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 [RFC3261] and contain references
to elements defined therein.
8.1. New Methods
This document describes two new SIP methods: SUBSCRIBE and NOTIFY.
8.1.1. 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.
8.1.2. NOTIFY Method
"NOTIFY" is added to the definition of the element "Method" in the
SIP message grammar.
The NOTIFY method is used to notify a SIP node that an event that has
been requested by an earlier SUBSCRIBE method has occurred. It may
also provide further details about the event.
8.2. New Header Fields
8.2.1. "Event" Header Field
Event is added to the definition of the element "message-header
field" in the SIP message grammar.
For the purposes of matching NOTIFY requests with SUBSCRIBE requests,
the event-type portion of the "Event" header field is compared byte
by byte, and the "id" parameter token (if present) is compared byte
by byte. An "Event" header field containing an "id" parameter never
matches an "Event" header field without an "id" parameter. No other
parameters are considered when performing a comparison. SUBSCRIBE
responses are matched per the transaction handling rules in
Note that the foregoing 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 field.
Multiple events per message are disallowed.
The "Event" header field is defined only for use in SUBSCRIBE and
NOTIFY requests and other requests whose definition explicitly calls
for its use. It MUST NOT appear in any other SIP requests and MUST
NOT appear in responses.
8.2.2. "Allow-Events" Header Field
"Allow-Events" is added to the definition of the element "general-
header field" in the SIP message grammar. Its usage is described in
User agents MAY include the "Allow-Events" header field in any
request or response, as long as its contents comply with the behavior
described in Section 4.4.4.
8.2.3. "Subscription-State" Header Field
"Subscription-State" is added to the definition of the element
"request-header" field in the SIP message grammar. Its usage is
described in Section 4.1.3. "Subscription-State" header fields are
defined for use in NOTIFY requests only. They MUST NOT appear in
other SIP requests or responses.
8.3. New Response Codes
8.3.1. 202 (Accepted) Response Code
For historical purposes, the 202 (Accepted) response code is added to
the "Success" header field definition.
This document does not specify the use of the 202 response code in
conjunction with the SUBSCRIBE or NOTIFY methods. Previous versions
of the SIP Events Framework assigned specific meaning to the 202
Due to response handling in forking cases, any 202 response to a
SUBSCRIBE request may be absorbed by a proxy, and thus it can never
be guaranteed to be received by the UAC. Furthermore, there is no
actual processing difference for a 202 as compared to a 200; a NOTIFY
request is sent after the subscription is processed, and it conveys
the correct state. SIP interoperability tests found that
implementations were handling 202 differently from 200, leading to
incompatibilities. Therefore, the 202 response is being deprecated
to make it clear there is no such difference and 202 should not be
handled differently than 200.
Implementations conformant with the current specification MUST treat
an incoming 202 response as identical to a 200 response and MUST NOT
generate 202 response codes to SUBSCRIBE or NOTIFY requests.
This document also updates [RFC4660], which reiterates the 202-based
behavior in several places. Implementations compliant with the
present document MUST NOT send a 202 response to a SUBSCRIBE request
and will send an alternate success response (such as 200) in its
8.3.2. 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.
8.4. Augmented BNF Definitions
The Augmented BNF [RFC5234] definitions for the various new and
modified syntax elements follows. The notation is as used in
[RFC3261], and any elements not defined in this section are as
defined in SIP and the documents to which it refers.
SUBSCRIBEm = %x188.8.131.52.184.108.40.206.45 ; SUBSCRIBE in caps
NOTIFYm = %x4E.4F.220.127.116.11 ; 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 / "-" / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
; The use of the "id" parameter is deprecated; it is included
; for backwards-compatibility purposes only.
[RFC4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent URIs (GRUUs) in the Session Initiation
Protocol (SIP)", RFC 5627, October 2009.
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change
Process for the Session Initiation Protocol (SIP) and
the Real-time Applications and Infrastructure Area",
BCP 67, RFC 5727, March 2010.
9.2. Informative References
[RFC2779] Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
"Instant Messaging / Presence Protocol Requirements",
RFC 2779, February 2000.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session
Initiation Protocol (SIP) "Replaces" Header", RFC 3891,
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3911] Mahy, R. and D. Petrie, "The Session Initiation Protocol
(SIP) "Join" Header", RFC 3911, October 2004.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session
Initiation Protocol (SIP)", RFC 4235, November 2005.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288,
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for
Authors of Extensions to the Session Initiation Protocol
(SIP)", RFC 4485, May 2006.
[RFC4538] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol
(SIP)", RFC 4538, June 2006.
[RFC4660] Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
Requena, "Functional Description of Event Notification
Filtering", RFC 4660, September 2006.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[RFC5839] Niemi, A. and D. Willis, "An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
Notification", RFC 5839, May 2010.
[CERT1998a] CERT, "CERT Advisory CA-1998-01: Smurf IP Denial-of-
Service Attacks", 1998,
Appendix A. 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 first draft version, and the authors of the "SIP Extensions
for Presence" document for their input to SUBSCRIBE and NOTIFY
I also owe a debt of gratitude to all the implementors who have
provided feedback on areas of confusion or difficulty in the original
specification. In particular, Robert Sparks' Herculean efforts
organizing, running, and collecting data from the SIPit events have
proven invaluable in shaking out specification bugs. Robert Sparks
is also responsible for untangling the dialog usage mess, in the form
of RFC 5057 [RFC5057].
