HTTP Streaming: What You Need to Know
[This article originally appeared in the 2010 流媒体源手册.]
在流媒体世界里, there are many terms that create confusion, 从“流媒体”这个词开始," which is often used interchangeably with the term "progressive download.“甚至编解码器这个词, 格式, 系统, and wrapper/container are often used interchangeably, 尽管不正确.
所有的项, 虽然, the one that seems to have the most uncertainty around it is HTTP streaming. 就在最近的2007年, the term was easily dismissed as a mistake because HTTP was a protocol that allowed content to be routed through port 80 while streaming often required a separate port.
在过去的两年里, 然而, HTTP streaming has become a reality, both for live and on-demand content. 是现在, 反过来, often confused with another technology that can accompany HTTP streaming—adaptive bitrate streaming.
在本文中, I’ll address the differences between HTTP streaming, 通用的流, 渐进式下载. I’ll also touch a bit on adaptive bitrate technologies, even 虽然 these technologies are a bit nascent and several of the major players have yet to fully firm up the technologies’ integration into HTTP streaming.
定义HTTP流
As such, a set of definitions is in order from the outset. First, let’s look at progressive downloads, streaming media, and hybrid solutions.
渐进式下载
A progressive download is a pre-encoded file that has been set to begin playing prior to the completion of the full download. 在过去, a file would completely download before it began playing, which meant the user had to wait several minutes (or hours) before he or she could watch the video.
The reason for the confusion of progressive downloading with streaming is that progressive downloaded file playback behaves somewhat like a streaming file, in that it often begins playing within 3–10 seconds of the time the file is selected. 流式传输通常需要一个缓冲区, 渐进下载也是如此, to address network inconsistencies; playback begins only after an initial playback buffer is loaded.
与流, a user can click on any location along a file’s timeline and begin to watch the video after a few seconds of buffering. 渐进式下载, 另一方面, require the file to be downloaded linearly, meaning that a user’s bandwidth may constrain the user from jumping ahead several minutes, at least until that portion of the video file has been downloaded.
To address this limitation in progressive downloads and to allow commands such as fast-forward, 倒带, 暂停, 或者定位搜索, some progressive download solutions offer "trick mode" requests that will divert downloading to the portion of the file that the user has requested. 要做到这一点, 需要对文件进行分段, which we’ll later see is critical to the development of adaptive bitrate streaming and HTTP streaming delivery.
流媒体
不像渐进式下载, which can often be downloaded from an HTTP server just as a PDF document can be delivered via HTTP, 流媒体内容, 直到现在, required the use of a separate protocol to deliver streaming packets.
The long-standard real-time protocol (RTP), which was used by videoconferencing and voice-over-IP solutions, 有同伴加入吗, streaming-specific protocol (RTSP) as a way to differentiate traffic from HTTP content, which most often used port 80 and was not as time-sensitive as streaming content. 不幸的是, the use of a port other than port 80 for streaming also meant it was easily identified, which 反过来 meant that streaming content could be blocked at the router by IT departments intent on keeping streaming content from "damaging" their network.
Besides the trick modes previously mentioned, there is another innovation in streaming that allows content to be rapidly launched. 被称为“快速启动”或“暗示”," this rapid streaming innovation allowed the streaming server to allow the player to begin playback without a buffer, almost guaranteeing that streaming content would begin playing back faster than progressive download content.
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