• A sends unicast messages to B
• A's send-window for B is full: all sender threads sending messages to B block
• B dropped one or more ACKs (e.g. because `TransferQueueBundler`'s queue was full and `drop_when_full==true`)
• B will not send further ACKs to A --> A will be blocked forever or until the send-window to B is closed

Solutions

1. B sends its highest ACK to A every `xmit_interval` ms
2. A blocks for a given time and sends a `SEND-ACK` message to B if no ACKs have been received. This is similar to `FlowControl.max_block_time`

Investigate whether NAKACK4 is also affected by this (it probably is)