IPIP-328: JSON and CBOR Response Formats on HTTP Gateways

IPIP/0328-gateway-json-cbor-response-format.md

@@ -0,0 +1,169 @@
+# IPIP-328: JSON and CBOR Response Formats on HTTP Gateways
+
+- Start Date: 2022-10-07
+- Related Issues:
+  - [ipfs/in-web-browsers/issues/182]
+  - [ipfs/specs/pull/328]
+  - [ipfs/kubo/issues/8823]
+  - [ipfs/kubo/pull/9335]
+  - [ipfs/go-ipfs/issues/7552]
+
+## Summary
+
+Add support for the [DAG-JSON], [DAG-CBOR], JSON and CBOR response formats in
+the [HTTP Gateway](../http-gateways/).
+
+## Motivation
+
+Currently, the gateway supports requesting data in the [DAG-PB], RAW, [CAR] and
+TAR formats. In addition, it allows for traversing of links encoded through CBOR
+Tag 42, as long as they are intermediate links, and not the final document.
+It works on both DAG-CBOR, and its JSON representation, DAG-JSON. However, it
+should be possible to download deserialized versions of data other than UnixFS,
+such as JSON and CBOR.
+
+The main functional gap in the IPFS ecosystem is the lack of support for
+non-UnixFS DAGs on HTTP gateways. Users are able to create custom DAGs based on
+traversable DAG-CBOR thanks to [CBOR tag 42 being reserved for CIDs][cbor-42]
+and DAG-JSON documents, but they are unable to load deserialized documents from
+a local gateway, which is severely decreasing the utility of non-UnixFS DAGs.
+
+Adding new responses types will also benefit UnixFS. DAG-PB has a
+[logical format][dag-pb-format] which makes it possible to represent a DAG-PB
+directory as a [DAG-JSON] document. This means that, if we support DAG-JSON in
+the gateway, then we would support
+[JSON responses for directory listings][ipfs/go-ipfs/issues/7552], which has been
+requested by our users in the past.
+
+In addition, this functionality is already present on the current Kubo CLI. By
+bringing it to the gateways, we provide users with more power when it comes
+to storing and fetching CBOR and JSON in IPFS.
+
+## Detailed design
+
+The solution is to allow the Gateway to support serializing data as [DAG-JSON],
+[DAG-CBOR], JSON and CBOR by requesting them using either the `Accept` HTTP header
+or the `format` URL query. In addition, if the resolved CID is of one of the
+aforementioned types, the gateway should be able to resolve them instead of
+failing with `node type unknown`.
+
+## Test fixtures
+
+- [`bafybeiegxwlgmoh2cny7qlolykdf7aq7g6dlommarldrbm7c4hbckhfcke`][f-dag-pb] is a
+  DAG-PB directory.
+- [`bafkreidmwhhm6myajxlpu7kofe3aqwf4ezxxn46cp5fko7mb6x74g4k5nm`][f-dag-pb-json]
+  is the aforementioned DAG-PB directory's [Logical DAG-JSON representation][dag-pb-format] that
+  is expected to be returned when using `?format=dag-json`.
+
+## Design rationale
+
+The current gateway already supports different response formats via the
+`Accept` HTTP header and the `format` URL query. This IPIP proposes adding
+more supported formats to that list.
+
+In addition, the current gateway already supports traversing through DAG-CBOR
+and DAG-JSON links if they are intermediary documents. With this IPIP, we aim
+to be able to download the DAG-CBOR, DAG-JSON, JSON and CBOR documents
+themselves.
+
+### User benefit
+
+The user benefits from this change as they will now be able to retrieve
+content encoded in the traversable DAG-JSON and DAG-CBOR formats. This is
+something that has been [requested before][ipfs/go-ipfs/issues/7552].
+
+In addition, both UX and DX are significantly improved, since every block can
+now be inspected in a regular web browser via `?format=json`. This can remove the
+need for additional tools, decoders, and/or libraries.
+
+### Compatibility
+
+This IPIP adds new response types and does not modify existing ones,
+making it a backwards-compatible change.
+
+### Security
+
