Absrracr-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 60th less than two percent, most nodes learn of an update within 100 ms, and the algorithm detects congestion and routes packets around congested areas. * Ira Richer (S'S8-M'63) received the B.E.E. degree from Rensselaer Polytechnic Institute. Troy, NY. in 1959, and the M.S. and Ph.D. degrees in electrical engineering from the California Institute of Technology. Pasadena. in 1960 and 1964. respectively. After holding postdoctoral positions at the Technical University of Denmark. Lyngby. and at Cal Tech, Pasadena, he joined M.I.T. Lincoln Laboratory, Lexington, MA, where he was involved in a number of advanced communications projects that spanned orders of magnitude in both the frequency and altitude domains (ELF to UHF and submarine to satellite). In 1977 he joined Bolt Beranek and Newman (BBN) Inc., Cambridge, MA. As a Senior Scientist at BBN, he consults to both commercial and government organizations on a variety of network and communications topics.
BEFin COMPLETO Yom I. REIPORT NUM69R I. GOVTlr A GI, S NO 9. &1aeCPIENT'S CATALOG NUMO4EI .3RE)RNUN Ar-Aol-33'? 4. TITLE (IN.d"dile). TYPE OF REPORT a P"O@ COVERED *-Report No. 3940 Bolt Beranek and Newman Inc.
The original routing algorithm of the ARPANET, in service for over a decade, has recently been removed from the ARPANET and replaced with a new and different algorithm. Although the new algorithm, like the old, is a distributed, adaptive routing algorithm, it is not similar to the old in any other important respect. In the new algorithm, each node maintains a data base describing the delay on each network line. A shortest-path computation is run in each node which explicitly computes the minimum-delay paths (based on the delay entries in the data base) from that node to all other nodes in the network. The average delay on each network line is measured periodically by the nodes attached to the lines. These measured delays are broadcast to all network nodes, so that all nodes use the same data base for performing their shortest-path computations. The new routing algorithm was extensively tested on the ARPANET before being released. This paper describes the algorithm and summarizes the results of these tests.
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