Dear David,
There is some information about the message data regarding to id filed in the text output format:
The interconnect events are traced for sockets and shared memory. Each interconnect trace message includes a message id unique across all ranks for the lifetime of the run. The message id in the form id={src.dest.ix} e.g., id={0.1.3} indicates that this is the 3<sup>rd</sup> message sent from rank 0 to rank 1.
Please let me know if you need more info.
Thanks,
James