wifi calling - in depth

[tonjg]

I have a samsung galaxy S8 and I'm trying to understand the mechanism behind wifi calling. firstly I realise the phone has to register with the wifi-calling-server first before the phone can make wifi calls. how often does the phone ping the O2 server when it is first trying to register? is there a way on my phone to see this happening?

[MI5]

@tonjg 

Your phone is constantly pinging the server in the same way that it does for a regular cell signal.

The only way to see it would be to root the phone and run an app to see the data.

However, the S8 is known to be flaky with O2 wifi calling due to it's age.

[tonjg]

yes I know my phone is constantly pinging but whats the interval between pings to the wifi-calling-server?

[MI5]

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@MI5 wrote:

@tonjg 

The only way to see it would be to root the phone and run an app to see the data.

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[madasaf1sh · Accepted solution]

@tonjg 

So lets this clear, your phone is not constantly pinging the o2 VoWiFI Service. There maybe an ACK SYN or a keep alive packet sent. 

When your SIM is enabled for VoWIFI and your phone is enabled to use it, then your phone when connected to WiFi automatically connects to the o2 ePDG (Evolved Packet Data Gateway) this secures the service from nefarious actors and rogue devices.

Your phone does a handshake with the ePDG and then creates and IPSEC tunnel aka VPN this is all done by DNS queries.

Once the tunnel is up, then an authentication process is done, usually via a variant of EAP.

The IPSEC tunnel is kept alive for as long as the WiFi connection is up, or until you move to Mobile Data.

If your phone cant register with the ePDG then depending on your phone it might retry every 90seconds until "n" then give up.

The only way you really are going to see the requests is a device with Wireshark and do some deep IP packet analysis you might also be able to see this on an enterprise level router or a none ISP provided one..

[tonjg]

@madasaf1sh thankyou for this detailed and informative reply. I knew I would have to use a packet sniffer of some kind and i'm also familiar with doing this with an enterprise level router. I just wanted to be more sure what was going on when wifi calling was not working and i'm not convinced the problem is always with my phone. the O2 service where I live is so bad if i don't have wifi calling my phone is bricked. must say tho 90 seconds is a long time between pings...

[madasaf1sh]

@tonjg 

I was using it as an example and some heartbeats are check every 12hours, as you will be aware... so 90seconds between Keep Alive Packets would be about right, to keep network traffic to a minumum, and prevent devices DDoS'ing the service..

The issues could be your router, your ISP (have seen that before) as well as the phone.. 

But TCP and UDP packet tear downs are the way forward, just be aware the S8 is an old phone, and is not the best for WiFi Calling..