SIP J. Rosenberg
Internet-Draft Cisco Systems
Expires: April 1, 2006 September 28, 2005
Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the
Session Initiation Protocol (SIP)
draft-ietf-sip-gruu-05
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Several applications of the Session Initiation Protocol (SIP) require
a user agent (UA) to construct and distribute a URI which can be used
by anyone on the Internet to route a call to that specific UA
instance. A URI which routes to a specific UA instance is called a
Globally Routable UA URI (GRUU). This document describes an
extension to SIP for obtaining a GRUU from a server, and for
communicating a GRUU to a peer within a dialog.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Defining a GRUU . . . . . . . . . . . . . . . . . . . . . . . 4
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 REFER . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Conferencing . . . . . . . . . . . . . . . . . . . . . . . 6
4.3 Presence . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
6. Creation of a GRUU . . . . . . . . . . . . . . . . . . . . . . 9
7. Obtaining a GRUU . . . . . . . . . . . . . . . . . . . . . . . 13
7.1 Through Registrations . . . . . . . . . . . . . . . . . . 13
7.1.1 User Agent Behavior . . . . . . . . . . . . . . . . . 13
7.1.2 Registrar Behavior . . . . . . . . . . . . . . . . . . 15
7.2 Administratively . . . . . . . . . . . . . . . . . . . . . 17
8. Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1 Sending a Message Containing a GRUU . . . . . . . . . . . 18
8.2 Sending a Message to a GRUU . . . . . . . . . . . . . . . 19
8.3 Receiving a Request Sent to a GRUU . . . . . . . . . . . . 20
8.4 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 20
8.4.1 Request Targeting . . . . . . . . . . . . . . . . . . 20
8.4.2 Record Routing . . . . . . . . . . . . . . . . . . . . 22
9. The opaque SIP URI Parameter . . . . . . . . . . . . . . . . . 25
10. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 27
11. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 27
12. Example Call Flow . . . . . . . . . . . . . . . . . . . . . 28
13. Security Considerations . . . . . . . . . . . . . . . . . . 33
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 34
14.1 Header Field Parameter . . . . . . . . . . . . . . . . . . 34
14.2 URI Parameters . . . . . . . . . . . . . . . . . . . . . . 34
14.3 Media Feature Tag . . . . . . . . . . . . . . . . . . . . 35
14.4 SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 35
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
16.1 Normative References . . . . . . . . . . . . . . . . . . . 36
16.2 Informative References . . . . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 39
A. Example GRUU Construction Algorithms . . . . . . . . . . . . . 39
A.1 Instance ID in opaque URI Parameter . . . . . . . . . . . 39
A.2 Encrypted Instance ID and AOR . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . 41
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1. Introduction
The Session Initiation Protocol, RFC 3261 [1] is used to establish
and maintain a dialog between a pair of user agents in order to
manage a communications session. Messages within the dialog are sent
from one user agent to another using a series of proxy hops called
the route set, eventually being delivered to the remote target - the
user agent on the other side of the dialog. This remote target is
identified by a SIP URI obtained from the value of the Contact header
field in INVITE requests and responses.
RFC 3261 mandates that a user agent populate the Contact header field
in INVITE requests and responses with a URI that is global (meaning
that it can be used from any element connected to the Internet), and
that routes to the user agent which inserted it. RFC 3261 also
mandates that this URI be valid for requests sent outside of the
dialog in which the Contact URI was inserted.
In practice, these requirements have proven very difficult to meet.
Few endpoints have a hostname that is is present in DNS. Many
endpoints have an IP address that is private because the client is
behind a NAT. Techniques like the Simple Traversal of UDP Through
NAT (STUN) [15] can be used to obtain IP addresses on the public
Internet. However, many firewalls will prohibit incoming SIP
requests from reaching a client unless they first pass through a
proxy sitting in the DMZ of the network. Thus URIs using STUN-
obtained IP addresses often do not work.
Because of these difficulties, most clients have actually been
inserting URIs into the Contact header field of requests and
responses with the form sip:<IP-address>. These have the property of
routing to the client, but they are generally only reachable from the
proxy to which the user is directly connected. This limitation does
not prevent SIP calls to an Address-of-Record (AOR) from proceeding,
since the user's proxy can usually reach these private addresses, and
the proxy itself is generally reachable over the public network.
However, this issue has impacted the ability of several other SIP
mechanisms and applications to work properly.
An example of such an application is call transfer [25], based on the
REFER method [7]. Another application is the usage of endpoint-
hosted conferences within the conferencing framework [17]. Both of
these mechanisms require the endpoint to be able to construct a URI
that not only routes to that user agent, but is usable by other
entities anywhere on the Internet as a target for new SIP requests.
This specification formally defines a type of URI called a Globally
Routable User Agent URI (GRUU) which has the properties of routing to
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the UA and being reachable from anywhere. Furthermore, it defines a
new mechanism by which a client can obtain a GRUU from its SIP
provider, allowing it to use that URI in the Contact header fields of
its dialog forming requests and responses. Since the GRUU is
provided by the user's SIP provider, the GRUU properties can be
guaranteed by the provider. As a result, the various applications
which require the GRUU property, including transfer, presence, and
conferencing, can work reliably.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [5] and
indicate requirement levels for compliant implementations.
This specification also defines the following additional terms:
contact: The term "contact", when used in all lowercase, refers to a
URI that is bound to an AOR or GRUU by means of a registration. A
contact is usually a SIP URI, and is bound to the AOR and GRUU
through a REGISTER request by appearing as the value of the
Contact header field.
remote target: The term "remote target" refers to a URI that a user
agent uses to identify itself for receipt of both mid-dialog and
out-of-dialog requests. A remote target is established by placing
a URI in the Contact header field of a dialog forming request or
response.
Contact header field: The term "Contact header field", with a
capitalized C, refers to the header field which can appear in
REGISTER requests and responses, redirects, or in dialog creating
requests and responses. Depending on the semantics, the Contact
header field sometimes conveys a contact, and sometimes conveys a
remote target.
3. Defining a GRUU
URIs have properties. Those properties are granted to the URI based
on the policies of the domain that owns the URI, and those properties
are not visible by inspection of the URI. In this context, the
domain that owns the URI is the one indicated in the host part of the
SIP URI. Some of the properties that a domain can confer upon a URI
are:
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The AOR property: A URI has the Address of Record (AOR) property if a
domain will allow it to appear in the To header field of REGISTER
request.
The alias property: A URI is an alias if its treatment by the domain
is identical to another URI.
The service treatment property: A URI has the service treatment
property if the domain will apply applications, features, and
services to calls made by, or made to, that URI, possibly based on
associating that URI with a user that has "subscribed" to various
features.
The anonymous property: A URI has the anonymous property when it is
not possible, by inspection of the URI, to discern the user with
whom the URI is associated.
The identity property: A URI is considered an identity when it is one
that the domain will authorize as a valid value in the From header
field of a request, such that an authentication service will sign
a request with that URI [20].
This specification focuses on a property, called the Globally
Routable User Agent URI (GRUU) property. A URI possesses this
property when the following is true:
Global: It can be used by any UAC connected to the Internet. In that
regard, it is like the address-of-record (AOR) property. A URI
with the AOR property (for example, sip:joe@example.com), is meant
to be used by anyone to reach that user. The same is true for a
URI with the GRUU property.
Routes to a Single Instance: A request sent to that URI will be
routed to a specific UA instance. In that regard, it is unlike
the address-of-record property. When a request is sent to a URI
with the AOR property, routing logic is applied in proxies to
deliver the request to one or more UAs. That logic can result in
a different routing decision based on the time-of-day, or the
identity of the caller. However, when a request is made to a URI
with the GRUU property, the routing logic is dictated by the GRUU
property. The request has to be delivered to a very specific UA
instance. That UA instance has to be the same UA instance for all
requests sent to that URI.
Long Lived: The URI with the GRUU property persists for relatively
long periods of time, ideally being valid for the duration of
existence of the AOR itself. This property cannot be completely
guaranteed, but providers are supposed to do their best to make
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sure that a GRUU remains viable indefinitely.
