27 Mar
2021
27 Mar
'21
6:59 p.m.
On Mar 27, 2021, at 11:06 AM, Mark Berryman <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.
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