Calculation of RTO in tcp and related open source implementation (Linux kernel, unbound) --- rtt, rttvar, cwnd, ssthresh

1. Concept

        RTT: Round trip delay. 

        RTO: After tcp sends a data packet, it will start a retransmission timer, RTO is the retransmission time of this timer.

        Since RTO refers to the estimated timeout time of sending the current data packet this time, then RTO needs a good statistical method to better predict the timeout time of this time.

        SRTT: smoothed tound-trip time

        RTTVAR: round-trip time variation

Second, we can think of the calculation method (take the average)

        For example, the first RTT is 500 milliseconds and the second is 800 milliseconds, then the RTO should be 650 milliseconds when it is sent for the third time.

3. RFC793 calculation method

        Similar to taking the average.

3.1 Method
        SRTT = ( ALPHA * SRTT ) + ( ( 1-ALPHA ) * RTT )

        RTO = min [ UBOUND, max [ LBOUND, ( BETA * SRTT ) ] ]

3.2 Parameter meaning

        UBOUND is the maximum value of RTO;

        LBOUND is the minimum value of RTO;

        The value of ALPHA is 0.8 ~ 0.9;

        The value of BETA is 1.3 ~ 2.0.

RFC793: http://tools.ietf.org/html/rfc793

4. Calculation method of paper "Congestion Avoidance and Control"

        Compared with RFC793, mean deviation is increased.

4.1 Method

        Err ≡ m?a
        a ← a + gErr
        v ← v + g (| Err | ?v)

4.2 Parameter meaning

        m is RTT;

        a is SRTT;

        v is the mean difference;

        g is a factor.

4.3 Method changes

Since there may be floating point numbers in the presence of g, we only need to make a change so that g = 1/2 ^ n, then the above equation becomes the following

        2 ^ na ← 2 ^ na + Err 
        2 ^ nv ← 2 ^ nv + g (| Err | ?v)

4.4 Pseudocode

/? update Average estimator ?/
m ? = (sa >> 3); 
sa + = m;
/? update Deviation estimator ?/
if (m <0)
   m = ?m; 
m ? = (sv >> 2);
sv + = m;
rto = (sa >> 3) + sv;

paper "Congestion Avoidance and Control": http://ee.lbl.gov/papers/congavoid.pdf

Five, RFC6298 calculation method

5.1 Calculation method

At the first RTT:

        SRTT <- R
        RTTVAR <- R/2
        RTO <- SRTT + max (G, K*RTTVAR)

During subsequent RTT:

        RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R‘|
        SRTT <- (1 - alpha) * SRTT + alpha * R‘
        RTO <- SRTT + max (G, K*RTTVAR)

5.2 Parameter meaning

R and R ‘are RTT;

        alpha = 1/8;

        beta = 1/4;

        K = 4.

5.3 Changed calculation method (this method is the same as the calculation method above)

delta = RTT - SRTT
SRTT = SRTT + g * delta  
rttvar = rttvar + h(|delta| - rttvar) 
RTO = srtt + 4*rttvar

among them:

g = 1/8

h = 1/4

RFC6298: http://tools.ietf.org/html/rfc6298

6. Some open source implementations of computing RTO

In the Linux kernel: net/ipv4/tcp_input.c tcp_rtt_estimator ()

Unbound: util/rtt.c

Reference materials:

RFC793: http://tools.ietf.org/html/rfc793

paper "Congestion Avoidance and Control": http://ee.lbl.gov/papers/congavoid.pdf

RFC6298: http://tools.ietf.org/html/rfc6298

Calculation of RTO in tcp and implementation under linux: http://www.cnblogs.com/hfww/archive/2012/08/01/2618109.html

TCP reliable transmission and flow control series 3: Retransmission time RTO calculation: http://zenhumany.blog.163.com/blog/static/1718066332010827104042708/