high RTT in 40 MHz channel width

Michal Kazior michal.kazior at tieto.com
Thu May 5 03:47:18 PDT 2016

On 29 April 2016 at 19:12, Iñaki Pascual <ipascual at cttc.cat> wrote:
> Hi,
> I am measuring TCP and UDP latency (actually RTT) and I am getting too high
> values when working with channels with 40MHz (also with 80MHz) width.
> I am using hping3 for testing and these are the RTT avg values:
> BW 20 MHz: TCP 0.9 UDP 1.1
> BW 40 MHz: TCP 7.7 UDP 1.3
> BW 80 MHz: TCP 3.5 UDP 1.3

Just 2cc. Did you try changing the a-msdu/a-mpdu limits in firmware?
You can tune it via debugfs file:

  echo 3 64 > /sys/kernel/debug/ieee80211/phy0/ath10k/htt_max_amsdu_ampdu

The "3" stands for A-MSDU limit, "64" stands for A-MPDU limit. "3 64"
is the default. You can test, e.g.

 - "1 64"
 - "3 8"
 - "1 1"
 - "1 8"

And see if this changes TCP RTT in any way.

You didn't really tell your setup (or I missed it). Are you using
ath10k as AP, client, or both (i.e. have two ath10k supported

Are you bridging traffic or is to locally generated? Perhaps there's a
problem with hw offloaded ip/tcp checksumming. You might want to
remove it from the driver (manually) to verify that or check tcpdump
to confirm whether OTA frames have correct checksums.


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