Internet Engineering Task Force                               Adam Roach
Internet Draft                                             Ericsson Inc.
Category: Standards Track                                  November 2001
                                                        Expires May 2002

                    SIP-Specific Event Notification

Status of this Memo

     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
     other groups may also distribute working documents as

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
     documents at any time. It is inappropriate to use Internet-Drafts
     as reference material or cite them other than as "work in

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at

     This document is an individual submission to the IETF. Comments
     should be directed to the authors.


     This document describes an extension to the Session Initiation
     Protocol (SIP). The purpose of this extension is to provide an
     extensible framework by which SIP nodes can request notification
     from remote nodes indicating that certain events have occurred.

     Concrete uses of the mechanism described in this document may be
     standardized in the future.

     Note that the event notification mechanisms defined herein are
     NOT intended to be a general-purpose infrastructure for all
     classes of event subscription and notification.

1. Table of Contents

    1.       Table of Contents...................................... 1
    2.       Introduction........................................... 3
    2.1.     Overview of Operation.................................. 4
    3.       Syntax................................................. 4

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    3.1.     New Methods............................................ 4
    3.1.1.   SUBSCRIBE method....................................... 5
    3.1.2.   NOTIFY method.......................................... 6
    3.2.     New Headers............................................ 6
    3.2.1.   "Event" header......................................... 6
    3.2.2.   "Allow-Events" Header.................................. 7
    3.2.3.   "Subscription-Expires" Header.......................... 7
    3.3.     New Response Codes..................................... 7
    3.3.1.   "202 Accepted" Response Code........................... 8
    3.3.2.   "489 Bad Event" Response Code.......................... 8
    4.       Node Behavior.......................................... 8
    4.1.     General................................................ 8
    4.1.1.   Route Handling......................................... 8
    4.1.2.   Detecting support for SUBSCRIBE and NOTIFY............. 9
    4.1.3.   CANCEL requests........................................ 9
    4.1.4.   State Agents and Notifier Migration.................... 9
    4.2.     Description of SUBSCRIBE Behavior...................... 10
    4.2.1.   Correlation to dialogs, calls, and terminals........... 10
    4.2.2.   Subscription duration.................................. 11
    4.2.3.   Identification of Subscribed Events and Event Classes.. 11
    4.2.4.   Additional SUBSCRIBE Header Values..................... 12
    4.2.5.   Subscriber SUBSCRIBE Behavior.......................... 12
    4.2.6.   Proxy SUBSCRIBE Behavior............................... 14
    4.2.7.   Notifier SUBSCRIBE Behavior............................ 14
    4.3.     Description of NOTIFY Behavior......................... 17
    4.3.1.   Correlation............................................ 17
    4.3.2.   Identification of reported events, event classes, and c 18
    4.3.3.   Notifier NOTIFY Behavior............................... 18
    4.3.4.   Proxy NOTIFY Behavior.................................. 20
    4.3.5.   Subscriber NOTIFY Behavior............................. 20
    4.4.     Polling Resource State................................. 21
    4.5.     Allow-Events header usage.............................. 21
    5.       Event Packages......................................... 21
    5.1.     Appropriateness of Usage............................... 22
    5.2.     Sub-packages........................................... 22
    5.3.     Amount of State to be Conveyed......................... 23
    5.3.1.   Complete State Information............................. 23
    5.3.2.   State Deltas........................................... 23
    5.4.     Event Package Responsibilities......................... 24
    5.4.1.   Event Package Name..................................... 24
    5.4.2.   Event Package Parameters............................... 24
    5.4.3.   SUBSCRIBE Bodies....................................... 24
    5.4.4.   Subscription Duration.................................. 25
    5.4.5.   NOTIFY Bodies.......................................... 25
    5.4.6.   Notifier processing of SUBSCRIBE requests.............. 25
    5.4.7.   Notifier generation of NOTIFY requests................. 25
    5.4.8.   Subscriber processing of NOTIFY requests............... 25
    5.4.9.   Handling of forked requests............................ 26
    5.4.10.  Rate of notifications.................................. 26
    5.4.11.  State Agents........................................... 26

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    5.4.12.  Examples............................................... 26
    6.       Security Considerations................................ 27
    6.1.     Access Control......................................... 27
    6.2.     Release of Sensitive Policy Information................ 27
    6.3.     Denial-of-Service attacks.............................. 27
    7.       IANA Considerations.................................... 27
    7.1.     Registration Template.................................. 28
    8.       Open Issues............................................ 29
    8.1.     CANCEL Handling........................................ 29
    8.2.     Version of SIP to reference............................ 29
    8.3.     Immediate NOTIFYs...................................... 30
    9.       Changes................................................ 30
    9.1.     Changes from draft-ietf-...-00......................... 30
    9.2.     Changes from draft-roach-...-03........................ 31
    9.3.     Changes from draft-roach-...-02........................ 33
    9.4.     Changes from draft-roach-...-01........................ 35
    10.      References............................................. 35
    11.      Acknowledgements....................................... 36
    12.      Author's Address....................................... 36

2. Introduction

     The ability to request asynchronous notification of events proves
     useful in many types of services for which cooperation between
     end-nodes is required. Examples of such services include
     automatic callback services (based on terminal state events),
     buddy lists (based on user presence events), message waiting
     indications (based on mailbox state change events), and PINT
     status (based on call state events).

     The methods described in this document allow a framework by which
     notification of these events can be ordered.

     The event notification mechanisms defined herein are NOT intended
     to be a general-purpose infrastructure for all classes of event
     subscription and notification. Meeting requirements for the
     general problem set of subscription and notification is far too
     complex for a single protocol. Our goal is to provide a
     SIP-specific framework for event notification which is not so
     complex as to be unusable for simple features, but which is still
     flexible enough to provide powerful services. Note, however, that
     event packages based on this framework may define arbitrarily
     complex rules which govern the subscription and notification for
     the events or classes of events they describe.

     This draft does not describe an extension which may be used
     directly; it must be extended by other drafts (herein referred to
     as "event packages.") In object-oriented design terminology, it
     may be thought of as an abstract base class which must be derived

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     into an instantiatable class by further extensions. Guidelines
     for creating these extensions are described in section 5.

2.1. Overview of Operation

     The general concept is that entities in the network can subscribe
     to resource or call state for various resources or calls in the
     network, and those entities (or entities acting on their behalf)
     can send notifications when those states change.

     A typical flow of messages would be:

     Subscriber          Notifier
         |-----SUBSCRIBE---->|     Request state subscription
         |<-------200--------|     Acknowledge subscription
         |<------NOTIFY----- |     Return current state information
         |<------NOTIFY----- |     Return current state information

     The subscriber and notifier entities need not necessarily be UAs,
     but often will be.

     Subscriptions are expired and must be refreshed in exactly the
     same manner as registrations (see RFC 2543 [1] ).

3. Syntax

     This section describes the syntax extensions required for event
     notification in SIP. Semantics are described in section 4.

3.1. New Methods

     This document describes two new SIP methods: "SUBSCRIBE" and

     This table expands on tables 4 and 5 in RFC 2543 [1] .

