Internet Engineering Task Force Adam Roach
Internet Draft Ericsson Inc.
Category: Standards Track November 2001
Expires May 2002
<draft-ietf-sip-events-01.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
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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.................................. 4
3. Syntax................................................. 4
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3.1. New Methods............................................ 4
3.1.1. SUBSCRIBE method....................................... 5
3.1.2. NOTIFY method.......................................... 6
3.2. New Headers............................................ 6
3.2.1. "Event" header......................................... 6
3.2.2. "Allow-Events" Header.................................. 7
3.2.3. "Subscription-Expires" Header.......................... 7
3.3. New Response Codes..................................... 7
3.3.1. "202 Accepted" Response Code........................... 8
3.3.2. "489 Bad Event" Response Code.......................... 8
4. Node Behavior.......................................... 8
4.1. General................................................ 8
4.1.1. Route Handling......................................... 8
4.1.2. Detecting support for SUBSCRIBE and NOTIFY............. 9
4.1.3. CANCEL requests........................................ 9
4.1.4. State Agents and Notifier Migration.................... 9
4.2. Description of SUBSCRIBE Behavior...................... 10
4.2.1. Correlation to dialogs, calls, and terminals........... 10
4.2.2. Subscription duration.................................. 11
4.2.3. Identification of Subscribed Events and Event Classes.. 11
4.2.4. Additional SUBSCRIBE Header Values..................... 12
4.2.5. Subscriber SUBSCRIBE Behavior.......................... 12
4.2.6. Proxy SUBSCRIBE Behavior............................... 14
4.2.7. Notifier SUBSCRIBE Behavior............................ 14
4.3. Description of NOTIFY Behavior......................... 17
4.3.1. Correlation............................................ 17
4.3.2. Identification of reported events, event classes, and c 18
4.3.3. Notifier NOTIFY Behavior............................... 18
4.3.4. Proxy NOTIFY Behavior.................................. 20
4.3.5. Subscriber NOTIFY Behavior............................. 20
4.4. Polling Resource State................................. 21
4.5. Allow-Events header usage.............................. 21
5. Event Packages......................................... 21
5.1. Appropriateness of Usage............................... 22
5.2. Sub-packages........................................... 22
5.3. Amount of State to be Conveyed......................... 23
5.3.1. Complete State Information............................. 23
5.3.2. State Deltas........................................... 23
5.4. Event Package Responsibilities......................... 24
5.4.1. Event Package Name..................................... 24
5.4.2. Event Package Parameters............................... 24
5.4.3. SUBSCRIBE Bodies....................................... 24
5.4.4. Subscription Duration.................................. 25
5.4.5. NOTIFY Bodies.......................................... 25
5.4.6. Notifier processing of SUBSCRIBE requests.............. 25
5.4.7. Notifier generation of NOTIFY requests................. 25
5.4.8. Subscriber processing of NOTIFY requests............... 25
5.4.9. Handling of forked requests............................ 26
5.4.10. Rate of notifications.................................. 26
5.4.11. State Agents........................................... 26
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5.4.12. Examples............................................... 26
6. Security Considerations................................ 27
6.1. Access Control......................................... 27
6.2. Release of Sensitive Policy Information................ 27
6.3. Denial-of-Service attacks.............................. 27
7. IANA Considerations.................................... 27
7.1. Registration Template.................................. 28
8. Open Issues............................................ 29
8.1. CANCEL Handling........................................ 29
8.2. Version of SIP to reference............................ 29
8.3. Immediate NOTIFYs...................................... 30
9. Changes................................................ 30
9.1. Changes from draft-ietf-...-00......................... 30
9.2. Changes from draft-roach-...-03........................ 31
9.3. Changes from draft-roach-...-02........................ 33
9.4. Changes from draft-roach-...-01........................ 35
10. References............................................. 35
11. Acknowledgements....................................... 36
12. Author's Address....................................... 36
2. Introduction
The ability to request asynchronous notification of events proves
useful in many types of services for which cooperation between
end-nodes is required. Examples of such services include
automatic callback services (based on terminal state events),
buddy lists (based on user presence events), message waiting
indications (based on mailbox state change events), and PINT
status (based on call state events).
The methods described in this document allow a framework by which
notification of these events can be ordered.
The event notification mechanisms defined herein are NOT intended
to be a general-purpose infrastructure for all classes of event
subscription and notification. Meeting requirements for the
general problem set of subscription and notification is far too
complex for a single protocol. Our goal is to provide a
SIP-specific framework for event notification which is not so
complex as to be unusable for simple features, but which is still
flexible enough to provide powerful services. Note, however, that
event packages based on this framework may define arbitrarily
complex rules which govern the subscription and notification for
the events or classes of events they describe.
This draft does not describe an extension which may be used
directly; it must be extended by other drafts (herein referred to
as "event packages.") In object-oriented design terminology, it
may be thought of as an abstract base class which must be derived
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into an instantiatable class by further extensions. Guidelines
for creating these extensions are described in section 5.
2.1. Overview of Operation
The general concept is that entities in the network can subscribe
to resource or call state for various resources or calls in the
network, and those entities (or entities acting on their behalf)
can send notifications when those states change.
A typical flow of messages would be:
Subscriber Notifier
|-----SUBSCRIBE---->| Request state subscription
|<-------200--------| Acknowledge subscription
|<------NOTIFY----- | Return current state information
|--------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. Syntax
This section describes the syntax extensions required for event
notification in SIP. Semantics are described in section 4.
3.1. New Methods
This document describes two new SIP methods: "SUBSCRIBE" and
"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 o -
From gc m m
Hide R o o
Max-Forwards R o o
Organization g o o
Priority R o o
Proxy-Authenticate 407 o o
Proxy-Authorization R o o
Proxy-Require R o o
Require R o o
Retry-After R - -
Retry-After 404,480,486 o o
Retry-After 503 o o
Retry-After 600,603 o o
Response-Key R o o
Record-Route R o o
Record-Route 2xx o o
Route R o o
Server r o o
Subject R o o
Timestamp g o o
To gc(1) m m
Unsupported 420 o o
User-Agent g o o
Via gc(2) m m
Warning r o o
WWW-Authenticate 401 o o
3.1.1. SUBSCRIBE method
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"SUBSCRIBE" is added to the definition of the element "Method" in
the SIP message grammar.
Like all SIP method names, the SUBSCRIBE method name is case
sensitive. The SUBSCRIBE method is used to request asynchronous
notification of an event or set of events at a later time.
