Optimizing Federated Presence with View Sharing
draft-ietf-simple-view-sharing-02

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Abstract

Presence federation refers to the exchange of presence information between systems. One of the primary challenges in presence federation is scale. With a large number of watchers in one domain obtaining presence for many presentities in another, the amount of notification traffic is large. This document describes an extension to the Session Initiation Protocol (SIP) event framework, called view sharing. View sharing can substantially reduce the amount of traffic, but requires a certain level of trust between domains. View sharing allows the amount of presence traffic between domains to achieve the theoretical lower bound on information exchange in any presence system.

Table of Contents

1.  Introduction
2.  Overview of Operation
3.  RLS Behavior
    3.1.  On Receipt of a Resource List Subscription Request
        3.1.1.  Find a Matching Back-End Subscription
        3.1.2.  Generating a Back-End Subscription
    3.2.  Processing NOTIFY Requests
        3.2.1.  Processing ACL-Infos
        3.2.2.  Processing State Documents
        3.2.3.  Processing Back-End Terminations
4.  Notifier Behavior
    4.1.  Authentication and Authorization
    4.2.  Processing Initial SUBSCRIBE Requests
    4.3.  SUBSCRIBE Refreshes
    4.4.  Policy Changes
    4.5.  Event State Changes
5.  ACL Format
    5.1.  Document Structure and Semantics
    5.2.  Trust Considerations when Construcing ACLs
    5.3.  Example Documents
    5.4.  Rule Determination Algorithm
    5.5.  XML Schema
6.  Performance Analysis
7.  Requirements Analysis
8.  Security Considerations
    8.1.  Privacy Considerations of the Serving Domain
    8.2.  Privacy Considerations of the Watched Resource
    8.3.  Interactions with S/MIME
9.  IANA Considerations
    9.1.  MIME Type Registration
    9.2.  URN Sub-Namespace Registration
    9.3.  Schema Registration
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

Presence refers to the ability, willingness and desire to communicate across differing devices, mediums and services [RFC2778] (Day, M., Rosenberg, J., and H. Sugano, “A Model for Presence and Instant Messaging,” February 2000.). Presence is described using presence documents [RFC3863] (Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, “Presence Information Data Format (PIDF),” August 2004.) [RFC4479] (Rosenberg, J., “A Data Model for Presence,” July 2006.), exchanged using a SIP-based event package [RFC3856] (Rosenberg, J., “A Presence Event Package for the Session Initiation Protocol (SIP),” August 2004.).

Presence federation refers to the interconnection of disparate systems for the purposes of sharing presence information. This interconnection involves passing of subscriptions from one system to another, and then the passing of notifications in the opposite direction. Federation can be occur between different domains, where it is referred to as inter-domain federation. However, federation can also occur within a domain, where it is referred to as intra-domain federation [I‑D.ietf‑simple‑intradomain‑federation] (Rosenberg, J., Houri, A., Smyth, C., and F. Audet, “Models for Intra-Domain Presence and Instant Messaging (IM) Bridging,” July 2009.).

[I‑D.ietf‑simple‑interdomain‑scaling‑analysis] (Houri, A., Aoki, E., Parameswar, S., Rang, T., Singh, V., and H. Schulzrinne, “Presence Interdomain Scaling Analysis for SIP/SIMPLE,” August 2009.) has analyzed the amount of traffic, in terms of messages and in terms of bytes, which flow between systems in various federated use cases. These numbers demonstrate that presence traffic can be a substantial source of overhead. The root cause of this scale challenge is the so-called multiplicative effect of presence data. If there are N users, each of which have B buddies on their buddy list, and each buddy changes state L times per hour, the amount of notification traffic is proportional to N*B*L. For example, in the case of two extremely large public IM providers that federate with each other (each with 20 million users), [I‑D.ietf‑simple‑interdomain‑scaling‑analysis] (Houri, A., Aoki, E., Parameswar, S., Rang, T., Singh, V., and H. Schulzrinne, “Presence Interdomain Scaling Analysis for SIP/SIMPLE,” August 2009.) shows that the amount of traffic due to these steady state notifications is 18.4 billion messages per day, an astoundingly large number. Overhead for subscription maintenance and refreshes brings the total to 25.6 billion per day.

The overhead for SIP-based presence can be reduced using SIP optimizations. In particular, [I‑D.ietf‑sip‑subnot‑etags] (Niemi, A., “An Extension to Session Initiation Protocol (SIP) Events for Conditional Event Notification,” March 2009.) can reduce the amount of traffic due to refreshes and polls. However, this optimization targets the overhead, and doesn't address the core scaling problem - the multiplicative effect of presence data.

For this reason, there is a clear need to improve the scale of SIMPLE in federated envrionments. [I‑D.ietf‑sipping‑presence‑scaling‑requirements] (Houri, A., Parameswar, S., Aoki, E., Singh, V., and H. Schulzrinne, “Scaling Requirements for Presence in SIP/SIMPLE,” January 2009.) has laid out a set of requirements for optimizations. The essence of these requirements are that the extension should improve performance, while being backwards compatible and supporting the privacy and policy requirements of users.

This document defines a mechanism called view sharing in support of those requirements. The key idea with view sharing is that when there are many watchers in one system to a single presentity in another system, each of which is actually going to get the exact same presence document, the watcher's system extends a single subscription to the system of the presentity, and the system of the presentity sends a single copy of the presence document back to the system of the watcher. Consequently, a "view" is a particular sequence of presence documents that come about as a consequence of a particular composition, authorization and privacy policy. Two watchers which share the same view will always receive the same presence document when the state of the presentity changes.

Though this mechanism can be applied intra-domain as well as inter-domain, the specification considers only the inter-domain case. In addition, though the principal application of view sharing is for presence, it is a general extension to the SIP events framework and specified in that way.

