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Mobility in Midcall (With replace Header)

During a session of midcall mobility, the real route set (set of SIP proxies that the SIP messages must traverse) must change. A re-INVITE cannot be used in midcall mobility.

Example, if a proxy is necessary for NAT/firewall traversal, then more than just the Contact URI must be changed, a new dialog must be created. The way out to this is to send a new INVITE with a Replaces header, which points out the existing session.

The call flow is similar to the call flow using re-INVITE except that a BYE is automatically produce to end the existing dialog when the INVITE with the Replaces is accepted.

Below are the details to points out in this following explanation:

1. The existing dialog between Bowditch and Laplace contains the old visited proxy server.

2. The new dialog making use of the new wireless network requires the addition of the new visited proxy server.

3. Therefore, an INVITE with Replaces is sent by Bowditch, which creates a new dialog that carries the new visited proxy server but not the old visited proxy server.

4. After Laplace accepts the INVITE, a BYE is automatically sent to end the old dialog that routes via the old visited proxy server that is now no longer involved in the session.

5. The resulting media session is established using Bowditch’s new IP address from the SDP in the INVITE.

Reference - Tutorials Point

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