draft-ietf-sip-events-04.txt   draft-ietf-sip-events-05.txt 
Internet Engineering Task Force Adam Roach A new Request for Comments is now available in online RFC libraries.
Internet Draft dynamicsoft
Category: Standards Track February 2002
Expires August 2002
<draft-ietf-sip-events-04.txt>
SIP-Specific Event Notification
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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 RFC 3265
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at Title: Session Initiation Protocol (SIP)-Specific Event
http://www.ietf.org/shadow.html Notification
Author(s): A. B. Roach
Status: Standards Track
Date: June 2002
Mailbox: adam@dynamicsoft.com
Pages: 38
Characters: 89005
Obsoletes: 2543
This document is an individual submission to the IETF. Comments I-D Tag: draft-ietf-sip-events-05.txt
should be directed to the authors.
Abstract URL: ftp://ftp.rfc-editor.org/in-notes/rfc3265.txt
This document describes an extension to the Session Initiation This document describes an extension to the Session Initiation
Protocol (SIP). The purpose of this extension is to provide an Protocol (SIP). The purpose of this extension is to provide an
extensible framework by which SIP nodes can request notification extensible framework by which SIP nodes can request notification
from remote nodes indicating that certain events have occurred. from remote nodes indicating that certain events have occurred.
Concrete uses of the mechanism described in this document may be Concrete uses of the mechanism described in this document may be
standardized in the future. standardized in the future.
Note that the event notification mechanisms defined herein are Note that the event notification mechanisms defined herein are
NOT intended to be a general-purpose infrastructure for all NOT intended to be a general-purpose infrastructure for all
classes of event subscription and notification. classes of event subscription and notification.
1. Table of Contents This document is a product of the Session Initiation Protocol Working
Group of the IETF.
1. Table of Contents...................................... 1
2. Introduction........................................... 3
2.1. Overview of Operation.................................. 4
2.2. Documentation Conventions.............................. 4
3. Definitions............................................ 4
4. Node Behavior.......................................... 5
4.1. Description of SUBSCRIBE Behavior...................... 5
4.1.1. Subscription Duration.................................. 6
4.1.2. Identification of Subscribed Events and Event Classes.. 6
4.1.3. Additional SUBSCRIBE Header Values..................... 7
4.1.4. Subscriber SUBSCRIBE Behavior.......................... 7
4.1.5. Proxy SUBSCRIBE Behavior............................... 9
4.1.6. Notifier SUBSCRIBE Behavior............................ 9
4.2. Description of NOTIFY Behavior......................... 12
4.2.1. Identification of Reported Events, Event Classes, and C 13
4.2.2. Notifier NOTIFY Behavior............................... 13
4.2.3. Proxy NOTIFY Behavior.................................. 15
4.2.4. Subscriber NOTIFY Behavior............................. 15
4.3. General................................................ 17
4.3.1. Detecting support for SUBSCRIBE and NOTIFY............. 17
4.3.2. CANCEL requests........................................ 18
4.3.3. Forking................................................ 18
4.3.4. Dialog creation and termination........................ 18
4.3.5. State Agents and Notifier Migration.................... 19
4.3.6. Polling Resource State................................. 20
4.3.7. Allow-Events header usage.............................. 20
4.3.8. PINT Compatibility..................................... 21
5. Event Packages......................................... 21
5.1. Appropriateness of Usage............................... 21
5.2. Event Template-packages................................ 22
5.3. Amount of State to be Conveyed......................... 22
5.3.1. Complete State Information............................. 22
5.3.2. State Deltas........................................... 23
5.4. Event Package Responsibilities......................... 23
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.................................. 24
5.4.5. NOTIFY Bodies.......................................... 24
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............................ 25
5.4.10. Rate of notifications.................................. 26
5.4.11. State Agents........................................... 26
5.4.12. Examples............................................... 26
5.4.13. Use of URIs to Retrieve State.......................... 27
6. Security Considerations................................ 27
6.1. Access Control......................................... 27
6.2. Notifier Privacy Mechanism............................. 27
6.3. Denial-of-Service attacks.............................. 27
6.4. Replay Attacks......................................... 28
6.5. Man-in-the middle attacks.............................. 28
6.6. Confidentiality........................................ 28
7. IANA Considerations.................................... 29
7.1. Registration Information............................... 29
7.2. Registration Template.................................. 30
7.3. Syntax................................................. 31
7.4. New Methods............................................ 31
7.4.1. SUBSCRIBE method....................................... 32
7.4.2. NOTIFY method.......................................... 33
7.5. New Headers............................................ 33
7.5.1. "Event" header......................................... 33
7.5.2. "Allow-Events" Header.................................. 34
7.5.3. "Subscription-State" Header............................ 34
7.6. New Response Codes..................................... 34
7.6.1. "202 Accepted" Response Code........................... 34
7.6.2. "489 Bad Event" Response Code.......................... 34
7.7. Augmented BNF Definitions.............................. 34
8. References............................................. 35
9. Acknowledgements....................................... 36
10. Author's Address....................................... 36
11. Notice Regarding Intellectual Property Rights.......... 36
12. Full Copyright Statement............................... 36
2. Introduction
The ability to request asynchronous notification of events proves
useful in many types of SIP 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 PSTN and
Internet Internetworking (PINT) [3] status (based on call state
events).
