Join Proxy for Bootstrapping of Constrained Network Elements

This document extends the constrained Bootstrapping Remote Secure Key Infrastructures (cBRSKI) onboarding protocol by adding a new network function, the constrained Join Proxy. This function can be implemented on a constrained node. The goal of the Join Proxy is to help new constrained nodes ("Pledges") securely onboard into a new IP network using the cBRSKI protocol. It acts as a circuit proxy for User Datagram Protocol (UDP) packets that carry the onboarding messages. The solution is extensible to support other UDP-based onboarding protocols as well. The Join Proxy functionality is designed for use in constrained networks, including IPv6 over Low- Power Wireless Personal Area Networks (6LoWPAN) based networks in which the onboarding authority server ("Registrar") may be multiple IP hops away from a Pledge. Despite this distance, the Pledge only needs to use link-local communication to complete cBRSKI onboarding. Two modes of Join Proxy operation are defined, stateless and stateful, to allow different trade-offs regarding resource usage, implementation complexity and security.