@miconda I think this is the relevant table in section 20 [link](https://datatracker.ietf.org/doc/html/rfc3261#section-20)
Seems to be pretty clear, if I read it correctly.
``` -: The header field is not applicable.
"Not applicable" means that the header field MUST NOT be present in a request. If one is placed in a request by mistake, it MUST be ignored by the UAS receiving the request. Similarly, a header field labeled "not applicable" for a response means that the UAS MUST NOT place the header field in the response, and the UAC MUST ignore the header field in the response. ```
``` Header field where proxy ACK BYE CAN INV OPT REG ___________________________________________________________ Contact R o - - m o o Contact 1xx - - - o - - Contact 2xx - - - m o o Contact 3xx d - o - o o o Contact 485 - o - o o o ```