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IETF RFC 5875
An Extensible Markup Language (XML) Configuration Access Protocol (XCAP) Diff Event Package
Last modified on Thursday, May 20th, 2010
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Internet Engineering Task Force (IETF) J. Urpalainen
Request for Comments: 5875 Nokia
Category: Standards Track D. Willis, Ed.
ISSN: 2070-1721 Softarmor Systems LLC
May 2010
An Extensible Markup Language (XML) Configuration Access Protocol (XCAP)
Diff Event Package
 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5875.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Urpalainen & Willis Standards Track PAGE 1 
RFC 5875 XCAP Diff Event May 2010
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . 13
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 . . . . . . . . . . . . . . . . 18
A.1. Initial Documents on an XCAP Server . . . . . . . . . . . 18
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
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RFC 5875 XCAP Diff Event May 2010
1. Introduction
The SIP events framework [RFC 3265] describes subscription and
notification conventions for the Session Initiation Protocol (SIP)
[RFC 3261]. The Extensible Markup Language (XML)
[W3C.REC-xml-20060816] Configuration Access Protocol (XCAP) [RFC 4825]
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 [RFC 3265] and the
XCAP diff format [RFC 5874], 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 documents in
one of three ways. The first mode only indicates the event type and
does not include document contents, so the subscriber uses HTTP
[RFC 2616] to retrieve the updated document. The second mode includes
document content changes in notification messages, using the XML-
Patch-Ops [RFC 5261] format with minimal notification size. The third
mode also includes document content changes in notification messages
with the same XML-Patch-Ops format, but is more verbose, and shows
the full HTTP version history.
Third, the client can subscribe to specific XML elements or
attributes (XCAP components) showing their existing 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.
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2. Terminology
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 RFC 2119, BCP 14
[RFC 2119] 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 [RFC 4825] and
some in WebDAV [RFC 4918].
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 an 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 [RFC 4918]. 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 all in-scope namespaces of an
element to be requested, 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 component contents that are part of the subscription. 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 package parameter,
covered in Section 4.3. Note that the existence of the "diff-
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RFC 5875 XCAP Diff Event May 2010
processing" parameter or its value has no influence on XCAP component
subscriptions.
4.2. Event Package Name
The name of this event package is "xcap-diff". As specified in
[RFC 3265], 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 doing so
optimizes network traffic within the capabilities of the receiver).
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. 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 made
by XCAP clients (via HTTP). The client receives the full (HTTP)
ETag change history of a document.
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 "xcap-patching" and "aggregate"
modes, and thus implement XML-Patch-Ops [RFC 5261] diff-generation,
because this can greatly reduce the required number of
notifications and overall transmissions.
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RFC 5875 XCAP Diff Event May 2010
If the subscription does not contain the "diff-processing" header
field parameter, the notifier MUST default to the "no-patching" mode.
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 [RFC 5234] of the "diff-processing" parameter is:
diff-processing = "diff-processing" EQUAL (
"no-patching" /
"xcap-patching" /
"aggregate" /
token )
where EQUAL and token are defined in RFC 3261 [RFC 3261].
4.4. SUBSCRIBE Bodies
The URI list is described by the XCAP resource list format [RFC 4826],
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 request
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.
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RFC 5875 XCAP Diff Event May 2010
The SUBSCRIBE request MAY contain the Suppress-If-Match header field
[RFC 5839], 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 (XCAP
component) 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
When subscribing to XCAP components, namespace prefixes of XCAP Node
Selectors MUST be properly resolved to namespace URIs. Section 6.4
of RFC 4825 [RFC 4825] describes the conventions when using prefixes
Urpalainen & Willis Standards Track PAGE 7
RFC 5875 XCAP Diff Event May 2010
in XCAP Node Selectors. If only XCAP Default Document Namespace is
used, just like in the previous example (where a <service> element is
selected), the query component of the "uri" value is not required.
4.5. Subscription Duration
The default expiration time for subscriptions within this package is
3600 seconds. As per RFC 3265 [RFC 3265], the subscriber MAY specify
an alternative expiration timer in the Expires header field.
4.6. NOTIFY Bodies
The format of the NOTIFY message body either is 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 [RFC 5874].
The XCAP diff format [RFC 5874] can include the subscribed XCAP
component contents. 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.
