About multicast

With IP multicast, data is broadcast to many IP addresses simultaneously.

At this time, multicast is best used for low-bandwidth coordination or service discovery and not a high-bandwidth solution.

Multicast traffic between OKD Pods is disabled by default. If you are using the OpenShift SDN default Container Network Interface (CNI) network provider, you can enable multicast on a per-project basis.

When using the OpenShift SDN network plug-in in networkpolicy isolation mode:

  • Multicast packets sent by a Pod will be delivered to all other Pods in the project, regardless of NetworkPolicy objects. Pods might be able to communicate over multicast even when they cannot communicate over unicast.

  • Multicast packets sent by a Pod in one project will never be delivered to Pods in any other project, even if there are NetworkPolicy objects that allow communication between the projects.

When using the OpenShift SDN network plug-in in multitenant isolation mode:

  • Multicast packets sent by a Pod will be delivered to all other Pods in the project.

  • Multicast packets sent by a Pod in one project will be delivered to Pods in other projects only if each project is joined together and multicast is enabled in each joined project.

Enabling multicast between Pods

You can enable multicast between Pods for your project.

Prerequisites
  • Install the OpenShift CLI (oc).

  • You must log in to the cluster with a user that has the cluster-admin role.

Procedure
  • Run the following command to enable multicast for a project. Replace <namespace> with the namespace for the project you want to enable multicast for.

    $ oc annotate netnamespace <namespace> \
        netnamespace.network.openshift.io/multicast-enabled=true
Verification steps

To verify that multicast is enabled for a project, complete the following procedure:

  1. Change your current project to the project that you enabled multicast for. Replace <project> with the project name.

    $ oc project <project>
  2. Create a Pod to act as a multicast receiver:

    $ cat <<EOF| oc create -f -
    apiVersion: v1
    kind: Pod
    metadata:
      name: mlistener
      labels:
        app: multicast-verify
    spec:
      containers:
        - name: mlistener
          image: registry.access.redhat.com/ubi8
          command: ["/bin/sh", "-c"]
          args:
            ["dnf -y install socat hostname && sleep inf"]
          ports:
            - containerPort: 30102
              name: mlistener
              protocol: UDP
    EOF
  3. Create a Pod to act as a multicast sender:

    $ cat <<EOF| oc create -f -
    apiVersion: v1
    kind: Pod
    metadata:
      name: msender
      labels:
        app: multicast-verify
    spec:
      containers:
        - name: msender
          image: registry.access.redhat.com/ubi8
          command: ["/bin/sh", "-c"]
          args:
            ["dnf -y install socat && sleep inf"]
    EOF
  4. Start the multicast listener.

    1. Get the IP address for the Pod:

      $ POD_IP=$(oc get pods mlistener -o jsonpath='{.status.podIP}')
    2. To start the multicast listener, in a new terminal window or tab, enter the following command:

      $ oc exec mlistener -i -t -- \
          socat UDP4-RECVFROM:30102,ip-add-membership=224.1.0.1:$POD_IP,fork EXEC:hostname
  5. Start the multicast transmitter.

    1. Get the Pod network IP address range:

      $ CIDR=$(oc get Network.config.openshift.io cluster \
          -o jsonpath='{.status.clusterNetwork[0].cidr}')
    2. To send a multicast message, enter the following command:

      $ oc exec msender -i -t -- \
          /bin/bash -c "echo | socat STDIO UDP4-DATAGRAM:224.1.0.1:30102,range=$CIDR"

      If multicast is working, the previous command returns the following output:

      mlistener