On 5/10/12 2:52 PM, Worley, Dale R (Dale) wrote:
>> From: [email protected] [[email protected]]
>>
>> 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.
I agree with what Dale says.
Note that there is no semantic difference between
- using one o/a to set the port in an m-line to zero,
and then a second one to reuse the m-line for something else
- using a single o/a to reuse an m-line for something else
The latter is just an optimization.
Its also far from clear that you can infer any particular relationship
between the stream established by an m-line and the one established when
the m-line is reused. The quote from 3264 doesn't help much because it
only *recommends* reuse.
As a result, the semantics of these cases are underspecified. Clearly
the end doing the offer has something in mind. But it may be a faint
hope that the other end will understand it the same way.
If the first o/a established only a voice stream, and the second o/a
establishes only a T.38 stream then there seems to be little alternative
but to assume the intent is to use T.38 and hence fax. That's true
whether the 2nd o/a has another m-line with port=0 or not, and
regardless of whether the T.38 m-line is first or 2nd or something else.
OTOH if the first o/a established only a voice stream, and the second
o/a establishes both voice stream and a T.38 stream (both active), then
it is far from clear what the answerer should assume. Probably best to
avoid this case.
Thanks,
Paul
_______________________________________________
Sip-implementors mailing list
[email protected]
https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
