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Internet Engineering Task Force                                SIMPLE WG
Internet Draft                                              J. Rosenberg
                                                               D. Willis
                                                               R. Sparks
                                                             B. Campbell
                                                          H. Schulzrinne
                                                             Columbia U.
                                                               J. Lennox
                                                             Columbia U.
                                                              C. Huitema
                                                                B. Aboba
                                                                D. Gurle
                                                                 D. Oran
March 1, 2002
Expires: September 2002

                      SIP Extensions for Presence


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see

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   This document describes the usage of SIP for subscriptions and
   notifications of user presence. User presence is defined as the
   willingness and ability of a user to communicate with other users on
   the network. Historically, presence has been limited to "on-line" and
   "off-line" indicators; the notion of presence here is broader.
   Subscriptions and notifications of user presence are supported by
   defining an event package within the general SIP event notification
   framework. This protocol is also compliant with the Common Presence
   and Instant Messaging (CPIM) framework.

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                           Table of Contents

   1          Introduction ........................................    4
   2          Terminology .........................................    4
   3          Definitions .........................................    4
   4          Overview of Operation ...............................    5
   5          Usage of Presence URLs ..............................    7
   6          Presence Event Package ..............................    8
   6.1        Package Name ........................................    8
   6.2        Event Package Parameters ............................    8
   6.3        SUBSCRIBE bodies ....................................    8
   6.4        Subscription Duration ...............................    9
   6.5        NOTIFY Bodies .......................................    9
   6.6        Notifier Processing of SUBSCRIBE Requests ...........    9
   6.6.1      Authentication ......................................   10
   6.6.2      Authorization .......................................   11
   6.7        Notifier Generation of NOTIFY Requests ..............   12
   6.8        Subscriber Processing of NOTIFY Requests ............   13
   6.9        Handling of Forked Requests .........................   14
   6.10       Rate of Notifications ...............................   14
   6.11       State Agents ........................................   14
   7          Publication .........................................   16
   7.1        Co-location .........................................   16
   7.2        REGISTER ............................................   16
   7.3        Uploading Presence Documents ........................   17
   8          Example message flow ................................   17
   9          Security considerations .............................   20
   9.1        Firewall and NAT Traversal ..........................   20
   9.2        Privacy .............................................   21
   9.3        Message integrity and authenticity ..................   21
   9.4        Outbound authentication .............................   22
   9.5        Replay prevention ...................................   22
   9.6        Denial of service attacks ...........................   22
   9.6.1      Smurf attacks through false contacts ................   22
   10         IANA Consideration ..................................   23
   11         Acknowledgements ....................................   23
   12         Authors Addresses ...................................   23
   13         Normative References ................................   25
   14         Informative References ..............................   26

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1 Introduction

   Presence is (indirectly) defined in RFC 2778 [8] as subscription to
   and notification of changes in the communications state of a user.
   This communications state consists of the set of communications
   means, communications address, and status of that user. A presence
   protocol is a protocol for providing such a service over the Internet
   or any IP network.

   This document proposes the usage of the Session Initiation Protocol
   (SIP) [1] for presence. This is accomplished through a concrete
   instantiation of the general event notification framework defined for
   SIP [2], and as such, makes use of the SUBSCRIBE and NOTIFY methods
   defined there. Specifically, this document defines an event package,
   as described in [2]. User presence is particularly well suited for
   SIP. SIP registrars and location services already hold aspects of
   user presence information; it is uploaded to these devices through
   REGISTER messages, and used to route calls to those users.
   Furthermore, SIP networks already route INVITE messages from any user
   on the network to the proxy that holds the registration state for a
   user. As this state is user presence, those SIP networks can also
   allow SUBSCRIBE requests to be routed to the same proxy. This means
   that SIP networks can be reused to establish global connectivity for
   presence subscriptions and notifications.

   This event package is based on the concept of a presence agent, which
   is a new logical entity that is capable of accepting subscriptions,
   storing subscription state, and generating notifications when there
   are changes in user presence. The entity is defined as a logical one,
   since it is generally co-resident with another entity.

   This event package is also compliant with the Common Presence and
   Instant Messaging (CPIM) framework that has been defined in [3]. This
   allows SIP for presence to easily interwork with other presence
   systems compliant to CPIM.

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
   indicate requirement levels for compliant implementations.

3 Definitions

   This document uses the terms as defined in RFC 2778 [8].
   Additionally, the following terms are defined and/or additionally

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        Presence User Agent (PUA): A Presence User Agent manipulates
             presence information for a presentity. This manipulation
             can be the side effect of some other action (such as
             sending a SIP REGISTER request to add a new Contact) or can
             be done explicitly through the publication of presence
             documents. We explicitly allow multiple PUAs per
             presentity. This means that a user can have many devices
             (such as a cell phone and PDA), each of which is
             independently generating a component of the overall
             presence information for a presentity. PUAs push data into
             the presence system, but are outside of it, in that they do
             not receive SUBSCRIBE messages, or send NOTIFY.

