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PROPOSED STANDARD
Network Working Group J. Peterson
Request for Comments: 3860 NeuStar
Category: Standards Track August 2004
Common Profile for Instant Messaging (CPIM)
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
At the time this document was written, numerous instant messaging
protocols were in use, and little interoperability between services
based on these protocols has been achieved. This specification
defines common semantics and data formats for instant messaging to
facilitate the creation of gateways between instant messaging
services.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Abstract Instant Messaging Service . . . . . . . . . . . . . . 4
3.1. Overview of Instant Messaging Service . . . . . . . . . 4
3.2. Identification of INSTANT INBOXes . . . . . . . . . . . 5
3.2.1. Address Resolution . . . . . . . . . . . . . . . 5
3.3. Format of Instant Messages . . . . . . . . . . . . . . . 5
3.4. The Messaging Service . . . . . . . . . . . . . . . . . 5
3.4.1. The Message Operation . . . . . . . . . . . . . 5
3.4.2. Looping . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. The IM URI Scheme. . . . . . . . . . . . . . . . . . . . 8
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
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A. IM URI IANA Registration Template . . . . . . . . . . . . . . 10
A.1. URI Scheme Name . . . . . . . . . . . . . . . . . . . . 10
A.2. URI Scheme Syntax . . . . . . . . . . . . . . . . . . . 10
A.3. Character Encoding Considerations . . . . . . . . . . . 10
A.4. Intended Usage . . . . . . . . . . . . . . . . . . . . . 10
A.5. Applications and/or Protocols which use this URI Scheme
Name . . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.6. Security Considerations . . . . . . . . . . . . . . . . 10
A.7. Relevant Publications . . . . . . . . . . . . . . . . . 11
A.8. Person & Email Address to Contact for Further
Information. . . . . . . . . . . . . . . . . . . . . . . 11
A.9. Author/Change Controller . . . . . . . . . . . . . . . . 11
A.10. Applications and/or Protocols which use this URI Scheme
Name . . . . . . . . . . . . . . . . . . . . . . . . . . 11
B. Issues of Interest . . . . . . . . . . . . . . . . . . . . . . 11
B.1. Address Mapping . . . . . . . . . . . . . . . . . . . . 11
B.2. Source-Route Mapping . . . . . . . . . . . . . . . . . . 11
C. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Instant messaging is defined in RFC2778 [5]. At the time this
document was written, numerous instant messaging protocols are in
use, and little interoperability between services based on these
protocols has been achieved. This specification defines semantics
and data formats for common services of instant messaging to
facilitate the creation of gateways between instant messaging
services: a common profile for instant messaging (CPIM).
Service behavior is described abstractly in terms of operations
invoked between the consumer and provider of a service. Accordingly,
each IM service must specify how this behavior is mapped onto its own
protocol interactions. The choice of strategy is a local matter,
providing that there is a clear relation between the abstract
behaviors of the service (as specified in this memo) and how it is
faithfully realized by a particular instant messaging service. For
example, one strategy might transmit an instant message as textual
key/value pairs, another might use a compact binary representation,
and a third might use nested containers.
The attributes for each operation are defined using an abstract
syntax. Although the syntax specifies the range of possible data
values, each IM service must specify how well-formed instances of the
abstract representation are encoded as a concrete series of bits.
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In order to provide a means for the preservation of end-to-end
features (especially security) to pass through instant messaging
interoperability gateways, this specification also provides
recommendations for instant messaging document formats that could be
employed by instant messaging protocols.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [1] and indicate requirement levels for
compliant implementations.
This memos makes use of the vocabulary defined in RFC 2778 [5].
Terms such as CLOSED, INSTANT INBOX, INSTANT MESSAGE, and OPEN are
used in the same meaning as defined therein.
The term 'gateway' used in this document denotes a network element
responsible for interworking between diverse instant messaging
protocols. Although the instant messaging protocols themselves are
diverse, under the model used in this document these protocols can
carry a common payload that is relayed by the gateway. Whether these
interworking intermediaries should be called 'gateways' or 'relays'
is therefore somewhat debatable; for the purposes of this document,
they are called 'CPIM gateways'.
