The TCN response header is used by a server to signal that the resource is transparently negotiated.

       TCN = "TCN" ":" #( response-type
                        | server-side-override-directive
                        | tcn-extension )
       response-type = "list" | "choice" | "adhoc"
       server-side-override-directive = "re-choose" | "keep"
       tcn-extension = token [ "=" ( token | quoted-string ) ]

If the resource is not transparently negotiated, a TCN header MUST NOT be included in any response. If the resource is transparently negotiated, a TCN header, which includes the response-type value of the response, MUST be included in every response with a 2xx status code or any 3xx status code, except 304, in which it MAY be included. A TCN header MAY also be included, without a response-type value, in other responses from transparently negotiated resources.

A server-side override directive MUST be included if the origin server performed a server-side override when choosing the response. If the directive is "re-choose", the server MUST include an Alternates header with the variant bound to the negotiable resource in the response, and user agent SHOULD use its internal variant selection algorithm to choose, retrieve, and display the best variant from this list. If the directive is "keep" the user agent SHOULD NOT renegotiate on the response, but display it directly, or act on it directly if it is a redirection response.

Clients SHOULD ignore all tcn-extensions they do not understand.