CoAP Management Interface
draft-vanderstok-core-comi-10
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| Authors | Peter Van der Stok , Andy Bierman , Michel Veillette , Alexander Pelov | ||
| Last updated | 2016-10-30 | ||
| Replaced by | draft-ietf-core-comi, draft-ietf-core-comi | ||
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draft-vanderstok-core-comi-10
CoRE P. van der Stok
Internet-Draft consultant
Intended status: Standards Track A. Bierman
Expires: May 3, 2017 YumaWorks
M. Veillette
Trilliant Networks Inc.
A. Pelov
Acklio
October 30, 2016
CoAP Management Interface
draft-vanderstok-core-comi-10
Abstract
This document describes a network management interface for
constrained devices and networks, called CoAP Management Interface
(CoMI). The Constrained Application Protocol (CoAP) is used to
access data resources specified in YANG, or SMIv2 converted to YANG.
CoMI uses the YANG to CBOR mapping and converts YANG identifier
strings to numeric identifiers for payload size reduction. CoMI
extends the set of YANG based protocols NETCONF and RESTCONF with the
capability to manage constrained devices and networks.
Note
Discussion and suggestions for improvement are requested, and should
be sent to core@ietf.org.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 3, 2017.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. CoMI Architecture . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Major differences between RESTCONF and CoMI . . . . . . . 7
2.2. Compression of data node instance identifier . . . . . . 8
3. Example syntax . . . . . . . . . . . . . . . . . . . . . . . 8
4. CoAP Interface . . . . . . . . . . . . . . . . . . . . . . . 8
5. /c Function Set . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Using the 'k' query parameter . . . . . . . . . . . . . . 11
5.2. Data Retrieval . . . . . . . . . . . . . . . . . . . . . 13
5.2.1. Using the 'c' query parameter . . . . . . . . . . . . 13
5.2.2. Using the 'd' query parameter . . . . . . . . . . . . 14
5.2.3. GET . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2.4. FETCH . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Data Editing . . . . . . . . . . . . . . . . . . . . . . 17
5.3.1. Data Ordering . . . . . . . . . . . . . . . . . . . . 17
5.3.2. POST . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.3. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3.4. iPATCH . . . . . . . . . . . . . . . . . . . . . . . 19
5.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 20
5.4. Full Data Store access . . . . . . . . . . . . . . . . . 20
5.4.1. Full Data Store examples . . . . . . . . . . . . . . 21
5.5. Notify functions . . . . . . . . . . . . . . . . . . . . 22
5.5.1. Notify Examples . . . . . . . . . . . . . . . . . . . 23
5.6. RPC statements . . . . . . . . . . . . . . . . . . . . . 23
5.6.1. RPC Example . . . . . . . . . . . . . . . . . . . . . 24
6. Access to MIB Data . . . . . . . . . . . . . . . . . . . . . 24
7. Use of Block . . . . . . . . . . . . . . . . . . . . . . . . 26
8. Resource Discovery . . . . . . . . . . . . . . . . . . . . . 26
9. Error Return Codes . . . . . . . . . . . . . . . . . . . . . 28
10. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 29
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11. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
14. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 32
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
15.1. Normative References . . . . . . . . . . . . . . . . . . 35
15.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. YANG example specifications . . . . . . . . . . . . 38
A.1. ietf-system . . . . . . . . . . . . . . . . . . . . . . . 39
A.2. server list . . . . . . . . . . . . . . . . . . . . . . . 40
A.3. interfaces . . . . . . . . . . . . . . . . . . . . . . . 40
A.4. Example-port . . . . . . . . . . . . . . . . . . . . . . 41
A.5. ipNetToMediaTable . . . . . . . . . . . . . . . . . . . . 42
Appendix B. Comparison with LWM2M . . . . . . . . . . . . . . . 43
B.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 43
B.2. Defining Management Resources . . . . . . . . . . . . . . 44
B.3. Identifying Management Resources . . . . . . . . . . . . 45
B.4. Encoding of Management Resources . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is designed for
Machine to Machine (M2M) applications such as smart energy and
building control. Constrained devices need to be managed in an
automatic fashion to handle the large quantities of devices that are
expected in future installations. The messages between devices need
to be as small and infrequent as possible. The implementation
complexity and runtime resources need to be as small as possible.
This draft describes the CoAP Management Interface which uses CoAP
methods to access structured data defined in YANG [RFC6020]. This
draft is complementary to the draft [I-D.ietf-netconf-restconf] which
describes a REST-like interface called RESTCONF, which uses HTTP
methods to access structured data defined in YANG.
The use of standardized data sets, specified in a standardized
language such as YANG, promotes interoperability between devices and
applications from different manufacturers. A large amount of
Management Information Base (MIB) [RFC3418] [mibreg] specifications
already exists for monitoring purposes. This data can be accessed in
RESTCONF or CoMI if the server converts the SMIv2 modules to YANG,
using the mapping rules defined in [RFC6643].
CoMI and RESTCONF are intended to work in a stateless client-server
fashion. They use a single round-trip to complete a single editing
transaction, where NETCONF needs up to 10 round trips.
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To promote small packets, CoMI uses a YANG to CBOR mapping
[I-D.ietf-core-yang-cbor] and an additional "data-identifier string-
to-number conversion" to minimize CBOR payloads and URI length.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Readers of this specification should be familiar with all the terms
and concepts discussed in [RFC3410], [RFC3416], and [RFC2578].
The following terms are defined in the NETCONF protocol [RFC6241]:
client, configuration data, datastore, and server.
The following terms are defined in the YANG data modelling language
[RFC6020]: container, data node, key, key leaf, leaf, leaf-list, and
list.
The following terms are defined in RESTCONF protocol
[I-D.ietf-netconf-restconf]: data resource, datastore resource, edit
operation, query parameter, and target resource.
The following terms are defined in this document:
data node instance: An instance of a data node specified in a YANG
module present in the server. The instance is stored in the
memory of the server.
Notification instance: An instance of a schema node of type
notification, specified in a YANG module present in the server.
The instance is generated in the server at the occurrence of the
corresponding event and appended to a stream.
YANG schema item identifier: Numeric identifier which replaces the
name identifying a YANG item ( see section 6.2 of [RFC7950]) (data
node, RPC, Action, Notification, Identity, Module name, Submodule
name, Feature).
list instance identifier: Handle used to identify a YANG data node
that is an instance of a YANG "list" specified with the values of
the key leaves of the list.
single instance identifier: Handle used to identify a specific data
node which can be instantiated only once. This includes data
nodes defined at the root of a YANG module or submodule and data
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nodes defined within a container. This excludes data nodes
defined within a list or any children of these data nodes.
instance identifier: List instance identifier or single instance
identifier.
data node value: Value assigned to a data node instance. Data node
values are encoded based on the rules defined in section 4 of
[I-D.ietf-core-yang-cbor].
set of data node instances: Represents the payload of CoAP methods
when a collection is sent or returned. There are two
possibilities, dependent on Request context :
1. CBOR array of pair(s) <instance identifier, data node value >
2. CBOR map of pair(s) <instance identifier, data node value >
The following list contains the abbreviations used in this document.
SID: YANG Schema Item iDentifier.
