+------------+
               |   Domain   | <-- Name
               +------------+
                    |     |
                    |   +------------+
                    |   |   Group    | <-- Name, IP
                    |   +------------+
                    |     |
               +------------+
               |  Endpoint  |  <-- Name, Scheme, IP, Port
               +------------+
                     |
                     |
               +------------+
               |  Resource  |  <-- Target, Parameters
               +------------+

Figure 2: The resource directory information hierarchy.

3.1. Use Case: Cellular M2M

Over the last few years, mobile operators around the world have focused on development of M2M solutions in order to expand the business to the new type of users: machines. The machines are connected directly to a mobile network using an appropriate embedded air interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short and wide range wireless interfaces. From the system design point of view, the ambition is to design horizontal solutions that can enable utilization of machines in different applications depending on their current availability and capabilities as well as application requirements, thus avoiding silo like solutions. One of the crucial enablers of such design is the ability to discover resources (machines — endpoints) capable of providing required information at a given time or acting on instructions from the end users.

In a typical scenario, during a boot-up procedure (and periodically afterwards), the machines (endpoints) register with a Resource Directory (for example EPs installed on vehicles enabling tracking of their position for fleet management purposes and monitoring environment parameters) hosted by the mobile operator or somewhere else in the network, periodically a description of its own capabilities. Due to the usual network configuration of mobile networks, the EPs attached to the mobile network may not always be efficiently reachable. Therefore, a remote server is usually used to provide proxy access to the EPs. The address of each (proxy) endpoint on this server is included in the resource description stored in the RD. The users, for example mobile applications for environment monitoring, contact the RD, look-up the endpoints capable of providing information about the environment using appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server) and then initiate interaction to obtain information that is finally processed, displayed on the screen and usually stored in a database. Similarly, fleet management systems provide the appropriate link parameters to the RD to look-up for EPs deployed on the vehicles the application is responsible for.

3.2. Use Case: Home and Building Automation

Home and commercial building automation systems can benefit from the use of M2M web services. The discovery requirements of these applications are demanding. Home automation usually relies on run-time discovery to commission the system, whereas in building automation a combination of professional commissioning and run-time discovery is used. Both home and building automation involve peer-to-peer interactions between endpoints, and involve battery-powered sleeping devices.

The exporting of resource information to other discovery systems is also important in these automation applications. In home automation there is a need to interact with other consumer electronics, which may already support DNS-SD, and in building automation larger resource directories or DNS-SD covering multiple buildings.

3.3. Use Case: Link Catalogues

Resources may be shared through data brokers that have no knowledge beforehand of who is going to consume the data. Resource Directory can be used to hold links about resources and services hosted anywhere to make them discoverable by a general class of applications.

For example, environmental and weather sensors that generate data for public consumption may provide the data to an intermediary server, or broker. Sensor data are published to the intermediary upon changes or at regular intervals. Descriptions of the sensors that resolve to links to sensor data may be published to a Resource Directory. Applications wishing to consume the data can use the Resource Directory lookup function set to discover and resolve links to the desired resources and endpoints. The Resource Directory service need not be coupled with the data intermediary service. Mapping of Resource Directories to data intermediaries may be many-to-many.

Metadata in link-format, link-format+cbor, or link-format+json representations are supplied by Resource Directories, which may be internally stored as triples, or relation/attribute pairs providing metadata about resource links. External catalogs that are represented in other formats may be converted to link-format, link-format+json, or link-format+cbor for storage and access by Resource Directories. Since it is common practice for these to be URN encoded, simple and lossless structural transforms will generally be sufficient to store external metadata in Resource Directories.

The additional features of Resource Directory allow domains to be defined to enable access to a particular set of resources from particular applications. This provides isolation and protection of sensitive data when needed. Resource groups may defined to allow batched reads from multiple resources.

4. Simple Directory Discovery

Not all endpoints hosting resources are expected to know how to implement the Resource Directory Function Set (see Section 5) and thus explicitly register with a Resource Directory (or other such directory server). Instead, simple endpoints can implement the generic Simple Directory Discovery approach described in this section. An RD implementing this specification MUST implement Simple Directory Discovery. However, there may be security reasons why this form of directory discovery would be disabled.

This approach requires that the endpoint makes available the hosted resources that it wants to be discovered, as links on its /.well-known/core interface as specified in [RFC6690].

The endpoint then finds one or more IP addresses of the directory server it wants to know about its resources as described in Section 4.1.

An endpoint that wants to make itself discoverable occasionally sends a POST request to the /.well-known/core URI of any candidate directory server that it finds. The body of the POST request is either

• empty, in which case the directory server is encouraged by this POST request to perform GET requests at the requesting server’s default discovery URI.

or

• a non-empty link-format document, which indicates the specific services that the requesting server wants to make known to the directory server.

The directory server integrates the information it received this way into its resource directory. It MAY make the information available to further directories, if it can ensure that a loop does not form. The protocol used between directories to ensure loop-free operation is outside the scope of this document.

The following example shows an endpoint using simple resource discovery, by simply sending a POST with its links in the body to a directory.

     EP                                               RD
     |                                                 |
     | -- POST /.well-known/core "</sen/temp>..." ---> |
     |                                                 |
     |                                                 |
     | <---- 2.01 Created   -------------------------  |
     |                                                 |

4.1. Finding a Directory Server

Endpoints that want to contact a directory server can obtain candidate IP addresses for such servers in a number of ways.

In a 6LoWPAN, good candidates can be taken from:

• specific static configuration (e.g., anycast addresses), if any,
• the ABRO option of 6LoWPAN-ND [RFC6775],
• other ND options that happen to point to servers (such as RDNSS),
• DHCPv6 options that might be defined later.

In networks with more inexpensive use of multicast, the candidate IP address may be a well-known multicast address, i.e. directory servers are found by simply sending GET requests to that well-known multicast address (see Section 5.1).

As some of these sources are just (more or less educated) guesses, endpoints MUST make use of any error messages to very strictly rate-limit requests to candidate IP addresses that don’t work out. For example, an ICMP Destination Unreachable message (and, in particular, the port unreachable code for this message) may indicate the lack of a CoAP server on the candidate host, or a CoAP error response code such as 4.05 “Method Not Allowed” may indicate unwillingness of a CoAP server to act as a directory server.

