core P. van der Stok
Internet-Draft consultant
Intended status: Informational B. Greevenbosch
Expires: July 13, 2014 Huawei Technologies
January 09, 2014

CoAp Management Interfaces


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.


Discussion and suggestions for improvement are requested, and should be sent to

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on July 13, 2014.

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Table of Contents

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.

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.

ASN.1 OBJECT-IDENTIFIER, which is used to uniquely identify MIB objects in the managed device.

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):, with path: /mg, where mg is short-hand for management. The mg resource has two sub-resources accessible with the paths: 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 mib resource provides access to the MIBs as described in

The profile of the management function set, with, is shown in the table below, following the guidelines of [I-D.ietf-core-interfaces]:

name path RT Data Type
Management /mg n/a
MIB /mg/mib application/cbor
XLAT /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: [RFC4088] describes a URI scheme to refer to a specific MIB instance.

In addition

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: Section 6.

The binding of the notification to a destination is discussed in

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:

ipv6InterfaceTable OBJECT-TYPE
  SYNTAX                      SEQUENCE OF Ipv6InterfaceEntry
  MAX-ACCESS                  not-accessible
  STATUS                      current
    "The table containing per-interface IPv6-specific
::= { ip 30 }

ipv6InterfaceEntry OBJECT-TYPE
  SYNTAX                      Ipv6InterfaceEntry
  MAX-ACCESS                  not-accessible
  STATUS current
    "An entry containing IPv6-specific information for a given
  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: Section 4.

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.

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

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.


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.

REQ: GET 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:


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:


RES: 2.05 Content (Content-Format: application/json)
   "ipNetTOMediaTable" : [
	"ipNetToMediaIfIndex" : 1,
	"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
	"ipNetToMediaNetAddress" : "",
	"ipNetToMediaType" : "static"
	"ipNetToMediaIfIndex" : 1,
	"ipNetToMediaPhysAddress" : "00:00::10:54:32:10",
	"ipNetToMediaNetAddress" : "",
	"ipNetToMediaType" : "dynamic"
	"ipNetToMediaIfIndex" : 2,
	"ipNetToMediaPhysAddress" : "00:00::10:98:76:54",
	"ipNetToMediaNetAddress" : "",
	"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 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 (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" : "",
	"ipNetToMediaType" : "static"
	"ipNetToMediaIfIndex" : 1,
	"ipNetToMediaPhysAddress" : "00:00::10:54:32:10",
	"ipNetToMediaNetAddress" : "",
	"ipNetToMediaType" : "dynamic"
	"ipNetToMediaIfIndex" : 2,
	"ipNetToMediaPhysAddress" : "00:00::10:98:76:54",
	"ipNetToMediaNetAddress" : "",
	"ipNetToMediaType" : "dynamic"

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:


RES: 2.05 Content (Content-Format: application/json)
{ "ipNetTOMediaTable" : [
	"ipNetToMediaIfIndex" : 1,
	"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
	"ipNetToMediaNetAddress" : "",
	"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:

REQ: GET application/json)

{ "_indexMIB" :
      "ipNetToMediaIfIndex" : 1,
      "ipNetToMediaNetAddress" : ""

RES: 2.05 Content (Content-Format: application/json)
{ "ipNetTOMediaTable" : [
	"ipNetToMediaIfIndex" : 1,
	"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
	"ipNetToMediaNetAddress" : "",
	"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 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 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.

xlatTable     : XLatTable;

*XLatTable {
  xlatId      : uint;
  xlatData    : map( uint, tstr );

The translation table is serialized to the payload in the following fashion:

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.

Conversion of YANG datatypes to CBOR
YANG type CBOR type Specification
int8, int16, int32, int64, uint16, uint32, uint64, decimal64 unsigned int (major type 0) or negative int (mayor type 1) The CBOR integer type depends on the sign of the actual value.
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 strings Each text string contains the name of a bit value that is set.
binary byte string (major type 2)
empty null (major type 7, simple value 22) TBD: This MAY not be applicable to true MIBs, as SNMP may not 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 type 4) The array is encapsulated in the map associated with the descriptor.
list map (major type 4) Like the higher level map, the lower level map contains descriptor number - value pairs of the elements in the list.
container map (major type 5) The map contains decriptor number - value pairs corresponding to the elements in the container.
smiv2:oid array of integers Each integer contains an element of the OID, the first integer in the array corresponds to the most left element in the OID.

4.2.3. Examples 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:

"ipNetToMediaTable" : {
	"ipNetToMediaEntry" : [
		"ipNetToMediaIfIndex" : 1.
		"ipNetToMediaPhysAddress" : "00:00::10:01:23:45",
		"ipNetToMediaNetAddress" : "",
		"ipNetToMediaType" : "static"
		"ipNetToMediaIfIndex " : 1,
		"ipNetToMediaPhysAddress " : "00:00::10:54:32:10",
		"ipNetToMediaNetAddress" : "",
		"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
           # ...