2. CoMI Architecture
This section describes the CoMI architecture to use CoAP for the
reading and modifying of instrumentation variables used for the
management of the instrumented node.
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Client
+--------------------------------------------------------------+
| +----------------+ +----------------+ |
| | SMIv2 | > | YANG | > COAP |
| |specification(2)| |specification(1)| Request(3) |
| +----------------+ +----------------+[ * |
+-----------------------------*-----------[---------*----------+
* [ *
* [ +-----------+
mapping * security[ | Network |
* (8) [ | packet(4) |
* [ +-----------+
Server * [ *
+-----------------------------*-----------[---------*----------+
| * [ * |
| * Retrieval, |
| * Modification(5) |
| \*/ * |
| +-------------------------------------------------*--------+ |
| | +--------------+ +------------+ | |
| | |configuration | |Operational | | |
| | | (6b) | | state(6a) | | |
| | +--------------+ +------------+ | |
| | datastore (6) * | |
| +-------------------------------------------------*--------+ |
| * |
| Variable |
| Instrumentation(7)|
+--------------------------------------------------------------+
Figure 1: Abstract CoMI architecture
Figure 1 is a high level representation of the main elements of the
CoAP management architecture. A client sends requests as payload in
packets over the network to a managed constrained node.
The different numbered components of Figure 1 are discussed according
to component number.
(1) YANG specification: contains a set of named and versioned
modules.
(2) SMIv2 specification: A named module specifies a set of variables
and "conceptual tables". There is an algorithm to translate SMIv2
specifications to YANG specifications.
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(3) CoAP request: The CoAP request needs a Universal Resource
Identifier (URI). The CoMI client sends request messages and
receives response messages.
(4) Network packet: The payload contains CBOR encoded YANG data node
instances.
(5) Retrieval, modification: The server needs to parse the CBOR
encoded message and identify the corresponding instances in the
datastore.
(6) Datastore: The store is composed of two parts: Operational state
and Configuration datastore.
(7) Variable instrumentation: This code depends on implementation of
drivers and other node specific aspects.
(8) Security: The server MUST prevent unauthorized users from
reading or writing any data resources. CoMI relies on the
security measures specified for CoAP such as DTLS [RFC6347].
2.1. Major differences between RESTCONF and CoMI
CoMI uses CoAP/UDP as transport protocol and CBOR as payload format
[I-D.ietf-core-yang-cbor]. RESTCONF uses HTTP/TCP as transport
protocol and JSON [RFC7159] or XML [XML] as payload formats. CoMI
encodes YANG identifier strings as numbers, where RESTCONF does not.
CoMI uses the methods FETCH and iPATCH, not used by RESTCONF.
RESTCONF uses the HTTP methods HEAD, and OPTIONS, which are not used
by CoMI.
CoAP servers MUST maintain the order of user-ordered data. CoMI does
not support insert-mode (first, last, before, after) and insertion-
point (before, after) which are supported by RESTCONF. Many CoAP
servers will not support date and time functions. For that reason
CoMI does not support the start, stop options for events.
CoMI servers only implement the efficient "trim" mode for default
values
The CoMI servers do not support the following RESTCONF functionality:
o The "fields" query parameter to query multiple instances.
o The 'filter' query that involves XML parsing, 'content', and
'depth', query parameters.
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2.2. Compression of data node instance identifier
In the YANG specification the nodes are identified with a name
string. The name string contains the module name, hierarchy of
container/list names, and the leaf name.
In order to significantly reduce the size of identifiers used in
CoMI, numeric object identifiers are used instead of these strings.
The specific encoding of the object identifiers is not hard-wired in
the protocol.
Examples of object identifier encoding formats are described in
[I-D.somaraju-core-sid].
3. Example syntax
This section presents the notation used for the examples. The YANG
specifications that are used throughout this document are shown in
Appendix A. The example specifications are taken over from existing
modules and annotated with SIDs. The values of the SIDs are taken
over from [yang-cbor].
CBOR is used for the payload of the request- and the return-packets.
The CBOR syntax of the YANG payloads is specified in [RFC7049]. The
payload examples are notated in Diagnostic notation (defined in
section 6 of [RFC7049]) that can be automatically converted to CBOR.
A YANG leaf (YANG item identifier, YANG item value) pair is mapped to
a CBOR(key, value) pair. The YANG leaf value is encoded as specified
in [I-D.ietf-core-yang-cbor]. The YANG leaf identifier can be a SID
or a CBOR array with the structure [SID, key1, key2], where SID is a
list identifier and the key values specify the list instance. The
YANG leaf value can be a simple value, a CBOR array, or a CBOR map.
Delta encoding is used for the SIDs. The notation +n is used when
the SID has the value PREC+n where PREC is the SID of the parent
container, or PREC is the SID of the preceding entity in a CBOR
array.
In all examples the resource path in the URI is expressed as a SID,
represented as a base64 number. SIDs in the payload are represented
as decimal numbers.
4. CoAP Interface
In CoAP a group of links can constitute a Function Set. The format of
the links is specified in [I-D.ietf-core-interfaces]. This note
specifies a Management Function Set. CoMI end-points that implement
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the CoMI management protocol support at least one discoverable
management resource of resource type (rt): core.c, with path: /c,
where c is short-hand for CoMI. The path root /c is recommended but
not compulsory (see Section 8).
The path prefix /c has resources accessible with the following three
paths:
/c: YANG-based data with path "/c" and using CBOR content encoding
format. This path represents a datastore resource which contains
YANG data resources as its descendant nodes. The data nodes are
identified with their SID with format /c/SID.
/c/mod.uri: URI identifying the location of the server module
information, with path "/c/mod.uri" and CBOR content format. This
YANG data is encoded with plain identifier strings, not YANG
encoded values. An Entity Tag MUST be maintained for this
resource by the server, which MUST be changed to a new value when
the set of YANG modules in use by the server changes.
/c/s: String identifying the default stream resource to which YANG
notification instances are appended. Notification support is
optional, so this resource will not exist if the server does not
support any notifications.
The mapping of YANG data node instances to CoMI resources is as
follows: A YANG module describes a set of data trees composed of YANG
data nodes. Every root of a data tree in a YANG module loaded in the
CoMI server represents a resource of the server. All data root
descendants represent sub-resources.
The resource identifiers of the instances of the YANG specifications
are encoded YANG identifier strings. When multiple instances of a
list node exist, instance selection is possible as described in
Section 5.2.4 and Section 5.2.3.1.
The profile of the management function set, with IF=core.c, is shown
in the table below, following the guidelines of
[I-D.ietf-core-interfaces]:
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+----------------+-------------+----------------+-------------------+
| name | path | rt | Data Type |
+----------------+-------------+----------------+-------------------+
| Management | /c | core.c | n/a |
| | | | |
| Data | /c | core.c.data | application/cbor |
| | | | |
| Module Set URI | /c/mod.uri | core.c.moduri | application/cbor |
| | | | |
| Events | /c/s | core.c.stream | application/cbor |
+----------------+-------------+----------------+-------------------+
5. /c Function Set
The /c Function Set provides a CoAP interface to manage YANG servers.
