER OPERATION
Client - Server Interface

• 3. For servers requiring NO input from the
  client
• a) Upon completing the accept, this type server
  has all the information about the client that
  it's ever going to get. No further input will
  come from the client, but the server has enough
  addressing information so that it knows how to
  respond to the client.
• b) The server sends back a string containing the
  result of its service to the client. This string
  has the following properties
• I. It is less than or equal to 2000 bytes (
  strlen(string)
• II. It is terminated with the Character.
• III. The server closes the connection with this
  client and prepares for an accept with the next
  client requester.

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CLIENT AND SERVER OPERATION
Client - Server Interface

• 4. For servers requiring input from the client
• a) Upon completing the accept, this type server
  has all the information about the client that it
  needs in order to respond to that client. The
  client will later send further information.
• b) The server sends back a string containing a
  help message. This message can be used in
  guiding the client as to what is required in
  order to obtain the service. This string has the
  following properties
• I. The help message begins with the 5-character
  ASCII string "Help". This is an indication to
  the client that it should go into "interactive
  mode", following a sequence of recv/send
  messages.
• II. The string is less than or equal to 2000
  bytes ( strlen(string)
• III. It is terminated with the one-character
  sequence .
• c) The server waits for a request from the
  client. The client string provides content as
  requested by the server. The format of this
  string is the same as in 4bII) and 4bIII).

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CLIENT AND SERVER OPERATION
Client - Server Interface

• d) The server and client communicate back and
  forth as necessary to provide the service. When
  the server has completed its service, it closes
  its connection. The client must be able to deal
  with this closed connection and also close the
  connection from its side.
• e) The user of the client can stop sending to the
  server by typing "quit". This causes the client
  to close the connection - thus the server must be
  able to handle a closed connection from the
  client.
• 5. Interoperability. Your client must operate on
  my interactive server. In addition, all of your
  servers must operate from my client. I will make
  available an executable of my client as well as
  my interactive server for your testing pleasure.
  I will be happy to discuss with you individually
  situations where you believe you must have some
  network functionality in a client that is not
  provided by the client I have prepared.

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INTERACTING WITH ORACLE
Oracle.h

• Oracle resides on machine 140.232.101.xxx
  iverson.clarku.edu at well-known UDP port
  netoracle . All communication with oracle is
  through UDP messages containing a structure
  called an om (for "oracle message", pronounced
  "ohhmm"), whose definition can be found in the
  file oracle.h in jbreecher/public/networks/Proje
  ct1/oracle.h. Thus the oracle, your client and
  your server all communicate via the same
  mechanism. The file is reproduced in the next
  few slides
• define luid 16 / Length of the username
  field
• define cchMaxServ 10
• define cchMaxDesc 40
• define verCur C'

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INTERACTING WITH ORACLE
Oracle.h

• enum cmd
• cmdErr, / An error occurred. See sbDesc for
  details /
• cmdGet, / Get the address of a service /
• cmdAckGet, / ACK for cmdGet message /
• cmdEnd, / Last response to a cmdGet message
  /
• cmdPut, / Register a new service /
• cmdAckPut, / ACK for cmdPut message /
• cmdClr, / Unregister a service /
• cmdAckClr / ACK for cmdClr message /

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INTERACTING WITH ORACLE
Oracle.h

• struct om / oracle message /
• char ver / version number of this
  structure /
• enum cmd cmd / command/reply code /
  
• char sbDesccchMaxDesc / desc. of service
  (or error) /
• char uidluid / id of
  requester/provider /
• char sbServcchMaxServ / name of service
  requested /
• struct sockaddr_in sa / socket where
  service is available /
• unsigned long ti / time of
  registration /
• define lom (sizeof (struct om))

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INTERACTING WITH ORACLE
Client - Oracle Interaction

