From an architectural standpoint the key feature of

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From an architectural standpoint, the key feature of the trust relationships is the chain of pairwise trust relationships stretching across principals. In other words there are no required trust dependencies between owners and users. Of course, the slice and management authorities are just agents, and owners and users are always free to bypass their agent authorities and forge direct trust relationships (e.g., a node owner can blacklist slices). However, this should not be necessary if the authority is doing its job. A secondary feature is that there is no inherent dependency between management and slice authorities. They are free to evolve independently of each other. 7
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This decoupling offers an important degree of freedom as the system evolves. For example, multiple autonomous regions can run their own management author- ities, while a single global slice authority grants service developers access to nodes across management boundaries. This distributes the node management problem without balkanizing slices. Similarly, there can be multiple slice authorities that may or may not be coupled with management domains. For example, there might remain a single “research” slice authority, corresponding to today’s PLC, along with a “public good” slice authority that approves only those service developers that offer network reports or [email protected] services, and a “commercial” slice authority that represents for-profit services. There can also be private slice authorities that allow users at a site to access site-specific resources not made available outside the site. In a sense, a slice authority is analogous to a virtual organization [1]. 4 Architectural Components This section describes the architectural elements that collectively define PlanetLab. These elements consist of two major pieces: (1) a set of software modules that run on each node, and (2) a collection of global mechanisms that implement PLC. 4.1 Node A node is a machine capable of hosting one or more virtual machines (VM). A node must have at least one non-shared (not NAT’ed) IP address. Each node is identified by a unique node id , which is bound to a set of attributes for the node; e.g., the hosting site (denoted by a network prefix), the node’s current IP address, the node’s MAC address, and so on. With the exception of the site network prefix, these attributes are allowed to change over time. This effectively allows the hardware that implements the node to be replaced, either incrementally or in whole, but a node (and its node id ) cannot migrate from one site to another. It is not a requirement that a node’s IP address be universally routable, but it all nodes must be reachable from the site at which PLC components run (see Section 4.7 and 4.8).
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