Network Working Group A. B. Roach

Network Working Group A. B. Roach
Request for Comments: 4662 B. Campbell
Category: Standards Track Estacado Systems
J. Rosenberg
Cisco Systems
August 2006

A Session Initiation Protocol (SIP) Event Notification Extension
for Resource Lists

Status of This Memo

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

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

This document presents an extension to the Session Initiation
Protocol (SIP)-Specific Event Notification mechanism for subscribing
to a homogeneous list of resources. Instead of sending a SUBSCRIBE
for each resource individually, the subscriber can subscribe to an
entire list and then receive notifications when the state of any of
the resources in the list changes.

Roach, et al. Standards Track [Page 1]

RFC 4662 SIP Event Lists August 2006

Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................4
3. Overview of Operation ...........................................4
4. Operation of List Subscriptions .................................5
4.1. Negotiation of Support for Resource Lists ..................6
4.2. Subscription Duration ......................................7
4.3. NOTIFY Bodies ..............................................7
4.4. RLS Processing of SUBSCRIBE Requests .......................7
4.5. RLS Generation of NOTIFY Requests ..........................7
4.6. Subscriber Processing of NOTIFY Requests ...................9
4.7. Handling of Forked Requests ...............................10
4.8. Rate of Notifications .....................................10
5. Using multipart/related to Convey Aggregate State ..............10
5.1. XML Syntax ................................................11
5.2. List Attributes ...........................................13
5.3. Resource Attributes .......................................14
5.4. Name Attributes ...........................................14
5.5. Instance Attributes .......................................14
5.6. Constructing Coherent Resource State ......................16
5.6.1. Processing Full State Notifications ................17
5.6.2. Processing Partial State Notifications .............17
6. Example ........................................................18
7. Security Considerations ........................................31
7.1. Authentication ............................................31
7.1.1. RLS and Subscriber in the Same Domain ..............31
7.1.2. RLS and Subscriber in Different Domains ............32
7.2. Risks of Improper Aggregation .............................33
7.3. Signing and Sealing .......................................33
7.4. Infinite Loops ............................................34
8. IANA Considerations ............................................34
8.1. New SIP Option Tag: eventlist .............................34
8.2. New MIME type for Resource List Meta-Information ..........34
8.3. URN Sub-Namespace .........................................35
9. Acknowledgements ...............................................36
10. References ....................................................36
10.1. Normative References .....................................36
10.2. Informative References ...................................37

Roach, et al. Standards Track [Page 2]

RFC 4662 SIP Event Lists August 2006

1. Introduction

The SIP-specific event notification mechanism [2] allows a user (the
subscriber) to request to be notified of changes in the state of a
particular resource. This is accomplished by the subscriber
generating a SUBSCRIBE request for the resource, which is processed
by a notifier that represents the resource.

In many cases, a subscriber has a list of resources they are
interested in. Without some aggregating mechanism, this will require
the subscriber to generate a SUBSCRIBE request for each resource
about which they want information. For environments in which
bandwidth is limited, such as wireless networks, subscribing to each
resource individually is problematic. Some specific problems are:

o Doing so generates substantial message traffic, in the form of the
initial SUBSCRIBE requests for each resource and the refreshes of
each individual subscription.

o The notifier may insist on low refresh intervals, in order to
avoid a long-lived subscription state. This means that the
subscriber may need to generate SUBSCRIBE refreshes faster than it
would like to or has the capacity to.

o The notifier may generate NOTIFY requests more rapidly than the
subscriber desires, causing NOTIFY traffic at a greater volume
than is desired by the subscriber.

To solve these problems, this specification defines an extension to
RFC 3265 [2] that allows for requesting and conveying notifications
for lists of resources. A resource list is identified by a URI, and
it represents a list of zero or more URIs. Each of these URIs is an
identifier for an individual resource for which the subscriber wants
to receive information. In many cases, the URI used to identify the
resource list will be a SIP URI [1]; however, the use of other
schemes (such as pres: [10]) is also foreseen.

The notifier for the list is called a "resource list server", or RLS.
In order to determine the state of the entire list, the RLS will act
as if it has generated a subscription to each resource in the list.

The resource list is not restricted to be inside the domain of the
subscriber. Similarly, the resources in the list are not constrained
to be in the domain of the resource list server.

Roach, et al. Standards Track [Page 3]

RFC 4662 SIP Event Lists August 2006

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].

The following terms are used throughout the remainder of this
document.

Back-End Subscription: Any subscription (SIP or otherwise) that an
RLS creates to learn of the state of a resource. An RLS will
create back-end subscriptions to learn of the state of a resource
about which the RLS is not an authority. For back-end
subscriptions, RLSes act as a subscriber.

List Subscription: A subscription to a resource list. In list
subscriptions, RLSes act as the notifier.

Resource: A resource is any logical entity that has a state or
states that can be subscribed to. Resources are identified by
URIs.

Resource List: A list of zero or more resources that can have their
individual states subscribed to with a single subscription.

RLMI: Resource List Meta-Information. RLMI is a document that
describes the state of the virtual subscriptions associated with a
list subscription.

RLS: Resource List Server. RLSes accept subscriptions to resource
lists and send notifications to update subscribers of the state of
the resources in a resource list.

Virtual Subscription: A Virtual Subscription is a logical construct
within an RLS that represents subscriptions to the resources in a
resource list. For each list subscription it services, an RLS
creates at least one virtual subscription for every resource in
the resource list being subscribed to. In some cases, such as
when the RLS is not the authority for the state of the resource,
this virtual subscription will be associated with a back-end
subscription. In other cases, such as when the RLS is the
authority for the state of the resource, the virtual subscription
will not have a corresponding back-end subscription.

3. Overview of Operation

This section provides an overview of the typical mode of operation of
this extension. It is not normative.

Roach, et al. Standards Track [Page 4]

RFC 4662 SIP Event Lists August 2006

When users wish to subscribe to the resource of a list of resources,
they can use the mechanisms described in this specification. The
first step is the creation of a resource list. This resource list is
represented by a SIP URI. The list contains a set of URIs, each of
which represents a resource for which the subscriber wants to receive
information. The resource list can exist in any domain. The list
could be manipulated through a web page, through a voice response
system, or through some other protocol. The specific means by which
the list is created and maintained is outside the scope of this
specification.

To learn the resource state of the set of elements on the list, the
user sends a single SUBSCRIBE request targeted to the URI of the
list. This will be routed to an RLS for that URI. The RLS acts as a
notifier, authenticates the subscriber, and accepts the subscription.

The RLS may have direct information about some or all of the
resources specified by the list. If it does not, it could subscribe
to any non-local resources specified by the list resource.

Note that subscriptions to non-local resources may or may not be SIP
subscriptions; any mechanism for determining such information may be
employed. This document uses the term "back-end subscription" to
refer to such a subscription, regardless of whether SIP is used to
establish and service it.

As the state of resources in the list change, the RLS generates
notifications to the list subscribers. The RLS can, at its
discretion, buffer notifications of resource changes and send the
resource information to the subscriber in batches, rather than
individually. This allows the RLS to provide rate limiting for the
subscriber.