        Presence Agent (PA): A presence agent is a SIP user agent which
             is capable of receiving SUBSCRIBE requests, responding to
             them, and generating notifications of changes in presence
             state. A presence agent must have knowledge of the presence
             state of a presentity. This means that it must have access
             to presence data manipulated by PUAs for the presentity.
             One way to do this is by co-locating the PA with the
             proxy/registrar, or the presence user agent of the
             presentity. However, this is not the only way, and this
             specification makes no recommendations about where the PA
             function should be located. A PA is always addressable with
             a SIP URI that uniquely identifies the presentity (i.e,
             sip:joe@example.com). There can be multiple PAs for a
             particular presentity, each of which handles some subset of
             the total subscriptions currently active for the
             presentity. A PA is also a notifier (defined in [2] that
             supports the presence events package.

        Presence Server: A presence server is a physical entity that can
             act as either a presence agent or as a proxy server for
             SUBSCRIBE requests. When acting as a PA, it is aware of the
             presence information of the presentity through some
             protocol means. When acting as a proxy, the SUBSCRIBE
             requests are proxied to another entity that may act as a

        Presence Client: A presence client is a presence agent that is
             co-located with a PUA. It is aware of the presence
             information of the presentity because it is co-located with
             the entity that manipulates this presence information.

4 Overview of Operation

   In this section, we present an overview of the operation of this
   event package. The overview describes behavior that is documented in

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   part here, in part within the SIP events framework [2], and in parth
   in the SIP specification [1], in order to provide clarity on this
   package for readers only casually familiar with those specifications.
   However, the detailed semantics of this package require the reader to
   be familiar with SIP events and the SIP specification itself.

   When an entity, the subscriber, wishes to learn about presence
   information from some user, it creates a SUBSCRIBE request. This
   request identifies the desired presentity in the request URI, using a
   SIP URI or a presence URL [3]. The subscription is carried along SIP
   proxies as any other request would be. It eventually arrives at a
   presence server, which can either terminate the subscription (in
   which case it acts as the presence agent for the presentity), or
   proxy it on to a presence client. If the presence client handles the
   subscription, it is effectively acting as the presence agent for the
   presentity. The decision at a presence server about whether to proxy
   or terminate the SUBSCRIBE is a local matter; however, we describe
   one way to effect such a configuration, using REGISTER.

   The presence agent (whether in the presence server or presence
   client) first authenticates the subscription, then authorizes it. The
   means for authorization are outside the scope of this protocol, and
   we expect that many mechanisms will be used. If authorized, a 200 OK
   response is returned. If authorization could not be obtained at this
   time, the subscription is considered "pending", and a 202 response is
   returned. In both cases, the PA sends an immediate NOTIFY message
   containing the state of the presentity and of the subscription. The
   presentity state may be bogus in the case of a pending subscription,
   indicating offline no matter what the state of the actual presentity,
   for example. This is to protect the privacy of the presentity, who
   may not want to reveal that they have not provided authorization for
   the subscriber. As the state of the presentity changes, the PA
   generates NOTIFYs for all subscribers with authorized subscriptions.
   The state of the subscription itself is carried in the Subscription-
   State header of the NOTIFY, and would typically indicate whether the
   subscription is active or pending.

   The SUBSCRIBE message establishes a "dialog" with the presence agent.
   A dialog is defined in [1], and it represents the SIP state between a
   pair of entities to facilitate peer-to-peer message exchanges. This
   state includes the sequence numbers for messages in both directions
   (SUBSCRIBE from the subscriber, NOTIFY from the presence agent), in
   addition to a route set and remote URI. The route set is a list of
   SIP (or SIPS) URIs which identify SIP proxy servers that are to be
   visited along the path of SUBSCRIBE refreshes or NOTIFY requests. The
   remote URI is the SIP or SIPS URI that identifies the target of the
   message - the subscriber, in the case of NOTIFY, or the presence
   agent, in the case of a SUBSCRIBE refresh.

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   SIP provides a procedure called record-routing that allows for proxy
   servers to request to be on the path of NOTIFY messages and/or
   SUBSCRIBE refreshes. This is accomplished by inserting a URI into the
   Record-Route header in the initial SUBSCRIBE request and/or response.

   The subscription persists for a duration that is negotiated as part
   of the initial SUBSCRIBE. The subscriber will need to refresh the
   subscription before termination, if they wish to continue. This is
   accomplished by sending a SUBSCRIBE refresh within the same dialog
   established by the initial SUBSCRIBE. This SUBSCRIBE is nearly
   identical to the initial one, but contains the dialog identifier,
   different sequence numbers, and a set of Route headers that identify
   the path of proxies the request is to take.

   The subscriber can terminate the subscription by sending a SUBSCRIBE,
   within the dialog, with an Expires header (which indicates duration
   of the subscription) of zero. This causes an immediate termination of
   the subscription. A NOTIFY request is generated by the presence agent
   with the most recent state. In fact, behavior of the presence agent
   for handing a SUBSCRIBE with Expires of zero is no different than for
   any other expiration value; all SUBSCRIBE requests result in a
   triggered NOTIFY with the current presentity and subscription state.

   The presence agent can terminate the subscription at any time. To do
   so, it sends a NOTIFY request with a Subscription-State header
   indicating that the subscription has been terminated. A reason
   parameter can be supplied which provides the reason.

5 Usage of Presence URLs

   A presentity is identified in the most general way through a presence
   URL [3], which is of the form pres:user@domain. These URLs are
   protocol independent. They are resolved to protocol specific URIs,
   such as a SIP or SIPS URI, through DNS procedures defined in [3].