A URI can have any combination of these properties. It is the
responsibility of the domain which mints the URI to determine what
properties are conferred upon that URI. This specification imposes
requirements on a domain that mints a URI with the GRUU property.
For convenience, a URI that possesses the GRUU property is also
referred to as a GRUU.
4. Use Cases
There are several use cases where the GRUU properties are truly
needed in order for a SIP application to operate.
4.1 REFER
Consider a blind transfer application [25]. User A is talking to
user B. User A wants to transfer the call to user C. So, user A
sends a REFER to user C. That REFER looks like, in part:
REFER sip:C@example.com SIP/2.0
From: sip:A@example.com;tag=99asd
To: sip:C@example.com
Refer-To: (URI that identifiers B's UA)
The Refer-To header field needs to contain a URI that can be used by
user C to place a call to user B. However, this call needs to route
to the specific UA instance which user B is using to talk to user A.
If it didn't, the transfer service would not execute properly. This
URI is provided to user A by user B. Because user B doesn't know who
user A will transfer the call to, the URI has to be usable by anyone.
Therefore, it needs to be a GRUU.
4.2 Conferencing
A similar need arises in conferencing [17]. In that framework, a
conference is described by a URI which identifies the focus of the
conference. The focus is a SIP UA that acts as the signaling hub for
the conference. Each conference participant has a dialog with the
focus. One case described in the framework is where a user A has
made a call to user B. User A puts user B on hold, and calls user C.
Now, user A has two separate dialogs for two separate calls - one to
user B, and one to user C. User A would like to conference them. To
do this, user A's user agent morphs itself into a focus. It sends a
re-INVITE or UPDATE [4] on both dialogs, and provides user B and user
C with an updated remote target that now holds the conference URI.
The URI in the Contact header field also has a callee capabilities
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[11] parameter which indicates that this URI is a conference URI.
User A proceeds to mix the media streams received from user B and
user C. This is called an ad-hoc conference.
At this point, normal conferencing features can be applied. That
means that user B can send another user, user D, the conference URI,
perhaps in an email. User D can send an INVITE to that URI, and join
the conference. For this to work, the conference URI used by user A
in its re-INVITE or UPDATE has to be usable by anyone, and it has to
route to the specific UA instance of user A that is acting as the
focus. If it didn't, basic conferencing features would fail.
Therefore, this URI has to be a GRUU.
4.3 Presence
In a SIP-based presence [27] system, the Presence Agent (PA)
generates notifications about the state of a user. This state is
represented with the Presence Information Document Format (PIDF)
[24]. In a PIDF document, a user is represented by a series of
tuples, each of which describes the services that the user has. Each
tuple also has a URI in the <contact> element, which is a SIP URI
representing that service. A watcher can make a call to that URI,
with the expectation that the call is routed to the service whose
presence is represented in the tuple.
In some cases, the service represented by a tuple may exist on only a
single user agent associated with a user. In such a case, the URI in
the presence document has to route to that specific UA instance.
Furthermore, since the presence document could be used by anyone who
subscribes to the user, the URI has to be usable by anyone. As a
result, it has to be a GRUU.
It is interesting to note that the GRUU may need to be constructed by
a presence agent, depending on how the presence document is computed
by the server.
5. Overview of Operation
This section is tutorial in nature, and does not specify any
normative behavior.
This extension allows a UA to obtain a GRUU, and to use a GRUU.
These two mechanisms are separate, in that a UA can obtain a GRUU in
any way it likes, and use the mechanisms in this specification to use
it. This specification defines two mechanisms for obtaining a GRUU -
through registrations, and through administrative operation. Only
the former requires protocol operations.
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A UA can obtain a GRUU by generating a normal REGISTER request, as
specified in RFC 3261 [1]. This request contains a Supported header
field with the value "gruu", indicating to the registrar that the UA
supports this extension. The UA includes a "+sip.instance" Contact
header field parameter of each contact for which a GRUU is desired.
This Contact parameter contains a globally unique ID that identifies
the UA instance. If the domain that the user is registering against
also supports GRUU, the REGISTER responses will contain the "gruu"
parameter in each Contact header field. This parameter contains a
GRUU which the domain guarantees will route to that UA instance. The
GRUU is associated with the UA instance. Should the client change
its contact, but indicate that it represents the same instance ID,
the server would provide the same GRUU. Furthermore, if the
registration for the contact expires, and the UA registers the
contact at a later time with the same instance identifier, the server
would provide the same GRUU.
Since the GRUU is a URI like any other, it can be handed out by a UA
by placing it in any header field which can contain a URI. A UA will
place the GRUU into the Contact header field of dialog creating
requests and responses it generates; RFC 3261 mandates that the
Contact header field have the GRUU property, and this specification
provides a reliable way for a UA to obtain one. In other words,
clients use the GRUU as a remote target. However, since the remote
target used by clients to date has typically not had the GRUU
properties, implementations have adapted their behaviors (oftentimes
in proprietary ways) to compensate. To facilitate a transition away
from these behaviors, it is necessary for a UA receiving the message
to know whether the remote target is a GRUU or not. To make this
determination, the UA looks for the presence of the Supported header
field in the request or response. If it is present with a value of
"gruu", it means that the remote target is a GRUU.
A domain can construct a GRUU in any way it chooses. However, it is
sometimes desirable to construct them in a way which allows for any
entity that receives the GRUU to determine the AOR for the subscriber
associated with the UA instance. To facilitate that, the GRUU can be
constructed such that it is identical to the subscribers AOR, but
includes the "opaque" URI parameter. The "opaque" URI parameter
provides a general facility to construct a URI (such as a GRUU or a
voicemail inbox for a user) that is related to an AOR, such that any
element can extract the AOR from the constructed URI by removing the
"opaque" parameter. For example:
AOR: sip:alice@example.com
GRUU: sip:alice@example.com;opaque="kjh29x97us97d"
When a proxy in the domain constructs the GRUU, it would set the
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value of the "opaque" URI parameter such that it includes the
instance ID. As such, when that proxy receives a request sent to the
GRUU, it can extract the AOR and instance ID, both of which are
needed to process the request.
When a UA uses a GRUU, it has the option of adding the "grid" URI
parameter to the GRUU. This parameter is opaque to the proxy server
handling the domain. However, when the server maps the GRUU to the
contact bound to it, the server will add the grid parameter into the
registered contact, and use the result in the Request URI. As a
result, when the UA receives the request, the Request URI will
contain the grid parameter it placed in the corresponding GRUU.
The "grid" and "opaque" URI parameters play similar roles, but
complement each other. The "opaque" parameter is added by the owner
of the domain to correlate the GRUU to its instance ID, and to easily
recognize that the URI has the GRUU property. The "grid" parameter
is added by the UA instance so that, when a request is received by
that instance, it can determine the context of the request.
6. Creation of a GRUU
A GRUU is a URI that is created and maintained by a server
authoritative for the domain in which the GRUU resides.
Independently of whether the GRUU is created as a result of a
registration or some other means, a server maintains certain
information associated with the GRUU. This information, and its
relationship with the GRUU, is modeled in Figure 3.
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+-----------+ +-----------+
| | associated | |
| |1 with n| |
| AOR |<----------------| GRUU |
| | | |
| | | |
+-----------+ +-----------+
^1 is ^^ |n
| bound //0..1 |
is| to// |associated
bound| // |with
to| // |
| // |
|0..n // V1
+-----------+ // +-----------+
| | / 0..n | |
| | | |
| contact |---------------->| Instance |
| |1 has 0..1| ID |
| | | |
+-----------+ +-----------+
Figure 3
The instance ID plays a key role in this specification. It is an
identifier, represented with a URN, that uniquely identifies a SIP
user agent amongst all other user agents associated with an AOR. For
hardware-based user agents, the instance ID would typically be burned
into the device at the factory, similar to the way a unique serial
number is encoded into each device. For software-based user agents,
each installation represents a unique instance. As such, the
identifier could be generated on installation and then stored on disk
for persistence.
