• 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
TCP
client
server
socket
socket
socket
Bind
bind
sendto
Listen
recvfrom recvfrom …
Sendto
close
close
client
socket
connect
…
close
#include <sys/socket.h>
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 <sys/select.h>
#include <sys/time.h>
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.