ARPANET

In the mid 1960s, at the height of the Cold War, the DoD wanted a commend and control network that could survive a nuclear war. Traditional circuit-switched telephone networks were considered too vulnerable, since the loss of one line or switch would certainly terminate all conversations using them and might even partition the network. To solve this problem, DoD turned to its research arm, ARPA (later DARPA, now ARPA again) – the Advanced Research Projects Agency.

After discussions with various experts, ARPA decided that the network the DoD needed should be a packet-switched network consisting of a subnet and host computers.

The subnets would consist of mini computers called IMPs (Interface Message Processors) connected by transmission lines. For high reliability, each IMP would be connected to at least two other IMPs. The subnet was to be such that if some lines and IMPs were destroyed, messages would automatically be re-routed along alternative paths.

Each node of the network was to consist of an IMP and a host, in the same room, connected by a short wire. A host could send messages of up to 8063 bits to its IMP, which would then break these up into packets of at most 1008 bits and forward them independently toward the destination. Each packet was received in its entirety before being forwarded, so the subnet was the first electronic store-and-forward packet-switching network.

ARPA then awarded a contract to build the subnet and write the subnet software. The contractor, BBN, used specially modified Honeywell DDP-316 mini computers with 12 K 16-bit words of memory as the IMPs. The IMPs did not have disks, since moving parts were considered unreliable. The IMPs were inter-connected by 56 kbps lines leased from telephone companies.

The software was split into two parts – subnet and host.

The subnet software consisted of…

• the IMP end of the host-IMP connection,
• the IMP-IMP protocol, and
• a source IMP to destination IMP protocol to improve reliability

The original ARPANET design was –

Outside the subnet, software was also needed, viz.

• the host end of the host-IMP connection,
• the host-host protocol, and
• the application software

The ARPANET grew rapidly and by early 1970s it spanned the United States.

Later the IMP software was changed to allow terminals to connect directly to a special IMP, called a TIP (Terminal Interface Processor).

Subsequent changes included…

• having multiple hosts per IMP - to save cost,
• hosts talking to multiple IMPs - to protect against IMP failures, and
• hosts and IMPs separated by a large distance – to accommodate hosts far from the subnet.

ARPA also promoted research on satellite networks and mobile packet radio networks. Experiments were performed to send messages across different types of networks, viz. radio, satellite and cable networks. These experiments demonstrated that the existing ARPANET protocols were not suitable for running over multiple networks. This observation led to more research on protocols, culminating with the invention of the TCP/IP model and protocols. TCP/IP was specifically designed to handle communication over internetworks, which had become more important as more and more networks were being hooked on to the ARPANET.

By 1983, the ARPANET was stable and successful with over 200 IMPs and hundreds of hosts. At this point, ARPA gave the management of the network to the Defense Communications Agency (DCA), to run it as an operational network. The first thing DCA did was to separate the military portion into a separate subnet – MILNET, with stringent gateways between MILNET and the remaining research subnet.

During the 1980s, additional networks, especially LANs, were connected to the ARPANET. As the scale increased, finding hosts became increasingly expensive, so DNS (Domain Naming System) was created to organize machines into domains and map host names onto IP addresses.