Kubernetes Networking Deep Dive: Surviving the Packet Jungle in Production

Let’s be honest. You didn't migrate to Kubernetes because you love managing iptables rules manually. You did it for the promise of self-healing infrastructure and seamless scaling. But then reality hit. You deployed a cluster, pods started flapping, and suddenly you're staring at a routing table at 3 AM trying to figure out why Service A can't talk to Service B across nodes.

In 2017, Kubernetes (k8s) is maturing rapidly with the release of 1.6 just days ago, but the networking layer remains a black box for many. The flat pod network model sounds simple on paper: "Every pod gets an IP." Achieving that in a restricted VPS environment without BGP access or direct Layer 2 switching? That's where the headaches begin.

I’ve seen clusters melt not because of CPU starvation, but because of poor network planning. Latency kills microservices. If your underlying infrastructure introduces jitter, your overlay network amplifies it. Let’s rip open the hood of the Container Network Interface (CNI) and fix your cluster before it wakes you up again.

The CNI Battlefield: Flannel vs. Calico

The first decision you make—often blindly—is your CNI plugin. This determines how packets move between hosts. In the Nordic hosting market, where we deal with strict data boundaries and high performance expectations, you cannot just pick the default.

Flannel is the common starter choice. It’s simple. It typically uses VXLAN to encapsulate Layer 2 frames inside UDP packets. It creates an overlay network that sits on top of your physical network.

The Trade-off: Encapsulation adds overhead. The CPU has to wrap and unwrap every packet. On a budget VPS with "noisy neighbors" stealing CPU cycles, this causes measurable latency spikes.

Calico takes a different approach. It uses pure Layer 3 routing (BGP). No encapsulation (unless you force IP-in-IP). It’s faster, leaner, and allows for Network Policies—crucial for GDPR compliance preparation as we look toward the 2018 enforcement date.

Configuring Calico for Performance

If you are running on CoolVDS, where we provide KVM isolation and a stable Layer 2 environment, you can push Calico to its limits. Here is how you define a `calico.yaml` to ensure it plays nice with your node's MTU:

# Calico Version v2.0 (2017 era configuration)
- name: CALICO_IPV4POOL_IPIP
  value: "Always" # Use 'CrossSubnet' if nodes are L2 connected for speed
- name: FELIX_IPINIPMTU
  value: "1440" # Critical: Account for physical interface MTU

Pro Tip: Never assume the MTU is 1500 inside your overlay. If your VPS provider uses QinQ or other VLAN tagging, the actual payload size drops. Set your CNI MTU to 1450 or lower to avoid packet fragmentation. Fragmentation is the silent killer of throughput.

The Iptables Nightmare: Kube-Proxy

Kubernetes uses `kube-proxy` to handle Service abstraction. In versions 1.5 and 1.6, the default mode is `iptables`. This is a massive improvement over the old userspace mode, but it has a scaling limit.

When you create a Service, `kube-proxy` writes rules to nat incoming traffic to the pod IPs. If you have 5,000 services, you have tens of thousands of iptables rules. Every packet has to traverse this list sequentially (O(n) complexity).

Here is what happens when you debug a node with just a moderate load. Run this and weep:

iptables-save | grep KUBE-SVC | wc -l
# If this number is over 20,000, you are adding latency to every connection.

If you see latency spiking on service discovery, check your connection tracking table. The Linux kernel has a limit on how many connections it tracks. When that table fills up, packets are dropped silently.

Tuning the Kernel for High Traffic

Don't deploy a production cluster on a standard Linux distro without tuning `sysctl.conf`. You need to increase the conntrack limits. On a CoolVDS instance, we recommend these settings for k8s nodes:

# /etc/sysctl.d/k8s.conf
net.netfilter.nf_conntrack_max = 131072
net.ipv4.tcp_keepalive_time = 600
net.ipv4.ip_forward = 1
net.bridge.bridge-nf-call-iptables = 1

Apply it with `sysctl -p`. The `bridge-nf-call-iptables` flag is non-negotiable; without it, bridged traffic (your pods) bypasses iptables, breaking your network policies and security.

The Latency Factor: Why Infrastructure Matters

You can optimize your CNI and kernel all day, but you cannot fix physics. Kubernetes is a distributed system. It relies heavily on etcd for state. Etcd is sensitive to disk write latency (fsync) and network round-trip time.

If your VPS provider oversells their storage or network, your etcd cluster will start timing out leader elections. The result? The API server stops responding. The cluster goes into a "split-brain" scenario.

Benchmarking Disk for Etcd

Before installing K8s, benchmark your disk I/O. We need low latency writes.

# Use fio to test sync write latency
fio --name=etcd_test --rw=write --ioengine=sync --fdatasync=1 \
    --size=100m --bs=2300 --runtime=60

If the 99th percentile latency is above 10ms, your cluster will be unstable. This is why CoolVDS uses pure NVMe storage. In our benchmarks, we consistently see write latencies under 1ms, ensuring etcd never chokes, even during heavy deployments.

Ingress: Exposing Services to the World

We are past the days of NodePort. You need an Ingress Controller. In 2017, the NGINX Ingress Controller is the standard. It terminates SSL and routes traffic based on host headers.

For a Norwegian e-commerce site targeting users in Oslo, every millisecond of TLS handshake matters. You should be using HTTP/2 (which NGINX supports) to multiplex requests.

Here is a snippet for your Ingress resource to enable SSL passthrough or optimization:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: web-ingress
  annotations:
    kubernetes.io/ingress.class: "nginx"
    ingress.kubernetes.io/ssl-redirect: "true"
    # Increase buffer size for large headers (common with OAuth)
    ingress.kubernetes.io/proxy-buffer-size: "16k"
spec:
  tls:
  - hosts:
    - shop.example.no
    secretName: tls-secret
  rules:
  - host: shop.example.no
    http:
      paths:
      - path: /
        backend:
          serviceName: web-svc
          servicePort: 80

Local Nuances: The Nordic Edge

Hosting in Norway isn't just about patriotism; it's about physics and law. With the EU Privacy Shield framework under scrutiny and the Datatilsynet (Norwegian Data Protection Authority) keeping a close watch, data sovereignty is critical. Keeping traffic inside the country or within the EEA reduces legal headaches.

Furthermore, routing traffic through NIX (Norwegian Internet Exchange) ensures that a request from a user in Bergen to your server in Oslo doesn't accidentally route through Frankfurt or London. That detour adds 30-40ms of latency.

CoolVDS peers directly at major Nordic exchanges. When you deploy a Kubernetes cluster on our infrastructure, you aren't just getting raw compute. You are getting a direct line to your customers.

Conclusion: Build on Solid Ground

Kubernetes networking is complex, but it obeys the laws of networking. Packets need routes. Overlays need CPU. Etcd needs fast disks.

Don't let a generic cloud provider throttle your innovation with high-latency storage or unstable networks. Your architecture deserves a foundation that respects the engineering rigor you put into it.

Ready to stabilize your production workloads? Stop fighting with noisy neighbors. Deploy a high-performance, NVMe-backed instance on CoolVDS today and see what sub-millisecond latency does for your cluster stability.