This hold SDP discussion is so common. It is actually an indicator of a common problem with understanding SIP and implementing SIP phones.
The truth of the matter is: there is no RIGHT way to do hold in SIP. UAC can send inactive SDP when it does not want to send or receive any media, and this can mean hold. It can also send SDP with c=0.0.0.0 and achieve the same thing. Alternatively, UAC can send send only SDP and start pumping music. Yet another alternative is to disable the microphone on the client and do not send anything via SIP. All of this is valid behavior as far as SIP is concerned, and for each model there is a device which implements it. What is dangerous, is that a lot of phone manufacturers read the specifications and understand that they MUST send a send only SDP to implement hold. Even if they do not to plan to send any media. This is just wrong, or at least inefficient. If the phone dose not want to send or receive media, it should send SDP with inactive or with c=0.0.0.0. Some phones get so confused, that in order to do hold they send send only SDP and do not disable the microphone. So that the remote party continues to hear you while you keep them on hold, but you don't hear the remote party. In general, if the phone does not generate its own music on hold, it should not send send only or receive only SDP, unless it receives a receive only or send only SDP. If the phone does this, it is doing something inefficiently or just wrong. ___________________________________ Roman Shpount, VP of Technology aTelo, Inc. -- www.atelo.com _______________________________________________ Sip-implementors mailing list [email protected] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
