File Fragmentation over an Unreliable Channel
It has been recently discovered that heavy-tailed file completion time can result from protocol interaction even when file sizes are light-tailed. A key to this phenomenon is the RESTART feature where if a file transfer is interrupted before it is completed, the transfer needs to restart from the beginning. In this paper, we show that independent or bounded fragmentation guarantees light-tailed file completion time as long as the file size is light-tailed, i.e., in this case, heavy-tailed file completion time can only originate from heavy-tailed file sizes. If the file size is heavy-tailed, then the file completion time is necessarily heavy-tailed. For this case, we show that when the file size distribution is regularly varying, then under independent or bounded fragmentation, the completion time tail distribution function is asymptotically upper bounded by that of the original file size stretched by a constant factor. We then prove that if the failure distribution has non-decreasing failure rate, the expected completion time is minimized by dividing the file into equal sized fragments; this optimal fragment size is unique but depends on the file size. We also present a simple blind fragmentation policy where the fragment sizes are constant and independent of the file size and prove that it is asymptotically optimal. Finally, we bound the error in expected completion time due to error in modeling of the failure process.
© 2010 IEEE. Issue Date: 14-19 March 2010, Date of Current Version: 06 May 2010. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings. This paper was presented as part of the main Technical Program at IEEE INFOCOM 2010. We thank Adam Wierman, Lijun Chen and Mani Chandy for helpful discussions. We acknowledge support of ARO through MURI Grant W911NF-08-1-0233, NSF through the NetSE grant, the Caltech Lee Center for Advanced Networking, and Australian Research Council grant DP0985322.
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