[HECnet] "Dropped by adjacent node" ....

[Johnny Billquist]

On 2014-05-28 15:33, Paul_Koning at Dell.com wrote:

On May 27, 2014, at 8:53 PM, Johnny Billquist <bqt at softjar.se> wrote:

On 2014-05-27 22:09, Paul_Koning at Dell.com wrote:

On May 27, 2014, at 4:04 PM, Johnny Billquist <bqt at softjar.se> wrote:

On 2014-05-27 21:48, Bob Armstrong wrote:
The way to run DECnet over a flaky long distance network is to use point
to point mode with a data link layer that deals with packet loss.

    Probably a good idea, but we don't have that option on HECnet.

Well, HECnet is not a static piece of equipment. Anything is possible...
My bridge emulates a simple ethernet segment. Good enough many times, but if we have a link like yours, that sometimes seems to drop packets, then maybe some other alternative should be considered.

Now, the question then becomes, what can we do in this case.

As far as I understand, links using Multinet are more broken, and still use UDP. The same would appear to possibly be the case for Cisco as well?

Do anyone run any links using TCP?

That would work.   DDCMP over UDP would work.

Really? UDP can cause packets to arrive in the wrong order, duplicated, or sometimes dropped. I was certain you wrote above "a data link layer that deals with packet loss". Or was that not meant to be read as that the underlaying transport should deal with it?

DDCMP is a data link layer that deals with packet loss.   It also deals with reordering, within reason, by treating it as packet loss, just as NSP did until Phase V.   It should be ok with limited duplication as well     though duplication is not one of the supposed characteristics of UDP.

Duplication is definitely a possible event with UDP. It happens from time to time. It's essentially because duplication is possible with IP. UDP is just IP with ports and a checksum (if you are lucky). And IP makes no promises at all. Packets might not arrive, might arrive out of order, become duplicated, delayed, or even corrupted.

Note that I meant the DDCMP protocol, not the DDCMP point to point service.   In other words, the UDP packets would carry DDCMP frames, with DDCMP header (including sequence number and all that).   I think SIMH has that right now, in V4.0 DMC emulation.

Ok. So if I understand correctly, DDCMP is guaranteeing the delivery of data between the two points. Data will arrive in order and without any loss or other confusion, as seen by the layers above DDCMP?

Seems like UDP could work then, but maybe TCP would be better?

	Johnny

-- 
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