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TFTP Detailed Operation and Messaging
(Page 3 of 3)
TFTP Write Process Steps
Here are the steps in the same process, but where the client is writing the file (see Figure 294):
- The client sends a write request to
the server, specifying the name of the file.
- The server sends back an acknowledgment.
Since this acknowledgment is prior to the receipt of any data, it uses
block number zero in the acknowledgment.
- The client sends a data message containing
block #1, 512 bytes of data.
- The server receives the data, sends
back an acknowledgment for block #1.
- The client sends block #2, 512 bytes
of data.
- The server receives the data, sends
back an acknowledgment for block #2.
- The client sends block #3, containing
176 bytes of data. It waits for an acknowledgment before terminating
the logical connection.
- The server receives block #3 and sends
an acknowledgment for it. Since this data message had fewer than 512
bytes, the transfer is done.
- The client receives the acknowledgment
for block #3 and knows the file write was completed successfully.
Figure 294: TFTP Write Process
This example shows the client sending the same 1,200-byte file to the server that it read in Figure 293. The client sends a write request to the server, which acknowledges it; it uses block #0 to represent acknowledgment of the request prior to receipt of any data. The client then sends blocks of data one at a time, each of which is acknowledged by the server. When the server receives block #3 containing fewer than 512 bytes of data, it knows it has received the whole file.
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Key Concept: A TFTP read operation begins with the client sending a Read Request message to the TFTP server; the server then sends the file in 512-byte Data messages, waiting after each one for the client to acknowledge receipt before sending the next. A TFTP write operation starts with a Write Request sent by the client to the server, which the server acknowledges. The client then sends the file in 512-byte Data blocks, waiting after each for the server to acknowledge receipt. In both cases there is no explicit means by which the end of a transfer is marked; the device receiving the file simply knows the transfer is complete when it receives a Data message containing fewer than 512 bytes.
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