SIP J. Urpalainen
Internet-Draft Nokia
Intended status: Standards Track D. Willis, Ed.
Expires: November 28, 2009 Softarmor Systems LLC
May 27, 2009
An Extensible Markup Language (XML) Configuration Access Protocol (XCAP)
Diff Event Package
draft-ietf-sip-xcapevent-07
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Abstract
This document describes an "xcap-diff" SIP (Session Initiation
Protocol) event package for the SIP Event Notification Framework,
which clients can use to receive notifications of changes to
Extensible Markup Language (XML) Configuration Access Protocol (XCAP)
resources. The initial synchronization information exchange and
document updates are based on the XCAP Diff format.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. XCAP-Diff Event Package . . . . . . . . . . . . . . . . . . . 4
4.1. Overview of Operation With Basic Requirements . . . . . . 4
4.2. Event Package Name . . . . . . . . . . . . . . . . . . . . 5
4.3. 'diff-processing' Event Package Parameter . . . . . . . . 5
4.4. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 6
4.5. Subscription Duration . . . . . . . . . . . . . . . . . . 8
4.6. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 8
4.7. Notifier Generation of NOTIFY Requests . . . . . . . . . . 8
4.8. Subscriber Processing of NOTIFY Requests . . . . . . . . . 11
4.9. Handling of Forked Requests . . . . . . . . . . . . . . . 12
4.10. Rate of Notifications . . . . . . . . . . . . . . . . . . 13
4.11. State Agents . . . . . . . . . . . . . . . . . . . . . . . 13
5. An Initial Example NOTIFY document . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Informative Examples . . . . . . . . . . . . . . . . 17
A.1. Initial documents on an XCAP server . . . . . . . . . . . 17
A.2. An Initial Subscription . . . . . . . . . . . . . . . . . 18
A.3. A Document Addition Into a Collection . . . . . . . . . . 19
A.4. A Series of XCAP Component Modifications . . . . . . . . . 20
A.5. An XCAP Component Subscription . . . . . . . . . . . . . . 23
A.6. A Conditional Subscription . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
The SIP Events framework [RFC3265] describes subscription and
notification conventions for the Session Initiation Protocol (SIP)
[RFC3261]. The Extensible Markup Language (XML)
[W3C.REC-xml-20060816] Configuration Access Protocol (XCAP) [RFC4825]
allows a client to read, write and modify XML-formatted application
usage data stored on an XCAP server.
While XCAP allows authorized users or devices to modify the same XML
document, XCAP does not provide an effective mechanism (beyond
polling) to keep resources synchronized between a server and a
client. This memo defines an "xcap-diff" event package that,
together with the SIP event notification framework [RFC3265] and the
XCAP diff format [I-D.ietf-simple-xcap-diff], allows a user to
subscribe to changes in an XML document, and to receive notifications
whenever the XML document changes.
There are three basic features that this event package enables:
First, a client can subscribe to a list of XCAP documents' URLs in a
collection located on an XCAP server. This allows a subscriber to
compare server resources with its local resources using the URLs and
the strong entity tag (ETag) values of XCAP documents, which are
shown in the XCAP Diff format, and to synchronize them.
Second, this event package can signal a change in those resources in
one of three ways. The first mode only indicates the event type and
does not include document contents, so the subscriber uses HTTP
[RFC2616] to retrieve the updated document. The second mode includes
document content or changes in notification messages, using the XML-
Patch-Ops [RFC5261] format with minimal notification size. The third
mode also includes document content or changes in notification
messages, but is more verbose, and shows the full HTTP version-
history.
Third, the client can subscribe to changes in specific XML element or
attribute contents (XCAP components) and receive element or attribute
contents in the resulting XCAP Diff format notification messages. If
the requested component does not exist but is later created, the
notifier sends a notification with the component's content. The
notifier also sends notifications when the subscribed XCAP components
are removed, for example after a successful HTTP DELETE request.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119, BCP 14
[RFC2119] and indicate requirement levels for compliant
implementations.
3. Definitions
The following terms are used in this document:
XCAP Component: An XML element or an attribute, which can be
updated, removed, or retrieved with XCAP.
Aggregating: An XCAP client can update only a single XCAP Component
at a time using HTTP. However, a notifier may be able to
aggregate a series of these modifications into a single
notification using XML-Patch- Ops semantics encoded in the XCAP-
Diff format.
This document reuses terminology mostly defined in XCAP [RFC4825] and
some in WebDAV [RFC4918].
4. XCAP-Diff Event Package
4.1. Overview of Operation With Basic Requirements
To receive "xcap-diff" event package features, the subscriber
indicates its interest in certain resources by including a URI list
in the subscription body to the notifier. Each URL in this list MUST
be a HTTP URL that identifies a collection, an XCAP document, or an
XCAP component. Collection URLs MUST have a trailing forward slash
"/", following the conventions of WebDAV [RFC4918]. A collection
selection includes all documents in that collection and recursively
all documents in sub-collections. The URL of an XCAP component
consists of the document URL with the XCAP Node Selector added.
