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 General Operation, Connection Establishment and Client/Server Communication 1 2 3 TFTP Options and Option Negotiation

TFTP Read Process Steps

Let's use an example to see how TFTP messaging works. Suppose the client wants to read a particular file that is 1,200 bytes long. Here are the steps in simplified form (also displayed in Figure 293):

1. The client sends a read request to the server, specifying the name of the file.
   
   
2. The server sends back a data message containing block #1, carrying 512 bytes of data.
   
   
3. The client receives the data, and sends back an acknowledgment for block #1.
   
   
4. The server sends block #2, with 512 bytes of data.
   
   
5. The client receives block #2, and sends back an acknowledgment for it.
   
   
6. The server sends block #3, containing 176 bytes of data. It waits for an acknowledgment before terminating the logical connection.
   
   
7. The client receives the data, sends an acknowledgment for block #3. Since this data message had fewer than 512 bytes, it knows the file is complete.
   
   
8. The server receives the acknowledgment, and knows the file was received successfully.
   

   Figure 293: TFTP Read Process

   In this example, the client starts the process of reading a file by sending a request for it to the server. The server acknowledges this request by immediately sending a DATA message carrying block #1, containing the first 512 bytes of the file. The client acknowledges this with an ACK message for block #1. The server then sends block #2, containing bytes 513 to 1,024, which the client acknowledges. When the client receives block #3, it realizes it has only 176 bytes, which marks it as the last block of the file.

   
   

TFTP General Operation, Connection Establishment and Client/Server Communication 1 2 3 TFTP Options and Option Negotiation