Multicast Basics

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Multicast FAQ

Q: What is multicast?
A: Multicast is an IP technology that allows for streams of data to be sent efficiently from
one to many destinations. Instead of setting up separate unicast sessions for each
destination, multicast will replicate packets at router hops where the path to different
multicast group members diverges. This allows a source to send a single copy of a stream
of data, while reaching any number of possible receivers.

Q: What are the advantages to using multicast?
A: Multicast is the most efficient method of delivering the same data to multiple receivers
at the same time. Servers send only one data stream to reach any number of end-users.
This greatly benefits content providers, who don't need as much raw bandwidth in order
to reach a large audience. Using unicast, a server would need to send out as many streams
as there are receivers. This increases the CPU load, as well as the amount of bandwidth
required to reach that audience. Multicast also benefits Internet providers, as there will
only be a single copy of the data streamed across any links. Utilizing multicast can
decrease bandwidth across a network, and increase the life of a current network without
constantly upgrading links and routers. End users probably won't see any immediate
benefits to multicast, but they should see content and network providers offering new
services due to the benefits they see from multicast.

Q: Which applications are best suited for multicast?
A: Many people associate multicast with multimedia as it is an excellent transmission
method for multimedia. This can be an Internet radio broadcast, television broadcast,
video conferencing, stock market tickers, slide presentations, etc. However, multicast is
also suited to a large number of other applications. Such applications include file
transfers to multiple locations, or dynamic web page updates. In addition, other
applications, such as online gaming, news feeds, chatrooms, and more are suited for
multicast.

Q: Can you provide an example of using multicast vs. unicast?
A: On May 18, 2000, over two million Internet users flocked to watch the Victoria's
Secret Fashion Show. Several different versions of the video stream were available: a 56
kbps unicast stream, 100 kbps unicast stream, and two multicast streams at 300 kbps and
700 kbps. The vast majority of these viewers did not have multicast available to them.
Even assuming that everyone was using the 56 kbps stream, that's over 100 Gbps of data

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being streamed, or the equivalent of 10 OC-48 connections. This is a huge expense,
especially for a 30 minute event. However, any number of users could have joined the
multicast streams, and the maximum amount of bandwidth that Victoria's Secret would
have needed for the event would have been 1 Mbps. A single T1 connection would have
served to reach the two million users, if multicast had been available to them, and it
provided higher quality video.

Q: What are the limitations of multicast?
A: Multicast has one major limitation. For multicast to work, every router between the
recipient and the source must be multicast enabled. Since multicast is a relatively new
technology, not all networks are multicast enabled. Sprint was the first ISP to completely
enable multicast technology natively across its entire backbone. As the technology is
embraced, more router vendors and ISPs will enable multicast. The cost benefits to
multicast and the increasing use of high bandwidth multimedia applications on the
Internet suggest that multicast will be ubiquitous very soon.

Joining Multicast Groups

Q: What do I need to do in order for my personal computer to receive multicast?
A: It's necessary for a computer system to support IGMP (Internet Group Management
Protocol), in order for you to join multicast groups. Most modern operating systems
already do, without needing any additional files to be added. This includes Windows
95/98/ME, Windows NT 4/2000/XP, MacOS 8 and later, Linux, Solaris (2.5 and later),
Net/Free/OpenBSD, etc. OS versions earlier than those listed here may also support
IGMP.

For more information about IGMP, please read RFC 2236.

In order to support SSM (Source Specific Multicast), a computer needs to support IGMP
version 3. Not as many operating systems support this yet. You can find patches for
FreeBSD and Linux to compile in your kernel, and Windows XP also has IGMPv3
support.

Q: What software applications are there for multicast?
A: Several. Many popular streaming applications, such as Microsoft Windows
MediaPlayer, Real Networks RealPlayer, and Apple QuickTime have multicast

capabilities. Several other multicast-only applications are appearing, such as Cisco's
IP/TV and Apple's MacTV.

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Numerous applications were developed in conjunction with the MBONE (an educational
multicast backbone research project). The most widely used of these is SDR (session
directory), which is a program that lists many of the multicast sessions occurring on the
MBONE. A host of helper applications are available for SDR, such as VIC (video), VAT
and RAT (audio), WB (white board), and more. These applications are available for
UNIX and Microsoft Windows platforms. Several other applications have been built as
clones of these programs for other operating systems.

You can find many MBONE applications for numerous platforms at the following site:
http://www.merit.edu/~mbone/index/titles.html

Q: How can I find out if my Internet provider offers multicast service?
A: Multicast Technologies maintains a list of active autonomous systems which are
routing multicast. This list will only show networks that utilize BGP routing, so your ISP
may not be listed. Ask you ISP or for INTRANET Multicasting talk with your network
administrators

DSL and cable modems could be capable of handling multicast (cable modems should
follow the DOCSIS 1.0 or 1.1 standards), depending on your ISPs network design.
Contact your provider for details.

