HTML & XHTML: The Definitive Guide, 6th Edition

Server-Push Documents

Netscape invented server-push dynamic documents. With the technology, the client/server connection remains open after an initial transfer of data, and the server periodically sends new data to the client, updating the document's display. Server-push is made possible by some special programming on the server side and is enabled by the multipart/mixed-media type feature of Multipurpose Internet Mail Extensions (MIME), the computer industry's standard for multimedia document transmission over the Internet.

The Multipart/Mixed Media Type

As we mentioned earlier in this chapter in the discussion of client-pull dynamic documents, the HTTP server sends a two-part transmission to the client browser: a header describing the document, followed by the document itself. The document's MIME type is part of the HTTP header field. Normally, the server includes "Content-Type: text/html" in an HTML document's header before sending its actual contents. By changing that content type to "multipart/mixed," you can send an HTML document or several documents in several pieces, rather than in a single chunk. Only Mozilla-based browsers, such as Netscape and Firefox, though, understand and respond to the multipart header field; other browsers either ignore additional parts or refuse the document altogether.

The general form of the MIME multipart/mixed-media Content-Type header looks like this:

Content-type: multipart/mixed;boundary="SomeRandomString"

This HTTP header component tells the ...