This document provides an overview of various ways to tunnel IPv6 packets over IPv4 networks. It covers mechanisms in current use, touches on several mechanisms that are now only of historic interest, and discusses some newer tunnel mechanisms that are not widely used at the time of publication. The goal of the document is helping people with an IPv6-in-IPv4 tunneling need to select the mechanisms that may apply to them. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7059.

Conventionally, interfaces of objects export a set of messages with their types, and suggest nothing about the order in which these services may be accessed. This leaves room for a large number of run-time errors or misbehaviours in type correct designs. To mend this, we introduce the notion of protocol, expressing offered and expected orderings of messages, along with a notion of protocol correctness. We do this by defining the Protocol Assuring Universal Language Paul, which describes protocol aspects of classes, and a semantics of in terms of CSP.

We propose an alternative notation and semantics for process models in object analysis, to resolve problems with current diagram languages. Our dynamic life cycles are communicating state diagrams. Our life cycles support polymorphic creation, and they are straightforwardly composed. We provide an operational semantics, and demonstrate how to interact with a system of life cycles.