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) | | |
| |                    +--------------+       +------------+ | |
| |                    variable store (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.

Objectives are:

• Equip a constrained node with a management server that provides information about the operational characteristics of the code running in the constrained node.
• The server provides this information in a variable store that contains values describing the performance characteristics and the code parameter values.
• The client receives the performance characteristics on a regular basis or on request.
• The client sets the parameter values in the server at bootstrap and intermittently when operational conditions change.
• The constrained network requires the payload to be as small as possible, and the constrained server memory requirements should be as small as possible.

For interoperability it is required that in addition to using the Internet Protocol for data transport:

• The names, type, and semantics of the instrumentation variables are standardized.
• The instrumentation variables are described in a standard language.
• The URI of the CoAP request is standardized.
• The format of the packet payload is standardized.
• The notification from server to client is standardized.

The different numbered components of Figure 1 are discussed according to component number.

(1) YANG specification:

contains a set of named and versioned modules. A module specifies a hierarchy of named and typed resources. A resource is uniquely identified by a sequence of its name and the names of the enveloping resources following the hierarchy order. The YANG specification serves as input to the writers of application and instrumentation code and the humans analying the returned values (arrow from YANG specification to Variable store). The specification can be used to check the correctness of the CoAP request and do the CBOR encoding.

(2) SMIv2 specification:

A named module specifies a set of variables and "conceptual tables". Named variables have simple types. Conceptual tables are composed of typed named columns. The variable name and module name identify the variable uniquely. There is an algorithm to translate SMIv2 specifications to YANG specifications.

(3) CoAP request:

The CoAP request needs a Universal Resource Identifier (URI) and the payload of the packet to send a request. The URI is composed of the schema, server, path and query and looks like coap://entry.example.com/<path>?<query>. Fragments are not supported. Allowed operations are PUT, PATCH, GET, DELETE, and POST. New variables can be created with POST when they exist in the YANG specification. The Observe option is used to return variable values regularly or on event occurrence (notification).

(3.1) CoAP <path>:

The path identifies the variable in the form "/mg/<hash-value>".

(3.2) CoAP <query>:

The query parameter is used to specify additional (optional) aspects like the container name, list instance, and others. The idea is to keep the path simple and put variations on variable specification in the query.

(3.3) CoAP discovery:

Discovery of the variables is done with standard CoAP resource discovery using /.well-known/core with ?rt=/core.mg.

(4) Network packet:

The payload contains the CBOR encoding of JSON objects. This object corresponds with the converted RESTCONF message payload.

(5) Retrieval, modification:

The server needs to parse the CBOR encoded message and identify the corresponding instances in the Variable store. In addition, this component includes the code for CoAP Observe and block options.

(6) Variable store:

The store is composed of two parts: Operational state and Configuration data-store (see Section 2.1). CoMI does not differentiate between variable store types. The Variable store contains data-node instances. Values are stored in the appropriate instances, and or values are returned from the instances into the payload of the packet.

(7) Variable instrumentation:

This code depends on implementation of drivers and other node specific aspects. The Variable instrumentation code stores the values of the parameters into the appropriate places in the operational code. The variable instrumentation code reads current execution values from the operational code and stores them in the appropriate instances.

(8) Security:

The server MUST prevent unauthorized users from reading or writing any data resources. CoMI relies on DTLS [RFC6347] which is specified to secure CoAP communication.

2.1. RESTCONF/YANG Architecture

CoMI adapts the RESTCONF architecture so data exchange and implementation requirements are optimized for constrained devices.

The RESTCONF protocol uses a unified data-store to edit conceptual data structures supported by the server. The details of transaction preparation and non-volatile storage of the data are hidden from the RESTCONF client. CoMI also uses a unified data-store, to allow stateless editing of configuration variables and the notification of operational variables.

The child schema nodes of the unified data-store include all the top-level YANG data nodes in all the YANG modules supported by the server. The YANG data structures represent a hierarchy of data resources. The client discovers the list of YANG modules, and important conformance information such as the module revision dates, YANG features supported, and YANG deviations required. The individual data nodes are discovered indirectly by parsing the YANG modules supported by the server.

The YANG data definition statements contain a lot of information that can help automation tools, developers, and operators use the data model correctly and efficiently. The YANG definitions and server YANG module capability advertisements provide an "API contract" that allow a client to determine the detailed server management capabilities very quickly. CoMI allows access to the same data resources as a RESTCONF server, except that messages are optimized to reduce identifier and payload size.

RESTCONF uses a simple algorithmic mapping from YANG to URI syntax to identify the target resource of a retrieval or edit operation. A client can construct operations or scripts using a predictable syntax, based on the YANG data definitions. The target resource URI can reference a data resource instance, or the data-store itself (to retrieve the entire data-store or create a top-level data resource instance). CoMI uses a compression algorithm to reduce the size of the data-node instance identifier (see Section 2.2.

2.2. Compression of data-node instance identifier

The RESTCONF protocol uses the full path of the desired data resource in the target resource URI. The JSON encoding will include the module name string to specify the YANG module. If a representation of the target resource is included in the request or response message in RESTCONF messages, then the data definition name string is used to identify each node in the message. The module namespace (or name) may also be present in these identifiers.

In order to greatly 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.

YANG Hash is the default encoding for object identifiers. This encoding in considered to be "unstructured" since the particular values for each object are determined by a hash algorithm. It is possible for 2 different objects to generate the same hash value. If this occurs, then the client and server will both need to rehash the colliding object identifiers to new unused hash values.

In order to eliminate the need for rehashing, CoMI allows for alternate "structured" object identifier encoding formats. Structured object identifier MUST be managed such that no object ID collisions are possible, and therefore no rehash procedures are needed. Structured object identifiers can also be selected to minimize the size of a subset of the object identifiers (e.g., the most requested objects).

3. 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 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 name /mg is recommended but not compulsory (see Section 4.5).

The path prefix /mg has resources accessible with the following five paths:

/mg:

YANG-based data with path "/mg" and using CBOR content encoding format. This path represents a data-store resource which contains YANG data resources as its descendant nodes. All identifiers referring to YANG data nodes within this path are encoded as YANG hash values (see Section 5.7).

/mg/mod.uri:

URI identifying the location of the server module information, with path "/mg/mod.uri" and CBOR content format. This YANG data is encoded with plain identifier strings, not YANG hash values. An EntityTag 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.

/mg/num.typ:

String identifying the object ID numbering scheme used by the CoMI server. The only value defined in this document is 'yanghash' to indicate that the YANG Hash numbering scheme defined in this document is used. It is possible for other object numbering schemes to be defined outside the scope of this document.

