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Re: [reSIProcate] Forking of INVITE request - regarding



On Nov 28, 2006, at 11:25 AM, Raj wrote:

Consider the following situation.

        Adam wants to call Bob, let’s say that Bob is
registered at three locations, that is, at home, at
head office and at his branch office. Hence, when Adam
makes a call, INVITE request is forked by the proxy
and it reaches all the three destinations. Hence,
three 200 OK SIP response will be sent back to Adam.

This will only happen when (as you say below) the three phones answer at the same time, where "same time" is the window (measured in milliseconds) between the first 200 arriving at the proxy and the CANCELs it sends to the other legs making it to the other two phones. So, realize when choosing how to deal with this that it is an edge condition. Unfortunately, studies in the traditional PSTN suggest that it's not that unusual to stimulate this particular edge.

Note also that early media (ringback or IVR media) can come from all three branches. In this case, there is no nice short window to save you from this edge condition, you'll have to figure out what to do
for the duration of the "ringing" time.


        Consider the situation, when Bob’s friends answer the
call at all the three locations at the same time, what
will happen? Either three media channel will be
established between them or which two will be
discarded.

        Can anyone please clear this?

So, the bad news is that the specs don't tell you what to do with this, and there is no current frontrunner for an "industry best practice". A lot of implementations will play media from whatever source got a packet to them first, discarding the other streams. Others have considered making each leg appear on individual lines, or mixing them into a local ad- hoc conference.

Finding a good solution is made more difficult by the lack of information that allows you to associate a leg with a branch. RTCP can eventually give you that, but there are a lot of phones out there now that aren't implementing RTCP yet. A lot of the work going into securing the media streams will make this problem
easier to solve.

It's a non-trivial problem and the more folks we have looking for good solutions, the better.
Check out the SIP/SIPPING mailing lists for the ongoing conversations.

RjS


With regards
Raj.




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