SIP J. Rosenberg
Internet-Draft dynamicsoft
Expires: August 15, 2004 February 15, 2004
Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in
the Session Initiation Protocol (SIP)
draft-ietf-sip-gruu-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Defining a GRUU . . . . . . . . . . . . . . . . . . . . . . 3
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1 REFER . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Conferencing . . . . . . . . . . . . . . . . . . . . . . . . 4
4.3 Presence . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Overview of Operation . . . . . . . . . . . . . . . . . . . 5
6. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 6
6.1 REGISTER Processing . . . . . . . . . . . . . . . . . . . . 6
6.2 Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . 7
7. Registrar Behavior . . . . . . . . . . . . . . . . . . . . . 8
7.1 Creation and Maintenance of GRUUs . . . . . . . . . . . . . 8
7.2 Providing GRUUs to User Agents . . . . . . . . . . . . . . . 11
8. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 12
9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12. Security Considerations . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 18
13.1 Header Field Parameter . . . . . . . . . . . . . . . . . . . 18
13.2 URI Parameter . . . . . . . . . . . . . . . . . . . . . . . 18
13.3 Media Feature Tag . . . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
Normative References . . . . . . . . . . . . . . . . . . . . 19
Informative References . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . 22
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1. Introduction
Several applications of the Session Initiation Protocol (SIP) [1]
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. An example of such an application is call transfer, based
on the REFER method [4]. Another application is the usage of
endpoint-hosted conferences within the conferencing framework [10].
We call these URIs Globally Routable UA URIs (GRUU). This
specification provides a mechanism for obtaining and using GRUUs.
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 [2] and
indicate requirement levels for compliant implementations.
3. Defining a GRUU
A GRUU is a SIP URI which has a specific set of characteristics:
Global: It can be used by any UAC connected to the Internet. In
that regard, it is like an address-of-record (AOR) for a user. The
address-of-record for a user, sip:joe@example.com, is meant to be
used by anyone to call that user. The same is true for a GRUU.
Temporally Scoped: It may be temporally scoped. In that regard,
its not like an AOR for a user. The general assumption is that an
AOR for a user is valid so long as the user resides within that
domain (of course, policies can be imposed to limit its validity,
but that is not the default case). However, a GRUU has a limited
lifetime by default. It can never be valid for longer than the
duration of the registration of the UA to which it is bound. For
example, if my PC registers to the SIP network, a GRUU for my PC
is only valid as long as my PC is registered. If the PC
unregisters, the GRUU is invalid; calls to it would result in a
404. If the PC comes back, the GRUU will be valid once more.
Furthermore, it will frequently be the case that the GRUU has a
lifetime shorter than the duration of the registration.
Instance Routing: It routes to a specific UA instance, and never
forks. In that regard, it is unlike an address-of-record. When a
call is made to a normal AOR which represents a user, routing
logic is applied in proxies to deliver the call 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
call is made to a GRUU, the routing logic is much more static. It
has to cause the call to be delivered to a very specific UA
instance. That UA instance has to be the same UA instance for any
request sent to that GRUU. This does not mean that a GRUU
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represents a fundamentally different type of URI; it only means
that the logic a proxy applies to a GRUU is going to generally be
simpler than that it applies to a normal AOR.
4. Use Cases
We have encountered several use cases for a GRUU.
4.1 REFER
Consider a blind transfer application [14]. User A is talking to user
B. A wants to transfer the call to user C. So, it 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 needs to contain a URI that can be used by C to
place a call to B. However, this call needs to route to the specific
UA instace which B is using to talk to A. If it didn't, the transfer
service would not execute. This URI is provided to A by B. Because B
doesn't know who A will transfer the call to, the URI has to be
usable by anyone. Therefore, it is a GRUU.
4.2 Conferencing
A similar need arises in conferencing [10]. In that framework, a
conference is described by a URI which identifies the focus of the
conference. The focus is a SIP UA at the center of a 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 B.
They then put B on hold, and call C. Now, A has two separate dialogs
for two separate calls - one to B, and one to C. A would like to
conference them. One model is that A morphs itself into a focus. It
sends a re-INVITE on each existing dialog, and provides both B and C
with an updated URI that now holds the conference URI. It also has a
callee capabilities [6] parameter which indicates that this URI is a
conference URI. A proceeds to mix the media streams from B and C.
