pathMTU tuning needs parameter tcp_mtu_probe_ceiling

[Bernhard Riegler]

I found the parameter tcp_mtu_probe_floor
I need an additional parameter tcp_mtu_probe_ceiling

why ?
If the tcp-connection goes only in a broadcast domain
 (no IPv4 router in between, sending ICMP frag needed)
the pathMTU algorithm goes crasy.

I have 2 nodes IPv4 10.0.0.2 (ethernet cable) and 10.0.0.4 (802.11n)
testing with "iperf3 -s -i 5" as server on Ubuntu 20.04 LTS (10.0.0.2)
and ANDROID12 "MAGIC iperf3" as client (iperf3 -c 10.0.0.2 -t 22 -i 5 -P 1 -Z -O 5)
(10.0.0.4)

monitoring with wireshark shows ip packets with ip.len 65212 and tcp flags push,ack.
this does not work with the HW buffers of the LANIC,
therefore a reqest a control with parameter "tcp_mtu_probe_ceiling"

[Bernhard Riegler (riegler-b)]

I repeated the tests with LANIC HW features Large ReceiveO ffload and TCP Segmentation Offload
turned OFF.

Now the IPv4 packets are stable with MTU=1500 and TCP MSS=1460 in a broadcast domain.
the IPv4 packets up to 64KiB are caused by the HW features of LANIC and not by pathMTU algorithm.
you can close this request.