In the case of a pair of large providers that are peering with each other, this mechanism can result in a significant savings. Assuming a symmetrical system whereby the average buddies per watcher is B and the average number of watchers for a user is also B, if most buddies are in one domain or the other, this optimization can reduce the overall subscription overhead and notification traffic by a factor of B/2. In cases where there are a large number of small domains, this mechanism is less useful. Of course, in such cases, the amount of traffic between any pair of domains is small anyway.

2.  Overview of Operation

The extension works in the environment shown in Figure 1 (Deployment Model). For explanatory purposes, the environment assumes two domains. There are some number of subscribers (W1 - W3) in the domain on the left, which we call the subscribing domain. All of those subscribers are interested in the state of a single resource P1 in the domain on the right, which we call the serving domain. The subscribers all make use of a resource list server (RLS) [RFC4662] (Roach, A., Campbell, B., and J. Rosenberg, “A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists,” August 2006.) which stores their resource lists and performs the list expansion. Consequently, when each subscriber subscribes to their resource list on the RLS, in absence of any optimizations, the RLS will generate three separate subscriptions to P1, each of which reaches the notifier in the serving domain.

                                     .
             +--------------+        .     +--------------+
             |              |        .     |              |
             |              |  SUB   .     |              |
             |              | -------.---> |              |
             |     RLS      |  NOT   .     |  Notifier    |
             |              | <------.---- |              |
             |              |        .     |              |
             |              |        .     |              |
             +--------------+        .     +--------------+
              ^      ^     ^         .            ^
       List   |      |     |         .            | PUB
       SUB    |      |     |         .            |
              |      |     |         .            |
          +----+  +----+  +----+     .          +----+
          |    |  |    |  |    |     .          |    |
          | W1 |  | W2 |  | W3 |     .          | P1 |
          |    |  |    |  |    |     .          |    |
          +----+  +----+  +----+     .          +----+
                                     .
                                     .
                                     .
             Subscribing             .        Serving
             Domain                  .        Domain
                                     .

Of course, in practice each domain will act as both a subscribing domain and a serving domain, thus implementing both sides of the system.

The initial SUBSCRIBE generated by the RLS includes a SIP option tag "view-share" in the Supported header field, indicating that the RLS supports the view sharing extension. If the notifier also supports the extension, it makes use of it and includes an indication of this fact in the Require header field in the SUBSCRIBE response and in NOTIFY requests it generates.

View sharing requires a level of trust between the two domains. Typically, TLS will be deployed between them, and the notifier uses it to determine if the subscribing domain is authorized.

If this is the first subscription from domain 1 for that particular resource, the notifier accepts the subscription (assuming the subscriber is authorized of course). The notifications sent to the RLS include two separate pieces of state. One is the actual state for the resource. The other is an Access Control List (ACL) document. This document describes the set of other subscribers from the originating domain, if any, who are authorized to see exactly the same document - in other words, the set of users that share the same view. Should one of those subscribers seek the state of that resource for the same event package, the RLS from the originating domain does not need to generate a back-end subscription; rather, it just uses the document it is receiving from the original subscription, and passes it to both subscribers. The ACL can also list users in the originating domain that are authorized to subscribe to that resource, but who will end up receiving a different view. Should one of those subscribers subscribe, the RLS knows that it must perform a back-end subscription to obtain that view. The ACL can also list subscribers in the originating domain that are not authorized at all, in which case the RLS could immediately reject their subscriptions. Finally, if the ACL says nothing about a particular subscriber, it means that the notifier has elected to say nothing about what view this subscriber will receive. Consequently, the RLS must perform a back-end subscription should that subscriber subscribe to the resource.

Other subsequent subscriptions to the same resource from the same originating domain are processed in a similar way. However, the notifier in the serving domain will keep track of the set of subscriptions to the same resource for the same event package from the same RLS which are to receive the same view. When a presence notification is to be sent, instead of sending it on all such subscriptions, the notification is sent on just a single subscription.

Should the authorization policies in the serving domain change, an updated ACL is sent, informing the subscribing domain of the new policies. This may require the subscribing domain to extend a back-end subscription to obtain a view, or may change the view an existing subscriber is receiving, and so on.

The ACL allows the serving domain a great deal of flexibility in the level of trust it imparts to the watching domain. The following are all possible approaches that the serving domain can utilize:

No Trust:

When a notifier receives the subscription, it elects not to use this mechanism at all using the negotiation techniques defined here.

Minimal Trust:

When a subscriber subscribes to a resource, the ACL generated for that subscription includes only that subscriber, along with an identifier for their view. Consequently, for each subscriber in domain 1 there will be a backend subscription to domain 2. However, should multiple subscribers share the same view, the notifier in domain 2 sends a single document on one of the subscriptions, and the RLS uses this for all of the other subscribers with the same view. With this approach, domain 2 never discloses the list of authorized subscribers ahead of time, and it has full knowledge of each subscriber that is subscribed. However, it gets the performance benefits of reducing the amount of notification traffic.

Partial Trust:

When a subscriber subscribes to a resource, the ACL generated for that subscription includes that subscriber and all other subscribers authorized for that same view. Consequently, there will only be one backend subscription from the RLS to the notifier for each view. However, the full set of authorized subscribers is not disclosed ahead of time, only those that will get the same view. With partial trust, the notifier will not know the full set of subscribers currently subscribed.

Full Trust:

When a subscriber subscribes to a resource, the ACL generated for that subscription includes that subscriber and all other subscribers that are authorized for that view, and all other views, along with a rule that says that all other subscribers get rejected. In this case, as with partial trust, there is only one backend subscription from the RLS to the notifier for each view. The full set of subscribers is disclosed ahead of time as well. The notifier will not know the full set of subscribers currently subscribed.

Depending on the level of trust, the mechanism trades off inter-domain messaging traffic for increased processing load in the RLS to handle the ACL documents.

