Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Enjoy The TCP/IP Guide? Get the complete PDF!
The TCP/IP Guide

Custom Search

Table Of Contents  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)

Previous Topic/Section
TFTP General Operation, Connection Establishment and Client/Server Communication
Previous Page
Pages in Current Topic/Section
Next Page
TFTP Options and Option Negotiation
Next Topic/Section

TFTP Detailed Operation and Messaging
(Page 1 of 3)

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.

Initial Message Exchange

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).

Note: There was originally a third file type option—mail mode—but TFTP was never really designed for transmitting mail and this option is now obsolete.

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.

Data Block Numbering

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.

Previous Topic/Section
TFTP General Operation, Connection Establishment and Client/Server Communication
Previous Page
Pages in Current Topic/Section
Next Page
TFTP Options and Option Negotiation
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $

Home - Table Of Contents - Contact Us

The TCP/IP Guide (
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.