The methods described in this document provide 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
elaborate rules which govern the subscription and notification
for the events or classes of events they describe.
This draft does not describe an extension which may be used
directly; it must be extended by other drafts (herein referred to
as "event packages".) In object-oriented design terminology, it
may be thought of as an abstract base class which must be derived
into an instantiatable class by further extensions. Guidelines
for creating these extensions are described in section 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------->|
Subscriptions are expired and must be refreshed by subsequent
SUBSCRIBE messages.
2.2. Documentation Conventions
There are several paragraphs throughout the document which
provide motivational or clarifying text. Such passages are
non-normative, and are provided only to assist with reader
comprehension. These passages are set off from the remainder of
the text by being indented thus:
This is an example of non-normative explanatory text. It does
not form part of the specification, and is used only for
clarification.
Numbers in square brackets (e.g. [1]) denote a reference to one
of the entries in the References section; see section 8.
The all-capital terms "MUST", "SHOULD", "MAY", "SHOULD NOT", and
"MUST NOT" are used as defined in RFC 2119 [7].
The use of quotation marks next to periods and commas follows the
convention used by the American Mathematical Society; although
contrary to traditional American English convention, this usage
lends clarity to certain passages.
3. Definitions
Event Package: An event package is an additional specification
which defines a set of state information to be reported by a
notifier to a subscriber. Event packages also define further
syntax and semantics based on the framework defined by this
document required to convey such state information.
Event Template-Package: An event template-package is a special
kind of event package which defines a set of state which may
be applied to all possible event packages, including itself.
Notification: Notification is the act of a notifier sending a
NOTIFY message to a subscriber to inform the subscriber of
the state of a resource.
Notifier: A notifier is a user agent which generates NOTIFY
requests for the purpose of notifying subscribers of the
state of a resource. Notifiers typically also accept
SUBSCRIBE requests to create subscriptions.
State Agent: A state agent is a notifier which publishes state
information on behalf of a resource; in order to do so, it
may need to gather such state information from multiple
sources. State agents always have complete state information
for the resource for which they are creating notifications.
Subscriber: A subscriber is a user agent which receives NOTIFY
requests from notifiers; these NOTIFY requests contain
information about the state of a resource in which the
subscriber is interested. Subscribers typically also generate
SUBSCRIBE requests and send them to notifiers to create
subscriptions.
Subscription: A subscription is a set of application state
associated with a dialog. This application state includes a
pointer to the associated dialog, the event package name, and
possibly an identification token. Event packages will define
additional subscription state information. By definition,
subscriptions exist in both a subscriber and a notifier.
Subscription Migration: Subscription migration is the act of
moving a subscription from one notifier to another notifier.
4. Node Behavior
4.1. Description of SUBSCRIBE Behavior
The SUBSCRIBE method is used to request current state and state
updates from a remote node.
4.1.1. Subscription Duration
SUBSCRIBE requests SHOULD contain an "Expires" header (defined in
SIP [1]). This expires value indicates the duration of the
subscription. In order to keep subscriptions effective beyond the
duration communicated in the "Expires" header, subscribers need
to refresh subscriptions on a periodic basis using a new
SUBSCRIBE message on the same dialog as defined in SIP [1].
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 but MUST NOT be longer than specified in the request. The
period of time in the response is the one which defines the
duration of the subscription.
An "expires" parameter on the "Contact" header has no semantics
for SUBSCRIBE and is explicitly not equivalent to an "Expires"
header in a SUBSCRIBE request or response.
A natural consequence of this scheme is that a SUBSCRIBE with an
"Expires" of 0 constitutes a request to unsubscribe from an
event.
In addition to being a request to unsubscribe, a SUBSCRIBE
message with "Expires" of 0 also causes a fetch of state; see
section 4.3.6.
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 4.2.2.
4.1.2. 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 per the request routing procedures outlined in
SIP [1]. 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@dynamicsoft.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 MAY
also contain an "id" parameter. This "id" parameter, if present,
contains an opaque token which identifies the specific
subscription within a dialog. An "id" parameter is only valid
within the scope of a single dialog.
If the event package to which the event token corresponds defines
behavior associated with the body of its SUBSCRIBE requests,
those semantics apply.
Event packages may also define parameters for the Event header;
if they do so, they must define the semantics for such
parameters.
4.1.3. Additional SUBSCRIBE Header Values
Because SUBSCRIBE requests create a dialog as defined in SIP [1],
they 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 SIP [1].
4.1.4. Subscriber SUBSCRIBE Behavior
4.1.4.1. Requesting a Subscription
SUBSCRIBE is a dialog-creating method, as described in SIP [1].
When a subscriber wishes to subscribe to a particular state for a
resource, it forms a SUBSCRIBE message. If the initial SUBSCRIBE
represents a request outside of a dialog (as it typically will),
its construction follows the procedures outlined in SIP [1] for
UAC request generation outside of a dialog.
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 no subscription or
dialog has 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 SIP [1].
A SUBSCRIBE request MAY include an "id" parameter in its "Event"
header to allow differentiation between multiple subscriptions in
the same dialog.