The list notifications contain a body of type multipart/related. The
root section of the multipart/related content is an XML document that
provides meta-information about each resource present in the list.
The remaining sections contain the actual state information for each
resource.

4. Operation of List Subscriptions

The event list extension acts, in many ways, like an event template
package. In particular, any single list subscription must be
homogeneous with respect to the underlying event package. In other
words, a single list subscription can apply only one event package to
all the resources in the resource list.

Roach, et al. Standards Track [Page 5]

RFC 4662 SIP Event Lists August 2006

Note that it is perfectly valid for an RLS to allow multiple
subscriptions to the same list to use differing event packages.

The key difference between a list subscription and templates in
general is that support for list subscriptions indicates support for
arbitrary nesting of list subscriptions. In other words, elements
within the list may be atomic elements, or they may be lists
themselves.

The consequence of this is that subscription to a URI that represents
a list actually results in several virtual subscriptions to a tree of
resources. The leaf nodes of this tree are virtual subscriptions of
the event type given in the "Event" header field; all other nodes in
the tree are list subscriptions that are serviced as described in
this section and its subsections.

Keep in mind that these virtual subscriptions are not literal SIP
subscriptions (although they may result in SIP subscriptions,
depending on the RLS implementation).

4.1. Negotiation of Support for Resource Lists

This specification uses the SIP option tag mechanism for negotiating
support for the extension defined herein. Refer to RFC 3261 [1] for
the normative description of processing of the "Supported" and
"Require" header fields and the 421 (Extension Required) response
code.

A non-normative description of the implications of the use of
option tags follows.
Any client that supports the event list extension will include an
option tag of "eventlist" in a "Supported" header field of every
SUBSCRIBE message for a subscription for which it is willing to
process a list. If the subscription is made to a URI that
represents a list, the RLS will include "eventlist" in a "Require"
header field of the response to the SUBSCRIBE, and in all NOTIFY
messages within that subscription.

Use of "Require: eventlist" in NOTIFY messages is applied by the
notifier to satisfy the RFC 3261 requirement that a UAC MUST
insert a Require header field into a request if the UAC wishes to
insist that a UAS understand an extension in order to process the
request. Because the NOTIFY would not be usable without applying
the eventlist option, the notifier is obligated to include it.

Including "eventlist" in a "Require" header field in a SUBSCRIBE
request serves no purpose except to break interoperability in certain
cases, and is consequently NOT RECOMMENDED.

Roach, et al. Standards Track [Page 6]

RFC 4662 SIP Event Lists August 2006

Sending of "Supported: eventlist" in a NOTIFY message is meaningless
and silly. Implementations SHOULD NOT include "Supported: eventlist"
in any requests except for SUBSCRIBE.

There is nothing in a SIP URI that indicates whether it represents a
list of resources or a single resource. Therefore, if a subscriber
sends a request to a URI that represents a list resource but does not
include a Supported header field listing the "eventlist" token, the
notifier will typically return a 421 (Extension Required) response
code. RFC 3261 [1] advises that servers avoid returning a 421 and
instead attempt to process the request without the extension.
However, in this case, the URI fundamentally represents a list
resource, and therefore the subscription cannot proceed without this
extension.

4.2. Subscription Duration

Since the primary benefit of the resource list server is to reduce
the overall messaging volume to a subscriber, it is RECOMMENDED that
the subscription duration to a list be reasonably long. The default,
when no duration is specified, is taken from the underlying event
package. Of course, the standard techniques [2] can be used to
increase or reduce this amount.

4.3. NOTIFY Bodies

An implementation compliant to this specification MUST support the
multipart/related and application/rlmi+xml MIME types. These types
MUST be included in an Accept header sent in a SUBSCRIBE message, in
addition to any other types supported by the client (including any
types required by the event package being used).

4.4. RLS Processing of SUBSCRIBE Requests

Once the subscriber is authenticated, the RLS performs authorization
per its local policy. In many cases, each resource list is
associated with a particular user (the one who created it and manages
the set of elements in it), and only that user will be allowed to
subscribe. Of course, this mode of operation is not inherent in the
use of resource lists, and an RLS can use any authorization policy it
chooses.

4.5. RLS Generation of NOTIFY Requests

This specification leaves the choice about how and when to generate
NOTIFY requests at the discretion of the implementor. One of the
differentiators between various RLS implementations is the means by
which they aggregate, rate-limit, or optimize the way in which

Roach, et al. Standards Track [Page 7]

RFC 4662 SIP Event Lists August 2006

notifications are generated. As a baseline behavior, the RLS MAY
generate a NOTIFY to the RLS subscriber whenever the state of any
resource on the list changes.

It is important to understand that any given subscription is a
subscription either to a single resource or to a list of resources.
This nature (single resource versus list of resources) cannot change
during the duration of a single subscription. In particular, this
means that RLSes MUST NOT send NOTIFY messages that do not contain
RLMI for a subscription if they have previously sent NOTIFY messages
in that subscription containing RLMI. Similarly, RLSes MUST NOT send
NOTIFY messages that do contain RLMI for a subscription if they have
previously sent NOTIFY messages in that subscription which do not.

List representations necessarily contain RLMI documents for two
reasons. Importantly, they identify the resource to which the
event state corresponds. Many state syntaxes do not fully
identify the resource to which the state applies, or they may
identify the resource in a different way than it is represented in
the list; for example, PIDF documents may contain resource URIs
that are not identical to the URI used to retrieve them. Further,
RLMI documents serve to disambiguate multiple instances of a
single resource.

See Section 5 for a detailed definition of the syntax used to convey
the state of resource lists. For the purposes of the following
discussion, it is important to know that the overall list contains
zero or more resources, and that the resources contain zero or more
instances. Each instance has a state associated with it (pending,
active, or terminating) representing the state of the virtual
subscription.

Notifications contain a multipart document, the first part of which
always contains meta-information about the list (e.g., membership,
state of the virtual subscription to the resource). Remaining parts
are used to convey the actual state of the resources listed in the
meta-information.

The "state" attribute of each instance of a resource in the
meta-information is set according to the state of the virtual
subscription. The meanings of the "state" attribute are described in
RFC 3265 [2].

If an instance of a resource was previously reported to the
subscriber but is no longer available (i.e., the virtual subscription
to that instance has been terminated), the resource list server
SHOULD include that resource instance in the meta-information in the
first NOTIFY message sent to the subscriber following the instance's

Roach, et al. Standards Track [Page 8]

RFC 4662 SIP Event Lists August 2006

unavailability. The RLS MAY continue to do so for future
notifications.

When sending information for a terminated resource instance, the RLS
indicates a state of "terminated" and an appropriate reason value.
Valid reason values and their meanings are described in RFC 3265 [2].
If the RLS will attempt to recover the resource state again at some
point in the future (e.g., when the reason in the meta-information is
"probation"), then the instance of the resource SHOULD remain in the
meta-information until the instance state is available, or until the
RLS gives up on making such state available.

