SIMPLE J. Rosenberg
Internet-Draft S. Donovan
Intended status: Standards Track K. McMurry
Expires: January 15, 2009 Cisco
July 14, 2008
Optimizing Federated Presence with View Sharing
draft-ietf-simple-view-sharing-01
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
3. RLS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. On Receipt of a Resource List Subscription Request . . . . 8
3.1.1. Authentication and Authorization . . . . . . . . . . . 8
3.1.2. No Active Back-End Subscription . . . . . . . . . . . 8
3.1.3. Active Back-End Subscription . . . . . . . . . . . . . 9
3.2. Processing NOTIFY Requests . . . . . . . . . . . . . . . . 10
3.2.1. Processing ACL-Infos . . . . . . . . . . . . . . . . . 10
3.2.2. Processing Presence Documents . . . . . . . . . . . . 11
3.2.3. Processing Back-End Terminations . . . . . . . . . . . 12
4. Presence Agent Behavior . . . . . . . . . . . . . . . . . . . 12
4.1. Authentication and Authorization . . . . . . . . . . . . . 12
4.2. Processing Initial SUBSCRIBE Requests . . . . . . . . . . 12
4.3. SUBSCRIBE Refreshes . . . . . . . . . . . . . . . . . . . 13
4.4. Policy Changes . . . . . . . . . . . . . . . . . . . . . . 13
4.5. Presence State Changes . . . . . . . . . . . . . . . . . . 15
5. ACL Format . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Document Structure and Semantics . . . . . . . . . . . . . 15
5.2. Trust Considerations when Construcing ACLs . . . . . . . . 17
5.3. Example Documents . . . . . . . . . . . . . . . . . . . . 18
5.4. Rule Determination Algorithm . . . . . . . . . . . . . . . 19
5.5. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Performance Analysis . . . . . . . . . . . . . . . . . . . . . 21
7. Requirements Analysis . . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9.1. MIME Type Registration . . . . . . . . . . . . . . . . . . 25
9.2. URN Sub-Namespace Registration . . . . . . . . . . . . . . 26
9.3. Schema Registration . . . . . . . . . . . . . . . . . . . 27
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
11.1. Normative References . . . . . . . . . . . . . . . . . . . 27
11.2. Informative References . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Introduction
Presence refers to the ability, willingness and desire to communicate
across differing devices, mediums and services [RFC2778]. Presence
is described using presence documents [RFC3863] [RFC4479], exchanged
using a SIP-based event package [RFC3856].
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.
[I-D.ietf-simple-interdomain-scaling-analysis] 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] 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] 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] 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 a domain to a single presentity in another
domain, each of which is actually going to get the exact same
presence document, the domain of the watchers extends a single
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subscription to the domain of the presentity, and the domain of the
presentity sends a single copy of the presence document back to the
domain of the watcher.
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 extensions works in the environment shown in Figure 1. The
environment assumes two domains. There are some number of watchers
(W1 - W3) in the domain on the left, which we call the watching
domain. All of those watchers are interested in the presence of a
single presentity P1 in the domain on the right, which we call the
serving domain. The watchers all make use of a resource list server
(RLS) [RFC4662] which stores their buddy lists and performs the buddy
list expansion. Consequently, when each watcher subscribes to their
buddy list on the RLS, in absence of any optimizations, the RLS will
generate three separate subscriptions to P1, each of which reaches
the presence server in the serving domain.
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.
+--------------+ . +--------------+
| | . | |
| | SUB . | |
| | -------.---> | Presence |
| RLS | NOT . | Server |
| | <------.---- | |
| | . | |
| | . | |
+--------------+ . +--------------+
^ ^ ^ . ^
List | | | . | PUB
SUB | | | . |
| | | . |
+----+ +----+ +----+ . +----+
| | | | | | . | |
| W1 | | W2 | | W3 | . | P1 |
| | | | | | . | |
+----+ +----+ +----+ . +----+
.
.
.
Watching . Serving
Domain . Domain
.
Figure 1: Deployment Model
Of course, in practice each domain will act as both a watching 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 presence server 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.
