[Asterisk-Dev] Early media in H.323 driver

[Chih-Wei Huang]

Hi all,
I finally solved the early media problem than I different way
I originally thinked. Originally I hope to connect the two channels
on the first early media packet arrives. But I can't find enough
information to call ast_channel_make_compatible.

Instead, I found a given media format is passed to ast_request.
I believe the format is the nativeformat of the calling party
(unverified). However, ast_request does not use the format to call the
channel driver for dialing. It uses the capabilities that the
driver supports:

  c = chan->tech->requester(type, capabilities, data, cause);
                                  ^^^^^^^^^^^^
It seems strange, because

 * The best codec format is calculated by ast_translator_best_choice
 and stored in variable fmt, but it is never be used later.
 * If we do want the driver to dial by the capabilities set it supports,
 we do not need to pass that as a parameter to the driver. Surely the
 driver knows what capabilities it supports.

So I believe it is a bug. It should be

  c = chan->tech->requester(type, fmt, data, cause);
                                  ^^^
In this way, we ask the drive to make the outgoing call by the best
codec format we chose (typically the format the calling party uses)
However, the H.323 driver doesn't really use the format to make
outgoing call. Fortunately, it's also a one-line fix.

In summary, here is the two-line fix for this problem.
(though it spent me at least 5 days to debug)


Index: channel.c
===================================================================
RCS file: /usr/cvsroot/asterisk/channel.c,v
retrieving revision 1.180
diff -u -r1.180 channel.c
--- channel.c   28 Mar 2005 20:48:24 -0000      1.180
+++ channel.c   30 Mar 2005 06:09:26 -0000
@@ -2013,7 +2013,7 @@
                        }
                        ast_mutex_unlock(&chlock);
                        if (chan->tech->requester)
-                               c = chan->tech->requester(type,
capabilities, data, cause);
+                               c = chan->tech->requester(type, fmt,
data, cause);
                        if (c) {
                                if (c->_state == AST_STATE_DOWN) {
                                        manager_event(EVENT_FLAG_CALL,
"Newchannel",

Index: channels/chan_h323.c
===================================================================
RCS file: /usr/cvsroot/asterisk/channels/chan_h323.c,v
retrieving revision 1.106
diff -u -r1.106 chan_h323.c
--- channels/chan_h323.c        17 Mar 2005 21:30:19 -0000      1.106
+++ channels/chan_h323.c        30 Mar 2005 06:09:27 -0000
@@ -1066,6 +1066,6 @@
        }
        /* Assign default capabilities */
-       pvt->capability = capability;
+	pvt->capability = format;
        pvt->dtmfmode = H323_DTMF_RFC2833;

        strncpy(tmp, dest, sizeof(tmp) - 1);


-- 
   ~     Chih-Wei Huang (cwhuang at citron.com.tw)
  'v'    CTO, Citron Network Inc. ( http://www.citron.com.tw/ )
 // \\   GnuGK Project : http://www.gnugk.org/    (Developer)
/(   )\  HomePage      : http://www.linux.org.tw/~cwhuang/
 ^`~'^