Surviving the Packet Storm: Kubernetes Networking & Latency in 2018

It is 3:00 AM. Your Prometheus alerts are firing. The latency on your microservices architecture just spiked from 20ms to 400ms, but CPU usage is normal. Welcome to the invisible hell of Kubernetes networking.

As we race toward the May 25th GDPR deadline, every CTO in Oslo is pushing to migrate legacy monoliths into containerized environments to ensure better compliance and isolation. But here is the reality check: Kubernetes (k8s) is not magic. It is a complex beast of distributed state management, and its networking model is often the first thing to break under load.

If you are deploying Kubernetes 1.10 on bare metal or VPS infrastructure, you are likely relying on `iptables` and overlay networks. In this deep dive, we are going to dissect how packets actually move between Pods, why your CNI plugin choice matters, and how to optimize for the Norwegian network topology.

The Hidden Cost of Overlay Networks (VXLAN)

Most default installations—whether you are using `kubeadm` or `kops`—will default to an overlay network. When a packet leaves Pod A destined for Pod B on a different node, it gets encapsulated. If you use Flannel with the VXLAN backend, your data is wrapped in a UDP packet.

This encapsulation costs CPU cycles. It also reduces the Maximum Transmission Unit (MTU). If your host interface (eth0) has an MTU of 1500, the CNI overlay usually claims 50 bytes for headers. That leaves your Pods with an MTU of 1450.

The Problem: If your legacy application tries to push a full 1500-byte packet, it gets fragmented or dropped if the DF (Don't Fragment) bit is set. I recently debugged a Magento cluster hosted in a data center near Bergen where the checkout page would hang indefinitely. The cause? MTU mismatch between the VPS interface and the Docker bridge.

Diagnosing MTU Issues

Don't guess. Check the interface configuration inside the container and on the host.

# On the Host Node
ip link show eth0 | grep mtu
# Output: mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000

# Inside the Pod
kubectl exec -it frontend-7689d96695-x4z2q -- ip link show eth0
# Output: mtu 1450 qdisc noqueue state UP mode DEFAULT group default qlen 1000

Pro Tip: If you are running high-throughput applications, consider using a CNI that supports Layer 3 routing via BGP, like Calico. It avoids the encapsulation overhead entirely by routing packets natively. However, this requires your underlying VPS provider to support BGP peering or disable source/destination checks on the network fabric.

Service Discovery: The `iptables` Bottleneck

In Kubernetes 1.10, `kube-proxy` defaults to `iptables` mode. For every Service you create, `kube-proxy` writes a set of rules to direct traffic to the backing Pods. This works fine for 50 services. It becomes a nightmare at 5,000 services.

`iptables` is an O(N) sequential list. Every packet has to traverse the chain to find a match. We have seen clusters where network latency degraded simply because the kernel was spending too much time traversing huge rule sets.

Here is how you can inspect the sheer volume of rules generated by a modest cluster:

# Count the number of K8s related rules
sudo iptables-save | grep KUBE-SVC | wc -l

# View the probability balancing (how K8s does round-robin load balancing)
sudo iptables -t nat -L KUBE-SVC-XYZ123 --line-numbers

If you see thousands of lines, you are hitting the scalability limit. While IPVS (IP Virtual Server) is currently in beta in 1.10, it is the future for high-scale clusters. For now, if you are stuck on `iptables`, ensure your underlying VPS has high single-core clock speeds to process these rules faster. This is where "budget" VPS providers fail—they give you noisy neighbors that steal CPU cycles, causing packet processing jitter.

Ingress Controllers: NGINX Performance Tuning

Once traffic enters your cluster, it usually hits an Ingress Controller. The community NGINX ingress is the industry standard. However, the default config is generic. For a low-latency setup targeting users in Scandinavia, you need to tune the buffers and keepalives.

Here is a snippet for a `ConfigMap` to optimize NGINX for high concurrency on a CoolVDS NVMe instance:

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-configuration
  namespace: ingress-nginx
  labels:
    app.kubernetes.io/name: ingress-nginx
    app.kubernetes.io/part-of: ingress-nginx
data:
  worker-processes: "auto"
  worker-connections: "10240"
  keep-alive: "75"
  upstream-keepalive-connections: "32"
  client-body-buffer-size: "16k"
  ssl-ciphers: "ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256"
  ssl-protocols: "TLSv1.2"

Note the explicit definition of `TLSv1.2` and modern ciphers. With GDPR enforcement starting in weeks, Datatilsynet (The Norwegian Data Protection Authority) will not look kindly on weak encryption standards.

The Hardware Reality: Why "Cloud" Isn't Enough

You can tune software all day, but you cannot code your way out of bad hardware. Kubernetes is I/O hungry. etcd (the cluster brain) requires extremely low write latency to maintain consensus. If `fsync` takes too long because your provider is using spinning rust or crowded SATA SSDs, your API server will start timing out.

At CoolVDS, we built our infrastructure on KVM because it provides the kernel isolation necessary for stable Kubernetes networking. We don't oversubscribe our CPU cores, meaning when your CNI plugin needs to encapsulate a packet, the cycle is available instantly.

Data Sovereignty and GDPR

Latency isn't just about speed; it's about geography. Routing traffic through Frankfurt or Amsterdam adds milliseconds. For a Norwegian user base, you want your packets terminating in Oslo. Furthermore, with the new privacy regulations, knowing exactly where your physical bytes reside is mandatory.

Hosting on US-owned hyper-scale clouds introduces legal complexities regarding the Cloud Act and data access. Running your K8s cluster on local, sovereign Norwegian infrastructure like CoolVDS simplifies your compliance posture significantly.

Final Thoughts

Building a robust Kubernetes cluster involves more than just `kubectl apply -f`. You must understand the path of the packet. From the MTU inside the VXLAN tunnel to the `iptables` rules on the host, every layer adds potential latency. Don't let your infrastructure be the bottleneck.

Need a K8s-ready foundation? Deploy a high-performance KVM instance with NVMe storage on CoolVDS today and keep your latency low and your data legal.