MLS Interim 2024-12-19

Cleaning up MLS / Application interface - Richard Barnes

• "MLS Extension" = "something that changes MLS behavior"
• A lot of the extensions we've discussed are just application logic
• App API Document: Expose safe APIs for application logic
• Introduces application "components" -- idea that single application
  may have multiple different integration points
• ApplicationData proposal type: opaque data gets included in MLS
  transcript, but doesn't affect MLS at all
• ApplicationDataDictionary: Single new extension that can go into
  various extension points (GroupContext, GroupInfo, etc) containing
  application data
• ApplicationDataUpdate: Proposal that supports sending patches of
  application data to GroupContext.application_data; helps when
  GroupContext.application_data is large

• Should AppDataUpdate diff or replace?

  • Rohan: In either case, you need an application callback to
    verify end-state application data. So there's not a lot of gain
    to not allowing diffs. Should be up to the application whether
    diff or whole replacement.

• Do we even need GCEDiff? Is there any place where the data is large
  enough to justify diffs?

  • Rohan: As an example: external senders, which could contain
    multiple credentials. I could see why someone would want that,
    but I don't see why it's necessary to have diff semantics.

Is this the right framework?

• Raphael: Agree that things that don't change MLS behavior aren't
  really extensions. There's very little in mls-extensions that
  actually changes MLS behavior. Should we just rename the document?
  Someone can make a proposal
• Raphael: I'm not sure we totally agree on what does/doesn't change
  MLS behavior. Also would like to discuss with Konrad before we throw
  a lot of the extensions document away.
• Britta: Like general direction. In terms of reducing complexity,
  terminology still needs to be improved.
• Rohan: Also in favor of this. My focus is on having something that
  replaces AppSync.
• Britta: In terms of feedback on app data containers, it's not clear
  how metadata will be handled.
• Richard: Next step would be for authors to flush out more
  comprehensive proposal and present in January maybe.