3.1.2. NOTIFY method
"NOTIFY" is added to the definition of the element "Method" in
the SIP message grammar.
The NOTIFY method is used to notify a SIP node that an event
which has been requested by an earlier SUBSCRIBE method has
occurred. It may also provide further details about the event.
3.2. New Headers
This table expands on tables 4 and 5 in RFC 2543 [1] , as amended
by the changes described in section 3.1.
Header field where proxy ACK BYE CAN INV OPT REG SUB NOT
-----------------------------------------------------------------
Allow-Events g o o o o o o o o
Allow-Events 489 - - - - - - m m
Event R - - - - - - m m
Subscription-Expires R - - - - - - - o
3.2.1. "Event" header
The following header is defined for the purposes of this
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 "request-header"
in the SIP message grammar.
This document does not define values for event-types. These
values will be defined by individual event packages, and MUST be
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registered with the IANA.
There must be exactly one event type listed per event header.
Multiple events per message are disallowed.
For the curious, the "o" short form is chosen to represent
"occurrence."
3.2.2. "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."
3.2.3. "Subscription-Expires" Header
The following header is defined for the purposes of this
specification.
Subscription-Expires = "Subscription-Expires" ":"
( SIP-date | delta-seconds )
*( ";" subexp-params )
subexp-params = "reason" "=" reason-code
| generic-param
reason-code = "migration"
| "maint"
| "refused"
| "timeout"
| reason-extension
reason-extension = token
Subscription-Expires is added to the definition of the element
"request-header" in the SIP message grammar.
3.3. New Response Codes
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3.3.1. "202 Accepted" Response Code
The 202 response is added to the "Success" header field
definition:
Success = "200" ; OK
| "202" ; Accepted
"202 Accepted" has the same meaning as that defined in HTTP/1.1
[5] .
3.3.2. "489 Bad Event" Response Code
The 489 event response is added to the "Client-Error" header
field definition:
Client-Error = "400" ; Bad Request
...
| "489" ; Bad Event
"489 Bad Event" is used to indicate that the server did not
understand the event package specified in a "Event" header field.
4. Node Behavior
4.1. General
Unless noted otherwise, SUBSCRIBE and NOTIFY requests follow the
same protocol rules governing the usage of tags, Route handling,
Record-Route handling, Via handling, and Contact handling as
INVITE; retransmission, reliability, CSeq handling and
provisional responses are the same as those defined for BYE.
For the purposes of this document, a "dialog" is defined as all
messages sharing the tuple of "To" (including tag), "From"
(including tag), and "Call-Id." As in INVITE-initiated dialogs,
requests containg no "To" tag are also considered to be part of
the same dialog as messages which contain a "To" tag but
otherwise match.
4.1.1. Route Handling
Route and Record-Route handling for SUBSCRIBE and NOTIFY dialogs
is generally the same as for INVITE and its subsequent responses.
The exact method for echoing Record-Route headers in responses
and using them to form Route headers in subsequent requests is
described in RFC2543 [1] . For the purposes of the following
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discussion, the "Contact" header is considered part of the
"Record-Route" set.
From a subscriber perspective, the "Record-Route" headers
received in a SUBSCRIBE response are stored locally and placed in
the "Route" headers for SUBSCRIBE refreshes. To support forking
of SUBSCRIBE requests, "Record-Route" headers received in NOTIFY
requests MUST be echoed back in the NOTIFY responses; if no route
for the dialog has been established, these "Record-Route" headers
MUST be stored locally and MUST be placed in the "Route" headers
for SUBSCRIBE refreshes.
From a notifier perspective, SUBSCRIBE request "Record-Route"
headers are echoed back in the SUBSCRIBE response and stored
locally. The locally stored copy of the "Record-Route" headers is
placed in the "Route" headers when generating NOTIFY requests.
The result of the forgoing rules is that proxies wishing to
remain on the signalling path for subsequent requests in the
dialog MUST include themselves in a "Record-Route" for all
requests, not just the initial SUBSCRIBE.
4.1.2. Detecting support for SUBSCRIBE and NOTIFY
Neither SUBSCRIBE nor NOTIFY necessitate the use of "Require" or
"Proxy-Require" headers; similarly, there is no token defined for
"Supported" headers. If necessary, clients may probe for the
support of SUBSCRIBE and NOTIFY using the OPTIONS request defined
in RFC2543 [1] .
The presence of the "Allow-Events" header in a message is
sufficient to indicate support for SUBSCRIBE and NOTIFY.
The "methods" parameter for Contact may also be used to
specifically announce support for SUBSCRIBE and NOTIFY messages
when registering. (See reference [8] for details on the "methods"
parameter).
4.1.3. CANCEL requests
For the purposes of generality, both SUBSCRIBE and NOTIFY MAY be
canceled; however, doing so is not recommended. Successfully
cancelled SUBSCRIBE and NOTIFY requests MUST be completed with a
"487 Request Cancelled" response; the server acts as if the
request were never received. In general, since neither SUBSCRIBE
nor NOTIFY are allowed to have protracted transactions, attempts
to cancel them are expected to fail.
4.1.4. State Agents and Notifier Migration
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When state agents (see section 5.4.11. ) are used, it is often
useful to allow migration of subscriptions between state agents
and the nodes for which they are providing state aggregation (or
even among various state agents). Such migration may be effected
by sending a "NOTIFY" with an "Subscription-Expires" header of
"0," and a reason parameter of "migration." This NOTIFY request
is otherwise normal, and is formed as described in section 4.3.3.
Upon receipt of this NOTIFY message, the subscriber SHOULD
attempt to re-subscribe (as described in the following sections).
The resulting SUBSCRIBE message can then be proxied or redirected
to the node to which notification responsibility is passing.
4.2. Description of SUBSCRIBE Behavior
The SUBSCRIBE method is used to request current state and state
updates from a remote node.
4.2.1. Correlation to dialogs, calls, and terminals
A subscription is uniquely identified by the combination of the
To, From, and Call-ID fields in the SUBSCRIBE request. Refreshes
of subscriptions SHOULD reuse the same Call-ID if possible, since
subscriptions are uniquely identified at presence servers using
the Call-ID. Two subscriptions from the same user, for the same
user, but with different Call-IDs, are considered different
subscriptions. Note this is exactly the same as usage of Call-ID
in registrations.
