CoAp Management Interfaces
draft-vanderstok-core-comi-02
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| Authors | Peter Van der Stok , Bert Greevenbosch | ||
| Last updated | 2014-01-09 | ||
| Replaced by | draft-ietf-core-comi, draft-ietf-core-comi | ||
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draft-vanderstok-core-comi-02
core P. van der Stok
Internet-Draft consultant
Intended status: Informational B. Greevenbosch
Expires: July 13, 2014 Huawei Technologies
January 9, 2014
CoAp Management Interfaces
draft-vanderstok-core-comi-02
Abstract
The draft describes an interface based on CoAP to manage constrained
devices via MIBs. The proposed integration of CoAP with SNMP reduces
the code- and application development complexity by accessing MIBs
via a standard CoAP server. The payload of the MIB request is CBOR
based on JSON. The mapping from SMI to CBOR is specified.
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 July 13, 2014.
Copyright Notice
Copyright (c) 2014 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
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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. CoAP Interface . . . . . . . . . . . . . . . . . . . . . . . 5
3. MIB Function Set . . . . . . . . . . . . . . . . . . . . . . 5
3.1. SNMP/MIB architecture . . . . . . . . . . . . . . . . . . 5
3.1.1. SNMP functions . . . . . . . . . . . . . . . . . . . 6
3.1.2. MIB structure . . . . . . . . . . . . . . . . . . . . 7
3.2. CoMI Function Set . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Single MIB values . . . . . . . . . . . . . . . . . . 9
3.2.2. multi MIB values . . . . . . . . . . . . . . . . . . 11
3.2.3. Table row . . . . . . . . . . . . . . . . . . . . . . 13
3.2.4. Error returns . . . . . . . . . . . . . . . . . . . . 14
4. Mapping SMI to CoMI payload . . . . . . . . . . . . . . . . . 14
4.1. Mapping strings to CBOR . . . . . . . . . . . . . . . . . 15
4.2. Mapping SMI to CBOR . . . . . . . . . . . . . . . . . . . 16
4.2.1. General overview . . . . . . . . . . . . . . . . . . 16
4.2.2. Conversion from YANG datatypes to CBOR datatypes . . 16
4.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . 18
4.2.4. 6LoWPAN MIB . . . . . . . . . . . . . . . . . . . . . 20
5. MIB discovery . . . . . . . . . . . . . . . . . . . . . . . . 23
6. Trap functions . . . . . . . . . . . . . . . . . . . . . . . 24
7. MIB access management . . . . . . . . . . . . . . . . . . . . 24
7.1. Notify destinations . . . . . . . . . . . . . . . . . . . 24
7.2. Conversion tables . . . . . . . . . . . . . . . . . . . . 25
8. Error handling . . . . . . . . . . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
13.1. Normative References . . . . . . . . . . . . . . . . . . 27
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Notational Convention for CBOR data . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
The Constrained RESTful Environments (CoRE) working group aims at
Machine to Machine (M2M) applications such as smart energy and
building control.
Small M2M devices need to be managed in an automatic fashion to
handle the large quantities of devices that are expected to be
installed in future installations. The management protocol of choice
for Internet is SNMP [RFC3410] as is testified by the large number of
Management Information Base (MIB) [RFC3418] specifications currently
published [STD0001]. More recently, the NETCONF protocol [RFC6241]
was developed with an extended set of messages using XML [XML] as
data format. The data syntax is specified with YANG [RFC6020] and a
mapping from Yang to XML is specified. In [RFC6643] SMIv2 syntax is
expressed in Yang. Contrary to SNMP and also CoAP, NETCONF assumes
persistent connections for example provided by SSH. The NETCONF
protocol provides operations to retrieve, configure, copy, and delete
configuration data-stores. Configuring data-stores distinguishes
NETCONF from SNMP which operates on standardized MIBs.
The CoRE Management Interface (CoMI) is intended to work on
standardized data-sets in a stateless client-server fashion and is
thus closer to SNMP than to NETCONF. Standardized data sets promote
interoperability between small devices and applications from
different manufacturers. Stateless communication is encouraged to
keep communications simple and the amount of state information small
in line with the design objectives of 6lowpan [RFC4944] [RFC6775],
RPL [RFC6650], and CoAP [I-D.ietf-core-coap].
The draft [I-D.bierman-netconf-restconf] describes a restful
interface to NETCONF data stores and approaches the CoMI approach.
CoMI uses SMI encoded in CBOR, and CoAP/UDP to access MIBs, where
restconf uses YANG encoded in JSON and HTTP/TCP to access NETCONF
data stores. CoMI is more low resource oriented than restconf is.
Currently, managed devices need to support two protocols: CoAP and
SNMP. When the MIB can be accessed with the CoAP protocol, the SNMP
protocol can be replaced with the CoAP protocol. This arrangement
reduces the code complexity of the stack in the constrained device,
and harmonizes applications development.
The objective of CoMI is to provide a CoAP based Function Set that
reads and sets values of MIB variables in devices to (1) initialize
parameter values at start-up, (2) acquire statistics during
operation, and (3) maintain nodes by adjusting parameter values
during operation.
