HomeDocsIETF standardsRFCRFC 4600 — 4699
Text version
Network Working Group
Request for Comments: 4660
Category: Standards Track
H. Khartabil
Telio
E. Leppanen
M. Lonnfors
J. Costa-Requena
Nokia
September 2006

Functional Description of Event Notification Filtering

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2006).

Abstract

The SIP event notification framework describes the usage of the Session Initiation Protocol (SIP) for subscriptions and notifications of changes to the state of a resource. The document does not describe a mechanism whereby filtering of event notification information can be achieved.

This document describes the operations a subscriber performs in order to put filtering rules associated with a subscription to event notification information in place. The handling, by the subscriber, of responses to subscriptions carrying filtering rules and the handling of notifications with filtering rules applied to them are also described. Furthermore, the document conveys how the notifier behaves when receiving such filtering rules and how a notification is constructed.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions .....................................................3
   3. Client Operation ................................................4
      3.1. Transport Mechanism ........................................4
      3.2. SUBSCRIBE Bodies ...........................................4
      3.3. Subscriber Generating of SUBSCRIBE Requests ................4
           3.3.1. Defining the Filtering Rules ........................4
           3.3.2. Request-URI vs. Filter URI ..........................5
           3.3.3. Changing Filters within a Dialog ....................5
           3.3.4. Subscriber Interpreting of SIP Responses ............6
      3.4. Subscriber Processing of NOTIFY Requests ...................6
   4. Resource List Server Behaviour ..................................7
      4.1. Request-URI vs. Filter URI .................................7
      4.2. Changing Filters within a Dialog ...........................9
   5. Server Operation ................................................9
      5.1. NOTIFY Bodies ..............................................9
      5.2. Notifier Processing of SUBSCRIBE Requests ..................9
           5.2.1. Request-URI vs. Filter URI .........................10
           5.2.2. Changing Filters within a Dialog ...................11
      5.3. Notifier Generating of NOTIFY Requests ....................11
           5.3.1. Generation of NOTIFY Contents ......................12
           5.3.2. Handling of Notification Triggering Rules ..........13
      5.4. Handling Abnormal Cases ...................................13
   6. XML Document Validation ........................................14
   7. Examples .......................................................14
      7.1. Presence Specific Examples ................................14
           7.1.1. Subscriber Requests Messaging-Related Information ..15
           7.1.2. Subscriber Fetches Information about "Open"
                  Communication Means ................................16
           7.1.3. Subscriber Requests Notifications When
                  Presentity's Status Changes ........................18
      7.2. Watcher Information Specific Examples .....................21
           7.2.1. Watcher Subscriber Makes Subscription to
                  Get All the Information about Active Watchers ......22
           7.2.2. Watcher Subscriber Requests Information of
                  Watchers with Specific Subscription Duration
                  Conditions .........................................23
           7.2.3. Watcher Subscriber Requests Specific
                  Watcher Info on Specific Triggers ..................24
   8. Security Considerations ........................................27
   9. IANA Considerations ............................................28
   10. Acknowledgements ..............................................28
   11. References ....................................................28
      11.1. Normative References .....................................28
      11.2. Informative References ...................................28

1. Introduction

SIP event notification is described in [3]. It defines a general framework for sending subscriptions and receiving notifications in SIP-based systems. It introduces the concept of event packages, which are concrete applications of the general event framework to a specific usage of events.

Filtering is a mechanism for controlling the content of event notifications. Additionally, the subscriber may specify the rules for when a notification should be sent to it. The filtering mechanism is expected to be particularly valuable to users of mobile wireless access devices. The characteristics of the devices typically include high latency, low bandwidth, low data processing capabilities, small display, and limited battery power. Such devices can benefit from the ability to filter the amount of information generated at the source of the event notification. However, implementers need to be aware of the computational burden on the source of the event notification. This is discussed further in Section 8.

It is stated in [3] that the notifier may send a NOTIFY at any time, but typically it is sent when the state of the resource changes. It also states that the notifications would contain the complete and current state of the resource authorized for a certain subscriber to see. The format of such resource state information is package specific. In this memo, we assume that the NOTIFY for any package contains an XML document.

This document, together with [5], presents a mechanism for filtering whereby a subscriber describes its preference of when notifications are to be sent to it and what they are to contain. It also describes how the notifier functions when generating notifications by taking into account filters and default functionality of the package/ service.

The XML format for defining the filter is described in [5].

2. Conventions

In this document, the key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' are to be interpreted as described in RFC 2119 [1] and indicate requirement levels for compliant implementations.

"Content" refers to the XML document that appears in a notification reflecting the state of a resource.

3. Client Operation

3.1. Transport Mechanism

Transportation of the filter to the server is achieved by inserting the XML document, as defined in [5], in the body of the SUBSCRIBE request. Alternatively, the XML document can be uploaded to the server using means outside the scope of this document.

3.2. SUBSCRIBE Bodies

SIP entities compliant with this specification MUST support the content type 'application/simple-filter+xml'.

3.3. Subscriber Generating of SUBSCRIBE Requests

This section presents additional functionality required from the subscriber when filters are used in the bodies of the SUBSCRIBE requests. Normal operations of services (e.g., as defined in [8], [10], and [4]) are otherwise followed.

As defined in [3], the SUBSCRIBE message MAY contain a body. This body would carry filtering information. Honouring those filters is at the discretion of the notifier and might depend on local policies.

No content in the body of a SUBSCRIBE indicates to the notifier that no filter is being requested, so the notifier is instructed to send all the NOTIFY requests using the notifier's own or service-specific policy. Note that, for example, in the list case [4], the filter might have been uploaded to the server beforehand (by means outside the scope of this document).

If the body of the SUBSCRIBE includes the filter, the body MUST be of the MIME-Type 'application/simple-filter+xml'.

3.3.1. Defining the Filtering Rules

Multiple filters MAY be included in one SUBSCRIBE. This is achieved by including multiple <filter> elements in the filter [5]. Each <filter> element may include a 'uri' attribute.

A SUBSCRIBE request destined to a list URI [4] MAY include multiple filters specific to individual resources. This is achieved by including multiple <filter> elements with different URIs of resources in each of those elements. This resource specific resource-specific filter are processed first before any list specific list-specific filter, if any. The list specific list-specific filter may or may not include a URI.

Furthermore, regardless of whether the SUBSCRIBE is destined to a list URI, there can only be one filter applicable to a single resource or domain within a single SUBSCRIBE. That is, each filter within a subscription MUST uniquely identify one resource or one domain.

A filter can be enabled and disabled using the 'enabled' attribute in the <filter> element, as described in [5].

3.3.2. Request-URI vs. Filter URI

The URI in the filter defines the target resource. For example, in the Presence service case, it is the presentity's presence information to which the filter is applied. The subscriber MAY choose to leave the URI in the filter undefined. If the URI is not defined within the filter, the filter applies to the resource identified in the Request-URI. Similarly, the subscriber MAY define a filter URI. If the Request-URI is a list URI [4], the filter URI MUST be the list URI, a sub-list URI, or resource whose URI is one of the URIs that result from a lookup, by a Resource List Server (RLS), on the Request-URI. If it is not, the filter may be ignored or may be rejected. URI matching is done according to the matching rules defined for a particular scheme (SIP URI matching rules are defined in RFC 3261 [2]).

A filter may also be addressed to a domain using the 'domain' attribute instead of the 'uri' attribute. In this case, the filter applies to resources in that domain. This can be used when a subscription is for a resource that is an event list with many resources from differing domains. If an individual resource-specific filter is present along with the domain filter, this resource-specific filter overrides any domain-specific filter, if any.

3.3.3. Changing Filters within a Dialog

The subscriber MAY reset or change the filter by re-issuing a new SUBSCRIBE request within the existing dialog. A SUBSCRIBE within the exiting dialog that does not contain a filter is assumed to maintain existing filters. This means that filters are persistent within a dialog and are only explicitly removed.

A subscriber requiring removal of a filter may do so by using the 'remove="true"' attribute, as defined in [5].

In the case where the URI in the filter is that of a list, a subscriber may override the existing filter with a filter for an individual resource that is part of the list subscribed to earlier by issuing a new SUBSCRIBE within the existing dialog and including a filter, specific for that individual resource, using a new filter ID. The new filter need not include the original filter since a filter is only removed in the manner indicated above.

A filter is replaced by the subscriber re-issuing the filter using the same filter ID and replacing the contents of the filter. Replacing a filter by changing the filter ID and keeping the resource URI is considered an error since this causes the server to assume that two filters are placed for the same resource.

Again, a filter can be disabled and re-enabled using the 'enabled' attribute in the <filter> element, as described in [5].

3.3.4. Subscriber Interpreting of SIP Responses

The SUBSCRIBE request will be confirmed with a final response. A 200-class response indicates that the subscription has been accepted and that a NOTIFY will be sent immediately. A "200" response indicates that the subscription has been accepted and that the filter is accepted. A "202" response merely indicates that the subscription has been understood, that the content type has been accepted, and that authorization may or may not have been granted. A "202" response also indicates that the filter has not been accepted yet. The acceptance of the filter MAY arrive in a subsequent NOTIFY.

A non-200 class final response indicates that no subscription or dialog has been created, and no subsequent NOTIFY message will be sent. All non-200 class final responses have the same meanings and handling as described in [2] and [3].

Specifically, a "415" response indicates that the MIME type 'application/simple-filter+xml' is not understood by the notifier. A "488" response indicates that the content type (filter) is understood but some aspects of it were either not understood or not accepted.

3.4. Subscriber Processing of NOTIFY Requests

If the 2xx response was returned for the SUBSCRIBE, the NOTIFY that follows MAY contain a body that describes the present state of the resource after the filters have been applied.

If the NOTIFY indicates that a subscription has been terminated [3], the subscription is assumed to be terminated. Behaviour in such events is also described in [3].

If the subscription is indicated as active, NOTIFY requests are handled as described in package-specific documents and in [3].

4. Resource List Server Behaviour

The Resource List Server is defined in [4]. This section describes how such an entity behaves in the presence of a filter in a subscription to a list.

4.1. Request-URI vs. Filter URI

If the URI is not defined within the filter, the filter applies to the resource list identified in the Request-URI of the SUBSCRIBE request. This results in the filters being applied to all the notifications that the RLS issues to this subscription. The same processing applies to a filter that defines a URI that matches the request-URI of the SUBSCRIBE request. That is, the filter applies to all notifications that the RLS issues to this subscription.