Initial SUBSCRIBE requests MUST contain a "tag" parameter (as
defined in RFC 2543 [1] ) in the "From" header, and MUST NOT
contain a "tag" parameter in the "To" header. Responses to
SUBSCRIBE requests MUST contain a "tag" parameter in the "To"
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
requests for re-subscription MUST contain "tag" parameters in
both the "To" and "From" headers (matching those previously
established for the dialog).
The relationship between subscriptions and (INVITE-initiated)
sessions sharing the same dialog correlation information is
undefined. Re-using dialog correlation information for
subscriptions is allowed, but sharing of such information does
not change the semantics of the INVITE session or the SUBSCRIBE
dialog.
Similarly, the relationship between a subscription in one
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direction (e.g. from node A to node B) and a subscription in the
opposite direction (from B to A) with the same dialog correlation
information is undefined. While re-using such information is
allowed, the sharing of such information does not change the
semantics of either SUBSCRIBE dialog.
4.2.2. Subscription duration
SUBSCRIBE requests SHOULD contain an "Expires" header. This
expires value indicates the duration of the subscription. The
formatting of these is described in RFC 2543. In order to keep
subscriptions effective beyond the duration communicated in the
"Expires" header, subscribers need to refresh subscriptions on a
periodic basis. This refreshing is performed in the same way as
REGISTER refreshes: the To, From, and Call-ID match those in the
SUBSCRIBE being refreshed, while the CSeq number is incremented.
If no "Expires" header is present in a SUBSCRIBE request, the
implied default is defined by the event package being used.
200-class responses to SUBSCRIBE requests also MUST contain an
"Expires" header. The period of time in the response MAY be
shorter or longer than specified in the request. The period of
time in the response is the one which defines the duration of the
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 4.3.3.
4.2.3. Identification of Subscribed Events and Event Classes
Identification of events is provided by three pieces of
information: Request URI, Event Type, and (optionally) message
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
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identify the resource for which event notification is desired,
but not necessarily enough information to uniquely identify the
nature of the event (e.g. "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 token which
indicates the type of state for which a subscription is being
requested. This token will be registered with the IANA and will
correspond to an event package which further describes the
semantics of the event or event class.
The "Event" header is considered mandatory for the purposes of
this document. However, to maintain compatibility with PINT (see
[3] ), servers MAY interpret a SUBSCRIBE request with no "Event"
header as requesting a subscription to PINT events. If the
servers do not support PINT, they SHOULD return "489 Bad Event"
to any SUBSCRIBE messages without an EVENT header.
If the event package to which the event token corresponds defines
behavior associated with the body of its SUBSCRIBE requests,
those semantics apply.
4.2.4. Additional SUBSCRIBE Header Values
Each SUBSCRIBE request MUST have exactly one "Contact:" header,
to be used as part of route handling, as described in section
4.1.1.
SUBSCRIBE requests MAY contain an "Accept" header. This header,
if present, indicates the body formats allowed in subsequent
NOTIFY requests. Event packages MUST define the behavior for
SUBSCRIBE requests without "Accept" headers; usually, this will
connote a single, default body type.
Header values not described in this document are to be
interpreted as described in RFC 2543 [1] .
4.2.5. Subscriber SUBSCRIBE Behavior
4.2.5.1. Requesting a Subscription
When a subscriber wishes to subscribe to a particular state for a
resource, it forms a SUBSCRIBE message.
The dialog correlation information is formed as if for an
original INVITE: the Call-ID is a new call ID with the syntax
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described in RFC 2543; the To: field indicates the subscribed
resource's persistent address (which will generally match the
Request URI used to form the message); and the From: field will
indicate the subscriber's persistent address (typically
sip:user@domain).
This SUBSCRIBE request will be confirmed with a final response.
200-class responses indicate that the subscription has been
accepted, and that a NOTIFY will be sent immediately. A 200
response indicates that the subscription has been accepted and
that the user is authorized to subscribe to the requested
resource. A 202 response merely indicates that the subscription
has been understood, and that authorization may or may not have
been granted.
The "Expires" header in a 200-class response to SUBSCRIBE
indicates the actual duration for which the subscription will
remain active (unless refreshed).
Non-200 class final responses indicate that the subscription has
not been created, and no subsequent NOTIFY message will be sent.
All non-200 class responses (with the exception of "489,"
described herein) have the same meanings and handling as
described in RFC 2543 [1] .
4.2.5.2. 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 except as described
below.
Subscription renewals will contain a "To" field matching the
"From" field in the first NOTIFY request for the dialog
containing the subscription to be refreshed. They will contain
the same "From" and "Call-ID" fields as the original SUBSCRIBE
request, and an incremented "CSeq" number from the original
SUBSCRIBE request. Route handling is as discussed in section
4.1.1.
If a SUBSCRIBE request to refresh a subscription receives a "481"
response, this indicates that the subscription has been
terminated and that the subscriber did not receive notification
of this fact. In this case, the subscriber should consider the
subscription invalid. If the subscriber wishes to re-subscribe to
the state, he does so by composing an unrelated initial SUBSCRIBE
request with a freshly-generated Call-ID and a new, unique "From"
tag (see section 4.2.5.1. )
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If a SUBSCRIBE request to refresh a subscription fails, the
original subscription is still considered valid for the duration
of the most recently known "Expires" value as negotiated by
SUBSCRIBE and its response, or as communicated by NOTIFY in
"Subscription-Expires," except as described above.
4.2.5.3. Unsubscribing
Unsubscribing is handled in the same way as refreshing of a
subscription, with the "Expires" header set to "0." Note that a
successful unsubscription will also trigger a final "NOTIFY".
4.2.5.4. 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 arrives, the
subscriber should consider the state of the subscribed resource
to be in a neutral state. Event packages which define new event
packages MUST define this "neutral state" in such a way that
makes sense for their application (see section 5.4.7. ).
Due to the potential for both out-of-order messages and forking,
the subscriber MUST be prepared to receive NOTIFY messages before
the SUBSCRIBE transaction has completed.
Except as noted above, processing of this NOTIFY is the same as
in section 4.3.5.
4.2.6. Proxy SUBSCRIBE Behavior
Proxies need no additional behavior beyond that described in RFC
2543 [1] to support SUBSCRIBE. If a proxy wishes to see all of
the SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
record-route all SUBSCRIBE and NOTIFY requests.
4.2.7. Notifier SUBSCRIBE Behavior
4.2.7.1. 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.
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The notifier SHOULD also perform any necessary authentication and
authorization per its local policy. See section 4.2.7.3.
