We've got hint right in the name of the challenge (t3am_vi3w3r).
I opened Wireshark then looked into Statistics -> Protocol Hierarchy.
protocol hierarchy
There, VNC (Virtual Network Computing) catched my eyes as it is a well-known protocol which allows to remote-control a computers, like RDP.
Exploitation
I used "vnc" as a filter then simply followed a TCP Stream and copy all the ASCII Content and paste it in notepad (or any editor).
Because VNC uses the RFB protocol and transmits binary framebuffer updates, non-printable bytes appear as dots in the ASCII view. Then got the flag with simple duplication encoding.
DDCCTTFF{{.....}}
Final Payload
We can basically decode it by typing each letter once, or with python:
