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ISO-PP: OSI Presentation Layer Protocol (X.226, ISO 8823)
The OSI presentation layer protocol (ISO-PP) is for the information transit between open systems using connection oriented or connectionless mode transmission at the presentation layer of the OSI 7 layer model . An application protocol is specified in terms of the transfer of presentation data values between application entities (PS users), using the User data parameter of presentation service primitives.
The Presentation Layer has two functions it carries out on behalf of PS users:
- negotiation of transfer syntaxes;
- transformation to and from transfer syntax.
The function of transfer syntax negotiation is supported by presentation protocols. Transformation of syntax is a function contained within a presentation entity and has no impact on presentation protocol design. For connectionless mode transmission, the sending presentation entity selects the transfer syntaxes. No transfer syntax negotiation occurs.
A set of presentation data value definitions associated with an application protocol constitutes an abstract syntax. For two application entities to communicate successfully they must have an agreement on the set of abstract syntaxes they intend to use. During the course of communication they may decide to modify this agreement. As a consequence, the set of abstract syntaxes in use may be changed. The abstract syntax specification identifies the information content of the set of presentation data values. It does not identify the transfer syntax to be used while presentation data values are transferred between presentation entities, nor is it concerned with the local representation of presentation data values.
The Presentation Layer exists to ensure that the information content of presentation data values is preserved during transfer. It is the responsibility of cooperating application entities to determine the set of abstract syntaxes they employ in their communication and inform the presentation entities of this agreement. Knowing the set of abstract syntaxes to be used by the application entities, the presentation entities are responsible for selecting mutually acceptable transfer syntaxes that preserve the information content of presentation data values.
For connectionless mode transmission, the abstract syntaxes used are determined by the sending application entity. For successful communication to take place, these must be acceptable to the receiving application entity.
For connectionless mode transmission, the presentation entities do not negotiate transfer syntaxes. The transfer syntaxes used are determined by the sending application entity. For successful communication to take place, these must be acceptable to the receiving application entity. The abstract syntaxes and the associated transfer syntaxes may be explicitly stated in the ?resentation context definition list?parameter as a user option.
Presentation entities support protocols that enhance the OSI session service in order to provide a presentation service. The PS user is provided with access to the session service which permits full use to be made of that service. This includes negotiation of and access to the session functional units. The role of the Presentation Layer in providing this access includes representation of presentation data values in the User data parameters of session service primitives.
Protocol Structure - ISO-PP: OSI Presentation Layer Protocol (X.226, X.216, ISO 8823, 8822) OSI Presentation Layer Protocol Primitives:
Connection Release Primitive
Token Handling Primitive
Presentation Exception Reporting Primitive
Activity Management Primitive
Synchronization Services Primitive
CLNP , CONP , TP0, TP1, TP2, TP3, TP4 , ISO Session Protocol , ACSE , ROSE
The OSIPresentation Layer protocol is defined in ISO (www.iso.org ) documents 8823 and 8822 and ITU (www.itu.org ) documents X.216, X.226. and X.236.
X.226: Information technology - Open Systems Interconnection - Connection-oriented Presentation protocol: Protocol specification
X.216: Information technology - Open Systems Interconnection - Presentation service definition
X.236: Information technology - Open Systems Interconnection - Connectionless Presentation protocol: Protocol specification