Secure Proxy ND Support for SEND

Secure Neighbor Discovery (SEND) specifies a method for securing Neighbor Discovery (ND) signaling against specific threats. As specified today, SEND assumes that the node advertising an address is the owner of the address and is in possession of the private key used to generate the digital signature on the message. This means that the Proxy ND signaling initiated by nodes that do not possess knowledge of the address owner's private key cannot be secured using SEND. This document extends the current SEND specification with support for Proxy ND, the Secure Proxy ND Support for SEND.