CDN is disabled for partial/range downloads (streaming_offset != 0)

[cdxf]

Commit: 89d7374 (Jan 8, 2020) File: td/telegram/files/FileDownloader.cpp:163-168

if (streaming_offset == 0) {
  cdn_supported = true;
}

The commit message says: "Temporarily disable CDN if streaming is enabled." — marked as temporary but has been in place for ~6.5 years with no follow-up.

cdn_supported is initialized to false and only set to true when streaming_offset == 0. Any download with offset > 0 or a bounded limit results in cdn_supported=false in the upload.getFile request, so the server never replies with upload_fileCdnRedirect. The download goes through the origin DC instead of a CDN edge.

Questions:

1. Protocol limitation or missing implementation? — Is there a fundamental MTProto restriction where CDN servers reject offset > 0 requests? The upload.getCdnFile TL schema accepts a plain int offset parameter, which suggests partial CDN reads should work?

2. Would accepting the redirect and then requesting the partial range from the CDN work? — If the initial upload_getFile(cdn_supported=true) returns an upload_fileCdnRedirect, the client could use upload_getCdnFile(offset, size) with the non-zero offset directly against the CDN. Is there any known server-side issue with this?

3. Is there intent to re-enable this?

[levlam]

Protocol limitation or missing implementation?

The latter. Current implementation doesn't support streaming of files from CDN.

Would accepting the redirect and then requesting the partial range from the CDN work?

Definitely, no. All data received from CDN must be verified before it can be used.