We present Elmo, a system that addresses the multicast scalability problem in multi-tenant datacenters. 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 bandwidth and end-host CPU overheads, 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 hypervisor switches, symmetric topology, and short paths in a datacenter. 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 27,000 hosts, Elmo supports a million multicast groups using an average packet-header size of 114 bytes (max. 325 bytes), requiring as few as 1,100 multicast group-table entries on average in leaf switches, and having a traffic overhead as low as 5% over ideal multicast.

中文翻译：

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Elmo：公共云的源路由多播

我们提出了Elmo，这是一个解决多租户数据中心中多播可伸缩性问题的系统。现代云应用程序经常表现出一对多的通信模式，并且同时要求毫秒级的延迟和高吞吐量。IP多播可以满足这些要求，但在控制和数据平面可伸缩性方面存在局限性，因此很难提供为数十万租户提供的服务 ，典型的云环境。因此，租户必须依靠基于单播的方法（ 例如 ，基于应用程序层或基于覆盖层）以支持其应用程序中的多播，从而带来带宽和最终主机CPU开销，并具有更高且不可预测的延迟。Elmo通过利用新兴的可编程交换机和数据中心网络的独特特性来扩展网络组播。特别是，虚拟机管理程序交换机，对称拓扑和数据中心中的短路径。Elmo在数据包本身中编码多播组信息，从而减少了在网络交换机中存储相同信息的需求。在具有27,000个主机的三层数据中心拓扑中，Elmo支持100万个多播组，其平均包头大小为114字节（最大325个字节），平均每个叶子中只有1,100个多播组表条目交换机，并且与理想组播相比，流量开销低至5％。