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I am using netem and tc to generate network impairments in my network. I have noticed that after I have upgraded my server from 16.04 to 18.04 the jitter functionality is not working properly. The time delay and the packet loss are both working correctly but not the jitter. This is my command:

sudo tc qdisc change dev eth0 root netem delay 200ms 100ms

And this is the output of the ping command:

$ ping 10.82.96.5 -c 10
PING 10.82.96.5 (10.82.96.5) 56(84) bytes of data.
64 bytes from 10.82.96.5: icmp_seq=4 ttl=64 time=258 ms
64 bytes from 10.82.96.5: icmp_seq=5 ttl=64 time=272 ms
64 bytes from 10.82.96.5: icmp_seq=1 ttl=64 time=4418 ms
64 bytes from 10.82.96.5: icmp_seq=6 ttl=64 time=230 ms
64 bytes from 10.82.96.5: icmp_seq=2 ttl=64 time=4704 ms
64 bytes from 10.82.96.5: icmp_seq=7 ttl=64 time=295 ms
64 bytes from 10.82.96.5: icmp_seq=3 ttl=64 time=4420 ms
64 bytes from 10.82.96.5: icmp_seq=9 ttl=64 time=240 ms
64 bytes from 10.82.96.5: icmp_seq=10 ttl=64 time=290 ms
64 bytes from 10.82.96.5: icmp_seq=8 ttl=64 time=4398 ms

--- 10.82.96.5 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9064ms
rtt min/avg/max/mdev = 230.158/1953.160/4704.921/2069.517 ms, pipe 5

I am expecting here to have RTT between 100ms and 300ms considering that the configured delay is 200ms and the jitter 100ms (200+/-100) but all the RTT values are 200 or higher.

If I generate only time delay or delay and packet loss all works fine:

sudo tc qdisc change dev eth0 root netem delay 200ms loss 10%

The output of the ping command:

$ ping 10.82.96.5 -c 10
PING 10.82.96.5 (10.82.96.5) 56(84) bytes of data.
64 bytes from 10.82.96.5: icmp_seq=1 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=2 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=3 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=5 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=6 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=7 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=8 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=9 ttl=64 time=200 ms
64 bytes from 10.82.96.5: icmp_seq=10 ttl=64 time=200 ms

--- 10.82.96.5 ping statistics ---
10 packets transmitted, 9 received, 10% packet loss, time 9038ms
rtt min/avg/max/mdev = 200.183/200.220/200.306/0.517 ms

I am pretty sure that these commands worked in the 16.04 LTS so I guess it is a bug.

As a matter of fact I am applying these network impairments on a bridged interface but this should not be that important.

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    This seems to be an issue with the kernel 4.15 or possible below and some above shipped with Ubuntu 18.04 (bug /1783822). Updating the kernel to 4.18 fixed this for me (kernel update). – Sidhant101 Oct 10 '18 at 12:45
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    I confirm that this bug is not present in Ubuntu 18.10 running kernel 4.18. The jitter is generated just fine in this version. – Georgi Stoyanov Oct 23 '18 at 10:07

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