6LoWPAN topology

6LoWPAN networks are mesh networks residing on the periphery of larger networks. The topologies are flexible, allowing for ad hoc and disjointed networks without any binding to the internet or other systems, or they can be connected to the backbone or the internet using edge routers. 6LoWPAN networks can be conjoined with multiple edge routers; this is called multi-homing. Additionally, ad-hoc networks can form without requiring an Internet connectivity of an edge router. These topologies are shown below:

An edge router (also known as border router) is necessary for a 6LoWPAN architecture as it has four functions:

• Handles the communication to the 6LoWPAN devices and relays data to the internet.
• Performs compression of IPv6 headers by reducing a 40-byte IPv6 header and 8-byte UDP headers for efficiency in a sensor network. A typical 40-byte IPv6 header can compress to two to 20-bytes depending on usage.
• Initiates the 6LoWPAN network.
• Exchanges data between devices on the 6LoWPAN network.

Edge routers form 6LoWPAN mesh networks on larger traditional network perimeters. They can also broker exchanges between IPV6 and IPV4 if necessary. Datagrams are handled in a similar manner as in an IP network, which has some advantages over proprietary protocols. All nodes within a 6LoWPAN network share the same IPv6 prefix that the edge router establishes. Nodes will register with the edge routers as part of the Network Discovery (ND) phase. 

ND controls how hosts and routers in the local 6LoWPAN mesh will interact with each other. Multi-homing allows for multiple 6LoWPAN edge routers to manage a network; for example, when there is a need for multiple media (4G and Wi-Fi) for failover or fault tolerance.  

There are three types of nodes within the 6LoWPAN mesh:

• Router nodes: These nodes marshal data from one 6LoWPAN mesh node to another. Routers can also communicate outward to the WAN and internet.
• Host nodes: Hosts in the mesh network cannot route data in the mesh and are simply endpoints consuming or producing data. Hosts are allowed to be in sleep states, occasionally waking to produce data or receive data cached by their parent routers. 
• Edge routers: As stated, these are the gateways and mesh controllers usually at a WAN edge. A 6LoWPAN mesh would be administered under the edge router.

Nodes are free to move and reorganize/reassemble in a mesh. For that matter, a node can move and associate with a different edge router in a multi-home scenario or even move between different 6LoWPAN meshes. These changes to the topology can be caused for various reasons, such as changes in signal strength or physical movement of nodes. When a topology change occurs, the IPv6 address of the associated nodes will also naturally change.