[Linux-ham] wrong packet size

[folkert]

Hi,

I have a radio link between two systems.
The maximum packet size in this link is 254 bytes.
On both sides this limit is set:

root at travelmate:~# uname -r
4.2.0-1-amd64

root at travelmate:~# ifconfig ax0
ax0       Link encap:AMPR AX.25  HWaddr FH4GOU-1  
          UP BROADCAST RUNNING  MTU:254  Metric:1
          RX packets:245 errors:0 dropped:0 overruns:0 frame:0
          TX packets:67 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:10 
          RX bytes:6492 (6.3 KiB)  TX bytes:8810 (8.6 KiB)

root at travelmate:~# grep ax0 /etc/ax25/axports 
ax0	FH4GOU-1	115200	254	1	AX25 over LoRa


root at tardis:/usr/src/sketches/mkiss# uname -r
4.1.18+

root at tardis:/usr/src/sketches/mkiss# ifconfig ax0
ax0       Link encap:AMPR AX.25  HWaddr TRDS01-1  
          UP BROADCAST RUNNING  MTU:254  Metric:1
          RX packets:569 errors:1 dropped:0 overruns:0 frame:0
          TX packets:212 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:10 
          RX bytes:14033 (13.7 KiB)  TX bytes:4987 (4.8 KiB)

root at tardis:/usr/src/sketches/mkiss# grep ax0 /etc/ax25/axports 
ax0     TRDS01-1        9600    254     1       AX.25 over LoRa


Transmitting/receiving packets < 254 bytes works fine; e.g. IDENT with
not too large texts and also "axcall"-sessions with small pieces of
texts.
But if I invoke "mheard" which returns quite a bit of text,
communication fails. I checked with wireshark and I saw that even
though there's that 254 bytes MTU, I still get packets way larger:

nr      time            source          target          protocol   length info
23	11.084320000	FH4GOU-01	TRDS01-01	AX.25-NoL3 271 Text

What can I do to prevent these overly large packets?