4. Node Behavior
4.1. Subscriber Behavior
4.1.1. Detecting Support for SIP Events
The extension described in this document does not make use of the
"Require" or "Proxy-Require" header fields; similarly, there is no
token defined for "Supported" header fields. Potential subscribers
may probe for the support of SIP events using the OPTIONS request
defined in [RFC3261].
The presence of "SUBSCRIBE" in the "Allow" header field of any
request or response indicates support for SIP events; further, in the
absence of an "Allow" header field, the simple presence of an "Allow-
Events" header field is sufficient to indicate that the node that
sent the message is capable of acting as a notifier (see
The "methods" parameter for Contact may also be used to
specifically announce support for SUBSCRIBE and NOTIFY requests
when registering. (See [RFC3840] for details on the "methods"
4.1.2. Creating and Maintaining Subscriptions
From the subscriber's perspective, a subscription proceeds according
to the following state diagram. Events that result in a transition
back to the same state are not represented in this diagram.
| init |<-----------------------+
Send SUBSCRIBE expires
V Timer N Fires; |
+-------------+ SUBSCRIBE failure |
+------------| notify_wait |-- response; --------+ |
| +-------------+ or NOTIFY, | |
| | state=terminated | |
| | | |
|| | | V | ||
|| Receive NOTIFY, Receive NOTIFY, +-------------+ ||
|| state=active state=pending | terminated | ||
|| | | +-------------+ ||
|| | | Re-subscription A A ||
|| | V times out; | | ||
|| | +-------------+ Receive NOTIFY, | | ||
|| | | pending |-- state=terminated; --+ | ||
|| | +-------------+ or 481 response | ||
|| | | to SUBSCRIBE | ||
|| | Receive NOTIFY, refresh | ||
|| | state=active | ||
|| | | Re-subscription | ||
|| | V times out; | ||
|| | +-------------+ Receive NOTIFY, | ||
|| +----------->| active |-- state=terminated; -----+ ||
|| +-------------+ or 481 response ||
|| to SUBSCRIBE ||
|| Subscription refresh ||
In the state diagram, "Re-subscription times out" means that an
attempt to refresh or update the subscription using a new SUBSCRIBE
request does not result in a NOTIFY request before the corresponding
Timer N expires.
Any transition from "notify_wait" into a "pending" or "active" state
results in a new subscription. Note that multiple subscriptions can
be generated as the result of a single SUBSCRIBE request (see
Section 4.4.1). Each of these new subscriptions exists in its own
independent state machine and runs its own set of timers.
18.104.22.168. Requesting a Subscription
SUBSCRIBE is a dialog-creating method, as described in [RFC3261].
When a subscriber wishes to subscribe to a particular state for a
resource, it forms a SUBSCRIBE request. If the initial SUBSCRIBE
request represents a request outside of a dialog (as it typically
will), its construction follows the procedures outlined in [RFC3261]
for User Agent Client (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 request will be sent immediately.
The "Expires" header field in a 200-class response to SUBSCRIBE
request indicates the actual duration for which the subscription will
remain active (unless refreshed). The received value might be
smaller than the value indicated in the SUBSCRIBE request but cannot
be larger; see Section 4.2.1 for details.
Non-200-class final responses indicate that no subscription or new
dialog usage has been created, and no subsequent NOTIFY request will
be sent. All non-200-class responses (with the exception of 489 (Bad
Event), described herein) have the same meanings and handling as
described in [RFC3261]. For the sake of clarity: if a SUBSCRIBE
request contains an "Accept" header field, but that field does not
indicate a media type that the notifier is capable of generating in
its NOTIFY requests, then the proper error response is 406 (Not
22.214.171.124. 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. The handling for
such a request is the same as for the initial creation of a
subscription except as described below.
If a SUBSCRIBE request to refresh a subscription receives a 404, 405,
410, 416, 480-485, 489, 501, or 604 response, the subscriber MUST
consider the subscription terminated. (See [RFC5057] for further
details and notes about the effect of error codes on dialogs and
usages within dialog, such as subscriptions). 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 126.96.36.199).
