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【正文】 Mechanisms for Congestion Control 隱式 顯式 Backpressure： 反壓 ?If node bees congested it can slow down or halt flow of packets from other nodes ?May mean that other nodes have to apply control on ining packet rates ?Propagates back to source ?Can restrict to logical connections generating most traffic ?Used in connection oriented that allow hop by hop congestion control (. ) ?Not used in ATM nor frame relay ?Only recently developed for IP Choke Packet： 阻流分組 ?Control packet ?Generated at congested node ?Sent to source node ?. ICMP source quench ?From router or destination ?Source cuts back until no more source quench message ?Sent for every discarded packet, or anticipated ?Rather crude mechanism Implicit Congestion Signaling ? delay may increase with congestion ? may be discarded ?Source can detect these as implicit indications of congestion ?Useful on connectionless (datagram) works ?. IP based ?(TCP includes congestion and flow control see chapter 17) ?Used in frame relay LAPF Explicit Congestion Signaling ?Network alerts end systems of increasing congestion ?End systems take steps to reduce offered load ?Backwards ?Congestion avoidance in opposite direction to packet required ?Forwards ?Congestion avoidance in same direction as packet required SOURCE NODE NODE NODE USER Categories of Explicit Signaling ?Binary—二進制 ?A bit set in a packet indicates congestion ?Credit based—基于信用值的 ?Indicates how many packets source may send ?Common for end to end flow control ?Rate based—基于速率 ?Supply explicit data rate limit ?. ATM Traffic Management： 通信量管理 ?Fairness—公平性 ?Quality of service—服務(wù)質(zhì)量 ?May want different treatment for different connections ?Reservations—預(yù)約 ?. ATM ?Traffic contract between user and work Congestion Control in Packet Switched Networks ?Send control packet to some or all source nodes ?Requires additional traffic during congestion ?Rely on routing information ?May react too quickly ?End to end probe packets ?Adds to overhead ?Add congestion info to packets as they cross nodes ?Either backwards or forwards Leaky Bucket Algorithm 漏桶算法 Leaky bucket algorithm Leaky bucket algorithm ?Each host is connected to the work by an interface containing a leaky b
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