   When subscribing to a presentity, the subscription can be addressed
   using the protocol independent form or the SIP or SIPS URI form. In
   the SIP context, "addressed" refers to the Request-URI. It is
   RECOMMENDED that if the entity sending a SUBSCRIBE is capable of
   resolving the protocol independent form to the SIP form, this
   resolution is done before sending the request. However, if the entity
   is incapable of doing this translation, the protocol independent form
   MAY be used in the Request-URI. Performing the translation as early
   as possible means that these requests can be routed by SIP proxies
   that are not aware of the presence namespace.

   SUBSCRIBE messages also contain logical identifiers that define the
   originator and recipient of the subscription (the To and From header

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   fields). These SHOULD contain SIP or SIPS URIs whenever possible, but
   MAY contain a pres URL if a SIP or SIPS URI is not known or

   The Contact, Record-Route and Route fields do not identify logical
   entities, but rather concrete ones used for SIP messaging. SIP [1]
   specifies rules for their construction.

6 Presence Event Package

   The SIP event framework [2] defines a SIP extension for subscribing
   to, and receiving notifications of, events. It leaves the definition
   of many additional aspects of these events to concrete extensions,
   also known as event packages. This document qualifies as an event
   package. This section fills in the information required by [2].

6.1 Package Name

   The name of this package is "presence". As specified in [2], this
   header appears in SUBSCRIBE and NOTIFY requests.


   Event: presence

6.2 Event Package Parameters

   The SIP Event Framework allows event packages to define additional
   parameters carried in the Event header for the specific package. This
   package, presence, does not define any additional parameters.

6.3 SUBSCRIBE bodies

   The body of a SUBSCRIBE request MAY contain a body. The purpose of
   the body depends on its type. Subscriptions will normally not contain
   bodies. The request URI, which identifies the presentity, combined
   with the event package name, is sufficient for user presence.

   We anticipate that document formats could be defined to act as
   filters for subscriptions. These filters would request that only
   certain user presence events that would generate notifies, or ask for
   a restriction on the set of data returned in NOTIFY requests. For
   example, a presence filter might specify that the notifications
   should only be generated when the status of the users instant message

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   inbox changes. It might also say that the content of these
   notifications should only contain the IM related information.

   Honoring of these filters is at the policy discretion of the PA.

   When no body is present, this specifies to the PA that no filter is
   being requested, so that the PA is being requested to send all NOTIFY
   requests that its own policy allows.

6.4 Subscription Duration

   User presence changes as a result of many events. Some examples are:

        o Turning on and off of a cell phone

        o Modifying the registration from a softphone

        o Changing the status on an instant messaging tool

   These events are usually triggered by human intervention, and occur
   with a frequency on the order of seconds to hours. As such,
   subscriptions should have an expiration in the middle of this range,
   which is roughly one hour. Therefore, the default expiration time for
   subscriptions within this package is 3600 seconds. As per [2], the
   subscriber MAY include an alternate expiration time.

6.5 NOTIFY Bodies

   As described in [2], the NOTIFY message will contain bodies that
   describe the state of the subscribed resource. This body is in a
   format listed in the Accept header of the SUBSCRIBE, or a package-
   specific default if the Accept header is omitted.

   In this event package, the body of the notification contains a
   presence document. This document describes the user presence of the
   presentity that was subscribed to. All subscribers MUST support the
   "application/cpim-pidf+xml" presence data format described in [5].
   The subscribe request MAY contain an Accept header. If no such header
   is present, it has a default value of "application/cpim-pidf+xml". If
   the header is present, it MUST include "application/cpim-pidf+xml",
   and MAY include any other types capable of representing user
   presence, such as "message/cpim" as described in [5].

6.6 Notifier Processing of SUBSCRIBE Requests

   Based on the proxy routing procedures defined in the SIP
   specification, the SUBSCRIBE request will arrive at a presence agent
   (PA). This subsection defines processing at the PA of a SUBSCRIBE

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   If a PA gets a SUBSCRIBE request, and the Request-URI identifies a
   user the PA is responsible for, but the To header does not, this
   means that the SUBSCRIBE was forwarded for some reason. Whether the
   PA is willing to accept subscriptions originally targeted to the user
   in the To field is a matter of local policy. If a PA decides not to,
   it SHOULD generate a 403 response.

   User presence is highly sensitive information. Because the
   implications of divulging presence information can be severe, strong
   requirements are imposed on the PA regarding subscription processing,
   especially related to authentication and authorization.

