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 the header block of a final response immediately after receiving a request. For example, the origin server might delegate a request to an upstream HTTP server running at a distant location, or the status code might depend on the result of a database query.¶

The dilemma here is that even though it is preferable for an origin server to send some header fields as soon as it receives a request, it cannot do so until the status code and the full header fields of the final HTTP response are determined.¶

HTTP/2 ([RFC7540]) server push can accelerate the delivery of resources, but only resources for which the server is authoritative. The other limitation of server push is that the response will be transmitted regardless of whether the client has the response cached. At the cost of spending one extra round-trip compared to server push in the worst case, delivering Link header fields in a timely fashion is more flexible and might consume less bandwidth.¶

This memo defines a status code for sending an informational response ([RFC7231], Section 6.2) that contains header fields that are likely to be included in the final response. A server can send the informational response containing some of the header fields 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 to the client that the server is likely to send a final response with the header fields included in the informational response.¶

Typically, a server will include the header fields sent in a 103 (Early Hints) response in the final response as well. However, there might be cases when this is not desirable, such as when the server learns that they are not correct before the final response is sent.¶

A client can speculatively evaluate the header fields 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, these header fields only provide hints to the client; they do not replace the header fields on the final response.¶

Aside from performance optimizations, such evaluation of the 103 (Early Hints) response’s header fields MUST NOT affect how the final response is processed. A client MUST NOT interpret the 103 (Early Hints) response header fields as if they applied to the informational response itself (e.g., as metadata about the 103 (Early Hints) response).¶

A server MAY use a 103 (Early Hints) response to indicate only some of the header fields that are expected to be found in the final response. A client SHOULD NOT interpret the nonexistence of a header field in a 103 (Early Hints) response as a speculation that the header field is unlikely to be part of the final response.¶

The following example illustrates a typical message exchange that involves a 103 (Early Hints) response.¶

As is the case with any informational response, a server might emit more than one 103 (Early Hints) response prior to sending a final response. This can happen for example when a caching intermediary generates a 103 (Early Hints) response based on the header fields of a stale-cached response, then forwards a 103 (Early Hints) response and a final response that were sent from the origin server in response to a revalidation request.¶

A server MAY emit multiple 103 (Early Hints) responses with additional header fields as new information becomes available while the request is being processed. It does not need to repeat the fields that were already emitted, though it doesn’t have to exclude them either. The client can consider any combination of header fields received in multiple 103 (Early Hints) responses when anticipating the list of header fields expected in the final response.¶

The following example illustrates a series of responses that a server might emit. In the example, the server uses two 103 (Early Hints) responses to notify the client that it is likely to send three Link header fields in the final response. Two of the three expected header fields are found in the final response. The other header field is replaced by another Link header field that contains a different value.¶

Some clients might have issues handling 103 (Early Hints), since informational responses are rarely used in reply to requests not including an Expect header field ([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 might 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 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 header fields 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 header fields using an informational response.¶