Can Wireshark decode DIAMETER packets without the IP or transport layer?

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Per Wireshark's DIAMETER documentation, a port and TCP/STCP will be used. Thus Wireshark will be expecting the transport layer (and by extension, the underlying network layer).

Wireshark reads various file types like pcap. If you create a pcap file with a user dlt and the rest diameter packet data wireshark can dissect that. If you have diameter packet data in a hex dump txt2pcap can convert that to a pcap file readable by wireshark. The exported pdu format could also be used.