With XCAP component subscriptions, XCAP Node Selectors can contain
namespace prefixes. A notifier MUST then resolve these prefixes to
namespace URIs according to RFC 4825 [RFC 4825] conventions. In other
words, notifiers MUST be aware of XCAP Default Document Namespaces
for Application Usages when they locate unprefixed qualified XCAP
elements. Note that the namespace resolving rules of Patch operation
elements <add>, <replace>, and <remove> are described in Section
4.2.1 of [RFC 5261].
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,
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RFC 5875 XCAP Diff Event May 2010
that is, waiting for this resource to be created (or read access
granted if XCAP Application Usages utilize dynamic access control
lists). 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
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.
Chronological order is preserved to simplify the required subscriber
implementation logic.
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 (such as when the XML prolog changes), the notifier
may not be able to provide patches with the XML-Patch-Ops [RFC 5261]
semantics.
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.
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RFC 5875 XCAP Diff Event May 2010
Note: The diff-generation may be disabled when the NOTIFY body
becomes impractically large or an intermediate error has happened.
As the subscriber loses track of the patching operations, it must
refresh to a "known good" state by downloading current documents.
Once it has done so, it can re-subscribe, for example, with the
"aggregate" mode.
In the "aggregate" mode, the notifier chooses how long to wait for
multiple patches to combine and how this combination is done.
In the "xcap-patching" mode, the notifier MAY try to optimize the
diff-generation, for example, by eliminating redundant information
since some XCAP clients will probably not have completely optimized
their HTTP PUT request.
Note: It is straightforward to change the XCAP client's change
requests: PUT and DELETE (sent via HTTP) to use XML-Patch-Ops
semantics. While XCAP does not support patching of all XML node
types -- for example, namespace declarations cannot be added
separately -- efficient utilization of XML-Patch-Ops can sometimes
significantly reduce the bandwidth requirements at the expense of
extra processing.
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 signaling 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. Even with rapidly changing resources, the
notifier MUST signal only the latest state: e.g., whether or not the
XCAP component exists.
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 [RFC 5839] 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 request 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.
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RFC 5875 XCAP Diff Event May 2010
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
will most likely fail (or worse, have unintended consequences). This
"xcap-diff" event package requires, similar to Partial-PIDF-Notify
RFC 5263 [RFC 5263], 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 have a child element that 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 and if the documents are changing
rapidly. It can happen that the version retrieved by HTTP is not the
same than what is indicated in the initial notification. A
subscriber can then chain the modification list for each document,
and locate the position where the previous ETag value is equal to
that retrieved via HTTP. If an ETag match is not found from the
first change, a subscriber MUST omit all changes up to the point
where it is the same. From that change onwards, the subscriber
applies all reported patches. 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.
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RFC 5875 XCAP Diff Event May 2010
This can happen since notifications are reported after HTTP changes
and preferably at some minimum intervals. Also, document removals
can be reported in notifications and/or HTTP retrievals may fail
because of unexisting resources (rapidly changing). In any case, the
subscriber can re-fetch the possible out-of-sync document, wait for
subsequent notifications or refresh the subscription (with "xcap-
patching"), and repeat the described "sync" algorithm until a "full"
sync is achieved.
If the notifier aggregates patches, the previous modification list
may not contain the ETag value retrieved by HTTP simply because of
aggregation optimizations. A similar out-of-sync cycle can happen
when new (subscribed) documents are created that change rapidly. To
avoid such difficulties, the subscriber MAY start the subscription
with the "xcap-patching" mode, and then refresh the subscription with
the "aggregate" mode after the initial sync is achieved. Naturally,
the subscriber can revert back to the "xcap-patching" mode from
"aggregate" at any time and vice versa.
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 [RFC 5261]), 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 [RFC 5839] 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.
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RFC 5875 XCAP Diff Event May 2010
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 [RFC 3265] for handling non-allowed forked
requests.
4.10. Rate of Notifications
Notifiers of an "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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RFC 5875 XCAP Diff Event May 2010
<?xml version="1.0" encoding="UTF-8"?>
<d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
xmlns:s="urn:ietf:params:xml:ns:rls-services"
xcap-root="http://xcap.example.com/root/">
<d:document new-etag="7ahggs"
sel="resource-lists/users/sip:joe@example.com/index"/>
<d: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:rl="urn:ietf:params:xml:ns:resource-lists"
><s:service uri="sip:marketing@example.com">
<s:list name="marketing">
<rl:entry uri="sip:joe@example.com"/>
<rl:entry uri="sip:sudhir@example.com"/>
</s:list>
<s:packages>
<s:package>presence</s:package>
</s:packages>
</s:service></d:element>
</d: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 privileges. The <service> element exists
within the rls-services "index" document and its content is shown.