A GRUU is associated, in a many-to-one fashion, with the combination
of an AOR and instance ID. This combination is referred to as an
instance ID/AOR pair. For each GRUU, there is one instance ID/AOR
pair, and for each instance ID/AOR pair, there can be one or more
GRUU. More than one GRUU might be defined in order to have aliases
or URI that are anonymous or have other URI properties. However,
this specification doesn't define any way for the client to learn
about or use more than a single GRUU for each instance ID/AOR pair.
The instance ID/AOR pair serves to uniquely identify a user agent
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instance servicing a specific AOR. The AOR identifies a resource,
such as a user or service within a domain, and the instance ID
identifies a specific UA instance servicing requests for that
resource.
It is important to understand that GRUU is associated with the
instance ID/AOR pair, not just the instance ID. For example, if a
user registered the contact sip:ua@pc.example.com to the AOR
sip:user@example.com, and included a +sip.instance="urn:foo:1"
parameter in the Contact header field, and also registered the
contact sip:ua-112@pc.example.com with the same +sip.instance Contact
header field parameter to a second AOR, say sip:boss@example.com,
each of those UA instances would have a different GRUU, since they
belong to different AORs. That is the reason why a single instance
ID can be associated with multiple GRUU; there would be one such
association for each AOR. The same goes for the association of AOR
to GRUU; there would be one such association for each instance ID.
The contacts that are bound to the GRUU are always the ones that have
an instance ID associated with that GRUU. If none of the contacts
bound to the AOR have the instance ID associated with the GRUU, then
there are no contacts bound to the GRUU. If a contact should become
registered to the AOR that has an instance ID equal to the one
associated with the GRUU, that contact also becomes bound to the
GRUU. If that contact should expire, it will no longer be bound to
the AOR, and similarly, it will no longer be bound to the GRUU. The
URI of the contact is irrelevant in determining whether it is bound
to a particular GRUU; only the instance ID and AOR are important.
This specification does not mandate a particular mechanism for
construction of the GRUU. Several example approaches are given in
Appendix A. However, the GRUU MUST exhibit the following properties:
o The domain part of the URI is an IP address present on the public
Internet, or, if it is a host name, the resolution procedures of
RFC 3263 [2], once applied, result in an IP address on the public
Internet.
o When a request is sent to the GRUU, it routes to a server that can
make sure the request is delivered to the UA instance. For GRUU
created through registrations, this means that the GRUU has to
route to a proxy server with access to registration data.
o A server in the domain can determine that the URI is a GRUU.
o For each GRUU, both the SIP and SIPS versions MUST exist. The
SIPS URI may not always work, particularly if the proxy cannot
establish a secure connection to the client. However, the SIPS
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URI always exists.
Section 8.4 defines additional behaviors that a proxy must exhibit on
receipt of a GRUU.
When a domain constructs a URI with the GRUU properties, it MAY
confer other properties upon this URI as a matter of domain policy.
A domain can elect to confer properties like identity, anonymity, and
service treatment. There is nothing in this specification that can
allow the recipient of the GRUU to determine which of these
properties besides the GRUU property itself have been conferred to
the URI.
The service treatment property merits further discussion. Typically,
the services a proxy executes upon receipt of a request sent to a
GRUU will be a subset of those executed when a request is sent to the
AOR. For requests that are outside of a dialog, it is RECOMMENDED to
apply screening types of functions, both automated (such as black and
white list screening) and interactive (such as interactive voice
response (IVR) applications which confer with the user to determine
whether to accept a call). In many cases, the new request is related
to an existing dialog, and may be an attempt to join it (using the
Join header field [30]) or replace it (using the Replaces header
field [31]). In such cases, the UA will typically make its own
authorization decisions, allowing the reuqest if the sender can prove
it knows the dialog identifiers [19]. In such cases, it might make
sense to bypass screening services, but network designers need to
carefully consider this, as it depends on the specific type of
screening service.
However, forwarding services, such as call forwarding, SHOULD NOT be
provided for requests sent to a GRUU. The intent of the GRUU is to
target a specific UA instance, and this is incompatible with
forwarding operations.
Mid-dialog requests will also be sent to GRUUs, as they are included
as the remote-target in dialog forming requests and responses. In
those cases, however, a proxy SHOULD only apply services that are
meaningful for mid-dialog requests generally speaking. This excludes
screening functions, as well as forwarding ones.
The "opaque" URI parameter, defined in Section 9, provides a means
for a domain to construct a GRUU such that the AOR associated with
the GRUU is readily extractable from the GRUU. Unless the GRUU is
meant to also possess the anonymity property, it is RECOMMENDED that
GRUUs be constructed using this parameter.
Since the GRUU is associated with both the instance ID and AOR, for
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any particular AOR there can be a potentially infinite number of
GRUU, and potentially more than one for each instance ID. However,
the instance IDs are only known to the network when an instance
actually registers with one. As a result, it is RECOMMENDED that a
GRUU be created at the time a contact with an instance ID is first
registered to an AOR (even if that registration indicates that the
registering UA doesnt even support GRUU), until the time that the AOR
is no longer valid in the domain. In this context, the GRUU exists
if the domain, upon receiving a request for that GRUU, recognizes it
as a GRUU, can determine the AOR and instance ID associated with it,
and translate the GRUU to a contact if there is one with that
instance ID currently registered. This property of the GRUU can be
difficult to achieve through software failures and power outages
within a network, and for this reason, the requirement is at
RECOMMENDED strength, and not MUST.
7. Obtaining a GRUU
A GRUU can be obtained in many ways. This document defines two -
through registrations, and through administrative operation.
7.1 Through Registrations
When a GRUU is associated with a user agent that comes and goes, and
therefore registers to the network to bind itself to an AOR, a GRUU
is provided to the user agent through SIP REGISTER messages.
7.1.1 User Agent Behavior
7.1.1.1 Generating a REGISTER Request
When a UA compliant to this specification generates a REGISTER
request (initial or refresh), it MUST include the Supported header
field in the request. The value of that header field MUST include
"gruu" as one of the option tags. This alerts the registrar for the
domain that the UA supports the GRUU mechanism.
Furthermore, for each contact for which the UA desires to obtain a
GRUU, the UA MUST include a "sip.instance" media feature tag as a UA
characteristic [11]. The instance ID MUST identify the UA that is
performing the registration. As described in [11], this media
feature tag will be encoded in the Contact header field as the
"+sip.instance" Contact header field parameter. The value of this
parameter MUST be a URN [10]. Usage of a URN is a MUST since it
provides a persistent and unique name for the UA instance, allowing
it to obtain the same GRUU over time. It also provides an easy way
to guarantee uniquess within the AOR. However, this specification
does not require a long-lived and persistent instance identifier to
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properly function, and in some cases, there may be cause to use an
identifier with weaker temporal persistence.
[11] defines equality rules for callee capabilities parameters, and
according to that specification, the "sip.instance" media feature tag
will be compared by case sensitive string comparison. This means
that the URN will be encapsulated by angle brackets "<" and ">" when
it is placed within the quoted string value of the +sip.instance
Contact header field parameter. The case sensitive matching rules
apply only to the generic usages defined there and in the caller
preferences specification [23]. When the instance ID is used in this
specification, it is effectively "extracted" from the value in the
"sip.instance" media feature tag, and thus equality comparisons are
performed using the rules for URN equality specific to the scheme in
the URN. If the element performing the comparisons does not
understand the URN scheme, it performs the comparisons using the
lexical equality rules defined in RFC 2141. Lexical equality may
result in two URN being considered unequal when they are actually
equal. In this specific usage of URNs, the only element which
provides the URN is the SIP UA instance identified by that URN. As a
result, the UA instance SHOULD provide lexically equivalent URNs in
each registration it generates. This is likely to be normal behavior
in any case; clients are not likely to modify the value of the
instance ID so that it remains functionally equivalent to previous
registrations, but lexigraphically different.
A registering UAC SHOULD use a UUID URN [28] in its "+sip.instance"
Contact header field parameter. The UUID URN allows for non-
centralized computation of a URN based on time, unique names (such as
a MAC address) or a random number generator. However, if a different
URN scheme is used, the URN MUST be selected such that the instance
can be certain that no other instance registering against the same
AOR would choose the same URN value. An example of a URN that would
not meet the requirements of this specification is the national
bibliographic number [16]. Since there is no clear relationship
between an SIP UA instance and a URN in this namespace, there is no
way a selection of a value can be performed that guarantees that
another UA instance doesn't choose the same value.
If a UA instance is registering against multiple AOR, it is
RECOMMENDED that a UA instance provide a different contact URI for
each AOR. This is needed for the UA to determine which GRUU to use
as the remote target in responses to incoming dialog forming
requests, as discussed in Section 8.1.
Besides the procedures discussed above, the REGISTER request is
constructed identically to the case where this extension was not
understood. Specifically, the contact in the REGISTER request SHOULD
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NOT contain the gruu Contact header field parameter, and the contact
URI itself SHOULD NOT contain the grid parameter defined below. Any
such parameters are ignored by the registrar, as the UA cannot
propose a GRUU for usage with the contact.
If a UA wishes to guarantee that the request is not processed unless
the domain supports and uses this extension, it MAY include a Require
header field in the request with a value that contains the "gruu"
option tag.
7.1.1.2 Processing the REGISTER Response
If the response is a 2xx, each Contact header field that contained
the "+sip.instance" Contact header field parameter may also contain a
"gruu" parameter. This parameter contains a SIP or SIPS URI that
represents a GRUU corresponding to the UA instance that registered
the contact. The URI will be a SIP URI if the To header field in the
REGISTER request contained a SIP URI, else it will be a SIPS URI if
the To header field in the REGISTER request contained a SIPS URI.
Any requests sent to the GRUU URI will be routed by the domain to a
contact with that instance ID. The GRUU will not normally change in
subsequent 2xx responses to REGISTER. Indeed, even if the UA lets
the contact expire, when it re-registers it at any later time, the
registrar will normally provide the same GRUU for the same address-
of-record and instance ID. However, as discussed above, this
property cannot be completely guaranteed, as network failures may
make it impossible to provide an identifier that persists for all
time. As a result, a UA MUST be prepared to receive a different GRUU
for the same instance ID/AOR pair in a subsequent registration
response.
A non-2xx response to the REGISTER request has no impact on any
existing GRUU previously provided to the UA. Specifically, if a
previously successful REGISTER request provided the UA with a GRUU, a
subsequent failed request does not remove, delete, or otherwise
invalidate the GRUU.
7.1.2 Registrar Behavior
A registrar MAY create a GRUU for a particular instance ID/AOR pair
at any time. Of course, if a UA requests a GRUU in a registration,
and the registrar has not yet created one, it will need to do so in
order to respond to the registration request. However, the registrar
can create the GRUU in advance of any request from a UA.
A registrar MUST create both the SIP and SIPS versions of the GRUU,
such that if the GRUU exists, both URI exist.
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7.1.2.1 Processing a REGISTER Request
A REGISTER request might contain a Require header field; this
indicates that the registration has to understand this extension in
order to process the request.
As the registrar is processing the contacts in the REGISTER request
according to the procedures of step 7 in Section 10.3 of RFC 3261,
the registrar checks whether each Contact header field in the
REGISTER message contains a "+sip.instance" header field parameter.
If present, the contact is processed further. If the registrar had
not yet created a GRUU for that instance ID/AOR pair, it MUST do so
at this time according to the procedures of Section 6. If the
contact contained a "gruu" Contact header field parameter, it MUST be
ignored by the registrar. A UA cannot suggest or otherwise provide a
GRUU to the registrar.
Registration processing then continues as defined in RFC 3261. If,
after that processing, that contact is bound to the AOR, it also
becomes bound to the GRUU associated with that instance ID/AOR pair.
If, after that processing, the contact was not bound to the AOR (due,
for example, to an expires value of zero), the contact is not bound
to the GRUU either. The registrar MUST store the instance ID along
with the contact.
When generating the 200 (OK) response to the REGISTER request, the
procedures of step 8 of Section 10.3 of RFC 3261 are followed.
Furthermore, for each Contact header field value placed in the
response, if the registrar has stored an instance ID associated with
that contact, that instance ID is returned as a Contact header field
parameter. If the REGISTER request contained a Supported header
field that included the "gruu" option tag, the server MUST add a
"gruu" Contact header field parameter to that Contact header field.
The value of the gruu parameter is a quoted string containing the URI
that is the GRUU for the associated instance ID/AOR pair. If the To
header field in the REGISTER request had contained a SIP URI, the SIP
version of the GRUU is returned. If the To header field in the
REGISTER request had contained a SIPS URI, the SIPS version of the
GRUU is returned.
If the REGISTER response contains a gruu Contact header field
parameter in any of the contacts, the REGISTER response MUST contain
a Require header field with the value "gruu". This is because the
client needs to extract its GRUU from the REGISTER response, and
utilize a GRUU as the remote target of dialog initiating requests and
responses.
Note that handling of a REGISTER request containing a Contact header
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field with value "*" and an expiration of 0 still retains the meaning
defined in RFC 3261 - all contacts, not just ones with a specific
instance ID, are deleted. This removes their binding to the AOR and
to any GRUU.
Inclusion of a GRUU in the "gruu" Contact header field parameter of a
REGISTER response is separate from the computation and storage of the
GRUU. It is possible that the registrar has computed a GRUU for one
UA, but a different UA that queries for the current set of
registrations doesn't understand GRUU. In that case, the REGISTER
response sent to that second UA would not contain the "gruu" Contact
header field parameter, even though the UA has a GRUU for that
contact.
7.1.2.2 Timing Out a Registration
When a registered contact expires, its binding to the AOR is removed
as normal. In addition, its binding to the GRUU is removed at the
same time.
7.2 Administratively
Administrative creation of GRUUs is useful when a UA instance is a
network server that is always available, and therefore doesn't
register to the network. Examples of such servers are voicemail
servers, application servers, and gateways.
There are no protocol operations required to administratively create
a GRUU. The proxy serving the domain is configured with the GRUU,
and with the contact it should be translated to. It is not strictly
necessary to also configure the instance ID and AOR, since the
translation can be done directly. However, they serve as a useful
tool for determining which resource and UA instance the GRUU is
supposed to map to.
In addition to configuring the GRUU and its associated contact in the
proxy serving the domain, the GRUU will also need to be configured
into the UA instance associated with the GRUU.
It is also reasonable to model certain network servers as logically
containing both a proxy and a UA instance. The proxy receives the
request from the network, and passes it internally to the UA
instance. In such a case, the GRUU routes directly to the server,
and there is no need for a translation of the GRUU to a contact. The
server itself would construct its own GRUU.
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8. Using the GRUU
8.1 Sending a Message Containing a GRUU
A UA first obtains a GRUU using the procedures of Section 7, or by
other means outside the scope of this specification.
A UA can use the GRUU in the same way it would use any other SIP or
SIPS URI. However, a UA compliant to this specification MUST use a
GRUU when populating the Contact header field of dialog-creating
requests and responses. In other words, a UA compliant to this
specification MUST use its GRUU as its remote target. This includes
the INVITE request and its 2xx response, the SUBSCRIBE [6] request,
its 2xx response, the NOTIFY request, and the REFER [7] request and
its 2xx response.
If the UA instance has obtained multiple GRUUs for different AOR as a
result of a registration, it MUST use one corresponding to the AOR
used to send or receive the request. For sending a request, this
means that the GRUU corresponds to the AOR present in the From header
field, and furthermore the credentials used for authentication of the
request correspond to the ones associated with that AOR. When
receiving a request, the GRUU in the response corresponds to the AOR
to which the original request was targeted. That AOR, however, will
be rewritten by the proxy to correspond to the UA's registered
contact. It is for this reason that different contacts are needed
for each AOR that an instance registers against. In this way, when
an incoming request arrives, the Request URI can be examined. It
will be equal to a registered contact. That contact can be used to
map directly to the AOR, and from there, the correct GRUU can be
selected.
