SIP J. Rosenberg
Internet-Draft Cisco Systems
Expires: February 1, 2007 July 31, 2006
Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the
Session Initiation Protocol (SIP)
draft-ietf-sip-gruu-10
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Several applications of the Session Initiation Protocol (SIP) require
a user agent (UA) to construct and distribute a URI that can be used
by anyone on the Internet to route a call to that specific UA
instance. A URI that 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
7.1.1.1. Generating a REGISTER Request . . . . . . . . . . 13
7.1.1.2. Processing the REGISTER Response . . . . . . . . . 14
7.1.2. Registrar Behavior . . . . . . . . . . . . . . . . . . 15
7.1.2.1. Processing a REGISTER Request . . . . . . . . . . 15
7.1.2.2. Timing Out a Registration . . . . . . . . . . . . 17
7.2. Through Administrative Operation . . . . . . . . . . . . . 17
8. Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. UA Behavior . . . . . . . . . . . . . . . . . . . . . . . 18
8.1.1. Sending a Message Containing a GRUU . . . . . . . . . 18
8.1.2. Sending a Message to a GRUU . . . . . . . . . . . . . 20
8.1.3. Receiving a Request Sent to a GRUU . . . . . . . . . . 20
8.2. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 21
8.2.1. Request Targeting for Requests Outside of a Dialog . . 21
8.2.2. Record-Routing . . . . . . . . . . . . . . . . . . . . 22
8.2.3. Request Targeting for Mid-Dialog Requests . . . . . . 23
9. The opaque SIP URI Parameter . . . . . . . . . . . . . . . . . 23
10. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 25
12. Example Call Flow . . . . . . . . . . . . . . . . . . . . . . 26
13. Security Considerations . . . . . . . . . . . . . . . . . . . 31
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
14.1. Header Field Parameter . . . . . . . . . . . . . . . . . . 33
14.2. URI Parameters . . . . . . . . . . . . . . . . . . . . . . 33
14.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 34
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
16.1. Normative References . . . . . . . . . . . . . . . . . . . 34
16.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Example GRUU Construction Algorithms . . . . . . . . 36
A.1. Instance ID in "opaque" URI Parameter . . . . . . . . . . 36
A.2. Encrypted Instance ID and AOR . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . . . 40
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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. They are eventually 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 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)
[13] 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
because 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 [21], based on the
REFER method [7]. Another application is the usage of endpoint-
hosted conferences within the conferencing framework [14]. Both of
these mechanisms require that the endpoint be able to construct a URI
that not only routes to that user agent, but is usable by 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 or target refresh requests and responses. Because
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
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].
This specification 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 and updated by target refresh requests.
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, which are granted to the URI based on the
policies of the domain that owns the URI. Those properties are not
necessarily 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 the
domain will authorize it as a valid value in the From header field
of a request, such that an authentication service will sign a
request with that URI [16].
This specification focuses on a property, called the Globally
Routable User Agent URI (GRUU) property. A URI possesses this
property when the following three properties are present:
Global: It can be used by any User Agent Client (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 [21]. 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 identifies 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 that user B is using to talk to user A.
If it doesn't, the transfer service will 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 [14]. In that framework, a
conference is described by a URI that 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
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C with an updated remote target that now holds the conference URI.
The URI in the Contact header field also has a callee capabilities
[8] 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 doesn't, basic conferencing features will fail.
Therefore, this URI has to be a GRUU.
4.3. Presence
In a SIP-based presence [23] system, the Presence Agent (PA)
generates notifications about the state of a user. This state is
represented with the Presence Information Document Format (PIDF)
[20]. 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 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
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-- through registrations and through administrative operation. Only
the former requires protocol operations.
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 parameter, defined in [11], contains a globally unique ID that
identifies the UA instance. If the domain that the user is
registering against also supports the GRUU specification, the
REGISTER responses will contain the "gruu" parameter in each Contact
header field. This parameter contains a URI which the domain
guarantees will route to that UA instance. This URI is tagged as a
GRUU through the inclusion of a "gruu" URI parameter, similar to the
way loose route URIs are tagged with the "lr" URI parameter. This
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 forming and
target refresh 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 property, implementations have adapted their behaviors
(oftentimes in proprietary ways) to compensate. To facilitate a
transition away from these behaviors, it is helpful for a UA
receiving the message to know whether the remote target is a GRUU or
not. This can be known to a remote target through the presence of
the "gruu" URI parameter.
