tions, Control of that ASPs are exchanged in the correct order within an operation, Control of format errors. 5 Protocol Network Modelling A duplicated invocation is the result either of transport errors, or because the ini- tiator assumes that a previous ASP has not been acted on. Duplicated invocations are detected by the FOH by storing previous Globlnvld values within a suitable window-size. The second aspect implies controlling that a request-ASP is responded to by a proper response-ASP. The purpose of this type of control is to protect an object from an erroneously behaving peer. This aspect depends on the operation classifi- cation system (for AML, see Sect. 7.4.4) that the language defines. ROSE, AML and IDL all have their own view on operation classes. The corresponding FOH functions are therefore not compatible. To perform dialogue control, FOHs must store information about each ASP instance and be able to exchange a special error message (ErrorMess), see Fig. 5.26. This message can also be used to report format errors, e.g. values that are out of range (see the size constraints that are defined in the operation and the message specifications of Figs. 5.25 and 5.26). Note that FOH error handling only com- prises "general errors", i.e. such errors that could occur for any operation. Errors that are related to a particular operation are reported by responding ASPs, see e.g. the causes for errors in the chAddrRefuse ASP in the operation specification, Fig. 5.25. To separate operation-specific causes for failed operations from general causes is, from an architectural point of view, a very important issue. This is because it creates distinct specifications, and because general error causes concern the management system, operation-specific causes do not. 5.4.4 Encoding An important observation that is implemented in the SM&AML message-han- dling model is that encoding of data to the bit-representation of messages, must be performed in every logical node. This is in contrast to the OSI RM layer model, where an 