Protocols: HTTP vs. RTMP
When you use an FLV or H.264 file as your distribution method for Web video content, you have two protocols for serving the video over the Internet: Hypertext Transfer Protocol (HTTP) and Real-Time Messaging Protocol (RTMP). You can use one or both protocols to serve video to your Internet audience via Flash Player.
Most content viewed on a Web site is served over HTTP. Any Web server, such as Apache or Microsoft Internet Information Services (IIS), can deliver video files. The best reasons to use a Web server with HTTP for hosting video content are simplicity and cost. If you know how to transfer files to a Web server using a File Transfer Protocol (FTP) client, for example, you can put video files on a Web site and make the content accessible to visitors. Another advantage of HTTP is cost: Most Web hosting providers offer cheap storage and large transfer quotas that allow you to host numerous media files and serve them to your visitors.
From a site visitor’s point of view, one advantage of using HTTP is access. Many corporate networks use firewalls to block specific content from entering. Popular methods of blocking are protocol and port restrictions. Some firewall rules allow only HTTP content served over port 80. Almost all Web servers use port 80 to serve content, but a Web server can be set up to serve HTTP content over custom ports such as 8080, 8081, or 8500. These ports are usually used by test or development servers. Some firewall rules allow only specific MIME types, such as text/html (HTML documents), and common image formats (image/gif, image/jpeg, and image/png). By far, video served over HTTP on port 80 has the best chance of being viewed by a visitor.
However, video content delivered using HTTP lacks intellectual property (IP) protection. When you use a Web browser to view Flash-based video content, there is no way to limit offline access to the video content served over HTTP: The Web browser caches video files in its temporary storage, which can be accessed by the user after browsing the site. As such, many content producers and licensees don’t distribute video content over HTTP; RTMP is the preferred protocol.
Another drawback to HTTP-delivered video is lack of access to all parts of the video file. With video served over HTTP, the video file progressively downloads to the Web browser’s cache. The viewer can access only the portion of the video that has already downloaded. For example, if you have a 30-minute training video and the viewer wants to view only the last 5 minutes of the video, they have to wait until the first 25 minutes have downloaded to the browser cache. For longer videos, you should consider using RTMP instead of HTTP.
You can serve video over the Internet to Flash Player using RTMP, a special protocol for real-time server applications ranging from instant messaging to collaborative data sharing to video streaming. Whereas HTTP-delivered video is referred to as progressive download video, RTMP-delivered video is called streaming video. However, because the term streaming is so often misused, I prefer the phrase real-time streaming video.
RTMP delivery of video is provided by a limited number of server technologies, including licensed server software from Adobe, notably Flash Media Server (FMS). FMS is installed on a networked server and manages streaming video separately from the Web server hosting the Flash movie (SWF) and other HTML content (see Figure 4.1). Licensing FMS for high-volume Web sites can be expensive, but you can find affordable shared hosting FMS services for less demanding sites from providers such as Influxis.com. Using a streaming video service provider is a better option for high-volume Flash-based video deployment, and I discuss this option later in the chapter.
Figure 4.1 When a user views real-time streaming video content in Flash Player, a Flash movie (SWF) is downloaded from a Web server (step 1), and a NetConnection instance created in the movie’s ActionScript code makes a connection to the FMS application or streaming video service provider (step 2). The video content (FLV or H.264) is then streamed packet by packet to Flash Player (step 3).
One of the benefits of RTMP delivery for the viewer is near-instantaneous playback of video, provided the video file is encoded with a bit rate appropriate to the viewer’s connection speed. Real-time streaming video can also be seeked to any point in the content. This feature is particularly advantageous for long-duration content because the viewer doesn’t have to wait for the video file to load before jumping ahead, as is the case for HTTP-delivered video.
Content producers who want to protect video from local playback can opt to use real-time streaming video—in any video format, not just Flash-based video formats, if they target other video player technologies such as Apple QuickTime or Microsoft Windows Media. When RTMP-based video streams into Flash Player, the audio and video data is stored only in Flash Player’s memory buffer. The entire video file is never copied or stored to the Web browser cache.
