Could you give an exemple? Is this something like paketdiag in http://blockdiag.com/en/nwdiag/index.html
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I think you answered it with the 3rd type of diagram
https://en.wikipedia.org/wiki/Real-time_Transport_Protocol#Packet_header
I ask the same in https://forum.plantuml.net/8787/nwdiag-package-includes-scripts-called-rackdiag-packetdiag
A workaound is to create an image with another tool like http://blockdiag.com/en/nwdiag/packetdiag-examples.html#structure-of-tcp-header
and intergrate it after in PlantUML