rfc2988：Computing TCP's Retransmission Timer

The Transmission Control Protocol (TCP) uses a retransmission   timer to ensure data delivery in the absence of any feedback from the   remote data receiver.  The duration of this timer is referred to as   RTO (retransmission timeout).
TCP依赖一个重传定时器来确保丢失的数据包能够被重发，这个定时时间就是 RTO。

要计算当前的RTO，TCP发送方需要维护两个状态变量，SRTT （平滑环回时间）和RTTVAR（环回时间变量）。另外，我们假设一个时钟间隔G秒。计算SRTT、RTTVAR和RTO的法则如下：(1) 在对一个收发双方之间所发出的一个包完成回环时间（RTT）测量之前，发送方应该将RTO设置为3秒。注意到有一些设施可能采用一种“心跳式”定时器，它能够产生一个介于2.5秒和3秒之间的值。因此，在该定时器绝不会在短于2.5秒的时间内超时的情况下，作为一个较低的2.5秒的步进也是可以接受的，使用间隔为G的心跳式定时器的设施，定时器的值不低于2.5 + G秒。(2) 当完成第一个RTT测量R时，宿主机必须设置            SRTT = R            RTTVAR = R/2            RTO = SRTT + max (G, K*RTTVAR)          其中K = 4。(3) 当完成一个并发的RTT测量R'时，宿主机必须设置            RTTVAR =(1 - beta) * RTTVAR + beta * |SRTT - R'|            SRTT = (1 - alpha) * SRTT + alpha * R'用来更新RTTVAR的SRTT的值，就是那个使用第二次分配来更新SRTT之前的SRTT值本身。就是说，更新RTTVAR和SRTT必须按照上述的顺序进行计算。上述计算应该使用alpha=1/8和beta=1/4进行计算。在计算之后，宿主机必须更新RTO = SRTT + max (G, K*RTTVAR)。
(4) 一旦RTO计算好，如果它小于1秒钟，则RTO应该补充到1秒。传统地，TCP设施使用粗间隔时钟来测量RTT并触发RTO，这使得应用于RTO上的最小值很大。研究表明，需要一个很大的最小RTO值来保持TCP的保守，以避免虚假重发。因此，RFC2988要求有一个很大的最小RTO值作为保守之需，而同时承认，在未来的某个时刻，研究可能表明一个小一些的最小RTO值是可接受的，或者，是优越的。(5) 可以给RTO使用最大值，如果该值至少有60秒。

void RTPBandwidthTracker::AddToRTTEstimate( SInt32 rttSampleMSecs ){//  qtss_printf("%d ", rttSampleMSecs);//  static int count = 0;//  if ((count++ % 10) == 0) qtss_printf("\n");        // this assert hits    Assert(fBytesInList < ((UInt32)fClientWindow + 2000)); //mainly just to catch fBytesInList wrapping below 0
    if ( fRunningAverageMSecs == 0 )        fRunningAverageMSecs = rttSampleMSecs * 8;  // init avg to cur sample, scaled by 2**3 
    SInt32 delta = rttSampleMSecs - fRunningAverageMSecs / 8; // scale average back to get cur delta from sample        // add 1/8 the delta back to the smooth running average    fRunningAverageMSecs = fRunningAverageMSecs + delta; // same as: rt avg = rt avg + delta / 8, but scaled        if ( delta < 0 )        delta = -1*delta;   // absolute value         /*            fRunningMeanDevationMSecs is kept scaled by 4                        so this is the same as                fRunningMeanDevationMSecs = fRunningMeanDevationMSecs + ( |delta| - fRunningMeanDevationMSecs ) /4;    */
    /*        rfc2988: The Basic Algorithm
        SRTT = (1 - 1/8) * SRTT + 1/8 * R'        RTTVAR = (1 - 1/4) * RTTVAR + 1/4 * |SRTT - R'|        RTO = SRTT + 4 * RTTVAR
        So,        SRTT = SRTT + 1/8 * (R' - SRTT)        RTTVAR = RTTVAR + 1/4 * (|SRTT - R'| - RTTVAR)        RTO = SRTT + 4 * RTTVAR
        And,        SRTT8 = SRTT8 + (R' - SRTT8 / 8)        RTTVAR4 = RTTVAR4 + (|SRTT8 / 8 - R'| - RTTVAR4 / 4)        RTO = SRTT8 / 8 + RTTVAR4
        (SRTT8: fRunningAverageMSecs, RTTVAR4: fRunningMeanDevationMSecs)     */
    fRunningMeanDevationMSecs += delta - fRunningMeanDevationMSecs / 4;            fUnadjustedRTO = fCurRetransmitTimeout = fRunningAverageMSecs / 8 + fRunningMeanDevationMSecs;        // rto should not be too low..    if ( fCurRetransmitTimeout < kMinRetransmitIntervalMSecs )          fCurRetransmitTimeout = kMinRetransmitIntervalMSecs;        // or too high...    if ( fCurRetransmitTimeout > kMaxRetransmitIntervalMSecs )        fCurRetransmitTimeout = kMaxRetransmitIntervalMSecs;//  qtss_printf("CurTimeout == %d\n", fCurRetransmitTimeout);}