Update architecture.md

contributors/design-proposals/architecture.md

@@ -176,12 +176,8 @@ available desired resources.
 The Kubernetes node has the services necessary to run application containers and
 be managed from the master systems.
 
-Each node runs a container runtime (like docker, rkt, or Hyper). The container
-runtime is responsible for downloading images and running containers.
-
 #### Kubelet
 
-
 The most important and most prominent controller in Kubernetes is the Kubelet, which is the
 primary implementer of the Pod and Node APIs that drive the container execution layer. Without
 these APIs, Kubernetes would just be a CRUD-oriented REST application framework backed by a
@@ -201,11 +197,21 @@ concerns, and migration from physical/virtual machines. The Pod primitive is key
 [primary benefits](https://kubernetes.io/docs/whatisk8s/#why-containers) of deployment on modern
 cloud platforms, such as Kubernetes.
 
+Kubelet also currently links in the [cAdvisor](https://github.com/google/cadvisor) resource monitoring
+agent.
+
 #### Container runtime
 
-TODO
+Each node runs a container runtime, which is responsible for downloading images and running containers.
+
+Kubelet does not link in the base container runtime. Instead, we're defining a [Container Runtime Interface]
+(container-runtime-interface-v1.md) to control the underlying runtime and facilitate pluggability of that layer.
+This decoupling is needed in order to maintain clear component boundaries, facilitate testing, and facilitate pluggability.
+Runtimes supported today, either upstream or by forks, include at least docker (for Linux and Windows), 
+[rkt](https://kubernetes.io/docs/getting-started-guides/rkt/),
+[cri-o](https://github.com/kubernetes-incubator/cri-o), and [frakti](https://github.com/kubernetes/frakti).
 
-#### `kube-proxy`
+#### Kube Proxy
 
 The [service](https://kubernetes.io/docs/user-guide/services/) abstraction provides a way to
 group pods under a common access policy (e.g., load-balanced). The implementation of this creates