Available-Dictionary

The HTTP Available-Dictionary request header signals to the server the client has a Compression dictionary available in storage and is ready to decompress responses encoded with the matching dictionary.

Table of Contents

Usage

Compression dictionaries reduce payload size by replacing repeated patterns with references to a shared dictionary. Both parties must have the same dictionary for this to work. The client stores dictionaries from previous responses marked with the Use-As-Dictionary header. When making future requests, the client includes Available-Dictionary to declare which dictionaries the client holds.

The header value is a Structured Field Byte Sequence containing the SHA-256 hash of the available dictionary encoded in Base64. The server checks the hash against dictionaries registered for the request path. When a match exists, the server compresses the response using the referenced dictionary and marks the encoding with Content-Encoding: dcb (dictionary Brotli) or dcz (dictionary Zstandard).

The compression dictionary transport framework defines the header format, negotiation flow, and security requirements. The mechanism operates only in secure contexts (HTTPS).

Example

The client sends Available-Dictionary with the SHA-256 hash of a previously stored dictionary. The hash identifies the dictionary the client holds for the requested resource.

Available-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:

The server matches the hash, compresses the response using the referenced dictionary, and indicates the encoding method with the dcb or dcz value in Content-Encoding.

GET /api/data HTTP/1.1
Host: example.re
Available-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:

HTTP/1.1 200 OK
Content-Encoding: dcb
Content-Length: 1456

Takeaway

The Available-Dictionary header announces the client has a specific Compression dictionary available, identified by SHA-256 hash, enabling the server to compress responses using the matching dictionary for reduced bandwidth usage.

See also

Last updated: March 6, 2026