Although the XCAP Node Selector allows requesting all in-scope
namespaces of an element, the client MUST NOT subscribe to
namespaces.
The notifier MUST support XCAP component subscriptions. The notifier
sends the first notification in response to the subscription, and
this first notification MUST contain the URLs of the documents and
XCAP components that are part of the subscription. The first
notification SHOULD contain the contents of subscribed XCAP documents
and components. The subsequent notifications MAY contain patches to
these documents. The subscriber can specify how the notifier will
signal the changes of documents by using the 'diff-processing' event
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package parameter, covered in the section Section 4.3
4.2. Event Package Name
The name of this event package is "xcap-diff". As specified in
[RFC3265], this value appears in the Event header field present in
SUBSCRIBE and NOTIFY requests.
4.3. 'diff-processing' Event Package Parameter
With the aid of the optional "diff-processing" Event header field
parameter, the subscriber indicates a preference as to how the
notifier SHOULD indicate change notifications of documents. The
possible values are "no-patching", "xcap-patching", and "aggregate".
All three modes provide information that allows the subscriber to
synchronize its local cache, but only the "xcap-patching" mode
provides intermediate states of the version history. The notifier
SHOULD use the indicated mode if it understands it (as such optimizes
network traffic within the capabilities of the receiver), but MAY use
"xcap-patching" as a matter of local policy, as all subscribers are
required to support that mode.
The "no-patching" value means that the notifier indicates only the
document and the event type (creation, modification, and removal)
in the notification. The notification does not necessarily
indicate the full HTTP ETag change history. Subscribers and
notifiers MUST support the "no-patching" mode as a base-line for
interoperability. The other, more complex modes are optional.
The "xcap-patching" value means that the notifier includes all
updated XCAP component contents and entity tag (ETag) changes.
The client receives the full (HTTP) ETag change history of a
document. This is equivalent to sending individual notifications
for each change.
The "aggregate" value means that the notifier MAY aggregate
several individual XCAP component updates into a single XCAP Diff
<document> element. The policy for determining whether or not to
apply aggregation or to determine how many updates to aggregate is
locally determined. The notifier SHOULD support the "aggregate"
mode and implement XML-Patch-Ops ( [RFC5261]) diff-generation,
because this can greatly reduce the number of notification
operations required.
If the subscription does not contain the "diff-processing" header
field parameter, the notifier SHOULD default to the "no-patching"
mode, as this is the only mode for which implementation is required
in both subscribers and notifiers.
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Note: To see the difference between "xcap-patching" and
"aggregate" modes, consider a document that has versions "a", "b"
and "c" with corresponding ETag values "1", "2" and "3". The
"xcap-patching" mode will include first the change from version
"a" to "b" with the versions' corresponding "1" and "2" ETags and
then the change from version "b" to "c" with their "2" and "3"
ETags. The "aggregate" mode optimizes the change and indicates
only a single aggregated change from "a" to "c" with the old "1"
and new "3" ETags. If these changes are closely related, that is,
the same element has been updated many times, the bandwidth
savings are larger.
This "diff-processing" parameter is a subscriber hint to the
notifier. The notifier may respond using a simpler mode, but not a
more complex one. Notifier selection of a mode is covered in
Section 4.7. During re-subscriptions, the subscriber MAY change the
diff-processing parameter.
The formal grammar [RFC5234] of the "diff-processing" parameter:
diff-processing = "diff-processing" EQUAL (
"no-patching" /
"xcap-patching" /
"aggregate" /
token )
where EQUAL and token are defined in RFC 3261 [RFC3261].
4.4. SUBSCRIBE Bodies
The URI list is described by the XCAP resource list format [RFC4826],
and is included as a body of the initial SUBSCRIBE request. Only a
simple subset of that format is required, a flat list of XCAP R-URIs.
The "uri" attribute of the <entry> element contains these URI values.
The subscriber MUST NOT use hierarchical lists or <entry-ref>
references, etc., (though in the future, semantics may be expanded
thanks to the functionality in the resource list format). In
subsequent SUBSCRIBE requests, such as those used for refreshing the
expiration timer, the subscribed URI list MAY change, in which case
the notifier MUST use the new list.
The SUBSCRIBE request MAY contain an Accept header field. If no such
header field is present, it has a default value of "application/
xcap-diff+xml". If the header field is present, it MUST include
"application/xcap-diff+xml", and MAY include any other types.
The SUBSCRIBE request MAY contain the Suppress-If-Match header field
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[I-D.ietf-sipcore-subnot-etags], which directs the notifier to
suppress either the body of a subsequent notification, or the entire
notification if the ETag value matches.
If the SUBSCRIBE body contains elements or attributes that the
notifier doesn't understand, the notifier MUST ignore them.
