What does option limit in tc netem mean and do?

Question

I'm trying to emulate slow net link with command tc. I use netem to emulate delay and packet loss and htb to emulate narrow bandwidth, but I find there is a limit option in netem, what does this option do? will it affect the final bandwidth?

I googled it and find something in http://manpages.ubuntu.com/manpages/raring/man8/tc-netem.8.html

which says:

limits the effect of selected options to the indicated number of next packets.

But I still can not understand what it does.

Answer 1

From https://lists.linuxfoundation.org/pipermail/netem/2007-March/001091.html

The "limit" parameter refers to the number of buffers allocated in the netem module.

The limit must be adjusted to support the number of frames delayed (500ms for e.g.) at a given data rate.

Yours sincerely,

Laurent MARIE

Answer 2

I don't know exactly what netem is doing, but I've found that if you don't set "limit" to a higher value, netem doesn't work correctly - i.e. it discards packets at higher speeds and possibly has other problems, essentially not accurately emulating a real network.

From the mailing list mentioned by CarlH, Stephen Hemminger said:

The limit value is in packets at least when using the default qdisc inside netem (tfifo). You can also use pfifo and configure it for packet limit, or bfifo same only bytes. The value 1000 is low, you want about 50% more than the max packet rate * delay, unless you are trying to emulate a router with a small queue.

So for a 1 Gbps link, 1 Gbps / 1500 bytes MTU * 100 ms * 1.5 = 12500.

Command:

sudo tc qdisc add dev eth1 root netem limit 12500 delay 100ms loss 1%

I've been using limit 100000, which seems to work fine, but it seems a lower value may be fine.

Answer 3

The updated documentation says:

limit packets
maximum number of packets the qdisc may hold queued at a time.