If the SUBSCRIBE request contains a tag parameter in the "To"
field, but the notifier has no record of the indicated dialog,
the notifier has two options. If the notifier is able and willing
to reconstruct subscription state, he may accept the subscription
as an initial subscription. If the notifier cannot or is not
willing to reconstitute such state, it should respond with a "481
Subscription does not exist."
If the notifier is able to immediately determine that it
understands the event package, that the authenticated subscriber
is authorized to subscribe, and that there are no other barriers
to creating the subscription, it creates the subscription and
returns a "200 OK" response, unless doing so would reveal
authorization policy in an undesirable fashion (see section 6.2.
).
If the notifier cannot immediately create the subscription (e.g.
it needs to wait for user input for authorization, or is acting
for another node which is not currently reachable), or wishes to
mask authorization policy, it will return a "202 Accepted"
response. This response indicates that the request has been
received and understood, but does not necessarily imply that the
subscription has been created yet.
The "Expires" values present in SUBSCRIBE 200-class responses
behave in the same way as they do in REGISTER responses: the
server MAY shorten or lengthen the interval.
200-class responses to SUBSCRIBE requests will not generally
contain any useful information beyond subscription duration;
their primary purpose is to serve as a reliability mechanism.
State information will be communicated via a subsequent NOTIFY
request from the notifier.
The other response codes defined in RFC 2543 may be used in
response to SUBSCRIBE requests, as appropriate.
4.2.7.2. Confirmation of Subscription Creation/Refreshing
Upon successfully accepting or refreshing of a subscription,
notifiers MUST send a NOTIFY message immediately to communicate
the current resource state to the subscriber. If the resource has
no meaningful state at the time that the SUBSCRIBE message is
processed, this NOTIFY message MAY contain an empty or neutral
body. See section 4.3.3. for further details on NOTIFY message
generation.
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Note that a NOTIFY message is always sent immediately after any
200-class response to a SUBSCRIBE request, regardless of whether
the subscription has already been authorized.
4.2.7.3. 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
via a "401" response, not a "407;" notifiers always act as a user
agents when accepting subscriptions and sending notifications.
If authorization fails based on an access list or some other
automated mechanism (i.e. it can be automatically authoritatively
determined that the subscriber is not authorized to subscribe),
the notifier SHOULD reply to the request with a "403 Forbidden"
or "603 Decline" response, unless doing so might reveal
information that should stay private; see section 6.2.
If the notifier owner is interactively queried to determine
whether a subscription is allowed, a "202 Accept" response is
returned immediately. Note that a NOTIFY message is still formed
and sent under these circumstances, as described in the previous
section.
If subscription authorization was delayed and the notifier wishes
to convey that such authorization has been declined, it may do so
by sending a NOTIFY message containting a "Subscription-Expires"
header with a value of "0" and a reason parameter of "refused."
4.2.7.4. Refreshing of Subscriptions
When a notifier receives a subscription refresh, assuming that
the subscriber is still authorized, the notifier updates the
expiration time for subscription. As with the initial
subscription, the server MAY shorten or increase the amount of
time until expiration. The final expiration time is placed in the
"Expires" header in the response.
If no refresh for a notification address is received before its
expiration time, the subscription is removed. When removing a
subscription, the notifier MAY send a NOTIFY message with a
"Subscription-Expires" value of "0" to inform it that the
subscription is being removed. If such a message is sent, the
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"Subscription-Expires" header SHOULD contain a "reason=timeout"
parameter.
4.3. Description of NOTIFY Behavior
NOTIFY messages are sent to inform subscribers of changes in
state to which the subscriber has a subscription. Subscriptions
are typically put in place using the SUBSCRIBE method; however,
it is possible that other means have been used.
If any non-SUBSCRIBE mechanisms are defined to create
subscriptions, it is the responsibility of the parties defining
those mechanisms to ensure that correlation of a NOTIFY message
to the corresponding subscription is possible. Designers of such
mechanisms are also warned to make a distinction between sending
a NOTIFY message to a subscriber who is aware of the
subscription, and sending a NOTIFY message to an unsuspecting
node. The latter behavior is invalid, and MUST receive a "481
Subscription does not exist" response (unless some other 400- or
500-class error code is more applicable), as described in section
4.3.5. In other words, knowlege of a subscription must exist in
both the subscriber and the notifier to be valid, even if
installed via a non-SUBSCRIBE mechanism.
A NOTIFY does not cancel its corresponding subscription; in other
words, a single SUBSCRIBE request may trigger several NOTIFY
requests.
4.3.1. Correlation
NOTIFY requests MUST contain the same Call-ID as the SUBSCRIBE
request which ordered them; the "To" field MUST match the "From"
field in the SUBSCRIBE that ordered them, and the "From" field
MUST match the "To" field that was sent in the 200-class response
to the SUBSCRIBE. In other words, NOTIFY requests MUST be in the
same dialog as the SUBSCRIBE that ordered them.
The From field of a NOTIFY request, like the "To" field of a
SUBSCRIBE response, MUST contain a tag; this allows for the
subscriber to differentiate between events from different
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.
If multiple NOTIFY messages are received in response to a single
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SUBSCRIBE message, they represent different destinations to which
the SUBSCRIBE request was forked. Unless the event package
specifies otherwise, the subscriber may either accept all such
notifications as representing different dialogs (which are then
refreshed separately), or send a 481 response to any NOTIFYs on
dialogs that it does not want to keep alive.
As expected, CSeq spaces are unique for each node; in other
words, the notifier uses a different CSeq space than the
subscriber and any other notifiers.
4.3.2. Identification of reported events, event classes, and current
state
Identification of events being reported in a notification is very
similar to that described for subscription to events (see section
4.2.3. ).
The Request URI of a NOTIFY request contains enough information
to route the request to the party which is subscribed to receive
notifications. It is derived from the "Contact" header present in
the corresponding SUBSCRIBE request.
If the same events for different resources are being subscribed
to, implementors are expected to use different dialogs in order
to be able to differentiate between notifications for them,
unless the body for the event contains enough information for
this correlation.
As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a
single token which identifies the event or class of events for
which a notification is being generated.
If the event package to which the event token corresponds defines
behavior associated with the body of its NOTIFY requests, those
semantics apply. This information is expected to provide
additional details about the nature of the event which has
occurred and the resultant resource state.
When present, the body of the NOTIFY request MUST be formatted
into one of the body formats specified in the "Accept" header of
the corresponding SUBSCRIBE request.