3.  RLS Behavior

This section defines the procedures that are to be followed by the RLS. It is important to note that, even though this specification defines an extension to the SIP events framework, the extension is only useful for the back-end subscriptions generated by an RLS. The extension defined here is not applicable or useful for individual users generating subscriptions. Indeed, if it were utilized by individual users, it has the potential for violations of user privacy. See Section 8 (Security Considerations) for a discussion.

3.1.  On Receipt of a Resource List Subscription Request

When the RLS receives a subscription to a resource list which includes some resource P in another domain or system, it follows the rules defined here. The processing depends on whether the RLS already has a backend subscription to the resource that is in the active state, and for which an ACL has been received.

3.1.1.  Find a Matching Back-End Subscription

First, the RLS determines if it has a back-end subscription in place whose view corresponds to that of the new subscriber. Let P be the target resource, E the desired event package, and W the identity of the subscriber.

Based on the procedures of Section 3.2.1 (Processing ACL-Infos), the RLS will keep, for each resource and event package, the list of the most recent ACLs received on each back-end subscription currently in place. This is called the current ACL list. Using this ACL list, the RLS performs the rule determination algorithm of Section 5.4 (Rule Determination Algorithm) to compute the rule ID R for the subscriber W. This represents the view that the subscriber is supposed to receive.

Next, the RLS goes through all subscriptions it currently has for resource P and event package E. For each one, it takes the identity of the subscriber for that actual subscription. The identity for the subscriber for that actual subscription is equal to the asserted identity included in the back-end subscription. For example, if SIP Identity [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.) is utilized, this would be the URI present in the From header field of the back-end SUBSCRIBE. Call the subscriber identity for each subscription Wj.

Next, the RLS computes the rule determination algorithm of Section 5.4 (Rule Determination Algorithm) to compute the rule ID Rj for the subscriber Wj on each subscription j. This represents the rule ID for the view being delivered on that subscription.

Then, processing depends on the values of R and Rj:

• If R is null, it means that no ACL in the list specifies the view for this subscriber. The RLS MUST generate a back-end subscription to resource P and event package E, and MUST use subscriber W as the identity in the back-end subscription.
• If R is not null, but the associated rule is blocked, it means that the subscriber will be rejected. The RLS SHOULD NOT perform another back-end subscription, and must act as if it had created a back-end subscription which was rejected.
• If R is not null, and there is at least one subscription j for which Rj = R, it means that subscription j is already generating notifications for the view that subscriber W is supposed to receive. In that case, the RLS SHOULD NOT generate a back-end subscription for P on behalf of W. Rather, it should treat the existing back end subscription j as if it were the back-end subscription for W, and follow the guidelines of RFC 4662 [RFC4662] (Roach, A., Campbell, B., and J. Rosenberg, “A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists,” August 2006.) based on that. Subscription j is called the generating subscription for subscriber W, and the actual subscriber associated with subscription j, Wj, is called the generating subscriber Wgen for subscriber W.
• If R is not null, but there is no subscription j for which Rj=R, it means that the RLS is not yet receiving the view that subscriber W requires. The RLS MUST generate a back-end subscription to resource P, and MUST use subscriber W as the identity in the back-end subscription.

3.1.2.  Generating a Back-End Subscription

If, based on the processing of the previous section, a new back-end subscription is needed, the rules in this section are followed.

The RLS MUST include a Supported header field in the request with the option tag "view-share". The Accept header field MUST be present in the request and MUST include the "application/viewshare-acl+xml" MIME type amongst the list of supported types. The RLS MUST include an +sip.instance Contact header field parameter [I‑D.ietf‑sip‑outbound] (Jennings, C., “Managing Client Initiated Connections in the Session Initiation Protocol (SIP),” June 2009.) to uniquely identify the RLS instance.

Note that it is possible that two subscribers, in a short period of time, both subscribe to their resource lists, both of which include resource P. This will cause the RLS to generate two back-end subscriptions at around the same time. The RLS is forced to generate the second back-end subscription because it doesn't have an active back-end subscription that has yet generated an ACL. Once both subscriptions become active and generate ACLs, if the subscribers are receiving the same view and both ACLs contain both subscribers, the RLS SHOULD terminate one of the back-end subscriptions.

3.2.  Processing NOTIFY Requests

If a NOTIFY request arrives with a Require header field that includes the "view-share" option tag, it MUST be processed according to the rules of this specification.

3.2.1.  Processing ACL-Infos

If the contents of the NOTIFY are of type "application/viewshare-acl+xml", the subscriber processes the body as described here.

For each resource that the RLS has at least one back-end subscription for, the RLS MUST remember the most recent viewshare-acl received on each back-end subscription. This is called the current ACL list for the resource. This set of viewshare-acl is used in the processing of subscription requests, as described in Section 3.1.1 (Find a Matching Back-End Subscription).

The serving domain can change policies at any time. When it does, it sends an updated ACL on one or more subscriptions. The RLS MUST store this ACL, replacing any previous ACL's received on this subscription. Furthermore, the ACL might impact the views being received by subscribers, and may impact the state of the back-end subscriptions.

The RLS computes the set of subscribers Wi which have a resource list subscription that includes the resource P for whom an updated ACL has just been received. For each Wi, it performs the view determination algorithm (see Section 5.4 (Rule Determination Algorithm) on the current ACL set. Let Ri be the view associated with subscriber Wi. If Ri has not changed from prior to the receipt of the new ACL, no action is taken. However, if it has changed, the RLS takes the set of current back-end subscriptions, and for each subscription j, computes the view determination algorithm for its associated subscriber Wj, to produce Rj. The action to take depends on what has changed:

• If Ri is now null, it means that the serving domain has changed the views associated with subscriber Wi, and this new view is not known to the RLS. The RLS MUST generate a new back-end subscription on behalf of subscriber Wi for resource P to obtain this view.
• If Ri is now a blocked rule, it means that the serving domain has now blocked Wi from obtaining the presence of the resource. The RLS must act as if it had a back-end subscription on behalf of subscriber Wi which was terminated.
• If Ri is not null and not blocked, and if there is an Rj which matches the new Ri, it means that the serving domain has changed the views associated with subscriber Wi, and changed them to another view already being received by the RLS. The RLS MUST treat this back-end subscription j as if it were the back-end subscription to resource P for subscriber Wi. If the most recent presence document received on this back-end subscription is not a semantic match for the presence document most recently delivered to Wi for resource P, the RLS MUST send this most recent presence document to subscriber Wi.
• If Ri is not null and not blocked, but there is no Rj which matches the new Ri, it means that the serving domain has changed the views associated with subscriber Wi, and this new view is not one currently being delivered to the RLS. The RLS MUST generate a new back-end subscription on behalf of subscriber Wi for resource P to obtain this view.

