Add Antrea architecture doc

Also add links to the architecture doc from REAME.md and
CONTRIBUTING.md.

CONTRIBUTING.md

@@ -11,6 +11,7 @@ Before getting started, go through the following:
 3. Familiarize yourself with the basic concepts of project Antrea.
 4. Set up necessary [accounts](#accounts-setup).
 5. Set up your [development environment](docs/manual-installation.md)
+6. Check out the [Architecture document](/docs/architecture.md) for the Antrea architecture and design.
 
 ## Accounts setup
 

README.md

@@ -95,3 +95,5 @@ and propose new ideas. We are waiting for your PRs!
 
 Check out the [Contributor Guide](CONTRIBUTING.md) for detailed
 information on how to effectively contribute to this project.
+
+Also check the [Architecture document](/docs/architecture.md) for the Antrea architecture and design.

docs/architecture.md

@@ -0,0 +1,213 @@
+# Antrea Architecture
+
+## Components
+
+In a Kubernetes cluster, Antrea creates a Deployment that runs Antrea
+Controller, and a DaemonSet that includes two containers to run Antrea Agent
+and OVS daemons respectively, on every worker Node. The DaemonSet also includes
+an init container that installs the CNI plugin - `antrea-cni` - on the Node and
+ensures that the OVS kernel module is loaded. All Antrea Controller, Agent, OVS
+daemons, and `antrea-cni` bits are included in a single Docker image. Antrea
+also has a command-line tool called `antctl`, and an [Octant](https://github.com/vmware-tanzu/octant)
+UI plugin.
+
+<img src="/docs/assets/arch.svg.png" width="600" alt="Antrea Architecture Overview">
+
+### Antrea Controller
+
+Antrea Controller watches NetworkPolicy, Pod, and Namespace resources from the
+Kubernetes API, computes NetworkPolicies and distributes the computed policies
+to all Antrea Agents. Right now Antrea Controller supports only a single
+replica. At the moment, Antrea Controller mainly exists for NetworkPolicy
+implementation. If you only care about connectivity between Pods but not
+NetworkPolicy support, you may choose not to deploy Antrea Controller at all.
+However, in the future, Antrea might support more features that require Antrea
+Controller.
+
+Antrea Controller leverages the [Kubernetes apiserver library](https://github.com/kubernetes/apiserver)
+to communicate updates to Antrea Agent. Antrea Agent connects to the Controller
+apiserver and watches the computed NetworkPolicy objects. Antrea Controller also
+leverages Kubernetes Service for:
+- API service discovery, and
+- API authentication and authorization.
+
+The Controller API endpoint is exposed through a Kubernetes ClusterIP type
+Service. Antrea Agent gets the Service's ClusterIP from the Service environment
+variable and connects to the Controller apiserver using the ClusterIP. Antrea
+Controller delegates authentication and authorization to the Kubernetes API -
+the Agent uses a Kubernetes ServiceAccount token to authenticate to the
+Controller and the Controller validates the token with the Kubernetes API.
+
+Antrea Controller also exposes REST API for `antctl` using the same HTTP
+endpoint as the API to Antrea Agent. See more information in the [`antctl` section](#antctl).
+
+### Antrea Agent
+
+Antrea Agent manages the OVS bridge and Pod interfaces and implements Pod
+networking with OVS on every Kubernetes Node.
+
+Antrea Agent exposes a gRPC service (`Cni` service) which is invoked by the
+`antrea-cni` binary to perform CNI operations. For each new Pod to be created on
+the Node, after getting the CNI `ADD` call from `antrea-cni`, the Agent creates
+the Pod's  network interface, allocates an IP address, connects the interface to
+the OVS bridge and installs the necessary flows in OVS.
+
+Antrea Agent includes two Kubernetes controllers:
+- The Node controller watches the Kubernetes API server for new Nodes, and
+creates an OVS (VXLAN or Geneve) tunnel to each remote Node.
+- The NetworkPolicy controller watches the computed NetworkPolicies from the
+Antrea Controller API, and installs OVS flows to implement the NetworkPolicies
+for the local Pods.
+
