Add documentation for noEncap and hybrid modes, and kube-router

docs/getting-started.md

@@ -135,8 +135,7 @@ provide your own certificates, please refer to [Securing Control Plane](securing
 
 ### Antctl: Installation and Usage
 
-To use antctl, the Antrea command-line tool, please refer to this
-[guide](antctl.md).
+To use antctl, the Antrea command-line tool, please refer to [this guide](antctl.md).
 
 ## Features
 
@@ -159,6 +158,17 @@ reference cookbook on how to visualize the exported network flows using Elastic
 Stack and Kibana dashboards. For more information, refer to the [network flow
 visibility document](network-flow-visibility.md).
 
+### NoEncap and Hybrid Traffic Modes
+
+Besides the default `Encap` mode, in which Pod traffic across Nodes will be
+encapsulated and sent over tunnels, Antrea also supports `NoEncap` and `Hybrid`
+traffic modes. In `NoEncap` mode, Antrea does not encapsulate Pod traffic, but
+relies on the Node network to route the traffic across Nodes. In `Hybrid` mode,
+Antrea encapsulates Pod traffic when the source Node and the destination Node
+are in different subnets, but does not encapsulate when the source and the
+destination Nodes are in the same subnet. Refer to [this guide](noencap-hybrid-modes.md)
+to learn how to configure Antrea with `NoEncap` or `Hybrid` mode.
+
 ### Octant UI
 
 Antrea ships with an Octant UI plugin which can show runtime information of Antrea