When the first SUBSCRIBE message for a particular subscription is
received by an RLS, the RLS will often not know state information for
all the resources specified by the resource list. For any resource
for which state information is not known, the corresponding "uri"
attribute will be set appropriately, and no <instance> elements will
be present for the resource.

For an initial notification, sections corresponding to resources for
which the RLS does have state will be populated with appropriate data
(subject, of course, to local policy decisions). This will often
occur if the resource list server is co-located with the server for
one or more of the resources specified on the list.

Immediate notifications triggered as a result of subsequent SUBSCRIBE
messages SHOULD include an RLMI document in which the full state is
indicated. The RLS SHOULD also include state information for all
resources in the list for which the RLS has state, subject to policy
restrictions. This allows the subscriber to refresh their state, and
to recover from lost notifications.

4.6. Subscriber Processing of NOTIFY Requests

Notifications for a resource list can convey information about a
subset of the list elements. This means that an explicit algorithm
needs to be defined in order to construct coherent and consistent
state.

The XML document present in the root of the multipart/related
document contains a <resource> element for some or all of the
resources in the list. Each <resource> element contains a URI that
uniquely identifies the resource to which that section corresponds.
When a NOTIFY arrives, it can contain full or partial state (as
indicated by the "fullState" attribute of the top-level <list>
element). If full state is indicated, then the recipient replaces
all state associated with the list with the entities in the NOTIFY
body. If full state is not indicated, the recipient of the NOTIFY

Roach, et al. Standards Track [Page 9]

RFC 4662 SIP Event Lists August 2006

updates information for each identified resource. Information for
any resources that are not identified in the NOTIFY is not changed,
even if they were indicated in previous NOTIFY messages. See
Section 5.6 for more information.

When full state is indicated, note that it applies only to the
RLMI document in which it occurs. In particular, one of the
<resource> elements in the document may in turn refer to another
list of resources. Any such sub-lists will be detailed in their
own RLMI documents, which may or may not have full state
indicated.

Further note that the underlying event package may have its own
rules for compositing partial state notification. When processing
data related to those packages, their rules apply (i.e., the fact
that they were reported as part of a list does not change their
partial notification semantics).

Finally, note that as a consequence of the way in which resource
list subscriptions work, polling of resource state may not be
particularly useful. While such polls will retrieve the resource
list, they will not necessarily contain state for some or all of
the resources on the list.

4.7. Handling of Forked Requests

Forking makes little sense with subscriptions to event lists, since
the whole idea is a centralization of the source of notifications.
Therefore, a subscriber to a list MUST NOT install multiple
subscriptions when the initial request is forked. If multiple
responses are received, they are handled using the techniques
described in Section 4.4.9 of RFC 3265 [2].

4.8. Rate of Notifications

One potential role of the RLS is to perform rate limitations on
behalf of the subscriber. As such, this specification does not
mandate any particular rate limitation, and rather leaves that to the
discretion of the implementation.

5. Using multipart/related to Convey Aggregate State

In order to convey the state of multiple resources, the list
extension uses the "multipart/related" mime type. The syntax for
multipart/related is defined in "The MIME Multipart/Related Content-
type" [4].

Roach, et al. Standards Track [Page 10]

RFC 4662 SIP Event Lists August 2006

5.1. XML Syntax

The root document of the multipart/related body MUST be a Resource
List Meta-Information (RLMI) document. It is of the type
"application/rlmi+xml". This document contains the meta-information
for the resources contained in the notification. The schema for this
XML document is given below.

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema targetNamespace="urn:ietf:params:xml:ns:rlmi"
elementFormDefault="qualified"
xmlns="urn:ietf:params:xml:ns:rlmi"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"/>
<xs:element name="list">
<xs:complexType>
<xs:sequence>
<xs:element ref="name" minOccurs="0"
maxOccurs="unbounded" />
<xs:element ref="resource" minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="uri" type="xs:anyURI" use="required" />
<xs:attribute name="version" type="xs:unsignedInt"
use="required" />
<xs:attribute name="fullState" type="xs:boolean"
use="required" />
<xs:attribute name="cid" type="xs:string" use="optional" />
<xs:anyAttribute processContents="lax" />
</xs:complexType>
</xs:element>
<xs:element name="resource">
<xs:complexType>
<xs:sequence>
<xs:element ref="name" minOccurs="0"
maxOccurs="unbounded" />
<xs:element ref="instance" minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="uri" type="xs:anyURI" use="required" />
<xs:anyAttribute processContents="lax" />
</xs:complexType>
</xs:element>
<xs:element name="instance">
<xs:complexType>
<xs:sequence>
<xs:any minOccurs="0" maxOccurs="unbounded"

Roach, et al. Standards Track [Page 11]

RFC 4662 SIP Event Lists August 2006

processContents="lax" />
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required" />
<xs:attribute name="state" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="active" />
<xs:enumeration value="pending" />
<xs:enumeration value="terminated" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="reason" type="xs:string"
use="optional" />
<xs:attribute name="cid" type="xs:string" use="optional" />
<xs:anyAttribute processContents="lax" />
</xs:complexType>
</xs:element>
<xs:element name="name">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute ref="xml:lang" use="optional"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>

An example of a document formatted using this schema follows.

<?xml version="1.0"?>
<list xmlns="urn:ietf:params:xml:ns:rlmi"
uri="sip:[email protected]"
version="7" fullState="true">
<name xml:lang="en">Buddy List</name>
<name xml:lang="fr">Liste d'amis</name>
<resource uri="sip:[email protected]">
<name>Bob Smith</name>
<instance id="juwigmtboe" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">
<name>Dave Jones</name>
<instance id="hqzsuxtfyq" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">

Roach, et al. Standards Track [Page 12]

RFC 4662 SIP Event Lists August 2006

<name>Jim</name>
<instance id="oflzxqzuvg" state="terminated"
reason="rejected" />
</resource>
<resource uri="sip:[email protected]">
<name>Ed</name>
<instance id="grqhzsppxb" state="pending"/>
</resource>
</list>

5.2. List Attributes

The <list> element present in a list notification MUST contain three
attributes.

The first mandatory <list> attribute is "uri", which contains the uri
that corresponds to the list. Typically, this is the URI to which
the SUBSCRIBE request was sent.

The second mandatory <list> attribute is "version", which contains a
number from 0 to 2^32-1. This version number MUST be 0 for the first
NOTIFY message sent within a subscription, and MUST increase by
exactly one for each subsequent NOTIFY sent within a subscription.

The third mandatory attribute is "fullState". The "fullState"
attribute indicates whether the NOTIFY message contains information
for every resource in the list. If it does, the value of the
attribute is "true" (or "1"); otherwise, it is "false" (or "0"). The
first NOTIFY sent in a subscription MUST contain full state, as must
the first NOTIFY sent after receipt of a SUBSCRIBE request for the
subscription.

Finally, <list> elements MAY contain a "cid" attribute. If present,
the "cid" attribute identifies a section within the multipart/related
body that contains aggregate state information for the resources
contained in the list. The definition of such aggregate information
is outside the scope of this document and will be defined on a per-
package basis, as needed. The cid attribute is the Content-ID for
the corresponding section in the multipart body.

