S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-3e. SIP PRACK request (UE1 to UE2):
from P-CSCF2 to UE2
(20 of 70)
: P-CSCF1 adds the port number negotiated during the security agreement and the comp=sigcomp parameter to its own entry in the Via header.