That list of questions is why I prefer retry and back-off to be separate from the internals of the http library.

As a library user I need to work around the peculiarities of the particular service endpoints I'm integrating with. Backoff and retry aren't specific to the application/transport protocol through which a service is consumed.

For example, (in the java world) the approach in libraries like hysterix, guava-retrying, and failsafe https://github.com/jhalterman/failsafe#retries