[HECnet] Connections?

[Paul Koning]

> On May 4, 2018, at 2:46 AM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> On 2018-05-04 05:55, Robert Armstrong wrote:
>>  I guess a more direct question for the DECnet experts out there would be, "can any DECnet circuit cope with multiple adjacent nodes on the same circuit?"  I always thought Ethernet circuits were special in that regard, but maybe not.  If any single circuit can have multiple adjacencies, then you could make a Multinet driver that accepted multiple connections.
> 
> There are a little bit more to it, but first of all, ethernet is special. The protocol DECnet uses over ethernet is a bit different than the protocol used over point-to-point links.
> 
> But in addition, you also have multipoint circuits, which is a variant of point-to-point circuits, but which have several endpoints/adjacent nodes. So ethernet is not the only one able to do that.

That's not exactly right.  There are a number of different objects at several layers.

The routing layer has two: "circuit" and "adjacency".  Circuit is the communication service provided by the datalink layer.  Adjacency is the information the routing layer has about a given neighbor.  If the circuit is point to point, there is exactly one adjacency per circuit.  If broadcast, there can be many (and they can be mixed: some endnode adjacencies, some routing adjacencies).

The data link layer also has two objects: "line" and "circuit".  Line corresponds to a physical wire.  Circuit is the communication service provided to the layers above.  I don't remember how X.25 links are modeled in the DECnet data link layer, never worked on that stuff.  Broadcast links like Ethernet have one circuit per line.  Point to point links like the DMC-11 also have one circuit per line.  Multipoint links such as a DMP-11 at the master end have as many circuits as there are configured drops; each circuit describes the connectivity to a particular multidrop slave station.

So from the routing layer point of view, there is NO difference between DMC and DMP -- in each case, routing sees point to point circuits.  The multiplexing for a multipoint link, if used, is in the data link layer, not in routing.  On the other hand, with Ethernet the multiplexing (for the different neighbors) is in the routing layer, NOT the data link layer.

It would certainly be plausible to model Multinet as a multidrop link, like a DMP-11.  If you do that, you'd still see multiple circuit objects in routing, each with its own name and state and all that.  Then you can make the TCP (or UDP) port number be a line attribute in the datalink layer.  That all fits.

I should look at the X.25 model more closely; it might be a better fit still.

Meanwhile, on an unrelated topic, are there any Multinet implementations that support IPv6?  My home IP connection supposedly is IPv6 capable, though I've never succeeded in getting that to work. 

	paul