@internal
@brief callback for data received messages.
@param data data retrieved from the socket or stream
@param len number of bytes available from this stream
@param user user-supplied pointer sent along with all callback messages
@returns number of bytes processed by the callee. The remaining bytes will
be sent in the next callback message, when more data is available.
@internal @brief callback for data received messages. @param data data retrieved from the socket or stream @param len number of bytes available from this stream @param user user-supplied pointer sent along with all callback messages @returns number of bytes processed by the callee. The remaining bytes will be sent in the next callback message, when more data is available.