RADIUS attribute from rfc3162

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Framed-Interface-Id

This Attribute indicates the IPv6 interface identifier to be
      configured for the user.  It MAY be used in Access-Accept packets.
      If the Interface-Identifier IPv6CP option  has been
      successfully negotiated, this Attribute MUST be included in an
      Access-Request packet as a hint by the NAS to the server that it
      would prefer that value.  It is recommended, but not required,
      that the server honor the hint.

   A summary of the Framed-Interface-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     |             Interface-Id
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                Interface-Id
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Interface-Id             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      96 for Framed-Interface-Id

   Length

      10

   Interface-Id

      The Interface-Id field is 8 octets.

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