MSC4083: Delta-compressed E2EE file transfers

proposals/4083-delta-compressed-file-transfers.md

@@ -0,0 +1,113 @@
+# MSC4083: Delta-compressed E2EE file transfers
+
+## Problem
+
+When collaborating on a large file of some kind, it's common to store that file in Matrix, and then need a way to
+express incremental changes to it.  For instance, in Third Room, you might store a large glTF scene graph as a GLB
+file, and then want to express a small change to it (e.g. using the editor to transform part of the scene graph).  Or
+you might want to store a change to a markdown or HTML file.
+
+Currently, your only option is to save a whole new copy of the file - or invent your own delta-compression scheme at
+the application layer. Instead, we could make Matrix itself aware of delta-compression, letting the content repository
+help users efficiently collaborate around updates to binary files, regardless of what the file is.
+
+## Solution
+
+When uploading a file, specify that it's a delta against a previous piece of content, using a given algorithm.
+
+ * `delta_base` is the mxc URL of the content the delta applies to
+ * `delta_format` is the file format of the binary diff
+   * This MSC defines `m.vcdiff.v1.gzip` to describe gzipped RFC3284 compatible binary VCDIFF payloads, picked for
+     computation efficiency rather than patch size (whereas bsdiff + bzip might provide better patch size at worse
+     computation complexity; other MSCs are welcome to propose different diff formats).
+
+Clients should upload a new snapshot of a piece of content if the sum of the deltas relative to the last snapshot
+is larger than 50% of the original piece of content
+
+For instance:
+
+`POST /_matrix/media/v3/upload?delta_base=mxc://matrix.org/b4s3v3rs10n&delta_format=m.vcdiff.v1.gzip`
+
+returning:
+```json
+{
+  "content_uri": "mxc://matrix.org/n3wv3rs10n"
+}
+```
+
+(or with the same parameters for MSC2246-style `POST /_matrix/media/v3/create`).
+
+The server tracks the graph of which deltas apply to which files, so it can only hand the relevant deltas to clients
+when they download them.
+
+For instance, when downloading a delta-compressed piece of content, a client might ask to pull in any delta dependencies
+it doesn't already have stored locally, relative to the last version that it has a full copy of:
+
+`GET /_matrix/media/v3/download/matrix/org/n3wv3rs10n?delta_base=mxc://matrix.org/b4s3v3rs10n`
+
+This would return an ordered multipart download of the deltas (once unencrypted, if needed) to apply to the base-version
+to get a copy of the new-version.
+
+## Alternatives
+
+### Track deltas on events rather than media repository
+
+Alternatively we could store the delta info on the `m.file` event itself as a mixin. This would allow us to shift the
+task of tracking deltas purely to clients, and protect the delta info within the E2EE payload.  However, this would then
+force the client to do many more roundtrips to spider the events (if needed) and files (if needed) one by one in order
+to calculate diffs, which would be O(N) latency with the number of diffs rather than O(1) for the above API.  Given the
+traffic pattern of these requests would reveal the delta graph to the server anyway, it's not clear that it provides a
+sufficient advantage. This would look like this:
+
+```json
+{
+  "content": {
+    "filename": "something-important.doc",
+    "info": {
+      "mimetype": "application/msword",
+      "size": 46144
+    },
+    "msgtype": "m.file",
+    "url": "mxc://example.org/n3wv3rs10n",
+    "delta_base": "$1235135aksjgdkg",
+    "delta_format": "m.vcdiff.v1.gzip"
+  },
+}
+```
+
+We could go even further down this path by defining an arbitrary CRDT for tracking these deltas, a bit like the
+(Saguaro CRDT-over-Matrix)[https://github.com/matrix-org/collaborative-documents/blob/main/docs/saguaro.md] proposal,
+with files decorating each event - effectively modelling the problem as a collaborative document problem (with binary
+diffs attached) rather than a binary file diffing problem.
+
+### Other alternatives
+
+We could use HTTP PATCH rather than POST when sending diffs.  This feels needlessly exotic, imo.
+
+Rather than having a delta_format field, we could use the MIME type of the upload to indicate that it's a patch to a
+given underlying MIME type. However, Matrix doesn't currently have to parse MIME types anywhere, so it's more matrixy
+to destructure this in JSON.
+
+For unencrypted files, the server could apply the diffs serverside as a convenience to clients who don't know
+how to apply the diffs themselves (or who don't have CPU to apply the diffs, or want to benefit from the server caching
+diff results).  This could be proposed as a separate MSC.
+
+## Security considerations
+
+This exposes the metadata of which file is a delta to which other file to the server.
+
+DoS by too many deltas
+
+DoS by using async uploads to create a cycle
+
+## Unstable prefix
+
+| Param        | Unstable prefixed param          | 
+| ------------ | -------------------------------- |
+| delta_base   | org.matrix.msc4083.delta_base    |
+| delta_format | org.matrix.msc4083.delta_format  |
+
+## Dependencies
+
+None. Although [MSC4016](https://github.com/matrix-org/matrix-spec-proposals/pull/4016) was sketched out at the same
+time and the two are siblings.