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View sharing requires a level of trust between the two domains.
Consequently, the connection between them utilizes TLS with mutual
authentication. The presence server verifies that the certificate
presented in the mutual authentication matches the domain of the
watcher.
If this is the first subscription from domain 1 for that particular
presentity, the presence server accepts the subscription (assuming
the watcher is authorized of course). The notifications sent to the
RLS include two separate pieces of state. One is the actual presence
state for the presentity. The other is an Access Control List (ACL)
document. This document describes the set of other watchers from the
originating domain, if any, who are authorized to see exactly the
same presence document - in other words, the set of users that share
the same view. Should one of those watchers seek the presence of
that presentity, the RLS from the originating domain does not need to
generate a back-end subscription; rather, it just uses the presence
document it is receiving from the original subscription, and passes
it to both watchers. The ACL can also list users in the originating
domain that are authorized to subscribe to that presentity, but who
will end up receiving a different view. Should one of those watchers
subscribe, the RLS knows that it must perform a back-end subscription
to obtain that view. The ACL can also list watchers 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 watcher, it means that the
presence server has elected to say nothing about what view this
watcher will receive. Consequently, the RLS must perform a back-end
subscription should that watcher subscribe to the presentity.
Other subsequent subscriptions to the same presentity from the same
originating domain are processed in a similar way. However, the
presence server in the serving domain will keep track of the set of
subscriptions to the same presentity 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 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 watching domain of the new
policies. This may require the watching domain to extend a back-end
subscription to obtain a view, or may change the view an existing
watcher 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:
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No Trust: When a presence server receives the subscription, it
elects not to use this mechanism at all using the negotiation
techniques defined here.
Minimal Trust: When a watcher subscribes to a presentity, the ACL
generated for that subscription includes only that watcher, along
with an identifier for their view. Consequently, for each watcher
in domain 1 there will be a backend subscription to domain 2.
However, should multiple watchers share the same view, the
presence server in domain 2 sends a single presence document on
one of the subscriptions, and the RLS uses this for all of the
other watchers with the same view. With this approach, domain 2
never discloses the list of authorized watchers ahead of time, and
it has full knowledge of each watcher that is subscribed.
However, it gets the performance benefits of reducing the amount
of notification traffic.
Partial Trust: When a watcher subscribes to a presentity, the ACL
generated for that subscription includes that watcher and all
other watchers authorized for that same view. Consequently, there
will only be one backend subscription from the RLS to the presence
server for each view. However, the full set of authorized
watchers is not disclosed ahead of time, only those that will get
the same view. With partial trust, the presence server will not
know the full set of watchers currently subscribed.
Full Trust: When a watcher subscribes to a presentity, the ACL
generated for that subscription includes that watcher and all
other watchers that are authorized for that view, and all other
views, along with a rule that says that all other watchers get
rejected. In this case, as with partial trust, there is only one
backend subscription from the RLS to the presence server for each
view. The full set of watchers is disclosed ahead of time as
well. The presence server will not know the full set of watchers
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, that extension is
only useful for the back-end subscriptions generated by an RLS. The
extension defined here is not applicable or useful for individual
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users generating subscriptions. Indeed, if it were utilized by
individual users, it has the potential for violations of user
privacy. See Section 8 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 presentity P in another domain, it follows the rules
defined here.
3.1.1. Authentication and Authorization
First, the RLS MUST check a configured list of peer domains for which
this extension is to be applied. Because of the potential privacy
disclosures involved in unauthorized use of this facility, it can
only be used between pairs of domains which have a pre-arranged
agreement to utilize it. If the domain of the presentity P matches
one of the configured list of peer domains, the RLS is permitted to
utilize this extension. If not, the extension MUST NOT be used.
Next, the RLS MUST send the SUBSCRIBE request over a mutually
authenticated TLS connection. The RLS MUST check that the
subjectAltName in the certificate of its peer contains a domain name
that is a match for the domain of the URI of the presentity. If they
are not a match, view sharing cannot be utilized for this
subscription.