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The payload of CoMI is encoded in CBOR [RFC7049] which similar to
JSON [JSON], but has a binary format and hence has more coding
efficiency. CoMI is intended for small devices. The JSON overhead
can be prohibitive. It is therefore chosen to transport CBOR in the
payload. CBOR, like BER used for SNMP, transports the data type in
the payload.
The end goal of CoMI is to provide information exchange over the CoAP
transport protocol in a uniform manner to approach the full
management functionality as specified in
[I-D.ersue-constrained-mgmt].
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 are required to be familiar with all
the terms and concepts discussed in [RFC3410], [RFC3416], and
[RFC2578].
Core Management Interface (CoMI) specifies the profile of Function
Sets which access MIBs with the purpose of managing the operation of
constrained devices in a network.
The following list defines the terms used in this document:
Managing Entity: An entity that manages one or more managed devices.
Within the CoMI framework, the managing entity acts as a CoAP
client for CoMI.
Managed Device: An entity that is being managed. The managed device
acts as a CoAP server for CoMI.
NOTE: It is assumed that the managed device is the most constrained
entity. The managing entity might be more capable, however this is
not necessarily the case.
The following list contains the abbreviations used in this document.
OID: ASN.1 OBJECT-IDENTIFIER, which is used to uniquely identify MIB
objects in the managed device.
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2. CoAP Interface
In CoRE 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
the CoMI management protocol support at least one discoverable
management resource of resource type (rt): core.mg, with path: /mg,
where mg is short-hand for management. The mg resource has two sub-
resources accessible with the paths:
o MIB with path /mg/mib and a CBOR content format.
o XLAT with path /mg/xlat and CBOR content format.
The mib resource provides access to the MIBs as described in
Section 3.2. The xlat resource provides access to a string to CBOR
identifier table as described in Section 4.1. The mib and xlat
resources are introduced as sub resources to mg to permit later
additions to CoMI mg resource.
The profile of the management function set, with IF=core.mg.mib, is
shown in the table below, following the guidelines of
[I-D.ietf-core-interfaces]:
+-----------------+-----------+---------------+-------------------+
| name | path | RT | Data Type |
+-----------------+-----------+---------------+-------------------+
| Management | /mg | core.mg | n/a |
| | | | |
| MIB | /mg/mib | core.mg.mib | application/cbor |
| | | | |
| XLAT | /mg/xlat | core.mg.xlat | application/cbor |
+-----------------+-----------+---------------+-------------------+
3. MIB Function Set
The MIB Function Set provides a CoAP interface to perform equivalent
functions to the ones provided by SNMP. Section 3.1 explains the
structure of SNMP Protocol Data Units (PDU), their transport, and the
structure of the MIB modules. An excellent overview of the documents
describing the SNMP/MIB architecture is provided in section 7 of
[RFC3410].
3.1. SNMP/MIB architecture
The architecture of the Internet Standard management framework
consists of:
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o A data definition language that is referred to as Structure of
Management Information (SMI)[RFC2578].
o The Management Information Base (MIB) which contains the
information to be managed and is defined for each specific
function to be managed [RFC3418].
o A protocol definition referred to as Simple Network Management
Protocol (SNMP) [RFC3416].
o Security and administration that provides SNMP message based
security on the basis of the user-based security model [RFC3414].
o A management domain definition where a SNMP entity has access to a
collection of management information called a "context" [RFC3411].
In addition [RFC4088] describes a URI scheme to refer to a specific
MIB instance.
Separation in modules was motivated by the wish to respond to the
evolution of Internet. The protocol part (SNMP) and data definition
part (MIB) are independent of each other. The separation has enabled
the progressive passage from SNMPv1 via SNMPv2 to SNMPv3. This draft
leverages this separation to replace the SNMP protocol with a CoAP
based protocol.
3.1.1. SNMP functions
The SNMP protocol supports seven types of access supported by as many
Protocol Data Unit (PDU) types:
o Get Request, transmits a list of OBJECT-IDENTIFIERs to be paired
with values.
o GetNext Request, transmits a list of OBJECT-IDENTIFIERs to which
lexicographic successors are returned for table traversal.
o GetBulk Request, transmits a list of OBJECT-IDENTIFIERs and the
maximum number of expected paired values.
o Response, returns an error or the (OBJECT-IDENTIFIER, value) pairs
for the OBJECT-IDENTIFIERs specified in Get, GetNext, GetBulk,
Set, or Inform Requests.
o Set Request, transmits a list of (OBJECT-IDENTIFIERs, value) pairs
to be set in the specified MIB object.
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o Trap, sends an unconfirmed message with a list of (OBJECT-
IDENTIFIERs, value) pairs to a notification requesting end-point.
o Inform Request, sends a confirmed message with a list of (OBJECT-
IDENTIFIERs, value) pairs to a notification requesting end-point.
The binding of the notification to a destination is discussed in
Section 6.
3.1.2. MIB structure
A MIB module is composed of MIB objects. MIB objects are
standardized by the IETF or by other relevant Standards Developing
Organizations (SDO).
