Understanding multicast and QoS in IP routers
Jul 1, 2007 12:00 PM, BY CIPRIAN POPOVICIU
The consolidation of the Internet Protocol as the foundation of most communications services has become indisputable. IP's simplicity, open standards development, relative low cost of deployment and, most importantly, a killer application — the World Wide Web — led to its extraordinary rapid uptake. Measured in terms of Internet adoption, the success of IP is unprecedented. It took 38 years to attract the first 50 million radio listeners and 13 years to attract 50 million television viewers. It only took four years for the Internet to attract its first 50 million users.
A large and rapidly increasing IP user base, at both individual and institutional levels, naturally led to the exploration of other applications and services that could leverage IP. These efforts were aided by advancements in media technologies, such as Wi-Fi, cable and xDSL, which led to availability of increased bandwidth at lower costs for broadband subscribers and enterprise users.
IP's ambitions focused on services such as telephony or video and audio content delivery, operating within a different paradigm than typical IP-based data exchange. With data exchange applications, the complete delivery and integrity of data is paramount. With the new services, the focus shifts to timely delivery of information even if some random, small amount is lost along the way. Moreover, while the original IP communications model targeted host-to-host exchange of information, the need to simultaneously deliver the same data to multiple hosts was also identified. IP evolved to support a wider range of services and applications.
This article is a follows-up to the Special Report in the August 2006 issue, “Understanding IP routers,” which analyzed the basic operation of a router and its generic architecture. It builds on that introduction to discuss some of the more advanced functions performed by routers today.
IP multicast
Figure 1. Conceptual representation of IP multicast operation
Click image to enlarge.
In its original version 4, IP was designed to support two types of communication:
- Unicast
One-to-one, with one source to one destination.
- Broadcast
One-to-all, with one source to all destinations within an IP subnet.
What happens, however, if several hosts in the same or different subnets are interested in the same packet at the same time? This scenario applies to the distribution of a software package within an enterprise or the delivery of a video program to broadband subscribers. Broadcasting the packet is not efficient because it bothers hosts that are not interested in the information. The packet could be sent via unicast to each destination, but in this case the resources of the source and network are dramatically taxed. A one-to-several communication mechanism, also called multicast, is more appropriate.
In a nutshell, the idea behind multicast is to enable the IP network to learn which hosts are interested in a given traffic (identified by a group address G) and to figure out how and where to optimally replicate that traffic. This way the source of the traffic (identified by its unicast IP address S) sends a single packet that will only reach all the listeners. Subscribers can be interested in the traffic regardless of the source (*, G) or in the traffic provided by a specific source (S, G). The IP routers providing network access to subscribers learn of their interest in a multicast group, and they signal that upstream.
Routers within the network collectively build a tree that enables them to optimally forward — over the shortest path between the source and the listeners — the traffic for a given multicast group. Due to the tree structure, it can avoid looping packets. While there is a lot more to the multicast operation (see Reference 1 on next page), this overview highlights the additional functions of a multicast-enabled router:
- Learn about listeners.
- Build the multicast tree.
- Replicate packets.
- Verify that packets are not looping.
Figure 2. IP QoS architectures
Click image to enlarge.
Figure 1 describes the process of a network building the knowledge necessary to forward multicast traffic, while Figure 2 shows the process of forwarding the traffic over the multicast tree.
In relation to router architecture, the multicast state build-up and maintenance is handled in the control plane, and it requires processing and memory resources. The multicast traffic forwarding and replication takes place in the data plane and can be performed either in software or in hardware depending on the router.
IP multicast has gained significant visibility over the past several years. Enterprises are leveraging it for internal applications while service providers are using it to provide audio and video content over IP. Multicast is now an essential feature of modern IP networks.
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