The Kubernetes networking model is based on a flat network space where every Pod can communicate with every other Pod directly, without Network Address Translation (NAT). This simplified networking model provides a consistent and predictable environment for applications, regardless of which node they are running on. To achieve this, Kubernetes imposes a few key requirements:
Every Pod gets its own IP address.
Pods can communicate with all other Pods without NAT.
Nodes can communicate with all Pods without NAT.
Here's a breakdown of how Pods communicate with each other across nodes in the Kubernetes networking model:
1. Pod IP Addresses:
Each Pod is assigned a unique IP address from a cluster-wide network. This IP address is used for all communication between Pods.
2. Container Network Interface (CNI):
Kubernetes uses the Container Network Interface (CNI) to configure networking for containers. CNI is a standard interface that allows different networking plugins to be used with Kubernetes. These plugins are responsible for setting up the network namespace for each Pod, assigning it an IP address, and configuring routing rules.
Common CNI plugins include:
Calico: Provides a secure networking solution with network policy enforcement.
Cilium: Uses eBPF to provide high-performance networking and security.
Weave Net: A simple and easy-to-use networking solution that works without requiring a separate datastore.
Flannel: A basic networking solution that uses a virtual network overlay.
3. Network Namespaces:
Each Pod has its own network namespace, which provides it with its own isolated network environment. This includes its own IP address, routing table, and network interfaces.
4. Virtual Ethernet (veth) Pairs:
CNI plugins typically use virtual ethernet (veth) pairs to connect the Pod's network namespace to the node's network namespace. One end of the veth pair resides in the Pod's network namespace, and the other end resides in the node's network namespace.
5. Routing Tables:
The CNI plugin configures routing tables on each node to ensure that traffic destined for a Pod's IP address is routed to the correct node. This is typically done by adding routes that direct traffic to the veth interface associated with the Pod.
6. Inter-Node Communication:
When a Pod on one node needs to communicate with a Pod on another node, the traffic is routed as follows:
The traffic originates from the source Pod and is sent to its default gateway, which is the veth interface in the Pod's network namespace.
The traffic is routed to the node's main network interface based on the node's routing table. The routing table contains entries that direct traffic destined for other Pods to the appropriate node.
The traffic is sent across the network to the destination node.
On the destination node, the traffic is routed to the veth interface associated with the destination Pod.
The traffic is delivered to the destination Pod.
Example:
Let's say you have two Pods:
Pod A: Running on Node 1 with IP address 10.1.0.10
Pod B: Running on Node 2 with IP address 10.2.0.20
When Pod A wants to communicate with Pod B, the following steps occur:
Pod A sends traffic to its default gateway (veth interface).
Node 1's routing table directs the traffic destined for 10.2.0.20 to Node 2's IP address.
The traffic is sent across the network to Node 2.
Node 2's routing table directs the traffic to the ....
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