Network Working Group P. Saint-Andre
Internet-Draft Cisco
Intended status: BCP July 5, 2011
Expires: January 6, 2012
Use of the "X-" Prefix in Application Protocols
draft-saintandre-xdash-01
Abstract
Many application protocols use named parameters to identify data.
Historically, protocol designers and implementers distinguished
between "standard" and "non-standard" parameters by prefixing the
latter with the string "X-" or similar constructions. On balance,
this "X-" convention has more costs than benefits, although it can be
appropriate in certain circumstances.
Status of this Memo
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described in the Simplified BSD License.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. Informative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Background
Many application protocols use named parameters to identify data
(media types, header fields in Internet mail messages and HTTP
requests, etc.). Historically, protocol designers and implementers
have often distinguished between "standard" and "non-standard"
parameters by prefixing the latter with the string "X-" or similar
constructions (e.g., "x."), where the "X" is commonly understood to
stand for "eXperimental" or "eXtension".
Although this usage is purely conventional and is not mandated by the
Internet Standards Process [BCP9] or IANA registration rules [BCP26],
some implementers, and even some RFCs, have interpreted the
convention in a more normative way (e.g., [RFC5451] states that
"result codes not beginning with 'x-' MUST be registered with the
Internet Assigned Numbers Authority (IANA) and published in an RFC").
The "X-" convention has been used for email header fields since at
least 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 (and subsequently
reinforced by [RFC2821] and [RFC5321]):
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 media types
([RFC2045], [RFC2046], [RFC2047]), HTTP headers ([RFC2068],
[RFC2616]), vCard parameters and properties ([RFC2426]), Uniform
Resource Names ([RFC3406]), LDAP field names ([RFC4512]), Session
Initiation Protocol "P-" headers ([RFC3427], obsoleted by [RFC5727]),
and other technologies.
Parameters prefaced with the "X-" string are currently used in
application protocols for two different purposes:
o Experiments that might be standardized in the future, if they are
successful.
o Extensions that will probably not be standardized because they are
intended only for use in implementation-specific applications or
on private networks.
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The remainder of this document analyzes the benefits and costs of the
"X-" convention and specifies when it is appropriate to apply the
convention in application protocols.
2. Analysis
The primary problem with the "X-" convention is that non-standard
parameters have a tendency to leak into the protected space of
standardized parameters (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 non-standard name has
become a de facto standard (thus obviating the need for segregation
of the name space into "standard" and "non-standard" in the first
place). As one example, we can see this phenomenon at work in
[RFC2068] (a similar example can be found in [RFC5064]):
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 non-standard 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, as explained in xref target='RFC4288'/>:
[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.
Furthermore, often standarization of a non-standard parameter or
protocol element leads to subtly different behavior (e.g., the
standardized version might have different security properties as a
result of security review provided during the standardization
process). If implementers treat the old, non-standard parameter and
the new, standard parameter as equivalent, interoperability and
security problems can ensue.
For similar considerations with regard to the "P-" convention in the
Session Initiation Protocol, see [RFC5727].
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In some situations, segregating the name space of parameters used in
a given application protocol can be justified:
1. When it is extremely unlikely that some parameters will ever be
standardized. However, in this case implementation-specific and
private-use parameters can be Uniform Resource Identifiers
[RFC3986] (e.g., "http://example.com/foo") or can be prepended
with a string that is derived from the name or primary domain
name of the organization that has defined the parameter (e.g.,
"Example-foo", "com.example.foo", or "VND.example.com.foo").
Similarly, truly experimental parameters can be given meaningless
names such as UUIDs [RFC4122].
2. When parameter names might have significant meaning. However,
this case is rare, since implementers can almost always find a
synonym for an existing term (e.g., "urgency" instead of
"priority") or simply invent a more creative name (e.g., "get-it-
there-fast").
3. When parameter names need to be very short (e.g., as in [RFC5646]
for language tags). However, in this case it can be more
efficient to assign numbers instead of human-readable names
(e.g., as in [RFC2939] for DCHP options) and to leave a certain
numeric range for implementation-specific extensions or private
use (e.g., as with the codec numbers used with the Session
Description Protocol [RFC4566]).
There are two primary objections to deprecating the "X-" convention
as a best practice for application protocols:
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
effectively meaningless.
o Collisions are undesirable. However, names are almost always
cheap, so an experimental, implementation-specific, or private-use
name of "foo" does not prevent a standards development
organization from issuing a similarly creative name such as "bar".
Furthermore, the existence of [BCP82] ("Assigning Experimental and
Testing Numbers Considered Useful") might appear to provide an
argument against deprecating the "X-" convention. However, BCP 82
addresses the need for protocol numbers when the pool of such numbers
is strictly limited (e.g., DHCP options) or when a number is
absolutely required even for purely experimental purposes (e.g., the
Protocol field of the IP header). In almost all application
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protocols that make use of protocol parameters (including email
headers, media types, HTTP headers, vCard parameters and properties,
URNs, and LDAP field names), the name space is not limited or
constrained in any way, so there is no need to assign a block of
names for private use or experimental purposes (see also [BCP26]).
Therefore it appears that segregating non-standard parameters into an
"X-" ghetto has few if any benefits, and has at least one significant
cost in terms of interoperability.
3. Recommendations
Based on the foregoing considerations, this document makes the
following recommendations:
1. Authors of application protocol specifications are discouraged
from imputing special meaning to parameters with the "X-" prefix.
2. Authors of application protocol specifications are discouraged
from mandating that all parameters without the "X-" prefix need
to be registered with the IANA.
3. Implementers wishing to experiment with parameters that have the
potential to be standardized are encouraged to generate names
without the "X-" prefix.
4. Implementers wishing to experiment with parameters that are
unlikely to be standardized are encouraged to generate
meaningless names such as UUIDs or the output of a hash function.
5. Implementers wishing to create parameters for use in
implementation-specific applications or on private networks are
encouraged to mint URIs or generate names that incorporate the
relevant organization's name or primary domain name.
4. Security Considerations
Interoperability and migration issues with security-critical
parameters can result in unnecessary vulnerabilities.
5. IANA Considerations
This document requests no action by the IANA.
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6. Acknowledgements
Thanks to Claudio Allocchio, Adam Barth, Nathaniel Borenstein, Eric
Burger, Al Constanzo, Dave Cridland, Dave Crocker, Martin Duerst,
Frank Ellermann, J.D. Falk, Tony Finch, Tony Hansen, Ted Hardie, Joe
Hildebrand, Alfred Hoenes, Paul Hoffman, Eric Johnson, John Klensin,
Graham Klyne, Murray Kucherawy, Eliot Lear, John Levine, Bill
McQuillan, Alexey Melnikov, Subramanian Moonesamy, Keith Moore, Ben
Niven-Jenkins, Mark Nottingham, Dirk Pranke, Randy Presuhn, Julian
Reschke, Doug Royer, Andrew Sullivan, Martin Thomson, Nicolas
Williams, and Kurt Zeilenga for their feedback.
7. Informative References
[BCP9] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[BCP82] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[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.
[RFC2426] Dawson, F. and T. Howes, "vCard MIME Directory Profile",
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RFC 2426, September 1998.
[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.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition
Mechanisms", BCP 66, RFC 3406, October 2002.
[RFC3427] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,
and B. Rosen, "Change Process for the Session Initiation
Protocol (SIP)", RFC 3427, December 2002.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512,
June 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5064] Duerst, M., "The Archived-At Message Header Field",
RFC 5064, December 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67,
RFC 5727, March 2010.
Author's Address
Peter Saint-Andre
Cisco
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
Email: psaintan@cisco.com
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