RADIUS attribute from rfc2866

---

Acct-Multi-Session-Id

This attribute is a unique Accounting ID to make it easy to link
      together multiple related sessions in a log file.  Each session
      linked together would have a unique Acct-Session-Id but the same
      Acct-Multi-Session-Id.  It is strongly recommended that the Acct-
      Multi-Session-Id contain UTF-8 encoded 10646  characters.

   A summary of the Acct-Session-Id 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

   50 for Acct-Multi-Session-Id.

   Length

   >= 3

   String

   The String field SHOULD contain UTF-8 encoded 10646  characters.

---