6.6.1 Authentication

   A presence agent MUST authenticate all subscription requests. This
   authentication can be done using any of the mechanisms defined in

   In single-domain systems, where the subscribers all have shared
   secrets with the PA in the domain, the combination of digest
   authentication over TLS provides a secure and workable solution for
   authentication. This use case is described in Section of

   In inter-domain scenarios, establishing an authenticated identity of
   the subscriber is harder. It is anticipated that authentication will
   often be established through transitive trust. Specifically, when
   user A generates a SUBSCRIBE for B@bar.com, his domain (say, foo.com)
   will use SIP proxy digest authentication, run over a TLS connection,
   to identify him (see Section of [1] for an example). The
   SUBSCRIBE is forwarded to the target domain over a secure connection,
   such as TLS (see Section of [1] for an example of TLS-based
   inter-domain security). The nature of the trust relationship between
   bar.com and foo.com is that bar.com trusts that foo.com has
   authenticated all subscribes it receives over that secure connection.
   As such, the bar.com server need only verify that the SUBSCRIBE came
   over the secure connection. The SIP extension for caller identity and
   privacy [9] can be used to allow one domain to provide the
   authenticated identity to another, even while maintaining privacy.
   These mechanisms apply equally well to SUBSCRIBE requests for

   A presentity can choose to represent itself with a SIPS URI. By
   "represent itself", it means that the user represented by the
   presentity hands out, on business cards, web pages, and so on, a SIPS
   URI for their presentity. The semantics associated with this URI, as

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   described in [1], will force TLS usage on each hop between the
   subscriber and the server in the domain of the URI. This provides
   additional assurances (but no absolute guarantees) that identity has
   been verified at each hop.

   Another mechanism for authentication is S/MIME. Its usage with SIP is
   described fully in [1]. If provides an end-to-end authentication
   mechanism that can be used for a PA to establish the identity of the

6.6.2 Authorization

   Once authenticated, the PA makes an authorization decision. A PA MUST
   NOT accept a subscription unless authorization has been provided by
   the presentity. The means by which authorization are provided are
   outside the scope of this document. Authorization may have been
   provided ahead of time through access lists, perhaps specified in a
   web page. Authorization may have been provided by means of uploading
   of some kind of standardized access control list document. Back end
   authorization servers, such as a DIAMETER [10], RADIUS [11], or COPS
   [12], can also be used. It is also useful to be able to query the
   user for authorization following the receipt of a subscription
   request for which no authorization information was present. The
   "watcherinfo" event sub-package for SIP [13] defines a means by which
   a presentity can become aware that a user has attempted to subscribe
   to it, so that it can then provide an authorization decision.

   Authorization decisions can be very complex. Ultimately, all
   authorization decisions can be mapped into one of three states:
   rejected, successful, and pending. Any subscription for which the
   client is authorized to receive information about some subset of
   presence state at some points in time is a successful subscription.
   Any subscription for which the client will never receive any
   information about any subset of the presence state is a rejected
   subscription. Any subscription for which it is not known yet whether
   it is successful or rejected is pending. Generally, pending occurs
   when the server cannot obtain authorization at the time of the
   subscription, and may be able to do so at a later time, when the
   presentity becomes available.

   The appropriate response codes for conveying a successful, rejected,
   or pending subscription (200, 403 or 603, and 202, respectively) are
   described in [2].

   The SIP events framework allows the initial NOTIFY to contain no body
   if the resource is not in a meaningful state. In the case of
   presence, that NOTIFY MAY contain a presence document. This document
   would indicate whatever presence state the subscriber has been

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   authorized to see; it is interpreted by the subscriber as the current
   presence state of the presentity. For pending subscriptions, the
   state of the presentity SHOULD include some kind of textual note that
   indicates a pending status.

   Polite blocking, as described in [14], is possible by generating a
   200 OK to the subscription even though it has been rejected (or
   marked pending). Of course, an immediate NOTIFY will still be sent.
   The contents of the presence document in such a NOTIFY are at the
   discretion of the implementor, but SHOULD be constructed in such a
   way as to not reveal to the subscriber that their request has
   actually been blocked. Typically, this is done by indicating
   "offline" or equivalent status for a single contact address.

   In many cases, it is useful for the response to the SUBSCRIBE to
   provide the logical identity of the PA which generated the response.
   This may not be the same as the user that the SUBSCRIBE was
   originally targeted at, because of request forwarding. SIP extensions
   have been defined to provide this capability [9].

6.7 Notifier Generation of NOTIFY Requests

   The SIP Events specification details the formatting and structure of
   NOTIFY messages. However, it leaves to packages the detailed
   information about what events cause a NOTIFY to be sent, how to
   compute the state information in the NOTIFY, how to generate neutral
   or fake state information to hide authorization delays and decisions
   from users, and whether state information is complete or deltas for

   A PA MAY send a NOTIFY at any time. Typically, it will send ones for
   successful subscriptions when the state of the presentity changes.
   The NOTIFY request MAY contain a body indicating the state of the
   presentity. The times at which the NOTIFY is sent for a particular
   subscriber, and the contents of the body within that notification,
   are subject to any rules specified by the authorization policy that
   governs the subscription. This protocol in no way limits the scope of
   such policies. As a baseline, a reasonable policy is to generate
   notifications when the state of any of the communications addresses
   changes. These notifications contain the complete and current
   presence state of the presentity as known to the presence agent.
   Future extensions can be defined that allow a subscriber to request
   that the notifications contain changes in presence information only,
   rather than complete state.

   In the case of a pending subscription, when final authorization is
   determined, a NOTIFY SHOULD be sent. If the result of the
   authorization decision was success, the NOTIFY SHOULD contain a

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   presence document with the current state of the presentity. If the
   subscription is rejected, a NOTIFY MAY be sent. As described in [2],
   the Subscription-State header can indicate the state of the

   The body of the NOTIFY MUST be sent using one of the types listed in
   the Accept header in the most recent SUBSCRIBE request, or using the
   type "application/cpim-pidf+xml" if no Accept header was present.

