The ACK request confirms that the client has received a final response to an INVITE request.
2xx responses acknowledged by client user agents, all other final responses by the first proxy or client user agent to receive the response.
the client user agent after a 2xx
response or the first proxy to receive a non-2xx final response. The
ACK request is forwarded as the corresponding INVITE request, based
on its Request-URI. See Section 10 for details.
proxy server receiving an ACK request after having sent a 3xx, 4xx,
5xx, or 6xx response must make a determination about whether the ACK
is for it, or for some user agent or proxy server further downstream.
2xx responses acknowledged by client user agents, all other final responses by the first proxy or client user agent to receive the response.
the client user agent after a 2xx
response or the first proxy to receive a non-2xx final response. The
ACK request is forwarded as the corresponding INVITE request, based
on its Request-URI. See Section 10 for details.
proxy server receiving an ACK request after having sent a 3xx, 4xx,
5xx, or 6xx response must make a determination about whether the ACK
is for it, or for some user agent or proxy server further downstream.