Default TFTP Server does not require any username and password
its all depends on the vendor of TFTP you using, look alternative one who can support username and password.
in this case i would suggest using SFTP for security reason.
Show the username and password from the packet capture file
i do not believe you able to see password if you seeing means that network have vulnerable for any attack.
Capture all the packet during the login process
this all depends on what device is this where you capturing ? device to device command syntax varies and it changes the outcome. so refer product configuration guide always.
