RADIUS attribute from rfc7268

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WLAN-HESSID

The WLAN-HESSID Attribute contains a MAC address that identifies
      the Homogenous Extended Service Set.  The HESSID is a globally
      unique identifier that, in conjunction with the SSID, encoded
      within the Called-Station-Id Attribute as described in ,
      may be used to provide network identification for a subscription
      service provider network (SSPN), as described in Section 8.4.2.94
      of .  Zero or one WLAN-HESSID Attribute is permitted
      within an Access-Request or Accounting-Request packet.

      A summary of the WLAN-HESSID 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     |          String...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      181

   Length

      19

   String

      The String field is encoded in uppercase ASCII characters with the
      octet values separated by dash characters, as described in, for example, "00-10-A4-23-19-C0".

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