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Computer Networks

Computer Networks. Project 2 Client Server Programming. 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.

harper-mckay
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Computer Networks

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  1. Computer Networks Project 2 Client Server Programming Project 2

  2. 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 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 Project 2

  3. 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. Project 2

  4. 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. Project 2

  5. 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. Project 2

  6. 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. 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. Project 2

  7. SERVER CLIENT OPERATING SYSTEM THE BIG PICTURE Getservbyname   Getservbyname  Oracle port returned. Oracle port returned  An Overview of How it works ORACLE UDP UDP Ask the oracle for the number of the server port. Register the server port number using the OM protocol  Client uses the server port to request result from server. Server returns result to client  Project 2

  8. User - Client Interface CLIENT AND SERVER OPERATION 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. Project 2

  9. Client - Server Connection CLIENT AND SERVER OPERATION 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)). Project 2

  10. Server - Client Connection CLIENT AND SERVER OPERATION 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) ). 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. Project 2

  11. Server - Client Connection CLIENT AND SERVER OPERATION 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) ). Project 2

  12. Client - Server Interface CLIENT AND SERVER OPERATION 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. At least one must be a server that requires input from the client. 2. You must provide each of these services in a separate program. Each server program, upon initialization, will register its service with the Oracle. Project 2

  13. Client - Server Interface A Server Requiring NO Client Interaction CLIENT AND SERVER OPERATION SERVER - - - - accept() send( Result of service - daytime”) close(); CLIENT - - - - a service is selected by the human user ( “connect daytime” ) connect() recv() close() Project 2

  14. Client - Server Interface A Server Requiring Client Interaction SERVER - - - - accept() send( “HELP: A useful help message”) while( recv( “request from client” ) ) { do useful work with client request send( “result back to client” ) } CLIENT AND SERVER OPERATION CLIENT - - - - a service is selected by the human user ( “connect evil” ) connect() recv() if ( "Help:" seen ) { read from standard in while ( "quit" not seen ) { send() recv() print out what server sent. read from standard in } } Project 2

  15. Client - Server Interface CLIENT AND SERVER OPERATION 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) <= 2000 ). II. It is terminated with the <NULL> Character. III. The server closes the connection with this client and prepares for an accept with the next client requester. Project 2

  16. Client - Server Interface CLIENT AND SERVER OPERATION 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) <= 2000 ). III. It is terminated with the one-character sequence <NULL>. 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). Project 2

  17. Client - Server Interface CLIENT AND SERVER OPERATION 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. Project 2

  18. Oracle.h INTERACTING WITH ORACLE Oracle resides on machine 140.232.101.129 osborne.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/Project2/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 username field # define cchMaxServ 10 # define cchMaxDesc 40 # define verCur ‘C' Project 2

  19. Oracle.h INTERACTING WITH ORACLE 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 */ }; All communication to and from the oracle will include one of these commands in the om. Project 2

  20. Oracle.h INTERACTING WITH ORACLE struct om { /* oracle message */ char ver; /* version number of this structure */ enum cmd cmd; /* command/reply code */ char sbDesc[cchMaxDesc]; /* desc. of service (or error) */ char uid[luid]; /* id of requester/provider */ char sbServ[cchMaxServ]; /* name of service requested */ struct sockaddr_in sa; /* socket where service is available */ unsigned long ti; /* time of registration */ }; # define lom (sizeof (struct om)) Project 2

  21. Client - Oracle Interaction INTERACTING WITH ORACLE 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: Project 2

  22. Client - Oracle Interaction INTERACTING WITH ORACLE 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. Project 2

  23. Server - Oracle Interaction INTERACTING WITH ORACLE 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. Project 2

  24. Server - Oracle Interaction INTERACTING WITH ORACLE 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) ). Project 2

  25. Server - Oracle Interaction INTERACTING WITH ORACLE It’s this variable that makes it UDP Used to convert machine name into ip address 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 ); } Used to convert service name into port number Project 2

  26. 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/Project2 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 it’s kind of an implementation standard. This client should be able to talk to your server. daytime – This is a simple server that behaves according to the spec. Once you get your client working, you should be able to talk to this server. evil – This is an interactive 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 oracle’s 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 osborne (using ps aux|grep jbreecher) are started up with a script with path ~jbreecher/start_oracle. You may find this of interest. Project 2

  27. The Steps To Follow GENERAL ADVICE 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. Project 2

  28. The Steps To Follow GENERAL ADVICE 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). Project 2

  29. The Steps To Follow GENERAL ADVICE 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. Project 2

  30. Good Citizenship GENERAL ADVICE 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 <your_user_id>” This will produce a list of process numbers and you can then go through these and utter “kill <process number>”. Project 2

  31. Plagiarism GENERAL ADVICE 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. Project 2

  32. HOW YOU WILL BE EVALUATED GENERAL ADVICE What follows is the sort of items I will be looking for when evaluating your Project I results. Please don’t think that this is an complete list, or that all these items will be there at the end. Having said that, it’s 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 student’s Server #1. 4. Student client works with student’s Server #2. 5. Student client works with my interactive server. 6. bclient works with student’s Server #1. 7. bclient works with student’s Server #2. 8. Relative complexity of student Server #1. 9. Relative complexity of student Server #2. Project 2

  33. HOW YOU WILL BE EVALUATED GENERAL ADVICE 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? Project 2

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