DLSw, originally a proprietary IBM protocol, was adopted by IETF as a standard. DSLw version 1 (DSLw v1) defines three primary functions:

• The Switch-to-Switch Protocol (SSP) is the protocol maintained between two DLSw nodes or routers.
• The termination of SNA data-link control (DLC) connections helps to reduce the likelihood of link layer timeouts across WANs.
• The local mapping of DLC connections to a DLSw circuit.

DLSw version 2 (DLSw v2) was introduced in 1997 in IETF, which provides the following enhancements to the version 1:

• IP multicast
• UDP unicast responses to DLSw broadcasts
• Enhanced peer-on-demand routing
• Expedited TCP connections

Each of these features enables DLSw as a scalable technology over WANs. In DLSw Version 1, transactions occur with TCP. As a result, many operations in a DLSw environment consumed circuits between peers. For example, a multicast required multiple TCP connections from the source to each peer. With DLSw Version 2, multicast is distributed using unreliable transport following traditional multicast methods.

Cisco supports a third version of DLSw called DLSw+. DLSw+ predates DLSw Version 2 and provides even further enhancements to basic DLSw.

• Version number - Set to 0x31 (ASCII 1) indicating a decimal value of 49. This is used to indicate DLSw version 1.
• Header length - Set to 0x48 for control messages and 0x10 for information and Independent Flow Control messages.
• Message length - Specifies the number of bytes within the data field following the header.
• Remote data link correlator - Works in tandem with the remote DLC port ID to form a 64-bit circuit ID that identifies the DLC circuit within a single DLSw node. The circuit ID is unique in a single DLSw node and is assigned locally. An end-to-end circuit is identified by a pair of circuit IDs that, along with the data-link IDs, uniquely identifies a single end-to-end circuit.
• Remote DLC port ID - Works in tandem with the remote data-link correlator to form a 64-bit circuit ID that identifies the DLC circuit within a single DLSw node. The contents of the DLC and DLC Port ID have local significance only. The values received from a partner DLSw must not be interpreted by the DLSw that receives them and should be echoed "as is" to a partner DLSw in subsequent messages.
• Message type - Indicates a specific DLSw message type. The value is specified in two different fields (offset 14 and 23 decimal) of the control message header. Only the first field is used when parsing a received SSP message. The second field is ignored by new implementations on reception, but it is retained for backward compatibility.
• Flow control byte - Carries the flow-control indicator, flow-control acknowledgment, and flow-control operator bits.