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Review: UDP: What is the difference between UDP and TCP? Why UDP? UDP service interface?

Review: UDP: What is the difference between UDP and TCP? Why UDP? UDP service interface?. #include <sys/socket.h> ssize_t recvfrom(int sockfd, void *buff, size_t nbytes, int flags, struct sockaddr *from, socklen_t *addrlen);

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Review: UDP: What is the difference between UDP and TCP? Why UDP? UDP service interface?

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  1. Review: • UDP: • What is the difference between UDP and TCP? • Why UDP? • UDP service interface?

  2. #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); Show lost of packets in UDP? What is the error rate? Why? Ethernet error rate: 10^-9 to 10^-12.

  3. Today’s topic: • Reliable communication over UDP. • Problems with UDP communication? • Need flow control and error control

  4. How to do flow control? • Control the speed of the sender – if the receiver does not have enough buffer, stop sending. • How to do it? • Example1.c and example2.c are primitive UDP sender and receiver, how to slow the sender down? • Adding delay (sleep???) • Performing handshaking – this technique is used in TCP. • Do example1.1.c and example2.1.c work?

  5. Example1.1.c and example2.1.c resolve the flow control problem. What would happen if some packet gets lost? • Solution. • Add timeout to the sender • If a packet gets lost, the sender should timeout and resend the packet. • How to implement a timeout mechanism? • See example1.2.c and example2.2.c

  6. Any problems with example1.2.c and example2.2.c? • Some packets get duplicated!! • How to resolve this problem? • The receiver must be able to recognize whether it has received a RIGHT packet – should throw away the duplicated packets!! • How to do this? • Add an identifier to each packet sent – sequence number. • The receiver should record what it receives. • Example1.3.c and example2.3.c implement this reliable protocol.

  7. The protocol implemented in example1.3.c and example2.3.c is called Positive Acknowledgement with Retransmission (PAR) protocol. • A reliable transmission protocol • But not the most efficient • The network (and end hosts) is idle sometimes • Sliding window protocols improve the performance of PAR

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