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Versions: 00 01 02 03 04 RFC 3265

Internet Engineering Task Force                               Adam Roach
Internet Draft                                             Ericsson Inc.
Category: Standards Track                                      July 2001
                                                    Expires January 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.................................. 3
    3.       Event Packages......................................... 4

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    3.1.     Appropriateness of Usage............................... 4
    3.2.     Additional Guidelines.................................. 4
    3.3.     Sub-packages........................................... 5
    3.4.     Event Package Responsibilities......................... 5
    3.4.1.   Event Package Name..................................... 6
    3.4.2.   Event Package Parameters............................... 6
    3.4.3.   SUBSCRIBE Bodies....................................... 6
    3.4.4.   Subscription Duration.................................. 6
    3.4.5.   NOTIFY Bodies.......................................... 6
    3.4.6.   Subscriber generation of SUBSCRIBE requests............ 7
    3.4.7.   Notifier processing of SUBSCRIBE requests.............. 7
    3.4.8.   Notifier generation of NOTIFY requests................. 7
    3.4.9.   Subscriber processing of NOTIFY requests............... 7
    3.4.10.  Handling of forked requests............................ 7
    3.4.11.  Rate of notifications.................................. 8
    3.4.12.  State Agents and Notifier Migration.................... 8
    3.4.13.  Examples............................................... 8
    4.       Syntax................................................. 8
    4.1.     New Methods............................................ 9
    4.1.1.   SUBSCRIBE method....................................... 10
    4.1.2.   NOTIFY method.......................................... 11
    4.2.     New Headers............................................ 11
    4.2.1.   "Event" header......................................... 11
    4.2.2.   "Allow-Events" Header.................................. 12
    4.3.     New Response Codes..................................... 12
    4.3.1.   "202 Accepted" Response Code........................... 12
    4.3.2.   "489 Bad Event" Response Code.......................... 12
    5.       Node Behavior.......................................... 13
    5.1.     Description of SUBSCRIBE Behavior...................... 13
    5.1.1.   Correlation to legs, calls, and terminals.............. 13
    5.1.2.   Subscription duration.................................. 14
    5.1.3.   Identification of Subscribed Events and Event Classes.. 14
    5.1.4.   Additional SUBSCRIBE Header Values..................... 15
    5.1.5.   Subscriber SUBSCRIBE Behavior.......................... 15
    5.1.6.   Proxy SUBSCRIBE Behavior............................... 17
    5.1.7.   Notifier SUBSCRIBE Behavior............................ 17
    5.2.     Description of NOTIFY Behavior......................... 19
    5.2.1.   Correlation............................................ 20
    5.2.2.   Identification of reported events, event classes, and c 20
    5.2.3.   Notifier NOTIFY Behavior............................... 21
    5.2.4.   Proxy NOTIFY Behavior.................................. 22
    5.2.5.   Subscriber NOTIFY Behavior............................. 22
    5.3.     Polling Resource State................................. 23
    5.4.     Allow-Events header usage.............................. 23
    6.       Security Considerations................................ 23
    7.       IANA Considerations.................................... 24
    7.1.     Registration Template.................................. 24
    8.       Open Issues............................................ 25
    8.1.     Denial-of-Service attacks.............................. 25
    8.2.     SUBSCRIBE Forking...................................... 26

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    9.       Changes................................................ 27
    9.1.     Changes from draft-roach-...-03........................ 27
    9.2.     Changes from draft-roach-...-02........................ 29
    9.3.     Changes from draft-roach-...-01........................ 30
    10.      References............................................. 31
    11.      Acknowledgements....................................... 32
    12.      Feedback and Discussion................................ 32
    13.      Author's Address....................................... 32

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
     extensions 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
     into an instantiatable class by further extensions. Guidelines
     for creating these extensions are described in section 3.

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:

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     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. Event Packages

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

3.1. 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" [3] 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.

3.2. Additional Guidelines

     When designing event packages, it is important to consider the

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     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.

     It is therefore suggested that event packages are designed so as
     to notify of new state 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
     sip:46@cs.xyz.int just logged off".)

3.3. 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
     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.

3.4. 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

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     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" [7] and "Guidelines for Authors of SIP Extensions"
     [3] , authors of event packages should take the following
     sections into consideration.

