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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)

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) 1 2 3 TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

Applications of TCP and UDP

Having two transport layer protocols with such complementary strengths and weaknesses provides considerable flexibility to the creators of networking software:

• TCP Applications: Most “typical” applications need the reliability and other services provided by TCP, and don't care about loss of a small amount of performance to overhead. For example, most applications that transfer files or important data between machines use TCP, because loss of any portion of the file renders the entire thing useless. Examples include such well-known applications as the Hypertext Transfer Protocol (HTTP) used by the World Wide Web (WWW), the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). I describe TCP applications in more detail in the TCP section.
  
  
• UDP Applications: I'm sure you're thinking: “what sort of application doesn't care if its data gets there, and why would I want to use it?” You might be surprised: UDP is used by lots of TCP/IP protocols. UDP is a good match for applications in two circumstances. The first is when the application doesn't really care if some of the data gets lost; streaming video or multimedia is a good example, since one lost byte of data won't even be noticed. The other is when the application itself chooses to provide some other mechanism to make up for the lack of functionality in UDP. Applications that send very small amounts of data, for example, often use UDP under the assumption that if a request is sent and a reply is not received, the client will just send a new request later on. This provides enough reliability without the overhead of a TCP connection. I discuss some common UDP applications in the UDP section.

In the next few sections we'll first examine the common transport layer addressing scheme used by TCP and UDP, and then look at each of the two protocols in detail. Following these sections is a summary comparison to help you see at a glance where the differences lie between TCP and UDP. Incidentally, if you want a good “real-world” illustration of why having both UDP and TCP is valuable, consider message transport under the Domain Name System (DNS), which actually uses UDP for certain types of communication and TCP for others.

Before leaving the subject of comparing UDP and TCP, I want to explicitly point out that even though TCP is often described as being slower than UDP, this is a relative measurement. TCP is a very well-written protocol that is capable of highly efficient data transfers. It is only slow compared to UDP because of the overhead of establishing and managing connections. The difference can be significant, but is not enormous, so keep that in mind.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) 1 2 3 TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets