An HTTP Status Code for Indicating Hints

This memo introduces an informational status code for HTTP that can be used for indicating hints to help a client start making preparations for processing the final response.¶

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 http://datatracker.ietf.org/drafts/current/.¶

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 September 30, 2017.¶

Copyright (c) 2017 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 (http://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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

Most if not all of the web pages processed by a web browser contain links to external resources that need to be fetched prior to rendering the documents. Therefore, it is beneficial to send such links as early as possible in order to minimize the time spent until the browser becomes possible to render the document. Link header of type “preload” ([Preload]) can be used to indicate such links within the response headers of an HTTP response.¶

However, it is not always possible for an origin server to send a response immediately after receiving a request. In fact, it is often the contrary. There are many deployments in which an origin server needs to query a database before generating a response. It is also not unusual for an origin server to 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 headers of the final HTTP response is determined.¶

HTTP/2 ([RFC7540]) push can be used as a solution to the issue, but has its own limitations. The resources 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 of the resources using HTTP/2 push. 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 ([RFC7231], 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 push at an caching intermediary.¶

This 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 headers ([RFC7230], section 6.1) in the informational response using the status code.¶

A client MAY speculatively evaluate the headers included in the informational response while waiting for the final response. For example, a client may recognize the link header of type preload and start fetching the resource. However, the evaluation MUST NOT affect how the final response is processed; the client must behave as if it had not seen the informational response. A client MUST NOT process the headers included in the response as if they belonged to the informational response.¶

An intermediary MAY drop the informational response. It MAY send HTTP/2 ([RFC7540]) push responses using the information found in the informational response.¶

Clients may have issues handling Early Hints, since informational response is rarely used for requests not including an Expect header ([RFC7231], section 5.1.1).¶

An HTTP/1.1 client that mishandles the informational response as a final response is likely to consider all the 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.¶

• 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.¶