Chapter 15

p. 209

TFTP is used at the University of Bath to install boot images in the machines in the Library and the computing laboratories. RFCs 2347-2349 update RFC 1350 by adding extra options.

p. 211

The ``sorcerer's apprentice'' happens as follows (assuming that the client is reading a file from the server).

• A packet (say 10) is delayed after leaving the TFTP process on the server (e.g. because of an Ethernet collision).
• The client times out and sends another ACK of 9, to prompt a re-send of 10.
• The client gets the original 10, and acknowledges it.
• The server sends 10 again, in response to the duplicate ACK of 9.
• The server sends 11, in response to the acknowledgement above.
• The client acknowledges the second copy of 10.
• The server sends 11 again, in response to the acknowledgement above.
• The client sends two acknowledgements for the two copies of 11.
• The server sends two copies of 12 in response to these acknowledgements.

The process continues doing double the required work until the file is received. Worse, the chance of collision is increased, so it is more likely that another packet will be delayed, and so on.

p. 212

The reason that it is important not to tie up the well-known port is that a TFTP server normally serves a family of machines, e.g. in a laboratory. These may well all want service at once, e.g. after a power failure or a change of class.