Local Area Transport (LAT) Specification Part No. AA-NL26A-TE A simple, efficient, transparent model for exchanging data between terminals c=r.· nected to terminal servers and host operating system processes is described. Tr.e model Is termed Local Area Transport (LAn. LAT Is carefully tailored to take ac­ vantage of the environment offered by Local Area Networks. such as the Ethernet data link, but maintains much of the simplicity of traditional methods of connec~ing terminals and hosts. Digital Equipment CorporatlonIProprietary and Confidential The information in this document is subject to change without notice and should not be construed as a commitment by Digital Equipment Corporation. DigitaJ Equipment Corporation assumes no responsibility for any errors that may appear in this document. June 1989 Copyright C1989 Digital Equipment Corporation All rights reserved. The following are trademarks of DIGITAL Equipment Corporation: DEC ~T RT pECmate MASSBUS ULTRIX DECnet PDP UNIBUS DECUS P/OS VAX DECWiiter Profe.sional VAXcluater DmOL Rainbow VMS !amDDma- RSTS VT DN,. RSX Work Processor This Inanuat was P§dUcec(!!i Networks and Communications Publicati~ ii f Thit document was prepared using VAX DOCUMENT, Version 1.1 Contents Preface 1 Introduction 1.1 Terminology . 1-5 2 Architecture Overview 2.1 Introduction ................................................ 2-1 2.2 SLOT LAYER - USER INTERFACE ............................. 2-5 2.2.1 Connecting to the Host Service .............................. 2-6 2.2.2 Connecting to the Terminal Server Service ..................... 2-7 2.3 VIRTUAL CIRCUIT LAYER ................................... 2-9 2.4 Product Considerations ...................................... 2-11 2.4.1 Host .................................... _./ ............•...... 2-11 2.4.2 Ternlinal Server ............................................ :' .. ·2-12 3 Naming and Translation 3.1 Naming Conventions .................... ~ ............•<i ••••.•• 3-1 3.1.1 Service Name Translation Process ......................... ;"'-.~ 3-1 3.1.2 Translation Process On A Source Node ....................... 3-4 3.1.3 Translation Process On A Destination Node .................... 3-5 3.2 Service Advertising Mechanisms ............................... 3~6 3.2.. 1 Host Advertising .......................................... 3-6 3.2.2 Terminal Server Advertising ................................. 3-8 3.3 Specification of Nanles ....................................... 3-9 3.4 Specification of Text ......................................... 3-11 III 4 Circuit and Session Layers 4.1 Architectural Model .......................................... 4-1 4.1.1 Slot Data ................................................. 4-1 4.1.2 Asynlnletry ............................................... 4-1 4.1.3 Virtual Circuit Service ...................................... 4-2 4.1.3.1 Virtual Circuit State .................................... 4-2 4.1.3.2 Architecturally Controlled Names and Variables ............. 4-4 4.1.3.3 Message Types ....................................... 4-11 4.1.3.4 Cirtual Circuit State Variables ........................... 4-12 4.1.3.5 Response Requested Flag and Balanced Mode .............. 4-13 4.1.3.6 Message Mapping Onto State Diagram ................... 4-13 4.1.4 User Connection Management And Data Flow ................ 4-19 4.1.4.1 Service Classes ..................................... .. 4-19 4.1.4.2 Host Session Management .............................. 4-19 4.1.4.3 Multiplexing Over A Virtual Circuit ...................... 4-20 4.1.4.4 Slot Ordering Within Messages .......................... 4-20 4.1.4.5 Slot State Variables .................................... 4-20 4.1.4.6 Ternlinal Server Slot Mapping Onto State Diagranl ......... 4-22 4.1.4.7 Terminal Server Slot State Table ......................... 4-23 4.1.4.8 Host Slot Mapping Onto State Diagram ................... 4-24 4.1.4.9 Host Slot State Table ................................... 4-25 4.2 Layer Illterfaces ............................................ 4-26 4.2.1 Data Types .............................................. 4-27 4.2.2 User/Slot Layer Interface ................................... 4-28 4.2.2.1 Summary Of Functions ................................. 4-28 4.2.2.2 Description Of Functions ............................... 4-29 4.2.3 Slot/Virtual Circuit Layer Interface ........................... 4-32 4.2.3.1 Summary Of Functions ................................. 4-32 4.2.3.2 Description Of Functions ............................... 4-33 4.3 Axioms And Algorithms ..., ........ '.......................... 4-35 4.3.1 Virtual Circuit Layer ....................................... 4-37 4.3.1.1 Circuit Starter (Terminal Server Only) ..................... 