An HTTP Status Code for Indicating Hints

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.¶

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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” ([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 ([RFC7540]) 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 ([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 server push at a caching intermediary.¶

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 ([RFC7230], 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 ([RFC7540]) 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 ([RFC7231], 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.¶

• 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.¶