MIB objects have a descriptor and an identifier: OBJECT-IDENTIFIER
(OID). The identifier, following the OSI hierarchy, is an ordered
list of non-negative numbers [RFC2578]. OID values are unique. Each
number in the list is referred as a sub-identifier. The descriptor
is unique within a module. Different modules may contain the same
descriptor. Consequently, a descriptor can be related to several
OIDs.
Many instances of an object type exist within a management domain.
Each instance can be identified within some scope or "context", where
there are multiple such contexts within the management domain.
Often, a context is a physical or logical device. A context is
always defined as a subset of a single SNMP entity. To identify an
individual item of management information within the management
domain, its contextName and contextEngineID must be identified in
addition to its object type and its instance. A default context is
assumed when no context is specified.
A MIB object is usually a scalar object. A MIB object may have a
tabular form with rows and columns. Such an object is composed of a
sequence of rows, with each row composed of a sequence of typed
values. The index is a subset (1-2 items) of the typed values in the
row. An index value identifies the row in the table.
In SMI, a table is constructed as a SEQUENCE OF its entries. For
example, the IpAddrTable from [RFC4293] has the following definition:
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ipv6InterfaceTable OBJECT-TYPE
SYNTAX SEQUENCE OF Ipv6InterfaceEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The table containing per-interface IPv6-specific
information."
::= { ip 30 }
ipv6InterfaceEntry OBJECT-TYPE
SYNTAX Ipv6InterfaceEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"An entry containing IPv6-specific information for a given
interface."
INDEX { ipv6InterfaceIfIndex }
::= { ipv6InterfaceTable 1 }
Ipv6InterfaceEntry ::= SEQUENCE {
ipv6InterfaceIfIndex InterfaceIndex,
ipv6InterfaceReasmMaxSize Unsigned32,
ipv6InterfaceIdentifier Ipv6AddressIfIdentifierTC,
ipv6InterfaceEnableStatus INTEGER,
ipv6InterfaceReachableTime Unsigned32,
ipv6InterfaceRetransmitTime Unsigned32,
ipv6InterfaceForwarding INTEGER
}
The descriptor (name) of the MIB table is used for the name of the
CoMI variable. However, there is no explicit mention of the names
"ipv6InterfaceEntry" and "Ipv6InterfaceEntry". Instead, the value of
the main CoMI variable consists of an array, each element of which
contains 7 CoMI variables: one element for "ipv6InterfaceIfIndex",
one for "ipv6InterfaceReasmMaxSize" and so on until
"ipv6InterfaceForwarding".
3.2. CoMI Function Set
Two types of interfaces are supported by CoMI:
single value Reading/Writing one MIB variable, specified in the URI
with path /mg/mib/descriptor or with path /mg/mib/OID.
multiple values Reading writing arrays or multiple MIB variables,
specified in the payload.
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The examples in this section use a JSON payload with one or more
entries describing the pair (descriptor, value), or (OID, value).
The CBOR syntax of the payloads is specified in Section 4.
3.2.1. Single MIB values
A request to read the value of a MIB variable is sent with a
confirmable CoAP GET message. The single MIB variable is specified
in the URI path with the OID or descriptor suffixing the /mg/mib/
path name. When the descriptor is used to specify the MIB value, the
same descriptor may be present in multiple module. To disambiguate
the descriptor the "mod" uri-query attribute specifies the enveloping
modules. A request to set the value of a MIB variable is sent with a
confirmable CoAP PUT message. The Response is piggybacked to the
CoAP ACK message corresponding with the Request.
TODO: for multicast send unconfirmed PUT
Using for example the same MIB from [RFC1213] as used in [RFC3416], a
request is sent to retrieve the value of sysUpTime specified in
module SNMPv2-MIB. The answer to the request returns a (descriptor,
value) pair.
For clarity of the examples, in this and all following examples the
payload is expressed in JSON, although the operational payload is
specified to be in CBOR, as described in Section 4.
REQ: GET example.com/mg/mib/sysUpTime?mod=SNMPv2-MIB
RES: 2.05 Content (Content-Format: application/json)
{
"sysUpTime" : 123456
}
Another way to express the descriptor of the required value is by
specifying the pair (descriptor or oid, null value) in the payload of
the request message.
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REQ: GET example.com/mg/mib/(Content-Format: application/json)
{
"SNMPv2-MIB.sysUpTime" : "null"
}
RES: 2.05 Content (Content-Format: application/json)
{
"SNMPv2-MIB.sysUpTime" : 123456
}
The module name SNMPv2-MIB can be omitted when there is no
possibility of ambiguity. The module.descriptor can of course be
replaced with the corresponding oid.