To see if you are already capable of receiving multicast streams, you can use the
Multicast Technologies multicast tester. Please make sure to read the instructions
carefully.

Q: What is the MBONE?
A: The MBONE was an educational research project to test new multicast applications.
Begun in 1992, the MBONE was orginally a collection of networks tunneled together to
support multicast. Today, MBONE refers to the general multicast backbone that is
deployed on the Internet. Many of the providers that connect to the MBONE use native
multicast routing protocols, instead of relying on tunnels.

Q: Can I still reach the MBONE ?
A: Yes. Some but not all Tier 1 Carriers have peerings to numerous other multicast
providers that make up the MBONE.

Multicast Routing


Q: What multicast protocols do ISPs use?
A: Most ISPs route multicast through their backbone with PIM-SM (Protocol
Independent Multicast - Sparse Mode). PIM-SM creates multicast trees from a central
core location, called a rendezvous point (RP). Tier 1 ISPs usually have several RPs
located geographically throughout their network. These RPs share the same IP address,
allowing our customers and peers to contact the nearest available RP (anycast RP).

RPs communicate with each other and RPs in other networks through MSDP (Multicast
Source Discovery Protocol). Routing is provided with either static routes, or MBGP
(Multicast extensions for Border Gateway Protocol).

Q: How does PIM-SM work?
A: PIM-SM is a multicast routing protocol belonging to the shared-tree family. It uses a
rendezvous point (RP), which multicast sources use to register their sessions. The RP
maintains a table with this source and group information. When a host wishes to join a
multicast session, it sends a join request to its gateway router for a multicast group. Since
the gateway router doesn't have information about the source address, it will build a path
back to the RP, which does contain the source information. The path that is chosen is
determined by the unicast routing protocol (such as IS-IS, OSPF, IGRP, EIGRP, or RIP)
running on the router. The RP will continue to build a tree from the destination back to
the source, and forward multicast packets to the destination. Once traffic has been
received from the source of the session, the gateway router can then build a tree back to
the source directly, which may be a more optimal path than going through the RP. PIM-
SM is defined in RFC 2362.

Q: What exactly does an RP do?
A: An RP serves as a central point for multicast sessions. A source will register its
session with an RP. This information is carried to other RPs through MSDP. When a host
joins a multicast group, it first joins the group at the RP. Once the host has received
traffic from the source, the host can switch to a tree rooted at the source.

Q: Do I need to have my own RP?
A: No. If you choose to, you can set up one of your own routers as an RP. However,
some ISPs have several routers geographically dispersed to provide the same service.

Q: What protocols does my router need to run?
A: Your router will need to be able to run PIM-SM and IGMP. If you have a Cisco
router, multicast functionality is available in almost all Cisco IOS versions 11.1 or later.
We strongly recommend using version 12.0 or later for best performance and reliability.
If you plan to use your own RP or run MBGP, we recommend version 12.0S or later.


JunOS 4.2 and later should support multicast, although we recommend using the latest
available version of JunOS.

Q: How difficult is it to configure a router for native multicast?
A: While the protocols that make multicast work can be extremely complicated,
configuring a router is very simple. The following is a very simplified sample
configuration for Cisco routers to show what is minimally needed to enable a router for
native multicast. In this example, the router is homed only to Sprint and is using Sprint's
RP.

ip multicast-routing
ip pim rp-address 144.232.187.198
ip pim accept-rp 144.232.187.198
!
int Ethernet0
ip pim sparse-mode
!
int Serial0
ip pim sparse-mode

Repeat for all interfaces through which multicast traffic should flow.

For further Cisco configuration examples, we strongly encourage you to visit
ftp://ftpeng.cisco.com/ipmulticast.html.

Q: What if I don't have a Cisco router?
A: You will need to check with your router vendor to find out if multicast is an available

option. Minimum requirements are the ability to support PIM-SM and IGMP.
Performance will be improved if your router supports PIM-SM version 2.

Q: What is IGMP?
A: The Internet Group Management Protocol (IGMP) is a protocol that controls group
membership individual hosts. This protocol only operates in a LAN setting, but is
required if you wish to be able to join a multicast group on a host. IGMP is defined in
RFC 2236.

Q: Do I need to have special addresses for multicast?
A: Multicast addresses are only needed for multicast sources. You don't need to obtain
any special addresses if you only wish to receive multicast traffic. Multicast sources use
Class D addresses (224.0.0.0 through 239.255.255.255). Addresses in this range are
controlled by IANA.


Many applications have been provided with ranges of addresses for their use, and will
dynamically assign a multicast address to sessions created using it. For instance, SDR
will automatically assign a multicast address to your session, if you announce the session
with SDR. More applications are expected to use this method of dynamically assigning
multicast addresses in the future.

