All network communication is handled by the main browser process. This is done not only so that the browser process can control each renderer's access to the network, but also so that we can maintain consistent session state across processes like cookies and cached data. It is also important because as a HTTP/1.1 user-agent, the browser as a whole should not open too many connections per host.
On Windows, we currently use WinHTTP [note: this is out of date; we have switched to our own custom network stack]. In the future, we plan to replace this with a custom HTTP implementation. Compared to WinInet (which we have a legacy implementation for):
- Both come with all Windows versions we plan to support (Win2K and later).
- WinHTTP is better documented. WinInet has a more difficult API and spotty documentation.
- WinHTTP has a cleaner API with the support we need. A number of WinInet functions that are necessary for us (for example, for SSL state) are undocumented and unsupported, even though IE uses them.
- WinHTTP allows us to implement our own cache. WinInet always shares a cache with IE. We would like to control our own caching, and also sharing a cache with IE can cause problems since some sites may serve browser-specific pages.
- WinInet is better battle tested on account of being used by IE. WinHTTP is not used very much, and was designed for server use. It has problems with non-ASCII URL encodings sent by the server and keepalive connections. We have to provide workarounds for these issues.
Our multi-process application can be viewed in three layers. At the lowest layer is the WebKit library which renders pages. Above that are the renderer process (simplistically, one-per-tab), each of which contains one WebKit instance. Managing all the renderers is the browser process, which controls all network accesses.
WebKit has a
ResourceLoader object which is responsible for fetching data. Each loader has a
ResourceHandle for performing the actual requests. The header file for this object is inside the WebKit code, and we are unwilling to fork it. Fortunately, it contains a
d member which we can define. We remove the old implementation of
ResourceHandleInternal and provide our own, located in
webkit/glue/resource_handle_win.cc. Despite the name, it has nothing to do with Win32.
ResourceHandleInternal implements the virtual interface
ResourceLoaderBridge::Peer, defined in
webkit/glue/resource_loader_bridge.h. This is the callback interface used by the renderer to dispatch data and other events to WebKit.
ResourceHandleInternal inside WebKit talks to the renderer (to initiate or cancel requests) via the
ResourceLoaderBridge virtual interface. An implementation of this interface is retrieved by calling the static function
ResourceLoaderBridge::Create, which the renderer implements. The test shell uses a different resource loader, so provides a different implementation, non-IPC version of
ResourceLoaderBridge, located in
The renderer's implementation of
IPCResourceLoaderBridge, is located in
renderer/resource_dispatcher. It uses the global
ResourceDispatcher singleton object (one for each renderer process) to create a unique request ID and forward the request to the browser via IPC. Responses from the browser will reference this request ID, which can then be converted back to the
ResourceLoaderBridge::Peer object (inside WebKit) by the resource dispatcher.
The conversion between resource requests and their data is all handled by
common/resource_loader_ipc. The renderer and the browser both share this code so that the conversions are in sync.
RenderProcessHost objects inside the browser receive the IPC requests from each renderer. It forwards these requests to the global
ResourceDispatcherHost, using a pointer to the render process host (specifically, an implementation of
ResourceDispatcherHost::Receiver) and the request ID generated by the renderer to uniquely identify the request.
Each request is then converted into a
URLRequest object, which in turn forwards it to it's internal
URLRequestJob that implements the specific protocol desired. When the
URLRequest generates notifications, its
ResourceDispatcherHost::Receiver and request ID are used to send the notification to the correct
RenderProcessHost for sending back to the renderer. Since the ID generated by the renderer is preserved, it is able to correlate all responses with a specific request first generated by WebKit.
All cookies are handled by our
CookieMonster object in
Pages can request cookies for a document via