[SIPForum-discussion] Reg: SDP offer

Nils Ohlmeier nils at ohlmeier.com
Thu Apr 9 06:49:20 UTC 2009

Am 08.04.2009 13:04 Uhr, schrieb prem ranjan:
> Hi,
> I have the following scenario:


> *v=0
> o=- 38000001 1239174761 IN IP4
> s=SDP Data
> c=IN IP4
> t=0 0
> m=audio 0 RTP/AVP 8
> a=ptime:10
> m=image 33815 udptl t38
> a=T38FaxVersion:0
> a=T38MaxBitRate:14400
> a=T38FaxRateManagement:transferredTCF
> a=T38FaxMaxBuffer:1800
> a=T38FaxMaxDatagram:260
> a=T38FaxUdpEC:t38UDPRedundancy
> *
> Now my question is, can we have the value "Zero" in the audio port. Is
> this RFC compliant???

Yes port zero is absolutely legal. And it is even required, because it 
is the only way how to reject m-lines if there are multiple of them. 
Note that the re-INVITE contains two m-lines. So the sender tells you 
that he does not want to do audio any longer, but wants to switch to 
image (T38 in this case) in this case.

Best regards
   Nils Ohlmeier

More information about the discussion mailing list