Application-Level Multicast
IP network-level multicast has not found a wide deployment on the Internet
for a number of technical and commercial reasons. To satisfy the need for mul-
ticast, various other techniques have emerged, such as:
■■ The distribution of applications on servers, such as reviewed in [7], [8].
■■ Application-level multicast on overlay networks [9], [10], and content
addressable networks (CAN) [11]. Overlay networks for real-time
communications are discussed in Chapter 20, “Peer-to-Peer SIP.”
Application-level multicast has not yet applied for real-time communica-
tions, and the technology is still in the research stage. For this reason, we will
not present it here, though we believe application-level multicast will signifi-
cantly disrupt the content distribution industry.Transport Protocols
Media streams (such as voice and video for real-time communications) use
UDP packet transport, since it makes no sense to wait for delayed media pack-
ets or for the retransmission for packets that were lost, as is done using TCP.
Lost media packets are discarded in favor of getting the shortest delivery time
possible. Media delivery using UDP over IP is sensitive to packet delay and
packet loss. Quality of IP service is, therefore, an important part for real-time
Internet multimedia communications.
As we will show in Chapter 18, “Quality of Service for Real-Time Internet
Communications,” packet loss on the Internet has been reduced in the last 1086 Chapter 5
IP UNICAST AND MULTICAST
In unicast IP packet forwarding, a packet stream is delivered to a single
destination. Multicast IP delivers a packet stream to a set of destinations. Note
that this is different from packet broadcast, which can be done on an Ethernet
LAN. Packet broadcast delivers the packet to every destination on the LAN. As a
result, packet broadcast does not scale outside the LAN, because it can
generate huge traffic loads. Multicast IP is different in that an endpoint must
join a multicast group before any of the multicast packets will be forwarded by
routers serving that user. This multicast group is identified by a multicast IP
address. This scalable architecture limits the distribution of multicast packets
only to users that are participating in the session.
IP multicast is generally not enabled on the public Internet. However, it is
available using an overlay network called the MBONE (Multicast Backbone). As
mentioned, some audiovisual IETF conference sessions were distributed over
the MBONE [6] starting in 1992 using multicast.