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     Header                    Where    SUB NOT
     ------                    -----    --- ---
     Accept                      R       o   o
     Accept-Encoding             R       o   o
     Accept-Language             R       o   o
     Allow                      200      -   -
     Allow                      405      o   o
     Authorization               R       o   o
     Call-ID                    gc       m   m
     Contact                     R       m   m
     Contact                    1xx      o   o
     Contact                    2xx      m   o
     Contact                    3xx      m   m
     Contact                    485      o   o
     Content-Encoding            e       o   o
     Content-Length              e       o   o
     Content-Type                e       *   *
     CSeq                       gc       m   m
     Date                        g       o   o
     Encryption                  g       o   o
     Expires                     g       o   -
     From                       gc       m   m
     Hide                        R       o   o
     Max-Forwards                R       o   o
     Organization                g       o   o
     Priority                    R       o   o
     Proxy-Authenticate         407      o   o
     Proxy-Authorization         R       o   o
     Proxy-Require               R       o   o
     Require                     R       o   o
     Retry-After                 R       -   -
     Retry-After            404,480,486  o   o
     Retry-After                503      o   o
     Retry-After              600,603    o   o
     Response-Key                R       o   o
     Record-Route                R       o   o
     Record-Route               2xx      o   o
     Route                       R       o   o
     Server                      r       o   o
     Subject                     R       o   o
     Timestamp                   g       o   o
     To                        gc(1)     m   m
     Unsupported                420      o   o
     User-Agent                  g       o   o
     Via                       gc(2)     m   m
     Warning                     r       o   o
     WWW-Authenticate           401      o   o

3.1.1. SUBSCRIBE method

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     "SUBSCRIBE" is added to the definition of the element "Method" in
     the SIP message grammar.

     Like all SIP method names, the SUBSCRIBE method name is case
     sensitive. The SUBSCRIBE method is used to request asynchronous
     notification of an event or set of events at a later time.

3.1.2. NOTIFY method

     "NOTIFY" is added to the definition of the element "Method" in
     the SIP message grammar.

     The NOTIFY method is used to notify a SIP node that an event
     which has been requested by an earlier SUBSCRIBE method has
     occurred. It may also provide further details about the event.

3.2. New Headers

     This table expands on tables 4 and 5 in RFC 2543 [1] , as amended
     by the changes described in section 3.1.

     Header field         where  proxy ACK BYE CAN INV OPT REG SUB NOT
     Allow-Events           g           o   o   o   o   o   o   o   o
     Allow-Events          489          -   -   -   -   -   -   m   m
     Event                  R           -   -   -   -   -   -   m   m
     Subscription-Expires   R           -   -   -   -   -   -   -   o

3.2.1. "Event" header

     The following header is defined for the purposes of this

     Event             =  ( "Event" | "o" ) ":" event-type
                          *(( ";" parameter-name
                          ["=" ( token | quoted-string ) ] )
     event-type        =  event-package *( "." event-subpackage )
     event-package     =  token-nodot
     event-subpackage  =  token-nodot
     token-nodot       =  1*( alphanum | "-"  | "!" | "%" | "*"
                              | "_" | "+" | "`" | "'" | "~" )

     Event is added to the definition of the element "request-header"
     in the SIP message grammar.

     This document does not define values for event-types. These
     values will be defined by individual event packages, and MUST be

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     registered with the IANA.

     There must be exactly one event type listed per event header.
     Multiple events per message are disallowed.

     For the curious, the "o" short form is chosen to represent

3.2.2. "Allow-Events" Header

     The following header is defined for the purposes of this

     Allow-Events =  ( "Allow-Events" | "u" ) ":" 1#event-type

     Allow-Events is added to the definition of the element
     "general-header" in the SIP message grammar.

     For the curious, the "u" short form is chosen to represent

3.2.3. "Subscription-Expires" Header

     The following header is defined for the purposes of this

     Subscription-Expires =  "Subscription-Expires" ":"
                              ( SIP-date | delta-seconds )
                               *( ";" subexp-params )

     subexp-params        =   "reason" "=" reason-code
                            | generic-param

     reason-code          =   "migration"
                            | "maint"
                            | "refused"
                            | "timeout"
                            | reason-extension

     reason-extension      =   token

     Subscription-Expires is added to the definition of the element
     "request-header" in the SIP message grammar.

3.3. New Response Codes

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3.3.1. "202 Accepted" Response Code

     The 202 response is added to the "Success" header field

     Success  = "200"  ;  OK
             |  "202"  ;  Accepted

     "202 Accepted" has the same meaning as that defined in HTTP/1.1
     [5] .

3.3.2. "489 Bad Event" Response Code

     The 489 event response is added to the "Client-Error" header
     field definition:

     Client-Error = "400"  ; Bad Request
                  | "489"  ; Bad Event

     "489 Bad Event" is used to indicate that the server did not
     understand the event package specified in a "Event" header field.

4. Node Behavior

4.1. General

     Unless noted otherwise, SUBSCRIBE and NOTIFY requests follow the
     same protocol rules governing the usage of tags, Route handling,
     Record-Route handling, Via handling, and Contact handling as
     INVITE; retransmission, reliability, CSeq handling and
     provisional responses are the same as those defined for BYE.

     For the purposes of this document, a "dialog" is defined as all
     messages sharing the tuple of "To" (including tag), "From"
     (including tag), and "Call-Id." As in INVITE-initiated dialogs,
     requests containg no "To" tag are also considered to be part of
     the same dialog as messages which contain a "To" tag but
     otherwise match.

4.1.1. Route Handling

     Route and Record-Route handling for SUBSCRIBE and NOTIFY dialogs
     is generally the same as for INVITE and its subsequent responses.
     The exact method for echoing Record-Route headers in responses
     and using them to form Route headers in subsequent requests is
     described in RFC2543 [1] . For the purposes of the following

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     discussion, the "Contact" header is considered part of the
     "Record-Route" set.

     From a subscriber perspective, the "Record-Route" headers
     received in a SUBSCRIBE response are stored locally and placed in
     the "Route" headers for SUBSCRIBE refreshes. To support forking
     of SUBSCRIBE requests, "Record-Route" headers received in NOTIFY
     requests MUST be echoed back in the NOTIFY responses; if no route
     for the dialog has been established, these "Record-Route" headers
     MUST be stored locally and MUST be placed in the "Route" headers
     for SUBSCRIBE refreshes.

     From a notifier perspective, SUBSCRIBE request "Record-Route"
     headers are echoed back in the SUBSCRIBE response and stored
     locally. The locally stored copy of the "Record-Route" headers is
     placed in the "Route" headers when generating NOTIFY requests.

     The result of the forgoing rules is that proxies wishing to
     remain on the signalling path for subsequent requests in the
     dialog MUST include themselves in a "Record-Route" for all
     requests, not just the initial SUBSCRIBE.

4.1.2. Detecting support for SUBSCRIBE and NOTIFY

     Neither SUBSCRIBE nor NOTIFY necessitate the use of "Require" or
     "Proxy-Require" headers; similarly, there is no token defined for
     "Supported" headers. If necessary, clients may probe for the
     support of SUBSCRIBE and NOTIFY using the OPTIONS request defined
     in RFC2543 [1] .

     The presence of the "Allow-Events" header in a message is
     sufficient to indicate support for SUBSCRIBE and NOTIFY.

     The "methods" parameter for Contact may also be used to
     specifically announce support for SUBSCRIBE and NOTIFY messages
     when registering. (See reference [8] for details on the "methods"

4.1.3. CANCEL requests

     For the purposes of generality, both SUBSCRIBE and NOTIFY MAY be
     canceled; however, doing so is not recommended. Successfully
     cancelled SUBSCRIBE and NOTIFY requests MUST be completed with a
     "487 Request Cancelled" response; the server acts as if the
     request were never received. In general, since neither SUBSCRIBE
     nor NOTIFY are allowed to have protracted transactions, attempts
     to cancel them are expected to fail.

