Multicast

Multicast delivers traffic to a group rather than to an individual destination. A sender transmits packets to a multicast address, and multicast-capable network devices replicate those packets where paths toward receivers diverge. Unlike broadcast, multicast targets participating group members rather than every host in a broadcast domain. Unlike repeated unicast, it does not require the sender to transmit a separate copy of every packet for every receiver.

The bandwidth advantage depends on where replication occurs. For example, sending a 20 Mbps stream to 50 receivers using separate unicast flows requires approximately 1 Gbps of sender egress, excluding protocol overhead. With multicast, the sender can transmit approximately 20 Mbps, while the network creates additional copies along receiver paths. Each receiver still consumes 20 Mbps, and links after replication points still carry the necessary copies. Multicast reduces redundant transmission on shared paths; it does not eliminate receiver bandwidth or guarantee lower cloud charges.

Group membership and multicast routing are separate functions. IPv4 hosts commonly use Internet Group Management Protocol, or IGMP, to signal membership to their local network. IPv6 uses Multicast Listener Discovery. In traditional routed networks, protocols such as Protocol Independent Multicast establish distribution paths between routers. IGMP snooping on switches can restrict forwarding to ports with interested receivers. Joining a group does not, by itself, establish an end-to-end multicast path.

Public cloud networks do not necessarily provide the multicast behavior of a physical Ethernet network. In AWS, an ordinary Amazon VPC does not natively forward IP multicast between instances. AWS Transit Gateway provides an explicit multicast capability for supported VPC attachments. A deployment requires a multicast-enabled transit gateway, a multicast domain, associated VPC attachment subnets, and receiver membership. Depending on the domain configuration, membership can use IGMPv2 or static registration of participating network interfaces. Transit Gateway multicast supports IPv4; do not assume that IPv6 multicast or arbitrary IGMP versions are supported.

Transit Gateway multicast is separate from ordinary unicast routing. Adding a route for a multicast address to a VPC or transit gateway route table does not create group membership or a distribution tree. Engineers must also validate supported instance types, source and receiver requirements, security groups, network ACLs, host firewalls, and application socket configuration. IGMP control traffic uses IP protocol 2, while application payloads commonly use UDP. Both control and data paths need appropriate permissions.

Hybrid deployments require particular care. AWS Direct Connect connections, AWS Site-to-Site VPN connections, and transit gateway peering attachments do not natively extend a Transit Gateway multicast domain to on-premises receivers. An established private connection therefore does not automatically provide multicast connectivity. A practical alternative is to use multicast-capable appliances or software gateways that encapsulate multicast inside unicast tunnels across the hybrid connection, or to relay an on-premises stream into a supported cloud multicast domain. Tunnel endpoints, multicast routing, MTU overhead, encryption, failure recovery, and supported interface behavior must be designed explicitly.

Multicast commonly carries UDP traffic and does not inherently provide acknowledgments, retransmission, ordering, or congestion control. Applications may require sequence numbers, jitter buffers, forward error correction, or a separate recovery channel. Test receiver joins and leaves, packet loss, source failover, and capacity limits. Group membership is also not an authorization boundary: sensitive streams still require access controls and, where appropriate, encryption.

Choose multicast when many receivers need the same simultaneous stream within a controlled, supported network. For public web delivery, Amazon CloudFront reduces origin load through caching and edge distribution, but viewer delivery remains unicast rather than IP multicast. For personalized responses, reliable independent downloads, or sparse audiences, unicast may be simpler and more economical. Exam scenarios often hinge on distinguishing simultaneous packet replication from caching, and on recognizing that hybrid connectivity alone does not enable multicast.