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Network Working Group G. Klyne
Request for Comments: 3862 Nine by Nine
Category: Standards Track D. Atkins
IHTFP Consulting
August 2004
Common Presence and Instant Messaging (CPIM): Message Format
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This memo defines the MIME content type 'Message/CPIM', a message
format for protocols that conform to the Common Profile for Instant
Messaging (CPIM) specification.
Klyne & Atkins Standards Track [Page 1]
RFC 3862 CPIM: Message Format August 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Terminology and Conventions . . . . . . . . . . . . . . 5
2. Overall Message Structure . . . . . . . . . . . . . . . . . . 5
2.1. Message/CPIM MIME Headers . . . . . . . . . . . . . . . 6
2.2. Message Headers . . . . . . . . . . . . . . . . . . . . 6
2.3. Character Escape Mechanism . . . . . . . . . . . . . . . 8
2.3.1. Escape Mechanism Usage . . . . . . . . . . . . . 8
2.4. Message Content . . . . . . . . . . . . . . . . . . . . 9
3. Message Header Syntax . . . . . . . . . . . . . . . . . . . . 10
3.1. Header Names . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Header Value . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Language tagging . . . . . . . . . . . . . . . . . . . . 10
3.4. Namespaces for Header Name Extensibility . . . . . . . . 11
3.5. Mandatory-to-Recognize Features . . . . . . . . . . . . 13
3.6. Collected Message Header Syntax . . . . . . . . . . . . 14
4. Header Definitions . . . . . . . . . . . . . . . . . . . . . . 16
4.1. The 'From' Header . . . . . . . . . . . . . . . . . . . 16
4.2. The 'To' Header . . . . . . . . . . . . . . . . . . . . 17
4.3. The 'cc' Header . . . . . . . . . . . . . . . . . . . . 18
4.4. The 'DateTime' Header . . . . . . . . . . . . . . . . . 18
4.5. The 'Subject' Header . . . . . . . . . . . . . . . . . . 19
4.6. The 'NS' Header . . . . . . . . . . . . . . . . . . . . 20
4.7. The 'Require' Header . . . . . . . . . . . . . . . . . . 20
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. An Example Message/CPIM Message . . . . . . . . . . . . 21
5.2. An Example Esing MIME multipart/signed . . . . . . . . . 22
6. Application Design Considerations . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7.1. Registration for Message/CPIM Content Type . . . . . . . 24
7.2. Registration for urn:ietf:params:cpim-headers . . . . . 25
8. Internationalization Considerations . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Normative References. . . . . . . . . . . . . . . . . . 26
11.2. Informative References. . . . . . . . . . . . . . . . . 27
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
13. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 30
Klyne & Atkins Standards Track [Page 2]
RFC 3862 CPIM: Message Format August 2004
1. Introduction
This memo defines the MIME content type 'Message/CPIM', a message
format for protocols that conform to the Common Profile for Instant
Messaging (CPIM) specification. This is a common message format for
CPIM-compliant messaging protocols [26].
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
address the questions raised in section 6 of this document.
1.1. Motivation
The Common Profile for Instant Messaging (CPIM) [26] specification
defines a number of operations to be supported and criteria to be
satisfied for interworking between 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 [20].
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, whereby signatures are calculated for end-to-end security.
The design of this message format is intended to enable security to
be applied, while itself remaining agnostic about the specific
security mechanisms that may be appropriate for a given application.
For CPIM instant messaging and presence, specific security protocols
are specified by the CPIM instant messaging [26] and CPIM presence
[27] specifications.
Also note that the message format described here is not itself a MIME
data format, although it may be contained within a MIME object, and
may contain MIME objects. See section 2 for more details.
1.2. Background
RFC 2779 requires that an instant message can carry a MIME payload
[1][2]; 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 RFC2822/MIME-like syntax [9], as
protocol implementations must already contain code to parse this.
Unfortunately, using pure RFC2822/MIME can be problematic:
Klyne & Atkins Standards Track [Page 3]
RFC 3862 CPIM: Message Format August 2004
o Irregular lexical structure -- RFC2822/MIME allows a number of
optional encodings and multiple ways to encode a particular value.
For example, RFC2822/MIME 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/MIME requires header values
to use 7-bit ASCII, which is problematic for encoding
international character sets. Mechanisms for language tagging in
RFC2822/MIME headers [16] are awkward to use and have limited
applicability.
