]> Telefonica

oscar.gonzalezdedios@telefonica.com

Telefonica

samier.barguilgiraldo.ext@telefonica.com

Orange

mohamed.boucadair@orange.com

Operations and Management netmod Internet-Draft RFC 8519 defines a YANG data model for Access Control Lists (ACLs). This document discusses a set of extensions that fix many of the limitations of the ACL model as initially defined in RFC 8519. Discussion Venues Discussion of this document takes place on the Network Modeling Working Group mailing list (netmod@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction defines Access control lists (ACLs) as a user-ordered set of filtering rules. The model targets the configuration of the filtering behaviour of a device. However, the model structure, as defined in , suffers from a set of limitations. This document describes these limitations and proposes an enhanced ACL structure. The YANG module in this document is solely based on augmentations to the ACL YANG module defined in . The motivation of such enhanced ACL structure is discussed in detail in . When managing ACLs, it is common for network operators to group match elements in pre-defined sets. The consolidation into group matches allows for reducing the number of rules, especially in large scale networks. If it is needed, for example, to find a match against 100 IP addresses (or prefixes), a single rule will suffice rather than creating individual Access Control Entries (ACEs) for each IP address (or prefix). In doing so, implementations would optimize the performance of matching lists vs multiple rules matching. The enhanced ACL structure is also meant to facilitate the management of network operators. Instead of entering the IP address or port number literals, using user-named lists decouples the creation of the rule from the management of the sets. Hence, it is possible to remove/add entries to the list without redefining the (parent) ACL rule. In addition, the notion of Access Control List (ACL) and defined sets is generalized so that it is not device-specific as per . ACLs and defined sets may be defined at network / administrative domain level and associated to devices. This approach facilitates the reusability across multiple network elements. For example, managing the IP prefix sets from a network level makes it easier to maintain by the security groups. Network operators maintain sets of IP prefixes that are related to each other, e.g., deny-lists or accept-lists that are associated with those provided by a VPN customer. These lists are maintained and manipulated by security expert teams. Note that ACLs are used locally in devices but are triggered by other tools such as DDoS mitigation or BGP Flow Spec . Therefore, supporting means to easily map to the filtering rules conveyed in messages triggered by these tools is valuable from a network operation standpoint.

Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in . The terminology for describing YANG modules is defined in . The meaning of the symbols in the tree diagrams is defined in . In addition to the terms defined in , this document makes use of the following terms:

• Defined set: Refers to reusable description of one or multiple information elements (e.g., IP address, IP prefix, port number, or ICMP type).

Problem Statement & Gap Analysis

Suboptimal Configuration: Lack of Support for Lists of Prefixes IP prefix related data nodes, e.g., "destination-ipv4-network" or "destination-ipv6-network", do not support handling a list of IP prefixes, which may then lead to having to support large numbers of ACL entries in a configuration file. The same issue is encountered when ACLs have to be in place to mitigate DDoS attacks (e.g., when a set of sources are involved in such an attack. The situation is even worse when both a list of sources and destination prefixes are involved. shows an example of the required ACL configuration for filtering traffic from two prefixes.

Example Illustrating Sub-optimal Use of the ACL Model with a Prefix List

Such a configuration is suboptimal for both: - Network controllers that need to manipulate large files. All or a subset for this configuration will need to be passed to the underlying network devices

• Devices may receive such a confirguration and thus will need to maintain it locally.

( depicts an example of an optimized structure:

Example Illustrating Optimal Use of the ACL Model in a Network Context.

Manageability: Impossibility to Use Aliases or Defined Sets The same approach as the one discussed for IP prefixes can be generalized by introduing the concept of "aliases" or "defined sets". The defined sets are reusable definitions across several ACLs. Each category is modelled in YANG as a list of parameters related to the class it represents. The following sets can be considered:

• Prefix sets: Used to create lists of IPv4 or IPv6 prefixes.
• Protocol sets: Used to create a list of protocols.
• Port number sets: Used to create lists of TCP or UDP port values (or any other transport protocol that makes uses of port numbers). The identity of the protocols is identified by the protocol set, if present. Otherwise, a set applies to any protocol.
• ICMP sets: Uses to create lists of ICMP-based filters. This applies only when the protocol is set to ICMP or ICMPv6.