If a SUBSCRIBE request to refresh a subscription fails with any error
code other than those listed above, the original subscription is
still considered valid for the duration of the most recently known
"Expires" value as negotiated by the most recent successful SUBSCRIBE
transaction, or as communicated by a NOTIFY request in its
"Subscription-State" header field "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
requests will be received.
When refreshing a subscription, a subscriber starts Timer N, set to
64*T1, when it sends the SUBSCRIBE request. If this Timer N expires
prior to the receipt of a NOTIFY request, the subscriber considers
the subscription terminated. If the subscriber receives a success
response to the SUBSCRIBE request that indicates that no NOTIFY
request will be generated -- such as the 204 response defined for use
with the optional extension described in [RFC5839] -- then it MUST
cancel Timer N.
Unsubscribing is handled in the same way as refreshing of a
subscription, with the "Expires" header field set to "0". Note that
a successful unsubscription will also trigger a final NOTIFY request.
The final NOTIFY request may or may not contain information about the
state of the resource; subscribers need to be prepared to receive
final NOTIFY requests both with and without state.
188.8.131.52. Confirmation of Subscription Creation
The subscriber can expect to receive a NOTIFY request from each node
which has processed a successful subscription or subscription
refresh. To ensure that subscribers do not wait indefinitely for a
subscription to be established, a subscriber starts a Timer N, set to
64*T1, when it sends a SUBSCRIBE request. If this Timer N expires
prior to the receipt of a NOTIFY request, the subscriber considers
the subscription failed, and cleans up any state associated with the
Until Timer N expires, several NOTIFY requests may arrive from
different destinations (see Section 4.4.1). Each of these requests
establishes a new dialog usage and a new subscription. After the
expiration of Timer N, the subscriber SHOULD reject any such NOTIFY
requests that would otherwise establish a new dialog usage with a 481
(Subscription does not exist) response code.
Until the first NOTIFY request arrives, the subscriber should
consider the state of the subscribed resource to be in a neutral
state. Event package specifications MUST define this "neutral state"
in such a way that makes sense for their application (see
Due to the potential for out-of-order messages, packet loss, and
forking, the subscriber MUST be prepared to receive NOTIFY requests
before the SUBSCRIBE transaction has completed.
Except as noted above, processing of this NOTIFY request is the same
as in Section 4.1.3.
4.1.3. Receiving and Processing State Information
Subscribers receive information about the state of a resource to
which they have subscribed in the form of NOTIFY requests.
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 usage are described in Section 4.4.1. Notifications for
subscriptions that were created inside an existing dialog match if
they are in the same dialog and the "Event" header fields match (as
described in Section 8.2.1).
If, for some reason, the event package designated in the "Event"
header field 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, such as
those described in Sections 22.2 and 23 of [RFC3261].
NOTIFY requests MUST contain "Subscription-State" header fields that
indicate the status of the subscription.
If the "Subscription-State" header field value is "active", it means
that the subscription has been accepted and (in general) has been
authorized. If the header field 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
field 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
MUST consider the subscription terminated. The "expires" parameter
has no semantics for "terminated" -- notifiers SHOULD NOT include an
"expires" parameter on a "Subscription-State" header field with a
value of "terminated", and subscribers MUST ignore any such
parameter, if present. 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 reason codes
defined by this document are:
deactivated: The subscription has been terminated, but the
subscriber 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 (as long as the resource's state
is still relevant to the client at that 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
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". This reason code is also associated with polling of
resource state, as detailed in Section 4.4.3.
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 that was being monitored no longer exists. Clients
SHOULD NOT attempt to re-subscribe. The "retry-after" parameter
has no semantics for "noresource".
invariant: The subscription has been terminated because the resource
state is guaranteed not to change for the foreseeable future.
This may be the case, for example, when subscribing to the
location information of a fixed-location land-line telephone.
When using this reason code, notifiers are advised to include a
"retry-after" parameter with a large value (for example, 31536000
-- or one year) to prevent older clients that are RFC 3265
compliant from periodically re-subscribing. Clients SHOULD NOT
attempt to re-subscribe after receiving a reason code of
"invariant", regardless of the presence of or value of a "retry-
Other specifications may define new reason codes for use with the
"Subscription-State" header field.
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 field.
Other responses defined in [RFC3261] 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.
