Trivial File Transfer Protocol (TFTP) is primarily used to exchange configuration or boot files between machines in an environment. It is relatively simple and provides no authentication mechanism. The TFTP server component can be installed and configured on a Windows or Linux machine.
There are many freeware TFTP servers on the Internet to choose from. I have deployed WinAgents TFTP server manager available at http://www.winagents.com/en/products/tftp-server/.
The installation of the TFTP server manager is pretty straightforward. Make sure it is installed on a Windows machine, which is in the same subnet as the intended management network for the ESXi hosts. Also make sure that the service is started before you begin.