Closing the Association



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Closing the Association

A-RELEASE is the ordinary, orderly release mechanism used to end an association. It is a confirmed service that protects against loss of data, a possible result if one side began the process of closing the association while the other side was still sending data. A-ABORT is an unconfirmed service that corresponds to an abrupt termination of the association, usually because of an error condition. Loss of data could occur as a result. The time line of Figure gif illustrates the orderly and abrupt termination situations. P-DATA is the basic data transmission service; it is available only while the association is in effect.

  
Figure: Orderly versus abrupt termination of an association, including possible data loss

We can think of this process of orderly closing of the association in terms of the usual ``protocol'' for terminating a telephone conversation. Ordinarily one party to the call says ``Good bye'', then waits for the other party to say ``Good bye'', then both hang up. If Colleen and Maggie are talking and Colleen says ``Good bye'' and hangs up without waiting for Maggie to respond, and if just as Colleen says ``Good bye'' Maggie says ``Oh, wait, I found that book you are looking for; I'll leave it on my desk for you to pick up'', Colleen will never get the information. If Colleen had waited for Maggie to say ``Good bye'', she would have heard the message about the book before Maggie's ``Good bye'' ended the conversation. The effectiveness of this protocol depends on reliable communication service. We assume the protocol data units (``Good bye'' in this case) will arrive. At the Application Layer, protocols depend on the reliable delivery of messages by lower layer network services.

A-ABORT may be issued by either participant in the association. When one side issues an A-ABORT.request, an A-ABORT.indication is delivered to the peer application entity. There is no response issued, all record of the association is deleted and no further exchange can occur. The parameters of A-ABORT include an optional user data field and a required abort source carried only in the indication primitive. The user-data field, if present, is delivered to the ACSE user and is likely to provide termination information for the user process. The abort source, carried in the indication, differentiates between an abort that originates with the ACSE user issuing A-ABORT.request, and one that originates with the ACSE service provider without a request ever used. This latter case would occur if the ACSE service provider detected an error and issued an indication of association termination (A-ABORT.indication) to both ACSE service users.

A-P-ABORT is a service that contains only an indication primitive. It is issued by the ACSE service provider to notify its users that an non-recoverable error has occurred. Its only parameter is the reason for this drastic action. The reason is provided by underlying service providers that detected the condition leading to termination of the association. Notice that the ACSE service provider has two vehicles for notifying its users of an error state that requires termination of the association: A-ABORT.indication generated by the ACSE service provider, and A-P-ABORT.indication. The difference is that A-ABORT.indication notifies the user of an error detected or encountered by the ACSE service provider. A-P-ABORT.indication allows ACSE to convey to its user the information that the connection with the remote process has been broken by the lower layer service providers. It is a way of passing on to the ACSE user the information given to the ACSE provider by the presentation service provider.



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Next: ACSE: The Protocol Up: ACSE: The Service Previous: Association Establishment



boots cassel
Wed Feb 7 10:22:57 EST 1996