HPACK

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Each HTTP transfer carries a set of headers that describe the transferred resource and its properties. In HTTP/1.x, this metadata is always sent as plain text and adds anywhere from 500–800 bytes of overhead per transfer, and sometimes kilobytes more if HTTP cookies are being used; see Measuring and Controlling Protocol Overhead. To reduce this overhead and improve performance, HTTP/2 compresses request and response header metadata using the HPACK compression format that uses two simple but powerful techniques:

It allows the transmitted header fields to be encoded via a static Huffman code, which reduces their individual transfer size.
It requires that both the client and server maintain and update an indexed list of previously seen header fields (i.e., establishes a shared compression context), which is then used as a reference to efficiently encode previously transmitted values.

Huffman coding allows the individual values to be compressed when transferred, and the indexed list of previously transferred values allows us to encode duplicate values (Figure 12-6) by transferring index values that can be used to efficiently look up and reconstruct the full header keys and values.