add blog document for feature ServiceNodePortStaticSubrange

content/en/blog/_posts/2023-03-08-nodeport-dynamic-and-static-allocation.md

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+---
+layout: blog
+title: "Kubernetes 1.27: Avoid Collisions Assigning ports to Nodeport Services"
+date: 2023-03-08
+slug: nodeport-dynamic-and-static-allocation
+---
+
+**Author:** xuzhenglun (Alibaba)
+
+In Kubernetes, the Service can be used to provide a unified traffic entry point for 
+applications running on a set of Pods. Clients can use the virtual IP address provided
+by the Service for access, and Kubernetes provides load balancing for traffic accessing
+different back-end Pods, but ClusterIP-type Service is limited to providing access to
+nodes within the cluster, and traffic from outside the cluster cannot be routed.
+One way to solve this problem is to use NodePort-type Service, which maps a VIP's port
+to a specific port of all nodes in the cluster, thus redirecting traffic from the
+outside to the inside of the cluster.
+
+## How NodePort's ports are allocated? 
+
+When a NodePort-type Service is created, its corresponding port is allocated in two ways.
+
+**Dynamic** : If the Service type is NodePort and the corresponding `service.spec.ports.nodePort`
+is not specified, the Kubernetes controlpalne will automatically allocat a unused port 
+to it at creation time.
+
+**Static** : In addition to the dynamic auto-assignment described above, you can also
+explicitly assign a port that is within the nodeport port configuration.
+
+the `service.spec.ports.nodePort` must be unique for each NodePort-type Service across the cluster.
+Attempting to create a NodePort-type Service with a port already allocated will return an error.
+
+## Why do you need to reserve ports of NodePort-type Service? 
+Sometimes, you may want to have the NodePort-type Service running on well-known ports
+so that other components and users inside and outside the cluster can use them.
+
+In some complex cluster deployments with a mix of K8S nodes and non-K8S nodes, it may be
+necessary to rely on some pre-defined port for communicating. In particular, some fundamental
+components cannot rely on the VIPs which provided by LoadBalancer-type  services,
+because the load balancing itself that provides the VIPs may also rely on these fundamental components.
+
+Now suppose we need to expose the Minio object storage service on K8S to services located on non-K8S nodes,
+and the agreed port is `30009`, we need to create a Service as follows:
+
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  labels:
+    k8s-app: minio
+  name: minio
+  namespace: kube-system
+spec:
+  ports:
+  - name: http
+    nodePort: 30009
+    port: 9000
+    protocol: TCP
+    targetPort: 9000
+  selector:
+    app: minio
+  type: NodePort
+```
+
+However, as mentioned before, if the port `30009` required for the minio Service is not reserved,
+and another NodePort Service is created and dynamically allocated before or concurrently with minio
+Service, `30009` can be allocated to those NodePort Service, and minio Service will fail to be
+created due to a port conflict.
+
+## How can you avoid NodePort-type Service port conflicts? 
+In the Kubernetes 1.24, Service ClusterIP had already divided the CIDRs into two blocks, using different
+allocation policies to reduce the risk of conflicts. In Kubernetes 1.27, a similar policy can be adopted
+for NodePort. You can enable a new featuregate `ServiceNodePortStaticSubrange`. Turning this on allows you
+to use a different port allocation strategy for NodePort-type Services, and reducing the risk of collision.
+
+The port range for `NodePort` will be divided, based on the formula `min(max(16, nodeport-size / 32), 128)`,
+which can be described as _never less than 16 or more than 128 with a graduated step between them_.
+
+Dynamic port assignment will use the upper band by default, once this has been exhausted it will use the lower range.
+This will allow users to use static allocations on the lower band with a low risk of collision.
+
+## Examples:
+
+### default range: 30000-32767
+service-node-port-range: 30000-32767
+Band Offset: `min(max(16, 2768/32), 128)` = `min(max(16, 86), 128)` = `min(86, 128)` = 86
+Static band start: 30000
+Static band end: 30085
+Range end: 32767
+
+{{< mermaid >}}
+pie showData
+    title 30000-32767
+    "Static" : 86
+    "Dynamic" : 2682
+{{< /mermaid >}}
+
+### very small range: 30000-30015
+service-node-port-range: 30000-30015
+Band Offset: 0
+Static band start: 30000
+Static band end: 30000
+Range end: 30015
+
+{{< mermaid >}}
+pie showData
+    title 30000-30015
+    "Static" : 0
+    "Dynamic" : 16
+{{< /mermaid >}}
+
+### small(lower boundary) range: 30000-30127
+service-node-port-range: 30000-30127
+Band Offset: `min(max(16, 128/32), 128)` = `min(max(16, 4), 128)` = `min(16, 128)` = 16
+Static band start: 30000
+Static band end: 30015
+Range end: 30127
+
+{{< mermaid >}}
+pie showData
+    title 30000-30127
+    "Static" : 16
+    "Dynamic" : 112
+{{< /mermaid >}}
+
+### large(upper boundary) range: 30000-34095
+service-node-port-range: 30000-34095
+Band Offset: `min(max(16, 4096/32), 128)` = `min(max(16, 128), 128)` = `min(128, 128)` = 128
+Static band start: 30000
+Static band end: 30127
+Range end: 34095
+
+{{< mermaid >}}
+pie showData
+    title 30000-34095
+    "Static" : 128
+    "Dynamic" : 3968
+{{< /mermaid >}}
+
+### very large range: 30000-38191
+service-node-port-range: 30000-38191
+Band Offset: `min(max(16, 8192/32), 128)` = `min(max(16, 256), 128)` =` min(256, 128)` = 128
+Static band start: 30000
+Static band end: 30127
+Range end: 38191
+
+{{< mermaid >}}
+pie showData
+    title 30000-38191
+    "Static" : 128
+    "Dynamic" : 3968
+{{< /mermaid >}}