Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 225
Trivial File Transfer Program (TFTP)

An alternative to the FTP program is the TFTP program. This is a simplex file transfer program and is primarily used to bootstrap diskless network workstations (the program is small enough to fit in a ROM chip on the diskless workstation to initiate a boot across the network) or even network components (network bridges and routers). The FTP program is an extremely robust and complex program, and situations exist that require file transfer capabilities without complexity. Hence, FTP is also a larger file. TFTP is a small file and provides a more restrictive file transfer (for example, no user authentication); it is also a smaller executable software program.

There are differences between FTP and TFTP. TFTP does not provide a reliable service; therefore, it uses the transport services of UDP instead of TCP. It also restricts its datagram size to 512 bytes, and every datagram must be acknowledged (no multiple-packet windowing). There are no windows for packets to be acknowledged. It could be said that it has a window of 1.

The protocol is very simple. The first packet transmitted from the client process to the server process is a control packet, which specifies the filename and whether it is to be read from or written to the remote workstation (GET or PUT command). Subsequent packets are data packets and file transfer is accomplished with 512 bytes transferred at one time. The initial data packet is specially marked with a number of 1. Each subsequent data is incremented by 1. This is the sequence numbering system for TFTP. The receiving station will acknowledge this packet immediately upon receipt, using this number. Any packet of less than 512 bytes in length signifies the end of the transfer. Error messages may be transmitted in place of the data in the data field, but any error message will terminate the transmission. Also notice that only one connection is made to the remote resource. FTP has one for data and one for control information. FTP is a very robust protocol, one that can handle many types of transfers over various (not so good) media types. The commands of GET and PUT are used the same as in the FTP program.

The sequencing of the data is accomplished through the TFTP application, not the transport-layer service of UDP. UDP provides only unreliable, connectionless service. TFTP keeps track of the sequencing of the blocks of data and the acknowledgments that should be received. For those readers familiar with NetWare, this is the same type of transaction accomplished between the NetWare Core Protocol (NCP, not using Burst mode) and its underlying delivery system, IPX.