



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.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

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


   W. Felix P. Handte (editor)
   Meta Platforms, Inc.
   380 W 33rd St
   New York, NY 10001
   United States of America
   Email: felixh@meta.com
