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Versions: 00 01 02 03 04 05 06 07 08 RFC 3862

Network Working Group                  D. Atkins, Telcordia Technologies
Internet Draft                          G. Klyne, Baltimore Technologies
                                                         5 November 2001
                                                       Expires: May 2002


         Common Presence and Instant Messaging: Message Format
                  <draft-ietf-impp-cpim-msgfmt-04.txt>


Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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Copyright Notice

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

Abstract

   This memo defines the mime type 'Message/CPIM', a message format for
   protocols that conform to the Common Profile for Instant Messaging
   (CPIM) specification.

Discussion of this document

   Please send comments to:  <impp@iastate.edu>.  To subscribe:  send a
   message with the body 'subscribe' to <impp-request@iastate.edu>.  The
   mailing list archive is at <http://www.imppwg.org>.




Atkins & Klyne                                                  [Page 1]


Internet Draft             CPIM Message Format             November 2001


Table of Contents

   1. INTRODUCTION
   1.1 Motivation
   1.2 Background
   1.3 Goals
   1.4 Terminology and conventions
   2. OVERALL MESSAGE STRUCTURE
   2.1 Message/CPIM MIME headers
   2.2 Message headers
   2.3 Character escape mechanism
   2.4 Message content
   3. MESSAGE HEADER SYNTAX
   3.1 Header names
   3.2 Header Value
   3.3 Language Tagging
   3.4 Namespaces for header name extensibility
   3.5 Mandatory-to-recognize features
   3.6 Collected message header syntax
   4. HEADER DEFINITIONS
   4.1 The 'From' header
   4.2 The 'To' header
   4.3 The 'cc' header
   4.4 The 'DateTime' header
   4.5 The 'Subject' header
   4.6 The 'NS' header
   4.7 The 'Require' header
   5. EXAMPLES
   5.1 An example Message/CPIM message
   5.2 An example using MIME multipart/signed
   6. APPLICATION DESIGN CONSIDERATIONS
   7. IANA CONSIDERATIONS
   7.1 Registration for Message/CPIM content type
   7.2 Registration for urn:ietf:params:cpim-headers:
   8. INTERNATIONALIZATION CONSIDERATIONS
   9. SECURITY CONSIDERATIONS
   10. ACKNOWLEDGEMENTS
   11. REFERENCES
   12. AUTHORS' ADDRESSES
   Appendix A: Amendment history
   Full copyright statement










Atkins & Klyne                                                  [Page 2]


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1. INTRODUCTION

   This memo defines the mime content-type 'Message/CPIM'.  This is a
   common message format for CPIM-compliant messaging protocols [14].

   While being prepared for CPIM, this format is quite general and may
   be reused by other applications with similar requirements.
   Application specifications that adopt this as a base format should
   answer the questions raised in section 6 of this document.

1.1 Motivation

   The Common Profile for Instant Messaging (CPIM) [14] specification
   defines a number of operations to be supported and criteria to be
   satisfied for interworking diverse instant messaging protocols.  The
   intent is to allow a variety of different protocols interworking
   through gateways to support cross-protocol messaging that meets the
   requirements of RFC 2779 [15].

   To adequately meet the security requirements of RFC 2779, a common
   message format is needed so that end-to-end signatures and encryption
   may be applied.  This document describes a common canonical message
   format that must be used by any CPIM-compliant message transfer
   protocol, and over which signatures are calculated for end-to-end
   security.

1.2 Background

   RFC 2779 requires that an instant message can carry a MIME payload
   [3,4];  thus some level of support for MIME will be a common element
   of any CPIM compliant protocol.  Therefore it seems reasonable that a
   common message format should use a MIME/RFC822 syntax, as protocol
   implementations must already contain code to parse this.

   Unfortunately, using pure RFC2822/MIME [2] can be problematic:

   o  Irregular lexical structure -- RFC2822 allows a number of optional
      encodings and multiple ways to encode a particular value.  For
      example RFC2822 comments may be encoded in multiple ways.  For
      security purposes, a single encoding method must be defined as a
      basis for computing message digest values.  Protocols that
      transmit data in a different format would otherwise lose
      information needed to verify a signature.

   o  Weak internationalization -- RFC2822 requires header values to use
      7-bit ASCII, which is problematic for encoding international
      character sets.  Mechanisms for language tagging in RFC2822
      headers [16] are awkward to use and have limited applicability.



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   o  Mutability -- addition, modification or removal of header
      information.  Because it is not explicitly forbidden, many
      applications that process MIME content (e.g. MIME gateways)
      rebuild or restructure messages in transit.  This obliterates most
      attempt at achieving security (e.g. signatures), leaving receiving
      applications unable to verify the received data.

   o  Message and payload separation -- there is not a clear syntactic
      distinction between message metadata and message content.

   o  Limited extensibility (X-headers are problematic).

   o  No support for structured information (text string values only).

   o  Some processors impose line length limitations The message format
      defined by this memo overcomes some of these difficulties by
      having a syntax that is generally compatible with the format
      accepted by MIME/RFC822 parsers, but simplified, and having a
      stricter syntax.  It also defines mechanisms to support some
      desired features not covered by the RFC822/MIME format
      specifications.