3.4.1. Event Package Name

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

3.4.2. 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.

3.4.3. 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
     specify that no event bodies are expected). It should point to
     detailed definitions of syntax and semantics for all referenced
     body types.

3.4.4. Subscription Duration

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

3.4.5. NOTIFY Bodies

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     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.

3.4.6. Subscriber generation of SUBSCRIBE requests

     This section of an event package describes the process by which
     the subscriber generates and sends a SUBSCRIBE request and
     processes the subsequent response. Such a section is optional,
     but encouraged for the sake of clarity.

3.4.7. 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

3.4.8. Notifier generation of NOTIFY requests

     This section of an event package describes the process by which
     the notifier generates and sends a NOTIFY request. It may
     optionally describe the behavior used to processes the subsequent
     response. Such a section is required.

3.4.9. Subscriber processing of NOTIFY requests

     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).

3.4.10. 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 leg is tied to a mutually
     exclusive state which is unaffected by the other legs; this must

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     be clearly stated if it is the case.

3.4.11. 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.

3.4.12. State Agents and Notifier Migration

     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

     When state agents are used, it may make sense to allow migration
     of subscriptions between state agents and the nodes for which
     they are providing state aggregation (or even among various state
     agents). Designers of packages using state agents are encouraged
     to include such a feature with detailed description of how such
     migration is performed.

     Note that the mechanism of sending a "NOTIFY" with an "Expires"
     header of "0" is an effective way to force a subscriber to
     re-subscribe, which may come in useful when designing a migration

3.4.13. Examples

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

4. Syntax

     This section describes the syntax extensions required for event
     notification in SIP. Semantics are described in section 5.

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4.1. 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       m   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

4.1.1. 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.

4.1.2. 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.

4.2. New Headers

     This table expands on tables 4 and 5 in RFC 2543 [1] , as amended
     by the changes described in section 4.1.

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

4.2.1. "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 "general-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
     registered with the IANA.

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     Experimental event types may be created by prepending an "x-"
     followed by the organization's internet domain, with the field
     order reversed, and "." characters replaced by dashes (e.g.
     "Event: x-com-ericsson-foo").

     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

4.2.2. "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

4.3. New Response Codes

4.3.1. "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
     [6] .

4.3.2. "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

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

5. Node Behavior

     Unless noted otherwise, SUBSCRIBE and NOTIFY requests follow the
     same protocol rules governing the usage of tags, Route,
     Record-Route, Via handling, retransmission, reliability, CSeq
     handling, Contact handling, provisional responses, and message
     formatting as those defined in RFC 2543 [1] for BYE.

     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. Note also that the presence of the "Allow-Events"
     header in a message is sufficient to indicate support for

     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.

5.1. Description of SUBSCRIBE Behavior

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

5.1.1. Correlation to legs, 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"
     header. 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

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     requests for re-subscription MUST contain "tag" parameters in
     both the "To" and "From" headers (matching those previously
     established for the leg).

     The relationship between subscriptions and (INVITE-initiated)
     sessions sharing the same call leg identification information is
     undefined. Re-using call leg information for subscriptions is

     Similarly, the relationship between a subscription in one
     direction (e.g. from node A to node B) and a subscription in the
     opposite direction (from B to A) with the same call leg
     identification information is undefined. Re-using subscription
     correlation information in two directions is discouraged.

5.1.2. Subscription duration

     SUBSCRIBE requests MUST 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.

     200-class responses to SUBSCRIBE requests also MUST contain an
     "Expires" header. The period of time in the response MAY be
     shorter than specified in the request, but MUST NOT be longer.
     The period of time in the response is the one which defines the
     duration of the subscription.

     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 5.2.3.

5.1.3. Identification of Subscribed Events and Event Classes

     Identification of events is provided by three pieces of

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     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
     identify the resource for which event notification is desired,
     but not necessarily enough information to uniquely identify the
     nature of the event (e.g. "sip:adam.roach@ericsson.com" 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 single opaque
     token which identifies the event or class of events 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
     [4] ), 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.

5.1.4. Additional SUBSCRIBE Header Values

     The "Contact:" header in a SUBSCRIBE message will contain
     information about where resulting NOTIFY requests are to be sent.
     Each SUBSCRIBE request must have exactly one "Contact:" header.

     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] .

