Details
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 caused a second copy of the previous packet to be generated, notionally 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:
- Computer S (source) sends data block X to computer D (destination)
- Computer D receives block X, and sends an acknowledgement for X back to S
- The packet containing the acknowledgement for X is delayed in the internetwork
- Computer S times out, and resends data block X to D
- Computer S receives the delayed acknowledgement for X, and sends data block X+1
- Computer D receives the second copy of block X, and sends another acknowledgement for X back to S
- Computer D receives block X+1, and sends an acknowledgement for X+1 back to S
- Computer S receives the second acknowledgement for X, and sends a second copy of data block X+1
- Computer S receives the acknowledgement for X+1, and sends data block X+2
- Computer D receives the second copy of block X+1, and sends another acknowledgement for X+1 back to S
- Computer D receives block X+2, and sends an acknowledgement for X+2 back to S
It will be seen that at this point the situation is now stable, and repeats; every packet from then on is duplicated (that is, two identical copies are sent across the internetwork).
Even worse, the increased number of packets being sent around the internetwork was likely to cause congestion, which was likely to cause a packet to be delayed past the timeout yet again, which would then cause yet another duplicate packet to be generated by a timeout, and from then on a third copy of each packet would be sent. Needless to say, at that point, the situation would usually snowball, and further copies would be generated — hence the name given to this pattern of behaviour.
For a small file, the transfer would complete, and the duplicate packets would eventually drain from the internetwork. If the file were large, however, congestive collapse would result, and only when the transfer failed would the mass of packets drain from the internetwork.
Read more about this topic: Sorcerer's Apprentice Syndrome
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