Internet DRAFT - draft-sparks-sipcore-refer-explicit-subscription
draft-sparks-sipcore-refer-explicit-subscription
Network Working Group R. Sparks
Internet-Draft Oracle
Intended status: Standards Track October 21, 2014
Expires: April 24, 2015
Explicit Subscriptions for the REFER Method
draft-sparks-sipcore-refer-explicit-subscription-02
Abstract
The Session Initiation Protocol (SIP) REFER request, as defined by
RFC3515, triggers an implicit SIP-Specific Event Notification
framework subscription. Conflating the start of the subscription
with handling the REFER request makes negotiating SUBSCRIBE
extensions impossible, and complicates avoiding SIP dialog sharing.
This document defines extensions to REFER to remove the implicit
subscription and, if desired, replace it with an explicit one.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Explicit Subscriptions . . . . . . . . . . . . . . . . . 3
3.2. No Subscriptions . . . . . . . . . . . . . . . . . . . . 4
4. The Explicit Subscription Extension . . . . . . . . . . . . . 5
4.1. Sending a REFER . . . . . . . . . . . . . . . . . . . . . 5
4.2. Processing a REFER Response . . . . . . . . . . . . . . . 5
4.3. Processing a Received REFER . . . . . . . . . . . . . . . 5
4.4. Subscribing to the 'refer' Event . . . . . . . . . . . . 6
4.5. Processing a Received SUBSCRIBE . . . . . . . . . . . . . 7
4.6. Sending a NOTIFY . . . . . . . . . . . . . . . . . . . . 7
4.7. Managing 'refer' Event State . . . . . . . . . . . . . . 7
4.8. The Refer-Events-At Header Field . . . . . . . . . . . . 8
5. The No Subscription Extension . . . . . . . . . . . . . . . . 8
5.1. Sending a REFER . . . . . . . . . . . . . . . . . . . . . 8
5.2. Processing a REFER Response . . . . . . . . . . . . . . . 8
5.3. Processing a Received REFER . . . . . . . . . . . . . . . 9
6. The 'explicitsub' and 'nosub' Option Tags . . . . . . . . . . 9
7. Updates to RFC 3515 . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9.1. Register the 'explicitsub' Option Tag . . . . . . . . . . 11
9.2. Register the 'nosub' Option Tag . . . . . . . . . . . . . 11
9.3. Register the 'Refer-Events-At' Header Field . . . . . . . 11
10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 12
10.2. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction
REFER as defined by [RFC3515] triggers an implicit SIP-Specific Event
Framework subscription. Sending a REFER within a dialog established
by an INVITE results in dialog reuse and the associated problems
described in [RFC5057]. The SIP-Specific Event Notification
framework definition [RFC6665] disallows such dialog reuse. Call
transfer, as defined in [RFC5589], thus requires sending a REFER
request on a new dialog, associating it with an existing dialog using
the 'Target-Dialog' mechanism defined in [RFC4538].
Because there is no explicit SUBSCRIBE request, the tools for
negotiating subscription details are unavailable for REFER
subscriptions. This includes negotiating subscription duration and
providing information through Event header field parameters. The use
of the SIP 'Supported' and 'Require' extension mechanisms [RFC3261]
is complicated by the implicit subscription. It is unclear whether
the extension applies to handling the REFER request itself, or to the
messages in the subscription created by the REFER, or both. Avoiding
this confusion requires careful specification in each extension.
Existing extensions do not provide this clarity.
This document defines two mechanisms that remove the implicit
subscription, one of which replaces it with an explicit one. The
benefits of doing so include:
o Allowing REFER to be used within INVITE-created dialogs without
creating dialog reuse.
o Allowing standard subscription parameter negotiation.
o Allowing standard negotiation of SIP extensions.
There are limitations on when it is appropriate to use the extension
that allows an explicit subscription, related directly to definition
of non-INVITE transaction handling SIP. These limitations are
discussed in Section 4.1.
3. Overview
This section provides a non-normative overview of the behaviors
defined in subsequent sections.
3.1. Explicit Subscriptions
A SIP User-Agent (UA) that wishes to issue a REFER request that will
not create an implicit subscription, but will allow an explicit one,
will include a new option tag, 'explicitsub', in the Require header
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field of the REFER request. This REFER could be sent either within
an existing dialog, or as an out-of-dialog request.
