TCP/IP Tutorial

Understanding TCP/IP is easier than you think. Find out all you need to know in this free 5 part tutorial from PC Network Advisor

This is a plain html version of the original TCP/IP tutorial. For the original version in PDF format complete with diagrams, click here.

Everyone knows that TCP/ IP is a network protocol used on LANs, WANs and the Internet, but not everyone who uses it understands how it works. It’s possible to use TCP/ IP with little more than a knowledge of how to configure the protocol stack, but a better understanding will give you a clearer picture of what is going on in your network and why the protocol needs to be set up in a particular way.

The aim of this multi- part TCP/IP tutorial is to explain the key concepts behind TCP/ IP.

TCP/ IP stands for Transmission Control Protocol/ Internet Protocol. If this leads you to think that it is not just one protocol, you’re right. In fact, it is not just two protocols, either. TCP/ IP is a suite of protocols. We’ll cover the most important ones in the course of this article.

Layered Protocol

Like most network protocols, TCP/ IP is a layered protocol. Each layer builds upon the layer below it, adding new functionality. The lowest level protocol is concerned purely with the business of sending and receiving data - any data - using specific network hardware. At the top are protocols designed specifically for tasks like transferring files or delivering email. In between are levels concerned with things like routing and reliability. The benefit that the layered protocol stack gives you is that, if you invent a new network application or a new type of hardware, you only need to create a protocol for that application or that hardware: you don’t have to rewrite the whole stack.

Link Layer

TCP/ IP is a four- layer protocol. The lowest level, the link layer, is implemented within the network adapter and its device driver. Like all the TCP/ IP protocols, it is defined by standards. The standards for generic Ethernet- type networks are defined by the IEEE 802 Committee: for example, IEEE 802.3 for Ethernet networks, or IEEE 802.5 for Token Ring networks.

Other link layer protocols that could be used include Serial Line IP (SLIP) or Point- to- Point Protocol (PPP), which are used when connecting to a network over an asynchronous dial- up link.

Since Ethernet is the most common type of network, we will look at it in a bit more detail. The Ethernet protocol is designed for carrying blocks of data called frames. A frame consists of a header containing 48- bit hardware destination and source addresses (which identify specific network adapters), a 2- byte length field, and some control fields. There follows the data, and then a trailer which is simply a 32- bit cyclic redundancy check (CRC) field. The data portion of an Ethernet frame must be at least 38 bytes long, so filler bytes are inserted if necessary.

All this means that frames are at least 64 bytes long, even if they carry only one byte of user data: a significant overhead in some types of application.

Frames also have a maximum size. Less headers, the maximum size for an Ethernet frame is 1492 bytes, which is the maximum transmission unit (MTU) for Ethernet. All link layer protocols have an MTU. It is one hardware characteristic that the higher- level protocol needs to be aware of, because larger blocks of data must be fragmented into chunks that fit within the MTU and then reassembled on arrival at their destination.

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