If the URI indicated by the filter is for one resource whose URI is one of the URIs that result from a lookup by the RLS on the Request-URI, the filter for that particular resource is extracted and propagated in the SUBSCRIBE request sent to that resource. It is possible to have more than one filter in a SUBSCRIBE request body, and therefore a filter specific to a resource MUST be extracted and only that one is propagated. For example, if the Request-URI in a SUBSCRIBE has the value "sip:mybuddies@example.com", where "bob@example.com" is a resource belonging to that list, and the URI in a filter is "sip:bob@example.com", the filter specific for Bob is extracted and placed in the body of the SUBSCRIBE sent to "bob@example.com".

If the URI indicated by the filter is for one resource whose URI is NOT under the RLS administrative control, the RLS propagates the filter to all the fanned out subscriptions. This is to accommodate the scenario where the subscriber knows that there are sub-lists in the event list that are under a different administrative domain from that where the original subscription was sent, and the subscriber wishes to set a filter for a resource in that sub-list.

If the URI indicated by the filter is for one resource whose URI is under the RLS administrative control but is not part of the resource list that the subscription was addressed to, the filter is not propagated. In this case, it is the RLS's responsibility to make sure that this filter is applied to notifications issued, if information about that resource is present.

For example: If we have 2 lists, each located on its own RLS: 

   List1 (list1@example.com) on RLS1 has: bob@example.com
   
   list2@biloxi.com
   
   List2 on RLS2 has: alice@biloxi.com sarah@example.com
   (Note: list2 is a resource in list1)

RLS1 receives the following SUBSCRIBE request (the SUBSCRIBE is addressed to list1 and contains 2 filters: one for sarah@example.com and the other for alice@biloxi.com): 

   SUBSCRIBE sip:List1@example.com SIP/2.0
   ...
   <?xml version="1.0" encoding="UTF-8"?>
   <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
     <ns-bindings>
       <ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
     </ns-bindings>
     <filter id="999" uri="sip:sarah@example.com">
       <what>
         <include type="namespace">
           urn:ietf:params:xml:ns:pidf</include>
         <exclude>
           //pidf:tuple/pidf:note</exclude>
       </what>
     </filter>
     <filter id="8439" uri="sip:alice@biloxi.com">
       <what>
         <include>
           //pidf:tuple/pidf:status/pidf:basic</include>
       </what>
     </filter>
   </filter-set>

RLS1 fans out subscriptions to resources on list1. The text above suggests that if a filter is destined to a resource that is not part of the list and is outside the administrative domain of an RLS, then that filter is propagated. The rest are consumed. In our example, only the filter to alice@biloxi.com is propagated since biloxi.com is not under the administrative domain of RLS1. The filter to sarah@example.com is consumed, and RLS1 needs to apply that filter to notifications it receives.

URI matching is done according to the matching rules defined for a particular scheme (SIP URI matching rules are defined in RFC 3261 [2]).

A filter may also be addressed to a domain using the 'domain' attribute instead of the 'uri' attribute. In this case, the filter applies to resources in that domain, and the RLS MUST NOT apply filters to any notifications it sends. Instead, it MUST forward the filter with all fanned-out subscriptions to the notifiers.

As indicated in Section 3.3.1, multiple filters can be present in a SUBSCRIBE request. Filters can also be added or modified as indicated in Section 3.3.3. In such circumstances, an RLS MUST check that there are no filters addressed to the same resource or domain, and if there are, it MUST reject the SUBSCRIBE request with a "488" error response.

4.2. Changing Filters within a Dialog

If an RLS receives a subscription refresh request with no filters specified (empty payload), the RLS assumes that the client does not wish to update the filters. If an RLS receives a subscription refresh with a filter containing the 'remove="true"' attribute, as defined in [5], the RLS assumes that the client is removing that filter identified by the filter ID.

If an RLS receives a subscription refresh request with a filter that already exists (i.e., having the same filter ID), the RLS interprets it as a replacement of the existing filter. Replacing a filter by changing the filter ID and keeping the resource URI is considered an error since this causes the RLS to assume that two filters are in place for the same resource.

A filter can be disabled and re-enabled using the 'enabled' attribute in the <filter> element, as described in [5].

5. Server Operation

5.1. NOTIFY Bodies

SIP entities compliant with this specification MUST support content-type 'application/simple-filter+xml'.

5.2. Notifier Processing of SUBSCRIBE Requests

This section presents additional functionality required from the notifier when filters are used in the bodies of the SUBSCRIBE requests. Normal package-specific functionality is otherwise followed.

The notifier will examine the Content-Type header field and will return a 415 response if it does not understand the content type 'application/simple-filter+xml'.

A 200-class response indicates that the subscription has been accepted, and the NOTIFY will be sent immediately. A "200" response indicates that the subscription has been accepted, the user is authorized, and the filter is accepted. A "202" response merely indicates that the subscription has been understood, but that the authorization may or may not have been granted. A "202" response also indicates that the filters have not been accepted yet. The acceptance of the filters MAY arrive in a subsequent NOTIFY.

Procedures described in Section 5.4 are followed if an error is encountered.

As indicated in Section 3.3.1, multiple filters can be present in a SUBSCRIBE request. Filters can also be added or modified as indicated in Section 3.3.3. In such circumstances, a server MUST check that there are no filters addressed to the same resource or domain, and if they are, it MUST reject the SUBSCRIBE request with a "488" error response.

5.2.1. Request-URI vs. Filter URI

The subscriber may have chosen to leave the URI in the filter undefined. If the URI is not defined within the filter, the filter applies to the resource identified in the Request-URI.

Similarly, the subscriber may have chosen to include a URI in the filter. In this case, the filter applies to all notifications sent with content associated with the resource with that URI for this subscription. If the Request-URI and the URI in the filter do not match, the filter may be ignored or rejected. URI matching is done according to the matching rules defined for a particular scheme (SIP URI matching rules are defined in RFC 3261 [2]).

A filter may also be addressed to a domain using the 'domain' attribute instead of the 'uri' attribute. In this case, the filter applies to resources in that domain. A notifier MUST ignore any filter using a 'domain' attribute containing a domain for which this notifier is not responsible. The notifier MUST NOT apply such a filter to any notification it sends. Notifiers belonging to the domain MUST apply the filter to all notifications it sends for that subscription, unless policy dictates otherwise.

5.2.2. Changing Filters within a Dialog

If a server receives a subscription refresh request with no filters specified (empty payload), it assumes that the client does not wish to update the filters. If it receives a subscription refresh with a filter containing the 'remove="true"' attribute, as defined in [5], the server assumes that the client is removing the filter identified by the filter ID.

If the server receives a subscription refresh request with a filter that already exists (i.e., having the same filter ID), it interprets it as a replacement of the existing filter. Replacing a filter by changing the filter ID and keeping the resource URI is considered an error since this causes the server to assume that two filters are placed for the same resource.

5.3. Notifier Generating of NOTIFY Requests

Upon receiving the SUBSCRIBE with the filter, the notifier SHOULD retain the filter as long as the subscription persists. The filter MAY be incorporated within an existing subscription (in an active dialog) by sending a re-SUBSCRIBE that includes the filter in the body.

If the response sent to the SUBSCRIBE was a "202" and the "202" was chosen because the filter could not be accepted that time, the NOTIFY MAY be used to terminate the subscription if the filter is found unacceptable.

As described in [3], the NOTIFY message MAY contain a body that describes the state of the resource. This body is in one of the formats listed in the Accept header field of the SUBSCRIBE, or in the package-specific default if the Accept header field is omitted.

Based on the contents of a filter, the following processing occurs:

  • A filter with only a <what> element will result in sending the requested resource state information in that <what> element whenever there is a change in the resource state.
  • A filter with only a <trigger> element will result in sending all resource state information whenever there is a change in the resource state that matches the triggers.
  • A filter with <what> and <trigger> elements will result in sending the requested resource state information in that <what> element whenever there is a change in the resource state that matches the triggers.

When a filter is disabled (by setting the 'enabled' attribute to "false"), it means the same thing as the absence of that filter. That is, all state and state changes are reported by issuing a notification to the subscriber (assuming there are no other filters).

When a filter is re-enabled (by setting the 'enabled' attribute to "true" or by omitting the 'enabled' attribute), the notifier behaves as if the filter has just been placed by the SUBSCRIBE request enabling it. Immediate NOTIFY rules, as stated in Section 5.3.1, apply.

5.3.1. Generation of NOTIFY Contents

If the NOTIFY being sent is the one sent immediately after a 2xx response to the original SUBSCRIBE, its contents MUST be populated according to the filter <what> element, unless the processing of the filters will take too long or the NOTIFY request is following a "202" response to the SUBSCRIBE request and is terminating the subscription. In the case that the filter is taking too long to process, the NOTIFY request being sent may be empty or may be populated with a pre-configured value as authorised to that subscriber. If applying the filter results in no content to be delivered, the NOTIFY MUST be sent with empty contents. If the filter contains <trigger> elements, the notifier ignores the trigger values when generating the first NOTIFY request. 

   The input to the content filter is a package-specific XML document
   (e.g., [7] and [9]) derived according to the package-specific
   specifications, (e.g., [8] and [10]).

The content is filtered according to the expressions in the <what> element of the filter. The expression indicates the delivered XML elements and/or attributes. Prefixes of the namespaces of the items of the XML document to be filtered must be expanded before applying the filter to the items.

The expression directly states the XML elements and attributes to be delivered in the NOTIFY, along with their values. In addition to the selected contents, the namespaces of all the selected items are also included in the NOTIFY. The XML elements and/or attributes indicated by the expression in the <what> element must be items that the subscriber is authorised to see. If they are not, the notifier policy dictates the behaviour of the notifier (which can ignore the filter, parts of the filter, or reject the filter completely). Implementers need to carefully consider such an implementation decision; the subscriber may not be aware of the authorised contents and therefore most likely will include a filter requesting unauthorised contents. It is therefore RECOMMENDED that notifiers just ignore the parts of the filter that are requesting unauthorised info (i.e., the filter in the <filter> element where the unauthorised contents are requested is ignored). If polite blocking is used by the notifier, the notifier may choose to deliver notifications containing bogus information in the unauthorised elements or attributes and applying the filter afterwards.

The resultant XML document MUST be well formed and valid according to the XML schema. This means that all mandatory elements and attributes, along with their values, MUST be included in the XML document regardless of the expression. In other words, if the result of applying a filter on an XML document is a non-valid XML document, the notifier MUST add elements and attributes, along with their values, from the original XML document into the newly formulated one in order for it to be valid.

5.3.2. Handling of Notification Triggering Rules

There can be several <trigger> elements inside one <filter> element. If the criteria for any of the <trigger> elements are satisfied, a NOTIFY SHOULD be generated.