+Serializers and deserializers for the JSON and CBOR must follow the security
+considerations of the original specifications, found in:
+
+- [RFC 8259 (JSON), Section 12][rfc8259-sec12]
+- [RFC 8949 (CBOR), Section 10][rfc8949-sec10]
+
+DAG-JSON and DAG-CBOR follow the same security considerations as JSON and CBOR.
+Note that DAG-JSON and DAG-CBOR are stricter subsets of JSON and CBOR, respectively.
+Therefore they must follow their specification and error if the payload is not
+strict enough:
+
+- [DAG-JSON Spec][dag-json-spec]
+- [DAG-CBOR Spec][dag-cbor-spec]
+
+### Alternatives
+
+If we do not introduce  DAG-JSON, DAG-CBOR, JSON and CBOR  response formats in
+the gateway, the usage of IPFS is constricted to files and directories represented
+by UnixFS (DAG-PB) codec. Therefore, if a user wants to store JSON and/or CBOR
+in IPFS, they have to wrap it as a UnixFS file in order to be able to fetch it
+through the gateway. That adds size and processing overhead.
+
+In addition, we could introduce only DAG-JSON and DAG-CBOR. However, not
+supporting the generic variants, JSON and CBOR, would lead to poor UX. The
+ability to retrieve DAG-JSON as `application/json` is an important step
+for the interoperability of the HTTP Gateway with web browsers and other tools
+that expect specific Content Types. Namely, `Content-Type: application/json` with
+`Content-Disposition: inline` allows for JSON preview to be rendered in a web browser.
+
+Finally, we considered supporting pathing within both DAG and non-DAG variants
+of the JSON and CBOR codecs. Pathing within these documents could lead to responses
+with extracts from the document. For example, if we have the document:
+
+```json
+{
+  "link" {
+    "to": {
+      "some": {
+        "cid2": <cbor tag 42 pointing at different CID>
+       }
+    }
+  }
+}
+```
+
+With CID `bafy`, and we navigate to `/ipfs/bafy/link/to`, we would be able to
+retrieve an extract from the document.
+
+```json
+{
+  "some": {
+    "cid2": <cbor tag 42 pointing at different CID>
+    }
+}
+```
+
+However, supporting this raises questions whose answers are not clearly defined
+or agreed upon yet. Right now, pathing is only supported over CID-based Links,
+such as Tag 42 in CBOR. In addition, some HTTP headers regarding caching are based
+on the CID, and adding extraction pathings would not be clear. Giving users the
+possibility to retrieve JSON, CBOR, DAG-JSON AND DAG-CBOR documents through the
+gateway is, in itself, a progress and will open the doors for new tools and explorations.
+
+### Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+
+[cbor-42]: https://github.com/core-wg/yang-cbor/issues/13#issuecomment-524378859
+[DAG-PB]: https://ipld.io/docs/codecs/known/dag-pb/
+[dag-pb-format]: https://ipld.io/specs/codecs/dag-pb/spec/#logical-format
+[DAG-JSON]: https://ipld.io/docs/codecs/known/dag-json/
+[DAG-CBOR]: https://ipld.io/docs/codecs/known/dag-cbor/
+[CAR]: https://ipld.io/specs/transport/car/
+[ipfs/in-web-browsers/issues/182]: https://github.com/ipfs/in-web-browsers/issues/182
+[ipfs/specs/pull/328]: https://github.com/ipfs/specs/pull/328
+[ipfs/kubo/issues/8823]: https://github.com/ipfs/kubo/issues/8823
+[ipfs/kubo/pull/9335]: https://github.com/ipfs/kubo/pull/9335
+[ipfs/go-ipfs/issues/7552]: https://github.com/ipfs/go-ipfs/issues/7552
+[f-dag-pb]: https://dweb.link/ipfs/bafybeiegxwlgmoh2cny7qlolykdf7aq7g6dlommarldrbm7c4hbckhfcke
+[f-dag-pb-json]: https://dweb.link/ipfs/bafkreidmwhhm6myajxlpu7kofe3aqwf4ezxxn46cp5fko7mb6x74g4k5nm
+[rfc8259-sec12]: https://datatracker.ietf.org/doc/html/rfc8259#section-12
+[rfc8949-sec10]: https://datatracker.ietf.org/doc/html/rfc8949#section-10
+[dag-json-spec]: https://ipld.io/specs/codecs/dag-json/spec/
+[dag-cbor-spec]: https://ipld.io/specs/codecs/dag-cbor/spec/

