YouTube

What Is YouTube?

YouTube is a video-sharing platform that has been operating since 2005, and was acquired by Google in 2006.

It is arguably the most recognized and widely used video site in the world.

More than 500 hours of video are reportedly uploaded to YouTube every minute — and all of it is delivered to billions of users around the globe with virtually no delay. It's fair to say YouTube runs some of the most sophisticated video processing and delivery infrastructure on the planet.

Uploading and Transcoding on YouTube

When a creator uploads a video to YouTube, it immediately undergoes transcoding (re-encoding) on YouTube's servers.

H.264 (AVC) is the recommended codec for uploads, but YouTube internally converts the video into multiple codecs and resolutions. This is also why you can upload in formats beyond MP4 — AVI, WMV, and others are accepted, because YouTube handles the conversion internally.

Supported upload formats include:

MOV, MPEG-1, MPEG-2, MPEG4, MP4, MPG, AVI, WMV, MPEGPS, FLV, 3GPP, WebM, DNxHR, ProRes, CineForm, HEVC (H.265)

Codecs Used Internally by YouTube

YouTube primarily uses three codecs internally:

H.264 (AVC)

A long-established, widely supported standard codec. Used for lower-resolution video and in contexts where broad compatibility is required.

VP9

An open-source, royalty-free codec developed by Google. It achieves the same visual quality as H.264 at lower bitrates. YouTube uses VP9 primarily for Full HD and above, and it has been the standard for 4K video since 2017.

AV1

A next-generation codec developed by the Alliance for Open Media. It is roughly 30% more efficient than VP9, delivering the same quality with less data. YouTube has been rolling out AV1 support, and users can opt into "Always prefer AV1" in their settings.

Resolution Variants

Every uploaded video is transcoded into multiple resolutions on YouTube's servers — 360p, 480p, 720p, 1080p, 4K, and more — so the appropriate resolution can be delivered automatically based on each viewer's network conditions.

YouTube Uses Adaptive Bitrate Streaming (ABR)

YouTube's streaming method is called Adaptive Bitrate Streaming (ABR).

In traditional download-based delivery, a video is retrieved as a single file from start to finish. With ABR, the video is split into short segments (a few seconds each), and the quality switches in real time based on the viewer's available bandwidth.

For example, a viewer might receive 1080p segments over Wi-Fi, but the moment they switch to a mobile connection, the player automatically switches to 480p segments. This minimizes buffering while maintaining the best possible quality at any given moment.

Streaming Protocols Used by YouTube

YouTube relies on two main protocols to implement ABR:

MPEG-DASH (Dynamic Adaptive Streaming over HTTP)

An internationally standardized, HTTP-based streaming protocol. It is codec-agnostic, and YouTube uses it as the backbone of its on-demand video delivery.

HLS (HTTP Live Streaming)

A streaming protocol developed by Apple, supporting both on-demand and live delivery. YouTube uses HLS for live streaming, including for ingesting video from encoders into YouTube's servers.

YouTube Delivers Video via CDN

To deliver video to viewers worldwide with minimal latency, YouTube relies on a CDN (Content Delivery Network).

A CDN is a network of edge servers distributed across the globe. When a video is uploaded to YouTube, its data is cached from the origin server to edge servers in multiple regions. When a viewer plays the video, content is served from the geographically nearest edge server, dramatically reducing the delay caused by physical distance.

Google operates its own proprietary CDN infrastructure, called the Google Global Cache (GGC), which underpins its worldwide video delivery.

How Playback Works on the Viewer's Side

Here's what happens when a viewer plays a YouTube video:

1. The browser or app accesses the video page and retrieves metadata — video info and a list of available resolutions.
2. The client-side ABR algorithm measures current network bandwidth and selects the appropriate resolution.
3. Segments at the selected resolution are fetched sequentially from the nearest CDN edge server and played back.
4. As network conditions change, the resolution switches automatically to maintain the buffer and keep playback going smoothly.

You can see this in action yourself: right-click the YouTube player and select Stats for nerds to view the current codec (vp09, avc1, etc.), resolution, and bitrate in real time.

For example, the stats above reveal the following about that particular video:

YouTube Video Storage URLs

When a video is uploaded to YouTube, a storage URL is generated in this format:

https://xxx.c.youtube.com/videoplayback?expire=xxx&ei=xxx&ip=xxx&cp=xxx&id=xxx&itag=xxx&source=xxx&requiressl=xxx&xpc=xxx&met=xxx&mh=xxx&mm=xxx&mn=xxx&ms=xxx&mv=xxx&mvi=xxx&pl=xxx&rms=xxx&sc=xxx&susc=xxx&mime=xxx&vprv=xxx&prv=xxx&rqh=xxx&ratebypass=xxx&dur=xxx&lmt=xxx&title=xxx&ext=xxx&mt=xxx&txp=xxx&sparams=xxx&sig=xxx&lsparams=xxx&lsig=xxx

In theory, replacing the xxx placeholders with valid values and sending a GET request to this URL would allow you to download the video — including private videos.

However, these URLs have an expiration time, so even if a download succeeds, it will stop working after a certain period.

The complexity of this design reflects YouTube's deliberate effort to protect its content.

Summary

YouTube's video delivery is made possible by the combination of these technologies:

What looks like a simple video playback is actually the result of multiple tightly integrated technologies working in concert — enabling simultaneous delivery to billions of viewers around the world.