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Capturing DUML packets can be tricky if you are new to the scene. This page is designed to list some of the easy ways to capture DUML. It will grow over time as other people document other methods.
sudo ifconfig XHC20 up
Capture from the XHC20 interface using Wireshark. USB capture filters are unsupported, so you will get lots of garbage mixed in with the duml.
To view only the interesting traffic, the following filter is what worked for me.
frame.protocols == "usb" && usb.darwin.request_type == 1 && usb.bInterfaceClass == 0x0a && frame.len > 32