Internet Engineering Task Force (IETF) N. Jaju, Ed.
Request for Comments: 9659 Google
Updates: 8878 W. F. Handte, Ed.
Category: Informational Meta Platforms, Inc.
ISSN: 2070-1721 September 2024
Window Sizing for Zstandard Content Encoding
Abstract
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, thereby causing interoperability issues. This document
updates the window size limit in RFC 8878 from a recommendation to a
requirement in HTTP contexts.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9659.
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Table of Contents
1. Introduction
2. Conventions and Definitions
3. Window Size
4. Security Considerations
5. IANA Considerations
5.1. Content Encoding
6. Normative References
Acknowledgments
Authors' Addresses
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 3.1.1.1.2), 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, where "<<" denotes a bitwise left shift, 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 3.1.1.1.2 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" HTTP 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].
Note that decoders still need to take into account that they can
receive oversized frames that do not follow the window size limit
specified in this document and fail decoding when such invalid frames
are received.
5. IANA Considerations
5.1. Content Encoding
This document updates the following entry in the "HTTP Content Coding
Registry" in the "Hypertext Transfer Protocol (HTTP) Parameters"
registry group (https://www.iana.org/assignments/http-parameters):
Name: zstd
Description: A stream of bytes compressed using the Zstandard
protocol with a Window_Size of not more than 8 MB.
Reference: This document and [RFC8878]
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8878] Collet, Y. and M. Kucherawy, Ed., "Zstandard Compression
and the 'application/zstd' Media Type", RFC 8878,
DOI 10.17487/RFC8878, February 2021,
<https://www.rfc-editor.org/info/rfc8878>.
Acknowledgments
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.
Authors' Addresses
Nidhi Jaju (editor)
Google
Shibuya Stream, 3 Chome-21-3 Shibuya, Shibuya City, Tokyo
150-0002
Japan
Email: nidhijaju@google.com