Virtual-Channel Flow Control

Interconnection network is characterized by its topology, routing, and flow control.

Topology: arrangement of nodes and channels into a graph

Routing: how a packet chooses a path in this graph

Flow control: allocation of channel and buffer resources to a packet as it traveses the path

2 types of resources: buffers and channels.

Typically, a single buffer is associated with each channel.

Virtual channels decouple resource allocation by providing multiple buffers for each channel in the network.

In addition to incresing throughput, wirtual channelss provide an additional degree of freedom in allocating resources to packets in the network. 

Virtual Channels:

vc for deadlock avoidance

output queueing or split input queues for partial resource decoupling.

Flow control protocol:

1) how resources (buffers and channel bandwidth) are allocated

2) how packet collisions over resources are resolved

Collisions:

1) blocking P (a packet) in place

2) buffering P in a node prior to where the collision occurs.

3) dropping P

4) misrouting P to a channel other than the one it requires.

Flits.

flits have no routing or sequencing information, the allocation must be  done in a manner that keeps the flits associated with a particular packet together.

Wormhole Routing:

advance each flit of a packet as soon as it arrives at a node (pipelining) and blocks packets in place when required resources are unavailable.

1) reduces the latency of message delivery compared to store and forward routing.

2) requires only a few flit buffers per node.