Internet DRAFT - draft-bormann-cbor-yang-standin
draft-bormann-cbor-yang-standin
Concise Binary Object Representation Maintenance and ExtensionsC. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Standards Track M. Matejka
Expires: 24 August 2024 CZ.NIC
21 February 2024
Stand-in Tags for YANG-CBOR
draft-bormann-cbor-yang-standin-00
Abstract
YANG (RFC 7950) is a data modeling language used to model
configuration data, state data, parameters and results of Remote
Procedure Call (RPC) operations or actions, and notifications.
YANG-CBOR (RFC 9254) defines encoding rules for YANG in the Concise
Binary Object Representation (CBOR) (RFC 8949). While the overall
structure of YANG-CBOR is encoded in an efficient, binary format,
YANG itself has its roots in XML and therefore traditionally encodes
some information such as date/times and IP addresses/prefixes in a
verbose text form.
This document defines how to use existing CBOR tags for this kind of
information in YANG-CBOR as a "stand-in" for the text-based
information that would be found in the original form of YANG-CBOR.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-cbor-yang-standin/.
Discussion of this document takes place on the CBOR (Concise Binary
Object Representation Maintenance and Extensions) Working Group
mailing list (mailto:cbor@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at
https://www.ietf.org/mailman/listinfo/cbor/.
Source for this draft and an issue tracker can be found at
https://github.com/cabo/yang-standin.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Stand-In Tags . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. ietf-yang-types: Tag 1 (Date/Time) and Tag 100 (Date) . . 4
3.2. ietf-inet-types: Tags 54 and 52 (IP addresses and
prefixes) . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Union handling . . . . . . . . . . . . . . . . . . . . . 8
4. Using Stand-In Tags . . . . . . . . . . . . . . . . . . . . . 8
4.1. Defining Stand-In Usage in Schema . . . . . . . . . . . . 8
4.2. Original stand-ins . . . . . . . . . . . . . . . . . . . 9
4.3. Legacy Round Trip . . . . . . . . . . . . . . . . . . . . 9
5. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. stand-in tags? . . . . . . . . . . . . . . . . . . . . . 10
7.2. media-type parameters . . . . . . . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
(see abstract)
2. Conventions and Definitions
The terminology of [RFC9254] applies.
Legacy representation: The (often text-based) representation for a
YANG data item as used in YANG-XML, YANG-JSON, and (unchanged)
YANG-CBOR.
Stand-in tag: A CBOR tag that can supply the information that is
equivalent to a legacy representation in a more efficient format
(e.g., using binary data).
Encoder: The party which generates (sends) CBOR data described by
YANG.
Intermediate Encoder: An encoder which isn't the original author of
the data, converting it from legacy representation.
Decoder: The party which receives and parses CBOR data described by
YANG.
Intermediate Decoder: A decoder which isn't the final recipient of
the data, converting it to legacy representation.
Data Transfer: A series of actions, generally beginning by data
origination, encoding, continuing by optional intermediate
transcoding, sending and receiving, and finally decoding and
consuming.
Round Trip: Part of a data transfer between an encoder generating
CBOR data with stand-in tags and a decoder parsing the data.
Legacy Round Trip: A Round Trip where the encoder is an intermediate
encoder or the decoder is an intermediate decoder and any of these
converts from or to the legacy representation.
Unambiguous Round Trip: A Legacy Round Trip that provides exactly
the same legacy representation (not just semantically equivalent).
The stand-in tag is also said to "unambiguously stand in" for the
legacy representation.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Stand-In Tags
This document defines two sets of stand-in tags. Where information
starts out in a legacy representation, these tags are only used when
an Unambiguous Round Trip can be achieved.
3.1. ietf-yang-types: Tag 1 (Date/Time) and Tag 100 (Date)
Section 3 of [I-D.schoenw-netmod-rfc6991-bis] defines the following
types in ietf-yang-types:
+=============+========+==================================+========+
| YANG type | base | specification | stand- |
| | type | | in |
+=============+========+==================================+========+
| date-and- | string | [RFC6021] | tag 1 |
| time | | | |
+-------------+--------+----------------------------------+--------+
| date-with- | string | [I-D.schoenw-netmod-rfc6991-bis] | (none) |
| zone-offset | | | |
+-------------+--------+----------------------------------+--------+
| date-no- | string | [I-D.schoenw-netmod-rfc6991-bis] | tag |
| zone | | | 100 |
+-------------+--------+----------------------------------+--------+
Table 1: Legacy representations in ietf-yang-types
Tag 1 (Section 3.4.2 of RFC 8949 [STD94]) can unambiguously stand in
for all date-and-time values that:
* do not specify a time zone (note that
[I-D.schoenw-netmod-rfc6991-bis] uses the legacy "-00:00" format
for time-zone-free date-times)
* are not an inserted leap second (23:59:60 or 23:59:61)
* do not have trailing zeroes in the fractional part of the seconds.
* do not have fractional parts of the seconds with a precision that
cannot be represented in floating-point tag content in a tag 1.
All other date-and-time values stay in legacy representation.
