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:
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:
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.
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
Some other Appendices, however, have evolved into a prescriptive
specification, as in the case of the ISO transport provider (see
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:
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 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