Elmo: Source-Routed Multicast for Cloud Services

TL;DR

Elmo is a scalable multicast system for cloud data centers that leverages programmable switches and packet encoding to support millions of groups with minimal overhead and high efficiency.

Contribution

Elmo introduces a novel source-routed multicast approach that encodes group info in packets, overcoming IP multicast scalability issues in cloud data centers.

Findings

Supports 1 million multicast groups in large data centers

Requires only 1.1K group-table entries per switch

Achieves as low as 5% traffic overhead

Abstract

We present Elmo, a system that addresses the multicast scalability problem in multi-tenant data centers. Modern cloud applications frequently exhibit one-to-many communication patterns and, at the same time, require sub-millisecond latencies and high throughput. IP multicast can achieve these requirements but has control- and data-plane scalability limitations that make it challenging to offer it as a service for hundreds of thousands of tenants, typical of cloud environments. Tenants, therefore, must rely on unicast-based approaches (e.g., application-layer or overlay-based) to support multicast in their applications, imposing overhead on throughput and end host CPU utilization, with higher and unpredictable latencies. Elmo scales network multicast by taking advantage of emerging programmable switches and the unique characteristics of data-center networks; specifically, the symmetric topology and short paths in a data center. Elmo encodes multicast group information inside packets themselves, reducing the need to store the same information in network switches. In a three-tier data-center topology with 27K hosts, Elmo supports a million multicast groups using a 325-byte packet header, requiring as few as 1.1K multicast group-table entries on average in leaf switches, with a traffic overhead as low as 5% over ideal multicast.