CS490T Advanced Tablet Platform Applications

1. CS490T Advanced Tablet Platform Applications Network Programming Evolution

2. Introduction • Programming network applications has evolved with programming languages • Tools have been created at each step to make distributed applications as easy as writing local applications.

3. BSD Sockets • Network sockets were introduced in the UNIX Berkeley Standard Distributions (BSD UNIX) • They provided a common API to the TCP/IP stacks • They were written generically enough to be used for other protocols different than TCP/IP. That made them awkward to use.

4. BSD Sockets • They were very successful because they provided a portability path for network applications. • An application written with sockets can be easily ported to other UNIX systems or Windows. • Sockets provide a • Stream interface for TCP • Message based interface for UDP

5. TCP Sockets • In the stream interface for TCP reading/writing to the other network peer will be similar to reading/writing to a file or pipe.

6. TCP Sockets Server Side • The server does: • Ms = Socket(…) – creates socket • Bind(…) – bind socket to a specific port • Listen(…) – puts socket in listen mode • While(1) { • ss = accept(ms, …) // Accept connection read(ss, …) // Read request • write(ss, …) // Write reply • }

7. TCP Sockets Client Side • The client does the following s= socket(…) // Creat socket bind(s, …) // Bind socket to a port connect(s) // Connect to server write(s, …) // Write requestread(s,…) // Read reply close(s)

8. Disadvantage of Socket Programming • Too complicated • Error prone. So many calls, so many possibilities of error. • Every communication detail has to be explicitly programmed.

9. Examples of Socket Applications • Lots of applications are implemented using sockets: • FTP, SMTP, Finger, HTTP, …

10. Remote Procedure Calls (RPC) • Abstract the network communication as a procedure call. • A program uses a method exported remotely as if it were local. • The interface of the procedure is described in a RPC description language. • The RPC description is processed with an RPC preprocessor that generates code for the client and the server.

11. RPCs Client Side • For the client side the code generated includes procedure stubs that • Defines the procedure with the same arguments • Connects to the remote server • Takes the arguments and send them through the network (marshaling). • Waits for a reply • Copies the result to the result of the procedure and returns. • The programmer uses these stubs transparently as if the procedures were executed locally.

12. RPC Server Side • The RPC processor generates for the server a dispatcher server code that • Initializes the sockets • Starts the serving loop waiting for a new request • When a request arrives it copies the data from the network to the argument buffer (unmarshalling) • It invokes the procedure corresponding to the RPC id. • It sends the results of the procedure to the other side (marshaling) • The programmer of the server only implements the procedures themselves. The communication part is generated by the RPC processor.

13. RPC Issues • The representation of numbers, strings etc. may be different in different machines. • In sockets that is not usually a problem since the communication is textually based in ASCII format. • In RPCs the data is sent in binary form. This saves processing time since no ASCII to decimal conversion is necessary but may cause problems in machines of different architecture. • The RPC generator also generates filters that convert the binary data if necessary.

14. RPC Issues • In SUN RPC, the data is always converted to an External Data Representation (XDR) before sending. • The data is converted back to the representation of the local machine when received. • In RPC’s arguments and results can be passed only by value (int, string, float etc) • It is not possible to send data by reference (Object *this) so it is not easy to write object oriented programs.

15. RPC Examples • The best known implementation of RPC’s is SUNRPC • SUNRPC defines an RPC language and provides an RPC processor called RPCgen • Applications such as NFS, rusers, mount, yellow pages are implemented in SUN RPC.

16. Remote Method Invocation (RMI) • It was created because of the limitations that RPC has handling references to objects. • In RMI a reference to a remote object is represented as a pointer to a local representation to the object called “Proxy”. • A method invocation to the proxy is forwarded to the remote object. • Invoking methods in the proxy will be transparent and it will look like we are invoking methods in the object as if it were local.

17. Proxy Creation/Deletion • When a reference to a remote object is received, a new proxy is created. • When the reference to the remote object is no longer needed, the proxy is deleted. • Some cooperation from the local garbage collector is needed to identify unneeded proxies. • Remote objects are removed when tere are no longer remote proxies pointing to it.

18. RMI Examples • The proxy code is generated by a processor from a RMI description. • Some examples of RMI are: Java RMI, CORBA Orbix, DCOM, Modula 3 Remote Objects.