[HECnet] Old protocols in new ones

[Johnny Billquist]

On 2021-03-27 21:51, Thomas DeBellis wrote:
> I was wondering whether the problem of running DDCMP over UDP might be 
> one of error timing.  If you blew it on a KMC or DUP, the hardware would 
> let you know pretty quick; milliseconds.  The problem with UDP is how 
> soon you declare an error.  If you have a packet going a long way, it 
> might take too long to declare the error.  It's a thought, but you can 
> get delays in TCP, too, so I'm not sure if the idea is half-baked.

No. It's not any timing problem.
It really is the fact that UDP packets can, and will get lost from time 
to time.
DDCMP is assuming that no packets get lost.
Reordered packets is sortof the same kind of problem.
As is duplicated packets.

All are bad, and DDCMP barely detects that it happens, and the only 
remedy is to tear down the link.

Run DDCMP over TCP instead, and all is good. Timing don't really matter 
that much.

   Johnny

> 
> On 3/27/2021 1:59 PM, John Forecast wrote:
>>
>>> 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.
>>
>>> 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
>>>
>>

-- 
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