The procedures followed by the RLS after this point depend on whether
the RLS already has a backend subscription to the presentity that is
in the active state, and for which an ACL has been received.
3.1.2. No Active Back-End Subscription
The RLS MUST generate a back-end subscription to obtain the state of
the presentity. 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/
aclinfo+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] to uniquely identify the RLS instance.
Note that it is possible that two watchers, in a short period of
time, both subscribe to their resource lists, both of which include
presentity 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 watchers are
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receiving the same view and both ACLs contain both watchers, the RLS
SHOULD terminate one of the back-end subscriptions.
3.1.3. Active Back-End Subscription
In this case, the RLS already has at least one back-end subscription
to the target presentity P, and it has received at least one ACL for
that presentity. It has received a resource list subscription from
watcher W which includes presentity P. Based on the procedures of
Section 3.2.1, the RLS will keep, for each presentity, the list of
the most recent ACLs received on each back-end subscription currently
in place. This is called the current ACL list.
For each ACL Ai in the current ACL list, the RLS performs the rule
determination algorithm of Section 5.4 to compute the rule ID R for
the watcher W. This represents the view that the watcher is supposed
to receive.
Next, the RLS goes through all subscriptions it currently has for
presentity P. For each one, it takes the identity of the watcher for
that actual subscription. The identity for the watcher for that
actual subscription is equal to the asserted identity included in the
back-end subscription. For example, if SIP Identity [RFC4474] is
utilized, this would be the URI present in the From header field of
the back-end SUBSCRIBE. Call the watcher identity for each
subscription Wj.
Next, the RLS computes the rule determination algorithm of
Section 5.4 to compute the rule ID Rj for the watcher 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:
o If R is null, it means that the ACL doesn't specify the view for
this watcher. The RLS MUST generate a back-end subscription to
presentity P, and MUST use watcher W as the identity in the back-
end subscription.
o If R is not null, but the associated rule is blocked, it means
that the watcher 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.
o 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 watcher W is supposed to receive.
In that case, the RLS SHOULD NOT generate a back-end subscription
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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] based on that.
Subscription j is called the generating subscription for watcher
W, and the actual watcher associated with subscription j, Wj, is
called the generating watcher Wgen for watcher W.
o 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 watcher W
requires. The RLS MUST generate a back-end subscription to
presentity P, and MUST use watcher W as the identity in the back-
end subscription.
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/aclinfo+xml",
the subscriber processes the body as described here.
For each presentity that the RLS has at least one back-end
subscription for, the RLS MUST remember the most recent aclinfo
received on each back-end subscription. This is called the current
ACL list for the presentity. This set of aclinfo is used in the
processing of subscription requests, as described in Section 3.1.3.
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. Furthermore, the ACL might impact the views being
received by watchers, and may impact the state of the back-end
subscriptions.
The RLS computes the set of watchers Wi which have a resource list
subscription that includes the presentity P for whom an updated ACL
has just been received. For each Wi, it performs the view
determination algorithm (see Section 5.4 on the current ACL set. Let
Ri be the view associated with watcher 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 watcher Wj, to produce Rj.
The action to take depends on what has changed:
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o If Ri is now null, it means that the serving domain has changed
the views associated with watcher Wi, and this new view is not
known to the RLS. The RLS MUST generate a new back-end
subscription on behalf of watcher Wi for presentity P to obtain
this view.
o If Ri is now a blocked rule, it means that the serving domain has
now blocked Wi from obtaining the presence of the presentity. The
RLS must act as if it had a back-end subscription on behalf of
watcher Wi which was terminated.
o 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 watcher 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
presentity P for watcher 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 presentity
P, the RLS MUST send this most recent presence document to watcher
Wi.
o 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 watcher 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 watcher Wi for presentity P to
obtain this view.
Furthermore, if there are now two back-end subscriptions j1 and j2
for which Aj1 = Aj2, the 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 Presence Documents
If the contents of the NOTIFY is a presence document, the RLS follows
the procedures defined here.
