Subject: Re: [Sip] SIP refer method

From: [email-masked]
Date: Mon, 12 May 2008 17:54:18 -0400

From: "ajay thakur" <[email-masked]>

   James,
   I will try to explain problem here.
   A---------|
	      |
	      |--------Router-----------------C
   B---------|
   On router access list is used to deny incoming calls from outside to
   internal network.
   A,B are internal network. And C is on WAN side.
   Outgoing calls are allowed through access lists.

   Now call from A to C works perfectly when A initiates call.
   Now when A wants to transfer call to other internal phone (B in this case),
   then A sends refer message to C.

   Now C tries to connect to B and C uses contact address of B to connect. But
   incoming calls from WAN side is not allowed. So what can be done to fix
   these kind of problems ?

This is the "transfer authorizaton" problem, where C is allowed to
contact B because A told it to.  A lot of effort in the sipX
open-source proxy has gone into solving this problem.  (You can search
at http://dir.gmane.org/gmane.comp.voip.sipx.devel.)  The general idea
is that when A sends its REFER to C, the proxy modifies the Refer-To
address to include a header-parameter.  This header parameter is
turned into a header in the INVITE from C to B, and the proxy (Router
in your scheme) ses that header as a signature that allows the call to
pass from C to B.  Of course, getting the details right is messy.

Dale
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