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CYCLADES was a ground-breaking, and extremely influential, early packet switching data network, done in France in the late 1970's. It was the key intermediate technical step between the ARPANET and the Internet.

CYCLADES was the first network to move the responsibility for the reliability of data sent across it into the hosts attached to the network. The technical chief of the project, Louis Pouzin, decided that the best way to build a network was to accept that the network itself was basically somewhat unreliable, and then move all the responsibility for reliability into the hosts on the ends (unlike in the ARPANET, where that responsibility was borne by the packet switches that made up the network, the IMPs).

They did that by using the now-standard mechanisms of sequence numbers, acknowledgements, timeouts, and retransmissions. In addition to making the packet switches in the network much simpler (a major gain, and one that would be crucial in the development of internets), it was also a step toward the end to end model of networking. It introduced the idea of 'fate-sharing', where the state necessary for the connection is co-located with the application, so generally 'they all go together if they go' (away).

The actual physical network (links and switches) was named CIGALE; CYCLADES includes the whole ensemble, including the hosts and higher-level protocols and applications.


That approach, of having the network provide only unreliable packets (sometimes called datagrams) has several huge advantages:

  • The network itself is much simpler, because it does not have to go to any extra work to guarantee that data is not lost. It just does its best (hence another name for the service it provides, 'best effort'), and focuses on its main job - moving data around.
  • This approach allows all the intermediate packet switches (routers, in an internet) in the overall network to carry no state about the connections travelling through them. This has been the single biggest reason why the Internet has been able to grow ('scale', in engineering jargon) to the stupendous size it has.

Further reading

  • Louis Pouzin (editor), The Cyclades Computer Network: Toward Layered Network Architectures (North-Holland, Amsterdam, 1982)