[Sip] INVITE client transaction

[feng zhang <y01317@njupt.edu.cn>]

Hello All,
	I have a doubt about the invite client transaction describe in SIP spec.
    In RFC3261 sec. 13.2.2.4 "2xx Responses", the last but one paragraph, it says:
" The UAC core considers the INVITE transaction completed 64*T1 seconds
   after the reception of the first 2xx response.  At this point all the
   early dialogs that have not transitioned to established dialogs are
   terminated.  Once the INVITE transaction is considered completed by
   the UAC core, no more new 2xx responses are expected to arrive.
"
   My question is:
   When recieving a 2xx response, the state of invite client transaction will transition to "Terminated" State, and the inv_client_transaction should be destroyed(17.1.1.2 RFC3261).But here in 13.2.2.4 , it says " The UAC core considers the INVITE transaction completed 64*T1 seconds after the reception of the first 2xx response.". Does the UAC core keeps the inv_client_transaction ALIVE for 64*T1 seconds? 
    And I want to know what "INVITE transaction completed" means? Does it mean the transaction is destroyed?  Why here uses 64*T1 seconds?
    I am doubted about it.Thank u very much to give me a clarification.

Best Regards.
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