FACK 重传

FACK 是 SACK 的一个激进版本，它拥有标准 SACK 算法的一切性质，除此之外，它假设网络不会使数据包乱序，因此收到最大的被 SACK 的数据包之前，FACK 均认为是丢失的。FACK 模式下，重传时机为 被 SACKed 的包数 + 空洞数 > dupthresh 同时dupack == dupthresh(3) 默认。
如下图所示，设 dupthresh = 3，FACKed_count = 12，从 unACKed 包开始的 FACKed_count
　　dupthresh 个数据包，即 9 个包会被标记为 LOST。

拥塞窗口状态

记分板状态如下，红色表示该数据包丢失.

 

FACK Design Goals

　　The requisite network state information can be obtained with accurate knowledge about the forward most data held by the receiver. By forward-most, we mean the correctly-received data with the highest sequence number. This is the origin of the name "forward acknowledgement."The goal of the FACK algorithm is to perform precise congestion control during recovery by keeping an accurate estimate of the amount of data outstanding in the network.

 

FACK  Algorithm

　　When a SACK block is received which acknowledges data with a higher sequence number than the current value of snd.fack, snd.fack is updated to reflect the highest sequence number known to have been received plus one. The FACK algorithm uses the additional information provided by the SACK option to keep an explicit measure of the total number of bytes of data outstanding in the network. In contrast, Reno and Reno + SACK both attempt to estimate this by assuming that each duplicate ACK received represents
one segment which has left the network. The FACK algorithm is able to do this in a staightforward way by introducing two new state variables, snd.fack and retran_data.
TCP's estimate of the amount of data outstanding in the network during recovery is given by:

awind = snd.nxt - snd.fack + retran_data

 

Triggering Recovery

　　 Reno invokes Fast Recovery by counting duplicate acknowledgements:

if ( dupacks == 3 ) {
...
}

　　This algorithm causes an unnecessary delay if several segments are lost prior to receiving three duplicate acknowledgements. In the FACK version, the cwnd adjustment and retransmission are also triggered when the receiver reports that the reassembley queue is longer than 3 segments:

if ( ( snd.fack - snd.una ) > (3*MSS) ) || (dupacks == 3) ) {
...
}

 

 

 

 

 

参考：https://blog.csdn.net/zhangskd/article/details/7236727