Subscribers need to appropriately populate the Request-URI of the
SUBSCRIBE request, typically set to the URI of the notifier. This
document does not constrain that URI. It is assumed that the
subscriber is provisioned with or has learned the URI of the notifier
of this event package.
The XCAP server will usually be co-located with the SIP notifier, so
the subscriber MAY use relative XCAP Request-URIs. Because relative
Request-URIs are allowed, the notifier MUST know how to resolve these
against the correct XCAP Root URI value.
Figure 1 shows a SUBSCRIBE request and body covering several XCAP
resources: a "resource-list" document, a specific element in a "rls-
services" document, and a collection in "pidf-manipulation"
application usage. The "Content-Type" header of this SUBSCRIBE
request is "application/resource-lists+xml".
SUBSCRIBE sip:tests@xcap.example.com SIP/2.0
...
Accept: application/xcap-diff+xml
Event: xcap-diff; diff-processing=aggregate
Content-Type: application/resource-lists+xml
Content-Length: [XXX]
Expires: 4200
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="resource-lists/users/sip:joe@example.com/index"/>
<entry uri="rls-services/users/sip:joe@example.com/index/
~~/*/service%5b@uri='sip:marketing@example.com'%5d"/>
<entry uri="pidf-manipulation/"/>
</list>
</resource-lists>
Figure 1: Example subscription body
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4.5. Subscription Duration
The default expiration time for subscriptions within this package is
3600 seconds. As per RFC 3265 [RFC3265], the subscriber MAY specify
an alternative expiration timer in the Expires header field.
4.6. NOTIFY Bodies
The format of the NOTIFY message body is either the default of
"application/xcap-diff+xml" or is a format listed in the Accept
header field of the SUBSCRIBE.
In this event package, notification messages contain an XCAP Diff
document .
The XCAP Diff format [I-D.ietf-simple-xcap-diff] can include the full
element and attribute content of XCAP documents and components. For
documents, the format can also include corresponding URIs, ETag
values, and patching instructions from version "a" to "b". Removal
events (of documents, elements, or attributes) can be identified too.
Except for collection selections, the "sel" selector values of the
XCAP-Diff format MUST be octet-by-octet equivalent to the relevant
"uri" parameter values of the <entry> element of the "resource-list"
document.
4.7. Notifier Generation of NOTIFY Requests
During the initial subscription, or if the URI list changes in
SUBSCRIBE refresh requests, the notifier MUST resolve the requested
XCAP resources and their privileges. If there are superfluous
resource selections in the requested URI list, the notifier SHOULD
NOT provide overlapping similar responses for these resources. A
resource for which an authenticated user does not have a read
privilege MUST NOT be included in the XCAP-Diff format. Note that an
XCAP component that could not be located with XCAP semantics does not
produce an error. Instead, the request remains in a "pending" state,
that is, waiting for this resource to be created (or read access
granted). Subscriptions to collections have a similar property: once
a new document is created into the subscribed collection, the
creation of a new resource is signaled with the next NOTIFY request.
After the notifier knows the list of authorized XCAP resources, it
generates the first NOTIFY, which contains URI references to all
subscribed, existing documents for which the subscriber has read
privileges, and typically XCAP component(s) of existing content.
After sending the initial notification, the notifier selects a diff-
processing mode for reporting changes. If the subscriber suggested a
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mode in the "diff-processing" parameter of the SUBSCRIBE, the
notifier MAY use that requested mode or MAY fall back to a simpler
operational mode, but the notifier MUST NOT use a more complex mode
than the one chosen by the subscriber. From least to most complex,
the order of the modes is the following: "no-patching", "xcap-
patching", "aggregate". Thus, the notifier may respond to an
"aggregate" request using any mode, but cannot reply to an "xcap-
patching" subscription using the "aggregate" mode. Naturally, the
notifier MUST handle a "no-patching" request with the "no-patching"
mode.
In all modes, the notifier MUST maintain the chronological order of
XCAP changes. If several changes to a given resource are presented
in a single notification, the chronological update order MUST be
preserved in the XML document order of the notification body.
Preservation of chronological order is required to produce a correct
document in the subscriber. If content modifications are made out-
of-order, an erroneous document would probably be formed.
While the "aggregate" mode uses bandwidth most efficiently, it
introduces other challenges. The initial synchronization might fail
with rapidly changing resources, because the "aggregate" mode
messages might not include the full version-history of a document and
the base XCAP protocol does not support version-history retrievals of
documents. When new documents are created in subscribed collections
and the notifier is aggregating patches, the same issue can occur.
In a corner case, the notifier may not be able to provide patches
with the XML-Patch-Ops [RFC5261] semantics. Therefore, if the
notifier has to temporarily disable diff generation and send only the
URI references of some changed documents to the subscriber, it MUST
continue with the "xcap-patching" mode afterwards for these
resources, if the initial subscription also started with the "xcap-
patching" mode. In other words, if the subscriber loses track of the
patching operations, the subscriber must refresh to a "known good"
state by downloading the current document. Once it has done so, it
can resume using xcap-patching.
