To-device messages have no ordering, so if they're delayed they can arrive out of order, breaking signalling.

[ara4n]

I haven't seen this in the wild, but there's nothing stopping it from happening if a client sets up quickly and blats out m.call.invite & candidates at precisely the same moment - there's a distinct chance the candidate will make it to the recipient before the invite does.

Same as https://github.com/matrix-org/matrix-doc/issues/3630

[ara4n]

Suggestion:

• each to-device event sent by a given device in a given call gets an incremental seq field, starting at 1.
• the receiver should buffer inbound messages if they arrive out of order.
• if messages go missing, the client should log a warning and discard the buffer after a ~10 minute timeout.

[SimonBrandner]

Suggestion:

• each to-device event sent by a given device in a given call gets an incremental seq field, starting at 1.
• the receiver should buffer inbound messages if they arrive out of order.
• if messages go missing, the client should log a warning and discard the buffer after a ~10 minute timeout.

TBH, that just feels like a temporary workaround and I'd rather see a proper solution though I am not really sure how easy fix would that be 🤔

[robertlong]

We now use a seq number and order the messages properly