Data Link Switching (DLSw) allows SNA and NetBIOS applications to communicate with other such applications running on a remote LAN. DLSw uses TCP/IP to forward packets between the two LANs. DLSw ensures that the TCP/IP connection is transparent, so that the applications communicate as if they were on the same LAN. The DLSw providers at each end of the TCP/IP link communicate with each other using the Switch-to-Switch Protocol (SSP) defined by the RFC 1795.
SNA and APPN Protocol Support
DLSw propagates SNA LLC Type 2 connections for SNA by intercepting the Type 1 frames used in session establishment (using promiscuous access to the LAN devices) and behaving to the local LLC2 node as if the DLSw provider was the other end of the connection. The LLC2 connection is terminated at the DLSw provider.
Once the connection is established, the two LLC2 nodes send data to each other in the form of LLC2 I-frames, unaware that the data is being
- intercepted at the local DLSw provider
- sent over TCP/IP in SSP format to the remote DLSw provider
- forwarded to the remote node.
A connection established across a pair of DLSw providers in this way is called a circuit.
NetBIOS Support
NetBIOS uses LLC Type 2 connections for its session support. It uses a name-based mechanism for connection establishment, as opposed to the raw MAC address-based mechanism used in LLC Type 2 for SNA.
DLSw permits the establishment of NetBIOS sessions across TCP/IP links in a similar way to LLC Type 2 for SNA, but with the initial explorer traffic being routed by name rather than MAC address.
NetBIOS connectionless frames are also routed by DLSw, including broadcast NetBIOS frames which are copied to all DLSw providers.
Comparison of DLSw with Bridging
DLSw offers similar functionality to bridging (from the perspective of an end node). The key difference between bridging and Data Link Switching is that the latter does not propagate RR/RNR frames across the WAN (utilising the guaranteed delivery feature of TCP/IP) and therefore
- improves the performance of the data path
- reduces link-level timeouts by reducing the round-trip delay on responses.
Coexistence with Bridging
In a general network configuration, two Token Rings connected via DLSw may also be bridged. However the current SSP protocols assume that if parallel bridging and DLSw are used, it is sufficient to let Token Ring Source Routing choose the route.
In such cases, the user must ensure that the Virtual Ring Number (VRN) is identical across all bridged DLSw providers. Failure to do so may result in broadcast frames, such as TEST frames, circulating forever.
If DLSw is used to connect Ethernet networks, the DLSw implementer (or anyone porting SNAP-DLSw) must take care to prevent similar loops by, for example, treating DLSw as one port to the spanning-tree functions used for other protocols within the router.