3.3.4.1 User Agent Operation

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To use the SIP.INSTANCE endpoint identification mechanism defined in this section, the user agent MUST add the +sip.instance parameter with an obtained UUID URN value, as described in section 3.3.3, to the Contact header field of the messages which carry the Contact header field because of SIP protocol requirements. [RFC3261] requires the addition of the Contact header field to the dialog creating requests and responses and a REGISTER request. The +sip.instance parameter syntax is defined in section 2.2.5.