Exploiting similarity for multi-source downloads using file handprints

Abstract

Many contemporary approaches for speeding up large file transfers attempt to download chunks of a data object from multiple sources. Systems such as BitTorrent quickly locate sources that have an exact copy of the desired object, but they are unable to use sources that serve similar but non-identical objects. Other systems automatically exploit cross-file similarity by identifying sources for each chunk of the object. These systems, however, require a number of lookups proportional to the number of chunks in the object and a mapping for each unique chunk in every identical and similar object to its corresponding sources. Thus, the lookups and mappings in such a system can be quite large, limiting its scalability. This paper presents a hybrid system that provides the best of both approaches, locating identical and similar sources for data objects using a constant number of lookups and inserting a constant number of mappings per object. We first demonstrate through extensive data analysis that similarity does exist among objects of popular file types, and that making use of it can sometimes substantially improve download times. Next, we describe handprinting, a technique that allows clients to locate similar sources using a constant number of lookups and mappings. Finally, we describe the design, implementation and evaluation of Similarity-Enhanced Transfer (SET), a system that uses this technique to download objects. Our experimental evaluation shows that by using sources of similar objects, SET is able to significantly out-perform an equivalently configured BitTorrent.