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RFC 10036
Internet Engineering Task Force (IETF) K. Oku
Request for Comments: 10036 Fastly
Category: Standards Track T. Pauly
ISSN: 2070-1721 Apple
M. Thomson
Mozilla
August 2026
Incremental Forwarding of HTTP Messages
Abstract
This document specifies the "Incremental" HTTP header field, which
instructs HTTP intermediaries to forward the HTTP message
incrementally.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc10036.
Copyright Notice
Copyright (c) 2026 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
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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. Conventions and Definitions
3. The Incremental Header Field
4. Security Considerations
4.1. Permanent Rejection
4.2. Temporary Rejection
4.3. Handling of Small Packets
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
HTTP [HTTP] permits receivers to begin processing portions of HTTP
messages as they arrive, rather than requiring them to wait for the
entire HTTP message to be received before acting.
Some applications are specifically designed to take advantage of this
capability.
For example, Server-Sent Events [SSE] uses a long-running HTTP
response, where the server continually sends notifications as they
become available.
In the case of Chunked Oblivious HTTP Messages [CHUNKED-OHTTP], the
client opens an HTTP request and incrementally sends application
data, while the server can start responding even before the HTTP
request is fully complete. In this way, the HTTP request-response
pair could create what is, in effect, a bidirectional communication
channel.
Applications that rely on incremental delivery of data are fragile
when HTTP intermediaries are involved. This is because HTTP
intermediaries are not only permitted but are frequently deployed to
buffer complete HTTP messages before forwarding them downstream
(Section 7.6 of [HTTP]).
If such a buffering HTTP intermediary exists between the client and
the server, these applications may fail to function as intended.
In the case of Server-Sent Events, an intermediary that tries to
buffer the HTTP response completely before forwarding it could be
left waiting indefinitely. A client might never receive any portion
of the response.
In the case of requests that involve any bidirectional exchange, an
intermediary that tries to buffer entire messages -- either request
or response -- prevents any data from being delivered.
To help avoid such behavior, this document specifies the
"Incremental" HTTP header field, which requests that HTTP
intermediaries begin forwarding the HTTP message downstream before
receiving the complete message.
This indication might not be supported by intermediaries.
Intermediaries that are unaware of this field will not change their
behavior. Intermediaries that support the field might choose instead
to reject a request; see Section 4.
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.
This document relies on structured field definitions of Item and
Boolean [STRUCTURED-FIELDS].
3. The Incremental Header Field
The Incremental HTTP header field expresses the sender's intent for
HTTP intermediaries to start forwarding the message downstream before
the entire message is received.
The Incremental header field is defined as a structured field
[STRUCTURED-FIELDS] of type Item. Only Boolean values (Section 3.3.6
of [STRUCTURED-FIELDS]) are valid; a recipient ignores the field if
it contains any other type.
Incremental: ?1
A true value ("?1") indicates that the sender requests intermediaries
to forward the message incrementally, as described below.
Incremental: ?0
A false value ("?0") indicates the default behavior defined in
[HTTP], where intermediaries might buffer the entire message before
forwarding it. However, this explicit signal might give an
intermediary greater confidence in choosing to buffer.
The Incremental HTTP header field applies to each HTTP message.
Therefore, if both the HTTP request and response need to be forwarded
incrementally, the Incremental HTTP header field MUST be set for both
the HTTP request and the response.
Upon receiving a header section that includes an Incremental header
field with a true value, HTTP intermediaries SHOULD NOT buffer the
entire message before forwarding it. Instead, intermediaries SHOULD
transmit the header section downstream and continuously forward the
bytes of the message content as they arrive. As the Incremental
header field indicates only how the message content is to be
forwarded, intermediaries can still buffer the entire header and
trailer sections of the message before forwarding them downstream.
If an intermediary decides outright to refuse forwarding the message
body incrementally, the intermediary MUST generate an error response
rather than buffering an entire message before forwarding. Typical
scenarios under which an intermediary might refuse are discussed in
Section 4.
The request to use incremental forwarding also applies to HTTP
implementations. Though most HTTP APIs provide the ability to
incrementally transfer message content, those that do not for any
reason SHOULD use the presence of the Incremental header field to
reduce or disable buffering.
The Incremental field might not be supported by intermediaries.
Intermediaries that are unaware of the field or that do not support
the field might buffer messages, even when explicitly requested
otherwise. Clients and servers therefore cannot expect all
intermediaries to understand and respect a request to deliver
messages incrementally. Clients that depend on support for
incremental forwarding can rely on prior knowledge or probe for
support on individual resources.
The Incremental header field facilitates the establishment of a
bidirectional byte channel over HTTP, as its presence in both
requests and responses requests that intermediaries forward early
responses (Section 7.5 of [HTTP]) and transmit message contents
incrementally in both directions. However, when developing
bidirectional protocols over HTTP, Extended CONNECT
[RFC8441][RFC9220] is generally more consistent with HTTP's
architecture.