Furthermore, if there are now two back-end subscriptions j1 and j2 which have identical ACLs, RLS SHOULD terminate one of those two subscriptions. Two ACL documents are considered equal if they enumerate the same set of rules with the same members for each rule.

3.2.2.  Processing State Documents

If the contents of the NOTIFY is a state document for the given event package, the RLS follows the procedures defined here.

Let Wj be the subscriber on the subscription j on which the document was just received. Let Rj be the results of running the rule determination algorithm on Wj using the current ACL set. Next, the RLS takes the set of subscribers Wi which have resource P on their resource lists. The RLS then runs the rule determination algorithm on each Wi using the current ACL set, producting Ri for each subscriber Wi. For each Ri that is equal to Rj, the RLS MUST utilize the document just received as if the back-end subscription j was in fact for subscriber Wi. This will typically cause that document to be sent in a NOTIFY request to each such subscriber, though each subscriber may have some kind of filtering policy which causes the RLS to modify the document prior to delivery.

3.2.3.  Processing Back-End Terminations

If the NOTIFY request from the serving domain terminates the back-end subscription, it may be because the subscriber Wj associated with that subscription is no longer permitted to view the state of the resource.

The ACL associated with the subscription MUST be removed from the current ACL set. The procedures of Section 3.2.1 (Processing ACL-Infos) MUST be performed to adjust back-end subscriptions, if needed.

4.  Notifier Behavior

When a notifier receives a SUBSCRIBE request containing a Supported header with the value "view-share", and it wishes to utilize view sharing for this subscription, it follows the procedures defined here.

4.1.  Authentication and Authorization

The principle concern of the notifier is to determine the domain of the RLS, and assess whether the subscribing entity is an RLS authorized to operate on behalf of that domain. In order to utilize view sharing, a notifier MUST determine both. This information is necessary in order to compute the ACL to be sent to that domain, and if done incorrectly, may reveal sensitive information to the watching domain.

To determine the domain of the subscribing RLS, TLS with mutual authentication SHOULD be used. In such a case, the notifier can determine the domain of the RLS from the subjectAltName in the certificate presented from its peer.

This specification does not define any automated mechanism for a notifier to determine whether the subscribing entity is, in fact, an RLS authorized to operate on behalf of the watching domain. Section 8 (Security Considerations) discusses why this determination is important. Absent an automated mechanism, notifiers SHOULD support a configuration option which allows the administrator to enumerate a set of domains for which it is known that an entity holding a certificate for that domain is an authorized RLS. In such a case, the subject from the certificate can be compared against that list, and if a match is found, view sharing can be utilized for this subscription.

4.2.  Processing Initial SUBSCRIBE Requests

First, the subscription is processed as it normally would be, including authorization and policy around the document to be delivered to the subscriber. Furthermore, if the notifier wishes to utilize view sharing for this subscription, it MUST include a Require header field in the first NOTIFY request (and indeed any subsequent ones) it sends confirming this subscription, and that NOTIFY MUST contain the "view-share" option tag. That option tag MUST NOT be present in the Require header field of notifications unless the corresponding dialog-forming SUBSCRIBE contained it in a Supported header field.

Furthermore, the initial state sent by the notifier MUST include an ACL document. It is formatted according to the rules and considerations in Section 5 (ACL Format).

The initial state sent by the notifier might include an actual state document. In particular, a state document MUST be sent if one of the following is true:

• There is only one subscription from the watching domain to this resource that has the view associated with the subscriber.
• There is more than one subscription from the watching domain to this resource with the same view, but the +sip.instance parameter for the remote target (as conveyed in the Contact header field of the SUBSCRIBE) differs. In other words, these subscriptions are from the same domain, but from different RLS in the watching domain. Each RLS in the watching domain needs to get their own copy of the view for a particular resource.

If one of these conditions is not true, the notifier SHOULD NOT send an initial state document on this subscription.

If an ACL and a state document are to be delivered, they MUST be delivered separate NOTIFY requests unless the subscriber indicated support for multipart, in which case the content MAY be included in a single NOTIFY with mulitpart content.

4.3.  SUBSCRIBE Refreshes

When the notifier receives a SUBSCRIBE refresh, it MUST send the most recent ACL document, and if state documents are being sent for this subscription, the most recent state document.

4.4.  Policy Changes

There are several different types of policy changes that can occur:

• If the definitions for a particular rule change, the notifier MUST assign a new rule ID for that rule. For each subscription to a resource which contained that rule, the notifier MUST send an updated ACL which includes a rule with this new rule ID.
• If some subscriber W was previously associated with rule X and is now associated with rule Y, the notifier checks if it has any subscriptions from subscriber W. If it does, it MUST send an updated ACL on that subscription. Based on the rules in Section 5 (ACL Format), this ACL will contain rule Y and will at least include W amongst the list of members. Furthermore, if there were subscriptions from other subscribers, but the notifier had previously sent an ACL on the subscription which was a match for W, it MUST send an updated ACL on that subscription. This updated ACL MAY omit a statement about rule Y or MAY include it. However, the updated ACL MUST NOT claim that subscriber W will receive rule X.
• If some subscriber W was previously associated with rule X and is now blocked, the notifier checks if it has any subscriptions from subscriber W. If it does, it MUST terminate the back-end subscription. If it doesn't, but it has a subscription from some other subscriber which had included a rule that was a match for W, the notifier MUST send an updated ACL on that subscription. This updated ACL MAY omit any statement about subscriber W, or MAY include them as part of a blocked rule in that ACL.
• If some subscriber W was previously blocked and is now permitted and associated with some rule X, the notifier checks if it had any subscriptions from some other subscriber which included a blocked rule that matched subscriber W. If it had, it MUST send an updated ACL on that subscription. That updated ACL MAY omit any statement about subscriber W, or MAY indicate that subscriber W is now associated with rule X.

