  An application that queries or consumes descriptions of VO resources must be
able to recognize a resource's support for standard IVOA protocols. This
specification describes how to describe a service that supports any of the four
fundamental data access protocols -- Simple Cone Search (SCS), Simple Image
Access (SIA), Simple Spectral Access (SSA), Simple Line Access (SLA) -- using
the VOResource XML encoding standard. A key part of this specification is the
set of VOResource XML extension schemas that define new metadata that are
specific to those protocols. This document describes in particular rules for
describing such services within the context of IVOA Registries and data
discovery as well as the VO Standard Interface (VOSI) and service
self-description. In particular, this document spells out the essential markup
needed to identify support for a standard protocol and the base URL required to
access the interface that supports that protocol.
