TFTP Detailed Operation and Messaging
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TFTP operation consists of three general steps: initial connection, data transfer, and connection termination. All operations are performed through the exchange of specific TFTP messages. Let's take a more detailed look now at these three phases of operation and the specifics of TFTP messaging functions.
The first message sent by the client to initiate TFTP is either a read request (RRQ) message or a write request (WRQ) message. This message serves to implicitly establish the logical TFTP connection, and to indicate if the file is to be sent from the server to the client (read request) or the client to the server (write request). The message also specifies the type of file transfer to be performed. TFTP supports two transfer modes: netascii mode (ASCII text files as used by the Telnet Protocol) and octet mode (binary files).
Assuming there was no problem with the request (such as a server problem, inability to find the file, etc.) the server will respond with a positive reply. In the case of a read request, the server will immediately send the first data message back to the client. In the case of a write request, the server will send an acknowledgment message to the client, telling it that it may proceed to send the first data message.
After the initial exchange, the client and server exchange data and acknowledgment messages in lock-step fashion as described in the previous topic. For a read, the server sends one data message and waits for an acknowledgment from the client before sending the next one. For a write, the client sends one data message and the server sends an acknowledgment for it, before the client sends the next data message.
If there is a problem at any stage of the connection establishment or transfer process, a device may reply with an error message instead of a data or acknowledgment message, as appropriate. An error message normally results in the failure of the data transfer; this is one of the prices paid for the simplicity of TFTP.
Each TFTP file transfer proceeds using one of the processes described below, which either reads or writes a single file. If another file needs to be sent or received, a new logical communication is established, in a manner analogous to how FTP creates data connections. The main difference is that TFTP has no persistent control connection like FTP does.
Each data message contains a block of between 0 and 512 bytes of data. The blocks are numbered sequentially, starting with 1. The number of each block is placed in the header of the data message carrying that block, and then used in the acknowledgment for that block so the original sender knows it was received. The device sending the data will always send 512 bytes of data at a time for as long as it has enough data to fill the message. When it gets to the end of the file and has fewer than 512 bytes to send, it will send only as many bytes as remain. Interestingly, this means that if the size of the file is an exact multiple of 512, the last message sent will have zero bytes of data!
The receipt of a data message with between 0 and 511 bytes of data signals that this is the last data message. Once this is acknowledged, this automatically signals the end of the data transfer. There is no need to explicitly terminate the connection, just as it was not necessary to explicitly establish it.
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