network protocols handbook.pdf

297Protocols GuideIBM ProtocolsProtocol NameDLSw: Data-Link SwitchingprotocolProtocol DescriptionData-link switching (DLSw) provides a forward mechanism fortransporting IBM Systems Network Architecture (SNA) and networkbasic input/output system (NetBIOS) traffic over an IP network.DLSw does not provide full routing, but instead providesswitching at the SNA Data Link layer (i.e., layer 2 in the SNAarchitecture) and encapsulation in TCP/IP for transport over theInternet.DLSw, originally a proprietary IBM protocol, was adopted byIETF as a standard. DSLw version 1 (DSLw v1) defines threeprimary functions:• The Switch-to-Switch Protocol (SSP) is the protocolmaintained between two DLSw nodes or routers.• The termination of SNA data-link control (DLC) connectionshelps to reduce the likelihood of link layer timeoutsacross WANs.• The local mapping of DLC connections to a DLSw circuit.DLSw version 2 (DLSw v2), which was introduced in 1997 inIETF, provides the following enhancements to the version 1:• IP multicast• UDP unicast responses to DLSw broadcasts• Enhanced peer-on-demand routing• Expedited TCP connectionsEach of these features enables DLSw as a scalable technologyover WANs. In DLSw Version 1, transactions occur with TCP.As a result, many operations in a DLSw environment consumecircuits between peers. For example, a multicast requires multipleTCP connections from the source to each peer. With DLSwVersion 2, multicast is distributed using unreliable transport followingtraditional multicast methods.Cisco supports a third version of DLSw called DLSw+. DLSw+predates DLSw Version 2 and provides even further enhancementsto basic DLSw.Protocol StructureVersionnumber8 16 24 32bitHeader LengthRemote data link correlatorRemote DLC port IDMessage LengthReserved FieldMessage typeFlow controlbyte• Version number - Set to 0x31 (ASCII 1) indicating adecimal value of 49. This is used to indicate DLSwversion 1.• Header length - Set to 0x48 for control messagesand 0x10 for information and Independent Flow Controlmessages.• Message length - Specifies the number of bytes withinthe data field following the header.• Remote data link correlator - Works in tandem withthe remote DLC port ID to form a 64-bit circuit ID thatidentifies the DLC circuit within a single DLSw node.The circuit ID is unique in a single DLSw node and isassigned locally. An end-to-end circuit is identified bya 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 remotedata-link correlator to form a 64-bit circuit IDthat identifies the DLC circuit within a single DLSwnode. The contents of the DLC and DLC Port ID havelocal significance only. The values received from apartner DLSw must not be interpreted by the DLSwthat receives them and should be echoed “as is” to apartner DLSw in subsequent messages.• Message type - Indicates a specific DLSw messagetype. The value is specified in two different fields(offset 14 and 23 decimal) of the control messageheader. Only the first field is used when parsing areceived SSP message. The second field is ignoredby new implementations on reception, but is retainedfor backward compatibility.• Flow control byte - Carries the flow-control indicator,flow-control acknowledgment, and flow-control operatorbits.Related protocolsSDLC, NetBIOS, TCP, SMP, Ethernet, Token Ring, SNASponsor SourceData-Link Switching (DSLw) was originated by IBM and adoptedas a standard by IETF.Referencehttp://www.javvin.com/protocol/rfc1795.pdfData Link Switching: Switch-to-Switch Protocol AIW DLSw RIG:DLSw Closed Pages, DLSw Standard Version 1.0http://www.javvin.com/protocol/rfc2166.pdfDLSw v2.0 Enhancements