4.3.3. Notifier NOTIFY Behavior
When a SUBSCRIBE request is successfully processed or a relevant
change in the subscribed state occurs, the notifier will
immediately construct and send a NOTIFY request to the
subscriber(s), per standard Route/Record-Route handling, as
described in section 4.1.1.
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If the notifier is able, through any means, to determine that the
subscriber is no longer available to receive notifications, it
MAY elect to not send a notification. An example of a method by
which such information may be known is the "SIP for Presence"
event set (see [4] ).
A NOTIFY request is considered failed if the response times out,
or a non-200 class response code is received which has no
"Retry-After" header and no implied further action which can be
taken to retry the request (e.g. "401 Authorization Required.")
If the NOTIFY request fails (as defined above) due to a timeout
condition, and the subscription was installed using a soft-state
mechanism (such as SIP signalling), the notifier SHOULD remove
the subscription.
If the NOTIFY request fails (as defined above) due to an error
response, and the subscription was installed using a soft-state
mechanism, the notifier MUST remove the corresponding
subscription.
If a NOTIFY request receives a 481 response, the notifier MUST
remove the corresponding subscription even if such subscription
was installed by non-SUBSCRIBE means (such as an administrative
interface).
NOTIFY requests SHOULD contain an "Subscription-Expires" header
which indicates the remaining duration of the subscription (such
a header is useful in case the SUBSCRIBE request forks, since
the response to a forked subscribe -- which contains the "Expire"
header that specifies the agreed-upon expiration time -- may not
be received by the subscriber). The notifier SHOULD use this
header to adjust the time remaining on the subscription; however,
this mechanism MUST not be used to lengthen a subscription, only
to shorten it. The notifier may inform a subscriber that a
subscription has been removed by sending a NOTIFY message with an
"Subscription-Expires" value of "0."
If the duration of a subscription has been shortened or
terminated by the "Subscription-Expires" header as compared to
the most recent 200-class SUBSCRIBE response sent, that header
SHOULD include a "reason" parameter indicating the reason that
such action was taken. Currently, four such values are defined:
"migration" indicates that the node acting as notifier is
transferring responsibility for maintaing such state information
to another node; this only makes sense when subscriptions are
terminated, not when they are shortened. "Maint" indicates that
the subscription is being truncated or terminated due to server
maintainance, and "refused" indicates that the subscription has
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been removed or shortened administratively (e.g. by a change in
ACL policy). Finally, if the notifier elects to send a NOTIFY
upon timeout of the subscription, they SHOULD include a
"Subscription-Expires" header with a value of "0" and a reason
parameter of "timeout."
4.3.4. Proxy NOTIFY Behavior
Proxies need no additional behavior beyond that described in RFC
2543 [1] to support NOTIFY. If a proxy wishes to see all of the
SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
record-route all SUBSCRIBE and NOTIFY requests.
4.3.5. Subscriber NOTIFY Behavior
Upon receiving a NOTIFY request, the subscriber should check that
it matches at least one of its outstanding subscriptions; if not,
it MUST return a "481 Subscription does not exist" response
unless another 400- or 500-class response is more appropriate.
If, for some reason, the event package designated in the "Event"
header of the NOTIFY request is not supported, the subscriber
will respond with a "489 Bad Event" response.
To prevent spoofing of events, NOTIFY requests MAY be
authenticated, using any defined SIP authentication mechanism.
NOTIFY requests SHOULD contain "Subscription-Expires" headers
which indicate the time remaining on the subscription. If this
header is present, the subscriber SHOULD take it as the
authoritative duration and adjust accordingly. If an expires
value of "0" is present, the subscriber should consider the
subscription terminated.
When the subscription is terminated or shortened using the
"Subscription-Expires" mechanism, there SHOULD be a reason
parameter present. If it is present and the subscriber is still
interested in receiving updates to the state information, the
subscriber SHOULD attempt re-subscribe upon expiration if it is
set to "migration," "timeout," is not present, or is set to an
unknown value. Such a resubscription will be completely
independant of the original subscription, and will not share a
dialog with it; it will be generated as described in section
4.2.5.1.
If the "reason" parameter on a "Subscription-Expires" header is
set to either "maint" or "refused," the subscriber SHOULD NOT
attempt re-subscription.
Once the notification is deemed acceptable to the subscriber, the
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subscriber SHOULD return a 200 response. In general, it is not
expected that NOTIFY responses will contain bodies; however, they
MAY, if the NOTIFY request contained an "Accept" header.
Other responses defined in RFC 2543 [1] may also be returned, as
appropriate.
4.4. Polling Resource State
A natural consequence of the behavior described in the preceding
sections is that an immediate fetch without a persistent
subscription may be effected by sending an appropriate SUBSCRIBE
with an "Expires" of 0.
Of course, an immediate fetch while a subscription is active may
be effected by sending an appropriate SUBSCRIBE with an "Expires"
greater than 0.
Upon receipt of this SUBSCRIBE request, the notifier (or
notifiers, if the SUBSCRIBE request was forked) will send a
NOTIFY request containing resource state to the address in the
SUBSCRIBE "Contact" field. Note that normal Route and
Record-Route handle still applies; see section 4.1.1.
4.5. Allow-Events header usage
The "Allow-Events" header, if present, includes a list of tokens
which indicates the event packages supported by the client (if
sent in a request) or server (if sent in a response). In other
words, a node sending an "Allow-Events" header is advertising
that it can process SUBSCRIBE requests and generate NOTIFY
requests for all of the event packages listed in that header.
Any node implementing one or more event packages SHOULD include
an appropriate "Allow-Events" header indicating all supported
events in INVITE requests and responses, OPTIONS responses, and
REGISTER requests. "Allow-Events" headers MAY be included in any
other type of request or response.
This information is very useful, for example, in allowing user
agents to render particular interface elements appropriately
according to whether the events required to implement the
features they represent are supported by the appropriate nodes.
Note that "Allow-Events" headers MUST NOT be inserted by proxies.
5. Event Packages
This section covers several issues which should be taken into
consideration when event packages based on SUBSCRIBE and NOTIFY
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are proposed.
5.1. Appropriateness of Usage
When designing an event package using the methods described in
this draft for event notification, it is important to consider:
is SIP an appropriate mechanism for the problem set? Is SIP being
selected because of some unique feature provided by the protocol
(e.g. user mobility), or merely because "it can be done?" If you
find yourself defining event packages for notifications related
to, for example, network management or the temperature inside
your car's engine, you may want to reconsider your selection of
protocols.
