Internet-Draft Zstd Window Size June 2024
Jaju & Handte Expires 14 December 2024 [Page]
Intended Status:
N. Jaju, Ed.
F. Handte, Ed.
Meta Platforms, Inc.

Window Sizing for Zstandard Content Encoding


Deployments of Zstandard, or "zstd", can use different window sizes to limit memory usage during compression and decompression. Some browsers and user agents limit window sizes to mitigate memory usage concerns, causing interoperability issues. This document updates the window size limit in RFC8878 from a recommendation to a requirement in HTTP contexts.

About This Document

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Table of Contents

1. Introduction

Zstandard, or "zstd", specified in [RFC8878], is a lossless data compression mechanism similar to gzip. When used with HTTP, the "zstd" content coding token signals to the decoder that the content is Zstandard-compressed.

An important property of Zstandard-compressed content is its Window_Size ([RFC8878], Section, which describes the maximum distance for back-references and therefore how much of the content must be kept in memory during decompression.

The minimum Window_Size is 1 KB. The maximum Window_Size is (1<<41) + 7*(1<<38) bytes, which is 3.75 TB. Larger Window_Size values tend to improve the compression ratio, but at the cost of increased memory usage.

To protect against unreasonable memory usage, some browsers and user agents limit the maximum Window_Size they will handle. This causes failures to decode responses when the content is compressed with a larger Window_Size than the recipient allows, leading to decreased interoperability.

[RFC8878], Section recommends that decoders support a Window_Size of up to 8 MB, and that encoders not generate frames using a Window_Size larger than 8 MB. However, it imposes no requirements.

This document updates [RFC8878] to enforce Window_Size limits on the encoder and decoder for the "zstd" content coding.

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Window Size

To ensure interoperability, when using the "zstd" content coding, decoders MUST support a Window_Size of up to and including 8 MB, and encoders MUST NOT generate frames requiring a Window_Size larger than 8 MB (see Section 5.1).

4. Security Considerations

This document introduces no new security considerations beyond those discussed in [RFC8878].

5. IANA Considerations

5.1. Content Encoding

This document updates the entry added in [RFC8878] to the "HTTP Content Coding Registry" within the "Hypertext Transfer Protocol (HTTP) Parameters" registry:




A stream of bytes compressed using the Zstandard protocol with a Window_Size of not more than 8 MB.


This document

6. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Collet, Y. and M. Kucherawy, Ed., "Zstandard Compression and the 'application/zstd' Media Type", RFC 8878, DOI 10.17487/RFC8878, , <>.


Zstandard was developed by Yann Collet.

The authors would like to thank Yann Collet, Klaus Post, Adam Rice, and members of the Web Performance Working Group in the W3C for collaborating on the window size issue and helping to formulate a solution. Also, thank you to Nick Terrell for providing feedback that went into RFC 8478 and RFC 8878.

Authors' Addresses

Nidhi Jaju (editor)
Shibuya Stream, 3 Chome-21-3 Shibuya, Shibuya City, Tokyo
W. Felix P. Handte (editor)
Meta Platforms, Inc.
380 W 33rd St
New York, NY 10001
United States of America