A domain can construct a GRUU in any way it chooses. However, it is
sometimes desirable to construct GRUUs so that any entity that
receives a GRUU can 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 subscriber's AOR, but
includes the "opaque" and "gruu" URI parameters. 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, so
that any element can extract the AOR from the constructed URI by
removing the "opaque" parameter. It is because of the desire to use
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"opaque" for construction of URI besides GRUUs, that both a "gruu"
flag and an "opaque" URI parameter are defined. For example:
AOR: sip:alice@example.com
GRUU: sip:alice@example.com;opaque="kjh29x97us97d";gruu
When a proxy in the domain constructs the GRUU, it would set the
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 determine that the request is a GRUU by the presence of
the "gruu" parameter, and then it can extract the AOR and instance
ID, both of which are needed to process the request.
When a UA uses a GRUU that routes to itself, 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 copy
the "grid" parameter and its value 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. If the GRUU did not contain a
"grid" URI parameter, the server will insert a "grid" parameter into
the Request-URI, but with no value. This signals to the UA that the
request was sent to a 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
potentially to help verify that the GRUU has not been forged. 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 |
| | | |
| | | |
+-----------+ +-----------+
^ 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. The instance
ID is defined in [11]. It is an identifier, represented with a URN,
that uniquely identifies a SIP user agent amongst all other user
agents associated with an AOR.
A GRUU is associated, in a many-to-one fashion, with the combination
of an instance ID and an AOR. 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
GRUUs. 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
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 a GRUU is associated with the
instance ID/AOR pair, not just the instance ID. For example, let's
say 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"
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parameter in the Contact header field. If the user 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 because they belong to different AORs. That is the
reason why a single instance ID can be associated with multiple
GRUUs; there would be one such association for each AOR. The same
goes for the association of an AOR to a 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
the instance ID that is 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 GRUUs
created through registrations, this means that the GRUU has to
route to a proxy server with access to registration data.
o The URI MUST include the "gruu" URI parameter.
o When a GRUU is assigned to an instance ID/AOR pair, both SIP and
SIPS GRUUs will be assigned. Only one will be returned, but both
will exist. The SIPS URI may not always work, particularly if the
proxy cannot establish a secure connection to the client.
o If the GRUU contains an "opaque" URI parameter, the URI that
results from stripping out the "opaque" and "gruu" URI parameters
MUST be equivalent to the AOR associated with the GRUU. Indeed,
the GRUU itself will also be equivalent to the AOR based on URI
comparison, since the "gruu" and "opaque" parameters will be
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ignored during comparison, being present in one URI (the GRUU) and
not the other (the AOR).
Section 8.2 defines additional behaviors that a proxy must exhibit on
receipt of a GRUU.
When a domain constructs a URI with the GRUU property, 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 that 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 [24]) or replace it (using the Replaces header
field [25]). In such cases, the UA will typically make its own
authorization decisions, allowing the request if the sender can prove
it knows the dialog identifiers [15]. In such cases, bypassing
screening services might make sense, but it needs to be carefully
considered by network designers, 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 and target refresh requests
and responses. However, in those cases, a proxy SHOULD only apply
services that are meaningful for mid-dialog requests, generally
speaking. This excludes screening functions, as well as forwarding
ones. A proxy can determine that a request is a mid-dialog request
based on the Route header field in the request it receives. If the
topmost URI matches one that the proxy placed into the Record-Route
header field of a dialog-forming request, then the request is a mid-
dialog request.
The "opaque" URI parameter, defined in Section 9, provides a means
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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.
