RADIUS attribute from rfc2865

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Proxy-State

This Attribute is available to be sent by a proxy server to
      another server when forwarding an Access-Request and MUST be
      returned unmodified in the Access-Accept, Access-Reject or
      Access-Challenge.  When the proxy server receives the response to
      its request, it MUST remove its own Proxy-State (the last Proxy-
      State in the packet) before forwarding the response to the NAS.

      If a Proxy-State Attribute is added to a packet when forwarding
      the packet, the Proxy-State Attribute MUST be added after any
      existing Proxy-State attributes.

      The content of any Proxy-State other than the one added by the
      current server should be treated as opaque octets and MUST NOT
      affect operation of the protocol.

      Usage of the Proxy-State Attribute is implementation dependent.  A
      description of its function is outside the scope of this
      specification.

   A summary of the Proxy-State 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      33 for Proxy-State.Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

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