Cluster Networking

Networking is a central part of Kubernetes, but it can be challenging to understand exactly how it is expected to work. There are 4 distinct networking problems to address:

Highly-coupled container-to-container communications: this is solved by pods and localhost communications.
Pod-to-Pod communications: this is the primary focus of this document.
Pod-to-Service communications: this is covered by services.
External-to-Service communications: this is covered by services.

Kubernetes is all about sharing machines between applications. Typically, sharing machines requires ensuring that two applications do not try to use the same ports. Coordinating ports across multiple developers is very difficult to do at scale and exposes users to cluster-level issues outside of their control.

Dynamic port allocation brings a lot of complications to the system - every application has to take ports as flags, the API servers have to know how to insert dynamic port numbers into configuration blocks, services have to know how to find each other, etc. Rather than deal with this, Kubernetes takes a different approach.

The Kubernetes network model

Every Pod gets its own IP address. This means you do not need to explicitly create links between Pods and you almost never need to deal with mapping container ports to host ports. This creates a clean, backwards-compatible model where Pods can be treated much like VMs or physical hosts from the perspectives of port allocation, naming, service discovery, load balancing, application configuration, and migration.

Kubernetes imposes the following fundamental requirements on any networking implementation (barring any intentional network segmentation policies):

pods on a node can communicate with all pods on all nodes without NAT
agents on a node (e.g. system daemons, kubelet) can communicate with all pods on that node

Note: For those platforms that support Pods running in the host network (e.g. Linux):

pods in the host network of a node can communicate with all pods on all nodes without NAT

This model is not only less complex overall, but it is principally compatible with the desire for Kubernetes to enable low-friction porting of apps from VMs to containers. If your job previously ran in a VM, your VM had an IP and could talk to other VMs in your project. This is the same basic model.

Kubernetes IP addresses exist at the Pod scope - containers within a Pod share their network namespaces - including their IP address. This means that containers within a Pod can all reach each other's ports on localhost. This also means that containers within a Pod must coordinate port usage, but this is no different from processes in a VM. This is called the "IP-per-pod" model.

How this is implemented is a detail of the particular container runtime in use.

It is possible to request ports on the Node itself which forward to your Pod (called host ports), but this is a very niche operation. How that forwarding is implemented is also a detail of the container runtime. The Pod itself is blind to the existence or non-existence of host ports.

How to implement the Kubernetes networking model

There are a number of ways that this network model can be implemented. This document is not an exhaustive study of the various methods, but hopefully serves as an introduction to various technologies and serves as a jumping-off point.

The following networking options are sorted alphabetically - the order does not imply any preferential status.

ACI

Cisco Application Centric Infrastructure offers an integrated overlay and underlay SDN solution that supports containers, virtual machines, and bare metal servers. ACI provides container networking integration for ACI. An overview of the integration is provided here.

Antrea

Project Antrea is an opensource Kubernetes networking solution intended to be Kubernetes native. It leverages Open vSwitch as the networking data plane. Open vSwitch is a high-performance programmable virtual switch that supports both Linux and Windows. Open vSwitch enables Antrea to implement Kubernetes Network Policies in a high-performance and efficient manner. Thanks to the "programmable" characteristic of Open vSwitch, Antrea is able to implement an extensive set of networking and security features and services on top of Open vSwitch.

AOS from Apstra

AOS is an Intent-Based Networking system that creates and manages complex datacenter environments from a simple integrated platform. AOS leverages a highly scalable distributed design to eliminate network outages while minimizing costs.

The AOS Reference Design currently supports Layer-3 connected hosts that eliminate legacy Layer-2 switching problems. These Layer-3 hosts can be Linux servers (Debian, Ubuntu, CentOS) that create BGP neighbor relationships directly with the top of rack switches (TORs). AOS automates the routing adjacencies and then provides fine grained control over the route health injections (RHI) that are common in a Kubernetes deployment.

AOS has a rich set of REST API endpoints that enable Kubernetes to quickly change the network policy based on application requirements. Further enhancements will integrate the AOS Graph model used for the network design with the workload provisioning, enabling an end to end management system for both private and public clouds.