Of course, a policy change will also potentially alter the state documents that are associated with a view. If so, the notifier MUST send an updated document on a subscription if one of the following is true:

• There is only one subscription from the watching domain to this resource that has the view associated with the subscriber.
• There is more than one subscription from the watching domain to this resource with the same view, but the +sip.instance Contact header field in the request differs between them.

If neither is true, the notifier MUST select one subscription amongst the several which share the same resource, view, and Contact +sip.instance header field parameter, and sent an updated notification on that subscription. The choice of subscriptions is arbitrary and MAY change for each notification.

4.5.  Event State Changes

If the state of some resource changes, the notifier may need to send an updated notification on a subscription. The notifier MUST send an update on a subscription if one of the following is true:

• There is only one subscription from the watching domain to this resource that has the view associated with the subscriber.
• There is more than one subscription from the watching domain to this resource with the same view, but the +sip.instance Contact header field in the request differs between them.

If neither is true, the notifier MUST select one subscription amongst the several which share the same resource, view, and Contact +sip.instance header field parameter, and sent an updated notification on that subscription. The choice of subscriptions is arbitrary and MAY change for each notification.

5.  ACL Format

An ACL document is an XML (Sperberg-McQueen, C., Maler, E., Paoli, J., and T. Bray, “Extensible Markup Language (XML) 1.0 (Second Edition),” October 2000.) [W3C.REC‑xml‑20001006] document that MUST be well-formed and MUST be valid according to schemas, including extension schemas, available to the validater and applicable to the XML document. ACL documents MUST be based on XML 1.0 and MUST be encoded using UTF-8. This specification makes use of XML namespaces for identifying ACL documents and document fragments. The namespace URI for elements defined by this specification is a URN (Moats, R., “URN Syntax,” May 1997.) [RFC2141], using the namespace identifier 'ietf' defined by RFC 2648 (Moats, R., “A URN Namespace for IETF Documents,” August 1999.) [RFC2648] and extended by RFC 3688 (Mealling, M., “The IETF XML Registry,” January 2004.) [RFC3688]. This URN is:

urn:ietf:params:xml:ns:viewshare-acl

5.1.  Document Structure and Semantics

An ACL document informs a watching domain of the set of views that can be received by that domain, and associates specific subscribers with specific views. It is very important to understand that the ACL document does not convey the actual processing that will be applied by the serving domain. It does not indicate, for example, whether geolocation is present in a presence document, or which rich presence [RFC4480] (Schulzrinne, H., Gurbani, V., Kyzivat, P., and J. Rosenberg, “RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF),” July 2006.) data elements will be conveyed. It merely provides grouping - indicating which subscribers from the subscribing domain will receive the same view.

Each ACL document starts with the enclosing root element <acl-list>. This contains the list of rules defined by the ACL. Each rule is represented by the <rule> element. Each rule represents a specific view, which is generated by the notifier based on its authorization, composition and filtering policies. Each rule is associated with a rule ID, which is a mandatory attribute of the <rule> element. This ID is scoped within a single resource. That is, the IDs for two rules for different presentities are unrelated.

The <rule> element also contains an optional "blocked" boolean attribute. If "true", it means that the rule specifies that the associated set of subscribers will be rejected, should they subscribe. This can be used by the watching domain to avoid performing back-end subscriptions to users which will only be blocked anyway.

Each <rule> contains the set of users that will receive the corresponding view. This can be described by an enumerated set or by a default. If it is an enumerated set, the <rule> is followed by a sequence of <member> elements, each of which contains a SIP URI for the subscriber that will receive that view.

The default view is specified by including a single child element for <rule> - <other>. The default view applies to all subscribers except those enumerated by other rules. For this reason, an ACL document which contains a default view MUST include the rule IDs and associated members for all other views that are delivered to subscribers. For example, consider a resource that has three views. View 1 is delivered to subscribers A and B. View 2 is delivered to subscriber C. View 3 is delivered to everyone else. An ACL document that includes the default view must also include views 1 and 2 with subscribers A, B, and C.

In contrast, an ACL document that does not include a default does not need to include all views, and it does not need to include all members for a particular view. Using the example above, it is valid to include an ACL document which includes only view 1 with subscriber 1.

If two URI are present within <member> elements within the same <rule>, it represents an indication by the notifier that both users MUST get the same view. Formally, if the notifier were to receive a subscription from each subscriber, both subscriptions would be accepted or both would be rejected, and if accepted, each subscription would receive semantically identical presence documents at approximately the same time.

Even if two users will receive the same view, a notifier MAY assign each to a different view ID. There is no requirement that two unique views actually contain different presence data. The only requirement is that, if two users are listed within the same rule, that they do in fact receive the same view.

An ACL document delivered in a subscription from subscriber W MUST include the view associated with subscriber W and MUST include subscriber W explicitly in a <member> element or implicitly by presence of an <other> element.

5.2.  Trust Considerations when Construcing ACLs

The semantics above give very little guidance about what a notifier should include in an ACL. The amount of information to convey depends on the level of trust between the subscribing and serving domains.

