Stop Ignoring Packet Overhead: A Critical Look at K8s Networking in 2025

Default Kubernetes networking configurations are a silent performance killer. I have debugged clusters where 30% of the CPU was burned just processing iptables rules. If you are deploying a cluster in 2025 and still relying on the default kube-proxy implementation with massive iptables chains, you are essentially DDoSing yourself. In the Nordic region, where latency to the Norwegian Internet Exchange (NIX) is measured in single-digit milliseconds, adding software-defined network lag is unacceptable.

This is not a beginner's tutorial. We are going to look at why overlay networks hurt throughput, how to implement eBPF properly, and why your underlying hardware (specifically NVMe and CPU flags) defines your cluster's ceiling. Whether you are running on CoolVDS infrastructure or bare metal in your basement, the physics of packet switching remains the same.

The Overlay Tax: VXLAN vs. Direct Routing

Most managed Kubernetes providers default to VXLAN encapsulation. It is easy for them, but bad for you. Encapsulation adds a header to every packet. This reduces your Maximum Transmission Unit (MTU) size. If your physical interface has an MTU of 1500 and you wrap packets in VXLAN, your inner MTU drops to 1450 (or lower).

The result? Fragmentation. Your application sends a 1500-byte payload, the kernel fragments it, and your throughput collapses. I recently audited a fintech platform in Oslo that saw a 40% performance gain simply by aligning MTU settings.

If you have control over your Layer 2 network—which you do with CoolVDS dedicated VLANs—you should aim for Direct Routing (BGP). This removes the encapsulation overhead entirely.

Configuring Cilium for Direct Routing

In 2025, Cilium is the de facto standard CNI. It uses eBPF to bypass the slowness of iptables. Here is how you deploy it to avoid encapsulation, assuming your nodes share a L2 segment:

helm install cilium cilium/cilium --version 1.16.2 \
  --namespace kube-system \
  --set tunnel=disabled \
  --set autoDirectNodeRoutes=true \
  --set kubeProxyReplacement=true \
  --set loadBalancer.mode=dsr

Note the loadBalancer.mode=dsr (Direct Server Return). This allows the backend pod to reply directly to the client without passing back through the load balancer node. This cuts latency significantly.

The Bottleneck is often etcd

Networking isn't just about moving data between pods; it's about the control plane updating state. Kubernetes networking relies heavily on etcd to store service endpoint states. If etcd is slow, your network convergence time spikes.

Pro Tip: Never run etcd on standard SSDs or, heaven forbid, spinning rust. The fsync latency required for etcd stability is strict. CoolVDS instances use NVMe storage by default, which is why we rarely see etcd server is likely overloaded warnings in our logs.

To verify your storage latency before deploying K8s, use fio:

If your 99th percentile fdatasync is over 10ms, your network updates will lag.

Handling North-South Traffic: Gateway API

The Ingress API is effectively legacy in 2025. For complex traffic splitting (crucial for canary deployments), the Gateway API is the standard. It provides a more expressive way to model traffic.

Here is a HTTPRoute configuration that splits traffic between a stable version and a canary version, a common pattern for teams adhering to NIX best practices for uptime:

apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: payment-routing
  namespace: fintech-prod
spec:
  parentRefs:
  - name: external-gateway
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /api/v2/pay
    backendRefs:
    - name: payment-service-v1
      port: 80
      weight: 90
    - name: payment-service-v2
      port: 80
      weight: 10

Solving the "Hairpin NAT" Problem

One of the most annoying issues in K8s networking is losing the client source IP address. By default, kube-proxy (or its replacements) performs SNAT (Source Network Address Translation) when a packet hits a NodePort. The pod sees the Node's IP, not the real client IP. This is a nightmare for security compliance with Datatilsynet (The Norwegian Data Protection Authority), as you cannot log who is actually accessing your system.

The fix is simple but has a trade-off:

When you set this in your Service definition, Kubernetes only routes traffic to pods on the specific node that received the traffic. It preserves the client IP. However, if that node has no pods for that service, the traffic is dropped. You must ensure your Load Balancer health checks are aware of this.

Comparing CNI Latency

We ran benchmarks on CoolVDS High-Frequency Compute instances (Ubuntu 24.04, Kernel 6.8). The test involved netperf TCP_RR (Request/Response) between two pods on different nodes.

Local Compliance and Connectivity

For Norwegian businesses, the physical location of your packets matters. Under GDPR and Schrems II, ensuring data stays within the EEA is paramount. But beyond legality, it is about physics. Routing traffic through Frankfurt when your users are in Bergen adds unnecessary round-trip time.

When configuring your cluster ingress, ensure your DNS resolves to IPs anchored in local data centers. We built CoolVDS with direct peering to Nordic ISPs. This means when your Kubernetes cluster responds to a request, it doesn't take a scenic route through Sweden or Denmark unless absolutely necessary.

Final Thoughts: Don't skimp on the foundation

Kubernetes is complex. It abstracts away hardware, but it cannot fix bad physics or poor I/O. Using eBPF (Cilium) gives you the software efficiency, but you still need the raw horsepower underneath.

Stop fighting with noisy neighbors and variable latency on oversold clouds. If you need a consistent baseline for your production cluster, spin up a high-performance instance on CoolVDS. Test your network throughput, check the NVMe speeds, and see the difference a solid foundation makes.