AOS supports the use of common vendor equipment from manufacturers including Cisco, Arista, Dell, Mellanox, HPE, and a large number of white-box systems and open network operating systems like Microsoft SONiC, Dell OPX, and Cumulus Linux.

Details on how the AOS system works can be accessed here: http://www.apstra.com/products/how-it-works/

AWS VPC CNI for Kubernetes

The AWS VPC CNI offers integrated AWS Virtual Private Cloud (VPC) networking for Kubernetes clusters. This CNI plugin offers high throughput and availability, low latency, and minimal network jitter. Additionally, users can apply existing AWS VPC networking and security best practices for building Kubernetes clusters. This includes the ability to use VPC flow logs, VPC routing policies, and security groups for network traffic isolation.

Using this CNI plugin allows Kubernetes pods to have the same IP address inside the pod as they do on the VPC network. The CNI allocates AWS Elastic Networking Interfaces (ENIs) to each Kubernetes node and using the secondary IP range from each ENI for pods on the node. The CNI includes controls for pre-allocation of ENIs and IP addresses for fast pod startup times and enables large clusters of up to 2,000 nodes.

Additionally, the CNI can be run alongside Calico for network policy enforcement. The AWS VPC CNI project is open source with documentation on GitHub.

Azure CNI for Kubernetes

Azure CNI is an open source plugin that integrates Kubernetes Pods with an Azure Virtual Network (also known as VNet) providing network performance at par with VMs. Pods can connect to peered VNet and to on-premises over Express Route or site-to-site VPN and are also directly reachable from these networks. Pods can access Azure services, such as storage and SQL, that are protected by Service Endpoints or Private Link. You can use VNet security policies and routing to filter Pod traffic. The plugin assigns VNet IPs to Pods by utilizing a pool of secondary IPs pre-configured on the Network Interface of a Kubernetes node.

Azure CNI is available natively in the [Azure Kubernetes Service (AKS)] (https://docs.microsoft.com/en-us/azure/aks/configure-azure-cni).

Big Cloud Fabric from Big Switch Networks

Big Cloud Fabric is a cloud native networking architecture, designed to run Kubernetes in private cloud/on-premises environments. Using unified physical & virtual SDN, Big Cloud Fabric tackles inherent container networking problems such as load balancing, visibility, troubleshooting, security policies & container traffic monitoring.

With the help of the Big Cloud Fabric's virtual pod multi-tenant architecture, container orchestration systems such as Kubernetes, RedHat OpenShift, Mesosphere DC/OS & Docker Swarm will be natively integrated alongside with VM orchestration systems such as VMware, OpenStack & Nutanix. Customers will be able to securely inter-connect any number of these clusters and enable inter-tenant communication between them if needed.

BCF was recognized by Gartner as a visionary in the latest Magic Quadrant. One of the BCF Kubernetes on-premises deployments (which includes Kubernetes, DC/OS & VMware running on multiple DCs across different geographic regions) is also referenced here.

Cilium

Cilium is open source software for providing and transparently securing network connectivity between application containers. Cilium is L7/HTTP aware and can enforce network policies on L3-L7 using an identity based security model that is decoupled from network addressing, and it can be used in combination with other CNI plugins.

CNI-Genie from Huawei

CNI-Genie is a CNI plugin that enables Kubernetes to simultaneously have access to different implementations of the Kubernetes network model in runtime. This includes any implementation that runs as a CNI plugin, such as Flannel, Calico, Romana, Weave-net.

CNI-Genie also supports assigning multiple IP addresses to a pod, each from a different CNI plugin.

cni-ipvlan-vpc-k8s

cni-ipvlan-vpc-k8s contains a set of CNI and IPAM plugins to provide a simple, host-local, low latency, high throughput, and compliant networking stack for Kubernetes within Amazon Virtual Private Cloud (VPC) environments by making use of Amazon Elastic Network Interfaces (ENI) and binding AWS-managed IPs into Pods using the Linux kernel's IPvlan driver in L2 mode.

The plugins are designed to be straightforward to configure and deploy within a VPC. Kubelets boot and then self-configure and scale their IP usage as needed without requiring the often recommended complexities of administering overlay networks, BGP, disabling source/destination checks, or adjusting VPC route tables to provide per-instance subnets to each host (which is limited to 50-100 entries per VPC). In short, cni-ipvlan-vpc-k8s significantly reduces the network complexity required to deploy Kubernetes at scale within AWS.