http-gateways/PATH_GATEWAY.md

@@ -79,6 +79,8 @@ where client prefers to perform all validation locally.
   - [Content resolution](#content-resolution)
     - [Finding the content root](#finding-the-content-root)
     - [Traversing remaining path](#traversing-remaining-path)
+    - [Traversing through UnixFS](#traversing-through-unixfs)
+    - [Traversing through DAG-JSON and DAG-CBOR](#traversing-through-dag-json-and-dag-cbor)
     - [Handling traversal errors](#handling-traversal-errors)
   - [Best practices for HTTP caching](#best-practices-for-http-caching)
   - [Denylists](#denylists)
@@ -182,10 +184,10 @@ For example:
 - [application/vnd.ipld.raw](https://www.iana.org/assignments/media-types/application/vnd.ipld.raw) – disables [IPLD codec deserialization](https://ipld.io/docs/codecs/), requests a verifiable raw [block](https://docs.ipfs.io/concepts/glossary/#block) to be returned
 - [application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car) – disables [IPLD codec deserialization](https://ipld.io/docs/codecs/), requests a verifiable [CAR](https://docs.ipfs.io/concepts/glossary/#car) stream to be returned
 - [application/x-tar](https://en.wikipedia.org/wiki/Tar_(computing)) – returns UnixFS tree (files and directories) as a [TAR](https://en.wikipedia.org/wiki/Tar_(computing)) stream. Returned tree starts at a root item which name is the same as the requested CID. Produces 400 Bad Request for content that is not UnixFS.
-<!-- TODO: https://github.com/ipfs/go-ipfs/issues/8823
-- application/vnd.ipld.dag-json OR application/json – requests IPLD Data Model representation serialized into [DAG-JSON format](https://ipld.io/docs/codecs/known/dag-json/)
-- application/vnd.ipld.dag-cbor OR application/cbor - requests IPLD Data Model representation serialized into [DAG-CBOR format](https://ipld.io/docs/codecs/known/dag-cbor/)
--->
+- [application/vnd.ipld.dag-json](https://www.iana.org/assignments/media-types/application/vnd.ipld.dag-json) – requests [IPLD Data Model](https://ipld.io/docs/data-model/) representation serialized into [DAG-JSON format](https://ipld.io/docs/codecs/known/dag-json/). If the requested CID already has `dag-json` (0x0129) codec, data is validated as DAG-JSON before being returned as-is. Invalid DAG-JSON produces HTTP Error 500.
+- [application/vnd.ipld.dag-cbor](https://www.iana.org/assignments/media-types/application/vnd.ipld.dag-cbor) – requests [IPLD Data Model](https://ipld.io/docs/data-model/) representation serialized into [DAG-CBOR format](https://ipld.io/docs/codecs/known/dag-cbor/). If the requested CID already has `dag-cbor` (0x71) codec,  data is validated as DAG-CBOR before being returned as-is. Invalid DAG-CBON produces HTTP Error 500.
+- [application/json](https://www.iana.org/assignments/media-types/application/json) – same as `application/vnd.ipld.dag-json`, unless the CID's codec already is `json` (0x0200). Then, the raw JSON block can be returned.
+- [application/cbor](https://www.iana.org/assignments/media-types/application/cbor) – same as `application/vnd.ipld.dag-cbor`, unless the CID's codec already is `cbor` (0x51). Then, the raw CBOR block can be returned.
 
 ### `Range` (request header)
 
@@ -246,11 +248,14 @@ parameter, if present)
 
 Optional, `format=<format>` can be used to request specific response format.
 
-This is a URL-friendly alternative to sending
-`Accept: application/vnd.ipld.<format>` header, see [`Accept`](#accept-request-header)
-for more details.
-
-In case of `Accept: application/x-tar`, the `?format=` equivalent is `tar`.
+This is a URL-friendly alternative to sending an [`Accept`](#accept-request-header) header. These are the equivalents:
+- `format=raw` → `Accept: application/vnd.ipld.raw`
+- `format=car` → `Accept: application/vnd.ipld.car`
+- `format=tar` → `Accept: application/x-tar`
+- `format=dag-json` → `Accept: application/vnd.ipld.dag-json`
+- `format=dag-cbor` → `Accept: application/vnd.ipld.dag-cbor`
+- `format=json` → `Accept: application/json`
+- `format=cbor` → `Accept: application/cbor`
 
 <!-- TODO Planned: https://github.com/ipfs/go-ipfs/issues/8769
 - `selector=<cid>`  can be used for passing a CID with [IPLD selector](https://ipld.io/specs/selectors)
@@ -584,24 +589,34 @@ A good practice is to always return it with HTTP error [status codes](#response-
 
 ## Response Payload
 
-Data sent with HTTP response depends on the type of requested IPFS resource:
+Data sent with HTTP response depends on the type of the requested IPFS resource, and the requested response type.
+The following response formats are selected according to the codec of the resolved CID:
 
