A few notes on IMS' per-session QoS


First, note that IMS' two-sided QoS signaling model is a major source of complexity.

Further complicating this picture are "services" (the whole point of IMS) such as toll-free sessions, reverse charging, and pre-paid services (etc). 

Consider what your network would have to do to support this
And how much per-flow state in the core is required?
And what kind of peering relationships/settlements/SLAs would be required?