Native SNA X.25 DTEs connect to IBM routers using the Qualified Logical Link Control (QLLC) transport protocol. The routers then propagate the QLLC traffic across the TCP/IP network using the XTP transport protocol. A peer router extracts the encapsulated QLLC packets from the XTP packets and forwards it on to a peer SNA X.25 DTE.
If the TCP/IP target is a host rather than a peer router, the origin router passes native SNA traffic across the TCP/IP network using the Downstream Logical Unit (DSLU) transport protocol. All QLLC processing is simulated by the origin router. The host resident TCP/IP stack extracts the SNA PIUs from the DSLU packets (see figure 1 of this document).
______________________________________________________________________ | | | Without X.25 HostNAS | | | | TCP/IP X.25 | | HOST NETWORK ROUTER NETWORK QLLC PAD | | _____________ _____________ _______ | | | | | | SNA | | | | QLLC | | | | | <-> <-> |<----------->| S | | Q |<---------->| |<-->| | | | | | | | N |\ | L | | | | | | | C | V | T | | A | \ | L | | | | | I | T | C | | | \| C | | | | | C | A | P | |---| | | | | | | S | M | I | | | /| P | SNA PU | | | | | | P |<-->| |<-->| X | / | A | _______ | | | | | | | | | | T |/ | D | | | | | | | | | | | | | P | | | | |<-->| | | | | | | | | | | | | | | | | D| |X | | S| |T | | L| |P | | U| | ROUTER QLLC PAD | | | | _____________ _______ | | | | | | | | QLLC | | | | | | | S | | Q |<---------->| |<-->| | | |<--+----| N |\ | L | | | | | | | | A | \ | L | | | | | | | \| C | | | | | |---| | | | | | | | | /| P | SNA PU | | | | | X | / | A | _______ | | | |<-->| T |/ | D | | | | | | | P | | | | |<-->| | | | | | | | | | | | | | | | | SNA device traffic is carried across the X.25 network in QLLC | | packets. The IBM router determines if its peer is a host or | | another router. If the peer is a router, the QLLC packets | | are encapsulated in XTP before they are passed to the TCP/IP | | network. If the peer is a host, the router takes the role of | | peer QLLC PAD and extracts the SNA data from the QLLC packets. | | The native SNA data is then encapsulated in DSLU packets | | before it is passed to the TCP/IP network. The host resident | | TCP/IP stack extracts the SNA data and passes it on to VTAM. | | |
Figure 1: Native SNA traffic in a hybrid X.25-TCP/IP Network
without HNAS
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Last Update - December 12, 2001