P2P caching

Peer-to-peer caching (P2P caching) is a computer network traffic management technology used by Internet Service Providers (ISPs) to accelerate content delivered over peer-to-peer (P2P) networks while reducing related bandwidth costs.

P2P caching is similar in principle to the content caching long used by ISPs to accelerate Web (HTTP) content. P2P caching temporarily stores popular content that is flowing into an ISP’s network. If the content requested by a subscriber is available from a cache, the cache satisfies the request from its temporary storage, eliminating data transfer through expensive transit links and reducing network congestion. This approach could make ISPs violate laws as P2P systems share files that infringe copyrights in significant portions.

P2P content responds well to caching because it has high reuse patterns reflecting a Zipf's-like distribution. P2P communities have different Zipf's parameters which determine what fraction of files is requested multiple times. For example, one P2P community may request 75% of content multiple times while another may request only 10%.

Some P2P caching devices can also accelerate HTTP video streaming traffic from YouTube, Facebook, RapidShare, MegaUpload, Google, AOL Video, MySpace and other web video-sharing sites.

P2P caching involves creating a cache or temporary storage space for P2P data, using specialized communications hardware, disk storage and associated software. This cache is placed in the ISP’s network, either co-located with the Internet transit links or placed at key aggregation points or at each cable head-end.

Once a P2P cache is established, the network will transparently redirect P2P traffic to the cache, which either serves the file directly or passes the request on to a remote P2P user and simultaneously caches that data for the next user. To what extent the caching is beneficial depends on how similar the content interests of ISP's customers. Due to relatively small number of content shared in P2P systems (compared to Web) and semantic, geographic, and organization interests of users sharing ratio in P2P can be significantly higher than HTTP/Web caching.

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