/mg/srv.typ:

String identifying the CoMI server type. The value 'ro' indicates that the server is a read-only server and no editing operations are supported. A read-only server is not required to provide YANG deviation statements for any writable YANG data nodes. The value 'rw' indicates that the server is a read-write server and editing operations are supported. A read-write server is required to provide YANG deviation statements for any writable YANG data nodes that are not fully implemented.

/mg/yh.uri:

URI indicating the location of the server YANG hash information if any objects needed to be re-hashed by the server. It has the path "/mg/yh.uri" and is encoded in CBOR format. The "yang-hash" container within the "ietf-yang-hash" module of Section 5.4 is used to define the syntax and semantics of this data structure. This YANG data is encoded with plain identifier strings, not YANG hash values. The server will only have this resource if there are any objects that needed to be re-hashed due to a hash collision. If a client requests a node that has been re-hashed, then a rehash error is returned, according to the procedure in Section 5.2.

/mg/stream:

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 YANG hash values, as described in Section 5.1. When multiple instances of a list node exist, the instance selection is described in Section 4.1.3.4

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

4. MG Function Set

The MG Function Set provides a CoAP interface to perform a subset of the functions provided by RESTCONF.

A subset of the operations defined in RESTCONF are used in CoMI:

4.1. Data Retrieval

4.1.1. GET

One or more instances of data resources are retrieved by the client with the GET method. The RESTCONF GET operation is supported in CoMI. The same constraints apply as defined in section 3.3 of [I-D.ietf-netconf-restconf]. The operation is mapped to the GET method defined in section 5.8.1 of [RFC7252].

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 [I-D.ietf-core-block] is used, as explained in more detail in Section 4.4.

There are two query parameters for the GET method. A CoMI server MUST implement the keys parameter and MAY implement the select parameter to allow common data retrieval filtering functionality.

The "keys" parameter is used to specify a specific instance of the list resource. When keys is not specified, all instances are returned. When no or one instance of the resource exists, the keys parameter is ignored.

4.1.2. Mapping of the 'select' Parameter

RESTCONF uses the 'select' parameter to specify an expression which can represent a subset of all data nodes within the target resource [I-D.ietf-netconf-restconf]. This parameter is useful for filtering sub-trees and retrieving only a subset that a managing application is interested in.

However, filtering is a resource intensive task and not all constrained devices can be expected to have enough computing resources such that they will be able to successfully filter and return a subset of a sub-tree. This is especially likely to be true with Class 0 devices that have significantly lesser RAM than 10 KiB [RFC7228]. Since CoMI is targeted at constrained devices and networks, only a limited subset of the 'select' parameter is used here.

Unlike the RESTCONF 'select' parameter, CoMI does not use object names in "XPath" or "path-expr" format to identify the subset that needs to be filtered. Parsing XML is resource intensive for constrained devices [management] and using object names can lead to large message sizes. Instead, CoMI utilizes the YANG hashes described in Section 5 to identify the sub-trees that should be filtered from a target resource. Using these hashes ensures that a constrained node can identify the target sub-tree without expending many resources and that the messages generated are also efficiently encoded.

The implementation of the 'select' parameter is already optional for constrained devices, however, even when implemented it is expected to be a best effort feature, rather than a service that nodes must provide. This implies that if a node receives the 'select' parameter specifying a set of sub-trees that should be returned, it will only return those that it is able to return.

4.1.3. Retrieval Examples

In all examples the path is expressed in readable names and as a hash value of the name (where the hash value in the payload is expressed as a hexadecimal number, and the hash value in the URL as a base64 number). CoMI payloads use the CBOR format. The CBOR syntax of the YANG payloads is specified in Section 5. The examples in this section use a JSON payload with extensions to approach the permissible CBOR payload. Appendix C shows the CBOR format of some of the examples.

4.1.3.1. Single instance retrieval

A request to read the values of instances of a management object or the leaf of an object is sent with a confirmable CoAP GET message. A single object is specified in the URI path prefixed with /mg.

Using for example the clock container from [RFC7317], a request is sent to retrieve the value of clock/current-datetime specified in module system-state. The answer to the request returns a (identifier, value) pair, transported as a CBOR map with a single item.

REQ: GET example.com/mg/system-state/clock/current-datetime

RES: 2.05 Content (Content-Format: application/cbor)
{
   "current-datetime" : "2014-10-26T12:16:31Z"
}

The YANG hash value for 'current-datetime' is calculated by constructing the schema node identifier for the object:

/ietf-system:system-state/clock/current-datetime

The 30 bit murmur3 hash value (see Section 5.1) is calculated on this string with hash: 0x047c468b and EfEaM. The request using this hash value is shown below:

REQ: GET example.com/mg/EfEaM

RES: 2.05 Content (Content-Format: application/cbor)
{
    0x047c468b : "2014-10-26T12:16:31Z"
}

The specified object can be an entire object. Accordingly, the returned payload is composed of all the leaves associated with the object. The payload is a CBOR map where each leaf is returned as a (YANG hash, value) pair. For example, the GET of the clock object, sent by the client, results in the following returned payload sent by the managed entity, transported as A CBOR map with two items:

REQ: GET example.com/mg/system-state/clock
   (Content-Format: application/cbor)

RES: 2.05 Content (Content-Format: application/cbor)
{
  "clock" : {
      "current-datetime" : "2014-10-26T12:16:51Z",
      "boot-datetime" : "2014-10-21T03:00:00Z"
  }
}

The YANG hash values for 'clock', 'current-datetime', and 'boot-datetime' are calculated by constructing the schema node identifier for the objects, and then calculating the 30 bit murmur3 hash values (shown in parenthesis):

/ietf-system:system-state/clock (0x021ca491 and CDKSQ)
/ietf-system:system-state/clock/current-datetime (0x047c468b)
/ietf-system:system-state/clock/boot-datetime (0x1fb5f4f8)

The request using the hash values is shown below:

REQ: GET example.com/mg/CDKSQ
   (Content-Format: application/cbor)

RES: 2.05 Content (Content-Format: application/cbor)
{
  0x021ca491 : {

      0x047c468b : "2014-10-26T12:16:51Z",
      0x1fb5f4f8 : "2014-10-21T03:00:00Z"
   }
}

The corresponding CBOR code can be found in Appendix C.

4.1.3.2. Multiple instance retrieval

A "list" node can have multiple instances. Accordingly, the returned payload is composed of all the instances associated with the list node. Each instance is returned as a (key, object) pair, where key and object are composed of one or more (identifier, value) pairs.

For example, the GET of the /interfaces/interface/ipv6/neighbor results in the following returned payload sent by the managed entity, transported as a CBOR map of 3 (key : object) pairs, where key and value are CBOR maps with one entry each. In this case the key is the "ip" attribute and the value is the "link-layer-address" attribute.

REQ: GET example.com/mg/interfaces/interface/ipv6/neighbor
   (Content-Format: application/cbor)

RES: 2.05 Content (Content-Format: application/cbor)
{
   "neighbor" :{
     {"ip" : "fe80::200:f8ff:fe21:67cf"}:
	{"link-layer-address" : "00:00::10:01:23:45"}
     ,
     {"ip" : "fe80::200:f8ff:fe21:6708"}:
	{"link-layer-address" : "00:00::10:54:32:10"}
     ,
     {"ip" : "fe80::200:f8ff:fe21:88ee"}:
	{"link-layer-address" : "00:00::10:98:76:54"}
   }
}

The YANG hash values for 'neighbor', 'ip', and 'link-layer-address' are calculated by constructing the schema node identifier for the objects, and then calculating the 30 bit murmur3 hash values (shown in parenthesis):

/ietf-interfaces:interfaces/interface/ietf-ip:ipv6/neighbor
    (0x2445e478 and kReR4)
/ietf-interfaces:interfaces/interface/ietf-ip:ipv6/neighbor/ip
    (0x2283ed40 and ig-la)
/ietf-interfaces:interfaces/interface/ietf-ip:ipv6/neighbor/
    link-layer-address  (0x3d6915c7)

The request using the hash values is shown below:

REQ: GET example.com/mg/kReR4
   (Content-Format: application/cbor)

RES: 2.05 Content (Content-Format: application/cbor)
{
   0x2445e478 : {
     {0x2283ed40 : "fe80::200:f8ff:fe21:67cf"}:
	{0x3d6915c7 : "00:00::10:01:23:45"}
     ,
     {0x2283ed40 : "fe80::200:f8ff:fe21:6708"}:
	{0x3d6915c7 : "00:00::10:54:32:10"}
     ,
     {0x2283ed40 : "fe80::200:f8ff:fe21:88ee"}:
	{0x3d6915c7 : "00:00::10:98:76:54"}
   }
}

4.1.3.3. Access to MIB Data

The YANG translation of the SMI specifying the ipNetToMediaTable [RFC4293] yields:

container IP-MIB {
  container ipNetToPhysicalTable {
    list ipNetToPhysicalEntry {
       key "ipNetToPhysicalIfIndex
            ipNetToPhysicalNetAddressType
            ipNetToPhysicalNetAddress";
       leaf ipNetToMediaIfIndex {
          type: int32;
       }
       leaf ipNetToPhysicalIfIndex {
         type if-mib:InterfaceIndex;
       }
       leaf ipNetToPhysicalNetAddressType {
         type inet-address:InetAddressType;
       }
       leaf ipNetToPhysicalNetAddress {
         type inet-address:InetAddress;
       }
       leaf ipNetToPhysicalPhysAddress {
         type yang:phys-address {
            length "0..65535";
         }
       }
       leaf ipNetToPhysicalLastUpdated {
         type yang:timestamp;
       }
       leaf ipNetToPhysicalType {
         type enumeration { ... }
       }
       leaf ipNetToPhysicalState {
         type enumeration { ... }
       }
       leaf ipNetToPhysicalRowStatus {
         type snmpv2-tc:RowStatus;
       }
    }
 }

The following example shows an "ipNetToPhysicalTable" with 2 instances, using JSON encoding as defined in [I-D.ietf-netmod-yang-json]:

{ 
  "IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry" : [
        {
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "10.0.0.51",
          "ipNetToPhysicalPhysAddress" : "00:00:10:01:23:45",
          "ipNetToPhysicalLastUpdated" : "2333943",
          "ipNetToPhysicalType" : "static",
          "ipNetToPhysicalState" : "reachable",
          "ipNetToPhysicalRowStatus" : "active"
        },
        {
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "9.2.3.4",
          "ipNetToPhysicalPhysAddress" : "00:00:10:54:32:10",
          "ipNetToPhysicalLastUpdated" : "2329836",
          "ipNetToPhysicalType" : "dynamic",
          "ipNetToPhysicalState" : "unknown",
          "ipNetToPhysicalRowStatus" : "active"
        }
      ]
    }
  }
}

The YANG hash values for 'ipNetToPhysicalEntry' and its child nodes are calculated by constructing the schema node identifier for the objects, and then calculating the 30 bit murmur3 hash values (shown in parenthesis):

/IP-MIB:IP-MIB/ipNetToPhysicalTable (0x0aba15cc and kuhXM)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry
   (0xo6aaddbc and Gqt28)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalIfIndex (0x346b3071)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalNetAddressType (0x3650bb64)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalNetAddress (0x06fd4d91)
/IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalPhysAddress (0x26180bcb)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalLastUpdated (0x3d6bbe90)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalType (0x35ecbb3d)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalState (0x13038bb5)
/IP-MIB:IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry/
   ipNetToPhysicalRowStatus (0x09e1fa37)

The following example shows a request for the entire ipNetToPhysicalTable. The payload is a CBOR map composed of two (key , object} pairs, where key and object are CBOR maps, composed of 3 and 5 (identifier, value) pairs respectively.

REQ: GET example.com/mg/Gqt28

RES: 2.05 Content (Content-Format: application/cbor)
{
    0x06aaddbc: {
        {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "10.0.0.51"}:
        {
          0x26180bcb : "00:00:10:01:23:45",
          0x3d6bbe90 : "2333943",
          0x35ecbb3d : "static",
          0x13038bb5 : "reachable",
          0x09e1fa37 : "active"
        },
        {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "9.2.3.4"}:
        {
          0x26180bcb : "00:00:10:54:32:10",
          0x3d6bbe90 : "2329836",
          0x35ecbb3d : "dynamic",
          0x13038bb5 : "unknown",
          0x09e1fa37 : "active"
        }
     }
}

The corresponding CBOR code can be found in Appendix C.

4.1.3.4. The 'keys' Query Parameter

There is a query parameter that MUST be supported by servers called "keys". This parameter is used to specify the key values for an instance of an object identified by a YANG hash value. All key leaf values of the instance are passed in order. The first key leaf in the top-most list is the first key encoded in the 'keys' parameter.

The key leafs from top to bottom and left to right are encoded as a comma-delimited list. If a key leaf value is missing then all values for that key leaf are returned.

Example: In this example exactly 1 instance is requested from the ipNetToPhysicalEntry (from a previous example). The CBOR payload is constructed as before.

