[Serusers] why mus use force_rport()??

["孙宗君" <zjsun@biigroup.com>]

Hi, All: 

 I just tested the SER+RTPProxy. My ser and rtpproxy
are located on the same machine with public ip address.
my SIP UA is behind the NAT device(Symmetric NAT). When
UA registered to SER, the SER send the response to the
port specified in the Via head parameters, not the
real port receiving the Registration request. After
I put the force_rport() in the ser.cfg, all will
works well. I wonder why the SER doesn't send the
response back to the receiving port. Whether that
conflicts the RFC3261 chapter 19.
  I am glad to hear your advice and help
 Best Regards 

Sun Zongjun