Network Working Group M. Nottingham
Internet-Draft 2011
Intended status: Informational

HTTP Browser Hints
draft-nottingham-http-browser-hints-01

Abstract

Over time, Web browsers have adapted how they use HTTP based upon common server configurations and behaviours. While this is necessary in the common case, it can be detrimental for performance and interoperability.

This document establishes a mechanism whereby origin servers can make available hints for browsers about their preferences and capabilities, without imposing overhead on their interactions or requiring support for them.

This is intended to allow browsers to safely optimise connections to servers.

Status of This Memo

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Copyright Notice

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Table of Contents

1. Introduction

HTTP [RFC2616] clients -- especially browsers -- typically use hardcoded values or heuristics to determine how many TCP connections to use to a server, based on common-case server behaviours and limitations.

Likewise, they often send voluminous request headers (e.g., in User-Agent and Allow) because they fear that changing those headers' values will break some sites that depend upon specific values.

These are just two examples of common, conservative behaviour by browsers that is good for interoperability, but potentially bad for performance in certain circumstances.

This memo proposes a mechanism whereby a HTTP server can advertise hints for browsers (and other clients), so that communication with them can be optimised.

It does so by defining a file format for such Browser Hints Section 3, and defining how clients can discover it for a given Web site Section 4. Finally, an extensible vocabulary of hints is defined Section 5.

Feedback for this draft should take place on the apps-discuss@ietf.org mailing list https://www.ietf.org/mailman/listinfo/apps-discuss.

2. Requirements

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].

3. A file format for Browser Hints

Browser hints are indicating using a JSON [RFC4627] formatted file, containing a single object whose member's names are browser hints, as defined by the registry Section 8.2.

For example;

          {
          "max-conns": 5,
          "small-hdrs": true
          }
        

By their nature, all browser hints are optional; i.e., browsers are free to ignore them.

4. Discovering Browser Hints for a Web site

The hints relevant to a given site can be determined by fetching the URI path "/.well-known/browser-hints" for that site.

Typically, clients (especially browsers) will not block other requests to a site while fetching the browser hints (because they're optional); instead, it will usually be done concurrently with other requests, or on idle connections for future use.

In this specification, "site" is scoped by the URI scheme and authority. As such, all of the following are considered to be different sites, and therefore have different browser hints:

Clients SHOULD follow HTTP 3xx redirects when retrieving hints.

A successful response is valid for its associated site for as long as it can be cached in HTTP.

If the response has a 200 status code but no explicit freshness (e.g., a Cache-Control: max-age or Expires: header), browsers SHOULD cache the response heuristically for a generous fixed period (e.g., 30 days).

5. Pre-defined Browser Hints

5.1. max-conns

5.2. pconn-ip

In other words, if both www.example.com and foo.example.org resolve to the address 192.0.2.5, and indicate this hint, then clients can send a request to www.example.com and then a request to foo.example.org on the same TCP connection to that address.

If any of the sites grouped together for the purposes of pconn-ip declare a max-conns hint, the max-conns value for that address is considered to be the maximum of the declared max-conn hints present.

5.3. max-pipeline-depth

5.4. small-hdrs

5.5. no-referer

5.6. send-ref

5.7. chunk-req-bodies

5.8. omit-cookies

6. The Ref HTTP Request Header

TBD: relative URI referer header

7. Security Considerations

TBD

8. IANA Considerations

8.1. The 'browser-hints' Well-Known URI

This document defines the "browser-hints" Well-Known URI [RFC5785].

8.2. The HTTP Browser Hints Registry

This document establishes the HTTP Browser Hints Registry.

New hints are registered First Come First Served (see [RFC5226]), by sending e-mail to mailto:iana@iana.org (or using other mechanisms, as established by IANA).

Registration requests MUST use the following template:

New hints MUST be optional; they cannot place requirements upon implementations.

Likewise, new hints MUST be relevant to browser use cases; other non-browsing hints and metadata would make the hints response undesirably large. However, note that non-browser clients MAY use them.

Finally, new hints MUST NOT make communication non-conformant with HTTP itself; i.e., this is not a mechanism for changing the HTTP protocol in incompatible ways. For example, if a hint indicates that browsers can compress request headers using GZIP, intermediaries that are interposed are likely to fail.

The initial contents of the registry are defined in Section 5.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, July 2006.

9.2. Informative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, April 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011.

Appendix A. Acknowledgements

Thanks to Mike Belshe, Poul-Henning Kamp, Anirban Kundu, Patrick McManus, and Steve Souders for their suggestions and feedback.

The author takes all responsibility for errors and omissions.

Author's Address

Mark Nottingham EMail: mnot@mnot.net URI: http://www.mnot.net/