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.

まず、h2 にある既存の HTTP/2 ステートマシン、フレーム解析、コールバック、設定、エラーコード処理を整理します。次に、hyperframe で必要となる関連作業を確認し、既存の動作を壊さずに拡張によってフレーム、遷移、設定、コールバック、エラーコードを追加する方法を決定します。両方のプロジェクトにおいて、拡張性の設計が定義され、それを支える実装が行われれば完了です。