2. YANG Boot Camp
The YANG Gang
IETF 71

3. NETCONF Modeling Language
NETCONF base protocol (rfc 4741)
RPC mechanism and operations
Left content and data models for future work
Operations allow any XML
<get>, <get-config> and <edit-config>
Mgmt Application
Devices
YANG modules

4. YANG is …. A NETCONF modeling language
Think SMI for NETCONF
Models semantics and data organization
Syntax falls out of semantics
Able to model config data, state data, RPCs, and notifications
Based on SMIng syntax
Text-based
, patch, and RFC friendly

5. YANG Concepts Standard Models Proprietary Models config data RPCs
notifications
containers
leafs
types

6. YANG .... Directly maps to XML content (on the wire) Extensible
Add new content to existing data models
Without changing the original model
Add new statements to the YANG language
Vendor extensions and future proofing
Preserves investment in SNMP MIBs
libsmi translates MIBs to YANG
See tools at

7. Semantics and syntax Semantic World YANG Syntactic World XML Schema
Relax-NG
Anything Else
Out
going
XML
Legacy tools
Validation

8. YANG values .... Readers and reviewers time and learning curve
Readability is highest priority
Limited Scope
Doesn't boil the ocean
Maximize utility within that scope
Can be extended in the future
Experience gained by existing implementations
Based on four proprietary modeling languages
Years of experience within multiple vendors
Quality draft backed by running code

9. Modules and submodules
Header statements
yang-version, namespace, prefix
Linkage statement
import and include
Meta information
organization, contact
Revision history
revision
Mod1
Import
Mod2
Include
SubA
Include
SubX
SubY
SubZ

10. module acme-module {
namespace "
prefix acme;
import "yang-types" {
prefix yang; }
include "acme-system";
organization "ACME Inc.";
contact
description "The module for entities
implementing the ACME products";
revision {
description "Initial revision."; …

11. The "leaf" Statement A leaf has one value no children one instance
YANG Example:
leaf host-name {
type string;
mandatory true;
config true;
description "Hostname for this system"; }
NETCONF XML Encoding:
<host-name>my.example.com</host-name>

12. The "leaf-list" Statement
A leaf-list has
one value
no children
multiple instances
YANG Example:
leaf-list domain-search {
type string;
ordered-by user;
description "List of domain names to search"; }
NETCONF XML Encoding:
<domain-search>high.example.com</domain-search>
<domain-search>low.example.com</domain-search>
<domain-search>everywhere.example.com</domain-search>

13. The "container" Statement
A container has
no value
holds related children
one instance
YANG Example:
container system {
container services {
container ssh {
presence "Enables SSH";
description "SSH service specific configuration";
// more leafs, containers and stuff here... }
May have specific meaning (presence)
Or may simply contain other nodes
NETCONF XML Encoding:
<system>
<services>
<ssh/>
</services>
</system>

14. The "must" Statement Constrains nodes by XPath expression
container timeout {
leaf access-timeout {
description "Maximum time without server response";
units seconds;
mandatory true;
type uint32; }
leaf retry-timer {
description "Period to retry operation";
must "$this < ../access-timeout" {
error-app-tag retry-timer-invalid;
error-message "The retry timer must be "
+ "less than the access timeout";

15. The "list" Statement A list is uniquely identified by key(s)
holds related children
no value
multiple instances
YANG Example:
list user {
key name;
leaf name {
type string; }
leaf uid {
type uint32;
leaf full-name {
leaf class {
default viewer;
NETCONF XML Encoding:
<user>
<name>glocks</name>
<full-name>Goldie</full-name>
<class>intruder</class>
</user>
<user>
<name>snowey</name>
<full-name>Snow</full-name>
<class>free-loader</class>
<name>rzull</name>
<full-name>Repun</full-name>

16. The "augment" Statement Extends data model Inserts nodes
Current or imported modules
Inserts nodes
Into an existing hierarchy
Nodes appear in current module's namespace
Original (augmented) module is unchanged
YANG Example:
augment system/login/user {
leaf expire {
type yang:date-and-time; }
NETCONF XML Encoding:
<user>
<name>alicew</name>
<class>drop-out</class>
<other:expire> T12:00:00</other:expire>
</user>

