Internet Engineering Task Force Adam Roach
Internet Draft Ericsson Inc.
Category: Standards Track February 2001
Expires August 2001
<draft-roach-sip-subscribe-notify-03.txt>
Event Notification in SIP
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or cite them other than as "work in
progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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 a
generic and 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.
1. Table of Contents
1. Table of Contents...................................... 1
2. Introduction........................................... 2
2.1. Overview of Operation.................................. 3
3. Extension Considerations............................... 3
3.1. Appropriateness of Usage............................... 4
3.2. Additional Guidelines.................................. 4
4. Syntax................................................. 5
Roach [Page 1]
Internet Draft Event Notification in SIP February 2001
4.1. New Methods............................................ 5
4.1.1. SUBSCRIBE method....................................... 6
4.1.2. NOTIFY method.......................................... 7
4.2. New Headers............................................ 7
4.2.1. "Event" header......................................... 7
4.2.2. "Allow-Events" Header.................................. 8
4.3. New Response Codes..................................... 8
4.3.1. "202 Accepted" Response Code........................... 8
4.3.2. "489 Bad Event" Response Code.......................... 8
5. Node Behavior.......................................... 8
5.1. Description of SUBSCRIBE Behavior...................... 9
5.1.1. Correlation to legs, calls, and terminals.............. 9
5.1.2. Subscription duration.................................. 9
5.1.3. Identification of Subscribed Events and Event Classes.. 10
5.1.4. Additional SUBSCRIBE Header Values..................... 11
5.1.5. Subscriber SUBSCRIBE Behavior.......................... 11
5.1.6. Proxy SUBSCRIBE Behavior............................... 12
5.1.7. Notifier SUBSCRIBE Behavior............................ 13
5.2. Description of NOTIFY Behavior......................... 15
5.2.1. Correlation............................................ 15
5.2.2. Identification of reported events, event classes, and c 15
5.2.3. Notifier NOTIFY Behavior............................... 16
5.2.4. Proxy NOTIFY Behavior.................................. 17
5.2.5. Subscriber NOTIFY Behavior............................. 17
5.3. Polling Resource State................................. 18
5.4. Allow-Events header usage.............................. 18
6. Security Considerations................................ 18
7. Open Issues............................................ 19
7.1. Event Agents........................................... 19
7.2. Event Throttling....................................... 19
7.3. Resource identification for out-of-band subscriptions.. 20
8. Changes................................................ 20
8.1. Changes from -02....................................... 20
8.2. Changes from -01....................................... 22
9. References............................................. 23
10. Credits................................................ 23
11. Feedback and Discussion................................ 23
12. Author's Address....................................... 23
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).
Roach [Page 2]
Internet Draft Event Notification in SIP February 2001
The methods described in this document allow a framework by which
notification of these events can be ordered.
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. 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 general 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. However,
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.
Note that this draft does not describe an extension which may be
used directly; it must be extended by other drafts (herein
referred to as "extension drafts" and "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:
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. Extension Considerations
This section covers several issues which should be taken into
Roach [Page 3]
Internet Draft Event Notification in SIP February 2001
consideration when SIP extensions based on SUBSCRIBE and NOTIFY
are proposed.
3.1. Appropriateness of Usage
When 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 SIP extensions 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 writing extensions based on SUBSCRIBE and NOTIFY, 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.
It is therefore suggested that extensions 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 extensions 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".)
Roach [Page 4]
Internet Draft Event Notification in SIP February 2001
4. Syntax
This section describes the syntax extensions required for event
notification in SIP. Semantics are described in section 5.
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] .
Roach [Page 5]
Internet Draft Event Notification in SIP February 2001
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 m 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
Roach [Page 6]
Internet Draft Event Notification in SIP February 2001
"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
extension.
Event = "Event" ":" event-type
*(( ";" parameter-name
["=" ( token | quoted-string ) ] )
event-type = token
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 in further extensions that take advantage
of the SUBSCRIBE and NOTIFY methods, and SHOULD be registered
with the IANA.
Note that experimental event types may be created by prepending
the organization's internet domain, with the field order reversed
Roach [Page 7]
Internet Draft Event Notification in SIP February 2001
(e.g. "Event: com.ericsson.foo").
Further note that there must be exactly one event type listed per
event header. Multiple events per message are disallowed.
4.2.2. "Allow-Events" Header
The following header is defined for the purposes of this
extension.
Allow-Events = "Allow-Events" ":" 1#event-type
Allow-Events is added to the definition of the element
"general-header" in the SIP message grammar.
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
"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.
Roach [Page 8]
Internet Draft Event Notification in SIP February 2001
Note that 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
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.
The relation between subscriptions and (INVITE-initiated)
sessions sharing the same call leg identification information is
undefined. Re-using session leg information for subscriptions 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
Roach [Page 9]
Internet Draft Event Notification in SIP February 2001
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
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 extension draft 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 instead return "400 Bad
Request."
If the extension draft to which the event token corresponds
defines behavior associated with the body of its SUBSCRIBE
Roach [Page 10]
Internet Draft Event Notification in SIP February 2001
requests, those semantics apply. It is expected that most, but
not all, extension drafts 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 extensions are strongly
encouraged to re-use existing MIME types for message bodies where
practical.
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. Extensions making use of SUBSCRIBE and NOTIFY
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
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
Roach [Page 11]
Internet Draft Event Notification in SIP February 2001
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
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. Extension drafts 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
Roach [Page 12]
Internet Draft Event Notification in SIP February 2001
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.
