27 Mar
2021
27 Mar
'21
8:06 p.m.
On 2021-03-27 18:59, John Forecast wrote:
Right. Smart controllers with DDCMP in firmware on the controller itself
was definitely not the first implementation.
But I don't have the full history of DDCMP as such. But it sounds
reasonable that the original DECnet could have been using DDCMP already.
But was that the first implementation of DDCMP? You mentioned V3.0,
which would imply that there were even older versions since before DECnet.
Did ANF-10 use DDCMP?
Johnny
On Mar 27, 2021, at 11:06 AM, Mark Berryman
<mark at theberrymans.com
<mailto:mark at theberrymans.com>> wrote:
DDCMP was originally designed to run over intelligent synchronous
controllers, such as the DMC-11 or the DMR-11, although it could also
be run over async serial lines. ?Either of these could be local or
remote. ?If remote, they were connected to a modem to talk over a
circuit provided by a common carrier and async modems had built in
error correction. ?From the DMR-11 user manual describing its features:
DDCMP implementation which handles message sequencing and error
correction by automatic retransmission
No. DDCMP was designed way before any of those intelligent controllers.
DDCMP V3.0 was refined during 1974 and released as part of DECnet Phase
I. The customer I was working with had a pair of PDP-11/40?s, each
having a DU-11 for DECnet communication at 9600 bps. DDCMP V4.0 was
updated in 1977 and released in 1978 as part of DECnet Phase II which
included DMC-11 support. The DMC-11/DMR-11 included an onboard
implementation of DDCMP to provide message sequencing and error
correction. Quite frequently, customers would have a DMC-11 on a system
communicating with a DU-11 or DUP-11 on a remote system.
? John.
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
In other words, DDCMP expected the underlying
hardware to provide
guaranteed transmission or be running on a line where the incidence of
data loss was very low. ?UDP provides neither of these.
DDCMP via UDP over the internet is a very poor choice and will result
in exactly what you are seeing. ?This particular connection choice
should be limited to your local LAN where UDP packets have a much
higher chance of surviving.
GRE survives much better on the internet than does UDP and TCP
guarantees delivery. ?If possible, I would recommend using one these
encapsulations for DECnet packets going to any neighbors over the
internet rather than UDP.
Mark Berryman
On Mar 27, 2021, at 4:40 AM, Keith Halewood
<Keith.Halewood at pitbulluk.org <mailto:Keith.Halewood at pitbulluk.org>>
wrote:
Hi,
I might have posted this to just Paul and Johnny but it?s probably
good for a bit of general discussion and it might enlighten me
because I often have a lot of difficulty in separating the layers and
functionality around tunnels of various types, carrying one protocol
on top of another.
I use Paul?s excellent PyDECnet and about half the circuits I have
connecting to others consist of DDCMP running over UDP. I feel as
though there?s something missing but that might be misunderstanding.
A DDCMP packet is encapsulated in a UDP one and sent. The receiver
gets it or doesn?t because that?s the nature of UDP. I?m discovering
it?s often the latter. A dropped HELLO or its response brings a
circuit down. This may explain why there?s a certain amount of
flapping between PyDECnet?s DDCMP over UDP circuits. I notice it a
lot between area 31 and me but but much less so with others.
In the old days, DDCMP was run over a line protocol (sync or async)
that had its own error correction/retransmit protocol, was it not? So
a corrupted packet containing a HELLO would be handled at the line
level and retransmitted usually long before a listen timer expired?
Are we missing that level of correction and relying on what happens
higher up in DECnet to handle missing packets?
I?m having similar issues (at least on paper) with an implementation
of the CI packet protocol over UDP having initially and quite fatally
assumed that a packet transmitted over UDP would arrive and therefore
wouldn?t need any of the lower level protocol that a real CI needed.
TCP streams are more trouble in other ways.
Just some thoughts
Keith