Standard slow start does not work well under large bandwidth-delay product (BDP) networks. We find two causes of this problemin existing three popular operating systems, Linux, FreeBSD and Windows XP. The first cause is that because of the exponential increase of cwnd during standard slow start, heavy packet losses occur. Recovering from heavy packet losses puts extremely high load on end systems which renders the end systems completely unresponsive for a long time, resulting in a long blackout period of no transmission. This problem commonly occurs with the three operating systems.The second cause is that some of proprietary protocol optimizations applied for slow start by these operating systems to relieve the system load happen to slowdown the loss recovery followed by slow start. To remedy this problem, we propose a new slow start algorithm, calledHybrid Start (HyStart) that finds a safe exit point of slow start at which slow start can finish and safely move to congestion voidance without causing any heavy packet losses. HyStart uses ACK trains and RTT delay samples to detect whether (1) the forward path is congestedor (2) the current size of congestion window has reached the available capacity of the forward path. HyStart is a plug-in to the TCP sender and does not require any change in TCP receivers. We implemented HyStart for TCP-NewReno andTCP-SACK in Linux and compare its performance with different slow start schemeswith the TCP receivers of the three different operating systems in the Internetand also in the lab testbeds. Our results indicate that HyStart works consistently well under diverse network environments including asymmetric links and high andlow BDP networks. Especially with different operating system receivers (Windows XPand FreeBSD), HyStart improves the start-up throughput of TCP more than 2 to 3times.
1、混合慢啓動設計的目的：
標準的慢啓動在BDP網絡環境下表現不好，不好的原因主要有兩個：
1） 標準慢啓動的擁塞窗口指數式的增長方式過於激進容易導致大量丟包，丟包恢復性能損耗太大。
2） 被優化過的慢啓動機制，丟包時在數據包重傳恢復的時候碰巧試圖去減小服務器的負載，導致數據包恢復慢。
總結這些原因都是因爲慢啓動過程過於盲目，不能及時的預測擁塞，導致了大量丟包，所以混合慢啓動機制的主要
功用是在慢啓動階段試圖找到“合理 ”的 退出慢啓動 進入擁塞避免狀態點（safe exit point）。
2、慢啓動是怎麼找到safe exit point
1）通過ACK train
2）通過RTT delay
3、混合滿慢啓動目前在cubic擁塞控制算法中引入，後面會具體分析cubic混合慢啓動實現的細節。