4.2. Third-party registration

For some applications, even Simple Directory Discovery may be too taxing for certain very constrained devices, in particular if the security requirements become too onerous.

In a controlled environment (e.g. building control), the Resource Directory can be filled by a third device, called an installation tool. The installation tool can fill the Resource Directory from a database or other means. For that purpose the scheme, IP address and port of the registered device is indicated in the Context parameter of the registration as well.

5. Resource Directory Function Set

This section defines the REST interfaces between an RD and endpoints, which is called the Resource Directory Function Set. Although the examples throughout this section assume the use of CoAP [RFC7252], these REST interfaces can also be realized using HTTP [RFC7230]. In all definitions in this section, both CoAP response codes (with dot notation) and HTTP response codes (without dot notation) are shown. An RD implementing this specification MUST support the discovery, registration, update, lookup, and removal interfaces defined in this section.

Resource directory entries are designed to be easily exported to other discovery mechanisms such as DNS-SD. For that reason, parameters that would meaningfully be mapped to DNS SHOULD be limited to a maximum length of 63 bytes.

5.1. Discovery

Before an endpoint can make use of an RD, it must first know the RD’s IP address, port and the path of its RD Function Set. There can be several mechanisms for discovering the RD including assuming a default location (e.g. on an Edge Router in a LoWPAN), by assigning an anycast address to the RD, using DHCP, or by discovering the RD using the CoRE Link Format (see also Section 4.1). This section defines discovery of the RD using the well-known interface of the CoRE Link Format [RFC6690] as the required mechanism. It is however expected that RDs will also be discoverable via other methods depending on the deployment.

Discovery is performed by sending either a multicast or unicast GET request to /.well-known/core and including a Resource Type (rt) parameter [RFC6690] with the value “core.rd” in the query string. Likewise, a Resource Type parameter value of “core.rd-lookup” is used to discover the RD Lookup Function Set. Upon success, the response will contain a payload with a link format entry for each RD discovered, with the URL indicating the root resource of the RD. When performing multicast discovery, the multicast IP address used will depend on the scope required and the multicast capabilities of the network.

HTTP does not support multicast and consequently discovery has no HTTP interface.

An RD implementation of this specification MUST support query filtering for the rt parameter as defined in [RFC6690].

The discovery request interface is specified as follows:

Interaction:

EP -> RD

Method:

GET

URI Template:

/.well-known/core{?rt}

URI Template Variables:

rt :=

Resource Type (optional). MAY contain the value “core.rd”, “core.rd-lookup”, “core.rd-group” or “core.rd*”

Content-Type:

application/link-format (if any)

Content-Type:

application/link-format+json (if any)

Content-Type:

application/link-format+cbor (if any)

The following response codes are defined for this interface:

Success:

2.05 “Content” with an application/link-format, application/link-format+json, or application/link-format+cbor payload containing one or more matching entries for the RD resource.

Failure:

4.04 “Not Found” is returned in case no matching entry is found for a unicast request.

Failure:

4.00 “Bad Request” is returned in case of a malformed request for a unicast request.

Failure:

No error response to a multicast request.

HTTP support :

NO

The following example shows an endpoint discovering an RD using this interface, thus learning that the base RD resource is, in this example, at /rd. Note that it is up to the RD to choose its base RD resource, although diagnostics and debugging is facilitated by using the base paths specified here where possible.

     EP                                               RD
     |                                                 |
     | ----- GET /.well-known/core?rt=core.rd* ------> |
     |                                                 |
     |                                                 |
     | <---- 2.05 Content "</rd>;rt="core.rd"  ------- |
     |                                                 |

Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*

Res: 2.05 Content
</rd>;rt="core.rd",
</rd-lookup>;rt="core.rd-lookup",
</rd-group>;rt="core.rd-group"

5.2. Registration

After discovering the location of an RD Function Set, an endpoint MAY register its resources using the registration interface. This interface accepts a POST from an endpoint containing the list of resources to be added to the directory as the message payload in the CoRE Link Format [RFC6690], JSON CoRE Link Format (application/link-format+json), or CBOR CoRE Link Format (application/link-format+cbor) [I-D.ietf-core-links-json], along with query string parameters indicating the name of the endpoint, its domain and the lifetime of the registration. All parameters except the endpoint name are optional. It is expected that other specifications will define further parameters (see Section 11.3). The RD then creates a new resource or updates an existing resource in the RD and returns its location. An endpoint MUST use that location when refreshing registrations using this interface. Endpoint resources in the RD are kept active for the period indicated by the lifetime parameter. The endpoint is responsible for refreshing the entry within this period using either the registration or update interface. The registration interface MUST be implemented to be idempotent, so that registering twice with the same endpoint parameter does not create multiple RD entries.

The registration request interface is specified as follows:

Interaction:

EP -> RD

Method:

POST

URI Template:

/{+rd}{?ep,d,et,lt,con}

URI Template Variables:

rd :=

RD Function Set path (mandatory). This is the path of the RD Function Set, as obtained from discovery. An RD SHOULD use the value “rd” for this variable whenever possible.

ep :=

Endpoint name (mandatory). The endpoint name is an identifier that MUST be unique within a domain. The maximum length of this parameter is 63 bytes.

d :=

Domain (optional). The domain to which this endpoint belongs. This parameter SHOULD be less than 63 bytes. Optional. When this parameter is elided, the RD MAY associate the endpoint with a configured default domain. The domain value is needed to export the endpoint to DNS-SD (see Section 9).

et :=

Endpoint Type (optional). The semantic type of the endpoint. This parameter SHOULD be less than 63 bytes. Optional.

lt :=

Lifetime (optional). Lifetime of the registration in seconds. Range of 60-4294967295. If no lifetime is included, a default value of 86400 (24 hours) SHOULD be assumed.

con :=

Context (optional). This parameter sets the scheme, address and port at which this server is available in the form scheme://host:port. Optional. In the absence of this parameter the scheme of the protocol, source IP address and source port of the register request are assumed. This parameter is mandatory when the directory is filled by a third party such as an installation tool.

