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SUSE Linux Enterprise Micro 5.0, K3s 1.21.2

Reference Implementation - K3s

The purpose of this document is to provide an overview and procedure of implementing SUSE (R) offerings for K3s, an official CNCF sandbox project that delivers a lightweight yet powerful certified Kubernetes distribution designed for production workloads across resource-restrained, remote locations or on Edge IoT devices.

Disclaimer: Documents published as part of the series SUSE Technical Reference Documentation have been contributed voluntarily by SUSE employees and third parties. They are meant to serve as examples of how particular actions can be performed. They have been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. SUSE cannot verify that actions described in these documents do what is claimed or whether actions described have unintended consequences. SUSE LLC, its affiliates, the authors, and the translators may not be held liable for possible errors or the consequences thereof.

Publication Date: 2021-11-26

1 Introduction

On the digital transformation journey to a full cloud native landscape, utilization of microservices becomes the main approach with the dominant technology for such container orchestration being Kubernetes.⁠[1] With its large community of developers and abundant features and capabilities, Kubernetes has become the de-facto standard and is included across most container-as-a-service platforms. With all of these technologies in place, both developer and operation teams can effectively deploy, manage and deliver functionality to their end users in a resilient and agile manner.

1.1 Motivation

As developers and organizations continue their journey from simple, containerized microservices towards having these workloads orchestrated and deployed where ever they need, being able to install, monitor and use such Kubernetes infrastructures is a core need. Such deployments, being Cloud Native Computing Foundation (CNCF⁠[2]) conformant and certified⁠[3] are essential for both development and production workloads.

For simplified scenarios, like edge, remote or IoT, this is where K3s leads the industry, being simple and secure.

1.2 Scope

The scope of this document is to provide a general reference implementation of K3s. This can be done in a variety of scenarios to create an edge-oriented, lightweight Kubernetes cluster deployment.

1.3 Audience

This document is intended for IT decision makers, architects, system administrators and technicians who are implementing a flexible, software-defined Kubernetes platform. One should still be familiar with the traditional IT infrastructure pillars — networking, computing and storage — along with the local use cases for sizing, scaling and limitations within each pillars' environments.

2 Business aspect

Agility is driving developers toward more cloud native methodologies that focus on microservices architectures and streamlined workflows. Container technologies, like Kubernetes, embody this agile approach and help enable cloud native transformation.

By unifying IT operations with Kubernetes, organizations realize key benefits like increased reliability, improved security and greater efficiencies with standardized automation. Therefore, Kubernetes infrastructure platforms are adopted by enterprises to deliver:

Cluster Operations

Improved Production and DevOps efficiencies with simplified cluster usage and robust operations

Security Policy & User Management

Consistent security policy enforcement plus advanced user management on any Kubernetes infrastructure

Access to Shared Tools & Services

A high level of reliability with easy, consistent access to a broad set of tools and services

2.1 Business problem

Kubernetes is the leading solution to address edge computing use cases in industry verticals such as manufacturing, transportation, power generation, healthcare, retail and banking. Typical edge systems that leverage Kubernetes to run complex workloads include energy meters, aircraft engines, gas & oil rigs, cruise ships, high-speed trains, retail scanners, wind turbine base stations, internet-connected cars, ATMs and much more.

For such target edge systems, which are often unattended, resource constrained and remote, orchestrating containerized workloads on Kubernetes deployments may seem overbearingly complex.

2.2 Business value

After two years of research and development in June 2020, K3s was donated to the CNCF. The donation is a testament of the commitment to the open source community and their mission to run Kubernetes everywhere.

Perfect for Edge

K3s is a highly available, certified Kubernetes distribution specifically designed for production workloads in unattended, resource-constrained, remote locations or inside IoT appliances.

Simplified & Secure

K3s is packaged as a tiny, single binary that reduces the dependencies and steps needed to install, run and auto-update a production Kubernetes cluster. For workloads, automated Manifest and Helm Chart managment deployments can be utilized. Also, multiple architectures, like x86_64, ARM64, and ARMv7, are supported with binaries and images available.