17. The "when" Statement Makes sparse augmentation
YANG Example:
augment system/login/user {
when "class = wheel";
leaf shell {
type string; }
Makes sparse augmentation
Nodes are only added when condition is true
"when" is XPath expression
NETCONF XML Encoding:
<user>
<name>alicew</name>
<class>wheel</class>
<other:shell>/bin/tcsh</other:shell>
</user>

18. Built-in types Category Types Integral {,u}int{8,16,32,64} String
string, enumeration, boolean
Binary Data
binary
Bit fields
bits
References
instance-identifier, keyref
Other
empty

19. Derived types Constraints
YANG Example:
typedef percent {
type uint16 {
range " "; }
description "Percentage";
leaf completed {
type percent;
Constraints
range
length
pattern
regex
A modules may use types imported from other modules
NETCONF XML Encoding:
<completed>20</completed>

20. The "union" type Allows a leaf to contain a superset of types
YANG Example:
leaf limit {
description "Number to allow";
type union {
type uint16 {
range " "; }
type enumeration {
enum none {
description "No limit";
NETCONF XML Encoding:
<limit>none</limit>
NETCONF XML Encoding:
<limit>20</limit>

21. The "grouping" Statement
Defines a reusable collection of nodes
Use multiple times
A modules may use groupings imported from other modules
Refinement
Use as structure, record, or object
YANG Example
grouping target {
leaf address {
type inet:ip-address;
description "Target IP address"; }
leaf port {
type inet:ip-port;
description "Target port number";
container peer {
container destination {
uses target;
NETCONF XML Encoding:
<peer>
<destination>
<address> </address>
<port>22</port>
</destination>
</peer>

22. The "choice" Statement
Allow only one member of the choice to exist in a valid config datastore
YANG Example:
choice transfer-method {
leaf transfer-interval {
description "Frequency at which file transfer happens";
type uint {
range " "; }
units minutes;
leaf transfer-on-commit {
description "Transfer after each commit";
type empty;
NETCONF XML Encoding:
<transfer-on-commit/>

23. The "anyxml" Statement
Allows arbitrary XML content to be carried in YANG-based models
Opaque
Limited operations
Bulk only
YANG Example:
anyxml software-version {
description "Number to allow"; }
NETCONF XML Encoding:
<software-version>
<base>A10.2</base>
<routing>B4.2</routing>
<snmp>C87.12</snmp>
</software-version>

24. The "rpc" Statement Defines RPC method names input parameters
rpc activate-software-image {
input {
leaf image-name {
type string; }
output {
leaf status {
Defines RPC
method names
input parameters
output parameters
<rpc xmlns="urn:mumble">
<activate-software-image>
<image-name>image.tgz</image-name>
</activate-software-image>
</rpc>

25. The "notification" Statement
Defines notification
Name
Content
YANG Example:
notification link-failure {
description "A link failure has been detected";
leaf if-index {
type int32 { range "1 .. max"; } }
leaf if-name {
type keyref {
path "/interfaces/interface/name";

26. Semantic Differentiators
Notice that YANG is modeling the semantics and data organization
Not just the syntax
Statement
Purpose
unique
Ensure unique values within list siblings
keyref
Ensure referential integrity
config
Indicate if a node is config data or not
default
Supply default value for leafs
error-app-tag
Define the tag used when constraint fails
error-message
Define the message used ....
mandatory
Node must exist in valid config datastore

27. Tools (yang-central.org)
pyang (python)
Validates YANG
Translates between YANG and YIN (XML)
Generates XSD
yangto (binary)
Generates XSD, dependencies, etc
libsmi
Translates SMI/SMIv2 MIBs to YANG
Other goodies
Emacs mode

28. What can you do to help? Read the draft Join the mailing list
There's a lot more in there
Join the mailing list
Try out the tools
Tutorial (this) at: