URI
1） 关于URI的一个误解。
A common misunderstanding of URIs is that they are only used to referto accessible resources. The URI itself only provides identification; access to the resource is neither guaranteed nor implied by the presence of a URI. Instead, any operation associated with a URI reference is defined by the protocol element, data format attribute, or natural language text in which it appears.
2） 格式
Percent-encoding. 使用%来转义（converse parsing）保留字段。
URI分为URL和URN。一个典型的URN的例子（ISBN 0-486-27557-4，urn:isbn:0-486-27557-4）。
A URL is a URI that, in addition to identifying a web resource, specifies the means of acting upon or obtaining the representation。
A uniform resource name (URN) is a URI that identifies a resource by name, in a particular namespace。URI does not suggest where or how to obtain an actual copy of it。
3） Syntax
The URI syntax consists of a URI scheme name (such as "http", "ftp", "mailto", "crid" or "file") followed by a colon character, and then by a scheme-specific part.
Example: <scheme name> : <hierarchical part> [ ? <query> ] [ # <fragment> ]