If the recipient of the REFER accepts the request, it will begin
managing the 'refer' event state described in RFC 3515, and will
provide a URI that will reach an event server that will service
subscriptions to that state. (In many cases, the recipient of the
REFER will perform the role of event server itself.) That URI is
returned in a new header field in the REFER response named 'Refer-
Events-At'.
The UA that issued the REFER can now subscribe to the 'refer' event
at the provided URI, using a SUBSCRIBE request with a new dialog
identifier. The full range of negotiation mechanisms is available
for its use in that request. As detailed in RFC 6665 and RFC 3515,
the event server accepting the subscription will send an immediate
NOTIFY with the current refer event state, additional NOTIFY messages
as the refer state changes, and a terminal NOTIFY message when the
referred action is complete. It is, of course, possible that the
initial NOTIFY is also the terminal NOTIFY.
It is possible that the referred action is completed before the
SUBSCRIBE arrives at the event server. The server needs to retain
the final refer event state for some period of time to include in the
terminal NOTIFY that will be sent for such subscriptions. It is also
possible that a SUBSCRIBE will never arrive.
This extension makes it possible to separate the event server that
will handle subscriptions from the UA that accepted the REFER. Such
a UA could use mechanisms such as PUBLISH [RFC3903] to convey the
refer event state to the event server. This extension also makes it
possible to allow more than one subscription to the refer event
state.
3.2. No Subscriptions
A UA that wishes to issue a REFER request that will not create an
implicit subscription, and tell the recipient that it is not
interested in creating an explicit subscription, will include a new
option tag, 'nosub', in the Require header field of the REFER
request. This REFER could be sent either within an existing dialog
or as an out-of-dialog request.
If the recipient of the REFER accepts the request, it knows not to
create an implicit subscription, and that no explicit subscription
will be forthcoming. The recipient will continue to process the
request indicated in the Refer-To header field as specified in RFC
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3515, but it can avoid the cost of preparing to handle any
subscriptions to the state of handling that request.
4. The Explicit Subscription Extension
4.1. Sending a REFER
To suppress the creation of any implicit subscription, and allow for
an explicit one, a UA forming a REFER request will include the option
tag 'explicitsub' in the "Require" header field of the request. The
REFER request is otherwise formed following the requirements of
[RFC3515]. Since this REFER has no chance of creating an implicit
subscription, the UA MAY send the REFER request within an existing
dialog or out-of-dialog.
Note that if the REFER forks (see [RFC3261]), only one final response
will be returned to the issuing UA. If it is important that the UA
be able to subscribe to any refer state generated by accepting this
request, the request needs to be formed to limit the number of places
that it will be accepted to one. This can be achieved by sending the
REFER request within an existing dialog, or by using the Target-
Dialog mechanism defined in [RFC4538]. If it is possible for the
request to be accepted in more than one location, and things would go
wrong if the UA did not learn about each location that the request
was accepted, using this extension is not appropriate.
4.2. Processing a REFER Response
The UA will process responses to the REFER request as specified in
[RFC3515] (and, consequently, [RFC3261]). In particular, if the
REFER was sent to an element that does not support or is unwilling to
use this extension, the response will contain a 420 Bad Extension
response code (see section 8.1.3.5 of [RFC3261]). As that document
states, the UA can retry the request without using this extension.
If the UA receives a 2xx-class response, it will contain a Refer-
Events-At header field (Section 4.8) with a single URI as its value.
If the UA is interested in the state of the referenced action, it
will subscribe to the 'refer' event at that URI.
4.3. Processing a Received REFER
An element receiving a REFER request requiring the 'explicitsub'
extension will use the same admissions policies that would be used
without the extension, with the addition that it is acceptable to
admit an in-dialog REFER request requiring this extension since it
can not create another usage inside that dialog. In particular, see
section 5.2 of [RFC3515].
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Accepting a REFER request that requires 'explicitsub' does not create
a dialog, or a new usage within an existing dialog. The element MUST
NOT create an implicit subscription when accepting the REFER request.
