Multicast
Multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to thousands of corporate recipients and homes. Applications that take advantage of multicast technologies include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.
Multicast Group Concept
Multicast is based on the concept of a group. A
multicast group is an arbitrary group of receivers that expresses an
interest in receiving a particular data stream. This group has no
physical or geographical boundaries—the hosts can be located anywhere on
the Internet or any private internetwork. Hosts that are interested in
receiving data flowing to a particular group must join the group using
IGMP (IGMP is discussed in the "Internet Group Management Protocol (IGMP)" section later in this document). Hosts must be a member of the group to receive the data stream.
IP Multicast Addresses
IP multicast addresses specify a "set" of IP
hosts that have joined a group and are interested in receiving multicast
traffic designated for that particular group. IPv4 multicast address
conventions are described in the following sections.
IP Class D Addresses
The Internet Assigned Numbers Authority (IANA)
controls the assignment of IP multicast addresses. IANA has assigned the
IPv4 Class D address space to be used for IP multicast. Therefore, all
IP multicast group addresses fall in the range from
224.0.0.0 through 239.255.255.255.
Note 
The
Class D address range is used only for the group address or destination
address of IP multicast traffic. The source address for multicast
datagrams is always the unicast source address.
The
Class D address range is used only for the group address or destination
address of IP multicast traffic. The source address for multicast
datagrams is always the unicast source address.
Table 1 gives a summary of the multicast address ranges discussed in this document.
Reserved Link Local Addresses
The IANA has reserved addresses in the range
224.0.0.0/24 to be used by network protocols on a local network segment.
Packets with these addresses should never be forwarded by a router.
Packets with link local destination addresses are typically sent with a
time-to-live (TTL) value of 1 and are not forwarded by a router.
Network protocols use these addresses for
automatic router discovery and to communicate important routing
information. For example, Open Shortest Path First (OSPF) uses the IP
addresses 224.0.0.5 and 224.0.0.6 to exchange link-state information. Table 2 lists some well-known link local IP addresses.
Globally Scoped Addresses
Addresses in the range from 224.0.1.0 through
238.255.255.255 are called globally scoped addresses. These addresses
are used to multicast data between organizations and across the
Internet.
Some of these addresses have been reserved for
use by multicast applications through IANA. For example, IP address
224.0.1.1 has been reserved for Network Time Protocol (NTP).
IP addresses reserved for IP multicast are defined in RFC 1112, Host Extensions for IP Multicasting. More information about reserved IP multicast addresses can be found at the following location:
http://www.iana.org/assignments/multicast-addresses.
http://www.iana.org/assignments/multicast-addresses.
Note You can find all RFCs and Internet Engineering Task Force (IETF) drafts on the IETF website (http://www.ietf.org).
You can find all RFCs and Internet Engineering Task Force (IETF) drafts on the IETF website (http://www.ietf.org). Source Specific Multicast Addresses
Addresses in the 232.0.0.0/8 range are reserved
for Source Specific Multicast (SSM). SSM is an extension of the PIM
protocol that allows for an efficient data delivery mechanism in
one-to-many communications. SSM is described in the "Source Specific Multicast (SSM)" section later in this document.
GLOP Addresses
RFC 2770, GLOP Addressing in 233/8,
proposes that the 233.0.0.0/8 address range be reserved for statically
defined addresses by organizations that already have an AS number
reserved. This practice is called GLOP addressing. The AS number of the
domain is embedded into the second and third octets of the 233.0.0.0/8
address range. For example, the AS 62010 is written in hexadecimal
format as F23A. Separating the two octets F2 and 3A results in 242 and
58 in decimal format. These values result in a subnet of 233.242.58.0/24
that would be globally reserved for AS 62010 to use.
Limited Scope Addresses
Addresses in the 239.0.0.0/8 range are called
limited scope addresses or administratively scoped addresses. These
addresses are described in RFC 2365, Administratively Scoped IP Multicast,
to be constrained to a local group or organization. Companies,
universities, or other organizations can use limited scope addresses to
have local multicast applications that will not be forwarded outside
their domain. Routers typically are configured with filters to prevent
multicast traffic in this address range from flowing outside of an
autonomous system (AS) or any user-defined domain. Within an autonomous
system or domain, the limited scope address range can be further
subdivided so that local multicast boundaries can be defined. This
subdivision is called address scoping and allows for address reuse
between these smaller domains.
Layer 2 Multicast Addresses
Historically, network interface cards (NICs) on
a LAN segment could receive only packets destined for their burned-in
MAC address or the broadcast MAC address. In IP multicast, several hosts
need to be able to receive a single data stream with a common
destination MAC address. Some means had to be devised so that multiple
hosts could receive the same packet and still be able to differentiate
between several multicast groups.
One method to accomplish this is to map IP
multicast Class D addresses directly to a MAC address. Today, using this
method, NICs can receive packets destined to many different MAC
addresses—their own unicast, broadcast, and a range of multicast
addresses.
The IEEE LAN specifications made provisions for
the transmission of broadcast and multicast packets. In the 802.3
standard, bit 0 of the first octet is used to indicate a broadcast or
multicast frame. Figure 2 shows the location of the broadcast or multicast bit in an Ethernet frame.
Figure 2 IEEE 802.3 MAC Address Format
This bit indicates that the frame is destined
for a group of hosts or all hosts on the network (in the case of the
broadcast address, 0xFFFF.FFFF.FFFF).
IP multicast makes use of this capability to send IP packets to a group of hosts on a LAN segment.
