container.training/slides/kube/kubenet.md

Kubernetes network model

• TL,DR:

  Our cluster (nodes and pods) is one big flat IP network.

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• In detail:

• all nodes must be able to reach each other, without NAT

• all pods must be able to reach each other, without NAT

• pods and nodes must be able to reach each other, without NAT

• each pod is aware of its IP address (no NAT)

• Kubernetes doesn't mandate any particular implementation

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Kubernetes network model: the good

• Everything can reach everything

• No address translation

• No port translation

• No new protocol

• Pods cannot move from a node to another and keep their IP address

• IP addresses don't have to be "portable" from a node to another

  (We can use e.g. a subnet per node and use a simple routed topology)

• The specification is simple enough to allow many various implementations

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Kubernetes network model: the less good

• Everything can reach everything

  • if you want security, you need to add network policies

  • the network implementation that you use needs to support them

• There are literally dozens of implementations out there

  (15 are listed in the Kubernetes documentation)

• Pods have level 3 (IP) connectivity, but services are level 4

  (Services map to a single UDP or TCP port; no port ranges or arbitrary IP packets)

• kube-proxy is on the data path when connecting to a pod or container,
  and it's not particularly fast (relies on userland proxying or iptables)

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Kubernetes network model: in practice

• The nodes that we are using have been set up to use Weave

• We don't endorse Weave in a particular way, it just Works For Us

• Don't worry about the warning about kube-proxy performance

• Unless you:

  • routinely saturate 10G network interfaces
  • count packet rates in millions per second
  • run high-traffic VOIP or gaming platforms
  • do weird things that involve millions of simultaneous connections
    (in which case you're already familiar with kernel tuning)

• If necessary, there are alternatives to kube-proxy; e.g. kube-router

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The Container Network Interface (CNI)

• The CNI has a well-defined specification for network plugins

• When a pod is created, Kubernetes delegates the network setup to CNI plugins

• Typically, a CNI plugin will:

  • allocate an IP address (by calling an IPAM plugin)

  • add a network interface into the pod's network namespace

  • configure the interface as well as required routes etc.

• Using multiple plugins can be done with "meta-plugins" like CNI-Genie or Multus

• Not all CNI plugins are equal

  (e.g. they don't all implement network policies, which are required to isolate pods)