4.1.4. Forking of SUBSCRIBE Requests
In accordance with the rules for proxying non-INVITE requests as
defined in [RFC3261], 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 that differ
from the "To:" tag received in the SUBSCRIBE 200-class response.
If multiple NOTIFY requests are received in different dialogs in
response to a single SUBSCRIBE request, each dialog represents a
different destination to which the SUBSCRIBE request was forked.
Subscriber handling in such situations varies by event package; see
Section 5.4.9 for details.
4.2. Notifier Behavior
4.2.1. Subscription Establishment and Maintenance
Notifiers learn about subscription requests by receiving SUBSCRIBE
requests from interested parties. Notifiers MUST NOT create
subscriptions except upon receipt of a SUBSCRIBE request. However,
for historical reasons, the implicit creation of subscriptions as
defined in [RFC3515] is still permitted.
[RFC3265] allowed the creation of subscriptions using means other
than the SUBSCRIBE method. The only standardized use of this
mechanism is the REFER method [RFC3515]. Implementation
experience with REFER has shown that the implicit creation of a
subscription has a number of undesirable effects, such as the
inability to signal the success of a REFER request while signaling
a problem with the subscription, and difficulty performing one
action without the other. Additionally, the proper exchange of
dialog identifiers is difficult without dialog reuse (which has
its own set of problems; see Section 4.5).
184.108.40.206. 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 [RFC3261]) 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 field 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 220.127.116.11.
The notifier MAY also check that the duration in the "Expires" header
field 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
Brief) error that contains a "Min-Expires" header field. The
"Min-Expires" header field is described in [RFC3261].
Once the notifier determines that it has enough information to create
the subscription (i.e., it understands the event package, the
subscription pertains to a known resource, and there are no other
barriers to creating the subscription), it creates the subscription
and a dialog usage, and returns a 200 (OK) response.
When a subscription is created in the notifier, it stores the event
package name 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 request 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
The other response codes defined in [RFC3261] may be used in response
to SUBSCRIBE requests, as appropriate.
18.104.22.168. Confirmation of Subscription Creation/Refreshing
Upon successfully accepting or refreshing a subscription, notifiers
MUST send a NOTIFY request immediately to communicate the current
resource state to the subscriber. This NOTIFY request 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 request is
processed, this NOTIFY request MAY contain an empty or neutral body.
See Section 4.2.2 for further details on NOTIFY request generation.
Note that a NOTIFY request is always sent immediately after any
200-class response to a SUBSCRIBE request, regardless of whether the
subscription has already been authorized.
22.214.171.124. 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
SIP authentication mechanisms are discussed in [RFC3261]. Note that,
even if the notifier node typically acts as a proxy, authentication
for SUBSCRIBE requests will always be performed via a 401
(Unauthorized) response, not a 407 (Proxy Authentication Required).
Notifiers always act as user agents when accepting subscriptions and
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 user agents and, as such,
perform user agent 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 200 (OK) response is returned immediately.
Note that a NOTIFY request 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 request containing a "Subscription-State" header
field with a value of "terminated" and a reason parameter of
126.96.36.199. 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
field in the response. If the duration specified in a SUBSCRIBE
request is unacceptably short, the notifier SHOULD respond with a 423
(Interval Too Brief) response.
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 request with a
"Subscription-State" value of "terminated" to inform it that the
subscription is being removed. If such a request is sent, the
"Subscription-State" header field SHOULD contain a "reason=timeout"
Clients can cause a subscription to be terminated immediately by
sending a SUBSCRIBE request with an "Expires" header field set to
'0'. Notifiers largely treat this the same way as any other
subscription expiration: they send a NOTIFY request containing a
"Subscription-State" of "terminated", with a reason code of
"timeout." For consistency with state polling (see Section 4.4.3)
and subscription refreshes, the notifier may choose to include
resource state in this final NOTIFY request. However, in some cases,
including such state makes no sense. Under such circumstances, the
notifier may choose to omit state information from the terminal
The sending of a NOTIFY request when a subscription expires allows
the corresponding dialog usage to be terminated, if appropriate.
