MTU issue on university's iotroam wifi network

[vgroenhuis]

I have a Pico 2W running a simple server, connected over wifi with university's iotroam network. HTTP response packets with payload larger than 1404 bytes disappear. ping -l 1416 to Pico 2W works, but ping -l 1417 (and higher) is unreachable. Clearly an issue with MTU limit. iotroam sysadmin explained that due to encapsulations the iotroam MTU is not the usual 1500, but 1472 bytes, and for wifi a bit lower. Connecting to mobile hotspot on my phone has zero issues. Is it the iotroam router's fault or an issue with Pico 2W's IP stack drivers?

[earlephilhower]

The something called path MTU discovery which might let a full fat IP stack like on your phone discover the maximum MTU, but that's not in LWIP as far as I know. So you'll need to manually set the MTU here.

Unfortunately it looks like you'll need to edit the source because there's not a nice exposed way to set the MTU on a WiFi interface. (The WiFi drivers differ from the wired ones by having a second level wrapper for all the link control... The wired Ethernet drivers can take MTU as part of the eth.begin() call).

arduino-pico/libraries/lwIP_Ethernet/src/LwipIntfDev.h

Line 50 in c2fe192

#define DEFAULT_MTU 1500

[JeroenvIS]

Thank you for explaining. Would it be possible to implement a simple fix that students or novice users can easily use?
For example:

• A way to override the MTU in the sketch
• A way to adjust the MTU somewhere in the code, eg at link setup?

...or would it be an option to lower the default MTU in the library to a "much safer" value, like 1400 bytes?

While we may be the first to ask, I can imagine that more users may run in to this issue - lots of edu networks are using tunneling mechanisms between their WiFi access points and central wireless controllers, and several of those can impact the MTU. Users hardly ever notice (because of PMTU discovery), but when clients don't do any PMTU discovery, perhaps it's wiser to not assume that the maximum "non jumbo" Ethernet MTU is available.

[JeroenvIS]

Looking at the context, it appears that the define is already in a conditional block:

arduino-pico/libraries/lwIP_Ethernet/src/LwipIntfDev.h

Lines 49 to 51 in c2fe192

	#ifndef DEFAULT_MTU
	#define DEFAULT_MTU 1500
	#endif

My last Arduino adventure was ~ 5 years ago so I'm a bit rusty, but wouldn't it be enough then to simply
#define DEFAULT_MTU 1400
...in the main sketch, before any imports?

[vgroenhuis]

#define DEFAULT_MTU 1400 in the main sketch does not work, but editing the source (as Earle suggested) seems to work. It is not too hard after all. It is in the file C:\Users\<user>\.platformio\packages\framework-arduinopico\libraries\lwIP_Ethernet\src\LwipIntfDev.h
I will investigate this further. Thanks for all comments!

[vgroenhuis]

I found an easier solution. In platformio.ini add the following line:
build_flags = -DDEFAULT_MTU=1400

It is then no longer needed to modify the source code.

[earlephilhower]

Great, I forgot about that! You can also use a platform.local.txt file to set that define.

The right thing to do, though, would be to add a way of setting the MTU before WiFi.begin(). WiFi::setMTU(int mtu);? I'd be happy to get a PR for that. From w/in WiFiClass you can get the WiFi netif and use netif->mtu = XXXX so it should be localised to that 1 file and not need delving into the lower-level drivers.