1.3 Goals

   This specification aims to satisfy the following goals:

   o  a securable end-to-end format for a message (a canonical message
      format for signature calculation)

   o  independent of any specific application

   o  capable of conveying a range of different address types

   o  assumes an 8-bit clean message-transfer protocol

   o  evolvable:  extensible by multiple parties

   o  to clearly separate message metadata from message content

   o  a simple, regular, easily parsed syntax

   o  a compact, low-overhead format for simple messages








Atkins & Klyne                                                  [Page 4]


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1.4 Terminology and conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

         NOTE:  Comments like this provide additional nonessential
         information about the rationale behind this document.
         Such information is not needed for building a conformant
         implementation, but may help those who wish to understand
         the design in greater depth.

   [[[Editorial comments and questions about outstanding issues are
   provided in triple brackets like this.  These working comments should
   be resolved and removed prior to final publication.]]]


2. OVERALL MESSAGE STRUCTURE

   The Message/CPIM format encapsulates an arbitrary MIME message
   content, together with message- and content-related metadata.  This
   can optionally be signed or encrypted using MIME security multiparts
   in conjunction with an appropriate security scheme.

   A Message/CPIM object is a multipart entity, where the first part
   contains the message metadata and the second part is the message
   content.  The two parts are syntactically separated by a blank line,
   to keep the message header information (with its more stringent
   syntax rules) separate from the MIME message content headers.

   Thus, the complete message looks something like this:

      m: Content-type: Message/CPIM
      s:
      h: (message-metadata-headers)
      s:
      e: (encapsulated MIME message-body)

   The end of the message body is defined by the framing mechanism of
   the protocol used.  The tags 'm:', 's:', 'h:', 'e:', and 'x:' are not
   part of the message format and are used here to indicate the
   different parts of the message, thus:

      m:  MIME headers for the overall message
      s:  a blank separator line
      h:  message headers
      e:  encapsulated MIME object containing the message content
      x:  MIME security multipart message wrapper



Atkins & Klyne                                                  [Page 5]


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2.1 Message/CPIM MIME headers

   The message MIME headers identify the message as a CPIM-formatted
   message.  The only required header is:

      Content-type: Message/CPIM

   Other MIME headers may be used as appropriate for the message
   transfer environment.

2.2 Message headers

   Message headers carry information relevant to the end-to-end transfer
   of the message from sender to receiver.  Message headers MUST NOT be
   modified, reformatted or reordered in transit, but in some
   circumstances they MAY be examined by a CPIM message transfer
   protocol.

   The message headers serve a similar purpose to RFC2822 message
   headers in email [2], and have a similar but restricted allowable
   syntax.

   The basic header syntax is:

      Key: Value

   where "Key" is a header name and "Value" is the corresponding header
   value.  The following considerations apply:

   o  The entire header MUST be contained on a single line.  The line
      terminator is not considered part of the header value.

   o  Only one header per line.  Multiple headers MUST NOT be included
      on a single line.

   o  Processors SHOULD NOT impose any line-length limitations.

   o  There MUST NOT be any whitespace at the beginning or end of a
      line.

   o  UTF-8 character encoding [21] MUST be used throughout.

   o  The character sequence CR,LF (13,10) MUST be used to terminate
      each line.

   o  The header name contains only US-ASCII characters (see later for
      the specific syntax)




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   o  The header MUST NOT contain any control characters (0-31).  If a
      header value needs to represent control characters then the escape
      mechanism described below MUST be used.

   o  There MUST be a single space character (32) following the header
      name and colon.

   o  Multiple headers using the same key (header name) are allowed.
      (Specific header semantics may dictate only one occurrence of any
      particular header.)

   o  Headers names MUST match exactly (i.e. "From:" and "from:" are
      different headers).

   o  If a header name is not recognized or not understood, the header
      should be ignored.  But see also the "Requires:" header.

   o  Interpretation (e.g. equivalence) of header values is dependent on
      the particular header definition.  Message processors MUST
      preserve exactly all octets of all headers (both name and value).

   o  Message processors MUST NOT change the order of message headers.

   Examples:

      To: Pooh Bear <im:pooh@100akerwood.com>
      From: <im:piglet@100akerwood.com>
      Date: 2001-02-02T10:48:54-05:00

2.3 Character escape mechanism

   This mechanism MUST be used to code control characters in a header,
   having Unicode code points in the range U+0000 to U+001f or U+007f.
   (The escape mechanism is as used by the Java programming language.)
   Note that the escape mechanism is applied to a UCS-2 character, NOT
   to the octets of its UTF-8 coding.  Mapping from/to UTF-8 coding is
   performed without regard for escape sequences or character coding.
   (The header syntax is defined so that octets corresponding to control
   characters other than CR and LF do not appear in the output.)












Atkins & Klyne                                                  [Page 7]


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   An arbitrary UCS-2 character is escaped using the form:

      \uxxxx

   where:

      \    is U+005c (backslash)
      u    is U+0075 (lower case letter U)
      xxxx is a sequence of exactly four hexadecimal digits
           (0-9, a-f or A-F) or
           (U+0030-U+0039, U+0041-U+0046, or U+0061-0066)

   The hexadecimal number 'xxxx' is the UCS code-point value of the
   escaped character.

