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

We saw in the overview of TCP that the protocol originally included the functions of both modern TCP and the Internet Protocol. TCP was split into TCP and IP to allow applications that didn't need TCP's complexity to bypass it, using the much simpler UDP instead as a transport layer protocol instead. This was an important step in the development of the TCP/IP protocol suite, since there are several important protocols for which UDP is ideally suited, and even some where TCP is more of a nuisance than a benefit.

Most commonly, however, UDP is only used in special cases. The UDP applications topic describes the two types of protocols that may be better suited to UDP than TCP: ones where speed is more important than reliability, and ones that send only short messages infrequently. The majority of TCP/IP applications do not fall into these categories. Thus, even though the layering of TCP and IP means most protocols aren't required to use TCP, most of them do anyway. The majority of the protocols that use it employ all or at least most of the features it provides. The establishment of a persistent connection is necessary for many interactive protocols, such as Telnet, as well as for ones that send commands and status replies, like HTTP. Reliability and flow control are essential for protocols that send large files, like FTP or the e-mail protocols.

Table 150 shows some of the more significant application protocols that run on TCP. I have shown for each the well-known or registered port number reserved for that protocol's server process (clients use ephemeral ports, not the port numbers in the table). I have also shown the special “keyword” shortcut for each port assignment, and provided brief comments on why the protocol is well-matched to TCP.

**Table 150: Common TCP Applications and Server Port Assignments**
Port #	Keyword	Protocol	Comments
20 and 21	ftp-data / ftp	File Transfer Protocol (FTP, data and control)	Used to send large files, so ideally suited for TCP.
23	telnet	Telnet Protocol	Interactive session-based protocol. Requires the connection-based nature of TCP.
25	smtp	Simple Mail Transfer Protocol (SMTP)	Uses an exchange of commands, and sends possibly large files between devices.
53	domain	Domain Name Server (DNS)	An example of a protocol that uses both UDP and TCP. For simple requests and replies, DNS uses UDP. For larger messages, especially zone transfers, TCP is used.
70	gopher	Gopher Protocol	A messaging protocol that has been largely replaced by the WWW.
80	http	Hypertext Transfer Protocol (HTTP / World Wide Web)	The classic example of a TCP-based messaging protocol.
110	pop3	Post Office Protocol (POP version 3)	E-mail message retrieval protocols; use TCP to exchange commands and data.
119	nntp	Network News Transfer Protocol (NNTP)	Used for transferring NetNews (USEnet) messages, which can be lengthy.
139	netbios-ssn	NetBIOS Session Service	A session protocol, clearly better suited to TCP than UDP.
143	imap	Internet Message Access Protocol (IMAP)	Another e-mail message retrieval protocol.
179	bgp	Border Gateway Protocol (BGP)	While interior routing protocols like RIP and OSPF use either UDP or IP directly, BGP runs over TCP. This allows BGP to assume reliable communication even as it sends data over potentially long distances.
194	irc	Internet Relay Chat (IRC)	IRC is like Telnet in that it is an interactive protocol that is strongly based on the notion of a persistent connection between a client and server.
2049	nfs	Network File System (NFS)	NFS was original implemented using UDP for performance reasons. Given that it is responsible for large transfers of files and UDP is unreliable, this was probably not the best idea, and TCP versions were created. The latest version of NFS uses TCP exclusively.
6000 - 6063	TCP	x11	Used for the X Window graphical system. Multiple ports are dedicated to allow many sessions.

A couple of the protocols in the table above use both TCP and UDP, to get the “best of both worlds”. Short, simple messages can be sent with UDP, while larger files are moved with TCP. Many of the protocols that use both TCP and UDP are actually utility/diagnostic protocols (such as Echo, Discard and the Time Protocol). These are a special case, because they were designed to use both UDP and TCP specifically to allow their use for diagnostics on both protocols.

I haven't included an exhaustive list of TCP applications above; see the topic on common TCP/IP applications and port numbers, which also contains a pointer to the full (massive) list of TCP well-known and registered server ports.