Cơ bản về TFTP

Usage: tftp [OPTION]... HOST [PORT]

Transfers a file from/to a tftp server using "octet" mode.

Options:
        -l FILE Local FILE.
        -r FILE Remote FILE.
        -g      Get file.
        -p      Put file.


So try next:
tftp -g -r filename.txt 20.20.20.1