HP-UX SNAplus2 R7 Administration Guide

SNA Terms and Concepts
Basic APPN Concepts
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.
Direct Connectivity
Direct connectivity enables session traffic to travel directly between two nodes without the need for an APPN network
node to route the session. In general, sessions between directly connected nodes can exchange data more quickly
than sessions for which data is routed through a network node. For nodes on a shared-access transport facility
(SATF)—for example, for nodes on a token ring as shown in Figure 1–9,
APPN Network Using a Shared-Access
Transport Facility—efficiency would be increased by defining links between every pair of nodes in your network.
However, this can be a difficult task—the number of link stations is n
(n−1), where n is the number of nodes in
the network.
An APPN network on a token ring is shown in Figure 1–9, APPN Network Using a Shared-Access Transport Facility.
Figure 1–9 APPN Network Using a Shared-Access Transport Facility
End Node 1 (EN1)
Link Definitions Needed for Node 1 EN1:
EN1 to NNA
EN1 to EN2
EN1 to EN3
EN1 to EN4
EN1 to EN5
End Node 5 (EN5)
End Node 4 (EN4)
End Node 3 (EN3)
End Node 2 (EN2)
Network Node A (NNA)
If Node EN1 has a link definition for each of the links in the network, it can establish a direct link to any node.
The link definitions needed to support direct links between Node EN1 and every other node in the APPN network
are shown in Figure 1–10, Definitions Needed for Direct Links from Node EN1 to Every Node in an APPN Network.
For a network that includes five other nodes, Node EN1 needs five link definitions:
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