REQ: GET example.com/mg/Gqt28?keys=1,ipv4,10.0.0.51

RES: 2.05 Content (Content-Format: application/cbor)
{
   0x06aaddbc: {
      {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "9.2.3.4"}:
        {
          0x26180bcb : "00:00:10:54:32:10",
          0x3d6bbe90 : "2329836",
          0x35ecbb3d : "dynamic",
          0x13038bb5 : "unknown",
          0x09e1fa37 : "active"
        }
    }
}

An example illustrates the syntax of keys query parameter. In this example the following YANG module is used:

  module foo-mod {
    namespace foo-mod-ns;
    prefix foo;

    list A {
      key "key1 key2";
      leaf key1 { type string; }
      leaf key2 { type int32; }
      list B {
        key "key3";
        leaf key3 { type string; }
        leaf col1 { type uint32; }
      }
    }
  }

The path identifier for the leaf "col1" is the following string:

   /foo-mod:A/B/col1

The YANG hash for this identifier string has values: 0x189295aa and YkpWq).

The following string represents the CoMI target resource identifier for the instance of the "col1" leaf with key values "top", 17, "group":

   /mg/YkpWq?keys="top",17,"group1"

4.1.3.5. The 'select' Query Parameter

The select parameter is used along with the GET method to provide a sub-tree filter mechanism. A list of YANG hashes that should be filtered is provided along with a list of keys identifying the instances that should be returned. When the keys parameter is used together with the select, the key values are added in brackets without using the "keys=" text.

The following example shows an "ipNetToPhysicalTable" (from a previous example) with 4 instances, using JSON encoding following [I-D.ietf-netmod-yang-json]:

{ 
  "IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry" : [
        {
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "10.0.0.51",
          "ipNetToPhysicalPhysAddress" : "00:00:10:01:23:45",
          "ipNetToPhysicalLastUpdated" : "2333943",
          "ipNetToPhysicalType" : "static",
          "ipNetToPhysicalState" : "reachable",
          "ipNetToPhysicalRowStatus" : "active"
        },
        {
          "ipNetToPhysicalIfIndex" : 3,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "9.2.3.4",
          "ipNetToPhysicalPhysAddress" : "00:00:10:54:32:10",
          "ipNetToPhysicalLastUpdated" : "2329836",
          "ipNetToPhysicalType" : "dynamic",
          "ipNetToPhysicalState" : "unknown",
          "ipNetToPhysicalRowStatus" : "active"
        },
        {
          "ipNetToPhysicalIfIndex" : 2,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "10.24.2.53",
          "ipNetToPhysicalPhysAddress" : "00:00:10:28:19:CA",
          "ipNetToPhysicalLastUpdated" : "2124368",
          "ipNetToPhysicalType" : "static",
          "ipNetToPhysicalState" : "unknown",
          "ipNetToPhysicalRowStatus" : "active"
        },
	{
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "192.168.2.12",
          "ipNetToPhysicalPhysAddress" : "00:00:10:29:11:32",
          "ipNetToPhysicalLastUpdated" : "1925384",
          "ipNetToPhysicalType" : "dynamic",
          "ipNetToPhysicalState" : "reachable",
          "ipNetToPhysicalRowStatus" : "active"
        }	  
      ]
    }
  }
}

Data may be retrieved using the select query parameter in the following way, transported as a CBOR maps of maps:

REQ: GET example.com/mg/?select=Gqt28(1,ipv4)

RES: 2.05 Content (Content-Format: application/cbor)
{
    0x06aaddbc: {
      {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "10.0.0.51"}:
        {
          0x26180bcb : "00:00:10:01:23:45",
          0x3d6bbe90 : "2333943",
          0x35ecbb3d : "static",
          0x13038bb5 : "reachable",
          0x09e1fa37 : "active"
        },
        {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "192.168.2.12"}:
        {
          0x26180bcb : "00:00:10:29:11:32",
          0x3d6bbe90 : "1925384",
          0x35ecbb3d : "dynamic",
          0x13038bb5 : "reachable",
          0x09e1fa37 : "active"
        }
   }
}

In this example exactly 2 instances are returned as response from the ipNetToPhysicalTable because both those instances match the provided keys.

Supposing there were multiple YANG hashes with their own sets of keys that were to be filtered, the select query parameter can be used to retrieve results from these in one go as well. The following string represents the CoMI target resource identifier when multiple YANG hashes, with their own sets of keys are queried:

       /mg/?select=hash1(hash1-key1,hash1-key2,...),hash2(hash2-key1)...
     

4.2. Data Editing

CoMI allows data-store contents to be created, modified and deleted using CoAP methods.

Data-editing is an optional feature. The server will indicate its editing capability with the "/core.mg.srv-type resource type. If the value is 'rw' then the server supports editing operations. If the value is 'ro' then the server does not support editing operations.

4.2.1. Data Ordering

A CoMI server is not required to support entry insertion of lists and leaf-lists that are ordered by the user (i.e., YANG statement "ordered-by user"). The 'insert' and 'point' query parameters from RESTCONF are not used in CoMI.

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.

4.2.2. POST

Data resource instances are created with the POST method. The RESTCONF POST operation is supported in CoMI, however it is only allowed for creation of data resources. The same constraints apply as defined in section 3.4.1 of [I-D.ietf-netconf-restconf]. The operation is mapped to the POST method defined in section 5.8.2 of [RFC7252].

There are no query parameters for the POST method.

4.2.3. PUT

Data resource instances are created or replaced with the PUT method. The PUT operation is supported in CoMI. A request to set the values of instances of an object/leaf is sent with a confirmable CoAP PUT message. The Response is piggybacked to the CoAP ACK message corresponding with the Request. The same constraints apply as defined in section 3.5 of [I-D.ietf-netconf-restconf]. The operation is mapped to the PUT method defined in section 5.8.3 of [RFC7252].

There are no query parameters for the PUT method.

4.2.4. PATCH

Data resource instances are partially replaced with the PATCH method [I-D.vanderstok-core-patch]. The PATCH operation is supported in CoMI. A request to set the values of instances of a subset of the values of the resource is sent with a confirmable CoAP PATCH message. The Response is piggybacked to the CoAP ACK message corresponding with the Request. The same constraints apply as defined in section 3.5 of [I-D.ietf-netconf-restconf]. The operation is mapped to the PATCH method defined in [I-D.vanderstok-core-patch].

The processing of the PATCH command is specified by the CBOR payload. The CBOR patch payload describes the changes to be made to target YANG data nodes. It follows closely the rules described in [RFC7396]. If the CBOR patch payload contains objects that are not present in the target, these objects are added. If the target contains the specified object, the contents of the objects are replaced with the values of the payload. Null values indicate the removal of existing values. The CBOR patch extends [RFC7396] by specifying rules for list elements.

For example consider the following YANG specification:

module foo {
  namespace "http://example.com/book";
  prefix "bo";
  revision 2015-06-07;

  list B {
      key key1;
      key key2;
      leaf key1 { type string; }
      leaf key2 {type string; }
      leaf col1 { type int32; }
      leaf counter1 { type uint32; }
  }

  container book {
    leaf title { type string; }
    container author {
      leaf  givenName {type string; }
      leaf  familyName {type string; }
    }
    leaf-list tags {type string; }
    leaf content{type string;}
    leaf phoneNumber {type string;}
  }

Consider the following target data nodes described with the JSON encoding of [I-D.ietf-netmod-yang-json].

