Tunneling Through Inner Space

It is common but perhaps misguided to provide extensibility for a layer X header in the layer X header. Strawman principle: In a middlebox world, it is both more principled and more pragmatic to extend the layer X header within layer X + 1 (i.e. within the payload encapsulated by the layer X header). For instance, extensions to the IP header should not be located in a chain of extensions to the IP header; they should be in an area of the payload that IP encapsulates (e.g. where you would expect UDP or TCP or an IPv6 destination option). Similarly, extensions to TCP should not be located in the TCP Options within the TCP header, they should be located within the TCP Data (i.e. in the space encapsulated by TCP that is intended for the app-layer). I’m not yet even convinced myself that this approach warrants promotion to the status of a principle. In the four examples this paper explores, it happened to be possible to make the approach work with some ingenious hackery, but it would not be easy in general. Therefore the message cannot be that existing protocols should be extended like this (because they were not designed to be extended in this way, so it will not always possible). The message is that we can at least try. And, in future, protocols would be more extensible in a middlebox world if they were designed so that they could be extended within their own encapsulation.