An element that accepts a REFER request with 'explicitsub' in its
Require header field MUST return a 200 response containing a sip: or
sips: URI that can be used to subscribe to the refer event state
associated with this REFER request. This URI MUST uniquely identify
this refer event state. The URI needs to reach the event server when
used in a SUBSCRIBE request from the element that sent the REFER.
One good way to ensure the URI provided has that property is to use a
GRUU [RFC5627] for the event server. As discussed in Section 8,
possession of this URI is often the only requirement for authorizing
a subscription to it. Implementations may wish to provide a URI
constructed in a way that is hard to guess. Again, using a GRUU is
one good way to achieve this property.
The accepting element will otherwise proceed with the processing
defined in [RFC3515].
The event server identified by the Refer-Events-At URI could receive
SUBSCRIBE requests at any point after the response containing the
Refer-Events-At header is sent. Implementations should take care to
ensure the event server is ready to receive those SUBSCRIBE requests
before sending the REFER response, but as with all non-INVITE
responses, the response should be sent as soon as possible (see
[RFC4321]). It is also possible that the referred action may
complete before any SUBSCRIBE request arrives. The event server will
need to maintain the final refer event state for a period of time
after the action completes in order to serve such subscriptions (see
Section 4.6).
4.4. Subscribing to the 'refer' Event
A UA that possesses a URI obtained from a Refer-Events-At header
field, MAY subscribe to the refer event state at that URI. It does
so following the requirements of [RFC6665], placing the token 'refer'
in the Event: header field and the URI in the Request-URI of the
SUBSCRIBE request. The SUBSCRIBE request MUST NOT reuse any existing
dialog identifiers.
Subsequent handling of the subscription MUST follow the requirements
of [RFC6665] and [RFC3515]. In particular, as discussed in section
2.4.6, the NOTIFY messages in the subscription might include an id
parameter in their Event header fields. Subsequent SUBSCRIBE
requests used to refresh or terminate this subscription MUST contain
this id parameter. Note that the rationale for the id parameter
provided in that section is not relevant when this extension is used.
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The URI returned in the Refer-Events-At header field uniquely
identifies appropriate state, making the id parameter redundant.
However, this behavioral requirement is preserved to reduce the
number of changes to existing implementations in order to support
this extension, and to make it more likely that existing diagnostic
tools will work with little or no modification.
4.5. Processing a Received SUBSCRIBE
An event server receiving a SUBSCRIBE request will process it
according to the requirements of [RFC6665]. The event server MAY
choose to authorize the SUBSCRIBE request based on the Request-URI
corresponding to existing refer event state. It MAY also require
further authorization as discussed in Section 8.
When accepting a subscription, the event server will establish the
initial subscription duration using the guidance in section 3.4 of
[RFC3515].
4.6. Sending a NOTIFY
NOTIFY messages within a subscription are formed and sent following
the requirements in [RFC3515]. See, in particular, section 2.4.5 of
that document.
4.7. Managing 'refer' Event State
As described in [RFC3515], an element creates the state for event
'refer' when it accepts a REFER request. It updates that state as
the referred request proceeds, ultimately reaching a state where the
request has completed, and the final state is known.
In RFC 3515 implementations, it was a reasonable design choice to
destroy the refer event state immediately after sending the NOTIFY
that terminated the implicit subscription. This is not the case when
using this extension. It is possible for the referenced request to
complete very quickly, perhaps sooner than the time it takes the
response to the REFER to traverse the network to the UA that sent the
request, and the time it takes that agent to send the SUBSCRIBE
request for the event state to the URI the response provides. Thus
the event server MUST retain the final refer event state for a
reasonable period of time, which SHOULD be at least 2*64*T1 (that is,
64 seconds), representing an upper-bound estimate of the time it
would take to complete two non-INVITE transactions: the REFER, and an
immediate SUBSCRIBE.
If an otherwise acceptable SUBSCRIBE arrives during this retention
period, the subscription would be accepted, and immediately
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terminated with a NOTIFY containing the final event state with a
Subscription-State of terminated with a reason value of "noresource".
