Program: TFTP UDP Client
This program implements the client half of the TFTP application protocol, a well-known service that has been used in the Unix world for network booting of workstations since before Windows 3.1. I chose this protocol because it’s widely implemented on the server side, so it’s easy to find a test server for it.
The TFTP protocol is a bit odd. The client contacts the server on the well-known UDP port number 69, from a generated port number, and the server responds to the client from a generated port number. Further communication is on the two generated port numbers.
Getting into more detail, as shown in Figure 15-1, the client initially sends a read request with the filename, and reads the first chunk of data. The read request consists of two bytes (a short) with the read request code (short integer with a value of 1, defined as OP_RRQ), two bytes for the sequence number, then the ASCII filename, null terminated, and the string octet, also null terminated. The server verifies that it can open the file and, if so, sends the first data packet (OP_DATA), and then reads again. This read-acknowledge cycle is repeated until all the data is read. Note that each packet is 516 bytes (512 bytes of data, plus 2 bytes for the packet type and 2 more for the packet number) except the last, which can be any length from 4 (zero bytes of data) to 515 (511 bytes of data). If there is a network I/O error, the packet is re-sent. If a given packet goes astray, both client ...