This is called an ad-hoc conference.
At this point, normal conferencing features can be applied. That
means that B can send another user, D, the conference URI, perhaps in
an email. D can send an INVITE to that URI, and join the conference.
For this to work, the conference URI used by A in its re-INVITE has
to be usable by anyone, and it has to route to the specific UA
instance of A that is acting as the focus. If it didn't, basic
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conferencing features would fail. Therefore, it is a GRUU.
4.3 Presence
In a SIP-based presence [15] system, the presence agent (PA)
generates notifications about the state of a user. This state is
represented with the Presence Information Document Format (PIDF)
[13]. In a PIDF document, a user is represented by a series of
tuples, each of which identifies the devices that the user has and
provides information about them. Each tuple also has a contact URI,
which is a SIP URI representing that device. A watcher can make a
call to that URI, with the expectation that the call is routed to the
device whose presence is represented in the tuple.
The URI in the presence document therefore has to route to the
specific UA instance whose presence was reported. 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 is 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
them. Similarly, a UA can obtain a GRUU but never use it.
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" media
feature tag in the Contact header field of each Contact for which a
GRUU is desired. This media feature tag 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 instace. That GRUU is guaranteed to remain valid for the
duration of the registration. The GRUU is bound to the UA instace.
Should the client change its Contact URI, 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
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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
normally place the GRUU into the Contact header field of dialog
creating requests and responses it generates. However, it is
important for the UA receiving the message to know whether the
Contact URI 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 Contact URI is a GRUU.
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
corresponding Contact URI, the server will copy the grid parameter
into the Contact URI. As a result, when the UA receives the request,
the Request URI will contain the grid parameter it placed in the
corresponding GRUU.
6. User Agent Behavior
User agent behavior is divided into two separate parts - REGISTER
processing, and GRUU usage.
6.1 REGISTER Processing
When a UA wishes to obtain a GRUU within the domain of its AOR, when
it 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 [6]. As described in [6], 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, as described in
Section 13.3, MUST be a globally unique identifier, and SHOULD remain
the same across all registrations generated from that particular UA
instance.
Besides the presence of the "gruu" option tag in the Supported header
field and the "+sip.instance" Contact header field parameter, the
REGISTER request is constructed identically to the case where this
extension was not understood. Specifically, the Contact URI in the
REGISTER request SHOULD NOT contain the gruu Contact header field
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parameter. Any such parameters are ignored by the registrar, as the
UA cannot propose a GRUU for usage with the Contact URI.
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.
If the response is a 2xx, each Contact header that contained the
"+sip.instance" Contact header field parameter may also contain a
"gruu" parameter. This parameter contains a SIP URI that represents a
GRUU corresponding to that UA instance. Any requests sent to the GRUU
URI will be routed by the domain to the Contact URI bound currently
bound to that instance ID. 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 the
UA instance ID. However, this property cannot be guaranteed, and a UA
MUST be prepared to receive a different GRUU in a subsequent
registration.
6.2 Using the GRUU
A UA first obtains a GRUU using the procedures of Section 6.1, 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 URI.
However, a UA compliant to this specification MUST use a GRUU when
populating the Contact header field of dialog-creating requests and
responses. This includes the INVITE request and its 2xx response, the
SUBSCRIBE [3] request, its 2xx response, and the NOTIFY request, and
the REFER [4] request and its 2xx response. Similarly, in those
requests and responses where the GRUU is used in the Contact header
field, 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 uses a GRUU before inserting
one into the Contact header field.
When placing a GRUU into the Contact header field of a request or
response, 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. Note that there are no limitations on the size of this
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 corresponding Contact URI. 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
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effectively manufacture an infinite supply of GRUU, 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 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. It is anticipated that this limitation will be
addressed in future specifications.
Once a UA knows that the Contact URI 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.
7. Registrar Behavior
A registrar compliant to this specification is responsible for the
creation and maintenance of GRUUs, and for providing those GRUU's to
a UA in response to a REGISTER request.