4.1.4. State Agents and Notifier Migration

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     When state agents (see section 5.4.11. ) are used, it is often
     useful to allow migration of subscriptions between state agents
     and the nodes for which they are providing state aggregation (or
     even among various state agents). Such migration may be effected
     by sending a "NOTIFY" with an "Subscription-Expires" header of
     "0," and a reason parameter of "migration." This NOTIFY request
     is otherwise normal, and is formed as described in section 4.3.3.

     Upon receipt of this NOTIFY message, the subscriber SHOULD
     attempt to re-subscribe (as described in the following sections).
     The resulting SUBSCRIBE message can then be proxied or redirected
     to the node to which notification responsibility is passing.

4.2. Description of SUBSCRIBE Behavior

     The SUBSCRIBE method is used to request current state and state
     updates from a remote node.

4.2.1. Correlation to dialogs, calls, and terminals

     A subscription is uniquely identified by the combination of the
     To, From, and Call-ID fields in the SUBSCRIBE request. Refreshes
     of subscriptions SHOULD reuse the same Call-ID if possible, since
     subscriptions are uniquely identified at presence servers using
     the Call-ID. Two subscriptions from the same user, for the same
     user, but with different Call-IDs, are considered different
     subscriptions. Note this is exactly the same as usage of Call-ID
     in registrations.

     Initial SUBSCRIBE requests MUST contain a "tag" parameter (as
     defined in RFC 2543 [1] ) in the "From" header, and MUST NOT
     contain a "tag" parameter in the "To" header. Responses to
     SUBSCRIBE requests MUST contain a "tag" parameter in the "To"

     The "tag" in the "To" header allows the subscriber to
     differentiate between NOTIFY requests from different clients in
     the case that the SUBSCRIBE request was forked. SUBSCRIBE
     requests for re-subscription MUST contain "tag" parameters in
     both the "To" and "From" headers (matching those previously
     established for the dialog).

     The relationship between subscriptions and (INVITE-initiated)
     sessions sharing the same dialog correlation information is
     undefined. Re-using  dialog correlation information for
     subscriptions is allowed, but sharing of such information does
     not change the semantics of the INVITE session or the SUBSCRIBE

     Similarly, the relationship between a subscription in one

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     direction (e.g. from node A to node B) and a subscription in the
     opposite direction (from B to A) with the same dialog correlation
     information is undefined. While re-using such information is
     allowed, the sharing of such information does not change the
     semantics of either SUBSCRIBE dialog.

4.2.2. Subscription duration

     SUBSCRIBE requests SHOULD contain an "Expires" header. This
     expires value indicates the duration of the subscription. The
     formatting of these is described in RFC 2543. In order to keep
     subscriptions effective beyond the duration communicated in the
     "Expires" header, subscribers need to refresh subscriptions on a
     periodic basis. This refreshing is performed in the same way as
     REGISTER refreshes: the To, From, and Call-ID match those in the
     SUBSCRIBE being refreshed, while the CSeq number is incremented.

     If no "Expires" header is present in a SUBSCRIBE request, the
     implied default is defined by the event package being used.

     200-class responses to SUBSCRIBE requests also MUST contain an
     "Expires" header. The period of time in the response MAY be
     shorter or longer than specified in the request. The period of
     time in the response is the one which defines the duration of the

     Similar to REGISTER requests, SUBSCRIBE requests may be renewed
     at any time to prevent them from expiring at the end of the
     "Expires" period. These renewals will contain a the same "To,"
     "From," and "Call-ID" as the original request, and an incremented
     "CSeq" number.

     Also similar to REGISTER requests, a natural consequence of this
     scheme is that a SUBSCRIBE with an "Expires" of 0 constitutes a
     request to unsubscribe from an event.

     Notifiers may also wish to cancel subscriptions to events; this
     is useful, for example, when the resource to which a subscription
     refers is no longer available. Further details on this mechanism
     are discussed in section 4.3.3.

4.2.3. Identification of Subscribed Events and Event Classes

     Identification of events is provided by three pieces of
     information: Request URI, Event Type, and (optionally) message

     The Request URI of a SUBSCRIBE request, most importantly,
     contains enough information to route the request to the
     appropriate entity. It also contains enough information to

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     identify the resource for which event notification is desired,
     but not necessarily enough information to uniquely identify the
     nature of the event (e.g. "" would be
     an appropriate URI to subscribe to for my presence state; it
     would also be an appropriate URI to subscribe to the state of my
     voice mailbox).

     Subscribers MUST include exactly one "Event" header in SUBSCRIBE
     requests, indicating to which event or class of events they are
     subscribing. The "Event" header will contain a token which
     indicates the type of state for which a subscription is being
     requested. This token will be registered with the IANA and will
     correspond to an event package which further describes the
     semantics of the event or event class.

     The "Event" header is considered mandatory for the purposes of
     this document. However, to maintain compatibility with PINT (see
     [3] ), servers MAY interpret a SUBSCRIBE request with no "Event"
     header as requesting a subscription to PINT events. If the
     servers do not support PINT, they SHOULD return "489 Bad Event"
     to any SUBSCRIBE messages without an EVENT header.

     If the event package to which the event token corresponds defines
     behavior associated with the body of its SUBSCRIBE requests,
     those semantics apply.

4.2.4. Additional SUBSCRIBE Header Values

     Each SUBSCRIBE request MUST have exactly one "Contact:" header,
     to be used as part of route handling, as described in section

     SUBSCRIBE requests MAY contain an "Accept" header. This header,
     if present, indicates the body formats allowed in subsequent
     NOTIFY requests. Event packages MUST define the behavior for
     SUBSCRIBE requests without "Accept" headers; usually, this will
     connote a single, default body type.

     Header values not described in this document are to be
     interpreted as described in RFC 2543 [1] .

4.2.5. Subscriber SUBSCRIBE Behavior Requesting a Subscription

     When a subscriber wishes to subscribe to a particular state for a
     resource, it forms a SUBSCRIBE message.

     The dialog correlation information is formed as if for an
     original INVITE: the Call-ID is a new call ID with the syntax

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     described in RFC 2543; the To: field indicates the subscribed
     resource's persistent address (which will generally match the
     Request URI used to form the message); and the From: field will
     indicate the subscriber's persistent address (typically

     This SUBSCRIBE request will be confirmed with a final response.
     200-class responses indicate that the subscription has been
     accepted, and that a NOTIFY will be sent immediately. A 200
     response indicates that the subscription has been accepted and
     that the user is authorized to subscribe to the requested
     resource. A 202 response merely indicates that the subscription
     has been understood, and that authorization may or may not have
     been granted.

     The "Expires" header in a 200-class response to SUBSCRIBE
     indicates the actual duration for which the subscription will
     remain active (unless refreshed).