4.8. The Refer-Events-At Header Field
The 'Refer-Events-At' header field is an extension-header as defined
by [RFC3261]. Its ABNF is as follows:
Refer-Events-At: "Refer-Events-At" HCOLON
LAQUOT ( SIP-URI / SIPS-URI ) RAQUOT
* ( SEMI generic-param )
See [RFC3261] for the definition of the elements used in that
production.
Note that this rule does not allow a full addr-spec as defined in RFC
3261, and it mandates the use of the angle brackets. That is:
Refer-Events-At: <sips:vPT3izGmo8NTxaPADRZvEAY22BKx@example.com;gr>
is well formed, but
Refer-Events-At: sip:wsXa9mkHtPcGu8@example.com
is invalid.
The 'Refer-Events-At' header field is only meaningful in a 200
response to a REFER request. If it appears in the header of any
other SIP message, its meaning is undefined and it MUST be ignored.
5. The No Subscription Extension
5.1. Sending a REFER
To suppress the creation of any implicit subscription, and signal
that no explicit subscription will be forthcoming, a UA forming a
REFER request will include the option tag 'nosub' in the "Require"
header field of the request. The REFER request is otherwise formed
following the requirements of [RFC3515]. Since this REFER has no
chance of creating an implicit subscription, the UA MAY send the
REFER request within an existing dialog or out-of-dialog.
5.2. Processing a REFER Response
The UA will process responses to the REFER request as specified in
[RFC3515] (and, consequently, [RFC3261]). In particular, if the
REFER was sent to an element that does not support or is unwilling to
use this extension, the response will contain a 420 Bad Extension
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response code (see section 8.1.3.5 of [RFC3261]). As that document
states, the UA can retry the request without using this extension.
5.3. Processing a Received REFER
An element receiving a REFER request requiring the 'nosub' extension
will use the same admissions policies that would be used without the
extension, with the addition that it is acceptable to admit an in-
dialog REFER request requiring this extension since it can not create
another usage inside that dialog. In particular, see section 5.2 of
[RFC3515].
Accepting a REFER request that requires 'nosub' does not create a
dialog, or a new usage within an existing dialog. The element MUST
NOT create an implicit subscription when accepting the REFER request.
Futhermore, the element accepting the REFER request is not required
to maintain any state for serving refer event subscriptions.
The accepting element will otherwise proceed with the processing
defined in [RFC3515].
6. The 'explicitsub' and 'nosub' Option Tags
This document defines the 'explicitsub' option tag, used to signal
the use of the extension defined in Section 4, and the 'nosub' option
tag, used to signal the use of the extension defined in Section 5.
The use of either option tag in a Require header field is only
defined when it appears in a REFER request. A UA MUST NOT include
the 'explicitsub' or 'nosub' option tag in the Require header field
of any request other than REFER. A UA MUST NOT include the
'explicitsub' or 'nosub' option tag in the Require header field of
any SIP response other than a 421 response to a REFER request.
The 'explicitsub' and 'nosub' option tags MAY appear in the Supported
header field of SIP messages, and in sip.extensions feature tag
defined in [RFC3840]. This signals only that the UA including the
value is aware of the extensions. In particular, a UA can only
invoke the use of one of the extensions in a request. A User-Agent
Server (UAS) that is processing a REFER request that lists
'explicitsub' or 'nosub' in its Supported header field and wishes to
use one of those extensions will return a 421 response indicating
which extension is required.
OPEN ISSUE: This description of the use of 421 is not yet perfectly
aligned with RFC3261's definition.
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7. Updates to RFC 3515
The requirement in section 2.4.4 of [RFC3515] to reject out-of-dialog
SUBSCRIBE requests to event 'refer' is removed. An element MAY
accept a SUBSCRIBE request to event 'refer', following the
requirements and guidance in this document. REFER is no longer the
only mechanism that can create a subscription to event 'refer'.
[RFC6665] section 8.3.1 deprecates the 202 Accepted response code.
New implementations of REFER, whether using the 'explicitsub'
extension or not, will never emit a 202 response code. Where RFC
3515 specifies using 202, new implementations MUST use 200 instead.
8. Security Considerations
The considerations of [RFC3515] all still apply to a REFER request
using this extension. The considerations there for the implicit
subscription apply to any explicit subscription for the 'refer'
event.
This update to RFC 3515 introduces a new authorization consideration.
