IMS: Why is IP sec tunnel established between UE and P-CSCF ?

Question

I was going through IMS call flow and came to know that an IP sec tunnel is established between UE and P-CSCF . I could not find out reasoning behind it.

Answer 1

I don't know the exact answer but want to put some points here.
- There could be one reason of having IP sec transport since SIP signalling messages may flow through from one network to another network. These SIP messages carry authentication/security keys . I think this may be the reason of establishing IP sec transport between IMS nodes.

Answer 2

On the SIP level, there are three headers that are of importance for the setup of these security associations. These are the Security-client, Security-server and Security-verify headers.
IP Seq used two protected ports, each are negotiated between the UE and the P-CSCF for all subsequent signaling traffic.
The IPSec is in tunneled mode between the UE and P-CSCF.
Between network nodes (P/I/S-CSCFs), IPSec is in transport mode.