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Multicast User Guide

Antrea supports multicast traffic in the following scenarios:

  1. Pod to Pod - a Pod that has joined a multicast group will receive the multicast traffic to that group from the Pod senders.
  2. Pod to External - external hosts can receive the multicast traffic sent from Pods, when the Node network supports multicast forwarding / routing to the external hosts.
  3. External to Pod - Pods can receive the multicast traffic from external hosts.

Table of Contents

Prerequisites

Multicast support was introduced in Antrea v1.5.0 as an alpha feature, and was graduated to beta in v1.12.0.

  • Prior to v1.12.0, a feature gate, Multicast must be enabled in the antrea-controller and antrea-agent configuration to use the feature.
  • Starting from v1.12.0, the feature gate is enabled by default, you need to set the multicast.enable flag to true in the antrea-agent configuration to use the feature.

There are three other configuration options -multicastInterfaces, igmpQueryVersions, and igmpQueryInterval for antrea-agent.

  antrea-agent.conf: |
    multicast:
      enable: true
      # The names of the interfaces on Nodes that are used to forward multicast traffic.
      # Defaults to transport interface if not set.
      multicastInterfaces: 
      # The versions of IGMP queries antrea-agent sends to Pods.
      # Valid versions are 1, 2 and 3.
      igmpQueryVersions:
      - 1
      - 2
      - 3
      # The interval at which the antrea-agent sends IGMP queries to Pods.
      # Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
      igmpQueryInterval: "125s"

Multicast NetworkPolicy

Antrea NetworkPolicy and Antrea ClusterNetworkPolicy are supported for the following types of multicast traffic:

  1. IGMP egress rules: applied to IGMP membership report and IGMP leave group messages.
  2. IGMP ingress rules: applied to IGMP query, which includes IGMPv1, IGMPv2, and IGMPv3.
  3. Multicast egress rules: applied to non-IGMP multicast traffic from the selected Pods to other Pods or external hosts.

Note, multicast ingress rules are not supported at the moment.

Examples: You can refer to the ACNP for IGMP traffic and ACNP for multicast egress traffic examples in the Antrea NetworkPolicy document.

Debugging and collecting multicast statistics

Antrea provides tooling to check multicast group information and multicast traffic statistics.

Pod multicast group information

The kubectl get multicastgroups command prints multicast groups joined by Pods in the cluster. Example output of the command:

$ kubectl get multicastgroups
GROUP       PODS
225.1.2.3   default/mcjoin, namespace/pod
224.5.6.4   default/mcjoin

Inbound and outbound multicast traffic statistics

antctl supports printing multicast traffic statistics of Pods. Please refer to the corresponding antctl user guide section.

Multicast NetworkPolicy statistics

The Antrea NetworkPolicyStats feature also supports multicast NetworkPolices.

Use case example

This section will take multicast video streaming as an example to demonstrate how multicast works with Antrea. In this example, VLC multimedia tools are used to generate and consume multicast video streams.

To start a video streaming server, we start a VLC Pod to stream a sample video to the multicast IP address 239.255.12.42 with TTL 6.

kubectl run -i --tty --image=quay.io/galexrt/vlc:latest vlc-sender -- --intf ncurses --vout dummy --aout dummy 'https://upload.wikimedia.org/wikipedia/commons/transcoded/2/26/Bees_on_flowers.webm/Bees_on_flowers.webm.120p.vp9.webm' --sout udp:239.255.12.42 --ttl 6 --repeat

You can verify multicast traffic is sent out from this Pod by running antctl get podmulticaststats in the antrea-agent Pod on the local Node, which indicates the VLC Pod is sending out multicast video streams.

You can also check the multicast routes on the Node by running command ip mroute, which should print the following route for forwarding the multicast traffic from the Antrea gateway interface to the transport interface.

$ ip mroute
(<POD IP>, 239.255.12.42)     Iif: antrea-gw0 Oifs: <TRANSPORT INTERFACES> State: resolved

We also create a VLC Pod to be the receiver with the following command:

kubectl run -i --tty --image=quay.io/galexrt/vlc:latest vlc-receiver -- --intf ncurses --vout dummy --aout dummy udp://@239.255.12.42 --repeat

It's expected to see inbound multicast traffic to this Pod by running antctl get podmulticaststats in the local antrea-agent Pod, which indicates the VLC Pod is receiving the video stream.

Also, the kubectl get multicastgroups command will show that vlc-receiver has joined multicast group 239.255.12.42.

Limitations

This feature is currently supported only for IPv4 Linux clusters. Support for Windows and IPv6 will be added in the future.

Encap mode

The configuration option multicastInterfaces is not supported with encap mode. Multicast packets in encap mode are SNATed and forwarded to the transport interface only.

Maximum number of receiver groups on one Node

A Linux host limits the maximum number of multicast groups it can subscribe to; the default number is 20. The limit can be changed by setting /proc/sys/net/ipv4/igmp_max_memberships. Users are responsible for changing the limit if Pods on the Node are expected to join more than 20 groups.

Traffic in local network control block

Multicast IPs in Local Network Control Block (224.0.0.0/24) can only work in encap mode. Multicast traffic destined for those addresses is not expected to be forwarded, therefore, no multicast route will be configured for them. External hosts are not supposed to send and receive traffic with those addresses either.

Linux kernel

If the following situations apply to your Nodes, you may observe multicast traffic is not routed correctly:

  1. Node kernel version under 5.4
  2. Node network doesn't support IGMP snooping

Antrea FlexibleIPAM

The configuration option multicastInterfaces is not supported with Antrea FlexibleIPAM. When Antrea FlexibleIPAM is enabled, multicast packets are forwarded to the uplink interface only.