The methods used by CoMI are:
+-----------+-----------------------------------------------------+
| Operation | Description |
+-----------+-----------------------------------------------------+
| GET | Retrieve the datastore resource or a data resource |
| | |
| FETCH | Retrieve partial data resources |
| | |
| POST | Create a data resource, invoke RPC |
| | |
| PUT | Create or replace a data resource |
| | |
| iPATCH | Idem-potently replace a data resource partially |
| | |
| DELETE | Delete a data resource |
+-----------+-----------------------------------------------------+
There is one query parameters for the GET, PUT, POST, and DELETE
methods.
+-----------------+------------------------------------+
| Query Parameter | Description |
+-----------------+------------------------------------+
| k | Select an instance of a list node |
+-----------------+------------------------------------+
This parameter is not used for FETCH and iPATCH, because their
request payloads support list instance selection.
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5.1. Using the 'k' query parameter
The "k" (key) parameter specifies the instance of a list node. The
SID in the URI is followed by the (?k=key1, key2,..). Where SID
identifies a list node, and key1, key2 are the values of the key
leafs that specify an instance of the list.
Key values are encoded using the rules defined in the following
table:
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+-----------------------+------------------+------------------------+
| YANG datatype | Binary | Text representation |
| | representation | |
+-----------------------+------------------+------------------------+
| uint8,uint16,unit32, | CBOR unsigned | int_to_text(number) |
| uint64 | integer | |
| | | |
| int8, int16,int32, | CBOR negative | Base64 (CBOR |
| int64 | integer | representation) |
| | | |
| | CBOR unsigned | |
| | integer | |
| | | |
| decimal64 | CBOR decimal | base64 (CBOR |
| | fractions | representation |
| | | |
| string | CBOR text or | text |
| | string | |
| | | |
| boolean | CBOR false or | "0" or "1" |
| | true | |
| | | |
| enumeration | CBOR unsigned | int_to_text (number) |
| | integer | |
| | | |
| bits | CBOR byte string | Base64 (CBOR |
| | | representation) |
| | | |
| binary | CBOR byte string | Base64 (CBOR |
| | | representation) |
| | | |
| identityref | CBOR unsigned | int_to_text (number) |
| | integer | |
| | | |
| union | | Base64 (CBOR |
| | | representation) |
| | | |
| List instance | CBOR unsigned | Base64 (CBOR |
| identifier | integer | representation) |
| | | |
| List instance | CBOR array | Base64 (CBOR |
| identifier | | representation) |
+-----------------------+------------------+------------------------+
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5.2. Data Retrieval
One or more data node instances can be retrieved by the client. The
operation is mapped to the GET method defined in section 5.8.1 of
[RFC7252] and to the FETCH method defined in section 2 of
[I-D.vanderstok-core-etch].
It is possible that the size of the payload is too large to fit in a
single message. In the case that management data is bigger than the
maximum supported payload size, the Block mechanism from [RFC7959] is
used, as explained in more detail in Section 7.
CoMI uses the FETCH payload for filtering sub-trees and retrieving
only a subset that a managing application is interested in.
There is one additional query parameters for the GET and FETCH
methods.
+-------------+-----------------------------------------------------+
| Query | Description |
| Parameter | |
+-------------+-----------------------------------------------------+
| c | Request to select configuration and non- |
| | configuration nodes (GET and FETCH) |
| | |
| d | Control retrieval of default values. |
+-------------+-----------------------------------------------------+
5.2.1. Using the 'c' query parameter
The 'c' (content) parameter controls how descendant nodes of the
requested data nodes will be processed in the reply.
The allowed values are:
+-------+------------------------------------------------------+
| Value | Description |
+-------+------------------------------------------------------+
| c | Return only configuration descendant data nodes |
| | |
| n | Return only non-configuration descendant data nodes |
| | |
| a | Return all descendant data nodes |
+-------+------------------------------------------------------+
This parameter is only allowed for GET and FETCH methods on datastore
and data resources. A 4.00 Bad Request error is returned if used for
other methods or resource types.
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If this query parameter is not present the default value is "a".
5.2.2. Using the 'd' query parameter
The "d" (with-defaults) parameter controls how the default values of
the descendant nodes of the requested data nodes will be processed.
The allowed values are:
+-------+-----------------------------------------------------------+
| Value | Description |
+-------+-----------------------------------------------------------+
| a | All data nodes are reported| Defined as 'report-all' in |
| | section 3.1 of [RFC6243]. |
| | |
| t | Data nodes set to the YANG default are not reported. |
| | Defined as 'trim' in section 3.2 of [RFC6243]. |
+-------+-----------------------------------------------------------+
If the target of a GET or FETCH method is a data node that represents
a leaf that has a default value, and the leaf has not been given a
value yet, the server MUST return the leaf.
If the target of a GET method is a data node that represents a
container or list that has any child resources with default values,
for the child resources that have not been given value yet, the
server MUST not return the child resource if this query parameter is
set to 't' and MUST return the child resource if this query parameter
is set to 'a'.
If this query parameter is not present, the default value is 't'.
5.2.3. GET
A request to read the values of instances of a management object is
sent with a confirmable CoAP GET message. A single object is
specified in the URI path prefixed with /c.
FORMAT:
GET /c/<instance identifier>
2.05 Content (Content-Format: application/cbor)
<data node value>
The specified object MUST be a complete object. Accordingly, the
returned payload is composed of all the leaves associated with the
object.
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The instance identifier is a SID or a SID followed by the "k" query
parameter.
5.2.3.1. GET Examples
Using for example the current-datetime leaf from Appendix A.1, a
request is sent to retrieve the value of system-state/clock/current-
datetime specified in container system-state. The ID of system-
state/clock/current-datetime is 1719, encoded in base64 this yields
a3. The answer to the request returns a <value>, transported as a
single CBOR string item.
REQ: GET example.com/c/a3
RES: 2.05 Content (Content-Format: application/cbor)
"2014-10-26T12:16:31Z"
For example, the GET of the clock node (ID = 1717; base64: a1), sent
by the client, results in the following returned value sent by the
server, transported as a CBOR map containing 2 pairs:
REQ: GET example.com/c/a1
RES: 2.05 Content (Content-Format: application/cbor)
{
+2 : "2014-10-26T12:16:51Z", # ID 1719
+1 : "2014-10-21T03:00:00Z" # ID 1718
}
A "list" node can have multiple instances. Accordingly, the returned
payload of GET is composed of all the instances associated with the
selected list node.
For example, look at the example in Appendix A.3. The GET of the
/interfaces/interface/ (with identifier 1533, base64: Bf0) results in
the following returned payload, transported as a CBOR array with 2
elements.
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REQ: GET example.com/c/Bf0
RES: 2.05 Content (Content-Format: application/cbor)
[
{+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type, (ID 1538)identity
# ethernetCsmacd (ID 1179)
+2 : true # enabled ( ID 1535)
},
{+4 : "eth1", # name
+1 : "Ethernet adaptor", # description
+5 : 1179, # type, identity ethernetCsmacd (ID 1179)
+2 : false # enabled
]
It is equally possible to select a leaf of one instance of a list or
a complete instance container with GET. The instance identifier is
the numeric identifier of the list followed by the specification of
the values for the key leafs that uniquely identify the list
instance. The instance identifier looks like: SID?k=key-value. The
key of "interface" is the "name" leaf. The example below requests
the description leaf of the instance with name="eth0" (ID=1534,
base64: Bf4). The value of the description leaf is returned.
