add blog for feature LoadBalancerIPMode

Author: RyanAoh

content/en/blog/_posts/2023-12-18-kubernetes-1-29-feature-loadbalancer-ip-mode-alpha.md

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+---
+layout: blog
+title: "Kubernetes 1.29: New (alpha) Feature, Load Balancer IP Mode for Services"
+date: 2023-12-18
+slug: kubernetes-1-29-feature-loadbalancer-ip-mode-alpha
+---
+
+**Author:** [Aohan Yang](https://github.com/RyanAoh)
+
+This blog introduces a new alpha feature in Kubernetes 1.29. 
+It provides a configurable approach to define how Service implementations, 
+exemplified in this blog by kube-proxy, 
+handle traffic from pods to the Service, within the cluster.
+
+## Background
+
+In older Kubernetes releases, the kube-proxy would intercept traffic that was destined for the IP
+address associated with a Service of `type: LoadBalancer`. This happened whatever mode you used
+for `kube-proxy`. 
+The interception implemented the expected behavior (traffic eventually reaching the expected
+endpoints behind the Service). The mechanism to make that work depended on the mode for kube-proxy;
+on Linux, kube-proxy in iptables mode would redirecting packets directly to the endpoint; in ipvs mode,
+kube-proxy would configure the load balancer's IP address to one interface on the node. 
+The motivation for implementing that interception was for two reasons:
+
+1. **Traffic path optimization:** Efficiently redirecting pod traffic - when a container in a pod sends an outbound
+   packet that is destined for the load balancer's IP address - 
+   directly to the backend service by bypassing the load balancer.
+  
+2. **Handling load balancer packets:** Some load balancers send packets with the destination IP set to 
+the load balancer's IP address. As a result, these packets need to be routed directly to the correct backend (which 
+might not be local to that node), in order to avoid loops.
+  
+## Problems
+
+However, there are several problems with the aforementioned behavior:
+
+1. **[Source IP](https://github.com/kubernetes/kubernetes/issues/79783):** 
+    Some cloud providers use the load balancer's IP as the source IP when 
+    transmitting packets to the node. In the ipvs mode of kube-proxy, 
+    there is a problem that health checks from the load balancer never return. This occurs because the reply packets 
+    would be forward to the local interface `kube-ipvs0`(where the load balancer's IP is bound to) 
+    and be subsequently ignored.
+  
+2. **[Feature loss at load balancer level](https://github.com/kubernetes/kubernetes/issues/66607):**
+    Certain cloud providers offer features(such as TLS termination, proxy protocol, etc.) at the
+    load balancer level.
+    Bypassing the load balancer results in the loss of these features when the packet reaches the service
+    (leading to protocol errors).
+  
+
+Even with the new alpha behaviour disabled (the default), there is a 
+[workaround](https://github.com/kubernetes/kubernetes/issues/66607#issuecomment-474513060) 
+that involves setting `.status.loadBalancer.ingress.hostname` for the Service, in order 
+to bypass kube-proxy binding. 
+But this is just a makeshift solution.
+
+## Solution
+
+In summary, providing an option for cloud providers to disable the current behavior would be highly beneficial.
+
+To address this, Kubernetes v1.29 introduces a new (alpha) `.status.loadBalancer.ingress.ipMode` 
+field for a Service.
+This field specifies how the load balancer IP behaves and can be specified only when 
+the `.status.loadBalancer.ingress.ip` field is also specified.
+
+Two values are possible for `.status.loadBalancer.ingress.ipMode`: `"VIP"` and `"Proxy"`.
+The default value is "VIP", meaning that traffic delivered to the node 
+with the destination set to the load balancer's IP and port will be redirected to the backend service by kube-proxy.
+This preserves the existing behavior of kube-proxy. 
+The "Proxy" value is intended to prevent kube-proxy from binding the load balancer's IP address 
+to the node in both ipvs and iptables modes. 
+Consequently, traffic is sent directly to the load balancer and then forwarded to the destination node. 
+The destination setting for forwarded packets varies depending on how the cloud provider's load balancer delivers traffic:
+
+- If the traffic is delivered to the node then DNATed to the pod, the destination would be set to the node's IP and node port;
+- If the traffic is delivered directly to the pod, the destination would be set to the pod's IP and port.
+
+## Usage
+
+Here are the necessary steps to enable this feature:
+
+- Download the [latest Kubernetes project](https://kubernetes.io/releases/download/) (version `v1.29.0` or later).
+- Enable the feature gate with the command line flag `--feature-gates=LoadBalancerIPMode=true` 
+on kube-proxy, kube-apiserver, and cloud-controller-manager.
+- For Services with `type: LoadBalancer`, set `ipMode` to the appropriate value. 
+This step is likely handled by your chosen cloud-controller-manager during the `EnsureLoadBalancer` process.
+
+## More information
+
+- Read [Specifying IPMode of load balancer status](/docs/concepts/services-networking/service/#load-balancer-ip-mode).
+- Read [KEP-1860](https://kep.k8s.io/1860) - [Make Kubernetes aware of the LoadBalancer behaviour](https://github.com/kubernetes/enhancements/tree/b103a6b0992439f996be4314caf3bf7b75652366/keps/sig-network/1860-kube-proxy-IP-node-binding#kep-1860-make-kubernetes-aware-of-the-loadbalancer-behaviour) _(sic)_.
+
+## Getting involved
+
+Reach us on [Slack](https://slack.k8s.io/): [#sig-network](https://kubernetes.slack.com/messages/sig-network), 
+or through the [mailing list](https://groups.google.com/forum/#!forum/kubernetes-sig-network).
+
+## Acknowledgments
+
+Huge thanks to [@Sh4d1](https://github.com/Sh4d1) for the original KEP and initial implementation code. 
+I took over midway and completed the work. Similarly, immense gratitude to other contributors 
+who have assisted in the design, implementation, and review of this feature (alphabetical order):
+
+- [@aojea](https://github.com/aojea)
+- [@danwinship](https://github.com/danwinship)
+- [@sftim](https://github.com/sftim)
+- [@tengqm](https://github.com/tengqm)
+- [@thockin](https://github.com/thockin)
+- [@wojtek-t](https://github.com/wojtek-t)