TFTP протоколындағы «Соқыр шекінің шәкірті» қатесі

Network protocol flaw in the original versions of TFTP

Sorcerer's Apprentice syndrome (SAS) is a network protocol flaw in the original versions of TFTP. It was named after Goethe's 1797 poem "Der Zauberlehrling" (popularized by the "Sorcerer's Apprentice" segment of the 1940 animated film Fantasia), because the details of its operation closely resemble the disaster that befalls the sorcerer's apprentice: the problem resulted in an ever growing replication of every packet in the transfer. The problem occurred because of a known failure mode of the internetwork which, through a mistake on the part of the TFTP protocol designers, was not taken into account when the protocol was designed; the failure mode interacted with several details of the mechanisms of TFTP to produce SAS.

TFTP operates in simple lock step: there is only ever one packet outstanding at any time, and every packet received by either party caused one packet to be sent in reply (until the termination of the transfer). The TFTP specification said that any time any packet was received, the receiver was required to send the appropriate reply packet. Thus, the receipt of a block of data triggered the sending of an acknowledgement, and the receipt of an acknowledgement triggered the sending of the next data block. TFTP also, like all protocols designed to operate across an unreliable network, includes timeouts. After sending a packet, it expects a reply, so it starts a timer. If the timer expires with no reply received, it takes some action; typically re sending the original packet.

The fix to SAS involved modifying the TFTP specification to break the loop.