Which protocol is the encapsulating protocol for mtrace packets?

“mtrace” is a diagnostic tool to trace the multicast path from a specified source to a destination for
a multicast group. It runs over IGMP protocol. Mtrace uses any information available to it to
determine a previous hop to forward the trace towards the source.
Reference.
http://www.brocade.com/downloads/documents/html_product_manuals/NI_05500c_MULTICAST/
wwhelp/wwhimpl/common/html/wwhelp.htm#context=NI_MCAST&file=IP_Multicast.3.04.html