Enterprise Edition transit routers support the multicast feature. After you connect an Enterprise Edition transit router to network instances, you can use the Enterprise Edition transit router to enable and manage multicast. The Enterprise Edition transit router also forwards multicast packets. This topic describes how multicast works, the billing rules for multicast, the limits of multicast, and how to use multicast.
What is multicast?
Multicast is a type of group communication where the same data packet is transmitted to a group of destination clients. Multicast is commonly used in live streaming, video conferencing, financial transaction data synchronization, online education, and streaming television.
Compared with unicast and broadcast, multicast can reduce workloads on servers and increase bandwidth utilization in one-to-many network communication scenarios.
In the following example, live streaming is used to show the differences between unicast and multicast, and the benefits of multicast.
The differences between broadcast and multicast are not discussed in this topic. You can research the differences if needed.
How unicast works
In unicast scenarios, the live streaming server must transmit two packets: Unicast Packet 1 and Unicast Packet 2. The router then forwards Unicast Packet 1 to Client 3 and Unicast Packet 2 to Client 4, The following figure shows the details.
How multicast works
In multicast scenarios, you can add the live streaming server and the clients to the same multicast group. When the server transmits a data packet to Client 3 and Client 4, only one data packet (Multicast Packet 1) needs to be transmitted to the multicast group. The packet is forwarded by the multicast routers and replicated on the multicast routers that are nearest to the clients. The number of replicas is determined by the number of clients in the multicast group. After the packet is replicated, the multicast routers forward the replicas to the clients, The following figure shows the details.
In unicast scenarios, the live streaming server must transmit a packet to every client. The routers must forward every packet. The workloads on the server and the bandwidth usage greatly increase as the number of clients grows. In multicast scenarios, the live streaming server and the clients can be added to the same multicast group. The server needs to transmit only one packet to the multicast group. The multicast routers need to forward only one packet. The packet is replicated on the multicast routers that are nearest to the clients. Multicast can reduce server workloads and bandwidth usage even if the number of clients is large.
Features and advantages
This section describes the use scenarios, advantages, and terms of multicast.
After you enable multicast for an Enterprise Edition transit router, you can create and manage multicast networks in the following scenarios:
Create a multicast network in a virtual private cloud (VPC), as shown in Multicast Network 3.
Create a multicast network among VPCs that are deployed in the same region, as shown in Multicast Network 2.
Create a multicast network among VPCs that are deployed in different regions, as shown in Multicast Network 1.
The multicast feature supported by Enterprise Edition transit routers is a cloud-native multicast feature developed by Alibaba Cloud. This feature helps you build multicast networks without additional physical devices or third-party software licenses. Multicast can be activated based on your business requirements and is billed on a pay-as-you-go basis.
Enterprise Edition transit routers support zone-disaster recovery, which prevents single points of failure and improves the availability of your multicast network.
Compared with traditional methods that use command-line interfaces (CLIs) for maintenance, Enterprise Edition transit routers simplify route configurations and multicast source and member management because CEN provides a visualized management console.
The following table describes the terms of multicast.
A multicast domain defines the scope of a multicast network in a region.
Only resources in the multicast domain can transmit or receive multicast packets.
You can create multiple multicast domains on an Enterprise Edition transit router. The multicast domains in the same region are isolated from each other. Multicast groups in multicast domains that are created in different regions can communicate with each other if the multicast groups have the same multicast IP address.
A multicast group consists of resources that transmit, replicate, and receive packets. A multicast group is identified by its IP address.
You can create multiple multicast groups in a multicast domain. Within the same multicast domain, the multicast IP address of each multicast group must be unique. You can assign the same multicast IP address to multicast groups that are created in different multicast domains.
Multicast packets from different multicast groups are isolated from each other. Multicast packets from the same multicast group are isolated in the same region but are not isolated across regions.
A multicast source in a multicast group transmits multicast packets.
A multicast member in a multicast group receives multicast packets.
After you enable multicast on an Enterprise Edition transit router, you are charged connection fees, data forwarding fees, and inter-region connection fees based on your network topology. For more information about connection fees and inter-region connection fees, see Billing rules.
The following section describes the billing rules for data forwarding fees in multicast scenarios:
Data forwarding is billed on a pay-as-you-go basis. The billing cycle is 1 hour. A bill is generated every hour. If you use multicast for less than 1 hour, the usage duration is rounded up to 1 hour.
In multicast scenarios: Data forwarding fee = Amount of data transfer × Unit price of data transfer (USD 0.02/GB)
The data transfer consists of the following items:
The total amount of data that each multicast source transmits to the Enterprise Edition transit routers.
The total amount of data that the Enterprise Edition transit routers forward to the multicast members.
The preceding figure shows how data forwarding is billed. As shown in the preceding figure, the Elastic Compute Service (ECS) instances in VPC1, VPC2, and VPC3 are added to Multicast Group 1. ECS1 in VPC1 is the multicast source, and the ECS instances in VPC2 and VPC3 are the multicast members. When the Enterprise Edition transit router forwards data, you are charged for two billable items, as described in the following table.
The total amount of data transfer from the multicast source to the Enterprise Edition transit router.
The total amount of data that the Enterprise Edition transit router forwards to ECS1, ECS2, ECS3, and ECS4 in VPC2, and to ECS1, ECS2, and ECS3 in VPC3.
Assume that Billable Item ① is 2 GB and Billable Item ② is 8 GB.
Data forwarding fee = (2 + 8) GB × USD 0.02/GB = USD 0.20.
Multicast is supported by Enterprise Edition transit routers only in the following regions: China (Hangzhou), China (Shanghai), China (Beijing), China (Shenzhen), China (Chengdu), China (Hong Kong), Japan (Tokyo), Singapore, Germany (Frankfurt), Australia (Sydney), UK (London), US (Virginia), and US (Silicon Valley).
Enterprise Edition transit routers support multicast only for VPCs.
The elastic network interfaces (ENIs) created by Enterprise Edition transit routers in VPCs cannot be added to multicast networks.
VPCs that do not support advanced VPC features cannot be added to multicast networks. vSwitches in such VPCs cannot be associated with multicast domains. For more information about advanced VPC features, see Advanced VPC features.
An elastic network interface (ENI) can be a multicast source and a multicast member in the same multicast group.
Only the primary private IP address of an ENI can be used as a multicast object. Other IP addresses of the ENI do not support multicast.
The following table describes the quota limits on each multicast resource.
The maximum number of multicast domains supported by a transit router
The maximum number of multicast members supported by a multicast group
The maximum number of vSwitches that can be associated with a multicast domain
The maximum number of multicast sources supported by a multicast group
The maximum number of inter-region multicast members supported by a multicast group created for a transit router
The maximum number of multicast groups that can be created in a multicast domain
The multicast feature cannot be disabled after it is enabled. The multicast feature does not affect other features.
For more information, see Create and manage a multicast network.