Debunking Multicast Urban Legends

Myth #4: There is No Means of Address Assignment
Unlike unicast addressing, which is handled by the Routing Registries and IANA (Internet Assigned Number Authority), initially there was no multicast address registration at all. Early MBone applications literally picked an address at random every time they started up, hoping that it was not in use by another device. While this might have been acceptable for short applications, it is not appropriate for more robust streaming scenarios.

In early 2000, GLOP address assignments were approved. Although GLOP isn’t an acronym, it is widely used to describe a block of 256 IP addresses. For practical purposes, GLOP is a means of giving every multi-homed service provider their own multicast address space. The GLOP mechanism has proven adequate up until the present for ASM. In SSM there is no need to worry about multicast address assignments, as only the full channel address (which includes the source address) has to be unique.

Myth #5: It is Possible to "Jam" Broadcasts by Multiple Use of the Same Group Address
While in ASM it is possible to interfere with a broadcast if two parties use the same group address and port number, in SSM this is not possible, or at least not nearly as easy. Since the source address is known, the only way to interfere with a transmission is to spoof the source address (which is fairly easy) and to inject undesirable (bogus) packets into the path between the source and the receiver (which is generally impossible).

Myth #6: Internet Service Providers (ISP’s) Think That it Will Steal Revenue and Therefore Won’t Support Multicast
While it is true that in the Dot-Com era some ISP’s resisted multicast deployment, thinking that they could thus ensure larger revenues by billing customers for unicast services, most of the Dot-Com streaming money has vanished along with most ISP profits. ISP’s currently seek revenues from whatever source possible.

Myth #7: Whither Multicast
Multicast is definitely not going away. Not only is it used in many enterprise applications, including internal enterprise webcasts, but the last three years multicast watchers have observed a steady improvement both in the technology and in its deployment in the Internet. Most backbone service providers now offer multicast, generally for free (as part of their basic service package) although you may have to ask for it.

It is also a fact that, unless an end user’s system is connected to the Internet 2 (that is, unless the viewer is a student at an I2-connected campus), it will probably not be able to receive a multicast from a source such as On-The-I.com or AmericaFree.TV. This is because most ISPs do not offer multicast service to end users. From extensive talks with ISP operators, the consensus is that this is due to a lack of content, not due to a lack of technology. There are currently no widespread attempts to provide multicast exclusive content, and thus no reason for an ISP to invest in supporting multicast. If the streaming industry wants to take advantage of the undeniable cost savings of multicast, then it will have to become aggressive about offering multicast content. DSL and Cable Internet providers, in particular, are eager to distinguish their service offerings and appear receptive to exclusive content deals.

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