RADIUS attribute from rfc7268

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Mobility-Domain-Id

A single Mobility-Domain-Id Attribute MAY be included in an
      Access-Request or Accounting-Request in order to enable the NAS to
      provide the RADIUS server with the Mobility Domain Identifier
      (MDID), defined in Section 8.4.2.49 of .  A summary
      of the Mobility-Domain-Id 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                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      177

   Length

      6

   Value

      The Value field is four octets, containing a 32-bit unsigned
      integer.  The two most significant octets MUST be set to zero by
      the sender and are ignored by the receiver; the two least
      significant octets contain the Mobility Domain Identifier (MDID)
      defined in Section 8.4.2.49 of .

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Reserved           |   Mobility Domain Identifier  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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