In the "aggregate" mode, the notifier chooses how long to wait for
multiple patches to combine. Even with rapidly changing resources
the notifier MUST signal only the latest state: e.g. whether the XCAP
component exists or not.
In the "xcap-patching" mode, the notifier MAY disable the diff-
generation temporarily for certain resources, for example when the
NOTIFY body becomes impractically large or an intermediate error has
happened. Some XCAP clients will probably not have completely
optimized their patch request, so even when acting in the "xcap-
patching" operational mode, the notifier MAY try to optimize the
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diff-generation, for example by eliminating redundant patch
operations. Otherwise said: the notifier is not required to send
patch operations exactly as received by clients; rather it MAY notify
with a more efficient patch operation that MUST produce the same
result as the series of patch operations produced by the XCAP client.
Note: It is straightforward to change the XCAP client's change
requests (sent via HTTP) to use XML-Patch-Ops semantics. While
XCAP does not support patching of all XML node types - for
example, namespace declarations can not be added separately -
utilization of XML-Patch-Ops can sometimes significantly reduce
the bandwidth requirements at the expense of extra processing.
Extension of XCAP for this utilization of patch-ops is outside the
scope of this document, but it is evident that XCAP clients that
produce efficient change requests using XML-Patch-Ops make it much
easier for the notifier to produce an efficient change
notification using XML-Patch-Ops.
After the notifier has reported the existence of an XCAP component,
it MUST also report its removal consistently. For example, the
removal of the parent element of the subscribed element requires the
same signalling since the subscribed element ceases to exist. To
signal the removal of an XCAP component, the notifier sets the
Boolean "exist" attribute value of the <element> or <attribute>
elements to false.
When the notifier receives a re-subscription, it MUST re-send the
current full XML-Diff content unless the subscriber has requested a
conditional subscription [I-D.ietf-sipcore-subnot-etags] by using the
header field Suppress-If-Match: [ETag value]. With a conditional re-
subscription, the notifier MUST also inspect the subscription body
when determining the current subscription state. Since the
subscription is based on a list of XCAP R-URIs, it is RECOMMENDED
that the notifier does not consider the order of these URIs when
determining the equivalence to "stored" previous states. If a match
to the previous state is not found, the NOTIFY message MUST contain
the full XML-Diff state (similar to the initial notification). The
notifiers SHOULD implement the conditional subscription handling with
this event package.
During re-subscriptions, the subscriber may change the value of the
diff-processing parameter. The value change influences only
subsequent notifications, not the notification (if generated)
followed immediately after the (re-)SUBSCRIBE request.
Event packages like this require reliable transfer of NOTIFY
messages. This means that all messages MUST successfully be
transferred or the document will become out of sync, and then patches
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will most likely fail (or worse, have unintended consequences). This
"xcap-diff" event package requires, similar to Partial-PIDF-Notify
RFC 5263 [RFC5263], that a notifier MUST NOT send a new NOTIFY
request to the same dialog unless a successful 200-response has been
received for the last sent NOTIFY request. If the NOTIFY request
fails due to a timeout, the notifier MUST remove the subscription.
Note: This requirement ensures that out-of-order events will not
happen or that the dialog will terminate after non-resolvable
NOTIFY request failures. In addition, some of the probable NOTIFY
error responses (for example, 401, 407, 413) can possibly be
handled gracefully without tearing down the dialog.
If, for example, the subscriber has selected too many elements to
which to subscribe, such that the notification body would be
impractically large (that is, an intermediate NOTIFY failure), the
notifier MAY discard the <element> element content. The existence of
elements is then indicated with an empty <element> element, and the
content is not shown for those resources. In other words, the
<element> element does not not have a child element which would show
the subscribed "full" element content.
4.8. Subscriber Processing of NOTIFY Requests
The first NOTIFY request will usually contain references to HTTP
resources including their strong ETag values. If the subscriber does
not have similar locally cached versions, it will typically start an
unconditional HTTP GET request for those resources. During this HTTP
retrieval time, the subscriber MAY also receive patches to these
documents (if it has requested them) if the documents are changing.
A subscriber can chain the modification list for the document and
aggregate the changes itself if the notifications contain patches and
indicate all atomic XCAP modifications with both previous and new
ETags of each resource ("xcap-patching" mode). If the version
received via HTTP is newer than any received via the notifications,
the subscriber may not find an equivalent match of an ETag value from
the chain of patches. This can happen since notifications are
reported after HTTP changes and preferably at some minimum intervals.
In such a case, the subscriber SHOULD either wait for subsequent
notifications or refresh the subscription and repeat the described
"sync" algorithm until a match is achieved.
