UDP Multicast Behaviour

The behavior you're experiencing with UDP multicast in AWS has several technical explanations:

Regarding TTL=0: In standard IP networking, a TTL of 0 should indeed prevent packets from leaving the host. However, in AWS networking environments, there may be some virtualization layer handling that allows these packets to be processed differently than on traditional hardware. The fact that your setup worked for over a year with TTL=0 suggests that AWS's networking implementation might be interpreting or handling the TTL differently than expected in a traditional network stack.

Regarding the local consumer issue: When you run a local consumer on the same EC2 instance as your producer, you're joining that interface to the multicast group. This likely changes how the Linux kernel's multicast routing handles those packets. With a local consumer present, the kernel may be enforcing the TTL=0 restriction more strictly, keeping those packets local to the host as technically should happen with TTL=0.

When you set TTL to 1 or higher, you're explicitly telling the network stack that these packets can leave the local host, which is why the behavior works regardless of whether a local consumer is present.

The Transit Gateway Multicast Domain is designed to handle multicast traffic across VPCs and subnets. When you register your EC2 as a source in the multicast domain, AWS is expecting properly configured multicast packets with appropriate TTL values. The interaction between your local interface joining a multicast group and the Transit Gateway's handling of your instance as a source is likely causing the behavior you're seeing.

To resolve this issue, the correct approach would be to set your TTL to at least 1 (or higher depending on how many network hops your multicast traffic needs to traverse). This would ensure proper multicast behavior regardless of local consumers.

The behavior isn’t a defect but a result of architectural layering. AWS’s virtual network behaves more like an overlay multicast distribution system that begins replication at the ENI boundary, not a physical router. Hence, any TTL-based limitation that works within host-local contexts can seemingly be overridden by design, unless the local OS forces the rule internally through active membership. Adjusting TTL to one or greater synchronizes both the AWS network layer and Linux’s behavior, ensuring expected multicast transmission flows even with local consumers present.

Sources
Amazon EC2 examples using AWS CLI - AWS SDK Code Examples

AWS Transit Gateway Multicast Overview

Running Multicast-Enabled Containers on AWS

Integrating External Multicast Services with AWS