7.1 Creation and Maintenance of GRUUs
A domain is responsible for creation and maintenance of a GRUU, along
with its association to instance IDs, AORs and Contact URIs. These
associations are modeled in the UML diagram in Figure 2.
+-------------+
| |
| |
| GRUU |----------------------+
| | |
| | |
+-------------+ |
| 1 |
| |
| associated-with |
| |
| |
| 1 |
+----------------+ |
| | |
+--------| instance ID/ |------+ |
| | AOR Pair | | |
| | | | |
| +----------------+ | |
| | |
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| | |
| | |translates
V V |to
+--------------+ +-----------+ |
| | | | |
| instance | | AOR | |
| ID | | | |
| | +-----------+ |
+--------------+ | |
^ | |
| | |
| | |
| |is-bound-to |
| +----------------+ | |
| | | | |
| | | | |
+--------| Contact URI |<-----+ |
| | 0..* |
| | |
+----------------+ |
0..1 ^ |
| |
+-----------------------------+
Figure 2
The combination of a UA instance ID and an AOR is referred to as an
instance ID/AOR pair. There is a one-to-one mapping between such a
pair and a GRUU; the GRUU is said to be associated with the pair, and
the pair is associated with the GRUU. As a result, if two instance
ID/AOR pairs are different, they each must be associated with a
different GRUU. If two GRUUs are different, they each must be
associated with a different instance ID/AOR pair. It is important to
understand that this uniqueness is over the instance ID/AOR pair, not
just the instance ID. For example, if a user registered the Contact
sip:ua@pc.example.com;+sip.instance="1", representing a device with
instance ID 1, to the AOR sip:user@example.com, and also registered
the same Contact, representing the same instance ID -
sip:ua@pc.example.com;+sip.instance="1" 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.
A GRUU translates to zero or one Contact URIs. If the instance ID
associated with the GRUU is the instance ID of a Contact URI
currently bound to the AOR associated with that GRUU, then the GRUU
translates to that Contact URI. If, however, the instance ID
associated with the GRUU is not an instance ID of a Contact URI
currently bound to the AOR associated with the GRUU (possibly because
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there are no Contact URIs bound to the AOR), the GRUU maps to no
Contact URI, and the GRUU is said to be invalid.
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.
This specification does not mandate a particular mechanism for
construction of the GRUU. 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, exists in the global DNS and
corresponds to an IP address present on the public Internet.
o When a request is sent to this URI, it routes to a proxy server in
the same domain as that of the registrar.
o A proxy server in the domain can determine that the URI is a GRUU.
o When a proxy server in this domain receives a request sent to a
URI that is a GRUU, that URI MUST be translated to the Contact URI
currently bound to the AOR associated with that GRUU whose
instance ID is the one associated with the GRUU.
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, though not required, 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 + instance ID + AOR)))
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. 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.
The benefit of this mechanism is that a server need not store
additional information on mapping a GRUU to its corresponding Contact
URI. 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,
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extract the currently registered Contacts, and find the one matching
the instance ID encoded in the request URI. The Contact URI whose
instance ID is that instance ID is then used as the translated
version of the URI. 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 one 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.
Once an association from an instance ID/AOR to a GRUU is created,
that mapping MUST remain in existence, and valid, as long as there
exists any Contact bound to that AOR whose instance ID is that
instance ID. If, through a de-registration or expiration, there is no
longer any Contact bound to that AOR whose instance ID is that
instance ID, the registrar MUST remove the mapping, and invalidate
the GRUU. However, at any time in the future, should a UA register a
Contact to that same AOR indicating that it represents that same
instance ID, the registrar SHOULD provide the UA the same GRUU
provided previously. Indeed, this requirement would ideally be a MUST
if it was achieveable, but even with the stateless algorithm
described above, key rotation or server failures may cause the GRUU
associated with an instance ID/AOR pair to change. The value of
associatig the GRUU with an instance ID/AOR pair, as opposed to a
Contact URI/AOR pair, is that the association can transcend
registrations. As a result, registrars SHOULD make every effort
possible to maintain the association for as long as possible.