Given its extensive Kubernetes capabilities, K3s can also be a suitable choice for:

  • embedded platforms,

  • continuous integration and continuous deployment platforms,

  • branch locations or individual developer deployments, and

  • even core or cloud production instances

Tip
Tip

When K3s is imported and combined with SUSE Rancher, organizations are equipped with an easy, complete and reliable management solution for Kubernetes at the edge.

3 Architectural overview

This section outlines the core elements of the K3s solution, along with the suggested target platforms and components.

3.1 Solution architecture

The figure below illustrates the high-level architecture of K3s:

ArchitectureOverview K3s
Figure 3.1: Architecture Overview - K3s
Container Runtime
  • Containerd & runc

  • Kine as a datastore shim that allows etcd to be replaced with other databases

Networking
  • Flannel for CNI

  • Kube-router for network policy

Services
  • CoreDNS

  • Metrics Server

  • Traefik for ingress

  • Klipper-lb as an embedded service load balancer provider

  • Local-path-provisioner for provisioning volumes using local storage

Workloads

Helm-controller to allow for CRD-driven deployment of helm manifests

Host utilities

iptables/nftables, ebtables, ethtool, and socat

Once set up, users can interact with K3s via

  • kubectl

    • directly on the K3s host or

    • remotely, leveraging the KUBECONFIG file of the K3s cluster’s deployment (/etc/rancher/k3s/k3s.yaml)

  • manual or automatic, manifest or Helm Chart based, workload deployments

4 Component model

This section describes the various components being used to create a K3s solution deployment.

4.1 Component overview

By utilizing:

  • Software

    • Kubernetes Platform - K3s

    • Linux Operating System - SUSE Linux Enterprise Micro

  • Compute Platform

one can create the necessary infrastructure and services. Further details for these components are described in the following sections.

4.2 Software - K3s

K3s is packaged as a single binary, which is about 50 megabytes in size. Bundled in that single binary is everything needed to run Kubernetes anywhere, including low-powered IoT and Edge-based devices. The binary includes:

  • the container runtime

  • important host utilities such as iptables, socat and du

The only OS dependencies are the Linux kernel itself and a proper dev, proc and sysfs mounts (this is done automatically on all modern Linux distributions). K3s bundles the Kubernetes components:

  • kube-apiserver,

  • kube-controller-manager,

  • kube-scheduler,

  • kubelet and

  • kube-proxy

into combined processes that are presented as a simple server and agent model, as represented in the following figure:

K3s Overview
Figure 4.1: Overview of K3s

K3s can run as a complete cluster on a single node or can be expanded into a multi-node cluster. Besides the core Kubernetes components, these are also included:

  • containerd,

  • Flannel,

  • CoreDNS,

  • ingress controller and

  • a simple host port-based service load balancer.

All of these components are optional and can be swapped out for your implementation of choice. With these included components, you get a fully functional and CNCF-conformant cluster so you can start running apps right away. K3s is now a CNCF Sandbox project, being the first Kubernetes distribution ever to be adopted into sandbox.

Learn more information about K3s at https://k3s.io

As K3s can be deployed on a single or multiple nodes, the next sections describe the suggested component layering approach.

4.3 Software - SUSE Linux Enterprise Micro

SUSE Linux Enterprise Micro combines the assurance of enterprise-grade security and compliance with the immutability and portability of a modern, lightweight operating system. The top 4 features are:

Immutable OS

Immutable design ensures the OS is not altered during runtime and runs reliably every single time. Security signed and verified transactional updates are easy to rollback if things go wrong.

Security and Compliance

Fully open source and built using open standards, SUSE Linux Enterprise Micro leverages SUSE Linux Enterprise common code base, to provide FIPS 140-2, DISA SRG/STIG, integration with CIS and Common Criteria certified configurations. Includes fully supported security framework (SELinux) with policies.

Architectural Flexibility

Both Arm and x86-64 architectures are supported so you can deploy edge applications with confidence across multiple architectures.

