MPLS in the SDN Era

Chapter 4. Internet Multicast Over MPLS

This chapter describes Global Internet Multicast, as opposed to Chapter 5, which focuses on Multicast VPN. Let’s begin with a basic multicast introduction that should also help for a better understanding of Chapter 5.

Multicast packets flow from a given source (S) to a group (G) of receivers, as compared to unicast packets, which are destined to a single receiver. The forwarding path used to transport multicast is typically modeled as a tree, with the source being the root and the receivers sitting at the leaves. In a multicast tree, the routers replicate the traffic at the branching points. The traffic flows from the root to the leaves, like sap in a tree. In that sense, the terms upstream and downstream defy gravity: take a picture of a tree and turn it upside down, with the root on top and the leaves at the bottom, and let the traffic flow down. With this image in mind it’s easier to understand why, in multicast land, upstream means toward the root (source) and downstream is toward the leaves (receivers). Multicast packets flow downstream in a point-to-multipoint manner.

Let’s get back to reality after this short imagination twister. In a nutshell, multicast technologies make it possible for a network to replicate one single packet to multiple destinations. A popular and typical multicast application is IP Television (IPTV), whereby many residential and mobile users can be watching the same live channel at the same time. If the source ...