|Title||Deprecating the "X-" Prefix and Similar Constructs in Application
|Author||P. Saint-Andre, D. Crocker, M. Nottingham
|Status:||BEST CURRENT PRACTICE
Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 6648 Cisco Systems, Inc.
BCP: 178 D. Crocker
Category: Best Current Practice Brandenburg InternetWorking
ISSN: 2070-1721 M. Nottingham
Deprecating the "X-" Prefix and Similar Constructs
in Application Protocols
Historically, designers and implementers of application protocols
have often distinguished between standardized and unstandardized
parameters by prefixing the names of unstandardized parameters with
the string "X-" or similar constructs. In practice, that convention
causes more problems than it solves. Therefore, this document
deprecates the convention for newly defined parameters with textual
(as opposed to numerical) names in application protocols.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Recommendations for Implementers of Application Protocols .......4
3. Recommendations for Creators of New Parameters ..................4
4. Recommendations for Protocol Designers ..........................4
5. Security Considerations .........................................5
6. IANA Considerations .............................................5
7. Acknowledgements ................................................5
Appendix A. Background ............................................6
Appendix B. Analysis ..............................................7
Normative References ...........................................10
Informative References .........................................10
Many application protocols use parameters with textual (as opposed to
numerical) names to identify data (media types, header fields in
Internet mail messages and HTTP requests, vCard parameters and
properties, etc.). Historically, designers and implementers of
application protocols have often distinguished between standardized
and unstandardized parameters by prefixing the names of
unstandardized parameters with the string "X-" or similar constructs
(e.g., "x."), where the "X" is commonly understood to stand for
"eXperimental" or "eXtension".
Under this convention, the name of a parameter not only identified
the data, but also embedded the status of the parameter into the name
itself: a parameter defined in a specification produced by a
recognized standards development organization (or registered
according to processes defined in such a specification) did not start
with "X-" or similar constructs, whereas a parameter defined outside
such a specification or process started with "X-" or similar
As explained more fully under Appendix A, this convention was
encouraged for many years in application protocols such as file
transfer, email, and the World Wide Web. In particular, it was
codified for email by [RFC822] (via the distinction between
"Extension-fields" and "user-defined-fields"), but then removed by
[RFC2822] based on implementation and deployment experience. A
similar progression occurred for SIP technologies with regard to the
"P-" header, as explained in [RFC5727]. The reasoning behind those
changes is explored under Appendix B.
In short, although in theory the "X-" convention was a good way to
avoid collisions (and attendant interoperability problems) between
standardized parameters and unstandardized parameters, in practice
the benefits have been outweighed by the costs associated with the
leakage of unstandardized parameters into the standards space.
This document generalizes from the experience of the email and SIP
communities by doing the following:
1. Deprecates the "X-" convention for newly defined parameters in
application protocols, including new parameters for established
protocols. This change applies even where the "X-" convention
was only implicit, and not explicitly provided, such as was done
for email in [RFC822].
2. Makes specific recommendations about how to proceed in a world
without the distinction between standardized and unstandardized
parameters (although only for parameters with textual names, not
parameters that are expressed as numbers, which are out of the
scope of this document).
3. Does not recommend against the practice of private, local,
preliminary, experimental, or implementation-specific parameters,
only against the use of "X-" and similar constructs in the names
of such parameters.
4. Makes no recommendation as to whether existing "X-" parameters
ought to remain in use or be migrated to a format without the
"X-"; this is a matter for the creators or maintainers of those
5. Does not override existing specifications that legislate the use
of "X-" for particular application protocols (e.g., the "x-name"
token in [RFC5545]); this is a matter for the designers of those
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
2. Recommendations for Implementers of Application Protocols
Implementations of application protocols MUST NOT make any
assumptions about the status of a parameter, nor take automatic
action regarding a parameter, based solely on the presence or absence
of "X-" or a similar construct in the parameter's name.
3. Recommendations for Creators of New Parameters
Creators of new parameters to be used in the context of application
1. SHOULD assume that all parameters they create might become
standardized, public, commonly deployed, or usable across
2. SHOULD employ meaningful parameter names that they have reason to
believe are currently unused.
3. SHOULD NOT prefix their parameter names with "X-" or similar
Note: If the relevant parameter name space has conventions about
associating parameter names with those who create them, a parameter
name could incorporate the organization's name or primary domain name
(see Appendix B for examples).
