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Web and Internet Transport HTTPBIS zstd zstandard compression content encoding application/zstd 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 The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the HTTPBIS Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Zstandard, or "zstd", specified in , is a lossless data compression mechanism similar to gzip. When used with HTTP, the "zstd" Content Encoding token signals to the decoder that the content is Zstandard-compressed. Deployments of Zstandard can use different window sizes to configure the maximum memory a decoder requires to decompress a frame. Larger window sizes tend to improve the compression ratio, but cause more memory to be used. provides a recommendation for decoders to support window sizes up to 8 MB, and for encoders to not generate frames requiring window sizes larger than 8 MB. However, it is just a recommendation (). To protect against unreasonable memory usage, some browsers and user agents limit the maximum window size allowed. This causes incompatibilities if the content is compressed with a larger limit, leading to decreased interoperability. This document updates to specify a window size limit associated with the "zstd" Content Encoding token.

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 when, and only when, they appear in all capitals, as shown here.

Window Size Section 3.1.1.1.2 of discusses window sizes in Zstandard. The window size provides guarantees about the minimum memory buffer required to decompress a frame. This information is important for decoders to allocate enough memory. The minimum window size is 1 KB. The maximum window size is (1<<41) + 7*(1<<38) bytes, which is 3.75 TB. In general, larger window size values tend to improve the compression ratio, but at the cost of increased memory usage. To properly decode compressed data, a decoder will need to allocate a buffer of at least the window size bytes. In order to protect decoders from unreasonable memory requirements, a decoder is allowed to reject a compressed frame that requests a memory size beyond the decoder's authorized range. To maintain interoperability of Zstandard in HTTP Content Encoding, decoders MUST support window sizes of up to and including 8 MB and encoders MUST NOT generate frames requiring a window size of larger than 8 MB, when using the "zstd" Content Encoding token (see ). Decoders are free to support higher or lower limits, depending on local limitations, if negotiated out-of-band. Many deployments of Zstandard operate in controlled, private environments and can directly communicate with their encoder and decoder to negotiate a higher or lower limit.

Security Considerations This document introduces no new security considerations beyond those discussed in .

IANA Considerations

Content Encoding This document updates the entry added in 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

Normative References Zstandard, or "zstd" (pronounced "zee standard"), is a lossless data compression mechanism. This document describes the mechanism and registers a media type, content encoding, and a structured syntax suffix to be used when transporting zstd-compressed content via MIME. Despite use of the word "standard" as part of Zstandard, readers are advised that this document is not an Internet Standards Track specification; it is being published for informational purposes only. This document replaces and obsoletes RFC 8478. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments zstd 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.