The protocol you use can affect your video deployment strategy. Be sure to carefully analyze your objectives before picking a protocol.
If you encode your video files with a compression tool, you should determine which protocol(s) to use before you create the video files. Because RTMP connections send video data from a remote server to a temporary player buffer, the video’s total data rate should be predictable throughout the entire duration of playback. This data rate consistency can be accomplished only with constant bit rate (CBR) encoding. Nearly all professional video encoders offer the option of selecting constant bit rate or variable bit rate (VBR) encoding. If your video-encoding tool doesn’t offer a choice, it’s probably using CBR encoding.
If your video content is encoded with a VBR setting, your file may have extreme data spikes that exceed the average bit rate of the video. These spikes can abruptly empty Flash Player’s buffer and result in temporarily stalled playback and an annoying pause-play-pause experience.
However, if your video content won’t be delivered over RTMP, you can safely use VBR encoding for standard HTTP delivery on a Web server. Because the video progressively downloads and is stored in the Web browser cache, data spikes probably won’t occur during playback—provided the bit rate you’re using is suitable to the viewer’s connection speed.
For security reasons, some internal networks, or intranets, control the type of content that can enter the network from remote locations. In large corporations, these intranets can block much of the content available on the Web. Firewalls and proxies that allow Internet traffic accept HTTP connections over port 80 from remote servers, so you’re nearly guaranteed that viewers behind these firewalls can access video delivered over HTTP.
However, this luxury isn’t always shared with RTMP-delivered video. The default port for RTMP connections is 1935, which may not be allowed on tight firewalls. If Flash Player’s first attempt to play video over port 1935 fails, Flash Player automatically tries to connect to the video stream with RTMP over port 80. If this second try fails, the last resort available for Flash Player is to try an HTTP-tunneled connection over port 80. A tunneled connection means the RTMP data packet is wrapped (or masked) in an HTTP data packet. Some firewalls allow this traffic, because the data packet appears to be normal HTTP Web traffic. If the firewall inspects the HTTP data, though, it still may reject the connection, and the video won’t play.
The bottom line: If you don’t want to deal with firewalls and proxies, you should serve your video from a Web server over HTTP.
Some video formats available over the Internet use DRM to control access. Microsoft Windows Media and Apple QuickTime content can use encrypted keys to limit many factors of media usage, including the number of computers that can view the content and how long the content can be viewed. Flash-based video hasn’t included such ironclad content protection measures in any past releases of Flash Player, including Flash Player 10. Adobe AIR applications, however, can now utilize DRM measures with Adobe FMRMS.
If you must control access to video content deployed to the Flash platform and you don’t want users to fish video files out of their Web browser cache, you must use real-time streaming video within the Web browser. Or you can use either progressive download or real-time streaming video with an AIR application and Adobe FMRMS. As soon as you put your video files in a publicly accessible location on a Web server, you’re allowing users to copy the video files and do whatever they want with them. Mind you, lots of video files on the Web—even from high-volume sites—aren’t real-time streaming video. Google’s Video search and YouTube.com are prime examples of sites offering browser-cacheable, do-as-you-please FLV files. However, real-time streaming video is usually the only viable option for companies and organizations (such as movie studios and broadcast networks) that need to control the way their content is viewed and accessed by the public.
Online or offline playback
If your Flash application is designed to run from a local source such as a user’s hard drive or fixed media (CD/DVD), you may want the video content to coexist with other local files used for the application. Unless you want to require an Internet connection for the Flash application to run, you don’t need to store video files on remote Web servers (HTTP) or real-time streaming servers (RTMP).
Duration of content
As the length of a video file increases, so should the likelihood of using real-time streaming video (RTMP) over a progressively downloaded video (HTTP). Regardless of the data rate used by the video file, longer files have larger file sizes. When an HTTP-delivered video starts to download into Flash Player, by default the file continues to download to the browser cache regardless of whether the user watches the content. If you host large video files on your Web server, the Web server’s data-transfer bytes will increase quickly, potentially raising the financial cost of hosting the files. If you host large video files on an FMS server (or a streaming video service provider), your data-transfer cost includes only the portions of the video watched by each user.