Many modern Internet services and applications operate on top of geographically distributed datacenters, which replicate large volumes of business data and other content, to improve performance and reliability. This leads to high volumes of continuous network traffic between datacenters, constituting the bulk of traffic exchanged over the Wide-Area Networks (WANs) that connect datacenters. Operators heavily rely on the efficient and timely delivery of such traffic, as it is key for the performance of distributed datacenter applications, and bulk inter-datacenter traffic also plays an important role for network capacity planning. In this article, we discuss the unique and salient features of bulk across-datacenter transfers and extensively review recent technologies and existing research solutions proposed in the literature for optimization of such traffic. Moreover, we discuss several challenges and interesting directions that call for substantial future research efforts.

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