APPSAWG                                                   P. Saint-Andre
Internet-Draft                                       Cisco Systems, Inc.
Intended status: BCP                                          D. Crocker
Expires: August 16, 2012                     Brandenburg InternetWorking
                                                           M. Nottingham
                                                       February 13, 2012

      Deprecating Use of the "X-" Prefix in Application Protocols


   Historically, designers and implementers of application protocols
   have often distinguished between "standard" and "non-standard"
   parameters by prefixing the latter with the string "X-" or similar
   constructions.  In practice, this convention causes more problems
   than it solves.  Therefore, this document deprecates the "X-"
   convention for textual parameters in application protocols.

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

   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 August 16, 2012.

Copyright Notice

   Copyright (c) 2012 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

Saint-Andre, et al.      Expires August 16, 2012                [Page 1]

Internet-Draft                The X- Prefix                February 2012

   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Recommendations for Implementers of Application Protocols  . .  3
   3.  Recommendations for Creators of New Parameters . . . . . . . .  3
   4.  Recommendations for Protocol Designers . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  5
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  5
   Appendix A.  Background  . . . . . . . . . . . . . . . . . . . . .  7
   Appendix B.  Analysis  . . . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

Saint-Andre, et al.      Expires August 16, 2012                [Page 2]

Internet-Draft                The X- Prefix                February 2012

1.  Introduction

   Many application protocols use parameters with textual 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 "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 in theory the "X-" convention was a good way to avoid
   collisions (and attendant interoperability problems) between standard
   parameters and non-standard parameters, in practice the benefits have
   been outweighed by the costs associated with the leakage of non-
   standard parameters into the standards space.  Therefore this
   document deprecates the "X-" convention for named parameters in
   application protocols and makes specific recommendations about how to
   proceed in a world without the distinction between standard and non-
   standard parameters.  Note that this document covers only parameters
   with textual names, not parameters that are expressed as numbers.  In
   addition, this document makes no recommendation as to whether
   existing "X-" parameters ought to remain in use or be migrated to a
   format without the "X-".

   See Appendix A for background information about the history of the
   "X-" convention, and Appendix B for the reasoning that led to the
   recommendations in this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Recommendations for Implementers of Application Protocols

   Implementers of application protocols MUST NOT treat the general
   categories of "standard" and "non-standard" parameters in
   programatically different ways within their applications.

3.  Recommendations for Creators of New Parameters

   Creators of new parameters to be used in the context of application

Saint-Andre, et al.      Expires August 16, 2012                [Page 3]

Internet-Draft                The X- Prefix                February 2012

   1.  SHOULD assume that all parameters they create might become
       standardized, public, commonly deployed, or used across multiple

   2.  SHOULD employ meaningful names that they have reason to believe
       are currently unused (without the "X-" prefix).

   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, if appropriate including both permanent and provisional

   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 identify a convention to allow local or implementation-
       specific extensions, and reserve delimeters for such uses as

   5.  SHOULD NOT prohibit parameters with the "X-" prefix from being
       registered with the IANA.

   6.  MUST NOT assume that a parameter with an "X-" prefix is non-

   7.  MUST NOT assume that a parameter without an "X-" prefix is

5.  Security Considerations

   Interoperability and migration issues with security-critical
   parameters can result in unnecessary vulnerabilities (see Appendix B
   for further discussion).

Saint-Andre, et al.      Expires August 16, 2012                [Page 4]

Internet-Draft                The X- Prefix                February 2012

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.

7.  Acknowledgements

   Thanks to Claudio Allocchio, Adam Barth, Nathaniel Borenstein, Eric
   Burger, Stuart Cheshire, Al Constanzo, Dave Cridland, 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, 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, Doug Royer, Andrew
   Sullivan, Martin Thomson, Matthew Wild, Nicolas Williams, Tim
   Williams, Mykyta Yevstifeyev, and Kurt Zeilenga for their feedback.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  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.

   [RFC691]   Harvey, B., "One more try on the FTP", RFC 691, June 1975.

   [RFC737]   Harrenstien, K., "FTP extension: XSEN", RFC 737,
              October 1977.

   [RFC743]   Harrenstien, K., "FTP extension: XRSQ/XRCP", RFC 743,
              December 1977.

Saint-Andre, et al.      Expires August 16, 2012                [Page 5]

Internet-Draft                The X- Prefix                February 2012

   [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,
              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",
              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,
              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

Saint-Andre, et al.      Expires August 16, 2012                [Page 6]

Internet-Draft                The X- Prefix                February 2012

              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.

   [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,
              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.

   [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.

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]:

Saint-Andre, et al.      Expires August 16, 2012                [Page 7]

Internet-Draft                The X- Prefix                February 2012

      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.

   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]), 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

Saint-Andre, et al.      Expires August 16, 2012                [Page 8]

Internet-Draft                The X- Prefix                February 2012

   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 non-standard
   parameters have a tendency to leak into the protected space of
   standard parameters (whether de jure or de facto), thus introducing
   the need for migration from the "X-" name to the standard 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 standard
   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"
   areas in the first place).

   We have already seen this phenomenon at work with regard to FTP in
   the quote from [RFC1123] in the previous section.  The HTTP community
   had the same experience with the "x-gzip" and "x-compressed" 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:

Saint-Andre, et al.      Expires August 16, 2012                [Page 9]

Internet-Draft                The X- Prefix                February 2012

      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
   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 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 non-standard parameter or
   protocol element leads to subtly different behavior (e.g., the
   standard 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].

   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.  However, 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], "") or primary domain name (e.g.,
       "" or a Uniform Resource Identifier [RFC3986] such
       as "").  In rare cases, truly experimental
       parameters could be given meaningless names such as nonsense
       words, the output of a hash function, or UUIDs [RFC4122].

   2.  When parameter names might have significant meaning.  However,
       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-

Saint-Andre, et al.      Expires August 16, 2012               [Page 10]

Internet-Draft                The X- Prefix                February 2012


   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 three primary objections to deprecating the "X-" convention
   as a best practice for application protocols:

   1.  Implementers are easily confused and can't be expected to know
       that a parameter is non-standard unless it contains the "X-"
       prefix.  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 without force.

   2.  Collisions are undesirable and it would be bad for both a
       standard parameter "foo" and a non-standard 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
   standard area and a non-standard area has few if any benefits, and
   has at least one significant cost in terms of interoperability.

Saint-Andre, et al.      Expires August 16, 2012               [Page 11]

Internet-Draft                The X- Prefix                February 2012

Authors' Addresses

   Peter Saint-Andre
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202

   Phone: +1-303-308-3282

   D. Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.

   Phone: +1.408.246.8253

   Mark Nottingham


Saint-Andre, et al.      Expires August 16, 2012               [Page 12]