Firstly, in all cases, any subscriber listed in a rule MUST be one that the subscribing RLS is authorized to perform subscriptions for. Typically, this is all of the subscribers in the domain of the RLS. For example, if a view-sharing subscription is received from example.com, only subscribers whose domain is example.com should be included in the ACL. However, in cases where view sharing is used between a clearinghouse provider and clearinghouse members, the ACL could include subscribers in other domains, based on the policy of the serving domain.

Optimal performance is achieved when the ACL document for a resource includes all views that the server might ever deliver to subscribers from the watching domain, and includes all members from that domain for each view, including any defaults and blocked rules. However, this informs the watching domain of the set of allowed and blocked subscribers from its own domain, and associated groupings amongst subscribers.

Slightly worse performance is achieved when the ACL document for a resource sent in a subscription from subscriber W includes only a single view - the one for subscriber W - along with the full set of subscribers from that domain which will also receive that view, assuming it is not the default view. If the view is the default view, the document can include just subscriber W. This approach will cause back-end subscriptions from every subscriber that will receive the default, but it discloses less information to the watching domain. In particular, the full set and number of views is never known by the watching domain. The fact that a view is default is never known by the watching domain. The full set of users that are permitted to view the state of the resource is never disclosed to the watching domain. The performance of this approach is still reasonably good when the default rule is blocked. However it is much less effective when the default is not blocked, and many subscribers receive the default.

Another choice for construction of ACL documents is to include, in a subscription from subscriber W, a single rule containing the rule ID for the view that subscriber W will receive, along with a single member - W. This approach will still result in a back-end subscription from each subscriber. However, a single notification is sent for each view, rather than one per subscriber. The benefit of this construction is that it provides the watching domain no additional information about the authorization policies of the resource than if this extension were not in use at all.

5.3.  Example Documents

The example document in Figure 2 (Example with Maximum Trust) shows the case when there is maximum trust between domains. The full set of subscribers, include a blocked default, is included.

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
    <member>sip:user2@example.com</member>
    <member>sip:user3@example.com</member>
    <member>sip:user4@example.com</member>
    <member>sip:user5@example.com</member>
  </rule>
  <rule id="3584">
    <member>sip:user6@example.com</member>
  </rule>
  <rule id="1735">
    <member>sip:user7@example.com</member>
    <member>sip:user8@example.com</member>
    <member>sip:user9@example.comm</member>
    <member>sip:user10@example.com</member>
    <member>sip:user11@example.com</member>
  </rule>
  <rule blocked="true" id="9433">
    <other />
  </rule>
</acl-list>

The example in Figure 3 (Example with Partial Trust) shows a moderate level of trust. This ACL only shows the view associated with the subscriber user1.

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
    <member>sip:user2@example.com</member>
    <member>sip:user3@example.com</member>
    <member>sip:user4@example.com</member>
    <member>sip:user5@example.com</member>
  </rule>
</acl-list>

The example in Figure 4 (Example with Minimal Trust) shows the minimal level of trust. This ACL would be sent in a subscription to user1.

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
  </rule>
</acl-list>

5.4.  Rule Determination Algorithm

Several steps in the processing of the ACL require that the RLS in the watching domain execute the rule determination algorithm for subscriber W on an ACL set. This algorithm is a simple algorithm which takes, as input, a subscriber W with a given SIP URI, and a set of ACL documents Ai, and returns as output, a rule ID R, which is the rule ID for the view that, according to the set of ACLs, subscriber W should receive.

The algorithm proceeds as follows. First, each Ai is matched to W. ACL Ai is a match for subscriber W if:

• ACL Ai contains a <member> tag whose URI is a match, based on URI equality, for W, or
• none of the <member> tags in Ai contain a URI that is a match, based on URI equality, for W, but there is an <other> element in Ai

If no ACL Ai matched, the algorithm returns a null result.

For each ACL Ai that matches based on the rules above, take the id of the enclosing <rule> element that contained the <member> or <other> element that caused the match. For ACL Ai, this is rule Ri. For example, consider the following ACL:

<?xml version="1.0" encoding="UTF-8"?>
<acl-list "xmlns=urn:ietf:params:xml:ns:viewshare-acl">
    <rule id="1">
      <member>sip:user1@example.com</member>
      <member>sip:user2@example.com</member>
    </rule>
    <rule id="2">
       <member>sip:user3@example.com</member>
    </rule>
    <rule id="3">
      <other/>
    </rule>
 </acl-list>

If this document is A1, and the subscriber is sip:user3@example.com, the associated rule R1 is 2. If the subscriber is sip:user1@example.com or sip:user2@example.com, the rule R1 is 1. If the subscriber is anyone else from example.com, such as sip:user4@example.com, the rule R1 is 3.

If all Ri are equal, denote R = Ri. Thus, R is the rule ID associated with this subscriber. Normally, all Ri will be equal. However, during transient periods of changes in authorization state, it is possible that inconsistent ACL documents exist. In that case, R is assigned the value Ri from the ACL Ai which is the most recently received amonst all ACLs.

5.5.  XML Schema

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="acl-list">
    <xs:complexType>
      <xs:sequence minOccurs="1" maxOccurs="unbounded">
        <xs:element name="rule">
          <xs:complexType>
            <xs:choice>
              <xs:element name="other" />
              <xs:sequence minOccurs="1" maxOccurs="unbounded">
                <xs:element name="member" type="xs:anyURI" />
              </xs:sequence>
            </xs:choice>
            <xs:attribute name="id" type="xs:integer" use="required" />
            <xs:attribute default="false" name="blocked" type="xs:boolean" use="optional" />
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

6.  Performance Analysis

This section considers the performance improvement of the mechanism when it is maximally exercised. The performance is examined in the context of an inter-domain presence federation. In this example, the full ACL, including blocked senders, is returned in the first subscription to a presentity. This analysis assumes there is a single, monolithic notifier serving each domain.

