Table of Contents

Chapter 1 Multicast Overview.. 1-1

1.1 Introduction to Multicast 1-1

1.1.1 Comparison of Information Transmission Techniques. 1-1

1.1.2 Features of Multicast 1-4

1.1.3 Common Notations in Multicast 1-5

1.1.4 Advantages and Applications of Multicast 1-6

1.2 Multicast Models. 1-6

1.3 Multicast Architecture. 1-7

1.3.1 Multicast Addresses. 1-7

1.3.2 Multicast Protocols. 1-12

1.4 Multicast Packets Forwarding Mechanism.. 1-14

 

Chapter 1  Multicast Overview

 

 

1.1  Introduction to Multicast

As a technique coexisting with unicast and broadcast, the multicast technique effectively addresses the issue of point-to-multipoint data transmission. By allowing high-efficiency point-to-multipoint data transmission over a network, multicast greatly saves network bandwidth and reduces network load.

With the multicast technology, a network operator can easily provide new value-added services, such as live Webcasting, Web TV, distance learning, telemedicine, Web radio, real-time videoconferencing, and other information services that have high demands on the bandwidth and real-time data communication.

1.1.1  Comparison of Information Transmission Techniques

I. Unicast

To deliver information to multiple receiver hosts over the IP network through unicast, the information source needs to send a separate copy of the information to each host that needs the information, as shown in Figure 1-1.

Figure 1-1 Unicast transmission

Assume that Host B, Host D and Host E need this information. The information source establishes a separate transmission channel for each of these hosts.

With unicast transmission, the traffic over the network is proportional to the number of hosts that need the information. If a large number of users need the information, the information source needs to send a copy of the same information to each of these users. This means a tremendous pressure on the information source and the network bandwidth.

As we can see from the information transmission process, unicast is not suitable for batch transmission of information.

II. Broadcast

When broadcast is used to deliver information, the information source sends information to all hosts on the network, even if some hosts do not need the information, as shown in Figure 1-2.

Figure 1-2 Broadcast transmission

Assume that only Host B, Host D, and Host E need the information. If the information source broadcasts the information, Host A and Host C also receive it. In addition to information security issues, this also causes traffic flooding on the same network.

Therefore, broadcast is disadvantageous in transmitting data to specific hosts; moreover, broadcast transmission is a significant usage of network resources.

III. Multicast

As discussed above, the unicast and broadcast techniques are unable to provide point-to-multipoint data transmissions with the minimum network resource consumption.

The multicast technique has solved this problem. When some hosts on the network need the information, the multicast source (namely, the information source) sends only one copy of the information. With tree-type routes established by leveraging some multicast routing protocol, the packets are replicated and distributed only where the tree branches, as shown in Figure 1-3:

Figure 1-3 Multicast transmission

Assume that Host B, Host D and Host E need the information. To receive the information correctly, these hosts need to join a receiver set known as a multicast group. The multicast routers on the network duplicate and forward the information based on the distribution of the receivers in this set. Finally, the information is correctly delivered to Hosts B, D, and E.

To sum up, multicast has the following advantages:

l           Over unicast: As multicast traffic flows to the node the farthest possible from the source before it is replicated and distributed, an increase of the number of hosts will not remarkably add to the network load.

l           Over broadcast: As multicast data is sent only to the receivers that need it, multicast uses the network bandwidth reasonably and brings no waste of network resources, and enhances network security.

1.1.2  Features of Multicast

Multicast has the following features:

l           A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join a multicast group to become members of the multicast group, before they can receive the multicast data addressed to that multicast group. Typically, a multicast source does not need to join a multicast group.

l           An information sender is referred to as a multicast source (Source in Figure 1-3). A multicast source can send data to multiple multicast groups at the same time, and multiple multicast sources can send data to the same multicast group at the same time.

l           All hosts that have joined a multicast group become members of the multicast group (Receiver in Figure 1-3). The group memberships are dynamic. Hosts can join or leave multicast groups at any time. Multicast groups are not subject to geographic restrictions.

l           Routers or Layer 3 switches that support Layer 3 multicast are called multicast routers or Layer 3 multicast devices. In addition to providing the multicast routing function, a multicast router can also manage multicast group memberships on stub subnets with attached group members.

For a better understanding of the multicast concept, you can assimilate multicast transmission to the transmission of TV programs, as shown in Table 1-1.

