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 mark-up needed to identify support
for a standard protocol and the base URL required to access
the interface that supports that protocol.
