Home > Articles

  • Print
  • + Share This
This chapter is from the book

YANG: The Data Modeling Language

YANG has been mentioned numerous times so far, but what is it exactly? YANG is an API contract language. This means that you can use YANG to write a specification for what the interface between a client and server should be on a particular topic, as shown in Figure 2-1. A specification written in YANG is referred to as a “YANG module,” and a set of YANG modules are collectively often called a “YANG model.” A YANG model typically focuses on the data that a client manipulates and observes using standardized operations, with a few actions and notifications sprinkled in. Note that, in the NETCONF and RESTCONF terms, the controller is the client and the network elements are the server, as the controller initiates the configuration session. It’s interesting to note that YANG is not an acronym—at least, it’s never expanded or referenced as an acronym in any documentation. However, there is a special meaning behind the term, as a kind of joke. Indeed, it stems from Yet Another Next Gen (data modeling language).

FIGURE 2.1

Figure 2.1 The Basic Model

Say you are designing the next cool server application. This application may or may not have some sort of interface for ordinary users, but it will almost certainly have a management interface, which the owner of the application can use to administrate and monitor the application. Obviously, this administrative interface needs to have a clear and concise API.

A YANG-based server publishes a set of YANG modules, which taken together form the system’s YANG model. These YANG modules declare what a client can do. The four areas listed next are the same for all applications, but the specific data and operations will vary. For the sake of clarity, let’s say that the application is a router. Other applications might have wildly different kinds of data and operations—it all comes down to what the YANG model contains.

  • Configure: For example, decide where the log files are stored, state which speed a network interface uses, and declare whether a particular routing protocol is disabled or enabled, and if so, which peers it will have.

  • Monitor status: For example, read how many lost packets there are on each network interface, check what the fan speeds are, and list which peers are actually alive in the network.

  • Receive notifications: For example, hear that a virtual machine is now ready for work, be warned of the temperature crossing a configured threshold, or be alerted of repeated login failures.

  • Invoke actions: For example, reset the lost packet counters, run a traceroute from the system to some address, or execute a system reboot.

As the author of the next cool application, it is up to you to decide what goes in the YANG module for your application. In the context of YANG, the application is an abstract service, such as a Layer 3 virtual private network (L3VPN) or access control service. Those applications often use networking devices, such as a router, load balancer, and base station controller. They could equally well be devices from other domains, such as a power distribution grid controller, a warehouse robot control system, and an office building control system. What else goes in your YANG module? In collaboration with the application operations engineer, you are perhaps able to select which authentication mechanism the application will use and where the authentication server sits. You may also want to provide the operations engineer with an operational status field indicating how many users are currently served by your application. Maybe the application will have a notification to report users abusing the application in some way? How about an action to produce a debug dump of the database contents for troubleshooting? In the end, your application would probably have a lot more that goes in the management interface.

The YANG model of a device is often called its “schema,” as in database schema or blueprint. A schema is basically the structure and content of messages exchanged between the application and the device. This is very different from the instance data—the actual configuration and monitoring data in the system. The instance data describes the current configuration and the current monitoring values. The schema describes the potential configuration, potential monitoring data, potential notifications, and potential actions a manager decides to execute.

The YANG language also incorporates a level of extensibility and flexibility not present in other model languages. New modules can augment the data hierarchies defined in other modules, seamlessly adding data at appropriate places in the existing data organization. YANG also permits new statements to be defined, allowing the language itself to be expanded in a consistent way. Note that the YANG model (keep in mind the API contract) does not change unless there is a software upgrade of the server, or perhaps if a license for a new feature is installed. The instance data changes as soon as a manager decides to change the configuration or if the system state changes due to internal or external events.

A small YANG model may declare just a dozen different elements, or even a single one (for example, a model that just defines the reboot operation). Some YANG models are very large, however, with thousands of elements. In a car or factory, each element would correspond to a button, control dial, indicator gauge, meter, or light. A modern car might have over a hundred elements on the dashboard and around the vehicle. A nuclear power plant might have over a thousand. Core routers are vastly more complicated, however, with well over 100,000 control interface elements.

Since the schema defines elements that can have many instances (for example, interfaces or access control rules), the instance data can be very much larger than the schema, with many millions of instances.

The YANG language itself is defined by the Internet Engineering Task Force (IETF).13 The latest version is YANG 1.1, defined in RFC 7950, and YANG 1.0 is an older version defined in RFC 6020. As of this writing, both YANG 1.0 and YANG 1.1 are in popular use. This new YANG 1.1 (RFC 7950) does not obsolete YANG 1.0 (RFC 6020). YANG 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. As a reference, YANG concepts are explained in Chapter 3. The extra YANG 1.1 capabilities are documented in Section 1.1 of RFC 7950. These days, YANG 1.1 should be the default when you’re writing YANG modules.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020