4.1.4.2. Refreshing of Subscriptions
At any time before a subscription expires, the subscriber may
refresh the timer on such a subscription by sending another
SUBSCRIBE request on the same dialog as the existing
subscription, and with the same "Event" header "id" parameter (if
one was present in the initial subscription). The handling for
such a request is the same as for the initial creation of a
subscription except as described below.
If the initial SUBSCRIBE message contained an "id" parameter
on the "Event" header, then refreshes of the subscription
must also contain an identical "id" parameter; they will
otherwise be considered new subscriptions in an existing
dialog.
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 4.1.4.1.)
If a SUBSCRIBE request to refresh a subscription fails with a
non-481 response, 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 the "Subscription-State" header "expires" parameter.
Note that many such errors indicate that there may be a
problem with the network or the notifier such that no further
NOTIFY messages will be received.
4.1.4.3. 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
message.
4.1.4.4. Confirmation of Subscription Creation
The subscriber can expect to receive a NOTIFY message from each
node which has processed 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 4.2.4.
4.1.5. Proxy SUBSCRIBE Behavior
Proxies need no additional behavior beyond that described in SIP
[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 the initial SUBSCRIBE and any dialog-establishing
NOTIFY requests. Such proxies SHOULD also record-route all other
SUBSCRIBE and NOTIFY requests.
Note that subscribers and notifiers may elect to use S/MIME
encryption of SUBSCRIBE and NOTIFY requests; consequently,
proxies cannot rely on being able to access any information
that is not explicitly required to be proxy-readable by SIP
[1].
4.1.6. Notifier SUBSCRIBE Behavior
4.1.6.1. Initial 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.
This requirement is imposed primarily to prevent the
non-INVITE transaction timeout timer F (see [1]) from firing
during the SUBSCRIBE transaction, since interaction with a
user would often exceed 64*T1 seconds.
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 4.1.6.3.
The notifier MAY also check that the duration in the "Expires"
header is not too small. If and only if the expiration interval
is greater than zero AND smaller than one hour AND less than a
notifier-configured minimum, the notifier MAY return a "423
Interval too small" error which contains a "Min-Expires" header
field. The "Min-Expires" header field is described in SIP [1].
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 a
dialog (if necessary), 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 authorized yet.
When a subscription is created in the notifier, it stores the
event package name and the "Event" header "id" parameter (if
present) as part of the subscription information.
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 lengthen it.
If the duration specified in a SUBSCRIBE message is
unacceptably short, the notifier may be able to send a 423
response, as described earlier in this section.
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 SIP [1] may be used in
response to SUBSCRIBE requests, as appropriate.
4.1.6.2. Confirmation of Subscription Creation/Refreshing
Upon successfully accepting or refreshing a subscription,
notifiers MUST send a NOTIFY message immediately to communicate
the current resource state to the subscriber. This NOTIFY message
is sent on the same dialog as created by the SUBSCRIBE response.
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 4.2.2. for further details
on NOTIFY message generation.
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.
4.1.6.3. Authentication/Authorization of SUBSCRIBE requests
Privacy concerns may require that notifiers apply policy to
determine whether a particular subscriber is authorized to
subscribe to a certain set of events. Such policy may be defined
by mechanisms such as access control lists or real-time
interaction with a user. In general, authorization of subscribers
prior to authentication is not particularly useful.
SIP authentication mechanisms are discussed in SIP [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.
Of course, when acting as a proxy, a node will perform normal
proxy authentication (using 407). The foregoing explanation
is a reminder that notifiers are always UAs, and as such
perform UA authentication.
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 containing a "Subscription-State"
header with a value of "terminated" and a reason parameter of
"rejected".
4.1.6.4. 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 the amount of time until
expiration, but MUST NOT increase it. The final expiration time
is placed in the "Expires" header in the response. If the
duration specified in a SUBSCRIBE message is unacceptably short,
the notifier SHOULD respond with a "423 Subscription Too Brief"
message.
If no refresh for a notification address is received before its
expiration time, the subscription is removed. When removing a
subscription, the notifier SHOULD send a NOTIFY message with a
"Subscription-State" value of "terminated" to inform it that the
subscription is being removed. If such a message is sent, the
"Subscription-State" header SHOULD contain a "reason=timeout"
parameter.
The sending of a NOTIFY when a subscription expires allows
the corresponding dialog to be terminated, if appropriate.
4.2. Description of NOTIFY Behavior
NOTIFY messages are sent to inform subscribers of changes in
state to which the subscriber has a subscription. Subscriptions
are typically put in place using the SUBSCRIBE method; however,
it is possible that other means have been used.
If any non-SUBSCRIBE mechanisms are defined to create
subscriptions, it is the responsibility of the parties defining
those mechanisms to ensure that correlation of a NOTIFY message
to the corresponding subscription is possible. Designers of such
mechanisms are also warned to make a distinction between sending
a NOTIFY message to a subscriber who is aware of the
subscription, and sending a NOTIFY message to an unsuspecting
node. The latter behavior is invalid, and MUST receive a "481
Subscription does not exist" response (unless some other 400- or
500-class error code is more applicable), as described in section
4.2.4. In other words, knowledge 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 terminate its corresponding subscription; in
other words, a single SUBSCRIBE request may trigger several
NOTIFY requests.
4.2.1. 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
4.1.2.).
