Routing

Encapsulation

When no configuration is provided, Cilium automatically runs in this mode as it is the mode with the fewest requirements on the underlying networking infrastructure.

In this mode, all cluster nodes form a mesh of tunnels using the UDP-based encapsulation protocols VXLAN or Geneve. All traffic between Cilium nodes is encapsulated.

Requirements on the network

  • Encapsulation relies on normal node to node connectivity. This means that if Cilium nodes can already reach each other, all routing requirements are already met.

  • The underlying network and firewalls must allow encapsulated packets:

    Encapsulation Mode Port Range / Protocol
    VXLAN (Default) 8472/UDP
    Geneve 6081/UDP

Advantages of the model

Simplicity
The network which connects the cluster nodes does not need to be made aware of the PodCIDRs. Cluster nodes can spawn multiple routing or link-layer domains. The topology of the underlying network is irrelevant as long as cluster nodes can reach each other using IP/UDP.
Addressing space
Due to not depending on any underlying networking limitations, the available addressing space is potentially much larger and allows to run any number of pods per node if the PodCIDR size is configured accordingly.
Auto-configuration
When running together with an orchestration system such as Kubernetes, the list of all nodes in the cluster including their associated allocation prefix node is made available to each agent automatically. New nodes joining the cluster will automatically be incorporated into the mesh.
Identity context
Encapsulation protocols allow for the carrying of metadata along with the network packet. Cilium makes use of this ability to transfer metadata such as the source security identity. The identity transfer is an optimization designed to avoid one identity lookup on the remote node.

Disadvantages of the model

MTU Overhead
Due to adding encapsulation headers, the effective MTU available for payload is lower than with native-routing (50 bytes per network packet for VXLAN). This results in a lower maximum throughput rate for a particular network connection. This can be largely mitigated by enabling jumbo frames (50 bytes of overhead for each 1500 bytes vs 50 bytes of overhead for each 9000 bytes).

Native-Routing

The native routing datapath is enabled with tunnel: disabled and enables the native packet forwarding mode. The native packet forwarding mode leverages the routing capabilities of the network Cilium runs on instead of performing encapsulation.

../../../_images/native_routing.png

In native routing mode, Cilium will delegate all packets which are not addressed to another local endpoint to the routing subsystem of the Linux kernel. This means that the packet will be routed as if a local process would have emitted the packet. As a result, the network connecting the cluster nodes must be capable of routing PodCIDRs.

Cilium automatically enables IP forwarding in the Linux kernel when native routing is configured.

Requirements on the network

  • In order to run the native routing mode, the network connecting the hosts on which Cilium is running on must be capable of forwarding IP traffic using addresses given to pods or other workloads.
  • The Linux kernel on the node must be aware on how to forward packets of pods or other workloads of all nodes running Cilium. This can be achieved in two ways:
    1. The node itself does not know how to route all pod IPs but a router exists on the network that knows how to reach all other pods. In this scenario, the Linux node is configured to contain a default route to point to such a router. This model is used for cloud provider network integration. See Google Cloud, AWS ENI, and Azure IPAM (beta) for more details.
    2. Each individual node is made aware of all pod IPs of all other nodes and routes are inserted into the Linux kernel routing table to represent this. If all nodes share a single L2 network, then this can be taken care of by enabling the option auto-direct-node-routes: true. Otherwise, an additional system component such as a BGP daemon must be run to distribute the routes. See the guide Using kube-router to run BGP on how to achieve this using the kube-router project.

Configuration

The following configuration options must be set to run the datapath in native routing mode:

  • tunnel: disabled: Enable native routing mode.
  • enable-endpoint-routes: true: Enable per-endpoint routing on the node
  • native-routing-cidr: x.x.x.x/y: Set the CIDR in which native routing can be performed.

AWS ENI

The AWS ENI datapath is enabled when Cilium is run with the option --ipam=eni. It is a special purpose datapath that is useful when running Cilium in an AWS environment.

Advantages of the model

  • Pods are assigned ENI IPs which are directly routable in the AWS VPC. This simplifies communication of pod traffic within VPCs and avoids the need for SNAT.
  • Pod IPs are assigned a security group. The security groups for pods are configured per node which allows to create node pools and give different security group assignments to different pods. See section AWS ENI for more details.

Disadvantages of this model

  • The number of ENI IPs is limited per instance. The limit depends on the EC2 instance type. This can become a problem when attempting to run a larger number of pods on very small instance types.
  • Allocation of ENIs and ENI IPs requires interaction with the EC2 API which is subject to rate limiting. This is primarily mitigated via the operator design, see section AWS ENI for more details.