In those requests and responses where a GRUU is used as the remote
target, the UA MUST include a Supported header field that contains
the option tag "gruu". However, it is not necessary for a UA to know
whether or not its peer in the dialog supports this specification
before using one as a remote target.
When using a GRUU as a remote target, a UA MAY add the "grid" URI
parameter to the GRUU. This parameter MAY take on any value
permitted by the grammar for the parameter. When a UA sends a
request to the GRUU, the proxy for the domain that owns the GRUU will
translate the GRUU in the Request-URI, replacing it with the contact
bound to that GRUU. However, it will retain the "grid" parameter
when this translation is performed. As a result, when the UA
receives the request, the Request-URI will contain the "grid" created
by the UA. This allows the UA to effectively manufacture an infinite
supply of GRUU, each of which differs by the value of the "grid"
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parameter. When a UA receives a request that was sent to the GRUU,
it will be able to tell which GRUU was invoked by looking at the
"grid" parameter.
An implication of this behavior is that all mid-dialog requests will
be routed through intermediate proxies. There will never be direct,
UA to UA signaling unless the UA is co-resident with the proxy (which
is the case for administratively constructed GRUU). It is
anticipated that this limitation will be addressed in future
specifications.
Once a UA knows that the remote target provided by its peer is a
GRUU, it can use it in any application or SIP extension which
requires a globally routable URI to operate. One such example is
assisted call transfer.
8.2 Sending a Message to a GRUU
There is no new behavior associated with sending a request to a GRUU.
A GRUU is a URI like any other. When a UA receives a request or
response, it can know that the remote target is a GRUU if the request
or response had a Supported header field that included the value
"gruu". The UA can take the GRUU, and send a request to it, and then
be sure that it is delivered to the UA instance which sent the
request or response.
If the GRUU contains the "opaque" URI parameter, a UA can obtain the
AOR for the user by stripping the parameter. The resulting URI is
the AOR. If the GRUU does not have the "opaque" URI parameter, there
is no mechanism defined for determining the AOR from the GRUU.
Extraction of the AOR from the GRUU is useful for call logs and other
accounting functions, where it is desirable to know the user to whom
the request was directed.
Since the instance ID is a callee capabilities parameter, a UA might
be tempted to send a request to the AOR of a user, and include an
Accept-Contact header field [23] which indicates a preference for
routing the request to a UA with a specific instance ID. Although
this would appear to have the same effect as sending a request to the
GRUU, it does not. The caller preferences expressed in the Accept-
Contact header field are just preferences. Its efficacy depends on a
UA constructing an Accept-Contact header field that interacts with
domain processing logic for an AOR, to cause it to route to a
particular instance. Given the variability in routing logic in a
domain (for example, time based routing to only selected contacts),
this doesn't work for many domain routing policies. However, this
specification does not forbid a client from attempting such a
request, as there may be cases where the desired operation truly is a
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preferential routing request.
8.3 Receiving a Request Sent to a GRUU
When a UAS receives a request sent to its GRUU, the incoming request
URI will be equal to the contact that was registered (through
REGISTER or some other action) by that UA instance. If the user
agent had previously handed out its GRUU with a grid parameter, the
incoming request URI may contain that parameter. This indicates to
the UAS that the request is being received as a result of a request
sent by the UAC to that GRUU/grid combination. This specification
makes no normative statements about when to use a grid parameter, or
what to do when receiving a request made to a GRUU/grid combination.
Generally, any differing behaviors are a matter of local policy.
It is important to note that, when a user agent receives a request,
and the request URI does not have a grid parameter, the user agent
cannot tell by inspection of the Request URI whether the request was
sent to the AOR or to the GRUU. The To header field might be
different, but due to forwarding cannot be depended on as a
differentiator. As such, the UAS will process such requests
identically. If a user agent needs to differentiate its behavior
based on these cases, it will need to use a grid parameter.
8.4 Proxy Behavior
Proxy behavior is fully defined in Section 16 of RFC 3261. GRUU
processing impacts that processing in two places - request targeting
and record-routing.
8.4.1 Request Targeting
When a proxy server receives a request, and the proxy owns the domain
in the Request URI, and the proxy is supposed to access a Location
Service in order to compute request targets (as specified in Section
16.5 of RFC 3261 [1]), the proxy examines the Request URI. If the
Request URI is an AOR against which there are multiple registered
contacts with the same instance ID parameter, the proxy MUST populate
the target set such that there is never more than one contact at a
time with a given instance ID. It is RECOMMENDED that the most
recently registered contact be used. It is expected that standards
track extensions will be developed that provide additional criteria
for selecting which contact to use [18].
The contact that is the most recently registered is the one that
has been bound to the AOR is the shortest period of time. This
corresponds to the minimum value for the "duration-registered"
attribute from the registration event package [29]. It is
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important to note that a refresh of the contact in a REGISTER
message does not reset the duration it has been registered to
zero. For example, if a softphone is started at 9am when a user
logs into their computer, and the softphone refreshes its
registration every hour, by 1230pm the contact has been registered
for three and a half hours.
If the request URI is within the domain of the proxy, and the URI has
been constructed by the domain such that the proxy is able to
determine that it has the form of a GRUU for an AOR that is unknown
within the domain, the proxy rejects the request with a 404. If the
request URI is within the domain of the proxy, and the URI has been
constructed by the domain such that the proxy is able to determine
that it has the form of a GRUU for an AOR that known within the
domain, but the instance ID is unknown, the proxy SHOULD generate a
480.
If the GRUU does exist, handling of the GRUU proceeds as specified in
RFC 3261 Section 16. For GRUUs, the abstract location service
described in Section 16.5 is utilized, producing a set of zero or
more contacts, each of which is associated with the same instance ID.
If there is more than one contact bound to the GRUU, the proxy MUST
populate the target set such that there is never more than one
contact at a time. It is RECOMMENDED that the most recently
registered contact be used. This produces zero or one contacts. The
server MUST copy the grid parameter from the Request URI, if present,
into the new target URI obtained from the registered contact. If the
grid was already present in the contact bound to the GRUU, it is
overwritten in this process. If no contacts were bound to the GRUU,
the lookup of the GRUU in the abstract location service will result
in zero target URI, eventually causing the proxy to reject the
request with a 480 (Temorarily Unavailable) response.
If the contact had been registered using a Path header field [3],
then that Path is used to construct the route set for reaching that
contact through the GRUU as well as through the AOR, using the
procedures specified in RFC 3327. Support for GRUU at a registrar
does not require support for RFC 3327, however.
A proxy MAY apply other processing to the request, such as execution
of called party features, as discussed in Section 6.
A request sent to a GRUU SHOULD NOT be redirected. In many
instances, a GRUU is used by a UA in order to assist in the traversal
of NATs and firewalls, and a redirection may prevent such a case from
working.
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8.4.2 Record Routing
As described above, a user agent uses its GRUU as a remote target.
This has an impact on the path taken by subsequent mid-dialog
requests. Depending on the desires of the proxies involved, this may
impact record route processing.
Two cases can be considered. The first is shown in Figure 4. In
this case, there is a single proxy in the user's domain. An incoming
INVITE request arrives for the users AOR (1) and is forwarded to the
user agent at its registered contact C1 (2). The proxy inserts a
Record-Route header field into the proxied request, with a value of
R1. The user agent generates a 200 OK to the request, using its GRUU
G1 as the remote target.
