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PROPOSED STANDARD
Errata Exist
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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