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Of course, TCP isn't the only user protocol in TCP/IP networking. Although suitable for applications like rlogin, the overhead involved is prohibitive for applications like NFS. Instead, it uses a sibling protocol of TCP called UDP, or User Datagram Protocol. Just like TCP, UDP also allows an application to contact a service on a certain port on the remote machine, but it doesn't establish a connection for this. Instead, you may use it to send single packets to the destination service-- hence its name.
Assume you have mounted the TeX directory hierarchy from the department's central NFS server, galois, and you want to view a document describing how to use LaTeX. You start your editor, who first reads in the entire file. However, it would take too long to establish a TCP connection with galois, send the file, and release it again. Instead, a request is made to galois, who sends the file in a couple of UDP packets, which is much faster. However, UDP was not made to deal with packet loss or corruption. It is up to the application-- NFS in this case-- to take care of this.
Thu Mar 7 23:22:06 EST 1996