4.2.2. Sending State Information to Subscribers
Notifiers use the NOTIFY method to send information about the state
of a resource to subscribers. The notifier's view of a subscription
is shown in the following state diagram. Events that result in a
transition back to the same state are not represented in this
| init |
V NOTIFY failure,
+-------------+ subscription expires,
+------------| resp_wait |-- or terminated ----+
| +-------------+ per local policy |
| | |
| | |
| | V
Policy grants Policy needed +-------------+
permission | | terminated |
| | +-------------+
| | A A
| V NOTIFY failure, | |
| +-------------+ subscription expires,| |
| | pending |-- or terminated -------+ |
| +-------------+ per local policy |
| | |
| Policy changed to |
| grant permission |
| | |
| V NOTIFY failure, |
| +-------------+ subscription expires, |
+----------->| active |-- or terminated ---------+
+-------------+ per local policy
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,
unless the state transition is caused by a NOTIFY transaction
failure. The sending of this NOTIFY message is also subject to
authorization, local policy, and throttling considerations.
If the NOTIFY request fails due to expiration of SIP Timer F
(transaction timeout), 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 [RFC3261]
(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 requests to refresh
potentially stale state information; such requests will reinstall
subscriptions in all relevant nodes.
If the NOTIFY transaction fails due to the receipt of a 404, 405,
410, 416, 480-485, 489, 501, or 604 response to the NOTIFY request,
the notifier MUST remove the corresponding subscription. See
[RFC5057] for further details and notes about the effect of error
codes on dialogs and usages within dialog (such as subscriptions).
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 sending of SUBSCRIBE requests to
refresh the expiration timer will reinstall subscription state
once the network problem has been resolved.
NOTIFY requests MUST contain a "Subscription-State" header field 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
If the value of the "Subscription-State" header field is "active" or
"pending", the notifier MUST also include in the "Subscription-State"
header field an "expires" parameter that 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
Including expiration information for active and pending
subscriptions is necessary in case the SUBSCRIBE request forks,
since the response to a forked SUBSCRIBE request may not be
received by the subscriber. [RFC3265] allowed the notifier some
discretion in the inclusion of this parameter, so subscriber
implementations are warned to handle the lack of an "expires"
parameter gracefully. Note well that this "expires" value is a
parameter on the "Subscription-State" header field NOT the
"Expires" header field.
The period of time for a subscription can be shortened to zero by
the notifier. In other words, it is perfectly valid for a
SUBSCRIBE request with a non-zero expires to be answered with a
NOTIFY request that contains "Subscription-Status:
terminated;reason=expired". This merely means that the notifier
has shortened the subscription timeout to zero, and the
subscription has expired instantaneously. The body may contain
valid state, or it may contain a neutral state (see
If the value of the "Subscription-State" header field 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 4.1.3.
4.2.3. PSTN/Internet Interworking (PINT) Compatibility
The "Event" header field is considered mandatory for the purposes of
this document. However, to maintain compatibility with PINT (see
[RFC2848]), notifiers MAY interpret a SUBSCRIBE request with no
"Event" header field as requesting a subscription to PINT events. If
a notifier does not support PINT, it SHOULD return 489 (Bad Event) to
any SUBSCRIBE requests without an "Event" header field.
4.3. Proxy Behavior
Proxies need no additional behavior beyond that described in
[RFC3261] to support SUBSCRIBE and NOTIFY transactions. If a proxy
wishes to see all of the SUBSCRIBE and NOTIFY requests for a given
dialog, it MUST add a "Record-Route" header field to the initial
SUBSCRIBE request and all NOTIFY requests. It MAY choose to include
"Record-Route" in subsequent SUBSCRIBE requests; however, these
requests cannot cause the dialog's route set to be modified.
Proxies that did not add a "Record-Route" header field to the initial
SUBSCRIBE request MUST NOT add a "Record-Route" header field to any
of the associated NOTIFY requests.
Note that subscribers and notifiers may elect to use Secure/
Multipurpose Internet Mail Extensions (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 [RFC3261].
4.4. Common Behavior
4.4.1. Dialog Creation and Termination
Dialogs usages are created upon completion of a NOTIFY transaction
for a new subscription, unless the NOTIFY request contains a
"Subscription-State" of "terminated."
