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These constraints will be discussed in Section 5. In order to further improve behavior for Internet-scale requirements, we add layered system constraints Figure As described in Section 3. By restricting knowledge of the system to a single layer, we place a bound on the overall system complexity and promote substrate independence. Layers can be used to encapsulate legacy services and to protect new services from legacy clients, simplifying components by moving infrequently used functionality to a shared intermediary.

Intermediaries can also be used to improve system scalability by enabling load balancing of services across multiple networks and processors. The primary disadvantage of layered systems is that they add overhead and latency to the processing of data, reducing user-perceived performance [ 32 ]. For a network-based system that supports cache constraints, this can be offset by the benefits of shared caching at intermediaries.

Placing shared caches at the boundaries of an organizational domain can result in significant performance benefits [ ]. Such layers also allow security policies to be enforced on data crossing the organizational boundary, as is required by firewalls [ 79 ]. The combination of layered system and uniform interface constraints induces architectural properties similar to those of the uniform pipe-and-filter style Section 3.

Although REST interaction is two-way, the large-grain data flows of hypermedia interaction can each be processed like a data-flow network, with filter components selectively applied to the data stream in order to transform the content as it passes [ 26 ]. Within REST, intermediary components can actively transform the content of messages because the messages are self-descriptive and their semantics are visible to intermediaries.

REST allows client functionality to be extended by downloading and executing code in the form of applets or scripts. This simplifies clients by reducing the number of features required to be pre-implemented. Allowing features to be downloaded after deployment improves system extensibility. However, it also reduces visibility, and thus is only an optional constraint within REST. The notion of an optional constraint may seem like an oxymoron. However, it does have a purpose in the architectural design of a system that encompasses multiple organizational boundaries.

It means that the architecture only gains the benefit and suffers the disadvantages of the optional constraints when they are known to be in effect for some realm of the overall system. For example, if all of the client software within an organization is known to support Java applets [ 45 ], then services within that organization can be constructed such that they gain the benefit of enhanced functionality via downloadable Java classes.

An optional constraint allows us to design an architecture that supports the desired behavior in the general case, but with the understanding that it may be disabled within some contexts.

REST consists of a set of architectural constraints chosen for the properties they induce on candidate architectures. Although each of these constraints can be considered in isolation, describing them in terms of their derivation from common architectural styles makes it easier to understand the rationale behind their selection. REST ignores the details of component implementation and protocol syntax in school homework help online to focus on the roles of components, the constraints upon their interaction with other components, and their interpretation of significant data elements.

It encompasses the fundamental constraints upon components, connectors, and data that define the basis of the Web architecture, and thus the essence of its behavior as a network-based application.

The rationale for this design can be seen in the nature of distributed hypermedia. When a link is selected, information needs to be moved from the location where it is stored to the location where it will be used by, in most cases, a human reader.

This is unlike many other distributed processing paradigms [ 650 ], where it is possible, and usually more efficient, to move the "processing agent" e. A distributed hypermedia architect has only three fundamental options: Each option has its advantages and disadvantages. Option 1, the traditional client-server style [ 31 ], allows all information about the true nature of the data to remain hidden within the sender, preventing assumptions from being made about the data structure and making client implementation easier.

However, it also severely restricts the functionality of the recipient and places most of the processing load on the sender, leading to scalability problems.

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Option 2, the mobile object style [ 50 ], provides information hiding while enabling specialized processing of the data via its unique rendering engine, but limits the functionality of the recipient to what is anticipated within that engine and may vastly increase the amount of data transferred. Option 3 allows the sender to remain simple and scalable while minimizing the bytes transferred, but loses the berkeley eecs phd thesis of information hiding and requires that both sender and recipient understand the same data types.

REST provides a hybrid of all three options by focusing on a shared understanding of data jama article submission with metadata, but limiting the scope of what is revealed to a standardized interface.

