[HECnet] Emulated XQ polling timer setting and data overrun

[Johnny Billquist]

On 2014-06-05 20:46, Mark Pizzolato - Info Comm wrote:
On Thursday, June 05, 2014 at 10:53 AM, Johnny Billquist wrote:
On 2014-06-05 19:23, Paul_Koning at Dell.com wrote:

On Jun 5, 2014, at 12:47 PM, Johnny Billquist <bqt at softjar.se> wrote:

Interesting. I still have real network issues with the latest simh
talking to a real PDP-11 sitting on the same physical network. So
you seem to have hit sime other kind of limitation or something...

I wouldn't think that traffic between PDP11 systems would put so much data in flight that all of the above issues would come into play.

Hmmm...Grind...Grind... I do seem have some vague recollection of an issue with some DEQNA devices not being able to handle back-to-back packets coming in from the wire.   This issue might have been behind DEC's wholesale replacement/upgrading of every DEQNA in the field, and it also may have had something to do with the DEQNA not being officially supported as a cluster device...

Hey, just do:

            sim> SET XQ TYPE=DELQA-T

and your all good.   :-)   Too bad you can't just upgrade real hardware like that.

Uh... It's the PDP-11 that have problems receiving packets, not simh. Also, my PDP-11 already have a DELQA-T. :-)
Finally, of course a simulated PDP-11 running on a fast machine will be able to output data at a high rate. Why would it not?
Two real PDP-11 systems do not get any problems.
Sending data from a real PDP-11 to the one in simh (or whatever) does not have any problems either.
It is only when you send lots of data from a simulated machine (be that a PDP-11 or a VAX) to a real PDP-11 that you get these issues. I would suspect you should be able to see similar issues if the receiving end was physical VAX as well.

The problem is also partly DECnet. DECnet do not seem to keep packets that arrive out of order. So if a packet in a sequence is lost, DECnet is going to retransmit all packets from that point forward. Meaning that when the session timer times out, the retransmission happens, and then you will yet again drop a packet in the whole sequence of packets that are sent. Each time the session timer times out, DECnet also do a backoff on the timeout time of that timer, until the session timer is about 2 minutes. So after a while you end up with DECnet sending a burst of packets, some of which are lost. It then takes about 2 minutes before a retransmission happens, at which point you get another 2 minute timeout. Thus, performance sucks.
TCP/IP is better (well, my TCP/IP anyway), in that when I loose a packet, I still keep whatever later packets I get, so after a while I get to a stable mode where TCP only sends one packet at a time, since the window is full. Only actually lost packets needs to be retransmitted, so I actually do get to this stable point.

	Johnny