   Further, the following special sequences introduced by "\" are used:

      \\   for \ (backslash, U+005c)
      \"   for " (double quote, U+0022)
      \'   for ' (single quote, U+0027)
      \b   for backspace (U+0008)
      \t   for tab (U+0009)
      \n   for linefeed (U+000a)
      \r   for carriage return (U+000d)

2.3.1 Escape mechanism usage

   When generating messages conformant with this specification:

   o  The special sequences listed above MUST be used to encode any
      occurrence of the following characters that appear anywhere in a
      header: backslash (U+005c), backspace (U+0008), tab (U+0009),
      linefeed (U+000a) or carriage return (U+000d).

   o  The special sequence \' MUST be used for any occurrence of a
      single quote (U+0027) that appears within a string delimited by
      single quotes.

   o  The special sequence \" MUST be used for any occurrence of a
      double quote (U+0022) that appears within a string delimited by
      double quotes.

   +  Quote characters that delimit a string value MUST NOT be escaped.

   o  The general escape sequence \uxxxx MUST be used for any other
      control character (U+0000 to U+0007, U+000b to U+000c, U+000e to
      U+001f or u+007f) that appears anywhere in a header.




Atkins & Klyne                                                  [Page 8]


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   o  All other characters MUST NOT be represented using an escape
      sequence.

   When processing a message based on this specification, the escape
   sequence usage described above MUST be recognized.

   Further, any other occurrence of any escape sequence described above
   SHOULD be recognized and treated as an occurrence of the
   corresponding Unicode character.

   Any backslash ('\') character SHOULD be interpreted as introducing an
   escape sequence.  Any unrecognized escape sequence SHOULD be treated
   as an instance of the character following the backslash character.
   An isolated backslash that is the last character of a header SHOULD
   be ignored.

2.4 Message content

   The final section of a Message/CPIM is the MIME-encapsulated message
   content, which follows standard MIME formatting rules [3,4].

   The MIME content headers MUST include at least a Content-Type header.
   The content may be any MIME type.

   Example:

      e: Content-Type: text/plain; charset=utf-8
      e: Content-ID: <1234567890@foo.com>
      e:
      e: This is my encapsulated text message content


3. MESSAGE HEADER SYNTAX

   A header is made of two parts, a name and a value, separated by a
   colon character (':') followed by a single space (32), and terminated
   by a sequence of CR,LF (13,10).

   Headers use UTF-8 character encoding thoughout, per RFC 2279 [21].

3.1 Header names

   The header name is a sequence of US-ASCII characters, excluding
   control characters, SPACE or separator characters.  Use of the
   character "." in a header name is reserved for a namespace prefix
   separator.





Atkins & Klyne                                                  [Page 9]


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   Separator characters are:

      SEPARATORS   = "(" / ")" / "<" / ">" / "@"
                   / "," / ";" / ":" / "
                   / "/" / "[" / "]" / "?" / "="
                   / "{" / "}" / SP

         NOTE:  the range of allowed characters was determined by
         examination of HTTP and RFC822 header name formats and
         choosing the more resticted.  The intent is to allow CPIM
         headers to follow a syntax that is compatible with the
         allowed syntax for both RFC 2822 [2] and HTTP [18]
         (including HTTP-derived protocols such as SIP).

3.2 Header Value

   A header value has a structure defined by the corresponding header
   specification.  Implementations that use a particular header must
   adhere to the format and usage rules thus defined when creating or
   processing a message containing that header.

   The other general constraints on header formats MUST also be followed
   (one line, UTF-8 character encoding, no control characters, etc.)

3.3 Language Tagging

   Full internationalization of a protocol requires that a language can
   be indicated for any human-readable text [6,19].

   A message header may indicate a language for its value by including
   ';lang=tag' after the header name and colon, where 'tag' is a
   language identifying token per RFC 3066 [7].

   Example:

      Subject:;lang=fr Objet de message

   If the language parameter is not applied a header, any human-
   readable text is assumed to use the language identified as
   'i-default' [19].

3.4 Namespaces for header name extensibility

         NOTE: this section defines a framework for header
         extensibility whose use is optional.  If no header
         extensions are allowed by an application then these
         structures may never be used.




Atkins & Klyne                                                 [Page 10]


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   An application that uses this message format is expected to define
   the set of headers that are required and allowed for that
   application.  This section defines a header extensibility framework
   that can be used with any application.

   The extensibility framework is based on that provided for XML [11] by
   XML namespaces [12].  All headers are associated with a "namespace",
   which is in turn associated with a globally unique URI.

   Within a particular message instance, header names are associated
   with a particular namespace through the presence or absence of a
   namespace prefix, which is a leading part of the header name followed
   by a period ("."); e.g.

      prefix.header-name: header-value

   Here, 'prefix' is the header name prefix, 'header-name' is the header
   name within the namespace associated with 'prefix', and
   'header-value' is the value for this header.

      header-name: header-value

   In this case, the header name prefix is absent, and the given
   'header-name' is associated with a default namespace.

   The Message/CPIM media type registration designates a default
   namespace  for any headers that are not more explicitly associated
   with any namespace.  In many cases, this default namespace is all
   that is needed.

   A namespace is identified by a URI.  In this usage, the URI is used
   simply as a globally unique identifier, and there is no requirement
   that it can be used for any other purpose.  Any legal globally unique
   URI MAY be used to identify a namespace.  (By "globally unique", we
   mean constructed according to some set of rules so that it is
   reasonable to expect that nobody else will use the same URI for a
   different purpose.)  A URI used as an identifier MUST be a full
   absolute-URI, per RFC 2396 [10].  (Relative URIs and URI- references
   containing fragment identifiers MUST NOT be used for this purpose.)