Those interested in extending the mechanism defined in this
document are urged to read "Guidelines for Authors of SIP
Extensions" [2] for further guidance regarding appropriate uses
of SIP.
Further, it is expected that this mechanism is not to be used in
applications where the frequency of reportable events is
excessively rapid (e.g. more than about once per second). A SIP
network is generally going to be provisioned for a reasonable
signalling volume; sending a notification every time a user's GPS
position changes by one hundreth of a second could easily
overload such a network.
5.2. Sub-packages
Normal event packages define a set of state applied to a specific
type of resource, such as user presence, call state, and
messaging mailbox state.
Sub-packages are a special type of package which define a set of
state applied to other packages, such as statistics, access
policy, and subscriber lists. Sub-packages may even be applied to
other sub-packages.
To extend the object-oriented analogy made earlier, sub-packages
can be thought of as templatized C++ packages which must be
applied to other packages to be useful.
The name of a sub-package as applied to a package is formed by
appending a period followed by the sub-package name to the end of
the package. For example, if a subpackage called "watcherinfo"
were being applied to a package called "presence," the event
token used in "Event" and "Allow-Events" would be
"presence.watcherinfo".
Sub-packages must be defined so that they can be applied to any
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arbitrary package. In other words, sub-packages cannot be
specifically tied to one or a few "parent" packages in such a way
that they will not work with other packages.
5.3. Amount of State to be Conveyed
When designing event packages, it is important to consider the
type of information which will be conveyed during a notification.
A natural temptation is to convey merely the event (e.g. "a new
voice message just arrived") without accompanying state (e.g. "7
total voice messages"). This complicates implementation of
subscribing entities (since they have to maintain complete state
for the entity to which they have subscribed), and also is
particularly susceptible to synchronization problems.
There are two possible solutions to this problem that event
packages may choose to implement.
5.3.1. Complete State Information
For packages which typically convey state information that is
reasonably small (on the order of 1 kb or so), it is suggested
that event packages are designed so as to send complete state
information when an event occurs.
In the circumstances that state may not be sufficient for a
particular class of events, the event packages should include
complete state information along with the event that occurred.
For example, "no customer service representatives available" may
not be as useful "no customer service representatives available;
representative sip:46@cs.xyz.int just logged off".
5.3.2. State Deltas
In the case that the state information to be conveyed is large,
the event package may choose to detail a scheme by which NOTIFY
messages contain state deltas instead of complete state.
Such a scheme would work as follows: any NOTIFY sent in immediate
response to a SUBSCRIBE contains full state information. NOTIFY
messages sent because of a state change will contain only the
state information that has changed; the subscriber will then
merge this information into its current knowledge about the state
of the resource.
Any event package that supports delta changes to states MUST use
a payload which contains a version number that increases by
exactly one for each NOTIFY message. Note that the state version
number appears in the body of the message, not in a SIP header.
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If a NOTIFY arrives that has a version number that is incremented
by more than one, the subscriber knows that a state delta has
been missed; it ignores the NOTIFY message containing the state
delta (except for the version number, which it retains to detect
message loss), and re-sends a SUBSCRIBE to force a NOTIFY
containing a complete state snapshot.
5.4. Event Package Responsibilities
Event packages are not required to re-iterate any of the behavior
described in this document, although they may choose to do so for
clarity or emphasis. In general, though, such packages are
expected to describe only the behavior that extends or modifies
the behavior described in this document.
Note that any behavior designated with "SHOULD" or "MUST" in this
document is not allowed to be changed by extension documents;
however, such documents may elect to strengthen "SHOULD"
requirements to "MUST" strength if required by their application.
In addition to the normal sections expected by "Instructions to
RFC Authors" [6] and "Guidelines for Authors of SIP Extensions"
[2] , authors of event packages MUST address each of the issues
detailed in the following subsections, whenever applicable.
5.4.1. Event Package Name
This mandatory section of an event package defines the token name
to be used to designate the event package. It MUST include the
information which appears in the IANA registration of the token.
For information on registering such types, see section 7.
5.4.2. Event Package Parameters
If parameters are to be used on the "Event" header to modify the
behavior of the event package, the syntax and semantics of such
headers must be clearly defined.
5.4.3. SUBSCRIBE Bodies
It is expected that most, but not all, event packages will define
syntax and semantics for SUBSCRIBE method bodies; these bodies
will typically modify, expand, filter, throttle, and/or set
thresholds for the class of events being requested. Designers of
event packages are strongly encouraged to re-use existing MIME
types for message bodies where practical.
This mandatory section of an event package defines what type or
types of event bodies are expected in SUBSCRIBE requests (or
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specify that no event bodies are expected). It should point to
detailed definitions of syntax and semantics for all referenced
body types.
5.4.4. Subscription Duration
It is recommended that event packages give a suggested range of
times considered reasonable for the duration of a subscription.
Such packages MUST also define a default "Expires" value to be
used if none is specified.
5.4.5. NOTIFY Bodies
The NOTIFY body is used to report state on the resource being
monitored. Each package must define a what type or types of event
bodies are expected in NOTIFY requests. Such packages must
specify or cite detailed specifications for the syntax and
semantics associated with such event body.
Event packages also need to define which MIME type is to be
assumed if none are specified in the "Accept" header of the
SUBSCRIBE request.
5.4.6. Notifier processing of SUBSCRIBE requests
This section describes the processing to be performed by the
notifier upon receipt of a SUBSCRIBE request. Such a section is
required.
Information in this section includes details of how to
authenticate subscribers and authorization issues for the
package. Such authorization issues may include, for example,
whether all SUBSCRIBE requests for this package are answered with
202 responses (see section 6.2. ).
5.4.7. Notifier generation of NOTIFY requests
This section of an event package describes the process by which
the notifier generates and sends a NOTIFY request. This includes
detailed information about what events cause a NOTIFY to be sent,
how to compute the state information in the NOTIFY, how to
generate "neutral" or "fake" state information to hide
authorization delays and decisions from users, and whether state
information is complete or deltas for notifications (see section
5.3. )
It may optionally describe the behavior used to processes the
subsequent response. Such a section is required.
5.4.8. Subscriber processing of NOTIFY requests
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This section of an event package describes the process followed
by the subscriber upon receipt of a NOTIFY request, including any
logic required to form a coherent resource state (if applicable).
