[HECnet] DECnet User Mode Router - Encapsulation Formats

[Paul_Koning at Dell.com]

On Jun 22, 2012, at 8:53 PM, Dave McGuire wrote:

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.

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

                      -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA

Yes, your suggestion is exactly the right one.   And the specification (the routing layer document) is organized that way, as a data link independent section and multiple data link dependent ones.   The best known data link dependent section nowadays would be the Ethernet one, but there's another one for point to point links (ddcmp,hdlc) and a third for switched links (X.25).   For that matter, IP has the same sort of approach, though described somewhat differently: Ethernet has MAC layer addressing and ARP to make that work, while point to point links (PPP and the like) have neither.

	paul