Re: Slow WAN traffic to FreeBSD hosts but not to Linux hosts---how to debug/fix?

From: Paul Mather <>
Date: Wed, 01 Feb 2023 16:26:24 UTC
On Jan 31, 2023, at 3:38 PM, Marek Zarychta <> wrote:

> W dniu 31.01.2023 o 19:31, Paul Mather pisze:
>>> While playing with different mod_cc(4) might bring some improvement, to get a real boost I'd suggest enabling tcp_rack(4) if feasible.
>> I am interested in trying this out, but believe it is more feasible in my case for the -STABLE and -CURRENT systems I am using, not so much for the -RELEASE systems that are kept up to date via binary freebsd-update updates.  My reading of the tcp_rack(4) man page is that you have to build a custom kernel as, unlike the cc_* congestion control algorithms, the loadable tcp_rack module is not built by default.  Is that an accurate reading?
> Yes, this gift from Netflix is probably better suited for -STABLE and -CURRENT as easier to set up there. There is an excellent, up-to-date article about it by Klara Systems writers[1]. From my experience tcp_rack(4) is well suited for congested, lossy or redundant network paths where loses, duplicated packets or races between packets occur. Not a panacea, but very performant TCP stack based on the _fair_  algorithm. In some instances, it might help you to saturate the bandwidth of the link. TCP algo can be loaded/unloaded/changed on the fly. In FreeBSD 14-CURRENT you can change it on an active socket with tcpsso(8) utility, in FreeBSD 12 and 13 you have to restart the app bound to the socket.
> Please feel free to play with TCP stacks and congestion algos with the help of benchmarks/iperf3 to find out what prevents the link from being saturated and give us some feedback here.
> [1]

Thank you, Marek, for the link to the Klara article about building and using RACK.  I'm building it now on a FreeBSD-CURRENT system and will test it out.