(1) + (2): initial INVITE
(2) + (3): mid-dialog request
(1) +-----------+ (2) +-----------+
------>| Proxy |--------------->| User |
| | | Agent |
(3) | | (4) | |
------>|R1 |--------------->|G1 |
| | | |
+-----------+ +-----------+
Figure 4
When a mid-dialog request shows up destined for the user agent
(message 3), it will arrive at the proxy in the following form:
INVITE G1
Route: R1
Since the top Route header field value identifies the proxy, the
proxy removes it. As there are no more Route header field values,
the proxy processes the request URI. However, the request URI is a
GRUU, and is therefore a domain under the control of the proxy. The
proxy will need to perform the processing of Section 8.4.1, which
will result in the translation of the GRUU into the contact C1,
followed by transmission of the request to the user agent (message
4).
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This sequence of processing in the proxy is somewhat unusual, in that
mid-dialog requests (that is, requests with a Route header field that
a proxy inserted as a result of a Record-Route operation) do not
normally cause a proxy to have to invoke a location service to
process the request URI. It is for this reason that this is called
out here.
The previous case assumed that there was a single proxy in the
domain. In more complicated cases, there can be two or more proxies
within a domain on the initial request path. This is shown in
Figure 6. In this figure, there is a home proxy, to which requests
targeted to the AOR are sent. The home proxy executes the abstract
location service and runs user features. The edge proxy acts as the
outbound proxy for users, performs authentication, manages TCP/TLS
connections to the client, and does other functions associated with
the transition from the provider proxy network to the client. This
specific division of responsibilities between home and edge proxy is
just for the purposes of illustration; the discussion applies to a
disaggregation of proxy logic into any number of proxies. In such a
configuration, registrations from the user agent would pass through
the edge proxy, which would insert a Path header field [3] for
itself.
(1) + (2) + (3): initial INVITE
(4) - (9): mid-dialog request
(1) +-----------+ (2) +-----------+ (3) +-----------+
---->| |------->| |-------->| |
(4) | | (5) | | | |
---->|H1 Home |------->|E1 Edge | | User |
(7) | Proxy | (8) | Proxy | (9) | Agent |
+-->|G1 |------->| |-------->| |
| +-----------+ +-----------+ +-----------+
| |
| |
+------------------------------+
(6)
Figure 6
When an incoming request arrives for the AOR (message 1), the home
proxy would look it up, discover the registered contact and Path, and
then send the request to the edge proxy as a result of the Route
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header field inserted with the Path value. The home proxy record
routes with the URI H1. The edge proxy would forward the request to
the request URI (which points to the client), and insert a Record-
Route header field value with the URI E1 (message 2). This request
is accepted by the user agent, which inserts its GRUU G1 as the
remote target.
When the peer in the dialog sends a mid-dialog request, it will have
the following form:
INVITE G1
Route: H1, E1
This request will arrive at the home proxy (due to H1 in the Route
header field) (message 4). The home proxy will forward it to the
edge proxy (due to E1 in the Route header field) (message 5). The
edge proxy, seeing no more Route header field values, sends the
request to the Request URI. This is a GRUU, and like an AOR, will
route to the home proxy. This causes the request to loop back around
(message 6). The home proxy performs the GRUU processing of
Section 8.4.1, causing the request to be forwarded to the edge proxy
a second time (this time, as a result of a Route header field value
obtained from the Path header in the registration) (message 7), and
then delivered to the client (message 8).
While this flow works, it is highly inefficient, as it causes each
mid-dialog request to spiral route. This behavior is not desirable.
To prevent it, the following procedures SHOULD be followed. When a
client generates a REGISTER request, this request passes through the
edge proxy on its way to the home proxy. The REGISTER request will
contain the AOR of the user (in the To header field). If the client
is registering an instance, there will be one or more Contact header
field values with the +sip.instance Contact header field parameter,
and a Supported header field that includes the value "gruu".
Normally there would be just one Contact header field value with an
instance ID, but it is possible for there to be more than one. All,
however, are required to identify the instance performing the
registration.
The proxy can decide to insert itself on the Path for a particular
AOR on a case by case basis. However, if it does so for one
registration, it SHOULD do so for all registrations for the same AOR
that contain an instance ID in the contact. The value of the Path
header field inserted by the proxy SHOULD be constructed so that it
indicates whether or not the proxy inserted itself on the Path for
this AOR.
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When a request arrives from the home proxy towards the client, the
proxy inspects the Route header field. This header field will
contain the URI the edge proxy had placed into the Path. If the
value indicates that the edge proxy had put itself on the Path for
the registration from this client, there is no need for the proxy to
retain its record-route in the response. The proxy MAY remove its
record-route value from the 200 OK response in this case. If the
value indicates that the proxy had not put itself on the Path, it
would retain the Record-Route in the response.
Similarly, if a request arrives from the client towards the home
proxy, the edge proxy would look at the identity of the sender of the
request. If the proxy knows that it is placing itself on the Path
for registrations from that AOR, and the request contains a Supported
header field with the value "gruu", the edge proxy would insert a
Record-Route into the request, and then remove it in the response.
Similarly, if the identity of the sender of the request is one for
which the client has not put itself on the Path, the edge proxy would
keep its Record-Route in the response.
Removing its Record-Route value from the response will result in a
different route set as seen by the caller and callee; the callee
(which is the user agent in the figure) will have a route set entry
for its edge proxy, while the caller will not. The caller will have
a route set entry for its edge proxy, while the callee will not.
In such a case, a mid-dialog request that arrives at the home proxy
will be of the form:
INVITE G1
Route: H1
This does the "right thing" and causes the request to be routed from
the home proxy to the edge proxy to the client, without the
additional spiral.
9. The opaque SIP URI Parameter
This specification defines a new SIP URI parameter, "opaque". The
"opaque" URI parameter is used to construct a URI (called the derived
URI) that is related to another URI (called the base URI, frequently
an AOR) in some way. In this specification, the parameter is used to
construct the GRUU (which is the derived URI) from the AOR (which is
the base URI). However, there are many other applications outside of
GRUU. It can be used, for example, to construct a URI for a
voicemail inbox (the derived URI) from a subscribers AOR (the base
URI), or the URI for a video service advertised via presence [26]
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(the derived URI) from the subscribers AOR (the base URI).
To construct a derived URI, the owner of the domain adds the "opaque"
URI parameter to the base URI, resulting in the derived URI. In
fact, these are the only semantics associated with the "opaque" URI
parameter: a URI containing the parameter MUST be related to another
URI, obtained by stripping the "opaque" URI parameter. Because the
"opaque" URI parameter implies a relationship, any element (including
ones outside of the domain that owns the URI) that receives a URI
with the "opaque" URI parameter will know definitively that it is a
derived URI, and can strip it to obtain the base URI.
The value of the "opaque" URI parameter is not relevant to anyone
except for the owner of the domain. It typically contains
information needed by the owner of the domain to correctly process a
request targeted to that URI according to the desired semantics of
the URI. As such, the parameter is a form of cookie. In the case of
a GRUU, the "opaque" URI parameter contains enough information for
the owner of the domain to determine the instance ID. Since the
structure of its value is not subject to standardization, it can only
be interpreted by the same proxy or cluster of proxies that created
the derived URI. For this reason, a proxy or cluster of proxies MUST
NOT create a derived URI unless a request sent to the base URI (and
consequently the derived URI) will be routed back to that same proxy
or cluster of proxies without any upstream proxies requiring
interpretation of the "opaque" URI parameter. Simply put, a request
sent to a derived URI has to get back to the same proxy farm that
created the derived URI.
The presence of the "opaque" URI parameter in a URI implies a
relationship between that URI and its base URI. However, the nature
of that relationship cannot be determined from inspection of the URI
alone. In some cases, there may be no way to know the relationship
outside of the domain that constructed the URI. In other cases, the
nature of the relationship is determined from the context in which
the URI was obtained. When used to construct a GRUU, it means that
the URI formed by stripping the "opaque" parameter corresponds to the
AOR associated with the GRUU. The recipient of a GRUU cannot
determine that it is a GRUU by direct examination of the URI.
However, the recipient may know if it received the GRUU in the
Contact header field of a SIP request or response that contained a
Supported header field with the option tag "gruu". If it knows its a
GRUU through such context, and the GRUU contains the "opaque"
parameter, the UA knows it can obtain the AOR by removing the
"opaque" parameter.
