Playing MIME Games

The Internet was not set up to handle audio and video broadcasting. It was built mainly for sending and receiving text messages and files. The standard for this is the Simple Mail Transfer Protocol (SMTP), the TCP/IP protocol that controls E-mail transmission. Somehow, the transmission of audio and video has to fit within that architecture.

Specifically, your browser may not have come with the capability to handle some of the audio and video streams now available over the Internet. No big surprise, since the formats for these data types are new. This is where Multipurpose Internet Mail Extensions (MIME) comes in. MIME is the mechanism for defining new formats to travel within the Internet's TCP/IP protocol. These are extensions to the SMTP format that define how to treat other types of data. The Internet Engineering Task Force developed MIME specifically for handling new media types on the Net. You want your browser to support MIME.

Built-in MIME capabilities already include provisions for seven types of data: plain text (with lines and messages of any length, but no word processor formatting); audio; video; still images; message (which can point to the actual message somewhere else); multipart messages (with each part possibly having a different type); and application-specific data (with lots of latitude to define subtypes).

Each of these types can have a number of subtypes, and this is where the action is. An application designer typically defines a new subtype, which essentially serves as a flag to use the application's own "viewer" for that subtype. While an audio application might define a subtype of the audio type, and a video application might define a subtype of the video type, t here is no hard and fast rule for this. Many applications simply define a new application subtype. For example, VocalTec defines an application/vocaltec-media-desc subtype, while Progressive Networks defines an audio/x-pn-realaudio subtype.