Network Working Group                                     P. Saint-Andre
Internet-Draft                                                     Cisco
Intended status: BCP                                       June 27, 2011
Expires: December 29, 2011

            Use of the "X-" Prefix in Application Protocols


   Many application protocols use named parameters to identity data.
   Historically, protocol designers and implementers distinguished
   between "standard" and "non-standard" parameters by prefixing the
   latter with the string "X-".  On balance, this "X-" convention has
   more costs than benefits, although it can be appropriate in certain

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   described in the Simplified BSD License.

Table of Contents

   1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Analysis  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Informative References  . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Background

   Many application protocols use named parameters to identity 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-", where the
   "X" stands for "eXperimental".

   This "X-" convention has been uses for email header fields at least
   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
      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]), email headers ([RFC2821],
   [RFC5321]), HTTP headers ([RFC2068], [RFC2616]), Uniform Resource
   Names ([RFC3406]), Session Initiation Protocol "P-" headers
   ([RFC3427], obsoleted by [RFC5727]), and other technologies.

   Parameters prefaced with the "X-" string (and similar constructions,
   such as "x.") are currently used in application protocols for two
   different purposes:

   o  Experiments that might lead to standardization in the future.
   o  Implementation-specific applications or private networks that are
      never intended to be standardized.

   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 produced by the IETF.

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

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   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 becomes
   a de facto standard (thus obviating the need for segregation of the
   name spaces in the first place).  As one example, we can see this
   phenomenon at work in [RFC2068] (similar examples can be found in

      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.

   [RFC4288] calls out one implication of non-standard 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.

   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].

   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.  In this case, private-use parameters can be URIs
       (e.g., "") 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" or "").  Similarly, truly experimental

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       parameters can be given meaningless names such as UUIDs

   2.  When parameter names might have significant meaning.  This case
       is rare, since implementers can almost always find a synonym
       (e.g., "urgency" instead of "priority") or simply invent a new

   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 DCHP options) and to leave a certain numeric range
       for 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
      meaningless to them.

   o  Collisions are undesirable.  However, names are almost always
      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".

   In addition, 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 protocols 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 (e.g., media types,
   email headers, HTTP headers, URIs), 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]).

   The foregoing considerations lead to the conclusion 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.
   Therefore, this document recommends against the creation of new names
   with the special "X-" prefix in application protocols produced within
   the IETF.

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3.  Security Considerations

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

4.  IANA Considerations

   This document requests no action by the IANA.

5.  Acknowledgements

   Thanks to Claudio Allocchio, Adam Barth, Nathaniel Borenstein, Eric
   Burger, Al Constanzo, Dave Cridland, Dave Crocker, Martin Duerst,
   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, Bill McQuillan, Alexey Melnikov,
   Subramanian Moonesamy, Keith Moore, Mark Nottingham, Randy Presuhn,
   Julian Reschke, Doug Royer, Andrew Sullivan, Martin Thomson, Nicolas
   Williams, and Kurt Zeilenga for feedback.

6.  Informative References

   [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",

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              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.

   [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.

   [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.

   [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.

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

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   [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
   1899 Wyknoop Street, Suite 600
   Denver, CO  80202

   Phone: +1-303-308-3282

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