Introduce pod-to-pod communication outline for edge docs

linkerd.io/content/2-edge/features/flat-network-multicluster.md

@@ -0,0 +1,45 @@
++++
+title = "Multi-cluster communication in flat networks"
+description = "Multi-cluster communication using pod-to-pod communication inshared network environments."
+aliases = [ "multicluster_support" ]
++++
+
+Linkerd's multi-cluster functionality allows pods to connect to Kubernetes
+services across cluster boundaries in a secure, and fully transparent way. In
+order to be environment agnostic, the multi-cluster extension uses an
+intermediary gateway in each cluster that exports a service to other clusters.
+
+Environments that can be categorized as having a flat network topology may
+choose to route traffic directly, instead of going through the intermediary
+hop. When clusters use a shared network, pods are able to establish TCP
+connections to each other, and consequently send traffic, without requiring any
+`LoadBalancer`-type services or ingress gateways. Linkerd's security,
+reliability and observability guarantees can be extended to environments that
+use a shared network by sending traffic directly to the pods in a multi-cluster
+setting.
+
+{{< fig
+  alt="An architectural diagram comparing hierarchical network mode with the new flat network mode"
+  src="/uploads/2023/07/[email protected]">}}
+
+Using a direct pod-to-pod communication model has a few advantages over the
+gateway model, all while preserving Linkerd's model of separate failure and
+security domains:
+
+* Improved latency for cross-cluster calls
+* Improved security, workload identity for mTLS is preserved across clusters
+  instead of being overridden with the gateway identity
+* Reduced operational costs by lowering the amount of traffic that has to be
+  routed through an ingress gateway
+
+## How does it work
+
+- Kubeconfig generated as part of link
+- Secret, RBAC
+- Controller watches for changes, reads from API server
+- Service can be exported in either mode
+
+## Namespace sameness
+
+- Explanation
+- Link

linkerd.io/content/2-edge/features/multicluster.md

@@ -51,6 +51,40 @@ networks](/2020/02/17/architecting-for-multicluster-kubernetes/#requirement-i-su
 Once these components are installed, Kubernetes `Service` resources that match
 a label selector can be exported to other clusters.
 
+## Multi-cluster for flat networks
+
+Linkerd's multi-cluster extension supports pod-to-pod communication in
+environments that use a shared, flat network. When clusters share the same
+network, pods may establish TCP connections and send traffic to each other
+across cluster boundaries. In such cases, it may be preferred to avoid the
+additional hop represented by the gateway intermediary.
+
+{{< fig
+  alt="An architectural diagram comparing hierarchical network mode with the new flat network mode"
+  src="/uploads/2023/07/[email protected]">}}
+
+Operating the multi-cluster extension in a direct pod-to-pod communication mode
+may provide several advantages over routing traffic through a gateway:
+
+* Improved latency, by avoiding an additional network hop
+* Reduced operational costs that stem from maintaing a `LoadBalancer`-type
+  service for the gateway
+* Finer grained multi-cluster authorization policies, cryptographic identity
+  can be preserved across cluster boundaries, allowing for more expressive
+  policies
+
+Direct pod-to-pod communication does not replace gateways, as a matter of fact,
+the two are not mutually exclusive. Routing configuration is expressed at the
+`Service` resource level. A label selector is used to export services to other
+clusters; the same label selector is shared by the two modes. Services that
+want to benefit from direct pod-to-pod routing can be exported with a
+`remote-discovery` mode, while services whose traffic should go through the
+gateway can continue to use the default label value.
+
+To read more about pod-to-pod communication, you can follow the [getting
+started with flat networks in multi-cluster](<placeholder>) guide, or consult
+the [multi-cluster reference page](../flat-network-multicluster).
+
 ## Headless services
 
 [headless-svc]: https://kubernetes.io/docs/concepts/services-networking/service/#headless-services
@@ -95,6 +129,8 @@ guide](../../tasks/multicluster/) for a walkthrough.
 ## Further reading
 
 * [Multi-cluster installation instructions](../../tasks/installing-multicluster/).
+* [Multi-cluster communication in flat networks](../flat-network-multicluster)
+* [Getting started with multi-cluster in flat networks](<placeholder-for-getting-started>)
 * [Multi-cluster communication with StatefulSets](../../tasks/multicluster-using-statefulsets/).
 * [Architecting for multi-cluster
   Kubernetes](/2020/02/17/architecting-for-multicluster-kubernetes/), a blog