4. Recommendations for Protocol Designers
Designers of new application protocols that allow extensions using
1. SHOULD establish registries with potentially unlimited value-
spaces, defining both permanent and provisional registries if
2. SHOULD define simple, clear registration procedures.
3. SHOULD mandate registration of all non-private parameters,
independent of the form of the parameter names.
4. SHOULD NOT prohibit parameters with an "X-" prefix or similar
constructs from being registered.
5. MUST NOT stipulate that a parameter with an "X-" prefix or
similar constructs needs to be understood as unstandardized.
6. MUST NOT stipulate that a parameter without an "X-" prefix or
similar constructs needs to be understood as standardized.
5. Security Considerations
Interoperability and migration issues with security-critical
parameters can result in unnecessary vulnerabilities (see Appendix B
for further discussion).
As a corollary to the recommendation provided under Section 2,
implementations MUST NOT assume that standardized parameters are
"secure" whereas unstandardized parameters are "insecure", based
solely on the names of such parameters.
6. IANA Considerations
This document does not modify registration procedures currently in
force for various application protocols. However, such procedures
might be updated in the future to incorporate the best practices
defined in this document.
Thanks to Claudio Allocchio, Adam Barth, Nathaniel Borenstein, Eric
Burger, Stuart Cheshire, Al Constanzo, Dave Cridland, Ralph Droms,
Martin Duerst, Frank Ellermann, J.D. Falk, Ned Freed, Tony Finch,
Randall Gellens, Tony Hansen, Ted Hardie, Joe Hildebrand, Alfred
Hoenes, Paul Hoffman, Eric Johnson, Scott Kelly, Scott Kitterman,
John Klensin, Graham Klyne, Murray Kucherawy, Eliot Lear, John
Levine, Bill McQuillan, Alexey Melnikov, Subramanian Moonesamy, Keith
Moore, Ben Niven-Jenkins, Zoltan Ordogh, Tim Petch, Dirk Pranke,
Randy Presuhn, Julian Reschke, Dan Romascanu, Doug Royer, Andrew
Sullivan, Henry Thompson, Martin Thomson, Matthew Wild, Nicolas
Williams, Tim Williams, Mykyta Yevstifeyev, and Kurt Zeilenga for
Appendix A. Background
The beginnings of the "X-" convention can be found in a suggestion
made by Brian Harvey in 1975 with regard to FTP parameters [RFC691]:
Thus, FTP servers which care about the distinction between Telnet
print and non-print could implement SRVR N and SRVR T. Ideally
the SRVR parameters should be registered with Jon Postel to avoid
conflicts, although it is not a disaster if two sites use the same
parameter for different things. I suggest that parameters be
allowed to be more than one letter, and that an initial letter X
be used for really local idiosyncracies [sic].
This "X" prefix was subsequently used in [RFC737], [RFC743], and
[RFC775]. This usage was noted in [RFC1123]:
FTP allows "experimental" commands, whose names begin with "X".
If these commands are subsequently adopted as standards, there may
still be existing implementations using the "X" form.... All FTP
implementations SHOULD recognize both forms of these commands, by
simply equating them with extra entries in the command lookup
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:
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]), Lightweight Directory Access Protocol
(LDAP) field names ([RFC4512]), and other application technologies.
However, use of the "X-" prefix in email headers was effectively
deprecated between the publication of [RFC822] in 1982 and the
publication of [RFC2822] in 2001 by removing the distinction between
the "extension-field" construct and the "user-defined-field"
construct (a similar change happened with regard to Session
Initiation Protocol "P-" headers when [RFC3427] was obsoleted by
Despite the fact that parameters containing the "X-" string have been
effectively deprecated in email headers, they continue to be used in
a wide variety of application protocols. The two primary situations
motivating such use are:
1. Experiments that are intended to possibly be standardized in the
future, if they are successful.
2. Extensions that are intended to never be standardized because
they are intended only for implementation-specific use or for
local use on private networks.
Use of this naming convention is not mandated by the Internet
Standards Process [BCP9] or IANA registration rules [BCP26]. Rather,
it is an individual choice by each specification that references the
convention or each administrative process that chooses to use it. In
particular, some Standards Track RFCs have interpreted the convention
in a normative way (e.g., [RFC822] and [RFC5451]).
