Internet Core Protocols: The Definitive Guide

UDP-Based Application Failures

Since UDP provides a datagram-based transport service that does not offer any of the reliability services found with TCP, UDP-based applications are more prone to failures in a noisy or loss-intensive network. If the network you are using to move IP packets across loses a lot of packets, then this failure will be felt with UDP-based applications before it is noticed by TCP-based protocols.

For example, Sun’s Network File Service (NFS) typically uses UDP for remote filesystem access, since it benefits from the low-overhead nature of UDP. In addition, NFS typically writes data in large chunks (such as eight-kilobyte blocks), which are then split into multiple IP fragments according to the MTU characteristics of the underlying topology. Once all of the fragments have been received by the destination system, the IP datagram is reassembled and the UDP/NFS message is read and processed. However, if the underlying network loses 20% of the fragments, then the entire IP datagram will not be received (and thus the NFS data will not be processed).

With TCP-based applications, any data that gets lost will be recognized and retransmitted, resulting in slower overall performance, but a functional connection. With UDP however, lost data is gone forever and must be recognized and dealt with by the application. If the network constantly loses data however (such as is seen with sustained congestion levels or 20% packet loss due to line problems), then UDP-based applications ...