[SR-Users] How to route on whether INVITE is for MSRP

[Alex Villací­s Lasso]

So far, I have a kamailio.cfg that routes INVITEs between Asterisk and the external networks. Now I want to start adding MSRP support. However, with my current configuration, the test client (Blink) sends an INVITE with a SDP payload that specifies msrps 
media. This gets routed to Asterisk, and it gets promptly rejected with "Not Acceptable Here".

I need to make Kamailio route the request by itself if the INVITE is for MSRP, and delegate to Asterisk for other INVITEs.

What is the most elegant way to test for the INVITE packet to see if it is MSRP? This testing should be at least mentioned in the documentation for the msrp.so module.