To avoid out-of-sync issues, the subscriber MAY start the
subscription with the "xcap-patching" mode, and then refresh the
subscription with the "aggregate" mode. Syncs are successful if the
received HTTP ETag matches with either the previous or new ETag of
the reported aggregated patch. If the subscription is started with
the "aggregate" mode, which doesn't necessarily show the full
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version-history information, the subscriber may not be able to
synchronize its local cache with the first notification. The
subscriber MAY resolve this issue by doing one of the following: re-
fetching the out-of-sync document, waiting for subsequent
notifications, or by refreshing the subscription. However, the same
issue may still repeat.
If the subscriber has received a "full" sync and it has detected that
some of the resources are being served with the "xcap-patching" mode
while others are in the "aggregate" mode, it SHOULD refresh the
subscription to the "aggregate" mode.
The notifier MAY at any time temporarily use the "no-patching" mode
for some resources so that the subscriber receives only URI
references of modifications. When the notifier is acting in this
mode, several cycles MAY be needed before an initial "full" sync is
achieved. As the notifier MAY change modes in the middle of a
dialog, the subscriber is always responsible for taking appropriate
actions. Also, as the last resort, the subscriber MAY always disable
the usage of diff-processing by setting the "diff-processing"
parameter to "no-patching".
If a diff format cannot be applied due to patch processing and/or
programming errors (for a list, see Section 5.1 of [RFC5261]), the
subscriber SHOULD refresh the subscription and disable patching by
setting the "diff-processing" parameter to "no-patching". The
subscriber SHOULD NOT reply with a non-200 response since the
notifier cannot make corrections.
During unconditional re-subscriptions, the subscriber MUST stamp the
received state of all previous resources as stale. However, if a
conditional [I-D.ietf-sipcore-subnot-etags] re-subscription is
successful, the subscriber MUST preserve the current state of
resources unless the subscribed URI list has changed. That is, the
subscriber MUST fetch the resource's state, for example, from some
local cache.
4.9. Handling of Forked Requests
This specification allows only a single dialog to be constructed from
an initial SUBSCRIBE request. If the subscriber receives forked
responses to a SUBSCRIBE, the subscriber MUST apply the procedures in
Section 4.4.9 of RFC 3265 [RFC3265] for handling non-allowed forked
requests.
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4.10. Rate of Notifications
Notifiers of "xcap-diff" event package SHOULD NOT generate
notifications for a single subscription at a rate of more than once
every five seconds.
4.11. State Agents
State agents play no role in this package.
5. An Initial Example NOTIFY document
Figure 2 shows an example initial XCAP Diff format document provided
by the first NOTIFY request to the SUBSCRIBE example in Figure 1.
The following is an example Event header field for this SUBSCRIBE
request:
Event: xcap-diff; diff-processing=aggregate
The subscriber requests that the notifier "aggregate" XCAP component
updates and anticipates that the subsequent notifications will
contain aggregated patches to these documents.
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<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/root/">
<document new-etag="7ahggs"
sel="resource-lists/users/sip:joe@example.com/index"/>
<document new-etag="30376adf"
sel="pidf-manipulation/users/sip:joe@example.com/index"/>
<d:element sel="rls-services/users/sip:joe@example.com/index/
~~/*/service%5b@uri='sip:marketing@example.com'%5d"
xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
xmlns="urn:ietf:params:xml:ns:rls-services"
xmlns:rl="urn:ietf:params:xml:ns:resource-lists">
<service uri="sip:marketing@example.com">
<list name="marketing">
<rl:entry uri="sip:joe@example.com"/>
<rl:entry uri="sip:sudhir@example.com"/>
</list>
<packages>
<package>presence</package>
</packages>
</service>
</d:element>
</xcap-diff>
Figure 2: An example initial XCAP Diff format document
Note that the resource-list "index" document included only the new
ETag value, as the document existed during the subscription time. In
the "pidf-manipulation" collection, there is only a single document
for which the user has read privilege. The <services> element exists
within the rls-services "index" document and its content is shown.
6. IANA Considerations
This specification instructs IANA to add a new event package to the
SIP Event Types Namespace registry. The new data to be added is:
Package Name Type Contact Reference
------------- -------- ------- ---------
xcap-diff package IETF SIP Working Group [RFCXXXX]
<sip@ietf.org>
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7. Security Considerations
This document defines a new SIP event package for the SIP event
notification framework specified in RFC 3265 [RFC3265]. As such, all
the security considerations of RFC 3265 apply. The configuration
data can contain sensitive information, and both the client and the
server need to authenticate each other. The notifiers MUST
authenticate the "xcap-diff" event package subscriber using the
normal SIP authentication mechanisms, for example Digest as defined
in Section 22 of RFC 3261 [RFC3261]. The notifiers MUST be aware of
XCAP User identities (XUI) and how to map the authenticated SIP
identities unambiguously with XUIs.