In some cases it is necessary to determine the "context" by
specifying a context name and a contextEngine identifier. The
context can be specified in the URI with the uri-query attribute
"con". Based on the example of figure 3 in section 3.3 of [RFC3411],
the context name, bridge1, and the context Engine Identifier,
800002b804616263, separated by an underscore, are specified in the
following example:
REQ: GET example.com/mg/mib/sysUPTime?con=bridge1_800002b804616263
RES: 2.05 Content (Content-Format: application/json)
{
"sysUpTime" : 123456
}
The specified object can be a table. The returned payload is
composed of all the rows associated with the table. Each row is
returned as a set of (column name, value) pairs. For example the GET
of the ipNetToMediaTable, sent by the managing entity, results in the
following returned payload sent by the managed entity:
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REQ: GET example.com/mg/mib/ipNetToMediaTable
RES: 2.05 Content (Content-Format: application/json)
{
"ipNetTOMediaTable" : [
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
"ipNetToMediaNetAddress" : "10.0.0.51",
"ipNetToMediaType" : "static"
},
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:54:32:10",
"ipNetToMediaNetAddress" : "9.2.3.4",
"ipNetToMediaType" : "dynamic"
},
{
"ipNetToMediaIfIndex" : 2,
"ipNetToMediaPhysAddress" : "00:00::10:98:76:54",
"ipNetToMediaNetAddress" : "10.0.0.15",
"ipNetToMediaType" : "dynamic"
}
]
}
It is possible that the size of the returned payload is too large to
fit in a single message.
CoMI gives the possibility to send the contents of the objects in
several fragments with a maximum size. The "sz" link-format
attribute [RFC6690] can be used to specify the expected maximum size
of the mib resource in (identifier, value) pairs. The returned data
MUST terminate with a complete (identifier, value) pair.
In the case that management data is bigger than the maximum supported
payload size, the Block mechanism from [I-D.ietf-core-block] is used.
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.
3.2.2. multi MIB values
A request to read multiple MIB variables is done by expressing the
pairs (MIB descriptor, null) in the payload of the GET request
message. A request to set multiple MIB variables is done by
expressing the pairs (MIB descriptor, null value) in the payload of
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the PUT request message. The key word _multiMIB is used as array
name to signal that the payload contains multiple MIB values as
separate _multiMIB array entries.
The following example shows a request that specifies to return the
values of sysUpTime and ipNetToMediaTable:
REQ: GET example.com/mg/mib (Content-Format: application/json)
{
"_multiMIB" : [
{ "sysUpTime" : "null"},
{ "ipNetToMediaTable" : "null" }
]
}
RES: 2.05 Content (Content-Format: application/json)
{
"_multiMIB" : [
{ "sysUpTime" : 123456},
{ "ipNetTOMediaTable" : [
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
"ipNetToMediaNetAddress" : "10.0.0.51",
"ipNetToMediaType" : "static"
},
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:54:32:10",
"ipNetToMediaNetAddress" : "9.2.3.4",
"ipNetToMediaType" : "dynamic"
},
{
"ipNetToMediaIfIndex" : 2,
"ipNetToMediaPhysAddress" : "00:00::10:98:76:54",
"ipNetToMediaNetAddress" : "10.0.0.15",
"ipNetToMediaType" : "dynamic"
}
]
}
]
}
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3.2.3. Table row
The managing entity MAY be interested only in certain table entries.
One way to specify a row is to specify its row number in the URI with
the "row" uri-query attribute. The specification of row=1 returns
row 1 values of the ipNetToMediaTable in the example:
REQ: GET example.com/mg/mib/ipNetToMediaTable?row=1
RES: 2.05 Content (Content-Format: application/json)
{ "ipNetTOMediaTable" : [
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
"ipNetToMediaNetAddress" : "10.0.0.51",
"ipNetToMediaType" : "static"
}
]
}
An alternative mode of selection is by specifying the value of the
INDEX attributes. Towards this end, the managing entity can include
the required entries in the payload of its "GET" request by
specifying the values of the index attributes. The key word
_indexMIB is used to specify the index value.
For example, to obtain a table entry from ipNetToMediaTable, the rows
are specified by specifying the index attributes: ipNetToMediaIfIndex
and ipNetToMediaNetAddress. The managing entity could have sent a
GET with the following payload:
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REQ: GET example.com/mg/mib/ipNetToMediaTable(Content-Format: application/json)
{ "_indexMIB" :
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaNetAddress" : "9.2.3.4"
}
}
RES: 2.05 Content (Content-Format: application/json)
{ "ipNetTOMediaTable" : [
{
"ipNetToMediaIfIndex" : 1,
"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
"ipNetToMediaNetAddress" : "9.2.3.4",
"ipNetToMediaType" : "static"
}
]
}
Constrained devices MAY support this kind of filtering. However, if
they don't support it, they MUST ignore the payload in the GET
request and handle the message as if the payload was empty.
It is advised to keep MIBs for constrained entities as simple as
possible, and therefore it would be best to avoid extensive tables.
TODO: Describe how the contents of the next lexicographical row can
be returned.
3.2.4. Error returns
When a variable with the specified name cannot be processed, CoAP
Error code 5.01 is returned. In addition, a MIB specific error can
be returned in the payload as specified in Section 8.
4. Mapping SMI to CoMI payload
The SMI syntax is mapped to CBOR necessary for the transport of MIB
data in the CoAP payload. This section first describes an additional
data reduction technique by creating a table that maps string values
to numbers used in CBOR encoded data.
The section continues by describing the mapping from SMI to CBOR.
