Reliable "virtual RF channel" between two grc applications (UDP Source/Sink?)

Hi,

Before going to hardware (and since my HW access is limited in Corona times) I want to build my "TRX1" with a behavioral model of "TRX2" (TRX2 will eventually be a simple hardware TRX on PCB, TRX1 a more signal processing heavy gnuradio application).

I thought UDP Source/Sink over localhost is the way to go but this works very unreliably here.

For TRX1 I use (simple demo):

Signal Source (fs=5MHz, f=1MHz) --> Throttle (5MHz) --> UDP Sink (127.0.0.1:1235)
|
\-----> |
| QT GUI Time Sink
UDP Source (127.0.0.1:1234) ------------> |

For TRX2 I use (simple demo):

UDP Source (127.0.0.1:1235) --> Throttle (5MHz) --> MultiplyConst (2) --> UDP Sink (127.0.0.1:1234)


I start TRX2 first and then TRX1. Initially this simple demo worked well with default values (payload_size=1500-20, sendnullpkt=false) but frequently I got the following error messages (from both TRX1 and TRX2):

Too much data; dropping packet.

This surprised me a lot because it's localhost which should be insanely fast. I run USRP with 200 MSps (10Gbe) on the same machine without a single packet drop!
Well, I checked the MTU of lo is not 1500 but 65536. Still baffled how this can happen with a ridiculously low sampe_rate=5M, I set payload_size=65516.
Now the error is largely gone. However, here and there I get a new nasty error message:

ERROR: send error: send_to: Message too long

This only seems to happen for TRX2.
After this error I have to restart both flowgraphs.
Unfortunately this only happens with more complicated stuff implemented within TRX1/TRX2 and not reproducibly.


Has anyone done something like this before and has a recommendation for a reliable setup?
Is UDP Source/Sink over localhost even the best way to go?

Best,
Lukas