REQ: GET example.com/c/Bf4?k="eth0"
RES: 2.05 Content (Content-Format: application/cbor)
"Ethernet adaptor"
5.2.4. FETCH
The FETCH is used to retrieve a list of data node values. The FETCH
Request payload contains a CBOR list of instance identifiers.
FORMAT:
FETCH /c/ Content-Format (application/YANG-fetch+cbor)
<CBOR array of instance identifiers>
2.05 Content (Content-Format: application/YANG-patch+cbor)
<CBOR array of data node values>
The instance identifier is a SID or a CBOR array containing the SID
followed by key values that identify the list instance (sec 5.13.1 of
[I-D.ietf-core-yang-cbor]. In the payload of the returned data node
values, delta encoding is used as described in
[I-D.ietf-core-yang-cbor].
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5.2.4.1. FETCH examples
The example uses the current-datetime leaf and the interface list
from Appendix A.1. In the following example the value of current-
datetime (ID 1719)and the interface list (ID 1533) instance
identified with name="eth0" are queried.
FETCH /c Content-Format (application/YANG-fetch+cbor)
[ 1719, # ID 1719
[-186, "eth0"] # ID 1533 with name = "eth0"
]
2.05 Content Content-Format (application/YANG-patch+cbor)
[
"2014-10-26T12:16:31Z",
{
+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type (ID 1538), identity ethernetCsmacd
+2 : true # enabled (ID 1535)
}
5.3. Data Editing
CoMI allows datastore contents to be created, modified and deleted
using CoAP methods.
5.3.1. Data Ordering
A CoMI server SHOULD preserve the relative order of all user-ordered
list and leaf-list entries that are received in a single edit
request. These YANG data node types are encoded as arrays so
messages will preserve their order.
5.3.2. POST
Data resource instances are created with the POST method. The CoAP
POST operation is used in CoMI for creation of data resources and the
invocation of "ACTION" and "RPC" resources. Refer to Section 5.6 for
details on "ACTION" and "RPC" resources.
A request to create the values of an instance of a container or leaf
is sent with a confirmable CoAP POST message. A single SID is
specified in the URI path prefixed with /c.
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FORMAT:
POST /c/<instance identifier> Content-Format(application/cbor)
<data node value>
2.01 Created (Content-Format: application/cbor)
If the data resource already exists, then the POST request MUST fail
and a "4.09 Conflict" status-line MUST be returned
The instance identifier is a SID or a SID followed by the "k" query
parameter.
5.3.2.1. Post example
The example uses the interface list from Appendix A.1. Example is
creating a new version of the container interface (ID = 1533):
FORMAT:
POST /c/Bf0 Content-Format(application/cbor)
{
+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type (ID 1538), identity
# ethernetCsmacd (ID 1179)
+2 : true # enabled (ID 1535)
}
2.01 Created (Content-Format: application/cbor)
5.3.3. PUT
Data resource instances are created or replaced with the PUT method.
The PUT operation is supported in CoMI. A request to set the value
of an instance of data node is sent with a confirmable CoAP PUT
message.
FORMAT:
PUT /c/<instance identifier> Content-Format(application/cbor)
<data node value>
2.01 Created
The instance identifier is a SID or a SID followed by the "k" query
parameter.
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5.3.3.1. PUT example
The example uses the interface list from Appendix A.1. Example is
renewing an instance of the list interface (ID = 1533) with key
name="eth0":
FORMAT:
PUT /c/Bf0?k="eth0" Content-Format(application/cbor)
{
+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type (ID 1538), identity
# ethernetCsmacd ( ID 1179)
+2 : true # enabled (ID 1535)
}
2.04 Changed
5.3.4. iPATCH
One or multiple data resource instances are replaced with the idem-
potent iPATCH method [I-D.vanderstok-core-etch]. A request to set
the values of instances of a subset of the values of the resource is
sent with a confirmable CoAP iPATCH message.
There are no query parameters for the iPATCH method.
The processing of the iPATCH command is specified by the CBOR
payload. The CBOR patch payload describes the changes to be made to
target YANG data nodes REF TO BE DEFINED. If the CBOR patch payload
contains data node instances that are not present in the target,
these instances are added or silently ignored dependent of the
payload information. If the target contains the specified instance,
the contents of the instances are replaced with the values of the
payload. Null values indicate the removal of existing values.
FORMAT:
iPATCH /c Content-Format(application/YANG-patch+cbor)
<set of data node instances>
2.04 Changed
5.3.4.1. iPATCH example
The example uses the interface list from Appendix A.3, and the
timezone-utc-offset leaf from Appendix A.1. In the example one leaf
(timezone-utc-offset ) and one container (interface) instance are
changed.
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iPATCH /c Content-Format(application/YANG-patch+cbor)
[
[1533, "eth0"] , # interface (ID = 1533)
{
+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type (ID 1538),
# identity ethernetCsmacd
+2 : true # enabled (ID 1535)
}
+203 , 60 # timezone-utc-offset (delta = 1736 - 1533)
]
2.04 Changed
5.3.5. DELETE
Data resource instances are deleted with the DELETE method. The
RESTCONF DELETE operation is supported in CoMI.
FORMAT:
Delete /c/<instance identifier>
2.02 Deleted
The instance identifier is a SID or a SID followed by the "k" query
parameter.
5.3.5.1. DELETE example
The example uses the interface list from Appendix A.3. Example is
deleting an instance of the container interface (ID = 1533):
FORMAT:
DELETE /c/Bf0?k="eth0"
2.02 Deleted
5.4. Full Data Store access
The methods GET, PUT, POST, and DELETE can be used to return,
replace, create, and delete the whole data store respectively.
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FORMAT:
GET /c
2.05 Content (Content-Format: application/cbor)
<array of data node instances>
PUT /c
(Content-Format: application/cbor)
<array of data node instances>
2.04 Changed
POST /c
(Content-Format: application/cbor)
<array of data node instances>
2.01 Created
DELETE /c
(Content-Format: application/cbor)
<array of data node instances>
2.02 Deleted
The array of data node instances represents an array of all root
nodes in the data store after the PUT, POST and GET method
invocations.
5.4.1. Full Data Store examples
The example uses the interface list and the clock container from
Appendix A.3. Assume that the data store contains two root objects:
the list interface (ID 1533) with one instance and the container
Clock (ID 1717). After invocation of GET an array with these two
objects is returned:
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GET /c
2.05 Content Content-Format (application/YANG-patch+cbor)
[
1717:
{ +1: "2016-10-26T12:16:31Z", # current-datetime (ID 501)
+2: "2014-10-05T09:00:00Z" # boot-datetime (ID 502)
}
-186: # clock (ID 1533)
{
+4 : "eth0", # name (ID 1537)
+1 : "Ethernet adaptor", # description (ID 1534)
+5 : 1179, # type (ID 1538), identity:
# ethernetCsmacd (ID 1179)
+2 : true # enabled (ID 1535)
}
]
5.5. Notify functions
Notification by the server to a selection of clients when an event
occurs in the server is an essential function for the management of
servers. CoMI allows events specified in YANG [RFC5277] to be
notified to a selection of requesting clients. The server appends
newly generated events to a stream. There is one, so-called
"default", stream in a CoMI server. The /c/s resource identifies the
default stream. The server MAY create additional stream resources.
