Subject: RE: [SIP] Media after INVITE

From: "Christian Huitema" <[email-masked]>
Date: Tue, 16 Jan 2001 09:22:56 -0800

> I was reading the thread about Early media replacement. My question,
> however, has not so much to do with the 183-to-be-or-not-to-be, but
> rather with the requirement that a UA should be able to receive media
> after the first INVITE is sent. This mean that the remote 
> user wouldn't
> have to send any response at all - instead he could start 
> sending media
> and would never be charged for it. 

Well, this is very much a fact of life. The moment you publish the RTP
address and port of the caller, anyone that get that address can start 
sending packets to it. Whether the packet are voiced out or dropped on
the floor depends on the caller. Presumably, if the caller is party
in a conspiracy to avoid taxes, the packets will not be dropped.

> My opinion is that SOME response (183, 200, or whatever will 
> be decided
> upon) should be received before the UA is required to start receiving
> media. I know that we then can get some other issues, ie. if 
> it takes a
> long time for the 200 response to reach the caller the first RTP media
> packets will be lost. But, again, at least the caller (or a possible
> intermediate proxy handling charging) will know that the media can be
> sent.

Well, merely sending a response does not achieve that. You only know
that the
caller is ready to receive packets if the caller tells you so. There are

basically two ways: after receiving the INVITE, or after receiving the 
ACK. Analysis of phone gateways shows that if the caller is not ready to
receive 
before the ringing or the announcement starts, i.e. after the INVITE in
the
current call flows, then some packets will be lost. 

To change that, you have to revise the call flows for phone
connections, so that there is a complete 3 ways handshake before you
start
propagating the call on the phone network. Sort of "receive INVITE, send
progress asking for permission to send early media, receive ack, then
and only then
start ringing the phone." 

Indeed, such a 3-ways handshake automatically precludes ringing several
destinations
at the same time.

-- Christian Huitema

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