[MLS] Two question regarding use of “reinit” in the RFC

[Vera Wesselkamp <[e-mail]>]

Hello,

I have two instances in the RFC where I struggle to understand whether 
the word reinit/reinitialization refers to the reinitialization 
operation defined in section 11.2.

The first is the following use in Section 12.4.3.2:

    /Applications for whom this distinction is salient can choose to
    disallow external commits that contain a Remove, or to allow such
    resync commits only if they contain a *"reinit"* PSK proposal that
    demonstrates the joining member's presence in a prior epoch of the
    group./

I read this as a resumption PSK of type /resumption/ and of usage 
/reinit/ as specified in section 7. However that is direct contradiction 
to the following sentence:

    /A PreSharedKey proposal with type resumption and usage reinit MUST
    BE be considered invalid./

Hence my question: Does the “reinit” PSK proposal actually refer to a 
/reinit/ PSK or would an /application/ PSK be more appropriate for the 
described mechanism?

The second question refers to two appearances of the term in section 
16.12 of the RFC and concerns group splitting:

    /Applications provide mechanisms for failed commits to be reported,
    so that group members who were not able to recognize the error
    themselves can *reinitialize the group* if necessary./

I do not understand how to use the reinitialization operation defined in 
the RFC in this context. My understanding is that the paragraph 
describes a situation in which some members processed a commit 
correctly, but a few members members cannot and are hence stuck in an 
older epoch. Reinitialization would send an reinit proposal to the 
current epoch of the group for the members that could process the 
commit. I believe that the “stuck” members, however, would not be able 
to parse the reinit proposal and following commit, and can hence not 
process the reinitialization.

Similarly a few paragraphs later:

    /These members will need to either add themselves back with an
    external Commit or *reinitialize the group* from scratch./

This sentence also refers to members stuck in an old epoch. If these 
members propose a reinitialization in the old group, the members that 
have already advance to other epochs should equally not be able to 
process such a reinitialization.

I also described a similar situation for a paragraph in the architecture 
file in this issue 
<https://github.com/mlswg/mls-architecture/issues/234> before.

My question is if the use of reinitialization in this sentence refers to 
the operation defined in section 11.2, or just another mechanism, i.e. 
re-joining the group at the “stuck” epoch using external commits. If it 
does refer to the operation, how could the process of reinitializing 
look like and how does it resolve the split group?

Thanks for your help!

Vera