Appendix B. Analysis
The primary problem with the "X-" convention is that unstandardized
parameters have a tendency to leak into the protected space of
standardized parameters, thus introducing the need for migration from
the "X-" name to a standardized name. Migration, in turn, introduces
interoperability issues (and sometimes security 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 unstandardized name has
become a de facto standard (thus obviating the need for segregation
of the name space into standardized and unstandardized areas in the
We have already seen this phenomenon at work with regard to FTP in
the quote from [RFC1123] in Appendix A. The HTTP community had the
same experience with the "x-gzip" and "x-compress" media types, as
noted 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.
A similar example can be found in [RFC5064], which defined the
"Archived-At" message header field but also found it necessary to
define and register the "X-Archived-At" field:
For backwards compatibility, this document also describes the
X-Archived-At header field, a precursor of the Archived-At header
field. The X-Archived-At header field MAY also be parsed, but
SHOULD NOT be generated.
One of the original reasons for segregation of name spaces into
standardized and unstandardized 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 explained in [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.
For some name spaces, another helpful practice has been the
establishment of separate registries for permanent names and
provisional names, as in [RFC4395].
Furthermore, often standardization of a unstandardized parameter
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, unstandardized parameter and the new, standardized
parameter as equivalent, interoperability and security problems can
ensue. Analysis of unstandardized parameters to detect and correct
flaws is, in general, a good thing and is not intended to be
discouraged by the lack of distinction in element names. If an
originally unstandardized parameter or protocol element is
standardized and the new form has differences that affect
interoperability or security properties, it would be inappropriate
for implementations to treat the old form as identical to the new
For similar considerations with regard to the "P-" convention in the
Session Initiation Protocol, see [RFC5727].
In some situations, segregating the parameter name space used in a
given application protocol can be justified:
1. When it is extremely unlikely that some parameters will ever be
standardized. In this case, implementation-specific and private-
use parameters could at least incorporate the organization's name
(e.g., "ExampleInc-foo" or, consistent with [RFC4288],
"VND.ExampleInc.foo") or primary domain name (e.g.,
"com.example.foo" or a Uniform Resource Identifier [RFC3986] such
as "http://example.com/foo"). In rare cases, truly experimental
parameters could be given meaningless names such as nonsense
words, the output of a hash function, or Universally Unique
Identifiers (UUIDs) [RFC4122].
2. When parameter names might have significant meaning. This case
too 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").
The existence of multiple similarly named parameters can be
confusing, but this is true regardless if there is an attempt to
segregate standardized and unstandardized parameters (e.g.,
"X-Priority" can be confused with "Urgency").
3. When parameter names need to be very short (e.g., as in [RFC5646]
for language tags). In this case, it can be more efficient to
assign numbers instead of human-readable names (e.g., as in
[RFC2939] for DHCP 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
There are three primary objections to deprecating the "X-" convention
as a best practice for application protocols:
1. Implementers might mistake one parameter for another parameter
that has a similar name; a rigid distinction such as an "X-"
prefix can make this clear. However, in practice, implementers
are forced to blur the distinction (e.g., by treating "X-foo" as
a de facto standard), so it inevitably becomes meaningless.
2. Collisions are undesirable, and it would be bad for both a
standardized parameter "foo" and a unstandardized parameter "foo"
to exist simultaneously. 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".
3. [BCP82] is entitled "Assigning Experimental and Testing Numbers
Considered Useful" and therefore implies that the "X-" prefix is
also useful for experimental parameters. 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 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 the parameter space into a
standardized area and a unstandardized area has few, if any, benefits
and has at least one significant cost in terms of interoperability.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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,
[BCP82] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC691] Harvey, B., "One more try on the FTP", RFC 691, June 1975.
[RFC737] Harrenstien, K., "FTP extension: XSEN", RFC 737,
[RFC743] Harrenstien, K., "FTP extension: XRSQ/XRCP", RFC 743,
[RFC775] Mankins, D., Franklin, D., and A. Owen, "Directory
oriented FTP commands", RFC 775, December 1980.
[RFC822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[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,
[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",
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.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
[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,
[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,
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512,
[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.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
[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.
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