Strange behaviour with iOS devices MTU woes

General questions
1

Hi All,
Firstly please forgive me if this seems like a simple question or has been asked before. I’ve done some digging and can’t find an answer, so hopefully someone with superior networking knowledge can point me in the right direction.

I recently moved over to VyOS and it’s been fantastic, Fallen in love with the CLI as soon as I started testing. Everything works fine, well everything that is not running iOS.

The WAN connection uses PPPoE to my ISP, I had this set to MTU 1500 and the physical interface MTU 1508. The ISP backhaul supports baby jumbo frames (so does the modem). All my Android/Linux & Windows devices work as expected without issue. However my other half pointed out that she was unable to visit any web pages on her iPhone (latest iOS version). Over LTE pages load fine without issue, on WiFi the phone just thinks about it.

I did some testing with simple network programs available in the app store. I confirmed that everything routes correctly and DNS is resolvable.

I then suspected an MTU issue as both the iPad and iPhone have the same issue. I changed the MTU size on the PPPoE device to 1492 and whoooosh, everything works on the iOS devices.

So maybe someone can help me understand why this is? What’s different about the iOS network stack that would cause an MTU of 1500 on the PPPoE device to stop correct operation?

Again, please forgive me if this is simple and I’m painfully missing something obvious.

Very Kind Regards
Jim

2

This was indeed an oversight on my part. It turns out the modem in bridge mode doesn’t support baby jumbo frames.

3

Do you have pmtud allowed? That way if a device in the path has a lower mtu, it sends it back with an icmp package and the host can auto scale. (Path MTU Discovery - Wikipedia)

1 Like
4

Hi,

I didn’t but I will go away and read up on pmtud :-). Thank you for the pointer.

5

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