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What Is Protocol Independent Multicast (PIM) and How Does It Work

Posted on Feb 3, 2024 by
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What Is Protocol Independent Multicast?

Before explaining PIM, let's briefly understand the concept of multicast. Multicast is a networking method where information can be sent from one source to many locations at the same time. This one-to-many approach excels in scenarios where the same data is sent to many recipients concurrently, such as broadcasting live video feeds or updating stock prices at the moment.

Protocol-Independent Multicast (PIM) is a protocol that helps devices in the network coordinate multicast communications. It is called "Protocol-Independent" because it does not rely on a specific unicast (one-to-one communication) routing protocol and can work with any existing unicast routing protocol (such as OSPF, RIP, or BGP). The PIM protocol enables data on the network to be delivered accurately and efficiently to all devices that wish to receive the data without interfering with devices that have not requested the information.

enterprise switch

How Does PIM Work?

PIM is not directly involved in transmitting data. It is mainly responsible for establishing and maintaining a structure called a multicast routing tree. This tree defines how data travels across the network from the sender (source) to the receiver (subscriber).

Working principle of PIM

The working principle of PIM can be simply divided into three steps:

  • Join a multicast group: When a device wants to receive data from a certain multicast group, it will send a message to tell nearby routers, "I am interested in this multicast group."

  • Establishing a multicast tree: Routers negotiate through PIM to determine how to most efficiently deliver multicast data from the sender to the receiver. This process is to establish a multicast tree.

  • Data delivery: Once the multicast tree is established, data can be delivered along the tree from the source (sender) to the destination (receiver). This avoids broadcasting to the entire network and improves efficiency. Working Modes of PIM

Working Modes of PIM

There are four working modes of PIM: PIM Sparse Mode, PIM Dense Mode, Bidirectional PIM and PIM Source-Specific Multicast.

  • PIM Sparse Mode (PIM-SM): Suitable for large networks, especially those with few multicast subscribers or very dispersed distribution. For example, a global company might use PIM-SM to send data to its offices around the world. It builds one-way shared routes that start from a central point called a rendezvous point (RP) for each group. Optionally, it can also create direct paths from the source to the receivers.

    Enterprise switch

  • PIM Dense Mode (PIM-DM): This mode is suitable for small networks or situations where multicast listeners are very dense. For example, if a school's teaching network wants to distribute real-time video lecture signals to classrooms throughout the school, it can use PIM-DM. Since nearly all classrooms require this service, PIM-DM spreads the message everywhere and then prunes the unnecessary parts to optimize delivery.

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  • Bidirectional PIM (Bidir-PIM): This mode is suitable for scenarios where data is transmitted from multiple sources to multiple targets, such as large-scale real-time interactive applications or distributed systems that communicate frequently with each other.

    Enterprise switch

  • PIM Source-Specific Multicast (PIM-SSM): Suitable for one-to-many communication mode, where messages are sent from a specific source to a clear set of recipients. This is excellent for live classes on pay TV services or online education platforms. Only subscribers who explicitly request data from this source will receive the data, ensuring efficiency and optimal utilization of bandwidth.

    Enterprise switch

Application of PIM in Switches

The application of PIM in switches usually involves the switch supporting IGMP (Internet Group Management Protocol) to manage host group membership on its local subnet, and using PIM to exchange multicast routing information with other routers or switches. Generally speaking, only enterprise switches support the PIM protocol.

To run PIM effectively on a switch, you typically need to perform the following steps:

  • Configure the switch interface to use PIM.

  • Specify the PIM operating mode (such as PIM-SM or PIM-DM).

  • For PIM-SM, set or discover an RP.

  • Periodically exchange PIM control messages to maintain the multicast routing tree.

The implementation of PIM ensures that data traffic can be efficiently delivered to each interested recipient through a shared network infrastructure, reducing network traffic redundancy and saving bandwidth. This is particularly important for real-time applications such as video conferencing, IPTV, and real-time stock quotes.

Solutions to Modern Network Challenges

Today, with the rapid development of the Internet, large-scale multicast and increasing network complexity have brought unprecedented challenges. Protocol-independent multicast (PIM), as a widely used multicast routing protocol, provides effective solutions to these challenges.

  • Large-scale multicast: PIM supports modes such as Sparse Mode and Source-Specific Multicast (SSM), which can dynamically build shared trees and source trees to effectively cope with large-scale multicast and ensure efficient data transmission.

  • Complex topology: The flexibility of PIM enables it to adapt to complex network topologies. Bidirectional PIM is particularly suitable for complex network designs because it builds bidirectional shared trees and is suitable for different types of topologies.

  • Dynamic network changes: By processing Join and Prune messages, PIM dynamically adjusts the multicast distribution tree to adapt to dynamic changes in members in the network, ensuring that data is only delivered to the required location.

  • Network scalability: The protocol independence of PIM enables it to work with various unicast routing protocols, which helps improve the scalability of the network and ensures that it can still operate effectively when the network scale is expanded.

The application of PIM in network design covers multiple key areas, from optimization of multicast traffic to flexible response to various challenges of modern networks. Its protocol independence, multi-mode support and dynamic construction of distribution trees make it an important protocol for efficient, flexible and reliable multicast communication.

Conclusion

Protocol-Independent Multicast (PIM) is emerging in the field of network protocols. Its working principle, protocol-agnostic nature, and seamless integration with switches make it a valuable asset for optimizing multicast communications. From enhancing network design to providing solutions to modern challenges, PIM demonstrates its versatility in the dynamic environment of the network. PIM stands out as a reliable and efficient solution shaping the future of multicast communications in contemporary networks.

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