This manual chiefly
focuses on the IP multicast technology and device operations. Unless otherwise
stated, the term “multicast” in this document refers to IP
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 bandwidth- and time-critical information services.
I. Unicast
In unicast, the information source sends a
separate copy of information to each host that needs the information, as shown
in Figure 1-1.
Figure 1-1 Unicast transmission
Assume that Hosts B, D and E need this
information. The information source establishes a separate transmission channel
for each of these hosts.
In 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.
In broadcast, 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 Hosts B, D, and E need the
information. If the information source broadcasts the information, Hosts A and
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 consumption.
The multicast technique has solved this
problem. When some hosts on the network need multicast information, the
multicast source (Source in the figure) sends only one copy of the information.
Multicast distribution threes are built for the multicast packets through
multicast routing protocols, and the packets are replicated only on nodes where
the trees branch, as shown in Figure 1-3:
Figure 1-3 Multicast transmission
Assume that Hosts B, D and E need the
information. To receive the information correctly, these hosts need to join a
receiver set, which is known as a multicast group. The 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.
The following roles are involved in
multicast transmission:
l
An information sender is referred to as a Multicast
Source (“Source” in Figure 1-3).
l
Each receiver is a Multicast Group Member
(“Receiver” in Figure
1-3).
l
All receivers interested in the same information
form a Multicast Group. Multicast groups are not subject to geographic
restrictions.
l
A router that supports Layer 3 multicast is
called multicast router or Layer 3 multicast device. In addition to providing
the multicast routing function, a multicast router can also manage multicast
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.
|
l
A multicast source does not necessarily belong
to a multicast group. Namely, a multicast source is not necessarily a multicast
data receiver.
l
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.
I. Advantages of multicast
Advantages of the multicast technique
include:
l
Enhanced efficiency: reduces the CPU load of information
source servers and network devices.
l
Optimal performance: reduces redundant traffic.
l
Distributive application: Enables point-to-multiple-point
applications at the price of the 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 other point-to-multiple-point data
distribution application.
Based on how the receivers treat the
multicast sources, there are two multicast models:
I. ASM model
In the ASM model, any sender can send
information to a multicast group as a multicast source, and numbers of receivers
can join a multicast group identified by a group address and obtain multicast
information addressed to that multicast group. In this model, receivers are not
aware of the position of multicast sources in advance. However, they can join
or leave the multicast group at any time.
II. SSM model
In the practical life, users may be
interested in the multicast data from only certain 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 model,
and dedicated multicast forwarding paths are established between receivers and
the specified multicast sources.
IP multicast addresses 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 from which the receivers
need to receive multicast data? (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.
To allow communication between multicast
sources and multicast group members, network-layer multicast addresses, namely,
multicast IP addresses must be provided. In addition, a technique must be
available to map multicast IP addresses to link-layer multicast MAC addresses.
Internet Assigned Numbers Authority (IANA) assigned
the Class D address space (224.0.0.0 to 239.255.255.255) for IPv4 multicast. The
specific address blocks and usages are 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
|
Reserved permanent group addresses. The
IP address 224.0.0.0 is reserved, and other IP addresses can be used by
routing protocols and for topology searching, 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 designated 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.
|
l
The membership of a group is dynamic. Hosts can
join or leave multicast groups at any time.
l
“Glop” is a mechanism for assigning
multicast addresses between different autonomous systems (ASs). By filling an
AS number into the middle two bytes of 233.0.0.0, you get 255 multicast
addresses for that AS.
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)
|
As defined in RFC 4291, the format of an
IPv6 multicast is as follows:
Figure 1-4 IPv6 multicast format
l
0xFF: 8 bits, indicating that this address is an
IPv6 multicast address.
l
Flags: 4 bits, of which the high-order flag is
reserved and set to 0; the definition and usage of the second bit can be found
in RFC 3956; and definition and usage of the third bit can be found in RFC 3306;
the low-order bit is the Transient (T) flag. When set to 0, the T flag
indicates a permanently-assigned multicast address assigned by IANA; when set
to 1, the T flag indicates a transient, or dynamically assigned 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
1-4.
l
Reserved: 80 bits, all set to 0 currently.
l
Group ID: 112 bits, identifying the multicast
group. For details about this field, refer to RFC 3306.
Table 1-4 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
|
|
|
