HTTP 103 - An HTTP Status Code for Indicating Hints (ietf.org)

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).

Comments

Perceived latency? Isn't waiting... waiting?

[adosch]

Maybe there is some bigger engineering brains, but if we're battling perceived latency by blocking/waiting for a database query or something upstream, whether is a REST service or what-the-F-ever, aren't we still blocking/waiting but now handling an intermediate response? Anything backend is going to have to prioritize shipping out constant '103's to clients, then how many to do handle and block for until we get our payload and '200'? Does this extend timeouts even longer? This actually changes ALOT of how any of us know the handshaking of HTTP is today if it's across the board and not for specific requests in header. Outside of that drivel of a thought, I can see a ton of man-in-the-middle hijacking or DDOS going on here with any sort of intermediate 'hey-hold-on-a-second' step in this handshake.

I'm like the rest of the opinions here, this seems like a vote to work-around how bloaty, complex and javascript laden and ad-network-revenue driven this is proposal is.

Adblocking escalation?

[fibonacci8]

To me this looks like an opportunity for a client to opt out of undesirable content, but in a way that a server can detect prior to sending the desired content. At first glance it's another vector for undesirable content purveyors to bypass local DNS policy.

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.

This part looks exactly like the "hints" are meant as an opportunity to avoid delivering content if the hints aren't properly "obeyed". If the "preload" directive doesn't happen and a third party doesn't relay that the undesirable content is at least transmitted, the first server can continue to wait until the demand is met.