4-37 4.3.1.2 Data_Volunteered ...................................... 4-37 4.3.1.3 Credits Returned ...................................... 4-38 4.3.1.4 Orcuit Ender ......................................... 4-38 4.3.1.5 Message Receiver ...................................... 4-38 4.3.1.6 Message Transnlitter ................................... 4-40 4.3.1.7 Circuit Tinler Policy .................................... 4-42 4.3.1.8 Buffering ............................................. 4-42 iv 4.3.2 Slot Layer ............................................... 4-43 4.3.2.1 Host Systenl Management .............................. 4-43 4.3.2.2 Terminal Server System Management ..................... 4-43 4.3.2~3 Session Starter (Tenninal Server) ........................ 4-44 4.3.2.4 Session Starter (Host) .................................. 4-44 4.3.2.5 Slot Deolultiplexer ..................................... 4-44 4.3.2.6 Slot Multiplexer ....................................... 4-45 4.3.2.7 Sessioll Ellder ......................................... 4-47 4.3.2.8 Flow Control ......................................... 4-47 4.3.2.9 Protocol Versions And ECO Control ...................... 4-48 4.3.3 Otl,er Processes .......................................... 4-49 4.3.3.1 Keep-Alive Process .................................... 4-49 4.3.3.2 Progress Process ...................................... 4-49 4.4 Message Forolats ........................................... 4-50 4.4.1 Virtual Circuit Message Header ............................. 4-51 4.4.1.1 Start Message Format .................................. 4-52 4.4.1.2 Run Message Forolat ................................... 4-55 4.4.1.3 Stop Message Fonnat .................................. 4-62 5 Connection Solicitation 5.1 Architectural Model .......................................... 5-1 5.1.1 Service Sharing ......•.................................... 5-2 5.1.1.1 Queue Coordination .................................... 5-4 5.1.1.2 Queue Access . .. 5-5 5.1.1.3 Queue Structure ........................................ 5-6 5.1.1.4 Queue Operations ...................................... 5-7 5.1.1.5 Concatenating The Status Entries ........................ 5-12 5.1.1.6 Retranslnission And TiIne-out Policies .................... 5-12 5.1.2 Connection Initiation ...................... , ............... 5-13 5.1.2.1 Solicitation Process Message Flow ........................ 5-14 5.1.2.2 Solicitation process state-tables .......................... 5-24 5.1.2.3 Name And Inforolation Field Presentation ................. 5-30 5.2 Message Formats ........................................... 5-31 5.2.1 Command Message ....................................... 5-32 5.2.2 Status Message ...................................... ',..... 5-36 v A Service Class 1 - Interactive And Application Terminals. A.l Local Area Directory Service .................................. A-l A.2 Service Access Control ...................................... A-2 A.3 Advertising Services Through Multicast Message ................. A-6 A.3.1 Host .................................................... A-6 A.3.1.l Initialization .......................................... A-6 A.3.1.2 Host Group Codes ..................................... A-7 A.3.1.3 Host Node Nanles ..................................... A-7 A.3.l.4 Multiple-Node Service Ratings. .. A-7 A.3.l.S Steady-State Operation ................................. A-7 A.3.1.6 System Shutdown .. .. A-8 A.3.2 Ternlillal Server .......................................... A-8 A.3.2.l Initialization .......................................... A-8 A.3.2.2 Building The Circuit Nanle Database ...................... A-9 A.4 Advertising Thr.ough Solicitation and Response Messages ........ A-11 A.4.l A Node Operating In Slave Mode .......................... A-12 A.4.2 A Node Operating In Master Mode ......................... A-13 A.4.3 Response Information Message Policy. .. A-14 A.S Service Class 1 Messages ................................... A -15 A.S.l Service Announcenlent Message ...................... .. A-1S A.S.2 Solicit Infornlation Message . .. A-19 A.S.3 Response Infornlation Message ............................ A-22 A.6 Service Class 1 Slot Format Extensions ........................ A-28 A.6.1 Start Slot Status Field .................................... A-29 A.6.2 Attention Slot Status Field ................................ A-31 A.6.3 Data_b Slot Extension .................................... A-33 A.6.3.1 Infornlation Exchange Using Data_b Slots ................ A-33 A.6.3.2 Data_b Slot Fornlat ................................... A-34 A.6.3.3 Guidelines And Reconlnlendations For Data_b Slot ProcessingA-38 B Compatibility and Implementation B.l Implementation Issues. .. B-1 B.l.l Possible