IP Multicast Basics - Basic Concepts of IP Multicast

Hello everyone,

Completed the knowledge introduction of HCIP-Datacom certification in 2021. Now we're going to introduce you to HCIP-Datacom. Today brings the basic concepts of IP multicast.

Concept

Source: The sender of multicast traffic. The multicast source does not need to run any multicast protocol, just simply sends the multicast data.

Receiver: Also known as a multicast group member, is a device that expects to receive traffic from a particular multicast group.

Multicast Group: A group identified using a multicast IP address.

Multicast Router: Network devices (routers, switches, firewalls) that support multicast and run multicast protocols.

First-Hop Router: A multicast router directly connected to the Source, which is responsible for forwarding the multicast packets sent by the Source to the multicast network.

Last-Hop Router: The multicast router of the directly connected group members. The router is responsible for forwarding multicast traffic to the directly connected network (to the receiver), and is also responsible for managing and maintaining the membership of the multicast group in the directly connected network segment.

Multicast IP address

IANA allocates the Class D address space for IPv4 multicast use, ranging from 224.0.0.0 to 239.255.255.255. 

Class D addresses are used to identify multicast groups and can only be used as destination addresses of multicast packets, not as source addresses.

The source address field of an IPv4 multicast packet is an IPv4 unicast address, which can use a class A, B, or C address, but cannot be a class D or class E address.

At the network layer, all user hosts that join the same multicast group can identify the same IPv4 multicast group address. Once a user on the network joins the multicast group, the user can receive IP packets with the group address as the destination address.

Table 1 Ranges and meanings of IPv4 multicast addresses

Table 2 List of common permanent group addresses

Multicast MAC address

When Ethernet transmits multicast data, its destination is no longer a specific receiver, but a group with uncertain members, so the multicast MAC address is used, that is, the multicast IP address is mapped to the address in the link layer.

According to IANA, the most significant 24 bits of an IPv4 multicast MAC address are 0x01005e, the 25th bit is 0, and the least significant 23 bits are the least significant 23 bits of an IPv4 multicast MAC address.

Figure 1 Mapping between multicast IPv4 addresses and multicast MAC addresses

The first four bits of an IPv4 multicast address are fixed at 1110 and correspond to the most significant 25 bits of the multicast MAC address. Only 23 bits of the last 28 bits are mapped to the MAC address. Therefore, five bits of the address information are lost. As a direct result, 32 IPv4 multicast addresses are mapped to the same MAC address. For example, the multicast MAC addresses of multicast groups with IP address 224.0. 1.1, 224.128. 1.1, 225.0. 1.1, and 239.128. 1.1 are all 01-00-5e-00-01-01.Network administrators must consider this when assigning addresses.

The multicast MAC address identifies a group of devices. The lowest bit of the first byte of this MAC address is 1, for example, 0100-5e-00ab.

A group of devices identified by a multicast MAC address has a common feature, that is, they all join the same multicast group, and these devices will listen to data frames whose destination MAC address is the multicast MAC address.

Multicast service model

The classification of the multicast service model is for the receiver host, and there is no difference for the multicast source. The multicast data sent by the multicast source always uses the IP address of the multicast source as the source address of the packet and the multicast group address as the destination address. The receiver host can select the source when receiving data, so two service models of ASM (Any-Source Multicast) and SSM (Source-Specific Multicast) are generated. The two service models use different multicast group address ranges.

ASM Model

The ASM model only provides multicast distribution for group addresses. A multicast group address is used as a collection of network services, and any source advertises data to the group address to obtain the same service. After the receiver host joins the multicast group, it can receive the data sent to the group by any source.

To improve security, you can configure filtering policies for multicast sources on routers to allow or prohibit packets from certain multicast sources from passing through. Ultimately, from the receiver's perspective, the data is filtered.

The ASM model requires that group addresses must be unique in the entire multicast network. "Unique" means that an ASM address can only be used by one multicast application at a time. If two different applications send data using the same ASM group address, their receivers will receive data from both sources at the same time. This will cause network traffic congestion on the one hand, and trouble the recipient host on the other hand.

SSM Model

The SSM model provides services for bound data streams of specific sources and groups. When a receiver host joins a multicast group, it can specify only which sources to receive data. After joining a multicast group, the host will only receive the data sent to the group by the specified source.

The SSM model no longer requires the group address to be unique on the entire network, but only requires each multicast source to be unique. "Unique" here means that different multicast applications on the same source must use different SSM addresses to distinguish them. The same group address can be used between different sources because an entry is generated for each (source, group) information in the SSM model. This saves multicast group addresses on the one hand and does not cause network congestion on the other hand.

In the next issue, I will explain the principle of multicast data forwarding.