Widely disparate applications are very often required to share and exchange relational data. Typically, this is accomplished by passing messages over a shared environment (e.g. computer network, file system, computing memory, etc.) in a well-defined, machine-processable format. In simple scenarios, when the relational schema is invariable, only the data need to be serialized and exchanged. The interacting applications are precisely customized for the particular schema: they share a custom serialization schema, they perform custom business logic on the data and, eventually, they have a custom user interface designed. As the relational schema evolves, the serialization schema has to be redefined and the interacting applications have to be reimplemented. In order to access evolving relational schema in real-time, it is necessary to serialize the schema using some metadata model and to exchange the schema together with the data. As a consequence, the user interface has to be inferred from the schema and generated on-the-fly instead of being designed and customized by the software vendor. This puts high demands on the metadata models to supply sufficient information for generating rich user interface from the provided metadata. This paper specifies the Relational Schema Protocol (RSP). RSP enables loosely coupled applications to share and exchange relational data. It provides access to heterogeneous relational data sources using a uniform set of operations and data types. It extends the standard relational meta-models such as SQL INFORMATION SCHEMA, OIM or CWM with additional metadata for rich user interface generation. It defines a set of operations for on-line schema exploration and efficient data manipulation in real-time.