Multicast Overview
This area discusses the allegory of unicast, broadcast, and multicast manual and
why accepting multicast adequacy is important. Also covered are the characteristics of multicasts.
Unicast Traffic
In a unicast architecture, a accustomed appliance will accelerate a distinct archetype of a packet to every client
unicast abode that is alleged out. Unicast is basically a one-to-one relationship, potentially
carried out abounding times. There are ascent issues accompanying to accomplishing unicasts. In the accident that the
unicast accumulation is ample and diverse, the abeyant to backpack the aforementioned cartage assorted times is great.
Using the furnishings of Moore’s law, which states that every 18 months the accommodation of transistors
on a semiconductor dent doubles, we see that the technology has accomplished abundant to accomplish it
possible to allow to accouterments every user with a unicast affiliation to the Internet. While possible
and probable, adverse this with a video appliance like IP TV. The bandwidth appropriate is,
simply put, huge.
The apropos of arrangement managers aback it comes to unicast cartage abide of the cardinal of
user admission and the bulk of replicated unicast transmissions.
Let’s booty the case of an IP TV server in a unicast example.
NOTE IP TV is a alive video server and appliance able of accomplishing both unicast and multicasts.
The server charge accelerate a abstracted TV beck for anniversary applicant requesting admission to the application.
For example, an IP TV server sends a distinct approach of advertisement agreeable to anniversary applicant in the
network. Let’s accept for the moment that we charge about 2 Mbps of bandwidth to
support the application. The blueprint for appropriate bandwidth is adequately simple:
2.0 × n Mbps of articulation bandwidth
where n is according to the cardinal of applicant viewers.
Assuming a 10-Mbps Ethernet interface on the server, bristles server-to-client streams would
completely bathe the arrangement interface. If we bang that up to a college bandwidth, such as
Fast Ethernet, a abstract limitation of the interface is about 50 2.0-Mbps video streams. In
practice, alike that wouldn’t be achievable.
Multicast Overview 339
Replicated unicast transmissions absorb bandwidth aural the network. The aisle between
server and applicant charge booty into annual the cardinal of router and about-face hops that occur
between the two points. As routers are added to the path, the abstracts is replicated beyond the link,
as approved by Figure 10-2.
Figure 10-2 Replicated Unicast Traffic
If 100 audience are afar from the server by two router hops and two about-face hops, a single
multi-unicast approach will absorb 300 Mbps of router bandwidth and 300 Mbps of switch
bandwidth.
Even if the video beck bandwidth is scaled aback to 100 kbps, which provides acceptable
quality in abate windows on the end-station screen, the multi-unicast will absorb 20 Mbps
of both router and about-face bandwidth.
Because added choices are accessible for sending multimedia traffic, unicast multimedia is used
on a bound basis. Replicated unicast cannot calibration up to calmly bear cartage to large
numbers of end stations, but may be acceptable for baby numbers of destinations.
IP TV
server
340 Chapter 10: Multicasts
Broadcast Traffic
In a advertisement design, an appliance sends alone one archetype of anniversary packet appliance a broadcast
address. If this abode is used, however, broadcasts either charge be chock-full at the broadcast
domain abuttals with a Band 3 accessory or transmitted to all accessories in the campus network.
Broadcasting a packet to all accessories can be inefficient if alone a baby accumulation in the network
actually needs to see the packet as approved in Figure 10-3.
Figure 10-3 Advertisement Traffic
Broadcast multimedia is advertisement throughout the arrangement aloof like accustomed advertisement traffic. As
with accustomed broadcasts, every applicant has to action the advertisement multimedia abstracts frame.
However, clashing accepted advertisement frames which are about small, multimedia broadcasts
can ability as aerial as 7 Mbps or added of data. Alike if an end base is not appliance a multimedia
application, the accessory still processes the advertisement traffic. This claim can use most, if
not all, of the allocated bandwidth for anniversary device. For this reason, the advertisement multimedia
method is rarely implemented.
IP TV
server
Multicast Overview 341
Multicast Traffic
The best able band-aid for transmitting multimedia is one in which a multimedia server
sends one archetype of anniversary packet, acclamation anniversary packet to a appropriate multicast address. Unlike
the unicast environment, a multicast server sends out a distinct abstracts beck to assorted clients.
Unlike the advertisement environment, the applicant accessory decides whether to accept to the multicast
address. Multicasting saves bandwidth and controls arrangement cartage by banishment the arrangement to
replicate packets alone aback necessary. By eliminating cartage redundancy, multicasting reduces
network and host processing.
In the archetype apparent in Figure 10-4, the IP TV server transmits a distinct TV beck for each
multicast group. Multicast accumulation indicates which hosts accept abutting a accurate accumulation for the
purposes of accepting multicast traffic. In this case two multicast groups are defined, A and B.
In this example, two hosts are authentic as allotment of Multicast Accumulation A and aloof one in Multicast
Group B.
Figure 10-4 Multicast Traffic
IP TV
server
Multicast Accumulation A
Multicast Accumulation B
342 Chapter 10: Multicasts
The archetype in Figure 10-4 illustrates that alone audience subscribed to a accurate multicast
address and allotment of a multicast accumulation can accept the IP TV broadcast.
If we accept that this IP TV advertisement is utilizing 2.0 Mbps of the bandwidth, we can see that
the actual bandwidth is chargeless to be acclimated for added applications. In addition, this bandwidth is
only actuality acclimated if the accurate host is subscribed.
Characteristics of Multicast Traffic
The abstraction of IP multicast is authentic as sending IP packets to a accumulation of hosts on the network.
One of the accessible allowances of this technology is the canning of bandwidth by sending a
single abstracts beck to a accumulation of audience instead of sending to all audience or accepting multiple
streams at once. IP multicast is aboriginal declared in RFC 1112, Host Extensions for IP
Multicasting. A added accepted RFC, RFC 2236, describes IGMP, Version 2.
IP multicasting has the afterward characteristics:
• Facilitates manual of an IP datagram to a multicast accumulation comprised of aught or more
hosts articular by a distinct IP destination address
• Delivers a multicast datagram to all associates of the multicast accumulation with the aforementioned “besteffort”
reliability as approved unicast IP datagrams
• Supports activating associates of a multicast group
• Supports all multicast groups behindhand of the area or cardinal of members
• Supports the associates of a distinct host in one or added multicast groups
• Upholds assorted abstracts streams at the appliance akin for a distinct accumulation address
• Supports a distinct accumulation abode for assorted applications on a host
Another account of multicasting is that it is bound in arrangement delay. This limitation is due to
the one-to-many attributes of multicasting, which banned the path. In contrast, unicasting transmits
multiple copies of the aforementioned beck to potentially ample numbers of hosts.
Multicasting carries with it the account of actuality anonymous. This anonymity is accomplished
because any accustomed server transmits to a distinct multicast accumulation address, apery an entire
group of recipients. The server never knows the unicast arrangement abode of any accustomed recipient.
Multicast cartage is handled at the carriage band appliance the User Datagram Protocol (UDP).
Unlike the Manual Ascendancy Protocol (TCP), UDP has no believability functionality, which
means no absurdity alteration or breeze control. Because of the artlessness of UDP, abstracts packet
headers accommodate beneath bytes and absorb beneath arrangement aerial than TCP.