Business Library

Transmission Control Protocol / Internet Protocol (TCP/IP)

The first three articles introduced some basic LAN concepts:

1. The Ethernet LAN
2. Cabling of Ethernet LAN
3. LAN Interconnection

These articles discussed how Ethernet local area networks (LANs) can be quickly and cheaply put together using both copper and fibre cabling and linked together to form complete networks. The LAN Interconnection article introduced the TCP/IP protocols in terms of its relevance to routing between LAN segments and across the Internet. This article considers TCP/IP in a little more depth - but not too much!

What is TCP/IP?

TCP/IP is a suite of communications protocols which allow computers of all shapes, sizes, and types to talk to each other. The original architecture was defined in an article by V.Cerf and R.Kahn in 1974 which stated the goals to be:

• Independence from underlying computer and network technology
• Universal any-to-any connectivity throughout the network
• End to end acknowledgement of traffic
• Standardised application protocols

It is the independence from any particular vendor's technology, taken for granted today, which was revolutionary. Up until that time, the type of network you had depended on your equipment vendor, as did it's capabilities - interfacing to the machines of a different vendor was extremely complex and often required 'black boxes' to provide protocol conversion.

So, TCP/IP was developed to provide an environment in which universal communications could occur - a prerequisite for the Internet we all know today. It is this universality which allows it to be called an Open technology.

For interest, TCP/IP continues to be developed - but relatively informally using a system of Request for Comments (RFC) in which anyone can suggest an enhancement to the base protocol or some additional application and have it considered for use in the standard. For this reason, TCP/IP functionality is often referred to by its RFC number. For example, RFC 894 defines "Standard for the Transmission of IP Datagrams over Ethernet Networks".

TCP/IP Layering

It is useful to consider network protocols in terms of layers - with each layer being responsible for a different aspect of the end to end communication. TCP/IP is no exception and is normally considered in terms of a 4-layer system:

1. Link Layer - which handles the problem of interfacing the computer to the network and getting the information across a link of some sort.
2. Network Layer - which looks after the movement of information around the communications network. This is where Internet Protocol (IP) is used. Note that this implies an involvement of a system other than just the two end systems - and you will remember that such a system is a called a Router in an IP network.
3. Transport Layer - which provides the end-to-end transfer of information between the two end computers. This is where Transmission Control Protocol (TCP) is used.
4. Application Layer - which handles the details of a particular application. For example, File Transfer Protocol (FTP), which is a standard TCP/IP application, operates at this layer. This allows complete files to be downloaded efficiently and is often used by software vendors to provide fixes and enhancements to products already shipped.

This can be viewed diagrammatically as follows:

This shows a typical application - a client computer (in other words an end user) accessing an FTP service (typically an unmanned machine) across an Ethernet LAN: The explanation which follows applies equally well to communications across the Internet - although in this case the Link layer uses a different protocol.

The Information to be sent is gathered in the Application layer. This is passed down to the TCP layer, then in turn to the IP layer. Finally it is passed to the Link layer for transmission across the physical network.

The receiver picks up the information from the network, passes it up to IP, which in turn passes it to TCP, which passes it to FTP. At each layer, a new communication may be generated which makes sense only to the same layer in the other computer - for example the TCP layer may detect an error in the information received and request a re-transmission - without passing the bad data to the Application layer. So the system works as a set of Peer protocols:

• The Application layer talks across the network to the Application layer
• The Transport Layer talks across the network to the Transport Layer
• The Network Layer talks across the network to the Network Layer
• The Link Layer talks across the network to (yep, you've guessed it) the Link Layer!

So how does this information get passed across the network?

The process used is one of Encapsulation - as data is passed down the layers, information is added to the front (and back at the lowest layer). So an application, for example FTP, obtains the data it wishes to transport ("user data") and adds its own header ("Appl Header"). Then it is passed to TCP which adds its own header ("TCP Header"), then IP ("IP Header"), then Ethernet (which also adds both an "Ethernet Header" and "Ethernet Trailer"). The Ethernet Frame, which the composite information is called, will transport up to 1500 bytes (or 'characters' if you like) of information across the physical Ethernet LAN - including the three underlying headers.

The information at some of the layers has its own buzz word:

• TCP Segment - TCP Header plus application information
• IP Datagram - IP Header plus TCP Segment
• Ethernet Frame - Ethernet Header and Trailer around IP Datagram

When the Ethernet Frame arrives at its destination, the Ethernet header and trailer are removed and the IP Datagram is passed to IP for processing. This checks the end to end integrity of the information and performs any processing of its own (such as sending an acknowledgement to the far end) prior to removing the IP Header and passing up to the next layer, TCP. And so on...

The Internet Engineering Task Force is responsible for the TCP/IP standards. If you are interested in finding out more about the Request for Comments process, take a look at the RFC Index.

Business Implications?

• TCP/IP is a universal communications medium.
• It can be found on practically every modern computer system - from PC to mainframe.
• It forms the basis for communicating with customers and suppliers without the need for specialised hardware or software.