Because the dialog usage is established by the NOTIFY request, the
route set at the subscriber is taken from the NOTIFY request itself,
as opposed to the route set present in the 200-class response to the
NOTIFY requests are matched to such SUBSCRIBE requests if they
contain the same "Call-ID", a "To" header field "tag" parameter that
matches the "From" header field "tag" parameter of the SUBSCRIBE
request, and the same "Event" header field. Rules for comparisons of
the "Event" header fields are described in Section 8.2.1.
A subscription is destroyed after a notifier sends a NOTIFY request
with a "Subscription-State" of "terminated", or in certain error
situations described elsewhere in this document. The subscriber will
generally answer such final requests with a 200 (OK) response (unless
a condition warranting an alternate response has arisen). Except
when the mechanism described in Section 4.5.2 is used, the
destruction of a subscription results in the termination of its
A subscriber may send a SUBSCRIBE request with an "Expires" header
field 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 transaction with a "Subscription-State" of "terminated"
4.4.2. Notifier Migration
It is often useful to allow migration of subscriptions between
notifiers. Such migration may be effected by sending a NOTIFY
request with a "Subscription-State" header field 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 request, 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 reuse
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
4.4.3. 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 a SUBSCRIBE with an "Expires" of 0.
Of course, an immediate fetch while a subscription is active may be
effected by sending a SUBSCRIBE request 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 requests triggered by SUBSCRIBE requests with
"Expires" header fields 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
When polling is used, subscribers SHOULD attempt to cache
authentication credentials between polls so as to reduce the number
of messages sent.
Due to the requirement on notifiers to send a NOTIFY request
immediately upon receipt of a SUBSCRIBE request, the state
provided by polling is limited to the information that the
notifier has immediate local access to when it receives the
SUBSCRIBE request. If, for example, the notifier generally needs
to retrieve state from another network server, then that state
will be absent from the NOTIFY request that results from polling.
4.4.4. "Allow-Events" Header Field Usage
The "Allow-Events" header field, if present, MUST include a
comprehensive and inclusive list of tokens that indicates the event
packages for which the user agent can act as a notifier. In other
words, a user agent sending an "Allow-Events" header field is
advertising that it can process SUBSCRIBE requests and generate
NOTIFY requests for all of the event packages listed in that header
Any user agent that can act as a notifier for one or more event
packages SHOULD include an appropriate "Allow-Events" header field
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.
On the other hand, it doesn't necessarily make much sense to
indicate supported events inside a dialog established by a NOTIFY
request if the only event package supported is the one associated
with that subscription.
Note that "Allow-Events" header fields MUST NOT be inserted by
The "Allow-Events" header field does not include a list of the event
template-packages supported by an implementation. If a subscriber
wishes to determine which event template-packages are supported by a
notifier, it can probe for such support by attempting to subscribe to
the event template-packages it wishes to use.
For example: to check for support for the templatized package
"presence.winfo", a client may attempt to subscribe to that event
package for a known resource, using an "Expires" header value of
0. If the response is a 489 error code, then the client can
deduce that "presence.winfo" is unsupported.
4.5. Targeting Subscriptions at Devices
[RFC3265] defined a mechanism by which subscriptions could share
dialogs with invite usages and with other subscriptions. The purpose
of this behavior was to allow subscribers to ensure that a
subscription arrived at the same device as an established dialog.
Unfortunately, the reuse of dialogs has proven to be exceedingly
confusing. [RFC5057] attempted to clarify proper behavior in a
variety of circumstances; however, the ensuing rules remain confusing
and prone to implementation error. At the same time, the mechanism
described in [RFC5627] now provides a far more elegant and
unambiguous means to achieve the same goal.
Consequently, the dialog reuse technique described in RFC 3265 is now
This dialog-sharing technique has also historically been used as a
means for targeting an event package at a dialog. This usage can be
seen, for example, in certain applications of the REFER method
[RFC3515]. With the removal of dialog reuse, an alternate (and more
explicit) means of targeting dialogs needs to be used for this type
of correlation. The appropriate means of such targeting is left up
to the actual event packages. Candidates include the "Target-Dialog"
header field [RFC4538], the "Join" header field [RFC3911], and the
"Replaces" header field [RFC3891], depending on the semantics
desired. Alternately, if the semantics of those header fields do not
match the event package's purpose for correlation, event packages can
devise their own means of identifying dialogs. For an example of
this approach, see the Dialog Event Package [RFC4235].
