Re: Something has changed dramatically

Steve Cohn (scohn@ALEXANDER.BBN.COM)
Fri, 17 Jul 87 09:18:13 -0500

Here's something that changed. BBN modified the metric used for calculating SPF
routes in the Arpanet on Sunday, 12 July. A detailed description and analysis
of how it works and a report on measured changes in performance in the Arpanet
will be forthcoming eventually. In brief, this change in the metric leads to
more stable routing behavior when the network is overloaded, thereby reducing
the tendency for congestion to spread from overloaded paths to the rest of the
network. This modification also tends to make satellite trunking more

Other potential sources for improvement include recent topology changes
including the addition of transcontinental VSAT trunks between MIT-77 and
SRI-51. These trunks became operational about 1 July.

The routing effort is still in the experimental/tuning stage. We expect that
performance on many PSN-to-PSN flows will be improved by this modification. It
is also possible that some PSN-to-PSN flows will receive degraded performance.
We at BBN would like to hear both the good news and the bad news. Please send
your observations about recent changes in internet/Arpanet performance to Fred
Serr (, Bob Pyle ( and myself ( and I
suppose to the list if your observation is of general interest.

Stephen Cohn
Director of Network Analysis
BBN Communications

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