Multicast Protocol: The "Group Chat Master" of the Internet World

Fans who love to think, have you ever had these confusions like me?

When participating in a multi-party video conference, why can we receive the conference image smoothly?

When watching the live broadcast of a gala, why can tens of thousands of viewers see the program at the same time?

When playing games online with friends, what ensures that all players can receive game information in real time?

…

In fact, all of these are inseparable from a magical network protocol - the multicast protocol.

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Today, I will take you to unveil the mystery of multicast protocol and explore the secret weapon of the Internet.

1. What is a multicast protocol?

The multicast protocol is a network communication protocol used for point-to-multipoint or multipoint-to-multipoint multicast communication.

We can use a diagram to distinguish unicast, broadcast, and multicast.

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IP unicast (Unicast), referred to as unicast

Unicast is a point-to-point communication method that requires a point-to-point network connection between the sender and each receiver.

IP Broadcast (Broadcast), referred to as broadcast

Broadcasting is a point-to-all communication. If a data packet is broadcast in an IP subnet, all hosts in the subnet will receive the data packet regardless of whether the hosts in the subnet are willing to receive the data packet.

IP Multicast, referred to as multicast

Multicast is between unicast and broadcast, and is a point-to-multipoint or multipoint-to-multipoint communication method. When a host sends information to a group of hosts, all hosts in a group can receive the information.

2. How does the multicast protocol work?

Before understanding the working principle of the multicast protocol, let us first understand the basic concepts of multicast.

• Multicast group: Use an IP multicast address (i.e., multicast group address) to identify. Any user host (or other receiving device) that joins this multicast group becomes a member of the group. The group member can identify and receive messages sent to the multicast address.
• Multicast source: An information source that sends IP multicast messages with the multicast group address as the destination address. A multicast source can send data to multiple multicast groups at the same time. Multiple multicast sources can send data to one multicast group at the same time.
• Multicast group members: may be widely distributed anywhere in the network, group members are dynamic, and user hosts in the network can join and leave the multicast group at any time. Multicast sources are usually not data receivers and are not members of the multicast group.
• Multicast router: A router in the network that supports multicast functions and can provide multicast routing functions and multicast group member management functions.
• Multicast distribution tree: A multicast distribution tree is a set of inbound and outbound interfaces of a series of routers, which determines a unique forwarding path between the subnet where the multicast source is located and all subnets containing group members.

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To understand these concepts more simply, we compare the multicast source to a teacher, the multicast group to a communication interest group, and the students in this group to the communication group members.

The multicast router can be a class committee member, such as the monitor, study committee member, etc., who can help the teacher to deliver messages, and is responsible for delivering the teacher's information more widely to every student in the communication interest group.

In a class, the transmission of information is often complicated. The teacher may first pass information to the class committee, and then the information is passed on to the students in the communication interest group. At the same time, the class monitor and the study committee member may also pass information to each other. This information transmission network composed of teachers and students is similar to a multicast distribution tree. In this tree structure, information is transmitted from the teacher (multicast source) to each student (multicast group member) along the shortest path, ensuring the effective dissemination of information.

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In the network, multicast protocols help routers and switches know which devices are interested in a specific "group" (that is, the multicast group address). When a device joins or leaves a multicast group, the device will notify the network, and the network will adjust the data transmission path accordingly to ensure that the information is only transmitted to the required devices.

Simply put, a multicast protocol allows a data sender to efficiently deliver information to multiple interested receivers without disturbing other network users who are not interested in the information.

The working process of the multicast protocol can be divided into the following parts:

1. Joining a Multicast Group When a host (receiver) wants to receive information from a specific multicast group, it sends a join request to its local router through some mechanism.

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2. When a multicast source (sender) wants to send data to a multicast group, it sets the destination IP address of the data packet to the corresponding multicast group address. This data packet will be sent to the network, but only members who have joined this multicast group will receive this data.
3. Transmitting multicast data The multicast router builds a multicast distribution tree based on the receiver's joining request through the multicast routing protocol, and copies and forwards the data packet to all receivers belonging to the multicast group.
4. A receiver receiving multicast data receives data packets from a multicast group address without knowing the existence of other receivers.

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5. Leaving a multicast group When a host no longer needs to receive data from a multicast group, it will notify the multicast router through the corresponding mechanism. The multicast router will update the multicast distribution tree and stop sending data to the host.

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3. What are the multicast protocols?

In the process of working on multicast protocol, we talked about some mechanisms and routing protocols, which constitute the system of multicast protocol.

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Group Membership Management Protocol

The group membership management protocol is a mechanism used by multicast group members to join or leave the multicast group. It is mainly used to manage and maintain the membership of the multicast group and ensure that multicast messages are sent only to receivers who are interested in the message.

Take the communication interest group mentioned above as an example. If you are a student who wants to join the interest group, you will tell the study committee member that you want to join the communication interest group through the group member management agreement. The study committee will record your information and notify the teacher. After receiving the "registration information", the teacher will update the list of members of the communication interest group. When the communication interest group has a new activity notification, the teacher will only send the notification to the interest group members including you.

