ht5-sdp.dox

最新osip源代码

/**
 * @ingroup libosip2 The GNU oSIP stack
 * @defgroup howto5_sdp How-To use the sdp negotiator.

 * @section howto5_1 Description.

The SDP offer/answer model is where most SIP interoperability issue
comes from.  The SDP specification (rfc2327.txt) is often not fully
respected. As an example, most SIP applications forget to add the
mandatory 's' field in the SDP packet. Another mistake is to assume
that an SDP packet don't need a 'p' and a 'e' field. Even if they
are both optional, at least of those is mandatory! I have never
seen ONE implementation that send at least one 'p' or 'e' field!!
For all those reasons, the negotiation happens to be a hard task

 * @section howto5_2 osip_negotiation.h VS osip_rfc3264.h

The include file osip_negotiation.h and osip_rfc3264.h both
offer the same feature. osip_negotiation.h is now made obsolete
as the result were poor. <B>PLEASE DON'T USE IT ANYMORE</B>

The new negotiator is more promising and nearly finished. You
can already use and test it, but <B>be aware that this is still
work in progress and even the API has not yet been fixed.</B>

 * @section howto5_3 Do you need to use a SDP negotiator?

Of course, a SDP negotiator is only needed for SIP endpoint.

Advanced applications may find it inappropriate, but as you can
modify the SDP answer after running the negotiation, I see no
reason why you should not use it. It will just simplify the amount
of code you'll need to negotiate medias.

If by using the new negotiator, you find some nice ideas to
improve it, I'll be glad to think about implementing them.
Trying to build a generic SDP negotiatior gave me headache...
I hope the new one will be suitable for all needs.

 * @section howto5_4 How-to initialize the SDP negotiator?

Here is the way to initialize the new negotiator:

<PRE>
	struct osip_rfc3264 *cnf;
	int i;
	
	i = osip_rfc3264_init(&cnf);
	if (i!=0)
	{
		fprintf(stderr, "Cannot Initialize Negotiator feature.\n");
		return -1;
	}
</PRE>

 * @section howto5_5 How-to add support for medias?

You then must add a set of known codecs. To simplify the
implementation, you add sdp_media_t elements. The next code
shows how you can add support for the G729 codec.

<PRE>
	sdp_media_t *med;
	sdp_attribute_t *attr;
	i = sdp_media_init(&med);
	med->m_proto = osip_strdup("RTP/AVP");
	med->m_media = osip_strdup("audio");
	osip_list_add(med->m_payloads, osip_strdup("18"), -1);
	
	i = sdp_attribute_init (&attr);
	attr->a_att_field = osip_strdup("rtpmap");
	attr->a_att_value = osip_strdup("G729/8000");
	osip_list_add (med->a_attributes, attr, -1);
	
	osip_rfc3264_add_audio_media(cnf, med, -1);
</PRE>

 * @section howto5_6 How-to execute the negotiation?

Running the negotiator is still a complex sequence of calls.
This is because I have oriented developments to make the
negotiator enough flexible for all situations. This makes
the negotiator quite complex to use but I hope you'll still
find it enough powerfull.

The test program is located in the source files of libosip2
in src/test/torture_rfc3262.c. You'll find a complete
negotiation process to build an answer to an offer.

When the negotiation is finished for all media lines, you
still MUST modify some elements missing in the SDP message.
<B>Especially, you must replace the IP addresses and port numbers
to reflect your configuration.</B>

Note that at the end of the negotiation process, you can still
modify the SDP message to make it closer to your own media
capabilities. For example, you can add attributes such as
"ptime" options for audio and video payloads.

*/