NonStop Systems Introduction for H-Series RVUs

Integrity NonStop NS-Series Server Architecture
NonStop Systems Introduction for H-Series RVUs—540083-001
7-10
Low-Latency Routing
provides fault tolerance by ensuring that if one path fails, continuous communication
can continue through the other path.
Either way, the existence of two paths to the I/O hardware ensures continuous
availability for input/output operations.
Low-Latency Routing
In Figure 7-5 on page 7-7 and Figure 7-6 on page 7-9, you can see that sometimes a
message needs to go through several routers to arrive at its destination. This
requirement means that there must be absolutely minimal delay in getting each packet
of a message from one router to the next.
This minimal delay, or low latency, is achieved by having the router first analyze the
routing information in the packet header. Then, before the rest of the packet even
arrives, the router sends the initial bytes of the packet on their way to the next router.
This is called wormhole routing and contrasts with the common store-and-forward
method of routing. (In store-and-forward routing, the full packet is accumulated at each
router before being forwarded to the next one.)
Wormhole routing, illustrated in Figure 7-7 on page 7-11, gets its name from the way a
worm moves through chunks of soil. The worm chews through one chunk of soil and
moves on to the next chunk before its tail has moved through the first chunk. In this
analogy, the ServerNet packet is the worm, and the routers are the chunks of soil.
It takes only three bytes of received information to allow the router to select the right
output port for forwarding the packet to the next router. A single packet could be
spanning several routers simultaneously.
Each router refers to a routing table that helps it determine the correct output port to
use. As shown in Figure 7-7, the router can connect any input port to any output port.
The ports can be used in full duplex mode. When necessary, such as in the case of
network congestion, routers can store up to a full packet (or a small amount more).