RADIUS attribute from rfc7268

---

Preauth-Timeout

This attribute sets the maximum number of seconds that pre-
      authentication state is required to be kept by the NAS without
      being utilized within a user session.  For example, when
       pre-authentication is used, if a user has not
      attempted to utilize the Pairwise Master Key (PMK) derived as a
      result of pre-authentication within the time specified by the
      Preauth-Timeout Attribute, the PMK MAY be discarded by the Access
      Point.  However, once the session is underway, the Preauth-Timeout
      Attribute has no bearing on the maximum session time for the user
      or the maximum time during which key state may be kept prior to
      re-authentication.  This is determined by the Session-Timeout
      Attribute, if present.A single Preauth-Timeout Attribute MAY be included within an
      Access-Accept or CoA-Request packet.  A summary of the Preauth-
      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

      178

   Length

      6

   Value

      The field is 4 octets, containing a 32-bit unsigned integer
      encoding the maximum time in seconds that pre-authentication state
      should be retained by the NAS.

---