MSC4041: http header Retry-After for http code 429 (#4041)

* http header Retry-After for http code 429

* MSC id

* Update proposals/4041-retry-after-header-rate-limiting.md

Thanks for the advice! What a stupid mistake!

Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>

* notes on backward compatibility

* fixes #4041 (comment)

* fixed typos

* made calculation of the Retry-After header value less explicit

* Update proposals/4041-retry-after-header-rate-limiting.md

Co-authored-by: Will Hunt <will@half-shot.uk>

* retry_after_ms gets deprecated

* explicit advice for server and client

* * moved deprecation into proposal
* made Retry-After is a delay

---------

Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>
Co-authored-by: Will Hunt <will@half-shot.uk>

proposals/4041-retry-after-header-rate-limiting.md

@@ -0,0 +1,50 @@
+# MSC4041: Use http header Retry-After to enable library-assisted retry handling
+
+The current Matrix Client-Server API (v1.7) recommends that home servers should protect themselves from
+being overloaded by enforcing rate-limits to selected API calls.
+If home servers limit access to an API they respond with an http error code 429 and a response body
+that includes a property `retry_after_ms` to indicate how long a client has to wait before retrying.
+
+Some http libraries (like [Ky](https://github.com/sindresorhus/ky), [got](https://github.com/sindresorhus/got
+and [urllib3](https://urllib3.readthedocs.io/en/stable/reference/urllib3.util.html#urllib3.util.Retry)) ease
+the burden of handling retries by honoring the http header `Retry-After`. As explained in 
+[RFC 9119 - HTTP Semantics](https://www.rfc-editor.org/rfc/rfc9110#field.retry-after) this header is optional
+and contains either a (relative) value for the delay in seconds or an (absolute) date.
+
+The current Matrix Client Server API specification does not take this header into account. This wastes the 
+potential that current http libraries offer in terms of automated retry handling.
+
+## Proposal
+
+In order to allow developers to make use of the automated retry handling capabilities of http libraries
+home servers shall use the http header `Retry-After` in case they respond with an http error 429.
+The value of `Retry-After` (in __seconds__) is meant to be a delay after receiving the response and must be 
+calculated in order to comply with the specification in [RFC 9119 - HTTP Semantics](https://www.rfc-editor.org/rfc/rfc9110#field.retry-after).
+
+With the introduction of the http header `Retry-After` the usage of the existing `retry_after_ms` property in the response body becomes deprecated.
+
+## Potential issues
+
+In order to maintain backward compatibility with existing client libraries home servers shall use both the `Retry-After` header and the
+`retry_after_ms` property in the response body.
+
+Client libraries shall use the values in this order:
+
+1) `Retry-After` http header.
+2) `retry_after_ms` property in the response body.
+
+## Alternatives
+
+N/A
+
+## Security considerations
+
+N/A
+
+## Unstable prefix
+
+Since this MSC is using a standard HTTP header, it will not use a unstable prefix.
+
+## Dependencies
+
+N/A