IPC via Sockets in CS 311: Understanding Server and Client Communication

1. CS 311 – Lecture 18 Outline • IPC via Sockets • Server side • socket() • bind() • accept() • listen() • Client side • connect() CS 311 - Operating Systems I

2. Sockets • Sockets are IPC mechanism which allows processes in same or different machines to communicate. • Printing a file on one machine from another machine • Transferring files between machines. • IPC via sockets follows a client-server model. (Server creates a socket and the clients talks over the socket) • Sockets connections are bidirectional. CS 311 - Operating Systems I

3. Server Sockets • A server socket listens on a given port • Many different clients may be connecting to that port • Ideally, you would like a separate file descriptor for each client connection Web Server Server Socket/Port Client 1 Client 2 Client 3 Client n CS 311 - Operating Systems I

4. Web Server New socket Server Socket/Port Server Sockets 1: Client requests connection 2: Server creates new file descriptor for client which is used for bi-directional communication 3: Other clients requesting connections can now be serviced. Client 1 Client 2 CS 311 - Operating Systems I

5. Socket types • Classified into • Domain • PF_LOCAL – client and server in same machine • PF_INET – client and server on different machines • PF_INET6 – client and server on IPv6 network • Type • SOCK_STREAM – sequenced, reliable, 2-way connection (TCP) • SOCK_DGRAM – connectionless, unreliable. (UDP) • Protocol • Specifies the low-level means by which the socket type is implemented. CS 311 - Operating Systems I

6. Server Sockets • How to use the socket API to listen for an accept connections • Start by describing a non-concurrent implementation of a server (only one thread of execution) • Procedure • Create network endpoint with socket() • Bind socket to a port - bind() • Start listening for connections - listen() • Loop and accept connections - accept() • Read and write data to client - send(), recv(), read(), write() CS 311 - Operating Systems I

7. Creating a socket • Using socket() system call • Prototype: int socket (int domain, int type, int protocol) • Creates an unnamed socket • Returns a server file descriptor CS 311 - Operating Systems I

8. Naming a socket • Ports allow multiple network processes on a machine with a single address • A server has to choose a port where clients can contact it • bind() associates the chosen port with a socket already created with the socket() command • Protoype: int bind (int fd, const struct sockaddr* address, size_t addressLen) CS 311 - Operating Systems I

9. structsockaddr_in and sockaddr_un • Both the structures are used to specify a port and an address for the socket. • structsockaddr_un (For local sockets) • sun_family - PF_LOCAL • sun_path - the full pathname of the socket (absolute or relative), up to 108 characters long • structsockaddr_in (For network sockets) • sin_family – address format (PF_INET or PF_LOCAL) • sin_port - port number of internet socket • sin_addr – a structure of type in_addr that holds the internet address CS 311 - Operating Systems I

10. Setting up an address • Server accepting connections: sin_family = PF_INET (or AF_INET); sin_port = htons(7000); (change machine byte order to network byte order) sin_addr.s_addr = INADDR_ANY; or inet_addr(“172.154.28.1”); • http://beej.us/guide/bgnet/output/html/multipage/htonsman.html • http://www.umiacs.com/inet_ntoaman.html CS 311 - Operating Systems I

11. Listening for Connections • The server will ignore any connection attempts until you tell the socket() to start listening • This is done with listen() • Prototype: int listen (int fd, int queueLength) • Queue length specifies maximum number of pending connections on a socket • If a client attempts a connection to a socket whose queue is full, it is denied. CS 311 - Operating Systems I

12. Loop and Accept • Servers generally run continually, waiting for clients to contact them • The accept() function takes the next connection off the listen queue or blocks the process until a connection arrives • Prototype: int accept (int fd, struct sockaddr* address, int* addressLen) • returns a new file descriptor that may be used to talk with the client; otherwise, it returns -1. • The address structure is filled with the address of the client. • http://www.retran.com/beej/sockaddr_inman.html CS 311 - Operating Systems I

13. Serving a client • This is the most common sequence of events that take place when a client connection succeeds • The server process forks • The parent process closes the newly formed client fds and loops back to accept() • The child process talks to the client using read() and write() or send() and recv(). • When the conversation is complete, the child process closes the client fds and exits. CS 311 - Operating Systems I

14. Client-side sockets • The client in order to connect to the server socket creates its own socket using the socket() system call • clientFd = socket(PF_LOCAL, SOCK_STREAM, PROTOCOL); • Client connects to server socket using the connect() system call • Prototype: int connect (intfd, structsockaddr* address, intaddressLen) • address should be the server socket address. • PF_LOCAL – sockaddr_un casted to sockaddr * • PF_INET – sockaddr_in casted to sockaddr * CS 311 - Operating Systems I