Kubernetes-Ready

You can easily combine SUSE Linux Enterprise Micro with the latest cloud-native technologies including SUSE Rancher, Rancher Kubernetes Engine, Longhorn persistent block storage, and K3s, the world’s most popular Kubernetes distribution for use in low resource, distributed edge locations.

As a result, you get an ultra-reliable infrastructure platform that is also simple to use and comes out-of-the-box with best-in-class compliance. Furthermore, SUSE’s flexible subscription model ensures enterprise assurance for any edge, embedded or IoT deployment without vendor lock-in. A free, evaluation copy can be downloaded or if the organization already has subscriptions, both install media and updates can be obtained from SUSE Customer Center.

4.4 Compute Platform

Leveraging the enterprise grade functionality of the operating system mentioned in the previous section, many compute platforms can be the foundation of the deployment:

  • Virtual machines on supported hypervisors or hosted on cloud service providers

  • Physical, baremetal or single-board computers, either on-premise or hosted by cloud service providers

Tip
Tip

Any SUSE YES certified platform can be used for the nodes of this deployment, as long as the certification refers to the major version of the underlying SUSE operating system required by its release.

5 Deployment

This section describes the process steps for the deployment of the K3s solution. It describes the process steps to deploy each of the component layers starting as a base functional proof-of-concept, having considerations on migration towards production, providing scaling guidance that is needed to create the solution.

5.1 Deployment overview

The deployment stack is represented in the following figure:

RI Deployment K3s SLEMicro
Figure 5.1: K3s Deployment Stack

and details are covered for each layer in the following sections.

Note
Note

The following section’s content is ordered and described from the bottom layer up to the top.

5.2 Compute Platform

Preparation(s)

For each node used in the deployment:

  • Validate the necessary CPU, memory, disk capacity, and network interconnect quantity and type are present for each node and its intended role. Refer to the recommended CPU/Memory/Disk/Networking requirements as noted in the SUSE Rancher Hardware Requirements.

  • Further suggestions

    • Disk : Use a pair of local, direct attached, mirrored disk drives is present on each node (SSDs are preferred); these will become the target for the operating system installation.

    • Network : Prepare an IP addressing scheme and optionally create both a public and private network, along with the respective subnets and desired VLAN designations for the target environment.

      • Baseboard Management Controller : If present, consider using a distinct management network for controlled access.

    • Boot Settings : BIOS/uEFI reset to defaults for a known baseline, consistent state or perhaps with desired, localized values.

    • Firmware : Use consistent and up-to-date versions for BIOS/uEFI/device firmware to reduce potential troubleshooting issues later

5.3 SUSE Linux Enterprise Micro

Utilize an enterprise-grade Linux operating system , like SUSE Linux Enterprise Micro, as the base software layer.

Preparation(s)

To meet the solution stack prerequisites and requirements, SUSE operating system offerings, like SUSE Linux Enterprise Micro can be utilized.

  1. Ensure these services are in place and configured for this node to use:

    • Domain Name Service (DNS) - an external network-accessible service to map IP Addresses to hostnames

    • Network Time Protocol (NTP) - an external network-accessible service to obtain and synchronize system times to aid in timestamp consistency

    • Software Update Service - access to a network-based repository for software update packages. This can be accessed directly from each node via registration to

      • the general, internet-based SUSE Customer Center (SCC) or

      • an organization’s SUSE Manager infrastructure or

      • a local server running an instance of Repository Mirroring Tool (RMT)

        Note
        Note

        During the node’s installation, it can be pointed to the respective update service. This can also be accomplished post-installation with the command-line tool named SUSEConnect.

Deployment Process

On the compute platform node, install the noted SUSE operating system, by following these steps:

  1. Download the SUSE Linux Enterprise Micro product (either for the ISO or Virtual Machine image)

  2. The installation process is described and can be performed with default values by following steps from the SUSE Linux Enterprise Micro product documentation, see Installation Quick Start

    Tip
    Tip

    Adjust both the password and the local network addressing setup to comply with local environment guidelines and requirements.

