Date: Tue, 8 Jan 2013 18:09:07 +0530 From: Debojyoti Roy <deb.roy.jyoti@gmail.com> To: freebsd-questions@freebsd.org Subject: Help to understand the behaviour of TCP-Cubic module in FreeBSD Message-ID: <CAG7KJ08zpaXch2OxXY9euaOsNi%2BJhgNbONHYA12RRMFUCUsqmA@mail.gmail.com>
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[-- Attachment #1 --] Hi Everyone, I am trying to develop TCP-CUBIC and using Free-BSD as a standard device for testing. To load the Cubic module in FreeBSD the following commands were executed : kldload ./cc_cubic.ko sysctl -w net.inet.tcp.cc.algorithm=cubic The test method has been attached which was used to understand the behaviour of TCP-Cubic. There are few questions regarding the behaviour of FreeBSD. Q1) After receiving Full Acknowledgement from the receiver, sender entered into Congestion Avoidance State. At that moment filghtsize was equal to 0 and CWND became 1. Now, sender will increase its CWND according to CUBIC algorithm but FreeBSD follows Slow Start Algorithm by incrementing CWND from 1 --> 2 --> 4 segments. As per my understanding, if FreeBSD follows TCP-Cubic functionality, it should be in TCP friendly region. So, CWND should be 7 in next RTT. Calculation has been provided below : *W_tcp(t) = W_Max * (1 - beta) + 3 * beta/(2 - beta) * t/RTT* *=> W_tcp(t) = 7 * *[W_Max = 8, congestion occurs = 18.874sec , current time = 18.878sec, therefore elapsed time t = 0.004 sec, RTT = (18.878 - 18.876) = 0.002 sec, beta = 0.2]* But I find that it incremented to 2 rather than 7. Could you please explain this behaviour ? Q2) If my tool acknowledges all the packets coming from FreeBSD after Fast Retransmit, the segment sequence was 1 --> 2 --> 4 --> 5 --> 6 --> 7 --> 8 --> 6 --> 6 --> 6... Why after sending 8 segments in congestion avoidance state, CWND unexpectedly sets to 6 ? Q3) What will be the unit of t and RTT (i.e seconds or miliseconds) ? Q4) In Cubic-draft both concave and convex use the same formula. Only difference in convex it depends on max_increment_rate. How does max_increment_rate variable can be calculated ? If I have any wrong understanding please rectify me and I will be very grateful if you could answer the questions as soon as possible. Thanks in advance, Debojyoti Roy [-- Attachment #2 --] The following method has been used to test the Cubic in congestion avoidance state : +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ | Test Action in | | Step No. the step Comments | +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ | 1 RECEIVER sends SYN packet Connection Estublishment | | 2 FreeBSD sends SYN-ACK packet | | 3 RECEIVER sends ACK packet | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 4 FreeBSD sends 4 segments From here we can infer that initial window size is 4. So, CWND = 4 | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 5 RECEIVER sends ACK for all 4 segments FreeBSD will follow the Slow Start Algorithm. Hence it should make CWND = 2 x 4 = 8 | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 6 FreeBSD sends 8 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 7 RECEIVER sends 3-Duplicate ACKs of last segment of step 5 FreeBSD realises the Congestion has occured | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 8 FreeBSD retransmits the lost segment FreeBSD in Fast Retransmit state | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 9 RECEIVER sends the ACK of the last segment of step 6 FreeBSD exits from Fast Recovery | | (i.e Full ACK) +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 10 FreeBSD sets CWND = 1 and sends 1 segment FreeBSD increases CWND in Slow Start manner. | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 11 RECEIVER sends ACK of the single segment | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 12 FreeBSD sets CWND = 2 x CWND or 2 and sends 2 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 13 RECEIVER sends ACK of 2 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 14 FreeBSD sets CWND = 2 x CWND or 4 and sends 4 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 15 RECEIVER sends ACK of 4 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 16 FreeBSD sets CWND = CWND + 1 or 5 and sends 5 segments FreeBSD increases CWND in Congestion Avoidence manner (RFC 2581) | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 17 RECEIVER sends ACK of 5 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 18 FreeBSD sets CWND = CWND + 1 or 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 19 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 20 FreeBSD sets CWND = CWND + 1 or 7 and sends 7 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 21 RECEIVER sends ACK of 7 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 22 FreeBSD sets CWND = CWND + 1 or 8 and sends 8 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 23 RECEIVER sends ACK of 8 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 24 FreeBSD sets CWND = 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 25 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 26 FreeBSD sets CWND = 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 27 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 28 FreeBSD sets CWND = 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 29 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 30 FreeBSD sets CWND = 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 31 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 32 FreeBSD sets CWND = 6 and sends 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 33 RECEIVER sends ACK of 6 segments | +-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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