As far as I know, AX.25 MTU is only take care of on transmitting.
There are no trunked buffer sizes on the rx-routines in the kernel.
=> larger packets, even beyond the ax25 protocol spec (>256 bytes)
could be handled.
On transmission, the MTU is respected.
But if you read my last posts on this list, that's not always true (IP):
I proposed a fix for packets leaving the interface that are much
larger than the configured MTU or / and the max-mtu of 256.
For normal PID=text packets, data sent with write() is adjusted to the
MTU, and data tried to send with syscall sendto() and a size > MTU are
rejected.
Btw, in your line:
> 23 11.084320000 FH4GOU-01 TRDS01-01 AX.25-NoL3 271 Text
=> 271 is beyond the standard. You'll might find many old software,
TNCs, etc. that would even not be capable to decode such a packet.
On the other hand I wonder what packet that was.
"NoL3" => I assume PID=text (not IP, ARP, etc.).
Sender is FH4GOU-1 (but that's not a call of one of the two sides in
your test setup). How does that callsign comes to the line and what
computer / os / pr-program / userspacesprogram sends this data?
vy 73,
- Thomas dl9sau
On Sat, Feb 27, 2016 at 12:42:04AM +0100, folkert wrote:
> 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@travelmate:~# uname -r
> 4.2.0-1-amd64
>
> root@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@travelmate:~# grep ax0 /etc/ax25/axports
> ax0 FH4GOU-1 115200 254 1 AX25 over LoRa
>
>
> root@tardis:/usr/src/sketches/mkiss# uname -r
> 4.1.18+
>
> root@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@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?
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