     Non-200 class final responses indicate that the subscription has
     not been created, and no subsequent NOTIFY message will be sent.
     All non-200 class responses (with the exception of "489,"
     described herein) have the same meanings and handling as
     described in RFC 2543 [1] . Refreshing of Subscriptions

     At any time before a subscription expires, the subscriber may
     refresh the timer on such a subscription by re-sending a
     SUBSCRIBE request. The handling for such a request is the same as
     for the initial creation of a subscription except as described

     Subscription renewals will contain a "To" field matching the
     "From" field in the first NOTIFY request for the dialog
     containing the subscription to be refreshed. They will contain
     the same "From" and "Call-ID" fields as the original SUBSCRIBE
     request, and an incremented "CSeq" number from the original
     SUBSCRIBE request. Route handling is as discussed in section

     If a SUBSCRIBE request to refresh a subscription receives a "481"
     response, this indicates that the subscription has been
     terminated and that the subscriber did not receive notification
     of this fact. In this case, the subscriber should consider the
     subscription invalid. If the subscriber wishes to re-subscribe to
     the state, he does so by composing an unrelated initial SUBSCRIBE
     request with a freshly-generated Call-ID and a new, unique "From"
     tag (see section )

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     If a SUBSCRIBE request to refresh a subscription fails, the
     original subscription is still considered valid for the duration
     of the most recently known "Expires" value as negotiated by
     SUBSCRIBE and its response, or as communicated by NOTIFY in
     "Subscription-Expires," except as described above. Unsubscribing

     Unsubscribing is handled in the same way as refreshing of a
     subscription, with the "Expires" header set to "0." Note that a
     successful unsubscription will also trigger a final "NOTIFY". Confirmation of Subscription Creation

     The subscriber can expect to receive a NOTIFY message from each
     node which has registered a successful subscription or
     subscription refresh. Until the first NOTIFY message arrives, the
     subscriber should consider the state of the subscribed resource
     to be in a neutral state. Event packages which define new event
     packages MUST define this "neutral state" in such a way that
     makes sense for their application (see section 5.4.7. ).

     Due to the potential for both out-of-order messages and forking,
     the subscriber MUST be prepared to receive NOTIFY messages before
     the SUBSCRIBE transaction has completed.

     Except as noted above, processing of this NOTIFY is the same as
     in section 4.3.5.

4.2.6. Proxy SUBSCRIBE Behavior

     Proxies need no additional behavior beyond that described in RFC
     2543 [1] to support SUBSCRIBE. If a proxy wishes to see all of
     the SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
     record-route all SUBSCRIBE and NOTIFY requests.

4.2.7. Notifier SUBSCRIBE Behavior SUBSCRIBE Transaction Processing

     In no case should a SUBSCRIBE transaction extend for any longer
     than the time necessary for automated processing. In particular,
     notifiers MUST NOT wait for a user response before returning a
     final response to a SUBSCRIBE request.

     The notifier SHOULD check that the event package specified in the
     "Event" header is understood. If not, the notifier SHOULD return
     a "489 Bad Event" response to indicate that the specified
     event/event class is not understood.

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     The notifier SHOULD also perform any necessary authentication and
     authorization per its local policy. See section

     If the SUBSCRIBE request contains a tag parameter in the "To"
     field, but the notifier has no record of the indicated dialog,
     the notifier has two options. If the notifier is able and willing
     to reconstruct subscription state, he may accept the subscription
     as an initial subscription. If the notifier cannot or is not
     willing to reconstitute such state, it should respond with a "481
     Subscription does not exist."

     If the notifier is able to immediately determine that it
     understands the event package, that the authenticated subscriber
     is authorized to subscribe, and that there are no other barriers
     to creating the subscription, it creates the subscription and
     returns a "200 OK" response, unless doing so would reveal
     authorization policy in an undesirable fashion (see section 6.2.

     If the notifier cannot immediately create the subscription (e.g.
     it needs to wait for user input for authorization, or is acting
     for another node which is not currently reachable), or wishes to
     mask authorization policy, it will return a "202 Accepted"
     response. This response indicates that the request has been
     received and understood, but does not necessarily imply that the
     subscription has been created yet.

     The "Expires" values present in SUBSCRIBE 200-class responses
     behave in the same way as they do in REGISTER responses: the
     server MAY shorten or lengthen the interval.

     200-class responses to SUBSCRIBE requests will not generally
     contain any useful information beyond subscription duration;
     their primary purpose is to serve as a reliability mechanism.
     State information will be communicated via a subsequent NOTIFY
     request from the notifier.

     The other response codes defined in RFC 2543 may be used in
     response to SUBSCRIBE requests, as appropriate. Confirmation of Subscription Creation/Refreshing

     Upon successfully accepting or refreshing of a subscription,
     notifiers MUST send a NOTIFY message immediately to communicate
     the current resource state to the subscriber. If the resource has
     no meaningful state at the time that the SUBSCRIBE message is
     processed, this NOTIFY message MAY contain an empty or neutral
     body. See section 4.3.3. for further details on NOTIFY message

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     Note that a NOTIFY message is always sent immediately after any
     200-class response to a SUBSCRIBE request, regardless of whether
     the subscription has already been authorized. Authentication/Authorization of SUBSCRIBE requests

     Privacy concerns may require that notifiers either use access
     lists or ask the notifier owner, on a per-subscription basis,
     whether a particular remote node is authorized to subscribe to a
     certain set of events. In general, authorization of users prior
     to authentication is not particularly useful.

     SIP authentication mechanisms are discussed in RFC2543 [1] . Note
     that, even if the notifier node typically acts as a proxy,
     authentication for SUBSCRIBE requests will always be performed
     via a "401" response, not a "407;" notifiers always act as a user
     agents when accepting subscriptions and sending notifications.

     If authorization fails based on an access list or some other
     automated mechanism (i.e. it can be automatically authoritatively
     determined that the subscriber is not authorized to subscribe),
     the notifier SHOULD reply to the request with a "403 Forbidden"
     or "603 Decline" response, unless doing so might reveal
     information that should stay private; see section 6.2.

     If the notifier owner is interactively queried to determine
     whether a subscription is allowed, a "202 Accept" response is
     returned immediately. Note that a NOTIFY message is still formed
     and sent under these circumstances, as described in the previous

     If subscription authorization was delayed and the notifier wishes
     to convey that such authorization has been declined, it may do so
     by sending a NOTIFY message containting a "Subscription-Expires"
     header with a value of "0" and a reason parameter of "refused." Refreshing of Subscriptions

     When a notifier receives a subscription refresh, assuming that
     the subscriber is still authorized, the notifier updates the
     expiration time for subscription. As with the initial
     subscription, the server MAY shorten or increase the amount of
     time until expiration. The final expiration time is placed in the
     "Expires" header in the response.

     If no refresh for a notification address is received before its
     expiration time, the subscription is removed. When removing a
     subscription, the notifier MAY send a NOTIFY message with a
     "Subscription-Expires" value of "0" to inform it that the
     subscription is being removed. If such a message is sent, the

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     "Subscription-Expires" header SHOULD contain a "reason=timeout"

4.3. Description of NOTIFY Behavior

     NOTIFY messages are sent to inform subscribers of changes in
     state to which the subscriber has a subscription. Subscriptions
     are typically put in place using the SUBSCRIBE method; however,
     it is possible that other means have been used.

     If any non-SUBSCRIBE mechanisms are defined to create
     subscriptions, it is the responsibility of the parties defining
     those mechanisms to ensure that correlation of a NOTIFY message
     to the corresponding subscription is possible. Designers of such
     mechanisms are also warned to make a distinction between sending
     a NOTIFY message to a subscriber who is aware of the
     subscription, and sending a NOTIFY message to an unsuspecting
     node. The latter behavior is invalid, and MUST receive a "481
     Subscription does not exist" response (unless some other 400- or
     500-class error code is more applicable), as described in section
     4.3.5. In other words, knowlege of a subscription must exist in
     both the subscriber and the notifier to be valid, even if
     installed via a non-SUBSCRIBE mechanism.

