SDCH (Shared Dictionary Compression for HTTP) is a data compression algorithm created by Google, based on VCDIFF (RFC 3284). It is supported natively in recent versions of Google Chrome, Chromium and Android, as well as on Google websites.
SDCH achieves its effectiveness by using pre-negotiated dictionaries to "warm up" its internal state prior to encoding or decoding. These may either be already stored locally, or uploaded from a source and then cached.
References
SDCH Wikipedia(Text) CC BY-SA