 "B": [
     {
      "key1" : "author1",
      "key2" : "book2",
      "col1" : 25,
      "counter1" : 4321
     },
    {
      "key1" : "author5",
      "key2" : "book6",
      "col1" : 2,
      "counter1" : 1234
     }
  ]


 "book": {
  "title" : "mytitle",
  "author": {
    "givenName" : "John",
    "familyName" : "Doe"
    }
  "tags" : [ "example", "sample"],
  "content" : "This will be unchanged"
}

The following changes are requested for the document (following the example from [RFC7396]: the title changes from "mytitle" to "favoured", the phoneNumber is added to the book container, the familyName is deleted, and "sample" is removed from the tags leaf-list. In addition author1, book1 item is removed, author5 counter1 is upgraded, and a new author is added in B list. The following CBOR Patch payload, represented in JSON is sent: (for the CBOR contents see Appendix C.

 { 
    "B":  {
     { "key1" : "author1",
      "key2" : "book2"}:
      { null : null},
     { "key1" : "author5"} :
       {"counter1" : 4444},
     { "key1" : "newauthor",
      "key2" : "newbook"}:
     { "col1" : 1,
       "counter1" : 1}
    },
  "book" : {
     "title" : "favoured",
     "author": {"familyName" : null},
     "tags" : [ "example"],
     "phoneNumber" : "+01-123-456-7890"
   }
}

In his example, the value "author5" specifies the entry uniquely. However, when several entries exist with the "author5" value for "key1", the outcome of the example Patch is undefined.

The processing of the Patch payload results in the following new target data nodes.

 "B": [
     {
      "key1" : "newauthor",
      "key2" : "newbook",
      "col1" : 1,
      "counter1" : 1
     },
    {
      "key1" : "author5",
      "key2" : "book6",
      "col1" : 2,
      "counter1" : 4444
     }
  ]

"book": {
  "title" : "favoured",
  "author": {
    "givenName" : "John"
    }
  "tags" : [ "example"],
  "content" : "This will be unchanged",
  "phoneNumber" : +01-123-456-7890"
}

There are no query parameters for the PATCH method.

4.2.5. DELETE

Data resource instances are deleted with the DELETE method. The RESTCONF DELETE operation is supported in CoMI. The same constraints apply as defined in section 3.7 of [I-D.ietf-netconf-restconf]. The operation is mapped to the DELETE method defined in section 5.8.4 of [RFC7252].

There are no optional query parameters for the DELETE method.

4.2.6. Editing Multiple Resources

Editing multiple data resources at once can allow a client to use fewer messages to make a configuration change. It also allows multiple edits to all be applied or none applied, which is not possible if the data resources are edited one at a time.

It is easy to add multiple entries at once. The "PATCH" method can be used to simply patch the parent node(s) of the data resources to be added. If multiple top-level data resources need to be added, then the data-store itself ('/mg') can be patched.

If other operations need to be performed, or multiple operations need to be performed at once, then the YANG Patch [I-D.ietf-netconf-yang-patch] media type can be used with the PATCH method. A YANG patch is an ordered list of edits on the target resource, which can be a specific data node instance, or the data-store itself. The resource type used by YANG Patch is 'application/yang.patch'. A status message is returned in the response, using resource type 'application/yang.patch.status'.

The following YANG tree diagram describes the YANG Patch structure, Each 'edit' list entry has its own operation, sub-resource target, and new value (if needed).

  +--rw yang-patch
      +--rw patch-id?   string
      +--rw comment?    string
      +--rw edit* [edit-id]
         +--rw edit-id      string
         +--rw operation    enumeration
         +--rw target       target-resource-offset
         +--rw point?       target-resource-offset
         +--rw where?       enumeration
         +--rw value        

The YANG Hash values for the YANG Patch request objects are calculated as follows:

0x2c3f93c7: /ietf-yang-patch:yang-patch
0x2fb8873e: /ietf-yang-patch:yang-patch/patch-id
0x011640f0: /ietf-yang-patch:yang-patch/comment
0x16804b72: /ietf-yang-patch:yang-patch/edit
0x2bd93228: /ietf-yang-patch:yang-patch/edit/edit-id
0x1959d8c9: /ietf-yang-patch:yang-patch/edit/operation
0x1346e0aa: /ietf-yang-patch:yang-patch/edit/target
0x0750e196: /ietf-yang-patch:yang-patch/edit/point
0x0b45277e: /ietf-yang-patch:yang-patch/edit/where
0x2822c407: /ietf-yang-patch:yang-patch/edit/value

Refer to [I-D.ietf-netconf-yang-patch] for more details on the YANG Patch request and response contents.

4.3. 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 /mg/stream resource identifies the default stream. The server MAY create additional streams. When a CoMI server generates an internal event, it is appended to the chosen stream, and the contents 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 [I-D.ietf-core-observe] function. The client subscribes to the notifications by sending a GET request with an "Observe" option, specifying the /mg/stream 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. After appending the instance, the contents of the instance is sent to all clients observing the modified stream.

Suppose the server generates the event specified with:

module example-port {
  ...
  prefix ep;
  ...
  notification example-port-fault {
    description
      "Event generated if a hardware fault on a
       line card port is detected";
    leaf port-name {
      type string;
      description "Port name";
    }
    leaf port-fault {
      type string;
      description "Error condition detected";
    }
  }
}

}

The YANG Hash values for this notification are assigned as follows:

0x3fe84d89: /example-port:example-port-fault
0x2921ba9e: /example-port:example-port-fault/port-name
0x2d452885: /example-port:example-port-fault/port-fault
0x11287619 (RKHUZ) : /stream


}

By executing a GET on the /mg/stream resource the client receives the following response:

REQ: GET example.com/mg/stream
    (observe option register)

RES: 2.05 Content (Content-Format: application/cbor)
{
      "example-port-fault":{
            "port-name" : "0/4/21",
            "port-fault" : "Open pin 2"
      }
}

Replacing the names by the hash values leads to:

REQ: GET example.com/mg/RKHUZ
    (observe option register)

RES: 2.05 Content (Content-Format: application/cbor)
{
    0x3fe84d89 : {
      0x2921ba9e : "0/4/21",
      0x2d452885 : "Open pin 2"
    }
}

In the example, the request returns a success response with the contents of the last generated event. 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 [I-D.ietf-core-observe].

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.

4.4. 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 [I-D.ietf-core-block] 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.

