In an IMS scenario, the
acting as a P-CSCF supports the outbound mechanism and NAT keep-alive mechanism (as defined in RFCs 5626 and 6223) by the User Agent (UA) when establishing a "flow" from behind a firewall or NAT. The purpose of the outbound and keep-alive mechanism is to maintain an active flow between the UA and P-CSCF to ensure the UA is able to receive incoming requests.
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