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RR Round Robin

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In computer networking, a routing protocol is a set of rules and algorithms that determine how data packets are forwarded from one network to another. One such routing protocol is the Reverse Path Forwarding (RPF) or Reverse Path Check (RPF) algorithm, which is commonly used in multicast routing.

In a multicast scenario, a router needs to determine the outgoing interface(s) through which it should forward multicast packets. This is where the RR (Requesting Router) comes into play. The RR is responsible for initiating the RPF check to ensure that the multicast traffic is being received on the correct interface.

Here's a detailed explanation of the RR and its role in the RPF process:

  1. RPF Check Overview: The RPF check is used to verify the validity of incoming multicast packets. It ensures that the packets arrive on the interface from the expected direction, preventing loops and ensuring efficient forwarding. The RPF check compares the source IP address of the multicast packet with the routing table to determine the correct interface.
  2. Requesting Router (RR): The RR is the router that initiates the RPF check for a particular multicast flow. It sends an RPF request to its neighboring routers to validate the incoming multicast traffic. The RR could be the designated router (DR) in a multicast group, or it can be any router along the path between the source and the receivers.
  3. RPF Request: When a router receives a multicast packet on an interface, it checks the source IP address and the multicast group address. If it determines that an RPF check is required for that packet, it becomes the RR for that particular flow. The RR then sends an RPF request message towards the source IP address.
  4. RPF Request Propagation: The RPF request message is propagated towards the source IP address, following the unicast routing table. Each router that receives the RPF request checks its own unicast routing table to determine the outgoing interface towards the source. It then sends the RPF request message out through that interface.
  5. RPF Response: When the RPF request reaches the source IP address, the source responds with an RPF response message. The response message contains information about the incoming interface through which the multicast packets should arrive.
  6. RPF Response Propagation: The RPF response message follows the reverse path back to the RR. Each router on the path receives the response, updates its RPF information, and forwards the response message towards the RR.
  7. RPF Table Update: As the RPF response message reaches the RR, it updates its RPF table with the information received. The RPF table contains the mapping of multicast sources to the incoming interfaces.
  8. Forwarding Decision: After the RR has obtained the RPF information for the multicast flow, it can make forwarding decisions for the received multicast packets. The RPF information helps determine the outgoing interface(s) for the multicast traffic based on the source IP address.

By using the RR and RPF mechanism, multicast routers can ensure that multicast traffic is forwarded efficiently and prevent loops in the network. The RR initiates the RPF check and collects the necessary information to determine the correct path for multicast traffic, enabling successful delivery to the intended receivers.