数据通讯与计算机网络讲义 24章.ppt

Chapter 24 拥塞控制与服务质量 （ Congestion Control Quality of Service ） (QOS: Reliability , Delay, Jitter, Throughput ) Data traffic —— 数据量随 t 的分布。 In congestion control, try to avoid traffic congestion. In QOS, try to give a good environment for the traffic. 24.1 Data Traffic Traffic Descriptor 系统有效带宽取决的三个参数 Traffic Profiles 电路交换网流量 主干网流量 拥塞的主要流量类型 拥塞 —— 进入网络的负载 网络的容量 拥塞控制 —— 保证负载低于容量的机制和措施 24.2 Congestion（拥塞） Queues in a router 若: 到达速率 处理速率 处理速率 输出速率 input queue full output queue full 拥塞出现在“分组交换网”中。 拥塞对网络性能的影响 24.3 Congestion Control Prevention alleviation Open-Loop Congestion Control Retransmission policy (sending policy) Window policy (accept policy) Acknowledgment policy (immediate, cumulative) Discarding policy (priority) Admission policy (reservation) —— Prevention of congestion Closed-Loop Congestion Control —— alleviation of congestion Choke packet Backpressure Refuse to accept dada ( Only used to virtual circuit networks ) Implicit signaling the source determines an occurrence of congestion by some implicit signal (e.g., timeout) Explicit signaling the congested router set a bit-pattern in a passing packet to indicate an occurrence of congestion backward signaling to set the packet moving in the direction opposite to the congestion, informing the source to reduce the outgoing rate. forward signaling to set the packet moving in the direction of the congestion informing the destination to slow down the ACK (or some other actions). TCP拥塞控制 24.4 Two Examples 拥塞窗口 发送窗口 = min ( rwnd, cwnd ) TCP拥塞控制策略 Slow start: exponential increase Congestion avoidance: additive increase Congestion detection: multiplicative decrease Congestion control example TCP congestion policy Slow start: exponential increase (2倍) (2倍) (2倍) Congestion avoidance: additive increase (加1) (加1) (加1) BECN 帧中继网络的显式拥塞控制 FECN 无滑窗机制 响应帧 Four cases of con