When a CoMI server generates an internal event, it is appended to the
chosen stream, and the content of a notification instance is ready to
be sent to all CoMI clients which observe the chosen stream resource.
Reception of generated notification instances is enabled with the
CoAP Observe [RFC7641] function. The client subscribes to the
notifications by sending a GET request with an "Observe" option,
specifying the /c/s resource when the default stream is selected.
Every time an event is generated, the chosen stream is cleared, and
the generated notification instance is appended to the chosen
stream(s). After appending the instance, the contents of the
instance is sent to all clients observing the modified stream.
FORMAT:
Get /<stream-resource>
Content-Format(application/YANG-patch+cbor) Observe(0)
2.05 Content Content-Format(application/YANG-patch+cbor)
<set of data node instances>
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TODO: addition of generic information
5.5.1. Notify Examples
Suppose the server generates the event specified in Appendix A.4. By
executing a GET on the /c/s resource the client receives the
following response:
GET /c/s Observe(0) Token(0x93)
2.05 Content Content-Format(application/YANG-patch+cbor)
Observe(12) Token(0x93)
{
2600 : # example-port-fault (ID 2600)
{
+1 : "0/4/21", # port-name (ID 2601)
+2 : "Open pin 2", # port-fault (ID 2602)
},
2600 : # example-port-fault (ID 2600)
{
+1 : "1/4/21", # port-name (ID 2601)
+2 : "Open pin 5", # port-fault (ID 2602)
}
}
In the example, the request returns a success response with the
contents of the last two generated events. Consecutively the server
will regularly notify the client when a new event is generated.
To check that the client is still alive, the server MUST send
confirmable notifications once in a while. When the client does not
confirm the notification from the server, the server will remove the
client from the list of observers [RFC7641].
In the registration request, the client MAY include a "Response-To-
Uri-Host" and optionally "Response-To-Uri-Port" option as defined in
[I-D.becker-core-coap-sms-gprs]. In this case, the observations
SHOULD be sent to the address and port indicated in these options.
This can be useful when the client wants the managed device to send
the trap information to a multicast address.
5.6. RPC statements
The YANG "action" and "RPC" statements specify the execution of a
Remote procedure Call (RPC) in the server. It is invoked using a
POST method to the "Action" or "RPC" identifier. The Request payload
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contains the values assigned to the input container when specified
with the action station. The Response payload contains the values of
the output container when specified with the action statement.
The returned success response code is 2.05 Content.
FORMAT:
POST /c/<instance identifier>
Content-Format(application/YANG-patch+cbor)
<input node value>
2.05 Content Content-Format (application/YANG-patch+cbor)
<output node value>
There "k" query parameter is allowed for the POST method when used
for RPC invocation.
5.6.1. RPC Example
The example is based on the YANG action specification of
Appendix A.2. A server list is specified and the action "reset",
that is part of a "server instance" with key value "myserver", is
invoked.
POST /c/B24?k="myserver"
Content-Format(application/YANG-patch+cbor)
{
+1 : "2016-02-08T14:10:08Z09:00" # reset-at (ID 1903)
}
2.05 Content Content-Format(application/YANG-patch+cbor)
{
+2 : "2016-02-08T14:10:08Z09:18" # reset-finished-at (ID 1904)
}
6. Access to MIB Data
Appendix A.5 shows a YANG specification mapped from the SMI
specification "ipNetToPhysicalTable". The following example shows
the YANG "ipNetToPhysicalTable" with 2 instances, using diagnostic
notation encoding and annotating the leaf names with SID numbers.
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{
"IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry" : # ID 302
[
{
"ipNetToPhysicalIfIndex" : 1, # ID 303
"ipNetToPhysicalNetAddressType" : "ipv4", # ID 304
"ipNetToPhysicalNetAddress" : "10.0.0.51", # ID 305
"ipNetToPhysicalPhysAddress" : "00:00:10:01:23:45", # ID 306
"ipNetToPhysicalLastUpdated" : "2333943", # ID 307
"ipNetToPhysicalType" : "static", # ID 308
"ipNetToPhysicalState" : "reachable", # ID 309
"ipNetToPhysicalRowStatus" : "active" # ID 310
},
{
"ipNetToPhysicalIfIndex" : 1, # ID 303
"ipNetToPhysicalNetAddressType" : "ipv4", # ID 304
"ipNetToPhysicalNetAddress" : "9.2.3.4", # ID 305
"ipNetToPhysicalPhysAddress" : "00:00:10:54:32:10",# ID 306
"ipNetToPhysicalLastUpdated" : "2329836", # ID 307
"ipNetToPhysicalType" : "dynamic", # ID 308
"ipNetToPhysicalState" : "unknown", # ID 309
"ipNetToPhysicalRowStatus" : "active" # ID 310
}
]
}
In the following example exactly one instance is requested from the
ipNetToPhysicalEntry. The CBOR payload, here represented with
diagnostic JSON, permits to transport the selected instance and
nothing more.
REQ: FETCH example.com/c/
(Content-Format: application/YANG-fetch+cbor)
[
302,1,"ipv4",9.2.3.4
]
RES: 2.05 Content (Content-Format: application/YANG-patch+cbor)
{
+1 : 1, ( ID 303)
+2 : "ipv4", ( ID 304)
+3 : "9.2.3.4", ( ID 305)
+4 : "00:00:10:54:32:10", ( ID 306)
+5 : "2329836", ( ID 307)
+6 : "dynamic", ( ID 308)
+7 : "unknown", ( ID 309)
+8 : "active" ( ID 310)
}
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In this example one instance of ipNetToPhysicalTable/
ipNetToPhysicalEntry that matches the key values (1,"ipv4",9.2.3.4)
is returned.
7. Use of Block
The CoAP protocol provides reliability by acknowledging the UDP
datagrams. However, when large pieces of text need to be transported
the datagrams get fragmented, thus creating constraints on the
resources in the client, server and intermediate routers. The block
option [RFC7959] allows the transport of the total payload in
individual blocks of which the size can be adapted to the underlying
fragment sizes such as: (UDP datagram size ~64KiB, IPv6 MTU of 1280,
IEEE 802.15.4 payload of 60-80 bytes). Each block is individually
acknowledged to guarantee reliability.
Notice that the Block mechanism splits the data at fixed positions,
such that individual data fields may become fragmented. Therefore,
assembly of multiple blocks may be required to process the complete
data field.
Beware of race conditions. Blocks are filled one at a time and care
should be taken that the whole data representation is sent in
multiple blocks sequentially without interruption. In the server,
values are changed, lists are re-ordered, extended or reduced. When
these actions happen during the serialization of the contents of the
variables, the transported results do not correspond with a state
having occurred in the server; or worse the returned values are
inconsistent. For example: array length does not correspond with
actual number of items. It may be advisable to use CBOR maps or CBOR
arrays of undefined length which are foreseen for data streaming
purposes.
8. Resource Discovery
The presence and location of (path to) the management data are
discovered by sending a GET request to "/.well-known/core" including
a resource type (RT) parameter with the value "core.c" [RFC6690].