Deployment Consideration(s)

To further optimize deployment factors, leverage the following practices:

  • Automation

    • To reduce user intervention, unattended deployments of SUSE Linux Enterprise Micro can be automated

      • for ISO-based installations, by referring to the AutoYaST Guide

      • for raw-image based installation, by configuring the Ignition and Combustion tooling as described in the Installation Quick Start

5.4 K3s

Utilize an enterprise-grade Linux operating system , like SUSE Linux Enterprise Micro, as the base software layer.

Preparation(s)

To meet the solution stack prerequisites and requirements, SUSE operating system offerings, like SUSE Linux Enterprise Micro can be utilized.

  1. Ensure these services are in place and configured for this node to use:

    • Domain Name Service (DNS) - an external network-accessible service to map IP Addresses to hostnames

    • Network Time Protocol (NTP) - an external network-accessible service to obtain and synchronize system times to aid in timestamp consistency

    • Software Update Service - access to a network-based repository for software update packages. This can be accessed directly from each node via registration to

      • the general, internet-based SUSE Customer Center (SCC) or

      • an organization’s SUSE Manager infrastructure or

      • a local server running an instance of Repository Mirroring Tool (RMT)

        Note
        Note

        During the node’s installation, it can be pointed to the respective update service. This can also be accomplished post-installation with the command-line tool named SUSEConnect.

  2. Identify the appropriate, desired version of the K3s binary (e.g. vX.YY.ZZ+k3s1), by reviewing the "Releases" on the Download web page.

Deployment Process

Perform the following steps to install the first K3s server on one of the nodes to be used for the Kubernetes control plane

  1. Set the following variable with the noted version of K3s, as found during the preparation steps.

    K3s_VERSION=""
  2. Install the version of K3s with embedded etcd enabled:

    curl -sfL https://get.k3s.io | \
    	INSTALL_K3S_VERSION=${K3s_VERSION} \
    	INSTALL_K3S_EXEC='server --cluster-init --write-kubeconfig-mode=644' \
    	sh -s -
    Tip
    Tip

    To address Availability and possible scaling to a multiple node cluster, etcd is enabled instead of using the default SQLite datastore.

    • Monitor the progress of the installation: watch -c "kubectl get deployments -A"

      • The K3s deployment is complete when elements of all the deployments (coredns, local-path-provisioner, metrics-server, and traefik) show at least "1" as "AVAILABLE"

      • Use Ctrl+c to exit the watch loop after all deployment pods are running

Deployment Consideration(s)

To further optimize deployment factors, leverage the following practices:

  • Availability

    • A full high-availability K3s cluster is recommended for production workloads. The etcd key/value store (aka database) requires an odd number of servers (aka master nodes) be allocated to the K3s cluster. In this case, two additional control-plane servers should be added; for a total of three.

      1. Deploy the same operating system on the new compute platform nodes, then log into the new nodes as root or as a user with sudo privileges.

      2. Execute the following sets of commands on each of the remaining control-plane nodes:

        • Set the following additional variables, as appropriate for this cluster

          # Private IP preferred, if available
          FIRST_SERVER_IP=""
          
          # From /var/lib/rancher/k3s/server/node-token file on the first server
          NODE_TOKEN=""
          
          # Match the first of the first server
          K3s_VERSION=""
        • Install K3s

          curl -sfL https://get.k3s.io | \
          	INSTALL_K3S_VERSION=${K3s_VERSION} \
          	K3S_URL=https://${FIRST_SERVER_IP}:6443 \
          	K3S_TOKEN=${NODE_TOKEN} \
          	K3S_KUBECONFIG_MODE="644" INSTALL_K3S_EXEC='server' \
          	sh -
        • Monitor the progress of the installation: watch -c "kubectl get deployments -A"

          • The K3s deployment is complete when elements of all the deployments (coredns, local-path-provisioner, metrics-server, and traefik) show at least "1" as "AVAILABLE"

          • Use Ctrl+c to exit the watch loop after all deployment pods are running

            Note
            Note

            This can be changed to the normal Kubernetes default by adding a taint to each server node. See the official Kubernetes documentation for more information on how to do that.

