Although all transport-layer protocols support these
characteristics, they vary in their level of support and/or
their interpretation and format. For example, there are
transport-level options which remain constant across all transport
providers while there are other options which are
transport-provider specific or have different values/names for
different transport providers.
The main Chapters in this specification describe interfaces, parameters and semantics constant across all transport providers. The remainder of the document consists of appendices that provide valuable information that is not an integral part of the main body bgcolor="#FFFFFF" since it is either descriptive or applies only to some transport providers.
Some appendices provide information pertinent to writing XTI applications over specific transport providers. The transport providers fall into three classes:
The ISO appendix (Appendix A) also describes a transport provider that uses RFC 1006 to compensate for the differences between ISO transport and TCP so that a TCP provider can present an ISO transport appearance.
While XTI gives transport users considerable independence from the underlying transport provider, the differences between providers are not entirely hidden. Appendix C includes guidelines for writing transport-provider-independent software, which can be done primarily by using only functions supported by all providers, avoiding option management, and using a provider-independent means of acquiring addresses.
While the transport-provider-specific appendices are intended mostly for transport users, they are also used by implementors of transport providers. For the purposes of the implementors, some of the appendices show how XTI services can be mapped to primitives associated with the specific providers. These are provided as guidance only and do not dictate anything about a given implementation.
Some of the appendices to the XTI specification are included as vehicles for communicating information needed by implementors, or guidelines to the use of the specification in question. The Guidelines for the use of XTI (Appendix C), Minimum OSI Functionality (Appendix H), SNA Transport provider (Appendix I) and comparison of XTI to TLI (Appendix E) belong to this category.
Some other appendices, however, have evolved into a prescriptive specification, as in the case of Appendix A for the ISO transport provider, Appendix B for the Internet transport provider and Appendix D for the NetBIOS transport provider. Since not every XTI implementor would find it relevant to implement the functionality of all of these appendices, they have been kept separate from the main XTI specification, thus becoming brandable XTI options. Support for these transport providers is declared by vendors through the XTI Conformance Statement Questionnaire.
An appendix may have a different status from the overall XTI specification. Thus the appendix for a particular transport provider may be a Preliminary Specification while the document is a CAE specification. When this is the case, the status of the appendix is clearly identified in its own introduction.
Topics beyond the scope of the XTI specification include:
Several functions have parameters for addresses. The structure of these addresses is beyond the scope of this document. Specific implementations specify means for transport users to get or construct addresses.
In order for applications to use XTI in a fully asynchronous manner, it will be necessary for the application to include facilities of an Event Management (EM) interface. Such EM facility may allow the application to be notified of a number of events over a range of active transport connections. For example, one event may denote a connection is flow-controlled. While Appendix C provides some guidelines for using EM in XTI applications, a complete specification defining an EM interface is beyond the scope of this document.
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