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TFTP General Operation, Connection Establishment and Client/Server Communication
(Page 2 of 3)
"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.)
Key Concept: Since TFTP uses UDP rather than TCP, there is no explicit concept of a connection as in FTP. A TFTP session instead uses the concept of a logical connection, which is opened when a client sends a request to a server to read or write a file. Communication between the client and server is performed in lock-step fashion: one device sends data messages and receives acknowledgments so it knows the data messages were received; the other sends acknowledgments and receives data messages so it knows the acknowledgments were received.
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Version 3.0 - Version Date: September 20, 2005
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