[HECnet] DECnet User Mode Router - Encapsulation Formats

[Rob Jarratt]

-----Original Message-----
From: owner-hecnet at Update.UU.SE [mailto:owner-hecnet at Update.UU.SE]
On Behalf Of Dave McGuire
Sent: 23 June 2012 01:54
To: hecnet at Update.UU.SE
Subject: Re: [HECnet] DECnet User Mode Router - Encapsulation Formats

On 06/22/2012 08:48 PM, Paul_Koning at Dell.com wrote:
Hmmm, so if we can use async serial then we have a chance to add that
to the router once I have had a chance to do the main part. It would
be good to do something like that. I don't know enough about DDCMP
yet to know what the implications are (can it work with a Version 2
router for example?).

Version 2?   Do you mean Level 2?   DDCMP works in DECnet phase 1
through 4, I should remember about 5 (a.k.a., DECnet/OSI) but I don't.
It's another datalink type, different from Ethernet, partly because it
doesn't have addresses (it's point to point) and partly because the
headers are different ("short" rather than "long"
headers).   But much of it is the same: routing exchanges work the
same, for example.   It's all spelled out in detail in the routing
layer spec.   The DDCMP protocol itself is in the DDCMP spec.   My first
exposure to its details was when I read the spec to implement the RSTS
driver, and the spec is good enough that I could simply do what it
says and end up with code that works.   (Most of the DECnet specs are
that good, actually.)   It's not exactly a hard protocol; maybe a few
thousand lines of assembly language code, so probably a few hundred if
done in C.   The point to point specific aspects of the routing layer
would actually be the larger part, though not all that much larger,
either.

  If it's that similar to DECnet over Ethernet, I would respectfully
submit that
the correct way to implement this would be to move as much of the DECnet
code as possible into a datalink type-independent layer and have datalink
layers below that, with a consistent API that serves as an abstraction
layer.


I agree and this is exactly what I am doing.

  Having the ability to route to DDCMP-over-async-connected hosts would be
really, really nice!

                        -Dave

--
Dave McGuire, AK4HZ
New Kensington, PA