    Network Working Group                                        J. Peterson
    Request for Comments: 3859                                       NeuStar
    Category: Standards Track                                    August 2004

                       Common Profile for Presence (CPP)

    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 presence protocols
       were in use (largely as components of commercial instant messaging
       services), and little interoperability between services based on
       these protocols has been achieved.  This specification defines common
       semantics and data formats for presence to facilitate the creation of
       gateways between presence services.

    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
       3.  Abstract Presence Service  . . . . . . . . . . . . . . . . . .  4
           3.1.  Overview of the Presence Service . . . . . . . . . . . .  4
           3.2.  Identification of PRESENTITIES and WATCHERS  . . . . . .  6
                 3.2.1.  Address Resolution . . . . . . . . . . . . . . .  6
           3.3.  Format of Presence Information . . . . . . . . . . . . .  6
           3.4.  The Presence Service . . . . . . . . . . . . . . . . . .  7
                 3.4.1.  The Subscribe Operation  . . . . . . . . . . . .  7
                 3.4.2.  The Notify Operation . . . . . . . . . . . . . .  8
                 3.4.3.  Subscribe Operation (with Zero Duration) . . . .  8
       4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
       5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
           5.1.  The PRES URI Scheme  . . . . . . . . . . . . . . . . . .  9
       6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10
       7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
           7.1.  Normative References . . . . . . . . . . . . . . . . . . 10
           7.2.  Informative References . . . . . . . . . . . . . . . . . 11

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       A.  PRES URI IANA Registration Template  . . . . . . . . . . . . . 12
           A.1.  URI Scheme Name  . . . . . . . . . . . . . . . . . . . . 12
           A.2.  URI Scheme Syntax  . . . . . . . . . . . . . . . . . . . 12
           A.3.  Character Encoding Considerations  . . . . . . . . . . . 12
           A.4.  Intended Usage . . . . . . . . . . . . . . . . . . . . . 12
           A.5.  Applications and/or Protocols which use this URI Scheme
                 Name . . . . . . . . . . . . . . . . . . . . . . . . . . 12
           A.6.  Interoperability Considerations  . . . . . . . . . . . . 13
           A.7.  Security Considerations  . . . . . . . . . . . . . . . . 13
           A.8.  Relevant Publications  . . . . . . . . . . . . . . . . . 13
           A.9.  Person & Email Address to Contact for Further
                 Information. . . . . . . . . . . . . . . . . . . . . . . 13
           A.10. Author/Change Controller . . . . . . . . . . . . . . . . 13
           A.11. Applications and/or Protocols which use this URI Scheme
                 Name . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       B.  Issues of Interest . . . . . . . . . . . . . . . . . . . . . . 13
           B.1.  Address Mapping  . . . . . . . . . . . . . . . . . . . . 13
           B.2.  Source-Route Mapping . . . . . . . . . . . . . . . . . . 13
       C.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 15

    1.  Introduction

       Presence is defined in RFC2778 [5].  At the time this document was
       written, numerous presence protocols are in use (largely as
       components of commercial instant messaging services), and little
       interoperability between services based on these protocols has been
       achieved.  This specification defines semantics and data formats for
       common services of presence to facilitate the creation of gateways
       between presence services: a common profile for presence (CPP).

       Service behavior is described abstractly in terms of operations
       invoked between the consumer and provider of a service.  Accordingly,
       each presence 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 presence service.   For example,
       one strategy might transmit presence information as key/value pairs,
       another might use a compact binary representation, and a third might
       use nested containers.

       The parameters for each operation are defined using an abstract
       syntax.  Although the syntax specifies the range of possible data
       values, each presence 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 presence
       interoperability gateways, this specification also provides
       recommendations for presence document formats that could be employed
       by presence 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 BCP 14, 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, PRESENCE, 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 presence protocols.
       Although the presence protocols themselves are diverse, under the
       model 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 'CPP gateways'.

       The term 'presence service' also derives from RFC 2778, but its
       meaning changes slightly due to the existence of gateways in the CPP
       model.  When a client sends an operation to a presence service, that
       service might either be an endpoint or an intermediary such as a CPP
       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
       presence protocol.  In order for a compliant protocol to interface
       with a presence gateway, it must support all of the operations
       described in this document (i.e., the presence protocol must have
       some message or capability that provides the function described by
       all given operations).  Similarly, the attributes defined for these
       operations must correspond to information available in the presence
       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 - the functions in a presence protocol that
       correspond to the operations described in this document can contain
       additional information that will not be mapped by CPP.

