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R2.  IMS Registration signalling flow —
Re-registration – User currently registered

 

 

 

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IMS R2-1a. SIP REGISTER request: from UE1 to P-CSCF1

R2-1b.  SIP REGISTER request: between P-CSCF1 and DNS  (3 of 10)

Based on the user's URI, P-CSCF1 determines that UE1 is registering from a visiting domain and performs the DNS queries to locate the I-CSCF in the home network. The look up in the DNS is based on the domain name specified in the Request URI. The DNS provides P-CSCF1 with the address of the I-CSCF in the home network.

NOTE: The P-CSCF must not use the I-CSCF address cached as a result of the previous registration.