     A NOTIFY does not cancel its corresponding subscription; in other
     words, a single SUBSCRIBE request may trigger several NOTIFY

4.3.1. Correlation

     NOTIFY requests MUST contain the same Call-ID as the SUBSCRIBE
     request which ordered them; the "To" field MUST match the "From"
     field in the SUBSCRIBE that ordered them, and the "From" field
     MUST match the "To" field that was sent in the 200-class response
     to the SUBSCRIBE. In other words, NOTIFY requests MUST be in the
     same dialog as the SUBSCRIBE that ordered them.

     The From field of a NOTIFY request, like the "To" field of a
     SUBSCRIBE response, MUST contain a tag; this allows for the
     subscriber to differentiate between events from different

     Successful SUBSCRIBE requests will receive only one 200-class
     response; however, due to forking, the subscription may have been
     accepted by multiple nodes. The subscriber MUST therefore be
     prepared to receive NOTIFY requests with "From:" tags which
     differ from the "To:" tag received in the SUBSCRIBE 200-class

     If multiple NOTIFY messages are received in response to a single

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     SUBSCRIBE message, they represent different destinations to which
     the SUBSCRIBE request was forked. Unless the event package
     specifies otherwise, the subscriber may either accept all such
     notifications as representing different dialogs (which are then
     refreshed separately), or send a 481 response to any NOTIFYs on
     dialogs that it does not want to keep alive.

     As expected, CSeq spaces are unique for each node; in other
     words, the notifier uses a different CSeq space than the
     subscriber and any other notifiers.

4.3.2. Identification of reported events, event classes, and current

     Identification of events being reported in a notification is very
     similar to that described for subscription to events (see section
     4.2.3. ).

     The Request URI of a NOTIFY request contains enough information
     to route the request to the party which is subscribed to receive
     notifications. It is derived from the "Contact" header present in
     the corresponding SUBSCRIBE request.

     If the same events for different resources are being subscribed
     to, implementors are expected to use different dialogs in order
     to be able to differentiate between notifications for them,
     unless the body for the event contains enough information for
     this correlation.

     As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a
     single  token which identifies the event or class of events for
     which a notification is being generated.

     If the event package to which the event token corresponds defines
     behavior associated with the body of its NOTIFY requests, those
     semantics apply. This information is expected to provide
     additional details about the nature of the event which has
     occurred and the resultant resource state.

     When present, the body of the NOTIFY request MUST be formatted
     into one of the body formats specified in the "Accept" header of
     the corresponding SUBSCRIBE request.

4.3.3. Notifier NOTIFY Behavior

     When a SUBSCRIBE request is successfully processed or a relevant
     change in the subscribed state occurs, the notifier will
     immediately construct and send a NOTIFY request to the
     subscriber(s), per standard Route/Record-Route handling, as
     described in section 4.1.1.

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     If the notifier is able, through any means, to determine that the
     subscriber is no longer available to receive notifications, it
     MAY elect to not send a notification. An example of a method by
     which such information may be known is the "SIP for Presence"
     event set (see [4] ).

     A NOTIFY request is considered failed if the response times out,
     or a non-200 class response code is received which has no
     "Retry-After" header and no implied further action which can be
     taken to retry the request (e.g. "401 Authorization Required.")

     If the NOTIFY request fails (as defined above) due to a timeout
     condition, and the subscription was installed using a soft-state
     mechanism (such as SIP signalling), the notifier SHOULD remove
     the subscription.

     If the NOTIFY request fails (as defined above) due to an error
     response, and the subscription was installed using a soft-state
     mechanism, the notifier MUST remove the corresponding

     If a NOTIFY request receives a 481 response, the notifier MUST
     remove the corresponding subscription even if such subscription
     was installed by non-SUBSCRIBE means (such as an administrative

     NOTIFY requests SHOULD contain an "Subscription-Expires" header
     which indicates the remaining duration of the subscription (such
     a header is  useful in case the SUBSCRIBE request forks, since
     the response to a forked subscribe -- which contains the "Expire"
     header that specifies the agreed-upon expiration time --  may not
     be received by the subscriber). The notifier SHOULD use this
     header to adjust the time remaining on the subscription; however,
     this mechanism MUST not be used to lengthen a subscription, only
     to shorten it. The notifier may inform a subscriber that a
     subscription has been removed by sending a NOTIFY message with an
     "Subscription-Expires" value of "0."

     If the duration of a subscription has been shortened or
     terminated by the "Subscription-Expires" header as compared to
     the most recent 200-class SUBSCRIBE response sent, that header
     SHOULD include a "reason" parameter indicating the reason that
     such action was taken. Currently, four such values are defined:
     "migration" indicates that the node acting as notifier is
     transferring responsibility for maintaing such state information
     to another node; this only makes sense when subscriptions are
     terminated, not when they are shortened. "Maint" indicates that
     the subscription is being truncated or terminated due to server
     maintainance, and "refused" indicates that the subscription has

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     been removed or shortened administratively (e.g. by a change in
     ACL policy). Finally, if the notifier elects to send a NOTIFY
     upon timeout of the subscription, they SHOULD include a
     "Subscription-Expires" header with a value of "0" and a reason
     parameter of "timeout."

4.3.4. Proxy NOTIFY Behavior

     Proxies need no additional behavior beyond that described in RFC
     2543 [1] to support NOTIFY. If a proxy wishes to see all of the
     SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
     record-route all SUBSCRIBE and NOTIFY requests.

4.3.5. Subscriber NOTIFY Behavior

     Upon receiving a NOTIFY request, the subscriber should check that
     it matches at least one of its outstanding subscriptions; if not,
     it MUST return a "481 Subscription does not exist" response
     unless another 400- or 500-class response is more appropriate.

     If, for some reason, the event package designated in the "Event"
     header of the NOTIFY request is not supported, the subscriber
     will respond with a "489 Bad Event" response.

     To prevent spoofing of events, NOTIFY requests MAY be
     authenticated, using any defined SIP authentication mechanism.

     NOTIFY requests SHOULD contain "Subscription-Expires" headers
     which indicate the time remaining on the subscription. If this
     header is present, the subscriber SHOULD take it as the
     authoritative duration and adjust accordingly. If an expires
     value of "0" is present, the subscriber should consider the
     subscription terminated.

     When the subscription is terminated or shortened using the
     "Subscription-Expires" mechanism, there SHOULD be a reason
     parameter present. If it is present and the subscriber is still
     interested in receiving updates to the state information, the
     subscriber SHOULD attempt re-subscribe upon expiration if it is
     set to "migration," "timeout," is not present, or is set to an
     unknown value. Such a resubscription will be completely
     independant of the original subscription, and will not share a
     dialog with it; it will be generated as described in section

     If the "reason" parameter on a "Subscription-Expires" header  is
     set to either "maint" or "refused," the subscriber SHOULD NOT
     attempt re-subscription.

     Once the notification is deemed acceptable to the subscriber, the

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     subscriber SHOULD return a 200 response. In general, it is not
     expected that NOTIFY responses will contain bodies; however, they
     MAY, if the NOTIFY request contained an "Accept" header.

     Other responses defined in RFC 2543 [1] may also be returned, as

4.4. Polling Resource State

     A natural consequence of the behavior described in the preceding
     sections is that an immediate fetch without a persistent
     subscription may be effected by sending an appropriate SUBSCRIBE
     with an "Expires" of 0.

     Of course, an immediate fetch while a subscription is active may
     be effected by sending an appropriate SUBSCRIBE with an "Expires"
     greater than 0.

