Intermediate routing

Intermediate routing enables an APPN network node to receive and route data destined for another node. The origin and destination of the data can be an end node, another network node, or a LEN.

Intermediate routing supports sessions between LUs that are not on adjacent nodes. After a route has been selected for a session, APPN network nodes in the route use intermediate routing to forward session data to the next node in the route.

Resource characteristics maintained by the topology database can include congestion status. If a network node becomes heavily congested, the network node can relay this information to other network nodes in the network, making the congested network node less likely to be included in session routes calculated for new sessions.

APPN provides two types of intermediate routing:

• In intermediate session routing (ISR), available in all network nodes, the network node keeps track of each intermediate session. Each intermediate node adjusts the pacing of session data to control the rate at which data flows between adjacent nodes. Each intermediate node can also perform segmentation and reassembly of segmented data. In ISR, once a session route has been established, all data on that session uses the same route. If part of the route fails, the session ends.
• In automatic network routing (ANR), available in network nodes that support APPN's High-Performance Routing (HPR) function, intermediate network nodes can dynamically reroute session traffic if part of the route fails. ANR does not provide intermediate session pacing or segmentation and reassembly.

ANR enables intermediate nodes to route session traffic much faster than is possible with traditional APPN ISR. However, ANR requires additional overhead at the RTP (Rapid Transport Protocol) endpoints. In routes with few intermediate nodes, an ANR route might actually be slower than an ISR route, due to processing time at the endpoints. For routes containing a larger number of intermediate nodes (hops), ANR routes are typically faster. The exact location of the break-even point depends on the efficiency of the RTP nodes.