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Instead of having one queue of size 2 (can only wait on the gray packet, can't even look at the blue packet), have multiples queues (in this case 2 queues of size 1) and look at both to see if either can make progress.


In this case, progress for both flits can only be made if both move in different directions (as in the case above). Otherwise only one flit will make forward progress.


Also, to add something mentioned by Hongyi during the final exam review session, having virtual channels doesn't allow you to send 2 different packets in different directions simultaneously. You can still only send one at a time but now you are not limited to serving all packets in the order they arrived in case a link is blocked.