High performance routing (HPR) is an addition to APPN that improves reliability, increases network performance, and was designed to exploit higher link speed technologies.

Intermediate session routing (ISR) requires significant processing for error control, flow control, and segmentation at each intermediate node. The significant processing causes significant latency in each node.

As higher speed connections evolved, the APPN architecture was required to introduce some changes and enhancements to allow switching in intermediate nodes to be done at higher speeds (that is, lower layers) thereby improving the throughput of data.

HPR addresses this by routing at layer 2 and 3 and changing the existing intermediate session routing (ISR) which is done in basic APPN at layer 5. HPR introduced new headers that HPR analyzes to determine the next hop to route the message. Inspecting headers of the higher layer requires more resources, and that affects the performance. As HPR is done in lower layers than ISR, the delay in each node along the path is shortened.

HPR has also shifted the error recovery to the end points, instead of individual lines. The two endpoints are the APPN nodes, end node or network node, that provide for the LU- LU session. With basic APPN, every network node was responsible for recovering from errors on the two links that were used to deliver the data to and from the network node. The error recovery consumed resources and affected performance.

With high speed networking, the reliability of the communication lines improved dramatically. Today the ratio of errors-to-traffic is in the range of 10-9. The probability for error is very low and moving the responsibility for error recovery to the end points improves performance and does not affect the integrity of the data.

HPR has also been designed to provide a non-disruptive path switch to route around failures. In simple words, non-disruptive path switching addressed one of the major deficiencies of SNA compared to other protocols. With non-disruptive path switching, a session is switched to another available path without affecting session availability to the end user.

In the following section we discuss the two major components of HPR, the rapid transport protocol (RTP) and automatic network routing (ANR).