The cid attribute MUST refer only to top-level parts of the
multipart/related document for which the RLMI document in which it
appears is the root. See Section 5.5 for an example.

Roach, et al. Standards Track [Page 13]

RFC 4662 SIP Event Lists August 2006

5.3. Resource Attributes

The resource list contains one <resource> element for each resource
being reported in the notification. These resource elements contain
attributes that identify meta-data associated with each resource.

The "uri" attribute identifies the resource to which the <resource>
element corresponds. Typically, this will be a SIP URI that, if
subscribed to, would return the state of the resource. This
attribute MUST be present.

5.4. Name Attributes

Each list and resource element contains zero or more name elements.
These name elements contain human-readable descriptions or names for
the resource list or resource. The contents of these elements are
somewhat analogous to the "Display Name" present in the SIP name-addr
element.

Name elements optionally contain the standard XML "xml:lang"
attribute. The "xml:lang" attribute, if present, specifies the
language of the human-readable name. If this attribute is present,
it MUST contain a valid language tag. Language tags are defined in
RFC 3066 [6]. The language tag assists applications in determining
which of potentially several name elements should be rendered to the
user.

5.5. Instance Attributes

Each resource element contains zero or more instance elements. These
instance elements are used to represent a single notifier for the
resource. For event packages that allow forking, multiple virtual
subscriptions may exist for a given resource. Multiple virtual
subscriptions are represented as multiple instance elements in the
corresponding resource element. For subscriptions in which forking
does not occur, at most one instance will be present for a given
resource.

The "id" attribute contains an opaque string used to uniquely
identify the instance of the resource. The "id" attribute is unique
only within the context of a resource. Construction of this string
is an implementation decision. Any mechanism for generating this
string is valid, as long as uniqueness within the resource is
assured.

The "state" attribute contains the subscription state for the
identified instance of the resource. This attribute contains one of
the values "active", "pending", or "terminated". The meanings for

Roach, et al. Standards Track [Page 14]

RFC 4662 SIP Event Lists August 2006

these values are as defined for the "Subscription-State" header field
in RFC 3265 [2].

If the "state" attribute indicates "terminated", then a "reason"
attribute MUST also be present. This "reason" attribute has the same
values and meanings as those given for the "reason" parameter on the
"Subscription-State" header field in RFC 3265 [2]. Note that the
"reason" attribute is included for informational purposes; the list
subscriber is not expected to take any automated actions based on the
reason value.

Finally, the "cid" attribute, which MUST be present if the "state"
attribute is "active", identifies the section within the
multipart/related body that contains the actual resource state. This
state is expressed in the content type defined by the event package
for conveying state. The cid attribute is the Content-ID for the
corresponding section in the multipart body.

The cid attribute MUST refer only to top-level parts of the
multipart/related document for which the RLMI document in which it
appears is the root.

Roach, et al. Standards Track [Page 15]

RFC 4662 SIP Event Lists August 2006

For example, consider a multipart/related document containing
three parts; we'll label these parts A, B, and C. Part A is type
application/rlmi+xml, part B is type multipart/related, and part C
is type application/pidf+xml. Part B is in turn a document
containing three parts: D, E, and F. Part D is of type
application/rlmi+xml, and parts E and F are of type
application/pidf+xml.

+-------------------------------------------+
| Top Level Document: multipart/related |
| |
| +---------------------------------------+ |
| | Part A: application/rlmi+xml | |
| +---------------------------------------+ |
| | Part B: multipart/related | |
| | | |
| | +-----------------------------------+ | |
| | | Part D: application/rlmi+xml | | |
| | +-----------------------------------+ | |
| | | Part E: application/pidf+xml | | |
| | +-----------------------------------+ | |
| | | Part F: application/pidf+xml | | |
| | +-----------------------------------+ | |
| | | |
| +---------------------------------------+ |
| | Part C: application/pidf+xml | |
| +---------------------------------------+ |
| |
+-------------------------------------------+

Any "cid" attributes in document A must refer only to parts B or
C. Referring to parts D, E, or F would be illegal. Similarly,
any "cid" attributes in document D must refer only to parts E or
F. Referring to any other parts would be illegal.
Also note that the subscription durations of any back-end
subscriptions are not propagated into the meta-information state
in any way.

5.6. Constructing Coherent Resource State

The resource list subscriber maintains a table for each resource
list. The table contains a row for each resource in the resource
list. Each row is indexed by the URI for that resource. That URI is
obtained from the "uri" attribute on each <resource> element. The
contents of each row contain the state of that resource as conveyed
in the resource document.

Roach, et al. Standards Track [Page 16]

RFC 4662 SIP Event Lists August 2006

For resources that provide versioning information (which is mandated
by [2] for any formats that allow partial notification), each row
also contains a resource state version number. The version number of
the row is initialized with the version specified in the first
document received, as defined by the corresponding event package.
This value is used when comparing versions of partial notifications
for a resource.

The processing of the resource list notification depends on whether
it contains full or partial state.

5.6.1. Processing Full State Notifications

If a notification contains full state, indicated by the <list>
attribute "fullState" set to "true", the notification is used to
update the table. A check is first made to ensure that the "version"
attribute of the <list> attribute in the received message is greater
than the local version number. If not, the received document is
discarded without any further processing. Otherwise, the contents of
the resource-list table are flushed and repopulated from the contents
of the document. A new row in the table is created for each
"resource" element.

5.6.2. Processing Partial State Notifications

If a notification contains partial state, indicated by the <list>
attribute "fullState" set to "false", a check is made to ensure that
no list notifications have been lost. The value of the local version
number (the "version" attribute of the <list> element) is compared to
the version number of the new document.

o If the value in the new document is exactly one higher than the
local version number, the local version number is increased by
one, and the document is processed as described below.

o If the version in the document is more than one higher than the
local version number, the local version number is set to the value
in the new document, and the document is processed as described
below. The list subscriber SHOULD also generate a refresh request
to trigger a full state notification.

o If the version in the document is less than or equal to the local
version, the document is discarded without any further processing.

For each resource listed in the document, the subscriber checks to
see whether a row exists for that resource. This check is done by
comparing the Resource-URI value with the URI associated with the
row. If the resource doesn't exist in the table, a row is added, and

Roach, et al. Standards Track [Page 17]

RFC 4662 SIP Event Lists August 2006

its state is set to the information from that "resource" element. If
the resource does exist, its state is updated to be the information
from that "resource" element, as described in the definition of the
event package. If a row is updated or created such that its state is
now "terminated," that entry MAY be removed from the table at any
time.

6. Example

This section gives an example call flow. It is not normative. If a
conflict arises between this call flow and the normative behavior
described in this or any other document, the normative descriptions
are to be followed.

In this particular example, we request a subscription to a nested
presence list. The subscriber's address-of-record is
"sip:[email protected]", and the name of the nested list
resource that we are subscribing to is called
"sip:[email protected]". The underlying event
package is "presence", described by [8].