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
attempts at achieving security (e.g., signatures), leaving
receiving applications unable to verify the data received.
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 because they
may not be standardized; this leads to situations where a header
starts out as experimental but then finds widespread application,
resulting in a common usage that cannot be standardized.)
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 simplified syntax that is generally
compatible with the format accepted by RFC2822/MIME parsers and
having a stricter syntax. It also defines mechanisms to support some
desired features not covered by the RFC2822/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 to serve as a basis for signature calculation, rather than
specified security mechanisms).
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o independence of any specific application
o capability of conveying a range of different address types
o assumption of an 8-bit clean message-transfer protocol
o evolvable: extensible by multiple parties
o a clear separation of message metadata from message content
o a simple, regular, easily parsed syntax
o a compact, low-overhead format for simple messages
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 BCP 14, RFC 2119 [4].
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.
2. Overall Message Structure
The CPIM message format encapsulates 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 two-part entity, where the first part
contains the message metadata and the second part is the message
content. The two parts are separated from the enclosing MIME header
fields and also from each other by blank lines. The message metadata
header information obeys more stringent syntax rules than the MIME
message content headers that may be carried within the message.
A complete message looks something like this:
m: Content-type: Message/CPIM
s:
h: (message-metadata-headers)
s:
e: (encapsulated MIME message-body)
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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
2.1. Message/CPIM MIME Headers
The message MIME headers identify the message as a CPIM-formatted
message.
The only required MIME 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 RFC 2822 message
headers in email [9], 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.
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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 [13] 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 section 3.1
and section 3.6 for the specific syntax).
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 Header 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 "Require:" header (section
4.7).
o Interpretation (e.g., equivalence) of header values is dependent
on the particular header definition. Message processors MUST
preserve all octets of all headers (both name and value) exactly.
o Message processors MUST NOT change the order of message headers.
Examples:
To: Pooh Bear <im:pooh@100akerwood.com>
From: <im:piglet@100akerwood.com>
DateTime: 2001-02-02T10:48:54-05:00
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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.
(Rather than invent something completely new, the escape mechanism
has been adopted from that 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.)
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).
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RFC 3862 CPIM: Message Format August 2004
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.
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 Single- or double-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.
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 an 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 [1][2].
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
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3. Message Header Syntax
A header contains two parts, a name and a value, separated by a colon
character (':') and single space (32). It is terminated by the
sequence CR,LF (13,10).
Headers use UTF-8 character encoding throughout, per RFC 3629 [13].
NOTE: in the descriptions that follow, header field names and other
specified text values MUST be used exactly as given, using exactly
the indicated upper- and lower- case letters. In this respect, the
ABNF usage differs from RFC 2234 [6].
3.1. Header Names
The header name is a sequence of US-ASCII characters, excluding
control, SPACE or separator characters. Use of the character "." in
a header name is reserved for a namespace prefix separator.
Separator characters are:
SEPARATORS = "(" / ")" / "<" / ">" / "@"
/ "," / ";" / ":" / "\" / DQUOTE
/ "/" / "[" / "]" / "?" / "="
/ "{" / "}" / SP
NOTE: The range of allowed characters was determined by examination
of HTTP and RFC 2822 header name formats and choosing the more
restricted. The intent is to allow CPIM headers to follow a syntax
that is compatible with the allowed syntax for both RFC 2822 [9] and
HTTP [18] (including HTTP-derived protocols such as SIP [21]).
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 [15][7].
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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 [10].
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' [7].
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.
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 [22] by
XML namespaces [23]. 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 most cases, this default namespace is all
that is needed.
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RFC 3862 CPIM: Message Format August 2004
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 [8]. (Relative URIs and URI-references
containing fragment identifiers MUST NOT be used for this purpose.)
Within a specific message, an '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
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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.
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 successfully 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.
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3.6. Collected Message Header Syntax
The following description of message header syntax uses ABNF, per RFC
2234 [6]. 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 as given, using exactly the
indicated upper- and lower-case letters. In this respect, the ABNF
usage here differs from RFC 2234 [6].
