wdiff draft-saintandre-xdash-considered-harmful-00.txt draft-saintandre-xdash-considered-harmful-01.txt

Network Working Group P. Saint-Andre
Internet-Draft Cisco Systems, Inc.
Intended status: BCP July 2, October 19, 2010
Expires: January 3, April 22, 2011

"X-" Considered Harmful
draft-saintandre-xdash-considered-harmful-00
draft-saintandre-xdash-considered-harmful-01

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" "standard" and "unofficial" "experimental" parameters
by prefixing unofficial experimental parameters with the string "X-". This
document argues that that, on balance 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
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This Internet-Draft will expire on January 3, April 22, 2011.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
3.
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
4.
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
6. Informative References . . . . . . . . . . . . . . . . . . . . 4 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 6

1. Argument Introduction

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" "standard" and "unofficial" "experimental" parameters
by prefixing unofficial experimental parameters with the string "X-" (often
pronounced "ex dash"). "X-", where the
"X" stands for "eXperimental". This document argues that on balance
the "X-" convention has more costs than benefits.

2. Argument

The "X-" prefix convention has been in use for email header fields since at least the
publication of
[RFC1154] [RFC822] in 1990, 1982, which set forth distinguished between
Extension-fields and user-defined-fields as follows:

The prefatory string "X-" will never be used in the following rule: 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
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 experimental or
implementation-specific parameters have a tendency to seep into the standardization
space, 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 unofficial experimental name is becomes a
standard
de facto (if not de jure). 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 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:

o Collisions are undesirable. However, names are usually cheap, so
an experimental or implementation-specific name of "foo" does not
prevent a standards development organization from issuing a
similarly creative name such as "bar".

Therefore, this document recommends against the creation of new names
with the special "X-" prefix in IETF protocols.

3. Security Considerations

Interoperability and migration issues with security-critical
paramaters can result in unnecessary vulnerabilities.

4. IANA Considerations

This document has no actions for the IANA.

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
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.

[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,
October 2008.

[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.

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