Atkins & Klyne                                                 [Page 11]


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   Within a specific message, a 'NS' header is used to declare a
   namespace prefix and associate it with a URI that identifies a
   namespace.  Following that declaration, within the scope of that
   message, the combination of namespace prefix and header name
   indicates a globally unique identifier for the header (consisting of
   the namespace URI and header name).  For example:

      NS: MyFeatures <mid:MessageFeatures@id.foo.com>
      MyFeatures.WackyMessageOption: Use-silly-font

   This defines a namespace prefix 'MyFeatures' associated with the
   namespace identifier 'mid:MessageFeatures@id.foo.com'.  Subsequently
   the prefix indicates that the WackyMessageOption header name
   referenced is associated with the identified namespace.

   A namespace prefix declaration MUST precede any use of that prefix.

   With the exception of any application-specific predefined namespace
   prefixes (see section 6), a namespace prefix is strictly local to the
   message in which it occurs.  The actual prefix used has no global
   significance.  This means that the headers:

      xxx.name: value
      yyy.name: value

   in two different messages may have exactly the same effect if
   namespace prefixes 'xxx' and 'yyy' are associated with the same
   namespace URI.  Thus the following have exactly the same meaning:

      NS: acme <http://id.acme.widgets/wily-headers/>
      acme.runner-trap: set

   and

      NS: widget <http://id.acme.widgets/wily-headers/>
      widget.runner-trap: set

   A 'NS' header without a header prefix name specifies a default
   namespace for subsequent headers;  that is a namespace that is
   associated with header names not having a prefix.  For example:

      NS: <http://id.acme.widgets/wily-headers/>
      runner-trap: set

   has the same meaning as the previous examples.






Atkins & Klyne                                                 [Page 12]


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   This framework allows different implementers to create extension
   headers without the worry of header name duplication;  each defines
   headers within their own namespace.

3.5 Mandatory-to-recognize features

   Sometimes it is necessary for the sender of a message to insist that
   some functionality is understood by the recipient.  By using the
   mandatory-to-recognize indicator, a sender is notifying the recipient
   that it MUST understand the named header or feature in order to
   properly understand the message.

   A header or feature is indicated as being mandatory-to-recognize by a
   'Require:' header.  For example:

      Require: MyFeatures.VitalMessageOption
      MyFeatures.VitalMessageOption: Confirmation-requested

   Multiple required header names may be listed in a single 'Require'
   header, separated by commas.

         NOTE:  indiscriminate use of 'Require:' headers could
         harm interoperability.  It is suggested that any
         implementer who defines required headers also publish the
         header specifications so other implementations can
         succesfully interoperate.

   The 'Require:' header MAY also be used to indicate that some non-
   header semantics must be implemented by the recipient, even when it
   does not appear as a header.  For example:

      Require: Locale.MustRenderKanji

   might be used to indicate that message content includes characters
   from the Kanji repertoire, which must be rendered for proper
   understanding of the message.  In this case, the header name is just
   a token (using header name syntax and namespace association) that
   indicates some desired behaviour.

3.6 Collected message header syntax

   The following description of message header syntax uses ABNF, per RFC
   2234 [17].  Most of this syntax can be interpreted as defining UCS
   character sequences or UTF-8 octet sequences.  Alternate productions
   at the end allow for either interpretation.

   NOTE:  specified text values MUST be used exactly as given, using
   exactly the indicated upper- and lower-case letters.  In this



Atkins & Klyne                                                 [Page 13]


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   respect, the ABNF usage here differs from RFC 2234.

      Header       = Header-name ":" *( ";" Parameter ) SP
                     Header-value
                     CRLF

      Header-name  = [ Name-prefix "." ] Name
      Name-prefix  = Name

      Parameter    = Lang-param / Ext-param
      Lang-param   = "lang=" Language-tag
      Ext-param    = Param-name "=" Param-value
      Param-name   = Name
      Param-value  = Token / Number / String

      Header-value = *HEADERCHAR

      Name         = 1*NAMECHAR
      Token        = 1*TOKENCHAR
      Number       = 1*DIGIT
      String       = DQUOTE *( Str-char / Escape ) DQUOTE
      Str-char     = %x20-21 / %x23-5B / %x5D-7E / UCS-high
      Escape       = "\" ( "u" 4(HEXDIG)    ; UCS codepoint
                         / "b"              ; Backspace
                         / "t"              ; Tab
                         / "n"              ; Linefeed
                         / "r"              ; Return
                         / DQUOTE           ; Double quote
                         / "'"              ; Single quote
                         / "\" )            ; Backslash

      Formal-name  = 1*( Token SP ) / String
      URI          = <defined as absolute-URI by RFC 2396>
      Language-tag = <defined by RFC 3066>

                   ; Any UCS character except CTLs, or escape
      HEADERCHAR   = UCS-no-CTL / Escape

                   ; Any US-ASCII char except ".", CTLs or SEPARATORS:
      NAMECHAR     = %21 / %23-26 / %2a-2b / %2d / %5e-60 / %7c / %7e
                   / ALPHA / DIGIT

