Summary

  • IGMPv3 and MLDv2 report a receiver’s multicast interest and source-filter state to a neighboring router; they do not acknowledge end-to-end packet delivery.
  • PIM-SM must separately turn that interest into shared-tree or source-tree state using a designated router, reverse-path information, an RP or SSM logic, and forwarding state.
  • A credible delivery verdict joins the listener record to the exact source and group, tree state, forwarding counters and packets observed by the intended application.

A clean join and a black screen

An operations console records a fresh IGMPv3 membership report from the correct access interface. The group address is right, the timer is current and the first-hop router shows an interested receiver. The video screen remains black.

That combination is not contradictory. It exposes two different claims. The report says that a system requested reception. It does not say that the expected source is active, that a PIM join reached the correct upstream state, that reverse-path forwarding chose a usable neighbor, that replication was installed, or that a packet reached the application socket.

Treating the membership report as a delivery receipt shortens the investigation at precisely the wrong boundary. It can turn a source-filter mismatch, stale RP mapping, failed tree transition, missing multicast forwarding entry or downstream application problem into an unexplained “receiver issue”.

Listener state ends at the neighboring router

RFC 3376 defines IGMPv3 as the mechanism by which IPv4 systems report multicast group memberships to neighboring multicast routers. Its important advance is source filtering. INCLUDE requests traffic only from listed sources; EXCLUDE requests traffic from all sources except those listed. The state is specific to a socket, interface, group, filter mode and source list.

The distinction within the host matters. RFC 3376 derives per-interface state from one or more socket requests, but a packet accepted on an interface can still be delivered to one listening socket and not another. Transport ports and other local conditions remain outside the membership report. RFC 3810 provides the analogous MLDv2 behavior for IPv6 listeners.

These protocols give the first-hop router an expression of interest. They do not carry a receipt from the source, every transit router or the receiving application. A current report is strong evidence about requested local state and weak evidence about the rest of the path.

PIM builds a different chain of state

RFC 7761 describes the next stage. A designated router acts for directly attached receivers and sends PIM Join messages toward a rendezvous point or source. Those messages travel hop by hop and instantiate (*,G), (S,G) or (S,G,rpt) state. The multicast routing information base supplies reverse-path information; the tree information base records distribution-tree state; an implementation will normally derive an efficient multicast forwarding information base for actual packet forwarding.

Each step can change the verdict. The RP mapping may be wrong or unreachable. The reverse-path neighbor can change with underlying routing. Join state can expire. A source may begin before or after the receiver joins. A receiver-side router may move from the shared tree to a shortest-path tree. On a multi-access LAN, duplicate traffic can appear until PIM Assert elects one forwarder.

None of these mechanisms makes an IGMP or MLD report unreliable. They show why it is only the first link in an evidence chain.

Source identity is part of delivery

RFC 4607 defines Source-Specific Multicast around an exact source and group pair. In SSM, interest in group G is not enough; the receiver asks for traffic from source S to G. A group-only dashboard can therefore look healthy while the requested (S,G) state is absent or a different source is sending.

Operators should carry source-filter mode and source list through the whole investigation. Compare the receiver request with the first-hop interface record, the PIM state, the RPF neighbor, the incoming interface, the outgoing-interface list and counters. Then observe packets at the intended receiver, not merely at an upstream tap.

Sources