5.4.9. Handling of forked requests
Each event package should specify whether forked SUBSCRIBE
requests are allowed to install multiple subscriptions. If such
behavior is not allowed, any NOTIFY messages not matching the
200-class response to the initial SUBSCRIBE message are responded
to with a 481.
In the case that multiple subscriptions are allowed, the event
package must specify whether merging of the notifications to form
a single state is required, and how such merging is to be
performed. Note that it is possible that some event packages may
be defined in such a way that each dialog is tied to a mutually
exclusive state which is unaffected by the other dialogs; this
must be clearly stated if it is the case.
5.4.10. Rate of notifications
Each event package is expected to define a requirement
(RECOMMENDED, SHOULD or MUST strength) which defines an absolute
maximum on the rate at which notifications are allowed to be
generated by a single notifier.
Such packages may further define a throttle mechanism which
allows subscribers to further limit the rate of notification.
5.4.11. State Agents
Designers of event packages should consider whether their package
can benefit from network aggregation points ("State Agents")
and/or nodes which act on behalf of other nodes. (For example,
nodes which provide state information about a resource when such
a resource is unable or unwilling to provide such state
information itself). An example of such an application is a node
which tracks the presence and availability of a user in the
network.
If state agents are to be used by the package, the package must
specify how such state agents aggregate information and how they
provide authentication and authorization.
5.4.12. Examples
Event packages should include several demonstrative message flow
diagrams paired with several typical, syntactically correct and
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complete messages.
It is recommended that documents describing event packages
clearly indicate that such examples are informative and not
normative, with instructions that implementors refer to the main
text of the draft for exact protocol details.
6. Security Considerations
6.1. Access Control
The ability to accept subscriptions should be under the direct
control of the user, since many types of events may be considered
sensitive for the purposes of privacy. Similarly, the notifier
should have the ability to selectively reject subscriptions based
on the calling party (based on access control lists), using
standard SIP authentication mechanisms. The methods for creation
and distribution of such access control lists is outside the
scope of this draft.
6.2. Release of Sensitive Policy Information
The mere act of returning a 200 or certain 4xx and 6xx responses
to SUBSCRIBE requests may, under certain circumstances, create
privacy concerns by revealing sensitive policy information. In
these cases, the notifier should always return a 202 response.
While the subsequent NOTIFY message may not convey true state, it
MUST appear to contain a potentially correct piece of data from
the point of view of the subscriber, indistinguishable from a
valid response. Information about whether a user is authorized to
subscribe to the requested state is never conveyed back to the
original user under these circumstances.
6.3. Denial-of-Service attacks
The current model (one SUBSCRIBE request triggers a SUBSCRIBE
response and one or more NOTIFY requests) is a classic setup for
an amplifier node to be used in a smurf attack.
Also, the creation of state upon receipt of a SUBSCRIBE request
can be used by attackers to consume resources on a victim's
machine, rendering it unusable.
To reduce the chances of such an attack, implementations of
notifiers SHOULD require authentication. Authentication issues
are discussed in RFC2543 [1] .
7. IANA Considerations
(This section is not applicable until this document is published
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as an RFC.)
This document defines an event-type namespace which requires a
central coordinating body. The body chosen for this coordination
is the Internet Assigned Numbers Authority (IANA).
There are two different types of event-types: normal event
packages, and event sub-packages; see section 5.2. To avoid
confusion, subpackage names and package names share the same
namespace; in other words, a sub-package MUST NOT share a name
with a package.
Following the policies outlined in "Guidelines for Writing an
IANA Considerations Section in RFCs" [7] , normal event package
identification tokens are allocated as First Come First Served,
and event sub-package identification tokens are allocated on a
IETF Consensus basis.
Registrations with the IANA MUST include the token being
registered and whether the token is a package or a subpackage.
Further, packages MUST include contact information for the party
responsible for the registration and/or a published document
which describes the event package. Sub-package token
registrations MUST include a pointer to the published RFC which
defines the sub-package.
Registered tokens to designate packages and sub-packages MUST NOT
contain the character ".", which is used to separate sub-packages
from packages.
7.1. Registration Template
As this document specifies no package or sub-package names, the
initial IANA registration for event types will be empty. The
remainder of the text in this section gives an example of the
type of information to be maintained by the IANA; it also
demonstrates all five possible permutations of package type,
contact, and reference.
The table below lists the event packages and sub-packages defined
in "SIP-Specific Event Notification" [RFC xxxx]. Each name is
designated as a package or a subpackage under "Type."
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Package Name Type Contact Reference
------------ ---- ------- ---------
example1 package [Roach]
example2 package [Roach] [RFC xxxx]
example3 package [RFC xxxx]
example4 sub-package [Roach] [RFC xxxx]
example5 sub-package [RFC xxxx]
PEOPLE
------
[Roach] Adam Roach <adam.roach@ericsson.com>
REFERENCES
----------
[RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX,
August 2002.
8. Open Issues
In addition to the three issues listed below, the BNF in this
document needs to be converted to explicit LWS to match the
latest bis draft; this change will be reflected in the next
version.
8.1. CANCEL Handling
This is actually a protocol-wide open issue which has impacts on
this specification: there hasn't been a clear consensus about
cancellation of non-INVITE requests yet. If non-INVITE requests
cannot be cancelled, we need to remove section 4.1.3.
8.2. Version of SIP to reference
Much of the handling in this document is rather different than
what is described in RFC2543; in fact, many of the recent changes
to this document have been tracking changes in the "bis" versions
of the SIP specification. We can continue to reference RFC2543
while pulling in huge chunks of the bis draft for compatibility
(for example, the Route handling would essentially be copied
word-for-word from the bis draft), or we can start referencing
the bis drafts.
Of course, referencing the bis drafts allows us to pick up
changes to protocol semantics "for free," while importing chunks
of it requires constant maintanance and runs the risk of getting
out of sync.
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On the other hand, placing a dependency on the bis draft pushes
the timeframe for this draft (and the drafts that depend on it)
out past the time that the next SIP RFC is published.
8.3. Immediate NOTIFYs
There has been discussion, but no consensus, on the issue of
whether each SUBSCRIBE must have an immediate NOTIFY message sent
in response. In attempts to follow the prevailing sentiment, this
draft had become internally inconsistent.
This version of this document has eliminated these
inconsistancies by requiring notifiers always to send a NOTIFY
immediately upon receiving a SUBSCRIBE. This decision does not
necessarily represent group consensus, and further discussion may
be warranted.
