Some text , as requested. Christer Holmberg wrote: ...
In section 6: When a forking proxy receives a non-2xx final response that it recognizes as terminating one or more early dialogs, if the proxy does not intend to forward the final response immediately (in accordance with rules for a forking proxy) and the UAC has indicated support for the 199 response code, the proxy SHOULD generate andsenda 199 response upstream for each early dialog terminated on the downstream side by the non-2xx final response, except for any early dialog for which the proxy has previously received and forwarded a 199 response.
...
My suggestion:
"When a forking proxy receives a non-2xx final response that it
recognizes as terminating one or more early dialogs, it SHOULD generate
and send a 199 response upstream for each such early dialog satisfying the
following conditions:
-- the proxy does not intend to forward the final response immediately
(in accordance with rules for a forking proxy);
-- the UAC has indicated support for the 199 response code;
-- the proxy has not already received and forwarded a 199 response for
that early dialogue."