The optimizations improve ramp-up, steady state, and termination message loads. In particular, each of those loads, without the optimization described here, is proportional to C04, the total number of federated presentities per watcher. If we assume symmetry, such that the number of federated presentities per watcher is equal to the number of watchers per federated presentity, then each of the load figures is reduced by C04. That is, the system behaves identically to the case where there is a single subscriber per federated presentity, and assuming symmetric, the same as if there is a single federated presentity per subscriber - e.g., C04 = 1.

Consider then the very large network peering model in [I‑D.ietf‑simple‑interdomain‑scaling‑analysis] (Houri, A., Aoki, E., Parameswar, S., Rang, T., Singh, V., and H. Schulzrinne, “Presence Interdomain Scaling Analysis for SIP/SIMPLE,” August 2009.). In this model, the assumption is two large peering domains with 20 million users each, with a value of 10 for C04. With this optimization, the number of steady state notifications due to presence state changes drops from 18.4 billion per day to 1.84 billion per day. The number of messages per second overall is reduced from 654,167 per second to 65,417 per second. Still a big number, of course, but it can't actually get much smaller.

Indeed, it can be readily shown that, assuming the federated domains do not actually share raw presence inputs and the actual policies that govern operation of their servers, no protocol can do better (constants, such as mesage size and the need for protocol responses and acknowledgements aside). Consider a domain with N presentities. Each resource changes state P times per hour. Every time the state changes, the domain applies its authorization and composition policies. The resulting presence document cannot be known to the watching domain. Thus, there must be at least one message from the serving to watching domain, per view, in order to inform it of that view. This means that the steady state rate of messages can never be better than N*P, and this is exactly the factor governing the rate of messages when this optimization is applied.

7.  Requirements Analysis

This section analyzes the requirements in [I‑D.ietf‑sipping‑presence‑scaling‑requirements] (Houri, A., Parameswar, S., Aoki, E., Singh, V., and H. Schulzrinne, “Scaling Requirements for Presence in SIP/SIMPLE,” January 2009.) to show how they are met by the mechanism proposed here.

REQ-001:

The solution should not hinder the ability of existing SIMPLE clients and/or servers from peering with a domain or client implementing the solution. No changes may be required of existing servers to interoperate. This requirement is met by usage of the Supported and Require mechanisms and SIP which negotiate its usage.

REQ-002:

It does NOT constrain any existing RFC functional or security requirements for presence. The mechanism does not change anything that is possible without it. It does, however, introduce new privacy considerations, described below in Section 8 (Security Considerations).

REQ-003:

Systems that are not using the new additions to the protocol should operate at the same level as they do today. This requirement is met by usage of the Supported and Require mechanisms in SIP.

REQ-004:

The solution does not limit the ability for presentities to present different views of presence to different watchers. This requirement is met by usage of the ACL document, which allows the serving domain to associate a subscriber with any view it likes, and to change it over time.

REQ-005:

The solution does not restrict the ability of a presentity to obtain its list of watchers. The mechanism does allow a presence server to know the list of subscribers, at the expense of non-optimal performance. In particular, it will receive a subscription from each subscriber. However, it only generates one notification per view on presence changes. The fully optimized solution will result in a loss of knowledge of the set of watchers. However, it is a policy decision at the presence agent about whether it would like to make this tradeoff.

REQ-006:

The solution MUST NOT create any new or make worse any existing privacy holes. This requirement is met, but only when carefully provisioned. See Section 8 (Security Considerations).

REQ-007:

It is highly desirable for any presence system (intra or inter-domain) to scale linearly as number of watchers and presentities increase linearly. When the most optimal technique is used, there is always one subscription per view per presentity, independent of the number of watchers in the remote domain or the number of averages buddies per buddy list. Since the number of views is not proportional to the number of users, the total traffic volume in a domain is linear with its number of presentities, and is independent of the number of users in the peering domain.

REQ-008:

The solution SHOULD NOT require significantly more state in order to implement the solution. The mechanism requires storage of the ACL, which has a size exactly equal to the number of subscriptions that would be required if the extension were not in place. Thus the memory usage is not worsened compared to the baseline.

REQ-009:

It MUST be able to scale to tens of millions of concurrent users in each domain and in each peer domain. The analysis in Section 6 (Performance Analysis) shows that, when fully utilized, this mechanism is the best that can possibly be achieved in any system that does not actually share policies and raw presence data.

REQ-010:

It MUST support a very high level of watcher/presentity intersections in various intersection models. The mechanism is optimized for this case.

REQ-011:

Protocol changes MUST NOT prohibit optimizations in different deployment models esp. where there is a high level of cross subscriptions between the domains. Since standard SIP techniques are utilized to negotiate the extension, other mechansims can be defined in the future.

REQ-012:

New functionalities and extensions to the presence protocol SHOULD take into account scalability with respect to the number of messages, state size and management and processing load. That is exactly what this extension targets.

REQ-013:

The solution SHOULD allow for arbitrary federation topologies including direct peering and intermediary routing. The mechanism is optimized for direct peering. It can work in intermediary routing cases as well.

8.  Security Considerations

The principal question with the specification is whether it alters the privacy characteristics of a non-optimized federated system. This can be considered for both the serving domain and the subscribed-to resource. In all cases, view sharing requires secure authentication and encryption between the domains that use it. This is provided by TLS.

8.1.  Privacy Considerations of the Serving Domain

Consider first the case where the serving domain is using the minimal trust model. In that case, the ACL provided to the subscribing domain does not carry any information that the subscribing domain doesn't already know. It merely points out when two subscribers share the same view. This is something that the subscribing domain could have already ascertained by comparing presence documents delivered to each subscriber. The ACL makes this task easier, but nonetheless the subscribing domain could have already ascertained it. Consequently, there is no change whatsoever in the level of privacy afforded by the optimization when this mode is used.

