The Sustainability HTTP Response Header Field
draft-besleaga-green-sustainability-header-00

GREEN                                                     A. N. Besleaga
Internet-Draft                                               Independent
Intended status: Informational                          27 February 2026
Expires: 31 August 2026

             The Sustainability HTTP Response Header Field
             draft-besleaga-green-sustainability-header-00

Abstract

   This document defines the Sustainability HTTP response header field.
   This field provides a mechanism for servers to report the
   environmental impact and carbon footprint metrics associated with the
   processing and delivery of an HTTP request.

   By defining this header as a Structured Field, this specification
   ensures high parsing reliability and compatibility with HTTP/2 and
   HTTP/3 header compression mechanisms, thereby mitigating the
   secondary environmental costs of transmitting the metadata itself.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  The Sustainability Header Field . . . . . . . . . . . . . . .   3
     2.1.  Example Usage . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Environmental Considerations (Mitigating the Rebound
           Effect) . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The digital economy consumes a significant and growing percentage of
   global electricity.  Historically, assessing the environmental impact
   of digital services has relied on annualized, aggregate reporting,
   guided by standards such as the Greenhouse Gas Protocol
   [GHG-PROTOCOL] and ISO 14064-1 [ISO14064-1].  However, emerging
   regulatory frameworks (e.g., the EU Corporate Sustainability
   Reporting Directive [EU-CSRD] and the Ecodesign for Sustainable
   Products Regulation [EU-DPP]) and carbon-accounting standards (such
   as IFRS S2 [IFRS-S2] and the Green Software Foundation's Software
   Carbon Intensity specification [GSF-SCI]) increasingly require
   transactional traceability.

   To enable real-time environmental accounting, clients and
   intermediaries need visibility into the carbon footprint of
   individual HTTP transactions.  This document defines the
   Sustainability HTTP response header field [RFC9110] to fulfill this
   need.  It allows servers to transmit structured environmental
   metadata - such as Scope 2 and Scope 3 emissions - directly in the
   HTTP response.  This work builds upon previous efforts to expose
   carbon metrics in HTTP, such as the expired Carbon-Emissions-Scope-2
   draft [MARTIN-DRAFT], and complements other energy footprint APIs
   like CAMARA EFN [CAMARA-EFN] and node-level exporters like Kepler
   [KEPLER].

1.1.  Requirements Language

   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.

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2.  The Sustainability Header Field

   The Sustainability header field is an HTTP response header that
   conveys the estimated environmental impact of fulfilling the request.

   The Sustainability header field is a Structured Field [RFC8941].  Its
   value MUST be a Dictionary.

   Sustainability = sf-dictionary

   The dictionary keys represent specific environmental metrics or
   dimensions, and the values represent the measurements.  The following
   keys are defined:

   *  scope-2: An sf-decimal indicating the Scope 2 carbon emissions.

   *  scope-3: An sf-decimal indicating the Scope 3 carbon emissions.

   *  unit: An sf-string indicating the unit of measurement (e.g.,
      "gCO2e").

2.1.  Example Usage

   A server reporting 0.005 grams of CO2 equivalent for Scope 2 and 0.12
   for Scope 3 would send:

   Sustainability: scope-2=0.005, scope-3=0.12, unit="gCO2e"

3.  Environmental Considerations (Mitigating the Rebound Effect)

   Introducing new headers to the HTTP ecosystem inherently increases
   the bandwidth required for every transaction.  If applied
   indiscriminately to all HTTP traffic, the energy required to
   transmit, process, and store this extra metadata (the "rebound
   effect") could exceed the environmental benefits of the transparency
   it provides.

   To mitigate this, implementers MUST adhere to the following
   guidelines:

   1.  *Header Compression:* The Sustainability header relies on
       Structured Fields [RFC8941], making it highly compressible.
       Servers SHOULD rely on HPACK (HTTP/2) and QPACK (HTTP/3) to
       minimize the wire footprint of this header.

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   2.  *Opt-In Behavior:* Servers SHOULD NOT include the Sustainability
       header in responses by default.  Clients that wish to receive
       this telemetry SHOULD signal their interest using an opt-in
       mechanism, such as the Prefer header (e.g., Prefer:
       return=sustainability).

   3.  *Proxy Aggregation:* Where calculating emissions per-thread is
       computationally prohibitive, edge proxies or API gateways MAY
       inject this header based on aggregate datacenter metrics rather
       than forcing the backend application to calculate it dynamically.

4.  IANA Considerations

   This document requests the following registration in the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" maintained at
   https://www.iana.org/assignments/http-fields.

   *  Field Name: Sustainability

   *  Status: provisional

   *  Structured Type: Dictionary

   *  Specification document(s): This document

   *  Comments:

5.  Security Considerations

   The Sustainability header field exposes internal operational metrics.
   While carbon metrics are generally not sensitive, highly granular,
   real-time reporting could theoretically allow an attacker to infer
   server load, backend architecture, or specific hardware
   configurations through timing and emission variations.

   Furthermore, generating accurate per-request carbon calculations may
   require significant CPU cycles.  If a server dynamically computes
   this value, an attacker could intentionally request this header at a
   high frequency to induce a Denial of Service (DoS) or artificially
   inflate the server's energy consumption.  Servers SHOULD cache these
   calculations or use aggregate approximations to mitigate this risk.

6.  References

6.1.  Normative References

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   [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>.

   [RFC8941]  Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/info/rfc8941>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

6.2.  Informative References

   [GHG-PROTOCOL]
              World Resources Institute and World Business Council for
              Sustainable Development, "The Greenhouse Gas Protocol: A
              Corporate Accounting and Reporting Standard", 2004.

   [MARTIN-DRAFT]
              Martin, B., "HTTP Response Header Field: Carbon-Emissions-
              Scope-2", Work in Progress, Internet-Draft, draft-martin-
              http-carbon-emissions-scope-2-00, April 2023,
              <https://datatracker.ietf.org/doc/html/draft-martin-http-
              carbon-emissions-scope-2-00>.

   [GSF-SCI]  Green Software Foundation, "Software Carbon Intensity
              (SCI) Specification, v1.0", December 2022.

   [EU-CSRD]  European Parliament and Council, "Directive (EU) 2022/2464
              as regards corporate sustainability reporting (CSRD)",
              December 2022.

   [EU-DPP]   European Parliament and Council, "Ecodesign for
              Sustainable Products Regulation (ESPR), Regulation (EU)
              2024/1781", 2024.

   [IFRS-S2]  International Sustainability Standards Board (ISSB), "IFRS
              S2 Climate-related Disclosures", June 2023.

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   [CAMARA-EFN]
              CAMARA Project, "Energy Footprint Notification API,
              v0.1.0", 2024.

   [KEPLER]   Linux Foundation, "Kubernetes-based Efficient Power Level
              Exporter (Kepler)", n.d..

   [ISO14064-1]
              International Organization for Standardization, "ISO
              14064-1:2018 Greenhouse gases - Part 1: Specification with
              guidance at the organization level for quantification and
              reporting of greenhouse gas emissions and removals",
              December 2018.

Author's Address

   Andrei Nicolae BESLEAGA
   Independent
   Email: andrei.besleaga@ieee.org

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