Contiv

Contiv provides configurable networking (native l3 using BGP, overlay using vxlan, classic l2, or Cisco-SDN/ACI) for various use cases. Contiv is all open sourced.

Contrail / Tungsten Fabric

Contrail, based on Tungsten Fabric, is a truly open, multi-cloud network virtualization and policy management platform. Contrail and Tungsten Fabric are integrated with various orchestration systems such as Kubernetes, OpenShift, OpenStack and Mesos, and provide different isolation modes for virtual machines, containers/pods and bare metal workloads.

DANM

DANM is a networking solution for telco workloads running in a Kubernetes cluster. It's built up from the following components:

A CNI plugin capable of provisioning IPVLAN interfaces with advanced features
An in-built IPAM module with the capability of managing multiple, cluster-wide, discontinuous L3 networks and provide a dynamic, static, or no IP allocation scheme on-demand
A CNI metaplugin capable of attaching multiple network interfaces to a container, either through its own CNI, or through delegating the job to any of the popular CNI solution like SRI-OV, or Flannel in parallel
A Kubernetes controller capable of centrally managing both VxLAN and VLAN interfaces of all Kubernetes hosts
Another Kubernetes controller extending Kubernetes' Service-based service discovery concept to work over all network interfaces of a Pod

With this toolset DANM is able to provide multiple separated network interfaces, the possibility to use different networking back ends and advanced IPAM features for the pods.

Flannel

Flannel is a very simple overlay network that satisfies the Kubernetes requirements. Many people have reported success with Flannel and Kubernetes.

Google Compute Engine (GCE)

For the Google Compute Engine cluster configuration scripts, advanced routing is used to assign each VM a subnet (default is /24 - 254 IPs). Any traffic bound for that subnet will be routed directly to the VM by the GCE network fabric. This is in addition to the "main" IP address assigned to the VM, which is NAT'ed for outbound internet access. A linux bridge (called cbr0) is configured to exist on that subnet, and is passed to docker's --bridge flag.

Docker is started with:

DOCKER_OPTS="--bridge=cbr0 --iptables=false --ip-masq=false"

This bridge is created by Kubelet (controlled by the --network-plugin=kubenet flag) according to the Node's .spec.podCIDR.

Docker will now allocate IPs from the cbr-cidr block. Containers can reach each other and Nodes over the cbr0 bridge. Those IPs are all routable within the GCE project network.

GCE itself does not know anything about these IPs, though, so it will not NAT them for outbound internet traffic. To achieve that an iptables rule is used to masquerade (aka SNAT - to make it seem as if packets came from the Node itself) traffic that is bound for IPs outside the GCE project network (10.0.0.0/8).

iptables -t nat -A POSTROUTING ! -d 10.0.0.0/8 -o eth0 -j MASQUERADE

Lastly IP forwarding is enabled in the kernel (so the kernel will process packets for bridged containers):

sysctl net.ipv4.ip_forward=1

The result of all this is that all Pods can reach each other and can egress traffic to the internet.

Jaguar

Jaguar is an open source solution for Kubernetes's network based on OpenDaylight. Jaguar provides overlay network using vxlan and Jaguar CNIPlugin provides one IP address per pod.

k-vswitch

k-vswitch is a simple Kubernetes networking plugin based on Open vSwitch. It leverages existing functionality in Open vSwitch to provide a robust networking plugin that is easy-to-operate, performant and secure.

Knitter

Knitter is a network solution which supports multiple networking in Kubernetes. It provides the ability of tenant management and network management. Knitter includes a set of end-to-end NFV container networking solutions besides multiple network planes, such as keeping IP address for applications, IP address migration, etc.

Kube-OVN

Kube-OVN is an OVN-based kubernetes network fabric for enterprises. With the help of OVN/OVS, it provides some advanced overlay network features like subnet, QoS, static IP allocation, traffic mirroring, gateway, openflow-based network policy and service proxy.

Kube-router

Kube-router is a purpose-built networking solution for Kubernetes that aims to provide high performance and operational simplicity. Kube-router provides a Linux LVS/IPVS-based service proxy, a Linux kernel forwarding-based pod-to-pod networking solution with no overlays, and iptables/ipset-based network policy enforcer.