Table 1-1 An analogy between TV transmission and multicast transmission

Step	TV transmission	Multicast transmission
1	A TV station transmits a TV program through a channel.	A multicast source sends multicast data to a multicast group.
2	A user tunes the TV set to the channel.	A receiver joins the multicast group.
3	The user starts to watch the TV program transmitted by the TV station via the channel.	The receiver starts to receive the multicast data that the source sends to the multicast group.
4	The user turns off the TV set or tunes to another channel.	The receiver leaves the multicast group or joins another group.

 

1.1.3  Common Notations in Multicast

Two notations are commonly used in multicast:

l           (*, G): Indicates a rendezvous point tree (RPT), or a multicast packet that any multicast source sends to multicast group G. Here “*” represents any multicast source, while “G” represents a specific multicast group.

l           (S, G): Indicates a shortest path tree (SPT), or a multicast packet that multicast source S sends to multicast group G. Here “S” represents a specific multicast source, while “G” represents a specific multicast group.

 

 

1.1.4  Advantages and Applications of Multicast

I. Advantages of multicast

Advantages of the multicast technique include:

l           Enhanced efficiency: reduces the CPU load of information sources and network devices.

l           Optimal performance: reduces redundant traffic.

l           Distributive application: enables point-to-multipoint applications at the price of minimum network resources.

II. Applications of multicast

Applications of the multicast technique include:

l           Multimedia and streaming applications, such as Web TV, Web radio, and real-time video/audio conferencing.

l           Communication for training and cooperative operations, such as distance learning and telemedicine.

l           Data warehouse and financial applications (stock quotes).

l           Any point-to-multipoint data distribution application.

1.2  Multicast Models

Based on the multicast source processing modes, there are two multicast models:

l           Any-Source Multicast (ASM)

l           Source-Specific Multicast (SSM)

I. ASM model

In the ASM model, any sender can send information to a multicast group address as a multicast source; numbers of receivers can join the multicast group identified by that group address to obtain multicast information addressed to the multicast group. In this model, the receivers are not ware of the position of a multicast source in advance. However, they can join or leave the multicast group at any time.

II. SSM model

In practice, users may be interested in the multicast data from particular multicast sources. The SSM model provides a transmission service that allows users to specify the multicast sources they are interested in at the client side.

The radical difference between the SSM model and the ASM model is that in the SSM model, receivers already know the locations of the multicast sources by some other means. In addition, the SSM model uses a multicast address range that is different from that of the ASM module, and dedicated multicast forwarding paths are established between receivers and the specified multicast sources.

1.3  Multicast Architecture

The purpose of an IPv6 multicast technology is to carry information, by multicast, from a multicast source to the receivers.

IP multicast involves the following questions:

l           Where should the multicast source transmit information to? (multicast addressing)

l           What receivers exist on the network? (host registration)

l           Where is the multicast source the receivers need to receive multicast data from? (multicast source discovery)

l           How should information be transmitted to the receivers? (multicast routing)

IP multicast falls in the scope of end-to-end service. The multicast architecture involves the following four parts:

1)         Addressing mechanism: Information is sent from a multicast source to a group of receivers through a multicast address.

2)         Host registration: Receiver hosts are allowed to join and leave multicast groups dynamically. This mechanism is the basis for group membership management.

3)         Multicast routing: A multicast distribution tree (namely a forwarding path tree for multicast data on the network) is constructed for delivering multicast data from a multicast source to receivers.

4)         Multicast applications: A software system that supports multicast applications, such as video conferencing, must be installed on multicast sources and receiver hosts, and the TCP/IP stack must support reception and transmission of multicast data.

1.3.1  Multicast Addresses

To allow communication between multicast sources and multicast group members, network-layer multicast addresses, namely, multicast IP addresses must be provided. When a unicast IP packet is transmitted over an Ethernet network, the destination MAC address is the MAC address of the receiver. When a multicast packet is transmitted over an Ethernet network, however, a multicast MAC address is used as the destination address because the packet is destined for a group formed by a number of receivers, rather than a specific receiver. Therefore, a technique must be available to map multicast IP addresses to link-layer multicast MAC addresses.

I. IPv4 multicast addresses

Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to 239.255.255.255) for IPv4 multicast, as shown in Table 1-2.