An element receiving an initial SUBSCRIBE request to the 'refer'
event needs to decide whether the subscriber should be allowed to see
the refer event state. In RFC 3515, this decision was conflated with
accepting the REFER request, and the only possible subscriber was the
element that sent the REFER. With this update, there may multiple
subscribers to any given refer event state.
This document allows an element to accept an initial SUBSCRIBE
request based on having a Request-URI that identifies existing refer
event state. (Such a URI will have previously been sent in the
Refer-Events-At header field in a successful REFER response). The
element retrieving that URI from the response, and any elements that
element shares the URI with are authorized to SUBSCRIBE to the event
state. Consequently, the URI should be constructed so that it is not
easy to guess, and should be protected against eavesdroppers when
transmitted. For instance, SIP messages containing this URI SHOULD
be sent using TLS or DTLS. An event server receiving a REFER request
over an unprotected transport can redirect the requester to use a
protected transport before accepting the request. A good way to
ensure that subscriptions use a protected transport is to only
construct sips: URIs. The event server can also require any of the
additional authorization mechanisms allowed for any SIP request. For
example, the event server could require a valid assertion of the
subscriber's identity using [RFC4474].
The URI provided in a 'Refer-Events-At' header field will be used as
the Request-URI of SUBSCRIBE requests. A malicious agent could take
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advantage of being able to choose this URI in ways similar to the
ways an agent sending a REFER request can take advantage of the
Refer-To URI, as described in the security considerations section of
RFC 3515. In particular, the malicious agent could cause a SIP
SUBSCRIBE to be sent as raw traffic towards a victim. If the victim
is not SIP aware, and the SUBSCRIBE is sent over UDP, there is (at
most) a factor of 11 amplification due to retransmissions of the
request. The potential for abuse in this situation is lower than
that of the Refer-To URI, since the URI can only have a sip: or sips:
scheme, and is only provided in a REFER response. A malicious agent
would have to first receive a REFER request to take advantage of
providing a Refer-Events-At URI.
9. IANA Considerations
9.1. Register the 'explicitsub' Option Tag
The option tag 'explicitsub' is registered in the 'Option Tag'
subregistry of the 'Session Initiation Protocol (SIP) Parameters'
registry by adding a row with these values:
Name: explicitsub
Description: This option tag identifies an extension to REFER to
suppress the implicit subscription, and provide a URI for an explicit
subscription.
Reference: (this document)
9.2. Register the 'nosub' Option Tag
The option tag 'nosub' is registered in the 'Option Tag' subregistry
of the 'Session Initiation Protocol (SIP) Parameters' registry by
adding a row with these values:
Name: nosub
Description: This option tag identifies an extension to REFER to
suppress the implicit subscription, and indicate that no explicit
subscription is forthcoming.
Reference: (this document)
9.3. Register the 'Refer-Events-At' Header Field
The header field described in Section 4.8 is registered in the
'Header Fields' subregistry of the 'Session Initiation Protocol (SIP)
Parameters' registry by adding a row with these values:
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Header Name: Refer-Events-At
compact: (none: the entry in this column should be blank)
Reference: (this document)
10. Changelog
RFC Editor - please remove this section when formatting this document
as an RFC.
10.1. 01 to 02
1. Added the 'nosub' option tag
2. Added text calling out the limitations on explicitsub when the
REFER might be accepted in more than one place.
10.2. 00 to 01
1. Replaced strawman proposal with a formal definition of the
mechanism. Added an overview, and detailed security
considerations.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[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.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665,
July 2012.
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11.2. Informative References
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC4321] Sparks, R., "Problems Identified Associated with the
Session Initiation Protocol's (SIP) Non-INVITE
Transaction", RFC 4321, January 2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4538] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
RFC 4538, June 2006.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[RFC5589] Sparks, R., Johnston, A., and D. Petrie, "Session
Initiation Protocol (SIP) Call Control - Transfer", BCP
149, RFC 5589, June 2009.
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent URIs (GRUUs) in the Session Initiation Protocol
(SIP)", RFC 5627, October 2009.
Author's Address
Robert Sparks
Oracle
7460 Warren Parkway
Suite 300
Frisco, Texas 75034
US
Email: rjsparks@nostrum.com
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