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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

TCP/IP Sockets and Socket Pairs: Process and Connection Identification 1 2 TCP/IP User Datagram Protocol (UDP)

The great popularity of the TCP/IP protocol suite has led to the development of literally thousands of different applications and protocols. Most of these use the client/server model of operation that we discussed earlier in this section. Server processes for a particular application are designed to use a particular reserved port number, with clients using an ephemeral (temporary) port number to initiate a connection to the server.

To ensure that everyone agrees on which port numbers server applications for each application should use, they are centrally managed by the Internet Assigned Numbers Authority (IANA). Originally, IANA kept the list of well-known and registered port numbers in a lengthy text document, along with all the many other parameters for which IANA was centrally responsible (such as IP Protocol field numbers, Type and Code field values for ICMP, and so on). These were published on a periodic basis in Internet (RFC) standards documents titled Assigned Numbers.

This system worked fine in the early days of the Internet, but by the mid-1990s, these values were changing so rapidly that using the RFC process was not feasible. It was too much work to keep publishing them, and the RFC was practically out of date the day after it was put out.

The last Assigned Numbers standard was RFC 1700, published in October 1994. After that time, IANA moved to a set of World Wide Web documents containing the parameters they manage. This allowed IANA to keep the lists constantly up to date, and for TCP/IP users to be able to get more current information. RFC 1700 was officially obsoleted in 2002.

The document mentioned above is the definitive list of all well-known and registered TCP and UDP port assignments. Each port number is assigned a short keyword, with a brief description of the protocol that uses it. There are two problems with this document. The first is that it is incredibly long: over 10,000 lines of text. Most of the protocols mentioned in those thousands of lines are for obscure applications that you have probably never heard of before (I certainly have never heard of most of them!) This makes it hard to easily see the port assignments for the protocols that are most commonly used.

The other problem with this document is that it shows the same port number as reserved for both TCP and UDP for an application. As I mentioned earlier, TCP and UDP port numbers are actually independent, so one could in theory assign TCP port 80 to one server application type and UDP port 80 to another. It was believed that this would lead to confusion, so with very few exceptions, the same port number is shown in the list for the same application for both TCP and UDP. This makes sense, but showing this in the list has a drawback: you can't tell which protocol the application actually uses, and which has just been reserved for consistency.

Given all that, I've decided to include a couple of summary tables here that show the well-known and registered port numbers for the most common TCP/IP applications, and indicated whether the protocol uses TCP, UDP or both.

TCP/IP Sockets and Socket Pairs: Process and Connection Identification 1 2 TCP/IP User Datagram Protocol (UDP)