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draft-ietf-httpbis-zstd-window-size.md

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title: Window Sizing for Zstandard Content Encoding abbrev: Zstd Window Size category: info

docname: draft-ietf-httpbis-zstd-window-size-latest submissiontype: IETF number: date: consensus: true updates: 8878 v: 3 area: Web and Internet Transport workgroup: HTTPBIS keyword:

author:

ins: N. Jaju
fullname: Nidhi Jaju
role: editor
organization: Google
street: Shibuya Stream, 3 Chome-21-3 Shibuya
region: Shibuya City, Tokyo
code: 150-0002
country: Japan
email: [email protected]
  • ins: F. Handte fullname: W. Felix P. Handte role: editor organization: Meta Platforms, Inc. street: 380 W 33rd St city: New York region: NY code: 10001 country: US email: [email protected]

normative:

informative:

--- 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, causing interoperability issues. This document updates the window size limit in RFC8878 from a recommendation to a requirement in HTTP contexts.

--- middle

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, 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.

Conventions and Definitions

{::boilerplate bcp14-tagged}

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 {{zstd-iana-token}}).

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.

IANA Considerations

Content Encoding {#zstd-iana-token}

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

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}}

--- back

Acknowledgments

{:numbered="false"}

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.