Note also that the <service> element was located using the Default
Document Namespace (no prefix in XCAP Node Selector value) although
it has an "s" prefix in the source document.
6. IANA Considerations
IANA has added a new event package to the SIP Event Types Namespace
registry as follows:
Package Name Type Contact Reference
------------- -------- ------- ---------
xcap-diff package IETF Real-time Applications [RFC 5875]
<rai@ietf.org>
Urpalainen & Willis Standards Track PAGE 14
RFC 5875 XCAP Diff Event May 2010
7. Security Considerations
This document defines a new SIP event package for the SIP event
notification framework specified in RFC 3265 [RFC 3265]. 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 [RFC 3261]. The notifiers MUST be aware of
XCAP User Identifiers (XUI) and how to map the authenticated SIP
identities unambiguously with XUIs.
Since XCAP [RFC 4825] 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 [RFC 3261], SHOULD be used. If that is not
available, it is RECOMMENDED that TLS [RFC 5246] be used between
elements to provide hop-by-hop authentication and encryption
mechanisms described in Sections 26.2.2 ("SIPS URI Scheme") and
26.3.2.2 ("Interdomain Requests") of RFC 3261 [RFC 3261].
8. Acknowledgments
The author would like to thank Jonathan Rosenberg for his valuable
comments and for 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
comments and cleaning up the technical English usage.
Urpalainen & Willis Standards Track PAGE 15
RFC 5875 XCAP Diff Event May 2010
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2616] 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.
[RFC 3261] 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.
[RFC 3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC 4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
[RFC 4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
for Representing Resource Lists", RFC 4826, May 2007.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC 5261] Urpalainen, J., "An Extensible Markup Language (XML) Patch
Operations Framework Utilizing XML Path Language (XPath)
Selectors", RFC 5261, September 2008.
[RFC 5839] Niemi, A. and D. Willis, "An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
Notification", RFC 5839, May 2010.
[RFC 5874] Rosenberg, J. and J. Urpalainen, "An Extensible Markup
Language (XML) Document Format for Indicating a Change in
XML Configuration Access Protocol (XCAP) Resources",
RFC 5874, May 2010.
Urpalainen & Willis Standards Track PAGE 16
RFC 5875 XCAP Diff Event May 2010
9.2. Informative References
[RFC 4918] Dusseault, L., "HTTP Extensions for Web
Distributed Authoring and Versioning
(WebDAV)", RFC 4918, June 2007.
[RFC 5263] 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] Paoli, J., Bray, T., Yergeau, F., Maler, E.,
and C. Sperberg-McQueen, "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>.
Urpalainen & Willis Standards Track PAGE 17
RFC 5875 XCAP Diff Event May 2010
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 request 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:
<?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 collection contents and it
shows only resources where the user has read privileges. 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>
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RFC 5875 XCAP Diff Event May 2010
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
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
privileges 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>
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RFC 5875 XCAP Diff Event May 2010
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"/>
</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>
Urpalainen & Willis Standards Track PAGE 20
RFC 5875 XCAP Diff Event May 2010
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>
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="*"
Urpalainen & Willis Standards Track PAGE 21
RFC 5875 XCAP Diff Event May 2010
><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
of the reference document, it would have needed to do an 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"
Urpalainen & Willis Standards Track PAGE 22
RFC 5875 XCAP Diff Event May 2010
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>
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),
the sync may then be achieved since the notifier provided the full
change history in the "xcap-patching" mode.
Last, 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" attribute 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.
Urpalainen & Willis Standards Track PAGE 23
RFC 5875 XCAP Diff Event May 2010
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 or 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>
The new HTTP ETag of the "index" document is now "dwawrrtyy".
Urpalainen & Willis Standards Track PAGE 24
RFC 5875 XCAP Diff Event May 2010
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>
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
Urpalainen & Willis Standards Track PAGE 25
RFC 5875 XCAP Diff Event May 2010
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>
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
Urpalainen & Willis Standards Track PAGE 26
RFC 5875 XCAP Diff Event May 2010
(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 [RFC 5839], 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
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
Urpalainen & Willis Standards Track PAGE 27
An Extensible Markup Language (XML) Configuration Access Protocol (XCAP) Diff Event Package
RFC TOTAL SIZE: 58307 bytes
PUBLICATION DATE: Thursday, May 20th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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