Overview

Bottleneck Bandwidth and Round-trip propagation time (BBR) is a TCP congestion control algorithm developed by Google in 2016. It helps significantly improve the throughput and the TCP connection latency of Linux servers. However, enabling BBR requires a Linux kernel version of 4.10 or later. If you use an earlier version, you need to upgrade your kernel. This document guides you through how to manually change the kernel and enable BBR on your Linux server.

Directions

Updating the kernel package

1. Run the following command to check the current kernel version.

   uname -r

2. Run the following command to update the software package.

   yum update -y

3. Run the following command to import the public key of ELRepo.

   rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org

4. Run the following command to install the yum repository of ELRepo.

   yum install https://www.elrepo.org/elrepo-release-7.0-4.el7.elrepo.noarch.rpm

Installing a new kernel

1. Run the following command to check the supported kernel package in the ELRepo repository.

   yum --disablerepo="*" --enablerepo="elrepo-kernel" list available

2. Run the following command to install the latest mainline stable kernel.

   yum --enablerepo=elrepo-kernel install kernel-ml

Modifying the grub configuration

1. Run the following command to open the /etc/default/grub file.

   vim /etc/default/grub

2. Press i to switch to the edit mode and change GRUB_DEFAULT=saved to GRUB_DEFAULT=0.
   
3. Press Esc and enter :wq to save and close the file.
4. Run the following command to generate the kernel configuration again.

   grub2-mkconfig -o /boot/grub2/grub.cfg

5. Run the following command to restart the server.

   reboot

6. Run the following command to check whether the modification is successful.

   uname -r

Deleting unnecessary kernels

1. Run the following command to view all kernels.

   rpm -qa | grep kernel

2. Run the following command to delete the older kernel.

   yum remove kernel-old_kernel_version

   For example:

   yum remove kernel-3.10.0-957.el7.x86_64

Enabling BBR

1. Run the following command to edit the /etc/sysctl.conf file.

   vim /etc/sysctl.conf

2. Press i to switch to the edit mode and enter the following:

   net.core.default_qdisc=fq
   net.ipv4.tcp_congestion_control=bbr

3. Press Esc and enter :wq to save and close the file.
4. Run the following command to load the kernel parameter settings to the /etc/sysctl.conf configuration file.

   sysctl -p

5. Run the following commands to verify whether BBR has been successfully enabled.

   sysctl net.ipv4.tcp_congestion_control
   # The following appears if the configuration succeeds:
   # net.ipv4.tcp_congestion_control = bbr

   sysctl net.ipv4.tcp_available_congestion_control
   # The following appears if the configuration succeeds:
   # net.ipv4.tcp_available_congestion_control = reno cubic bbr

6. Run the following commend to check whether the kernel module is loaded.

   lsmod | grep bbr

   If the following information is returned, BBR has been successfully enabled.