As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a
single event package name for which a notification is being
generated. The package name in the "Event" header MUST match the
"Event" header in the corresponding SUBSCRIBE message. If an "id"
parameter was present in the SUBSCRIBE message, that "id"
parameter MUST also be present in the corresponding NOTIFY
messages.
Event packages may define semantics associated with the body of
their NOTIFY requests; if they do so, those semantics apply.
NOTIFY bodies are 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. This body will contain
either the state of the subscribed resource or a pointer to such
state in the form of a URI.
4.2.2. Notifier NOTIFY Behavior
When a SUBSCRIBE request is answered with a 200-class response,
the notifier MUST immediately construct and send a NOTIFY request
to the subscriber. When a change in the subscribed state occurs,
the notifier SHOULD immediately construct and send a NOTIFY
request, subject to authorization, local policy, and throttling
considerations.
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 SUBSCRIBE), the notifier SHOULD remove the
subscription.
This behavior prevents unnecessary transmission of state
information for subscribers who have crashed or disappeared
from the network. Because such transmissions will be sent
multiple times, per the retransmission algorithm defined in
SIP [1] (instead of the typical single transmission for
functioning clients), continuing to service them when no
client is available to acknowledge them could place undue
strain on a network. Upon client restart or reestablishment
of a network connection, it is expected that clients will
send SUBSCRIBE messages to refresh potentially stale state
information; such messages will re-install subscriptions in
all relevant nodes.
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.
A notify error response would generally indicate that
something has gone wrong with the subscriber or with some
proxy on the way to the subscriber. If the subscriber is in
error, it makes the most sense to allow the subscriber to
rectify the situation (by re-subscribing) once the error
condition has been handled. If a proxy is in error, the
periodic SUBSCRIBE refreshes will re-install subscription
state once the network problem has been resolved.
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).
If the above behavior were not required, subscribers
receiving a notify for an unknown subscription would need to
send an error status code in response to the NOTIFY and also
send a SUBSCRIBE request to remove the subscription. Since
this behavior would make subscribers available for use as
amplifiers in denial of service attacks, we have instead
elected to give the 481 response special meaning: it is used
to indicate that a subscription must be cancelled under all
circumstances.
NOTIFY requests MUST contain a "Subscription-State" header with a
value of "active", "pending", or "terminated". The "active" value
indicates that the subscription has been accepted and has been
authorized (in most cases; see section 6.2.). The "pending" value
indicates that the subscription has been received, but that
policy information is insufficient to accept or deny the
subscription at this time. The "terminated" value indicates that
the subscription is not active.
If the value of the "Subscription-State" header is "active" or
"pending", the notifier SHOULD also include in the
"Subscription-State" header an "expires" parameter which
indicates the time remaining on the subscription. The notifier
MAY use this mechanism to shorten a subscription; however, this
mechanism MUST NOT be used to lengthen a subscription.
Including expiration information for active and pending
subscriptions is useful in case the SUBSCRIBE request forks,
since the response to a forked SUBSCRIBE may not be received
by the subscriber. Note well that this "expires" value is a
parameter on the "Subscription-State" header, NOT an
"Expires" header.
If the value of the "Subscription-State" header is "terminated",
the notifier SHOULD also include a "reason" parameter. The
notifier MAY also include a "retry-after" parameter, where
appropriate. For details on the value and semantics of the
"reason" and "retry-after" parameters, see section 4.2.4.
4.2.3. Proxy NOTIFY Behavior
Proxies need no additional behavior beyond that described in SIP
[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 the initial SUBSCRIBE and any dialog-establishing
NOTIFY requests. Such proxies SHOULD also record-route all other
SUBSCRIBE and NOTIFY requests.
Note that subscribers and notifiers may elect to use S/MIME
encryption of SUBSCRIBE and NOTIFY requests; consequently,
proxies cannot rely on being able to access any information
that is not explicitly required to be proxy-readable by SIP
[1].
4.2.4. 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.
The rules for matching NOTIFY requests with subscriptions that
create a new dialog are described in section 4.3.4. Notifications
for subscriptions which were created inside an existing dialog
match if they are in the same dialog and the "Event" headers
match (as described in section 7.5.1.)
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 SHOULD be
authenticated, using any defined SIP authentication mechanism.
NOTIFY requests MUST contain "Subscription-State" headers which
indicate the status of the subscription.
If the "Subscription-State" header value is "active", it means
that the subscription has been accepted and (in general) has been
authorized. If the header also contains an "expires" parameter,
the subscriber SHOULD take it as the authoritative subscription
duration and adjust accordingly. The "retry-after" and "reason"
parameters have no semantics for "active".
If the "Subscription-State" value is "pending", the subscription
has been received by the notifier, but there is insufficient
policy information to grant or deny the subscription yet. If the
header also contains an "expires" parameter, the subscriber
SHOULD take it as the authoritative subscription duration and
adjust accordingly. No further action is necessary on the part of
the subscriber. The "retry-after" and "reason" parameters have no
semantics for "pending".