The items (XML elements and/or attributes) indicated by the expression in the <changed> element, <added> element, or <removed> element must be items that the subscriber is authorised to access. If they are not, the notifier policy dictates the behaviour of the notifier (which can ignore the filter, parts of the filter, or reject the filter completely).

5.4. Handling Abnormal Cases

In case of an invalid filter definition where the XML document of the filter is not aligned with the XML schema of the filter format [5], the notifier rejects the SUBSCRIBE request with a "488" response. A Warning header field in the response may give a better indication as to why the filters were not accepted. If the subscription was accepted with a "202" response but the invalid filter was discovered after that, a NOTIFY with a subscription-state of value 'terminated' is sent. An event-reason-value "badfilter", introduced here, of subexp-params [3] MAY be included.

In case of an erroneous expression in the filter definition, the notifier either ignores the filter definition or terminates the subscription.

If a <what> or <trigger> element is empty, the notifier proceeds as if the element did not exist.

6. XML Document Validation

The subscriber of the filter MUST ensure that the XML document inserted as the SUBSCRIBE request body is well formed and valid. The subscriber MUST NOT insert any extension elements or attributes into the XML document unless it has access to the extension schema and can validate the XML document. The XML document notifier MAY validate the XML document according to the schemas, including extension schemas, to which it has access that are applicable to this XML document.

7. Examples

The following sections include filtering examples for Presence and Watcher Information. The format of filter is according to [5].

7.1. Presence Specific Examples

This section describes three use cases where the presence information filtering solution is utilised [8]. In the first use case, the watcher is interested in getting messaging-specific information of a certain presentity. In the second use case, the watcher is interested in getting information about the communication means and contact addresses on which the presentity is currently available for communication. The third case shows how a presentity can request triggers to receive notifications.

Below is the presentity's presence information in PIDF [7]. It includes two tuples: one for the instant messaging and another for the voice-related information.

<?xml version="1.0" encoding="UTF-8"?>

<presence xmlns="urn:ietf:params:xml:ns:pidf" 

                           xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
                           entity="sip:presentity@example.com">
            <tuple id="432sd">
               <status>
                  <basic>closed</basic>
               </status>
               <rpid:class>IM</rpid:class>
               <contact>im:presentity@example.com</contact>
            </tuple>
            <tuple id="thr76jk">
               <status>
                  <basic>open</basic>
               </status>
               <rpid:class>voice</rpid:class>
               <contact>tel:2224055555@example.com</contact>
            </tuple>
         </presence>

7.1.1. Subscriber Requests Messaging-Related Information

The subscriber initiates a subscription to the presentity's messaging (MMS, IM, and SMS) related presence information. The subscription includes the content limiting filter.

The filtered content is indicated with an expression. This expression selects the <basic> element and all the parent elements (i.e., the <status>, the <tuple>, and its root element), the <class> element, and the <contact> element. The filter matches if the <class> element contains "MMS", "SMS", or "IM".

In this case, the notification includes the contents of the tuple that has the value "IM" in its <class> element.

SUBSCRIBE request from the subscriber including filter: 

   SUBSCRIBE sip:presentity@example.com
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>
   From: <sip:watcher@example.com>;tag:12341111
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 3600
   Event: Presence
   Contact: <sip:watcher@client.example.com>
   Content-Type: application/simple-filter+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter"> 

     <ns-bindings>
       <ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
       <ns-binding prefix="rpid"
                          urn="urn:ietf:params:xml:ns:pidf:rpid"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <what>
         <include type="xpath">
           //pidf:tuple[rpid:class="IM" or rpid:class="SMS"
           or rpid:class="MMS"]/pidf:status/pidf:basic
       </include>
       <include type="xpath">
         //pidf:tuple[rpid:class="IM" or rpid:class="SMS"
         or rpid:class="MMS"]/rpid:class
   
       </include>
       <include type="xpath">
         //pidf:tuple[rpid:class="IM" or rpid:class="SMS"
         or rpid:class="MMS"]/pidf:contact
       </include>
       </what>
     </filter>
   </filter-set>

Notification to the subscriber: 

   NOTIFY sip:watcher@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: <sip:watcher@example.com>;tag:12341111
   From: <sip:presentity@example.com>;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Event: Presence
   Subscription-State: active; expires=3599
   Contact: sip:presentity@server.example.com
   Content-Type: application/pidf+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> 

      <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
      entity="sip:presentity@example.com">
         <tuple id="432sd">
            <status>
               <basic>closed</basic>
            </status>
            <rpid:class>IM</rpid:class>
            <contact>im:presentity@example.com</contact>
         </tuple>
      </presence>

7.1.2. Subscriber Fetches Information about "Open" Communication Means

The subscriber makes a subscription to the presentity's available communication means. The subscription includes the content-limiting filter.

The filtered content is indicated with an expression. This expression selects the <basic> element and all the parent elements (i.e., the <status>, the <tuple>, and its root element), the <class> element, and the <contact> element. The filter matches if the <basic> element's value is "open".

In this case, the notification returns the contents of the tuple that has the value "open" inside the <status> element.

SUBSCRIBE request from the subscriber including filter: 

   SUBSCRIBE sip:presentity@example.com SIP/2.0
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>
   From: <sip:watcher@example.com>;tag:12341111
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 3600
   Event: Presence
   Contact: <sip:watcher@client.example.com>
   Content-Type: application/simple-filter+xml
   Content-Length: ...
   
