> From: kumar.ramad...@wipro.com [kumar.ramad...@wipro.com] > > v=0 > o=- 200804293 1328781680 IN IP4 192.168.13.235 > s=SDP Data > c=IN IP4 192.168.13.235 > t=0 0 > m=image 51846 udptl t38 > a=T38FaxRateManagement:transferredTCF > a=T38FaxUdpEC:t38UDPRedundancy > a=ecan:fb on - > m=audio 49846 RTP/AVP 8 > a=ptime:10 > a=ecan:fb on - > > To my understanding of RFC 3264, the above SDP is not in-line with > Section 8 & 8.1, if the previously offered or answered SDP (exchanged > during the basic call setup) contains only "audio" media type with > same port number. Or is it still OK to send the "image" media line > before the "audio" media line for the fax call scenarios?
My belief was that such a change in a new SDP offer was not allowed. But I looked at RFC 3264 section 8.3.3, and it says: The media type (audio, video, etc.) for a stream MAY be changed. It is RECOMMENDED that the media type be changed (as opposed to adding a new stream), when the same logical data is being conveyed, but just in a different media format. This is particularly useful for changing between voiceband fax and fax in a single stream, which are both separate media types. To do this, the offerer creates a new media description, with a new media type, in place of the description in the previous SDP which is to be changed. So it is allowed to change a media stream in any way, it seems. Within that rule, what is being done in your example is that the first media stream is being changes from "audio" to "image" (fax), and a new "audio" stream is being added. That is probably not the best way to express what is desired, but it appears to be *valid*. About a week ago, I was examining the operation of a popular PSTN/SIP gateway device, and it changes the one media stream from "audio" to "image" when fax is detected. I thought that its behavior was incorrect, but now I see that it is correct. Dale _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
