The TCP/IP Guide - UDP Common Applications and Server Port Assignments

Table 148 shows some of the more interesting protocols that use UDP and the well-known and registered port numbers used for each one's server processes. It also provides a very brief description of why these protocols use UDP instead of TCP. See the sections or topics devoted to each application for more details:

**Table 148: Common UDP Applications and Server Port Assignments**
Port #	Keyword	Protocol	Comments
53	domain	Domain Name Server (DNS)	Uses a simple request/reply messaging system for most exchanges (but also uses TCP for longer ones).
67 and 68	bootps / bootpc	Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP)	Host configuration protocols that consist of short request and reply exchanges.
69	tftp	Trivial File Transfer Protocol (TFTP)	TFTP is a great example of a protocol that was specifically designed for UDP, especially when it is compared to regular FTP. The latter protocol uses TCP to establish a session between two devices, and then makes use of its own large command set and TCP's features to ensure reliable transfer of possibly very large files. In contrast, TFTP is designed for the quick and easy transfer of small files. It includes simple versions of some of TCP's features, such as acknowledgments, to avoid file corruption.
161 and 162	snmp	Simple Network Management Protocol	An administrative protocol that uses relatively short messages.
520 and 521	router / ripng	Routing Information Protocol (RIP-1, RIP-2, RIPng)	Unlike more complex routing protocols like BGP, RIP uses a simple request/reply messaging system, doesn't require connections, and does require multicasts/broadcasts. This makes it a natural choice for UDP. If a routing update is sent due to a request and is lost, it can be replaced by sending a new request. Routine (unsolicited) updates that are lost are replaced in the next cycle.
2049	nfs	Network File System	NFS is an interesting case. Since it is a file sharing protocol, one would think that it would use TCP instead of UDP, but it was originally designed to use UDP for performance reasons. There were many people who felt this was not the best design decision, and later versions moved to the use of TCP. The latest version of NFS uses only TCP.

There are some protocols that actually use both UDP and TCP. This is often the case either for utility protocols that are designed to accept connection using both transport layer protocols, or for applications that need the benefits of TCP in some cases, but not others.

The classic example of the latter is DNS, which normally uses UDP port 53 for simple requests and replies, which are usually short. Larger messages requiring reliable delivery, such as zone transfers, use TCP port 53 instead. Note that in the table above I have omitted some of the less-significant protocols, such as the ones used for diagnostic purposes (Echo, Discard, CharGen, etc.) For a full list of all common applications, see the topic on common TCP/IP applications and port numbers.

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