RFC 10036: Incremental Forwarding of HTTP Messages | RFC Editor

RFC Ê 10036 : Incremental Forwarding of HTTP Messages

• K. Oku , Ê
• T. Pauly , Ê
• M. Thomson

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 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. ¦

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 BCPÊ14 [ 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 ] . ¦

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: ¦

Incremental ¦

permanent ¦

Item ¦

This document ¦

None ¦

An HTTP Proxy Error Type has been registered in the "HTTP Proxy Error Types" registry as shown below: ¦

incremental_ refused ¦

The HTTP message contained the Incremental HTTP header field, but the intermediary refused to forward the message incrementally. ¦

none ¦

501 ¦

true ¦

This document ¦

6. References

6.1. Normative References

6.2. Informative References

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