Chapter 4 — Part 2

Commissionable Node Discovery

In the continuation of the fourth chapter and the subject of the secure channel, in this section we intend to have a more comprehensive look at the topic of Commissionable node detection.

The discovery section actually describes the advertising and discovery services for the Matter protocol and is used in the following areas:

• Commissionable Node Discovery

• Operational Discovery

• Commissioner Discovery

• User Directed Commissioning

Service Advertisement and Discovery for Matter uses standard DNS-based service discovery. The Matter protocol does not require any changes to the IETF standard DNS-SD. In fact, using DNS-SD means that the IPv6 unicast address and port of the provided service are discovered, and Matter does not need a pre-assigned fixed port. It is also possible to run several instances of the Matter software in one device; Because each instance will have its own dynamic port instead of trying to use the same pre-allocated static port.

Since Matter protocols must support at least IPv6, Matter software that discovers other Matter instances must process IPv6 address (DNS AAAA) records as well as IPv4 address DNS A records.

For this reason, where possible in the underlying service discovery API, Matter software that advertises the availability of a service MUST indicate that the service's announcements and responses only contain IPv6 address records, not IPv4 address records.

Where supported in the underlying service discovery API, Matter software using Multicast DNS to advertise the availability of a service SHOULD indicate that notifications and responses for this service should only be made over IPv6.

For Thread mesh networks, where excessive use of multicast is harmful [RFC 7558], DNS-SD uses Unicast DNS instead and uses the Thread Service Registry capabilities in draft-lemon-stub-networks.

Conceptually, the DNS-SD [RFC 6763] information being sent is the same as when DNS Multicast [RFC 6762] is used, except that the information is sent in multicast packets to and from every other node in the same domain rather than in multicast packets. Broadcasts are transmitted in unicast packets from a designated service registry.

Using the Service Registration Protocol (SRP) and an advertising proxy (AdProx) running on the Thread border router, Matter nodes in a Thread network can be reached by other Matter nodes on an adjacent Ethernet or Wi-Fi link, without the cost of using multicast. Discovered on Thread mesh.

When Matter nodes send short-term requests to its other nodes, the response is sent to the source IPv6 address and port of the request. This is despite the fact that by issuing long-term requests to other requester nodes, the IPv6 address or port has been changed, so the responder may discover the current IPv6 address and the initiator port to send.

Commissionable Node Discovery

The Matter protocol family supports UDP and TCP for setting up gateways that are already defined on the client's IP network, as well as for setting up the end device in connection with the Wi-Fi Soft-AP.

For these gateways, Matter bootable node discovery is performed using the IETF Standard DNS-Based Service Discovery (DNS SD) [RFC 6763].

To discover a Commissionable Matter node in cases such as already-on-network and Soft-AP, the DNS-SD instance name must be a 64-bit temporary, dynamically unique identifier, chosen as a fixed-length sixteen-character hexadecimal.

When a Node receives either the OpenCommissioningWindow or the OpenBasicCommissioningWindow command, the node only needs to be represented on the IP network using the corresponding DNS SD attributes.

Host Name Construction

In DNS-SD, in addition to the instance name, the hostname is also required. The target hostname must be constructed using one of the available link layer addresses, such as a 48-bit device MAC address for Ethernet and Wi-Fi or a 64-bit Extended MAC address for Thread.

If a privacy-preserving device performs MAC address randomization, the target hostname should use the privacy-preserving random version, and the hostname record SHOULD be updated each time the underlying link layer address is rotated. Note that it is legal to reuse the same hostname on more than one interface, even if the underlying link-layer address does not match the hostname on that interface, because the purpose of using a link-layer address is to ensure uniqueness.

Extended Discovery

A Matter Commissionee that advertises endpoint discovery service records is not necessarily in a state that allows startup, which is called the "in startup" mode. On the other hand, the end node discovery advertisement is known as "expanded discovery" when it is not in launch mode. Extended discovery is only allowed for DNS-SD advertisements and is not allowed for other forms of device discovery such as BLE device discovery and Wi-Fi ad hoc access point discovery. To protect customer privacy on public networks, Matter Gateway must provide a way for the customer to set a time period in device discovery (Extended Discovery) or otherwise disable it.

Commissioning Subtypes

1. _L<dddd>, where <dddd> provides the full 12-bit discriminator, encoded as a variable-length decimal number in ASCII text, omitting any leading zeroes.
2. _S<dd>, where <dd> provides the upper 4 bits of the discriminator, encoded as a variable-length
decimal number in ASCII text, omitting any leading zeroes.
3. _V<ddddd>, where <ddddd> provides the 16-bit Vendor ID, encoded as a variable-length decimal
number in ASCII text, omitting any leading zeroes.
4. _T<ddd>, where <ddd> provides the device type identifier for the device, encoded as a variable length decimal number in ASCII (UTF-8) text, omitting any leading zeroes. In case the device
combines multiple device types, the manufacturer SHOULD choose the device type identifier of the primary function of the device for which the device wishes to be discoverable.

5. _CM, which represents "currently in Commissioning Mode"

TXT Records

After discovery, IPv6 addresses are returned in the AAAA records and key/value pairs are returned in the DNS‑SD TXT record.

Nodes SHALL publish AAAA records for all available IPv6 addresses upon which they are willing to accept Matter commissioning messages.

TXT records available for Commissionable Node Discovery include the common TXT record
key/value pairs.

In this section, we learned how to discover a Commissionable node and its principles. In the next section, we will continue discussing the safe channel.