Tắc nghẽn sự sụp đổ bắt đầu với mức tăng ổn định trong tải trên mạng. Khi máy chủ gửi nhiều dữ liệu hơn, nhiều dữ liệu hơn là xếp hàng đợi trong bộ đệm của bộ định tuyến liên mạng. Khi điều này xảy ra, thời gian vận chuyển từ nguồn đến đích và từ đích để tăng nguồn. Vì vậy, RTT thực tế phát triển lớn hơn so với RTT | Chapter 13 Transmission Control Protocol TCP Retransmission and Time-Out 273 Congestion Collapse The proper measurement of the RTT and determination of the RTO for sent TCP segments are important to prevent a phenomenon of routed internetworks known as congestion collapse. Congestion collapse occurs when the buffers of the internetwork routers fill to capacity and the routers begin to discard packets. Congestion collapse begins with a steady increase in the load on the internetwork. As hosts send more data more data is queued in the buffers of the internetwork routers. As this occurs the transit time from the source to the destination and from the destination to the source increases. Therefore the actual RTT grows larger than the currently known RTT of sending hosts. The current RTO for sent segments is based on the currently known RTT. When the actual RTT increases to the extent that it is greater than the current RTO sent TCP segments have ACKs that arrive late. When the ACKs do not arrive in the time based on the current RTO the segments are retransmitted. There are then two copies of each retransmitted segment effectively doubling the load on the internetwork at a time when the load needs to be decreased. As more TCP segments are retransmitted eventually the buffers on the internetwork routers fill and the routers begin to discard packets. Congestion collapse can be avoided through the ongoing determination of the current RTT which is monitored on a per window or per segment basis. Changes in the current RTT are used to update the RTO. The recurrence of congestion collapse is avoided through the combination of the slow start and congestion avoidance algorithms of the sending host as discussed in Chapter 12 Transmission Control Protocol TCP Data Flow. When the RTO for a segment expires TCP assumes that RTO expiration is a result of the segment being discarded by a router experiencing congestion. Slow start and congestion avoidance are used to slowly scale the .