The term 'instant messaging service' also derives from RFC 2778, but
its meaning changes slightly due to the existence of gateways in the
CPIM model. When a client sends an operation to an instant messaging
service, that service might either be an endpoint or an intermediary
such as a CPIM gateway - in fact, the client should not have to be
aware which it is addressing, as responses from either will appear
the same.
This document defines operations and attributes of an abstract
instant messaging protocol. In order for a compliant protocol to
interface with an instant messaging gateway, it must support all of
the operations described in this document (i.e., the instant
messaging protocol must have some message or capability that provides
the function described by each of the given operations). Similarly,
the attributes defined for these operations must correspond to
information available in the instant messaging protocol in order for
the protocol to interface with gateways defined by this
specification. Note that these attributes provide only the minimum
possible information that needs to be specified for interoperability
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- the functions in an instant messaging protocol that correspond to
the operations described in this document can contain additional
information that will not be mapped by CPIM.
3. Abstract Instant Messaging Service
3.1. Overview of Instant Messaging Service
When an application wants to send a message to an INSTANT INBOX, it
invokes the message operation, e.g.,
+-------+ +-------+
| | | |
| appl. | -- message ------> | IM |
| | | svc. |
+-------+ +-------+
The message operation has the following attributes: source,
destination, MaxForwards and TransID. 'source' and 'destination'
identify the originator and recipient of an instant message,
respectively, and consist of an INSTANT INBOX identifier (as
described in Section 3.2). The MaxForwards is a hop counter to avoid
loops through gateways, with usage detailed defined in Section 3.4.2;
its initial value is set by the originator. The TransID is a unique
identifier used to correlate message operations to response
operations; gateways should be capable of handling TransIDs up to 40
bytes in length.
The message operation also has some content, the instant message
itself, which may be textual, or which may consist of other data.
Content details are specified in Section 3.3.
Note that this specification assumes that instant messaging protocols
provide reliable message delivery; there are no application-layer
message delivery assurance provisions in this specification.
Upon receiving a message operation, the service immediately responds
by invoking the response operation containing the same transaction-
identifier, e.g.,
+-------+ +-------+
| | | |
| appl. | <----- response -- | IM |
| | | svc. |
+-------+ +-------+
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The response operation contains the following attributes: TransID and
status. The TransID is used to correlate the response to a
particular instant message. Status indicates whether the delivery of
the message succeeded or failed. Valid status values are described
in Section 3.4.1.
3.2. Identification of INSTANT INBOXes
An INSTANT INBOX is specified using an instant messaging URI with the
'im:' URI scheme. The full syntax of the IM URI scheme is given in
Appendix A. An example would be: "im:fred@example.com"
3.2.1. Address Resolution
An IM service client determines the next hop to forward the IM to by
resolving the domain name portion of the service destination.
Compliant implementations SHOULD follow the guidelines for
dereferencing URIs given in [2].
3.3. Format of Instant Messages
This specification defines an abstract interoperability mechanism for
instant messaging protocols; the message content definition given
here pertains to semantics rather than syntax. However, some
important properties for interoperability can only be provided if a
common end-to-end format for instant messaging is employed by the
interoperating instant messaging protocols, especially with respect
to security. In order to maintain end-to-end security properties,
applications that send message operations to a CPIM gateway MUST
implement the format defined in MSGFMT [4]. Applications MAY support
other content formats.
CPIM gateways MUST be capable of relaying the content of a message
operation between supported instant messaging protocols without
needing to modify or inspect the content.
3.4. The Messaging Service
3.4.1. The Message Operation
When an application wants to send an INSTANT MESSAGE, it invokes the
message operation.
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When an instant messaging service receives the message operation, it
performs the following preliminary checks:
1. If the source or destination does not refer to a syntactically
valid INSTANT INBOX, a response operation having status "failure"
is invoked.