Let Wj be the watcher on the subscription j on which the presence
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 watchers Wi which have presentity P on their
buddy lists. The RLS then runs the rule determination algorithm on
each Wi using the current ACL set, producting Ri for each watcher Wi.
For each Ri that is equal to Rj, the RLS MUST utilize the presence
document just received as if the back-end subscription j was in fact
for watcher Wi. This will typically cause that presence document to
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be sent in a NOTIFY request to each such watcher, though each watcher
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 watcher Wj associated with that
subscription is no longer permitted to view the presence of the
presentity.
The ACL associated with the subscription MUST be removed from the
current ACL set. The procedures of Section 3.2.1 MUST be performed
to adjust back-end subscriptions, if needed.
4. Presence Agent Behavior
When a presence agent 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
First, the presence agent MUST have received the SUBSCRIBE request
over a mutually authenticated TLS connection. If it had not, view
sharing cannot be utilized for this subscription. The presence agent
MUST check that the subjectAltName in the certificate of its peer
contains a domain name that is a match for the domain of the URI of
the watcher. If they are not a match, view sharing cannot be
utilized for this subscription.
Assuming they are a match, the presence agent MUST check a configured
list of peer domains for which this extension is to be applied.
Because of the potential privacy disclosures involved in unauthorized
use of this facility, it can only be used between pairs of domains
which have a pre-arranged agreement to utilize it. If the domain of
the watcher W matches one of the configured list of peer domains, the
presence agent is permitted to utilize this extension. If not, the
extension MUST NOT be used.
4.2. Processing Initial SUBSCRIBE Requests
First, the subscription is processed as it normally would be,
including authorization and policy around the presence document to be
delivered to the watcher. Furthermore, if the presence agent wishes
to utilize view sharing for this subscription, it MUST include a
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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.
Furthermore, the initial state sent by the presence agent MUST
include an ACL document. It is formatted according to the rules and
considerations in Section 5.
The initial state sent by the presence agent might include an actual
presence document. In particular, a presence document MUST be sent
if one of the following is true:
o There is only one subscription from the watching domain to this
presentity that has the view associated with the watcher.
o There is more than one subscription from the watching domain to
this presentity 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 presentity.
If one of these conditions is not true, the presence agent SHOULD NOT
send an initial presence document on this subscription.
If an ACL and a presence document are to be delivered, they MUST be
delivered in a separate NOTIFY request (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 presence agent receives a SUBSCRIBE refresh, it MUST send
the most recent ACL document, and if presence documents are being
sent for this subscription, the most recent presence document.
4.4. Policy Changes
There are several different types of policy changes that can occur:
o If the definitions for a particular rule change, the presence
agent MUST assign a new rule ID for that rule. For each
subscription to a presentity which contained that rule, the
presence agent MUST send an updated ACL which includes a rule with
this new rule ID.
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o If some watcher W was previously associated with rule X and is now
associated with rule Y, the presence agent checks if it has any
subscriptions from watcher W. If it does, it MUST send an updated
ACL on that subscription. Based on the rules in Section 5, this
ACL will contain rule Y and will at least include W amongst the
list of members. Furthermore, if there were subscriptions from
other watchers, but the presence agent 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 watcher W will receive rule X.
o If some watcher W was previously associated with rule X and is now
blocked, the presence agent checks if it has any subscriptions
from watcher W. If it does, it MUST terminate the back-end
subscription. If it doesn't, but it has a subscription from some
other watcher which had included a rule that was a match for W,
the presence agent MUST send an updated ACL on that subscription.
This updated ACL MAY omit any statement about watcher W, or MAY
include them as part of a blocked rule in that ACL.
o If some watcher W was previously blocked and is now permitted and
associated with some rule X, the presence agent checks if it had
any subscriptions from some other watcher which included a blocked
rule that matched watcher W. If it had, it MUST send an updated
ACL on that subscription. That updated ACL MAY omit any statement
about watcher W, or MAY indicate that watcher W is now associated
with rule X.
