22.10 Summary

There are applications that want to know the destination IP address and the received interface for a UDP datagram. The IP_RECVDSTADDR and IP_RECVIF socket options can be enabled to return this information as ancillary data with each datagram. Similar information, along with the received hop limit, can be returned for IPv6 sockets by enabling the IPV6_RECVPKTINFO socket option.

Despite all the features provided by TCP that are not provided by UDP, there are times to use UDP. UDP must be used for broadcasting or multicasting. UDP can be used for simple request-reply scenarios, but some form of reliability must then be added to the application. UDP should not be used for bulk data transfer.

We added reliability to our UDP client in Section 22.5 by detecting lost packets using a timeout and retransmission. We modified our retransmission timeout dynamically by adding a timestamp to each packet and kept track of two estimators: the RTT and its mean deviation. We also added a sequence number to verify that a given reply was the one expected. Our client still employed a simple stop-and-wait protocol, but that is the type of application for which UDP can be used.