Appendix B. Changes from RFC 3265
This document represents several changes from the mechanism
originally described in RFC 3265. This section summarizes those
changes. Bug numbers refer to the identifiers for the bug reports
kept on file at http://bugs.sipit.net/.
B.1. Bug 666: Clarify use of "expires=xxx" with "terminated"
Strengthened language in Section 4.1.3 to clarify that "expires"
should not be sent with "terminated", and must be ignored if
B.2. Bug 667: Reason code for unsub/poll not clearly spelled out
Clarified description of "timeout" in Section 4.1.3. (n.b., the text
in Section 4.4.3 is actually pretty clear about this).
B.3. Bug 669: Clarify: SUBSCRIBE for a duration might be answered with
Added clarifying text to Section 4.2.2 explaining that shortening a
subscription to zero seconds is valid. Also added sentence to
Section 3.1.1 explicitly allowing shortening to zero.
B.20. Eliminate Implicit Subscriptions
Added text to Section 4.2.1 explaining some of the problems
associated with implicit subscriptions, and added normative language
prohibiting them. Removed language from Section 3.2 describing "non-
SUBSCRIBE" mechanisms for creating subscriptions. Simplified
language in Section 4.2.2, now that the soft-state/non-soft-state
distinction is unnecessary.
B.21. Deprecate Dialog Reuse
Moved handling of dialog reuse and "id" handling to Section 4.5.2.
It is documented only for backwards-compatibility purposes.
B.22. Rationalize Dialog Creation
Section 4.4.1 has been updated to specify that dialogs should be
created when the NOTIFY arrives. Previously, the dialog was
established by the SUBSCRIBE 200 or by the NOTIFY transaction. This
was unnecessarily complicated; the newer rules are easier to
implement (and result in effectively the same behavior on the wire).
B.23. Refactor Behavior Sections
Reorganized Section 4 to consolidate behavior along role lines
(subscriber/notifier/proxy) instead of method lines.
B.24. Clarify Sections That Need to Be Present in Event Packages
Added sentence to Section 5 clarifying that event packages are
expected to include explicit sections covering the issues discussed
in this section.
B.25. Make CANCEL Handling More Explicit
Text in Section 4.6 now clearly calls out behavior upon receipt of a
CANCEL. We also echo the "...SHOULD NOT send..." requirement from
B.26. Remove "State Agent" Terminology
As originally planned, we anticipated a fairly large number of event
packages that would move back and forth between end-user devices and
servers in the network. In practice, this has ended up not being the
case. Certain events, like dialog state, are inherently hosted at
end-user devices; others, like presence, are almost always hosted in
the network (due to issues like composition, and the ability to
deliver information when user devices are offline). Further, the
concept of State Agents is the most misunderstood by event package
authors. In my expert review of event packages, I have yet to find
one that got the concept of State Agents completely correct -- and
most of them start out with the concept being 100% backwards from the
way RFC 3265 described it.
Rather than remove the ability to perform the actions previously
attributed to the widely misunderstood term "State Agent", we have
simply eliminated this term. Instead, we talk about the behaviors
required to create state agents (state aggregation, subscription
notification) without defining a formal term to describe the servers
that exhibit these behaviors. In effect, this is an editorial change
to make life easier for event package authors; the actual protocol
does not change as a result.
The definition of "State Agent" has been removed from Section 2.
Section 4.4.2 has been retooled to discuss migration of subscription
in general, without calling out the specific example of state agents.
Section 5.4.11 has been focused on state aggregation in particular,
instead of state aggregation as an aspect of state agents.
B.27. Miscellaneous Changes
The following changes are relatively minor revisions to the document
that resulted primarily from review of this document in the working
group and IESG, rather than implementation reports.
o Clarified scope of "Event" header field parameters. In RFC 3265,
the scope is ambiguous, which causes problems with the registry in
RFC 3968. The new text ensures that "Event" header field
parameters are unique across all event packages.
o Removed obsoleted language around IANA registration policies for
event packages. Instead, we now cite RFC 5727, which updates RFC
3265, and is authoritative on event package registration policy.
o Several editorial updates after input from working group,
including proper designation of "dialog usage" rather than
"dialog" where needed.
o Clarified two normative statements about subscription termination
by changing from plain English prose to RFC2119 language.
o Removed "Table 2" expansions, per WG consensus on how SIP Table 2
is to be handled.
o Removed 202 response code.
o Clarified that "Allow-Events" does not list event template-
o Added clarification about proper response when the SUBSCRIBE
indicates an unknown media type in its "Accept" header field.
o Minor clarifications to "Route" and "Record-Route" behavior.
o Added non-normative warning about the limitations of state
o Added information about targeting subscriptions at specific
o Added RFC 3261 to list of documents updated by this one (rather
than the "2543" indicated by RFC 3265).
o Clarified text in Section 3.1.1 explaining the meaning of
o Changed text in definition of "probation" reason code to indicate
that subscribers don't need to re-subscribe if the associated
state is no longer of use to them.
o Specified that the termination of a subscription due to a NOTIFY
transaction failure does not require sending another NOTIFY
o Clarified how order of template application affects the meaning of
an "Event" header field value (e.g., "foo.bar.baz" is different
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