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.
ICRQ
| Attribute | Meaning | |
|---|---|---|
| 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 |
ICCN
| Attribute | Meaning | |
|---|---|---|
| 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 |
Limitations
- 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.
Connect Speed
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 10Mb/s and even the source code had it hard coded to 10Mb/s.
This means that it is asking for a 1Mb/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 100Mb/s at our side.