A candidate structure is shown in :

Examples of Defined Sets.

Bind ACLs to Devices, Not Only Interfaces In the context of network management, an ACL may be enforced in many network locations. As such, the ACL module should allow for binding an ACL to multiple devices, not only (abstract) interfaces. The ACL name must, thus, be unique at the scale of the network, but the same name may be used in many devices when enforcing node-specific ACLs.

Partial or Lack of IPv4/IPv6 Fragment Handling does not support fragment handling capability for IPv6 but offers a partial support for IPv4 by means of 'flags'. Nevertheless, the use of 'flags' is problematic since it does not allow a bitmask to be defined. For example, setting other bits not covered by the 'flags' filtering clause in a packet will allow that packet to get through (because it won't match the ACE). Defining a new IPv4/IPv6 matching field called 'fragment' is thus required to efficiently handle fragment-related filtering rules.

Suboptimal TCP Flags Handling supports including flags in the TCP match fields, however that structure does not support matching operations as those supported in BGP Flow Spec. Defining this field to be defined as a flag bitmask together with a set of operations is meant to efficiently handle TCP flags filtering rules.

Rate-Limit Action specifies that forwarding actions can be 'accept' (i.e., accept matching traffic), 'drop' (i.e., drop matching traffic without sending any ICMP error message), or 'reject' (i.e., drop matching traffic and send an ICMP error message to the source). However, there are situations where the matching traffic can be accepted, but with a rate-limit policy. Such capability is not currently supported by .

Payload-based Filtering Some transport protocols use existing protocols (e.g., TCP or UDP) as substrate. The match criteria for such protocols may rely upon the 'protocol' under 'l3', TCP/UDP match criteria, part of the TCP/UDP payload, or a combination thereof. does not support matching based on the payload. Likewise, the current version of the ACL model does not support filtering of encapsulated traffic.

Reuse the ACLs Content Across Several Devices Having a global network view of the ACLs is highly valuable for service providers. An ACL could be defined and applied following the hierarchy of the network topology. So, an ACL can be defined at the network level and, then, that same ACL can be used (or referenced to) in several devices (including termination points) within the same network. This network/device ACLs differentiation introduces several new requirements, e.g.:

• An ACL name can be used at both network and device levels.
• An ACL content updated at the network level should imply a transaction that updates the relevant content in all the nodes using this ACL.
• ACLs defined at the device level have a local meaning for the specific node.
• A device can be associated with a router, a VRF, a logical system, or a virtual node. ACLs can be applied in physical and logical infrastructure.

Overall Module Structure

Enhanced ACL

Enhanced ACL tree ../../../../defined-sets/port-sets/port-set/name +--rw destination-tcp-port-set? -> ../../../../defined-sets/port-sets/port-set/name augment /ietf-acl:acls/ietf-acl:acl/ietf-acl:aces/ietf-acl:ace /ietf-acl:matches/ietf-acl:l4/ietf-acl:udp: +--rw source-udp-port-set? | -> ../../../../defined-sets/port-sets/port-set/name +--rw destination-udp-port-set? -> ../../../../defined-sets/port-sets/port-set/name augment /ietf-acl:acls/ietf-acl:acl/ietf-acl:aces/ietf-acl:ace /ietf-acl:matches/ietf-acl:l4/ietf-acl:icmp: +--rw icmp-set? leafref augment /ietf-acl:acls/ietf-acl:acl/ietf-acl:aces/ietf-acl:ace /ietf-acl:actions: +--rw rate-limit? decimal64 ]]>

Defined sets The augmented ACL structure includes several containers to manage reusable sets of elements that can be matched in an ACL entry. Each set is uniquely identified by a name, and can be called from the relevant entry. The following sets are defined:

• IPv4 prefix set: It contains a list of IPv4 prefixes. A match will be considered if the IP address (source or destination, depending on the ACL entry) is contained in any of the prefixes.
• IPv6 prefix set: It contains a list of IPv6 prefixes. A match will be considered if the IP address (source or destination, depending on the ACL entry) is contained in any of the prefixes.
• Port sets: It contains a list of port numbers to be used in TCP / UDP entries. The ports can be individual port numbers, a range of ports, and an operation.
• Protocol sets: It contains a list of protocol values. Each protocol can be identified either by a number (e.g., 17) or a name (e.g., UDP).
• ICMP sets: It contains a list of ICMP types, each of them identified by a type value, optionally the code and the rest of the header.

