The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP General File Transfer Protocols (FTP and TFTP)                      9  Trivial File Transfer Protocol (TFTP)

TFTP Overview, History and Standards 1 2 3 TFTP Detailed Operation and Messaging

"Lock-Step" Client/Server Messaging

After the initial exchange, the client and server exchange data and acknowledgment messages in “lock-step” fashion. Each device sends a message for each message it receives: one device sends data messages and waits for acknowledgments, the other sends acknowledgments and waits for data messages. This form of rigid communication is less efficient than allowing the transmitter to “fire away” with one data message after another, but is important because it keeps TFTP simple when it comes to an important issue: retransmissions.

Like all protocols using the unreliable UDP, TFTP has no assurances that any messages sent will in fact arrive at their destination, so it must use timers to detect lost transmissions and resend them. What is different about TFTP is that both clients and servers perform retransmission. The device that is sending data messages will resend the data message if it doesn't receive an acknowledgment in a reasonable period of time; the device sending the acknowledgments will resend the acknowledgment if it doesn't receive the next data message promptly. The “lock-step” communication discussed above greatly simplifies this process, since each device only needs to keep track of one “outstanding” message at a time. It also eliminates the need to deal with complications such as reorganizing blocks received out of order (which protocols like FTP rely on TCP to manage.)

TFTP Overview, History and Standards 1 2 3 TFTP Detailed Operation and Messaging