     Upon receipt of this SUBSCRIBE request, the notifier (or
     notifiers, if the SUBSCRIBE request was forked) will send a
     NOTIFY request containing resource state to the address in the
     SUBSCRIBE "Contact" field. Note that normal Route and
     Record-Route handle still applies; see section 4.1.1.

4.5. Allow-Events header usage

     The "Allow-Events" header, if present, includes a list of tokens
     which indicates the event packages supported by the client (if
     sent in a request) or server (if sent in a response). In other
     words, a node sending an "Allow-Events" header is advertising
     that it can process SUBSCRIBE requests and generate NOTIFY
     requests for all of the event packages listed in that header.

     Any node implementing one or more event packages SHOULD include
     an appropriate "Allow-Events" header indicating all supported
     events in INVITE requests and responses, OPTIONS responses, and
     REGISTER requests. "Allow-Events" headers MAY be included in any
     other type of request or response.

     This information is very useful, for example, in allowing user
     agents to render particular interface elements appropriately
     according to whether the events required to implement the
     features they represent are supported by the appropriate nodes.

     Note that "Allow-Events" headers MUST NOT be inserted by proxies.

5. Event Packages

     This section covers several issues which should be taken into
     consideration when event packages based on SUBSCRIBE and NOTIFY

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     are proposed.

5.1. Appropriateness of Usage

     When designing an event package using the methods described in
     this draft for event notification, it is important to consider:
     is SIP an appropriate mechanism for the problem set? Is SIP being
     selected because of some unique feature provided by the protocol
     (e.g. user mobility), or merely because "it can be done?" If you
     find yourself defining event packages for notifications related
     to, for example, network management or the temperature inside
     your car's engine, you may want to reconsider your selection of

     Those interested in extending the mechanism defined in this
     document are urged to read "Guidelines for Authors of SIP
     Extensions" [2] for further guidance regarding appropriate uses
     of SIP.

     Further, it is expected that this mechanism is not to be used in
     applications where the frequency of reportable events is
     excessively rapid (e.g. more than about once per second). A SIP
     network is generally going to be provisioned for a reasonable
     signalling volume; sending a notification every time a user's GPS
     position changes by one hundreth of a second could easily
     overload such a network.

5.2. Sub-packages

     Normal event packages define a set of state applied to a specific
     type of resource, such as user presence, call state, and
     messaging mailbox state.

     Sub-packages are a special type of package which define a set of
     state applied to other packages, such as statistics, access
     policy, and subscriber lists. Sub-packages may even be applied to
     other sub-packages.

     To extend the object-oriented analogy made earlier, sub-packages
     can be thought of as templatized C++ packages which must be
     applied to other packages to be useful.

     The name of a sub-package as applied to a package is formed by
     appending a period followed by the sub-package name to the end of
     the package. For example, if a subpackage called "watcherinfo"
     were being applied to a package called "presence," the event
     token used in "Event" and "Allow-Events" would be

     Sub-packages must be defined so that they can be applied to any

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     arbitrary package. In other words, sub-packages cannot be
     specifically tied to one or a few "parent" packages in such a way
     that they will not work with other packages.

5.3. Amount of State to be Conveyed

     When designing event packages, it is important to consider the
     type of information which will be conveyed during a notification.

     A natural temptation is to convey merely the event (e.g. "a new
     voice message just arrived") without accompanying state (e.g. "7
     total voice messages"). This complicates implementation of
     subscribing entities (since they have to maintain complete state
     for the entity to which they have subscribed), and also is
     particularly susceptible to synchronization problems.

     There are two possible solutions to this problem that event
     packages may choose to implement.

5.3.1. Complete State Information

     For packages which typically convey state information that is
     reasonably small (on the order of 1 kb or so), it is suggested
     that event packages are designed so as to send complete state
     information when an event occurs.

     In the circumstances that state may not be sufficient for a
     particular class of events, the event packages should include
     complete state information along with the event that occurred.
     For example, "no customer service representatives available" may
     not be as useful "no customer service representatives available;
     representative just logged off".

5.3.2. State Deltas

     In the case that the state information to be conveyed is large,
     the event package may choose to detail a scheme by which NOTIFY
     messages contain state deltas instead of complete state.

     Such a scheme would work as follows: any NOTIFY sent in immediate
     response to a SUBSCRIBE contains full state information. NOTIFY
     messages sent because of a state change will contain only the
     state information that has changed; the subscriber will then
     merge this information into its current knowledge about the state
     of the resource.

     Any event package that supports delta changes to states MUST use
     a payload which contains a version number that increases by
     exactly one for each NOTIFY message. Note that the state version
     number appears in the body of the message, not in a SIP header.

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     If a NOTIFY arrives that has a version number that is incremented
     by more than one, the subscriber knows that a state delta has
     been missed; it ignores the NOTIFY message containing the state
     delta (except for the version number, which it retains to detect
     message loss), and re-sends a SUBSCRIBE to force a NOTIFY
     containing a complete state snapshot.

5.4. Event Package Responsibilities

     Event packages are not required to re-iterate any of the behavior
     described in this document, although they may choose to do so for
     clarity or emphasis. In general, though, such packages are
     expected to describe only the behavior that extends or modifies
     the behavior described in this document.

     Note that any behavior designated with "SHOULD" or "MUST" in this
     document is not allowed to be changed by extension documents;
     however, such documents may elect to strengthen "SHOULD"
     requirements to "MUST" strength if required by their application.

     In addition to the normal sections expected by "Instructions to
     RFC Authors" [6] and "Guidelines for Authors of SIP Extensions"
     [2] , authors of event packages MUST address each of the issues
     detailed in the following subsections, whenever applicable.

5.4.1. Event Package Name

     This mandatory section of an event package defines the token name
     to be used to designate the event package. It MUST include the
     information which appears in the IANA registration of the token.
     For information on registering such types, see section 7.

5.4.2. Event Package Parameters

     If parameters are to be used on the "Event" header to modify the
     behavior of the event package, the syntax and semantics of such
     headers must be clearly defined.

5.4.3. SUBSCRIBE Bodies

     It is expected that most, but not all, event packages will define
     syntax and semantics for SUBSCRIBE method bodies; these bodies
     will typically modify, expand, filter, throttle, and/or set
     thresholds for the class of events being requested. Designers of
     event packages are strongly encouraged to re-use existing MIME
     types for message bodies where practical.

     This mandatory section of an event package defines what type or
     types of event bodies are expected in SUBSCRIBE requests (or

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     specify that no event bodies are expected). It should point to
     detailed definitions of syntax and semantics for all referenced
     body types.

5.4.4. Subscription Duration

     It is recommended that event packages give a suggested range of
     times considered reasonable for the duration of a subscription.
     Such packages MUST also define a default "Expires" value to be
     used if none is specified.

5.4.5. NOTIFY Bodies

     The NOTIFY body is used to report state on the resource being
     monitored. Each package must define a what type or types of event
     bodies are expected in NOTIFY requests. Such packages must
     specify or cite detailed specifications for the syntax and
     semantics associated with such event body.

     Event packages also need to define which MIME type is to be
     assumed if none are specified in the "Accept" header of the
     SUBSCRIBE request.

5.4.6. Notifier processing of SUBSCRIBE requests

     This section describes the processing to be performed by the
     notifier upon receipt of a SUBSCRIBE request. Such a section is

     Information in this section includes details of how to
     authenticate subscribers and authorization issues for the
     package. Such authorization issues may include, for example,
     whether all SUBSCRIBE requests for this package are answered with
     202 responses (see section 6.2. ).

