RADIUS attribute from rfc2869

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Acct-Interim-Interval

This attribute indicates the number of seconds between each
      interim update in seconds  for this specific session. This value
      can only appear in the Access-Accept message.

   A summary of the Acct-Interim-Interval 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

      85 for Acct-Interim-Interval.

   Length

      6

   Value

      The Value field contains the number of seconds between each
      interim update to be sent from the NAS for this session. The value
      MUST NOT be smaller than 60.  The value SHOULD NOT be smaller than
      600, and careful consideration should be given to its impact on
      network traffic.

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