5.1.5. Subscriber SUBSCRIBE Behavior Requesting a Subscription

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     When a subscriber wishes to subscribe to (or refresh a
     subscription to) an event class, he forms a SUBSCRIBE message.

     The call leg information is formed as if for an original INVITE:
     the Call-ID is a new call ID with the syntax 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 sip:user@machine for
     UAs, or sip:machine for other entities).

     This SUBSCRIBE request will be confirmed with a final response.
     200-class responses indicate that the subscriber will be
     receiving a confirmation of subscription in the form of a NOTIFY
     message. A 200 response can be interpreted to mean that the
     requested subscription has succeeded and that a NOTIFY is to be
     expected immediately. A 202 response indicates that there may be
     a sizable delay before a notification is received, pending the
     actual creation of the subscription. For most implementations,
     there will be no difference in handling these two response codes.

     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, with the exception
     that these renewals will contain a the same "To," "From," and
     "Call-ID" as the original SUBSCRIBE request, and an incremented
     "CSeq" number.

     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. Unsubscribing

     Unsubscribing is handled in the same way as refreshing of a

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     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(s) arrive,
     the subscriber should consider the state of the subscribed
     resource to be in an undefined state. Event packages which define
     new event packages MUST define this "undefined state" in such a
     way that makes sense for their application.

     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 5.2.5.

5.1.6. Proxy SUBSCRIBE Behavior

     Proxies need no additional behavior beyond that described in RFC
     2543 [1] to support SUBSCRIBE. Note that SIP proxies may also act
     as subscribers or notifiers, as appropriate; under these
     circumstances, they will act as described in 5.1.5. and 5.1.7.

5.1.7. 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.

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

     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 subscriptions, it creates the subscription and
     returns a "200 OK" response.

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     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), it will
     return a "202 Accepted" response. This response indicates that
     the request has been received and understood, but that no action
     has yet taken place.

     The "Expires" values present in SUBSCRIBE 200-class responses
     behave in the same way as they do in REGISTER responses: the
     server MAY shorten the interval, but MUST not increase it.

     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 successful creation or refreshing of a subscription,
     notifiers MUST send a NOTIFY message as soon as practical 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
     body. See section 5.2.3. for further details on NOTIFY message

     If the response to the SUBSCRIBE message was 202, this initial
     NOTIFY will serve as indication that the subscription has finally
     been processed. In the case that the subscription has not been
     created (e.g. the notifier was waiting for authorization and such
     authorization failed), the notifier SHOULD indicate to the
     subscriber that the subscription does has not been created by
     setting the "Expires" header to "0" in this initial NOTIFY
     response. 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

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     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, as appropriate. Depending on the
     situation, such a response may have security implications; see
     section 6.

     If the notifier owner is interactively queried to determine
     whether a subscription is allowed, a "202 Accept" response is
     returned immediately, and the subsequent NOTIFY request is
     suppressed until the notifier owner responds. Refreshing of Subscriptions

     When a notifier receives a subscription refresh, assuming that
     the subscriber is still authorized, the notifier updates the
     expiration time for the "Contact:" address present in the
     SUBSCRIBE. As with the initial subscription, the server MAY lower
     the amount of time until expiration, but MUST NOT increase it.
     The final expiration time is placed in the Expires header in the

     If no refresh for a notification address is received before its
     expiration time, that address is removed from the list of
     addresses. When removing a contact, the notifier MAY send a
     NOTIFY message to that contact with an "Expires" value of "0" to
     inform it that the subscription is being removed. If all
     notification addresses are removed, the entire subscription is

5.2. 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

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     500-class error code is more applicable), as described in section
     5.2.5. In other words, subscriptions 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

5.2.1. Correlation

     NOTIFY requests MUST contain the same Call-ID, local URI, and
     remote URI as the SUBSCRIBE request which ordered them. This is
     the same set of criteria that define a call leg.

     The From field of a NOTIFY request MUST contain a tag; this
     allows for the subscriber to differentiate between events from
     different notifiers.

     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

     Handling of the situation in which multiple distinct NOTIFY
     requests are received for a SUBSCRIBE is still an open issue; see
     section 8.2.

     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.

5.2.2. 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
     5.1.3. ).

     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 "Call Legs" (To,
     From, Call-ID) in order to be able to differentiate between
     notifications for them, unless the body for the event contains

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     enough information for this correlation.