However, when an ACL is provided that includes other users besides the actual subscriber, this provides additional information to the subscribing domain. This is, however, information that the subscribing domain could find out anyway. If it generated a subscription from each of its users to the resource it would be able to determine who from its domain is allowed to subscribe and what view they would receive. This would be an expensive operation to be sure, but it is possible. Consequently, the optimization doesn't really provide anything new to the originating domain, even in this case.

However, there is an attack possible when the information is divulged to an end user. Consider a subscribing domain that doesn't actually implement this extension at all. A user within the domain uses a client that generates a subscription to a resource in a remote domain. This subscription uses an outbound proxy in the watching domain. The outbound proxy is just a proxy, and therefore doesn't remove or modify the Supported header field in the request. The serving domain accepts the subscription and sends an ACL that contains the full set of subscribers that are permitted in the originating domain. The original subscriber now knows the set of other authorized buddies within their own domain, and what views they will see. While this is information that the domain overall would have access to, it is not information an end user would normally have access to. Consequently, this is a more serious privacy violation.

It is for this reason that this specification requires that both sides of the federated link be explicitly provisioned to utilize this optimization. In the attack above, the subscribing domain would not have set up a peering relationship with the serving domain. If it had, it would have an RLS and would not have permitted the user to directly subscribe in this way. Thus, when the subscription is received by the serving domain, it will find that it has no agreement with the originating domain, and would not utilize view sharing. This thwarts the attack.

This remedy is not optimal because it requires on provisioning to prevent. There does not appear to be any easy cryptographic means to prevent it, however.

8.2.  Privacy Considerations of the Watched Resource

The principle security concern for the watched resource is whether the documents shown to subscriber meet its privacy policies. This is particularly a concern for presence. These privacy policies can be violated if presence documents are shown to subscribers to whom the resource has not granted permission, or if they contain content that the resource has not allowed the subscriber to see.

Based on the mechanisms defined in this specification, view sharing gives clear guidance to the watching RLS about which additional subscribers can see a particular presence document. Consequently, under normal operating conditions, the system ensures that the privacy policies of the resource are met. It is possible that a buggy implementation might accidentally redistribute presence documents to unauthorized subscribers. Implementors SHOULD be careful to implement the ACL mechanism carefully to avoid this. A malicious RLS or domain could ignore the ACL documents defined by this document, and distribute the presence documents to unauthorized subscribers. However, such an attack is already possible in the normal operation of an RLS, and is not worsened by the view sharing mechanism defined here.

8.3.  Interactions with S/MIME

The SIP and SIMPLE specifications do allow state documents to be signed and/or encrypted with S/MIME. When S/MIME is used strictly for message integrity, view sharing is fully compatible with S/MIME. However, when presence documents are encrypted using S/MIME, this causes an interaction with view sharing. The serving domain will send out only a single document to the watching domain for each view. This document needs to be decryptable by each authorized subscriber. Consequently, that group must either share a single key, or the serving domain needs to encrypt the content using the keys from each of the authorized subscribers. In the latter case, view sharing and S/MIME cannot be used together if the set of authorized subscribers is wildcarded.

9.  IANA Considerations

There are several IANA considerations associated with this specification.

9.1.  MIME Type Registration

This specification requests the registration of a new MIME type according to the procedures of RFC 2048 (Freed, N., Klensin, J., and J. Postel, “Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures,” November 1996.) [RFC2048] and guidelines in RFC 3023 (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.) [RFC3023].

MIME media type name: application

MIME subtype name: viewshare-acl+xml

Mandatory parameters: none

Optional parameters: Same as charset parameter application/xml as specified in RFC 3023 (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.) [RFC3023].

Encoding considerations: Same as encoding considerations of application/xml as specified in RFC 3023 (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.) [RFC3023].

Security considerations: See Section 10 of RFC 3023 (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.) [RFC3023] and Section 8 (Security Considerations) of RFC XXXX [[NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this specification]].

Interoperability considerations: none.

Published specification: RFC XXXX [[NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this specification]]

Applications which use this media type: This document type has been used to support subscriptions to lists of users (Roach, A., Campbell, B., and J. Rosenberg, “A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists,” August 2006.) [RFC4662] for SIP-based presence (Rosenberg, J., “A Presence Event Package for the Session Initiation Protocol (SIP),” August 2004.) [RFC3856].

Additional Information:

Magic Number: None

File Extension: .acl

Macintosh file type code: "TEXT"

Personal and email address for further information: Jonathan Rosenberg, jdrosen@jdrosen.net

Intended usage: COMMON

Author/Change controller: The IETF.

9.2.  URN Sub-Namespace Registration

This section registers a new XML namespace, as per the guidelines in RFC 3688 (Mealling, M., “The IETF XML Registry,” January 2004.) [RFC3688].

URI: The URI for this namespace is urn:ietf:params:xml:ns:viewshare-acl.

Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org), Jonathan Rosenberg (jdrosen@jdrosen.net).

XML:

             BEGIN
             <?xml version="1.0"?>
             <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                       "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
             <html xmlns="http://www.w3.org/1999/xhtml">
             <head>
               <meta http-equiv="content-type"
                  content="text/html;charset=iso-8859-1"/>
               <title>ACL Info Namespace</title>
             </head>
             <body>
               <h1>Namespace for ACL Info</h1>
               <h2>urn:ietf:params:xml:ns:viewshare-acl</h2>
               <p>See <a href="[URL of published RFC]">RFCXXXX [NOTE
TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this
specification.]</a>.</p>
             </body>
             </html>
             END

9.3.  Schema Registration

This section registers an XML schema per the procedures in [RFC3688] (Mealling, M., “The IETF XML Registry,” January 2004.).

URI: urn:ietf:params:xml:schema:viewshare-acl

Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org), Jonathan Rosenberg (jdrosen@jdrosen.net).

The XML for this schema can be found as the sole content of Section 5.5 (XML Schema).

10.  Acknowledgements

The authors would like to thank Avshalom Houri, Richard Barnes, and Michael Froman for their comments on this document.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses

Full Copyright Statement

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