[SIPForum-discussion] Reg: SDP offer

Garron, James jgarron at sonusnet.com
Wed Apr 8 19:36:41 UTC 2009

Prem, Looking at RFC3264 I believe that this is indicating that the
audio RTP stream (port 18990) is being removed, and that a new RTP
stream (port 33815) is being opened for fax transmission.  /Jim



8.2 Removing a Media Stream 

Existing media streams are removed by creating a new SDP with the port
number for that stream set to zero. The stream description MAY omit all
attributes present previously, and MAY list just a single media format. 

A stream that is offered with a port of zero MUST be marked with port
zero in the answer. Like the offer, the answer MAY omit all attributes
present previously, and MAY list just a single media format from amongst
those in the offer. 

Removal of a media stream implies that media is no longer sent for that
stream, and any media that is received is discarded. In the case of RTP,
RTCP transmission also ceases, as does processing of any received RTCP
packets. Any resources associated with it can be released. The user
interface might indicate that the stream has terminated, by closing the
associated window on a PC, for example.




From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of prem ranjan
Sent: Wednesday, April 08, 2009 7:05 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Reg: SDP offer


I have the following scenario:
A calls B
A sends INVITE with the following SDP:

o=780 409699 409699 IN IP4
c=IN IP4
t=0 0
m=audio 41270 RTP/AVP 8 18 4 96
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15

B answers the following SDP in 180 Ringing:

o=- 38000001 1239174750 IN IP4
s=SDP Data
c=IN IP4
t=0 0
m=audio 18990 RTP/AVP 8 96
a=rtpmap:96 TELEPHONE-EVENT/8000
a=cdsc:1 image udptl t38
a=cpar: a=T38FaxVersion:0
a=cpar: a=T38MaxBitRate:14400
a=cpar: a=T38FaxRateManagement:transferredTCF
a=cpar: a=T38FaxMaxBuffer:280
a=cpar: a=T38FaxMaxDatagram:260
a=cpar: a=T38FaxUdpEC:t38UDPRedundancy

After this Call is successful.
Now B wants to send a FAX. For this it creates a Re-Invite with the
following SDP:

o=- 38000001 1239174761 IN IP4
s=SDP Data
c=IN IP4
t=0 0
m=audio 0 RTP/AVP 8
m=image 33815 udptl t38

Now my question is, can we have the value "Zero" in the audio port. Is
this RFC compliant???

Kind Regards,
Prem Ranjan Behra 

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