< draft-saintandre-xdash-considered-harmful-00.txt		draft-saintandre-xdash-considered-harmful-01.txt >
	Network Working Group P. Saint-Andre		Network Working Group P. Saint-Andre
	Internet-Draft Cisco Systems, Inc.		Internet-Draft Cisco
	Intended status: BCP July 2, 2010		Intended status: BCP October 19, 2010
	Expires: January 3, 2011		Expires: April 22, 2011
	"X-" Considered Harmful		"X-" Considered Harmful
	draft-saintandre-xdash-considered-harmful-00		draft-saintandre-xdash-considered-harmful-01
	Abstract		Abstract
	Many application protocols use named parameters to represent data		Many application protocols use named parameters to represent data
	(for example, header fields in Internet mail messages and HTTP		(for example, header fields in Internet mail messages and HTTP
	requests). Historically, protocol designers and implementers have		requests). Historically, protocol designers and implementers have
	often differentiated between "official" and "unofficial" parameters		often differentiated between "standard" and "experimental" parameters
	by prefixing unofficial parameters with the string "X-". This		by prefixing experimental parameters with the string "X-". This
	document argues that on balance the "X-" convention has more costs		document argues that, on balance, the "X-" convention has more costs
	than benefits.		than benefits.
	Status of this Memo		Status of this Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 3, 2011.		This Internet-Draft will expire on April 22, 2011.
	Copyright Notice		Copyright Notice
	Copyright (c) 2010 IETF Trust and the persons identified as the		Copyright (c) 2010 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Security Considerations . . . . . . . . . . . . . . . . . . . . 4		2. Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4		3. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
	4. Informative References . . . . . . . . . . . . . . . . . . . . 4		4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5		5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
			6. Informative References . . . . . . . . . . . . . . . . . . . . 5
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
	1. Argument		1. Introduction
	Many application protocols use named parameters to represent data		Many application protocols use named parameters to represent data
	(for example, header fields in Internet mail messages and HTTP		(for example, header fields in Internet mail messages and HTTP
	requests). Historically, protocol designers and implementers have		requests). Historically, protocol designers and implementers have
	often differentiated between "official" and "unofficial" parameters		often differentiated between "standard" and "experimental" parameters
	by prefixing unofficial parameters with the string "X-" (often		by prefixing experimental parameters with the string "X-", where the
	pronounced "ex dash"). This document argues that on balance the "X-"		"X" stands for "eXperimental". This document argues that on balance
	convention has more costs than benefits.		the "X-" convention has more costs than benefits.
	The "X-" prefix has been in use since at least the publication of		2. Argument
	[RFC1154] in 1990, which set forth the following rule:
			The "X-" convention has been in use for email header fields since the
			publication of [RFC822] in 1982, which distinguished between
			Extension-fields and user-defined-fields as follows:
			The prefatory string "X-" will never be used in the names of
			Extension-fields. This provides user-defined fields with a
			protected set of names.
			That rule was restated by [RFC1154] as follows:
	Keywords beginning with "X-" are permanently reserved to		Keywords beginning with "X-" are permanently reserved to
	implementation-specific use. No standard registered encoding		implementation-specific use. No standard registered encoding
	keyword will ever begin with "X-".		keyword will ever begin with "X-".
	This convention continued with various specifications for MIME		This convention continued with various specifications for MIME
	[RFC2045] [RFC2046] [RFC2047], email [RFC2821] [RFC5321], HTTP		[RFC2045] [RFC2046] [RFC2047], email [RFC2821] [RFC5321], HTTP
	[RFC2068] [RFC2616], and other technologies.		[RFC2068] [RFC2616], and other technologies.
			The primary problem with the "X-" convention is that experimental or
			implementation-specific parameters have a tendency to become
			standardized (whether de jure or de facto), thus 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 experimental name becomes a
			de facto standard (thus obviating the need for segregation of the
			name spaces in the first place). 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.
			One of the original reasons for segregation of name spaces into
			standard and experimental areas was the perceived difficulty of
			registering names. However, the solution to that problem has been
			simpler registration rules, such as those provided by [RFC3864] and
			[RFC4288], as well as separate registries for permanent and
			provisional names. Indeed, [RFC4288] explicitly calls out the
			implications for experimental names:
			[W]ith the simplified registration procedures described above for
			vendor and personal trees, it should rarely, if ever, be necessary
			to use unregistered experimental types. Therefore, use of both
			"x-" and "x." forms is discouraged.