   The means by which the PA learns the state of the presentity are also
   outside the scope of this recommendation. Registrations can provide
   one way, although the means (if any) by which a PA uses registrations
   to construct a presence document are an implementation choice. If a
   PUA wishes to explicitly inform the presence agent of its presence
   state, it should explicitly upload the presence document (or its
   piece of it) rather than attempting to manipulate their registrations
   to achieve the desired result.

   For reasons of privacy, it will frequently be necessary to encrypt
   the contents of the notifications. This can be accomplished using
   S/MIME. The encryption can be performed using the key of the
   subscriber as identified in the From field of the SUBSCRIBE.
   Similarly, integrity of the notifications is important to
   subscribers. As such, the contents of the notifications MAY be
   provide authentication and message integrity using S/MIME. Since the
   NOTIFY are generated by the presence server, which may not have
   access to the key of the user represented by the presentity, it will
   frequently be the case that the NOTIFY are signed by a third party.
   It is RECOMMENDED that the signature be by an authority over domain
   of the presentity. In other words, for a user pres:user@example.com,
   the signator of the NOTIFY SHOULD be the authority for example.com.

6.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework [2] leaves it to event packages to describe
   the process followed by the subscriber upon receipt of a NOTIFY
   request, including any logic required to form a coherent resource

   In this specification, each NOTIFY contains either no presence
   document, or a document representing the complete and coherent state
   of the presentity. The presence document in the NOTIFY request with
   the highest CSeq value is the current one. When no document is
   present in that NOTIFY, the presence document present in the NOTIFY
   with the next highest CSeq value is used. Extensions which specify
   the use of partial state for presentities will need to dictate how
   coherent state is achieved.

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6.9 Handling of Forked Requests

   The SIP Events framework [2] requires each package to describe
   handling of forked SUBSCRIBE requests.

   This specification only allows a single dialog to be constructed as a
   result of emitting an initial SUBSCRIBE request. This guarantees that
   only a single PA is generating notifications for a particular
   subscription to a particular presentity. The result of this is that a
   presentity can have multiple PAs active, but these should be
   homogeneous, so that each can generate the same set of notifications
   for the presentity. Supporting heterogeneous PAs, each of which
   generated notifications for a subset of the presence data, is complex
   and difficult to manage. Doing so would require the subscriber to act
   as the aggregator for presence data. This aggregation function can
   only reasonably be performed by agents representing the presentity.
   Therefore, if aggregation is needed, it MUST be done in a PA present
   in a network server representing the presentity that has access to
   the total set of user presence to be aggregated.

   The required processing to guarantee that only a single dialog is
   established is described in Section 5.4.9 of the SIP Events framework

6.10 Rate of Notifications

   For reasons of congestion control, it is important that the rate of
   notifications not become excessive. As a result, it is RECOMMENDED
   that the PA not generate notifications for a single presentity at a
   rate faster than once every 5 seconds.

6.11 State Agents

   It is important to realize that the PA function can be colocated with
   several elements:

        o It can be co-located with the SIP registrar handling
          registrations for the presentity (the co-location of the PA
          within the proxy/registrar is known as a presence server). In
          this way, the presence server knows the presence of the user
          through registrations or other means.

        o It can be co-located with a PUA for that presentity (the co-
          location of the PA within the PUA is known as a presence
          client). In the case of a single PUA per presentity, the PUA
          knows the state of the presentity by sheer nature of its co-

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        o It can be co-located in any proxy along the request path. That
          proxy can learn the presence state of the presentity by
          generating its own SUBSCRIBE in order to determine it. In this
          case, the PA is effectively a B2BUA. For this mechanism to be
          effective, PUA need to act as PA. Therefore, it is RECOMMENDED
          that all PUA be capable of acting as a PA for the state that
          they manipulate, and that they authorize subscriptions that
          can be authenticated as coming from the domain of the

   On occassion, it makes sense for the PA function to migrate from one
   of these places to another. For example, for reasons of scale, the PA
   function may reside in the presence server when the PUA is not
   running, but when the PUA connects to the network, the PA decides to
   migrate subscriptions to it in order to reduce state in the network.
   The mechanism for accomplishing the migration is described in Section
   4.3.5 of [2]. However, packages need to define under what conditions
   such a migration would take place.

   A PA MAY choose to migrate subscriptions at any time, through
   configuration, or through dynamic means. One dynamic means for a
   presence server to discover that the function can migrate to a PUA is
   through the REGISTER message. Specifically, if a PUA wishes to
   indicate support for the PA function, it SHOULD include a contact
   address in its registration with a caller preferences "methods"
   parameter listing SUBSCRIBE [6]. This indicates that it is capable of
   terminating and processing SUBSCRIBE, and therefore can act as a PA.
   However, just because a PUA indicates it can accept subscriptions,
   does not mean a PA should migrate the subscriptions there. In
   particular, a PA SHOULD NOT migrate the subscription if it is
   composing aggregated presence documents from state received from
   several PUA.

   When the PA sends notifications to migrate subscriptions, it should
   be wary of the load that this may cause. A PA SHOULD rate limit the
   notifications, in order to avoid a flood of simultaneous re-
   SUBSCRIBEs from all subscribers.

