Network Stack Use in Chromium
Chromium is the primary consumer of the Chromium network stack. While there are
other consumers, the main one is the Chromium browser process. For the most
part, the network stack runs in the browser process's IO thread, and interfaces
with the Chromium network stack via
The Chromium browser process uses several instances of
ChromeURLRequestContext, a subclass of
For a given profile, there are a number of
- The "main"
ChromeURLRequestContext. The majority of
URLRequests are associated with this
- The "media"
ChromeURLRequestContext. This one uses a different
[HttpCache](/developers/design-documents/network-stack/disk-cache)instance that is optimized for large media requests (video). It shares many objects, such as the
[CookieMonster](/developers/design-documents/network-stack/cookiemonster), etc. with the main
- The "extension"
ChromeURLRequestContext. It shares many objects with the main
ChromeURLRequestContext. This instance helps service chrome-extension:// requests.
There are also some other
ChromeURLRequestContexts, such as sync's
ChromeURLRequestContext, etc. These are not tied to the profile, although with
a multiple profile Chromium, sync's
HttpBridge::RequestContext likely would
have to be tied to a profile.
Note that the "off the record" (Incognito) profile uses a special OTR
ChromeURLRequestContext, in addition to the media and extension contexts, that
shares some objects, although notably not the
that are important to be separated for an off the record profile. The
HostResolver is shared.
Many of the objects in
scoped_refptrs to share
ownership. This causes a lot of problems with object destruction ordering and
reference cycles. Many of these members are being moved towards being owned
externally, by the
IOThread, although that choice may have to change to
ProfileImpl or something to support multiple profiles in Chromium.
Note that currently ChromeURLRequestContext contains a lot of extra members that
don't exist in
URLRequestContext. These are being moved out, because they have
nothing to do with
URLRequestContext and were only placed in
ChromeURLRequestContext because it was a convenient per-profile object, and
originally there weren't many ChromeURLRequestContext objects.
ChromeURLRequestContext is constructed and destroyed on the same thread.
However, sometimes other threads need to hold references to them, even before
they get constructed.
ChromeURLRequestContextGetter is a handle for
ChromeURLRequestContext that will lazily construct it on the first attempt to
use it. Accesses to the contained
ChromeURLRequestContext are only allowed on
the IO thread.
URLRequestswhen the renderer or plugin processes issue resource requests. It retains pointers to all the
URLFetcheris another frontend for
URLRequest. It allows any thread with access to the appropriate
ChromeURLRequestContextGetterto proxy requests to the IO thread. It provides a simplified delegate interface that returns the full response body as a single string. There are many users of
URLFetcherin the Chromium browser process, including the omnibox, extensions, sync (which uses its own
ChromeURLRequestContext), etc. Usually these objects use the "main" ChromeURLRequestContextGetter object.
- IntranetRedirectDetector - Lets us probe the network to see if ISPs are trying to redirect requests to non-existent hosts.
- GoogleURLTracker - Lets us probe Google to see the appropriate domain for the search engine.
URLRequest::Interceptor to hook into all
URLRequests to watch them
come and go, so it can analyze the referers to learn the subdomains used in
subsequent requests. It uses this knowledge to predict network requests. For
likely requests, it keeps around the pointer to the "main"
HostResolver, so it can perform DNS prefetching.
For requests that are almost guaranteed to happen, it goes even further and uses
HttpStreamFactory to preconnect a