MSC: Request max body size

proposals/4154-request-max-body-size.md

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+# MSC4154: Request max body size
+
+When making requests to the homeserver, clients have a lot of freedom to include large strings and
+similar. Homeserver implementations may not be prepared for long text fields, for example, and may
+encounter issues or present errors to clients.
+
+This proposal aims to place a *minimum* expectation into the specification on request sizes rather
+than trying to limit individual field types to certain lengths. The obvious disadvantage is that
+most endpoints accept only a few fields, so whatever limit is chosen for the whole request will need
+to also apply to the database level - an API with a single field which ends up in the database will
+need that column to support nearly the entire body size, as a user could feasibly pad their request
+out to meet the limit.
+
+## Proposal
+
+A request body size limit of 1,000,000 bytes (1 MB) MUST be expected by clients on *all* Client-Server
+API endpoints. If a request exceeds this limit, the client SHOULD be presented with a `413 M_TOO_LARGE`
+error. The `M_TOO_LARGE` error code [already exists](https://spec.matrix.org/v1.10/client-server-api/#other-error-codes).
+
+Servers MAY raise this limit to higher than 1 MB, but SHOULD NOT lower it below 1 MB.
+
+This limit is applied before any endpoint-specific considerations, such as
+[event size limits](https://spec.matrix.org/v1.10/client-server-api/#size-limits).
+
+*Note*: The 1 MB requirement is fairly arbitrary. It's just what
+[nginx does by default](https://nginx.org/en/docs/http/ngx_http_core_module.html#client_max_body_size).
+Critically, the limit *must* be more than the 65,536 bytes permitted for events at a minimum.
+
+Response sizes from servers are *not* affected by this proposal. For example, a `/sync` response may
+exceed 1 MB.
+
+## Potential issues
+
+None perceived. By using a default already found in popular reverse proxy software, clients will be
+subject to these limits unintentionally today. This proposal improves the error response in these
+cases, hopefully.
+
+## Alternatives
+
+Per-field or field type size restrictions may be a better fit, though open a can of worms on what
+the 'right' size should be. Limiting strings to 512 KB (for example) may mean some APIs generate way
+more than 1 MB of JSON, running into the nginx default request body limit. This also doesn't address
+clients including off-spec fields in their requests, such as a `/login` containing namespaced fields:
+can the client send an infinite number of 512 KB strings, or is there a limit? What is that limit?
+
+## Security considerations
+
+This proposal technically fixes a security concern regarding unbounded requests.
+
+## Unstable prefix
+
+It is difficult to implement this MSC in a namespaced way. Clients may already be encountering `413`
+errors without the Matrix standardized error response, and so should generally expect to receive such
+errors.
+
+## Dependencies
+
+None relevant.