low TCP throughput with 80MHz channel width

[Iñaki Pascual]

Hi everyone,

we have configured two boxes with 802.11ac NICs to work in channel 48 
(5240 MHz), ad-hoc mode, 80MHz channel width.
For UDP traffic we get 300 Mbps bandwith but for TCP we are around 30 Mbps.

We have tried other channels with similar results.
Throughput for 20 and 40 MHz channel width are ok (20MHz: TCP 21Mbps, 
UDP 27Mbps, 40MHz: TCP 82Mbps, UDP 144Mbps).

Testing with iperf we have also tried different congestion windows with 
no or low improvements.

Below are config details and the sequence of commands we are using to 
configure the interface.

Any ideas on what we may be doing wrong?

Bests,

Iñaki

iw outputs:

root at sc227:/home/monet# iw wlan2 info
Interface wlan2
     ifindex 6
     wdev 0x200000001
     addr 04:f0:21:18:26:a2
     ssid SC226SC227
     type IBSS
     wiphy 2
     channel 48 (5240 MHz), width: 80 MHz, center1: 5210 MHz

root at sc227:/home/monet# iw wlan2 station dump
Station 04:f0:21:18:26:a8 (on wlan2)
     inactive time:    86 ms
     rx bytes:    467732521
     rx packets:    364249
     tx bytes:    1394558
     tx packets:    16200
     tx retries:    0
     tx failed:    0
     signal:      -51 dBm
     signal avg:    -51 dBm
     tx bitrate:    351.0 MBit/s VHT-MCS 4 80MHz VHT-NSS 2
     rx bitrate:    526.5 MBit/s VHT-MCS 4 80MHz VHT-NSS 3
     authorized:    yes
     authenticated:    yes
     preamble:    long
     WMM/WME:    yes
     MFP:        no
     TDLS peer:    no
     connected time:    3169 seconds


config commands:

ifconfig wlan2 down
iw wlan2 set type ibss
ifconfig wlan2 up
ifconfig wlan2 10.2.1.7 netmask 255.255.255.0
iw wlan2 ibss join SC226SC227 5240 80MHz

iperf:
root at sc227:/home/monet# iperf -c 10.2.1.6
------------------------------------------------------------
Client connecting to 10.2.1.6, TCP port 5001
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[  3] local 10.2.1.7 port 48548 connected with 10.2.1.6 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  33.1 MBytes  27.7 Mbits/sec