An HTTP Status Code for Indicating Hints
draft-kazuho-early-hints-status-code-latest

Abstract

This memo introduces an informational status code for HTTP that can be used for indicating hints to help a client start making preparations for processing the final response.

Status of This Memo

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This Internet-Draft will expire on May 4, 2017.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. 103 Early Hints
• 3. Interoperatibility Issues
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

Most if not all of the web pages processed by a web browser contain links to external resources that need to be fetched prior to rendering the documents. Therefore, it is beneficial to send such links as early as possible in order to minimize the time spent until the browser becomes possible to render the document. Link header of type “preload” ([Preload]) can be used to indicate such links within the response headers of an HTTP response.

However, it is not always possible for an origin server to send a response immediately after receiving a request. In fact, it is often the contrary. There are many deployments in which an origin server needs to query a database before generating a response. It is also not unusual for an origin server to 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 headers of the final HTTP response is determined.

HTTP/2 ([RFC7540]) push can be used as a solution to the issue, but has its own limitations. The resources 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 of the resources using HTTP/2 push. 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 push at an caching intermediary.

1.1. Notational Conventions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

2. 103 Early Hints

This 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 headers ([RFC7230], section 6.1) in the informational response using the status code.

A client MAY speculatively evaluate the headers included in the informational response while waiting for the final response. For example, a client may recognize the link header of type preload and start fetching the resource. However, the evaluation MUST NOT affect how the final response is processed; the client must behave as if it had not seen the informational response.

An intermediary MAY drop the informational response. It MAY send HTTP/2 ([RFC7540]) push responses using the information found in the informational response.

3. Interoperatibility Issues

Clients may have issues handling Early Hints, since informational response is rarely used for requests not including an Expect header ([RFC7231], section 5.1.1). Therefore, it is desirable to negotiate the capability to use the status code.

4. Security Considerations

TBD

5. IANA Considerations

If Early Hints is standardized, the HTTP Status Codes Registry should be updated with the following entries:

• Code: 103
• Description: Early Hints
• Specification: this document

6. Acknowledgements

Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending the link headers using an informational response.

7. References

7.1. Normative References

7.2. Informative References

Author's Address