The mapping is inspired by the mapping from SMI to JSON via YANG
[RFC6020], as described in [RFC6643] defining a mapping from SMI to
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YANG, and [I-D.lhotka-netmod-yang-json] defining a mapping from YANG
to JSON.
Notice that such conversion chain MAY be virtual only, as SMI could
be converted directly to JSON by combining the rules from the above
documents.
4.1. Mapping strings to CBOR
Because descriptors may be rather long and may occur repeatedly, CoMI
allows for association of a string with an integer, henceforth called
"string number". The association between string and string number is
done through a translation table, leveraging CBOR encoding.
Using the notational convention from Appendix A, the CBOR data has
the following syntax:
cBorMIB : CBorMIB;
*CBorMIB {
xlatTableID : uint;
mibString : map( uint, . );
}
The main structure consist of an array of two elements: "xlatTableID"
and "mibString".
The values of the MIB strings are stored in the "mibString" field.
This field consist of integer-value pairs. The integers correspond
to the string numbers, whereas the values contain the actual value of
the associated string.
The "xlatTableID" contains an integer that is used to indentify the
translation table. The translation table can be acquired as follows:
GET /mg/xlat/[xlatTableID]
where "[xlatTableID]" is replaced by the the value of xlatId from the
CBorMIB structure, encoded as a hexidecimal integer without leading
zeros.
The maintenance of the table is described in Section 7.2.
The use of the table is to initialize devices with the strings which
will be frequently used, such as the strings of the descriptors in
the MIB variables. The transmitted CBOR data will contain the string
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numbers and not the entire descriptor strings, leading to appreciable
data reduction.
It is important that sender and receiver have identical versions of
the table.
The translation table is serialized to the payload in the following
fashion:
xlatTable : XLatTable;
*XLatTable {
xlatId : uint;
xlatData : map( uint, tstr );
}
where "xlatId" has the same value as "xlatId" in the CBorMIB
structure, and "xlatData" is a CBOR map between the string number and
associated variable descriptor.
4.2. Mapping SMI to CBOR
4.2.1. General overview
Starting from the intermediate conversion from SMI to YANG as defined
in [RFC6643], This section defines how to convert the resulting YANG
structure to CBOR [RFC7049]. The actual conversion code from SMI to
YANG and subsequently YANG to CBOR MAY be direct conversion code from
SMI to CBOR or a sequence of existing SMI to YANG conversion code
followed by YANG to CBOR conversion code.
4.2.2. Conversion from YANG datatypes to CBOR datatypes
Table 1 defines the mapping between YANG datatypes and CBOR
datatypes.
Elements of types not in this table, and of which the type cannot be
inferred from a type in this table, are ignored in the CBOR encoding
by default. Examples include the "description" and "key" elements.
However, conversion rules for some elements to CBOR MAY be defined
elsewhere.
+-------------+------------------+----------------------------------+
| YANG type | CBOR type | Specification |
+-------------+------------------+----------------------------------+
| int8, | unsigned int | The CBOR integer type depends on |
| int16, | (major type 0) | the sign of the actual value. |
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| int32, | or negative int | |
| int64, | (mayor type 1) | |
| uint16, | | |
| uint32, | | |
| uint64, | | |
| decimal64 | | |
| | | |
| boolean | either "true" | |
| | (major type 7, | |
| | simple value 21) | |
| | or "false" | |
| | (major type 7, | |
| | simple value 20) | |
| | | |
| string | text string | |
| | (major type 3) | |
| | | |
| enumeration | unsigned int | |
| | (major type 0) | |
| | | |
| bits | array of text | Each text string contains the |
| | strings | name of a bit value that is set. |
| | | |
| binary | byte string | |
| | (major type 2) | |
| | | |
| empty | null (major type | TBD: This MAY not be applicable |
| | 7, simple value | to true MIBs, as SNMP may not |
| | 22) | support empty variables... |
| | | |
| union | | Similar ot the JSON |
| | | transcription from |
| | | [I-D.lhotka-netmod-yang-json], |
| | | the elements in a union MUST be |
| | | determined using the procedure |
| | | specified in section 9.12 of |
| | | [RFC6020]. |
| | | |
| leaf-list | array (major | The array is encapsulated in the |
| | type 4) | map associated with the |
| | | descriptor. |
| | | |
| list | map (major type | Like the higher level map, the |
| | 4) | lower level map contains |
| | | descriptor number - value pairs |
| | | of the elements in the list. |
| | | |
| container | map (major type | The map contains decriptor |
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| | 5) | number - value pairs |
| | | corresponding to the elements in |
| | | the container. |
| | | |
| smiv2:oid | array of | Each integer contains an element |
| | integers | of the OID, the first integer in |
| | | the array corresponds to the |
| | | most left element in the OID. |
+-------------+------------------+----------------------------------+
Table 1: Conversion of YANG datatypes to CBOR
4.2.3. Examples
4.2.3.1. ipNetToMediaTable to JSON/CBOR
The YANG translation of the SMI specifying the
ipNetToMediaTable yields:
container ipNetToMediaTable {
list ipNetToMediaEntry {
leaf ipNetToMediaIfIndex {
type: int32;
}
leaf ipNetToPhysAddress {
type: phys-address;
}
leaf ipNetToMediaNetAddress {
type: ipv4-address;
}
leaf ipNetToMediaType {
type: int32;
}
}
}
The coresponding JSON looks like:
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{
"ipNetToMediaTable" : {
"ipNetToMediaEntry" : [
{
"ipNetToMediaIfIndex" : 1.
