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rfc:rfc3860

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

Peterson Standards Track [Page 1] RFC 3860 CPIM August 2004

 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.

Peterson Standards Track [Page 2] RFC 3860 CPIM August 2004

 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

Peterson Standards Track [Page 3] RFC 3860 CPIM August 2004

  1. 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. |
 +-------+                    +-------+

Peterson Standards Track [Page 4] RFC 3860 CPIM August 2004

 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.

Peterson Standards Track [Page 5] RFC 3860 CPIM August 2004

 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.

Peterson Standards Track [Page 6] RFC 3860 CPIM August 2004

 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.

Peterson Standards Track [Page 7] RFC 3860 CPIM August 2004

 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)

Peterson Standards Track [Page 8] RFC 3860 CPIM August 2004

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.

Peterson Standards Track [Page 9] RFC 3860 CPIM August 2004

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.

Peterson Standards Track [Page 10] RFC 3860 CPIM August 2004

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.

Peterson Standards Track [Page 11] RFC 3860 CPIM August 2004

 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

Peterson Standards Track [Page 12] RFC 3860 CPIM August 2004

Full Copyright Statement

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 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Peterson Standards Track [Page 13]

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