Google, LLC Liquid Intelligent Technologies Cisco Cisco There is a trend towards documents describing protocols that are only intended to be used within "limited domains". These documents often do not clearly define how the boundary of the limited domain is implemented and enforced, or require that operators of these limited domains //perfectly// implement filters to protect the rest of the global Internet from these protocols and vice-versa. This document discusses the concepts of "fail-open" versus "fail- closed" protocols and limited domains, and specifies a layer-2 mechanism that can be used for designing limited domain protocols that are safer to deploy.