docs/noencap-hybrid-modes.md

@@ -0,0 +1,150 @@
+# NoEncap and Hybrid Traffic Modes of Antrea
+
+Besides the default `Encap` mode, in which Pod traffic across Nodes will be
+encapsulated and sent over tunnels, Antrea also supports `NoEncap` and `Hybrid`
+traffic modes. In `NoEncap` mode, Antrea does not encapsulate Pod traffic, but
+relies on the Node network to route the traffic across Nodes. In `Hybrid` mode,
+Antrea encapsulates Pod traffic when the source Node and the destination Node
+are in different subnets, but does not encapsulate when the source and the
+destination Nodes are in the same subnet. This document describes how to
+configure Antrea with the `NoEncap` and `Hybrid` modes.
+
+## Hybrid Mode
+
+Let us start from `Hybrid` mode which is simpler to configure. `Hybrid` mode
+does not encapsulate Pod traffic when the source and the destination Nodes are
+in the same subnet. Thus it requires the Node network to allow Pod IP addresses
+to be sent out from the Nodes' NICs. This requirement is not supported in all
+the networks and clouds, or in some cases it might require specific
+configuration of the Node network. For example:
+
+* On AWS, the source/destination checks must be disabled on the EC2 instances of
+the Kubernetes Nodes, as described in the
+[AWS documentation](https://docs.aws.amazon.com/vpc/latest/userguide/VPC_NAT_Instance.html#EIP_Disable_SrcDestCheck).
+
+* On Google Compute Engine, IP forwarding must be enabled on the VM instances as
+described in the [Google Cloud documentation](https://cloud.google.com/vpc/docs/using-routes#canipforward).
+
+* On Azure, there is no way to let VNet forward unknown IPs, hence Antrea
+`Hybrid` mode cannot work on Azure.
+
+If the Node network does allow Pod IPs sent out from the Nodes, you can
+configure Antrea to run in the `Hybrid` mode by setting the `trafficEncapMode`
+config option of `antrea-agent` to `hybrid`. The `trafficEncapMode` config
+option is defined in `antrea-agent.conf` of the `antrea` ConfigMap in the
+[Antrea deployment YAML](https://github.com/vmware-tanzu/antrea/blob/main/build/yamls/antrea.yml).
+
+```yaml
+  antrea-agent.conf: |
+    ... ...
+    trafficEncapMode: hybrid
+    ... ...
+```
+
+After changing the config option, you can deploy Antrea in `Hybrid` mode with
+the usual command:
+
+```bash
+kubectl apply -f antrea.yml
+```
+
+## NoEncap Mode
+
+In `NoEncap` mode, Antrea never encapsulates Pod traffic. Just like `Hybrid`
+mode, the Node network needs to allow Pod IP addresses sent out from Nodes. When
+the Nodes are not in the same subnet, `NoEncap` mode additionally requires the
+Node network be able to route the Pod traffic from the source Node to the
+destination Node. There are two possibilities to enable this routing by Node
+network:
+
+* Leverage Route Controller of [Kubernetes Cloud Controller Manager](https://kubernetes.io/docs/tasks/administer-cluster/running-cloud-controller).
+The Kubernetes Cloud Providers that implement Route Controller can add routes
+to the cloud network routers for the Pod CIDRs of Nodes, and then the cloud
+network is able to route Pod traffic between Nodes. This Route Controller
+functionality is supported by the Cloud Provider implementations of the major
+clouds, including: [AWS](https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/legacy-cloud-providers/aws),
+[Azure](https://github.com/kubernetes-sigs/cloud-provider-azure),
+[GCE](https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/legacy-cloud-providers/gce),
+and [vSphere (with NSX-T)](https://github.com/kubernetes/cloud-provider-vsphere).
+
+* Run a routing protocol or even manually configure routers to add routes to
+the Node network routers. For example, Antrea can work with [kube-router](https://www.kube-router.io)
+and leverage kube-router to advertise Pod CIDRs to routers using BGP. Section
+[Using kube-router for BGP](#using-kube-router-for-bgp) describes how to
+configure Antrea and kube-router to work together.
+
+When the Node network can support forwarding and routing of Pod traffic, Antrea
+can be configured to run in the `NoEncap` mode, by setting the `trafficEncapMode`
+config option of `antrea-agent` to `noEncap`. By default, Antrea performs SNAT
+(source network address translation) for the outbound connections from a Pod to
+outside of the Pod network, using the Node's IP address as the SNAT IP. In the
+`NoEncap` mode, as the Node network knows about Pod IP addresses, the SNAT by
+Antrea might be unnecessary. In this case, you can disable it by setting the
+`noSNAT` config option to `true`. The `trafficEncapMode` and `noSNAT` config
+options are defined in `antrea-agent.conf` of the `antrea` ConfigMap in the
+[Antrea deployment YAML](https://github.com/vmware-tanzu/antrea/blob/main/build/yamls/antrea.yml).
+
+```yaml
+  antrea-agent.conf: |
+    ... ...
+    trafficEncapMode: noEncap
+
+    noSNAT: false # Set to true to disable Antrea SNAT for external traffic
+    ... ...
+```
+
+After changing the options, you can deploy Antrea in `noEncap` mode by applying
+the deployment YAML.
+
+### Using kube-router for BGP
+
+We can run kube-router in advertisement-only mode to advertise Pod CIDRs to the
+peered routers, so the routers can know how to route Pod traffic to the Nodes.
+To deploy kube-router in advertisement-only mode, first download the
+[kube-router DaemonSet template](https://raw.githubusercontent.com/cloudnativelabs/kube-router/v0.4.0/daemonset/generic-kuberouter-only-advertise-routes.yaml):
+
+```bash
+curl -LO https://raw.githubusercontent.com/cloudnativelabs/kube-router/v0.4.0/daemonset/generic-kuberouter-only-advertise-routes.yaml
+```
+
+Then edit the YAML file and set the following kube-router arguments:
+
+```yaml
+- "--run-router=true"
+- "--run-firewall=false"
+- "--run-service-proxy=false"
+- "--enable-cni=false"
+- "--enable-ibgp=false"
+- "--enable-overlay=false"
+- "--enable-pod-egress=false"
+- "--peer-router-ips=<CHANGE ME>"
+- "--peer-router-asns=<CHANGE ME>"
+```
+
+The BGP peers should be configured by specifying the `--peer-router-asns` and
+`--peer-router-ips` parameters. Note, the ASNs and IPs must match the
+configuration on the peered routers. For example:
+
+```yaml
+- "--peer-router-ips=192.168.1.99,192.168.1.100
+- "--peer-router-asns=65000,65000"
+```
+
+Then you can deploy the kube-router DaemonSet with:
+
+```bash
+kubectl apply -f generic-kuberouter-only-advertise-routes.yaml
+```
+
+You can verify that the kube-router Pods are running on the Nodes of your
+Kubernetes cluster by (the cluster in the following example has only two Nodes):
+
+```bash
+$ kubectl -n kube-system get pods -l k8s-app=kube-router
+NAME                READY     STATUS    RESTARTS   AGE
+kube-router-rn4xc   1/1       Running   0          1m
+kube-router-vhrf5   1/1       Running   0          1m
+```
+
+Antrea can be deployed either before or after kube-router, with the `NoEncap`
+mode.

hack/.notableofcontents

@@ -23,6 +23,7 @@ docs/kind.md
 docs/maintainers/build-kubemark.md
 docs/maintainers/getting-started-gif.md
 docs/maintainers/release.md
+docs/noencap-hybrid-modes.md
 docs/octant-plugin-installation.md
 docs/os-issues.md
 docs/ovs-offload.md