Network Working Group P. Saint-Andre Internet-Draft Cisco Systems, Inc. Intended status: BCP July 2, 2010 Expires: January 3, 2011 "X-" Considered Harmful draft-saintandre-xdash-considered-harmful-00 Abstract Many application protocols use named parameters to represent data (for example, header fields in Internet mail messages and HTTP requests). Historically, protocol designers and implementers have often differentiated between "official" and "unofficial" parameters by prefixing unofficial parameters with the string "X-". This document argues that on balance the "X-" convention has more costs than benefits. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 3, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Saint-Andre Expires January 3, 2011 [Page 1] Internet-Draft X- Considered Harmful July 2010 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 4. Informative References . . . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 Saint-Andre Expires January 3, 2011 [Page 2] Internet-Draft X- Considered Harmful July 2010 1. Argument Many application protocols use named parameters to represent data (for example, header fields in Internet mail messages and HTTP requests). Historically, protocol designers and implementers have often differentiated between "official" and "unofficial" parameters by prefixing unofficial parameters with the string "X-" (often pronounced "ex dash"). This document argues that on balance the "X-" convention has more costs than benefits. The "X-" prefix has been in use since at least the publication of [RFC1154] in 1990, which set forth the following rule: Keywords beginning with "X-" are permanently reserved to implementation-specific use. No standard registered encoding keyword will ever begin with "X-". This convention continued with various specifications for MIME [RFC2045] [RFC2046] [RFC2047], email [RFC2821] [RFC5321], HTTP [RFC2068] [RFC2616], and other technologies. The primary objections to discarding the "X-" convention are: o Implementers are easily confused. However, implementers already are quite flexible about using both prefixed and non-prefixed names based on what works in the field, so the distinction between de facto names (e.g., "X-foo") and de jure names (e.g., "foo") is meaningless to them. o Collisions are undesirable. However, names are cheap, so an implementation-specific name of "foo" does not prevent a standards development organization from issuing a similarly creative name such as "bar". The primary problem with the "X-" convention is that implementation- specific parameters have a tendency to seep into the standardization space, introducing the need for migration from the "X-" name to the standardized name. Migration, in turn, introduces interoperability issues because older implementations will support only the "X-" name and newer implementations might support only the standardized name. To preserve interoperability, newer implementations simply support the "X-" name forever, which means that the unofficial name is a standard de facto (if not de jure). We can see this phenomenon at work in [RFC2068]: For compatibility with previous implementations of HTTP, applications should consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively. Saint-Andre Expires January 3, 2011 [Page 3] Internet-Draft X- Considered Harmful July 2010 Therefore, segregating implementation-specific parameters into an "X-" ghetto has few if any benefits and at least one significant interoperability cost. The practice is at best useless and at worst harmful. 2. Security Considerations Interoperability and migration issues with security-critical paramaters can result in unnecessary vulnerabilities. 3. IANA Considerations This document has no actions for the IANA. 4. Informative References [RFC1154] Robinson, D. and R. Ullmann, "Encoding header field for internet messages", RFC 1154, April 1990. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January 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. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. Saint-Andre Expires January 3, 2011 [Page 4] Internet-Draft X- Considered Harmful July 2010 Author's Address Peter Saint-Andre Cisco Systems, Inc. 1899 Wyknoop Street, Suite 600 Denver, CO 80202 USA Phone: +1-303-308-3282 Email: psaintan@cisco.com Saint-Andre Expires January 3, 2011 [Page 5]