   In the case where the subscription has migrated to the presence
   server, the presence server will simply act as a PA for these new
   subscriptions. In the case where the subscription has migrated from
   the presence server to the PUA, the presence server MUST operate like
   a proxy. Furthermore, it SHOULD implement the SIP Caller preferences
   extension [6]. Because of the existence of a registered Contact with
   a "methods" parameter containing SUBSCRIBE, the caller preferences
   extension will cause the proxy to send the SUBSCRIBEs to that
   Contact. Assuming it accepts, a 2xx is generated and forwarded to the
   subscriber. The subscriber will now receive and accept notifications

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   from that PA. Because the "methods" parameter does not convey the set
   of event packages for which the PUA can accept SUBSCRIBE, it is
   possible that the PUA doesn't understand the presence event package,
   and will therefore reject the subscription with a 489. In this case,
   the presence server SHOULD NOT migrate any other subscriptions to
   this PUA.

   Migration of subscriptions will still work if the proxy does not
   support the caller preferences extension. However, the proxy will
   instead fork the SUBSCRIBE, possibly to Contacts which have not
   indicated that they support SUBSCRIBE. The result will be 405
   responses from those UAS. However, the one UAS which does support the
   method will generate a 2xx class response (assuming the subscription
   is accepted), and this will be correctly forwarded towards the
   subscriber based on proxy response processing rules [1]. The penalty
   of not supporting caller preferences is the additional unneeded SIP

7 Publication

   The user presence for a presentity can be obtained from any number of
   different ways. None of these mechanisms are mandated by this
   specification. The discussion here is for informational purposes

7.1 Co-location

   When the PA function is co-located with the PUA, user presence is
   known directly by the PA.


   Baseline SIP defines a method that is used by all SIP clients - the
   REGISTER method. This method allows a UA to inform a SIP network of
   its current communications addresses (ie., Contact addresses) .
   Furthermore, multiple UA can independently register Contact addresses
   for the same SIP URL. These Contact addresses can be SIP URLs, or
   they can be any other valid URL.

   Usage of REGISTER information to construct presence is only possible
   if the PA is co-located with, or shares information with, the SIP
   registrar. In this case, the combined PA/registrar/proxy is known as
   a presence server.

   Using the register information for presence is straightforward. The
   address of record in the REGISTER (the To field) identifies the
   presentity. The Contact headers define communications addresses that
   describe the state of the presentity. The use of the SIP caller

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   preferences extension [6] is RECOMMENDED for use with UAs that are
   interested in presence. It provides additional information about the
   Contact addresses that can be used to construct a richer presence

   The presence of a registered Contact with a "methods" parameter [6]
   listing the MESSAGE method implies that the presentity supports
   instant messaging as a communications means.

   The q values from the Contact header [1] can be used to establish
   priorities amongst the various communications addresses in the
   Contact headers.

   The application of registered contacts to presence increases the
   requirements for authenticity. Therefore, REGISTER requests used by
   presence user agents SHOULD be authenticated using either SIP
   authentication mechanisms, or a hop-by-hop mechanism.

7.3 Uploading Presence Documents

   If a means exists to upload presence documents from PUA to the PA,
   the PA can act as an aggregator and redistributor of those documents.
   The PA, in this case, would take the presence documents received from
   each PUA for the same presentity, and merge the communications means
   across all of those PUA into a single presence document. Typically,
   this aggregation would be accomplished through administrator or user
   defined policies about how the aggregation should be done.

   The specific means by which a presence document are uploaded to a
   presence agent are outside the scope of this specification. When a
   PUA wishes to have direct manipulation of the presence that is
   distributed to subscribers, direct uploading of presence documents is

8 Example message flow

   This message flow illustrates how the presence server can be the
   responsible for sending notifications for a presentity. This flow
   assumes that the watcher has previously been authorized to subscribe
   to this resource at the server.

   In this flow, the PUA informs the server about the updated presence
   information though some non-SIP means.

   When the value of the Content-Length header is "..." this means that
   the value should be whatever the computed length of the body is.

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   Watcher             Server                 PUA
      | F1 SUBSCRIBE      |                    |
      |------------------>|                    |
      | F2 200 OK         |                    |
      |<------------------|                    |
      | F3 NOTIFY         |                    |
      |<------------------|                    |
      | F4 200 OK         |                    |
      |------------------>|                    |
      |                   |                    |
      |                   |   Update presence  |
      |                   |<------------------ |
      |                   |                    |
      | F5 NOTIFY         |                    |
      |<------------------|                    |
      | F6 200 OK         |                    |
      |------------------>|                    |

   Message Details

   F1 SUBSCRIBE   watcher->example.com server

      SUBSCRIBE sip:resource@example.com SIP/2.0
      Via: SIP/2.0/UDP watcherhost.example.com;branch=z9hG4bKnashds7
      To: <sip:resource@example.com>
      From: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      CSeq: 1 SUBSCRIBE
      Max-Forwards: 70
      Event: presence
      Accept: application/cpim-pidf+xml
      Contact: <sip:user@watcherhost.example.com>
      Expires: 600
      Content-Length: 0

   F2 200 OK   example.com server->watcher

      SIP/2.0 200 OK
      Via: SIP/2.0/UDP watcherhost.example.com;branch=z9hG4bKnashds7

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      To: <sip:resource@example.com>;tag=ffd2
      From: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      CSeq: 1 SUBSCRIBE
      Event: presence
      Expires: 600
      Contact: sip:example.com
      Content-Length: 0

   F3 NOTIFY  example.com server-> watcher

      NOTIFY sip:user@watcherhost.example.com SIP/2.0
      Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna998sk
      From: <sip:resource@example.com>;tag=ffd2
      To: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      Event: presence
      Subscription-State: active;expires=599
      Max-Forwards: 70
      CSeq: 1 NOTIFY
      Content-Type: application/cpim-pidf+xml
      Content-Length: ..