Content-Type:

application/link-format

Content-Type:

application/link-format+json

Content-Type:

application/link-format+cbor

The following response codes are defined for this interface:

Success:

2.01 “Created” or 201 “Created”. The Location header MUST be included with the new resource entry for the endpoint. This Location MUST be a stable identifier generated by the RD as it is used for all subsequent operations on this registration. The resource returned in the Location is only for the purpose of the Update (POST) and Removal (DELETE), and MUST NOT implement GET or PUT methods.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support:

YES

The following example shows an endpoint with the name “node1” registering two resources to an RD using this interface. The resulting location /rd/4521 is just an example of an RD generated location.

    EP                                                RD
     |                                                 |
     | --- POST /rd?ep=node1 "</sensors..." ------->   |
     |                                                 |
     |                                                 |
     | <-- 2.01 Created Location: /rd/4521 ----------  |
     |                                                 |

Req: POST coap://rd.example.com/rd?ep=node1
Content-Format: 40
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"

Res: 2.01 Created
Location: /rd/4521

Req: POST /rd?ep=node1 HTTP/1.1
Host : example.com
Content-Type: application/link-format
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"

Res: 201 Created
Location: /rd/4521

5.3. Update

The update interface is used by an endpoint to refresh or update its registration with an RD. To use the interface, the endpoint sends a POST request to the resource returned in the Location option in the response to the first registration. An update MAY update the lifetime or context parameters if they have changed since the last registration or update. Parameters that have not changed SHOULD NOT be included in an update. Upon receiving an update request, the RD resets the timeout for that endpoint and updates the scheme, IP address and port of the endpoint (using the source address of the update, or the context parameter if present).

An update MAY optionally add or replace links for the endpoint by including those links in the payload of the update as a CoRE Link Format document. Including links in an update message greatly increases the load on an RD and SHOULD be done infrequently. A link is replaced only if both the target URI and relation type match (see Section 10.1)

The update request interface is specified as follows:

Interaction:

EP -> RD

Method:

POST

URI Template:

/{+location}{?lt,con}

URI Template Variables:

location :=

This is the Location path returned by the RD as a result of a successful earlier registration.

lt :=

Lifetime (optional). Lifetime of the registration in seconds. Range of 60-4294967295. If no lifetime is included, a default value of 86400 (24 hours) SHOULD be assumed.

con :=

Context (optional). This parameter sets the scheme, address and port at which this server is available in the form scheme://host:port. Optional. In the absence of this parameter the scheme of the protocol, source IP address and source port used to register are assumed. This parameter is compulsory when the directory is filled by a third party such as an installation tool.

Content-Type:

application/link-format (optional)

Content-Type:

application/link-format+json (optional)

Content-Type:

application/link-format+cbor (optional)

The following response codes are defined for this interface:

Success:

2.04 “Changed” or 204 “No Content” in the update was successfully processed.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support:

YES

The following example shows an endpoint updating its registration at an RD using this interface.

     EP                                                RD
     |                                                 |
     | --- POST /rd/4521  -------------------------->  |
     |                                                 |
     |                                                 |
     | <-- 2.04 Changed  ----------------------------  |
     |                                                 |

Req: POST /rd/4521

Res: 2.04 Changed

5.4. Removal

Although RD entries have soft state and will eventually timeout after their lifetime, an endpoint SHOULD explicitly remove its entry from the RD if it knows it will no longer be available (for example on shut-down). This is accomplished using a removal interface on the RD by performing a DELETE on the endpoint resource.

The removal request interface is specified as follows:

Interaction:

EP -> RD

Method:

DELETE

URI Template:

/{+location}

URI Template Variables:

location :=

This is the Location path returned by the RD as a result of a successful earlier registration.

The following responses codes are defined for this interface:

Success:

2.02 “Deleted” or 204 “No Content” upon successful deletion

Failure:

4.00 “Bad Request” or 400 “Bad request”. Malformed request.

Failure:

4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support: YES

The following examples shows successful removal of the endpoint from the RD.

    EP                                                RD
     |                                                 |
     | --- DELETE /rd/4521  ------------------------>  |
     |                                                 |
     |                                                 |
     | <-- 2.02 Deleted  ----------------------------  |
     |                                                 |

Req: DELETE /rd/4521

Res: 2.02 Deleted

5.5. Read Endpoint Links

Some endpoints may wish to manage their links as a collection, and may need to read the current set of links in order to determine link maintenance operations.

One or more links MAY be selected by using query filtering as specified in [RFC6690] Section 4.1

The read request interface is specified as follows:

Interaction:

EP -> RD

Method:

GET

URI Template:

/{+location}{?href,rel,rt,if,ct}

URI Template Variables:

location :=

This is the Location path returned by the RD as a result of a successful earlier registration.

href,rel,rt,if,ct := link relations and attributes specified in the query in order to select particular links based on their relations and attributes. “href” denotes the URI target of the link. See [RFC6690] Sec. 4.1

The following responses codes are defined for this interface:

Success:

2.05 “Content” or 200 “OK” upon success with an application/link-format, application/link-format+cbor, or application/link-format+json payload.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support: YES

The following examples show successful read of the endpoint links from the RD.

    EP                                                RD
     |                                                 |
     | --- GET /rd/4521  ------------------------>  |
     |                                                 |
     |                                                 |
     | <-- 2.05 Content </sensors... ----------------  |
     |                                                 |

Req: GET /rd/4521

Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"

5.6. Update Endpoint Links

A PATCH update adds, removes or changes links for the endpoint by including link update information in the payload of the update as a merge-patch+json format [RFC7396] document.

One or more links are selected for update by using query filtering as specified in [RFC6690] Section 4.1

The query filter selects the links to be modified or deleted, by matching the query parameter values to the values of the link attributes.

When the query parameters are not present in the request, the payload specifies links to be added to the target document. When the query parameters are present, the attribute names and values in the query parameters select one or more links on which to apply the PATCH operation.

If an attribute name specified in the PATCH document exists in any the set of selected links, all occurrences of the attribute value in the target document MUST be updated using the value from the PATCH payload. If the attribute name is not present in any selected links, the attribute MUST be added to the links.