                   ; Any UCS char except CTLs or SEPARATORS:
      TOKENCHAR    = NAMECHAR / "." / UCS-high







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      SEPARATORS   = "(" / ")" / "<" / ">" / "@"    ; 28/29/3c/3e/40
                   / "," / ";" / ":" / "\" / <">    ; 2c/3b/3a/5c/22
                   / "/" / "[" / "]" / "?" / "="    ; 2f/5b/5d/3f/3d
                   / "{" / "}" / SP                 ; 7b/7d/20
      CTL          = <Defined by RFC 2234 -- %x0-%x1f, %x7f>
      CRLF         = <Defined by RFC 2234 -- CR, LF>
      SP           = <defined by RFC 2234 -- %x20>
      DIGIT        = <defined by RFC 2234 -- '0'-'9'>
      HEXDIG       = <defined by RFC 2234 -- '0'-'9', 'A'-'F', 'a'-'f'>
      ALPHA        = <defined by RFC 2234 -- 'A'-'Z', 'a'-'z'>
      DQUOTE       = <defined by RFC 2234 -- %x22>

   To interpret the syntax in a general UCS character environment, use
   the following productions:

      UCS-no-CTL   = %x20-7e / UCS-high
      UCS-high     = %x80-ffffffff

   To interpret the syntax as defining UTF-8 coded octet sequences, use
   the following productions:

      UCS-no-CTL   = UTF8-no-CTL
      UCS-high     = UTF8-multi
      UTF8-no-CTL  = %x20-7e / UTF8-multi
      UTF8-multi   = %xC0-DF %x80-BF
                   / %xE0-EF %x80-BF %x80-BF
                   / %xF0-F7 %x80-BF %x80-BF %x80-BF
                   / %xF8-FB %x80-BF %x80-BF %x80-BF %x80-BF
                   / %xFC-FD %x80-BF %x80-BF %x80-BF %x80-BF %x80-BF


4. HEADER DEFINITIONS

   This specification defines a core set of headers that are defined and
   available for use by applications:  the application specification
   must indicate the headers that may be used, those that must be
   recognized and those that must appear in any message (see section 6).

   The header definitions that follow fall into two categories:

   (a) those that are part of the CPIM format extensibility framework,
       and

   (b) some that have been based on similar headers in RFC 2822 [2],
       specified here with corresponding semantics.

   Header names and syntax are described without a namespace
   qualification, and the associated namespace URI is listed as part of



Atkins & Klyne                                                 [Page 15]


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   the header specification.  Any of the namespace associations already
   mentioned (implied default namespace, explicit default namespace or
   implied namespace prefix or explicit namespace prefix declaration)
   may be used to identify the namespace.

   All headers defined here are associated with the namespace URI
   <urn:ietf:params:cpim-headers:>, which is defined according to [22].

   NOTE:  Header names and other text MUST be used exactly as given,
   using exactly the indicated upper- and lower-case letters.  In this
   respect, the ABNF usage here differs from RFC 2234 [17].

4.1 The 'From' header

   Indicates the sender of a message.

   Header name:   From

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      From-header = "From" ": " [ Formal-name ] "<" URI ">"

   Description:

      Indicates the sender or originator of a message.

      If present, the 'Formal-name' identifies the person or "real
      world" name for the originator.

      The URI indicates an address for the originator.

   Examples:

      From: Winnie the Pooh <im:pooh@100akerwood.com>

      From: <im:tigger@100akerwood.com>













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4.2 The 'To' header

   Specifies an intended recipient of a message.

   Header name:   To

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      To-header = "To" ": " [ Formal-name ] "<" URI ">"

   Description:

      Indicates the recipient of a message.

      If present, the 'Formal-name' identifies the person or "real
      world" name for the recipient.

      The URI indicates an address for the recipient.

      Multiple recipients may be indicated by including multiple 'To'
      headers.

   Examples:

      To: Winnie the Pooh <im:pooh@100akerwood.com>

      To: <im:tigger@100akerwood.com>

4.3 The 'cc' header

   Specifies a non-primary recipient ("courtesy copy") for a message.

   Header name:   cc

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      Cc-header   = "cc" ": " [ Formal-name ] "<" URI ">"

   Description:

      Indicates a courtesy copy recipient of a message.

      If present, the 'Formal-name', if present, identifies the person
      or "real world" name for the recipient.



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      The URI indicates an address for the recipient.

      Multiple courtesy copy recipients may be indicated by including
      multiple 'cc' headers.

   Examples:

      cc: Winnie the Pooh <im:pooh@100akerwood.com>

      cc: <im:tigger@100akerwood.com>

4.4 The 'DateTime' header

   Specifies the date and time a message was sent.

   Header name:   Date

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax:

      DateTime-header = "DateTime" ": " date-time

      (where the syntax of 'date-time' is a profile of ISO8601, defined
      in "Date and Time on the Internet" [23])

   Description:

      The 'Date' header supplies the current date and time at which the
      sender sent the message.

      One purpose of the this header is to provide for protection
      against a replay attack, by allowing the recipient to know when
      the message was intended to be sent.  The value of the date header
      is the current time at the sender when the message was
      transmitted, using ISO 8601 date and time format as profiles in
      "Date and Time on the Internet: Timestamps" [23].