Of course, a policy change will also potentially alter the presence
documents that are associated with a view. If so, the presence agent
MUST send an updated document on a subscription if one of the
following is true:
o There is only one subscription from the watching domain to this
presentity that has the view associated with the watcher.
o There is more than one subscription from the watching domain to
this presentity with the same view, but the User-Agent header
field in the request differs between them.
If neither is true, the presence agent MUST select one subscription
amongst the several which share the same presentity, view, and User-
Agent header field, and sent an updated notification on that
subscription. The choice of subscriptions is arbitrary and MAY
change for each notification.
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4.5. Presence State Changes
If the state of some presentity changes, the presence agent may need
to send an updated notification on a subscription. The presence
agent MUST send an update on a subscription if one of the following
is true:
o There is only one subscription from the watching domain to this
presentity that has the view associated with the watcher.
o There is more than one subscription from the watching domain to
this presentity with the same view, but the User-Agent header
field in the request differs between them.
If neither is true, the presence agent MUST select one subscription
amongst the several which share the same presentity, view, and User-
Agent header field, 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 [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 [RFC2141],
using the namespace identifier 'ietf' defined by RFC 2648 [RFC2648]
and extended by RFC 3688 [RFC3688]. This URN is:
urn:ietf:params:xml:ns:aclinfo
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 watchers 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] data elements will be conveyed. It merely provides
grouping - indicating which watchers from the watching domain will
receive the same view.
Each ACL document starts with the enclosing root element <acl-list>.
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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 presence server 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 presentity. 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 watchers 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 watcher 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 watchers 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
watchers. For example, consider a presentity that has three views.
View 1 is delivered to watchers A and B. View 2 is delivered to
watcher C. View 3 is delivered to everyone else. An ACL document
that includes the default view must also include views 1 and 2 with
watchers 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 watcher 1.
If two URI are present within <member> elements within the same
<rule>, it represents a contract by the presence server that both
users MUST get the same view. Formally, if the presence server were
to receive a subscription from each watcher, 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 presence server 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,
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that they do in fact receive the same view.
An ACL document delivered in a subscription from watcher W MUST
include the view associated with watcher W and MUST include watcher 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 presence
server should include in an ACL. The amount of information to convey
depends on the level of trust between the watching and serving
domains.
Optimal performance is achieved when the ACL document for a
presentity includes all views that the server might ever deliver, and
includes all members for each view, including any defaults and
blocked rules. However, this informs the watching domain of the set
of allowed and blocked watchers, and associated groupings amongst
watchers.
Slightly worse performance is achieved when the ACL document for a
presentity sent in a subscription from watcher W includes only a
single view - the one for watcher W - along with the full set of
watchers 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 watcher W. This approach will cause back-end
subscriptions from every watcher 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 presence of the presentity 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 watchers receive the
default.
Another choice for construction of ACL documents is to include, in a
subscription from watcher W, a single rule containing the rule ID for
the view that watcher W will receive, along with a single member - W.
This approach will still result in a back-end subscription from each
watcher. However, a single notification is sent for each view,
rather than one per watcher. The benefit of this construction is
that it provides the watching domain no additional information about
the authorization policies of the presentity than if this extension
were not in use at all.
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5.3. Example Documents
The example document in Figure 2 shows the case when there is maximum
trust between domains. The full set of watchers, 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>
Figure 2: Example with Maximum Trust
The example in Figure 3 shows a moderate level of trust. This ACL
only shows the view associated with the watcher user1.
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<?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>
Figure 3: Example with Partial Trust
The example in Figure 4 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>
Figure 4: Example with Minimal Trust
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
watcher W on an ACL set. This algorithm is a simple algorithm which
takes, as input, a watcher 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, watcher W should
receive.
The algorithm proceeds as follows. First, each Ai is matched to W.
ACL Ai is a match for watcher W if:
o ACL Ai contains a <member> tag whose URI is a match, based on URI
equality, for W, or
o 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
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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:aclinfo">
<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 watcher is sip:user3@example.com, the
associated rule R1 is 2. If the watcher is sip:user1@example.com or
sip:user2@example.com, the rule R1 is 1. If the watcher 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 watcher. 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 ACL.
