YANG in a Nutshell

1
YANG in a Nutshell

• The YANG Gang
• IETF 71

2
YANG has...

• A reasonable self-contained specification
• A focus on readers and reviewers
• Text-based
• Email, patch, and RFC friendly
• Limited Scope, but extensible
• Ability to model config data, state data, RPCs,
  and notifications
• Experience gained by existing implementations
• Design is based on four proprietary modeling
  languages

3
Support for YANG

• Running code (multiple implementations)
• People outside the YANG team using it in other
  WGs in the IETF
• Backing of several NETCONF implementers
• Growing group of fans and admirers

4
Semantics and syntax

• Models semantics and data organization
• Syntax falls out of semantics
• Think SMI for NETCONF

Semantic World
YANG
Syntactic World
Anything Else
XML Schema
Relax-NG
Out going XML
Legacy tools
Validation
5
Easy to read, easy to learn

• Grok factor
• Direct encoding of data hierarchy

container system leaf host-name
type inethost description "Name of this
host" container services
container ssh presence "Enables
SSH" description "SSH service
specific configuration" // more
leafs, containers and stuff here...

ltsystemgt lthost-namegtfredlt/host-namegt
ltservicesgt ltssh/gt lt/servicesgt lt/systemgt
6
Model-Based Infrastructure
Management application

• YANG can form the infrastructure for the
  management plane inside the device
• Not required, but common wisdom
• Three implementations do this
• Ease the maintenance, stability, and
  enhance-ability of the device

YANG Modules
CLI
NETCONF Server
YANG Modules
Management Plane
S1
S2
S3
Subcomponents
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Constraints using "must"

• 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" units seconds
type uint32 must "this lt
../access-timeout" error-app-tag
retry-timer-invalid error-message
"The retry timer must be "
"less than the access timeout"

• Also
• keyref
• unique
• range
• pattern

8
Extensibility with "augment"

• Extends data model
• Current or imported modules
• Inserts nodes
• Into an existing hierarchy
• Nodes appear in current module's namespace
• Original (augmented) module is unchanged
• Allows sparse augmentation
• Nodes are only added when condition is true
• "when" is XPath expression

augment interfaces/interface when "type
'ethernet'" container ether-options
leaf auto-negotiation type empty

ltinterfacesgt ltinterfacegt ltnamegtfe-0/0/0lt/nam
egt lttypegtethernetlt/typegt ltether-optionsgt
ltauto-negotiation/gt lt/ether-optionsgt
lt/interfacegt lt/interfacesgt
9
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 inetip-address
description "Target IP address" leaf
port type inetip-port
description "Target port number"
container peer container destination
uses target
NETCONF XML Encoding ltpeergt ltdestinationgt
ltaddressgt192.0.2.1lt/addressgt ltportgt22lt/portgt
lt/destinationgt lt/peergt
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The "rpc" Statement
rpc activate-software-image input
leaf image-name type string
output leaf status
type string

• Defines RPC
• method names
• input parameters
• output parameters

ltrpc xmlns"urnmumble"gt ltactivate-software-imag
egt ltimage-namegtimage.tgzlt/image-namegt
lt/activate-software-imagegt lt/rpcgt
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Tools (yang-central.org)

• pyang (python)
• Validates YANG
• Translates between YANG and YIN (XML)
• Generates XSD
• yangto (binary)
• Validates YANG
• Generates XSD, dependencies, etc
• libsmi
• Translates SMI/SMIv2 MIBs to YANG
• Other goodies
• Emacs mode

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YANGsters Unite!

• Read the draft
• There's a lot more in there
• Join the mailing list
• yang_at_ietf.org
• https//www.ietf.org/mailman/listinfo/yang
• Try out the tools
• www.yang-central.org
• Tutorial at
• http//www.yang-central.org/twiki/bin/view/Main/Ya
  ngTutorials