Because the GRUU is associated with both the instance ID and AOR, for
any particular AOR there can be a potentially infinite number of
GRUUs, 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 doesn't even support GRUUs), 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 (existing from the time the first registration until removal of
the AOR) can be difficult to achieve through software failures and
power outages within a network, and for this reason, providing the
property 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
registers itself to the network to bind a contact 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 [11] as
a UA characteristic [8], whose value MUST be the instance ID that
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identifies the UA instance being registered.
If a UA instance is registering against multiple AORs, 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 8.1.1.
If a UA instance is trying to register multiple contacts for the same
instance for the purposes of redundancy, it MUST use the procedures
defined in [11].
Besides the procedures discussed above, the REGISTER request is
constructed as it is in the case where this extension was not
understood. Specifically, if a contact contains an instance ID
parameter, then it SHOULD NOT contain the gruu Contact header field
parameter, and the contact URI itself SHOULD NOT contain the "grid"
or "gruu" URI parameters defined below. Any such parameters are
ignored by the registrar and not stored. Typically, the contact URI
is not itself a GRUU and will not contain these URI parameters.
There are use cases where a contact can contain a GRUU, however. A
typical use case is when a user wanted calls to one AOR to be
delivered to a specific UA instance associated with another AOR. If
a client does include a contact which contains a GRUU, it MUST NOT
contain the instance ID parameter. This will ensure that the contact
URI does not get bound to any GRUU, avoiding an infinite loop which
might otherwise occur.
A UA MAY perform third party registrations (registrations where the
entity performing the registration is not the same as the AOR in the
To header field of the registration), and as in the above paragraph,
MAY register contacts that do not point to the UA performing the
registration. In addition, a UA MAY register contacts which omit the
"+sip.instance" Contact header field parameter, in which case they
would not be associated with any GRUUs.
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. This is in addition to the presence of the Supported
header field.
7.1.1.2. Processing the REGISTER Response
The 2xx response to REGISTER returns all the contacts bound to the
AOR. This may include contacts bound by other UAs. The UA MUST
extract the contacts it has just registered, and perform the
processing on those contacts as described here.
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If the response is a 2xx, each Contact header field that contained
the "+sip.instance" Contact header field parameter may also contain a
"gruu" Contact header field parameter (which is distinct from the
"gruu" URI 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 (if the To
header field in the REGISTER request contained a SIPS URI) it will be
a SIPS URI. Any requests sent to the GRUU URI will be routed by the
domain to a contact with that instance ID. Normally, the GRUU will
not 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.
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
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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. If the contact contained a "grid" URI
parameter or "gruu" URI parameter, they MUST be discarded by the
registrar and MUST NOT be stored as part of the URI. Note that these
restrictions only apply to contacts that contain the "+sip.instance"
header field parameter. Contacts omitting the "+sip.instance"
parameter (and therefore not processed by the rules in this
specification) might contain the "gruu" and "grid" URI parameters,
and would be stored as part of the URI.
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.
In addition, if that contact had callee capabilities associated with
it [8], those capabilities continue to be associated with the contact
as it is bound to the GRUU. If, after that processing, the contact
was not bound to the AOR (due, for example, to an expiration of
zero), the contact is not bound to the GRUU either.
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 contains a SIP URI, the SIP
version of the GRUU is returned. If the To header field in the
REGISTER request contains a SIPS URI, the SIPS version of the GRUU is
returned.
Note that handling of a REGISTER request containing a Contact header
field with value "*" and an expiration of 0 still retains the meaning
defined in RFC 3261 -- all contacts, not just those with a specific
instance ID, are deleted. This removes the binding of each contact
to the AOR and the binding of each contact to a 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 the GRUU specification. In that
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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. Similarly, a UA might send a REGISTER request
with a contact containing a "+sip.instance" Contact header field but
no "gruu" option tag in the Supported header field. The registrar
can still assign a GRUU, and indeed, a subscriber to the registration
event package could learn the GRUU from the notification [26] [28].
There is no need for inclusion of either a Require or Supported
header field in the response with the "gruu" option tag.
7.1.2.2. Timing Out a Registration
When a registered contact expires, its binding to the AOR is removed
as usual. In addition, its binding to the GRUU is removed at the
same time.
7.2. Through Administrative Operation
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 to which it should be translated. It is not
strictly necessary to also configure the instance ID and AOR, since
the translation can be done directly. However, they serve as useful
tools for determining to which resource and UA instance the GRUU is
supposed to map.
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 (such as PSTN
gateways and media 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. When such a server constructs a GRUU, it MUST include the
"gruu" URI parameter in it.
8. Using the GRUU
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8.1. UA Behavior
8.1.1. 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. If a UA is
using registrations to obtain GRUUs, it MUST have an active
registration prior to using a GRUU. It cannot simply reuse a
previously cached GRUU, even if the URI itself doesn't ever change.
This is because mid-dialog requests, which will be targeted at a
GRUU, cannot be processed correctly at the home proxy unless a
registration is active.
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 SHOULD use a
GRUU when populating the Contact header field of dialog-forming and
target refresh requests and responses. In other words, a UA
compliant to this specification SHOULD use its GRUU as its remote
target. This includes the INVITE request, 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 AORs as
a result of a registration, it SHOULD 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. Furthermore, this means that the credentials used for
authentication of the request correspond to the ones associated with
that AOR. When a peer sends a request to a UA, that request will
arrive at the home proxy, and be forwarded to the UA. The GRUU
placed into the Contact header field of the response SHOULD
correspond to the AOR to which the request was targeted by the peer.
Unfortunately, this AOR is not directly present in the request when
it is received by the UA. It would have been placed into the
Request-URI of the request when sent by the peer. However, upon
reaching the home proxy of the UA, the Request-URI is rewritten with
the registered contact. It is for this reason that different
contacts are needed for each AOR that an instance registers against.
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 addition, if the UA is making use of an outbound proxy, the GRUU
that is used by the UA MUST be one obtained from a domain that will
be visited by the request prior to being routed towards the target in
the Request-URI. A request can visit multiple domains if, in
addition to an outbound proxy, a UA asks that a request visit
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additional proxies, identified by additional Route header fields in
the request. This can happen when a UA has learned a service route
using the mechanisms in RFC 3608 [27]. When using a service route,
this requirement (that the GRUU be obtained from a domain that will
be visited by the request) it is sufficient to use the GRUU learned
in the same registration that delivered the service route. If there
is no service route, and the request will visit only the single
specified outbound proxy, the UA needs to make sure that the GRUU it
used was obtained from the same domain as the outbound proxy.
Barring dynamic mechanisms to discover an outbound proxy, such a
relationship needs to be configured into the UA.
Note that this specification does not require the use of an outbound
proxy; it merely introduces the previous requirements in cases where
one is used.
When using a GRUU as a remote target (for example, when populating
the Contact header field of an INVITE or 200 OK response), the UA
placing the GRUU into the request or response MAY add the "grid" URI
parameter to the GRUU, and if one is added, MUST include a value for
it. This parameter MAY take any value permitted by the grammar
defined in Section 10. 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, the proxy will retain the "grid" parameter and its value
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 GRUUs, each of which differs by the value of the "grid"
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
can be the case for administratively constructed GRUUs, for example).
When a UA requires a URI with the GRUU property in order to reach a
peer for a particular SIP application (such as assisted call
transfer), it uses the URI in the Contact header field of a request
or response from that peer if it contains a GRUU. This is trivially
determined by the presence of the "gruu" URI parameter.
As per RFC 3261, a UA SHOULD include a Supported header with the
option tag "gruu" in requests it generates.
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8.1.2. 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 knows that the remote target is a GRUU by the presence
of the "gruu" URI parameter. The UA can take the GRUU, send a
request to it, and then be sure that the request 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 "opaque" and "gruu" URI parameters.
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.
Because 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 [19] that 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 a request 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
preferential routing request.
8.1.3. Receiving a Request Sent to a GRUU
When a User Agent Server (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. If the UA had not included a
"grid" parameter in the GRUU, the incoming request will still have a
"grid" parameter, but with no value.
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It is important to note that, when a user agent receives a request,
the presence or absence of the "grid" parameter will inform the user
agent whether the request was sent to the AOR or to the GRUU. If the
parameter is absent, it means that the request was sent to the AOR.