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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. In that case, the full ACL,
including blocked senders, is returned in the first subscription to a
presentity. This analysis assumes there is a single, monolithic
presence server 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 watcher per federated presentity,
and assuming symmetric, the same as if there is a single federated
presentity per watcher - e.g., C04 = 1.
Consider then the very large network peering model in
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[I-D.ietf-simple-interdomain-scaling-analysis]. 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 presentity 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] 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.
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.
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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 watcher 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 watchers, at the expense of
non-optimal performance. In particular, it will receive a
subscription from each watcher. 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.
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 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.
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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.
Consider first the case where the serving domain is using the minimal
trust model. In that case, the ACL provided to the watching
information does not carry any information that the watching domain
doesn't already know. It merely points out when two watchers share
the same view. This is something that the watching domain could have
already ascertained by comparing presence documents delivered to each
watcher. The ACL makes this task easier, but nonetheless the
watching 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 watcher, this provides additional information to the
watching domain. This is, however, information that the watching
domain could find out anyway. If it generated a subscription from
each of its users to the presentity 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 watching domain that doesn't actually
implement this extension at all. A user within the domain uses a
client that generates a subscription to a presentity in a remote
domain. This subscription uses an outbound proxy in the watching
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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 watchers that are permitted in the
originating domain. The original watcher 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 watching 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.
In all cases, view sharing requires secure authentication and
encryption between the domains that use it. This is provided by TLS.
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 [RFC2048] and guidelines in
RFC 3023 [RFC3023].
MIME media type name: application
MIME subtype name: aclinfo+xml
Mandatory parameters: none
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Optional parameters: Same as charset parameter application/xml as
specified in RFC 3023 [RFC3023].
Encoding considerations: Same as encoding considerations of
application/xml as specified in RFC 3023 [RFC3023].
Security considerations: See Section 10 of RFC 3023 [RFC3023] and
Section 8 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 [RFC4662] for
SIP-based presence [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 [RFC3688].
URI: The URI for this namespace is urn:ietf:params:xml:ns:aclinfo.
Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
Jonathan Rosenberg (jdrosen@jdrosen.net).
XML:
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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:aclinfo</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].
URI: urn:ietf:params:xml:schema:aclinfo
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.
10. Acknowledgements
The authors would like to thank Avshalom Houri and Michael Froman for
their comments on this document.
11. References
11.1. Normative References
[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
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Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
August 1999.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[W3C.REC-xml-20001006]
Maler, E., Paoli, J., Bray, T., and C. Sperberg-McQueen,
"Extensible Markup Language (XML) 1.0 (Second Edition)",
World Wide Web Consortium FirstEdition REC-xml-20001006,
October 2000,
<http://www.w3.org/TR/2000/REC-xml-20001006>.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-15 (work in progress), June 2008.
11.2. Informative References
[RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC 3863, August 2004.
[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479,
July 2006.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC4480] Schulzrinne, H., Gurbani, V., Kyzivat, P., and J.
Rosenberg, "RPID: Rich Presence Extensions to the Presence
Information Data Format (PIDF)", RFC 4480, July 2006.
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[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",
draft-ietf-simple-interdomain-scaling-analysis-04 (work in
progress), February 2008.
[I-D.ietf-sip-subnot-etags]
Niemi, A., "An Extension to Session Initiation Protocol
(SIP) Events for Conditional Event Notification",
draft-ietf-sip-subnot-etags-02 (work in progress),
February 2008.
[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",
draft-ietf-sipping-presence-scaling-requirements-00 (work
in progress), February 2008.
Authors' Addresses
Jonathan Rosenberg
Cisco
Edison, NJ
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Steve Donovan
Cisco
Richardson, TX
US
Email: stdonova@cisco.com
Kathleen McMurry
Cisco
Richardson, TX
US
Email: kmcmurry@cisco.com
Rosenberg, et al. Expires January 15, 2009 [Page 29]
Internet-Draft Presence View Sharing July 2008
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Rosenberg, et al. Expires January 15, 2009 [Page 30]
