Handbook of Local Area Networks, 1998 Edition:Advanced LAN Interconnectivity Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next CONGESTION CONTROL SCHEMES A congestion control loop between the network and the user is required to support ABR service. In closed-loop control, the network requires a feedback mechanism to tell each source how much data to send. The feedback information occurs through the headers of data cells, and some through control cells. The data and control traffic of a typical closed-loop congestion control scheme is illustrated in Exhibit 4-7-5. Exhibit 4-7-5.  Typical Closed-loop Congestion Control Scheme In this example, two virtual connections (VC1 and VC2) share the link between switches 1 and 2. For implementation of closed-loop control, two kinds of schemes have been debated in the ATM Forum. They are classified as credit-based or rate-based. Previous Congestion Control Schemes To design an efficient congestion control scheme, it is helpful to understand previous approaches and their limitations. In this section, congestion control is briefly described by classifying protocols as open-loop or closed-loop. Before discussing the two main approaches seriously considered by the ATM Forum, other methods that were proposed to solve the congestion control problem will be examined. Fast Resource Management A fast resource management method was proposed by France Telecom. Before actually sending data cells, a source sends a resource management (RM) cell in order to request the desired bandwidth. When a switch receives an RM cell from the source, it passes the RM cell on to the next switch if it can satisfy the request. A switch simply drops the RM cell if it cannot grant the request. The source then resends a request when it times out. Upon receiving an RM cell, the destination returns the RM cell back to the source, which can then transmit the data cell. Therefore, the data cell has to wait for at least one round trip delay at the source even if the network is idle. To avoid this delay, an immediate transmission (IT) mode was proposed in which a data cell is transmitted immediately following an RM cell. If a switch cannot satisfy the request, it simply drops the RM cell and data cell. Then, the switch sends an indication to the source. The main problem in this method is excessive delay during normal operation or excessive loss during congestion. Early Packet Discard The early packet discard method is based on the observation that a packet consists of several cells. The method uses a bit in the cell header to indicate end of message (EOM). A switch looks for the EOM marker and drops all future cells of the VC until the EOM marker is seen again when its queues start getting full because it is better to drop all cells of one packet than randomly drop cells belonging to different packets. This method can be used without any standardization because it does not require any inter-switch or source-switch communication. However, this method may not be fair in the sense that the cell arriving at a full buffer may not belong to the VC causing the congestion. Delay-Based Rate Control A delay-based rate control method requires the source to monitor the round trip delay in order to control the congestion. In this method, a source periodically sends an RM cell that contains a timestamp. When a destination receives an RM cell from the source, it returns it to the source. Upon receiving an RM cell from the destination, the source uses the timestamp in the RM cell to measure the round trip delay and to deduce the level of congestion. This approach has the advantage of no explicit feedback from the network. Therefore, it will work even if the path contains non-ATM networks. Link Window with End-to-End Binary Rate The link window with end-to-end binary rate method uses window flow control on every link and explicit forward congestion indication (EFCI)-based binary end-to-end rate control. The method is a merger of a rate-based scheme with a credit-based scheme. It is scaleable in terms of number of VCs because the window control is per-link and not per-VC. It also guarantees zero cell loss as in credit-based scheme. However, neither the credit-based nor the rate-based camp found it acceptable because it contained elements from both camps. 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