Therefore the application layer is basically a

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Unformatted text preview: dialog session if the message is a connection request message. The other services of the session layer include quarantine service, dialog control, and priority management. The quarantine service buffers a group of messages on the receiving side until the session layer on the sending side explicitly releases them. This is useful in database applications where a transaction (consisting of a group of messages) needs to be an atomic unit. The dialog control is useful for dialog sessions in which the user primitives used for sending and receiving messages are of the nonblocking type. In this case, the user may have multiple requests outstanding on the same session, and replies may come back in an order different from that in which the requests were sent. The dialog control reorders replies according to the order of requests. The priority management service is useful for giving priority to important and time-bound messages over normal, less-important messages. The session layer is not required for connectionless communication. Presentation Layer The purpose of this layer is to represent message information to communicating application layer entities in a way that preserves meaning while resolving syntax differences. For this, the presentation layer may perform one or more of the following types of transformations on message data: 1. A message usually contains structured information that may include any of the data types used in programming languages - integers, characters, arrays, records, and so on, including user-defined data types. Translation is therefore required where language systems or application programs in the source and destination computers use different representations for these data types. 2. Data format conversions are also needed to transfer data between computers when the hardware of the sending and receiving computers uses different data representations. In this case, the presentation layer software in the sending computer transforms message data from the formats used in its own computer to a set of standard network representations called eXternal Data Representation {XDR) before t...
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This document was uploaded on 04/07/2014.

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