        • (Optional) In cases where agent nodes are desired, execute the following sets of commands, using the same "K3s_VERSION", "FIRST_SERVER_IP", and "NODE_TOKEN" variable settings as above, on each of the agent nodes to add it to the K3s cluster:

          curl -sfL https://get.k3s.io | \
          	INSTALL_K3S_VERSION=${K3s_VERSION} \
          	K3S_URL=https://${FIRST_SERVER_IP}:6443 \
          	K3S_TOKEN=${NODE_TOKEN} \
          	K3S_KUBECONFIG_MODE="644" \
          	sh -

After this successful deployment of the K3s solution, review the product documentation for details on how to directly utilize this Kubernetes cluster. Furthermore, by reviewing the SUSE Rancher product documentation this solution can also be:

  • imported (refer to sub-section "Importing Existing Clusters"), then

  • managed (refer to sub-section "Cluster Administration") and

  • accessed (refer to sub-section "Cluster Access") to address orchestration of workloads, maintaining security and many more functions are readily available.

6 Summary

Using components and offerings from SUSE and the Rancher portfolio streamlines the ability to quickly and effectively engage in a digital transformation, taking advantage of cloud native resources and disciplines. Using such technology approaches lets you deploy and leverage transformations of infrastructure into a durable, reliable enterprise-grade environment.

Simplify

Simplify and optimize your existing IT environments

  • Using K3s enables you to quickly and simply deploy a Kubernetes cluster in a wide array of locations, across edge, branch, core and cloud.

Modernize

Bring applications and data into modern computing

  • With K3s, the digital transformation to containerized applications can progress since both developers and production can leverage these deployments for the actual workloads.

Accelerate

Accelerate business transformation through the power of open source software

  • Given the open source nature of K3s and the minimal underlying software components, you can expand into a very distributed ecosystem, bringing computing to where the data exists or arrives, to answer the necessary business needs.

7 References

White Papers
Websites

Glossary

  • Document Scope

    Reference Implementation

    A guide with the basic steps to deploy the highlighted components of the SUSE portfolio, including generalized pointers to other layers and elements. This is considered an introductory approach and a basis for other tested variations.

    Reference Architectures⁠[4]

    A guide with the general steps to deploy and validate the structured solution components from both the SUSE and partner portfolios. This provides a shareable template of consistency for consumers to leverage for similar production ready solutions, including design considerations, implementation suggestions and best practices.

    Best Practice

    Information that can overlap both the SUSE and partner space. It can either be provided as a standalone guide that provides reliable technical information not covered in other product documentation, based on real-life installation and implementation experiences from subject matter experts or complementary, embedded sections within any of the above documentation types describing considerations and possible steps forward.

  • Factor(s)

    Automation⁠[5]

    Infrastructure automation enables speed through faster execution when configuring the infrastructure and aims at providing visibility to help other teams across the enterprise work quickly and more efficiently. Automation removes the risk associated with human error, like manual misconfiguration; removing this can decrease downtime and increase reliability. These outcomes and attributes help the enterprise move towards implementing a culture of DevOps, the combined working of development and operations.

    Availability⁠[6]

    The probability that an item operates satisfactorily, without failures or downtimes, under stated conditions as a function of its reliability, redundancy and maintainability attributes. Some major objectives to achieve a desired service level objectives are:

    • Preventing or reducing the likelihood and frequency of failures via design decisions within the allowed cost of ownership

    • Correcting or coping with possible component failures via resiliency, automated failover and disaster-recovery processes

    • Estimating and analyzing current conditions to prevent unexpected failures via predictive maintenance

    Integrity⁠[7]

    Integrity is the maintenance of, and the insurance of the accuracy and consistency of a specific element over its entire lifecycle. Both physical and logical aspects must be managed to ensure stability, performance, re-usability and maintainability.

