Multicast - Upstream and Downstream

In Multicast communication, particularly in the directionality of data flow is often discussed in terms of "downstream" and "upstream."

PIM Sparse Mode

In PIM sparse mode, multicast traffic flows from a source to multiple receivers through a distribution tree structure. This tree can be either a Source-Specific Tree (SST) or a Rendezvous Point Tree (RPT). These are also known as the shortest path tree (SPT) and the root path tree (RPT) respectively. With reference to these trees, traffic in a PIM-SM arrangement always moves downstream:

  1. Source-Specific Tree (SST): In this tree, data flows directly from the source to the receivers. The direction from the source towards the receivers is referred to as downstream. PIM-SM initially builds an SST, called a Shortest Path Tree (SPT), to optimize the path from the source to the receivers.

  2. Rendezvous Point Tree (RPT): This tree initially guides the multicast traffic through a central point called the Rendezvous Point (RP). The RP serves as a common point where all multicast data is sent before being distributed to the receivers. Here, the term "downstream" refers to the direction from the RP towards the multicast receivers.

PIM Dense Mode

Similarly, in PIM Dense Mode the concept of "downstream" is still relevant, but the operational mechanics differ significantly from Sparse Mode (PIM-SM).

In PIM-DM, the downstream direction also refers to the flow of multicast traffic towards the receivers, similar to PIM-SM. However, the way this flow is established is different.

  • Flood Phase: Initially, the multicast traffic is flooded to all downstream links. This ensures that all potential receivers have a chance to receive the multicast traffic.
  • Prune Phase: Routers that do not have downstream receivers (i.e., no interested hosts on their subnetworks) send Prune messages upstream. This action stops the unnecessary flow of multicast traffic to those segments of the network. Therefore, the traffic that remains is only flowing downstream to areas with active receivers.
  • Grafting: If a previously pruned router receives a new join request for the multicast group, it sends a Graft message upstream to rejoin the multicast tree, re-establishing the downstream flow of traffic to that segment.

Bidirectional PIM

Bidirectional PIM, is used to optimize multicast distribution in scenarios where sources and receivers are located throughout the network, and the traffic pattern is bidirectional. Unlike PIM-SM and PIM-DM, Bidir-PIM builds a single, shared tree rooted at an RP to facilitate both upstream and downstream traffic flows.

  • Bidir-PIM constructs a single shared tree that is bidirectional, allowing multicast data to flow from any source to the RP and then to any receiver. This tree is often referred to as a bidirectional shared tree.
  • The RP is a central point in the network that serves as the root of the shared tree. All sources send their multicast traffic towards the RP, and the RP forwards this traffic to all interested receivers.
  • In Bidir-PIM, the concepts of upstream and downstream are relative to the RP. "Upstream" refers to the direction towards the RP, while "downstream" refers to the direction away from the RP towards the receivers.
    • Sources send their multicast traffic upstream towards the RP.
    • The RP then distributes the multicast traffic downstream to the receivers.

https://networklessons.com/multicast/multicast-bidirectional-pim/

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