News11 Apr 2006: Non-PlanetLab hosts begin running OASIS replicas: Thanks OverCite!29 Mar 2006: Posted paper on OASIS, to appear in NSDI '06. 31 Jan 2006: Third-party services begin using OASIS: Thanks OpenDHT! 15 Nov 2005: OASIS begins alpha testing on PlanetLab. OverviewMany Internet services are distributed across a collection of servers that handle client requests. For example, high-volume web sites are typically replicated at multiple locations for performance and availability. Content distribution networks amplify a website's capacity by serving clients through a large network of web proxies. File-sharing and VoIP systems use rendezvous servers to bridge hosts behind NATs.The performance and cost of such systems depend highly on the servers that clients select. For example, file download times can vary greatly based on the locality and load of the chosen replica. Furthermore, a service provider's costs may depend on the load spikes that the server-selection mechanism produces, as many data centers charge customers based on the 95th-percentile usage over all five-minute periods in a month. Unfortunately, common techniques for replica selection produce sub-optimal results. Asking human users to select the best replica is both inconvenient and inaccurate. Round-robin and other primitive DNS techniques spread load, but do little for network locality. More recently, sophisticated techniques for server-selection have been developed. When a legacy client initiates an anycast request, these techniques typically probe the client from a number of vantage points, and then use this information to find the closest server. While efforts, such as virtual coordinate systems (GNP, Vivaldi, etc.) and on-demand probing overlays (Meridian), seek to reduce the probing overhead, the savings in overhead comes at the cost of accuracy of the system. Nevertheless, significant on-demand probing is still necessary for all these techniques, and this overhead is reincurred by every new deployed service. While on-demand probing potentially offers greater accuracy, it has several drawbacks that we have experienced first-hand when deploying CoralCDN. First, probing adds latency, which can be significant for small web requests. Second, performing several probes to a client often triggers intrusion-detection alerts, resulting in abuse complaints. This mundane problem can pose real operational challenges for a deployed system. OASIS (Overlay-based Anycast Service InfraStructure) is a shared locality-aware server selection infrastructure. OASIS is organized as an infrastructure overlay, providing high availability and scalability. OASIS allows a service to register a list of servers, then answers the query, ``Which server should the client contact?'' Selection is primarily optimized for network locality, but also incorporates liveness and load. OASIS can, for instance, be used by CGI scripts to redirect clients to an appropriate web mirror. It can locate servers for IP anycast proxies, or it can select distributed SMTP servers in large email services. To eliminate on-demand probing when clients make anycast requests, OASIS probes clients in the background. One of OASIS's main contributions is a set of techniques that makes it practical to measure the entire Internet in advance. By leveraging the locality of the IP prefixes, OASIS probes only each prefix, not each client; in practice, IP prefixes from BGP dumps are used as a starting point. OASIS delegates measurements to the service replicas themselves, thus amortizing costs (approximately 2--10~GB/week) across multiple services, resulting in an acceptable per-node cost. To share OASIS across services and to make background probing feasible, OASIS requires stable network coordinates for maintaining locality information. Unfortunately, virtual coordinates tend to drift over time. Thus, since OASIS seeks to probe an IP prefix as infrequently as once a week, virtual coordinates would not provide sufficient accuracy. Instead, OASIS stores the geographic coordinates of the replica closest to each prefix it maps. OASIS is publicly deployed on PlanetLab and has already been adopted by a number of services Currently, we have implemented a DNS redirector that performs server selection upon hostname lookups, thus supporting a wide range of unmodified client applications. We also provide an HTTP and RPC interface to expose its anycast and locality-estimation functions to OASIS-aware hosts. Experiments from our deployment have shown rather surprisingly that the accuracy of OASIS is competitive with Meridian, currently the best on-demand probing system. In fact, OASIS performs better than all replica-selection schemes we evaluated across a variety of metrics, including resolution and end-to-end download times for simulated web sessions, while incurring much less network overhead. For more detailed technical information, please see our publications page.
If you are interested in using OASIS for your service, please contact us.
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