7.2 Providing GRUUs to User Agents
When a registrar compliant to this specification receives a REGISTER
request, it checks for the presence of the Require header field in
the request. If present, and if it contains the "gruu" option tag,
the registrar MUST follow the procedures in the next paragraph for
inclusion of the "gruu" parameter in a 2xx response to REGISTER. If
not present, but a Supported header field was present with the "gruu"
option tag, the registrar SHOULD follow the procedures in the next
paragraph for inclusion of the "gruu" parameter in a 2xx response to
REGISTER. If the Supported header field was not present, or it if was
present but did not contain the value "gruu", the registrar SHOULD
NOT follow the procedures of the next paragraph for inclusion of the
"gruu" parameter in a 2xx response to REGISTER.
If the register request contained any "gruu" Contact header field
parameters, these MUST be ignored by the registrar. A UA cannot
suggest or otherwise provide a GRUU to the registrar.
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A GRUU is provided to a UA by including it in the "gruu" Contact
header field parameter for each Contact URI that contains a
"+sip.instance" Contact header field parameter. 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 server does not currently
have a GRUU associated with the instance ID/AOR, one is created
according to the procedures of Section 7.1. Otherwise, if a GRUU
already exists for that instance ID/AOR pair, the GRUU associated
with that pair MUST be placed into the "gruu" Contact header field
parameter of the REGISTER response.
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.
8. Proxy Behavior
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 MUST check if the Request URI is a
GRUU created by that domain.
If the URI is a GRUU, the proxy MUST determine if there is still a
Contact URI bound to AOR associated with the GRUU, whose instance ID
is the instance ID associated with the GRUU. If that AOR no longer
has any contacts bound to it, or if it does have contacts bound to
it, but none of them have an instance ID equal to the instance ID
associated with the GRUU, the proxy MUST generate a 404 (Not Found)
response to the request.
Otherwise, the proxy MUST populate the target set with a single URI.
This URI MUST be equal to the Contact URI that is translated from the
GRUU. Furthermore, if the GRUU contained a "grid" URI parameter, the
URI in the target set MUST also contain the same parameter with the
same value.
A proxy MAY apply other processing to the request, such as execution
of called party features. In particular, it is RECOMMENDED that
non-routing called party features, such as call logging and
screening, that are associated with the AOR are also applied to
requests for all GRUUs associated with that AOR.
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In many cases, a proxy will record-route an initial INVITE request,
and the user agents will insert a GRUU into the Contact header field.
When this happens, a mid-dialog request will arrive at the proxy with
a Route header field that was inserted by the proxy, and a
Request-URI that represents a GRUU. Proxies follow normal processing
in this case; they will strip the Route header field, and then
process the Request URI as described above.
The procedures of RFC 3261 are then followed to proxy the request.
The request SHOULD NOT be redirected in this case. In many instances,
a GRUU is used by a UA in order to assist in the traversal of NATs,
and a redirection may prevent such a case from working.
9. Grammar
This specification defines two new Contact header field parameters,
gruu and +sip.instance, and a new URI parameter, grid. The grammar
for string-value is obtained from [6].
contact-params = c-p-q / c-p-expires / c-p-gruu / cp-instance
/ contact-extension
c-p-gruu = "gruu" EQUAL DQUOTE SIP-URI DQUOTE
cp-instance = "+sip.instance" EQUAL LDQUOT string-value RDQUOT
uri-parameter = transport-param / user-param / method-param
/ ttl-param / maddr-param / lr-param / grid-param
/ other-param
grid-param = "grid=" pvalue
10. 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 a UA
instance needs a different URI for each conference it is hosting.
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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.
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 instace 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 instace bound to an AOR
in that domain. In other words, the proxy can construct a GRUU
too. This is needed for the presence application.
11. Examples
The following call flow shows a basic registration and call setup,
followed by a subscription directed to the GRUU.
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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 |
| |-------------------->|
The Callee supports the GRUU extension. As such, its REGISTER (1)
looks like:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl
Supported: gruu
To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
CSeq: 1 REGISTER
Contact: <sip:callee@client.example.com>;+sip.instance="<hffua-ssdfff877>"
Content-Length: 0
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The REGISTER response would look like:
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
CSeq: 1 REGISTER
Contact: <sip:callee@client.example.com>
;gruu="sip:hha9s8d=-999a@example.com"
;+sip.instance="<hffua-ssdfff877>"
Content-Length: 0
Note how the Contact header field in the REGISTER response contains
the gruu parameter with the URI sip:hha9s8d=-999a@example.com. This
represents a GRUU that translates to the Contact URI
sip:callee@client.example.com.