   Example:

      Date: 2001-02-01T12:16:49-05:00










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4.5 The 'Subject' header

   Contains a description of the topic of the message.

   Header name:   Subject

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      Subject-header = "Subject" ":" [ ";" lang-param ] SP *HEADERCHAR

   Description:

      The 'Subject' header supplies the sender's description of the
      topic or content of the message.

      The sending agent should specify the language parameter if it has
      any reasonable knowledge of the language used by the sender to
      describe the message.

   Example:

      Subject:;lang=en Eeyore's feeling very depressed today

4.6 The 'NS' header

   The "NS" header is used to declare a local namespace prefix.

   Header name:   NS

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      NS-header = "NS" ": " [ Name-prefix ] "<" URI ">"

   Description:

      Declares a namespace prefix that may be used in subsequent header
      names.  See section 3.4 for more details.

   Example:

      NS: MyAlias <mid:MessageFeatures@id.foo.com>
      MyAlias.MyHeader: private-extension-data





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4.7 The 'Require' header

   Specify a header or feature that must be implemented by the receiver
   for correct message processing.

   Header name:   Require

   Namespace URI: <urn:ietf:params:cpim-headers:>

   Syntax: (see also section 3.6)

      Require-header = "Require" ": " Header-name *( "," Header-name )

   Description:

      Declares a namespace prefix that may be used in subsequent header
      names. See section 3.5 for more details.

      Note that there is no requirement that the required header
      actually be used, but for brevity it is recommended that an
      implemention not use issue require header for unused headers.

   Example:

      Require: MyAlias.VitalHeader


5. EXAMPLES

   The examples in the following sections use the following per-line
   tags to indicate different parts of the overall message format:

      m:  MIME headers for the overall message
      s:  a blank separator line
      h:  message headers
      e:  encapsulated MIME object containing the message content
      x:  MIME security multipart message wrapper

   The following examples also assume that <urn:ietf:params:cpim-
   headers:> is the implied default namespace for the application
   concerned.










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5.1 An example Message/CPIM message

   The following example shows a Message/CPIM message:

      m: Content-type: Message/CPIM
      s:
      h: From: MR SANDERS <im:piglet@100akerwood.com>
      h: To: Depressed Donkey <im:eeyore@100akerwood.com>
      h: Date: 2000-12-13T13:40:00-08:00
      h: Subject: the weather will be fine today
      h: Subject:;lang=fr beau temps prevu pour aujourd'hui
      h: NS: MyFeatures <mid:MessageFeatures@id.foo.com>
      h: Require: MyFeatures.VitalMessageOption
      h: MyFeatures.VitalMessageOption: Confirmation-requested
      h: MyFeatures.WackyMessageOption: Use-silly-font
      s:
      e: Content-type: text/xml; charset=utf-8
      e: Content-ID: <1234567890@foo.com>
      e:
      e: <body>
      e: Here is the text of my message.
      e: </body>

5.2 An example using MIME multipart/signed

   In order to secure a Message/CPIM, an application or implementation
   should use RFC 1847 and some appropriate cryptographic scheme.

   Using S/MIME and pkcs7, the above message would look like this:

      x: Content-Type: multipart/signed; boundary=next;
                       MDALG=SHA-1; type=application/pkcs
      x:
      x: --next
      m: Content-Type: Message/CPIM
      s:
      h: From: MR SANDERS <im:piglet@100akerwood.com>
      h: To: Dopey Donkey <im:eeyore@100akerwood.com>
      h: Date: 2000-12-13T13:40:00-08:00
      h: Subject: the weather will be fine today
      h: Subject:;lang=fr beau temps prevu pour aujourd'hui
      h: NS: MyFeatures <mid:MessageFeatures@id.foo.com>
      h: Require: MyFeatures.VitalMessageOption
      h: MyFeatures.VitalMessageOption: Confirmation-requested
      h: MyFeatures.WackyMessageOption: Use-silly-font
      s:





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      e: Content-type: text/xml; charset=utf-8
      e: Content-ID: <1234567890@foo.com>
      e:
      e: <body>
      e: Here is the text of my message.
      e: </body>
      x: --next
      x: Content-Type: application/pkcs7
      x:
      x: (signature stuff)
          :
      x: --next--


6. APPLICATION DESIGN CONSIDERATIONS

   As defined, the 'Message/CPIM' content type uses a default namespace
   URI 'urn:ietf:params-cpim-headers:', and does not define any other
   implicit namespace prefixes.  Applications that have different
   requirements should define and register a different MIME media type,
   specify the required default namespace URI and define any implied
   namespace prefixes as part of the media type specification.

   Applications using this specification must also specify:

   o  all headers that must be recognized by implementations of the
      application

   o  any headers that must be present in messages created by that
      application.

   o  any headers that may appear more than once in a message, and how
      they are to be interpreted (e.g. how to interpret multiple
      'subject:' headers with different language parameter values).

   Within a network of message transfer agents, an intermediate gateway
   MUST NOT change the Message/CPIM content in any way.  This implies
   that headers cannot be changed or reordered, transfer encoding cannot
   be changed, languages cannot be changed, etc.

   Because Message/CPIM messages are immutable, any transfer agent that
   wants to modify the message should create a new Message/CPIM message
   with the modified header and containing the original message as its
   content.  (This approach is similar to real-world bill-of-lading
   handling, where each person in the chain attaches a new sheet to the
   message.  Then anyone can validate the original message and see what
   was changed and who changed it by following the trail of amendments.
   Another metaphor is including the old message in a new envelope.)