"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
"ipNetToMediaNetAddress" : "10.0.0.51",
"ipNetToMediaType" : "static"
},
{
"ipNetToMediaIfIndex " : 1,
"ipNetToMediaPhysAddress " : "00:00::10:54:32:10",
"ipNetToMediaNetAddress" : "9.2.3.4",
"ipNetToMediaType " : "dynamic"
}
]
}
}
An example CBOR instance of the MIB can be found in Figure 1. The
names "ipNetToMediaTable", "ipNetToMediaEntry", and
"ipNetToMediaIfIndex" are represented with the string numbers 00, 01,
and 02 as described in Section 4.1.
82 # two element array
19 43 A1 # translation table ID 43A1
BF # indefinite length map
00 # descriptor number related to
# ipNetToMediaTable
BF # indefinite length map related to
# ipNetToMediaTable
01 # descriptor number related to
# ipNetToMediaEntry
BF # map related to ipNetToMediaEntry
02 # descriptor number associated with
# ipNetToMediaIfIndex
1A 00 00 00 01 # associated value as 32-bit integer
# ...
FF
FF
FF
Figure 1: Example CBOR encoding for ifTable
The associated "descriptor string" to "string number" translation
table is given in Figure 2.
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82 # two element array
19 43 A1 # translation table ID 43A1
BF # indefinite length map
00 # descriptor number related to
# ipNetToMediaTable
72 69 70 50 65 74 57
6F 51 65 64 61 57 61
62 6C 65 # "ipNetToMediaTable"
01 # descriptor number related to
# ipNetToMediaEntry
72 69 70 50 65 74 57
6F 51 65 64 61 45 6E
74 72 78 # "ipNetToMediaEntry"
02 # descriptor number related to
# ipNetToMediaIfIndex
75 69 70 50 65 74 57
6F 51 65 64 61 61 49
66 49 6E 64 65 77 # "ipNetToMediaIfIndex"
# ...
FF
Figure 2: Translation table for ifTable
4.2.4. 6LoWPAN MIB
A MIB for 6LoWPAN is defined in [I-D.schoenw-6lowpan-mib]. The
document also provides an example JSON representation in its
Appendix A. Figure 3 shows the associated CBOR representation with
string number, and Figure 4 shows the corresponding string to string
number conversion table.
82 # two element array
1A 8B 47 88 F3 # translation table ID 8B4788F3
BF # indefinite length map
00 # "LOWPAN-MIB:LOWPAN-MIB"
BF # indefinite length map related to ifTable
01 # "lowpanReasmTimeout"
14 # 20
02 # "lowpanInReceives"
18 2A # 42
03 # "lowpanInHdrErrors"
00 # 0
04 # "lowpanInMeshReceives"
08 # 8
05 # "lowpanInMeshForwds"
00 # 0
06 # "lowpanInMeshDelivers"
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00 # 0
07 # "lowpanInReasmReqds"
16 # 22
08 # "lowpanInReasmFails"
02 # 02
09 # "lowpanInReasmOKs"
14 # 20
0A # "lowpanInCompReqds"
10 # 16
0B # "lowpanInCompFails"
02 # 2
0C # "lowpanInCompOKs"
0E # 14
0D # "lowpanInDiscards"
01 # 01
0E # "lowpanInDelivers"
0C # 12
0F # "lowpanOutRequests"
0C # 12
10 # "lowpanOutCompReqds"
00 # 0
11 # "lowpanOutCompFails"
00 # 0
12 # "lowpanOutCompOKs"
00 # 0
13 # "lowpanOutFragReqds"
05 # 5
14 # "lowpanOutFragFails"
00 # 0
15 # "lowpanOutFragOKs"
05 # 5
16 # "lowpanOutFragCreates"
08 # 8
17 # "lowpanOutMeshHopLimitExceeds"
00 # 0
18 18 # "lowpanOutMeshNoRoutes"
00 # 0
18 19 # "lowpanOutMeshRequests"
00 # 0
18 1A # "lowpanOutMeshForwds"
00 # 0
18 1B # "lowpanOutMeshTransmits"
00 # 0
18 1C # "lowpanOutDiscards"
00 # 0
18 1D # "lowpanOutTransmits"
0F # 15
FF
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FF
Figure 3: Example CBOR encoding for the 6LoWPAN MIB
82 # two element array
1A 8B 47 88 F3 # translation table ID 8B4788F3
BF # indefinite length map
00
75 # "LOWPAN-MIB:LOWPAN-MIB"
01 #
72 ... # "lowpanReasmTimeout"
02
70 ... # "lowpanInReceives"
03
71 ... # "lowpanInHdrErrors"
04
74 ... # "lowpanInMeshReceives"
05
72 ... # "lowpanInMeshForwds"
06
74 ... # "lowpanInMeshDelivers"
07
72 ... # "lowpanInReasmReqds"
08
72 ... # "lowpanInReasmFails"
09
70 ... # "lowpanInReasmOKs"
0A
71 ... # "lowpanInCompReqds"
0B
71 ... # "lowpanInCompFails"
0C
6F ... # "lowpanInCompOKs"
0D
70 ... # "lowpanInDiscards"
0E
70 ... # "lowpanInDelivers"
0F
71 ... # "lowpanOutRequests"
10
72 ... # "lowpanOutCompReqds"
11
72 ... # "lowpanOutCompFails"
12
70 ... # "lowpanOutCompOKs"
13
72 ... # "lowpanOutFragReqds"
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14
72 ... # "lowpanOutFragFails"
15
70 ... # "lowpanOutFragOKs"
16
74 ... # "lowpanOutFragCreates"
17
78 1B ... # "lowpanOutMeshHopLimitExceeds"
18 18
75 ... # "lowpanOutMeshNoRoutes"
18 19
75 ... # "lowpanOutMeshRequests"
18 1A
73 ... # "lowpanOutMeshForwds"
18 1B
76 ... # "lowpanOutMeshTransmits"