     As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a
     single opaque 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. The formatting rules and
     behavior when no "Accept" header is present are expected to be
     defined by the document which describes the relevant event

5.2.3. Notifier NOTIFY Behavior

     When a SUBSCRIBE request is successfully processed or a relevant
     change in the subscribed state occurs, the notifier will
     construct and send a NOTIFY request to the subscriber(s), as
     specified in the "Contact" field of the SUBSCRIBE request. Such a
     message should be sent in as timely a manner as is practical.

     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 [5] ).

     If the original subscription contained a "Record-Route" header,
     notifications are sent according to the rules outlined in RFC
     2543 [1] , as if the SUBSCRIBE were an INVITE, and the NOTIFY
     were any subsequent message (e.g. BYE).

     Notify requests MUST contain a "Contact" header. This contact
     header is used by the subscriber in building "Route" headers for
     subsequent subscriptions (i.e. refreshes).

     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), the notifier MUST
     remove the contact from the appropriate subscription. If removal
     of the contact leaves no remaining contacts, the entire

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     subscription is removed.

     NOTIFY requests MAY contain an "Expires" header which indicates
     the remaining duration of the subscription. The notifier MAY 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 "Expires" value of "0."

5.2.4. Proxy NOTIFY Behavior

     Proxies need no additional behavior beyond that described in RFC
     2543 [1] to support NOTIFY.

5.2.5. 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 may contain "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. Note that
     this does not prevent the subscriber from re-sending a SUBSCRIBE
     if he wishes to re-initiate the subscription.

     Once the notification is deemed acceptable to the subscriber, the
     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

     Event packages should describe appropriate handling for the
     situation in which NOTIFY requests are received from multiple
     notifiers. In general, such handling will involve a simple
     merging of the received notifications into a single, overall

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5.3. 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.

5.4. 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.

6. Security Considerations

     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), and/or
     using standard SIP authentication mechanisms. The methods for
     creation and distribution of such access control lists is outside
     the scope of this draft.

     The mere act of returning a "403 Forbidden" or "603 Decline"
     response code to a SUBSCRIBE request may, under certain very rare

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     circumstances, create privacy concerns. Similarly, a delay in the
     initial notification may create the same concerns. In these
     cases, the notifier may elect to return an immediate 200 or 202
     response and send a NOTIFY message with (possibly erroneous)
     state. Note that this behavior is a rare exception, and should
     not be exhibited without justification.

7. IANA Considerations

     (This section is not applicable until this document is published
     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 3.3. 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" [8] , 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. Package names beginning with "x-" are
     experimental, and are reserved for Private Use. such names MUST
     be formed according to the rules outlined in section 4.2.1.

     Note that the naming scheme allows a certain level of
     Hierarchical Allocation for experimental types. Organizations may
     choose to centrally coordinate allocation of names within the
     scope of the experimental namespace designated by their internet
     domain name. Assignment of such authority is not in the scope of
     this document, and will not be provided by the IANA.

     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

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     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."

     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 <adam.roach@ericsson.com>

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

8. Open Issues

8.1. 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.

     These problems can be mitigated by requiring that all SUBSCRIBE
     requests be authenticated (and that unauthenticated SUBSCRIBE
     requests maintain zero state), but this doesn't actually solve
     the problem, as much as it makes it somewhat less likely to be

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     Suggestions for improvements in this area are solicited, and
     should be taken on the mailing list (see section 12. ).

8.2. SUBSCRIBE Forking

     Forking poses an interesting problem for SUBSCRIBE requests.

     At first glance, everything would seem to work okay; a forked
     SUBSCRIBE which successfully reaches more than one notifier will
     install a subscription in all of the notifier nodes. Generally,
     several 200 class responses will be received by the forking
     proxy, and the first one will be returned to the subscriber.

     Upon receipt of the 200 response, the subscriber could correctly
     deduce that the subscription has been successfully created in at
     least one node. Once the NOTIFY responses begin arriving, it is
     trivial to differentiate between the notifiers using the "To" tag
     values. If the subscriber is happy having multiple outstanding
     subscriptions, he can accept each of them, and refresh them
     independently. If multiple subscriptions don't make sense for the
     event package, or introduce a level of complexity that the
     subscriber implementor doesn't want to worry about, all
     subscriptions with correlation information (i.e. "To" tags)
     differing from those received in the 200-class response may be
     rejected with a 481 response (which will remove the subscription
     from the notifiers).

