多播的回归:为什么它在实时线性世界中成功
多年来多次在 流媒体 I’ve been given free rein to indulge myself writing about my favorite of all topics in the streaming space: IP 多播.
IP多播提供了显著的优势, 在某些情况下, 超过传统的单播和广播.
在现场直播中, 每个连接的用户都有他或她自己从服务器返回的带宽需求, 因此，服务器需要一个足够大的互联网连接来服务流的所有请求. 这是真的，即使同样的数据给了很多不同的人. While essential for personalized services, unicast doesn’t scale well for large live online events.
在广播中没有直接连接, but some of the network is allocated to the broadcast; in effect, the content is streamed to all users even if they don’t watch it—internet routers essentially do not forward broadcast packets, 所以广播在路由IP网络之间是不可能的.
在组播中, 终端用户通过在自己的路由器上注册他们对流的兴趣来收听, 然后依次注册所有的路由器回到原点. The routers then work out that they only need to send one copy of the packet of data to the next router (rather than one for each user). 最后一台路由器只需向最终接入网络发送一次数据包, 但目前确实如此, 所有需要该数据包的接收器都同时接收. 因此，无论收件人是一个还是数千个(就像在企业网络中一样), 负载看起来仍然像单个终端用户.
This means that many different broadcasts can be delivered to many different people using the internet all at the same time, and audience sizes are no longer restricted by the size of a central server (or in the case of the traditional pureplay CDN, 一个可以共享负载的分布式服务器网络).
组播简史
我自己在多播方面的活动始于90年代末. 我一直在流式传输音频，主要使用Live365的一个小hack.获得免费的CDN服务, 然后更专业地使用Akamai, 真实广播网络, 和其他U.S. CDNs.
我转向了视频，并部署了很多Windows Media项目, 当我学习各种界面的时候, I kept reading about the Windows Media 多播 capability and the scale of live audiences it could enable. 我深入挖掘，感觉好像找到了一段被遗忘的历史, one that just needed a kick to start up and change how anyone could broadcast a live data stream to all those who wanted to hear it, 比任何其他方法都更有效. I realized that scale could be reached with existing equipment—it just needed a coherent deployment configuration. 有几个标准，没有明确的业务, 多播还需要获得相关玩家的支持.
完全享受了互联网时代, 之前还做过俱乐部推广, 我都意识到MP3是如何在瞬间改变了整个内容产业的动态, and how IP 多播 would similarly (if it ever escaped into the wild) change the dynamic of the broadcast industry.
尽管面临挑战, I decided to take an early position myself; in 2001 I joined with one of the authors of the early IP 多播 protocols to create the Global-多播 Internet eXchange (Global-MIX). Global-MIX背后的想法是通过isp提供结构化的特定于源的多播, 与我们一起管理直播电视和广播编码器的聚合来源, 以及多播服务器. We used MBGP (in whose creation my partner was also involved) to deliver the 多播 video to ISPs who wanted to forward it to their users.
Since we reverted to unicast in the event 多播 was not possible (which was the case most of the time) we could offer a comprehensive CDN live service to TV and radio broadcasters that were simulcasting. 我们在哪里实现了多播, 我们获得了惊人的利润, 这使我们能够在由大型美国公司主导的市场中占有一席之地.S. CDNs.
一切都很顺利，直到Windows Media失宠, and Adobe didn’t really get its Fusion product together well enough to replace the simplicity with which Windows Media could be deployed.
那一刻就这样过去了. Global-MIX的遗产作为MBGP对等网络在伦敦互联网交换中心继续存在, 它仍然有一些功能, 但它是一头复杂的野兽. 传统上以家电为主，资本支出(capex)密集的网络投资, most network services need a minimum level of traffic before they can garner political and financial support internally, 由于IP多播降低了每个人的成本, any successful long-term strategy was vulnerable to a counterargument of “that didn’t increase our revenue.” Worse, 这种效率可能会减少收入或降低过境购买力, 这可能会更广泛地影响业务. 有时候低效率是有价值的.
这种情况多年来一直保持不变. 但事情不是一成不变的. 一直以来, 注意到在他们自己的后院运营的不断增长的CDN市场, 运营商一直在制定自己的网络分销策略. Over the past 10 years we have seen the pureplay CDNs provide a competency that has typically been outside of the operators’ own domain: a knowledge of how to run media services and manage the application layer software on top of the IP routes.
今天多播
现在，这一切都在改变. Operators often see that value as something they want to take in-house; they want to be able to offer CDN-like services among the suites of propositions that they offer to their wide range of clients. VPNs, proxies, 语音电话, and an abundance of data services all present architecturally similar problems to a network operator. 它们都需要工作流、网络和处理，以及最终的计费.
正如我最近写的 软件定义网络和网络功能虚拟化的出现在美国，运营商正在定义新一代的架构. They are adopting a forward-looking and more dynamic approach to their network functions deployment, 允许他们在市场上以更快的周期工作.
For an operator to deploy an on-net CDN a couple of years ago was a significant capex consideration. 现在的计划是一次性部署通用计算基础设施, 并确保应用程序(包括cdn)可以在该计算基础设施上运行. 从那里, 他们可以通过软件改变他们正在做的事情，几乎没有增加成本, 而不是通过“滚动的卡车”和燃烧成吨的资本.
在接下来的一两年, as the operators finish that fundamental architecture and replace routers with compute power and routing software, it becomes easy for them to roll out both very small and very large service models in dynamically. 在这一点上, they will take over the problems of network optimization for particular client traffic entirely with their own CDN architectures, 它们将开始从cdn的托管服务中分离出来. 当然, operators might initially license software from today’s CDNs to get them started—and we can see Akamai, 聚光灯下, and others already in the process of providing licensed software models—but over time many will develop new CDN capabilities themselves, 利用自己的网络百家乐软件和技能, 并提供它们作为一个关键的区别. 但这还没有到来.
然而，还有别的东西在锅里, 伴随着这种新的计算机化和软件驱动的网络模式, 它将给这个汤添加一些重要的味道:IP多播的回归.
这一次, audience adoption of live linear streaming is reaching a critical mass; the value of the audience outweighs the revenue lost by replacing inefficient unicast with highly efficient IP 多播. 突然间，电信公司开始支持增加用户, 而不是向内容提供商收取增加受众的费用. 现在，电信公司从用户端开始参与价值链, 而不是简单地从发行商那里获取收益. 这意味着多播的优化增加了利润, 而不是简单地降低服务的价值，电信公司可以卖给别人.
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