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ccna pdf in tamil.101000 cs5 (CS) 011110 af33 (AF)110000 cs6 (CS) 100010 af41 (AF)111000 cs7 (CS) 100100 af42 (AF)001010 afll (AF) 100110 af43 (AF)001100 afl2 (AF) 101110 ef (EF)001110 afl3 (AF)21.3.2 Congestion AvoidanceWhen the length of the queue reaches the specified maximum length, the later arriving packets are dropped tail. If a WFQ queue is used, it may not be tail-dropped. For TCP packets, if a large number of packets are dropped will cause a TCP timeout, which triggers a slow start of TCP and TCP reduces packet delivery. When the queue drops packets from multiple TCP connections at the same time, it will cause multiple TCP connections to go into slow start and congestion avoidance at the same time, a situation called TCP global synchronization. The packets sent to the queue from multiple TCP connections will be reduced simultaneously and the link bandwidth is not fully utilized; then the multiple TCP connections slowly increase the packets sent at the same time and the queue overflows again. This way the flow of packets in the queue is always large and small, so that the traffic on the link always fluctuates between very little and full, and the average link bandwidth utilization decreases, which also affects the delay and jitter of specific traffic. Tail drop causes uneven bandwidth allocation among TCP traffic, so that some "greedy" traffic will take up most of the bandwidth, while ordinary TCP traffic will "starve" because it cannot be allocated bandwidth. In particular, when there is both TCP and UDP traffic in the network, the TCP traffic releases bandwidth because of the sliding window mechanism (tail drop causes the sliding window to decrease), while the UDP traffic does not use the sliding window mechanism, so the UDP traffic will quickly take up the bandwidth released by TCP, which eventually causes the UDP traffic to take up all the bandwidth and the TCP traffic to "starve" because there is no bandwidth allocation. "starving".As shown in Figure 21.2, TCP appears synchronized and the average link utilization is low.Bandwidth