Google Toolbar for Internet Explorer implemented Shared Dictionary Compression over HTTP (SDCH) that speeds up loading web pages. “retrieving a set of HTML pages with the same header, footer, inlined JavaScript and CSS requires the retransmission of the same data multiple times. [SDCH is] a compression technique that leverages this crosspayload redundancy,” explain Google's proposal (PDF). A Google help page provides a way to remove the feature by disabling the "Google Dictionary Compression sdch" add-on. The developer versions of Google Chrome implement SDCH as well.

Source:→ Google System