Mobile L2TP Technical
An L2TP tunnel is established using the pre-agreed IP addresses, hostname and shared secret. Note that if your LNS rejects the tunnel connection, or fails to respond to the SCCRQ or a subsequent HELLO, then it will be black listed for 5 minutes allowing further connections to go to the fall-back LNSs.
Once the tunnel is established, a session is established for each connecting data SIM.
|Assigned Session ID||14||Session ID|
|Call Serial Number||15||Present if received from mobile network|
|Calling Number||22||19 digit ICCID of SIM, starting 8944|
|Framing Type||19||Always 1 (PPP)|
|Initial Received LCP CONFREQ||26||As received / negotiated by proxy|
|Last Sent LCP CONFREQ||27||As sent / negotiated by proxy|
|Last Received LCP CONFREQ||28||As received / negotiated by proxy|
|Proxy Authen Type||29||Will always be CHAP|
|Proxy Authen Name||30||User name from mobile device, typically blank|
|Proxy Authen Challenge||31||Chap challenge|
|Proxy Authen ID||32||Chap challenge ID|
|Proxy Authen Response||33||Chap response hash|
|Tx connect speed||24||Present if advised by mobile network|
|Rx connect speed||38||Present if advised by mobile network|
- Only available on data SIMs.
- Called Number is a fixed APN and not the one entered on the phone, which is ignored.
- Reduced MTU of 1450 is in operation when L2TP is run over the internet. This restriction can be removed to allow full 1500 byte MTU where direct *Baby-jumbo frame links are available from us to your LNS.
- User-Name passed is what is provided by the mobile which is usually blank. Do not use the password 'password' as this will not be passed through.
- IPv6 is not available yet, but is planned for the future if possible.
We've seen various devices give odd Connect Speed settings when they connect. EG, OSX sends:
Connect Speed: 1000000
And a Pfsense has been seen to send 10Mbit/s and even the source code had it hard coded to 10Mbit/s.
This means that it is asking for a 1Mbit/s connection. This does not looks like it's configurable either.
From May 2016 we ignore the Connect Speed, and simply limit the speed to 100Mbit/s at our side.