Upon success, the return payload will contain the root resource of
the management data. It is up to the implementation to choose its
root resource, but it is recommended that the value "/c" is used,
where possible. The example below shows the discovery of the
presence and location of management data.
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REQ: GET /.well-known/core?rt=core.c
RES: 2.05 Content </c>; rt="core.c"
Management objects MAY be discovered with the standard CoAP resource
discovery. The implementation can add the encoded values of the
object identifiers to /.well-known/core with rt="core.c.data". The
available objects identified by the encoded values can be discovered
by sending a GET request to "/.well-known/core" including a resource
type (RT) parameter with the value "core.c.data". Upon success, the
return payload will contain the registered encoded values and their
location. The example below shows the discovery of the presence and
location of management data.
REQ: GET /.well-known/core?rt=core.c.data
RES: 2.05 Content </c/BaAiN>; rt="core.c.data",
</c/CF_fA>; rt="core.c.data"
Lists of encoded values may become prohibitively long. It is
discouraged to provide long lists of objects on discovery.
Therefore, it is recommended that details about management objects
are discovered by reading the YANG module information stored in the
"ietf-YANG-library" module [I-D.ietf-netconf-restconf]. The resource
"/c/mod.uri" is used to retrieve the location of the YANG module
library.
TODO: additional references using SIDs
The module list can be stored locally on each server, or remotely on
a different server. The latter is advised when the deployment of
many servers are identical.
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Local in example.com server:
REQ: GET example.com/c/mod.uri
RES: 2.05 Content (Content-Format: application/cbor)
{
"mod.uri" : "example.com/c/modules"
}
Remote in example-remote-server:
REQ: GET example.com/c/mod.uri
RES: 2.05 Content (Content-Format: application/cbor)
{
"moduri" : "example-remote-server.com/c/group17/modules"
}
Within the YANG module library all information about the module is
stored such as: module identifier, identifier hierarchy, grouping,
features and revision numbers.
9. Error Return Codes
The RESTCONF return status codes defined in section 7 of
[I-D.ietf-netconf-restconf] are used in CoMI error responses, except
they are converted to CoAP error codes.
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+-------------------------------+------------------+
| RESTCONF Status Line | CoAP Status Code |
+-------------------------------+------------------+
| 100 Continue | none? |
| | |
| 200 OK | 2.05 |
| | |
| 201 Created | 2.01 |
| | |
| 202 Accepted | none? |
| | |
| 204 No Content | 2.04 Changed |
| | |
| 304 Not Modified | 2.03 |
| | |
| 400 Bad Request | 4.00 |
| | |
| 403 Forbidden | 4.03 |
| | |
| 404 Not Found | 4.04 |
| | |
| 405 Method Not Allowed | 4.05 |
| | |
| 409 Conflict | none? |
| | |
| 412 Precondition Failed | 4.12 |
| | |
| 413 Request Entity Too Large | 4.13 |
| | |
| 414 Request-URI Too Large | 4.00 |
| | |
| 415 Unsupported Media Type | 4.15 |
| | |
| 500 Internal Server Error | 5.00 |
| | |
| 501 Not Implemented | 5.01 |
| | |
| 503 Service Unavailable | 5.03 |
+-------------------------------+------------------+
10. Error Handling
In case a request is received which cannot be processed properly, the
CoMI server MUST return an error message. This error message MUST
contain a CoAP 4.xx or 5.xx response code, and SHOULD include
additional information in the payload.
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Such an error message payload is encoded in CBOR, using the following
structure:
errorMsg : ErrorMsg;
*ErrorMsg {
errorCode : uint;
?errorText : tstr;
}
The variable "errorCode" has one of the values from the table below,
and the OPTIONAL "errorText" field contains a human readable
explanation of the error.
TODO: Alternatives?
+----------------+----------------+---------------------------------+
| CoMI Error | CoAP Error | Description |
| Code | Code | |
+----------------+----------------+---------------------------------+
| 0 | 4.00 | General error |
| | | |
| 1 | 4.00 | Malformed CBOR data |
| | | |
| 2 | 4.00 | Incorrect CBOR datatype |
| | | |
| 3 | 4.00 | Unknown MIB variable |
| | | |
| 4 | 4.00 | Unknown conversion table |
| | | |
| 5 | 4.05 | Attempt to write read-only |
| | | variable |
| | | |
| 0..2 | 5.01 | Access exceptions |
| | | |
| 0..18 | 5.00 | SMI error status |
+----------------+----------------+---------------------------------+
The CoAP error code 5.01 is associated with the exceptions defined in
[RFC3416] and CoAP error code 5.00 is associated with the error-
status defined in [RFC3416].
11. Security Considerations
For secure network management, it is important to restrict access to
configuration variables only to authorized parties. This requires
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integrity protection of both requests and responses, and depending on
the application encryption.
CoMI re-uses the security mechanisms already available to CoAP as
much as possible. This includes DTLS [RFC6347] for protected access
to resources, as well suitable authentication and authorization
mechanisms.
Among the security decisions that need to be made are selecting
security modes and encryption mechanisms (see [RFC7252]). This
requires a trade-off, as the NoKey mode gives no protection at all,
but is easy to implement, whereas the X.509 mode is quite secure, but
may be too complex for constrained devices.
In addition, mechanisms for authentication and authorization may need
to be selected.
CoMI avoids defining new security mechanisms as much as possible.
However some adaptations may still be required, to cater for CoMI's
specific requirements.
12. IANA Considerations
'rt="core.c"' needs registration with IANA.
'rt="core.c.data"' needs registration with IANA.
'rt="core.c.moduri"' needs registration with IANA.
'rt="core.c.stream"' needs registration with IANA.
Content types to be registered:
o application/YANG-patch+cbor
o application/YANG-fetch+cbor
13. Acknowledgements
We are very grateful to Bert Greevenbosch who was one of the original
authors of the CoMI specification and specified CBOR encoding and use
of hashes.
Mehmet Ersue and Bert Wijnen explained the encoding aspects of PDUs
transported under SNMP. Carsten Bormann has given feedback on the
use of CBOR.
Timothy Carey has provided the text for Appendix B.
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The draft has benefited from comments (alphabetical order) by Rodney
Cummings, Dee Denteneer, Esko Dijk, Michael van Hartskamp, Juergen
Schoenwaelder, Anuj Sehgal, Zach Shelby, Hannes Tschofenig, Michael
Verschoor, and Thomas Watteyne.
14. Changelog
Changes from version 00 to version 01
o Focus on MIB only
o Introduced CBOR, JSON, removed BER
o defined mappings from SMI to xx
o Introduced the concept of addressable table rows
Changes from version 01 to version 02
o Focus on CBOR, used JSON for examples, removed XML and EXI
o added uri-query attributes mod and con to specify modules and
contexts
o Definition of CBOR string conversion tables for data reduction
o use of Block for multiple fragments
o Error returns generalized
o SMI - YANG - CBOR conversion
Changes from version 02 to version 03
o Added security considerations
Changes from version 03 to version 04
o Added design considerations section
o Extended comparison of management protocols in introduction
o Added automatic generation of CBOR tables
o Moved lowpan table to Appendix
Changes from version 04 to version 05
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o Merged SNMP access with RESTCONF access to management objects in
small devices
o Added CoMI architecture section
o Added RESTCONf NETMOD description
o Rewrote section 5 with YANG examples
o Added server and payload size appendix
o Removed Appendix C for now. It will be replaced with a YANG
example.