The INVITE from the caller is a normal SIP INVITE. The 200 OK
generated by the callee, however, now contains a GRUU in the Contact
header field. The UA has also chosen to include a grid URI parameter
into the GRUU.
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
Contact: <sip:hha9s8d=-999a@example.com;grid=99a>
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 [11] at that specific UA. To do that, it
generates a SUBSCRIBE request (message 9), but directs it towards the
GRUU contained in the Contact header field.
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SUBSCRIBE sip:hha9s8d=-999a@example.com;grid=99a SIP/2.0
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:caller@example.com>;tag=kkaz-
To: Callee <sip:callee@example.com>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:bad998asd8asd0000a0@example.com>
Content-Length: 0
In this example, the caller itself supports the GRUU extension, and
is using its own GRUU to populate the Contact header field of the
SUBSCRIBE.
This request is routed to the proxy, which proceeds to perform a
location lookup on the request URI. It is translated into the Contact
URI of that GRUU, and then proxied there (message 10). Note how the
grid parameter is maintained.
SUBSCRIBE sip:callee@client.example.com;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-
To: Callee <sip:callee@example.com>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:bad998asd8asd0000a0@example.com>
Content-Length: 0
12. Security Considerations
Since GRUUs do not reveal information about the identity of the
associated address-of-record or Contact URI, they provide routability
without identity. However, GRUUs do not provide a complete or
reliable 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 [9].
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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 when
registering.
13. IANA Considerations
This specification defines a new Contact header field parameter, URI
parameter and media feature tag.
13.1 Header Field Parameter
This specification defines a new header field parameter, as per the
registry created by [7]. 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.]]
13.2 URI Parameter
This specification defines a new SIP URI parameter, as per the
registry created by [8].
Name of the Parameter grid
RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
13.3 Media Feature Tag
This section registers a new media feature tag, per the procedures
defined in RFC 2506 [5]. The tag is placed into the sip tree, which
is defined in [6].
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 that indicates a unique identifier associated
with the UA instance registering the Contact. This identifier is
globally unique, and remains bound to the UA instance for as long
as is achievable. For UA instances that are implemented as
hardware, such as an IP phone, the instance ID would ideally be
burned into firmware when the device is manufactured. For
software, the instance ID would generally be randomly created at
installation time.
Values appropriate for use with this feature tag: String.
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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 [12] 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. Acknowledgements
The author would like to thank Rohan Mahy, Paul Kyzivat, Alan
Johnston, and Cullen Jennings for their contributions to this work.
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] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[5] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.
[6] Rosenberg, J., "Indicating User Agent Capabilities in the
Session Initiation Protocol (SIP)",
draft-ietf-sip-callee-caps-03 (work in progress), January 2004.
[7] Camarillo, G., "The Internet Assigned Number Authority Header
Field Parameter Registry for the Session Initiation Protocol",
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draft-ietf-sip-parameter-registry-01 (work in progress),
November 2003.
[8] Camarillo, G., "The Internet Assigned Number Authority Universal
Resource Identifier Parameter Registry for the Session
Initiation Protocol", draft-ietf-sip-uri-parameter-reg-01 (work
in progress), November 2003.
Informative References
[9] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[10] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-01 (work in
progress), October 2003.
[11] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
Event Package for the Session Initiation Protocol (SIP)",
draft-ietf-sipping-dialog-package-03 (work in progress),
October 2003.
[12] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
draft-ietf-sip-callerprefs-10 (work in progress), October 2003.
[13] Sugano, H. and S. Fujimoto, "Presence Information Data Format
(PIDF)", draft-ietf-impp-cpim-pidf-08 (work in progress), May
2003.
[14] Sparks, R. and A. Johnston, "Session Initiation Protocol Call
Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
progress), February 2003.
[15] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
in progress), January 2003.
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Author's Address
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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