Review: How to create a TCP end point? What is the right format for sin_port and sin_addr in the sockaddr_in data structure? How many different ways we can bind a socket?

How to specify the maximum number of connections for a socket? How to find out the remote machine information?

Today’s topic: Introduction to UDP Some server design alternatives Select

TCP: Reliable byte stream service. –Different ways to build client/servers –How to get around blocking I/O –Assumption: whatever sent will eventually be received!! UDP: Unreliable datagram service. Data may get lost – application may need to deal with more details in the communication.

Why UDP: –Applications that do not need 100% reliability communication. E.g VoIP, video stream, DNS servers. –Applications care a lot about performance: high performance computing (TCP cares too much about fairness). –Applications that need multicast or broadcast (TCP only supports point to point communication).

Basic UDP service interface: –Socket, bind, sendto, recvfrom, close UDP server client socket bind sendto recvfrom Sendto close TCP server client socket Bind connect Listen … … close close

#include ssize_t recvfrom(int sockfd, void *buff, size_t nbytes, int flags, struct sockaddr *from, socklen_t *addrlen); ssize_t sendto(int sockfd, void *buff, size_t nbytes, int flags, const struct sockaddr *to, socklen_t addrlen); See udpsender.c/udprecv.c for communication using UDP.

Server design alternatives: concurrent and multiplexed server. Concurrent server (see lect3/example5.c): Use a new child process to handle a new connection requests.

Multiplexed Server: The use of select. I/O multiplexing – check the file descriptor before performing a blocking operation (what happen to the client when a concurrent server clushs?).

–The select function that allows: To detect any of the descriptors in the read set are ready for reading. To detect any of the descriptors in the write set are ready for writing To detect any of the descriptors in the error set have exception conditions pending. To wait for a period for something to happen.

#include int select (int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset, struct timeval *timeout)

–Set the timeout value: Struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ } Wait forever (blocking select): timeout = NULL Non blocking select (return right away: tv_sec = 0; tv_usec = 0; Wait for a certain amount of time: tv_sec and tv_usec

–Set the set of file descriptors value: void FD_ZERO(fd_set *fdset) void FD_SET(int fd, fd_set *fdset) void FD_CLR(int fd, fd_set *fdset) void FD_ISSET(int fd, fd_set *fdset)

–Maxfdp1: the maximum file descriptor number plus 1. (can just specify a big number (64) if unknown). –Select clears the uninteresting file descriptors in the fd_sets – always reset the fd_sets before calling select.

When is a socket ready for read? The number of bytes in the socket is more than the low-water mark (can be set, default 1 for TCP/UDP socket) Half of the connection is closed Listening socket with nonzero of completed connections Socket error.

When is a socket ready for write? The available buffer space is larger than the low-water mark Half the connection is closed Error pending Exception? Out of band data exists.

A multiplexed server (multiserv.c) –A single process to handle everything including connection request and data processing. –Using select the check on all descriptors that might need communication. Response to whatever from the clients.