DTJ Number 3 September 1987 - Digital Technical Journals

The Extended Local Area Network Architecture and LANBridge 100Bridge ResourcesThe major resources of concern in a bridge arethe buffering required to store and forwardframes , the table space for the forwardingdatabase, and the CPU cycles to executethe algorithm. Note that CPU cycles are alsorequired to perform network management. Typically,any bridge implementation must guaranteethat network management commands are eventuallyexecuted . For example, suppose a bridgewas heavily loaded because of a slow outboundIAN. A network manager wanting to disconnectthat bridge may be unable to do so if all receivedframes are being dropped because of buffer congestion.Therefore , one important aspect ofimplementing a network management architectureis that some amount of buffering must bepreallocated to handle those messages . Moreover,scheduling must be accomplished so thatthe network management process in the bridge isguaranteed to make progress . This guarantee is amatter of correctness and therefore should bestated in any effort to make the architecture astandard.Buffers are also required to hold frames whilethey are waiting to be either processed or fo r­warded. As depicted in Figure 3, bridge can bemodeled as a queuing system in which the servicecenters represent the forwarding processand the outbound LANs. Congestion can occur atthree places:1. Upon reception, owing to the lack of receivebuffers2. After reception, owing to queuing for theforwarding process3. After the forwarding process, because of congestionon the outbound LANProper bridge design can solve the first twosources of congestion. The third problem, however,is a general one for bridges, routers, andany store-and-forward device. 20 There are severalways that the bridge designer can address thisproblem. We first make a general observationabout the required service rate of the servicecenters in a queuing network. Steady-state congestionat the forwarding process can be avoidedcompletely if the network can always make forwardingdecisions faster than the summation ofthe interarrival times of the smallest framesacross all the inbound LANs. The forwardingdatabase must be consulted for each frame onwhich a forwarding decision is made. There aremany ways to do that very efficiently.The table discussed earlier is really only acache of station address-to-MAC entity associations;a search of that table is required to locatean entry. If the table is ordered, then a binarysearch can locate the entry in question. There areother alternative search methods, such as segmentedhashing. The implementation of this pro-BRIDGE,-------------------------------------------IIIIIIBUFFERST. DISCARDL--------------------------------------------j..L...L-L....I,IIIIIIFigure 3The Two Port Bridge Resource Model64Digital TecbnicalJournalNo. 3 September 1986