Figure 1: Example CBOR encoding for ifTable

The associated "descriptor string" to "string number" translation table is given in Figure 2.

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"
      # ...

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"
        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

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
      75                   # "LOWPAN-MIB:LOWPAN-MIB"
      01                   # 
      72 ...               # "lowpanReasmTimeout"
      70 ...               # "lowpanInReceives"
      71 ...               # "lowpanInHdrErrors"
      74 ...               # "lowpanInMeshReceives"
      72 ...               # "lowpanInMeshForwds"
      74 ...               # "lowpanInMeshDelivers"
      72 ...               # "lowpanInReasmReqds"
      72 ...               # "lowpanInReasmFails"
      70 ...               # "lowpanInReasmOKs"
      71 ...               # "lowpanInCompReqds"
      71 ...               # "lowpanInCompFails"
      6F ...               # "lowpanInCompOKs"
      70 ...               # "lowpanInDiscards"
      70 ...               # "lowpanInDelivers"
      71 ...               # "lowpanOutRequests"
      72 ...               # "lowpanOutCompReqds"
      72 ...               # "lowpanOutCompFails"
      70 ...               # "lowpanOutCompOKs"
      72 ...               # "lowpanOutFragReqds"
      72 ...               # "lowpanOutFragFails"
      70 ...               # "lowpanOutFragOKs"
      74 ...               # "lowpanOutFragCreates"
      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"

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

5. MIB discovery

MIB objects are discovered like resources with the standard CoAP resource discovery. Performing a GET on "/.well-known/core" with 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 and


RES: 2.05 Content (Content-Format: application/text)

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 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.

CoMI Error Code CoAP Error Code Description
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=""' needs registration with IANA.

Content types to be registered:

  • application/comi+json
  • 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 extensively from Ladislav Lhotka's description on conversion from YANG to JSON.

12. Changelog

Changes from version 00 to version 01

  • Focus on MIB only
  • Introduced CBOR, JSON, removed BER
  • defined mappings from SMI to xx
  • Introduced the concept of addressable table rows

Changes from version 01 to version 02

  • Focus on CBOR, used JSON for examples, removed XML and EXI
  • added uri-query attributes mod and con to specify modules and contexts
  • Definition of CBOR string conversion tables for data reduction
  • use of Block for multiple fragments
  • Error returns generalized
  • 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", Internet-Draft draft-becker-core-coap-sms-gprs-04, August 2013.
[I-D.ietf-core-block] Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP", Internet-Draft draft-ietf-core-block-11, March 2013.
[I-D.ietf-core-coap] Shelby, Z., Hartke, K. and C. Bormann, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-14, March 2013.
[I-D.ietf-core-observe] Hartke, K., "Observing Resources in CoAP", Internet-Draft draft-ietf-core-observe-08, February 2013.
[I-D.ietf-json-rfc4627bis] Bray, T., "The JSON Data Interchange Format", Internet-Draft draft-ietf-json-rfc4627bis-10, December 2013.
[I-D.lhotka-netmod-yang-json] Lhotka, L., "Modeling JSON Text with YANG", Internet-Draft draft-lhotka-netmod-yang-json-02, 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., Perkins, D. and J. Schoenwaelder, "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.
[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.
[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", Internet-Draft draft-ietf-core-groupcomm-05, February 2013.
[I-D.ietf-core-interfaces] Shelby, Z. and M. Vial, "CoRE Interfaces", Internet-Draft draft-ietf-core-interfaces-00, June 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", Internet-Draft draft-ersue-constrained-mgmt-03, 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)", Internet-Draft draft-schoenw-6lowpan-mib-03, February 2013.
[I-D.bierman-netconf-restconf] Bierman, A., Bjorklund, M., Watsen, K. and R. Fernando, "RESTCONF Protocol", Internet-Draft draft-bierman-netconf-restconf-03, December 2013.
[STD0001]Official Internet Protocols Standard", Web, .
[XML]Extensible Markup Language (XML)", Web, .
[JSON]JavaScript Object Notation (JSON)", Web, .

Appendix A. Notational Convention for CBOR data

To express CBOR structures [RFC7049], this document uses the following conventions:

   name : datatype;

A declaration of a CBOR variable has the form:

The name of the variable has no encoding in the CBOR data.

"datatype" can be a CBOR primitive such as:

A text string (major type 3)
An unsigned integer (major type 0)
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: URI:
Bert Greevenbosch Huawei Technologies Co., Ltd. Huawei Industrial Base Bantian, Longgang District Shenzhen, 518129 P.R. China EMail: