Extensibility framework

HTTP/2 is extendable: it's one of its great strengths. It would be really nice if H2 was relatively easily extensible. We should aim to start supporting pluggable HTTP/2 extensions.

What do we need to do this? Well, the answer is: what can extensions do?

• They can add new frames. This means we need to be able to add:
  • new frame types that can be successfully parsed;
  • new transitions in the state machine, both connection and stream, that occur when a new frame is received;
  • specific frame callbacks, both connection and stream, that allow manipulating the connection and stream objects;
  • allow new frames to follow the state machine transitions and callbacks of a pre-existing frame;
• They can add new state machine transitions in the absence of new frames, or adjust the existing state machine transitions;
• They can add new settings. We need to be able to inject useful representations of settings, and allow callbacks for those settings if they're changed;
• They can add new error codes.

This will also require work in hyperframe.

Start by mapping the existing HTTP/2 state machine, frame parsing, callbacks, settings, and error-code handling in h2. Then review the related work required in hyperframe and determine how extensions could add frames, transitions, settings, callbacks, and error codes without breaking existing behavior. Done means a defined extensibility design and supporting implementation across both projects.