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wit Internet Engineering Task Force This document describes a mechanism for introducing dynamic backpressure into an HTTP-based system, to aid in controlling server resources while minimizing tradeoffs.

Introduction Operating an HTTP service can get expensive. One of the ways services can manage the costs is by providing backpressure. Traditionally, HTTP backpressure is implemented variously by refusing connections after a limit, holding a connection open and slowly trickling data into it, among other techniques. This document describes a simple and widely-deployed mechanism for implementing backpressure by telling the client when to make new requests.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Background: Why use dynamic backpressure Consider the scenario: a service that is billed by requests, and an Atom feed that gains new entries every 5 minutes. This service is composed of app servers and caching load balancers, with a cache policy of 5 minutes. Yet, the service doesn't want Atom clients to send a request every 5 minutes. This scenario is exactly where dynamic backpressure would shine: it doesn't try to slow down or throttle the connections (not that it would be able to, due to the caches) or anything like that, it simply tells the client "hey, I would appreciate it if you waited N seconds before sending another request, thanks!".

Solution: The Refresh HTTP header The Refresh header is composed of a non-negative integer number of seconds to wait until the next request, and an optional URI to move future requests to, according to the following syntax described in Augmented BNF , borrowing 'URI-reference' from : The value of 'wait-seconds' SHOULD NOT be shorter than the value of any caching policy applied to the document. The client SHOULD wait at least 'wait-seconds' before making new requests. This syntax is in alignment with . Below is a diagram of how the Refresh header can be used in a generic control loop, with requests per second as input and wait-seconds as output.

Control loop (+)-->| Control function |----+-------> ^- +-------------------+ | | +-------------------+ | +----| Feedback function |<---+ +-------------------+ ]]>

Rejected alternatives The Refresh header is strongly disliked by basically everyone, but it may well be the only mechanism that provides the desired properties. One may argue the Retry-After header would be a better substitute, but the Retry-After header isn't really specified for successful content requests such as those used by Atom feeds, nor is it as widely deployed as the Refresh header.

IANA Considerations This document requests that the IANA update the registration for the Refresh header to also point to this document.

Security Considerations This document should not affect the security of the Internet. After all, the Refresh header is already widely deployed.

References Normative References Informative References WHATWG Living Standard

Acknowledgements The authors would like to acknowledge the true power of the Refresh header. It had to be very powerful to survive this long.