In this example, the RLS has information to service some of the
resources on the list, but must consult other servers to retrieve
information for others. The implementation of the RLS in this
example uses the SIP SUBSCRIBE/NOTIFY mechanism to retrieve such
information.

Roach, et al. Standards Track [Page 18]

RFC 4662 SIP Event Lists August 2006

Terminal pres.vancouver.example.com pres.stockholm.example.org
| | pres.dallas.example.net |
1 |---SUBSCRIBE--->| | |
2 |<-----200-------| | |
3 |<----NOTIFY-----| | |
4 |------200------>| | |
5 | |---SUBSCRIBE--->| |
6 | |<-----200-------| |
7 | |<----NOTIFY-----| |
8 | |------200------>| |
9 | |------------SUBSCRIBE----------->|
10| |<--------------200---------------|
11| |<-------------NOTIFY-------------|
12| |---------------200-------------->|
13|<----NOTIFY-----| | |
14|------200------>| | |

1. We initiate the subscription by sending a SUBSCRIBE message to
our local RLS. (There is no reason that the RLS we contact has
to be in our domain, of course). Note that we must advertise
support for application/rlmi+xml and multipart/related because
we support the eventlist extension, and that we must advertise
application/pidf+xml because we are requesting a subscription to
presence.

Terminal -> Local RLS

SUBSCRIBE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP terminal.vancouver.example.com;
branch=z9hG4bKwYb6QREiCL
Max-Forwards: 70
To: <sip:[email protected]>
From: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 322723822 SUBSCRIBE
Contact: <sip:terminal.vancouver.example.com>
Event: presence
Expires: 7200
Supported: eventlist
Accept: application/pidf+xml
Accept: application/rlmi+xml
Accept: multipart/related
Accept: multipart/signed
Accept: application/pkcs7-mime
Content-Length: 0

Roach, et al. Standards Track [Page 19]

RFC 4662 SIP Event Lists August 2006

2. The Local RLS completes the SUBSCRIBE transaction. Note that
authentication and authorization would normally take place at
this point in the call flow. Those steps are omitted for
brevity.

Local RLS -> Terminal

SIP/2.0 200 OK
Via: SIP/2.0/TCP terminal.vancouver.example.com;
branch=z9hG4bKwYb6QREiCL
To: <sip:[email protected]>;tag=zpNctbZq
From: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 322723822 SUBSCRIBE
Contact: <sip:pres.vancouver.example.com>
Expires: 7200
Require: eventlist
Content-Length: 0

3. As is required by RFC 3265 [2], the RLS sends a NOTIFY
immediately upon accepting the subscription. In this example,
we are assuming that the local RLS is also an authority for
presence information for the users in the
"vancouver.example.com" domain. The NOTIFY contains an RLMI
document describing the entire buddy list (initial notifies
require full state), as well as presence information for the
users about which it already knows. Note that, since the RLS
has not yet retrieved information for some of the entries on the
list, those <resource> elements contain no <instance> elements.

Local RLS -> Terminal

NOTIFY sip:terminal.vancouver.example.com SIP/2.0
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKMgRenTETmm
Max-Forwards: 70
From: <sip:[email protected]>;tag=zpNctbZq
To: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 997935768 NOTIFY
Contact: <sip:pres.vancouver.example.com>
Event: presence
Subscription-State: active;expires=7200
Require: eventlist
Content-Type: multipart/related;type="application/rlmi+xml";
start="<[email protected]>";
boundary="50UBfW7LSCVLtggUPe5z"
Content-Length: 1560

Roach, et al. Standards Track [Page 20]

RFC 4662 SIP Event Lists August 2006

--50UBfW7LSCVLtggUPe5z
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/rlmi+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<list xmlns="urn:ietf:params:xml:ns:rlmi"
uri="sip:[email protected]"
version="1" fullState="true">
<name xml:lang="en">Buddy List at COM</name>
<name xml:lang="de">Liste der Freunde an COM</name>
<resource uri="sip:[email protected]"">
<name>Bob Smith</name>
<instance id="juwigmtboe" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">
<name>Dave Jones</name>
<instance id="hqzsuxtfyq" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">
<name>Ed at NET</name>
</resource>
<resource uri="sip:[email protected]">
<name xml:lang="en">My Friends at ORG</name>
<name xml:lang="de">Meine Freunde an ORG</name>
</resource>
</list>

--50UBfW7LSCVLtggUPe5z
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="sg89ae">
<status>
<basic>open</basic>
</status>
<contact priority="1.0">sip:[email protected]</contact>
</tuple>
</presence>

--50UBfW7LSCVLtggUPe5z
Content-Transfer-Encoding: binary

Roach, et al. Standards Track [Page 21]

RFC 4662 SIP Event Lists August 2006

Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="slie74">
<status>
<basic>closed</basic>
</status>
</tuple>
</presence>

--50UBfW7LSCVLtggUPe5z--

4. The terminal completes the transaction.

Terminal -> Local RLS

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKMgRenTETmm
From: <sip:[email protected]>;tag=zpNctbZq
To: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 997935768 NOTIFY
Contact: <sip:terminal.vancouver.example.com>
Content-Length: 0

5. In order to service the subscription, the local RLS subscribes
to the state of the resources. In this step, the RLS attempts
to subscribe to the presence state of the resource
"sip:[email protected]". Since the local RLS knows how to
receive notifications for list subscriptions, it includes the
"Supported: eventlist" header field in its request. Although
the linkage between this subscription and the one sent by the
terminal is left up to the application, this message
demonstrates some reasonable behavior by including "Accept"
header fields for all the body types it knows the subscriber
(Terminal) supports. This is safe to do, since the local RLS
will only pass these formats through to the subscriber and does
not need to actually understand them.

Local RLS -> Presence Server in dallas.example.net

SUBSCRIBE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKMEyGjdG1LH

Roach, et al. Standards Track [Page 22]

RFC 4662 SIP Event Lists August 2006

Max-Forwards: 70
To: <sip:[email protected]>
From: <sip:[email protected]>;tag=aM5icQu9
Call-ID: [email protected]
CSeq: 870936068 SUBSCRIBE
Contact: <sip:pres.vancouver.example.com>
Identity: Tm8sIHRoaXMgaXNuJ3QgYSByZWFsIGNlcnQuIFlvdSBvn
Zpb3VzbHkgaGF2ZSB0aW1lIHRvIGtpbGwuIEkKc3VnZ2V
zdCBodHRwOi8vd3d3LmhvbWVzdGFycnVubmVyLmNvbS8K
Identity-Info: https://vancouver.example.com/cert
Event: presence
Expires: 3600
Supported: eventlist
Accept: application/pidf+xml
Accept: application/rlmi+xml
Accept: multipart/related
Accept: multipart/signed
Accept: application/pkcs7-mime
Content-Length: 0

6. The Presence Server in dallas.example.net completes the
SUBSCRIBE transaction. Note that authentication would normally
take place at this point in the call flow. This step is omitted
for brevity.