Since XCAP [RFC4825] provides a basic authorization policy for
resources and since notifications contain content similar to XCAP
resources, the security considerations of XCAP also apply. The
notifiers MUST obey the XCAP authorization rules when signalling
resource changes. In practice, this means following the read
privilege rules of XCAP resources.
Denial-of-Service attacks against notifiers deserve special mention.
The following can cause denial of service due to intensive
processing: subscriptions to a long list of URIs, "pending"
subscriptions to non-existent documents or XCAP components, and diff-
generation algorithms that try to optimize the required bandwidth
usage to extremes.
The mechanism used for conveying xcap-diff event information MUST
ensure integrity and SHOULD ensure confidentially of the information.
An end-to-end SIP encryption mechanism, such as S/MIME described in
Section 26.2.4 of RFC 3261 [RFC3261], SHOULD be used. If that is not
available, it is RECOMMENDED that TLS [RFC5246] be used between
elements to provide hop-by-hop authentication and encryption
mechanisms described in Section 26.2.2 "SIPS URI Scheme" and Section
26.3.2.2 "Interdomain Requests" of RFC 3261 [RFC3261].
8. Acknowledgments
The author would like to thank Jonathan Rosenberg for his valuable
comments and providing the initial event package, and Aki Niemi,
Pekka Pessi, Miguel Garcia, Pavel Dostal, Krisztian Kiss, Anders
Lindgren, Sofie Lassborn, Keith Drage, Stephen Hinton, Byron Campen,
Avshalom Houri, Ben Campbell, Paul Kyzivat, Spencer Dawkins, Pasi
Eronen and Chris Newman for their valuable comments. Lisa Dusseault
critiqued the document during IESG review, raising numerous issues
that resulted in improved document quality. Further, technical
writer A. Jean Mahoney devoted countless hours to integrating Lisa's
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comments and cleaning up the technical English usage.
9. References
9.1. Normative References
[I-D.ietf-simple-xcap-diff]
Rosenberg, J. and J. Urpalainen, "An Extensible Markup
Language (XML) Document Format for Indicating A Change in
XML Configuration Access Protocol (XCAP) Resources",
draft-ietf-simple-xcap-diff-09 (work in progress),
May 2008.
[I-D.ietf-sipcore-subnot-etags]
Niemi, A., "An Extension to Session Initiation Protocol
(SIP) Events for Conditional Event Notification",
draft-ietf-sipcore-subnot-etags-02 (work in progress),
April 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
[RFC4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
for Representing Resource Lists", RFC 4826, May 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5261] Urpalainen, J., "An Extensible Markup Language (XML) Patch
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Operations Framework Utilizing XML Path Language (XPath)
Selectors", RFC 5261, September 2008.
9.2. Informative References
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[RFC5263] Lonnfors, M., Costa-Requena, J., Leppanen, E., and H.
Khartabil, "Session Initiation Protocol (SIP) Extension
for Partial Notification of Presence Information",
RFC 5263, September 2008.
[W3C.REC-xml-20060816]
Sperberg-McQueen, C., Paoli, J., Bray, T., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
Edition)", World Wide Web Consortium FirstEdition REC-xml-
20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml-20060816>.
Appendix A. Informative Examples
These examples illustrate the basic features of the xcap-diff event
package. Only the relevant header fields are shown. Note also that
the SIP R-URIs of these examples don't correspond to reality.
A.1. Initial documents on an XCAP server
The following documents exist on an XCAP server (xcap.example.com)
with an imaginary "tests" application usage (there's no default
document namespace defined in this imaginary application usage).
http://xcap.example.com/tests/users/sip:joe@example.com/index:
<?xml version="1.0" encoding="UTF-8"?>
<doc>
<note>This is a sample document</note>
</doc>
and then
http://xcap.example.com/tests/users/sip:john@example.com/index:
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<?xml version="1.0" encoding="UTF-8"?>
<doc>
<note>This is another sample document</note>
</doc>
A.2. An Initial Subscription
The following demonstrates the listing of a collection contents and
it shows only resources where the user has read privilege. The user
Joe, whose XUI is "sip:joe@example.com", sends an initial
subscription:
SUBSCRIBE sip:tests@xcap.example.com SIP/2.0
...
Accept: application/xcap-diff+xml
Event: xcap-diff; diff-processing=aggregate
Content-Type: application/resource-lists+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="tests/users/sip:joe@example.com/"/>
</list>
</resource-lists>
In addition to the 200 (OK) response, the notifier sends the first
NOTIFY:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<document new-etag="7ahggs"
sel="tests/users/sip:joe@example.com/index"/>
</xcap-diff>
The subscriber learns that the document on this "tests" application
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usage is equivalent to its locally cached version, so it does not
act. If the local version had been different, the subscriber would
most likely re-fetch the document.
