The package will contain two main things:

• A set of networking protocols
• Network applications which use the networking protocols

The TCP/IP protocols provide the ability to connect machines regardless of the underlying network cabling and also regardless of the operating systems in use.

The main feature of these protocols is that they provide an internetworking capability.

The network applications are called services. TCP/IP provides the three core services:

• File transfer
• Remote login
• Electronic mail

Internetworking

Most networks are established to server the needs of a particular group. The groups will choose a hardware technology appropriate to their communication needs. Some might choose slow links over great distances others would choose fast links over shorter distances...

Internetworking is the technology which allows the connection of separate physical networks. One of the main goals of TCP/IP was to provide an internetworking architecture. The connection of a number of separate networks results in an Internet.

Four Layers of TCP/IP

For a long time all communications have been layered in their architecture. A simple layering might involve just two layers:

• Software
• Hardware

A more sophisticated model would divide the hardware layer into two (resulting in three layers):

• Software
• Network card
• Cable

Each layer performs a discrete task. The layers are often called protocols. The layers sit on top of each other.

When data is sent over the network it is passed down through all the layers and then when it reaches its destination the data is passed up through all the corresponding layers. (What comes down, must go up!)

TCP/IP actually comes in four layers. A set of layers is often called a protocol stack. The TCP/IP stack contains the TCP/IP layers.

With layering comes three main fundamental concepts:

• Encapsulation
• Demultiplexing
• Fragmentation

Encapsulation

Each layer takes data from above and encapsulates it into the data area of its own "packet". An analogy is that each layer will take the data and envelope from above and place it in its own envelope which in turn is passed to the layer below.

Demultiplexing

This is the reverse of multiplexing. When a packet arrives at a host the layers must pass the packet up to the layer above. It is not untypical to have more than one layer sitting on top of a layer. In this case the lower layer must decide which layer to pass the information up to. In other words some form of Demultiplexing is required.

Fragmentation

Briefly fragmentation is where the data in one layer is split up into smaller units so that the lower layers can handle the data correctly. This will be explored in detail later.

History

Beginnings

In the late 1960s The Advanced Research Project Agency (ARPA) wished to connect its computers.

Note: ARPA later became known as the Defence Advanced Research Project Agency (DARPA).

The network produced became known as the ARPANET. This network linked universities and government agencies together. It is important to remember that the ARPANET was essentially a hardware project. The American Department Of Defence (DOD) was heavily involved in funding at this stage. The initial protocol used by the ARPANET was called NCP. No thought had been given to expansion.

By the mid 1970s NCP could no longer cope with the size of the network and was therefore replaced with the Internet Protocol Suite. The Internet Protocol Suite was later named as TCP/IP after its two main protocols.

From January 1983 all computers wishing to connect to the ARPANET were required to use the TCP/IP protocols. Also in 1983, The Department of Defence separated the network into separate networks:

• ARPANET For experimental research
• MILNET For military use.

Berkeley

In the early 1980s Berkeley University ported the TCP/IP protocols to their version of UNIX. This made TCP/IP ever more popular and also ensured that TCP/IP became the main method of connecting UNIX machines. As well as porting the protocols Berkeley also added UNIX like TCP/IP applications.

The Internet

From the ARPANET came The Internet. The researchers who developed the Internet architecture thought of the ARPANET as a dependable wide area backbone around which The Internet could be built.

The Internet began around 1980 when DARPA started converting machines attached to its research networks to the TCP/IP protocols.

Today the ARPANET has been replaced by new technologies but MILNET still forms the backbone of military communications. The success of the TCP/IP and the Internet led other groups to adopt it. The National Science Foundation took an active role in enabling TCP/IP to connect as many scientists as possible.

At the time the ARPANET was declining a new backbone network was produced. This new backbone was called NSFNET. NSFNET is now the main backbone of the Internet.

 The Internet

The Internet is an example internet. It consists of over 5000 LANs and is based on TCP/IP. Many problems on the Internet result in developments in TCP/IP to overcome these problems.

The Internet connect most of the US research institutions. The Internet expands across the world and is not just limited to the US. The Internet has been described as a large research project to which anyone can contribute by way of RFCs.

The IAB and RFCs

TCP/IP did not arise from a particular vendor or recognised standards body. TCP/IP is "controlled" by the Internet Activities Board (IAB). The main role of the IAB is:

• to set the technical direction of TCP/IP
• Standardise relevant protocols.

Documentation for TCP/IP comes in the shape of documents called Request For Comments (RFCs). Prior to RFCs the documentation was known as Internet Engineering Notes (IENs).

A funded group called the Network Information Centre (NIC) distributes RFCs to the world at large. RFC 1261 gives the address of NIC as:

  Government Systems Inc.
  Attn: Network Information Centre
  14200 Park Meadow Drive
  Suite 200
  Chantilly, VA 22021
  Help Desk number: 1-800-365-3642

All RFCs are numbered. An update to an RFC will result in a new number and the old RFC being obsolete.

There are two main types of RFC:

• Information and discussion
  For example: RFC 1118 Hitchhikers guide to the Internet
• Protocols
  Not all of these are standard. The standard protocols are referenced in RFC1100 IAB Official protocol standards.

The OSI Reference Model During the late 1970s the International Standards Organisation (ISO) set up committees to define an architecture for further development of standards in the networking world.

This architecture became known as the Open Systems Interconnect Reference Model. The OSI/RM consists of 7 layers (The author prefers 7½). The model defines a layered peer to peer networking architecture.

The model is often split into two main parts:

• Communications: Layers 1-4 are responsible for transferring data between two systems.
• Applications: Layers 5-7 provide application oriented services.

A short description of the seven layers follows:

Layer 1: Physical
Sends/receives bits along a medium.

Layer 2 Data Link
Performs the actual sending. Detects errors in transfer.

Layer 3 Network
Connect networks. Provides routing through intermediary systems if necessary.

Layer 4 Transport
Provides data transfer between end processes.

Layer 5 Session
Manages the comms session from the application side.

Layer 6 Presentation
Ensures that data is represented in the appropriate format for different machines.

Layer7 Application
Not the actual application itself but the part dealing with the network.

There are two ways to use the OSI Reference Model

• To implement it.
• To use it as a reference to compare different protocols.

There are not that many implementations of OSI. The main use of the model is as a reference. Note that it is a stated aim of the Internet to migrate to OSI at some stage. The author believes this time span to be in the region of 3-50 years time.