Xiaomi 11T Pro - Wi-Fi calling disconnects every 50 minutes exactly

[PhilipL]

I have the Xiaomi 11T Pro on an EE monthly SIM.  All works great except for Wi-Fi calling.

Wi-Fi calling will connect back to EE servers but at exactly 50 minutes (that time span must mean something to someone technical) since connecting it drops the connection.  The phone reconnects itself after 10 or 15 seconds, only to drop again 50 minutes later.

I can monitor this on my router in real time and see the connection on port 4500 to EE drop at the point it starts exchanging ISAKMP packets for CREATE_CHILD_SA.  

The same SIM in other Android devices will connect to Wi-Fi calling for tens of hours.  I've a second device connected to EE and can watch that remain connected whilst the Xiaomi drops.

Suffice to say if I'm on a call at that 50 minute boundary the call drops.

I had a similar issue with a Pixel 2 XL started dropping the connection every 25 minutes exactly which was eventually fixed by EE.

I would be very grateful if this could be passed to someone in the tech team perhaps.  I can provide packet capture logs if that helps.  Of course it may well be a problem with the Xiaomi.  I'm probably not alone with this issue with these devices but most people will just think it is a random drop without realising it is always on 50 minute boundaries.

Many thanks.

[James_B]

Thanks for letting us know @PhilipL

Please get back in touch with Customer Care so our Tech Gurus can escalate this for you.

James

[PhilipL]

@James_B They said because it isn't a device they sell or sold they wouldn't be able to look into it, and the resetting of the network settings on my account was just a finger in the air lets give it a go type of thing.

[James_B]

Hi @PhilipL,

It sounds like they think it's a software issue, rather than network. I'd recommend contacting the manufacturer directly for support.

James

[PhilipL]

So I've been doing some debugging and investigation into this 50 minute drop, this possibly affects a lot of devices using standard Qualcomm supplied modem configuration binaries as I've seen the same settings in many modem.bin files for different manufacturers on Qualcomm phones.

The issue is the settings in the /modem_pr/mcfg/configs/mcfg_sw/generic/EU/EE/Commercial mcfg_sw.mbn file, this has various configuration settings but the ones in question are in the XML for <IWLAN_S2B_CONFIG>.

There are two elements in this XML file called <esp_rekey_time> and <ikev2_sa_rekey_timer>, the esp_rekey_timer is set to 3000 seconds for soft and 3600 for hard. Now 3000 seconds is 50 minutes!

At the 50 minute mark the initial keys are exchanged for new ones, this is failing, I think because no group or hash algorithms are in this configuration file (they are in other carrier configuration settings).

A work around I have tested is to simply set these timeout values both to soft:64800/hard:64900 (as seen in other operator config files, e.g. Vodafone), this means the initial keys will last around 45 days, in other words under normal usage the Wi-Fi calling VPN will never get to that age as the connection is dropped and reset on moving around and leaving the Wi-Fi network, but now it will no longer drop every 50 minutes as it never attempts to 'rekey' and so fail. However to implement these changes requires a rooted device and a Qualcomm PST debug port enabled, along with the Qualcomm PST software.

I hope this information can be fed back to the network team who may be able to get Qualcomm to update their standard operator settings, either using the large timeout value or fixing the issue why the rekey fails.  

At least now I have VoWiFi working and not dropping.

[PhilipL]

Just an update to previous post.  My calculation on extending the seconds to a larger value was wrong, as its not 50 days but just 18 hours before it times out!  So the VPN Wi-Fi calling connection dropped after 18 hours, but considerably better than 50 minutes.

So I've gone back to his and have now successfully added in the missing settings, so when it times out the keys are successfully renewed as intended and the connection remains up.

For some reason EE have quite short time outs between creating new keys compared to other operators which all seem to be > 18 hours.  I suspect the rekey values might be quite low as EE was one of the first operators to get Wi-Fi calling and perhaps these were early test configurations that got added as standard to Android that have never been updated?