If present, it means that the request was sent to the GRUU. This is
true regardless of whether the UA itself uses the "grid" parameter,
since the home proxy will insert one into the Request-URI when
receiving a request sent to a GRUU.
8.2. Proxy Behavior
Proxy behavior is fully defined in Section 16 of RFC 3261 [1]. GRUU
processing impacts that processing in two places -- request targeting
and record routing.
8.2.1. Request Targeting for Requests Outside of a Dialog
The processing in this section applies to requests that are received
outside of a dialog. These requests are identified by their lack of
a Route header field when received by the proxy.
When a proxy server receives a request, owns the domain in the
Request-URI, and 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 URI contains the
"gruu" URI parameter, but the URI does not refer to a GRUU known
within the domain, the proxy rejects the request with a 404 (Not
Found). If the Request-URI is within the domain of the proxy,
contains a "gruu" URI parameter, and the GRUU is known within the
domain and refers to a valid AOR within the domain, but the instance
ID is unknown, the proxy SHOULD generate a 480 (Temporarily
Unavailable).
Otherwise, 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 the proxy
supports the caller preferences specification [19], and the request
contained Accept-Contact or Reject-Contact header fields, caller
preferences are applied to the contact set. If there are more than
one contact with the same instance ID, and those contact were
registered using the procedures of [11], those procedures are used to
select one. Otherwise, the most recently updated contact is 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 there was no "grid"
parameter in the Request-URI, the proxy MUST insert a "grid"
parameter into the new target URI obtained from the registered
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contact, and MUST omit a value for the parameter. Note that the
"gruu" URI parameter is not copied. If no contacts were bound to the
GRUU, the lookup of the GRUU in the abstract location service will
result in zero target URIs, eventually causing the proxy to reject
the request with a 480 (Temporarily Unavailable) response.
If the contact was registered using a Path header field [3], then
that Path is used to construct the route set for reaching the contact
through the GRUU, as well as through the AOR, using the procedures
specified in RFC 3327 [3]. However, support for GRUUs at a registrar
does not require support for RFC 3327.
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.
8.2.2. Record-Routing
The proxy that accesses the location service (called the home proxy
here) MUST record-route under two circumstances. Firstly, if the
home proxy receives a dialog forming request from a UA in its own
domain (an originating request) whose Contact header field contains a
GRUU (indicated by the presence of the "gruu" URI parameter) that is
also in that domain, the proxy MUST record-route. This, of course,
assumes that the home proxy is in the request path for such
originating requests. If it is not, some other proxy that is in the
same domain as the home proxy, but which is on the request path, MUST
generate a record-route which will route to the home proxy. The
rules in Section 8.1.1 ensure that at least one proxy in the same
domain of the home proxy will be visited. A home proxy can use
mechanisms such as Service-Route [27] or UA configuration to ensure
that it, or a proxy in the same domain, is on the outgoing request
path.
Secondly, if the home proxy receives a dialog-forming request
targeted to an AOR or GRUU within the domain of the home proxy (a
terminating request), and it translates the Request URI into a
contact that is associated with an instance ID to which a GRUU has
been assigned, the proxy MUST record-route. The URI placed into the
record-route MUST cause the request to be routed to a proxy that can
access the location service for that AOR.
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8.2.3. Request Targeting for Mid-Dialog Requests
When a mid-dialog request is sent to a UA which used its GRUU as the
remote target, this mid-dialog request will arrive at the home proxy.
As a consequence of the record-routing procedures in Section 8.2.2,
this request will arrive with a Request-URI equal to the GRUU, and
the topmost Route header field equal to the URI placed into the
Record-Route previously.
Proxy processing of this request is nearly identical to that of
Section 8.2.1. The proxy MUST look up the GRUU in the location
service, and translate it to the registered contacts. If, based on
the procedures of Section 8.2.1, this lookup fails or produces no
contacts, the request MUST be rejected as described there. If the
lookup produces a single registered contact, that contact is placed
into the Request-URI. As with requests outside of a dialog, the
"grid" URI parameter is placed into the translated URI (having either
been copied from the Request-URI or placed there with no value
otherwise). If there are multiple registered contacts (which happens
when the client registers multiple flows using [11]), the proxy
chooses one arbitrarily. The actual one that is chosen is not
relevant; the Request-URI will not be used by an edge proxy compliant
to [11] to deliver the request to the UA.