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10. Grammar
This specification defines two new Contact header field parameters,
gruu and +sip.instance, and two new URI parameters, "grid" and
"opaque". The grammar for string-value is obtained from [11], and
the grammar for uric is defined in RFC 3986 [9].
contact-params = c-p-q / c-p-expires / c-p-gruu / cp-instance
/ contact-extension
c-p-gruu = "gruu" EQUAL DQUOTE (SIP-URI / SIPS-URI) DQUOTE
cp-instance = "+sip.instance" EQUAL LDQUOT "<"
instance-val ">" RDQUOT
uri-parameter = transport-param / user-param / method-param
/ ttl-param / maddr-param / lr-param / grid-param
/ opaque-param / other-param
grid-param = "grid=" pvalue ; defined in RFC3261
opaque-param = "opaque=" pvalue ; defined in RFC3261
instance-val = *uric ; defined in RFC 2396
11. Requirements
This specification was created in order to meet the following
requirements:
REQ 1: When a UA invokes a GRUU, it MUST cause the request to be
routed to the specific UA instance to which the GRUU refers.
REQ 2: It MUST be possible for a GRUU to be invoked from anywhere on
the Internet, and still cause the request to be routed
appropriately. That is, a GRUU MUST NOT be restricted to use
within a specific addressing realm.
REQ 3: It MUST be possible for a GRUU to be constructed without
requiring the network to store additional state.
REQ 4: It MUST be possible for a UA to obtain a multiplicity of
GRUUs, each one of which routes to that UA instance. This is
needed to support ad-hoc conferencing, for example, where a UA
instance needs a different URI for each conference it is hosting.
REQ 5: When a UA receives a request sent to a GRUU, it MUST be
possible for the UA to know the GRUU which was used to invoke the
request. This is necessary as a consequence of requirement 4.
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REQ 6: It MUST be possible for a UA to add opaque content to a GRUU,
which is not interpreted or altered by the network, and used only
by the UA instance to whom the GRUU refers. This provides a basic
cookie type of functionality, allowing a UA to build a GRUU with
state embedded within it.
REQ 7: It MUST be possible for a proxy to execute services and
features on behalf of a UA instance represented by a GRUU. As an
example, if a user has call blocking features, a proxy may want to
apply those call blocking features to calls made to the GRUU in
addition to calls made to the user's AOR.
REQ 8: It MUST be possible for a UA in a dialog to inform its peer of
its GRUU, and for the peer to know that the URI represents a GRUU.
This is needed for the conferencing and dialog reuse applications
of GRUUs, where the URIs are transferred within a dialog.
REQ 9: When transferring a GRUU per requirement 8, it MUST be
possible for the UA receiving the GRUU to be assured of its
integrity and authenticity.
REQ 10: It MUST be possible for a server, authoritative for a domain,
to construct a GRUU which routes to a UA instance bound to an AOR
in that domain. In other words, the proxy can construct a GRUU
too. This is needed for the presence application.
12. Example Call Flow
The following call flow shows a basic registration and call setup,
followed by a subscription directed to the GRUU. It then shows a
failure of the callee, followed by a re-registration. The
conventions of [22] are used to describe representation of long
message lines.
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Caller Proxy Callee
| |(1) REGISTER |
| |<--------------------|
| |(2) 200 OK |
| |-------------------->|
|(3) INVITE | |
|-------------------->| |
| |(4) INVITE |
| |-------------------->|
| |(5) 200 OK |
| |<--------------------|
|(6) 200 OK | |
|<--------------------| |
|(7) ACK | |
|-------------------->| |
| |(8) ACK |
| |-------------------->|
|(9) SUBSCRIBE | |
|-------------------->| |
| |(10) SUBSCRIBE |
| |-------------------->|
| |(11) 200 OK |
| |<--------------------|
|(12) 200 OK | |
|<--------------------| |
| |(13) NOTIFY |
| |<--------------------|
|(14) NOTIFY | |
|<--------------------| |
|(15) 200 OK | |
|-------------------->| |
| |(16) 200 OK |
| |-------------------->|
| | |Crashes,
| |(17) REGISTER | Reboots
| |<--------------------|
| |(18) 200 OK |
| |-------------------->|
The Callee supports the GRUU extension. As such, its REGISTER (1)
looks like:
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REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl
Supported: gruu
To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
CSeq: 1 REGISTER
Contact: <sip:callee@192.0.2.1>
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
Content-Length: 0
The REGISTER response would look like:
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Require: gruu
Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
CSeq: 1 REGISTER
<allOneLine>
Contact: <sip:callee@192.0.2.1>
;gruu="sip:callee@example.com;
opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
Content-Length: 0
Note how the Contact header field in the REGISTER response contains
the gruu parameter with the URI sip:callee@
example.com;opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6.
This represents a GRUU that translates to the contact
sip:callee@192.0.2.1.
The INVITE from the caller is a normal SIP INVITE. The 200 OK
generated by the callee (message 5), however, now contains a GRUU as
the remote target. The UA has also chosen to include a grid URI
parameter into the GRUU.
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SIP/2.0 200 OK
Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKnaa8
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a
From: Caller <sip:caller@example.com>;tag=n88ah
To: Callee <sip:callee@example.com> ;tag=a0z8
Call-ID: 1j9FpLxk3uxtma7@host.example.com
CSeq: 1 INVITE
Supported: gruu
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
<allOneLine>
Contact:
<sip:callee@example.com
;opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a>
</allOneLine>
Content-Length: --
Content-Type: application/sdp
[SDP Not shown]
At some point later in the call, the caller decides to subscribe to
the dialog event package [21] at that specific UA. To do that, it
generates a SUBSCRIBE request (message 9), but directs it towards the
remote target, which is a GRUU:
<allOneLine>
SUBSCRIBE sip:callee@example.com;opaque=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
SIP/2.0
</allOneLine>
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:caller@example.com>;tag=kkaz-
<allOneLine>
To: sip:callee@example.com;opaque=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
</allOneLine>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:caller@example.com;opaque=hdg7777ad7aflzig8sf7>
Content-Length: 0
In this example, the caller itself supports the GRUU extension, and
is using its own GRUU to populate its remote target.
This request is routed to the proxy, which proceeds to perform a
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location lookup on the request URI. It is translated into the
contact for that instance, and then proxied there (message 10 below).
Note how the grid parameter is maintained.
SUBSCRIBE sip:callee@192.0.2.1;grid=99a SIP/2.0
Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK9555
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:caller@example.com>;tag=kkaz-
<allOneLine>
To: sip:callee@example.com;opaque=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
</allOneLine>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:caller@example.com;opaque=hdg7777ad7aflzig8sf7>
Content-Length: 0
At some point after message 16 is received, the callee's machine
crashes and recovers. It obtains a new IP address, 192.0.2.2.
Unaware that it had previously had an active registration, it creates
a new one (message 17 below). Notice how the instance ID remains the
same, as it persists across reboot cycles:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbba
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=ha8d777f0
Supported: gruu
To: Callee <sip:callee@example.com>
Call-ID: hf8asxzff8s7f@192.0.2.2
CSeq: 1 REGISTER
Contact: <sip:callee@192.0.2.2>
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
Content-Length: 0
The registrar notices that a different contact, sip:callee@192.0.2.1,
is already associated with the same instance ID. It registers the
new one too and returns both in the REGISTER response. Both have the
same GRUU. However, only this new contact (the most recently
registered one) will be used by the proxy for population in the
target set. It then generates the following response:
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SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbba
From: Callee <sip:callee@example.com>;tag=ha8d777f0
To: Callee <sip:callee@example.com>;tag=99f8f7
Require: gruu
Call-ID: hf8asxzff8s7f@192.0.2.2
CSeq: 1 REGISTER
<allOneLine>
Contact: <sip:callee@192.0.2.2>
;gruu="sip:callee@example.com;opaque=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
Contact: <sip:callee@192.0.2.1>
;gruu="sip:callee@example.com;opaque=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=400
</allOneLine>
Content-Length: 0
13. Security Considerations
It is important for a UA to be assured of the integrity of a GRUU
when it is given one in a REGISTER response. If the GRUU is tampered
with by an attacker, the result could be denial of service to the UA.