Architecture

Ingress

  1. Traffic is received on one of the ENIs attached to the instance which is represented on the node as interface ethN.

  2. An IP routing rule ensures that traffic to all local pod IPs is done using the main routing table:

    20:      from all to 192.168.105.44 lookup main
    
  3. The main routing table contains an exact match route to steer traffic into a veth pair which is hooked into the pod:

    192.168.105.44 dev lxc5a4def8d96c5
    
  4. All traffic passing lxc5a4def8d96c5 on the way into the pod is subject to Cilium’s BPF program to enforce network policies, provide service reverse load-balancing, and visibility.

Egress

  1. The pod’s network namespace contains a default route which points to the node’s router IP via the veth pair which is named eth0 inside of the pod and lxcXXXXXX in the host namespace. The router IP is allocated from the ENI space, allowing for sending of ICMP errors from the router IP for Path MTU purposes.

  2. After passing through the veth pair and before reaching the Linux routing layer, all traffic is subject to Cilium’s BPF program to enforce network policies, implement load-balancing and provide networking features.

  3. An IP routing rule ensures that traffic from individual endpoints are using a routing table specific to the ENI from which the endpoint IP was allocated:

    30:      from 192.168.105.44 to 192.168.0.0/16 lookup 92
    
  4. The ENI specific routing table contains a default route which redirects to the router of the VPC via the ENI interface:

    default via 192.168.0.1 dev eth2
    192.168.0.1 dev eth2
    

Configuration

The AWS ENI datapath is enabled by setting the following option:

  • ipam: eni Enables the ENI specific IPAM backend and indicates to the datapath that ENI IPs will be used.
  • blacklist-conflicting-routes: "false" disables blacklisting of local routes. This is required as routes will exist covering ENI IPs pointing to interfaces that are not owned by Cilium. If blacklisting is not disabled, all ENI IPs would be considered used by another networking component.
  • enable-endpoint-routes: "true" enables direct routing to the ENI veth pairs without requiring to route via the cilium_host interface.
  • auto-create-cilium-node-resource: "true" enables the automatic creation of the CiliumNode custom resource with all required ENI parameters. It is possible to disable this and provide the custom resource manually.
  • egress-masquerade-interfaces: eth+ is the interface selector of all interfaces which are subject to masquerading. Masquerading can be disabled entirely with masquerade: "false".

See the section AWS ENI for details on how to configure ENI IPAM specific parameters.

Google Cloud

When running Cilium on Google Cloud via either Google Kubernetes Engine (GKE) or self-managed, it is possible to utilize the Google Cloud’s networking layer with Cilium running in a Native-Routing configuration. This provides native networking performance while benefiting from many additional Cilium features such as policy enforcement, load-balancing with DSR, efficient NodePort/ExternalIP/HostPort implementation, extensive visibility features, and so on.

../../../_images/gke_datapath.png
Addressing
Cilium will assign IPs to pods out of the PodCIDR assigned to the specific Kubernetes node. By using Alias IP ranges, these IPs are natively routable on Google Cloud’s network without additional encapsulation or route distribution.
Masquerading
All traffic not staying with the native-routing-cidr (defaults to the Cluster CIDR) will be masqueraded to the node’s IP address to become publicly routable.
Load-balancing
ClusterIP load-balancing will be performed using BPF for all version of GKE. Starting with >= GKE v1.15 or when running a Linux kernel >= 4.19, all NodePort/ExternalIP/HostPort will be performed using a BPF implementation as well.
Policy enforcement & visibility
All NetworkPolicy enforcement and visibility is provided using BPF.

Configuration

The following configuration options must be set to run the datapath on GKE:

  • gke.enabled: true: Enables the Google Kubernetes Engine (GKE) datapath. Setting this to true will enable the following options:
    • ipam: kubernetes: Enable Kubernetes Host Scope IPAM
    • tunnel: disabled: Enable native routing mode
    • enable-endpoint-routes: true: Enable per-endpoint routing on the node
    • blacklist-conflicting-routes: false: Disable blacklisting of IPs which collide with a local route
    • enable-local-node-route: false: Disable installation of the local node route
  • native-routing-cidr: x.x.x.x/y: Set the CIDR in which native routing is supported.

See the getting started guide Installation on Google GKE to install Cilium on Google Kubernetes Engine (GKE).