Summary

  • RFC 1075’s Non-Membership Report said that a sending router had no descendant members for a listed multicast group; it gave an upstream router a time-bounded reason to stop forwarding that group down that interface.
  • The report was neither a host’s leave message nor a proof that the group was empty across the Internet: local membership, reverse-path tree state, downstream scope and application delivery remained different facts.
  • A matching NMR Cancel withdrew a prior negative report; expiry and cancellation were part of the evidence, not administrative clutter.

A negative answer with an address on it

RFC 1075, the 1988 experimental Distance Vector Multicast Routing Protocol, was trying to make multicast forwarding economical without pretending that every branch needed every packet. DVMRP built forwarding decisions around reverse paths to a multicast source. In that arrangement, a router needed a way to tell its upstream neighbor that continuing to send a particular group into one downstream direction served nobody it knew about.

The Non-Membership Report, or NMR, supplied that narrow message. For each listed multicast address, it carried a hold-down time. The document says the sending router has no descendant group members, and that the receiver can stop forwarding the named group’s datagrams to that router for the stated interval. Each important word limits the claim. “Descendant” describes a relation inside a forwarding tree, not a population of all possible receivers. “Can stop” describes a forwarding consequence for the receiving router, not a global command. The hold-down time makes the claim perishable by design.

That is more cautious than everyday language makes it sound. “No members below” does not tell us whether a host elsewhere belongs to the group, whether another branch carries receivers, whether the source is still sending, whether the upstream route is unchanged, or whether any application has received useful content. It does not decide whether a group is legitimate, whether a sender is allowed to use it, or whether a packet would have arrived correctly. It records one router’s bounded downstream knowledge at a time.

A host joining was not the whole tree speaking

The separation becomes clearer beside RFC 1112. That document specifies host extensions for IP multicast, including joining and leaving host groups, while saying that algorithms within and between multicast routers are transparent to hosts and belong in separate specifications. A host’s local membership mechanism and an upstream router’s tree decision therefore answer related but non-identical questions.

An NMR was not simply a translated host leave. A downstream router could summarize the absence of descendant members for a group for its upstream peer. The peer could suppress that branch until the report expired. A later NMR Cancel could revoke the previous report. RFC 1075 even says a cancel with no corresponding report should be ignored for that address. The protocol thus kept negative state associated with a prior claim rather than letting any bare “start again” message rewrite a branch blindly.

Expiry prevented a silence from becoming a constitution

This use of time is the decisive historical detail. A hold-down was not proof that no new listener could appear after the report, nor a promise that the tree would remain the same. It was an interval during which an upstream router could act on a downstream absence report. At the end of that interval, the report stopped being valid according to its own grammar. If an operator later saw no packets on a branch, that observation would still need a source, group, interface, neighbor, routing epoch, membership evidence and collection time before it could be explained.

Later multicast protocols retained the need to separate state rather than erase it. RFC 4601 describes PIM-SM with distinct local-membership, interface join/prune, neighbor-liveness and reverse-path state. It is not DVMRP and should not be projected backward into 1988. It is a useful reminder of what RFC 1075’s NMR already refused to collapse: an absence report, a path choice, a neighbor relation and a delivery result are not one fact merely because they concern the same group.

Sources and limits

RFC 1075 supports the experimental DVMRP status, source-rooted forwarding context, NMR, hold-down and cancel mechanics. RFC 1112 supports the host/router boundary, and RFC 4601 is a later scope comparison only. None establishes a current DVMRP deployment, a named implementation, a multicast incident, a complete global receiver census, authentication, authorization, integrity or application outcome. The inference that an expiring branch report should retain its tree and time context follows from those documented boundaries.