Collected syntax:
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
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RFC 3862 CPIM: Message Format August 2004
; Any US-ASCII char except ".", CTLs or SEPARATORS:
NAMECHAR = %x21 / %x23-27 / %x2a-2b / %x2d
/ %x5e-60 / %x7c / %x7e
/ ALPHA / DIGIT
; Any UCS char except CTLs or SEPARATORS:
TOKENCHAR = NAMECHAR / "." / UCS-high
SEPARATORS = "(" / ")" / "<" / ">" / "@" ; 28/29/3c/3e/40
/ "," / ";" / ":" / "\" / DQUOTE ; 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 = %x80-7fffffff
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
NOTE: the above syntax comes from an older version of UTF-8, and is
included for compatibility with UTF-8 software based on the earlier
specifications. Applications generating this message format SHOULD
generate UTF-8 that matches the more restricted specification in RFC
3629 [13].
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4. Header Definitions
This specification defines a core set of headers that are available
for use by applications: an 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) those that have been based on similar headers in RFC 2822 [9],
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
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 [12].
NOTE: Header names and other text MUST be used as given, using
exactly the indicated upper- and lower-case letters. In this
respect, the ABNF usage here differs from RFC 2234 [6].
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 ">"
; "From" is case-sensitive
Description:
Indicates the sender or originator of a message.
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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>
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 ">"
; "To" is case-sensitive
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>
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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 ">"
; "cc" is case-sensitive
Description:
Indicates a courtesy copy 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 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: DateTime
Namespace URI:
<urn:ietf:params:cpim-headers:>
Syntax:
(see also section 3.6)
DateTime-header = "DateTime" ": " date-time
; "DateTime" is case-sensitive
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(where the syntax of 'date-time' is a profile of ISO8601 [24]
defined in "Date and Time on the Internet" [11])
Description:
The 'DateTime' header supplies the 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 senders's current time when the message was transmitted,
using ISO 8601 [24] date and time format as profiled in "Date and
Time on the Internet: Timestamps" [11].
Example:
DateTime: 2001-02-01T12:16:49-05:00
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
; "Subject" is case-sensitive
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
indicate the message subject.
Example:
Subject:;lang=en Eeyore's feeling very depressed today
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4.6. The 'NS' Header
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 ">"
; "NS" is case-sensitive
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
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 )
; "Require" is case-sensitive
Description:
Indicates a header or feature that must be implemented or
understood by the receiver for correct message processing. See
section 3.5 for more details.
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Note that the required header or feature does not have to be used
in the message, but for brevity it is recommended that an
implementation does not issue the 'Required' header for unused
features.
Example:
Require: MyAlias.VitalHeader
5. Examples
The examples in the following sections use the per-line tags below 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 <urn:ietf:params:cpim-headers:> is
the implied default namespace for the application.
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: DateTime: 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>
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5.2. An Example Esing MIME multipart/signed
In order to secure a Message/CPIM, an application or implementation
may use RFC 1847 [14], and some appropriate security protocols (e.g.,
S/MIME [19] or openPGP [17]), and cryptographic scheme.
Using S/MIME [19] and pkcs7, the above message would look like this:
x: Content-Type: multipart/signed; boundary=next;
micalg=sha1;
protocol=application/pkcs7-signature
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: DateTime: 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>
x: --next
x: Content-Type: application/pkcs7-signature
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.
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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 all 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).
o Security mechanisms and crytography schemes to be used with the
application, including any mandatory-to-implement security
provisions.
The goal of providing a definitive message format to which security
mechanisms can be applied places some constraints on the design of
applications that use this message format:
o 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.
o 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 with 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 has changed and who changed it by following
the trail of amendments. Another metaphor is including the old
message in a new envelope.)
In chosing security mechanisms for an applications, the following IAB
survey documents may be helpful:
o Security Mechanisms for the Internet [28]
o A Survey of Authentication Mechanisms [29].
7. IANA Considerations
This memo calls for two new IANA registrations:
o A new MIME content-type value, Message/CPIM, per RFC 2048 [3].
The registration template can be found in section 7.1 below.
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o A new IANA URN sub-namespace, urn:ietf:params:cpim-headers:, per
RFC 3553 [12]. The registration template can be found in 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
[9]).
Interoperability considerations:
This content format is intended to be used to exchange possibly-
secured messages between different instant messaging protocols.
Very strict adherence to the message format (including whitespace
usage) may be needed to achieve interoperability.
Published specification: RFC 3862
Applications which use this media type: Instant messaging
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Additional information:
The default namespace URI associated with this content-type is
'urn:ietf:params:cpim-headers:'. (See RFC 3862 for further
details.)