18 1C
71 ... # "lowpanOutDiscards"
18 1D
72 ... # "lowpanOutTransmits"
FF
Figure 4: Translation table for the 6LoWPAN MIB
In this example, a GET to /mg/mib/lowpanOutFragFails would give:
82 # two element array
1A 8B 47 88 F3 # translation table ID 8B4788F3
BF # indefinite length map
14 # "lowpanOutFragFails"
00 # 0
FF
5. MIB discovery
MIB objects are discovered like resources with the standard CoAP
resource discovery. Performing a GET on "/.well-known/core" with
rt=core.mg.mib returns all MIB descriptors and all OIDs which are
available on this device. For table objects there is no further
possibility to discover the row descriptors. For example, consider
there are two MIB objects with descriptors "sysUpTime" and
"ipNetToMediaTable" associated with OID 1.3.6.1.2.1.1.3 and
1.3.6.1.2.1.4.22
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REQ: GET example.com/.well-known/core?rt=core.mg.mib
RES: 2.05 Content (Content-Format: application/text)
</mg/mib/sysUpTime>;rt="core.mg.mib";oid="1.3.6.1.2.1.1.3";mod="SNMPv2-MIB"
</mg/mib/ipNetToMediaTable>;rt="core.mg.mib";oid="1.3.6.1.2.1.4.22";mod="ipMIB"
The link format attribute 'oid' is used to associate the name of the
MIB resource with its OID. The OID is written as a string in its
conventional form.
Notice that a MIB variable normally is associated with a descriptor
and an OID. The OID is unique, whereas the descriptor is unique in
combination with the module name.
The "mod", "con", and "rt" attributes can be used to filter resource
queries as specified in [RFC6690].
6. Trap functions
A trap can be set through the CoAP Observe [I-D.ietf-core-observe]
function. As regular with Observe, the managing entity subscribes to
the variable by sending a GET request with an "Observe" option.
TODO: Observe example
In the registration request, the managing entity 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 managing entity wants the
managed device to send the trap information to a multicast address.
7. MIB access management
Two topics are relevant: (1) the definition of the destination of
Notify messages, and (2) the creation and maintenance of "string to
number" tables.
7.1. Notify destinations
The destination of notifications need to be communicated to the
applications sending them. Draft [I-D.ietf-core-interfaces]
describes the binding of end-points to end-points on remote devices.
The object with type "binding table" contains a sequence of bindings.
The contents of bindings contains the methods, location, the interval
specifications, and the step value as suggested in
[I-D.ietf-core-interfaces]. The method "notify" has been added to
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the binding methods "poll", "obs" and "push", to cater for the
binding of notification source to the receiver.
TODO: describe interface for NOTIFY destination definition.
7.2. Conversion tables
POST is used to initialize a conversion table. At the arrival of the
POST, all existing tables are removed and new tables as specified by
the payload are created with the contents specified in the payload.
When the payload of the POST is empty, no table is created.
PUT is used to create new entries in an existing table and overwrite
existing entries. When the payload of the PUT contains a non
existing table, a new table with the new identity is created. When
the payload of the PUT contains a table with an already existing
identifier, two possiblities exist:
exiting string value the contents of the existing pair is
overwritten with the pair in the payload.
new string value A new pair is created in the table with the pair in
the payload.
8. Error handling
In case a request is received which cannot be processed properly, the
managed entity 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.
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 readible
explanation of the error.
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+----------------+----------------+---------------------------------+
| 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 translation 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 associted with the exceptions defined in
[RFC3416] and CoAP error code 5.00 is associated with the error-
status defined in [RFC3416].
9. Security Considerations
TODO: follows CoAP security provisioning.
10. IANA Considerations
'rt="core.mg.mib"' needs registration with IANA.
