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... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 176 Length >=3 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.