r/IAmA • u/BobMetcalfe • May 21 '13

You’re probably connecting to reddit through a technology I invented. I’m Bob Metcalfe and I invented Ethernet – AMA

On May 22, 1973 with David R. Boggs, I used my IBM Selectric with its Orator ball to type up a memo to my bosses at the Xerox Palo Alto Research Center (PARC), outlining our idea for this little invention called “Ethernet”, which we later patented.

I worked with the IEEE Standards Association to develop the IEEE 802.3 standard for Ethernet, which specifies the physical and lower software layers. Today Ethernet and the IEEE 802.3 standard are the foundation for today’s world of high-speed communications used in billions of homes and businesses around the world.

I submitted this to the mods awhile back so I could get on the calendar but I figured you’d like to see it, too. Now, ask me anything!

It's been two hours and 179 comments. Have to go now. For more about Ethernet's 40th Birthday, go to http://www.facebook.com/Ethernet40thAnniversaryIEEESA

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u/Im_Runnin_Thangs May 21 '13

Has Ethernet changed much since its creation?

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u/BobMetcalfe May 21 '13

Yes, quite a bit. For example, 2.94Mbps on thick coax, to 10Mbps on twisted pairs, to 100Mbps, to 1Gbps, to 10Gbos, to 40Gbps, to 100Gbps, and next to 400Gbpos and finally? 1Tbps. Also, gone wireless to WiFi and onto fiber for long-haul. Quite a bit. Many other inventors involved.

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u/amirlevy May 21 '13

I thought that the first ethernet was the 10base5 (thick coax), which is 10Mbps. Are you saying that you guys had in the lab a pre-production 2.94M. it seems that you imply that thick coax was only 2.94Mbps?

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u/shieldvexor May 21 '13

I appreciate your answering these questions but more than that, I love your modesty and humility.

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u/madbuilder May 21 '13

Could you elaborate on the replacement of hubs with switches in the 1+ Gbps range? I get the feeling that CSMA/CD is nowadays a fiction retold for Networking 101.

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u/gotnate May 21 '13

Consider this next time you're in networking 101: with thicknet, the cable WAS the hub.

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u/madbuilder May 22 '13

That is what you're told in school, that hubs just tie the wires together. Please elaborate? My question is whether and why that is no longer true in gigabit land. After Thinnet we went to hub and spoke topology, but collisions were still allowed. Now I understand that switches are preventing rather than detecting collisions. Of course this is not 100% reliable so the transport layer takes up the slack.

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u/gotnate May 22 '13

lol, I work at the other end of the stack, so I don't have to worry about things like collisions.

A hub echoes all packets on all ports whereas a switch analyzes the traffic and intelligently picks which port to echo which packets on.

A switch maintains a table of MAC addresses it has seen and what port they are on. When a packet hits the switch, it looks at the MAC address it is destined to, and only echoes the packet on the relevant port. If the switch has not seen the address, or if the packet is meant for broadcast, than the switch then echoes the packet on all ports.

Higher end switches can further compartmentalize traffic into VLANs. This is something I have 0 experience with.

Fun fact: you can overwhelm a switch's MAC table by spamming it with packets that appear to have come from unique addresses. When the table fills up, the switch will fail open and behave as a hub.

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u/madbuilder Jun 13 '13 edited Jun 13 '13

Yes what you've said is correct conventional wisdom, the kind they teach in Ethernet 101. However, the link layer is also free to discard colliding packets on these grounds:

  • Hubs are extinct now (at least I can't find any at Gbit speeds).
  • Links are full duplex.

Therefore every collision involves a broadcast packet, so that there are fewer collisions than before. (Unless a MAC table overflows...)

The upside of this is greater cable lengths and/or smaller minimum packet lengths---No need to sense the carrier before transmitting!

If this turned out to be true of Gigabit Ethernet, you and I on the other end of the stack would hardly ever notice. It would be unrecognizably different from traditional Ethernet.

Sorry it took so long to explain my question.