Models for Open System Protocol Development : A Technical Report (1994) - UDT Series on Data Communication Technologies and Standards for Libraries

*UDT Series on Data Communication Technologies and Standards for Libraries* Models for Open System Protocol Development : A Technical Report (1994) 8. Architecture and Mapping In the Upper Layer architecture of OSI, Application Layer Protocol APDUs of an Application service Element (ASE) are mapped onto the services of underlying supporting services, such as the Association Control Service Element (ACSE), the Remote Operations Service Element (ROSE), the Presentation service , and the Session service for transmission to the peer ASE. In addition, the APDUs of one ASE may be interspersed with APDUs of another ASE, such as CCR, that provides additional services. 8.1 ACSE Support The Association Control Service Element (ACSE) is a fundamental building block in an application-entity (AE). The ACSE service is defined in ISO 8649; the ACSE protocol is specified in ISO 8650. ACSE services establish and terminate application-associations. Once established, these application associations can support communication between other application service elements (ASEs) in the AE. ACSE can operate in either a connection-oriented or a connection-less mode. In connection-oriented mode, ACSE provides for the establishment of an application-association, and the termination of that application-association in either an orderly or an abrupt manner. The connection-oriented ACSE services are: Association-Establishment service Association-Release service Association-Abort service The first two services are confirmed services, consisting of a request and a response. The Abort service is unconfirmed. The parameters allow for communication about application names, addresses, contexts, authentication, and the acceptance or rejection of requests. In connection-oriented mode, the Association-Establishment service is mapped onto the Connection-Oriented Presentation Connection Establishment service. Once the Presentation Connection and the Application Association are established, other ASEs (such as SR) can map their APDUs directly onto the Presentation Data service for transmission to their peer. The Association-Release service is also mapped onto the Presentation Release service, and the Association-Abort service is mapped on the Presentation Abort service. In Connectionless mode, ACSE consists of a single service, the Association-Data service, which is mapped onto the Connectionless Presentation-Data service. APDUs from any other ASEs are carried in the user data parameter of ACSE. As is the case with all Connectionless services, the Association-Data service is unconfirmed, and data my be lost without any notification of that happening. Association-Data service Association-Data request 8.2 Remote Operations Service Element (ROSE) The Remote Operations Service Element (ROSE) (ISO 9072) provides a remote procedure call (RPC) service. (Note: this is not the same as the service being defined in the OSI Remote Procedure Call standard 11578.) It enables an application to invoke an operation to be performed by a remote peer application and to receive a report of the results. ROSE may map these requests and responses directly on ACSE and Presentation services, or may optionally use the Reliable Transfer (RTSE). Some protocol developers, such as the developers of the Directory protocol, have found it helpful to use ROSE. ROSE uses a request-response paradigm. It provides a set of ASN.1 macros which the protocol developer can tailor to carry the semantics of a particular application. The ROSE protocol itself is only a vehicle for conveying the arguments and results of the operation as defined by the application. 8.3 Commitment, Concurrency, And Recovery (CCR) Service The Commitment, Concurrency, and Recovery (CCR) service (ISO 9804 and 9805) provides builds upon the synchronization facilities of the Session service to enable an application to define an activity as an atomic action. An atomic action is defined as a specific set of operations of a distributed application that have the properties of atomicity, consistency, isolation, and durability (the ACID properties). CCR services deal with operations on a single association. Referencing applications, such as Transaction Processing, can manage the use of CCR services across multiple associations. CCR uses the three-phase commit paradigm. One participant in the association acts in the role of superior, the other of subordinate. The superior can request that the subordinate offer commitment, order the subordinate to commit, order the subordinate to roll back an action, or perform recovery after an application or communication failure. The subordinate may offer commitment to the superior, inform the superior that rollback has been accomplished, or perform recovery after an application or communication failure. 8.4 Distributed Transaction Processing (TP) Service Transaction Processing service (ISO 10026) enables an application to send a unit of work, called a transaction, to another system for processing. The service enables the communicating systems to insure that the properties of atomicity, consistency, isolation, and durability of the transaction are preserved. Atomicity is the property that all of the operations of the transaction are performed or none of them are performed. Consistency is the property that if the operations of the transaction are performed they are performed accurately, correctly, and with validity. Isolation is the property that partial results of the transaction are not accessible. Durability is the property that all effects of a completed transaction are not altered by any sort of failure. These are known as the ACID properties. When multiple systems are involved in various parts of a transaction this can all become very complicated. The Transaction Action Processing standards organize the communication between systems into a Dialog, and communications among systems into a Transaction Tree. The beginning of a Dialog signals the beginning of a Transaction Branch; the end of a Dialog signals the end of a Transaction Branch. Multiple dialogues can occur serially on a single Application Association, and the Transaction Processing Application can utilize a pool of associations if necessary. The Distributed Transaction Processing service is applicable to situations where there is a large number of transactions among multiple systems and where consistency of data across the systems (the ACID properties) is essential to maintain. Banking and airline reservations systems are prime examples of the use of distributed transaction processing. The Distributed Transaction Processing service makes use of both CCR and ACSE. 8.5 Presentation Service The Presentation service (ISO 8822) manages the negotiation of supported data types (abstract syntax) and their representation (encoding) during communication between systems. The encoding of an abstract syntax results in a concrete syntax used to transfer data between systems. In its simplest form, the Presentation service provides for agreement at the establishment of the connection for form of all data to be transferred during the life of the connection. In more complex forms the service can support changes in the agreed definitions during the course of the connection. The context restoration facility allows the state of these agreements to be restored after a temporary failure of the connection. 8.6 Session Service The Session service (ISO 8326) provides for the exchange of tokens to manage dialog control, synchronization, checkpointing, and resynchronization of the data flow over a connection. The Session service also deals with a multitude of collision cases during release of a connection to insure that no data in transmission is lost. The Session service is divided into a number of functional units so that a user can select those functions needed without necessarily taking everything.


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