[MLS] Message ordering

["Alexey Ermishkin" <[e-mail]>]

I'd like to discuss the practical part of the requirement that MLS puts on
all group participants.
Hope that I haven't missed such discussion already.

All messages (not only state change messages) must have a counter field
which must be unique among all the messages and server must reject messages
that have this field duplicated.
This might be ok for a group of 3. But as MLS targets groups up to 50k
users, I believe  the percent of rejected messages will dramatically
increase and affect user experience.
I know that ART or TreeKem is, in the end, supposed to be bound to double
ratchet's KDF chain which have one sequence of message numbers per "epoch"
but that clearly won't work for large groups where participants will
constantly have to work on some "consensus" during communication.
I believe there's better solution to this problem. Maybe we should consider
making a unique KDF chain (prefix?) for each group member and perform
timestamp-based ordering, I'm not sure.

Regards, 
Alex