Why does URL encoding exist for ASCII character se

问题:

It is clearly stated in W3Schools that

URLs can only be sent over the Internet using the ASCII character-set.

Why does URL encoding exist for ASCII characters like a , b , c when it can be sent over the internet without any URL encoding ???

Eg: Why encode 'a' when it can send over as 'a'

What are the possible reasons to encode ASCII characters ?? The only reason i can think of are hackers who are trying to make their URL as unreadable as possible to carry out XSS attacks

回答1:

STD 66, Percent-Encoding:

A percent-encoding mechanism is used to represent a data octet in a component when that octet's corresponding character is outside the allowed set or is being used as a delimiter of, or within, the component.

So percent-encoding is a kind of escape mechanism: Some characters have a special meaning in URI components (→ they are reserved). If you want to use such a character without its special meaning, you percent-encode it.

Unreserved characters (like a, b, c, …) can always be used directly, but it’s also allowed to percent-encode them. Such URIs would be equivalent:

URIs that differ in the replacement of an unreserved character with its corresponding percent-encoded US-ASCII octet are equivalent: they identify the same resource.

Why it’s allowed to percent-encode unreserved characters in the first place? The obsolete RFC 2396 contains (bold by me):

Unreserved characters can be escaped without changing the semantics of the URI, but this should not be done unless the URI is being used in a context that does not allow the unescaped character to appear.

I can’t think of an example for such a "context", but this sentence suggests that there may be some.

Also, maybe some people/implementations like to simply percent-encode everything (except for delimiters etc.), so they don’t have to check if/which characters would need percent-encoding in the corresponding component.

回答2:

URL encoding exists for the full range of ASCII because it was easier to define an encoding that works for all characters than to define one that only works for the set of characters with special meanings.

回答3:

URL encoding allows for characters that have special meaning in a URL to be included in a segment, without their special meaning. There are many examples, but the most common ones to require encoding include " ", "?", "=" and "&"

回答4:

URL encoding was designed so it can encode any ASCII character.

While = is encoded as %3d, ? is encoded as %3f and & is encoded as %26, it makes sense for a to be encoded as %61 and b to be encoded as %62, as the hex number after the % represents the ASCII code of the character.