[OpenSER-Users] newbie questions

[Alex Balashov]

Doug,

Doug McLetchie wrote:

> For inbound calls (calls coming from another carrier to my big expensive 
> gateway & destined for a specific PBX),  I'd like my gateway to send the 
> call to the proxy, who will determine which PBX to send the call to, and 
> then send the call to to correct PBX via the Gateway.  I don't 
> understand how to set up OpenSER to send the call via the Gateway.  If I 
> provision the static ip address of the PBX in the proxy, won't it try to 
> send directly to that IP address instead of sending it to the Gateway?  
> I think that the feature that I'm looking for is something like an 
> outbound proxy, for the proxy. (does that make sense?)

OpenSER can certainly do what you are trying to accomplish.

SIP routing is done by URI, which consists of a "user" part and a 
"domain" part.  The "user" part is the number (or alphanumeric 
identification, in the case of pure-VoIP peering) and the "domain" part 
is the IP "place" at which the "user" part is reachable.

When you route a call to some URI, what you are really saying is, "Here, 
domain, you must know what to do with this 'user' part - i.e. have 
reachability information for it (a SIP contact bound from a 
registration, for example)."

A proxy by itself isn't enough.  If you need to reach these PBXs, you 
clearly need to know how to reach them.  This requires a SIP registrar 
somewhere, so that the PBXs can register against it and say, "Here, you 
can reach me at such and such IP and port."  Or, I suppose, you can 
define these contacts statically with a database interface from the 
proxy, in which case you don't need to use a registrar.

I don't know what this Big Expensive Gateway is, but if it's something 
like a Session Border Controller, it should be able to forward SIP 
REGISTER requests to your proxy/registrar.  Or do they register against 
the gateway?

Either way, you can perform this resolution with OpenSER.

The problem you *might* run into is sending the same logical call leg 
back to the Gateway.  Depending on what it is, it may not like that and 
perceive a call routing loop, because the call that went through it to 
the proxy is the "same" call (in terms of SIP Call-ID, and other things 
that make up a logical call "leg") that is now being sent back around to 
it.  This problem is often solved with the introduction of a 
back-to-back user agent which can re-originate a different call leg.

-- 
Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
Tel    : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
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