DTJ Number 3 September 1987 - Digital Technical Journals

New Productsshould have the added advantage of requiringno management intervention when previouslydisjoint extended IANs are merged.• A general mesh topology including backupbridges and IANs should be supponed transparentto the end stations for increased reliabilityand availability.• End-to-end data integrity should be providedacross the extended IAN for all normal andmulticast/broadcast frames when the MACsare the same type. This capability holds acrossany connected subset of the topology that isthe same MAC type. Thus the frames are notmodified and the MAC frame check sequence(FCS) has end-to-end significance for theextended IAN.Architectural OverviewWe used the goals above to develop a uniquearchitecture for the extended IAN concept. Thisarchitecture is described in this section. Followingthat description are sections on bridge performanceand resources. These are two important,but often neglected, topics that should beconsidered when specifying an architecture. Performanceis especially critical to the properfunctioning of products that are eventually builtto use the architecture. For the first time, performanceanalyses were included as an integral panof the architectural specification.The algorithm used in the bridge is very simple.The algorithm maintains an associationbetween the end-station MAC address and theMAC entity on the bridge through which that stationhas been observed. The associations arestored in a table, also called a forwarding database,in the bridge . The bridge maintains thattable by observing traffic on the IANs to whichthe bridge is attached, operating in "promiscuous"mode. In this mode the bridge monitors allframes that appear on the IAN. For each framereceived , the bridge notes the source MACaddress and the MAC entity on which the framewas seen. The bridge also searches in the tablefor the destination MAC address. If that address isfound, the frame will be forwarded on the MACentity indicated in the table. Of course, if theindicated MAC entity is the same one thatreceived it, the frame will be dropped since thedestination is known to be on that "side" of thebridge. If no association is found, the frame willbe forwarded on each MAC entity except the onethat received the frame.Frames with group addresses (i.e., multicastaddresses) are always forwarded on each MACentity since the bridge has; no way of knowingwhich end stations should receive the framesthat are addressed to grdups. This concept,called protocol regionalization, can effectivelylimit the propagation of these messages throughthe extended IAN, thus allowing cenain applicationprotocols to be confined to variousregions.14 This confinement is done for reasonsof performance, management convenience, andprivacy.· IThe table is simply a cace of station addressto-MACentity associations! for stations that arecommunicating. As with any caching scheme,the problem of stale data exists. Therefore, thetable entries are aged out on a time scale that islong enough to minimize. overhead, yet shortenough to capture station rp.ovements.The algorithm learns the location of end stationsdynamically and assumes that few of themIsimply receive traffic without ever sendingIreplies. If the station location is not known, thenframes directed to it are forwarded on all MACentities. Our experience shows that in a typicaloperation only one frame from a station isrequired for most, if not all, bridges in theextended IAN to learn the stion's location. Typicalhigher-layer protocols more than satisfy thisrequirement. )The initialization phase qf a bridge is specifiedin a panicular fashion. The ' bridge is powered onand then passively observes· the traffic on its MACentities for a number of seconds. During thistime the bridge accumulates associations in itsforwarding database, after which it comes on lineand begins forwarding oprations. This initialpassive learning period prdvents the bridge fromunduly flooding the extened IAN with framesdestined to stations it hasn't yet heard from. Aswith all parameters in the algorithm, the durationof this learning period during power-up isnot critical. It should simply be long enough towitness frames from a large percentage of theactive stations.As specified so far, the algorithm will not modifyframes as they are passed through a bridgebetween IANs of the samejtype. This restrictionprovides the additional benefit of end-to-end FCScoverage for normal and broadcast frames withinDigital TecbntcaiJournalNo. 3 September 198661