Overview
The APPN AIW developed Data Link Switching (DLSw) for tunneling unroutable, non-IP based protocol such as IBM SNA over a TCP/IP network.
Figure 75 shows the DLSw mechanism.
Figure 75: DLSw mechanism
The local DLSw router converts the LLC2 frames from the local SNA device into SSP frames that can be encapsulated in TCP packets.
The local DLSw router forwards the SSP frames across the WAN over a TCP connection to the remote router.
The remote router converts the SSP frames back into LLC2 frames and sends them to the peer SNA device.
DLSw enables the SNA devices to communicate with each other as if they were on the same network.
Different from transparent bridging, DLSw does not forward LLC2 frames transparently to the peer. Instead, it converts LLC2 frames into SSP frames for data encapsulation in TCP packets. The local termination mechanism of DLSw eliminates the requirement for link layer acknowledgments and keepalive messages to flow across a WAN. It also solves the data link control timeout problem.
DLSw also enables transmission of SDLC traffic across a TCP/IP WAN by first converting SDLC frames to LLC2 frames, and then transporting them to the remote end through DLSw. DLSw can interconnect LAN and SDLC media.
The following DLSw versions are available: version 1.0 and version 2.0. DLSw v1.0 is implemented based on RFC 1795, and DLSw v2.0 is implemented based on RFC 2166 and is intended to improve product maintainability and to reduce network cost. In addition, DLSw v2.0 provides enhancements by means of UDP explorer frames sent in multicast and unicast modes. When the peer is also running DLSw v2.0, the two ends can use UDP packets to explore reachability, and a TCP connection is established only when data transmission is required.
SDLC is an IBM data link layer protocol, for use in IBM SNA networks.
For more information on LLC, see the IEEE 802.2 standard.