As a result, it is RECOMMENDED that a UA use the SIPS URI scheme in
the Request-URI when registering. Proxies and registrars MUST
support the sips URI and MUST support TLS. Note that this does not
represent a change from the requirements in RFC 3261.
The example GRUU construction algorithm in Appendix A.1 makes no
attempt to create a GRUU that hides the AOR and instance ID
associated with the GRUU. In general, determination of the AOR
associated with a GRUU is considered a good property, since it allows
for easy tracking of the target of a particular call. Learning the
instance ID provides little benefit to an attacker. To register or
otherwise impact registrations for the user, an attacker would need
to obtain the credentials for the user. Knowing the instance ID is
insufficient.
The example GRUU construction algorithm in Appendix A.1 makes no
attempt to create a GRUU that prevents users from guessing a GRUU
based on knowledge of the AOR and instance ID. A user that is able
to do that will be able to direct a new request at a particular
instance. However, this specification recommends that service
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treatment be given to requests that are sent to a GRUU, including
screening features in particular. That treatment will make sure that
the GRUU does not provide a back door for attackers to contact a user
that has tried to block the attacker.
GRUUs do not provide a solution for privacy. In particular, since
the GRUU does not change during the lifetime of a registration, an
attacker could correlate two calls as coming from the same source,
which in and of itself reveals information about the caller.
Furthermore, GRUUs do not address other aspects of privacy, such as
the addresses used for media transport. For a discussion of how
privacy services are provided in SIP, see RFC 3323 [14].
14. IANA Considerations
This specification defines a new Contact header field parameter, two
SIP URI parameters, a media feature tag and a SIP option tag.
14.1 Header Field Parameter
This specification defines a new header field parameter, as per the
registry created by [12]. The required information is as follows:
Header field in which the parameter can appear: Contact
Name of the Parameter: gruu
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
14.2 URI Parameters
This specification defines two new SIP URI parameters, as per the
registry created by [13].
Name of the Parameter: grid
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
Name of the Parameter: opaque
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
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14.3 Media Feature Tag
This section registers a new media feature tag, per the procedures
defined in RFC 2506 [8]. The tag is placed into the sip tree, which
is defined in [11].
Media feature tag name: sip.instance
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature tag
contains a string containing a URN that indicates a unique
identifier associated with the UA instance registering the
Contact.
Values appropriate for use with this feature tag: String.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application, for
describing the capabilities of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a specific device.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
Security Considerations: This media feature tag can be used in ways
which affect application behaviors. For example, the SIP caller
preferences extension [23] allows for call routing decisions to be
based on the values of these parameters. Therefore, if an
attacker can modify the values of this tag, they may be able to
affect the behavior of applications. As a result of this,
applications which utilize this media feature tag SHOULD provide a
means for ensuring its integrity. Similarly, this feature tag
should only be trusted as valid when it comes from the user or
user agent described by the tag. As a result, protocols for
conveying this feature tag SHOULD provide a mechanism for
guaranteeing authenticity.
14.4 SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
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Name: gruu
Description: This option tag is used to identify the Globally
Routable User Agent URI (GRUU) extension. When used in a
Supported header, it indicates that a User Agent understands the
extension, and has included a GRUU in the Contact header field of
its dialog initiating requests and responses. When used in a
Require header field of a REGISTER request, it indicates that the
registrar should assign a GRUU to the Contact URI.
15. Acknowledgements
The author would like to thank Rohan Mahy, Paul Kyzivat, Alan
Johnston, and Cullen Jennings for their contributions to this work.
16. References
16.1 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] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[3] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002.
[4] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[7] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[8] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.
[9] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
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January 2005.
[10] Moats, R., "URN Syntax", RFC 2141, May 1997.
[11] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[12] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[13] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Uniform Resource Identifier (URI) Parameter Registry for the
Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
December 2004.
16.2 Informative References
[14] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[15] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[16] Hakala, J., "Using National Bibliography Numbers as Uniform
Resource Names", RFC 3188, October 2001.
[17] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-05 (work in
progress), May 2005.
[18] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-00 (work in progress), July 2005.
[19] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
draft-ietf-sip-target-dialog-01 (work in progress), July 2005.
[20] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-05 (work in progress), May 2005.
[21] Rosenberg, J., "An INVITE Inititiated Dialog Event Package for
the Session Initiation Protocol (SIP)",
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draft-ietf-sipping-dialog-package-06 (work in progress),
April 2005.
[22] Sparks, R., "Session Initiation Protocol Torture Test
Messages", draft-ietf-sipping-torture-tests-07 (work in
progress), May 2005.
[23] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[24] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)",
RFC 3863, August 2004.
[25] Sparks, R., "Session Initiation Protocol Call Control -
Transfer", draft-ietf-sipping-cc-transfer-05 (work in
progress), July 2005.
[26] Rosenberg, J., "A Data Model for Presence",
draft-ietf-simple-presence-data-model-04 (work in progress),
August 2005.
[27] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[28] Leach, P., Mealling, M., and R. Salz, "A Universally Unique
IDentifier (UUID) URN Namespace", RFC 4122, July 2005.
[29] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[30] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[31] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
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Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Appendix A. Example GRUU Construction Algorithms
The mechanism for constructing a GRUU is not subject to
specification. This appendix provides two examples that can be used
by a registar. Others are, of course, permitted, as long as they
meet the constraints defined for a GRUU.
A.1 Instance ID in opaque URI Parameter
The most basic approach for constructing a GRUU is to utilize the
"opaque" URI parameter. The user and domain portions of the URI are
equal to the AOR, and the "opaque" parameter is populated with the
instance ID.
A.2 Encrypted Instance ID and AOR
In many cases, it will be desirable to construct the GRUU in such a
way that it will not be possible, based on inspection of the URI, to
determine the Contact URI that the GRUU translates to. It may also
be desirable to construct it so that it will not be possible to
determine the instance ID/AOR pair associated with the GRUU. Whether
or not a GRUU should be constructed with this property is a local
policy decision.
With these rules, it is possible to construct a GRUU without
requiring the maintenance of any additional state. To do that, the
URI would be constructed in the following fashion:
user-part = "GRUU" | BASE64(E(K, (salt | " " | AOR | " " |
instance ID)))
Where E(K,X) represents a suitable encryption function (such as AES
with 128 bit keys) with key K applied to data block X, and the "|"
operator implies concatenation. The single space (" ") between
components is used as a delimeter, so that the components can easily
be extracted after decryption. Salt represents a random string that
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prevents a client from obtaining pairs of known plaintext and
ciphertext. A good choice would be at least 128 bits of randomness
in the salt.
This mechanism uses the user-part of the SIP URI to convey the
encrypted AOR and instance ID. The user-part is used instead of the
"opaque" URI parameter because of the desired anonymity properties.
The benefit of this mechanism is that a server need not store
additional information on mapping a GRUU to its corresponding
contact. The user part of the GRUU contains the instance ID and AOR.
Assuming that the domain stores registrations in a database indexed
by the AOR, the proxy processing the GRUU would look up the AOR,
extract the currently registered contacts, and find the one matching
the instance ID encoded in the request URI. The contact whose
instance ID is that instance ID is then used as the translated
version of the GRUU. Encryption is needed to prevent attacks whereby
the server is sent requests with faked GRUU, causing the server to
direct requests to any named URI. Even with encryption, the proxy
should validate the user part after decryption. In particular, the
AOR should be managed by the proxy in that domain. Should a UA send
a request with a fake GRUU, the proxy would decrypt and then discard
it because there would be no URI or an invalid URI inside.
While this approach has many benefits, it has the drawback of
producing fairly long GRUUs.
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Intellectual Property Statement
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Rosenberg Expires April 1, 2006 [Page 41]