L2 networks and linux bridging

If you have a "dumb" L2 network, such as a simple switch in a "bare-metal" environment, you should be able to do something similar to the above GCE setup. Note that these instructions have only been tried very casually - it seems to work, but has not been thoroughly tested. If you use this technique and perfect the process, please let us know.

Follow the "With Linux Bridge devices" section of this very nice tutorial from Lars Kellogg-Stedman.

Multus (a Multi Network plugin)

Multus is a Multi CNI plugin to support the Multi Networking feature in Kubernetes using CRD based network objects in Kubernetes.

Multus supports all reference plugins (eg. Flannel, DHCP, Macvlan) that implement the CNI specification and 3rd party plugins (eg. Calico, Weave, Cilium, Contiv). In addition to it, Multus supports SRIOV, DPDK, OVS-DPDK & VPP workloads in Kubernetes with both cloud native and NFV based applications in Kubernetes.

OVN4NFV-K8s-Plugin (OVN based CNI controller & plugin)

OVN4NFV-K8S-Plugin is OVN based CNI controller plugin to provide cloud native based Service function chaining(SFC), Multiple OVN overlay networking, dynamic subnet creation, dynamic creation of virtual networks, VLAN Provider network, Direct provider network and pluggable with other Multi-network plugins, ideal for edge based cloud native workloads in Multi-cluster networking

NSX-T

VMware NSX-T is a network virtualization and security platform. NSX-T can provide network virtualization for a multi-cloud and multi-hypervisor environment and is focused on emerging application frameworks and architectures that have heterogeneous endpoints and technology stacks. In addition to vSphere hypervisors, these environments include other hypervisors such as KVM, containers, and bare metal.

NSX-T Container Plug-in (NCP) provides integration between NSX-T and container orchestrators such as Kubernetes, as well as integration between NSX-T and container-based CaaS/PaaS platforms such as Pivotal Container Service (PKS) and OpenShift.

Nuage Networks VCS (Virtualized Cloud Services)

Nuage provides a highly scalable policy-based Software-Defined Networking (SDN) platform. Nuage uses the open source Open vSwitch for the data plane along with a feature rich SDN Controller built on open standards.

The Nuage platform uses overlays to provide seamless policy-based networking between Kubernetes Pods and non-Kubernetes environments (VMs and bare metal servers). Nuage's policy abstraction model is designed with applications in mind and makes it easy to declare fine-grained policies for applications.The platform's real-time analytics engine enables visibility and security monitoring for Kubernetes applications.

OpenVSwitch

OpenVSwitch is a somewhat more mature but also complicated way to build an overlay network. This is endorsed by several of the "Big Shops" for networking.

OVN (Open Virtual Networking)

OVN is an opensource network virtualization solution developed by the Open vSwitch community. It lets one create logical switches, logical routers, stateful ACLs, load-balancers etc to build different virtual networking topologies. The project has a specific Kubernetes plugin and documentation at ovn-kubernetes.

Project Calico

Project Calico is an open source container networking provider and network policy engine.

Calico provides a highly scalable networking and network policy solution for connecting Kubernetes pods based on the same IP networking principles as the internet, for both Linux (open source) and Windows (proprietary - available from Tigera). Calico can be deployed without encapsulation or overlays to provide high-performance, high-scale data center networking. Calico also provides fine-grained, intent based network security policy for Kubernetes pods via its distributed firewall.

Calico can also be run in policy enforcement mode in conjunction with other networking solutions such as Flannel, aka canal, or native GCE, AWS or Azure networking.

Romana

Romana is an open source network and security automation solution that lets you deploy Kubernetes without an overlay network. Romana supports Kubernetes Network Policy to provide isolation across network namespaces.

Weave Net from Weaveworks

Weave Net is a resilient and simple to use network for Kubernetes and its hosted applications. Weave Net runs as a CNI plug-in or stand-alone. In either version, it doesn't require any configuration or extra code to run, and in both cases, the network provides one IP address per pod - as is standard for Kubernetes.

What's next

The early design of the networking model and its rationale, and some future plans are described in more detail in the networking design document.

Black lives matter.

Documentation