TCP Flags Handling The augmented ACL structure includes a new leaf 'flags-bitmask' to better handle flags. Clients that support both 'flags-bitmask' and 'flags' matching fields MUST NOT set these fields in the same request. shows an example of a request to install a filter to discard incoming TCP messages having all flags unset.

Example to Deny TCP Null Attack Messages

Fragments Handling The augmented ACL structure includes a new leaf 'fragment' to better handle fragments. Clients that support both 'fragment' and 'flags' matching fields MUST NOT set these fields in the same request. shows the content of a POST request to allow the traffic destined to 198.51.100.0/24 and UDP port number 53, but to drop all fragmented packets. The following ACEs are defined (in this order):

• "drop-all-fragments" ACE: discards all fragments.
• "allow-dns-packets" ACE: accepts DNS packets destined to 198.51.100.0/24.

Example Illustrating Candidate Filtering of IPv4 Fragmented Packets.

shows an example of the body of a POST request to allow the traffic destined to 2001:db8::/32 and UDP port number 53, but to drop all fragmented packets. The following ACEs are defined (in this order):

• "drop-all-fragments" ACE: discards all fragments (including atomic fragments). That is, IPv6 packets that include a Fragment header (44) are dropped.
• "allow-dns-packets" ACE: accepts DNS packets destined to 2001:db8::/32.

Example Illustrating Candidate Filtering of IPv6 Fragmented Packets.

Rate-Limit Traffic In order to support rate-limiting (see ), a new action called "rate-limit" is defined. (#example_5) shows an ACL example to rate-limit incoming SYNs during a SYN flood attack.

