Network Working Group I. Petrov, Ed.
Internet-Draft Acklio
Intended status: Informational November 04, 2019
Expires: May 7, 2020
YANG Object Universal Parsing Interface
draft-petrov-t2trg-youpi-01
Abstract
YANG Object Universal Parsing Interface (YOUPI) specification
describes generic way to encode and decode binary data based on a
YANG model for use of constrainted devices. YOUPI is a generic
mechanism designed for great flexibility, so that it can be adapted
for any of the constainted devices.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. YOUPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. YANG extentions . . . . . . . . . . . . . . . . . . . . . 3
2.2. Position . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Bit positions . . . . . . . . . . . . . . . . . . . . 5
2.2.2. Cursor . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Absolute position . . . . . . . . . . . . . . . . . . 5
2.2.4. Relative position . . . . . . . . . . . . . . . . . . 5
2.3. FieldIndex . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Multiplier . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Offset . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.6. Units-subject . . . . . . . . . . . . . . . . . . . . . . 6
2.7. Data definitions . . . . . . . . . . . . . . . . . . . . 6
2.7.1. Supported built-in type . . . . . . . . . . . . . . . 6
2.7.2. Leafs . . . . . . . . . . . . . . . . . . . . . . . . 7
2.7.3. Type min/max values . . . . . . . . . . . . . . . . . 7
2.7.4. Type fraction digits . . . . . . . . . . . . . . . . 7
2.7.5. Containers . . . . . . . . . . . . . . . . . . . . . 8
2.7.6. Condition . . . . . . . . . . . . . . . . . . . . . . 8
2.7.7. Lists . . . . . . . . . . . . . . . . . . . . . . . . 9
2.7.8. Enumerations as mappings . . . . . . . . . . . . . . 10
2.7.9. Groupings . . . . . . . . . . . . . . . . . . . . . . 10
2.7.10. Typedefs . . . . . . . . . . . . . . . . . . . . . . 10
3. Security Considerations . . . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Normative References . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Complete examples . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
A huge number of very constraint IoT devices are expected to be
coming to the market. They are very constraint in terms of the MTU
(sometimes as small as 10b per message). As they are expected to be
running for many years without the need for external energy, energy
efficiency which is directly linked to the size of the payloads that
need to be sent, is also very important. For those devices JSON and
even CBOR formats might be too wasteful in terms of payload size.
The reality of the ecosystem is that currently a great number of
applications use proprietary binary formats for exchanging
information. A significant problem exists if those systems are to be
interacting in a purely M2M fashion. While there are a number of
possibilities to resolve those issues, due to the constraints it is
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mandatory to have a way to extract and encode information from/to the
binary payload and be able to annotate it with semantic metadata.
While binary formats can be rather complicated to parse and sometimes
even context dependent (some entity needs to keep context in order to
parse a message), for most cases a simple description format could be
sufficient.
A good solution should not be bounded to the output format. It
should be a data modeling language like YANG [RFC7950] that simply
describes the structure of the obtained data and that allows
different serialization formats afterwards.
1.1. Terminology
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.
2. YOUPI
YOUPI provides a number of yang extentions as defined in Section 2.1.
Thanks to that additional information in the YANG definitions, it is
possible to decode binary data and then transform it to a different
easier to parse format like JSON, XML or CBOR. Additionally it
defines extensions that allow meta information to be added so that
JSON-LD is generated. This draft is not describing how the data is
formatted as JSON or other format. For information how this could be
done, please refer to RESTCONF, NETCONF or CORECONF.
The opposite process is also possible - generating binary packets
from parsed data that comes from JSON or other format.
2.1. YANG extentions
The definitions of the YANG extensions.
<CODE BEGINS> file "petrov-youpi-file@2019-07-22.yang"
module youpi {
namespace "http://ackl.io/youpi";
prefix "youpi";
organization
"Acklio";
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contact
"Ivaylo Petrov
<mailto:ivaylo@ackl.io>";
description
"This module defines the extentions used by youpi.";
revision 2019-07-22 {
description "Initial revision.";
}
/**
*
* Extension for Binary data to CBOR mapping.
*
**/
extension position {
argument object;
}
extension fieldIndex {
argument object;
}
extension condition {
argument object;
}
extension multiplier {
argument object;
}
extension offset {
argument object;
}
extension units-subject {
argument object;
}
extension js {
argument object;
}
}
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2.2. Position
Information about which bits need to be used in order to find the
value of a field.
2.2.1. Bit positions
If the position is not present or is empty, the value contains 0 bits
and has a default value of 0 (or equivalent for the given type).
Could be useful if a field needs to be the result of arithmetic
operations from different fields.
It is possible to have a single bit read by giving only its value in
the position extension.
If continuous bits need to be used to obtain the value of a given
field, this can be achieved using the ".." syntax. For example
"0..3" means bits 0, 1, 2 and 3.
