Status: Last call for comments
When a user agent is to fetch a resource or URL, optionally from an origin origin, and optionally with a synchronous flag and/or a manual redirect flag, the following steps must be run. (When a URL is to be fetched, the URL identifies a resource to be obtained.)
Generate the address of the resource from which Request-URIs
are obtained as required by HTTP for the Referer (sic) header from the
document's current address of the appropriate
Document as given by the following list. [HTTP]
Document.Remove any <fragment> component from the generated address of the resource from which Request-URIs are obtained.
If the origin of the appropriate
Document is not a scheme/host/port tuple, then the
Referer (sic) header must be
omitted, regardless of its value.
If the algorithm was not invoked with the synchronous flag, perform the remaining steps asynchronously.
This is the main step.
If the resource is identified by an absolute URL,
and the resource is to be obtained using an idempotent action
(such as an HTTP GET or
equivalent), and it is already being downloaded for other
reasons (e.g. another invocation of this algorithm), and this
request would be identical to the previous one (e.g. same Accept and Origin headers), and the user agent is
configured such that it is to reuse the data from the existing
download instead of initiating a new one, then use the results of
the existing download instead of starting a new one.
Otherwise, at a time convenient to the user and the user agent,
download (or otherwise obtain) the resource, applying the
semantics of the relevant specifications (e.g. performing an HTTP
GET or POST operation, or reading the file from disk, dereferencing javascript: URLs,
etc).
For the purposes of the Referer (sic) header, use the
address of the resource from which Request-URIs are
obtained generated in the earlier step.
For the purposes of the Origin
header, if the fetching algorithm was
explicitly initiated from an origin, then the origin that initiated the HTTP request is origin. Otherwise, this is a request from
a "privacy-sensitive" context. [ORIGIN]
If the resource is identified by the URL
about:blank, then the resource is
immediately available and consists of the empty string, with no
metadata.
If there are cookies to be set, then the user agent must run the following substeps:
Wait until ownership of the storage mutex can be taken by this instance of the fetching algorithm.
Take ownership of the storage mutex.
Update the cookies. [COOKIES]
Release the storage mutex so that it is once again free.
If the fetched resource is an HTTP redirect or equivalent, then:
Continue, using the fetched resource (the redirect) as the result of the algorithm.
First, apply any relevant requirements for redirects (such as showing any appropriate prompts). Then, redo main step, but using the target of the redirect as the resource to fetch, rather than the original resource.
The HTTP specification requires that 301, 302, and 307 redirects, when applied to methods other than the safe methods, not be followed without user confirmation. That would be an appropriate prompt for the purposes of the requirement in the paragraph above. [HTTP]
If the algorithm was not invoked with the synchronous flag: When the resource is available, or if there is an error of some description, queue a task that uses the resource as appropriate. If the resource can be processed incrementally, as, for instance, with a progressively interlaced JPEG or an HTML file, additional tasks may be queued to process the data as it is downloaded. The task source for these tasks is the networking task source.
Otherwise, return the resource or error information to the calling algorithm.
If the user agent can determine the actual length of the resource
being fetched for an instance of this
algorithm, and if that length is finite, then that length is the
file's size. Otherwise, the
subject of the algorithm (that is, the resource being fetched) has
no known size. (For
example, the HTTP Content-Length header might
provide this information.)
The user agent must also keep track of the number of bytes downloaded for each instance of this algorithm. This number must exclude any out-of-band metadata, such as HTTP headers.
The application cache processing model introduces some changes to the networking model to handle the returning of cached resources.
The navigation processing model handles redirects itself, overriding the redirection handling that would be done by the fetching algorithm.
Whether the type sniffing rules apply to the fetched resource depends on the algorithm that invokes the rules — they are not always applicable.
Status: Last call for comments
User agents can implement a variety of transfer protocols, but this specification mostly defines behavior in terms of HTTP. [HTTP]
The HTTP GET method is equivalent to the default retrieval action of the protocol. For example, RETR in FTP. Such actions are idempotent and safe, in HTTP terms.
The HTTP response codes are equivalent to statuses in other protocols that have the same basic meanings. For example, a "file not found" error is equivalent to a 404 code, a server error is equivalent to a 5xx code, and so on.
The HTTP headers are equivalent to fields in other protocols that have the same basic meaning. For example, the HTTP authentication headers are equivalent to the authentication aspects of the FTP protocol.
Status: Last call for comments
Anything in this specification that refers to HTTP also applies
to HTTP-over-TLS, as represented by URLs
representing the https scheme.
User agents should report certificate errors to the user and must either refuse to download resources sent with erroneous certificates or must act as if such resources were in fact served with no encryption.
User agents should warn the user that there is a potential problem whenever the user visits a page that the user has previously visited, if the page uses less secure encryption on the second visit.
Not doing so can result in users not noticing man-in-the-middle attacks.
If a user connects to a server with a self-signed certificate, the user agent could allow the connection but just act as if there had been no encryption. If the user agent instead allowed the user to override the problem and then displayed the page as if it was fully and safely encrypted, the user could be easily tricked into accepting man-in-the-middle connections.
If a user connects to a server with full encryption, but the page then refers to an external resource that has an expired certificate, then the user agent will act as if the resource was unavailable, possibly also reporting the problem to the user. If the user agent instead allowed the resource to be used, then an attacker could just look for "secure" sites that used resources from a different host and only apply man-in-the-middle attacks to that host, for example taking over scripts in the page.
If a user bookmarks a site that uses a CA-signed certificate, and then later revisits that site directly but the site has started using a self-signed certificate, the user agent could warn the user that a man-in-the-middle attack is likely underway, instead of simply acting as if the page was not encrypted.
Status: Last call for comments
The Content-Type metadata of a resource must be obtained and interpreted in a manner consistent with the requirements of the Content-Type Processing Model specification. [MIMESNIFF]
The sniffed type of a resource must be found in a manner consistent with the requirements given in the Content-Type Processing Model specification for finding the sniffed-type of the relevant sequence of octets. [MIMESNIFF]
The rules for sniffing images specifically and the rules for distingushing if a resource is text or binary are also defined in the Content-Type Processing Model specification. Both sets of rules return a MIME type as their result. [MIMESNIFF]
It is imperative that the rules in the Content-Type Processing Model specification be followed exactly. When a user agent uses different heuristics for content type detection than the server expects, security problems can occur. For more details, see the Content-Type Processing Model specification. [MIMESNIFF]
The algorithm for extracting an encoding from a Content-Type, given a string s, is as follows. It either returns an encoding or nothing.
Find the first seven characters in s
that are an ASCII case-insensitive match for the word
"charset". If no such match is found, return
nothing.
Skip any U+0009, U+000A, U+000C, U+000D, or U+0020
characters that immediately follow the word "charset" (there might not be any).
If the next character is not a U+003D EQUALS SIGN ('='), return nothing and abort these steps.
Skip any U+0009, U+000A, U+000C, U+000D, or U+0020 characters that immediately follow the equals sign (there might not be any).
Process the next character as follows:
This requirement is a willful violation of the HTTP specification, motivated by the need for backwards compatibility with legacy content. [HTTP]