This document does not define any parameters for the Incremental
header field value, but future documents might define parameters.
Receivers MUST ignore unknown parameters.
4. Security Considerations
When receiving a request or response that asks for incremental
forwarding, intermediaries might reject the HTTP request due to
security concerns. The following subsections explore typical
scenarios under which the intermediaries might reject requests.
Note that rejecting requests based on the value of the Incremental
field only occurs when an intermediary understands the field.
4.1. Permanent Rejection
Some intermediaries inspect the content of HTTP messages and forward
them only if their content is deemed safe. Any feature that depends
on seeing the entirety of the message in this way is incompatible
with incremental delivery.
When an intermediary is asked to incrementally forward a message and
cannot -- whether that message is a request or a response -- due to
security concerns about the message content, the intermediary SHOULD
respond with a 501 (Not Implemented) error with an
incremental_refused Proxy-Status response header field (Section 5).
4.2. Temporary Rejection
To conserve resources required to handle HTTP requests or
connections, it is common for intermediaries to impose limits on the
maximum number of concurrent HTTP requests that they forward, while
buffering requests that exceed this limit.
Such intermediaries could apply a more restrictive concurrency limit
to requests marked as incremental to ensure that capacity remains
available for non-incremental requests, even when the maximum number
of incremental requests is reached. This approach helps balance the
processing of different types of requests and maintains service
availability across all requests.
When rejecting incremental requests due to reaching the concurrency
limit, intermediaries SHOULD respond with a 429 (Too Many Requests)
error (Section 4 of [EXTRA-STATUS]), accompanied by a
connection_limit_reached Proxy-Status response header field
(Section 2.3.12 of [PROXY-STATUS]).
4.3. Handling of Small Packets
For performance and efficiency reasons, a small amount of buffering
might be used by intermediaries, even for incremental messages.
Immediate forwarding might be exploited to cause an intermediary to
waste effort on many small packets. Enabling incremental delivery
might instead set limits on the number of bytes that are buffered or
the length of time that buffers are held before forwarding. Any
buffering could adversely affect application latency, even if it
improves efficiency. In all cases, intermediaries cannot hold data
in buffers indefinitely, so data needs to be forwarded when either
the time limit or the byte limit is reached.
5. IANA Considerations
An HTTP field named Incremental has been registered in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" following the
procedures in Section 18.4 of [HTTP]. The following values are
registered:
Field Name: Incremental
Status: permanent
Structured Type: Item
Reference: This document
Comments: None
An HTTP Proxy Error Type has been registered in the "HTTP Proxy Error
Types" registry as shown below:
Name: incremental_refused
Description: The HTTP message contained the Incremental HTTP header
field, but the intermediary refused to forward the message
incrementally.
Extra Parameters: none
Recommended HTTP Status Code: 501
Response Only Generated By Intermediaries: true
Reference: This document
6. References
6.1. Normative References
[EXTRA-STATUS]
Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/info/rfc6585>.
[HTTP] 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>.
[PROXY-STATUS]
Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
Response Header Field", RFC 9209, DOI 10.17487/RFC9209,
June 2022, <https://www.rfc-editor.org/info/rfc9209>.
[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>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
<https://www.rfc-editor.org/info/rfc9651>.
6.2. Informative References
[CHUNKED-OHTTP]
Pauly, T. and M. Thomson, "Chunked Oblivious HTTP
Messages", Work in Progress, Internet-Draft, draft-ietf-
ohai-chunked-ohttp-08, 18 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-ohai-
chunked-ohttp-08>.
[RFC8441] McManus, P., "Bootstrapping WebSockets with HTTP/2",
RFC 8441, DOI 10.17487/RFC8441, September 2018,
<https://www.rfc-editor.org/info/rfc8441>.
[RFC9220] Hamilton, R., "Bootstrapping WebSockets with HTTP/3",
RFC 9220, DOI 10.17487/RFC9220, June 2022,
<https://www.rfc-editor.org/info/rfc9220>.
[SSE] WHATWG, "HTML - Server-Sent Events", WHATWG Living
Standard, <https://html.spec.whatwg.org/multipage/server-
sent-events.html>. Commit snapshot:
<https://html.spec.whatwg.org/commit-
snapshots/6f84b26bd6eb8bd0e0e8df9819e43e901867166b/>
Acknowledgments
The authors would like to thank many members of the IETF HTTP Working
Group for their discussions and feedback on this specification. In
particular, the authors would like to thank Mark Thomas, Piotr
Sikora, Thibault Meunier, Marius Kleidl, Ben Schwartz, Willy Tarreau,
Will Hawkins, Mark Nottingham, and Lucas Pardue for close review and
suggested changes.
Authors' Addresses
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Additional contact information:
奥 一穂
Fastly
Tommy Pauly
Apple
Email: tpauly@apple.com
Martin Thomson
Mozilla
Email: mt@lowentropy.net