RADIUS attribute from rfc2866

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Acct-Session-Id

This attribute is a unique Accounting ID to make it easy to match
      start and stop records in a log file.  The start and stop records
      for a given session MUST have the same Acct-Session-Id.  An
      Accounting-Request packet MUST have an Acct-Session-Id.  An
      Access-Request packet MAY have an Acct-Session-Id; if it does,
      then the NAS MUST use the same Acct-Session-Id in the Accounting-
      Request packets for that session.

      The Acct-Session-Id SHOULD contain UTF-8 encoded 10646 
      characters.For example, one implementation uses a string with an 8-digit
      upper case hexadecimal number, the first two digits increment on
      each reboot (wrapping every 256 reboots) and the next 6 digits
      counting from 0 for the first person logging in after a reboot up
      to 2^24-1, about 16 million.  Other encodings are possible.

   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     |  Text ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      44 for Acct-Session-Id.

   Length

      >= 3

   String

      The String field SHOULD be a string of UTF-8 encoded 10646 
      characters.

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