The update request interface is specified as follows:

Interaction:

EP -> RD

Method:

PATCH

URI Template:

/{+location}{?href,rel,rt,if,ct}

URI Template Variables:

location :=

This is the Location path returned by the RD as a result of a successful earlier registration.

href,rel,rt,if,ct := link relations and attributes specified in the query in order to select particular links based on their relations and attributes. “href” denotes the URI target of the link. See [RFC6690] Sec. 4.1

Content-Format:

application/merge-patch+json (mandatory)

The following response codes are defined for this interface:

Success:

2.04 “Changed” 0r 204 “No Content” in the update was successfully processed.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

4.04 “Not Found” or 404 “Not Found”. Registration resource does not exist (e.g. may have expired).

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support: YES

The following examples show an endpoint adding </sensors/humid>, modifying </sensors/temp>, and removing </sensors/light> links in RD using the Update Endpoint Links function.

The following example shows an EP adding the link </sensors/humid>;ct=41;rt=”humidity-s”;if=”sensor” to the collection of links at the location /rd/4521.

     EP                                                RD
     |                                                 |
     | --- PATCH /rd/4521--------------------------->  |
     |                                                 |
     |                                                 |
     | <-- 2.04 Changed  ----------------------------  |
     |                                                 |

Req: PATCH /rd/4521

Payload: 
[{"href":"/sensors/humid","ct": 41, "rt": "humidity-s", "if": "sensor"}]

Content-Format:
application/merge-patch+json

Res: 2.04 Changed

The following example shows an EP modifying all links at the location /rd/4521 which are identified by href=”/sensors/temp”, from the initial link-value of </sensors/temp>;rt=”temperature” to the new link-value </sensors/temp>;rt=”temperature-c”;if=”sensor” by changing the value of the link attribute “rt” and adding the link attribute if=”sensor” using the PATCH operation with the supplied merge-patch+json document payload.

     EP                                                RD
     |                                                 |
     | --- PATCH /rd/4521?href="/sensors/temp"  ---->  |
     |                                                 |
     |                                                 |
     | <-- 2.04 Changed  ----------------------------  |
     |                                                 |

Req: PATCH /rd/4521?href="/sensors/temp"

Payload: 
{"rt": "temperature-c", "if": "sensor"},

Content-Format:
application/merge-patch+json

Res: 2.04 Changed

This example shows an EP removing all links at the location /rd/4521 which are identified by href=”/sensors/light”.

     EP                                                RD
     |                                                 |
     | --- PATCH /rd/4521?href="/sensors/light"  ---->  |
     |                                                 |
     |                                                 |
     | <-- 2.04 Changed  ----------------------------  |
     |                                                 |

Req: PATCH /rd/4521?href="/sensors/light"

Payload: 
{null}

Content-Format:
application/merge-patch+json

Res: 2.04 Changed

6. Group Function Set

This section defines a function set for the creation of groups of endpoints for the purpose of managing and looking up endpoints for group operations. The group function set is similar to the resource directory function set, in that a group may be created or removed. However unlike an endpoint entry, a group entry consists of a list of endpoints and does not have a lifetime associated with it. In order to make use of multicast requests with CoAP, a group MAY have a multicast address associated with it.

6.1. Register a Group

In order to create a group, a management entity used to configure groups, makes a request to the RD indicating the name of the group to create (or update), optionally the domain the group belongs to, and optionally the multicast address of the group. The registration message includes the list of endpoints that belong to that group. If an endpoint has already registered with the RD, the RD attempts to use the context of the endpoint from its RD endpoint entry. If the client registering the group knows the endpoint has already registered, then it MAY send a blank target URI for that endpoint link when registering the group. Configuration of the endpoints themselves is out of scope of this specification. Such an interface for managing the group membership of an endpoint has been defined in [RFC7390].

The registration request interface is specified as follows:

Interaction:

Manager -> RD

Method:

POST

URI Template:

/{+rd-group}{?gp,d,con}

URI Template Variables:

rd-group :=

RD Group Function Set path (mandatory). This is the path of the RD Group Function Set. An RD SHOULD use the value “rd-group” for this variable whenever possible.

gp :=

Group Name (mandatory). The name of the group to be created or replaced, unique within that domain. The maximum length of this parameter is 63 bytes.

d :=

Domain (optional). The domain to which this group belongs. The maximum length of this parameter is 63 bytes. Optional. When this parameter is elided, the RD MAY associate the endpoint with a configured default domain. The domain value is needed to export the endpoint to DNS-SD (see Section 9)

con :=

Context (optional). This parameter is used to set the IP multicast address at which this server is available in the form scheme://multicast-address:port. Optional. In the absence of this parameter no multicast address is configured. This parameter is compulsory when the directory is filled by an installation tool.

Content-Type:

application/link-format

Content-Type:

application/link-format+json

Content-Type:

application/link-format+cbor

The following response codes are defined for this interface:

Success:

2.01 “Created” or 201 “Created”. The Location header MUST be included with the new group entry. This Location MUST be a stable identifier generated by the RD as it is used for delete operations on this registration.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support:

YES

The following example shows an EP registering a group with the name “lights” which has two endpoints to an RD using this interface. The resulting location /rd-group/12 is just an example of an RD generated group location.

    EP                                                RD
     |                                                 |
     | - POST /rd-group?gp=lights "<>;ep=node1..." --> |
     |                                                 |
     |                                                 |
     | <---- 2.01 Created Location: /rd-group/12 ----  |
     |                                                 |

Req: POST coap://rd.example.com/rd-group?gp=lights
Payload:
<>;ep="node1",
<>;ep="node2"

Res: 2.01 Created
Location: /rd-group/12

Req: POST /rd-group?gp=lights HTTP/1.1
Host: example.com
Accept: application/link-format
Payload:
<>;ep="node1",
<>;ep="node2"

Res: 201 Created
Location: /rd-group/12

6.2. Group Removal

A group can be removed simply by sending a removal message to the location returned when registering the group. Removing a group MUST NOT remove the endpoints of the group from the RD.

The removal request interface is specified as follows:

Interaction:

Manager -> RD

Method:

DELETE

URI Template:

/{+location}

URI Template Variables:

location :=

This is the Location path returned by the RD as a result of a successful group registration.