   <?xml version="1.0" encoding="UTF-8"?>
   <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
     <ns-bindings>
       <ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
       <ns-binding prefix="rpid"
                          urn="urn:ietf:params:xml:ns:pidf:rpid"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <what>
         <include type="xpath">
           //pidf:tuple/pidf:status[pidf:basic="open"]/pidf:basic
         </include>
         <include type="xpath">
           //pidf:tuple[pidf:status/pidf:basic="open"]/rpid:class
         </include>
         <include type="xpath">
           //pidf:tuple[pidf:status/pidf:basic="open"]/pidf:contact
         </include>
       </what>
     </filter>
   </filter-set>

Notification to the subscriber: 

   NOTIFY sip:watcher@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: <sip:watcher@example.com>;tag:12341111
   From: <sip:presentity@example.com>;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Event: Presence
   Subscription-State: active; expires=3599
   Contact: sip:presentity@server.example.com
   Content-Type: application/pidf+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> 

      <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
      entity="sip:presentity@example.com">
         <tuple id="thr76jk">
            <status>
               <basic>open</basic>
            </status>
               <rpid:class>voice</rpid:class>
               <contact>tel:2224055555@example.com</contact>
         </tuple>
      </presence>

7.1.3. Subscriber Requests Notifications When Presentity's Status

Changes

The subscriber subscribes to the presentity, specifying in the filter that it wants notifications only when the <basic> element has changed to value "open".

SUBSCRIBE request from the subscriber including filter: 

   SUBSCRIBE sip:presentity@example.com
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>
   From: <sip:watcher@example.com>;tag:12341111
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 3600
   Event: Presence
   Contact: <sip:watcher@client.example.com>
   Content-Type: application/simple-filter+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter"> 

     <ns-bindings>
       <ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <trigger>
         <changed from="closed" to="open">
           /pidf:presence/pidf:tuple/pidf:status/pidf:basic
         </changed>
       </trigger>
     </filter>
   </filter-set>

At some point during the subscription, a second PIDF document is created with both tuples having a status of "closed":

<?xml version="1.0" encoding="UTF-8"?> 

      <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
      entity="sip:presentity@example.com">
         <tuple id="432sd">
            <status>
              <basic>closed</basic>
            </status>
               <rpid:class>IM</rpid:class>
               <contact>im:presentity@example.com</contact>
         </tuple>
         <tuple id="thr76jk">
            <status>
               <basic>closed</basic>
            </status>
            <rpid:class>voice</rpid:class>
            <contact>tel:2224055555@example.com</contact>
         </tuple>
      </presence>

A NOTIFY is not sent to the subscriber in this case.

Now, a third PIDF document is created when the IM status changes to "open":

<?xml version="1.0" encoding="UTF-8"?>

<presence xmlns="urn:ietf:params:xml:ns:pidf" xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid" entity="sip:presentity@example.com"> 

         <tuple id="432sd">
            <status>
               <basic>open</basic>
            </status>
            <rpid:class>IM</rpid:class>
            <contact>im:presentity@example.com</contact>
         </tuple>
         <tuple id="thr76jk">
            <status>
               <basic>closed</basic>
            </status>
            <rpid:class>voice</rpid:class>
            <contact>tel:2224055555@example.com</contact>
         </tuple>
      </presence>

Notification containing both tuples is sent to the subscriber in this case: 

   NOTIFY sip:watcher@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: <sip:watcher@example.com>;tag:12341111
   From: <sip:presentity@example.com>;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Event: Presence
   Subscription-State: active; expires=3599
   Contact: sip:presentity@server.example.com
   Content-Type: application/pidf+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> 

      <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
      entity="sip:presentity@example.com">
         <tuple id="432sd">
            <status>
               <basic>closed</basic>
            </status>
            <rpid:class>IM</rpid:class>
            <contact>im:presentity@example.com</contact>
         </tuple>
         <tuple id="thr76jk">
            <status>
               <basic>open</basic>
            </status>
            <rpid:class>voice</rpid:class>
            <contact>tel:2224055555@example.com</contact>
         </tuple>
      </presence>

7.2. Watcher Information Specific Examples

The examples in this section use the winfo template-package with the presence event package [10].

Watcher information to a Presentity: 

      <?xml version="1.0"?>
        <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
      version="0" state="full">
        <watcher-list resource="sip:presentity@example.com"
                      package="presence">
            <watcher status="active"
               id="sr8fdsj"
               duration-subscribed="509"
               expiration="20"
               event="approved">sip:watcherA@example.com"</watcher>
            <watcher status="pending"
               id="sr8fdsj"
               duration-subscribed="501"
               expiration="100"
               event="subscribe">sip:watcherB@example.com"</watcher>
            <watcher status="terminated"
               id="sr8fdsj"
               duration-subscribed="500"
               expiration="0"
               event="rejected">sip:watcherC@example.com"</watcher>
            <watcher status="active"
               id="sr8fdsj"
               duration-subscribed="20"
               expiration="30"
               event="approved">sip:watcherD@example.com"</watcher>
        </watcher-list>
        </watcherinfo>

7.2.1. Watcher Subscriber Makes Subscription to Get All the Information

about Active Watchers

SUBSCRIBE request from the presentity including the filter: 

   SUBSCRIBE sip:presentity@example.com
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>
   From: <sip:presentity@example.com>;tag:12341111
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 3600
   Event: Presence.winfo
   Contact: sip:presentity@client.example.com
   Content-Type: application/simple-filter+xml
   Content-Length: ...
   
