Protocol Independent Multicast - Dense Mode (PIM-DM) is designed for networks where multicast group members are densely distributed. It operates on a push model (also known as flood-and-prune), where multicast traffic is initially flooded to all parts of the network under the assumption that every subnet has at least one receiver. This flooding process builds source-based distribution trees (also called Shortest Path Trees or SPTs), rooted at the source of the multicast stream ($S, G$). Routers that do not have active receivers for a specific group must explicitly signal their upstream neighbors to stop forwarding traffic by sending Prune messages; therefore, PIM-DM uses prune mechanisms to remove branches of the tree that do not need the traffic.

In contrast, PIM Sparse Mode (PIM-SM) uses a pull model, shared trees (*, G), and relies on a Rendezvous Point (RP).

Adams, A., Nicholas, J., & Siadak, W. (2005). Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised) (RFC 3973). Internet Engineering Task Force.

Section 1 (Introduction): Describes PIM-DM as using a "push model" where traffic is flooded to the entire network.

Section 1.1: Confirms the mechanism of "Prune messages" used to stop traffic flow on specific interfaces.

Section 2 (Protocol Overview): "In PIM-DM, a source-based tree is built...".

Cisco Systems. (2024). IP Multicast: PIM Configuration Guide, Cisco IOS XE 17.x.

Chapter: PIM Dense Mode: States, "PIM dense mode (PIM-DM) uses a push model to flood multicast traffic to every corner of the network." and "This method creates a source-based distribution tree."

Williamson, B. (2000). Developing IP Multicast Networks, Volume 1. Cisco Press.

Chapter 6 (PIM Dense Mode): Details the "Flood and Prune" behavior, characterizing it as a push model that utilizes source distribution trees (S,G) exclusively, distinguishing it from the shared trees used in Sparse Mode.