Dictionary-ID

The HTTP Dictionary-ID request header carries an opaque identifier for a Compression dictionary the client has cached, echoing the identifier the server assigned via the Use-As-Dictionary response header.

Table of Contents

Usage

Compression dictionaries enable smaller payload sizes by referencing shared data patterns between requests. A server advertising a dictionary via Use-As-Dictionary provides the dictionary content and assigns the dictionary an identifier. The client caches the dictionary and stores the identifier alongside the dictionary.

On subsequent requests matching the dictionary's scope, the client sends Dictionary-ID alongside the Available-Dictionary request header. The Dictionary-ID value echoes the identifier the server originally assigned, letting the server confirm which dictionary version the client holds. The server then applies dictionary-based compression to the response body if the content benefits from compression.

The header works alongside Use-As-Dictionary and Available-Dictionary to coordinate dictionary-based compression with schemes like Brotli and Zstandard.

Values

The Dictionary-ID header accepts a Structured Field String representing the server's internal identifier for the dictionary resource. Servers generate identifiers based on their own dictionary storage and versioning mechanisms. The value must not exceed 1024 characters and must match the id value from the original Use-As-Dictionary response.

Dictionary-ID: "v42"

The identifier remains stable as long as the dictionary content remains unchanged. When the server updates the dictionary, the server assigns a new identifier via Use-As-Dictionary, and the client updates its cached value accordingly.

Example

A server sends a response with Use-As-Dictionary assigning a dictionary identifier. The client caches both the dictionary and the identifier.

Use-As-Dictionary: match="/api/*", id="dict-2024-08"

On a subsequent request matching the pattern, the client echoes the stored identifier back to the server alongside the dictionary hash.

Available-Dictionary: :abc123hash:
Dictionary-ID: "dict-2024-08"

When the server updates the dictionary, a new identifier is issued. The client fetches the updated dictionary and sends the new identifier on future requests.

Dictionary-ID: "dict-2024-09"

Takeaway

The Dictionary-ID request header lets the client echo back the server-assigned dictionary identifier, enabling the server to confirm which dictionary version the client holds and apply matching compression to the response.

See also

Last updated: March 6, 2026