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Sorcerer's Apprentice Syndrome
Sorcerer's Apprentice Syndrome (SAS) is a particularly bad network protocol flaw, discovered in the original versions of TFTP. It was named after 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.
==Technical background==
TFTP operates in a 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 may sound fairly harmless, but it led to disaster.
TFTP also, like all protocols designed to operate across an unreliable network, includes timeouts. For example, when it does something to which it expects a reply from the party at the other end (such as sending it a packet), it starts a timer, and if the timer goes off and the reply has not been received, it takes some action; usually, the response is to re-send the original packet.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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