Internet Engineering Task Force Adam Roach
Internet Draft Ericsson Inc.
Category: Standards Track July 2001
Expires January 2002
<draft-ietf-sip-events-00.txt>
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
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progress".
The list of current Internet-Drafts can be accessed at
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Abstract
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
|--------200------->|
|<------NOTIFY----- | Return current state information
|--------200------->|
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
protocols.
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
"presence.watcherinfo".
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
required.
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
network.
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
scheme.
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
"NOTIFY."
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
specification.
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
"occurrence."
4.2.2. "Allow-Events" Header
The following header is defined for the purposes of this
specification.
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
"understands."
4.3. New Response Codes
4.3.1. "202 Accepted" Response Code
The 202 response is added to the "Success" header field
definition:
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
SUBSCRIBE and NOTIFY.
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
discouraged.
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
body.
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
5.1.5.1. 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] .
5.1.5.2. 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.
5.1.5.3. 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".
5.1.5.4. 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
5.1.7.1. 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 5.1.7.3.
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.
5.1.7.2. 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
generation.
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.
5.1.7.3. 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.
5.1.7.4. 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
response.
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
deleted.
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
requests.
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
response.
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
state
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
package.
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
appropriate.
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
state.
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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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Internet Draft SIP-Specific Event Notification July 2001
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]
PEOPLE
------
[Roach] Adam Roach <adam.roach@ericsson.com>
REFERENCES
----------
[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
exploited.
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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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5.2.
Removed from open issues.
- Made "Contact" header optional for SUBSCRIBE 1xx responses.
- Added description clarifying tag handling (section
5.1.1.
)
- 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
nodes.
- Added preliminary IANA Considerations section
- Changed syntax for experimental event tokens to avoid
possibly ambiguity between experimental tokens and
sub-packages.
- 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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Internet Draft SIP-Specific Event Notification July 2001
- 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
purpose.
- 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
cancelled.
- 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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- 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
subscriptions.
- 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
progress.
[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
USA
Phone: +1 972 583 7594
Fax: +1 972 669 0154
E-Mail: adam.roach@ericsson.com
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