REST components communicate by transferring a representation of a resource in a format matching one of an evolving set of standard data types, selected dynamically based on the capabilities or desires of the recipient and the nature of the resource. Whether the representation is in the same format as the raw source, or is derived from the source, remains hidden behind the interface. The benefits of the mobile object style are approximated by sending a representation that consists of instructions in the standard data format of an encapsulated rendering engine e.

REST therefore gains the separation of concerns of the client-server style without the server scalability problem, allows information hiding through a generic interface to enable encapsulation and evolution of services, and provides for a diverse set of functionality through downloadable feature-engines. The key abstraction of information in REST is a resource. Any information that can be named can be a resource: A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time.

More precisely, a resource R is a temporally varying membership function M R twhich for time t maps to a set of entities, or values, which are equivalent. A resource can map to the empty set, which allows references to be made to a concept before any realization of that concept exists -- a notion that was foreign to most hypertext systems prior to the Web [ 61 ].

Some resources are static in the sense that, when examined at any time after their creation, they always correspond to the same value set. Others have a high degree of variance in their value over time. The only thing that is required to be static for a resource is the semantics of the mapping, since the semantics is what distinguishes one resource from another. These are two distinct resources, even if they both map to the same value at some point in time.

The distinction is necessary so that both resources can be identified and referenced independently. A similar example from software engineering is the separate identification of a version-controlled source code file when referring to the "latest revision", "revision number 1.

This abstract definition of a resource enables key features of the Web architecture. First, it provides generality by encompassing many sources of information without artificially distinguishing them by type or implementation.

Second, it allows late binding of the reference to a representation, enabling content negotiation to take place based on characteristics of the buy essay online reviews. Finally, it allows an author to reference the concept rather than some singular representation of that concept, thus removing the need to change all existing links whenever the representation changes assuming the author used the right identifier.

REST uses a resource identifier to identify the particular resource involved in an interaction between components. REST connectors provide a generic interface for accessing and manipulating the value set of a resource, regardless of how the membership function is defined or the type of software that is handling the request. The naming authority that assigned the resource identifier, making it possible to reference the resource, is responsible for maintaining the semantic validity of the mapping over time i.

Traditional hypertext systems [ 61 ], which typically operate in a closed or local environment, use unique node or document identifiers that change every time the information changes, relying on link servers to maintain references separately from the content [ ].

Since centralized link servers are an anathema to the immense scale and multi-organizational domain requirements of the Web, REST relies instead on the author choosing a resource identifier that best fits the nature of the concept being identified.

Naturally, the quality of an identifier is often proportional to the amount of money spent to retain its validity, which leads to broken links as ephemeral or poorly supported information moves or disappears over time. REST components perform actions on a resource by using a representation to capture the current or intended state of that resource and transferring that representation between components.

A representation is a sequence of bytes, plus representation metadata to describe those bytes. Other commonly used but less precise names for a representation include: A representation consists of data, metadata describing the data, and, on occasion, metadata to describe the metadata usually for the purpose of verifying message integrity. Response messages may include both representation metadata and resource metadata: Control data defines the purpose of a message between components, such as the action being requested or the meaning of a response.

It is also used to parameterize requests and override the default behavior of some connecting elements. For example, cache behavior can be modified by control data included in the request or response message. For example, remote authoring of a resource requires descriptive essays about a person the author send a representation to the server, thus establishing a value for title generator for essay resource that can be retrieved by later requests.

If the value set of a resource at a given time consists of multiple representations, content negotiation may be used to select the best representation for inclusion in a given message. The data format of a representation is known as a media type [ 48 ]. A representation can be included in a message and processed by the recipient according to the control data of the message and dissertation educational research nature of the media type.

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Some media types are intended for automated processing, some are intended to be rendered for viewing by a user, and a few are capable of both. Composite media types can be used to enclose multiple representations in a single message. The design of a media type can directly impact the user-perceived performance of a distributed hypermedia system.

