Let's take a look at how Thread identifies each device in the network, and what types of addresses they use to communicate with each other.
Key Term: In this primer, the term “interface” is used to identify an endpoint of a Thread device within a network. Typically, a single Thread device has a single Thread interface.
There are three scopes in a Thread network for unicast addressing:
The first two scopes correspond to prefixes designated by a Thread network. Link-Local have prefixes of fe80::/16, while Mesh-Local have prefixes of fd00::/8.
There are multiple IPv6 unicast addresses that identify a single Thread device. Each has a different function based on the scope and use case.
Before we detail each type, let's learn more about a common one, called the Routing Locator (RLOC). The RLOC identifies a Thread interface, based on its location in the network topology.
All devices are assigned a Router ID and a Child ID. Each Router maintains a table of all their Children, the combination of which uniquely identifies a device within the topology. For example, consider the highlighted nodes in the following topology, where the number on a Router (pentagon) is the Router ID, and the number on an End Device (circle) is the Child ID:
Each Child's Router ID corresponds to their Parent (Router). Because a Router is not a Child, the Child ID for a Router is always 0. Together, these values are unique for each device in the Thread network, and are used to create the RLOC16, which represents the last 16 bits of the RLOC.
For example, here's how the RLOC16 is calculated for the upper-left node (Router ID = 1 and Child ID = 1):
The RLOC16 is part of the Interface Identifier (IID), which corresponds to the last 64 bits of the IPv6 address. Some IIDs can be used to identify some types of Thread interfaces. For example, the IID for RLOCs is always of the form 0000:00ff:fe00:RLOC16.
The IID, combined with a Mesh-Local Prefix, results in the RLOC. For example, using a Mesh-Local Prefix of fde5:8dba:82e1:1::/64, the RLOC for a node where RLOC16 = 0x401 is:
This same logic can be used to determine the RLOC for all highlighted nodes in the sample topology above:
However, because the RLOC is based on the location of the node in the topology, the RLOC of a node can change as the topology changes.
For example, perhaps node 0x400 is removed from the Thread network. Nodes 0x401 and 0x402 establish new links to different Routers, and as a result they are each assigned a new RLOC16 and RLOC:
The RLOC is just one of many IPv6 unicast addresses a Thread device can have. Another category of addresses are called Endpoint Identifiers (EIDs), which identify a unique Thread interface within a Thread network partition. EIDs are independent of Thread network topology.
Common unicast types are detailed below.
Multicast is used to communicate information to multiple devices at once. In a Thread network, specific addresses are reserved for multicast use with different groups of devices, depending on the scope.
| IPv6 Address | Scope | Delivered to |
|---|---|---|
ff02::1 | Link-Local | All FTDs and MEDs |
ff02::2 | Link-Local | All FTDs |
ff03::1 | Mesh-Local | All FTDs and MEDs |
ff03::2 | Mesh-Local | All FTDs |
Key Point: A major difference between FTDs and MTDs are that FTDs subscribe to the ff03::2 multicast address. MTDs do not.
You might notice that Sleepy End Devices (SEDs) are not included as a recipient in the multicast table above. Instead, Thread defines link-local and realm-local scope unicast prefix-based IPv6 multicast address used for All Thread Nodes, including SEDs. These multicast addresses vary by Thread network, because it is built on the unicast Mesh-Local prefix (see RFC 3306 for more details on unicast-prefix-based IPv6 multicast addresses).
Arbitrary scopes beyond those already listed are also supported for Thread devices.
Anycast is used to route traffic to a Thread interface when the RLOC of a destination is not known. An Anycast Locator (ALOC) identifies the location of multiple interfaces within a Thread partition. The last 16 bits of an ALOC, called the ALOC16, is in the format of 0xfcXX, which represents the type of ALOC.
For example, an ALOC16 between 0xfc01 and 0xfc0f is reserved for DHCPv6 Agents. If the specific DHCPv6 Agent RLOC is unknown (perhaps because the network topology has changed), a message can be sent to a DHCPv6 Agent ALOC to obtain the RLOC.
Thread defines the following ALOC16 values:
| ALOC16 | Type |
|---|---|
0xfc00 | Leader |
0xfc01 – 0xfc0f | DHCPv6 Agent |
0xfc10 – 0xfc2f | Service |
0xfc30 – 0xfc37 | Commissioner |
0xfc40 – 0xfc4e | Neighbor Discovery Agent |
0xfc38 – 0xfc3f0xfc4f – 0xfcff | Reserved |
What you've learned:
To learn more about Thread's IPv6 addressing, see sections 5.2 and 5.3 of the Thread Specification.