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
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.