The 233.0.0.0/8 block has been set aside for multicast use for anyone with their own
autonomous system number (ASN). The ASN can be written as a 16-bit binary number
(left-padded with zeroes), and then inserted into the middle two octets of this address
block. For example, Sprint has ASN 1239. Written as a 16-bit binary number, this value
is 00000100 11010111. By inserting this value into the middle two octets of the
233.0.0.0/8 block, Sprint can then use 233.4.215.0/24 for multicast sessions. Anyone with
their own ASN can use this method to create a /24 block for multicast use. You can find a
JavaScript calculator that will figure out a block for you at
http://www.sprint.net/multicast/addresses.html.

The 239.0.0.0/8 block is administratively scoped for internal use only. You can use this
block within your own network similar to the way that the 10.0.0.0/8, 172.16.0.0/12, and
192.168.0.0/16 blocks are used.

If you decide that you require static multicast addresses and do not have your own ASN,
you can request them from IANA at this site: http://www.iana.net/cgi-bin/multicast.pl. In
rare occassions where a customer is unable to obtain addresses from IANA, your ISP
may be able to provide a limited number of addresses for use.

Q: Will multicast work through my firewall?
A: You will need to check with your firewall vendor to determine how to enable
multicast traffic through a firewall. In addition, you may want to read RFC 2588: IP
Multicast and Firewalls.

Q: I have multicast enabled. How can I make sure that everything is working correctly?
A: There are a number of tests you can perform to make sure that multicast is working.
First, you should verify that the router is set up correctly. On a Cisco router, you should
be able to perform the command: mtrace rp.sprintlink.net and see a result similar to the
following:

sl-gw10-dc#mtrace rp.sprintlink.net
mtrace rp.sprintlink.net
Translating "rp.sprintlink.net"...domain server (204.117.214.10) [OK]

Type escape sequence to abort.
Mtrace from 144.232.187.198 to 144.228.20.10 via RPF
From source (rp.sprintlink.net) to destination (sl-gw10-
dc-0-0-0.sprintlink.net)
Querying full reverse path...
0 sl-gw10-dc-0-0-0.sprintlink.net (144.228.20.10)
-1 sl-gw10-dc-0-0-0.sprintlink.net (144.228.20.10) PIM/MBGP
[144.232.187.198/32]


-2 sl-gw13-dc-4-0-0.sprintlink.net (144.228.20.13) PIM/MBGP
[144.232.187.198/32]
-3 sl-bb22-rly-2-0.sprintlink.net (144.232.25.217) PIM
[144.232.187.198/32]
-4 rp.sprintlink.net (144.232.187.198)

The key here is to see the mtrace terminate to rp.sprintlink.net. Because we have multiple
RPs in the network, you may see the trace take a different route than the one shown here.

You can also pull up a web browser, and use that to verify that multicast is working. A
multicast tester is available that uses an applet to test your connectivity. Please use the
contact information available on that page to report any issues.)

Users who have Microsoft Windows Media Player can attempt to join one of two
multicast streams.

• Video - 363 kbps - suitable for cable modem, DSL, and T1 or greater
• Audio - 22 kbps - suitable for modem users

Alternatively, multicast streams are also available for users of Real Network's RealOne
player, or a MP3 player capable of receiving RTP-encapsulated data, such as Zinf.

• RealOne can use this URL: http://mcast2.sprintlink.net:8080/Mussorgsky.sdp
• In Zinf, open the following URL. rtp://233.19.86.123:51042

These streams are only available via multicast. If you can view or hear the stream you've
selected, then multicast is working.

Additional Information

Q: Where can I find out more information?
A: For more information about multicasting, you might want to check out the following
sources:

Edwards, Brian M. et al. Interdomain Multicast Routing: Practical Juniper Networks and
Cisco Systems Solutions Addison-Wesley, Inc. 2002. ISBN: 0201746123
Parkhurst, William R. Cisco Multicast Routing and Switching. McGraw Hill Text. 1999.
ISBN: 0071346473
Williamson, Beau. Developing IP Multicast Networks. Cisco Press. 2000. ISBN:
1578700779
Maufer, Thomas A. Deploying IP Multicast in the Enterprise. Prentice Hill PTR. 1998.
ISBN: 0138976872

The following RFC's and drafts contain useful information about multicast:


• RFC 2236: Internet Group Management Protocol, Version 2
http://www.ietf.org/rfc/rfc2236.txt
• RFC 2283: Multiprotocol Extensions for BGP-4
• RFC 2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)
• RFC 2588: IP Multicast and Firewalls
• RFC 2858: Multiprotocol Extensions for BGP-4
• RFC 3180: GLOP Addressing In 233/8

The following sites also contain useful information about multicast:

• ftp://ftpeng.cisco.com/ipmulticast.html


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