14.1 Multicast Backbone


Intel, Microsoft, Cisco Systems, and children's favorite Toys-R-Us are all companies that have deployed cost-saving and business-enabling multicast solutions. These are just companies with large enterprise networks and even ISPs are activating multicast on their production networks.

Multicast is a unique aspect of routing in networks that has been steadily growing in use across the Internet. However, many engineers have yet to acknowledge its growing role. Certainly your everyday Internet consumer is completely oblivious to its existence as an overlay on the Internet known as the multicast backbone ( mbone ).

In a typical IPv4 unicast routing scenario, delivery, ordering, and delay are not guaranteed . Although multicast employs techniques to use bandwidth more efficiently , there is still no guarantee of delay and ordering. Thus, in addition to multicast techniques, proper application deployment and internal application correction methods need to be implemented to take advantage of multicast fully. Do not misunderstand ”multicast does have some issues associated with it, the largest being that network engineers do not seem to believe in it as a solution, it has no built-in network congestion avoidance mechanism, there are packet duplication issues, and because multicast commonly uses UDP as the transport mechanism, packet delivery can be unreliable at times.

As multimedia in all its forms more strongly dominates the realm of networking in general and the Internet in particular, the use of multicast becomes ever more important. Juniper Networks has a strong multicast implementation that enables the issues associated with multicast to be better addressed through the support of many of the newer multicast protocols designed to cope with these issues.

Mbone was at its inception like many fundamental stalwarts of the Internet today ”an experiment. The IETF wanted to broadcast its meetings to those unable to be present who were located across the globe. The idea at the time was to create a volunteer-based experiment to explore how to successfully transmit audio and video from one source to many destinations. The technology to provide this type of host and network capabilities evolved into several new technologies known collectively as multicasting.

Mbone is an interconnected set of subnetworks of routers that support multicast. These routers are grouped together into multicast islands that are then overlaid onto the Internet as shown in Figure 14-2. An island is connected to another island over the Internet via a tunnel (a virtual point-to-point link). These tunnels allow multicast traffic to pass undisturbed through the portions of the Internet that are not multicast-enabled.

Figure 14-2. Multicast Islands in Mbone Architecture

graphics/14fig02.gif

The deployment and continued growth of mbone is important to multicast and its use, as well as its evolution. However, the need, or more specifically , the commercial and business factors involved in driving growth in multicast is even more important.

In conclusion mbone is winding down, and the experiment is over. The experiment was extremely successful and demonstrated that multicast and tunneling could be accomplished. Today and in the future, we will see many large ISPs, including Sprint, Level3, and WorldCom, continue to incorporate multicast into their production networks. These networks, in general, are using PIM as their multicast protocol of choice, instead of DVMRP, through either MBGP or MSDP. Later, this chapter will introduce and discuss these protocols.

The standards relating to multicast are as follows :

  • RFC 1112, "Host Extensions for IP Multicasting" (defines IGMP Version 1)

  • RFC 2236, "Internet Group Management Protocol, Version 2"

  • RFC 2327, "SDP: Session Description Protocol"

  • RFC 2362, "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification"

  • RFC 2365, "Administratively Scoped IP Multicast"

  • "Protocol Independent Multicast Version 2 Dense Mode Specification," Internet draft, draft-ietf-pim-v2-dm-03.txt

  • "Distance Vector Multicast Routing Protocol," Internet draft, draft-ietf-idmr-dvmrp-v3-07

  • "SAP: Session Announcement Protocol," Internet draft, draft-ietf-mmusic-sap-00

  • "MSDP," Internet draft, draft-ietf-msdp-spec-01.txt

  • "Anycast RP Mechanism Using PIM and MSDP," Internet draft, draft-ietf-mboned-anycast-rp-05.txt

To access Internet RFCs and drafts, go to the IETF Web site at www.ietf.org. These standards are current in their status at the time of this writing. It is always good to look beyond the standards to how they have been interpreted. The following additional resources can assist:

  • ftp://parcftp.xerox.com/pub/net-research/mbone/maps/mbone-map.pdf

  • www.savetz.com/mbone/

  • www-itg.lbl.gov/mbone/

14.1.1 Benefits of Multicasting

Before this chapter delves into the details of how multicasting works, the following list describes some of the benefits of multicasting. We will look at these benefits first from a network engineer's perspective and then from a cost-profit business perspective.

  • Provides economic benefits through savings in costs and server resources

  • Allows increased users in a multicast group without an exponential networkwide increase in bandwidth use

  • Reduces the load on the sending server, which no longer has to support many sequential or concurrent unicast sessions (this benefit can be just as significant as the bandwidth savings)

  • Decreases LAN traffic, thus reducing potential bottlenecks and scaling issues

  • Reduces WAN traffic, which has a positive impact on overall network management

Events in the arenas of both Internet politics and personal Internet use are also contributing to the continued growth of multicast. Consider the following more specific snapshot of factors and business or personal uses for multicast:

  • Deployment of new applications that can save your company money or possibly become a source of income

  • Multimedia conferencing, including audio and video, electronic whiteboards , and Microsoft's Net Meeting

  • Data distribution and sharing information, such as files and data, Gnuetella and Bear Share (file sharing applications), and Napster or clone

  • Real-time data, like stock tickers and news reports (certainly it will not be long before sporting events and concerts are multicast regularly over the Internet)

  • The hosting and selling subscriptions to participate in Internet games like Doom, Quake, Dungeon Siege, and Everquest and military warfare simulations

As the Internet has evolved and information sharing has increased, many different models to share information via the Internet have been developed. Some have gotten the rest of the world really stirred up, but the point here is that multicasting applications are on the rise and this will continue! Are you ready?

14.1.2 JUNOS Multicast Implementations

Multicast routing is the key to multicast and is the framework upon which multicast packets are routed to recipients. Some of the concepts that we will introduce here will be explained in other sections, but it is good to get comfortable with the proper multicast terminology at the outset. JUNOS implements the following protocols to support IP multicast routing:

  • Internet Group Management Protocol (IGMP), versions 1, 2, and 3 ”This protocol is used by routers to solicit from their directly connected hosts if they wish to participate in a multicast group.

  • DVMRP ”This is the first true multicast routing protocol that operates in dense mode and determines path information via a distance vector algorithm. It has been many years since DVMRP was introduced, and it is now considered a legacy multicast protocol, like RIPv1.

  • PIM ”PIM supports two modes of operation (dense and sparse) and is used for efficiently routing to multicast groups that might span wide-area and interdomain internetworks. It is referred to as protocol independent because it is not dependent on any particular unicast routing protocol. JUNOS also supports dense-sparse mode, which is a combination of features from both modes.

  • MSDP ”This multicast routing protocol is used by PIM when in sparse mode to span multicast domains to discover other multicast sources in other domains.

  • MBGP ”BGP was extended to carry multicast family information, allowing the sharing of multicast information between ASs. MSDP will not function properly without MBGP as we will see later in this chapter.

  • Session Announcement Protocol (SAP) and Session Description Protocol (SDP) ”These protocols handle conference session announcements.

It is commonly understandood that when discussing multicast and BGP, the "m" in MBGP stands for "multicast," even though the RFC defines it as "multiprotocol."



Juniper Networks Reference Guide. JUNOS Routing, Configuration, and Architecture
Juniper Networks Reference Guide: JUNOS Routing, Configuration, and Architecture: JUNOS Routing, Configuration, and Architecture
ISBN: 0201775921
EAN: 2147483647
Year: 2002
Pages: 176

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