RADIUS attribute from rfc7268


The EAP-Server-Id Attribute contains a Server-Id generated by the
      EAP method.  Exactly how this name is used depends on the link
      layer in question.  See  for more discussion.  The EAP-
      Server-Id Attribute is only allowed in Access-Request, Access-
      Accept, and Accounting-Request packets.  More than one EAP-Server-Id Attribute MUST NOT be included in an Access-Request; one or
      more EAP-Server-Id Attributes MAY be included in an Access-Accept.

      It should be noted that not all link layers use this name, and
      existing EAP method implementations do not generate it.  Since the
      NAS operates as a pass-through in EAP , it cannot know
      the EAP-Server-Id before receiving it from the RADIUS server.  As
      a result, an EAP-Server-Id Attribute sent in an Access-Request
      MUST contain only a single NUL character.  A home RADIUS server
      receiving an Access-Request with an EAP-Server-Id Attribute
      containing anything other than a single NUL character MUST
      silently discard the attribute.  In addition, the home RADIUS
      server SHOULD include this attribute in an Access-Accept only if
      an EAP-Server-Id Attribute was present in the Access-Request.  A
      summary of the EAP-Server-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       |            String...






      The String field is one or more octets, containing an EAP Server-
      Id exported by the EAP method.  For details, see.  A robust implementation SHOULD support the field as
      undistinguished octets.