The acknowledgemet packets have a length of 4 bytes. Because each data packet should be 512 (data) bytes long, the last packet will have between 0 and 511 data bytes. Acknowledgement packets will be answered with the next data packet. If a packet is not acknowledged in time (some seconds), the sender will repeat the data packet automatically until it is acknowledged. TFTP will use a block number for each data packet, which has to be acknowledged. Therefore, the combination of the server and the client TIDs will be used as a 'virtual channel'. UDP does not provide a streaming functionality by itself. If something goes wrong, the server will send an error packet. Request Packetĭepending on the type of request, a data packet for RRQ or an acknowledgement packet for WRQ will follow. The TIDs are constant while the transfer is active.Įach request packet will contain the Opcode, the filename terminated by a zero, and the transfer mode terminated by a zero. The next packet from the client will be sent to the server using the server's TID as the destination port and vice versa.

The source port of the client packet is the client side TID, and the source port of the server side is the server's TID (transfer ID). The server will either answer with the first data packet (RRQ) or an acknowledgement packet (WRQ). Each session will start with a request (read / write) packet from the client which will be sent directly to the servers port (e.g., 69).

Windows tftp client