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    3.  Abstract Presence Service

    3.1.  Overview of the Presence Service

       When an application wants to subscriber to the presence information
       associated with a PRESENTITY (in order to receive periodic
       notifications of presence information), it invokes the subscribe
       operation, e.g.,

                 +-------+                    +-------+
                 |       |                    |       |
                 | appl. | -- subscribe ----> | pres. |
                 |       |                    | svc.  |
                 +-------+                    +-------+

       The subscribe operation has the following attributes: watcher,
       target, duration, SubscriptID and TransID.  The 'watcher' and
       'target' identify the WATCHER and PRESENTITY, respectively, using the
       identifiers described in Section 3.2.  The duration specifies the
       maximum number of seconds that the SUBSCRIPTION should be active
       (which may be zero, in which case this is a one-time request for
       presence information).  The SubscriptID creates a reference to the
       SUBSCRIPTION that is used when unsubscribing.  The TransID is a
       unique identifier used to correlate the subscribe operation with a
       response operation.  Gateways should be capable of handling TransIDs
       and SubscriptIDs up to 40 bytes in length.

       Upon receiving a subscribe operation, the service immediately
       responds by invoking the response operation containing the same
       TransID, e.g.,

                 +-------+                    +-------+
                 |       |                    |       |
                 | appl. | <----- response -- | pres. |
                 |       |                    | svc.  |
                 +-------+                    +-------+

       The response operation has the following attributes: status, TransID,
       and duration.  'status' indicates whether the subscribe operation has
       succeeded or failed.  The TransID of the response operation
       corresponds to the TransID of the subscription operation to which it
       is responding.  The 'duration' attribute specifies the number of
       seconds for which the subscription will be active (which may differ
       from the value requested in the subscribe operation).

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       If the response operation indicates success, the service immediately
       invokes the notify operation to communicate the presence information
       to the WATCHER, e.g.,

                 +-------+                    +-------+
                 |       |                    |       |
                 | appl. | <------- notify -- | pres. |
                 |       |                    | svc.  |
                 +-------+                    +-------+

       The notify operation has the following attributes: watcher, target,
       and TransID.  The values of 'watcher' and 'target' are identical to
       those given in the subscribe operation that triggered this notify
       operation.  The TransID is a unique identifier for this notification.

       The notify operation also has content, namely PRESENCE INFORMATION.
       Content details are specified in Section 3.3.

       If the duration parameter is non-zero, then for up to the specified
       duration, the service invokes the notify operation whenever there are
       any changes to the PRESENTITY's presence information.  Otherwise,
       exactly one notify operation is invoked, achieving a one-time poll of
       the presence information.  Regardless, there is no application
       response to the notify operation (i.e., the application does not
       invoke a response operation when a notify operation occurs) defined
       in CPP.

       The application may prematurely cancel a subscription by re-invoking
       the subscribe operation (as described above) with a duration of 0 and
       the same SubscriptID as the original subscribe operation , e.g.,

                 +-------+                    +-------+
                 |       |                    |       |
                 | appl. | -- subscribe 0 --> | pres. |
                 |       |                    | svc.  |
                 +-------+                    +-------+

       Note that a notify operation will be invoked when a subscription is
       prematurely canceled in this fashion; this notification may be
       discarded by the watcher.

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       The service immediately responds by invoking the response operation
       containing the same TransID; e.g.,

                 +-------+                    +-------+
                 |       |                    |       |
                 | appl. | <----- response -- | pres. |
                 |       |                    | svc.  |
                 +-------+                    +-------+

       Note that this specification assumes that CPP-compliant presence
       protocols provide reliable message delivery; there are no
       application-layer message delivery assurance provisions in this
       specification.

    3.2.  Identification of PRESENTITIES and WATCHERS

       A PRESENTITY is specified using the PRES URI scheme, which is further
       described in Appendix A.  An example would be:
       "pres:fred@example.com"

       WATCHERs identify themselves in the same manner as PRESENTITIES; that
       is, with a pres URI.

    3.2.1.  Address Resolution

       A presence service client determines the next hop to forward an
       operation 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 Presence Information

       This specification defines an abstract interoperability mechanism for
       presence 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 presence is employed by the interoperating
       presence protocols, especially with respect to security.  In order to
       maintain end-to-end security properties, applications that send
       notification operations through a CPP gateway MUST support the format
       defined in PIDF [4].  Applications MAY support other content formats.

       CPP gateways MUST be capable of relaying the body of a notification
       operation between supported presence protocols without needing to
       modify or inspect the content.

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    3.4.  The Presence Service

       An implementation of the service must maintain information about both
       presence information and continual operations (like periodic
       notification) in persistent storage.

       Note that the subscription-identifier attribute used by the subscribe
       operation is potentially long-lived.  Accordingly, the values
       generated for this parameter should be unique across a significant
       duration of time.  The SubscriptID parameter should be intrinsically
       globally unique over time, not merely unique among operations sent to
       or from a particular WATCHER and PRESENTITY.

    3.4.1.  The Subscribe Operation

       When an application wants to subscribe to the presence information
       associated with a PRESENTITY, it invokes the subscribe operation.

       When the service is informed of the subscribe operation, it performs
       these steps:

       1.  If the watcher or target parameter does not refer to a valid
           PRESENTITY, a response operation having status "failure" is
           invoked.

       2.  If access control does not permit the application to request this
           operation, a response operation having status "failure" is
           invoked.