Any data that must be received before the recipient can begin rendering the representation adds to the latency of an interaction. A data format that places the most important rendering information up front, such that the initial information can be incrementally rendered while the rest of the information is being received, results in much better user-perceived performance than a data format that must be entirely received before rendering can begin.

For example, a Web browser that can incrementally render a large HTML document while it is being received provides significantly better user-perceived performance than one that waits until the entire document is completely received prior to rendering, even though the network performance is the same.

Note that the rendering ability of a representation can also be impacted by the choice of content. If the dimensions of dynamically-sized tables and embedded objects must be determined before they can be rendered, their occurrence within the viewing area of a hypermedia page will increase its latency. REST uses various connector types, summarized in Tableto encapsulate the activities of accessing resources and transferring resource representations.

The connectors present an abstract interface for component communication, enhancing simplicity by providing a clean separation of concerns and hiding the underlying implementation of resources and communication mechanisms. The generality of the interface also enables substitutability: Since a connector manages network communication for a component, information can be shared across multiple interactions in order to improve efficiency and responsiveness.

All REST interactions are stateless. That is, each request contains all of the information necessary for a connector to understand the request, independent of any requests that may have preceded it. This restriction accomplishes four functions: The connector interface is similar to procedural invocation, but with important differences in the passing of parameters and results.

The in-parameters consist of request control data, a resource identifier indicating the target of the request, and an optional representation. The out-parameters consist of response control data, optional resource metadata, and an optional representation.

From an abstract viewpoint the invocation is synchronous, but both in and out-parameters can be passed as data streams. In other words, processing can be invoked before the value of the parameters is completely known, thus avoiding the latency of batch processing large data transfers.

The primary connector types are client and server. The essential difference between the two is that a client initiates communication by making a request, whereas a server listens for connections and responds to cheap resume writers melbourne in order to supply access to its services. A component may include both client and server connectors.

A third connector type, the cache connector, can be located on the interface to a client or server connector in order to save cacheable responses to pay for dissertation writing interactions so that they can be reused for later requested interactions.

A cache may be used by a client to avoid repetition of network communication, or by a server to avoid repeating the process of generating a response, with both cases serving to reduce interaction latency.

phd thesis dissertation network

A cache is typically implemented within the address space of the connector that uses it. Some cache connectors are shared, meaning that its cached responses may be used in answer to a client other than the one for which the response was originally obtained. However, shared caching can also lead to errors if the cached response does not match what would have been obtained by a new request.

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REST attempts to balance the desire for transparency in cache behavior with the desire for efficient use of the network, rather than assuming that absolute transparency is always required. A cache is able to determine the cacheability of a response because the interface is generic rather than specific to each resource.

By default, the response to a retrieval request is cacheable and the responses to other requests are non-cacheable. If some form of user authentication is part of the request, or if the response indicates that it should not be shared, then the response is only cacheable by a non-shared cache. A component can override these defaults by including control data that marks the interaction as cacheable, non-cacheable or cacheable for only a limited time.

A resolver translates partial or complete resource identifiers into the network address information needed to establish an inter-component connection.

In order to initiate a request, a Web browser will extract the psychology assignments from the URI and make use of a DNS resolver to obtain the Internet Protocol address for that authority.

Another example is that some identification schemes e. Use of one or more intermediate resolvers can improve the longevity of resource references through indirection, though doing so adds to the request latency.

The final form of connector type is a tunnel, which simply relays communication across a connection boundary, such as a firewall or lower-level network gateway. The only reason it is modeled as part of REST and not abstracted away as part of the network infrastructure is that some REST components may dynamically switch from active component behavior to that of a tunnel. The tunnel disappears when both ends terminate their communication. REST components, summarized in Tableare typed by their roles in an overall application action.

A user agent uses a client connector to initiate a request and becomes the ultimate recipient of the response. The most common example is a Web browser, which provides access to information services and renders service responses according to the application needs.

An origin server uses a server connector to govern the namespace for a requested resource. It is the definitive source for representations of its resources and must be the ultimate recipient of any request that intends to modify the value of its resources.

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