If the "Subscription-State" value is "terminated", the subscriber
should consider the subscription terminated. The "expires"
parameter has no semantics for "terminated". If a reason code is
present, the client should behave as described below. If no
reason code or an unknown reason code is present, the client MAY
attempt to re-subscribe at any time (unless a "retry-after"
parameter is present, in which case the client SHOULD NOT attempt
re-subscription until after the number of seconds specified by
the "retry-after" parameter). The defined reason codes are:
deactivated: The subscription has been terminated, but the client
SHOULD retry immediately with a new subscription. One primary
use of such a status code is to allow migration of
subscriptions between nodes. The "retry-after" parameter has
no semantics for "deactivated".
probation: The subscription has been terminated, but the client
SHOULD retry at some later time. If a "retry-after" parameter
is also present, the client SHOULD wait at least the number
of seconds specified by that parameter before attempting to
re-subscribe.
rejected: The subscription has been terminated due to change in
authorization policy. Clients SHOULD NOT attempt to
re-subscribe. The "retry-after" parameter has no semantics
for "rejected".
timeout: The subscription has been terminated because it was not
refreshed before it expired. Clients MAY re-subscribe
immediately. The "retry-after" parameter has no semantics for
"timeout".
giveup: The subscription has been terminated because the notifier
could not obtain authorization in a timely fashion. If a
"retry-after" parameter is also present, the client SHOULD
wait at least the number of seconds specified by that
parameter before attempting to re-subscribe; otherwise, the
client MAY retry immediately, but will likely get put back
into pending state.
noresource: The subscription has been terminated because the
resource state which was being monitored no longer exists.
Clients SHOULD NOT attempt to re-subscribe. The "retry-after"
parameter has no semantics for "noresource".
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 SIP [1] may also be returned, as
appropriate. In no case should a NOTIFY transaction extend for
any longer than the time necessary for automated processing. In
particular, subscribers MUST NOT wait for a user response before
returning a final response to a NOTIFY request.
4.3. General
4.3.1. 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 SIP[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 [6] for details on
the "methods" parameter).
4.3.2. CANCEL requests
No semantics are associated with cancelling SUBSCRIBE or NOTIFY.
4.3.3. Forking
In accordance with the rules for proxying non-INVITE requests as
defined in SIP [1], 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
SUBSCRIBE message, they represent different destinations to which
the SUBSCRIBE request was forked. For information on subscriber
handling in such situations, see section 5.4.9.
4.3.4. Dialog creation and termination
If an initial SUBSCRIBE request is not sent on a pre-existing
dialog, the subscriber will wait for a response to the SUBSCRIBE
request or a matching NOTIFY.
Responses are matched to such SUBSCRIBE requests if they contain
the same the same "Call-ID", the same "From" header "tag", and
the same "CSeq". Rules for the comparison of these headers are
described in SIP [1]. If a 200-class response matches such a
SUBSCRIBE request, it creates a new subscription and a new dialog
(unless they have already been created by a matching NOTIFY
request; see below).
NOTIFY requests are matched to such SUBSCRIBE requests if they
contain the same "Call-ID", a "To" header "tag" parameter which
matches the "From" header "tag" parameter of the SUBSCRIBE, and
the same "Event" header field. Rules for comparisons of the
"Event" headers are described in section 7.5.1. If a matching
NOTIFY request contains a "Subscription-State" of "active" or
"pending", it creates a new subscription and a new dialog (unless
they have already been created by a matching response, as
described above).
If an initial SUBSCRIBE is sent on a pre-existing dialog, a
matching 200-class response or successful NOTIFY request merely
creates a new subscription associated with that dialog.
Multiple subscriptions can be associated with a single dialog.
Subscriptions may also exist in dialogs associated with
INVITE-created application state and other application state
created by mechanisms defined in other specifications. These sets
of application state do not interact beyond the behavior
described for a dialog (e.g. route set handling).
A subscription is destroyed when a notifier sends a NOTIFY
request with a "Subscription-State" of "terminated".
A subscriber may send a SUBSCRIBE request with an "Expires"
header of 0 in order to trigger the sending of such a NOTIFY
request; however, for the purposes of subscription and dialog
lifetime, the subscription is not considered terminated until
the NOTIFY with a "Subscription-State" of "terminated" is
sent.
If a subscription's destruction leaves no other application state
associated with the dialog, the dialog terminates. The
destruction of other application state (such as that created by
an INVITE) will not terminate the dialog if a subscription is
still associated with that dialog.
Note that the above behavior means that a dialog created with
an INVITE does not necessarily terminate upon receipt of a
BYE. Similarly, in the case that several subscriptions are
associated with a single dialog, the dialog does not
terminate until all the subscriptions in it are destroyed.
4.3.5. State Agents and Notifier Migration
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 message with a "Subscription-State" header of
"terminated", and a reason parameter of "deactivated". This
NOTIFY request is otherwise normal, and is formed as described in
section 4.2.2.
Upon receipt of this NOTIFY message, the subscriber SHOULD
attempt to re-subscribe (as described in the preceding sections).
Note that this subscription is established on a new dialog, and
does not re-use the route set from the previous subscription
dialog.
The actual migration is effected by making a change to the policy
(such as routing decisions) of one or more servers to which the
SUBSCRIBE request will be sent in such a way that a different
node ends up responding to the SUBSCRIBE request. This may be as
simple as a change in the local policy in the notifier from which
the subscription is migrating so that it serves as a proxy or
redirect server instead of a notifier.
Whether, when, and why to perform notifier migrations may be
described in individual event packages; otherwise, such decisions
are a matter of local notifier policy, and are left up to
individual implementations.
