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William Stallings Data and Computer Communications 7 th Edition

William Stallings Data and Computer Communications 7 th Edition. Chapter 1 3 Congestion in Data Networks. Fig.1.3 A Networking Configuration. 13.1 What Is Congestion?.

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William Stallings Data and Computer Communications 7 th Edition

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  1. William StallingsData and Computer Communications7th Edition Chapter 13 Congestion in Data Networks

  2. Fig.1.3 A NetworkingConfiguration

  3. 13.1 What Is Congestion? • Congestion occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network • Congestion control aims to keep number of packets below level at which performance falls off dramatically • Data network is a network of queues • Finite queues mean data may be lost

  4. Fig.13.1 Queues at a Node

  5. Interaction of Queues

  6. Flow control vs Congestion control • Flow control means preventing the source from sending data that the sink will end up dropping because it runs out of buffer space.终端将因缓存溢出而最终性能下降。 • This is fairly easy with a sliding window protocol- • Congestion control means preventing the source from sending data that will end up getting dropped by a router because its queue is full.最终因路由器队列已满而导致性能开始下降。 • This is more complicated, because packets from different sources travelling different paths can converge on the same queue.

  7. Fig.13.3 Ideal Performance 归一化 吞吐量 Normalized throughput Normalized load Delay 时延 Normalized load Power Normalized load

  8. Effects of Congestion -No Control Normalized throughput 归一化 吞吐量 load 载荷（负载） Delay 时延 load

  9. 13.2 Mechanisms for Congestion Control P401 隐式拥塞信令 (时延,丢弃) 监管 反压 阻流分组 显式拥塞信令 (二进制,信用,速率)

  10. (1)Backpressure • If node becomes congested it can slow down or halt flow of packets from other nodes • Propagates back to source • E.g. X.25: Used in connection oriented that allow hop by hop congestion control

  11. (2)Choke Packet 阻流分组 • Control packet • Generated at congested node • Sent to source node • E.g. ICMP(Internet Control Message Protocol) 网络控制报文协议 Source Quench packet 源端抑制报文

  12. IP协议族中的ICMP Source-quench format 源端抑制报文格式

  13. Internetworking Protocols

  14. IP协议族中的ICMP A router cannot detect all problems that prevent the delivery of a packet. There is no flow-control mechanism in the IP protocol.

  15. IP协议族中的ICMP A source-quench message informs the source that a datagram has been discarded due to congestion in a router or the destination host. The source must slow down the sending of datagrams until the congestion is relieved.

  16. IP协议族中的ICMP One source-quench message should be sent for each datagram that is discarded due to congestion.

  17. (3)Implicit Congestion Signaling隐式 • Transmission delay may increase with congestion • Packet may be discarded • Source can detect these as implicit indications of congestion • Reduce flow • E.g. TCP

  18. 拥塞窗口大小 TCP协议中的拥塞控制 发送的报文 Slow start 慢启动 Additive increase 加法增加 Multiplicative decrease 乘法减小

  19. (4)Explicit Congestion Signaling显式 • 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 • E.g. Frame relay

  20. Frame Formats of LAPF p332

  21. Categories of Explicit Signaling显式拥塞控制的类别 • Binary • 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 • Supply explicit data rate limit

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