In the sense of switching, head of line blocking is a typical issue that happens mostly in input queuing instead of output queuing. Input/Output queuing means the buffer resides in the front/back of the switching fabric of a certain router/switch. However, here in this example, head of line blocking is given a similar meaning but in a different context.
We cannot fix this problem by making the flit buffer bigger. Even if all the blue and gray packets end up in the bottom right flit buffer, the front of the queue will still be a gray packet which is blocked, and so the blue packets cannot go to the right.
Head-of-line blocking is a problem associated with cut-through flow control (packed-based approach). In this case, the blue packet cannot proceed until the grey packets are gone.
@cmusam No it is actually associated with wormhole flow-control in this slide