Network Working GroupG. Klyne
Internet-DraftMIMEsweeper Group
Expires: March 20, 2002Sep 19, 2001

A URN sub-namespace for media feature tags

Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.


1 This specification defines a form of URI to be used to identify email and MIME message headers, defined by RFC 2822 and related documents.


Table of Contents


1. Introduction

2 This specification defines a form of URI to be used to identify email and MIME message headers, defined by RFC 2822 [19] and related documents. The URI scheme used is 'urn:' as defined by RFC 2141 [11], and the URN namespace used is 'ietf', defined by RFC 2648 [18] and extended by RFC ???? [25] to include names for IETF protocol parameters.

1.1 Background

3 RFC 2822 [19] defines a general syntax for Internet message headers. It also defines a number of headers to be used with Internet mail. Other specifications define additional headers that can appear in an Internet message: RFC 1036 [1], RFC 2156 [12], RFC 1496 [2], RFC 1766 [4], RFC 2183 [13], RFC 1864 [5], RFC 2421[15] and RFC 2045 [6], to name a few.

4 Internet message headers convey a range of message metadata information that can be useful to applications other than message transfer programs. Many programs are being developed to use XML [22] as their main data exchange format. Using XML namespace URIs [23] and a URI+local part concatenation convention introduced by RDF [24], each element- or attribute-name in an XML document can be identified with a URI.

5 A goal of this specification is to define a URI form for Intenet message headers that allows them to be incorporated into XML formatted data, as in "An XML format for mail and other messages" [26]. It also provides a URI form for use with non-XML specifications that employ the URI-based XML namespace extensibility model, such as the commpon profile for instant messaging format specification [27].


2. Registration template

6 The URN sub-namespace for message headers is defined as follows.

Registry name:
7 rfc822
8 RFC 2822 [16].
9 The primary definition of Internet message headers is RFC 2822 [16]. Currently, there is no IANA registry for Internet message headers, and additional header names are defined in a variety of standards-track RFC documents, including: RFC 1036 [1], RFC 1496 [2], RFC 1505 [3], RFC 1766 [4], RFC 1864 [5], RFC 2156 [12], RFC 2183 [13], RFC 2045 [6], RFC 2110 [8], RFC 2298 [9], RFC 2369 [10], RFC 2421 [15], RFC 2912 [20] and RFC 2919 [21].

A summary of defined message headers can be found in RFC 2076 [7], and updates [28].
Index value:
10 The header name is the index value. RFC 2822 allows a header name to contain any printable, non-space US-ASCII character (i.e., characters that have US-ASCII codes between 33 and 126, inclusive), except colon (":"). Header names are case-insensitive.
URN formation:
11 The URN for a media feature tag is formed as: "urn:ietf:params:rfc822:<header-name>", where <header-name> is the message header index value.

RFC 2141 [11] defines the format of URNs. Allowable characters include upper- or lower-case ASCII letters, decimal digits, "(", ")", "+", ",", "-", ".", ":", "=", "@", ";", "$", "_", "!", "*" and "'". Any other character that appears as part of a header name should be replaced by its %-encoded equivalent (per URI specification [14]); e.g. '%' is represented as '%25', and '/' as '%2f'.

URNs are defined by RFC 2141 [11] as lexically equivalent if they are identical following case normalization of the urn scheme name, the namespace name and any %-escaping used. Header names are defined such that upper- and lower-case ASCII characters are not distinguished. Thus, in forming a URN, all ASCII characters in the header name must be expressed in lower case.


3. Examples

12 This table lists some header names, and the corresponding urn: URIs.

13 urn:ietf:params:rfc822:from
14 urn:ietf:params:rfc822:to
15 urn:ietf:params:rfc822:x-envelope-from
16 urn:ietf:params:rfc822:content-md5


4. IANA considerations

17 This document calls for the creation of a new IETF sub-namespace, per RFC???? [25]. Registration details are in the preceding section.


5. Security considerations

18 No security considerations are introduced by the specification beyond those already inherrent in use of media feature tags.



[1] Horton, M. and R. Adams, "Standard for interchange of USENET messages", RFC 1036, December 1987.
[2] Alvestrand, H., Jordan, K. and J. Romaguera, "Rules for downgrading messages from X.400/88 to X.400/84 when MIME content-types are present in the messages", RFC 1496, August 1993.
[3] Costanzo, A., Robinson, D. and R. Ullmann, "Encoding Header Field for Internet Messages", RFC 1505, August 1993.
[4] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995.
[5] Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864, October 1995.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.
[7] Palme, J., "Common Internet Message Headers", RFC 2076, February 1997.
[8] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)", RFC 2110, March 1997.
[9] Fajman, R., "An Extensible Message Format for Message Disposition Notifications", RFC 2298, March 1998.
[10] Baer, J. and G. Neufeld, "The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields", RFC 2369, July 1998.
[11] Moats, R., "URN Syntax", RFC 2141, May 1997.
[12] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME", RFC 2156, January 1998.
[13] Moore, K., Troost, R. and S. Dorner, "Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field", RFC 2183, August 1997.
[14] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[15] Parsons, G. and G. Vaudreuil, "Voice Profile for Internet Mail - version 2", RFC 2421, September 1998.
[16] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag Registration Procedure", BCP 31, RFC 2506, March 1999.
[17] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC 2533, March 1999.
[18] Moats, R., "A URN Namespace for IETF Documents", RFC 2648, August 1999.
[19] Resnick, P. and , "Internet Message Format", RFC 2822, April 2001.
[20] Klyne, G., "Indicating Media Features for MIME Content", RFC 2912, September 2000.
[21] Chandhok, R. and G. Wenger, "List-Id: A Structured Field and Namespace for the Identification of Mailing Lists", RFC 2919, April 2001.
[22] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C Recommendation xml, October 2000.
[23] Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", W3C Recommendation xml-names, January 1999.
[24] Lassila, O. and R. Swick, "Resource Description Framework (RDF) Model and Syntax Specification", W3C Recommendation rdf-syntax, February 1999.
[25] Mealling, M., Masinter, L., Hardie, T. and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", Internet draft draft-mealling-iana-urn-01, August 2001.
[26] Klyne, G., "An XML format for mail and other messages", Internet draft draft-klyne-message-rfc822-xml-02, September 2001.
[27] Klyne, G., "Common Presence and Instant Messaging: Message Format", Internet draft draft-ietf-impp-cpim-msgfmt-03, June 2001.
[28] Palme, J., "Common Internet Message Header Fields", Internet draft draft-palme-mailext-headers-05, May 2001.


Author's Address

  Graham Klyne
  MIMEsweeper Group
  1310 Waterside
  Arlington Business Park
  Theale, Reading RG7 4SA
Phone:  +44 118 903 8000
Fax:  +44 118 903 9000


Appendix A. Revision history

56 (This section to be removed on final publication)

A.1 draft-klyne-urn-ietf-rfc822-00


58 19-Sep-2001: document initially created.


Appendix B. Todo

59 (This section to be removed on final publication)


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