Alright, I can somewhat reliably reproduce this and it's definitely an issue for large data transfer. It looks like all I need to do is send as much data to fill the buffer of the remote peer. In our case, the remote peer (the Threema app) is uploading data elsewhere in a blocking manner which will therefore completely stall the flow of incoming data. Michael, I will send you a packet trace and Firefox log in private since this may contain sensitive information (even though I've done this on a test device).
Bug 1646715 Comment 4 Edit History
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Alright, I can somewhat reliably reproduce this and it's definitely an issue for large data transfer. It does look like the large packet is indeed coming from usrsctp and all I need to do to trigger it is send as much data to fill the buffer of the remote peer. In our case, the remote peer (the Threema app) is uploading data elsewhere in a blocking manner which will therefore completely stall the flow of incoming data. Michael, I will send you a packet trace and Firefox log in private since this may contain sensitive information (even though I've done this on a test device).