Presence Server in dallas.example.net -> Local RLS

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKMEyGjdG1LH
To: <sip:[email protected]>;tag=e45TmHTh
From: <sip:[email protected]>;tag=aM5icQu9
Call-ID: [email protected]
CSeq: 870936068 SUBSCRIBE
Contact: <sip:dallas.example.net>
Expires: 3600
Content-Length: 0

7. In this example, we assume that the server at dallas.example.net
doesn't have enough authorization information to reject or
accept our subscription. The initial notify, therefore,
contains a "Subscription-State" of "pending". Presumably, the
party responsible for accepting or denying authorization for the
resource is notified of this change; however, those steps are
not included in this call flow for brevity.

Roach, et al. Standards Track [Page 23]

RFC 4662 SIP Event Lists August 2006

Presence Server in dallas.example.net -> Local RLS

NOTIFY sip:pres.vancouver.example.com SIP/2.0
Via: SIP/2.0/TCP pres.dallas.example.net;
branch=z9hG4bKfwpklPxmrW
Max-Forwards: 70
From: <sip:[email protected]>;tag=e45TmHTh
To: <sip:[email protected]>;tag=aM5icQu9
Call-ID: [email protected]
CSeq: 1002640632 NOTIFY
Contact: <sip:dallas.example.net>
Subscription-State: pending;expires=3600
Event: presence
Require: eventlist
Content-Length: 0

8. The local RLS completes the NOTIFY transaction. Note that, at
this point, the Local RLS has new information to report to the
subscriber. Whether it chooses to report the information
immediately or spool it up for later delivery is completely up
to the application. For this example, we assume that the RLS
will wait for a short period of time before doing so, in order
to allow the subscriptions it sent out sufficient time to
provide useful data.

Local RLS -> Presence Server in dallas.example.net

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.dallas.example.net;
branch=z9hG4bKfwpklPxmrW
From: <sip:[email protected]>;tag=e45TmHTh
To: <sip:[email protected]>;tag=aM5icQu9
Call-ID: [email protected]
CSeq: 1002640632 NOTIFY
Contact: <sip:pres.vancouver.example.com>
Content-Length: 0

9. The Local RLS subscribes to the state of the other non-local
resource.

Local RLS -> RLS in stockholm.example.org

SUBSCRIBE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKFSrAF8CZFL
Max-Forwards: 70
To: <sip:[email protected]>
From: <sip:[email protected]>;tag=a12eztNf

Roach, et al. Standards Track [Page 24]

RFC 4662 SIP Event Lists August 2006

Call-ID: [email protected]
CSeq: 980774491 SUBSCRIBE
Contact: <sip:pres.vancouver.example.com>
Identity: Tm90IGEgcmVhbCBzaWduYXR1cmUsIGVpdGhlci4gQ2VydGFp
bmx5IHlvdSBoYXZlIGJldHRlcgp0aGluZ3MgdG8gYmUgZG9p
bmcuIEhhdmUgeW91IGZpbmlzaGVkIHlvdXIgUkxTIHlldD8K
Identity-Info: https://vancouver.example.com/cert
Event: presence
Expires: 3600
Supported: eventlist
Accept: application/pidf+xml
Accept: application/rlmi+xml
Accept: multipart/related
Accept: multipart/signed
Accept: application/pkcs7-mime
Content-Length: 0

10. The RLS in stockholm.example.org completes the SUBSCRIBE
transaction. Note that authentication would normally take place
at this point in the call flow. This step is omitted for
brevity.

RLS in stockholm.example.org -> Local RLS

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bKFSrAF8CZFL
To: <sip:[email protected]>;tag=JenZ40P3
From: <sip:[email protected]>;tag=a12eztNf
Call-ID: [email protected]
CSeq: 980774491 SUBSCRIBE
Contact: <sip:stockholm.example.org>
Expires: 3600
Content-Length: 0

11. In this example, we assume that the RLS in stockholm.example.org
is also an authority for presence information for the users in
the "stockholm.example.org" domain. The NOTIFY contains an RLMI
document describing the contained buddy list, as well as
presence information for those users. In this particular case,
the RLS in stockholm.example.org has chosen to sign [14] the
body of the NOTIFY message. As described in RFC 3851, signing
is performed by creating a multipart/signed document that has
two parts. The first part is the document to be signed (in this
example, the multipart/related document that describes the list
resource states), while the second part is the actual signature.

Roach, et al. Standards Track [Page 25]

RFC 4662 SIP Event Lists August 2006

RLS in stockholm.example.org -> Local RLS

NOTIFY sip:pres.vancouver.example.com SIP/2.0
Via: SIP/2.0/TCP pres.stockholm.example.org;
branch=z9hG4bKmGL1nyZfQI
Max-Forwards: 70
From: <sip:[email protected]>;tag=JenZ40P3
To: <sip:[email protected]>;tag=a12eztNf
Call-ID: [email protected]
CSeq: 294444656 NOTIFY
Contact: <sip:stockholm.example.org>
Event: presence
Subscription-State: active;expires=3600
Require: eventlist
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1;boundary="l3WMZaaL8NpQWGnQ4mlU"
Content-Length: 2038

--l3WMZaaL8NpQWGnQ4mlU
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: multipart/related;type="application/rlmi+xml";
start="<[email protected]>";
boundary="tuLLl3lDyPZX0GMr2YOo"

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/rlmi+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<list xmlns="urn:ietf:params:xml:ns:rlmi"
uri="sip:[email protected]" version="1"
fullState="true">
<name xml:lang="en">Buddy List at COM</name>
<name xml:lang="de">Liste der Freunde an COM</name>
<resource uri="sip:[email protected]">
<name>Joe Thomas</name>
<instance id="1" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">
<name>Mark Edwards</name>
<instance id="1" state="active"
cid="[email protected]"/>
</resource>
</list>

Roach, et al. Standards Track [Page 26]

RFC 4662 SIP Event Lists August 2006

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="x823a4">
<status>
<basic>open</basic>
</status>
<contact priority="1.0">sip:[email protected]</contact>
</tuple>
</presence>

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="z98075">
<status>
<basic>closed</basic>
</status>
</tuple>
</presence>

--tuLLl3lDyPZX0GMr2YOo--

--l3WMZaaL8NpQWGnQ4mlU
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pkcs7-signature

[PKCS #7 signature here]

--l3WMZaaL8NpQWGnQ4mlU--

Roach, et al. Standards Track [Page 27]

RFC 4662 SIP Event Lists August 2006

12. The Local RLS completes the NOTIFY transaction.

Local RLS -> RLS in stockholm.example.org

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.stockholm.example.org;
branch=z9hG4bKmGL1nyZfQI
From: <sip:[email protected]>;tag=JenZ40P3
To: <sip:[email protected]>;tag=a12eztNf
Call-ID: [email protected]
CSeq: 294444656 NOTIFY
Contact: <sip:pres.vancouver.example.com>
Content-Length: 0

13. At this point, the Local RLS decides it has collected enough
additional information to warrant sending a new notification to
the user. Although sending a full notification would be
perfectly acceptable, the RLS decides to send a partial
notification instead. The RLMI document contains only
information for the updated resources, as indicated by setting
the "fullState" parameter to "false". To avoid corrupting the
S/MIME signature on the data received from the RLS in
stockholm.example.org, the local RLS copies the entire
multipart/signed body as-is into the notification that it sends.

