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General HTTP Internet-Draft This memo introduces an informational HTTP status code that can be used to convey hints that help a client make preparations for processing the final response. Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group information can be found at https://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/early-hints.

It is common for HTTP responses to contain links to external resources that need to be fetched prior to their use; for example, rendering HTML by a Web browser. Having such links available to the client as early as possible helps to minimize perceived latency. The “preload” () link relation can be used to convey such links in the Link header field of an HTTP response. However, it is not always possible for an origin server to generate a response header block immediately after receiving a request. For example, the origin server might need to query a database before generating a response, or it might delegate a request to an upstream HTTP server running at a distant location. The dilemma here is that even though it is preferable for an origin server to send some headers as soon as it receives a request, it cannot do so until the status code and the full headers of the final HTTP response are determined. HTTP/2 () server push can be used as a solution to this issue, but has its own limitations. The responses that can be pushed using HTTP/2 are limited to those belonging to the same origin. Also, it is impossible to send only the links using server push. Finally, sending HTTP responses for every resource is an inefficient way of using bandwidth, especially when a caching server exists as an intermediary. This memo defines a status code for sending an informational response (, section 6.2) that contains headers that are likely to be included in the final response. A server can send the informational response containing some of the headers to help the client start making preparations for processing the final response, and then run time-consuming operations to generate the final response. The informational response can also be used by an origin server to trigger HTTP/2 server push at a caching intermediary.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in .

The 103 (Early Hints) informational status code indicates the client that the server is likely to send a final response with the headers included in the informational response. A server MUST NOT include Content-Length, Transfer-Encoding, or any hop-by-hop header fields (, section 6.1) in a 103 (Early Hints) response. A client MAY speculatively evaluate the headers included in a 103 (Early Hints) response while waiting for the final response. For example, a client might recognize a Link header field value containing the relation type “preload” and start fetching the target resource. However, this MUST NOT affect how the final response is processed; when handling it, the client MUST behave as if it had not seen the informational response. In particular, a client MUST NOT process the headers included in the final response as if they belonged to the informational response, or vice versa. An intermediary MAY drop the informational response. It MAY send HTTP/2 () server pushes using the information found in the 103 (Early Hints) response.

Some clients may have issues handling 103 (Early Hints), since informational responses are rarely used in reply to requests not including an Expect header (, section 5.1.1). In particular, an HTTP/1.1 client that mishandles an informational response as a final response is likely to consider all responses to the succeeding requests sent over the same connection to be part of the final response. Such behavior may constitute a cross-origin information disclosure vulnerability in case the client multiplexes requests to different origins onto a single persistent connection. Therefore, a server might refrain from sending Early Hints over HTTP/1.1 unless when the client is known to handle informational responses correctly. HTTP/2 clients are less likely to suffer from incorrect framing since handling of the response headers does not affect how the end of the response body is determined.

The HTTP Status Codes Registry will be updated with the following entry: Code: 103 Description: Early Hints Specification: [this document]

Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending the link headers using an informational response.

Editorial changes.

Forbid processing the headers of a 103 response as part of the informational response.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP’s existing semantics remain unchanged.