[OpenSIPS-Users] Mediaproxy ver 2.3.4 - Conntrack meets Music on Hold

[Dan Pascu]

On 6 Jul 2009, at 19:47, Alex Hermann wrote:

> On Thursday 02 July 2009 12:33:14 Dan Pascu wrote:
>> Another false assumption is that the receiving endpoint would  
>> accept a
>> media stream coming from a source that was not negotiated in the
>> current session.
>
> How is the source IP:port negotiated in SDP?
>
> AFAIK only the destination IP:port is negotiated, an UA only  
> specifies the
> address is it willing to _receive_ RTP on.


The RTP stream contains both a sequence number and a signature. The  
signature is the same for the life of the stream, while the sequence  
number is incremented with each packet. The destination may choose to  
learn the signature from the first packet and discard any packet not  
having the same signature, so a newly negotiated RTP stream outside of  
the dialog will fail. It will also ignore any packet with a sequence  
number that is lower than the last sequence number, because it thinks  
it already did process that chunk. The destination may even employ  
security measures similar to mediaproxy: it can learn the IP address  
from the 1st packet and refuse any packet from a different IP until  
the stream is renegotiated using a re-INVITE.

Overall, all these issues do not exist if the MOH is delivered through  
a stream negotiated inside the same dialog using a re-INVITE, but they  
pose a problem if the MOH comes via a stream from a different dialog.  
It's lottery: it may work with some particular devices, it may not  
with others.

--
Dan