Policy tree multicast routing

Abstract

Bandwidth-sensitive multicast delivery controlled by routing criteria pertinent to the actual traffic flow is very costly in terms of router state and control overhead and it scales poorly towards larger, wide-area networks. PIM-SM (Protocol-independent Multicast - Sparse Mode) has been introduced as a simple, flexible and scalable concept for Internet-wide multicasting. Yet, PIM's efficiency potential (like that of alternative wide-area multicast concepts) can only be fully exploited if it is based on Reverse Path Forwarding (RPF), a low-cost mechanism, which however does not select prescribed delivery paths if link parameters or routing policies are asymmetric. This paper presents an extension to protocols like PIM-SM, called Policy Tree Multicast Routing (PTMR). This concept leads to multicast delivery trees which, even under asymmetric conditions, readily comply with imposed macroscopic policies and moreover enable support of shortest path and QoS criteria. PTMR gives no consideration to how the policy-sensitive paths have been established; they may be imposed by the network itself but also by providers, recipients and even sources. PTMR amends the PIM-SM mechanism with a macroscopic control layer which marks (pegs) domain border routers on policy-sensitive forward paths. In a given domain, a pegged ingress border router is then joined by local group members as well as by its child pegs. This results in optimally fusing transit paths into local distribution trees. PTMR performance and policy control potential are restricted primarily: by the congestion of control messages at the multicast sources, introduced by source-originating tree construction; by the possible extent of policy-sensitive path aggregation; and by the intra-domain (transit) delivery conditions. PTMR is a single-layer protocol, appending PIM-SM with a policy dedicated delivery mode. Ist primary design target is to forward multicast traffic in accordance with any underlying multicast-relevant routing, including comprehensive policy routing. In comparison, Border Gateway Multicast Protocol (BGMP) is a proposal for an intra-domain MR protocol based on BGP-type routing, which focuses on fusing heterogenous multicast routing domains and which allows ASs to control multicast transit traffic.