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I think the minimum is 64 Bytes. Why is that minimum necessary?

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up vote 12 down vote accepted

Doing some quick reading, it seems it is related to the Collision Detection part of CSMA/CD. If frames were too small on old broadcast media, then some collisions would be undetectable. Continuing my theme of automobile analogies today, it's for the same reason that we don't allow bicycles on high-speed highways - it's just not safe for them.

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+1 , I did not know that about bicycles ... –  Kyle Brandt Jun 24 '10 at 18:43
+1 Collision detection is the reason. 64 bytes is 0.04ms at 10Mb ethernet speeds. Any smaller and collisions would go undetected (in 1982). –  sysadmin1138 Jun 24 '10 at 18:48
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In addition to mfinni's (absolutely correct) answer, setting a minimum frame size allows you to spend multiple receive cycles verifying your frames' checksums. In Ye Olde Days, one can easily imagine a chip that processes one bit per cycle, but takes many cycles to compute a checksum on a dedicated pathway that runs parallel to the receive pathway. Receiving many short messages could result in this checksum logic becoming garbled by having multiple simultaneous operations triggered in it. Discarding anything below a certain size threshold allows you to avoid this issue in a simple way.

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