stewart@XYPLEX.COM (Bob Stewart) (10/05/90)
>In the DEC VAX environment the unique Ethernet address on each board is >overridden by DECNET when it starts to use that board. The address is set >to four bytes of a constant value plus two bytes which contain the DECNET >area and node numbers. Lots of opportunity for duplication! >Does anyone know why DEC chose this scheme? > > - Bob Fillmore FILLMORE@EMRCAN.BITNET I believe it was to keep down the size of routing tables. Since the Ethernet address can be calculated from the DECnet address, you don't have to keep a 48-bit MAC address in your mapping from Network layer to Data Link layer. Bob ----------- Bob Stewart (rlstewart@eng.xyplex.com) Xyplex, Boxborough, Massachusetts (508) 264-9900
fillmore@emrcan.BITNET (10/05/90)
In the DEC VAX environment the unique Ethernet address on each board is overridden by DECNET when it starts to use that board. The address is set to four bytes of a constant value plus two bytes which contain the DECNET area and node numbers. Lots of opportunity for duplication! Does anyone know why DEC chose this scheme? - Bob Fillmore FILLMORE@EMRCAN.BITNET
jqj@HOGG.CC.UOREGON.EDU (10/05/90)
Having the same MAC-level address for all interfaces (phrased differently, associating the MAC-level address with the NODE rather than with the INTERFACE) is part of the spec for both DECNET IV and XNS. It falls out of a design that has an algorithmic link between the MAC address and the protocol address. Many people believe that having a protocol address that belongs to the node rather than to the interface is a Good Idea, and that doing it the other way was a bad decision in the IP suite. I don't have a strong opinion, but I do know that the XNS/DECNET way of doing things makes routing to multihomed hosts easier. In the IP world there is no unambiguous way to decide whether 2 IP addresses refer to the same host; sure I can look in the DNS, but there is no guarantee that either or both addresses will be in the in-addr.arpa tree. Other people like the idea of algorithmic mapping from protocol to MAC address. It means that you don't need something like ARP, but it also limits your flexibility substantially, especially on physical networks like arcnet or 3mb Ethernet where the range of MAC addresses is very limited or hardwired in the interface. And it of course fails to generalize to cases where one needs to support multiple protocol stacks that each want to change the MAC address based on the protocol address!
oberman@rogue.llnl.gov (10/06/90)
In article <5A0A050B012801FE-MTAEMR1*fillmore@emrcan>, fillmore@emrcan.BITNET writes: > In the DEC VAX environment the unique Ethernet address on each board is > overridden by DECNET when it starts to use that board. The address is set > to four bytes of a constant value plus two bytes which contain the DECNET > area and node numbers. Lots of opportunity for duplication! > Does anyone know why DEC chose this scheme? Basic answer- They messed up! DECnet Phase V will abandon this ill concieved idea. It was a clever idea to allow a direct and unambiguous translation from DECnet address to Ehternet address which eliminates the need for ARP or some similar method of Ethernet address resolution. They have since learned the folly of this and do the reverse in Phase V. They code the 6 bytes of the Ethernet address into the system's NSAP (OSI address). R. Kevin Oberman Lawrence Livermore National Laboratory Internet: oberman@icdc.llnl.gov (415) 422-6955 Disclaimer: Don't take this too seriously. I just like to improve my typing and probably don't really know anything useful about anything.
