To Download (verb) is to receive information to a local device from a computer. The word download is also sometimes used as a noun, in which case this refers to one or more item which was downloaded. The Internet is little more than an infrastructure which allows for the convenient download and upload of information from one device to another.
When data is being downloaded, a request packet is sent to the serving (uploading) host. If all goes well, the host responds to that packet by sending packets which contain the requested information back to the requesting device. In most cases (that is, with protocols which use TCP), the recipient will send acknowledgment packets (ACK) back to the host, to indicate that each portion has been received properly. If any packet is not received and acknowledged properly, it will be resent. Technically, this means that most downloads do not involve a completely passive recipient, but the is a small amount of upload taking place as well, as the confirmation messages are sent back to the sender.
Since this method of download is reliable, it is used for almost everything, including:
- Website browsing
- File and image downloading
- E-mail processing
- Online office software such as Microsoft Word Online
- Internet Relay Chat and other instant messaging
- Software updates
- almost everything else
In a few cases (with protocols that use UDP) confirmation responses are not used. After the recipient requests data, the host will simply send it all, without caring whether it is received or not. Although a true download in which the recipient is passive, this method is risky to use in most situations, since important content can be lost. However, this method works much more quickly, so it is very useful when the focus is on quantity over quality. While a garbled sentence in an e-mail would be unacceptable, a few discolored pixels in one frame of a streamed video will not matter. For this reason, UDP is primarily used for:
- Video streaming (Netflix, YouTube, DailyMotion, etc.)
- Music streaming (Pandora, Spotify, etc.)
- Video conferencing (Skype, etc.)
- Voice over IP (VoIP)
Although UDP is less versatile and reliable, it is quite common among Internet communication. In fact, a 2013 article reported that half of the Internet traffic in the United States is streaming from Netflix and YouTube. This means that if this report is still accurate, over half of the Internet traffic is using UDP.