    Security⁠[8]

    Security is about ensuring freedom from or resilience against potential harm, including protection from destructive or hostile forces. To minimize risks, one mus manage governance to avoid tampering, maintain access controls to prevent unauthorized usage and integrate layers of defense, reporting and recovery tactics.

  • Deployment Flavor(s)

    Proof-of-Concept⁠[9]

    A partial or nearly complete prototype constructed to demonstrate functionality and feasibility for verifying specific aspects or concepts under consideration. This is often a starting point when evaluating a new, transitional technology. Sometimes it starts as a Minimum Viable Product (MVP⁠[10]) that has just enough features to satisfy an initial set of requests. After such insights and feedback are obtained and potentially addressed, redeployments may be utilized to iteratively branch into other realms or to incorporate other known working functionality.

    Production

    A deployed environment that target customers or users can interact with and rely upon to meet their needs, plus be operationally sustainable in terms of resource utilization and economic constraints.

    Scaling

    The flexibility of a system environment to either vertically scale-up, horizontally scale-out or conversely scale-down by adding or subtracting resources as needed. Attributes like capacity and performance are often the primary requirements to address, while still maintaining functional consistency and reliability.

A Appendix

The following sections provide a bill of materials listing for the respective component layer(s) of the described deployment.

A.1 Compute platform bill of materials

Recommended set of computing platform models, components and resources.

RoleQtySKUComponentNotes

System

1-3

n/a

  • Virtual Machine,

  • Single Board Computer (SBC) or

  • Industry Standard Server

Configuration

A.2 Software bill of materials

Recommended set of software, support and services.

RoleQtySKUComponentNotes

Operating System

1-3

874-007864

SUSE Linux Enterprise Micro,

  • x86_64,

  • Priority Subscription,

  • 1 Year

Configuration:

  • per node (up to 16 cores, stackable)

Kubernetes Management

1

R-0001-PS1

SUSE Rancher,

  • x86-64,

  • Priority Subscription,

  • 1 Year

Configuration:

  • per deployed instance

Rancher Management

2

R-0004-PS1

Rancher 10 Nodes

  • x86-64 or aarch64,

  • Priority Subscription,

  • 1 Year,

Configuration:

  • requires priority server subscription

Consulting and Training

1

R-0001-QSO

Rancher Quick Start,

  • Go Live Services

 
Note
Note

For the software components, other durations of support terms are also available.

A.3 Documentation configuration / attributes

This document was built using the following AsciiDoc and DocBook Authoring and Publishing Suite (DAPS) attributes:

Appendix=1 ArchOv=1 Automation=1 Availability=1 BP=1 BPBV=1 CompMod=1 DepConsiderations=1 Deployment=1 FCTR=1 FLVR=1 GFDL=1 Glossary=1 HWComp=1 HWDepCfg=1 Integrity=1 LN=1 PoC=1 Production=1 RA=1 RI=1 References=1 Requirements=1 SWComp=1 SWDepCfg=1 Scaling=1 Security=1 env-daps=1 focusK3s=1 iK3s=1 iRKE1=1 iRKE2=1 iRMT=1 iRancher=1 iSLEMicro=1 iSLES=1 iSUMa=1 layerK3s=1 layerSLEMicro=1

9 GNU Free Documentation License

Copyright © 2000, 2001, 2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

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This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

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If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

  1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.

  2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.

  3. State on the Title page the name of the publisher of the Modified Version, as the publisher.

  4. Preserve all the copyright notices of the Document.

  5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.

  6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.

  7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.

  8. Include an unaltered copy of this License.

  9. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.

  10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.

  11. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.

  12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.

  13. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version.

  14. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section.

  15. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties—​for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements".

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

Copyright (c) YEAR YOUR NAME.
   Permission is granted to copy, distribute and/or modify this document
   under the terms of the GNU Free Documentation License, Version 1.2
   or any later version published by the Free Software Foundation;
   with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
   A copy of the license is included in the section entitled “GNU
   Free Documentation License”.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “ with…​Texts.” line with this:

with the Invariant Sections being LIST THEIR TITLES, with the
   Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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