The New Routing Algorithm for the ARPANET

TLDR

Each node maintains a full topology and delay database, periodically updates it with measured line delays, and uses the updated data to build minimum‑delay trees that route traffic efficiently across the ARPANET. Tests on the ARPANET show that the algorithm keeps line and CPU overhead below two percent, propagates updates to most nodes within 100 ms, and successfully detects congestion to reroute packets around congested areas.

Abstract

The new ARPANET routing algorithm is an improvement over the old procedure in that it uses fewer network resources, operates on more realistic estimates of network conditions, reacts faster to important network changes, and does not suffer from long-term loops or oscillations. In the new procedure, each node in the network maintains a database describing the complete network topology and the delays on all lines, and uses the database describing the network to generate a tree representing the minimum delay paths from a given root node to every other network node. Because the traffic in the network can be quite variable, each node periodically measures the delays along its outgoing lines and forwards this information to all other nodes. The delay information propagates quickly through the network so that all nodes can update their databases and continue to route traffic in a consistent and efficient manner. An extensive series of tests were conducted on the ARPANET, showing that line overhead and CPU overhead are both less than two percent, most nodes learn of an update within 100 ms, and the algorithm detects congestion and routes packets around congested areas.