docs/usecases/contiv/K8s_Overview.rst

Contiv/VPP Kubernetes Network Plugin
====================================

Overview
--------

Kubernetes is a container orchestration system that efficiently manages
Docker containers. The Docker containers and container platforms provide
many advantages over traditional virtualization. Container isolation is
done on the kernel level, which eliminates the need for a guest virtual
operating system, and therefore makes containers much more efficient,
faster, and lightweight. The containers in Contiv/VPP are referred to as
PODs.

Contiv/VPP is a Kubernetes network plugin that uses `FD.io
VPP <https://fd.io/>`__ to provide network connectivity between PODs in
a k8s cluster (k8s is an abbreviated reference for kubernetes). It
deploys itself as a set of system PODs in the ``kube-system`` namespace,
some of them (``contiv-ksr``, ``contiv-etcd``) on the master node, and
some of them (``contiv-cni``, ``contiv-vswitch``, ``contiv-stn``) on
each node in the cluster.

Contiv/VPP is fully integrated with k8s via its components, and it
automatically reprograms itself upon each change in the cluster via k8s
API.

The main component of the `VPP <https://fd.io/technology/#vpp>`__
solution, which runs within the ``contiv-vswitch`` POD on each node in
the cluster. The VPP solution also provides POD-to-POD connectivity
across the nodes in the cluster, as well as host-to-POD and
outside-to-POD connectivity. This solution also leverages VPP’s fast
data processing that runs completely in userspace, and uses
`DPDK <https://dpdk.org/>`__ for fast access to the network IO layer.

Kubernetes services and policies are also a part of the VPP
configuration, which means they are fully supported on VPP, without the
need of forwarding packets into the Linux network stack (Kube Proxy),
which makes them very effective and scalable.

Architecture
------------

Contiv/VPP consists of several components, each of them packed and
shipped as a Docker container. Two of them deploy on Kubernetes master
node only:

-  `Contiv KSR <#contiv-ksr>`__
-  `Contiv ETCD <#contiv-etcd>`__

The rest of them deploy on all nodes within the k8s cluster (including
the master node):

-  `Contiv vSwitch <#contiv-vswitch>`__
-  `Contiv CNI <#contiv-cni>`__
-  `Contiv STN <#contiv-stn-daemon>`__

The following section briefly describes the individual Contiv
components, which are displayed as orange boxes on the picture below:

.. figure:: ../../_images/contiv-arch.png
   :alt: Contiv/VPP Architecture

   Contiv/VPP Architecture

Contiv KSR
~~~~~~~~~~

Contiv KSR (Kubernetes State Reflector) is an agent that subscribes to
k8s control plane, watches k8s resources and propagates all relevant
cluster-related information into the Contiv ETCD data store. Other
Contiv components do not access the k8s API directly, they subscribe to
Contiv ETCD instead. For more information on KSR, read the `KSR
Readme <https://github.com/contiv/vpp/blob/master/cmd/contiv-ksr/README.md>`__.

Contiv ETCD
~~~~~~~~~~~

Contiv/VPP uses its own instance of the ETCD database for storage of k8s
cluster-related data reflected by KSR, which are then accessed by Contiv
vSwitch Agents running on individual nodes. Apart from the data
reflected by KSR, ETCD also stores persisted VPP configuration of
individual vswitches (mainly used to restore the operation after
restarts), as well as some more internal metadata.

Contiv vSwitch
~~~~~~~~~~~~~~

vSwitch is the main networking component that provides the connectivity
to PODs. It deploys on each node in the cluster, and consists of two
main components packed into a single Docker container: VPP and Contiv
VPP Agent.

**VPP** is the data plane software that provides the connectivity
between PODs, host Linux network stack, and data-plane NIC interface
controlled by VPP:

-  PODs are connected to VPP using TAP interfaces wired between VPP, and
   each POD network namespace.
-  host network stack is connected to VPP using another TAP interface
   connected to the main (default) network namespace.
-  data-plane NIC is controlled directly by VPP using DPDK. Note, this
   means that this interface is not visible to the host Linux network
   stack, and the node either needs another management interface for k8s
   control plane communication, or [STN (Steal The
   NIC)](SINGLE_NIC_SETUP.html) deployment must be applied.

**Contiv VPP Agent** is the control plane part of the vSwitch container.
It is responsible for configuring the VPP according to the information
gained from ETCD, and requests from Contiv STN. It is based on the
`Ligato VPP Agent <https://github.com/ligato/vpp-agent>`__ code with
extensions that are related to k8s.

For communication with VPP, it uses VPP binary API messages sent via
shared memory using `GoVPP <https://wiki.fd.io/view/GoVPP>`__. For
connection with Contiv STN, the agent acts as a GRPC server serving CNI
requests forwarded from the Contiv CNI.

Contiv CNI
~~~~~~~~~~

Contiv CNI (Container Network Interface) is a simple binary that
implements the `Container Network
Interface <https://github.com/containernetworking/cni>`__ API and is
being executed by Kubelet upon POD creation and deletion. The CNI binary
just packs the request into a GRPC request and forwards it to the Contiv
VPP Agent running on the same node, which then processes it
(wires/unwires the container) and replies with a response, which is then
forwarded back to Kubelet.

Contiv STN Daemon
~~~~~~~~~~~~~~~~~

This section discusses how the Contiv [STN (Steal The
NIC)](SINGLE_NIC_SETUP.html) daemon operation works. As already
mentioned, the default setup of Contiv/VPP requires two network
interfaces per node: one controlled by VPP for data facing PODs, and one
controlled by the host network stack for k8s control plane
communication. In case that your k8s nodes do not provide two network
interfaces, Contiv/VPP can work in the single NIC setup, when the
interface will be “stolen” from the host network stack just before
starting the VPP and configured with the same IP address on VPP, as well
as on the host-VPP interconnect TAP interface, as it had in the host
before it. For more information on STN setup, read the [Single NIC Setup
README](./SINGLE_NIC_SETUP.html)