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Tag 1 uses an integer tag content for all date-and-time values
without fractional seconds and a floating-point tag content for
values that have fractional seconds given.
Tag 100 [RFC8943] can unambiguously stand in for all date-no-zone
values.
3.2. ietf-inet-types: Tags 54 and 52 (IP addresses and prefixes)
Section 4 of [I-D.schoenw-netmod-rfc6991-bis] defines in ietf-inet-
types:
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+=============+============+================================+======+
|YANG type |base type |specification |stand-|
| | | |in |
+=============+============+================================+======+
|ip-address |union |[RFC6021] |(see |
| | | |union)|
+-------------+------------+--------------------------------+------+
|ipv6-address |string |[RFC6021] |tag 54|
+-------------+------------+--------------------------------+------+
|ipv4-address |string |[RFC6021] |tag 52|
+-------------+------------+--------------------------------+------+
|ip-address- |union |RFC 6991 |(see |
|no-zone | | |union)|
+-------------+------------+--------------------------------+------+
|ipv6-address-|ipv6-address|RFC 6991 |tag 54|
|no-zone | | | |
+-------------+------------+--------------------------------+------+
|ipv4-address-|ipv4-address|RFC 6991 |tag 52|
|no-zone | | | |
+-------------+------------+--------------------------------+------+
|ip-address- |union |[I-D.schoenw-netmod-rfc6991-bis]|(see |
|link-local | | |union)|
+-------------+------------+--------------------------------+------+
|ipv6-address-|ipv6-address|[I-D.schoenw-netmod-rfc6991-bis]|tag 54|
|link-local | | | |
+-------------+------------+--------------------------------+------+
|ipv4-address-|ipv4-address|[I-D.schoenw-netmod-rfc6991-bis]|tag 52|
|link-local | | | |
+-------------+------------+--------------------------------+------+
|ip-prefix |union |[RFC6021] |(see |
| | | |union)|
+-------------+------------+--------------------------------+------+
|ipv6-prefix |string |[RFC6021] |tag 54|
+-------------+------------+--------------------------------+------+
|ipv4-prefix |string |[RFC6021] |tag 52|
+-------------+------------+--------------------------------+------+
|ip-address- |union |[I-D.schoenw-netmod-rfc6991-bis]|(see |
|and-prefix | | |union)|
+-------------+------------+--------------------------------+------+
|ipv6-address-|string |[I-D.schoenw-netmod-rfc6991-bis]|tag 54|
|and-prefix | | | |
+-------------+------------+--------------------------------+------+
|ipv4-address-|string |[I-D.schoenw-netmod-rfc6991-bis]|tag 52|
|and-prefix | | | |
+-------------+------------+--------------------------------+------+
Table 2: Legacy representations in ietf-yang-types
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An intermediate encoder MAY normalize IPv6 addresses and prefixes
that do not comply with [RFC5952] but can be converted into the
stand-in representation. For example, IPv6 address written as
2001:db8:: is the same as 2001:0db8::0:0 and both would be converted
to 54(h'20010db8000000000000000000000000'), anyway only the first one
complies with [RFC5952]. The encoder MAY refuse to convert the
latter one.
If the schema specifies ip-prefix, an intermediate encoder MAY
normalize prefixes with non-zero bits after the prefix end. For
example, if the legacy representation of ipv6-prefix is
2001:db8:1::/40, the encoder may either refuse it as malformed or
convert it to 2001:db8::/40 and represent as 54([40, h'20010db8']).
The encoder implementation should be clear about which normalizations
are employed and how.
Adapted examples from [RFC9164]:
Stand-in representation of IPv6 address
2001:db8:1234:deed:beef:cafe:face:feed is
54(h'20010db81234deedbeefcafefacefeed').
CBOR encoding of stand-in (19 bytes):
cbor-pretty D8 36 # tag(54) 50 # bytes(16)
20010DB81234DEEDBEEFCAFEFACEFEED
CBOR encoding of legacy representation (40 bytes):
cbor-pretty 78 26 # text(38) 323030313A6462383A313233343A646565643A62
6565663A636166653A666163653A66656564
Stand-in representation of IPv6 prefix 2001:db8:1234::/48 is 54([48,
h'20010db81234']).
CBOR encoding of stand-in (12 bytes):
cbor-pretty D8 36 # tag(54) 82 # array(2) 18 30 # unsigned(48) 46 #
bytes(6) 20010DB81234 # " \u0001\r\xB8\u00124"
CBOR encoding of legacy representation (19 bytes):
cbor-pretty 72 # text(18) 323030313A6462383A313233343A3A2F3438 #
"2001:db8:1234::/48"
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Stand-in representation of IPv6 link-local address
fe80::0202:02ff:ffff:fe03:0303/64%eth0 is
54([h'fe8000000000020202fffffffe030303', 64, 'eth0']).