   <?xml version="1.0" encoding="UTF-8"?>
   <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
     <ns-bindings>
       <ns-binding prefix="wi"
                          urn="urn:ietf:params:xml:ns:watcherinfo"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <what>
         <include>
           /wi:watcherinfo/wi:watcher-list[@package="presence"]/
           wi:watcher[@status="active"]
         </include>
   </what>
   </filter>
   </filter-set>

Notification to the subscriber: 

   NOTIFY sip:presentity@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: sip:presentity@example.com;tag:12341111
   From: sip:presentity@example.com;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Contact: sip:presentity@server.example.com
   Event: Presence.winfo
   
   Content-Type: application/watcherinfo+xml
   Content-Length: ...
   <?xml version="1.0"?>
     <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
   version="0" state="full">
     <watcher-list resource="sip:presentity@example.com"
                   package="presence">
         <watcher status="active"
            id="sr8fdsj"
            duration-subscribed="509"
            expiration="20"
            event="approved">sip:watcherA@example.com"</watcher>
         <watcher status="active"
            id="sr8fdsj"
            duration-subscribed="20"
            expiration="30"
            event="approved">sip:watcherD@example.com"</watcher>
     </watcher-list>
     </watcherinfo>

7.2.2. Watcher Subscriber Requests Information of Watchers with

Specific Subscription Duration Conditions

SUBSCRIBE request from the presentity including the filter: 

   SUBSCRIBE sip:presentity@example.com
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>;tag:12341111
   From: <sip:presentity@example.com>
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 0
   Event: Presence.winfo
   Contact: <sip:presentity@client.example.com>
   Content-Type: application/simple-filter+xml
   Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?> <filter-set xmlns="urn:ietf:params:xml:ns:simple-filter"> 

     <ns-bindings>
       <ns-binding prefix="wi"
                          urn="urn:ietf:params:xml:ns:watcherinfo"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <what>
         <include>
           /wi:watcherinfo/wi:watcher-list[@package="presence"]/
           wi:watcher[@duration-subscribed>500]
         </include>
       </what>
     </filter>
   </filter-set>

Notification to the subscriber: 

   NOTIFY sip:presentity@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: sip:presentity@example.com;tag:12341111
   From: sip:presentity@example.com;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Contact: sip:presentity@server.example.com
   Event: Presence.winfo
   
   Content-Type: application/watcherinfo+xml
   Content-Length: ...
   
   <?xml version="1.0"?>
     <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
   version="0" state="full">
     <watcher-list resource="sip:presentity@example.com"
                   package="presence">
         <watcher status="active"
            id="sr8fdsj"
            duration-subscribed="509"
            expiration="20"
            event="approved">sip:watcherA@example.com"</watcher>
         <watcher status="pending"
            id="sr8fdsj"
            duration-subscribed="501"
            expiration="100"
            event="subscribe">sip:watcherB@example.com"</watcher>
     </watcher-list>
     </watcherinfo>

7.2.3. Watcher Subscriber Requests Specific Watcher Info on Specific

Triggers

This filter selects watcher information notifications [9] to be sent when the pending subscription status has changed from "pending" to "terminated". In the notification, only the watchers that have a status of "terminated" and an event of "rejected" are included.

SUBSCRIBE request from the Watcher Subscriber including the filter: 

   SUBSCRIBE sip:presentity@example.com
   Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
   To: <sip:presentity@example.com>;tag:12341111
   From: <sip:presentity@example.com>
   Call-ID: 32432udfidfjmk342
   Cseq: 1 SUBSCRIBE
   Expires: 0
   Event: Presence.winfo
   Contact: <sip:presentity@client.example.com>
   Content-Type: application/simple-filter+xml
   Content-Length: ...
   
   <?xml version="1.0" encoding="UTF-8"?>
   <filter-set xmlns="urn:ietf:params:xml:ns:simple-winfo-filter">
     <ns-bindings>
       <ns-binding prefix="wi"
                          urn="urn:ietf:params:xml:ns:watcherinfo"/>
     </ns-bindings>
     <filter id="123" uri="sip:presentity@example.com">
       <what>
         <include>
           /wi:watcherinfo/wi:watcher-list[@package="presence"]/
           wi:watcher[@status="terminated" and @event="rejected"]
         </include>
       </what>
       <trigger>
         <changed from="pending"
                                             to="terminated">
           //@status
         </changed>
       </trigger>
     </filter>
   </filter-set>

At some point during the subscription, a second Winfo document is created due to some change:

<?xml version="1.0"?> 

        <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
      version="0" state="full">
        <watcher-list resource="sip:presentity@example.com"
                      package="presence">
            <watcher status="active"
               id="sr8fdsj"
               duration-subscribed="509"
               expiration="20"
               event="approved">sip:watcherA@example.com"</watcher>
            <watcher status="terminated"
               id="sr8fdsj"
               duration-subscribed="501"
               expiration="100"
               event="rejected">sip:watcherB@example.com"</watcher>
            <watcher status="terminated"
               id="sr8fdsj"
               duration-subscribed="500"
               expiration="0"
               event="rejected">sip:watcherC@example.com"</watcher>
            <watcher status="active"
               id="sr8fdsj"
               duration-subscribed="20"
               expiration="30"
               event="approved">sip:watcherD@example.com"</watcher>
       </watcher-list>
        </watcherinfo>

Notification to the subscriber is created, taking into account the <trigger> and <what> elements: 

   NOTIFY sip:presentity@client.example.com SIP/2.0
   Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
   To: sip:presentity@example.com;tag:12341111
   From: sip:presentity@example.com;tag:232321
   Call-ID: 32432udfidfjmk342
   Cseq: 1 NOTIFY
   Contact: sip:presentity@server.example.com
   Event: Presence.winfo
   
   Content-Type: application/watcherinfo+xml
   Content-Length: ...
   
