DTJ Number 3 September 1987 - Digital Technical Journals

New Productsthat may be used. Rather than requiring that eachterminal server possess this information a priori,LAT provides a mechanism that permits eachserver to "learn" about the configuration.To accomplish this learning process, an additionalmessage type is used, the "service advertisement."This message is multicast from eachslave node to all master nodes and gives thenames of all services that the slave node is currentlyoffering. (A multicast message is a singlemessage addressed to and received by multiplenodes.) An advertisement is transmitted periodically,typically every 60 seconds. Thus on startup,a server can "listen" for service advertisementsand build a directory of available services.This directory can then be presented to the user,on demand, enabling him to choose whicheverservices he wants from those available when aconnection to a host system is desired.Service names, the names used to gain accessto the appropriate service access points, are notlimited to the name of the node on which the serviceis offered. Indeed, there is no restrictionthat any node may offer just one single service.Instead, LAT allows a given node to offer multipleservices.One common use for multiple service names isin a VAXcluster environment. Here the clustermanager can choose to offer as a service a namerepresenting the logical name of the cluster, inaddition to (or instead of) each individual nodename. When a user requests a connection to theservice name representing the cluster, the terminalserver can select one of the available nodes.In this case all nodes offering the same servicewill be presumed to be offering identical capabilitiesto the user.To assist the terminal server in choosing anode, the service nodes provide a "rating" associatedwith each service offered. The rating is anumeric value from 0 to 255 that representssome measure of the resources available to applyto that service. For example, the current VMSLTDRIVER implementation takes into accountthe most recent CPU idle time, the CPU type, theamount of memory, and the number of remaininginteractive job slots. VMS LTDRIVER alsoallows the system manager to specify a rating.The terminal server can then choose, at anyinstant, the node that offers a requested servicewith the highest rating and use that node asthe one to which to form the connection. Thischoice ensures that the load can be shared amongthe nodes in a VAXcluster system. The users neednot be aware of the current configuration of thecluster in order to form a connection.By carefully managing the service advertisements,the server makes the service directoriesreflect the current service list and their associatedratings. If a server fails o hear from a serviceprovider for some period, t?e server can assumethat the service provider has failed, or crashed.IThe server can then remove the service from itsdirectory of available servies.Note that this multicast naming service is alsoasymmetric; the master nodes do not send multicastadvertisements to the slave nodes. A recentaddition to the LAT protocol allows a slave to utilizea different multicast message to determine ifa given node name exists on the LAN . This techniqueis used so that host sytems can find terminalservers (in order to solibt connections fromtheir ports, described later) I by knowing only thename, not the specific Ethernet address, of theserver.Some details of this naming service deservefurther discussion. For example, the LAT "loadbalancing"and "fail-over" features are mostoften associated with VAXcluster systems. However,although they enhance Digital's VAXclusteroffering, these LAT features are independentof it.i"Equivalent services" my also be offered byImultiple nodes using the directory service. Considerservices that are net\vork based, such asvideotext and dial-out modems. With an EthernetLAN, many independent nodes might offer suchservices; typically, however, users can access theservice only through nodes on which they haveaccounts. If a user's system is down, he is deniedaccess to the service, even though the serviceremains available on other nodes. For example,consider a videotext-basbd service, such asLIVE_ WIRE (an in-house j electronic bulletinboard) , that can be offered by many independentLAT host systems. If a LAT user connects toLIVE_WIRE, the terminal server software willdetect that the service is offered from multiplesources. The software will then make a connectionto the source believed to be currently offeringthe best level of service . If that serviceshould fail (i.e., stops sending Ethernet LAT messages), the terminal server s 1bftware will automaticallyreconnect the user toi an alternate providerof the same service if one1 exists; this action isknown as fail-over.Digital TecbnicalJounaalNo. 3 September 198681