Chapter 24. Getting started with Multipath TCP


Multipath TCP (MPTCP) is an extension to the original TCP protocol (single-path). MPTCP enables a transport connection to operate across multiple paths simultaneously, and brings network connection redundancy to user endpoint devices.

24.1. Understanding MPTCP

The Multipath TCP (MPTCP) protocol provides simultaneous usage of multiple paths between connection endpoints. The protocol design improves connection stability and also brings other benefits compared to the single-path TCP.

Note

In MPTCP terminology, links are considered as paths.

The following are some of the advantages of using MPTCP:

  • It allows a connection to simultaneously use multiple network interfaces.
  • In case a connection is bound to a link speed, the usage of multiple links can increase the connection throughput. Note, that in case of the connection is bound to a CPU, the usage of multiple links causes the connection slowdown.
  • It increases the resilience to link failures.

You can configure MultiPath TCP (MPTCP) by using the nmcli command to permanently establish multiple subflows between a source and destination system.

The subflows can use different resources, different routes to the destination, and even different networks. Such as Ethernet, cellular, wifi, and so on. As a result, you achieve combined connections, which increase network resilience and throughput.

The server uses the following network interfaces in our example:

  • enp4s0: 192.0.2.1/24
  • enp1s0: 198.51.100.1/24
  • enp7s0: 192.0.2.3/24

The client uses the following network interfaces in our example:

  • enp4s0f0: 192.0.2.2/24
  • enp4s0f1: 198.51.100.2/24
  • enp6s0: 192.0.2.5/24

Prerequisites

  • You configured the default gateway on the relevant interfaces.

Procedure

  1. Enable MPTCP sockets in the kernel:

    # echo "net.mptcp.enabled=1" > /etc/sysctl.d/90-enable-MPTCP.conf
    # sysctl -p /etc/sysctl.d/90-enable-MPTCP.conf
    Copy to Clipboard Toggle word wrap
  2. Optional: The RHEL kernel default for subflow limit is 2. If you require more:

    1. Create the /etc/systemd/system/set_mptcp_limit.service file with the following content:

      [Unit]
      Description=Set MPTCP subflow limit to 3
      After=network.target
      
      [Service]
      ExecStart=ip mptcp limits set subflows 3
      Type=oneshot
      
      [Install]
      WantedBy=multi-user.target
      Copy to Clipboard Toggle word wrap

      The oneshot unit executes the ip mptcp limits set subflows 3 command after your network (network.target) is operational during every boot process.

      The ip mptcp limits set subflows 3 command sets the maximum number of additional subflows for each connection, so 4 in total. It is possible to add maximally 3 additional subflows.

    2. Enable the set_mptcp_limit service:

      # systemctl enable --now set_mptcp_limit
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  3. Enable MPTCP on all connection profiles that you want to use for connection aggregation:

    # nmcli connection modify <profile_name> connection.mptcp-flags signal,subflow,also-without-default-route
    Copy to Clipboard Toggle word wrap

    The connection.mptcp-flags parameter configures MPTCP endpoints and the IP address flags. If MPTCP is enabled in a NetworkManager connection profile, the setting will configure the IP addresses of the relevant network interface as MPTCP endpoints.

    By default, NetworkManager does not add MPTCP flags to IP addresses if there is no default gateway. If you want to bypass that check, you need to use the also-without-default-route flag.

Verification

  1. Verify that you enabled the MPTCP kernel parameter:

    # sysctl net.mptcp.enabled
    net.mptcp.enabled = 1
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  2. Verify that you set the subflow limit correctly, in case the default was not enough:

    # ip mptcp limit show
    add_addr_accepted 2 subflows 3
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  3. Verify that you configured the per-address MPTCP setting correctly:

    # ip mptcp endpoint show
    192.0.2.1 id 1 subflow dev enp4s0
    198.51.100.1 id 2 subflow dev enp1s0
    192.0.2.3 id 3 subflow dev enp7s0
    192.0.2.4 id 4 subflow dev enp3s0
    ...
    Copy to Clipboard Toggle word wrap

24.3. Configuring mptcpd

The mptcpd service is a component for configuring Multipath TCP (MPTCP) endpoints. It automatically creates and dynamically updates a list of subflow endpoints for each IP address on the host, providing an alternative to the ip utility for enabling multiple paths.

Prerequisites

  • The mptcpd package installed

Procedure

  1. Enable mptcp.enabled option in the kernel with the following command:

    # echo "net.mptcp.enabled=1" > /etc/sysctl.d/90-enable-MPTCP.conf
    # sysctl -p /etc/sysctl.d/90-enable-MPTCP.conf
    Copy to Clipboard Toggle word wrap
  2. Enable and start the mptcpd service:

    # systemctl enable --now mptcp.service
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  3. To configure mptcpd service manually, modify the /etc/mptcpd/mptcpd.conf configuration file.

    Note, that the endpoint, which mptcpd service creates, lasts till the host shutdown.

Verification

  • Verify endpoint creation:

    # ip mptcp endpoint
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