Example Rate-Limit Incoming TCP SYNs

YANG Modules

Enhanced ACL WG List: Author: Mohamed Boucadair Author: Samier Barguil Author: Oscar Gonzalez de Dios "; description "This module contains YANG definitions for enhanced ACLs. Copyright (c) 2022 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX; see the RFC itself for full legal notices."; revision 2022-10-24 { description "Initial revision."; reference "RFC XXXX: Extensions to the Access Control Lists (ACLs) YANG Model"; } feature match-on-payload { description "Match based on a pattern is supported."; } identity offset-type { description "Base identity for payload offset type."; } identity layer3 { base offset-type; description "IP header."; } identity layer4 { base offset-type; description "Transport header (e.g., TCP or UDP)."; } identity payload { base offset-type; description "Transport payload. For example, this represents the beginning of the TCP data right after any TCP options."; } typedef operator { type bits { bit not { position 0; description "If set, logical negation of operation."; } bit match { position 1; description "Match bit. This is a bitwise match operation defined as '(data & value) == value'."; } bit any { position 2; description "Any bit. This is a match on any of the bits in bitmask. It evaluates to 'true' if any of the bits in the value mask are set in the data, i.e., '(data & value) != 0'."; } } description "Specifies how to apply the defined bitmask. 'any' and 'match' bits must not be set simultaneously."; } description "How to apply the defined bitmask."; } typedef fragment-type { type bits { bit df { position 0; description "Don't fragment bit for IPv4. Must be set to 0 when it appears in an IPv6 filter."; } bit isf { position 1; description "Is a fragment."; } bit ff { position 2; description "First fragment."; } bit lf { position 3; description "Last fragment."; } } description "Different fragment types to match against."; } grouping tcp-flags { description "Operations on TCP flags."; leaf operator { type operator; default "match"; description "How to interpret the TCP flags."; } leaf bitmask { type uint16; description "The bitmask matches the last 4 bits of byte 12 and byte 13 of the TCP header. For clarity, the 4 bits of byte 12 corresponding to the TCP data offset field are not included in any matching."; } } grouping fragment-fields { description "Operations on fragment types."; leaf operator { type operator; default "match"; description "How to interpret the fragment type."; } leaf type { type fragment-type; description "What fragment type to look for."; } } grouping payload { description "Operations on payload match."; leaf offset { type identityref { base offset-type; } description "Indicates the payload offset."; } leaf offset-end { type uint64; description "Indicates the number of bytes to cover when performing the prefix match."; } leaf operator { type operator; default "match"; description "How to interpret the prefix match."; } leaf prefix { type binary; description "The pattern to match against."; } } augment "/ietf-acl:acls/ietf-acl:acl" { description "add a new container to store sets (prefix sets, port sets, etc"; container defined-sets { description "Predefined sets of attributes used in policy match statements."; container ipv4-prefix-sets { description "Data definitions for a list of IPv4 or IPv6 prefixes which are matched as part of a policy."; list prefix-set { key "name"; description "List of the defined prefix sets."; leaf name { type string; description "Name of the prefix set -- this is used as a label to reference the set in match conditions."; } leaf description { type string; description "Defined Set description."; } leaf-list prefix { type inet:ipv4-prefix; description "List of IPv4 prefixes to be used in match conditions."; } } } container ipv6-prefix-sets { description "Data definitions for a list of IPv6 prefixes which are matched as part of a policy."; list prefix-set { key "name"; description "List of the defined prefix sets."; leaf name { type string; description "Name of the prefix set -- this is used as a label to reference the set in match conditions."; } leaf description { type string; description "A textual description of the prefix list."; } leaf-list prefix { type inet:ipv6-prefix; description "List of IPv6 prefixes to be used in match conditions."; } } } container port-sets { description "Data definitions for a list of ports which can be matched in policies."; list port-set { key "name"; description "List of port set definitions."; leaf name { type string; description "Name of the port set -- this is used as a label to reference the set in match conditions."; } list port { key "id"; description "Port numbers along with the operator on which to match."; leaf id { type string; description "Identifier of the list of port numbers."; } choice port { description "Choice of specifying the port number or referring to a group of port numbers."; container port-range-or-operator { description "Indicates a set of ports."; uses packet-fields:port-range-or-operator; } } } } } container protocol-sets { description "Data definitions for a list of protocols which can be matched in policies."; list protocol-set { key "name"; description "List of protocol set definitions."; leaf name { type string; description "Name of the protocols set -- this is used as a label to reference the set in match conditions."; } leaf-list protocol { type union { type uint8; type string; //Check if we can reuse an IANA-maintained module } description "Value of the protocl set."; } } } container icmp-type-sets { description "Data definitions for a list of ICMP types which can be matched in policies."; list icmp-type-set { key "name"; description "List of ICMP type set definitions."; leaf name { type string; description "Name of the ICMP type set -- this is used as a label to reference the set in match conditions."; } list types { key "type"; description "Includes a list of ICMP types."; uses packet-fields:acl-icmp-header-fields; } } } } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches" { description "Add a new match types."; choice payload { description "Match a prefix pattern."; container prefix-pattern { if-feature "match-on-payload"; description "Rule to perform payload-based match."; uses payload; } } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches/ietf-acl:l3/ietf-acl:ipv4" { description "Handle non-initial and initial fragments for IPv4 packets."; container ipv4-fragment { description "Indicates how to handle IPv4 fragments."; uses fragment-fields; } leaf source-ipv4-prefix-list { type leafref { path "../../../../defined-sets/ipv4-prefix-sets/prefix-set/name"; } description "A reference to a prefix list to match the source address."; } leaf destination-ipv4-prefix-list { type leafref { path "../../../../defined-sets/ipv4-prefix-sets/prefix-set/name"; } description "A reference to a prefix list to match the destination address."; } leaf next-header-set { type leafref { path "../../../../defined-sets/protocol-sets/protocol-set/name"; } description "A reference to a protocol set to match the next-header field."; } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches/ietf-acl:l3/ietf-acl:ipv6" { description "Handles non-initial and initial fragments for IPv6 packets."; container ipv6-fragment { description "Indicates how to handle IPv6 fragments."; uses fragment-fields; } leaf source-ipv6-prefix-list { type leafref { path "../../../../defined-sets/ipv6-prefix-sets/prefix-set/name"; } description "A reference to a prefix list to match the source address."; } leaf destination-ipv6-prefix-list { type leafref { path "../../../../defined-sets/ipv6-prefix-sets/prefix-set/name"; } description "A reference to a prefix list to match the destination address."; } leaf protocol-set { type leafref { path "../../../../defined-sets/protocol-sets/protocol-set/name"; } description "A reference to a protocol set to match the protocol field."; } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches/ietf-acl:l4/ietf-acl:tcp" { description "Handles TCP flags and port sets."; container flags-bitmask { description "Indicates how to handle TCP flags."; uses tcp-flags; } leaf source-tcp-port-set { type leafref { path "../../../../defined-sets/port-sets/port-set/name"; } description "A reference to a port set to match the source port."; } leaf destination-tcp-port-set { type leafref { path "../../../../defined-sets/port-sets/port-set/name"; } description "A reference to a port set to match the destination port."; } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches/ietf-acl:l4/ietf-acl:udp" { description "Handle UDP port sets."; leaf source-udp-port-set { type leafref { path "../../../../defined-sets/port-sets/port-set/name"; } description "A reference to a port set to match the source port."; } leaf destination-udp-port-set { type leafref { path "../../../../defined-sets/port-sets/port-set/name"; } description "A reference to a port set to match the destination port."; } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:matches/ietf-acl:l4/ietf-acl:icmp" { description "Handle ICMP type sets."; leaf icmp-set { type leafref { path "../../../../defined-sets/icmp-type-sets/icmp-type-set/name"; } description "A reference to an ICMP type set to match the ICMP type field."; } } augment "/ietf-acl:acls/ietf-acl:acl/ietf-acl:aces" + "/ietf-acl:ace/ietf-acl:actions" { description "Rate-limit action."; leaf rate-limit { when "../ietf-acl:forwarding = 'ietf-acl:accept'" { description "Rate-limit valid only when accept action is used."; } type decimal64 { fraction-digits 2; } units "bytes per second"; description "Indicates a rate-limit for the matched traffic."; } } } ]]>