4.5.1. Using GRUUs to Route to Devices
Notifiers MUST implement the Globally Routable User Agent URI (GRUU)
extension defined in [RFC5627], and MUST use a GRUU as their local
target. This allows subscribers to explicitly target desired
If a subscriber wishes to subscribe to a resource on the same device
as an established dialog, it should check whether the remote contact
in that dialog is a GRUU (i.e., whether it contains a "gr" URI
parameter). If so, the subscriber creates a new dialog, using the
GRUU as the Request URI for the new SUBSCRIBE request.
Because GRUUs are guaranteed to route to a specific device, this
ensures that the subscription will be routed to the same place as
the established dialog.
4.5.2. Sharing Dialogs
For compatibility with older clients, subscriber and notifier
implementations may choose to allow dialog sharing. The behavior of
multiple usages within a dialog are described in [RFC5057].
Subscribers MUST NOT attempt to reuse dialogs whose remote target is
Note that the techniques described in this section are included
for backwards-compatibility purposes only. Because subscribers
cannot reuse dialogs with a GRUU for their remote target, and
because notifiers must use GRUUs as their local target, any two
implementations that conform to this specification will
automatically use the mechanism described in Section 4.5.1.
Further note that the prohibition on reusing dialogs does not
exempt implicit subscriptions created by the REFER method. This
means that implementations complying with this specification are
required to use the "Target-Dialog" mechanism described in
[RFC4538] when the remote target is a GRUU.
If a subscriber wishes to subscribe to a resource on the same device
as an established dialog and the remote contact is not a GRUU, it MAY
revert to dialog-sharing behavior. Alternately, it MAY choose to
treat the remote party as incapable of servicing the subscription
(i.e., the same way it would behave if the remote party did not
support SIP events at all).
If a notifier receives a SUBSCRIBE request for a new subscription on
an existing dialog, it MAY choose to implement dialog sharing
behavior. Alternately, it may choose to fail the SUBSCRIBE request
with a 403 (Forbidden) response. The error text of such 403
responses SHOULD indicate that dialog sharing is not supported.
To implement dialog sharing, subscribers and notifiers perform the
following additional processing:
o When subscriptions exist in dialogs associated with INVITE-created
application state and/or other subscriptions, these sets of
application state do not interact beyond the behavior described
for a dialog (e.g., route set handling). In particular, multiple
subscriptions within a dialog expire independently and require
independent subscription refreshes.
o 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. This means that, when dialogs are
reused, 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
o Subscribers MAY include an "id" parameter in a SUBSCRIBE request's
"Event" header field to allow differentiation between multiple
subscriptions in the same dialog. This "id" parameter, if
present, contains an opaque token that identifies the specific
subscription within a dialog. An "id" parameter is only valid
within the scope of a single dialog.
o If an "id" parameter is present in the SUBSCRIBE request used to
establish a subscription, that "id" parameter MUST also be present
in all corresponding NOTIFY requests.
o When a subscriber refreshes the subscription timer, the SUBSCRIBE
request MUST contain the same "Event" header field "id" parameter
as was present in the SUBSCRIBE request that created the
subscription. (Otherwise, the notifier will interpret the
SUBSCRIBE request as a request for a new subscription in the same
o When a subscription is created in the notifier, it stores any
"Event" header field "id" parameter as part of the subscription
information (along with the event package name).
o If an initial SUBSCRIBE request is sent on a pre-existing dialog,
a matching NOTIFY request merely creates a new subscription
associated with that dialog.
4.6. CANCEL Requests for SUBSCRIBE and NOTIFY Transactions
Neither SUBSCRIBE nor NOTIFY requests can be canceled. If a User
Agent Server (UAS) receives a CANCEL request that matches a known
SUBSCRIBE or NOTIFY transaction, it MUST respond to the CANCEL
request, but otherwise ignore it. In particular, the CANCEL request
MUST NOT affect processing of the SUBSCRIBE or NOTIFY request in any
UACs SHOULD NOT send CANCEL requests for SUBSCRIBE or NOTIFY