The following responses codes are defined for this interface:

Success:

2.02 “Deleted” or 204 “No Content” upon successful deletion

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

4.04 “Not Found” or 404 “Not Found”. Group does not exist.

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support:

YES

The following examples shows successful removal of the group from the RD.

    EP                                                RD
     |                                                 |
     | --- DELETE /rd-group/412  ------------------->  |
     |                                                 |
     |                                                 |
     | <-- 2.02 Deleted  ----------------------------  |
     |                                                 |

Req: DELETE /rd-group/12

Res: 2.02 Deleted

7. RD Lookup Function Set

In order for an RD to be used for discovering resources registered with it, a lookup interface can be provided using this function set. This lookup interface is defined as a default, and it is assumed that RDs may also support lookups to return resource descriptions in alternative formats (e.g. Atom or HTML Link) or using more advanced interfaces (e.g. supporting context or semantic based lookup).

This function set allows lookups for domains, groups, endpoints and resources using attributes defined in the RD Function Set and for use with the CoRE Link Format. The result of a lookup request is the list of links (if any) corresponding to the type of lookup. Using the Accept Option, the requester can control whether this list is returned in CoRE Link Format (application/link-format, default) or its alternate content-formats (application/link-format+json or application/link-format+cbor).
The target of these links SHOULD be the actual location of the domain, endpoint or resource, but MAY be an intermediate proxy e.g. in the case of an HTTP lookup interface for CoAP endpoints. Multiple query parameters MAY be included in a lookup, all included parameters MUST match for a resource to be returned. The character ‘*’ MAY be included at the end of a parameter value as a wildcard operator.

The lookup interface is specified as follows:

Interaction:

Client -> RD

Method:

GET

URI Template:

/{+rd-lookup-base}/{lookup-type}{?d,ep,gp,et,rt,page,count,resource-param}

URI Template Variables:

rd-lookup-base :=

RD Lookup Function Set path (mandatory). This is the path of the RD Lookup Function Set. An RD SHOULD use the value “rd-lookup” for this variable whenever possible.

lookup-type :=

(“d”, “ep”, “res”, “gp”) (mandatory) This variable is used to select the kind of lookup to perform (domain, endpoint, resource, or group).

ep :=

Endpoint name (optional). Used for endpoint, group and resource lookups.

d :=

Domain (optional). Used for domain, group, endpoint and resource lookups.

page :=

Page (optional). Parameter can not be used without the count parameter. Results are returned from result set in pages that contains ‘count’ results starting from index (page * count).

count :=

Count (optional). Number of results is limited to this parameter value. If the parameter is not present, then an RD implementation specific default value SHOULD be used.

rt :=

Resource type (optional). Used for group, endpoint and resource lookups.

et :=

Endpoint type (optional). Used for group, endpoint and resource lookups.

resource-param :=

Link attribute parameters (optional). Any link attribute as defined in Section 4.1 of [RFC6690], used for resource lookups.

The following responses codes are defined for this interface:

Success:

2.05 “Content” or 200 “OK” with an application/link-format, application/link-format+cbor, or application/link-format+json payload containing matching entries for the lookup.

Failure:

4.04 “Not Found” or 404 “Not Found” in case no matching entry is found for a unicast request.

Failure:

No error response to a multicast request.

Failure:

4.00 “Bad Request” or 400 “Bad Request”. Malformed request.

Failure:

5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support:

YES

The examples in this section assume a CoAP host with IP address FDFD::123 and a default CoAP port 61616. HTTP hosts are possible and do not change the nature of the examples. The following example shows a client performing a resource lookup:

   Client                                                          RD
     |                                                             |
     | ----- GET /rd-lookup/res?rt=temperature ----------------->  |
     |                                                             |
     |                                                             |
     | <-- 2.05 Content <coap://[FDFD::123]:61616/temp>;---------  |
     |                                  rt="temperature" --------  |
     |                                                             |

Req: GET /rd-lookup/res?rt=temperature

Res: 2.05 Content
<coap://[FDFD::123]:61616/temp>;rt="temperature"

The following example shows a client performing an endpoint type lookup:

   Client                                                          RD
     |                                                             |
     | ----- GET /rd-lookup/ep?et=power-node --------------------> |
     |                                                             |
     |                                                             |
     | <-- 2.05 Content <coap://[FDFD::123]:61616>;ep="node5" ---- |
     |                                                             |

Req: GET /rd-lookup/ep?et=power-node

Res: 2.05 Content
<coap://[FDFD::123]:61616>;ep="node5",
<coap://[FDFD::123]:61616>;ep="node7"

The following example shows a client performing a domain lookup:

   Client                                                          RD
     |                                                             |
     | ----- GET /rd-lookup/d ---------------------------------->  |
     |                                                             |
     |                                                             |
     | <-- 2.05 Content </rd>;d=domain1,</rd>;d=domain2 ---------- |
     |                                                             |

Req: GET /rd-lookup/d

Res: 2.05 Content
</rd>;d="domain1",
</rd>;d="domain2"

The following example shows a client performing a group lookup for all groups:

   Client                                                          RD
     |                                                             |
     | ----- GET /rd-lookup/gp --------------------------------->  |
     |                                                             |
     |                                                             |
     | <-- 2.05 Content </rd-group/12>;gp="lights1"; ------------- |
     |                               d="example.com" ------------- |
     |                                                             |

Req: GET /rd-lookup/gp

Res: 2.05 Content
</rd-group/12>;gp="lights1";d="example.com"

The following example shows a client performing a lookup for all endpoints in a particular group:

   Client                                                          RD
     |                                                             |
     | ----- GET /rd-lookup/ep?gp=lights1----------------------->  |
     |                                                             |
     |                                                             |
     | <-- 2.05 Content <coap://[FDFD::123]:61616>;ep="node1" ---- |
     |                                                             |

Req: GET /rd-lookup/ep?gp=lights1

Res: 2.05 Content
<coap://[FDFD::123]:61616>;ep="node1",
<coap://[FDFD::123]:61616>;ep="node2",

The following example shows a client performing a lookup for all groups an endpoint belongs to:

