协议，解决方案。
https://en.wikipedia.org/wiki/Uniform_Resource_Identifier
While most URI schemes were originally designed to be used with a particular protocol, and often have the same name, they are semantically different from protocols. For example, the scheme http is generally used for interacting with web resources using HTTP, but the scheme file has no protocol.
Examples of popular schemes include http(s), ftp, mailto, file, data, and irc. URI schemes should be registered with the Internet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.[b]