• To find a service, your client program fills in
  the fields of the om structure as follows
• ver Always verCur in all messages.
• cmd cmdGet .
• uid The user id of the user offering
  the service - user who started the server
  process. If the client doesn't care which uid to
  use, it sets this field to the NULL string (NULL
  character in first byte).
• sbServ The name of the service being searched
  for (filled in by the client). To get a listing
  of all available) services, set sbServ to the
  NULL string (NULL character in first byte). In
  response to a cmdGet message, oracle returns two
  or more messages. Response messages have a cmd
  type of cmdAckGet, and the end of cmdAckGet
  responses is signaled by a cmdEnd message.
  CmdEnd messages do not contain the name of a
  service they simply signal the end the last
  response. If only one service matches the
  client's request, the server will return two
  messages a cmdAckGet, followed by a cmdEnd .
  Each cmdAck message contains the following fields

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INTERACTING WITH ORACLE
Client - Oracle Interaction

• As a return to cmdGet, the oracle program fills
  in the fields of the om structure as follows
• sbDesc A sentence describing the service. Your
  server uses this field when registering a
  service, and the oracle returns it in response to
  cmdGet queries. When locating a service, the
  field should contain all zeros.
• sa The transport address at which the service
  resides.
• ti The time at which the service was
  registered.

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INTERACTING WITH ORACLE
Server - Oracle Interaction

• When your server wishes to register a service
  with the oracle, it sends an om message with the
  following fields
• cmd cmdPut, to register a service.
• uid The login id of the user registering the
  service (see getuid(2) and getpwent(3) ).
• sbServ One word name of the service (e.g.,
  "daytime").
• sbDesc A brief description of the service.
• sa The transport level address at which the
  server can be reached.
• In response to a cmdPut message, oracle returns a
  message of type cmdAckPut if the registration
  succeeds. In the case of errors, the oracle
  returns a message of type cmdErr, and the sets
  the field sbDesc to contain a short explanation
  of the error.

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INTERACTING WITH ORACLE
Server - Oracle Interaction

• Here are the steps to follow
• 1. Create a UDP socket (see socket(2) ).
• 2. Get the Internet address and port number of
  the oracle (see gethostbyname(3) and
  getservbyname(3) ).
• 3. Open a UDP connection to the oracle (see
  connect(2)).
• 4. Send an om message of the appropriate type to
  the oracle (see send(2) ).
• 5. Wait for an om reply from the oracle (see
  recv(2) ).

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INTERACTING WITH ORACLE
Server - Oracle Interaction

• In a more schematic form, these steps look like
  this
• sock ConnectUdp (hostOracle, portOracle)
• send (sock, om, lom, 0)
• recv (sock, om, lom, 0)
• int ConnectUdp ( char sbHost, char sbPort)
• fd socket (AF_INET, SOCK_DGRAM, 0)
• phe gethostbyname (sbHost))
• na ( (struct in_addr ) (phe-h_addr))
• sa.sin_family AF_INET
• sa.sin_addr na
• pse getservbyname (sbPort, "udp")
• sa.sin_port pse-s_port
• connect (fd, (struct sockaddr )sa, lsa )
• return( fd )

Its this variable that makes it UDP
Used to convert machine name into ip address
Used to convert service name into port number
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TOOLS AVAILABLE TO YOU

• In the course of doing this project, it would be
  nice to get all the help you can.
• In the directory jbreecher/public/networks/Proj
  ect1 are the following documents and executables.
• oracle.h The include file that contains
  structures, etc. allowing you to communicate with
  the oracle.
• bclient This is the Breecher Client. It
  behaves according to the functionality defined in
  this document. So its kind of an implementation
  standard. This client should be able to talk to
  your server.
• checksum_server This is a server that behaves
  according to the spec. Once you get your client
  working, you should be able to talk to this
  server.
• oracle_log contains the history of transactions
  from the oracles point of view. So if you send
  a request to the oracle, and you get back
  garbage, you can look in this log and see what
  the oracle thought was happening.
• The processes you see running on iverson (using
  ps auxgrep jbreecher) are started up with a
  script with path jbreecher/start_oracle. You
  may find this of interest.