   Client                                                         RD
     |                                                            |
     | ----- GET /rd-lookup/gp?ep=node1 ----------------------->  |
     |                                                            |
     |                                                            |
     |< 2.05 Content <coap://[FDFD::123]:61616>;gp="lights1"; --  |
     |                                          ep="node1" ------ |
     |                                                            |

Req: GET /rd-lookup/gp?ep=node1

Res: 2.05 Content
<coap://[FDFD::123]:61616>;gp="lights1";ep="node1",

8. New Link-Format Attributes

When using the CoRE Link Format to describe resources being discovered by or posted to a resource directory service, additional information about those resources is useful. This specification defines the following new attributes for use in the CoRE Link Format [RFC6690]:

   link-extension    = ( "ins" "=" quoted-string ) ; Max 63 bytes
   link-extension    = ( "exp" )

8.1. Resource Instance attribute ‘ins’

The Resource Instance “ins” attribute is an identifier for this resource, which makes it possible to distinguish it from other similar resources. This attribute is similar in use to the <Instance> portion of a DNS-SD record (see Section 9.1, and SHOULD be unique across resources with the same Resource Type attribute in the domain it is used. A Resource Instance might be a descriptive string like “Ceiling Light, Room 3”, a short ID like “AF39” or a unique UUID or iNumber. This attribute is used by a Resource Directory to distinguish between multiple instances of the same resource type within the directory.

This attribute MUST be no more than 63 bytes in length. The resource identifier attribute MUST NOT appear more than once in a link description.

8.2. Export attribute ‘exp’

The Export “exp” attribute is used as a flag to indicate that a link description MAY be exported by a resource directory to external directories.

The CoRE Link Format is used for many purposes between CoAP endpoints. Some are useful mainly locally, for example checking the observability of a resource before accessing it, determining the size of a resource, or traversing dynamic resource structures. However, other links are very useful to be exported to other directories, for example the entry point resource to a functional service.

9. DNS-SD Mapping

CoRE Resource Discovery is intended to support fine-grained discovery of hosted resources, their attributes, and possibly other resource relations [RFC6690]. In contrast, service discovery generally refers to a coarse-grained resolution of an endpoint’s IP address, port number, and protocol.

Resource and service discovery are complementary in the case of large networks, where the latter can facilitate scaling. This document defines a mapping between CoRE Link Format attributes and DNS-Based Service Discovery [RFC6763] fields that permits discovery of CoAP services by either means.

9.1. DNS-based Service discovery

DNS-Based Service Discovery (DNS-SD) defines a conventional method of configuring DNS PTR, SRV, and TXT resource records to facilitate discovery of services (such as CoAP servers in a subdomain) using the existing DNS infrastructure. This section gives a brief overview of DNS-SD; see [RFC6763] for a detailed specification.

DNS-SD service names are limited to 255 octets and are of the form:

Service Name = <Instance>.<ServiceType>.<Domain>.

The service name is the label of SRV/TXT resource records. The SRV RR specifies the host and the port of the endpoint. The TXT RR provides additional information.

The <Domain> part of the service name is identical to the global (DNS subdomain) part of the authority in URIs that identify servers or groups of servers.

The <ServiceType> part is composed of at least two labels. The first label of the pair is the application protocol name [RFC6335] preceded by an underscore character. The second label indicates the transport and is always “_udp” for UDP-based CoAP services. In cases where narrowing the scope of the search may be useful, these labels may be optionally preceded by a subtype name followed by the “_sub” label. An example of this more specific <ServiceType> is “lamp._sub._dali._udp”.

The default <Instance> part of the service name may be set at the factory or during the commissioning process. It SHOULD uniquely identify an instance of <ServiceType> within a <Domain>. Taken together, these three elements comprise a unique name for an SRV/ TXT record pair within the DNS subdomain.

The granularity of a service name MAY be that of a host or group, or it could represent a particular resource within a CoAP server. The SRV record contains the host name (AAAA record name) and port of the service while protocol is part of the service name. In the case where a service name identifies a particular resource, the path part of the URI must be carried in a corresponding TXT record.

A DNS TXT record is in practice limited to a few hundred octets in length, which is indicated in the resource record header in the DNS response message. The data consists of one or more strings comprising a key=value pair. By convention, the first pair is txtver=<number> (to support different versions of a service description).

9.2. mapping ins to <Instance>

The Resource Instance “ins” attribute maps to the <Instance> part of a DNS-SD service name. It is stored directly in the DNS as a single DNS label of canonical precomposed UTF-8 [RFC3629] “Net-Unicode” (Unicode Normalization Form C) [RFC5198] text. However, to the extent that the “ins” attribute may be chosen to match the DNS host name of a service, it SHOULD use the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].

The <Instance> part of the name of a service being offered on the network SHOULD be configurable by the user setting up the service, so that he or she may give it an informative name. However, the device or service SHOULD NOT require the user to configure a name before it can be used. A sensible choice of default name can allow the device or service to be accessed in many cases without any manual configuration at all. The default name should be short and descriptive, and MAY include a collision-resistant substring such as the lower bits of the device’s MAC address, serial number, fingerprint, or other identifier in an attempt to make the name relatively unique.

DNS labels are currently limited to 63 octets in length and the entire service name may not exceed 255 octets.

9.3. Mapping rt to <ServiceType>

The resource type “rt” attribute is mapped into the <ServiceType> part of a DNS-SD service name and SHOULD conform to the reg-rel-type production of the Link Format defined in Section 2 of [RFC6690]. The “rt” attribute MUST be composed of at least a single Net-Unicode text string, without underscore ‘_’ or period ‘.’ and limited to 15 octets in length, which represents the application protocol name. This string is mapped to the DNS-SD <ServiceType> by prepending an underscore and appending a period followed by the “_udp” label. For example, rt=”dali” is mapped into “_dali._udp”.

The application protocol name may be optionally followed by a period and a service subtype name consisting of a Net-Unicode text string, without underscore or period and limited to 63 octets. This string is mapped to the DNS-SD <ServiceType> by appending a period followed by the “_sub” label and then appending a period followed by the service type label pair derived as in the previous paragraph. For example, rt=”dali.light” is mapped into “light._sub._dali._udp”.

