The answer is found in 4.2.2:
2xx responses are acknowledged by client user agents, all other
final responses by the first proxy or client user agent to receive
the response.
And 6.15 states that INVITE 2xx responses "... MUST insert a Contact
response
header field indicating the SIP address under which it is reachable most
directly
for future SIP requests, such as ACK..."
Christian Jansson
Hotsip
www.hotsip.com
> Hello,
>
>
> In the SIP specification is written :
>
> " The ACK request is forwarded as the corresponding INVITE request, based
> on its Request-URI and thus MAY take a different path than the
> original INVITE request,.. "
>
> and
>
> "The ACK request does not generate responses for any transport protocol."
>
>
> Assume that the ACK is not sent directly from a user agent client to a
user agent server,
> but sent to a Proxy and the Record header is not present. In this case the
ACK is processed
> in the same way as the original INVITE.
> Assume that the proxy contacted a Redirect Server for the INVITE, it may
contact the Redirect Server
> for the ACK too. If the Redirect follows the rules (see above), it does
not generate a response.
> The ACK will never reach the user agent server and the call setup will not
be successfull.
>
>
> Regarding this problem there arise several questions :
>
> (1) Is this a realistic scenario in existing networks ?
> (2) MUST the client send the ACK directly to the user agent server, if it
is not willing (ignore the
> Record-Route header) or no able (Record-Route header is not present
in the 200 OK)
> to insert the Route header into the ACK ?
> (3) Is it acceptable, that the call setup is terminated, if the user agent
client does not follow the rules of (2) ?
> (4) What is the real behavior of a Redirect Server ?
> Shouldn't the Redirect Server proxy the ACK, if it is not destined
for itself ?
> Or should it answer the ACK, because the Redirect Server is
considered to be a database
> the Proxy Server can use for look up ?
> (5) Should the Proxy provide any additional function to handle the problem
?
> Should the Proxy discard ACK without Record header, instead of
forwarding it ?
>
>
> Thanks,
> Elke Neum�rker
>
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