Content types to be registered:
o application/comi+json
o application/comi+cbor
11. Acknowledgements
Mehmet Ersue and Bert Wijnen explained the encoding aspects of PDUs
transported under SNMP. Carsten Bormann has given feedback on the
use of CBOR. Juergen Schoenwalder has commented on inconsistencies
and missing aspects of SNMP in earlier versions of the draft. The
draft has benefited from comments by Thomas Watteyne, Dee Denteneer,
Esko Dijk, and Michael van Hartskamp. The CBOR encoding borrows
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extensively from Ladislav Lhotka's description on conversion from
YANG to JSON.
12. 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
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
[I-D.becker-core-coap-sms-gprs]
Becker, M., Li, K., Poetsch, T., and K. Kuladinithi,
"Transport of CoAP over SMS", draft-becker-core-coap-sms-
gprs-04 (work in progress), August 2013.
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[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-14 (work in progress), October 2013.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-11 (work in progress), October 2013.
[I-D.ietf-json-rfc4627bis]
Bray, T., "The JSON Data Interchange Format", draft-ietf-
json-rfc4627bis-10 (work in progress), December 2013.
[I-D.lhotka-netmod-yang-json]
Lhotka, L., "Modeling JSON Text with YANG", draft-lhotka-
netmod-yang-json-02 (work in progress), September 2013.
13.2. Informative References
[RFC1213] McCloghrie, K. and M. Rose, "Management Information Base
for Network Management of TCP/IP-based internets:MIB-II",
STD 17, RFC 1213, March 1991.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2863] McCloghrie, K. and F. Kastenholz, "The Interfaces Group
MIB", RFC 2863, June 2000.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
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[RFC3416] Presuhn, R., "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62, RFC
3416, December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62, RFC
3418, December 2002.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4088] Black, D., McCloghrie, K., and J. Schoenwaelder, "Uniform
Resource Identifier (URI) Scheme for the Simple Network
Management Protocol (SNMP)", RFC 4088, June 2005.
[RFC4113] Fenner, B. and J. Flick, "Management Information Base for
the User Datagram Protocol (UDP)", RFC 4113, June 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4293] Routhier, S., "Management Information Base for the
Internet Protocol (IP)", RFC 4293, April 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6643] Schoenwaelder, J., "Translation of Structure of Management
Information Version 2 (SMIv2) MIB Modules to YANG
Modules", RFC 6643, July 2012.
[RFC6650] Falk, J. and M. Kucherawy, "Creation and Use of Email
Feedback Reports: An Applicability Statement for the Abuse
Reporting Format (ARF)", RFC 6650, June 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
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[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-18 (work in progress), December
2013.
[I-D.ietf-core-interfaces]
Shelby, Z. and M. Vial, "CoRE Interfaces", draft-ietf-
core-interfaces-01 (work in progress), December 2013.
[I-D.ersue-constrained-mgmt]
Ersue, M., Romascanu, D., and J. Schoenwaelder,
"Management of Networks with Constrained Devices: Problem
Statement, Use Cases and Requirements", draft-ersue-
constrained-mgmt-03 (work in progress), February 2013.
[I-D.schoenw-6lowpan-mib]
Schoenwaelder, J., Sehgal, A., Tsou, T., and C. Zhou,
"Definition of Managed Objects for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", draft-
schoenw-6lowpan-mib-03 (work in progress), February 2013.
[I-D.bierman-netconf-restconf]
Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
"RESTCONF Protocol", draft-bierman-netconf-restconf-02
(work in progress), October 2013.
[STD0001] "Official Internet Protocols Standard", Web
http://www.rfc-editor.org/rfcxx00.html, .
[XML] "Extensible Markup Language (XML)", Web
http://www.w3.org/xml, .
[JSON] "JavaScript Object Notation (JSON)", Web
http://www.json.org, .
Appendix A. Notational Convention for CBOR data
To express CBOR structures [RFC7049], this document uses the
following conventions:
A declaration of a CBOR variable has the form:
name : datatype;
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where "name" is the name of the variable, and "datatype" its CBOR
datatype.
The name of the variable has no encoding in the CBOR data.
"datatype" can be a CBOR primitive such as:
tstr: A text string (major type 3)
uint: An unsigned integer (major type 0)
map(x,y): A map (major type 5), where each first element of a pair
is of datatype x, and each second element of datatype y. A '.'
character for either x or y means that all datatypes for that
element are valid.
A datatype can also be a CBOR structure, in which case the variable's
"datatype" field contains the name of the CBOR structure. Such CBOR
structure is defined by a character sequence consisting of first its
name, then a '{' character, then its subfields and finally a '}'
character.
A CBOR structure can be encapsulated in an array, in which case its
name in its definition is preceeded by a '*' character. Otherwise
the structure is just a grouping of fields, but without actual
encoding of such grouping.
The name of an optional field is preceded by a '?' character. This
means, that the field may be omitted if not required.
Authors' Addresses
Peter van der Stok
consultant
Phone: +31-492474673 (Netherlands), +33-966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
Bert Greevenbosch
Huawei Technologies Co., Ltd.
Huawei Industrial Base
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: bert.greevenbosch@huawei.com
van der Stok & GreevenboscExpires July 13, 2014 [Page 31]