CODEWITHSUNDEEP
linuxnetwork-programmingtrafficshapingipfw
user308917
user308917

Reputation:

Traffic shaping with tc is inaccurate with high bandwidth and delay

I'm using tc with kernel 2.6.38.8 for traffic shaping. Limit bandwidth works, adding delay works, but when shaping both bandwidth with delay, the achieved bandwidth is always much lower than the limit if the limit is >1.5 Mbps or so.

Example:

tc qdisc del dev usb0 root
tc qdisc add dev usb0 root handle 1: tbf rate 2Mbit burst 100kb latency 300ms
tc qdisc add dev usb0 parent 1:1 handle 10: netem limit 2000 delay 200ms

Yields a delay (from ping) of 201 ms, but a capacity of just 1.66 Mbps (from iperf). If I eliminate the delay, the bandwidth is precisely 2 Mbps. If I specify a bandwidth of 1 Mbps and 200 ms RTT, everything works. I've also tried ipfw + dummynet, which yields similar results.

I've tried using rebuilding the kernel with HZ=1000 in Kconfig -- that didn't fix the problem. Other ideas?

Upvotes: 1

Views: 3620

Answers (2)

Kal
Kal

Reputation: 421

Think of it like this:

Suppose you set your latency to 1 hour, and your speed to 2 Mbit/s.

2 Mbit/s requires (for example) 50 Kbit/s for TCP ACKs. Because the ACKs take over a hour to reach the source, then the source can't continue sending at 2 Mbit/s because the TCP window is still stuck waiting on the first acknowledgement.

Latency and bandwidth are more related than you think (in TCP at least. UDP is a different story)

Upvotes: 1

Alex
Alex

Reputation: 81

It's actually not a problem, it behaves just as it should. Because you've added a 200ms latency, the full 2Mbps pipe isn't used at it's full potential. I would suggest you study the TCP/IP protocol in more detail, but here is a short summary of what is happening with iperf: your default window size is maybe 3 packets (likely 1500 bytes each). You fill your pipe with 3 packets, but now have to wait until you get an acknowledgement back (this is part of the congestion control mechanism). Since you delay the sending for 200ms, this will take a while. Now your window size will double in size and you can next send 6 packets, but will again have to wait 200ms. Then the window size doubles again, but by the time your window is completely open, the default 10 second iperf test is close to over and your average bandwidth will obviously be smaller.

Upvotes: 8

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