-- UnixFS (implicit default)
+- UnixFS, either `dag-pb` (0x70) or `raw` (0x55)
   - File
     - Bytes representing file contents
   - Directory
     - Generated HTML with directory index (see [additional notes here](#generated-html-with-directory-index))
     - When `index.html` is present, gateway can skip generating directory index and return it instead
-- Raw block
-  - Opaque bytes, see [application/vnd.ipld.raw](https://www.iana.org/assignments/media-types/application/vnd.ipld.raw)
+- JSON (0x0200)
+  - Bytes representing a JSON file, see [application/json](https://www.iana.org/assignments/media-types/application/json).
+- CBOR (0x51)
+  - Bytes representing a CBOR file, see [application/cbor](https://www.iana.org/assignments/media-types/application/cbor)
+- DAG-JSON (0x0129)
+  - If the `Accept` header includes `text/html`: generated HTML with options to download DAG-JSON as-is, or converted to DAG-CBOR.
+  - Bytes representing a DAG-JSON file, see [application/vnd.ipld.dag-json](https://www.iana.org/assignments/media-types/application/vnd.ipld.dag-json)
+- DAG-CBOR (0x71)
+  - If the `Accept` header includes `text/html`: generated HTML with options to download DAG-CBOR as-is, or converted to DAG-JSON.
+  - Bytes representing a DAG-CBOR file, see [application/vnd.ipld.dag-cbor](https://www.iana.org/assignments/media-types/application/vnd.ipld.dag-cbor)
+
+The following response types are opt-in, can only be requested with [`format`](#format-request-query-parameter) query parameter or [`Accept`](#accept-request-header) header:
+
+- Raw Block
+  - Opaque bytes, see [application/vnd.ipld.raw](https://www.iana.org/assignments/media-types/application/vnd.ipld.raw).
 - CAR
-  - Arbitrary DAG as a verifiable CAR file or a stream, see [application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car)
+  - Arbitrary DAG as a verifiable CAR file or a stream, see [application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car).
 - TAR
-  - Deserialized UnixFS files and directories as a TAR file or a stream, see [application/x-tar](https://en.wikipedia.org/wiki/Tar_(computing))
-<!-- TODO: https://github.com/ipfs/go-ipfs/issues/8823
-- dag-json / dag-cbor
-  - See [https://github.com/ipfs/go-ipfs/issues/8823](https://github.com/ipfs/go-ipfs/issues/8823)
--->
+  - Deserialized UnixFS files and directories as a TAR file or a stream, see [IPIP-288](https://github.com/ipfs/specs/pull/288)
 
 # Appendix: notes for implementers
 
@@ -627,13 +642,32 @@ and [DNSLINK_GATEWAY.md](./DNSLINK_GATEWAY.md)).
 
 ### Traversing remaining path
 
-UnixFS pathing over files and directories is the implicit default used for
-resolving content paths that start with `/ipfs/` and `/ipns/`. It allows for
-traversal based on link names,  which provides a better user experience than
-low level logical pathing from IPLD:
+After the content route is found, the remaining of the path should be traversed
+and resolved. Depending on the data type, that may occur through UnixFS pathing,
+or DAG-JSON, and DAG-CBOR pathing.
+
+### Traversing through UnixFS
+
+UnixFS is an abstraction over the low level [logical DAG-PB pathing][dag-pb-format]
+from IPLD, providing a better user experience:
 
 - Example of UnixFS pathing: `/ipfs/cid/dir-name/file-name.txt`
 
+For more details regarding DAG-PB pathing, please read the "Path Resolution" section
+of [this document](https://ipld.io/design/tricky-choices/dag-pb-forms-impl-and-use/#path-resolution).
+
+### Traversing through DAG-JSON and DAG-CBOR
+
+Traversing through [DAG-JSON][dag-json] and [DAG-CBOR][dag-cbor] is possible
+through fields that encode a link:
+
+- DAG-JSON: link are represented as a base encoded CID under the `/` reserved
+namespace, see [specification](https://ipld.io/specs/codecs/dag-json/spec/#links).
+- DAG-CBOR: links are tagged with CBOR tag 42, indicating that they encode a CID,
+see [specification](https://ipld.io/specs/codecs/dag-cbor/spec/#links).
+
+Note: pathing into [IPLD Kind](https://ipld.io/docs/data-model/kinds/) other than Link (CID) is not supported at the moment. Implementations should return HTTP 501 Not Implemented when fully resolved content path has any remainder left.  This feature may be specified in a future IPIP that introduces data onboarding  and [IPLD Patch](https://ipld.io/specs/patch/) semantics.
+
 ### Handling traversal errors
 
 Gateway MUST respond with HTTP error when it is not possible to traverse the requested content path:
@@ -693,9 +727,8 @@ It should be always fast, even when a directory has 10k of items.
 The usual optimizations involve:
 
 - Skipping size and type resolution for child UnixFS items, and using `Tsize`
-  from [logical format](https://ipld.io/specs/codecs/dag-pb/spec/#logical-format)
-  instead, allows gateway to respond much faster, as it no longer need to fetch
-  root nodes of child items.
+  from [logical format][dag-pb-format] instead, allows gateway to respond much
+  faster, as it no longer need to fetch root nodes of child items.
   - Additional information about child nodes can be fetched lazily
     with JS, but only for items in the browser's viewport.
 
@@ -705,3 +738,7 @@ The usual optimizations involve:
     limiting the cost of a single page load.
   - The downside of this approach is that it will always be slower than
     skipping child block resolution.
+
+[dag-pb-format]: https://ipld.io/specs/codecs/dag-pb/spec/#logical-format
+[dag-json]: https://ipld.io/specs/codecs/dag-json/spec/
+[dag-cbor]: https://ipld.io/specs/codecs/dag-cbor/spec/