Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

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 Client (Ephemeral) Ports and Client/Server Application Port Use 1 2 Common TCP/IP Applications and Assigned Well-Known and Registered Port Numbers

Socket Pairs: Connection Identification

So, the exchange of data between a pair of devices consists of a series of messages sent from a socket on one device to a socket on the other. Each device will normally have multiple such simultaneous conversations going on. In the case of TCP, a connection is established for each pair of devices for the duration of the communication session. These connections must be managed, and this requires that they be uniquely identified. This is done using the pair of socket identifiers for each of the two devices that are connected.

Let's return to the example we used in the previous topic (Figure 199). We are sending an HTTP request from our client at 177.41.72.6 to the Web site at 41.199.222.3. The server for that Web site will use well-known port number 80, so its socket is 41.199.222.3:80, as we saw before. We have been ephemeral port number 3,022 for our Web browser, so the client socket is 177.41.72.6:3022. The overall connection between these devices can be described using this socket pair:

(41.199.222.3:80, 177.41.72.6:3022)

For much more on how TCP identifies connections, see the topic on TCP ports and connection identification in the section on TCP fundamentals.

Unlike TCP, UDP is a connectionless protocol, so it obviously doesn't use connections. The pair of sockets on the sending and receiving devices can still be used to identify the two processes exchanging data, but since there are no connections the socket pair doesn't have the significance that it does in TCP.

TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use 1 2 Common TCP/IP Applications and Assigned Well-Known and Registered Port Numbers