[HECnet] DECnet et al

[Paul Koning]

On Jul 17, 2011, at 2:48 PM, Johnny Billquist wrote:

On 2011-07-17 20.35, hvlems at zonnet.nl wrote:
DECnet phase 4 is about as old as ethernet and VMS.   Ethernet was the main driving force for phase 4 owing to the large number of adjacent nodes and the sheer number.

Not really. Phase IV is about as old as ethernet, yes. VMS is older...

Phase IV was invented for Ethernet support.   Originally the plan was to do what became Phase V, but there was a lot of resistance against anything that complicated back in the early 1980s.   So "Phase 3E" was invented, a small generalization of Phase 3, and that was subsequently renamed.


Phase 3 does not have areas and only recognizes 255 hosts max. It did have circuit routing (of course)

Yes. About the same thing as L1 routing. I think it might even be partially compatible, within the restrictions on the number of nodes...

Three major differences: Phase IV adds Ethernet support, including "long packet format".   That, by the way, was intended to be compatible with the original "phase IV" which used link state routing.   But that never saw the light of day; while DECnet Phase V uses link state routing also, and a number of the innovations carried over (like the sequence numbering), the packet format is completely different because of OSI.   So the "long packet format" is effectively an orphan, a bit of extra complexity that ended up serving no purpose.

The second difference is that Phase IV adds hierarchical routing.

The third is that it adds partial routing messages.   The routing table contains entries for every node in the area (L1) or every area (L2) but the corresponding routing messages can contain just a couple of entries, if only those changed.   By contrast, Phase III always sends the entire routing table (up to 255 entries). 

	paul