Review:

1

• 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?

2

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

3

• Todays topic
• Introduction to UDP
• Some server design alternatives
• Select

4

• 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.

5

• 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).
  

6

• Basic UDP service interface
• Socket, bind, sendto, recvfrom, close

UDP server client socket
socket bind sendto recvfrom
recvfrom Sendto close
TCP server client socket socket Bind
connect Listen
close close
7

• include ltsys/socket.hgt
• 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.

8

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

9

• 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?).

10

• 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.

11

• include ltsys/select.hgt
• include ltsys/time.hgt
• int select (int maxfdp1, fd_set readset, fd_set
  writeset, fd_set exceptset, struct timeval
  timeout)

12

• 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

13

• 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)

14

• 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.

15

• 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.

16

• 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.

17

• 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.