Need to get RTP sequence number from payload dissector

I have written a Lua payload dissector for a dynamic RTP payload (https://github.com/FOXNEOAdvancedTechnology/RFC4175-dissector)

However now I would like to get access to the RTP sequence number (rtp.seq) of the RTP packet where my payload lives, but do so inside my payload dissector (the RTP sequence number needs to be added to an "extended sequence number" in the payload for display in the dissector).

How can you do this? Thanks!