     On closer examination, there appears to be a minor problem with
     proxies inserting "Record-Route" headers: specifically, the
     200-class response to the SUBSCRIBE can only carry one route; the
     routes to the other notifiers appears to be effectively lost.
     This problem is rather trivial to overcome; in particular, the
     newest versions of SIP have a "SHOULD" strength requirement that
     proxies wishing to stay in the path include "Record-Route"
     headers in all requests. This means that the incoming NOTIFYs
     themselves will contain this routing information for proxies that
     comply with the newer SIP specification.

     Since the draft you are currently reading technically references
     RFC 2543 (which has no such provision), we can describe this
     behavior in here. Proxies which have no notion of what
     "SUBSCRIBE" and "NOTIFY" mean don't know that "SUBSCRIBE" has a
     long-running leg associated with it. Record-Routing a "SUBSCRIBE"
     without knowing what it means should cause no problems, but those
     proxies certainly won't know to expect "NOTIFY" messages. On the
     other hand, proxies wishing to track subscriptions and
     notifications are doubtless aware of this draft; if we include a
     provision that proxies interested in tracking these types of legs
     MUST include Record-Route headers in all NOTIFY requests, it
     solves our routing problem -- and it's completely compatible with

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     the remainder of SIP, since we're just strengthening a
     requirement already presented in the newer SIP specification.

     So, there's a rather airtight technical solution to the problem;
     currently, no one seems to be disputing that fact. However, there
     are some theory-of-knowledge type philosophical arguments that
     claim that installing multiple subscriptions with one
     subscription request is a fundamentally flawed concept.

     The arguments, if I understand them correctly, roughly state that
     a subscription is to a particular single state, and that only one
     node in the network can possibly be considered the authoritative
     source of that state. I would counterargue that for certain event
     packages -- like user presence -- this is absolutely correct.
     Those packages should mandate that all but one NOTIFY is rejected
     with a 481. In circumstances where the node reached is the
     authoritative source for one instance of a set of state (such as
     terminal state), it makes a lot of sense to have the ability to
     install a subscription into every end-node reached.

     Of course, the forgoing discussion reflects the author's
     viewpoint; others would certainly cast the situation in different
     light. In any case, without a group consensus on this topic, it
     is considered an open issue.

9. Changes

9.1. 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.

     - 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

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     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

     - 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.

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

     - Several minor non-semantic editorial changes.

9.2. 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).

     - 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.

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     - 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

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

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

9.3. Changes from draft-roach-...-01

     - Multiple contacts per SUBSCRIBE message disallowed.

     - Contact header now required in NOTIFY messages.

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Internet Draft      SIP-Specific Event Notification            July 2001

     - 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.

     [2] Adam Roach, "Automatic Call Back Service in SIP", Internet
         Draft <draft-roach-sip-acb-00.txt>, IETF; March 2000. Work in

     [3] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
         Extensions", <draft-ietf-sip-guidelines-01.txt>, IETF; July
         2000. Work in progress.

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

     [5] J. Rosenberg et. al., "SIP Extensions for Presence",
         <draft-rosenberg-impp-presence-00.txt>, IETF; June 2000. Work
         in progress.

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Internet Draft      SIP-Specific Event Notification            July 2001

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

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

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

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, and the authors of
     the "SIP Extensions for Presence" draft for their input to
     SUBSCRIBE and NOTIFY request semantics.

12. Feedback and Discussion

     Comments regarding this draft are welcomed at the author's
     address listed below.

     General-purpose discussion of asynchronous event topics,
     including this draft, should be taken on the sip-events mailing
     list (and NOT the general-purpose SIP mailing list). To
     subscribe, send mail to "sip-events@standards.ericsson.net" with
     the word "SUBSCRIBE" in the body.

13. Author's Address

     Adam Roach
     Ericsson Inc.
     Mailstop L-04
     851 International Pkwy.
     Richardson, TX 75081
     Phone: +1 972 583 7594
     Fax: +1 972 669 0154
     E-Mail: adam.roach@ericsson.com

Roach                                                          [Page 32]

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