      [PIDF Document]

   F4 200 OK watcher-> example.com server

      SIP/2.0 200 OK
      Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna998sk
      From: <sip:resource@example.com>;tag=ffd2
      To: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      CSeq: 1 NOTIFY
      Content-Length: 0

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   F5 NOTIFY example.com server -> watcher

      NOTIFY sip:user@watcherhost.example.com SIP/2.0
      Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna998sl
      From: <sip:resource@example.com>;tag=ffd2
      To: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      CSeq: 2 NOTIFY
      Event: presence
      Subscription-State: active;expires=543
      Max-Forwards: 70
      Content-Type: application/cpim-pidf+xml
      Content-Length: ...

      [New PIDF Document]

   F6 200 OK

      SIP/2.0 200 OK
      Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna998sl
      From: <sip:resource@example.com>;tag=ffd2
      To: <sip:user@example.com>;tag=xfg9
      Call-ID: 2010@watcherhost.example.com
      CSeq: 2 NOTIFY
      Content-Length: 0

9 Security considerations

   There are numerous security considerations for presence. Many are
   outlined above; this section considers them issue by issue.

9.1 Firewall and NAT Traversal

   It is anticipated that presence services will be used by clients and
   presentities that are connected to proxy servers on the other side of
   firewalls and NATs. Fortunately, since the SIP presence messages do
   not establish independent media streams, as INVITE does, firewall and

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   NAT traversal is straightforward.

   Specifically, the SIP extensions for NAT traversal [15] are directly
   applicable to SUBSCRIBE and NOTIFY, and will allow operation of the
   protocol through NATs. Firewall administrators can set policies that
   allow or disallow communications services by opening or closing
   access to port 5060 (the default SIP port).

9.2 Privacy

   Privacy encompasses many aspects of a presence system:

        o Subscribers may not want to reveal the fact that they have
          subscribed to certain users

        o Users may not want to reveal that they have accepted
          subscriptions from certain users

        o Notifications (and fetch results) may contain sensitive data
          which should not be revealed to anyone but the subscriber

   Privacy is provided through a combination of hop-by-hop encryption
   and end-to-end encryption. The hop-by-hop mechanisms provide scalable
   privacy services, disable attacks involving traffic analysis, and
   hide all aspects of presence messages. However, they operate based on
   transitivity of trust, and they cause message content to be revealed
   to proxies. The end-to-end mechanisms do not require transitivity of
   trust, and reveal information only to the desired recipient. However,
   end-to-end encryption cannot hide all information, and is susceptible
   to traffic analysis. Strong end to end authentication and encryption
   also requires that both participants have public keys, which is not
   generally the case. Thus, both mechanisms combined are needed for
   complete privacy services.

   SIP allows any hop by hop encryption scheme, but TLS is mandatory to
   implement for servers. Therefore, it is RECOMMENDED that TLS [7] be
   used between elements to provide this function.  The details for
   usage of TLS for server-to-server, and client-to-server security are
   detailed in Section 26.3.2 of SIP [1].

   SIP encryption, using S/MIME, MAY be used end-to-end for the
   transmission of both SUBSCRIBE and NOTIFY requests.

9.3 Message integrity and authenticity

   It is important for the message recipient to ensure that the message
   contents are actually what was sent by the originator, and that the
   recipient of the message be able to determine who the originator

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   really is. This applies to both requests and responses of SUBSCRIBE
   and NOTIFY. This is supported in SIP through end-to-end
   authentication and message integrity. SIP provides http digest for
   authentication, and S/MIME for authentication and integrity.

9.4 Outbound authentication

   When local proxies are used for transmission of outbound messages,
   proxy authentication is RECOMMENDED. This is useful to verify the
   identity of the originator, and prevent spoofing and spamming at the
   originating network.

9.5 Replay prevention

   To prevent the replay of old subscriptions and notifications, all
   signed SUBSCRIBE and NOTIFY requests and responses MUST contain a
   Date header covered by the message signature. Any message with a date
   older than several minutes in the past, or more than several minutes
   into the future, SHOULD be discarded.

   Furthermore, all signed SUBSCRIBE and NOTIFY requests MUST contain a
   Call-ID and CSeq header covered by the message signature. A user
   agent or presence server MAY store a list of Call-ID values, and for
   each, the higest CSeq seen within that Call-ID. Any message that
   arrives for a Call-ID that exists, whose CSeq is lower than the
   highest seen so far, is discarded.

