动态超时计时器的计算算法

基本算法——

We assume initial round trip time i.e PRTT.
On sending out each packet TOT = 2 * PRTT. 
The next round trip time is calculated using
    PRTTn+1 = α × PRTTn + (1 - α) × ARTTn
where PRTT = predicted round trip time
      ARTT = actual round trip time
      α = smoothing factor such that 0<= α <=1 

TOT = 2 * PRTT1 = 20ms 
Let ARTT1 = 15ms
Then,
PRTT2 = (0.5 * 10) + (0.5 * 15) = 12.5ms
TOT = 2 * 12.5 = 25ms
Let ARTT2 = 20ms
PRTT3 = (0.5 * 12.5) + (0.5 * 20) = 16.25ms
TOT = 2 * 16.25 = 32.5ms
And so on TOT is calculated.

• 比静态的TOT要好得多。
• TOT对动态往返时间具有灵活性。
• 在计算出新的PRTT值时，会考虑所有的数据包。

• TOT = 2 * PRTT，这一数值用于为返回确认消息提供一定的延迟时间。
• 这太浪费了。

2. Jacobson算法 –

We assume initial round trip time i.e. PRTT.
PRTTn+1 = α * PRTTn + (1 - α) * ARTn 
PDn+1 = α × PDn + (1 – α) × ADn 
where ADn = |PRTTn – ARTTn|
AD = Actual deviation
PD = predicted deviation
On sending out each packet, TOT = (4 * PD) + PRTT。 

第1轮迭代
Given α = 0.5, PRTT1 = 10ms, PD1 = 5ms and ARTT1 = 20ms
TOT = (4 * 5) + 10 = 30ms
AD1 = |10 - 20| = 10ms

第二次迭代
PRTT2 = α PRTT1 + (1 - α)ARTT1
      = (0.5 * 10) + (0.5 * 20) = 15ms
PD2 = α PD1 + (1 - α)AD1
      = (0.5 * 5) + (0.5 * 10) = 7.5ms
TOT = (4 * 7.5) + 15 = 45ms
Given ARTT2 = 30ms
AD2 = |15 - 30| = 15ms

第3轮迭代
PRTT3 = α PRTT2 + (1 - α)ARTT2
      = (0.5 * 15) + (0.5 * 30) = 22.5ms
PD3 = α PD2 + (1 - α)AD2
      = (0.5 * 7.5) + (0.5 * 15) = 11.25ms
TOT = (4 * 11.25) + 22.5 = 67.5ms
Given ARTT3 = 10ms
AD2 = |22.5 - 10| = 12.5ms
And so on TOT is calculated.

3. Karn的修改方式 –