If non-continuous bits need to be used, one can use the concatenation
of bit ranges using the "|" operator. For example "0..1 | 3".
2.2.2. Cursor
Starts at 0 and changes with each read to the last bit index that was
read. Used in Section 2.2.4 to determine where the read will start
from. Section 2.2.3 is not affected by it, but changes its value.
2.2.3. Absolute position
The default one if no keyword is used. Alternatively "absolute"
keyword can be provided to explicitly request such position.
Example:
leaf temp {
type uint8;
default -19;
description "The temperature";
youpi:position "0..6";
}
2.2.4. Relative position
Example:
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leaf temp {
type uint8;
default -19;
description "The temperature";
youpi:position "relative 1..7";
}
This means that the value starts 1 bit after the current cursor and
will read up to 7 bits after the current cursor position, including
that 7th bit.
2.3. FieldIndex
Can be used to change the order in which fields are processed. By
default the order in which fields appear in the document is the order
in which they are processed.
2.4. Multiplier
A value or another field by which a given field needs to be
multiplied before the final value is obtained. The operations are
executed in the order of appearance (this includes "offset" extension
defined in Section 2.5).
2.5. Offset
A value or another field to which a given field needs to be added
before the final value is obtained. The operations are executed in
the order of appearance (this includes "offset" extension defined in
Section 2.5).
2.6. Units-subject
Meta information used to compute JSON-LD.
2.7. Data definitions
2.7.1. Supported built-in type
o binary
o enumeration
o int8
o int16
o int32
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o int64
o string
o uint8
o uint16
o uint32
o uint64
2.7.2. Leafs
Simple fields like integers and strings are represented by leafs in
YOUPI.
2.7.3. Type min/max values
"range" attribute can be used for giving a "min"/"max" acceptable
value for a type. If the value is outside of the defined range, it
is silently excluded from the final result.
Example:
typedef temp {
type int8 {
range "-20 .. 107";
}
}
2.7.4. Type fraction digits
It is possible to specify how many fraction digits are expected for a
value to have.
Example:
leaf temp {
type decimal64 {
fraction-digits 2;
}
}
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2.7.5. Containers
Complex fields like objects are represented by containers in YOUPI.
2.7.6. Condition
2.7.6.1. Choice
Inside a choice statement, the condition extension gives information
based on what value the choice will be decided.
For example considering that there is a value "mode" with the value
of btn inside the model
leaf mode {
...
}
choice data {
case _btn {
container button-data {
...
}
}
case _temp {
container temperature-data {
...
}
}
youpi:condition "../mode";
}
Then the button-data container will be used to parse the data.
2.7.6.2. When
With when statement it is possible to link the presence of some piece
of data to a value of another field. For example it is possible to
have button-data or temperature-data depending of the value of the
mode field.
container button-data {
when "../mode[.=1]"
...
}
container temperature-data {
when "../mode[.=2]"
...
}
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2.7.7. Lists
List statements are supported and they generate an array of a given
composite type.
2.7.7.1. With explicit length
A list of minimum and maximum temperatures can be defined as:
leaf temperature-len {
type int32;
}
list temperatures {
youpi:length "../temperatures-len";
leaf min-value {
type int32;
}
leaf max-value {
type int32;
}
}
2.7.7.2. Until the end of input
The list as defined in Section 2.7.7.1 can omit the length extension
statement if all the remaining bytes in the payload are part of the
list.
2.7.7.3. Until a specific value
The list as defined in Section 2.7.7.1 can also omit the length if it
has a defined key and if it only has one leaf or container in the
list apart from the key and it is a subject to when statement that
defines a stop value for the key.
list temperatures {
key option-id;
leaf option-id {
type int32;
}
container value {
when "../option-id[.!=0xffffffff]";
...
}
}
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2.7.8. Enumerations as mappings
Enumerations can be used inside a typedef in order to restrict a
field only to a set of acceptable values or in order to accomplish
mapping between some values and other values (for example 0 stands
for "temperature", 1 stands for "humidity", etc).
Example:
typedef mode-type {
type enumeration {
enum temp {
value 0;
}
enum humidity {
value 1;
}
enum light {
value 2;
}
...
}
...
}
2.7.9. Groupings
Groupings can be used for better reuse of definitions. They don't
affect the generated output.
2.7.10. Typedefs
Typedefs can be used to provide extra information about the type of a
field, including semantic information about it.
3. Security Considerations
The YANG file should be valid.
Segmentation faults might result from invalid data being provided
with a given YANG model.
Resource exhaustion can be looked for.
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4. IANA Considerations
This document registers a YANG model.
Acknowledgements
Contributors
7. Normative References
[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/info/rfc2119>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[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/info/rfc8174>.
Appendix A. Complete examples
Author's Address
Ivaylo Petrov (editor)
Acklio
1137A avenue des Champs Blancs
Cesson-Sevigne, Bretagne 35510
France
Email: ivaylo@ackl.io
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