If the subscriber had requested the "tests/users/" collection, the
notification body would have been the same since Joe has no read
privilege to John's resources (XCAP default behavior).
If the Expires header field had a value "0", the request would be
similar to the PROPFIND method of WebDAV. The syntax and responses
differ, however.
A.3. A Document Addition Into a Collection
Let's say that Joe adds a new document to his collection, using
either the same client or another client running on a different
device. He does an HTTP PUT to his application usage collection:
PUT /tests/users/sip:joe@example.com/another_document HTTP/1.1
Host: xcap.example.com
....
Content-Type: application/xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<doc>
<note>This is another sample document</note>
</doc>
This HTTP PUT request results in the XCAP client receiving a strong
HTTP ETag "terteer" for this new document.
Then the subscriber receives a notification afterwards:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<document new-etag="terteer"
sel="tests/users/sip:joe@example.com/another_document"/>
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</xcap-diff>
Note that the result is "additive"; it doesn't indicate the already
indicated "index" document. Only the initial (or refreshed)
notification contains all document URI references.
If Joe's client both modifies the documents and refreshes the
subscriptions, it would typically ignore this notification, since its
modifications had caused the notification. If the client that
received this NOTIFY hadn't submitted the document change, it would
probably fetch this new document.
If Joe's client refreshes the subscription with the same request body
as in the initial subscription, the result will include these two
documents: "index" and "another_document" with their ETags.
A.4. A Series of XCAP Component Modifications
Now Joe's client uses its XCAP patching capability by doing the
following:
PUT /tests/users/sip:joe@example.com/index/~~/doc/foo HTTP/1.1
Host: xcap.example.com
....
Content-Type: application/xcap-el+xml
Content-Length: [XXX]
<foo>this is a new element</foo>
Since the insertion of the element is successful, Joe's client
receives the new HTTP ETag "fgherhryt3" of the updated "index"
document.
Immediately thereafter, Joe's client issues another HTTP request
(this request could even be pipe-lined):
PUT /tests/users/sip:joe@example.com/index/~~/doc/bar HTTP/1.1
Host: xcap.example.com
....
Content-Type: application/xcap-el+xml
Content-Length: [XXX]
<bar>this is a bar element
</bar>
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The reported new HTTP ETag of "index" is now "dgdgdfgrrr".
And Joe's client issues yet another HTTP request:
PUT /tests/users/sip:joe@example.com/index/~~/doc/foobar HTTP/1.1
Host: xcap.example.com
....
Content-Type: application/xcap-el+xml
Content-Length: [XXX]
<foobar>this is a foobar element</foobar>
The reported new ETag of "index" is now "63hjjsll".
After awhile, Joe's client receives a notification with an embedded
patch since it has requested "aggregate" diff-processing and the
notifier is capable of producing them:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<d:document previous-etag="7ahggs3"
sel="tests/users/sip:joe@example.com/index"
new-etag="63hjjsll">
<d:add sel="*">
<foo>this is a new element</foo>
<bar>this is a bar element
</bar>
<foobar>this is a foobar element</foobar>
</d:add>
</d:document>
</d:xcap-diff>
Joe's client applies this patch to the locally cached "index"
document, detects the ETag update, and stores the last ETag value.
Note how several XCAP component modifications were aggregated.
Note also that, if Joe's client did not have a locally cached version
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of the reference document, it would have needed to do a HTTP GET
request after the initial notification. If the ETag of the received
resource by HTTP did not match either the previous or new ETag of
this aggregated patch, an out-of-sync condition would be probable.
This issue is not typical, but it can happen. To resolve the issue,
the client could re-fetch the "index" document and/or wait for
subsequent notifications to detect a match. A better and simpler way
to avoid the issue is to refresh the subscription with the "xcap-
patching" mode and later refresh with the "aggregate" mode.
Alternatively, if the notifier's operational mode been "xcap-
patching", the NOTIFY could have been the following:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<d:document previous-etag="7ahggs"
sel="tests/users/sip:joe@example.com/index"
new-etag="fgherhryt3">
<d:add sel="*">
<foo>this is a new element</foo>
</d:add>
</d:document>
<d:document previous-etag="fgherhryt3"
sel="tests/users/sip:joe@example.com/index"
new-etag="dgdgdfgrrr">
<d:add sel="*">
<bar>this is a bar element
</bar>
</d:add>
</d:document>
<d:document previous-etag="dgdgdfgrrr"
sel="tests/users/sip:joe@example.com/index"
new-etag="63hjjsll">
<d:add sel="*">
<foobar>this is a foobar element</foobar>
</d:add>
</d:document>
</d:xcap-diff>
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If the client had to re-fetch the "index" document after the initial
notification, it could have skipped some or all of these patches,
depending on whether the HTTP ETag matched some of these ETags in the
chain of patches. If the HTTP ETag did not match and the received
HTTP version is a newer version indicated in later notification(s)
then the sync may then be achieved since the notifier provided the
full change history in the "xcap-patching" mode.