Local RLS -> Terminal

NOTIFY sip:terminal.vancouver.example.com SIP/2.0
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bK4EPlfSFQK1
Max-Forwards: 70
From: <sip:[email protected]>;tag=zpNctbZq
To: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 997935769 NOTIFY
Contact: <sip:pres.vancouver.example.com>
Event: presence
Subscription-State: active;expires=7200
Require: eventlist
Content-Type: multipart/related;type="application/rlmi+xml";
start="<[email protected]>";
boundary="TfZxoxgAvLqgj4wRWPDL"
Content-Length: 2862

--TfZxoxgAvLqgj4wRWPDL
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/rlmi+xml;charset="UTF-8"

Roach, et al. Standards Track [Page 28]

RFC 4662 SIP Event Lists August 2006

<?xml version="1.0" encoding="UTF-8"?>
<list xmlns="urn:ietf:params:xml:ns:rlmi"
uri="sip:[email protected]" version="2"
fullState="false">
<name xml:lang="en">Buddy List at COM</name>
<name xml:lang="de">Liste der Freunde an COM</name>
<resource uri="sip:[email protected]">
<name>Ed at NET</name>
<instance id="sdlkmeopdf" state="pending"/>
</resource>
<resource uri="sip:[email protected]">
<name xml:lang="en">My Friends at ORG</name>
<name xml:lang="de">Meine Freunde an ORG</name>
<instance id="cmpqweitlp" state="active"
cid="[email protected]"/>
</resource>
</list>

--TfZxoxgAvLqgj4wRWPDL
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1;boundary="l3WMZaaL8NpQWGnQ4mlU"

--l3WMZaaL8NpQWGnQ4mlU
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: multipart/related;type="application/rlmi+xml";
start="<[email protected]>";
boundary="tuLLl3lDyPZX0GMr2YOo"

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/rlmi+xml;charset="UTF-8"
<?xml version="1.0" encoding="UTF-8"?>
<list xmlns="urn:ietf:params:xml:ns:rlmi"
uri="sip:[email protected]" version="1"
fullState="true">
<name xml:lang="en">Buddy List at ORG</name>
<name xml:lang="de">Liste der Freunde an ORG</name>
<resource uri="sip:[email protected]">
<name>Joe Thomas</name>
<instance id="1" state="active"
cid="[email protected]"/>
</resource>
<resource uri="sip:[email protected]">

Roach, et al. Standards Track [Page 29]

RFC 4662 SIP Event Lists August 2006

<name>Mark Edwards</name>
<instance id="1" state="active"
cid="[email protected]"/>
</resource>
</list>

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="x823a4">
<status>
<basic>open</basic>
</status>
<contact priority="1.0">sip:[email protected]</contact>
</tuple>
</presence>

--tuLLl3lDyPZX0GMr2YOo
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pidf+xml;charset="UTF-8"

<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
entity="sip:[email protected]">
<tuple id="z98075">
<status>
<basic>closed</basic>
</status>
</tuple>
</presence>
--tuLLl3lDyPZX0GMr2YOo--

--l3WMZaaL8NpQWGnQ4mlU
Content-Transfer-Encoding: binary
Content-ID: <[email protected]>
Content-Type: application/pkcs7-signature

[PKCS #7 signature here]

--l3WMZaaL8NpQWGnQ4mlU--

--TfZxoxgAvLqgj4wRWPDL--

Roach, et al. Standards Track [Page 30]

RFC 4662 SIP Event Lists August 2006

14. The terminal completes the NOTIFY transaction.

Terminal -> Local RLS

SIP/2.0 200 OK
Via: SIP/2.0/TCP pres.vancouver.example.com;
branch=z9hG4bK4EPlfSFQK1
From: <sip:[email protected]>;tag=zpNctbZq
To: <sip:[email protected]>;tag=ie4hbb8t
Call-ID: [email protected]
CSeq: 997935769 NOTIFY
Contact: <sip:terminal.vancouver.example.com>
Content-Length: 0

7. Security Considerations

Note that the mechanisms for obtaining state information for
resources in a list are generally left to the RLS implementor. Some
of the security issues below are specific to the circumstance in
which a SIP back-end subscription is used for such a purpose. Non-
SIP mechanisms for obtaining state information of resources in a list
will typically have their own security issues associated with doing
so; however, exhaustively enumerating such access methods is not
possible in this document. Implementors using such mechanisms must
analyze their chosen access methods for relevant security issues.

7.1. Authentication

If back-end subscriptions are required to retrieve resource state
information, the end user is no longer the direct subscriber to the
state of the resource. This means that direct authentication of the
user is no longer possible.

7.1.1. RLS and Subscriber in the Same Domain

It is expected that the most common deployment of RLSes entails that
the subscribers to the RLS will be in the same domain as the RLS.
When this is the case, the RLS then has the ability to act as an
authentication service. The role of authentication service is
defined in "Enhancements for Authenticated Identity Management in the
Session Initiation Protocol (SIP)" [7].

At a high level, under this system, the RLS authenticates the
subscriber and then includes an "Identity" header field in all of the
back-end subscriptions performed on behalf of that authenticated
user. This "Identity" header field cryptographically asserts that
the request has been authorized to be made on behalf of the user
indicated in the "From" header field.

Roach, et al. Standards Track [Page 31]

RFC 4662 SIP Event Lists August 2006

Because the ability to authenticate requests is central to the proper
functioning of the network, any RLS that uses SIP back-end
subscriptions to acquire information about the resources in a
resource list MUST be able to act as an authentication service as
defined in [7], provided that local administrative policy allows it
to do so.

In other words, all RLS implementations that support back-end SIP
subscriptions also must include the ability to be configured to
act as an authentication service. Whether any given administrator
chooses to activate such a feature is completely up to them. Of
course, lacking the ability to act as an identity server, any RLS
so configured will behave as described in the following section,
since it is effectively acting as if it were in a different domain
than the user.

7.1.2. RLS and Subscriber in Different Domains

In the general case, the SIP Authenticated Identity extensions do not
provide a means for the RLS to securely assert that subscriptions are
being performed on the end user's behalf. Specifically, when the
subscriber and the RLS are in different domains, the RLS will have no
means by which it can vouch for the user's identity. Mechanisms by
which back-end subscriptions in such circumstances can be
authenticated are left for future study.

Until such general solutions are developed, RLSes that are in a
different domain than the subscriber on whose behalf they are
creating back-end subscriptions SHOULD subscribe to the resources
using their own identity. By doing so, the RLS will generally obtain
only the resource information that is made publicly available.

Absent such general solutions, implementations of subscriber user
agents MAY attempt direct subscriptions to resources in the resource
list when subscribing to an RLS outside of their domain (either
directly or by way of another resource list subscription). The
resources to be subscribed to will be those indicated in the "uri"
attribute of the <resource> elements present in the RLMI document
returned by the RLS. Directly subscribing to the resources allows
proper authentication of the user to take place, which will generally
authorize them to receive more complete state information.
Implementations that choose to perform such direct subscriptions
SHOULD use the data retrieved instead of any information about the
resource obtained via the list subscription.

