## 3 Related Technologies
### Lightweight Secure Neighbor Discovery for 6 LoWPAN (LSEND)  
In traditional IPv6 networks, both nodes and routers use the neighbor discovery protocol [5] and the stateless address autoconfiguration [32], which, together, are referred to as the neighbor discovery protocols (NDP). These protocols enable the following functions: (i) learning prefixes and configuration parameters related to address configuration, (ii) locating neighborhood routers, (iii) maintaining reachability information on active neighbors and (iv) detecting duplicate addresses. Note that NDP was proposed for unconstrained node devices. Moreover, NDP for IPv6 networks uses multicast to exchange most of the protocol messages and was designed considering that routers and nodes are always active. Given the resource constraints of sensor nodes, the resource inefficiency associated to multicast based mechanisms, the low duty-cycle and multi-hop support, NDP on 6 LoWPAN networks requires a different approach. Neighbor discovery optimization for low power and lossy networks [6] is a work in progress at IETF 6 LoWPAN working group. It proposes optimizations to NDP, header compression context information dissemination, auto configuration addressing mechanisms and duplicate address detection for low power networks. The NDP signaling was changed by replacing the standard address resolution mechanism (based on multicast messages between hosts) with an address registration mechanism. Moreover, some multicast messages associated to node address configuration were replaced by unicast messages, providing host-initiated request for router advertisements (RA) and eliminating in this way the need for periodic router advertisement multicasting. In this way, NDP for 6 LoWPAN is more suitable for multi-hop sensor networks and is independent of the selected routing approach. The 6 LBR is responsible for interconnecting the LoWPAN to the Internet and for disseminating IPv6 prefixes and header compression context information across the LoWPAN. The 6 LBR also maintains a network cache of all IPv6 addresses and EUI-64 identifiers. In this way, 6 LBR is able to make layer-two address resolution and to perform duplicate address detection (DAD). Figure 1: 6 LoWPAN network architecture. Besides the Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS) and Neighbor Advertisement (NA) message types which were already defined for IPv6 networks, NDP for 6 LoWPAN [6] defines two new ICMPv6 message types to implement DAD on multi-hop networks: Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) and some new options on the previous message types. When a 6 LN interface is initialized, a link-local address is formed based on the EUI-64 [33]. Next, the 6 LN multicasts a RS message indicating its source link-layer address. All 6 LRs with direct connectivity reply with a unicast RA message indicating the available IPv6 prefix(es). Once an address has been configured, the following messages exchange is shown in Figure 2. First, a unicast NS message is sent to the selected 6 LR to register its configured address (if 6 LN receives RA messages from different 6 LRs, it should attempt to register its address in more than one 6 LN to increase the network resilience). Then, the 6 LR sends a unicast DAR message to the 6 LBR to check if the IPv6 address is already in use and the 6 LBR replies to the 6 LR with a DAC message indicating the status of the registration. The status indicates either a successful registration or a failure due to a duplicated address (or any other reason like, for example, the routers registration cache exhaustion). Finally, the 6 LR sends a unicast NA message to the 6 LN indicating the same status as received from the 6 LBR in the DAC message. The information contained on NA messages have an associated lifetime and the address registration process is repeated before the lifetime expires.