Python in a Nutshell, 4th Edition

Chapter 18. Networking Basics

Connection-oriented protocols work like making a telephone call. You request a connection to a particular network endpoint (equivalent to dialing somebody’s phone number), and your party either answers or doesn’t. If they do, you can talk to them and hear them talking back (simultaneously, if necessary), and you know that nothing is getting lost. At the end of the conversation you both say goodbye and hang up, so it’s obvious something has gone wrong if that closing event doesn’t occur (for example, if you just suddenly stop hearing the other party). The Transmission Control Protocol (TCP) is the main connection-oriented transport protocol of the internet, used by web browsers, secure shells, email, and many other applications.

Connectionless or datagram protocols work more like communicating by sending postcards. Mostly, the messages get through, but if anything goes wrong you have to be prepared to cope with the consequences—the protocol doesn’t notify you whether your messages have been received, and messages can arrive out of order. For exchanging short messages and getting answers, datagram protocols have less overhead than connection-oriented ones, as long as the overall service can cope with occasional disruptions. For example, a Domain Name Service (DNS) server may fail to respond: most DNS communication was until recently connectionless. The User Datagram Protocol (UDP) is the main connectionless transport protocol for internet communications. ...