See also the Common Profile for Instant Messaging (CPIM) [26].
Person & email address to contact for further information:
G. Klyne, <GK-IETF@ninebynine.org>
Intended usage: LIMITED USE
Author/Change controller: IETF
7.2. Registration for urn:ietf:params:cpim-headers
Registry name: cpim-headers
Specification:
RFC 3862. Additional values may be defined by standards track
RFCs that update or obsolete RFC 3862.
Repository:
http://www.iana.org/assignments/cpim-headers
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
defined above.
URN Formation:
The URI for a header is formed from its name by:
a) replacing any non-URN characters (as defined by RFC 2141 [5])
with the corresponding '%hh' escape sequence (per RFC 2396
[8]); 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'.
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8. Internationalization Considerations
Message headers use UTF-8 character encoding throughout; hence, they
can convey the full UCS-4 (Unicode [30], ISO/IEC 10646 [25])
character repertoire.
Language tagging is provided for message headers using the "Lang"
parameter (section 3.3).
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 option. This implies that the
Message/CPIM must be encapsulated by the message transfer system and
unencapsulated at the receiving end of the tunnel.
The resulting message must not have 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.
10. Acknowledgements
The authors thank the following for their helpful comments: Harald
Alvestrand, Walter Houser, Leslie Daigle, Mark Day, Brian Raymor.
11. References
11.1. Normative References
[1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[2] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
Klyne & Atkins Standards Track [Page 26]
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[3] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Moats, R., "URN Syntax", RFC 2141, May 1997.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[7] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
[8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[9] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[10] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[11] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[12] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF
URN Sub-namespace for Registered Protocol Parameters", BCP 73,
RFC 3553, June 2003.
[13] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
11.2. Informative References
[14] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC 1847, October 1995.
[15] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson,
R., Crispin, M., and P. Svanberg, "The Report of the IAB
Character Set Workshop held 29 February - 1 March, 1996", RFC
2130, April 1997.
[16] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
Extensions: Character Sets, Languages, and Continuations", RFC
2231, November 1997.
Klyne & Atkins Standards Track [Page 27]
RFC 3862 CPIM: Message Format August 2004
[17] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[18] 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.
[19] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[20] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements", RFC 2779, February
2000.
[21] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[22] Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
"Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
Recommendation xml, October 2000,
<http://www.w3.org/TR/2000/REC-xml-20001006>.
[23] Bray, T., Hollander, D., and A. Layman, "Namespaces in XML", W3C
Recommendation xml-names, January 1999,
<http://www.w3.org/TR/REC-xml-names>.
[24] International Organization for Standardization, "Data elements
and interchange formats - Information interchange -
Representation of dates and times", ISO Standard 8601, June
1988.
[25] International Organization for Standardization, "Information
Technology - Universal Multiple-octet coded Character Set (UCS)
- Part 1: Architecture and Basic Multilingual Plane", ISO
Standard 10646-1, May 1993.
[26] Peterson, J., "Common Profile for Instant Messaging (CPIM)", RFC
3860, August 2004.
[27] Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
August 2004.
[28] Bellovin, S., Kaufman, C., and J. Schiller, "Security Mechanisms
for the Internet", RFC 3631, December 2003.
[29] Rescorla, E., "A Survey of Authentication Mechanisms", Work in
Progress, March 2004.
Klyne & Atkins Standards Track [Page 28]
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[30] The Unicode Consortium, "The Unicode Standard, Version 4.0",
Addison-Wesley, Boston, MA. ISBN 0-321-18578-1, April 2003,
<http://www.unicode.org/unicode/standard/versions/
enumeratedversions.html#Unicode_4_0_0>.
12. Authors' Addresses
Graham Klyne
Nine by Nine
EMail: GK-IETF@ninebynine.org
URI: http://www.ninebynine.net/
Derek Atkins
IHTFP Consulting
6 Farragut Ave
Somerville, MA 02144
USA
Phone: +1 617 623 3745
EMail: derek@ihtfp.com, warlord@alum.mit.edu
Klyne & Atkins Standards Track [Page 29]
RFC 3862 CPIM: Message Format August 2004
13. Full Copyright Statement
Copyright (C) The Internet Society (2004). 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 assignees.
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.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Klyne & Atkins Standards Track [Page 30]