Changes from version 04 to version 05
o Extended examples with hash representation
o Added keys query parameter text
o Added select query parameter text
o Better separation between specification and instance
o Section on discovery updated
o Text on rehashing introduced
o Elaborated SMI MIB example
o YANG library use described
o use of BigEndian/LittleEndian in Hash generation specified
Changes from version 05 to version 06
o Hash values in payload as hexadecimal and in URL in base64 numbers
o Streamlined CoMI architecture text
o Added select query parameter text
o Data editing optional
o Text on Notify added
o Text on rehashing improved with example
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Changes from version 06 to version 07
o reduced payload size by removing JSON hierarchy
o changed rehash handling to support small clients
o added LWM2M comparison
o Notification handling as specified in YANG
o Added Patch function
o Rehashing completely reviewed
o Discover type of YANG name encoding
o Added new resource types
o Read-only servers introduced
o Multiple updates explained
Changes from version 07 to version 08
o Changed YANG Hash algorithm to use module name instead of prefix
o Added rehash bit to allow return values to identify rehashed nodes
in the response
o Removed /c/mod.set resource since this is not needed
o Clarified that YANG Hash is done even for unimplemented objects
o YANG lists transported as CBOR maps of maps
o Adapted examples with more CBOR explanation
o Added CBOR code examples in new appendix
o Possibility to use other than default stream
o Added text and examples for Patch payload
o Repaired some examples
o Added appendices on hash clash probability and hash clash storage
overhead
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Changes from version 08 to version 09
o Removed hash and YANG to CBOR sections
o removed hashes from examples.
o Added RPC
o Added content query parameter.
o Added default handling.
o Listed differences with RESTCONF
Changes from version 09 to version 10. This is the merge of cool-01
with comi-09.
o Merged with CoOL SIDs
o Introduced iPATCH, PATCH and FETCH
o Update of LWM2M comparison
o Added appendix with module examples
o Removed introductory text
o Removed refernces
15. References
15.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
<http://www.rfc-editor.org/info/rfc5277>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<http://www.rfc-editor.org/info/rfc6020>.
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[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<http://www.rfc-editor.org/info/rfc7950>.
[I-D.becker-core-coap-sms-gprs]
Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
"Transport of CoAP over SMS", draft-becker-core-coap-sms-
gprs-05 (work in progress), August 2014.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<http://www.rfc-editor.org/info/rfc7959>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
[I-D.ietf-netconf-restconf]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", draft-ietf-netconf-restconf-18 (work in
progress), October 2016.
[I-D.vanderstok-core-etch]
Stok, P., Bormann, C., and A. Sehgal, "Patch and Fetch
Methods for Constrained Application Protocol (CoAP)",
draft-vanderstok-core-etch-00 (work in progress), March
2016.
[I-D.somaraju-core-sid]
Somaraju, A., Veillette, M., Pelov, A., Turner, R., and A.
Minaburo, "Structure Identifier (SID)", draft-somaraju-
core-sid-01 (work in progress), July 2016.
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[I-D.ietf-core-yang-cbor]
Veillette, M., Pelov, A., Somaraju, A., Turner, R., and A.
Minaburo, "CBOR Encoding of Data Modeled with YANG",
draft-ietf-core-yang-cbor-02 (work in progress), July
2016.
15.2. Informative References
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578,
DOI 10.17487/RFC2578, April 1999,
<http://www.rfc-editor.org/info/rfc2578>.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410,
DOI 10.17487/RFC3410, December 2002,
<http://www.rfc-editor.org/info/rfc3410>.
[RFC3416] Presuhn, R., Ed., "Version 2 of the Protocol Operations
for the Simple Network Management Protocol (SNMP)",
STD 62, RFC 3416, DOI 10.17487/RFC3416, December 2002,
<http://www.rfc-editor.org/info/rfc3416>.
[RFC3418] Presuhn, R., Ed., "Management Information Base (MIB) for
the Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, DOI 10.17487/RFC3418, December 2002,
<http://www.rfc-editor.org/info/rfc3418>.
[RFC4293] Routhier, S., Ed., "Management Information Base for the
Internet Protocol (IP)", RFC 4293, DOI 10.17487/RFC4293,
April 2006, <http://www.rfc-editor.org/info/rfc4293>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC6243] Bierman, A. and B. Lengyel, "With-defaults Capability for
NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
<http://www.rfc-editor.org/info/rfc6243>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
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[RFC6643] Schoenwaelder, J., "Translation of Structure of Management
Information Version 2 (SMIv2) MIB Modules to YANG
Modules", RFC 6643, DOI 10.17487/RFC6643, July 2012,
<http://www.rfc-editor.org/info/rfc6643>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[I-D.ietf-core-interfaces]
Shelby, Z., Vial, M., Koster, M., and C. Groves, "Reusable
Interface Definitions for Constrained RESTful
Environments", draft-ietf-core-interfaces-06 (work in
progress), October 2016.
[XML] "Extensible Markup Language (XML)",
Web http://www.w3.org/xml.
[OMA] "OMA-TS-LightweightM2M-V1_0-20131210-C", Web
http://technical.openmobilealliance.org/Technical/
current_releases.aspx.
[OMNA] "Open Mobile Naming Authority (OMNA)", Web
http://http://technical.openmobilealliance.org/Technical/
technical-information/omna.
[netconfcentral]
"NETCONF Central: library of YANG modules",
Web http://www.netconfcentral.org/modulelist.
[mibreg] "Structure of Management Information (SMI) Numbers (MIB
Module Registrations)", Web
http://www.iana.org/assignments/smi-numbers/
smi-numbers.xhtml/.
[yang-cbor]
"yang-cbor Registry", Web https://github.com/core-wg/yang-
cbor/tree/master/registry/.
Appendix A. YANG example specifications
This appendix shows 5 YANG example specifications taken over from as
many existing YANG modules. The YANG modules are available from
[netconfcentral]. Each YANG item identifier is accompanied by its
SID shown after the "#" character, taken from [yang-cbor].
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A.1. ietf-system
Taken over from the module ietf-system.
module ietf-system {
container system-state{ # ID 1716
container clock { # ID 1717
leaf current-datetime{ # ID 1719
type YANG:date-and-time
}
leaf boot-datetime{ # ID 1718
type YANG:date-and-time
}
...
container system {
leaf timezone-name
leaf timezone-utc-offset{ # ID 1736
type int16
}
...
container ntp { # ID 1750
leaf enabled { # ID 1751
type boolean;
}
list server { # ID 1752
key name;
leaf name { # ID 1755
type string;
}
choice transport {
case udp {
container udp { # ID 1757
leaf address { # ID 1758
type inet:host;
}
leaf port { # ID 1759
type inet:port-number;
}
}
}
}
leaf association-type { # ID 1753
type enumeration {
enum server {}
enum peer {}
enum pool {}
}
}
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leaf iburst { # ID 1754
type boolean;
}
leaf prefer { # ID 1756
type boolean;
}
}
}
...
