Avalanche


David C. Feldmeier (dcf@COMET.LCS.MIT.Edu)
Thu, 29 May 86 23:39:28 edt


I am doing measurements of the interpacket arrival times for the gateways on
a 10 Mbit token ring at MIT, especially our gateway to the ARPANET because
it carries the most traffic. While doing these measurements, I noticed that
about 35% of the packets arriving at the gateway were only 300 microseconds
after the previous packet. Since this is faster than the gateway receives
or a single host transmits, I was curious about what was causing it.

The problem turned out to be with the 30 VAXs on the ring that run Berkeley
4.2 Unix. A gateway would send a routing packet to the broadcast address
and all of the VAXs would not recognize the destination address and forward
the packet to the default gateway, which is the gateway to the ARPANET. As
soon as the broadcast packet was transmitted, all of the VAXs would
simultaneously begin forwarding packets to the gateway as quickly as
possible until they got through. The token ring has a data link layer
packet acknowledgment, so the VAXs would simply continue in order around the
ring with successful host dropping out. Eventually, all would succeed
(about 0.13 seconds) and the net would return to normal. The interarrival
time of 300 microseconds is because the transmission time of the routing
packet is 300 microseconds. The ring was 100% loaded with back-to-back
packets for the entire 0.13 seconds. I suspect that on an Ethernet, this
would be a bigger mess because of the multiple collisions.

                                        -Dave Feldmeier



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