Table 1-2 Class D IP address blocks and description

Address block	Description
224.0.0.0 to 224.0.0.255	Permanent group addresses. 224.0.0.0 is reserved, and other IP addresses are used by routing protocols and for topology discovery, protocol maintenance, and so on. Commonly used permanent group addresses are listed in Table 1-3. A packet destined for an address in this block will not be forwarded beyond the local subnet regardless of the Time to Live (TTL) value in the IP header.
224.0.1.0 to 238.255.255.255	Globally scoped group addresses. This block includes two types of special group addresses: l      232.0.0.0/8: SSM group addresses, and l      233.0.0.0/8: Glop group addresses;  for details, see RFC 2770.
239.0.0.0 to 239.255.255.255	Administratively scoped multicast addresses. These addresses are considered to be locally rather than globally unique, and can be reused in domains administered by different organizations without causing conflicts. For details, refer to RFC 2365.

 

 

Table 1-3 Some reserved multicast addresses

Address	Description
224.0.0.1	All systems on this subnet, including hosts and routers
224.0.0.2	All multicast routers on this subnet
224.0.0.3	Unassigned
224.0.0.4	Distance Vector Multicast Routing Protocol (DVMRP) routers
224.0.0.5	Open Shortest Path First (OSPF) routers
224.0.0.6	OSPF designated routers/backup designated routers
224.0.0.7	Shared Tree (ST) routers
224.0.0.8	ST hosts
224.0.0.9	Routing Information Protocol version 2 (RIPv2) routers
224.0.0.11	Mobile agents
224.0.0.12	Dynamic Host Configuration Protocol (DHCP) server / relay agent
224.0.0.13	All Protocol Independent Multicast (PIM) routers
224.0.0.14	Resource Reservation Protocol (RSVP) encapsulation
224.0.0.15	All Core-Based Tree (CBT) routers
224.0.0.16	Designated Subnetwork Bandwidth Management (SBM)
224.0.0.17	All SBMs
224.0.0.18	Virtual Router Redundancy Protocol (VRRP)

 

II. IPv6 Multicast Addresses

As defined in RFC 4291, the format of an IPv6 multicast address is as follows:

Figure 1-4 IPv6 multicast address format

Referring to Figure 1-4, the meanings of the fields of an IPv6 multicast address are as follows:

l           0xFF: The most significant 8 bits are 11111111, indicating that this address is an IPv6 multicast address.

Figure 1-5 format of the Flags field

l           Flags: Referring to Figure 1-5, the following table describes the 4 bits of the Flags field.

Table 1-4 Description on the bits of the Flags fields

Bit	Description
0	Reserved, set to 0
R	l      When set to 0, it indicates that this address is an IPv6 multicast address without an embedded RP address l      When set to 1, it indicates that this address is an IPv6 multicast address with an embedded RP address (The P and T bits must also be set to 1)
P	l      When set to 0, it indicates that this address is an IPv6 multicast address not based on a unicast prefix l      When set to 1, it indicates that this address is an IPv6 multicast address based on a unicast prefix (the T bit must also is set to 1)
T	l      When set to 0, it indicates that this address is an IPv6 multicast address permanently-assigned by IANA l      When set to 1, it indicates that this address is a transient, or dynamically assigned IPv6 multicast address

 

l           Scope: 4 bits, indicating the scope of the IPv6 internetwork for which the multicast traffic is intended. Possible values of this field are given in Table 1-5.

Table 1-5 Values of the Scope field

Value	Meaning
0, 3, F	Reserved
1	Node-local scope
2	Link-local scope
4	Admin-local scope
5	Site-local scope
6, 7, 9 through D	Unassigned
8	Organization-local scope
E	Global scope

 

l           Group ID: 112 bits, IPv6 multicast group identifier that uniquely identifies an IPv6 multicast group in the scope defined by the Scope field.

III. IPv4 multicast MAC addresses

As defined by IANA, the high-order 24 bits of a multicast MAC address are 0x01005e, bit 25 is 0x0, and the low-order 23 bits are the low-order 23 bits of the corresponding multicast IP address. The IPv4-to-MAC mapping relation is shown in Figure 1-6.

