General Introduction to the XTI

The X/Open Transport Interface (XTI) specification defines an independent transport-service interface that allows multiple users to communicate at the transport level of the OSI reference model. The specification describes transport-layer characteristics that are supported by a wide variety of transport-layer protocols. Supported characteristics include:

• connection establishment

• state change support

• event handling

• data transfer

• option manipulation.

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 Part 2: XTI specification describe interfaces, parameters and semantics constant across all transport providers. Several Appendices provide information that is not an integral part of the main body 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:

• Those corresponding to traditional transport providers, such as:

  • ISO Transport (connection-mode or connectionless-mode)

  • TCP

  • UDP

  • NetBIOS.

• Those corresponding to commonly used subsets of higher-layer protocols that provide transport-like services, such as:

  • minimal functionality OSI (mOSI), that is, OSI ACSE/Presentation with the kernel and duplex functional units

  • SNA LU6.2 subset.

• Mixed-protocol providers that provide the appearance of one protocol over a different protocol such as:

  • ISO transport appearance (connection-mode) over TCP.

    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. Guidelines for Use of XTI 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 (see Guidelines for Use of XTI), Minimum OSI Functionality (see Minimum OSI Functionality), (Appendix H), SNA transport provider (see SNA Transport Provider), SPX/IPX transport provider (see IPX/SPX Transport Provider), and Comparison of XTI to TLI (see XTI and TLI) all belong to this category.

Some other Appendices, however, have evolved into a prescriptive specification, as in the case of the ISO transport provider (see Use of XTI with ISO Transport Protocols), and the NetBIOS transport provider (see Use of XTI to Access NetBIOS).

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 definitions for XTI. Thus they are readily identifable as brandable XTI options. Support for these transport providers is declared in Branding documentation through the XTI Conformance Statement Questionnaire.

Topics beyond the scope of the XTI specification include:

• Address parameters

  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.

• Event management

  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 interface. Such an event management 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 Event Management provides some guidelines for using event management in XTI applications, a complete specification defining an event management interface is beyond the scope of this document.