Roach, et al. Standards Track [Page 32]

RFC 4662 SIP Event Lists August 2006

7.2. Risks of Improper Aggregation

A resource list server typically serves information to multiple
subscribers at once. In many cases, resources may be present in
several lists; additionally, it is quite possible that resource list
servers will have two users subscribe to the same list.

In these cases, misguided RLS implementations may attempt to minimize
network load by maintaining only one back-end subscription to a
resource in a list and presenting the result of such a subscription
to more than one user. Of course, doing so circumvents any
authorization policy that the notifier for the resource maintains.
Keep in mind that authorization is often much more than a simple
binary "allowed/not allowed" decision; resources may render very
different -- and even conflicting -- resource states, depending on
the identity of the subscribing user.

To prevent the transmission of event information to anyone other than
the intended recipient, implementations MUST NOT present the result
of one back-end subscription to more than one user, unless:

a. The RLS has adequate access to the complete authorization policy
associated with the resource to which the back-end subscription
has been made, AND

b. The RLS can and has determined that presenting the information to
more than one user does not violate such policy.

Note that this is a very difficult problem to solve correctly. Even
in the cases where such access is believed possible, this mode of
operation is NOT RECOMMENDED.

7.3. Signing and Sealing

Implementors should keep in mind that any section of the MIME body
may be signed and/or encrypted as necessary. Resource List Servers
should take care not to modify any MIME bodies they receive from any
back-end subscriptions, and should not generally rely on being able
to read them.

In order to facilitate security, resource list servers SHOULD pass
along indication for support of "multipart/signed" and "application/
pkcs7-mime" content types to any SIP back-end subscriptions, if the
subscriber includes them in the initial SUBSCRIBE message. Not doing
so may actually result in resources refusing to divulge state (if
notifier policy requires encryption, but the RLS fails to convey
support), or subscribers discarding valid state (if subscriber policy
requires a signature, but the RLS fails to convey support).

Roach, et al. Standards Track [Page 33]

RFC 4662 SIP Event Lists August 2006

Note that actual implementation of encryption and signing by the RLS
is not necessary to be able to pass through signed and/or encrypted
bodies.

7.4. Infinite Loops

One risk introduced by the ability to nest resource lists is the
possibility of creating lists that ultimately contain themselves as a
sub-list. Detection and handling of such a case is trivial when the
RLS services all the virtual subscriptions internally. When back-end
subscriptions are created to service virtual subscriptions, however,
detection of such situations becomes a more difficult problem.

Implementors of RLSes that create back-end subscriptions MUST
implement safeguards to prevent such nestings from creating an
infinite loop of subscriptions. Typically, such mechanisms will
require support in the back-end subscription protocol. In
particular, applying filters to the back-end subscriptions can be an
effective way to preclude such problems.

8. IANA Considerations

8.1. New SIP Option Tag: eventlist

This section defines a new option tag for the registry established by
Section 27.1 of RFC 3261[1].

Option Tag Name: eventlist

Description: Extension to allow subscriptions to lists of resources.

Published specification: RFC 4662

8.2. New MIME type for Resource List Meta-Information

MIME Media Type Name: application

MIME subtype name: rlmi+xml

Required parameters: None

Optional parameters: charset

See RFC 3023 [12] for a discussion of the charset parameter on
XML-derived MIME types. Since this MIME type is used exclusively
in SIP, the use of UTF-8 encoding is strongly encouraged.

Encoding considerations: 8-bit text

Roach, et al. Standards Track [Page 34]

RFC 4662 SIP Event Lists August 2006

Security considerations: Security considerations specific to uses of
this MIME type are discussed in RFC 4662. RFC 1874 [11] and RFC
3023 [12] discuss security issues common to all uses of XML.

Interoperability considerations: The use of this MIME body is
intended to be generally interoperable. No unique considerations
have been identified.

Published specification: RFC 4662

Applications that use this media type: This media type is used to
convey meta-information for the state of lists of resources within
a Session Initiation Protocol (SIP) subscription.

Additional information:
Magic Number(s): None.
File Extension(s): None.
Macintosh File Type Code(s): None.
Object Identifier(s) or OID(s): None.

Intended usage: Limited Use

Other Information/General Comment: None.

Person to contact for further information:
Name: Adam Roach
E-Mail: [email protected]
Author/Change Controller: The specification of this MIME type is
a work product of the SIMPLE working group and was authored by
Adam Roach, Jonathan Rosenberg, and Ben Campbell. The IETF has
change control over its specification.

8.3. URN Sub-Namespace

URI: urn:ietf:params:xml:ns:rlmi

Description: This is the XML namespace URI for XML elements defined
by RFC 4662 to describe information about subscriptions when such
subscriptions are aggregated within a single SIP subscription. It
is used in the application/rlmi+xml body type.

Registrant Contact:
Name: Adam Roach
E-Mail: [email protected]
Author/Change Controller: The specification of this MIME type is
a work product of the SIMPLE working group and was authored by
Adam Roach, Jonathan Rosenberg, and Ben Campbell. The IETF has
change control over its specification.

Roach, et al. Standards Track [Page 35]

RFC 4662 SIP Event Lists August 2006

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=utf-8"/>
<title>Namespace for SIP Event Resource List
Meta-Information</title>
</head>
<body>
<h1>Namespace for SIP Event Resource List
Meta-Information</h1>
<h2>application/rlmi+xml</h2>
<p>See <a href="[http://www.rfc-editor.org/rfc/rfc4662.txt]">
RFC4662</a>.</p>
</body>
</html>
END

9. Acknowledgements

Thanks to Sean Olson for a review of and corrections to the usage of
XML in this protocol.

Thanks also to Hisham Khartabil, Paul Kyzivat, Keith Drage, and
Robert Sparks for their careful reviews of and comments on this
document.

10. References

10.1. Normative References

[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[2] Roach, A. B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.

[3] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.

Roach, et al. Standards Track [Page 36]

RFC 4662 SIP Event Lists August 2006

[4] Levinson, E., "The MIME Multipart/Related Content-type", RFC
2387, August 1998.

[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[6] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.

[7] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.

10.2. Informative References

[8] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.

[9] Burger, E., "A Mechanism for Content Indirection in Session
Initiation Protocol (SIP) Messages", RFC 4483, May 2006.

[10] Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
August 2004.

[11] Levinson, E., "SGML Media Types", RFC 1874, December 1995.

[12] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.

[13] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.

[14] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC 1847, October 1995.

[15] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

Roach, et al. Standards Track [Page 37]

RFC 4662 SIP Event Lists August 2006

Authors' Addresses

Adam Roach
Estacado Systems
US

EMail: [email protected]

Ben Campbell
Estacado Systems
US

EMail: [email protected]

Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054-2711
US

EMail: [email protected]

Roach, et al. Standards Track [Page 38]

RFC 4662 SIP Event Lists August 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

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

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

Intellectual Property

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

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

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].

Acknowledgement

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

Roach, et al. Standards Track [Page 39]