+Antrea Agent also exposes REST API on a local HTTP endpoint for `antctl`.
+
+### OVS daemons
+
+The two OVS daemons - `ovsdb-server` and `ovs-vswitchd` run in a separate
+container, called `antrea-ovs`, of the Antrea Agent DaemonSet.
+
+### `antrea-cni`
+
+`antrea-cni` is the [CNI](https://github.com/containernetworking/cni) plugin
+binary of Antrea. It is executed by `kubelet` for each CNI command. It is a
+simple gRPC client which issues an RPC to Antrea Agent for each CNI command. The
+Agent performs the actual work (sets up networking for the Pod) and returns the
+result or an error to `antrea-cni`.
+
+### `antctl`
+
+`antctl` is a command-line tool for Antrea. At the moment, it can show basic
+runtime information for both Antrea Controller and Antrea Agent, for debugging
+purposes.
+
+When accessing the Controller, `antctl` invokes the Controller API to query the
+required information. Antrea leverages [Kubernetes API aggregation](https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/)
+to enable `antctl` to reach the Antrea Controller API through the Kubernetes
+API - `antctl` connects and authenticates to the Kubernetes API, which will
+proxy the `antctl` API requests to the Antrea Controller. In this way, `antctl`
+can be executed on any machine that can reach the Kubernetes API, and it can
+also leverage the `kubectl` configuration (`kubeconfig` file) to discover the
+Kubernetes API and authentication information. `antctl` can be executed through
+`kubectl` as a `kubectl` plugin as well.
+
+When accessing the Agent, `antctl` connects to the Agent's local REST endpoint,
+and can only be executed locally in the Agent's container.
+
+### Octant UI plugin
+
+Antrea also implements an Octant plugin which can show the Controller and
+Agent's health and basic runtime information in the Octant UI. The Octant plugin
+gets the Controller and Agent's information from the `AntreaControllerInfo` and
+`AntreaAgentInfo` CRDs (Custom Resource Definition) in the Kubernetes API. The
+CRDs are created by the Antrea Controller and each Antrea Agent to populate
+their health and runtime information.
+
+## Pod Networking
+
+### Pod interface configuration and IPAM
+
+On every Node, Antrea Agent creates an OVS bridge (named `br-int` by default),
+and creates a veth pair for each Pod, with one end being in the Pod's network
+namespace and the other connected to the OVS bridge. On the OVS bridge, Antrea
+Agent also creates an internal port - `gw0` by default - to be the gateway of
+the Node's subnet, and a tunnel port `tun0` which is for creating VXLAN / Geneve
+tunnels to other Nodes.
+
+<img src="/docs/assets/node.svg.png" width="300" alt="Antrea Node Network">
+
+Each Node is assigned a single subnet, and all Pods on the Node get an IP from
+the subnet. Antrea leverages Kubernetes' `NodeIPAMController` for the Node
+subnet allocation, which sets the `podCIDR` field of the Kubernetes Node spec
+to the allocated subnet. Antrea Agent retrieves the subnets of Nodes from the
+`podCIDR` field. It reserves the first IP of the local Node's subnet to be the
+gateway IP and assigns it to the `gw0` port, and invokes the
+[host-local IPAM plugin](https://github.com/containernetworking/plugins/tree/master/plugins/ipam/host-local)
+to allocate IPs from the subnet to all local Pods. A local Pod is assigned an IP
+when the CNI ADD command is received for that Pod.
+
+For every remote Node, Antrea Agent adds an OVS flow to send the traffic to that
+Node through the appropriate VXLAN or Geneve tunnel. The flow matches the
+packets' destination IP against each Node's subnet.
+
+### Traffic walk
+
+<img src="/docs/assets/traffic_walk.svg.png" width="600" alt="Antrea Traffic Walk">
+
+* ***Intra-node traffic*** Packets between two local Pods will be forwarded by
+the OVS bridge directly.
+
+* ***Inter-node traffic*** Packets to a Pod on another Node will be first
+forwarded to the `tun0` port, encapsulated (with VXLAN or Geneve), and sent to
+the destination Node through the tunnel; then they will be decapsulated,
+injected through the `tun0` port to the OVS bridge, and finally forwarded to the
+destination Pod.
+
+* ***Pod to external traffic*** Packets sent to an external IP or the Nodes'
+network will be forwarded to the `gw0` port (as it is the gateway of the local
+Pod subnet), and will be routed (based on routes configured on the Node) to the
+appropriate network interface of the Node (e.g. a physical network interface for
+a baremetal Node) and sent out to the Node network from there. Antrea Agent
+creates an iptables (MASQUERADE) rule to perform SNAT on the packets from Pods,
+so their source IP will be rewritten to the Node's IP before going out.
+
+### ClusterIP Service
+
+<img src="/docs/assets/service_walk.svg.png" width="600" alt="Antrea Service Traffic Walk">
+
+At the moment, Antrea leverages `kube-proxy` to serve traffic for ClusterIP and
+NodePort type Services. The packets from a Pod to a Service's ClusterIP will be
+forwarded through the `gw0` port, then `kube-proxy` will select one Service
+backend Pod to be the connection's destination and DNAT the packets to the Pod's
+IP and port. If the destination Pod is on the local Node the packets will be
+forwarded to the Pod directly; if it is on another Node the packets will be sent
+to that Node via the VXLAN / Geneve tunnel.
+
+`kube-proxy` can be used in any supported mode: user-space iptables, or IPVS.
+See the [Kubernetes Service documentation](https://kubernetes.io/docs/concepts/services-networking/service)
+for more details.
+
+In the future, Antrea will also implement ClusterIP Service with OVS, and make
+it a configurable option. This should help achieve better performance than
+`kube-proxy` when `kube-proxy` is used in the user-space or iptables modes.
+
+### NetworkPolicy
+
+An important design choice Antrea took regarding the NetworkPolicy
+implementation is centralized policy computation. Antrea Controller watches
+NetworkPolicy, Pod, and Namespace resources from the Kubernetes API. It
+processes podSelectors, namespaceSelectors, and ipBlocks as follows:
+- PodSelectors directly under the NetworkPolicy spec (which define the Pods to
+which the NetworkPolicy is applied) will be translated to member Pods.
+- Selectors (podSelectors and namespaceSelectors) and ipBlocks in rules (which
+define the ingress and egress traffic allowed by this policy) will be mapped to
+Pod IP addresses / IP address ranges.
+
+Each Antrea Agent receives only the computed policies which affect Pods running
+locally on its Node, and directly uses the IP addresses computed by the
+Controller to create OVS flows enforcing the specified NetworkPolicies.
+
+We see the following major benefits of the centralized computation approach:
+
+* Only one Antrea Controller instance needs to receive and process all
+NetworkPolicy, Pod, and Namespace updates, and compute podSelectors and
+namespaceSelectors. This has a much lower overall cost compared to watching
+these updates and performing the same complex policy computation on all Nodes.
+
+* It could enable scale-out of Controllers, with multiple Controllers working
+together on the NetworkPolicy computation, each one being responsible for a
+subset of NetworkPolicies (though at the moment Antrea supports only a single
+Controller instance).
+
+* Antrea Controller is the single source of NetworkPolicy computation. It is
+much easier to achieve consistency among Nodes and easier to debug the
+NetworkPolicy implementation.
+
+As described earlier, Antrea Controller leverages the Kubernetes apiserver
+library to build the API and communication channels to Agents. The apiserver
+implementation is customized and optimized for publishing the computed
+NetworkPolicies to Agents. Antrea Controller keeps all the state in in-memory
+caches and does not require a datastore to persist the data. It supports sending
+incremental updates to the computed NetworkPolicy objects to Agents, and only
+send the NetworkPolicy objects to those Nodes that need to apply the
+NetworkPolicies locally. Messages between Controller and Agent are serialized
+using the Protobuf format for reduced size and higher efficiency.