Understanding how networking functions within a Kubernetes cluster is crucial for managing and troubleshooting applications. One key aspect is how communication happens between different software containers, known as pods, that are running on the same physical or virtual server, referred to as a node. This type of traffic is called intra-node networking.
While complex networking solutions (CNIs) handle routing traffic across different nodes, a simpler mechanism is often used for traffic staying within a single node. Tools like Kindnet, a CNI plugin often utilized in lightweight Kubernetes environments like Kind (Kubernetes in Docker), demonstrate this simpler approach effectively.
Kindnet employs a technique often described using the concept of PTP, or Point-to-Point virtual links. In the context of Linux networking, this is typically implemented using veth pairs. Imagine a veth pair as a virtual network cable with two ends. One end of this cable is placed inside the network namespace of a specific pod, often appearing as the pod’s main network interface (like eth0). The other end of this virtual cable is connected to a virtual bridge on the node.
Kindnet sets up a central Linux bridge on each node (for example, named kindnet0). As each new pod is created on that node, Kindnet creates a veth pair. One end goes into the pod, and the other end is “plugged into” this central virtual bridge.
Now, consider how traffic flows intra-node. If Pod A needs to send a packet to Pod B, and both pods are running on the same node:
- The packet leaves Pod A via its network interface (
eth0inside the pod). - It travels across the veth pair connected to Pod A.
- The packet arrives at the other end of that veth pair, which is connected to the virtual bridge (
kindnet0). - The virtual bridge examines the destination IP address (which belongs to Pod B).
- Because Pod B is also connected to the same virtual bridge via its own veth pair, the bridge knows exactly which virtual port (the other end of Pod B’s veth pair) to forward the packet to.
- The packet travels across Pod B’s veth pair and arrives at Pod B’s network interface.
This process is highly efficient for intra-node communication because the traffic never needs to leave the node’s local network stack. It simply traverses the virtual bridge created by the CNI plugin like Kindnet, moving directly from one pod’s virtual interface to another via the bridge. This reliance on veth pairs and a virtual bridge provides a simple, performant, and easy-to-understand method for handling pod-to-pod communication when those pods reside together on the same node.
Source: https://itnext.io/inside-intra-node-pod-traffic-in-kubernetes-how-kindnet-with-ptp-moves-packets-ffbbc07612b7?source=rss—-5b301f10ddcd—4
