A media stream is declined by setting the port number to zero for the
corresponding media field in the SDP response. In the following
example, the caller Harpreet wants to set up an audio and video call
with two possible audio codecs and a video codec in the SDP carried in
the initial INVITE:
v=0
o=Harpreet 2880944526 2880944526 IN IP4 lab.high-voltage.org
s=-
c=IN IP4 100.101.102.103
t=0 0
m=audio 49170 RTP/AVP 0 8
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
m=video 49172 RTP/AVP 32
a=rtpmap:32 MPV/90000
The codecs are referenced by the RTP/AVP profile numbers 0, 8, and 32.
The called party Priyanka answers the call, chooses the second codec
for the first media field and declines the second media field, only
wanting a PCM A-Law audio session.
v=0
o=Priyanka 2880944526 2880944526 IN IP4 tower.radio.org
s=-
c=IN IP4 200.201.202.203
t=0 0
m=audio 60000 RTP/AVP 8
a=rtpmap:8 PCMA/8000
m=video 0 RTP/AVP 32
If this audio-only call is not acceptable, then Harpreet would send an
ACK then a BYE to cancel the call. Otherwise, the audio session would
be established and RTP packets exchanged.
Hope this helps ...
Harpreet Juneja
------------------------------------------------------
Harpreet Singh Juneja
New Delhi, India
--- somesh s <[EMAIL PROTECTED]> wrote:
> Hi All,
>
> I have the following doubt in SDP.
>
> UA1 ========================> UA2
> Invite(SDP) ------------------>
>
> >From UA1:
> SDP has both audio & video capabilities.
>
> At UA2:
> UA2 doesn't have audio capability & so it doesn't
> know any codec. But according to RFC 3264, it MUST
> reply something for audio. We have two choices -
>
> 1) Reply with port = 0
> --> But we don't know the codecs?
>
> 2) Reply with inactive
> --> But we don't know the codecs?
>
> Is it feasible to use either of the approach?
> Please correct me if I am wrong?
> is it valid to construct m= lines in SDP without
> any codec?
>
> With regards
> Somesh S. Shanbhag
>
>
>
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