}
}
A.2. server list
Taken over from module
list server # ID = 1901
{
key name;
leaf name {
type string;
}
action reset { # ID = 1902
input {
leaf reset-at { # ID = 1903
type YANG:date-and-time;
mandatory true;
}
}
output {
leaf reset-finished-at { # ID = 1904
type YANG:date-and-time;
mandatory true;
}
}
}
}
A.3. interfaces
Taken over from module ietf-interfaces.
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container interfaces {
list interface { # ID = 1533
key "name";
leaf name { # ID = 1537
type string;
}
leaf description { # ID = 1534
type string;
}
leaf type { # ID = 1538
type identityref {
base interface-type;
}
mandatory true;
}
leaf enabled { # ID = 1535
type boolean;
default "true";
}
leaf link-up-down-trap-enable {
if-feature if-mib;
type enumeration {
enum enabled {
value 1;
}
enum disabled {
value 2;
}
}
} } } }
A.4. Example-port
Taken over from module example-port.
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module example-port {
...
prefix ep;
...
notification example-port-fault { # ID 2600
description
"Event generated if a hardware fault on a
line card port is detected";
leaf port-name { # ID 2601
type string;
description "Port name";
}
leaf port-fault { # ID 2601
type string;
description "Error condition detected";
}
}
}
A.5. ipNetToMediaTable
The YANG translation of the SMI specifying the
ipNetToMediaTable [RFC4293], extended with example SID numbers,
yields:
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container IP-MIB {
container ipNetToPhysicalTable { # ID 301
list ipNetToPhysicalEntry { # ID 302
key "ipNetToPhysicalIfIndex
ipNetToPhysicalNetAddressType
ipNetToPhysicalNetAddress";
leaf ipNetToMediaIfIndex { # ID 303
type: int32;
}
leaf ipNetToPhysicalIfIndex { # ID 304
type if-mib:InterfaceIndex;
}
leaf ipNetToPhysicalNetAddressType { # ID 305
type inet-address:InetAddressType;
}
leaf ipNetToPhysicalPhysAddress { # ID 306
type YANG:phys-address {
length "0..65535";
}
}
leaf ipNetToPhysicalLastUpdated { # ID 307
type YANG:timestamp;
}
leaf ipNetToPhysicalType { # ID 308
type enumeration { ... }
}
leaf ipNetToPhysicalState { # ID 309
type enumeration { ... }
}
leaf ipNetToPhysicalRowStatus { # ID 310
type snmpv2-tc:RowStatus;
}
}
}
Appendix B. Comparison with LWM2M
B.1. Introduction
CoMI and LWM2M [OMA], both, provide RESTful device management
services over CoAP. Differences between the designs are highlighted
in this section.
The intent of the LWM2M protocol is to provide a single protocol to
control and manage IoT devices. This means the IoT device implements
and uses the same LWM2M agent function for the actuation and sensing
features of the IoT device as well as for the management of the IoT
device. The intent of CoMI Interface as described in the Abstract
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section of this document is to provide management of constrained
devices and devices in constrained networks using RESTCONF and YANG.
This implies that the device, although reusing the CoAP protocol,
would need a separate CoAP based agent in the future to control the
actuation and sensing features of the device and another CoMI agent
that performs the management functions.
It should be noted that the mapping of a LWM2M server to YANG is
specified in [YANGlwm2m]. The converted server can be invoked with
CoMI as specified in this document.
For the purposes of managing IoT devices the following points related
to the protocols compare how management resources are defined,
identified, encoded and updated.
B.2. Defining Management Resources
Management resources in LWM2M (LWM2M objects) are defined using a
standardized number. When a new management resource is defined,
either by a standards organization or a private enterprise, the
management resource is registered with the Open Mobile Naming
Authority [OMNA] in order to ensure different resource definitions do
not use the same identifier. CoMI, by virtue of using YANG as its
data modeling language, allows enterprises and standards
organizations to define new management resources (YANG nodes) within
YANG modules without having to register each individual management
resource. Instead YANG modules are scoped within a registered name
space. As such, the CoMI approach provides additional flexibility in
defining management resources. Likewise, since CoMI utilizes YANG,
existing YANG modules can be reused. The flexibility and reuse
capabilities afforded to CoMI can be useful in management of devices
like routers and switches in constrained networks. However for
management of IoT devices, the usefulness of this flexibility and
applicability of reuse of existing YANG modules may not be warranted.
The reason is that IoT devices typically do not require complex sets
of configuration or monitoring operations required by devices like a
router or a switch. To date, OMA has defined approximately 15
management resources for constrained and non-constrained mobile or
fixed IoT devices while other 3rd Party SDOs have defined another 10
management resources for their use in non-constrained IoT devices.
Likewise, the Constrained Object Language [I-D.somaraju-core-sid]
which is used by CoMI when managing constrained IoT devices uses YANG
schema item identifiers, which are registered with IANA, in order to
define management resources that are encoded using CBOR when
targeting constrained IoT Devices.
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B.3. Identifying Management Resources
As LWM2M and CoMI can similarly be used to manage IoT devices,
comparison of the CoAP URIs used to identify resources is relevant as
the size of the resource URI becomes applicable for IoT devices in
constrained networks. LWM2M uses a flat identifier structure to
identify management resources and are identified using the LWM2M
object's identifier, instance identifier and optionally resource
identifier (for access to and object's attributes). For example,
identifier of a device object (object id = 3) would be "/3/0" and
identification of the device object's manufacturer attribute would be
"/3/0/0". Effectively LWM2M identifiers for management resources are
between 4 and 10 bytes in length.
CoMI is expected to be used to manage constrained IoT devices. CoMI
utilizes the YANG schema item identifier[SID] that identify the
resources. CoMI recommends that IoT device expose resources to
identify the data stores and event streams of the CoMI agent.
Individual resources (e.g., device object) are not directly
identified but are encoded within the payload. As such the
identifier of the CoMI resource is smaller (4 to 7 bytes) but the
overall payload size isn't smaller as resource identifiers are
encoded on the payload.
B.4. Encoding of Management Resources
LWM2M provides a separation of the definition of the management
resources from how the payloads are encoded. As of the writing of
this document LWM2M encodes LWM2M encodes payload data in Type-
length-value (TLV), JSON or plain text formats. JSON encoding is the
most common encoding scheme with TLV encoding used on the simplest
IoT devices. CoMI's use of CBOR provides a more efficient transfer
mechanism [RFC7049] than the current LWM2M encoding formats.
In situations where resources need to be modified, CoMI uses the CoAP
PATCH operation resources only require a partial update. LWM2M does
not currently use the CoAP PATCH operation but instead uses the CoAP
PUT and POST operations which are less efficient.
Authors' Addresses
Peter van der Stok
consultant
Phone: +31-492474673 (Netherlands), +33-966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
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Andy Bierman
YumaWorks
685 Cochran St.
Suite #160
Simi Valley, CA 93065
USA
Email: andy@yumaworks.com
Michel Veillette
Trilliant Networks Inc.
610 Rue du Luxembourg
Granby, Quebec J2J 2V2
Canada
Phone: +14503750556
Email: michel.veillette@trilliantinc.com
Alexander Pelov
Acklio
2bis rue de la Chataigneraie
Cesson-Sevigne, Bretagne 35510
France
Email: a@ackl.io
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