gtps2m1oACF/SNA Data Communications Reference

Benefits of Using HPR Support

The following table shows some of the important benefits of HPR support by comparing it to traditional SNA (PU 5 and base PU 2.1 support).

Table 5. Comparing Traditional SNA Support to HPR Support

Traditional SNA HPR Support
If any node along the LU-LU session path fails, the session fails. If any node along the LU-LU session path fails, a path switch is automatically performed to obtain a new route for the session. No data is lost during the path switch, no operator intervention is required, and the whole process is transparent to end users.
In PU 5 networking, flow control is end-to-end using fixed-size window-based pacing. In PU 2.1 networking, flow control is performed on a hop-by-hop basis using an adaptive-sized, window-based pacing algorithm. Flow control is performed on an end-to-end basis using an adaptive method that is time-based rather than window-based. This method is much more effective than window-based flow control mechanisms and is better at reacting to change to prevent network congestion.
Intermediate nodes along the session path have full session awareness. Storage for session control blocks must be allocated on every intermediate node for every session passing through the node. Intermediate nodes have no session awareness and, therefore, do not require any storage for session control blocks.
There is reliable hop-to-hop delivery. Each node along the path is responsible for making sure that the data has been received by the next node along the route, the data arrives in the correct order, and no duplicate data is sent. If data is too large to be sent across the next hop, the node will segment the data into smaller pieces and the adjacent node will reassemble the data into one message again. There is reliable end-to-end delivery. Intermediate nodes no longer examine the data. Instead, intermediate nodes simply forward the data to the next node and are not concerned whether the adjacent node receives the data. Intermediate nodes are no longer responsible for detecting lost data, segmentation and reassembly of messages, or any other aspect of reliable delivery. These functions are performed only by the two end points rather than by every node along the path, which greatly improves end-to-end throughput and performance.

Traditional SNA support is connection oriented. The primary weakness of connection-oriented protocols is single points of failure in the network. While protocols that are not connection oriented do not suffer from single points of failure, they lack the correct flow control mechanisms, especially in high-volume networks. HPR support is a mixture of connection-oriented and connectionless protocols. In doing so, the architecture incorporated the best features of both protocol types.