- The Networking Services (XNS), Issue 5 specification describes the industry-standard open systems interfaces to communication services. These include two APIs to transport-level process-to-process communications Sockets and XTI. Both Sockets and XTI are specified for use over Internet protocols (TCP, UDP, and IP) and ISO Transport protocols. They also include a set of Internet address resolution interfaces which are commonly used in conjunction with Sockets. XTI support for many other protocols is described in appendixes included in this specification. This document forms part of the Single UNIX Specification, Version 2 (T912) required for UNIX 98. It includes revisions to align with IEEE Standard 1003.1 1990 (Portable Operating System Interface).