o other links. The Neighbor Discovery Proxy node keeps a track of all nodes connected to all interfaces used for packet proxying. This allows optional selective multicast forwarding only to the link, or links, that the Neighbor Discovery Proxy node knows has the right receivers. The selective forwarding is useful when resources Proxied Router Router Advertisement with Advertisement with 2001:db8:42::66 2001:db8:42::66 Shared link Point-to-point Proxy Router (+possible filtering) Link-layer using Link-layer using multicast unicast Figure 3.14 Illustration of Neighbor Discovery Proxy. Introduction to IPv6 on the interfaces need to be conserved and when simple forwarding would only consume resources, such as energy and bandwidth, without bringing any benefits. For example, in Figure 3.14 the point-to-point link could be an energy-consuming cellular connection. The multicast packets of special interest for proxying are ICMPv6 Router Advertisements, Redirects, Neighbor Solicitations, and Neighbor Advertisements. These four messages transport link-layer addresses in their payloads and hence require special attention from the Neighbor Discovery Proxy. All other multicast traffic is always proxied as it is to all interfaces. In order to further optimize, for example to conserve power, the central node may implement additional selective multicast filtering and forwarding policies. When proxying, with few exceptions, the Neighbor Discovery Proxy leaves the packet otherwise intact, but the source link-layer address is changed to the one owned by the proxy node. The destination's link layer address will be looked on from the Neighbor Discovery Proxy's Neighbor Cache and it will be the one corresponding to the destina- tion IPv6 address. Furthermore, link-layer addresses included in the protocol payload of Router Solicitation, Redirects, Neighbor Solicitations, and Neighbor Advertisements will be replaced with Neighbor Discovery Proxy's link-layer address. It is worth noting that the Hop Limit field is never updated, hence th