APPN relies on LLC2 links.
IBM offers an extension to APPN that can optionally operate without LLC2: High Performance Routing (HPR). HPR can operate without an underlying [reliable] data link protocol. Retransmission and flow control is performed end-to-end by a higher layer protocol, similar to TCP within the TCP/IP protocol suite.
HPR traffic that does not operate on top of an LLC2 link can support transmission across Frame Relay links without encountering the issues associated with reliable links, such as SDLC or LLC2. An example of reliable link issues not found with HPR is an SDLC or LLC2 poll timeout.
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