Talk:DECnet
From Wikipedia, the free encyclopedia
http://it.slashdot.org/comments.pl?sid=155038&cid=12998126 http://it.slashdot.org/comments.pl?sid=155038&cid=12996714
Seems, that while Phase V was aimed to outperform Phase IV, results were doubtfull, if not just opposite :(
Quote from comp.os.vms Phase IV manuals
- Gee, since phase IV is still used at plenty of sites. It's too bad it's considered "older". Why can't Compaq admit phase V is a bust and support the customers?
Isto Ylisirkka 08:01, 6 October 2005 (UTC)
[edit] MOP, layers, "proprietary"
MOP is not a network layer protocol. Its spec shows it in the "network management modules", which isn't really a layer (it's better viewed as something "to the side of" the regular stack.
DECnet was never viewed as an 8 layer model. It is a 7 layer model. I suspect the statement about "8 layers" is a misinterpretation of the picture in the Phase IV protocol specs; as I mentioned above, the "network management modules" shown in the structural model diagram isn't a layer in the OSI sense. In current terminology it would be part of the "control plane", and "to the side of" the data stack.
The use of the term "proprietary" is misleading. (See also Legitimacy of standards). While DECnet was developed by DEC, the specifications were available to all parties at no charge, and third party implementations were not just permitted but encouraged. Several companies did so, and for that matter there is at least one open source implementation (in Linux). Paul Koning 15:25, 23 March 2007 (UTC)
[edit] Router-less LAN operation?
The Ethernet implementation was unusual in that the software changed the physical address of the Ethernet interface on the network to AA-00-04-00-xx-yy where xx-yy reflected the DECnet network address of the host. This allowed router-less LAN operation because the LAN address could be deduced from the DECnet address
Changing the MAC address means that there is no need for a network-layer-address to link-layer-address mapping protocol, like ARP for IPv4. However this has nothing to do with routers. Royhills 12:22, 30 March 2007 (UTC)
- That's partly true but there is more to the story than that. DECnet has hello messages, from which a network layer to data link layer address mapping table could be built. The "hiord" hack was implemented because of concern at the time that maintaining such a table would be too expensive. That probably was silly at the time, though perhaps plausible at least for PDP-11 based routers (which were the majority then). The other point, which does tie into the "routerless" issue, is that in DECnet only routers listen to "hello" messages; end nodes do not. So while routers could do the mapping between addresses, end nodes could not unless they also listen to hello messages and maintain a table of who is out there. The whole point of end nodes was to avoid having per-node state -- the very most we wanted to have was per-router state. After all, the smallest endnode implementation was RT-11, which would run comfortably on a 56 kbyte machine (that's for everything: DECnet, OS, application! Paul Koning 20:03, 30 March 2007 (UTC)