			In some limited situations, segregating a name space can be
			justified; for example, when the names need to be very small (as in
			[RFC5646]) or when the names have significant meaning. However, in
			general, segregating experimental or implementation-specific
			parameters into an "X-" ghetto has few if any benefits, and has at
			least one significant interoperability cost. The practice is at best
			useless and at worst harmful.
	The primary objections to discarding the "X-" convention are:		The primary objections to discarding the "X-" convention are:
	o Implementers are easily confused. However, implementers already		o Implementers are easily confused. However, implementers already
	are quite flexible about using both prefixed and non-prefixed		are quite flexible about using both prefixed and non-prefixed
	names based on what works in the field, so the distinction between		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		de facto names (e.g., "X-foo") and de jure names (e.g., "foo") is
	meaningless to them.		meaningless to them.
	o Collisions are undesirable. However, names are cheap, so an		o Collisions are undesirable. However, names are usually cheap, so
	implementation-specific name of "foo" does not prevent a standards		an experimental or implementation-specific name of "foo" does not
	development organization from issuing a similarly creative name		prevent a standards development organization from issuing a
	such as "bar".		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.
	Therefore, segregating implementation-specific parameters into an		Therefore, this document recommends against the creation of new names
	"X-" ghetto has few if any benefits and at least one significant		with the special "X-" prefix in IETF protocols.
	interoperability cost. The practice is at best useless and at worst
	harmful.
	2. Security Considerations		3. Security Considerations
	Interoperability and migration issues with security-critical		Interoperability and migration issues with security-critical
	paramaters can result in unnecessary vulnerabilities.		paramaters can result in unnecessary vulnerabilities.
	3. IANA Considerations		4. IANA Considerations
	This document has no actions for the IANA.		This document has no actions for the IANA.
	4. Informative References		5. Acknowledgements
			Thanks to Adam Barth, Dave Crocker, Martin Duerst, Paul Hoffman,
			Graham Klyne, Alexey Melnikov, Mark Nottingham, and Randy Presuhn for
			feedback.
			6. Informative References
			[RFC822] Crocker, D., "Standard for the format of ARPA Internet
			text messages", STD 11, RFC 822, August 1982.
	[RFC1154] Robinson, D. and R. Ullmann, "Encoding header field for		[RFC1154] Robinson, D. and R. Ullmann, "Encoding header field for
	internet messages", RFC 1154, April 1990.		internet messages", RFC 1154, April 1990.
	[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail		[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
	Extensions (MIME) Part One: Format of Internet Message		Extensions (MIME) Part One: Format of Internet Message
	Bodies", RFC 2045, November 1996.		Bodies", RFC 2045, November 1996.
	[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail		[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
	Extensions (MIME) Part Two: Media Types", RFC 2046,		Extensions (MIME) Part Two: Media Types", RFC 2046,
	skipping to change at page 4, line 47 ¶		skipping to change at page 5, line 42 ¶
	Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",		Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
	RFC 2068, January 1997.		RFC 2068, January 1997.
	[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,		[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext		Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
	Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.		Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
	[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,		[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
	April 2001.		April 2001.
			[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
			Procedures for Message Header Fields", BCP 90, RFC 3864,
			September 2004.
			[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
			Registration Procedures", BCP 13, RFC 4288, December 2005.
	[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,		[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
	October 2008.		October 2008.
			[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
			Languages", BCP 47, RFC 5646, September 2009.
	Author's Address		Author's Address
	Peter Saint-Andre		Peter Saint-Andre
	Cisco Systems, Inc.		Cisco
	1899 Wyknoop Street, Suite 600		1899 Wyknoop Street, Suite 600
	Denver, CO 80202		Denver, CO 80202
	USA		USA
	Phone: +1-303-308-3282		Phone: +1-303-308-3282
	Email: psaintan@cisco.com		Email: psaintan@cisco.com
End of changes. 17 change blocks.
	47 lines changed or deleted		98 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/