2. If the destination of the operation cannot be resolved by the
recipient, and the recipient is not the final recipient, a
response operation with the status "failure" is invoked.
3. If access control does not permit the application to request this
operation, a response operation having status "failure" is
invoked.
4. Provided these checks are successful:
If the instant messaging service is able to successfully
deliver the message, a response operation having status
"success" is invoked.
If the service is unable to successfully deliver the message,
a response operation having status "failure" is invoked.
If the service must delegate responsibility for delivery
(i.e., if it is acting as a gateway or proxying the
operation), and if the delegation will not result in a future
authoritative indication to the service, a response operation
having status "indeterminant" is invoked.
If the service must delegate responsibility for delivery, and
if the delegation will result in a future authoritative
indication to the service, then a response operation is
invoked immediately after the indication is received.
When the service invokes the response operation, the transID
parameter is identical to the value found in the message operation
invoked by the application.
3.4.2. Looping
The dynamic routing of instant messages can result in looping of a
message through a relay. Detection of loops is not always obvious,
since aliasing and group list expansions can legitimately cause a
message to pass through a relay more than one time.
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This document assumes that instant messaging protocols that can be
gatewayed by CPIM support some semantic equivalent to an integer
value that indicates the maximum number of hops through which a
message can pass. When that number of hops has been reached, the
message is assumed to have looped.
When a CPIM gateway relays an instant message, it decrements the
value of the MaxForwards attribute. This document does not mandate
any particular initial setting for the MaxForwards element in instant
messaging protocols, but it is recommended that the value be
reasonably large (over one hundred).
If a CPIM gateway receives an instant message operation that has a
MaxForwards attribute of 0, it discards the message and invokes a
failure operation.
4. Security Considerations
Detailed security considerations for instant messaging protocols are
given in RFC 2779 [6] (in particular, requirements are given in
section 5.4 and some motivating discussion with 8.1).
CPIM defines an interoperability function that is employed by
gateways between instant messaging protocols. CPIM gateways MUST be
compliant with the minimum security requirements of the instant
messaging protocols with which they interface.
The introduction of gateways to the security model of instant
messaging in RFC 2779 also introduces some new risks. End-to-end
security properties (especially confidentiality and integrity)
between instant messaging user agents that interface through a CPIM
gateway can only be provided if a common instant message format (such
as the format described in MSGFMT [4]) is supported by the protocols
interfacing with the CPIM gateway.
When end-to-end security is required, the message operation MUST use
MSGFMT, and MUST secure the MSGFMT MIME body with S/MIME [8], with
encryption (CMS EnvelopeData) and/or S/MIME signatures (CMS
SignedData).
The S/MIME algorithms are set by CMS [9]. The AES [11] algorithm
should be preferred, as it is expected that AES best suits the
capabilities of many platforms. Implementations MAY use AES as an
encryption algorithm, but are REQUIRED to support only the baseline
algorithms mandated by S/MIME and CMS.
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When IM URIs are placed in instant messaging protocols, they convey
the identity of the sender and/or the recipient. Certificates that
are used for S/MIME IM operations SHOULD, for the purposes of
reference integrity, contain a subjectAltName field containing the IM
URI of their subject. Note that such certificates may also contain
other identifiers, including those specific to particular instant
messaging protocols. In order to further facilitate interoperability
of secure messaging through CPIM gateways, users and service
providers are encouraged to employ trust anchors for certificates
that are widely accepted rather than trust anchors specific to any
particular instant messaging service or provider.
In some cases, anonymous messaging may be desired. Such a capability
is beyond the scope of this specification.
5. IANA Considerations
The IANA has assigned the "im" scheme.
5.1. The IM URI Scheme
The Instant Messaging (IM) URI scheme designates an Internet
resource, namely an INSTANT INBOX.
The syntax of an IM URI is given in Appendix A.
6. Contributors
Dave Crocker edited earlier versions of this document.