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7. IANA CONSIDERATIONS

   This memo calls for two new IANA registrations:

   o  A new MIME content-type value, Message/CPIM, per RFC 2048 [5].
      The registration template is at section 7.1 below.

   o  A new IANA URN sub-namespace, urn:ietf:params:cpim-headers:, per
      RFC [[[XXXX]]] [22].  The registration template is at section 7.2
      below.


7.1 Registration for Message/CPIM content type

   To:      ietf-types@iana.org
   Subject: Registration of MIME media type Message/CPIM

   MIME media type name:
         Message

   MIME subtype name:
         CPIM

   Required parameters:
         (None)

   Optional parameters:
         (None)

   Encoding considerations:
         Intended to be used in 8-bit clean environments, with non-
         transformative encoding (8-bit or binary, according to the
         content contained within the message;  the CPIM message headers
         can be handled in an 8-bit text environment).

         This content type could be used with a 7-bit transfer
         environment if appropriate transfer encoding is used.  NOTE
         that for this purpose, enclosed MIME content MUST BE treated as
         opaque data and encoded accordingly.  Any encoding must be
         reversed before any enclosed MIME content can be accessed.

   Security considerations:
         The content may contain signed data, so any transfer encoding
         MUST BE exactly reversed before the content is processed.

         See also the security considerations for email messages (RFC
         2822 [2]).




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   Interoperability considerations:
         This content format is intended to be used to exchange
         possibly-secured messages between different intant messaging
         protocols.  Very strict adherence to the message format
         (including whitespare usage) may be needed to achieve
         interoperability.

   Published specification:
         RFC XXXX [this document]

   Applications which use this media type:
         Instant messaging

   Additional information:
         The default namespace URI associated with this content-type is
         'urn:ietf:params:cpim-headers:'.  (See RFC XXXX [this document]
         for further details.)

         See also the Common Profile for Instant Messaging (CPIM) [14]

   Person & email address to contact for further information:
         G. Klyne, GK@ACM.ORG

   Intended usage:
         LIMITED USE

   Author/Change controller:
         IETF


7.2 Registration for urn:ietf:params:cpim-headers:


   Registry name:
         cpim-headers

   Specification:
         RFC XXXX [this document] Additional values may be defined by
         standards track RFCs that update or obsolete RFC XXXX [this
         document].

   Repository:
         ftp://ftp.isi.edu/in-notes/rfcXXXX.txt [this document]

   Index value:
         The index value is a CPIM message header name, which may
         consist of a sequence from a restricted set of US-ASCII
         characters, as define above.



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         The URI for a header is formed from its name by:

         (a) replacing any non-URN characters (as defined by RFC 2141
             [24]) with the corresponding '%hh' escape sequence (per RFC
             2396 [10]), and

         (b) prepending the resulting string with 'urn:ietf:params:cpim-
             headers:'.


         Thus, the URI corresponding to the CPIM message header 'From:'
         would be 'urn:ietf:params:cpim-headers:From'.  The URI
         corresponding to the (putative) CPIM message header 'Top&Tail'
         would be 'urn:ietf:params:cpim-headers:Top%26Tail'.



8. INTERNATIONALIZATION CONSIDERATIONS

   Message headers use UTF-8 character encoding throughout, so can
   convey the full UCS-4 (Unicode, ISO/IEC 10646) character repertoire.

   Language tagging is provided for message headers using the "Language"
   parameter.

   Message content is any MIME-encapsulated content, and normal MIME
   content internationalization considerations apply.


9. SECURITY CONSIDERATIONS

   The Message/CPIM format is designed with security in mind.  In
   particular it is designed to be used with MIME security multiparts
   for signatures and encryption.  To this end, Message/CPIM messages
   must be considered immutable once created.

   Because Message/CPIM messages are binary messages (due to UTF-8
   encoding), if they are transmitted across non-8-bit-clean transports
   then the transfer agent must tunnel the entire message.  Changing the
   message data encoding is not an allowable option.  This implies that
   the Message/CPIM must be encapsulated by the message tranfer system
   and unencapsulated at the receiving end of the tunnel.

   The resulting message must have no data loss due to the encoding and
   unencoding of the message.  For example, an application may choose to
   apply the MIME base64 content-transfer-encoding to the Message/CPIM
   object to meet this requirement.




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10. ACKNOWLEDGEMENTS

   The authors thank the following for their helpful comments:  Harald
   Alvestrand, Walter Houser, Leslie Daigle, [[[....]]]


11. REFERENCES

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

   [2]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [3]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [4]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046 November
        1996.

   [5]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
        Mail Extensions (MIME) Part Four: Registration Procedures", RFC
        2048, BCP 13, November 1996.

   [6]  Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson,
        R., Crispin, M., Svanberg, P., "Report from the IAB Character
        Set Workshop", RFC 2130, April 1997.

   [7]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        3066, January 2001.  (Defines Content-language header.)

   [8]  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
        2633, June 1999.

   [9]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
        Message Format", RFC 2440, November 1998.

   [10] Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform
        Resource Identifiers (URI): Generic Syntax", RFC 2396, August
        1998.