Figure 1-6 IPv4-to-MAC address mapping

The high-order four bits of a multicast IPv4 address are 1110, indicating that this address is a multicast address, and only 23 bits of the remaining 28 bits are mapped to a MAC address, so five bits of the multicast IPv4 address are lost. As a result, 32 multicast IPv4 addresses map to the same MAC address. Therefore, in Layer 2 multicast forwarding, a device may receive some multicast data addressed for other IPv4 multicast groups, and such redundant data needs to be filtered by the upper layer.

IV. IPv6 multicast MAC addresses

The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the low-order 32 bits are the low-order 32 bits of the corresponding multicast IPv6 address. Figure 1-7 shows an example of mapping an IPv6 multicast address, FF1E::F30E:0101, to a MAC address.

Figure 1-7 An example of IPv6-to-MAC address mapping

1.3.2  Multicast Protocols

 

 

I. Layer 3 multicast protocols

Layer 3 multicast protocols include multicast group management protocols and multicast routing protocols. Figure 1-8 describes where these multicast protocols are in a network.

Figure 1-8 Positions of Layer 3 multicast protocols

1)         Multicast management protocols

Typically, the internet group management protocol (IGMP) or multicast listener discovery protocol (MLD) is used between hosts and Layer 3 multicast devices directly connected with the hosts. These protocols define the mechanism of establishing and maintaining group memberships between hosts and Layer multicast devices.

2)         Multicast routing protocols

A multicast routing protocol runs between Layer 3 multicast devices to establish and maintain multicast routes and forward multicast packets correctly and efficiently. A multicast route is a loop-free data transmission path from a data source to multiple receivers. Namely, it is a multicast distribution tree.

In the ASM model, multicast routes come in intra-domain routes and inter-domain routes.

l           An intra-domain multicast routing protocol is used to discover multicast sources and build multicast distribution trees with an autonomous system (AS) so as to deliver multicast data to receivers. Among a variety of mature intra-domain multicast routing protocols, protocol independent multicast (PIM) is a most popular one. It delivers information to receivers by discovering the multicast source and establishing multicast distribution trees. Based on the forwarding mechanism, PIM comes in two modes – dense mode (PIM-DM) and sparse mode (PIM-SM).

l           An inter-domain multicast routing protocol is used for delivery of multicast information between two ASs. So far, mature solutions include multicast source discovery protocol (MSDP).

For the SSM model, multicast routes are not divided into inter-domain routes and intra-domain routes. Since receivers know the position of the multicast source, channels established through PIM-SD are sufficient for multicast information transport.

II. Layer 2 multicast protocols

Layer 2 multicast protocols include IGMP Snooping/MLD Snooping and multicast VLAN. Figure 1-9 shows where these protocols are in the network.

Figure 1-9 Positions of Layer 2 multicast protocols

1)         IGMP Snooping/MLD Snooping

Running on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) and Multicast Listener Discovery Snooping (MLD Snooping) are multicast constraining mechanisms that manage and control multicast groups by listening to and analyzing IGMP or MLD messages exchanged between the hosts and Layer 3 multicast devices, thus effectively controlling the flooding of multicast data in a Layer 2 network.

2)         Multicast VLAN

In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device need multicast information, the upstream Layer 3 device needs to forward a separate copy of the multicast data to each VLAN of the Layer 2 device. With the multicast VLAN feature enabled on the Layer 2 device, the Layer 3 multicast device needs to send only one copy of multicast to the multicast VLAN on the Layer 2 device. This avoids waste of network bandwidth and extra burden on the Layer 3 device.

1.4  Multicast Packets Forwarding Mechanism

In a multicast model, a multicast source sends information to a host group, which is identified by a multicast group address in the destination address field of the IP packets. Therefore, to deliver multicast packets to receivers located in different parts of the network, multicast routers on the forwarding path usually need to forward multicast packets received on one incoming interface to multiple outgoing interfaces. Compared with a unicast model, a multicast model is more complex in the following aspects.

l           To ensure multicast packet transmission in the network, unicast routing tables or multicast-dedicated routing tables must be used as guidance for multicast forwarding.

l           To process the same multicast information from different peers received on different interfaces of the same device, every multicast packet is subject to a reverse path forwarding (RPF) check on the incoming interface. The result of the RPF check determines whether the packet will be forwarded or discarded. The RPF check mechanism is the basis for most multicast routing protocols to implement multicast forwarding.