An important benefit of client/server based window systems is the added protection and reliability that comes from isolating the window system server from the window system clients. A faulty client should not compromise the window system. Microsoft's Windows is moving in this direction. X has always had this type of protection.
Protection in X11 is achieved through isolating the window system server from its clients by putting the clients and server in separate address spaces. This isolation incurs a significant cost in cache misses, context switching, protocol packing and unpacking, and transport overhead.
A client/server architecture using separate processes is not the only means to achieve protection. D11 provides the same measure of protection as client/server based window systems but does so using the active context facility described in Section 3 and detailed in Section 5. D11 does not isolate the window system kernel from clients using isolated address spaces; instead D11 uses virtual memory techniques to allow the client to in effect become the window system kernel through controlled interfaces without a heavyweight process switch and change of address space.