   [11] Tim Bray, Jean Paoli, and C. M. Sperberg-McQueen, "Extensible
        Markup Language (XML) 1.0", W3C recommendation:
        <http://www.w3.org/TR/REC-xml>, 10 February 1998.






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   [12] Tim Bray, Dave Hollander, and Andrew Layman, "Namespaces in
        XML", W3C recommendation: <http://www.w3.org/TR/REC-xml-names>,
        14 January 1999.

   [13] "Data elements and interchange formats - Information interchange
        - Representation of dates and times", ISO 8601:1988(E),
        International Organization for Standardization, June 1988.

   [14] Crocker, D.H., Diacakis, A., Mazzoldi, F., Huitema, C., Klyne,
        G., Rose, M.T., Rosenberg, J., Sparks, R. and H. Sugano, "A
        Common Profile for Instant Messaging (CPIM)", draft-thenine-im-
        common-00 (work in progress), August 2000.

   [15] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
        Messaging / Presence Protocol Requirements", RFC 2779, February
        2000.

   [16] N. Freed, K. Moore, "MIME Parameter Value and Encoded Word
        Extensions: Character Sets, Languages, and Continuations", RFC
        2231, November 1997.

   [17] D. Crocker, P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [18] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
        Leach, T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [19] Alvestrand, H, "IETF Policy on Character Sets and Languages",
        RFC 2277, BCP 18, January 1998.

   [20] Freed, N., and J. Postel, "IANA Charset Registration
        Procedures", BCP 19, RFC 2278, January 1998.

   [21] F. Yergeau, "UTF-8, a transformation format of ISO 10646", RFC
        2279 January 1998.

   [22] M. Mealling, L. Masinter, T. Hardie, G. Klyne, "An IETF URN Sub-
        namespace for Registered Protocol Parameters", draft-mealling-
        iana-urn-02.txt (work in progress), October 2001

   [23] C. Newman, G. Klyne, "Date and Time on the Internet:
        Timestamps", draft-ietf-impp-datetime-03.txt (work in progress),
        May 2001.

   [24] R. Moats, "URN Syntax", RFC 2141, May 1997.





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12. AUTHORS' ADDRESSES

   Derek Atkins
   Telcordia Technologies
   6 Farragut Ave
   Somerville, MA 02144
   USA.
   Telephone: +1 617 623 3745
   E-mail: warlord@research.telcordia.com
   E-mail: warlord@alum.mit.edu

   Graham Klyne
   MIMEsweeper Group,
   1310 Waterside,
   Arlington Business Park
   Theale
   Reading, RG7 4SA
   United Kingdom.
   Telephone: +44 118 903 8000
   Facsimile: +44 118 903 9000
   E-mail:    GK@ACM.ORG

Appendix A: Amendment history

   [[[RFC editor:  please remove this appendix on publication.]]]


   00a 01-Feb-2001 Memo initially created.

   00b 06-Feb-2001 Editorial review.  Reworked namespace framework
                   description.  Deferred specification of mandatory
                   headers to the application specification, allowing
                   this document to be less application-dependent.
                   Expanded references.  Replaced some text with ABNF
                   syntax descriptions.  Reordered some major sections.

   00c 07-Feb-2001 Folded in some review comments.  Fix up some syntax
                   problems.  Other small editorial changes.  Add some
                   references.

   01a 29-Mar-2001 Incorporate review comments.  State (simply) that
                   this is a canonical end-to-end format for the purpose
                   of signature calculation.  Defined escape mechanism
                   for control characters.  Header name parameters
                   placed after the ":".  Changed name of Date: header
                   to DateTime:.  Revised syntax to separate character-
                   level syntax from UTF-8 octet- level syntax.




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   01b 30-Mar-2001 State explicitly that unrecognized header names
                   should be ignored.  Remove text about
                   (non)significance of header order:  simply say that
                   order must be preserved.

   02a 30-May-2001 Updated reference to date/time draft.  Editorial
                   changes.

   03a 13-Jun-2001 Tighten up application of escape sequences.

   04a 05-Nov-2001 Add registration templates for Message/CPIM content
                   type, and urn:ietf:params:cpim-headers: URN space.
                   Update namespace identifier.  Revised application
                   design considerations to indicate that a default
                   namespace URI and any implicit prefixes are to be
                   specified by the MIME media type specification.
                   Noted variation of ABNF usage from RFC 2234 (exact
                   case only).  Add reference to URN syntax (RFC 2141).


   TODO:

   o  Remove remaining [[[editorial comments]]].

   o  Replace XXXX with assigned RFC numbers.  (Note that this memo
      depends on [22] progressing to RFC status.)


   REVIEW CHECKLIST:

   (Points to be checked or considered more widely on or before final
   review.)

   o  The desirability of a completely rigid syntax.

   o  Escape mechanism details.















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Full copyright statement

   Copyright (C) The Internet Society 2001.  All Rights Reserved.  This
   document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.

   However, this document itself may not be modified in any way, such as
   by removing the copyright notice or references to the Internet
   Society or other Internet organizations, except as needed for the
   purpose of developing Internet standards in which case the procedures
   for copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.  This
   document and the information contained herein is provided on an "AS
   IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
   FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
   LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL
   NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY
   OR FITNESS FOR A PARTICULAR PURPOSE.

























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