RADIUS attribute from rfc2867
This Attribute indicates the identifier assigned to the tunnel session. It SHOULD be included in Accounting-Request packets which contain an Acct-Status-Type attribute having the value Start, Stop or any of the values described above. This attribute, along with the Tunnel-Client-Endpoint and Tunnel- Server-Endpoint attributes , may be used to provide a means to uniquely identify a tunnel session for auditing purposes. A summary of the Acct-Tunnel-Connection Attribute format is shown below. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | String ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 68 for Acct-Tunnel-Connection Length >= 3 String The format of the identifier represented by the String field depends upon the value of the Tunnel-Type attribute . For example, to fully identify an L2TP tunnel connection, the L2TP Tunnel ID and Call ID might be encoded in this field. The exact encoding of this field is implementation dependent.