5.4.7. Notifier generation of NOTIFY requests

     This section of an event package describes the process by which
     the notifier generates and sends a NOTIFY request. This includes
     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 notifications (see section
     5.3. )

     It may optionally describe the behavior used to processes the
     subsequent response. Such a section is required.

5.4.8. Subscriber processing of NOTIFY requests

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     This section of an event package describes the process followed
     by the subscriber upon receipt of a NOTIFY request, including any
     logic required to form a coherent resource state (if applicable).

5.4.9. Handling of forked requests

     Each event package should specify whether forked SUBSCRIBE
     requests are allowed to install multiple subscriptions. If such
     behavior is not allowed, any NOTIFY messages not matching the
     200-class response to the initial SUBSCRIBE message are responded
     to with a 481.

     In the case that multiple subscriptions are allowed, the event
     package must specify whether merging of the notifications to form
     a single state is required, and how such merging is to be
     performed. Note that it is possible that some event packages may
     be defined in such a way that each dialog is tied to a mutually
     exclusive state which is unaffected by the other dialogs; this
     must be clearly stated if it is the case.

5.4.10. Rate of notifications

     Each event package is expected to define a requirement
     (RECOMMENDED, SHOULD or MUST strength) which defines an absolute
     maximum on the rate at which notifications are allowed to be
     generated by a single notifier.

     Such packages may further define a throttle mechanism which
     allows subscribers to further limit the rate of notification.

5.4.11. State Agents

     Designers of event packages should consider whether their package
     can benefit from network aggregation points ("State Agents")
     and/or nodes which act on behalf of other nodes. (For example,
     nodes which provide state information about a resource when such
     a resource is unable or unwilling to provide such state
     information itself). An example of such an application is a node
     which tracks the presence and availability of a user in the

     If state agents are to be used by the package, the package must
     specify how such state agents aggregate information and how they
     provide authentication and authorization.

5.4.12. Examples

     Event packages should include several demonstrative message flow
     diagrams paired with several typical, syntactically correct and

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     complete messages.

     It is recommended that documents describing event packages
     clearly indicate that such examples are informative and not
     normative, with instructions that implementors refer to the main
     text of the draft for exact protocol details.

6. Security Considerations

6.1. Access Control

     The ability to accept subscriptions should be under the direct
     control of the user, since many types of events may be considered
     sensitive for the purposes of privacy. Similarly, the notifier
     should have the ability to selectively reject subscriptions based
     on the calling party (based on access control lists), using
     standard SIP authentication mechanisms. The methods for creation
     and distribution of such access control lists is outside the
     scope of this draft.

6.2. Release of Sensitive Policy Information

     The mere act of returning a 200 or certain 4xx and 6xx responses
     to SUBSCRIBE requests may, under certain circumstances, create
     privacy concerns by revealing sensitive policy information. In
     these cases, the notifier should always return a  202 response.
     While the subsequent NOTIFY message may not convey true state, it
     MUST appear to contain a potentially correct piece of data from
     the point of view of the subscriber, indistinguishable from a
     valid response. Information about whether a user is authorized to
     subscribe to the requested state is never conveyed back to the
     original user under these circumstances.

6.3. Denial-of-Service attacks

     The current model (one SUBSCRIBE request triggers a SUBSCRIBE
     response and one or more NOTIFY requests) is a classic setup for
     an amplifier node to be used in a smurf attack.

     Also, the creation of state upon receipt of a SUBSCRIBE request
     can be used by attackers to consume resources on a victim's
     machine, rendering it unusable.

     To reduce the chances of such an attack, implementations of
     notifiers SHOULD require authentication. Authentication issues
     are discussed in RFC2543 [1] .

7. IANA Considerations

     (This section is not applicable until this document is published

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     as an RFC.)

     This document defines an event-type namespace which requires a
     central coordinating body. The body chosen for this coordination
     is the Internet Assigned Numbers Authority (IANA).

     There are two different types of event-types: normal event
     packages, and event sub-packages; see section 5.2. To avoid
     confusion, subpackage names and package names share the same
     namespace; in other words, a sub-package MUST NOT share a name
     with a package.

     Following the policies outlined in "Guidelines for Writing an
     IANA Considerations Section in RFCs" [7] , normal event package
     identification tokens are allocated as First Come First Served,
     and event sub-package identification tokens are allocated on a
     IETF Consensus basis.

     Registrations with the IANA MUST include the token being
     registered and whether the token is a package or a subpackage.
     Further, packages MUST include contact information for the party
     responsible for the registration and/or a published document
     which describes the event package. Sub-package token
     registrations MUST include a pointer to the published RFC which
     defines the sub-package.

     Registered tokens to designate packages and sub-packages MUST NOT
     contain the character ".", which is used to separate sub-packages
     from packages.

7.1. Registration Template

     As this document specifies no package or sub-package names, the
     initial IANA registration for event types will be empty. The
     remainder of the text in this section gives an example of the
     type of information to be maintained by the IANA; it also
     demonstrates all five possible permutations of package type,
     contact, and reference.

     The table below lists the event packages and sub-packages defined
     in "SIP-Specific Event Notification" [RFC xxxx]. Each name is
     designated as a package or a subpackage under "Type."

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     Package Name      Type         Contact      Reference
     ------------      ----         -------      ---------
     example1          package      [Roach]
     example2          package      [Roach]      [RFC xxxx]
     example3          package                   [RFC xxxx]
     example4          sub-package  [Roach]      [RFC xxxx]
     example5          sub-package               [RFC xxxx]

     [Roach] Adam Roach <>

     [RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX,
                August 2002.

8. Open Issues

     In addition to the three issues listed below, the BNF in this
     document needs to be converted to explicit LWS to match the
     latest bis draft; this change will be reflected in the next

8.1. CANCEL Handling

     This is actually a protocol-wide open issue which has impacts on
     this specification: there hasn't been a clear consensus about
     cancellation of non-INVITE requests yet. If non-INVITE requests
     cannot be cancelled, we need to remove section 4.1.3.

8.2. Version of SIP to reference

     Much of the handling in this document is rather different than
     what is described in RFC2543; in fact, many of the recent changes
     to this document have been tracking changes in the "bis" versions
     of the SIP specification. We can continue to reference RFC2543
     while pulling in huge chunks of the bis draft for compatibility
     (for example, the Route handling would essentially be copied
     word-for-word from the bis draft), or we can start referencing
     the bis drafts.

     Of course, referencing the bis drafts allows us to pick up
     changes to protocol semantics "for free," while importing chunks
     of it requires constant maintanance and runs the risk of getting
     out of sync.

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     On the other hand, placing a dependency on the bis draft pushes
     the timeframe for this draft (and the drafts that depend on it)
     out past the time that the next SIP RFC is published.

8.3. Immediate NOTIFYs

     There has been discussion, but no consensus, on the issue of
     whether each SUBSCRIBE must have an immediate NOTIFY message sent
     in response. In attempts to follow the prevailing sentiment, this
     draft had become internally inconsistent.

     This version of this document has eliminated these
     inconsistancies by requiring notifiers always to send a NOTIFY
     immediately upon receiving a SUBSCRIBE. This decision does not
     necessarily represent group consensus, and further discussion may
     be warranted.

9. Changes

9.1. Changes from draft-ietf-...-00

     - Fixed confusing typo in section describing correlation
       of SUBSCRIBE requests

     - Added explanitory text to clarify tag handling when
       generating re-subscriptions

     - Expanded general handling section to include specific
       discussion of Route/Record-Route handling.

