DLSw: Data-Link Switching protocol
Data-link switching (DLSw) provides a forward mechanism of transporting IBM Systems Network Architecture (SNA) and network basic input/output system (NetBIOS) traffic over an IP network. DLSw does not provide full routing, but instead provides switching at the SNA Data Link layer (i.e., layer 2 in the SNA architecture) and encapsulation in TCP/IP for transport over the Internet.
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.
Protocol Structure - DLSw: Data-Link Switching protocol Header
| 8 | 16 | 24 | 32 bit |
| Version number | Header Length | Message Length | |
| Remote data link correlator | |||
| Remote DLC port ID | |||
| Reserved Field | Message type | Flow control byte | |
- 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.
Related Protocols
SDLC , NetBIOS , TCP , SMP , Ethernet , Token Ring , SNA
Sponsor Source
Data-Link Switching (DSLw) was originated by IBM and adopted as a standard by IETF.
Reference
http://www.javvin.com/protocol/rfc1795.pdf : Data Link Switching: Switch-to-Switch Protocol AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1.0
http://www.javvin.com/protocol/rfc2166.pdf : DLSw v2.0 Enhancements
http://www.javvin.com/protocol/rfc2166.pdf : DLSw v2.0 Enhancements
