On 8/19/16 6:07 PM, Manoj Priyankara wrote:
Hi Folks,
Is it normal to have two active m= lines in an offer, one for audio and one
for T.38(image) fax? It's my understanding that this confuses the receiving
party.... However the RFC3264(below) provides provision for it. What's your
thoughts on this?
I don't understand what sort of confusion you have. Multiple m-lines in
an offer means multiple (concurrent) media streams are being offered.
(They are not *alternatives*.) The typical example is audio+video, or
video+audio. But they can also include multiple streams of the same type
(e.g. three audio), or other sorts of media (e.g. text).
If you have a device that can't handle all the media that is being
offered, then you are free to accept what you want and refuse the rest.
I am guessing that you are thinking of this from the perspective of a
fax machine? Such a thing might *start* with real audio, where you talk
and confirm that you want to send a fax, followed by a change to t.38 in
order to send the fax.
In that case, you *may* simply send a new new offer where you *replace*
the audio m-line with a t.38 m-line. Or you may send a new offer where
you *add* a t.38 m-line while retaining the audio. And you may do that
even if you don't have the capability to render (or gateway, if that is
what you are doing) both media concurrently. You could simply cease
sending (and ignore incoming) audio once you start receiving t.38. And
you could then send another reinvite to reject (port=0) the audio.
But from a sip perspective you are free to continue your conversation in
audio while the fax is transmitting, as long as you have a functional
audio m-line.
Thanks,
Paul
8.1 Adding a Media Stream
New media streams are created by new additional media descriptions
below the existing ones, or by reusing the "slot" used by an old
media stream which had been disabled by setting its port to zero.
Reusing its slot means that the new media description replaces the
old one, but retains its positioning relative to other media
descriptions in the SDP. New media descriptions MUST appear below
any existing media sections. The rules for formatting these media
descriptions are identical to those described in Section 5.
When the answerer receives an SDP with more media descriptions than
the previous SDP from the offerer, or it receives an SDP with a media
stream in a slot where the port was previously zero, the answerer
knows that new media streams are being added. These can be rejected
or accepted by placing an appropriately structured media description
in the answer. The procedures for constructing the new media
description in the answer are described in Section 6.
Cheers!
Manoj
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