回到基础:HTTP流

“回归基础”是一种新的流媒体.com column that will feature 文章 from various authors attempting to clarify and analyze common—and sometimes commonly misunderstood—topics in the online video industry. Got an idea for a topic you'd like to see written about, or that you'd like to write about yourself? 电子邮件编辑Eric Schumacher-Rasmussen在 erics@riyutraining.com.

One topic of conversation at 流媒体欧洲 was the subject of streaming through firewalls. 许多公司讨论了这个概念, especially around Flash and the significant Silverlight plug-in deployments that have occurred during major political and sporting events this summer.

HTTP流总是在对话中出现, but there were quite a few claims that didn't seem to be quite right, so I started digging into this complex topic and came up with 信息 that might be valuable to helping 流媒体 readers understand what's meant by HTTP streaming.

通过HTTP下载和渐进式下载
First of all, let's look at the claims about HTTP streaming from a web server. Some content delivery network (CDN) vendors use the term "HTTP streaming" to apply to their HTTP video file downloads or HTTP progressive downloads. Do a Google search for HTTP streaming and you'll come up with numerous instances where HTTP streaming is used for delivery from a web server.

Adding a video file to a web page is fairly simple and good for sites with modest traffic, so potential CDN customers might ask why they should go to the trouble of paying for a CDN service. 毕竟, delivering a progressive download via HTTP is as simple as creating a file such as a Windows Media (wmv or wma); uploading the file to a web server; creating an HTML reference file with an HTTP link to the file; and publishing the link. The file will begin playing as it is downloaded (cached to the client machine) in either a stand-alone player or an embedded player.

For those customers who understand the limitations of HTTP progressive download, 并且有最小的交通流量, CDN可能不是正确的答案. 一些发布商, 虽然, 提供基本的HTTP渐进式下载交付服务, 而不调用它们HTTP流, and charge less for this service than true streaming delivery. This allows potential customers to easily migrate to start simply on a CDN and embrace more sophisticated techniques over time.

真正的HTTP流与Windows媒体服务
So now that we've looked at HTTP progressive downloads, what about HTTP streaming? Since streaming in its truest form is delivery without caching, can an enterprise or a site with significant traffic deliver streams via HTTP?

Yes, al虽然 true HTTP streaming requires a complex network of video servers.

HTTP streaming via Windows Media Services is straightforward, 包括直播, 使用Windows Media HTTP流协议. In explaining the HTTP Streaming Protocol, Microsoft notes the best use for this type of delivery. Besides the ability to push video streaming content through port 80, which is open on many firewalls to allow traditional web traffic that is delivered via HTTP, the Microsoft HTTP Streaming Protocol "is suitable for streaming delivery at a fixed rate or at a data rate correlating closely to rate at which the video will be displayed by the receiver."

这个协议, 像其他流媒体协议一样, also allows back-channel communications between the server and the client, allowing the client player to send feedback to the server to change the transmission rate to a secondary stream, which is referred to as "intelligent streaming" or "adaptive streaming."

除了, 关于Windows Media文件, HTTP streaming can also be used to send commands like fast-forward, 倒带, 暂停, 或位置查找(也称为点击字节或范围请求). 这些术语被称为“欺骗模式”请求. It's true that some CDNs using HTTP progressive downloads have done enhancements to their service to simulate these "trick modes" to mimic the seeking capabilities, but these are often proprietary and won’t work with all available players. 除了, 可以使用HTTP发送控件, 而流本身是通过UDP发送的, 尽管微软推荐其他方法来处理这个问题.

HTTP流与Flash
Flash呢?? When someone refers to Flash HTTP streaming they actually mean HTTP tunneling. Flash服务端协议为RTMP, 它被设计成在TCP上传递, rather than the UDP delivery of the more traditional RTSP steaming protocol. 考虑到RTMP通过TCP交付的能力, Adobe通过HTTP集成了RTMP隧道, 哪种方式对直播和点播都适用, al虽然 the tunneling causes a bit of extra delay that might not be acceptable in all situations.

总之, ignoring the equally big issue of protocol rollover to HTTP (which I'll cover in a future article), pay careful attention to services that called HTTP streaming. It took a decent amount of research and talking to others to understand the differences. Don't be afraid to ask a potential CDN partner questions about their service, and ask that the CDN provide someone who can explain their HTTP streaming in simple, 一致的条款, 不是营销用语.