Technical Standard: Networking Services (XNS), Issue 5.2 Draft 2.0
Copyright © 1999 The Open Group

XTI and TLI

XTI is based on the System V Interface Definitions (SVID) Issue 2, Volume III, Networking Services Extensions (see Referenced Documents).

XTI provides refinement of the Transport Level Interface (TLI) where such refinement is considered necessary. This refinement takes the form of:

additional commentary or explanatory text, in cases where the TLI text is either ambiguous or not sufficiently detailed
modifications to the interface, to cater for service and protocol problems which have been fully considered. In this case, it must be emphasised that such modifications are kept to an absolute minimum, and are intended to avoid any fundamental changes to the interface defined by TLI
the removal of dependencies on specific UNIX versions and specific transport providers.

It is important to bear in mind the following points when considering the use of XTI:

It was stated that XTI "recommends" a subset of the total set of functions and facilities defined in TLI, and also that XTI introduces modifications to some of these functions and/or facilities where this is considered essential. For these reasons, an application which is written in conformance to XTI may not be immediately portable to work over a provider which has been written in conformance to TLI.
XTI does not address management aspects of the interface, that is:
- how addressing may be done in such a way that an application is truly portable
- no selection and/or negotiation of service and protocol characteristics.
For addressing, the same is also true for TLI. In this case, it is envisaged that addresses will be managed by a higher-level directory function. For options selection and/or negotiation, XTI attempts to define a basic mechanism by which such information may be passed across the transport service interface, although again, this selection/negotiation may be done by a higher-level management function (rather than directly by the user). Since address structure is not currently defined, the user protocol address is system-dependent.

The following features can be considered as XTI extensions to the System V Release 3 version of TLI:

Some functions may return more error types. The use of the [TOUTSTATE] error is generalised to almost all protocol functions.
The transport provider identifier has been generalised to remove the dependence on a device driver implementation.
Additional events have been defined to help applications make full use of the asynchronous features of the interface.
Additional features have been introduced to t_snd(), t_sndrel() and t_rcvrel() to allow fuller use of TCP transport providers.
Usage of options for certain types of transport service has been defined to increase application portability.
Because most XTI functions require read/write access to the transport provider, the usage of flags O_RDONLY and O_WRONLY has been withdrawn from the XTI.
XTI checks the value of qlen and prevents an application from waiting forever when issuing t_listen().
XTI allows an application to call t_accept() with a resfd which is not bound to a local address.
XTI provides the additional utility functions t_strerror() and t_getprotaddr().