     - Included use of "methods" parameter on Contact as
       a means for detecting support for SUBSCRIBE and NOTIFY.

     - Added definition of term "dialog"; changed "leg" to
       "dialog" everwhere.

     - Added syntax for "Subscription-Expires" header.

     - Changed NOTIFY messages to refer to "Subscription-Expires"
       everywhere (instead of "Expires.")

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     - Added information about generation and handling of
       481 responses to SUBSCRIBE requests

     - Changed having Expires header in SUBSCRIBE from
       MUST to SHOULD; this aligns more closely with
       REGISTER behavior

     - Removed experimental/private event package names,
       per list consensus

     - Cleaned up some legacy text left over from very early
       drafts that allowed multiple contacts per subscription

     - Strengthened language requiring the removal of subscriptions
       if a NOTIFY request fails with a 481. Clarified that such
       removal is required for all subscriptions, including
       administrative ones.

     - Removed description of delaying NOTIFY requests until
       authorization is granted. Such behavior was inconsistent
       with other parts of this document.

     - Moved description of event packages to later in document,
       to reduce the number of forward references.

     - Minor editorial and nits changes

     - Added new open issues to open issues section. All
       previous open issues have been resolved.

9.2. Changes from draft-roach-...-03

     - Added DOS attacks section to open issues.

     - Added discussion of forking to open issues

     - Changed response to PINT request for notifiers who don't
       support PINT from 400 to 489.

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     - Added sentence to security section to call attention to
       potential privacy issues of delayed NOTIFY responses.

     - Added clarification: access control list handling is out
       of scope.

     - (Hopefully) Final resolution on out-of-band subscriptions:
       mentioned in section
     Removed from open issues.

     - Made "Contact" header optional for SUBSCRIBE 1xx responses.

     - Added description clarifying tag handling (section

     - Removed event throttling from open issues.

     - Editorial cleanup to remove term "extension draft" and
       similar; "event package" is now (hopefully) used consistently
       throughout the document.

     - Remove discussion of event agents from open issues.
       This is covered in the event packages section now.

     - Added discussion of forking to open issues.

     - Added discussion of sub-packages

     - Added clarification that, upon receiving a "NOTIFY"
       with an expires of "0", the subscriber can re-subscribe.
       This allows trivial migration of subscriptions between

     - Added preliminary IANA Considerations section

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     - Changed syntax for experimental event tokens to avoid
       possibly ambiguity between experimental tokens and

     - Slight adjustment to "Event" syntax to accommodate sub-packages.

     - Added section describing the information which is to be
       included in documents describing event packages.

     - Made 481 responses mandatory for unexpected notifications
       (allowing notifiers to remove subscriptions in error cases)

     - Several minor non-semantic editorial changes.

9.3. Changes from draft-roach-...-02

     - Clarification under "Notifier SUBSCRIBE behavior" which
       indicates that the first NOTIFY message (sent immediately
       in response to a SUBSCRIBE) may contain an empty body, if
       resource state doesn't make sense at that point in time.

     - Text on message flow in overview section corrected

     - Removed suggestion that clients attempt to unsubscribe
       whenever they receive a NOTIFY for an unknown event.
       Such behavior opens up DOS attacks, and will lead to
       message loops unless additional precautions are taken.
       The 481 response to the NOTIFY should serve the same

     - Changed processing of non-200 responses to NOTIFY from
       "SHOULD remove contact" to "MUST remove contact" to support
       the above change.

     - Re-added discussion of out-of-band subscription mechanisms
       (including open issue of resource identification).

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Internet Draft      SIP-Specific Event Notification        November 2001

     - Added text specifying that SUBSCRIBE transactions are not
       to be prolonged. This is based on the consensus that non-INVITE
       transactions should never be prolonged; such consensus within
       the SIP working group was reached at the 49th IETF.

     - Added "202 Accepted" response code to support the above
       change. The behavior of this 202 response code is a
       generalization of that described in the presence draft.

     - Updated to specify that the response to an unauthorized
       SUBSCRIBE request is 603 or 403.

     - Level-4 subheadings added to particularly long sections to
       break them up into logical units. This helps make the
       behavior description seem somewhat less rambling. This also
       caused some re-ordering of these paragraphs (hopefully in a
       way that makes them more readable).

     - Some final mopping up of old text describing "call related"
       and "third party" subscriptions (deprecated concepts).

     - Duplicate explanation of subscription duration removed from
       subscriber SUBSCRIBE behavior section.

     - Other text generally applicable to SUBSCRIBE (instead of just
       subscriber handling of SUBSCRIBE) moved to parent section.

     - Updated header table to reflect mandatory usage of "Expires"
       header in SUBSCRIBE requests and responses

     - Removed "Event" header usage in responses

     - Added sentence suggesting that notifiers may notify
       subscribers when a subscription has timed out.

     - Clarified that a failed attempt to refresh a subscription
       does not imply that the original subscription has been

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     - Clarified that 489 is a valid response to "NOTIFY" requests.

     - Minor editorial changes to clean up awkward and/or unclear
       grammar in several places

9.4. Changes from draft-roach-...-01

     - Multiple contacts per SUBSCRIBE message disallowed.

     - Contact header now required in NOTIFY messages.

     - Distinction between third party/call member events removed.

     - Distinction between call-related/resource-related events removed.

     - Clarified that subscribers must expect NOTIFY messages before
       the SUBSCRIBE transaction completes

     - Added immediate NOTIFY message after successful SUBSCRIBE;
       this solves a myriad of issues, most having to do with forking.

     - Added discussion of "undefined state" (before a NOTIFY arrives).

     - Added mechanism for notifiers to shorten/cancel outstanding

     - Removed open issue about appropriateness of new "489" response.

     - Removed all discussion of out-of-band subscriptions.

     - Added brief discussion of event state polling.

10. References

     [1] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP:
         Session Initiation Protocol", RFC 2543, IETF; March 1999.

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     [2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
         Extensions", <draft-ietf-sip-guidelines-02.txt>, IETF; March
         2001. Work in progress.

     [3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848,
         IETF; June 2000.

     [4] J. Rosenberg et. al., "SIP Extensions for Presence",
         <draft-ietf-simple-presence-03.txt>, IETF; September 2001.
         Work in progress.

     [5] R. Fielding et. al., "Hypertext Transfer Protocol --
         HTTP/1.1", RFC2068, IETF, January 1997.

     [6] J. Postel, J. Reynolds, "Instructions to RFC Authors",
         RFC2223, IETF, October 1997.

     [7] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, IETF, October 1998.

     [8] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee
         Capabilities", <draft-ietf-sip-callerprefs-04.txt>, IETF;
         June 2001. Work in progress.

11. Acknowledgements

     Thanks to the participants in the Events BOF at the 48th IETF
     meeting in Pittsburgh, as well as those who gave ideas and
     suggestions on the SIP Events mailing list. In particular, I wish
     to thank Henning Schulzrinne of Columbia University for coming up
     with the final three-tiered event identification scheme, Sean
     Olson of Ericsson for miscellaneous guidance, Jonathan Rosenberg
     for a thorough scrubbing of the -00 draft, and the authors of the
     "SIP Extensions for Presence" draft for their input to SUBSCRIBE
     and NOTIFY request semantics.

12. Author's Address

     Adam Roach
     Ericsson Inc.
     Mailstop L-04
     740 E. Campbell Rd.
     Richardson, TX 75081
     Phone: +1 972 583 7594
     Fax: +1 972 669 0154

Roach                                                          [Page 36]