The resulting string is used to form labels for DNS-SD records which are stored directly in the DNS.

9.4. Domain mapping

DNS domains may be derived from the “d” attribute. The domain attribute may be suffixed with the zone name of the authoritative DNS server to generate the domain name. The “ep” attribute is prefixed to the domain name to generate the FQDN to be stored into DNS with an AAAA RR.

9.5. TXT Record key=value strings

A number of [RFC6763] key/value pairs are derived from link-format information, to be exported in the DNS-SD as key=value strings in a TXT record ([RFC6763], Section 6.3).

The resource <URI> is exported as key/value pair “path=<URI>”.

The Interface Description “if” attribute is exported as key/value pair “if=<Interface Description>”.

The DNS TXT record can be further populated by importing any other resource description attributes as they share the same key=value format specified in Section 6 of [RFC6763].

9.6. Importing resource links into DNS-SD

Assuming the ability to query a Resource Directory or multicast a GET (?exp) over the local link, CoAP resource discovery may be used to populate the DNS-SD database in an automated fashion. CoAP resource descriptions (links) can be exported to DNS-SD for exposure to service discovery by using the Resource Instance attribute as the basis for a unique service name, composed with the Resource Type as the <ServiceType>, and registered in the correct <Domain>. The agent responsible for exporting records to the DNS zone file SHOULD be authenticated to the DNS server. The following example shows an agent discovering a resource to be exported:

    Agent                                                          RD
      |                                                             |
      | --- GET /rd-lookup/res?exp ------------------------------>  |
      |                                                             |
      |                                                             |
      | <-- 2.05 Content "<coap://[FDFD::1234]:5683/light/1>;exp;  |
      |                   rt="dali.light";ins="Spot";               |
      |                   d="office";ep="node1"                     |
      |                                                             |

   Req: GET /rd-lookup/res?exp

   Res: 2.05 Content
   <coap://[FDFD::1234]:5683/light/1>;
     exp;rt="dali.light";ins="Spot";
               d="office";ep="node1"

The agent subsequently registers the following DNS-SD RRs, assuming a zone name “example.com” prefixed with “office”:

node1.office.example.com.          IN AAAA        FDFD::1234
_dali._udp.office.example.com      IN PTR
                          Spot._dali._udp.office.example.com
light._sub._dali._udp.example.com  IN PTR
                          Spot._dali._udp.office.example.com
Spot._dali._udp.office.example.com IN SRV  0 0 5683
                          node1.office.example.com.
Spot._dali._udp.office.example.com IN TXT
                          txtver=1;path=/light/1

In the above figure the Service Name is chosen as Spot._dali._udp.office.example.com without the light._sub service prefix. An alternative Service Name would be: Spot.light._sub._dali._udp.office.example.com.

10. Security Considerations

The security considerations as described in Section 7 of [RFC5988] and Section 6 of [RFC6690] apply. The /.well-known/core resource may be protected e.g. using DTLS when hosted on a CoAP server as described in [RFC7252]. DTLS or TLS based security SHOULD be used on all resource directory interfaces defined in this document.

10.1. Endpoint Identification and Authentication

An Endpoint is determined to be unique by an RD by the Endpoint identifier parameter included during Registration, and any associated TLS or DTLS security bindings. An Endpoint MUST NOT be identified by its protocol, port or IP address as these may change over the lifetime of an Endpoint.

Every operation performed by an Endpoint or Client on a resource directory SHOULD be mutually authenticated using Pre-Shared Key, Raw Public Key or Certificate based security. Endpoints using a Certificate MUST include the Endpoint identifier as the Subject of the Certificate, and this identifier MUST be checked by a resource directory to match the Endpoint identifier included in the Registration message.

10.2. Access Control

Access control SHOULD be performed separately for the RD Function Set and the RD Lookup Function Set, as different endpoints may be authorized to register with an RD from those authorized to lookup endpoints from the RD. Such access control SHOULD be performed in as fine-grained a level as possible. For example access control for lookups could be performed either at the domain, endpoint or resource level.

10.3. Denial of Service Attacks

Services that run over UDP unprotected are vulnerable to unknowingly become part of a DDoS attack as UDP does not require return routability check. Therefore, an attacker can easily spoof the source IP of the target entity and send requests to such a service which would then respond to the target entity. This can be used for large-scale DDoS attacks on the target. Especially, if the service returns a response that is order of magnitudes larger than the request, the situation becomes even worse as now the attack can be amplified. DNS servers have been widely used for DDoS amplification attacks. Recently, it has been observed that NTP Servers, that also run on unprotected UDP have been used for DDoS attacks (http://tools.cisco.com/security/center/content/CiscoSecurityNotice/CVE-2013-5211) since there is no return routability check and can have a large amplification factor. The responses from the NTP server were found to be 19 times larger than the request. A Resource Directory (RD) which responds to wild-card lookups is potentially vulnerable if run with CoAP over UDP. Since there is no return routability check and the responses can be significantly larger than requests, RDs can unknowingly become part of a DDoS amplification attack. Therefore, it is RECOMMENDED that implementations ensure return routability. This can be done, for example by responding to wild card lookups only over DTLS or TLS or TCP.

11. IANA Considerations

11.1. Resource Types

“core.rd”, “core.rd-group” and “core.rd-lookup” resource types need to be registered with the resource type registry defined by [RFC6690].

11.2. Link Extension

The “exp” attribute needs to be registered when a future Web Linking link-extension registry is created (e.g. in RFC5988bis).

11.3. RD Parameter Registry

This specification defines a new sub-registry for registration and lookup parameters called “RD Parameters” under “CoRE Parameters”. Although this specification defines a basic set of parameters, it is expected that other standards that make use of this interface will define new ones.

Each entry in the registry must include the human readable name of the parameter, the query parameter, validity requirements if any and a description. The query parameter MUST be a valid URI query key [RFC3986].