Lastly, the notifier could (temporarily) fall back to the "no-
patching" mode, which allows the notifier to keep the dialog alive
when there are too many updates:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<document previous-etag="7ahggs3"
sel="tests/users/sip:joe@example.com/index"
new-etag="63hjjsll"/>
</xcap-diff>
At any time, the notifier may fall back to the "no-patching" mode for
some or all of the subscribed documents.
A.5. An XCAP Component Subscription
The user Joe sends an initial subscription for the "id" affribute of
a <doc> element. The "index" document exists, but the <doc> root
element does not contain the "id" attribute at the time of the
subscription.
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SUBSCRIBE sip:tests@xcap.example.com SIP/2.0
...
Accept: application/xcap-diff+xml
Event: xcap-diff
Content-Type: application/resource-lists+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="tests/users/sip:joe@example.com/index/~~/doc/@id"/>
</list>
</resource-lists>
The first NOTIFY looks like the following since there is nothing to
indicate:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/"/>
Note that if the "index" document hadn't existed, the first NOTIFY
request would have been the same. The XCAP Diff document format
doesn't indicate reasons for non-existing resources.
Afterwards Joe's client updates the whole document root element
including the attribute "id" (not a typical XCAP operation nor a
preferred one, just an illustration here):
PUT /tests/users/sip:joe@example.com/index/~~/doc HTTP/1.1
Host: xcap.example.com
....
Content-Type: application/xcap-el+xml
Content-Length: [XXX]
<doc id="bar">This is a new root element</doc>
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The new HTTP ETag of the "index" document is now "dwawrrtyy".
Then Joe's client gets a notification:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<attribute
sel="tests/users/sip:joe@example.com/index/~~/doc/@id">
bar
</attribute>
</xcap-diff>
Note that the HTTP ETag value of the new document is not shown as it
is irrelevant for this use-case.
Then Joe's client removes the "id" attribute:
DELETE /tests/users/sip:joe@example.com/index/~~/doc/@id HTTP/1.1
Host: xcap.example.com
....
Content-Length: 0
And the subscriber gets a notification:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<attribute sel="tests/users/sip:joe@example.com/index/~~/doc/@id"
exists="0"/>
</xcap-diff>
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The notification indicates that the subscribed attribute was removed
from the document. Naturally attributes are "removed" if the element
where they belong is removed, for example by an HTTP DELETE request.
The component selections indicate only the existence of attributes or
elements.
A.6. A Conditional Subscription
The last example is a conditional subscription where a full refresh
can be avoided when there are no changes in resources. Joe's client
sends an initial subscription:
SUBSCRIBE sip:tests@xcap.example.com SIP/2.0
...
Accept: application/xcap-diff+xml
Event: xcap-diff; diff-processing=xcap-patching
Content-Type: application/resource-lists+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="tests/users/sip:joe@example.com/"/>
</list>
</resource-lists>
Since there are now two documents in the repository, the first NOTIFY
looks like the following:
NOTIFY sip:joe@userhost.example.com SIP/2.0
...
Event: xcap-diff
SIP-ETag: xggfefe54
Content-Type: application/xcap-diff+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
xcap-root="http://xcap.example.com/">
<document new-etag="63hjjsll"
sel="tests/users/sip:joe@example.com/index"/>
<document new-etag="terteer"
sel="tests/users/sip:joe@example.com/another_document"/>
</xcap-diff>
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Note that the NOTIFY request contains the SIP-ETag "xggfefe54". This
SIP-ETag is placed in the Suppress-If-Match header field of the
conditional subscription. The "diff-processing" mode also is changed
(or is requested to change):
SUBSCRIBE sip:tests@xcap.example.com SIP/2.0
...
Suppress-If-Match: xggfefe54
Accept: application/xcap-diff+xml
Event: xcap-diff; diff-processing=aggregate
Content-Type: application/resource-lists+xml
Content-Length: [XXX]
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="tests/users/sip:joe@example.com/"/>
</list>
</resource-lists>
If the notifier finds a match to the previous stored state when it
evaluates this request, it responds with 204 (No Notification). If
there are no reportable changes as per
[I-D.ietf-sipcore-subnot-etags], NOTIFY request generation is
suppressed. When the notifier can aggregate several modifications,
this re-subscription enables the processing of that mode thereafter.
Indeed, the re-subscription may be quite process-intensive,
especially when there are a large number of relevant reported
resources.
Authors' Addresses
Jari Urpalainen
Nokia
Itamerenkatu 11-13
Helsinki 00180
Finland
Phone: +358 7180 37686
Email: jari.urpalainen@nokia.com
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Dean Willis (editor)
Softarmor Systems LLC
3100 Independence Pk #311-164
Plano, TX 75075
USA
Phone: +1 214 504 19876
Email: dean.willis@softarmor.com
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