Data Link Switching (DLSw) is a routing method developed by IBM and described in RFC 1434. Using DLS switching, SNA data and NetBIOS can be encapsulated in the Transmission Control Protocol (TCP) and dynamically routed in the WAN area.
One goal of the DLS specification was to make the SNA architecture routable without having to change the data link software in the end devices. DLS switching terminates the LLC session at the LAN-WAN transition using two-way local polls and acknowledgments. Data Link Switching supports SNA PU types 2, 4, and 2.1 and optionally Netbios.
An important aspect for SNA routing is the Synchronous Data Link Control (SDLC) used in the SNA architecture, which mandates Source Route Bridging (SRB) when messages are to be distributed in a network. In this process, messages are sent to all network stations, which leads to bottlenecks.
Before a single data packet can even be exchanged between the communication partners, the intermediate data link switches must have established a TCP connection. For this purpose, a data link switch keeps a list of all data link switches known to it.