<p></p>
<p dir="auto"><a class="user-mention notranslate" data-hovercard-type="user" data-hovercard-url="/users/miconda/hovercard" data-octo-click="hovercard-link-click" data-octo-dimensions="link_type:self" href="https://github.com/miconda">@miconda</a> I think this is the relevant table in section 20 <a href="https://datatracker.ietf.org/doc/html/rfc3261#section-20" rel="nofollow">link</a></p>
<p dir="auto">Seems to be pretty clear, if I read it correctly.</p>
<pre class="notranslate"><code class="notranslate">      -: 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.
</code></pre>
<pre class="notranslate"><code class="notranslate">      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
</code></pre>

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