[Serusers] How does UA1 knows another UA2 at another SER sip server?

[Alexander Philipp Lintenhofer]

Look at SIP Trapezoid at 
http://www.iptel.org/ser/doc/sip_intro/figures/trapezoid.png (codeword 
"DNS-Query")
or read RFC 3261: http://www.ietf.org/rfc/rfc3261.txt
 :-)

>Dear ALL:
>
>Question:
>How can UA1 know UA2 under SER2 if UA1 registers to SER1 sip
>server(domainA.com)
>and another UA2 registers to SER2 sip server(domainB.com) ?
>
>UA1 ==> SER1(domainA.com) <== ?? ==> SER2(domainB.com) ==> UA2
>
>Is there any middleware like freeradius or mysql to let UA1 know the
>location of UA2 ?
>
>
>Thank you for your kind to answer my question.
>
>Best Regard.
>
>Charles
>
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>