Initial entries in this sub-registry are as follows:

Req: POST coap://[FDFD::ABCD:0]/rd
  ?ep=lm_R2-4-015_wndw&con=coap://[FDFD::ABCD:1]
Payload:
</light/left>;rt="light";
  d="R2-4-015";ins="lamp4444";exp,
</light/middle>;rt="light";
  d="R2-4-015";ins="lamp5555";exp,
</light/right>;rt="light";
  d="R2-4-015";ins="lamp6666";exp

Res: 2.01 Created
Location: /rd/4521

Req: POST coap://[FDFD::ABCD:0]/rd
  ?ep=lm_R2-4-015_door&con=coap://[FDFD::ABCD:2]
Payload:
</light/left>;rt="light";
  d="R2-4-015";ins="lamp1111";exp,
</light/middle>;rt="light";
  d="R2-4-015";ins="lamp2222";exp,
</light/right>;rt="light";
  d="R2-4-015";ins="lamp3333";exp

Res: 2.01 Created
Location: /rd/4522

Req: POST coap://[FDFD::ABCD:0]/rd
  ?ep=ps_R2-4-015_door&con=coap://[FDFD::ABCD:3]
Payload:
</ps>;rt="p-sensor";
  d="R2-4-015";ins="pres1234";exp

Res: 2.01 Created
Location: /rd/4523

Req: POST
 coap://[FDFD::ABCD:1]/light/grp1
 (content-type:application/link-format)<light/middle>,<light/left>

Res: 2.04 Changed

Req: POST
 coap://[FDFD::ABCD:2]/light/grp1
(content-type:application/link-format)<light/right>

Res: 2.04 Changed

Req: POST coap://[FDFD::ABCD:0]/rd-group
?gp=grp_R2-4-015;con="coap//[FF05::1]";exp;ins="grp1234"
Payload:
<>ep=lm_R2-4-015_wndw,
<>ep=lm_R2-4-015_door,
<>ep=ps_R2-4-015_door

Res: 2.01 Created
Location: /rd-group/501

Req: GET coap://[FDFD::ABCD:0]/rd-lookup/ep
  ?d=R2-4-015;rt=light

Res: 2.05 Content
<coap://[FDFD::ABCD:1]>;
  ep="lm_R2-4-015_wndw",
<coap://[FDFD::ABCD:2]>;
   ep="lm_R2-4-015_door"

Req: GET coap://[FDFD::ABCD:0]/rd-lookup/gp
  ?ep=lm_R2-4-015_wndw

Res: 2.05 Content
<coap://[FF05::1]>;gp="grp_R2-4-015"

Req: POST //[FDFD::ABCD:1]/coap-group
          Content-Format: application/coap-group+json
       { "a": "[FF05::1]" }
       { "n": "grp_R2-4-015"}

Res: 2.01 Created
Location-Path: /coap-group/1

lm_R2-4-015_wndw.bc.example.com.        IN AAAA      FDFD::ABCD:1
lm_R2-4-015_door.bc.example.com.        IN AAAA      FDFD::ABCD:2
ps_R2-4-015_door.bc.example.com.        IN AAAA      FDFD::ABCD:3
_light._udp.bc.example.com              IN PTR
                           lamp1111._light._udp.bc.example.com
_light._udp.bc.example.com              IN PTR
                           lamp2222._light._udp.bc.example.com
_light._udp.bc.example.com              IN PTR
                           lamp3333._light._udp.bc.example.com
_light._udp.bc.example.com              IN PTR
                           lamp4444._light._udp.bc.example.com
_light._udp.bc.example.com              IN PTR
                           lamp5555._light._udp.bc.example.com
_light._udp.bc.example.com              IN PTR
                           lamp6666._light._udp.bc.example.com
_p-sensor._udp.bc.example.com           IN PTR
                       pres12324._p-sensor._udp.bc.example.com
lamp1111._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_door.bc.example.com.
lamp1111._light._udp.bc.example.com     IN TXT
                                     txtver=1;path=/light/left
lamp2222._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_door.bc.example.com.
lamp2222._light._udp.bc.example.com     IN TXT
                                   txtver=1;path=/light/middle
lamp3333._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_door.bc.example.com.
lamp3333._light._udp.bc.example.com     IN TXT
                                    txtver=1;path=/light/right
lamp4444._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_wndw.bc.example.com.
lamp4444._light._udp.bc.example.com     IN TXT
                                     txtver=1;path=/light/left
lamp5555._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_wndw.bc.example.com.
lamp5555._light._udp.bc.example.com     IN TXT
                                   txtver=1;path=/light/middle
lamp6666._light._udp.bc.example.com     IN SRV  0 0 5683
                              lm_R2-4-015_wndw.bc.example.com.
lamp6666._light._udp.bc.example.com     IN TXT
                                    txtver=1;path=/light/right
pres1234._p-sensor._udp.bc.example.com  IN SRV  0 0 5683
                              ps_R2-4-015_door.bc.example.com.
pres1234._p-sensor._udp.bc.example.com  IN TXT
                                             txtver=1;path=/ps

R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp1111._light._udp.bc.example.com
R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp2222._light._udp.bc.example.com
R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp3333._light._udp.bc.example.com
R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp4444._light._udp.bc.example.com
R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp5555._light._udp.bc.example.com
R2-4-015._light._udp.bc.example.com              IN PTR
                           lamp6666._light._udp.bc.example.com

   Req: GET /rd-lookup/gp?exp

   Res: 2.05 Content
   <coap://[FF05::1]/>;exp;gp="grp_R2-4-015;ins="grp1234";
ep="lm_R2-4-015_wndw";
ep="lm_R2-4-015_door

grp_R2-4-015.bc.example.com.        IN AAAA            FF05::1
_group._udp.bc.example.com          IN PTR
                            grp1234._group._udp.bc.example.com
grp1234._group._udp.bc.example.com  IN SRV  0 0 5683
                             grp_R2-4-015_door.bc.example.com.
grp1234._group._udp.bc.example.com  IN TXT
                                     txtver=1;path=/light/grp1

/1/0/1

ep - Endpoint Name
lt - registration lifetime

et - Endpoint Type 
con - Context

</1>,</1/0>,</3/0>,</5>

</my_lwm2m>;rt="oma.lwm2m",</my_lwm2m/1>,<my_lwm2m/1/0>,<my_lwm2m/5>

lt - Registration Lifetime
b - Protocol Binding
sms - MSISDN
link payload - new or modified links