Security Considerations (TBC) The YANG modules specified in this document define a schema for data that is designed to be accessed via network management protocol such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS . The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability:

• TBC

Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability:

• TBC

IANA Considerations

URI Registration This document requests IANA to register the following URI in the "ns" subregistry within the "IETF XML Registry" :

YANG Module Name Registration This document requests IANA to register the following YANG module in the "YANG Module Names" subregistry within the "YANG Parameters" registry.

References Normative References This document defines a data model for Access Control Lists (ACLs). An ACL is a user-ordered set of rules used to configure the forwarding behavior in a device. Each rule is used to find a match on a packet and define actions that will be performed on the packet. "Dissemination of Flow Specification Rules" (RFC 8955) provides a Border Gateway Protocol (BGP) extension for the propagation of traffic flow information for the purpose of rate limiting or filtering IPv4 protocol data packets. This document extends RFC 8955 with IPv6 functionality. It also updates RFC 8955 by changing the IANA Flow Spec Component Types registry. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). This document describes a method for invoking and running the Network Configuration Protocol (NETCONF) within a Secure Shell (SSH) session as an SSH subsystem. This document obsoletes RFC 4742. [STANDARDS-TRACK] This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletes RFC 6536. This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas. YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS-TRACK] Informative References This document specifies the Distributed Denial-of-Service Open Threat Signaling (DOTS) signal channel, a protocol for signaling the need for protection against Distributed Denial-of-Service (DDoS) attacks to a server capable of enabling network traffic mitigation on behalf of the requesting client. A companion document defines the DOTS data channel, a separate reliable communication layer for DOTS management and configuration purposes. This document obsoletes RFC 8782. This document defines a Border Gateway Protocol Network Layer Reachability Information (BGP NLRI) encoding format that can be used to distribute (intra-domain and inter-domain) traffic Flow Specifications for IPv4 unicast and IPv4 BGP/MPLS VPN services. This allows the routing system to propagate information regarding more specific components of the traffic aggregate defined by an IP destination prefix. It also specifies BGP Extended Community encoding formats, which can be used to propagate Traffic Filtering Actions along with the Flow Specification NLRI. Those Traffic Filtering Actions encode actions a routing system can take if the packet matches the Flow Specification. This document obsoletes both RFC 5575 and RFC 7674. This document captures the current syntax used in YANG module tree diagrams. The purpose of this document is to provide a single location for this definition. This syntax may be updated from time to time based on the evolution of the YANG language.

Acknowledgements Many thanks to Jon Shallow and Miguel Cros for the discussion when preparing this document.