RADIUS attribute from rfc2865

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Framed-MTU

This Attribute indicates the Maximum Transmission Unit to be
      configured for the user, when it is not negotiated by some other
      means (such as PPP).  It MAY be used in Access-Accept packets.  It
      MAY be used in an Access-Request packet as a hint by the NAS to
      the server that it would prefer that value, but the server is not
      required to honor the hint.

   A summary of the Framed-MTU 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

      12 for Framed-MTU.

   Length

      6

   Value

      The Value field is four octets.  Despite the size of the field,
      values range from 64 to 65535.

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