gavron@alpha.sunquest.com (Ehud Gavron) (10/06/90)
In article <5A0A050B012801FE-MTAEMR1*fillmore@emrcan>, fillmore@emrcan.BITNET writes... #In the DEC VAX environment the unique Ethernet address on each board is #overridden by DECNET when it starts to use that board. The address is set #to four bytes of a constant value plus two bytes which contain the DECNET #area and node numbers. Lots of opportunity for duplication! #Does anyone know why DEC chose this scheme? On one ethernet, the duplication only occurs if two nodes have the same DECnet address -- which is not only a no-no but since both would have terrible problems, this condition would not persist long. There is therfore not "lots of opportunity for duplication." It also is completely immaterial why DEC chose this scheme. Suffice to say that the next major DECnet release (now targeted for '92) will be completely different. # # - Bob Fillmore FILLMORE@EMRCAN.BITNET ^^^^^^ Ehud /----------------------------------------------------------------------------\ | Ehud Gavron, Systems analyst | gavron@vesta.sunquest.com (Internet) | | Sunquest Information Systems | uunet!sunquest!gavron (UUCP) | | 930 N. Finance Center Drive | gavron@lampf (BITNET) | | Tucson, Arizona, 85710 | (602)722-7546/885-7700 x.2546 (AT&Tnet) | |----------------------------------------------------------------------------| | your cute quote here | \----------------------------------------------------------------------------/
system@LNS61.TN.CORNELL.EDU (10/06/90)
Bob, >In the DEC VAX environment the unique Ethernet address on each board is >overridden by DECNET when it starts to use that board. The address is set >to four bytes of a constant value plus two bytes which contain the DECNET >area and node numbers. Lots of opportunity for duplication! >Does anyone know why DEC chose this scheme? The "four constant bytes" are reserved to DEC (for DECnet) just as the high-order butes of the physical hardware Ethernet addresses in those Ethernet boards' address ROMs are also reserved to DEC. In a given DECnet wide-area network, all DECnet node addresses are required to be unique. As a result, there is no opportunity for duplication on a local Ethernet either. (I am ignoring the complications introduced by setting limits on the address ranges passed by routers.) One reason that DEC chose this scheme is that it allows "end-node" DECnet systems, which have no routing information whatsoever, to be able to send messages to other end-node systems on the same Ethernet even when there are no DECnet routers present. They just calculate what the Ethernet address must be and blindly transmit the message into the ether. Of course, the fact that DECnet addresses are limited to 16 bits means that larger networks are limited to a maximum of about 64K systems. My guess is that there are fewer than a dozen networks for which this causes problems. This address limitation in Phase IV DECnet is one of the reasons that DEC is moving to OSI for DECnet Phase V. I hope this helps. Selden E. Ball, Jr. (Wilson Lab's network and system manager) Cornell University Voice: +1-607-255-0688 Laboratory of Nuclear Studies FAX: +1-607-255-8062 Wilson Synchrotron Lab BITNET: SYSTEM@CRNLNS Judd Falls & Dryden Road Internet: SYSTEM@LNS61.TN.CORNELL.EDU Ithaca, NY, USA 14853-8001 HEPnet/SPAN: LNS61::SYSTEM = 44283::SYSTEM
romkey@ASYLUM.SF.CA.US (John Romkey) (10/06/90)
Date: 5 Oct 90 19:09:22 GMT From: decwrl!lll-winken.llnl.gov!rogue.llnl.gov!oberman References: <5A0A050B012801FE-MTAEMR1*fillmore@emrcan> Sender: tcp-ip-relay@nic.ddn.mil They code the 6 bytes of the Ethernet address into the system's NSAP (OSI address). What a great idea. So if you change your ethernet card, your machine's address changes too? - john romkey USENET/UUCP: romkey@asylum.sf.ca.us Internet: romkey@ftp.com
clw@MERIT.EDU (Chris Weider) (10/10/90)
In a recent message (10/5), Selden E. Ball, Jr. states that: >Of course, the fact that DECnet addresses are limited to 16 bits >means that larger networks are limited to a maximum of about 64K >systems. My guess is that there are fewer than a dozen networks >for which this causes problems. I think this estimate is conservative. :) On CICNet, we've had to jerry-rig our routers to get a true heirarchical DECNet architechture; I suspect that a lot of other regionals have had to also. I'm eagerly awaiting DECnet Phase 5. Chris Weider ----------------------------------------------------------------------------- Chris Weider | -Geteilte Freude ist MERIT's CICNet Technical Rep. | doppelte Freude email: clw@merit.edu | -Goethe Phone: (313) 936-2090 or | -Party on, dudes! 1-800-66-MERIT | -Abraham Lincoln, "Bill and Ted"