SIP (Session Initiation Protocol) is used as a signaling protocol in session setup and maintenance in IMS (IP Multimedia Subsystem). During the session setup, P-CSCF needs to query DNS server to obtain ICSCF address and I-CSCF needs to query HSS to get SCSCF address. All of these queries will cause the session path longer than the media path and increase the session setup delay. This paper investigates the performance of the IMS session setup and proposes a caching scheme which can speed up the session setup procedure when user roams. The result of data analysis illustrates that the performance of this optimized scheme is better than the primitive one.
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