CODEWITHSUNDEEP
linuxunixudpmulticast
Herodotus
Herodotus

Reputation: 31

If there are no subscribers, does udp multicast put anything on the wire?

Suppose an application is writing to a udp multicast, and all subscribers quit (or perhaps no processes ever register to read the multicast). Does anything go out on the wire?

Upvotes: 3

Views: 627

Answers (2)

dbush
dbush

Reputation: 224522

The packet will always be sent out. IGMP messages, which contain information about hosts joining/leaving multicast groups, are typically only processed by routers so they know where to route multicast traffic. So hosts generally don't have that information.

Even then, routers may not forward IGMP messages but may have static multicast routes set up to forward certain traffic anyway. In that situation, multicast traffic could pass through routers to an intended destination even in the absence of IGMP.

Regarding which interface(s) the source host sends on, that's application defined behavior. The sending socket sets the IP_MULTICAST_IF or IPV6_MULTICAST_IF socket option to dictate which interface multicast traffic is sent out on. If this option is not set, the system chooses a default interface to send multicast packets out on.

Upvotes: 1

user207421
user207421

Reputation: 311018

The source host always sends the datagram. It is up to the router to decide whether there are group members on the other side, and if so to forward the datagram, otherwise drop it.

Upvotes: 3

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