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October 31, 2017 04:00 pm

HTTP 103 - An HTTP Status Code for Indicating Hints

The Internet Task Engineering Group (IETF) has approved the new HTTP status code 103. The new status code is intended to "minimize perceived latency." From the circular: 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. [...] 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).

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