   Finally, HTTP digest authentication MAY be used to prevent replay

9.6 Denial of service attacks

   Denial of service attacks are a critical problem for an open, inter-
   domain, presence protocol. Here, we discuss several possible attacks,
   and the steps we have taken to prevent them.

9.6.1 Smurf attacks through false contacts

   Unfortunately, presence is a good candidate for smurfing attacks
   because of its amplification properties. A single SUBSCRIBE message
   could generate a nearly unending stream of notifications, so long as
   a suitably dynamic source of presence data can be found. Thus, a
   simple way to launch an attack is to send subscriptions to a large
   number of users, and in the Contact header (which is where
   notifications are sent), place the address of the target.

   The only reliable way to prevent these attacks is through
   authentication and authorization. End users will hopefully not accept

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   subscriptions from random unrecognized users. Also, the presence
   client software could be programmed to warn the user when the Contact
   header in a SUBSCRIBE is from a domain which does not match that of
   the From field (which identifies the subscriber).

   Also, note that as described in [2], if a NOTIFY is not acknowledged
   or was not wanted, the subscription that generated it is removed.
   This eliminates the amplification properties of providing false
   Contact addresses.

10 IANA Consideration

   This specification registers an event package, based on the
   registration procedures defined in [2]. The following is the
   information required for such a registration:

        Package Name: presence

        Package or Template-Package: This is a package.

        Published Document: RFC XXXX (Note to RFC Editor: Please fill in
             XXXX with the RFC number of this specification).

        Person to Contact: Jonathan Rosenberg, jdrosen@jdrosen.net.

11 Acknowledgements

   We would like to thank the following people for their support and
   comments on this draft:

   Rick Workman     Nortel
   Adam Roach       dynamicsoft
   Sean Olson       Ericsson
   Billy Biggs      University of Waterloo
   Stuart Barkley   UUNet
   Mauricio Arango  Sun
   Richard Shockey  Neustar
   Jorgen Bjorkner  Hotsip
   Henry Sinnreich  MCI Worldcom
   Ronald Akers     Motorola

12 Authors Addresses

   Jonathan Rosenberg

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   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Dean Willis
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024
   email: dwillis@dynamicsoft.com

   Robert Sparks
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024
   email: rsparks@dynamicsoft.com

   Ben Campbell
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024
   email: bcampbell@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu

   Jonathan Lennox
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: lennox@cs.columbia.edu

   Christian Huitema
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052-6399
   email: huitema@microsoft.com

   Bernard Aboba
   Microsoft Corporation

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   One Microsoft Way
   Redmond, WA 98052-6399
   email: bernarda@microsoft.com

   David Gurle
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052-6399
   email: dgurle@microsoft.com

   David Oran
   Cisco Systems
   170 West Tasman Dr.
   San Jose, CA 95134
   email: oran@cisco.com

   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

13 Normative References

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   [1] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] A. Roach et al.  , "SIP-specific event notification," Internet
   Draft, Internet Engineering Task Force, Feb. 2002.  Work in progress.

   [3] D. Crocker et al.  , "A common profile for instant messaging
   (CPIM)," Internet Draft, Internet Engineering Task Force, Nov. 2001.
   Work in progress.

   [4] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [5] H. Sugano, S. Fujimoto, et al.  , "CPIM presence information data
   format," Internet Draft, Internet Engineering Task Force, Oct. 2001.
   Work in progress.

   [6] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
   callee capabilities," Internet Draft, Internet Engineering Task
   Force, Nov. 2001.  Work in progress.

   [7] T. Dierks and C. Allen, "The TLS protocol version 1.0," Request
   for Comments 2246, Internet Engineering Task Force, Jan. 1999.

14 Informative References

   [8] M. Day, J. Rosenberg, and H. Sugano, "A model for presence and
   instant messaging," Request for Comments 2778, Internet Engineering
   Task Force, Feb.  2000.

   [9] W. Marshall et al.  , "SIP extensions for caller identity and
   privacy," Internet Draft, Internet Engineering Task Force, Nov. 2001.
   Work in progress.

   [10] P. Calhoun, H. Akhtar, J. Arkko, E. Guttman, A. Rubens, and G.
   Zorn, "Diameter base protocol," Internet Draft, Internet Engineering
   Task Force, Nov.  2001.  Work in progress.

   [11] C. Rigney, S. Willens, A. Rubens, and W. Simpson, "Remote
   authentication dial in user service (RADIUS)," Request for Comments
   2865, Internet Engineering Task Force, June 2000.

   [12] J. Boyle, R. Cohen, D. Durham, S. Herzog, R. Rajan, and A.
   Sastry, "The COPS (common open policy service) protocol," Request for
   Comments 2748, Internet Engineering Task Force, Jan. 2000.

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   [13] J. Rosenberg, "A SIP event sub-package for watcher information,"
   Internet Draft, Internet Engineering Task Force, July 2001.  Work in

   [14] M. Day, S. Aggarwal, G. Mohr, and J. Vincent, "Instant messaging
   / presence protocol requirements," Request for Comments 2779,
   Internet Engineering Task Force, Feb. 2000.

   [15] J. Rosenberg, J. Weinberger, and H. Schulzrinne, "SIP extensions
   for NAT traversal," Internet Draft, Internet Engineering Task Force,
   Nov. 2001.  Work in progress.

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