Internet DRAFT - draft-jaju-httpbis-zstd-window-size
draft-jaju-httpbis-zstd-window-size
HTTPBIS N. Jaju
Internet-Draft Google
Intended status: Informational W. F. P. Handte
Expires: 5 September 2024 Meta Platforms, Inc.
4 March 2024
Window Sizing for Zstandard Content Encoding
draft-jaju-httpbis-zstd-window-size-00
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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://nidhijaju.github.io/draft-zstd-window-size/draft-jaju-
httpbis-zstd-window-size.html. Status information for this document
may be found at https://datatracker.ietf.org/doc/draft-jaju-httpbis-
zstd-window-size/.
Discussion of this document takes place on the HTTPBIS Working Group
mailing list (mailto:ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/.
Source for this draft and an issue tracker can be found at
https://github.com/nidhijaju/draft-zstd-window-size.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Jaju & Handte Expires 5 September 2024 [Page 1]
Internet-Draft Zstd Window Size March 2024
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 September 2024.
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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Window Size . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5.1. Content Encoding . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . 4
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
Zstandard, or "zstd", specified in [RFC8878], 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. [RFC8878] 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 ([RFC8878], Section 3.1.1.1.2).
Jaju & Handte Expires 5 September 2024 [Page 2]
Internet-Draft Zstd Window Size March 2024
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 [RFC8878] to specify a window size limit
associated with the "zstd" Content Encoding token.
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
Section 3.1.1.1.2 of [RFC8878] 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
Section 5.1).
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.
Jaju & Handte Expires 5 September 2024 [Page 3]
Internet-Draft Zstd Window Size March 2024
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:
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
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/rfc/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/rfc/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/rfc/rfc8878>.
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.
Authors' Addresses
Jaju & Handte Expires 5 September 2024 [Page 4]
Internet-Draft Zstd Window Size March 2024
Nidhi Jaju
Google
Shibuya Stream, 3 Chome-21-3 Shibuya, Shibuya City, Tokyo
150-0002
Japan
Email: nidhijaju@google.com
W. Felix P. Handte
Meta Platforms, Inc.
380 W 33rd St
New York, NY 10001,
United States of America
Email: felixh@meta.com
Jaju & Handte Expires 5 September 2024 [Page 5]