| AVP | No. | Incoming | Outgoing |
| Message Type | 0 | Value 1 | Value 1 |
| Protocol Version | 2 | Mandatory, value 1 expected | Value 1 |
| Framing Capabilities | 3 | Ignored | Value 3 |
| Bearer Capabilities | 4 | Ignored | Not sent |
| Tie Breaker | 5 | Ignored as FireBrick only accepts connections for inbound calls | Not sent |
| Firmware Revision | 6 | Ignored | FireBrick s/w version string |
| Host Name | 7 | Used to select which incoming L2TP configuration applies. | As per config/RADIUS request |
| Vendor Name | 8 | Ignored | FireBrick Ltd |
| Assigned Tunnel ID | 9 | Mandatory | Mandatory, our tunnel ID |
| Receive Window Size | 10 | Accepted, assumed 4 if not present or less than 4 is specified | Value 4 |
| Challenge | 11 | Accepted if a configured secret is defined, a response is sent in the SCCRP | Not sent at present |
