Wormhole Switching - Example

Example

A wormhole flow control transmission may work as follows. Each node contains a router that will determine which path the packet will take through the network and holds the virtual channel state information:

1. A packet, P_a at an upstream node, say N₁, attempts to allocate an input virtual channel on a downstream node, N₂, to reach its destination, N₃. The input VC (Virtual Channel) at each side of each node (call them N,S,E,W) will hold flit buffers and, in this case, will specify whether this input virtual channel is waiting, idle, or active. It will also specify which output virtual channel we are attempting to acquire. An output VC will hold information about only which input virtual channel it is reserved by.
2. P_a's header flit arrives at N₂'s West input VC, which happens to be in the idle state, so assuming we can buffer two flits, P_a's header flit and first body flit are buffered.
3. P_a wants to use N₂'s East output VC to reach N₃ so it specifies that in the VC state, but this output VC is currently being used by some other packet, P_b coming from the North. P_a is now blocked, so the West input VC on N₂ will enter the wait state. Note that N₂'s East output VC will specify that it is reserved by the North input VC. N₁ cannot send any more flits to N₂ now because the flit buffer is full.
4. P_b finishes transmitting and N₂'s East output VC becomes available.
5. P_a can now transmit to N₃ so the West input VC enters the active state, and the East output VC specifies that it is reserved by W.
6. P_a continues this transmission process until it reaches its destination.

Note that when P_a was blocked by P_b, the upstream node could not transmit any more packets downstream. This may extend upstream all the way to the source node as flit buffers fill up due to the blocking. This is an example of backpressure.