9. Changes
9.1. Changes from draft-ietf-...-00
- Fixed confusing typo in section describing correlation
of SUBSCRIBE requests
- Added explanitory text to clarify tag handling when
generating re-subscriptions
- Expanded general handling section to include specific
discussion of Route/Record-Route handling.
- Included use of "methods" parameter on Contact as
a means for detecting support for SUBSCRIBE and NOTIFY.
- Added definition of term "dialog"; changed "leg" to
"dialog" everwhere.
- Added syntax for "Subscription-Expires" header.
- Changed NOTIFY messages to refer to "Subscription-Expires"
everywhere (instead of "Expires.")
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- Added information about generation and handling of
481 responses to SUBSCRIBE requests
- Changed having Expires header in SUBSCRIBE from
MUST to SHOULD; this aligns more closely with
REGISTER behavior
- Removed experimental/private event package names,
per list consensus
- Cleaned up some legacy text left over from very early
drafts that allowed multiple contacts per subscription
- Strengthened language requiring the removal of subscriptions
if a NOTIFY request fails with a 481. Clarified that such
removal is required for all subscriptions, including
administrative ones.
- Removed description of delaying NOTIFY requests until
authorization is granted. Such behavior was inconsistent
with other parts of this document.
- Moved description of event packages to later in document,
to reduce the number of forward references.
- Minor editorial and nits changes
- Added new open issues to open issues section. All
previous open issues have been resolved.
9.2. Changes from draft-roach-...-03
- Added DOS attacks section to open issues.
- Added discussion of forking to open issues
- Changed response to PINT request for notifiers who don't
support PINT from 400 to 489.
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- Added sentence to security section to call attention to
potential privacy issues of delayed NOTIFY responses.
- Added clarification: access control list handling is out
of scope.
- (Hopefully) Final resolution on out-of-band subscriptions:
mentioned in section
4.3.
Removed from open issues.
- Made "Contact" header optional for SUBSCRIBE 1xx responses.
- Added description clarifying tag handling (section
4.2.1.
)
- 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
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- 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.
- Made 481 responses mandatory for unexpected notifications
(allowing notifiers to remove subscriptions in error cases)
- Several minor non-semantic editorial changes.
9.3. Changes from draft-roach-...-02
- Clarification under "Notifier SUBSCRIBE behavior" which
indicates that the first NOTIFY message (sent immediately
in response to a SUBSCRIBE) may contain an empty body, if
resource state doesn't make sense at that point in time.
- Text on message flow in overview section corrected
- Removed suggestion that clients attempt to unsubscribe
whenever they receive a NOTIFY for an unknown event.
Such behavior opens up DOS attacks, and will lead to
message loops unless additional precautions are taken.
The 481 response to the NOTIFY should serve the same
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).
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- Added text specifying that SUBSCRIBE transactions are not
to be prolonged. This is based on the consensus that non-INVITE
transactions should never be prolonged; such consensus within
the SIP working group was reached at the 49th IETF.
- Added "202 Accepted" response code to support the above
change. The behavior of this 202 response code is a
generalization of that described in the presence draft.
- Updated to specify that the response to an unauthorized
SUBSCRIBE request is 603 or 403.
- Level-4 subheadings added to particularly long sections to
break them up into logical units. This helps make the
behavior description seem somewhat less rambling. This also
caused some re-ordering of these paragraphs (hopefully in a
way that makes them more readable).
- Some final mopping up of old text describing "call related"
and "third party" subscriptions (deprecated concepts).
- Duplicate explanation of subscription duration removed from
subscriber SUBSCRIBE behavior section.
- Other text generally applicable to SUBSCRIBE (instead of just
subscriber handling of SUBSCRIBE) moved to parent section.
- Updated header table to reflect mandatory usage of "Expires"
header in SUBSCRIBE requests and responses
- Removed "Event" header usage in responses
- Added sentence suggesting that notifiers may notify
subscribers when a subscription has timed out.
- Clarified that a failed attempt to refresh a subscription
does not imply that the original subscription has been
cancelled.
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Internet Draft SIP-Specific Event Notification November 2001
- Clarified that 489 is a valid response to "NOTIFY" requests.
- Minor editorial changes to clean up awkward and/or unclear
grammar in several places
9.4. Changes from draft-roach-...-01
- Multiple contacts per SUBSCRIBE message disallowed.
- Contact header now required in NOTIFY messages.
- Distinction between third party/call member events removed.
- Distinction between call-related/resource-related events removed.
- Clarified that subscribers must expect NOTIFY messages before
the SUBSCRIBE transaction completes
- Added immediate NOTIFY message after successful SUBSCRIBE;
this solves a myriad of issues, most having to do with forking.
- Added discussion of "undefined state" (before a NOTIFY arrives).
- Added mechanism for notifiers to shorten/cancel outstanding
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.
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Internet Draft SIP-Specific Event Notification November 2001
[2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
Extensions", <draft-ietf-sip-guidelines-02.txt>, IETF; March
2001. Work in progress.
[3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848,
IETF; June 2000.
[4] J. Rosenberg et. al., "SIP Extensions for Presence",
<draft-ietf-simple-presence-03.txt>, IETF; September 2001.
Work in progress.
[5] R. Fielding et. al., "Hypertext Transfer Protocol --
HTTP/1.1", RFC2068, IETF, January 1997.
[6] J. Postel, J. Reynolds, "Instructions to RFC Authors",
RFC2223, IETF, October 1997.
[7] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, IETF, October 1998.
[8] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee
Capabilities", <draft-ietf-sip-callerprefs-04.txt>, IETF;
June 2001. Work in progress.
11. Acknowledgements
Thanks to the participants in the Events BOF at the 48th IETF
meeting in Pittsburgh, as well as those who gave ideas and
suggestions on the SIP Events mailing list. In particular, I wish
to thank Henning Schulzrinne of Columbia University for coming up
with the final three-tiered event identification scheme, Sean
Olson of Ericsson for miscellaneous guidance, Jonathan Rosenberg
for a thorough scrubbing of the -00 draft, and the authors of the
"SIP Extensions for Presence" draft for their input to SUBSCRIBE
and NOTIFY request semantics.
12. Author's Address
Adam Roach
Ericsson Inc.
Mailstop L-04
740 E. Campbell Rd.
Richardson, TX 75081
USA
Phone: +1 972 583 7594
Fax: +1 972 669 0154
E-Mail: adam.roach@ericsson.com
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