RADIUS attribute from rfc2869

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NAS-Port-Id

This Attribute contains a text string which identifies the port of
      the NAS which is authenticating the user.  It is only used in
      Access-Request and Accounting-Request packets.  Note that this is
      using "port" in its sense of a physical connection on the NAS, not
      in the sense of a TCP or UDP port number.

      Either NAS-Port or NAS-Port-Id SHOULD be present in an Access-
      Request packet, if the NAS differentiates among its ports.  NAS-
      Port-Id is intended for use by NASes which cannot conveniently
      number their ports.

   A summary of the NAS-Port-Id Attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Text...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      87 for NAS-Port-Id.

   Length

      >= 3

   Text

      The Text field contains the name of the port using UTF-8 encoded
      10646  characters.

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