Commonly used group membership management protocols include IGMP (Internet Group Management Protocol) and MLD (Multicast Listener Discovery).

IGMP and MLD serve IPv4 and IPv6 networks respectively. They have similar functions but differ in application scenarios, protocol details, and network environments.

• IGMP: A multicast group membership management protocol for IPv4 networks. IGMP includes three versions: IGMPv1, IGMPv2, and IGMPv3. The newer versions are fully compatible with the older versions. Currently, IGMPv2 is the most widely used.
• MLD: Multicast group membership management protocol for IPv6 networks. MLD includes two versions, MLDv1 and MLDv2. The function of MLDv1 is similar to that of IGMPv2, and the function of MLDv2 is similar to that of IGMPv3.

Multicast Routing Protocol

The multicast routing protocol is a protocol that runs between multicast routers. It is mainly used to control the transmission path of multicast data streams between multicast routers, ensuring that multicast messages are only sent to network nodes that request and can handle this specific multicast traffic, thereby optimizing the use of network resources and reducing unnecessary bandwidth consumption.

Multicast routing protocols are divided into intra-domain multicast routing protocols and inter-domain multicast routing protocols.

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Taking interest groups as an example, information transmission between different interest groups in the same class uses the intra-domain multicast routing protocol, and information transmission between different interest groups in different classes uses the inter-domain multicast routing protocol.

• Intra-domain multicast protocols are used to discover multicast sources in an AS (Autonomous System) and build a multicast distribution tree to deliver information to receivers. Intra-domain multicast routing protocols are divided into sparse mode and dense mode according to the distribution of multicast users in the network. Dense mode The premise of dense mode multicast routing protocols is that multicast users in the network are densely distributed and bandwidth is relatively abundant. Dense mode multicast routing protocols build and maintain multicast trees by flooding multicast packets to the entire network in stages. Routers running multicast routing protocols flood received multicast packets on all other interfaces. DVMRP (Distance Vector Multicast Routing Protocol), MOSPF (Multicast Open Shortest Path First), and PIM-DM (Protocol Independent Multicast - Dense Mode) are all dense mode multicast routing protocols.

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• Sparse Mode Sparse mode multicast routing protocol is suitable for situations where multicast receivers are sparsely distributed in the network. In this case, if the same flooding method as dense mode is used to build a multicast routing tree, it will greatly waste bandwidth. In sparse mode, a network device must apply to join the multicast routing tree if it wants to receive multicast packets. CBT (Core-Based Trees) and PIM-SM (Protocol Independent Multicast - Sparse Mode) are sparse mode multicast routing protocols. Due to the protocol itself, CBT is not suitable for use in network-wide multicast applications, so the intra-domain multicast routing protocols mainly use PIM-SM, PIM-DM, and DVMRP protocols.
• The inter-domain multicast routing protocol is used to transmit multicast source information between ASs, thereby establishing multicast routing across domains and sharing multicast resources between domains. Since different ASs may belong to different operators, a mechanism is needed to discover multicast sources in other domains, which requires the inter-domain routing protocol MSDP (Multicast Source Discovery Protocol) and the multicast extension MBGP (Multiprotocol Border Gateway Protocol, a multi-protocol extension of BGP). MSDP is a typical inter-domain multicast routing protocol, which usually works in conjunction with MBGP. MSDP is suitable for the case where PIM-SM runs in each domain.

4. Advantages and Challenges of Multicast Protocols

Multicast protocols have many advantages in practical applications:

• Bandwidth savings: Multicast protocols avoid the need to send separate packets to each recipient, thereby saving network bandwidth, especially in scenarios such as video conferencing and online live streaming that require large amounts of data to be transmitted simultaneously.
• Reduce network load: Since the replication and distribution of data packets are carried out in the network, the load on the source node is greatly reduced, improving the efficiency and stability of the overall network.
• Improve transmission efficiency: Multicast protocols allow data to reach multiple receivers more quickly, reducing latency and transmission time.

Although multicast protocols have many advantages, they still face some challenges in actual deployment:

• Complexity of multicast routing: Multicast routing protocols (such as PIM-SM and DVMRP) need to maintain a complex multicast tree structure in the network to ensure that data packets can reach all receivers efficiently.
• Security issues: Data packets in multicast communications may be intercepted by unauthorized receivers, so encryption and authentication measures are needed to ensure data security.
• Limitations of network infrastructure: Not all network equipment and service providers support multicast protocols, which may limit the application scope of multicast technology.

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As a key technology to improve network communication efficiency, multicast protocol has demonstrated unique advantages in many fields. With the continuous development of Internet technology and the increasing application demand, multicast protocol will continue to play an important role in promoting the efficient and stable development of network communication. In the future, with the further maturity of technology and the continuous improvement of infrastructure, multicast protocol is expected to be widely used in more scenarios, bringing a better network experience.

After this study, I believe everyone has learned about multicast protocols. What other potential application scenarios do you think multicast protocols have? Or which multicast protocol do you want to learn more about?