RADIUS attribute from rfc2867

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Acct-Tunnel-Connection

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.

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