The block size is specified as exponents of the power 2. The SZX exponent value can have 7 values ranging from 0 to 6 with associated block sizes given by 2**(SZX+4); for example SZX=0 specifies block size 16, and SZX=3 specifies block size 128.

The block number of the block to transmit can be specified. There are two block options: Block1 option for the request payload transported with PUT, POST or PATCH, and the block2 option for the response payload with GET. Block1 and block2 can be combined. Examples showing the use of block option in conjunction with observer options are provided in [I-D.ietf-core-block].

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 are foreseen for data streaming purposes.

4.5. 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.mg" [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 "/mg" is used, where possible. The example below shows the discovery of the presence and location of management data.

  REQ: GET /.well-known/core?rt=core.mg

  RES: 2.05 Content </mg>; rt="core.mg"
  

Management objects MAY be discovered with the standard CoAP resource discovery. The implementation can add the hash values of the object identifiers to /.well-known/core with rt="core.mg.data". The available objects identified by the hash values can be discovered by sending a GET request to "/.well-known/core" including a resource type (RT) parameter with the value "core.mg.data". Upon success, the return payload will contain the registered hash 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.mg.data

  RES: 2.05 Content </mg/BaAiN>; rt="core.mg.data",
  </mg/CF_fA>; rt="core.mg.data"

Lists of hash 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 "/mg/mod.uri" is used to retrieve the location of the YANG module library.

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.

  Local in example.com server:

  REQ: GET example.com/mg/mod.uri

  RES: 2.05 Content (Content-Format: application/cbor)
  {
    "mod.uri" : "example.com/mg/modules"
  }


  Remote in example-remote-server:

  REQ: GET example.com/mg/mod.uri

  RES: 2.05 Content (Content-Format: application/cbor)
  {
    "moduri" : "example-remote-server.com/mg/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.

The hash identifier is obtained as specified in Section 5.1. When a collision occurred in the name space of the target server, a rehash is executed as explained in Section 5.2.

4.6. Error Return Codes

The RESTCONF return status codes defined in section 6 of the RESTCONF draft are used in CoMI error responses, except they are converted to CoAP error codes.

TODO: assign an error code for a rehash-error.

5. Mapping YANG to CoMI payload

A mapping for the encoding of YANG data in CBOR is necessary for the efficient transport of management data in the CoAP payload. Since object names may be rather long and may occur repeatedly, CoMI allows for association of a given object path identifier string value with an integer, called a "YANG hash".

5.1. YANG Hash Generation

The association between string value and string number is done through a hash algorithm. The 30 least significant bits of the "murmur3" 32-bit hash algorithm are used. This hash algorithm is described online at [murmur3]. Implementations are available online [murmur-imp]. When converting 4 input bytes to a 32-bit integer in the hash algorithm, the Little-Endian convention MUST be used.

The hash is generated for the string representing the object path identifier. A canonical representation of the path identifier is used.

• The module name is used to identify the namespace of the object node. The prefix cannot be used because it is allowed to change over time. The module name is never allowed to change.
• The module name MUST be present in the identifier for the first node in the object path identifier.
• If a child node in the object path identifier is from the same module namespace as its parent, then the module-name MUST NOT be used in the identifier.
• If a child node in the object path identifier is not from the same module namespace as its parent, then the module-name MUST be used in the identifier.
• Choice and case node names are not included in the path expression. Only 'container', 'list', 'leaf', 'leaf-list', and 'anyxml' nodes are listed in the path expression.
• The YANG Hash value is calculated for all data nodes in the module, even if the server only implements a subset of these objects. This includes all "data-def", "rpc", "notification", and external data nodes derived from "augment" statements.

The "murmur3_32" hash function is executed for the entire path string. The value '42' is used as the seed for the hash function. The YANG hash is subsequently calculated by taking the 30 least significant bits.

The resulting 30-bit number is used by the server, unless the value is already being used for a different object by the server. In this case, the re-hash procedure in the following section is executed.

Example: the following identifier is for the 'mtu' leaf in the ietf-interfaces module:

   /ietf-interfaces:interfaces/interface/mtu

Example: the following identifier is for the 'ipv4' container in the ietf-ip module, which augments the 'interface' list in the ietf-interfaces module:

   /ietf-interfaces:interfaces/interface/ietf-ip:ipv4

5.2. Re-Hash Error Procedure

In most cases, the hash function is expected to produce unique values for all the node names supported by a constrained device. Given a known set of YANG modules, both server and client can calculate the YANG hashes independently, and offline.

Even though collisions are expected to happen rather rarely, they need to be considered. Collisions can be detected before deployment, if the vendor knows which modules are supported by the server, and hence all YANG hashes can be calculated. Collisions are only an issue when they occur at the same server. The client needs to discover any re-hash mappings on a per server basis.

If the server needs to re-hash any object identifiers, then it MUST create a "rehash" entry for all its rehashed node names, as described in the following YANG module.

A re-hashed object identifier has the rehash bit set in the identifier, every time it is sent from the server to the client. This allows the client to identify nodes for which a "reverse rehash" entry needs to be retrieved. A client does not need to retrieve the rehash map before retrieving or altering data nodes.

If any node identifier provided by the client is not available because it has been rehashed, the server MUST return a rehash error, containing the 'rehash' entries for all the invalid nodes which were specified by the client.

It is possible that none of the node identifiers provided by the client in a GET method are invalid and rehashed, but rather one or more descendant nodes within the selected subtree(s) has been rehashed. In this case, a rehash error is not returned. Instead the requested subtree(s) are returned, and the rehash bit is set for any descendant node(s) that have been rehashed. The client will strip off the rehash bit and retrieve the 'revhash' entry for these nodes (if not already done).

5.3. Reverse Re-Hash Error Procedure

A hash collision occurs if two different path identifier strings have the same hash value. If the server has over 30,000 node names in its YANG modules, then the probability of a collision is 10% or higher, see Appendix E. If a hash collision occurs on the server, then the node name that is causing the conflict has to be altered, such that the new hash value does not conflict with any value already in use by the server.

5.4. ietf-yang-hash YANG Module

The "ietf-yang-hash" YANG module is used by the server to report any objects that have been mapped to produce a new hash value that does not conflict with any other YANG hash values used by the server.

