Multicast Basics


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Multicast Basics

  1. 1. VIDEO STREAMING SERVICES, LLC 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 WWW.VIDEOSSC.COM all rights reserved 2005
  2. 2. 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. WWW.VIDEOSSC.COM All Rights Reserved 2005
  3. 3. 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: 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 WWW.VIDEOSSC.COM All Rights Reserved 2005
  4. 4. 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. WWW.VIDEOSSC.COM All Rights Reserved 2005
  5. 5. 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 ip pim accept-rp ! 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 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 ( through Addresses in this range are controlled by IANA. WWW.VIDEOSSC.COM All Rights Reserved 2005
  6. 6. 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 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 block, Sprint can then use 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 The block is administratively scoped for internal use only. You can use this block within your own network similar to the way that the,, and 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: 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 and see a result similar to the following: sl-gw10-dc#mtrace mtrace Translating ""...domain server ( [OK] Type escape sequence to abort. Mtrace from to via RPF From source ( to destination (sl-gw10- Querying full reverse path... 0 ( -1 ( PIM/MBGP [] WWW.VIDEOSSC.COM All Rights Reserved 2005
  7. 7. -2 ( PIM/MBGP [] -3 ( PIM [] -4 ( The key here is to see the mtrace terminate to 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: • In Zinf, open the following URL. rtp:// 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: WWW.VIDEOSSC.COM All Rights Reserved 2005
  8. 8. • RFC 2236: Internet Group Management Protocol, Version 2 • 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: • WWW.VIDEOSSC.COM All Rights Reserved 2005