Advanced UNIX programming

1
Advanced UNIX programming

• Fall 2002
• Instructor Ashok Srinivasan
• Lecture 27

Acknowledgements The syllabus and power point
presentations are modified versions of those by
T. Baker and X. Yuan
2
Announcements

• Reading assignment
• Chapter 20
• Sections 20.4 and 20.5
• Chapter 14

3
Week 10 Topics

• Advanced UDP sockets
• Reliable communication over UDP
• Time out
• Sequence numbers
• Unix domain protocols
• Address structure
• Client-server
• socketpair

4
Reliable communication over UDP

• Flow control
• Error control

5
Flow control

• Control the speed of the sender
• If the receiver does not have enough buffer, stop
  sending
• How do we accomplish this?
• example1.c and example2.c are primitive UDP
  sender and receiver
• How can we slow down the sender?
• Add delay (sleep)
• example1a.c and example2.c
• Perform handshaking
• example1b.c and example2a.c

6
Time out

• example1b.c and example2a.c resolve the flow
  control problem
• What will happen if some packet gets lost?
• example1b.c and example2b.c
• Solution
• Add timeout to the sender
• If a packet gets lost, the sender should timeout
  and resend the packet
• How can we implement a timeout?
• See example1c.c and example2b.c

7
Sequence numbers

• Problems with example1c.c and example2b.c
• Some packets get duplicated
• How can we resolve this problem?
• The receiver must be able to recognize whether it
  has received a packet
• It should discard the duplicate
• How can we accomplish this?
• Add an identifier to each packet sent a
  sequence number
• The receiver should record sequence numbers that
  is has received
• example1d.c and example2c.c implement this

8
A reliable protocol

• Positive Acknowledgement with Retransmission
  (PAR) protocol
• The protocol implemented in example1d.c and
  example2c.c is PAR
• A reliable transmission protocol
• But not the most efficient
• The network and end hosts are idle sometimes
• Sliding window protocols improve the performance
  of PAR