RADIUS attribute from rfc2865

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Idle-Timeout

This Attribute sets the maximum number of consecutive seconds of
      idle connection allowed to the user before termination of the
      session or prompt.  This Attribute is available to be sent by the
      server to the client in an Access-Accept or Access-Challenge.

   A summary of the Idle-Timeout Attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      28 for Idle-Timeout.

   Length

      6

   Value

      The field is 4 octets, containing a 32-bit unsigned integer with
      the maximum number of consecutive seconds of idle time this user
      should be permitted before being disconnected by the NAS.

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