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 Book is Here... and Now On Sale!

Searchable, convenient, complete TCP/IP information.
The TCP/IP Guide

Custom Search







Table Of Contents  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 Transmission Control Protocol (TCP)
                     9  TCP Fundamentals and General Operation

Previous Topic/Section
TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
Previous Page
Pages in Current Topic/Section
1
2
Next Page
TCP Common Applications and Server Port Assignments
Next Topic/Section

TCP Ports, Connections and Connection Identification
(Page 1 of 2)

The two TCP/IP transport layer protocols, TCP and UDP, play the same architectural role in the protocol suite, but do it in very different ways. In fact, one of the few functions that the two have in common is providing a method of transport-layer addressing and multiplexing. Through the use of ports, both protocols allow the data from many different application processes to be aggregated and sent through the IP layer, and then returned up the stack to the proper application process on the destination device. This is all explained in detail in the section describing ports and sockets.

Despite this commonality, TCP and UDP diverge somewhat even in how they deal with processes. UDP is a connectionless protocol, which of course means devices do not set up a formal connection before sending data. UDP doesn't have to use sliding windows, or keep track of how long it has been since a transmission was sent and so forth. When the UDP layer on a device receives data it just sends it to the process indicated by the destination port, and that's that. It can seamlessly handle any number of processes sending it messages because they are all handled identically.

Connection-Handling Responsibilities

Since TCP is connection-oriented, it has many more responsibilities. Each TCP software layer needs to be able to support connections to several other TCPs simultaneously. The operation of each connection is separate from of each other connection, and the TCP software must manage each independently. It must be sure not only that data is routed to the right process, but that data transmitted on each connection is managed without any overlap or confusion.

Connection Identification

The first consequence of this is that each connection must be uniquely identified. This is done by using the pair of socket identifiers corresponding to the two endpoints of the connection, where a socket is simply the combination of the IP address and the port number of each process. This means a socket pair contains four pieces of information: source address, source port, destination address. Thus, TCP connections are sometimes said to be described by this addressing quadruple.

I introduced this in the general topic on TCP/IP sockets, where I gave the example of an HTTP request sent from a client at 177.41.72.6 to a 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. If the client has been assigned ephemeral port number 3,022 for the Web browser, 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)

Previous Topic/Section
TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
Previous Page
Pages in Current Topic/Section
1
2
Next Page
TCP Common Applications and Server Port Assignments
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.