CBOR encoding of stand-in (27 bytes):
cbor-pretty D8 36 # tag(54) 83 # array(3) 50 # bytes(16)
FE8000000000020202FFFFFFFE030303 18 40 # unsigned(64) 44 # bytes(4)
65746830 # "eth0"
CBOR encoding of legacy representation (40 bytes):
cbor-pretty 78 26 # text(38) 666538303A3A303230323A303266663A66666666
3A666530333A303330332F36342565746830
TO DO: adapt more examples from [RFC9164]
TO DO: Check how the unions in [RFC6021] and
[I-D.schoenw-netmod-rfc6991-bis] interact with this. E.g., the union
ip-address needs to be parsed to decide between tag 54 and tag 52.
3.3. Union handling
When the schema specifies a union data type for a node, there are
additional requirements on the encoder and decoder.
An encoder which is fully aware of data semantics MUST use the
appropriate data type, even though it isn't formally specified by the
schema.
If an intermediate encoder doesn't fully understand the data
semantics, it needs to find out which type the data actually is to
choose the right stand-in. If more types are possible, it MAY choose
any of these which allow for an Unambiguous Round Trip, otherwise it
SHOULD keep the legacy representation.
If a decoder receives data for a union-typed node, it MUST accept any
data type of the union, even though it may violate additional
constraints outside the schema.
4. Using Stand-In Tags
4.1. Defining Stand-In Usage in Schema
TO DO: formally define the YANG extension
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4.2. Original stand-ins
The simplest situation is when no intermediate encoders and decoders
are involved in the data transfer, therefore the round trip is not
legacy. In this case, no conversions are involved and data is
validated using the schema extension from the previous section.
4.3. Legacy Round Trip
Producing a stand-in MUST be triggered by schema usage. Intermediate
encoders MUST NOT encode stand-ins when no schema is available.
It's generally not recommended to do a legacy round trip where both
the encoder and decoder are converting from and to the legacy
representation.
5. Negotiation
Introducing stand-in tags in YANG-CBOR requires some form of consent
between the producer and the consumer of YANG-CBOR information:
* A producer that creates YANG-CBOR containing stand-in tags needs
to know whether the consumer supports stand-in tags, and,
possibly, which specific stand-in tags it supports. We speak
about the _capability_ of a consumer to consume stand-in tags. A
producer MUST NOT employ stand-in tags unless it knows about the
capabilities of the consumer. A consumer SHOULD indicate its
capabilities for consuming stand-in tags.
* A consumer may not want to implement certain legacy text-based
representations where more efficient (and easy to implement)
stand-in tags are available. This places a _requirement_ on the
producer (which needs to have the _capability_ to produce YANG-
CBOR where those stand-in tags are used, in place of legacy
representations). A producer MUST NOT employ legacy
representations where stand-in tags are _required_ by the
consumer. A consumer that has requirements for only receiving
stand-in tags in place of legacy representations, MUST indicate
this to the producer.
ISSUE: Where do we put those two aspects of negotiation?
* NETCONF negotiation
* yang-library
* media-type parameters
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* ?
6. Security Considerations
TODO Security
7. IANA Considerations
7.1. stand-in tags?
ISSUE: Do we want to have a separate registry for stand-in tags?
They already are CBOR tags and thus in the in the registry, but might
get lost in the bulk of that (and are only identified as YANG-CBOR
stand-in Tags in the specification).
7.2. media-type parameters
ISSUE: Should the use of stand-in tags be mentioned in the various
YANG-CBOR-based media types (as a media type parameter)? Compare how
application/yang-data+cbor can use id=name/id=sid to indicate another
encoding decision.
8. Normative References
[I-D.schoenw-netmod-rfc6991-bis]
Schönwälder, J., "Common YANG Data Types", Work in
Progress, Internet-Draft, draft-schoenw-netmod-rfc6991-
bis-01, 11 March 2019,
<https://datatracker.ietf.org/doc/html/draft-schoenw-
netmod-rfc6991-bis-01>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/rfc/rfc5952>.
[RFC6021] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6021, DOI 10.17487/RFC6021, October 2010,
<https://www.rfc-editor.org/rfc/rfc6021>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8943] Jones, M., Nadalin, A., and J. Richter, "Concise Binary
Object Representation (CBOR) Tags for Date", RFC 8943,
DOI 10.17487/RFC8943, November 2020,
<https://www.rfc-editor.org/rfc/rfc8943>.
[RFC9164] Richardson, M. and C. Bormann, "Concise Binary Object
Representation (CBOR) Tags for IPv4 and IPv6 Addresses and
Prefixes", RFC 9164, DOI 10.17487/RFC9164, December 2021,
<https://www.rfc-editor.org/rfc/rfc9164>.
[RFC9254] Veillette, M., Ed., Petrov, I., Ed., Pelov, A., Bormann,
C., and M. Richardson, "Encoding of Data Modeled with YANG
in the Concise Binary Object Representation (CBOR)",
RFC 9254, DOI 10.17487/RFC9254, July 2022,
<https://www.rfc-editor.org/rfc/rfc9254>.
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, December 2020.
<https://www.rfc-editor.org/info/std94>
Acknowledgments
TODO acknowledge.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Maria Matejka
CZ.NIC
Milesovska 1136/5
13000 Praha
Czechia
Email: maria.matejka@nic.cz, mq@jmq.cz
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