YANG tree diagram for "ietf-yang-hash" module:

  +--ro yang-hash
      +--ro rehash* [hash]
         +--ro hash      uint32
         +--ro object*
            +--ro module     string
            +--ro newhash    uint32
            +--ro path?      string

<CODE BEGINS> file "ietf-yang-hash@2015-06-06.yang"

module ietf-yang-hash {
  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-hash";
  prefix "yh";

  organization
    "IETF CORE (Constrained RESTful Environments) Working Group";

  contact
    "WG Web:   <http://tools.ietf.org/wg/core/>
     WG List:  <mailto:core@ietf.org>

     WG Chair: Carsten Bormann
               <mailto:cabo@tzi.org>

     WG Chair: Andrew McGregor
               <mailto:andrewmcgr@google.com>

     Editor:   Peter van der Stok
               <mailto:consultancy@vanderstok.org>

     Editor:   Andy Bierman
               <mailto:andy@yumaworks.com>

     Editor:   Juergen Schoenwaelder
               <mailto:j.schoenwaelder@jacobs-university.de>

     Editor:   Anuj Sehgal
               <mailto:s.anuj@jacobs-university.de>";

  description
    "This module contains re-hash information for the CoMI protocol.

     Copyright (c) 2015 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Simplified BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (http://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX; see
     the RFC itself for full legal notices.";

  // RFC Ed.: replace XXXX with actual RFC number and remove this
  // note.

  // RFC Ed.: remove this note
  // Note: extracted from draft-vanderstok-core-comi-08.txt

  // RFC Ed.: update the date below with the date of RFC publication
  // and remove this note.
  revision 2015-09-24 {
    description
      "Initial revision.";
    reference
      "RFC XXXX: CoMI Protocol.";
  }

  container yang-hash {
    config false;
    description
      "Contains information on the YANG Hash values used by
       the server.";

    list rehash {
      key hash;
      description
        "Each entry describes an re-hash mapping in use by
         the server.";

      leaf hash {
        type uint32;
        description 
          "The hash value that has a collision.  This hash value
           cannot be used on the server.  The rehashed
           value for each affected object must be used instead.";
      }

      list object {
        min-elements 2;

        description
          "Each entry identifies one of the objects involved in the
           hash collision and contains the rehash information for
           that object.";

        leaf module {
          type string;
          mandatory true;
          description
            "The module name identifying the module namespace
             for this object.";
        }

        leaf newhash {
          type uint32;
          mandatory true;
          description
            "The new hash value for this object. The rehash bit is
             not set in this value.";
        }

        leaf path {
          type string;
          description
            "The object path identifier string used in the original
             YANG hash calculation. This object MUST be included for
             any objects in the rehash entry with the same 'module'
             value.";
        }
      }
    }
  }

}

<CODE ENDS>

5.5. YANG Re-Hash Examples

In this example there are two YANG modules, "foo" and "bar".

module foo {
  namespace "http://example.com/ns/foo";
  prefix "f";
  revision 2015-06-07;

  container A {
    list B {
      key name;
      leaf name { type string; }
      leaf counter1 { type uint32; }
    }
  }
}

module bar {
  namespace "http://example.com/ns/bar";
  prefix "b";
  import foo { prefix f; }
  revision 2015-06-07;

  augment /f:A/f:B {
    leaf counter2 { type uint32; }
  }
}

This set of 3 YANG modules containing a total of 7 objects produces the following object list. Note that actual hash values are not shown, since these modules do not actually cause the YANG Hash clashes described in the examples.

   Object    Path                    Hash

foo:

   container /foo:A                  h1
   list      /foo:A/B                h2
   leaf      /foo:A/B/name           h3
   leaf      /foo:A/B/counter1       h4

bar:

  leaf      /foo:A/B/bar1:counter2   h5

5.5.1. Multiple Modules

In this example, assume that the 'B' and 'counter2' objects produce the same hash value, so 'h2' and 'h5' both have the same value (e.g. '1234'):

The client might retrieve an entry from the list "/foo:A/B", which would cause this subtree to be returned. Instead, the server will return a message with the resource type "core.mg.yang-hash", representing the "yang-hash" data structure. Only the entry for the requested identifier is returned, even if multiple 'rehash' list entries exist.

REQ: GET example.com/mg/h2?keys="entry2"

RES: 4.00 "Bad Request" (Content-Format: application/cbor)
{
   "ietf-yang-hash:yang-hash" : {
     "rehash" : [
        {
          "hash" : 1234,
           "object" : [
             {
               "module" : "foo",
               "newhash" : 5678
             },
             {
               "module" : "bar",
               "newhash" : 8182
             }
           ]
        }
     ]
  }
}

5.5.2. Same Module

In this example, assume that the 'B', 'counter1', and 'counter2' objects produce the same hash value, so 'h2', 'h4', and 'h5' objects all have the same value (e.g. '1234'):

The client might retrieve an entry from the list "/foo:A/B", which would cause this subtree to be returned. Instead, the server will return a message with the resource type "core.mg.yang-hash", representing the "yang-hash" data structure. Only the entry for the requested identifier is returned, even if multiple 'rehash' list entries exist.

REQ: GET example.com/mg/h2?keys="entry2"

RES: 4.00 "Bad Request" (Content-Format: application/cbor)
{
   "ietf-yang-hash:yang-hash" : {
     "rehash" : [
        {
          "hash" : 1234,
           "object" : [
             {
               "module" : "foo",
               "newhash" : 5678,
               "path" : "/foo:A/B"
               
             },
             {
               "module" : "foo",
               "newhash" : 2134,
               "path" : "/foo:A/B/counter1"
             },
             {
               "module" : "bar",
               "newhash" : 8182,
               "path" : "/foo:A/B/bar:counter2"
             }
           ]
        }
     ]
  }
}

5.6. Retrieval of Rehashed Data

In this example, assume that the 'B', 'counter1', and 'counter2' objects produce the same hash value, so 'h2', 'h4', and 'h5' objects all have the same value (e.g. '1234'):

The client might retrieve the top-level container "/foo:A", which would cause this subtree to be returned. Since the identifier (h1) has not been re-hashed, the server will return the requested data. The new hashes for 'h2', 'h4',and 'h5' will be returned, except the rehash bit will be set for these identifiers.

The notation "R+" indicates that the rehash bit is set.

REQ: GET example.com/mg/h1

RES: 2.05 Content (Content-Format: application/cbor)
{
     h1 : {
        R+5678 : {
           { h3 : "entry1"}:
             {R+2134: 615,
              R+8182: 7},
           { h3 : "entry2"}:
             {R+2134: 491,
              R+8182: 26}
         }
      }
}

The client will notice that the rehash bit is set for 3 nodes. The client will need to retrieve the full "yang-hash" container at this point, if that has not already been done. The rehashed identifiers will be in "rehash" list, contained in the "newhash" leaf for the "object" list.

5.7. YANG Hash in URL

When a URL contains a YANG hash, it is encoded using base64url "URL and Filename safe" encoding as specified in [RFC4648].

The hash H is represented as a 30-bit integer, divided into five 6-bit integers as follows:

B1 = (H & 0x3f000000) >> 24
B2 = (H & 0xfc0000) >> 18
B3 = (H & 0x03f000) >> 12
B4 = (H & 0x000fc0) >> 6
B5 =  H & 0x00003f

Subsequently, each 6-bit integer Bx is translated into a character Cx using Table 2 from [RFC4648], and a string is formed by concatenating the characters in the order C1, C2, C3, C4, C5.