Once the proxy finishes its processing, it will pop the topmost Route
header field value. If there were additional Route header field
values beyond the one pointing to the home proxy, these are not
touched or modified in any way by the procedures defined here. Any
Path values that may have been registered are not used. If there
were no additional Route header field values beyond the one pointing
to the home proxy, any Path values that were registered MUST be used
as if this was a request sent outside of any existing dialog.
The request is then forwarded based on the rules in RFC 3261. This
will use any Route header field values if present, else will use the
Request-URI. If the Request-URI is being used, the request gets
delivered using the procedures of [11] if the contact was registered
using those mechanisms.
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 (the derived URI) from the AOR (the base URI).
However, there are many other applications outside of GRUUs. It can
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be used, for example, to construct a URI for a voicemail inbox (the
derived URI) from a subscriber's AOR (the base URI), or the URI for a
video service advertised via presence [22] (the derived URI) from the
subscriber's 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
those outside 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, as
with GRUUs, the nature of the relationship can be determined from the
URI. When any element receives a URI with the "gruu" URI parameter,
and that URI contains the "opaque" URI parameter, the URI formed by
stripping the "opaque" and "gruu" URI parameter is the AOR associated
with the GRUU.
10. Grammar
This specification defines one new Contact header field parameter
("gruu") by extending the grammar for "contact-params" as defined in
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RFC 3261. It also defines three new SIP URI parameters ("grid",
"gruu" and "opaque") by extending the grammar for "uri-parameter" as
defined in RFC 3261.
contact-params =/ c-p-gruu
c-p-gruu = "gruu" EQUAL LDQUOT (SIP-URI / SIPS-URI) RDQUOT
uri-parameter =/ grid-param / opaque-param / gruu-param
grid-param = "grid" ["=" pvalue] ; defined in RFC3261
opaque-param = "opaque=" pvalue ; defined in RFC3261
gruu-param = "gruu"
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
that each route to that UA instance. For example, this is needed
to support ad-hoc conferencing 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 that was used to invoke the
request. This is necessary as a consequence of REQ 4.
REQ 6: It must be possible for a UA to add opaque content to a GRUU.
This content is not interpreted or altered by the network, and is
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 the state embedded.
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
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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 REQ 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 that is 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, shown in Figure 5, 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 [18] 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 |
| |-------------------->|
Figure 5
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 (message 2) 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
Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
CSeq: 1 REGISTER
<allOneLine>
Contact: <sip:callee@192.0.2.1>
;gruu="sip:callee@example.com;gruu;
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;gruu;
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 (message 3) is a normal SIP INVITE.
However, the 200 OK generated by the callee (message 5) 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, SUBSCRIBE
<allOneLine>
Contact:
<sip:callee@example.com;gruu
;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 [17] 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;gruu;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;gruu>
</allOneLine>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK, NOTIFY
Contact: <sip:caller@example.com;gruu;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 to that contact. Note
how the "grid" parameter is maintained, and the "gruu" parameter is
no longer present.
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;gruu>
</allOneLine>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK, NOTIFY
Contact: <sip:caller@example.com;opaque=hdg7777ad7aflzig8sf7>
Content-Length: 0
The SUBSCRIBE generates a 200 response (message 11), which is
followed by a NOTIFY (message 13 and 14) and its response (message 15
and 16). 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
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registered one) will be used by the proxy for population in the
target set. The registrar then generates the following response:
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
Call-ID: hf8asxzff8s7f@192.0.2.2
CSeq: 1 REGISTER
<allOneLine>
Contact: <sip:callee@192.0.2.2>
;gruu="sip:callee@example.com;gruu;opaque=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
<allOneLine>
Contact: <sip:callee@192.0.2.1>
;gruu="sip:callee@example.com;gruu;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
given 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
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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
treatment (in particular, screening features) be given to requests
that are sent to a GRUU. 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 [12].
