10 May 2020 – T.I.L

I have reading this new post from Cloudflare about their congestion control implementations for QUIC.

Reading the article I wanted to check the TCP CCA (Congestion Control Algorithm) available in my laptop (Debian 1o Testing).

So I searched a bit and found a couple of useful links like this:

For checking your current TCP CCA:

# sysctl net.ipv4.tcp_congestion_control
net.ipv4.tcp_congestion_control = cubic

$ cat /proc/sys/net/ipv4/tcp_congestion_control
cubic

For checking the available TCP CCAs:

# sysctl net.ipv4.tcp_available_congestion_control
net.ipv4.tcp_available_congestion_control = reno cubic

As well, you can see via “ss” the CCA per connection:

$ ss -ti
...
tcp   ESTAB      0       0                                     192.168.1.158:60238                     169.54.204.232:https       
	 cubic wscale:7,7 rto:320 rtt:116.813/2.428 ato:40 mss:1448 pmtu:1500 rcvmss:1448 advmss:1448 cwnd:10 bytes_sent:4366 bytes_acked:4367 bytes_received:7038 segs_out:98 segs_in:183 data_segs_out:91 data_segs_in:93 send 991.7Kbps lastsnd:1260 lastrcv:1260 lastack:1140 pacing_rate 2.0Mbps delivery_rate 102.2Kbps delivered:92 app_limited busy:10632ms rcv_space:14480 rcv_ssthresh:64088 minrtt:113.391
...

If you want to change your TCP CCA, this is a good link:

Check the modules installed:

$ ls -la /lib/modules/$(uname -r)/kernel/net/ipv4

Check the kernel config:

$ grep TCP_CONG /boot/config-$(uname -r)
CONFIG_TCP_CONG_ADVANCED=y
CONFIG_TCP_CONG_BIC=m
CONFIG_TCP_CONG_CUBIC=y
CONFIG_TCP_CONG_WESTWOOD=m
CONFIG_TCP_CONG_HTCP=m
CONFIG_TCP_CONG_HSTCP=m
CONFIG_TCP_CONG_HYBLA=m
CONFIG_TCP_CONG_VEGAS=m
CONFIG_TCP_CONG_NV=m
CONFIG_TCP_CONG_SCALABLE=m
CONFIG_TCP_CONG_LP=m
CONFIG_TCP_CONG_VENO=m
CONFIG_TCP_CONG_YEAH=m
CONFIG_TCP_CONG_ILLINOIS=m
CONFIG_TCP_CONG_DCTCP=m
CONFIG_TCP_CONG_CDG=m
CONFIG_TCP_CONG_BBR=m
CONFIG_DEFAULT_TCP_CONG="cubic"

We can see that “cubic” is the default TCP CCA and we have for example BBR available as a module.

So let’s change to BBR (rfc, github, blog) based on this link:

Check the kernel supports BBR:

$ cat /boot/config-$(uname -r) | grep 'CONFIG_TCP_CONG_BBR'
CONFIG_TCP_CONG_BBR=m
$ cat /boot/config-$(uname -r) | grep 'CONFIG_NET_SCH_FQ'
CONFIG_NET_SCH_FQ_CODEL=m
CONFIG_NET_SCH_FQ=m

Enable TCP BBR:

# vi /etc/sysctl.conf
net.core.default_qdisc=fq
net.ipv4.tcp_congestion_control=bbr

Apply the changes:

# sysctl --system

And check:

$ cat /proc/sys/net/ipv4/tcp_congestion_control
bbr

So we have moved from CUBIC to BBR. Let’s see how is the experience in the following days.

Day: 10 May 2020

TCP Congestion Control and Recovery