4.3.6. 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 a SUBSCRIBE with an
"Expires" of 0.
Of course, an immediate fetch while a subscription is active may
be effected by sending a SUBSCRIBE with an "Expires" equal to the
number of seconds remaining in the subscription.
Upon receipt of this SUBSCRIBE request, the notifier (or
notifiers, if the SUBSCRIBE request was forked) will send a
NOTIFY request containing resource state in the same dialog.
Note that the NOTIFY messages triggered by SUBSCRIBE messages
with "Expires" headers of 0 will contain a "Subscription-State"
value of "terminated", and a "reason" parameter of "timeout".
Polling of event state can cause significant increases in load on
the network and notifiers; as such, it should be used only
sparingly. In particular, polling SHOULD NOT be used in
circumstances in which it will typically result in more network
messages than long-running subscriptions.
When polling is used, subscribers SHOULD attempt to cache
authentication credentials between polls so as to reduce the
number of messages sent.
4.3.7. 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 all methods which initiate dialogs and their responses
(such as INVITE) and OPTIONS responses.
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.
4.3.8. PINT Compatibility
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 a server
does not support PINT, it SHOULD return "489 Bad Event" to any
SUBSCRIBE messages without an "Event" header.
5. Event Packages
This section covers several issues which should be taken into
consideration when event packages based on SUBSCRIBE and NOTIFY
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 follow the development of "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. Event Template-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.
Event template-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. Event
template-packages may even be applied to other event
template-packages.
To extend the object-oriented analogy made earlier, event
template-packages can be thought of as templatized C++ packages
which must be applied to other packages to be useful.
The name of an event template-package as applied to a package is
formed by appending a period followed by the event
template-package name to the end of the package. For example, if
a template-package called "winfo" were being applied to a package
called "presence", the event token used in "Event" and
"Allow-Events" would be "presence.winfo".
Event template-packages must be defined so that they can be
applied to any arbitrary package. In other words, event
template-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 some circumstances, conveying the current state alone may be
insufficient for a particular class of events. In these cases,
the event packages should include complete state information
along with the event that occurred. For example, conveying "no
customer service representatives available" may not be as useful
as conveying "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
include a version number that increases by exactly one for each
NOTIFY message in a subscription. Note that the state version
number appears in the body of the message, not in a SIP header.
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 weakened 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 in
standards-track RFCs and SIP extension documents, authors of
event packages need to address each of the issues detailed in
the following subsections, whenever applicable.
5.4.1. Event Package Name
This section, which MUST be present, 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
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 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 MUST 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. Such a
section is required.
This section may optionally describe the behavior used to process
the subsequent response.
5.4.8. Subscriber processing of NOTIFY requests
This section of an event package describes the process followed
by the subscriber upon receipt of a NOTIFY request, including any
logic required to form a coherent resource state (if applicable).
5.4.9. Handling of forked requests
Each event package MUST specify whether forked SUBSCRIBE requests
are allowed to install multiple subscriptions.
If such behavior is not allowed, the first potential
dialog-establishing message will create a dialog. All subsequent
NOTIFY messages which correspond to the SUBSCRIBE message (i.e.
match "To", "From", "From" header "tag" parameter, "Call-ID",
"CSeq", "Event", and "Event" header "id" parameter) but which do
not match the dialog would be rejected with a 481 response. Note
that the 200-class response to the SUBSCRIBE can arrive after a
matching NOTIFY has been received; such responses might not
correlate to the same dialog established by the NOTIFY. Except as
required to complete the SUBSCRIBE transaction, such non-matching
200-class responses are ignored.
If installing of multiple subscriptions by way of a single forked
SUBSCRIBE is allowed, the subscriber establishes a new dialog
towards each notifier by returning a 200-class response to each
NOTIFY. Each dialog is then handled as its own entity, and is
refreshed independent of the other dialogs.
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 (SHOULD or
MUST strength) which defines an absolute maximum on the rate at
which notifications are allowed to be generated by a single
notifier.
Each package 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.
Event packages MAY also outline specific scenarios under which
notifier migrations take place.
5.4.12. Examples
Event packages SHOULD include several demonstrative message flow
diagrams paired with several typical, syntactically correct and
complete messages.
It is RECOMMENDED that documents describing event packages
clearly indicate that such examples are informative and not
normative, with instructions that implementors refer to the main
text of the draft for exact protocol details.
5.4.13. Use of URIs to Retrieve State
Some types of event packages may define state information which
is potentially too large to reasonably send in a SIP message. To
alleviate this problem, event packages may include the ability to
convey a URI instead of state information; this URI will then be
used to retrieve the actual state information.
The precise mechanisms for conveying such URIs are out of the
scope of this document.
6. Security Considerations
6.1. Access Control
The ability to accept subscriptions should be under the direct
control of the notifier's 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 subscriber identity (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. Notifier Privacy Mechanism
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.
Individual packages and their related drafts for which such a
mode of operation makes sense can further describe how and why to
generate such potentially correct data. For example, such a mode
of operation is mandated by RFC 2779 [8] for user presence
information.
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 SIP [1].
6.4. Replay Attacks
Replaying of either SUBSCRIBE or NOTIFY can have detrimental
effects.
