Transparent Content Negotiation in HTTP

HTTP allows web site authors to put multiple versions of the same information under a single URL. Transparent content negotiation (TCN) is an extensible negotiation mechanism, layered on top of HTTP, for automatically selecting the best version when the URL is accessed. This enables the smooth deployment of new web data formats and markup tags.

Current status

• The transparent content negotiation specifications have been frozen as experimental RFCs.
• There are currently three (known) server-side implementations.
• Feature tag registration procedures have been defined in an RFC. The feature tags from the transparent content negotiation RFCs are now officially called 'media feature tags'. A tag registry has been set up, though at this time the 'global tree' registration mechanism is not yet in place.
• The IETF has formed a Content Negotiation Working Group (conneg) to cover content negotiation inside and outside of HTTP.
  (Last updated 5 Jul 1999)

Specifications

• RFC2295, Transparent Content Negotiation in HTTP
  Text version / HTML version

• RFC2296, HTTP Remote Variant Selection Algorithm -- RVSA/1.0
  Text version / HTML version

• RFC2506, BCP 31, Media Feature Tag Registration Procedure
  Text version at www.imc.org

Implementations

• Apache (starting version 1.3.4) has server-side support for transparent content negotiation and remote variant selection. Feature tags are not supported.

• The EmWeb embedded web server architecture includes support for transparent content negotiation and remote variant selection. Feature tags are not supported.

• SRE-http, a free HTTP/1.1 compliant web server for OS/2, implements transparent content negotiation, and can do remote variant selection with its own custom algorithm. Currently the implementation is in 'stable beta'. RFC2295 is supported from version 1.3g on, some earlier versions supported pre-RFC2295 semantics. Feature tags are not supported.

Test implementations

• A transparently negotiated page of web statistics

• Content negotiating user agent extension (conneg-uax)
  Unix software: requires perl 4, awk, libwww-perl-0.4.
  • conneg-uax-v2.0.tar.gz archive (implements client side of RFC 2295 and RFC 2296)

  • README file

  • Example of page negotiated by conneg-uax

Feature tag registry

• IANA Media feature tag registry

IETF Content Negotiation Working Group

• Working group home page and mail archive

Historical

• Archive of internet drafts related to transparent content negotiation

• Negotiation in HTTP (1992, Mar 1993)

Related documents

• HTTP-WG home page

• Content Negotiation Techniques (talk at WWW9 conference)