As a consequence of this specification, a UA will begin using GRUUs
in the dialog forming and target refresh requests and responses it
emits. These GRUUs will be passed to other UA (called the
correspondent), which then use them in requests that they emit.
These UA might be malicious, and attempt to remove the "gruu",
"grid", or "opaque" parameters from the URI before using it.
Consequently, consideration must be given to the effect of such
removal.
If a malicious correspondent removes the "gruu" URI parameter, the
request will be routed to the home proxy. Despite the absence of the
"gruu" parameter, the home proxy will still recognize the URI as a
GRUU, based on the presence and value of the "opaque" parameter.
Consequently, a home proxy SHOULD NOT rely solely on the presence of
the "gruu" parameter to determine that a URI is a GRUU. If a
malicious correspondent removes both the "gruu" and "opaque" URI
parameters, the resulting URI will, in many cases, look identical to
an AOR, and thus receive the same treatment as an AOR. If this is
done in a mid-dialog request, the proxy might translate the AOR to a
registered contact. If this registered contact points to a different
UA instance than the one in the dialog, the request might be
misrouted to that instance. Since the dialog doesn't exist there,
the request is rejected. This has no harmful effects to anyone
except for the malicious correspondent. If a malicious correspondent
removes the "grid" parameter, the request will be delivered to the
UA, but contain an empty "grid" parameter inserted by the home proxy.
If a UA requires the "grid" parameter to process the request, then it
SHOULD always insert a "grid" parameter into all GRUUs it hands out,
with a different value for each. Consequently, if a request should
arrive with an empty "grid" parameter, the UA will know that the
parameter had been stripped by a malicious correspondent, and it can
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reject the request if desired.
14. IANA Considerations
This specification defines a new Contact header field parameter,
three SIP URI parameters, and a SIP option tag.
14.1. Header Field Parameter
This specification defines a new header field parameter, as per the
registry created by [9]. 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 three new SIP URI parameters, as per the
registry created by [10].
Name of the Parameter: grid
Predefined Values: none
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
Name of the Parameter: opaque
Predefined Values: none
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
Name of the Parameter: gruu
Predefined Values: none
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
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14.3. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
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. When used in a Require header field of a REGISTER
request, it indicates that the registrar shouldn't process the
registration unless it supports the GRUU extension.
15. Acknowledgements
The author would like to thank Rohan Mahy, Paul Kyzivat, Alan
Johnston, Ya-Ching Tan, Dale Worley, Jeroen van Bemmel, Fredrik
Thulin and Cullen Jennings for their comments and 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
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Method", RFC 3515, April 2003.
[8] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[9] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[10] 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.
[11] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006.
16.2. Informative References
[12] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[13] 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.
[14] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol (SIP)", RFC 4353, February 2006.
[15] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
RFC 4538, June 2006.
[16] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06 (work in progress), October 2005.
[17] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[18] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and
H. Schulzrinne, "Session Initiation Protocol (SIP) Torture Test
Messages", RFC 4475, May 2006.
[19] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
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Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[20] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)",
RFC 3863, August 2004.
[21] Sparks, R., "Session Initiation Protocol Call Control -
Transfer", draft-ietf-sipping-cc-transfer-06 (work in
progress), March 2006.
[22] Rosenberg, J., "A Data Model for Presence",
draft-ietf-simple-presence-data-model-07 (work in progress),
January 2006.
[23] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[24] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[25] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
[26] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[27] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003.
[28] Kyzivat, P., "Registration Event Package Extension for Session
Initiation Protocol (SIP) Globally Routable User Agent URIs
(GRUU)", draft-ietf-sipping-gruu-reg-event-06 (work in
progress), May 2006.
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. Of course, others are 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
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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
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 signifies concatenation. The single space (" ") between
components is used as a delimiter, so that the components can easily
be extracted after decryption. Salt represents a random string that
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 it has 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 that
matches 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 fake GRUUs, 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
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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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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
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Rosenberg Expires February 1, 2007 [Page 40]
