sip call hold

In many mailing lists I read of problems with SIP signaling protocol and 
call on hold. In the actual implementation, "hold" is not different from 
"mute".
This problem also occurred during my work.
I wrote this draft: 
http://tools.ietf.org/id/draft-giordano-sip-call-hold-revision-00.txt
What's your opinion.
Thanks. 

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