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
Roach [Page 13]
Internet Draft Event Notification in SIP February 2001
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
Note that 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, 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
Roach [Page 14]
Internet Draft Event Notification in SIP February 2001
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
Note that 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.
Note that 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.
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
Roach [Page 15]
Internet Draft Event Notification in SIP February 2001
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
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 extension draft 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.
Note that NOTIFY requests MAY be sent without a matching
SUBSCRIBE under certain circumstances. It may make sense, for
example, to set up a subscription using an out-of-band mechanism
(e.g. HTTP, static provisioning). A subscriber which is designed
to operate in this fashion MUST be prepared to receive NOTIFY
requests without a corresponding call leg.
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
Roach [Page 16]
Internet Draft Event Notification in SIP February 2001
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 response to a NOTIFY request fails, the notifier MUST
remove the contact from the appropriate subscription. If removal
of the contact leaves no remaining contacts, the entire
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 SHOULD return a "481 Call leg/transaction does not exist"
response.
A notable exception to the above behavior will occur when clients
are designed to receive NOTIFY messages for subscriptions set up
via any means other than a SUBSCRIBE message (e.g. HTTP requests,
static provisioning). Such clients will need to, under certain
circumstances, process unmatched NOTIFY requests as if they had
previous knowledge of the subscription.
If, for some reason, the event package designated in the "Event"
header of the NOTIFY request is not supported, the subscriber
should 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
Roach [Page 17]
Internet Draft Event Notification in SIP February 2001
subscriber should consider the subscription terminated.
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.
Extension drafts 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.
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 indicate the event packages supported by the client (if
sent in a request) or server (if sent in a response).
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
Roach [Page 18]
Internet Draft Event Notification in SIP February 2001
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 user agent
should have the ability to selectively reject subscriptions based
on the calling party (using either a white-list or black-list
functionality), and/or using standard SIP authentication
mechanisms.
The mere act of returning a "403 Forbidden" or "603 Decline"
response code to a SUBSCRIBE request may, under certain very rare
circumstances, pose privacy concerns. In these cases, the
notifier may elect to return a 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. Open Issues
7.1. Event Agents
The SIP for Presence draft (draft-rosenberg-impp-presence-00.txt)
describes a mechanism by which presentities having access to
registration information can accept registrations on behalf of
user agents incapable of processing SUBSCRIBE requests. This is a
very useful concept; however, it does not seem to be
generalizable to all classes of events. Should this draft make
explicit provisions for this capability, or should it remain
defined in the SIP for Presence draft as behavior specific to the
"presence" event package?
The only comments I've received on this issue so far favor
deciding that this premise is not generally applicable, removing
it as an open issue.
7.2. Event Throttling
Is the concept of throttling events (e.g. "never inform me of
events more frequently than once every n seconds, no matter
what") useful enough across all event types that we should define
a top-level mechanism for this, or do we let extension drafts
that might benefit from this sort of scheme define their own
throttles?
The comments I've received on this topic are split between
suggesting that event packages should define their own throttles,
if appropriate, and suggesting that a general-purpose throttle
mechanism would save event-package creators unnecessary
re-invention of the same concepts. Supporting arguments for both
viewpoints should be taken to the sip-events mailing list, please
Roach [Page 19]
Internet Draft Event Notification in SIP February 2001
(see section 11. )
7.3. Resource identification for out-of-band subscriptions
In this draft, we explicitly allow subscriptions to be put into
place via a mechanism other than a SUBSCRIBE request. Many people
believe that sanctioning of such behavior in the base draft is
important. It raises an interesting issue, however, that is
probably not completely appropriate for this draft to solve. For
documentation purposes, the problem is this: In a SUBSCRIBE
request, the request URI is used to identify the resource
(although not the event) to which a subscription is requested; If
there is no explicit SUBSCRIBE, this information doesn't really
exist anywhere.
I get the general feeling that this problem isn't well understood
by the community, and that it deserves a great deal more thought
than it's receiving.
8. Changes
8.1. Changes from -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).
Roach [Page 20]
Internet Draft Event Notification in SIP February 2001
- Added text specifying that SUBSCRIBE transactions are not
to be prolonged. This is based on the consensus that non-INVITE
transactions should never be prolonged; such consensus within
the SIP working group was reached at the 49th IETF.
- Added "202 Accepted" response code to support the above
change. The behavior of this 202 response code is a
generalization of that described in the presence draft [5] .
- 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.
Roach [Page 21]
Internet Draft Event Notification in SIP February 2001
- 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
8.2. Changes from -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.
Roach [Page 22]
Internet Draft Event Notification in SIP February 2001
9. 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.
[6] R. Fielding et. al., "Hypertext Transfer Protocol --
HTTP/1.1", RFC2068, IETF, January 1997.
10. Credits
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.
11. Feedback and Discussion
Comments regarding this draft are welcomed at the 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, go to http://groups.yahoo.com/group/sip-events
12. Author's Address
Adam Roach
Ericsson Inc.
Mailstop L-04
851 International Pkwy.
Roach [Page 23]
Internet Draft Event Notification in SIP February 2001
Richardson, TX 75081
USA
Phone: +1 972 583 7594
Fax: +1 972 669 0154
E-Mail: adam.roach@ericsson.com
Roach [Page 24]