Original text:
the sender will not have the keys necessary to construct an MLSCiphertext object. <del> 13.1.9.1. </del>  The "external_senders" extension is a group context extension that contains credentials of senders that are permitted to send external

Reviewer comments:
The way the spec is written now, clients need to implement all proposal types, and they must react to appropriately to all valid proposals, including AppAck. AppAck in a large, busy group could result in a situation where clients with longer latency connections to the DS (perhaps in a different country, using satellite, or using some wireless networks) might be effectively prevented from sending any messages, or worse could be prevented from sending AppAcks because their proposals are always a few hundred milliseconds late. I think AppAcks still have a place is relatively small groups with low message volumes, but I have a few suggestions: 1) Describe this issue either in the AppAck proposal section or in the security considerations section 2) I think it is reasonable for a group to consider some proposal types invalid. If there is consensus with this position, I'd like to make this clear with an extra sentence or two. If that is not the consensus, then I will propose that only Add, Remove, and Update are assumed to be supported by each group, and other proposal types have to be explicitly added.
My team had similar comments. I agree that this shouldn't be a 100% necessary thing to support.
I would differentiate between "support" in the sense of "if you receive one, do something sensible with it" vs. "send this under some circumstances". My understanding was that the requirement here was the former, and even that the definition of "sensible" was up to the implementation. The thinking being that if an application wanted to get some anti-suppression properties, it could send and verify these on some schedule, but when exactly they would be sent would be up to the application. Does that alleviate some of the concern here? If so, I would totally be open to clarifying that. To NAME point about latency, it seems like we could add an parameter to AppAck so that you could report on past epochs' messages in a future epoch. On considering proposal types invalid -- I think there has always been a notion that applications might have policies on which proposals are acceptable. Not just at the proposal type level, but at even finer granularities. Think about things like groups where only certain members are allowed to add/remove participants. I would also be fine laying this out more explicitly, though maybe it would be a more natural fit in the architecture document.
Sorry, meant to delete a comment, closed the issue instead.
In the past I've expressed reservations about AppAck being included in the spec. I don't think it's sufficiently general (it only handles a one specific pattern of message loss and won't work for others, per NAME comment), and I don't like that we don't specify how to handle dropped messages in any way. The AppAck functionality is likely to just be re-invented at the application-level because of this. For example, AppAck can't be used to implement read receipts It doesn't help detect the suppression of handshake messages It doesn't handle cases where some messages must be delivered and others may be dropped So instead I would propose removing AppAck from the spec.
I've opened a PR to that effect in .
Virtual interim 2022-05-19 Agreement that the bar for support is quite low ... but you don't get a security property unless members actually do something more than the minimum That sort of implies that you want the group to opt in to this mechanism So we should have a new proposal type in a separate I-D
(To be clear, AppAck should move from the main spec here to its own I-D.)
One comment that renumbering the groupcontextextensions proposal was probably a bad idea from a process perspective which we should avoid doing in the future. In this case it seems unlikely that many people have implemented it who could not make a breaking change.

Revised text:
the sender will not have the keys necessary to construct an MLSCiphertext object.  <ins> 13.1.8.1. </ins> The "external_senders" extension is a group context extension that contains credentials of senders that are permitted to send external