For example, the YANG hash 0x29abdcca is encoded as "pq9zK".

6. Mapping YANG to CBOR

6.1. High level encoding

When encoding YANG variables in CBOR, the CBOR encodings entry is a map, composed of (key, value) pairs. The key is the YANG hash of entry variable, whereas the value contains its value.

For encoding of the variable values, a CBOR datatype is used. Section 6.2 provides the mapping between YANG datatypes and CBOR datatypes.

6.2. 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.

errorMsg     : ErrorMsg;

*ErrorMsg {
  errorCode  : uint;
  ?errorText : tstr;
}

   name : datatype;

--------------------------JSON example -----------------------
{
  0x021ca491 : {
      0x047c468b : "2014-10-26T12:16:51Z",
      0x1fb5f4f8 : "2014-10-21T03:00:00Z"
   }
}
-----------------------------CBOR code -----------------------
a1                                      # map(1)
   1a 021ca491                          # unsigned(35431569)
   a2                                   # map(2)
      1a 047c468b                       # unsigned(75253387)
      74                                # text(20)
         323031342d31302d32365431323a31363a35315a # "2014-10-26T12:16:51Z"
      1a 1fb5f4f8                       # unsigned(532018424)
      74                                # text(20)
         323031342d31302d32315430333a30303a30305a # "2014-10-21T03:00:00Z"

--------------------------JSON example -----------------------
{
    0x06aaddbc: {
        {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "10.0.0.51"}:
        {
          0x26180bcb : "00:00:10:01:23:45",
          0x3d6bbe90 : "2333943",
          0x35ecbb3d : "static",
          0x13038bb5 : "reachable",
          0x09e1fa37 : "active"
        },
        {
          0x346b3071 : 1,
          0x3650bb64 : "ipv4",
          0x06fd4d91 : "9.2.3.4"}:
        {
          0x26180bcb : "00:00:10:54:32:10",
          0x3d6bbe90 : "2329836",
          0x35ecbb3d : "dynamic",
          0x13038bb5 : "unknown",
          0x09e1fa37 : "active"
        }
     }
}
-----------------------------CBOR code -----------------------
a1                                      # map(1)
   1a 06aaddbc                          # unsigned(111861180)
   a2                                   # map(2)
      a3                                # map(3)
         1a 346b3071                    # unsigned(879439985)
         01                             # unsigned(1)
         1a 3650bb64                    # unsigned(911260516)
         64                             # text(4)
            69707634                    # "ipv4"
         1a 06fd4d91                    # unsigned(117263761)
         69                             # text(9)
            31302e302e302e3531          # "10.0.0.51"
      a5                                # map(5)
         1a 26180bcb                    # unsigned(639110091)
         71                             # text(17)
            30303a30303a31303a30313a32333a3435 # "00:00:10:01:23:45"
         1a 3d6bbe90                    # unsigned(1030471312)
         67                             # text(7)
            32333333393433              # "2333943"
         1a 35ecbb3d                    # unsigned(904706877)
         66                             # text(6)
            737461746963                # "static"
         1a 13038bb5                    # unsigned(318999477)
         69                             # text(9)
            726561636861626c65          # "reachable"
         1a 09e1fa37                    # unsigned(165804599)
         66                             # text(6)
            616374697665                # "active"
      a3                                # map(3)
         1a 346b3071                    # unsigned(879439985)
         01                             # unsigned(1)
         1a 3650bb64                    # unsigned(911260516)
         64                             # text(4)
            69707634                    # "ipv4"
         1a 06fd4d91                    # unsigned(117263761)
         67                             # text(7)
            392e322e332e34              # "9.2.3.4"
      a5                                # map(5)
         1a 26180bcb                    # unsigned(639110091)
         71                             # text(17)
            30303a30303a31303a35343a33323a3130 # "00:00:10:54:32:10"
         1a 3d6bbe90                    # unsigned(1030471312)
         67                             # text(7)
            32333239383336              # "2329836"
         1a 35ecbb3d                    # unsigned(904706877)
         67                             # text(7)
            64796e616d6963              # "dynamic"
         1a 13038bb5                    # unsigned(318999477)
         67                             # text(7)
            756e6b6e6f776e              # "unknown"
         1a 09e1fa37                    # unsigned(165804599)
         66                             # text(6)
            616374697665                # "active"

--------------------------JSON example -----------------------
 { 
     {
      0x200a297b : "author1",
      0x35e1c1da : "book2"}:
      { null : null},
     {
      0x200a297b : "author5"} :
       {0x26c3ca7e : 4444},
     {
      0x200a297b : "newauthor",
      0x35e1c1da : "newbook"}:
     { 0x0119ddec : 1,
       0x26c3ca7e : 1},

  0x5cc7979 : "favoured",
  0x2935c912 : null,
  0x3f83c748 : [ "example"],
  0x324d9526 : "+01-123-456-7890"
}
-----------------------------CBOR code -----------------------
a7                                     # map(7)
   a2                                  # map(2)
      1a 200a297b                      # unsigned(537536891)
      67                               # text(7)
         617574686f7231                # "author1"
      1a 35e1c1da                      # unsigned(903987674)
      65                               # text(5)
         626f6f6b32                    # "book2"
   a1                                  # map(1)
      00                               # unsigned(0)
      f6                               # primitive(22)
   a1                                  # map(1)
      1a 200a297b                      # unsigned(537536891)
      67                               # text(7)
         617574686f7235                # "author5"
   a1                                  # map(1)
      1a 26c3ca7e                      # unsigned(650365566)
      19 115c                          # unsigned(4444)
   a2                                  # map(2)
      1a 200a297b                      # unsigned(537536891)
      69                               # text(9)
         6e6577617574686f72            # "newauthor"
      1a 35e1c1da                      # unsigned(903987674)
      67                               # text(7)
         6e6577626f6f6b                # "newbook"
   a2                                  # map(2)
      1a 0119ddec                      # unsigned(18472428)
      01                               # unsigned(1)
      1a 26c3ca7e                      # unsigned(650365566)
      01                               # unsigned(1)
   1a 05cc7979                         # unsigned(97286521)
   68                                  # text(8)
      6661766f75726564                 # "favoured"
   1a 2935c912                         # unsigned(691390738)
   f6                                  # primitive(22)
   1a 3f83c748                         # unsigned(1065600840)
   81                                  # array(1)
      67                               # text(7)
         6578616d706c65                # "example"
   1a 324d9526                         # unsigned(843945254)
   70                                  # text(16)
      2b30312d3132332d3435362d37383930 # "+01-123-456-7890"