Network Working Group P. Saint-Andre
Internet-Draft Cisco
Intended status: Best Current Practice 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 circumstances.

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Table of Contents

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:

That rule was restated by [RFC1154] as follows:

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:

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 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 [RFC5064]):

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:

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 parameters can be given meaningless names such as UUIDs [RFC4122].

  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 name.

  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:

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.

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. 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.H., "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.S. 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.
[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.
[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: