Recently, several friends doing online education complained to me, saying that their video lessons are stuck in PPT at the peak of the evening, and user complaints can burst the customer service mailbox. I took a look at the background, good guy, all mobile networks and weak network environment playback failure. Their first reaction is to add bandwidth, but the cost is directly on the sky. In fact, these days the problem has long been not in the source station bandwidth, the real antidote isUse the right high-defense CDN and eat through HLS multi-code rate adaptation。
Some people open their mouths to “HLS is not .m3u8 plus .ts slicing well”, but really want to carry DDoS and ensure that the HD smooth, the pit here can be buried in an operation and maintenance team. I have tested three mainstream high-defense CDN vendors, found that even boasts “full protocol support”, the level of optimization of multi-bit rate HLS can be out of ten streets.
Let's start with the conclusion:Serious high-defense CDNs definitely support the HLS protocol, but support ≠ good use. Some vendors can just help you forward traffic, while others have done in-depth optimization from slice caching, code rate switching algorithms to anti-theft chain. For example, CDN5 nodes will automatically preload adjacent code rate slices, weak network environment switching latency can be pressed to 200ms, while some small manufacturers do not move on the lag for more than 3 seconds - the experience of the user does not run is strange.
Why is HLS so difficult to adapt? First of all, it is inherently “fragmented”: a video is sliced into hundreds of .ts files, each of which may be requested by users with different network conditions. A high-defense CDN not only has to withstand CC attacks, but also has to intelligently cache these slices. I have seen a certain misconfigured CDN, edge nodes crazy back to the source to pull slices, directly to the source station dragged down. At this time it is necessary to look at the CDN'sIs the caching policy tuned for HLS?。
For example, 08Host's solution is a bit of a thing: their edge nodes dynamically choose the cache slice bitrate based on the user's network speed. When the user's bandwidth is detected to drop, not only automatically reduce the bit rate, but also low bit rate slices will be cached in advance to the nearest node to the user. This requires CDN vendors in the TCP acceleration and caching algorithms on the pile of material, not a simple set of nginx can handle.
When it comes to multi-bitrate adaptation, don't believe those “one-click open multi-bitrate” propaganda. I picked up the log of a CDN and found that their so-called bit rate switching is entirely the client's own judgment, the server did not do any intervention. As a result, the user in the network jitter crazy code rate switching, but caused continuous lag. A truly reliable program needsCollaborative decision making between server and client。
Taking CDN07 as an example, their HLS optimization scheme plays like this: edge nodes will monitor the user network quality in real time, and when the packet loss rate is found to be more than 2%, they will take the initiative to push low bit rate streams to the client. At the same time, the next slice is sent in advance through HTTP/2 Server Push to reduce waiting time. This set of combinations of punches down, even in the 4G network can realize seamless switching.
Technical details should pay attention to the handling of the manifest file (.m3u8). High-defense CDN must be able to dynamically modify the inside of the slice URL, otherwise anti-stealing chain and node scheduling all have to rest. Share a tested and usable nginx configuration snippet:
The key thing about this configuration is thatsub_filterThat line - it ensures that client requests for slicing go directly to the current CDN node instead of stupidly going back to the source. Some vendors charge extra for even this basic feature, so be sure to ask when selecting a model.
Another bloody lesson: HLS security links should never be checked with just a Referer! It can be forged casually. I recommend usingTimestamp+IP+Key Signature Scheme, generate secure links like this:
CDN5 has done a particularly good job of this: each of their nodes supports key rotation, so even if a node's key is compromised, it won't affect the entire network. The other two I have seen share the same key for all nodes, this level of security is about the same as setting up a table in front of the hacker's door.
The test session is the easiest to roll over. A lot of people take a local 100 megabit network to test 4K streams and say it's fine, but the result is that the whole thing collapses when you go online. The real test isweak network simulation: Performance in 2G/3G/Congested WiFi scenarios. I'm used to generating multi-bitrate test streams with ffmpeg:
Then take the generated master.m3u8 to different network environments to test. Pay attention to see whether the bit rate switching is smooth, there is no problem of audio and video out of sync. 08Host debugging interface directly integrated network simulation function, can save a lot of things.
Lastly, some vendors use HLS support as a gimmick, but in reality, they don't even support BASIC authentication. You really want to go online only to find that you have to pay more money to buy the enterprise version to open the anti-theft chain function. So before signing up, you must askTest Account Hands-on ExerciseThe main point is to see whether the console can customize the caching strategy, whether there is a real-time bandwidth chart, and which anti-theft chain methods are supported. Don't wait until your business is online to realize that you've been trapped.
Now you know why the same labeling support HLS high defense CDN, the price can be three times the difference, right? Good things are in the details: between the nodes slice synchronization efficiency, code rate switching algorithm, security policy granularity ...... these are the key to determine the user experience. The next selection, directly take the technical points in this article to ask the vendor to ensure that they do not dare to casually fool you.
By the way, if you guys use a cloud vendor's CDN, watch out for traffic fee traps. A well-known cloud vendor once had a bug: HLS each slice of a separate calculation of the number of requests, a small video can generate thousands of requests, the bill directly exploded. Or CDN07 really, they are billed according to the actual length of video playback, watch 4K and watch 144p a price - these days this kind of conscientious vendors are not much.
In short, technology selection is like a blind date, just look at the promotional materials must fall into the pit. Be sure to hand test, take the real business scenarios to the death test. The day you find three o'clock in the morning the server is not alarmed, that's when you really found a reliable HLS high defense CDN.
