Sorcerer's Apprentice Syndrome

Sorcerer's Apprentice Syndrome (SAS) is a particularly bad flaw, discovered in the original versions of . It was named after Goethe's poem "Der Zauberlehrling" (popularized by the "Sorcerer's Apprentice" segment of the 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. This sounds fairly harmless at first, but it leads to disaster.

TFTP was 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.

SAS occurred when a packet was not lost in the internetwork, but rather simply delayed, and later successfully delivered, after a timeout had occurred (on either side).

The timeout causes a second copy of the previous packet to be sent to replace the 'lost' packet. However, the first copy was not lost, and since, according to the TFTP specification, receipt of any packet always forced the generation of a reply packet, two replies were generated (one to each copy). Those forced the generation of two replies to them, and so on. A typical scenario was as follows:

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