       3.  If the duration parameter is non-zero, and if the watcher and
           target parameters refer to an in-progress subscribe operation for
           the application, a response operation having status "failure" is
           invoked.

       4.  Otherwise, if the service is able to successfully deliver the
           message:

             A response operation having status "success" is immediately
             invoked.  (If the service chooses a different duration for the
             subscription then it conveys this information in the response
             operation.)

             A notify operation, corresponding to the target's presence
             information, is immediately invoked for the watcher.

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             For up to the amount of time indicated by the duration
             parameter of the notify operation (measured from the time that
             the subscribe operation was received), if the target's presence
             information changes, and if access control allows, a notify
             operation is invoked for the watcher.

       Note that if the duration parameter is zero-valued, then the
       subscribe operation is making a one-time poll of the presence
       information.  Accordingly, the final step above (continued
       notifications for the duration of the subscription) does not occur.

       When the service invokes a response operation as a result of this
       processing, the transID parameter is identical to the value found in
       the subscribe operation invoked by the application.

    3.4.2.  The Notify Operation

       The service invokes the notify operation whenever the presence
       information associated with a PRESENTITY changes and there are
       subscribers requesting notifications for that PRESENTITY.

       There is no application response to the notify operation.

    3.4.3.  Subscribe Operation (with Zero Duration)

       When an application wants to terminate a subscription, it issues a
       SUBSCRIBE 0 with the SubscriptID of an existing subscription.  Note
       that a notify operation will be invoked by the presentity when a
       subscription is canceled in this fashion; this notification can be
       discarded by the watcher.  There is no independent UNSUBSCRIBE
       operation.

       When an application wants to directly request presence information to
       be supplied immediately without initiating any persistent
       subscription, it issues a SUBSCRIBE 0 with a new SubscriptID.  There
       is no independent FETCH operation.

    4.  Security Considerations

       Detailed security considerations for presence protocols given in RFC
       2779 [6] (in particular, requirements are given in sections 5.1
       through 5.3 with some motivating discussion in 8.2).

       CPP defines an interoperability function that is employed by gateways
       between presence protocols.  CPP gateways MUST be compliant with the
       minimum security requirements of the presence protocols with which
       they interface.

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       The introduction of gateways to the security model of presence in RFC
       2779 also introduces some new risks.  End-to-end security properties
       (especially confidentiality and integrity) between presentities and
       watchers that interface through a CPP gateway can only be provided if
       a common presence format (such as the format described in [4]) is
       supported by the protocols interfacing with the CPP gateway.

       When end-to-end security is required, the notify operation MUST use
       PIDF, and MUST secure the PIDF 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.

       When PRES URIs are used in presence protocols, they convey the
       identity of watchers and/or presentities.  Certificates that are used
       for S/MIME presence operations SHOULD, for the purposes of reference
       integrity, contain a subjectAltName field containing the PRES URI of
       their subject.  Note that such certificates may also contain other
       identifiers, including those specific to particular presence
       protocols.  In order to further facilitate interoperability of secure
       presence services through CPP 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
       presence service or provider.

       In some cases, anonymous presence services may be desired.  Such a
       capability is beyond the scope of this specification.

    5.  IANA Considerations

       The IANA has assigned the "pres" URI scheme.

    5.1.  The PRES URI Scheme

       The Presence (PRES) URI scheme designates an Internet resource,
       namely a PRESENTITY or WATCHER.

       The syntax of a PRES URI is given in Appendix A.

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    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)

    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]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
            J. Peterson, "Presence Information Data Format (PIDF)", RFC
            3863, 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.

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       [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, July 2003.

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    Appendix A.  PRES URI IANA Registration Template

       This section provides the information to register the pres: presence
       URI .

    A.1.  URI Scheme Name

       pres

    A.2.  URI Scheme Syntax

       The syntax follows the existing mailto: URI syntax specified in RFC
       2368.  The ABNF is:

       PRES-URI         = "pres:" [ 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 pres: URI follows closely usage of the mailto: URI.  That
       is, invocation of an PRES 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.

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    A.6.  Interoperability Considerations

       The underlying exchange protocol used to send an instant message may
       vary from service to service.  Therefore complete, Internet-scale
       interoperability cannot be guaranteed.  However, a service conforming
       to this specification permits gateways to achieve interoperability
       sufficient to the requirements of RFC 2779.

    A.7.  Security Considerations

       See Section 4.

    A.8.  Relevant Publications

       RFC 2779, RFC 2778

    A.9.  Person & Email Address to Contact for Further Information

       Jon Peterson [mailto:jon.peterson@neustar.biz]

    A.10.  Author/Change Controller

       This scheme is registered under the IETF tree.  As such, IETF
       maintains change control.

    A.11.  Applications and/or Protocols which use this URI Scheme Name

       Instant messaging service; presence 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 RFC2846 [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.

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       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.

       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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    Full Copyright Statement

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       Funding for the RFC Editor function is currently provided by the
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