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
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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