The following individuals made substantial textual contributions to
this document:
Athanassios Diacakis (thanos.diacakis@openwave.com)
Florencio Mazzoldi (flo@networkprojects.com)
Christian Huitema (huitema@microsoft.com)
Graham Klyne (gk@ninebynine.org)
Jonathan Rosenberg (jdrosen@dynamicsoft.com)
Robert Sparks (rsparks@dynamicsoft.com)
Hiroyasu Sugano (suga@flab.fujitsu.co.jp)
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7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC 2119, March 1997.
[2] Peterson, J., "Address Resolution for Instant Messaging and
Presence", RFC 3861, August 2004.
[3] Resnick, P., "Internet Message Format", STD 11, RFC 2822, April
2001.
[4] Atkins, D. and G. Klyne, "Common Presence and Instant Messaging:
Message Format", RFC 3862, August 2004.
[5] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.
[6] Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[7] Allocchio, C., "GSTN Address Element Extensions in Email
Services", RFC 2846, June 2000.
[8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[9] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
July 2004.
[10] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
7.2. Informative References
[11] Schaad, J., "Use of the Advanced Encryption Standard (AES)
Encryption Algorithm and in Cryptographic Message Syntax (CMS)",
RFC 3565, August 2003.
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Appendix A. IM URI IANA Registration Template
This section provides the information to register the im: instant
messaging URI.
A.1. URI Scheme Name
im
A.2. URI Scheme Syntax
The syntax follows the existing mailto: URI syntax specified in RFC
2368. The ABNF is:
IM-URI = "im:" [ to ] [ headers ]
to = mailbox
headers = "?" header *( "&" header )
header = hname "=" hvalue
hname = *uric
hvalue = *uric
Here the symbol "mailbox" represents an encoded mailbox name as
defined in RFC 2822 [3], and the symbol "uric" denotes any character
that is valid in a URL (defined in RFC 2396 [10]).
A.3. Character Encoding Considerations
Representation of non-ASCII character sets in local-part strings is
limited to the standard methods provided as extensions to RFC 2822
[3].
A.4. Intended Usage
Use of the im: URI follows closely usage of the mailto: URI. That
is, invocation of an IM URI will cause the user's instant messaging
application to start, with destination address and message headers
fill-in according to the information supplied in the URI.
A.5. Applications and/or Protocols which use this URI Scheme Name
It is anticipated that protocols compliant with RFC 2779, and meeting
the interoperability requirements specified here, will make use of
this URI scheme name.
A.6. Security Considerations
See Section 4.
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A.7. Relevant Publications
RFC 2779, RFC 2778
A.8. Person & Email Address to Contact for Further Information
Jon Peterson [mailto:jon.peterson@neustar.biz]
A.9. Author/Change Controller
This scheme is registered under the IETF tree. As such, IETF
maintains change control.
A.10. Applications and/or Protocols which use this URI Scheme Name
Instant messaging service
Appendix B. Issues of Interest
This appendix briefly discusses issues that may be of interest when
designing an interoperation gateway.
B.1. Address Mapping
When mapping the service described in this memo, mappings that place
special information into the im: address local-part MUST use the
meta-syntax defined in RFC 2846 [7].
B.2. Source-Route Mapping
The easiest mapping technique is a form of source-routing and usually
is the least friendly to humans having to type the string. Source-
routing also has a history of operational problems.
Use of source-routing for exchanges between different services is by
a transformation that places the entire, original address string into
the im: address local part and names the gateway in the domain part.
For example, if the destination INSTANT INBOX is "pepp://example.com/
fred", then, after performing the necessary character conversions,
the resulting mapping is:
im:pepp=example.com/fred@relay-domain
where "relay-domain" is derived from local configuration information.
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Experience shows that it is vastly preferable to hide this mapping
from end-users - if possible, the underlying software should perform
the mapping automatically.
Appendix C. Acknowledgments
The author would like to acknowledge John Ramsdell for his comments,
suggestions and enthusiasm. Thanks to Derek Atkins for editorial
fixes.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
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