In the case of SUBSCRIBE messages, attackers may be able to
install any arbitrary subscription which it witnessed being
installed at some point in the past. Replaying of NOTIFY messages
may be used to spoof old state information (although a good
versioning mechanism in the body of the NOTIFY messages may help
mitigate such an attack). Note that the prohibition on sending
NOTIFY messages to nodes which have not subscribed to an event
also aids in mitigating the effects of such an attack.
To prevent such attacks, implementations SHOULD require
authentication with anti-replay protection. Authentication issues
are discussed in SIP [1].
6.5. Man-in-the middle attacks
Even with authentication, man-in-the-middle attacks using
SUBSCRIBE may be used to install arbitrary subscriptions, hijack
existing subscriptions, terminate outstanding subscriptions, or
modify the resource to which a subscription is being made. To
prevent such attacks, implementations SHOULD provide integrity
protection across "Contact", "Route", "Expires", "Event", and
"To" headers of SUBSCRIBE messages, at a minimum. If SUBSCRIBE
bodies are used to define further information about the state of
the call, they SHOULD be included in the integrity protection
scheme.
Man-in-the-middle attacks may also attempt to use NOTIFY messages
to spoof arbitrary state information and/or terminate outstanding
subscriptions. To prevent such attacks, implementations SHOULD
provide integrity protection across the "Call-ID", "CSeq", and
"Subscription-State" headers and the bodies of NOTIFY messages.
Integrity protection of message headers and bodies is discussed
in SIP [1].
6.6. Confidentiality
The state information contained in a NOTIFY message has the
potential to contain sensitive information. Implementations MAY
encrypt such information to ensure confidentiality.
While less likely, it is also possible that the information
contained in a SUBSCRIBE message contains information that users
might not want to have revealed. Implementations MAY encrypt such
information to ensure confidentiality.
To allow the remote party to hide information it considers
sensitive, all implementations SHOULD be able to handle encrypted
SUBSCRIBE and NOTIFY messages.
The mechanisms for providing confidentiality are detailed in SIP
[1].
7. IANA Considerations
(This section is not applicable until this document is published
as an RFC.)
This document defines an event-type namespace which requires a
central coordinating body. The body chosen for this coordination
is the Internet Assigned Numbers Authority (IANA).
There are two different types of event-types: normal event
packages, and event template-packages; see section 5.2. To avoid
confusion, template-package names and package names share the
same namespace; in other words, an event template-package MUST
NOT share a name with a package.
Following the policies outlined in "Guidelines for Writing an
IANA Considerations Section in RFCs"[5], normal event package
identification tokens are allocated as First Come First Served,
and event template-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
template-package. Further, packages MUST include contact
information for the party responsible for the registration and/or
a published document which describes the event package. Event
template-package token registrations MUST include a pointer to
the published RFC which defines the event template-package.
Registered tokens to designate packages and template-packages
MUST NOT contain the character ".", which is used to separate
template-packages from packages.
7.1. Registration Information
As this document specifies no package or template-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 template-packages
defined in "SIP-Specific Event Notification" [RFC xxxx]. Each
name is designated as a package or a template-package under
"Type".
Package Name Type Contact Reference
------------ ---- ------- ---------
example1 package [Roach]
example2 package [Roach] [RFC xxxx]
example3 package [RFC xxxx]
example4 template [Roach] [RFC xxxx]
example5 template [RFC xxxx]
PEOPLE
------
[Roach] Adam Roach <adam@dynamicsoft.com>
REFERENCES
----------
[RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX,
August 2002.
7.2. Registration Template
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package Name:
(Package names must conform to the syntax described in
section 7.5.1.)
Is this registration for a Template Package:
(indicate yes or no)
Published Specification(s):
(Template packages require a published RFC. Other packages
may reference a specification when appropriate).
Person & email address to contact for further information:
7.3. Syntax
This section describes the syntax extensions required for event
notification in SIP. Semantics are described in section 4. Note
that the formal syntax definitions described in this document are
expressed in the ABNF format used in SIP [1], and contain
references to elements defined therein.
7.4. New Methods
This document describes two new SIP methods: SUBSCRIBE and
NOTIFY.
This table expands on tables 2 and 3 in SIP [1].
Header Where SUB NOT
------ ----- --- ---
Accept R o o
Accept 2xx - -
Accept 415 o o
Accept-Encoding R o o
Accept-Encoding 2xx - -
Accept-Encoding 415 o o
Accept-Language R o o
Accept-Language 2xx - -
Accept-Language 415 o o
Alert-Info R - -
Alert-Info 180 - -
Allow R o o
Allow 2xx o o
Allow r o o
Allow 405 m m
Authentication-Info 2xx o o
Authorization R o o
Call-ID c m m
Contact R m m
Contact 1xx o o
Contact 2xx m o
Contact 3xx m m
Contact 485 o o
Content-Disposition o o
Content-Encoding o o
Content-Language o o
Content-Length t t
Content-Type * *
CSeq c m m
Date o o
Error-Info 300-699 o o
Expires o -
Expires 2xx m -
From c m m
In-Reply-To R - -
Max-Forwards R m m
Min-Expires 423 m -
MIME-Version o o
Organization o -
Priority R o -
Proxy-Authenticate 407 m m
Proxy-Authorization R o o
Proxy-Require R o o
RAck R - -
Record-Route R o o
Record-Route 2xx,401,484 o o
Reply-To - -
Require o o
Retry-After 404,413,480,486 o o
Retry-After 500,503 o o
Retry-After 600,603 o o
Route R c c
RSeq 1xx o o
Server r o o
Subject R - -
Supported R o o
Supported 2xx o o
Timestamp o o
To c(1) m m
Unsupported 420 o o
User-Agent o o
Via c m m
Warning R - o
Warning r o o
WWW-Authenticate 401 m m
7.4.1. SUBSCRIBE method
"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.