   <?xml version="1.0"?>
     <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
   version="0" state="full">
     <watcher-list resource="sip:presentity@example.com"
                   package="presence">
         <watcher status="terminated"
            id="sr8fdsj"
            duration-subscribed="501"
            expiration="100"
            event="rejected">sip:watcherB@example.com"</watcher>
         <watcher status="terminated"
            id="sr8fdsj"
            duration-subscribed="500"
            expiration="0"
            event="rejected">sip:watcherC@example.com"</watcher>
     </watcher-list>
     </watcherinfo>

8. Security Considerations

The presence of filters in the body of a SIP message has a significant effect on the ways in which the request is handled at a server. As a result, it is especially important that messages containing this extension be authenticated and authorized. Authentication can be achieved using the Digest Authentication mechanism described in [2]. The authorisation decision is based on the permissions that the resource (notifier) has given to the watcher. An example of such auhorisation policy can be found in [11].

Processing of requests and looking up filters requires set operations and searches, which can require some amount of computation. This enables a DoS attack whereby a user can send requests with substantial numbers of messages with large contents, in the hopes of overloading the server. To counter this, the server can establish a limit on the number of occurrences of the <what>, <changed>, <added>, and <removed> elements that are allowed in the filters. A default limit of 40 is RECOMMENDED; however, servers may raise or lower the limit depending upon their specific engineered capacity.

Requests can reveal sensitive information about a User Agent's (UA's) capabilities. If this information is sensitive, it SHOULD be encrypted using SIP S/MIME capabilities [6]. All package-specific security measures MUST be followed.

Propagating filters in SUBSCRIBE requests to foreign domains reveals sensitive information about a user's resource lists. It is therefore required that an RLS does not forward a filter if that filter is addressed to a resource that is under the administrative domain of the RLS, but that is not on the resource list. Section 4.1 shows an example where such a scenario can occur.

Note that a filtered document located at a subscriber may project false reality. For example, if a subscriber asked to be notified when a resource has changed his presence state from "closed" to "open" but not from "open" to "closed", then the subscriber may afterwards be under the false impression that the resource's presence state is "open", even long after the resource has changed it to "closed". Therefore, subscribers need to be sure what they put in a filter, understand what they asked for, and be prepared to be out of sync with the real state of a resource.

9. IANA Considerations

A new event-reason-value "badfilter" is defined to represent the event where the filter is not well formed and/or not accepted. No IANA registration is required for this value.

10. Acknowledgements

The authors would like to thank George Foti, Tim Moran, Sreenivas Addagatla, Juha Kalliokulju, Jari Urpalainen, and Mary Barnes for their valuable input.

11. References

11.1. Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.
   
   [2]  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.
   
   [3]  Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.
   
   [4]  Roach, A.B., Campbell, B., and J. Rosenberg, "A Session
        Initiation Protocol (SIP) Event Notification Extension for
        Resource Lists", RFC 4663, September 2006.
   
   [5]  Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-Requena,
        "An Extensible Markup Language (XML)-Based Format for Event
        Notification Filtering", RFC 4661, September 2006.
   
   [6]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
        (S/MIME) Version 3.1 Message Specification", RFC 3851, July
        2004.

11.2. Informative References

   [7]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
        J. Peterson, "Presence Information Data Format (PIDF)", RFC
        3863, August 2004.
   
   [8]  Rosenberg, J., "A Presence Event Package for the Session
        Initiation Protocol (SIP)", RFC 3856, August 2004.
   
   [9]  Rosenberg, J., "An Extensible Markup Language (XML) Based Format
        for Watcher Information", RFC 3858, August 2004.
   
   [10] Rosenberg, J., "A Watcher Information Event Template-Package for
        the Session Initiation Protocol (SIP)", RFC 3857, August 2004.
   
   [11] Rosenberg, J., "Presence Authorization Rules", Work in Progress,
        June 2006.

Authors' Addresses

Hisham Khartabil
Telio
P.O. Box 1203 Vika
Oslo
Norway 

   Phone: +47 2167 3544
   EMail: hisham.khartabil@telio.no

Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland 

   Phone: +358 7180 77066
   EMail: eva-maria.leppanen@nokia.com

Mikko Lonnfors
Nokia
P.O BOX 321
Helsinki
Finland 

   Phone: + 358 71800 8000
   EMail: mikko.lonnfors@nokia.com

Jose Costa-Requena
Nokia
P.O. Box 321
FIN-00045 NOKIA GROUP
FINLAND 

   Phone: +358 71800 8000
   EMail: jose.costa-requena@nokia.com

Full Copyright Statement

Copyright © The Internet Society (2006).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.

Acknowledgement

Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).