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GENERAL ADVICE
The Steps To Follow

• 1. Start small. Write a procedure that sends a
  datagram to the oracle, and receives a datagram
  response from the oracle. If you made a mistake,
  the oracle will return an om with cmd set to
  cmdErr, and sbDesc will contain a brief
  description of the problem.
• 2. Write a client that connects to a server
  registered with the oracle. I will register a
  few simple commands "daytime" and "finger" seem
  to be staples that you can use to test your
  client.
• 3. Once you have the client working, begin work
  on the server.
• 4. Nearly all system calls and library routines
  return some form of error code if the operation
  was not successful. You must check the return
  value from every routine for an error code.

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GENERAL ADVICE
The Steps To Follow

• 5. As an aid to debugging, as well as to trace
  use of your server, you should print trace output
  when connections are made. For example, you
  might print the host name and port number of
  every user that connects to your server, along
  with the time of the connection.
• 6. Requests to the oracle are actually a bit
  more general than we've described so far. Regular
  expressions can be used as service names or users
  ids to effect a kind of "wild-search" for
  services. For example, specifying a service name
  of "." matches all services. Specifying a user
  name of "..." matches all services provided by
  users with three-character ids. The format of
  regular expressions is the same as that of ed(1).

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GENERAL ADVICE
The Steps To Follow

• 7. Use your imagination and creativity in
  designing server programs. You will earn extra
  credit by implementing a "neat" server.
• The following library procedures and system calls
  will be helpful clearerr(3), gethostname(2),
  listen(2), close(2), getpwent(3), getuid(2),
  send(2), recv(2), getservbyname(3),
  gethostbyname(3), gethostbyaddr(3),
  getsockname(2), bind(2), socket(2), connect(2),
  accept(2), listen(2) and select(2).
• For more information on how the networking
  related calls, use the recommended text for the
  course.

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GENERAL ADVICE
Good Citizenship

• By now you understand that your job is to create
  processes.
• Problems will occur however when the whole class
  is creating new processes, not terminating
  erroneous processes, and not cleaning up after
  themselves. What happens is that the machine
  grinds to a halt.
• The net effect is to use up the maximum number of
  processes available on the machine, and to slow
  down the machine dramatically. Eventually the
  system Gods must intervene and reboot the
  machine.
• Please get in the habit of cleaning up before you
  log off the easiest way to do that is ps aux
  grep
• This will produce a list of process numbers and
  you can then go through these and utter kill
  .

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GENERAL ADVICE
Plagiarism

• Feel free to discuss high level aspects of this
  project with other class members.
• But be careful it is unethical to share code OR
  concrete implementation details with anyone else,
  either a member of the class or otherwise.
• Failure to follow these guidelines will result in
  severe action on my part.

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GENERAL ADVICE
HOW YOU WILL BE EVALUATED

• What follows is the sort of items I will be
  looking for when evaluating your Project I
  results. Please dont think that this is an
  complete list, or that all these items will be
  there at the end. Having said that, its
  certainly a good starting point - you would be
  wise to ensure that your project can accomplish
  all these items.
• 1. Client and servers build correctly.
• 2. Client and two servers start.
• 3. Student client works with students Server
  1.
• 4. Student client works with students Server
  2.
• 5. Student client works with my interactive
  server.
• 6. bclient works with students Server 1.
• 7. bclient works with students Server 2.
• 8. Relative complexity of student Server 1.
• 9. Relative complexity of student Server 2.

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GENERAL ADVICE
HOW YOU WILL BE EVALUATED

• Checkoffs Human nature being what it is,
  procrastination is avoided by having weekly
  check-ins. The schedule for this project looks
  like this
• Week 1 Check-in. Your client can talk to the
  oracle. It correctly can list the servers
  available via the oracle, and it can connect and
  obtain information from the sample servers
  available to you.
• Week 2 You have written a server that registers
  with the oracle. That server can receive a
  simple message from your client.
• Week 3 Final project Due.
• How will this project be turned in?