[Sip-implementors] Question on draft-ietf-sipping-nat-scenarios-00!

[Rahul Gulati]

Hi, All,   I am again reposting this query as I didnt get reply to the previous one.   From draft-ietf-sipping-nat-scenarios-00, page 47-48:   In the example configuration shown in Figure 22, the signaling path is established when the SIP UA's register with the corresponding  Proxy/Register (Signaling server). *The SIP REGISTER message contains  an extension tag carried in the Proxy-Require header, indicating to  the Proxy that the client is behind a NAT* (Note: the client or the Proxy does not care about the type of NAT, as the same solution applies to all types). This packet is NAT-ed by the Enterprise NAT with a new source IP address and port. If the Proxy supports the extension tag, it creates an association between the IP address and port from which the packet arrived and the actual address of the user. The response to the Registration message is sent to this IP address and port (not to the Contact address in the REGISTER).   My question here is :   *Why do we need this extension tag?* --We can know if a UA is behind the NAT by verifying if the Src IP address ( NATed incase of NAT ) is the same as the IP address mentioned in the Contact header of the REGISTER message.   Best Regards' Rahul