7.4.2. 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.
7.5. New Headers
This table expands on tables 2 and 3 in SIP [1], as amended by
the changes described in section 7.4.
Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
-----------------------------------------------------------------
Allow-Events R o o - o o o o o o
Allow-Events 2xx - o - o o o o o o
Allow-Events 489 - - - - - - - m m
Event R - - - - - - - m m
Subscription-State R - - - - - - - - m
7.5.1. "Event" header
Event is added to the definition of the element "message-header"
in the SIP message grammar.
For the purposes of matching responses and NOTIFY messages with
SUBSCRIBE messages, the event-type portion of the "Event" header
is compared byte-by-byte, and the "id" parameter token (if
present) is compared byte-by-byte. An "Event" header containing
an "id" parameter never matches an "Event" header without an "id"
parameter. No other parameters are considered when performing a
comparison.
Note that the forgoing text means that "Event: foo; id=1234"
would match "Event: foo; param=abcd; id=1234", but not
"Event: foo" (id does not match) or "Event: Foo; id=1234"
(event portion does not match).
This document does not define values for event-types. These
values will be defined by individual event packages, and MUST be
registered with the IANA.
There MUST be exactly one event type listed per event header.
Multiple events per message are disallowed.
7.5.2. "Allow-Events" Header
Allow-Events is added to the definition of the element
"general-header" in the SIP message grammar. Its usage is
describe in section 4.3.7.
7.5.3. "Subscription-State" Header
Subscription-State is added to the definition of the element
"request-header" in the SIP message grammar. Its usage is
described in section 4.2.4.
7.6. New Response Codes
7.6.1. "202 Accepted" Response Code
The 202 response is added to the "Success" header field
definition. "202 Accepted" has the same meaning as that defined
in HTTP/1.1 [4].
7.6.2. "489 Bad Event" Response Code
The 489 event response is added to the "Client-Error" header
field definition. "489 Bad Event" is used to indicate that the
server did not understand the event package specified in a
"Event" header field.
7.7. Augmented BNF Definitions
The Augmented BNF definitions for the various new and modified
syntax elements follows. The notation is as used in SIP [1], and
any elements not defined in this section are as defined in SIP
and the documents to which it refers.
SUBSCRIBEm = %x53.55.42.53.43.52.49.42.45 ; SUBSCRIBE in caps
NOTIFYm = %x4E.4F.54.49.46.59 ; NOTIFY in caps
extension-method = SUBSCRIBEm / NOTIFYm / token
Event = ( "Event" / "o" ) HCOLON event-type
*( SEMI event-param )
event-type = event-package *( "." event-template )
event-package = token-nodot
event-template = token-nodot
token-nodot = 1*( alphanum / "-" / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
event-param = generic-param / ( "id" EQUAL token )
Allow-Events = ( "Allow-Events" / "u" ) HCOLON event-type
*(COMMA event-type)
Subscription-State = "Subscription-State" HCOLON substate-value
*( SEMI subexp-params )
substate-value = "active" / "pending" / "terminated"
/ extension-substate
extension-substate = token
subexp-params = ("reason" EQUAL reason-value)
/ ("expires" EQUAL delta-seconds)
/ ("retry-after" EQUAL delta-seconds)
/ generic-param
reason-value = "deactivated"
/ "probation"
/ "rejected"
/ "timeout"
/ "giveup"
/ "noresource"
/ reason-extension
reason-extension = token
8. References
NOTE: Non-normative references are so labeled.
[1] J. Rosenberg et. al., "SIP: Session Initiation Protocol",
<draft-ietf-sip-rfc2543bis-07>, IETF; February 2002. Work in
progress.
[2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
Extensions", <draft-ietf-sip-guidelines-03.txt>, IETF;
November 2001. Work in progress. Non-normative.
[3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848,
IETF; June 2000.
[4] R. Fielding et. al., "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, IETF, June 1999.
[5] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, IETF, October 1998.
[6] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee
Capabilities", <draft-ietf-sip-callerprefs-05.txt>, IETF;
November 2001. Work in progress. Non-normative.
[7] S. Bradner, "Key words for use in RFCs to indicate
requirement levels", RFC 2119, IETF, March 1997
[8] M. Day et. al., "Instant Messaging/Presence Protocol
Requirements", RFC 2779, IETF, February 2000
9. 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 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.
10. Author's Address
Adam Roach
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
USA
E-Mail: <adam@dynamicsoft.com>
Voice: <sip:adam@dynamicsoft.com>
11. Notice Regarding Intellectual Property Rights This is now a Proposed Standard Protocol.
The IETF has been notified of intellectual property rights This document specifies an Internet standards track protocol for
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