In fact, to summarize my issue, I see that :
At Termination, PCSCF is asking PCRF to setup QoS for a flow between the RTP Port of the offer it received and the RTP Port of the anwer it received. Whole it should ask to set QoS for a flow between the RTP port it allocated (after rewriting SDP Offer) and the RTP Port of the answer it got from B.
Same kind of issue at Origination.
Best Regards