network protocols handbook.pdf

257Protocols GuideISO Protocols - Application LayerProtocol NameISO-SP: OSI Session LayerProtocolProtocol DescriptionThe OSI Session Layer Protocol (ISO-SP) provides sessionmanagement, e.g. opening and closing of sessions. In case ofa connection loss it tries to recover the connection. If a connectionis not used for a longer period, the session layer may closeit down and re-open it for next use. This happens transparentlyto the higher layers. The Session layer provides synchronizationpoints in the stream of exchanged packets.The Session Protocol Machine (SPM), an abstract machine thatcarries out the procedures specified in the session layer protocol,communicates with the session service user (SS-user)through an session-service-access-point (SSAP) by means ofthe service primitives. Service primitives will cause or be theresult of session protocol data unit exchanges between the peerSPMs using a transport connection. These protocol exchangesare effected using the services of the transport layer.Session connection endpoints are identified in end systems byan internal, implementation dependent, mechanism so that theSS-user and the SPM can refer to each session connection.The functions in the Session Layer are those necessary to bridgethe gap between the services available from the Transport Layerand those offered to the SS-users.The functions in the Session Layer are concerned with dialoguemanagement, data flow synchronization, and data flow resynchronization.These functions are described below; the descriptions aregrouped into those concerned with the connection establishmentphase, the data transfer phase, and the release phase.Protocol StructureISO Session Layer Protocol Messages:KernelFunctional unit SPDU code SPDU nameCNOACDOACRFFNDNABAADTPRCONNECTOVERFLOW ACCEPTCONNECT DATA OVERFLOWACCEPTREFUSEFINISHDISCONNECTABORTABORT ACCEPTDATA TRANSFERPREPARENegotiated releaseHalf-duplexDuplexNFGTPTGTPTNOT FINISHEDGIVE TOKENSPLEASE TOKENSGIVE TOKENSPLEASE TOKENSNo additional associatedSPDUsExpedited data EX EXPEDITED DATATyped data TD TYPED DATACapability dataexchangeMinor synchronizeSymmetric synchronizeData separationMajor synchronizeResynchronizeExceptionsActivity managementCDCDAMIPMIAGTPTMIPMIAMAPMAAPRGTPTRSRAPREREDASARAIAIAADADAAEAEAPRGTPTGTCGTARelated protocolsISO-TP, ISO-PP, CONP, CLNPCAPABILITY DATACAPABILITY DATA ACKMINOR SYNC POINTMINOR SYNC ACKGIVE TOKENSPLEASE TOKENSMINOR SYNC POINTMINOR SYNC ACKNo additional associatedSPDUsMAJOR SYNC POINTMAJOR SYNC ACKPREPAREGIVE TOKENSPLEASE TOKENSRESYNCHRONIZERESYNCHRONIZE ACKPREPAREEXCEPTION REPORTEXCEPTION DATAACTIVITY STARTACTIVITY RESUMEACTIVITY INTERRUPTACTIVITY INTERRUPT ACKACTIVITY DISCARDACTIVITY DISCARD ACKACTIVITY ENDACTIVITY END ACKPREPAREGIVE TOKENSPLEASE TOKENSGIVE TOKENS CONFIRMGIVE TOKENS ACKSponsor SourceThe ISO-SP (OSI Session Layer protocol) is defined in ISO(www.iso.org) documents 8326 and 8327 and ITU (www.itu.org)documents X.215, X.225, X.235.