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
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.
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
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 [RFC5874], 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 [RFC2616] to retrieve the updated document. The second mode includes document content changes in notification messages, using the XML- Patch-Ops [RFC5261] 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.
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 [RFC2119] and indicate requirement levels for compliant implementations.
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].
4.1. 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 [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 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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processing" parameter or its value has no influence on XCAP component subscriptions.
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 [RFC5261] diff-generation, because this can greatly reduce the required number of notifications and overall transmissions.
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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 [RFC5234] of the "diff-processing" parameter is:
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 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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The SUBSCRIBE request MAY contain the Suppress-If-Match header field [RFC5839], 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".
When subscribing to XCAP components, namespace prefixes of XCAP Node Selectors MUST be properly resolved to namespace URIs. Section 6.4 of RFC 4825 [RFC4825] describes the conventions when using prefixes
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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.
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 [RFC5874].
The XCAP diff format [RFC5874] 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 [RFC4825] 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 [RFC5261].
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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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 [RFC5261] 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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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 [RFC5839] 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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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 [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 have a child element that would show the subscribed "full" element content.
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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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 [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 [RFC5839] 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.
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.
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.
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.
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 Identifiers (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 Sections 26.2.2 ("SIPS URI Scheme") and 18.104.22.168 ("Interdomain Requests") of RFC 3261 [RFC3261].
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.
[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] 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>.
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.
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:firstname.lastname@example.org", sends an initial subscription:
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.
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.
><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>
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:
<d:document previous-etag="7ahggs" sel="tests/users/sip:email@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:firstname.lastname@example.org/index" new-etag="dgdgdfgrrr"> <d:add sel="*" ><bar>this is a bar element </bar></d:add></d:document>
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sel="tests/users/sip:email@example.com/index" new-etag="63hjjsll"> <d:add sel="*" ><foobar>this is a foobar element</foobar></d:add></d:document>
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:
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.
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
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 [RFC5839], 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.
Jari Urpalainen Nokia Itamerenkatu 11-13 Helsinki 00180 Finland