Problem with HTB bandwidth slicing when using TCP traffic

Linux Advanced Routing and Traffic Control

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Hi to all,

I am using CentOS 6.4 and have problems when I want to limit bandwidth
of TCP flow to some value smaller than 15 Mbit/s. Namely, I used iperf
to generate TCP traffic and limited bandwidth (with tc command) for
that flow on  6 Mbit/s. I got approximately 6 Mbit/s in average but
iperf, which I adjusted to report me statistics every second, showed
that in one second flow got 10 Mbit/s or more and than for few
consecutive seconds 0 Mbit/s. With UDP traffic everything works fine.
I expected that TCP bandwidth will fluctuate because of congestion
mechanism but not like this. When I reserve more than 15 Mbit/s
situation is pretty much OK.

Also I had similar problem when I tried to split link bandwidth (which
I had throttled previously on 10 Mbit/s with tc) between two TCP
flows. On the other side, TCPvsUDP and UDPvsUDP slicing works fine.

I had updated kernel version to 2.6.32-431 recently. I don't know did
this cause a problem because I didn't use tc htb mechanism in the
older version.

Do you have any idea why is this happening and how I can fix it?

These are commands I had used to create htb classes:

tc class add dev eth0 parent 1: classid 1:1 htb rate 10000kbps ceil 10000kbps
tc class add dev eth0 parent 1:1 classid 1:10 htb rate 4000kbps ceil 4000kbps
tc class add dev eth0 parent 1:1 classid 1:11 htb rate 6000kbps ceil 6000kbps

I will appreciate any help!

Slavica
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