One document matched: draft-ietf-sip-gruu-12.txt
Differences from draft-ietf-sip-gruu-11.txt
SIP J. Rosenberg
Internet-Draft Cisco
Intended status: Standards Track March 5, 2007
Expires: September 6, 2007
Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the
Session Initiation Protocol (SIP)
draft-ietf-sip-gruu-12
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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 registrar and for
communicating a GRUU to a peer within a dialog.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
3.1. Structure of GRUUs . . . . . . . . . . . . . . . . . . . . 5
3.1.1. GRUUs Which Expose the Underlying AOR . . . . . . . . 6
3.1.2. GRUUs Which Hide the Underlying AOR . . . . . . . . . 6
3.2. Obtaining a GRUU . . . . . . . . . . . . . . . . . . . . . 7
3.3. Using a GRUU . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Dereferencing a GRUU . . . . . . . . . . . . . . . . . . . 8
4. User Agent Behavior . . . . . . . . . . . . . . . . . . . . . 8
4.1. Generating a REGISTER Request . . . . . . . . . . . . . . 8
4.2. Learning GRUUs from REGISTER Responses . . . . . . . . . . 9
4.3. Constructing a Self-Made GRUU . . . . . . . . . . . . . . 10
4.4. Using Ones Own GRUUs . . . . . . . . . . . . . . . . . . . 11
4.4.1. Considerations for Multiple AORs . . . . . . . . . . . 11
4.5. Dereferencing a GRUU . . . . . . . . . . . . . . . . . . . 12
4.6. Rendering GRUUs on a User Interface . . . . . . . . . . . 13
5. Registrar Behavior . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Processing a REGISTER Request . . . . . . . . . . . . . . 13
5.2. Generating a REGISTER Response . . . . . . . . . . . . . . 14
5.3. Timing Out a Registration . . . . . . . . . . . . . . . . 15
5.4. Creation of a GRUU . . . . . . . . . . . . . . . . . . . . 15
5.5. Registration Event Support . . . . . . . . . . . . . . . . 17
6. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Request Targeting . . . . . . . . . . . . . . . . . . . . 18
6.2. Record-Routing . . . . . . . . . . . . . . . . . . . . . . 19
7. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Example Call Flow . . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10.1. Outside Attacks . . . . . . . . . . . . . . . . . . . . . 27
10.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . . . 27
10.3. Privacy Considerations . . . . . . . . . . . . . . . . . . 28
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11.1. Header Field Parameter . . . . . . . . . . . . . . . . . . 30
11.2. URI Parameter . . . . . . . . . . . . . . . . . . . . . . 30
11.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
13.1. Normative References . . . . . . . . . . . . . . . . . . . 31
13.2. Informative References . . . . . . . . . . . . . . . . . . 32
Appendix A. Example GRUU Construction Algorithms . . . . . . . . 33
A.1. Public GRUU Algorithm . . . . . . . . . . . . . . . . . . 33
A.2. Temporary GRUU . . . . . . . . . . . . . . . . . . . . . . 33
Appendix B. Network Design Considerations . . . . . . . . . . . . 35
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
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Intellectual Property and Copyright Statements . . . . . . . . . . 37
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1. Introduction
In the Session Initiation Protocol (SIP), RFC 3261 [1], the basic
unit of reference is the Address-Of-Record (AOR). However, in SIP
systems a single user can have a number of user agents (handsets,
softphones, voicemail accounts, etc.) which are all referenced by the
same AOR. There are a number of contexts in which it is desirable to
have an identifier which addresses a single user agent rather than
the group of user agents indicated by an AOR.
As an example, consider a blind transfer application [18]. 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. For
example, if A provides C with B's AOR, the call might be routed to
B's voice mail rather than B's current handset.
In order to enable this functionality, User B provides an instance-
specific URI to User A in the Contact header of their SIP exchange.
This URI refers only the the user agent B is currently using and can
be provided to user C. Because user B doesn't know in advance who
user A will transfer the call to, the URI has to be usable by anyone.
Many current clients attempt to meet the need for an instance-
specific identifier by using explicit IP addresses in the values they
provide in the Contact header field. However, this interacts poorly
with NATs and firewalls, and as a practical matter these URIs cannot
be used by arbitrary external clients. Similarly, usage of hostnames
has proven problematic for similar reasons. In addition, many SIP
clients do not have or cannot obtain a hostname for themselves at
all.
This specification describes a mechanism for providing a unique user-
agent identifier which is still globally routable. This identifier
is called a Globally Routable User Agent (UA) URI (GRUU).
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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 [4].
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 and 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 a value of the Contact
header field. The contact URI identifies a specific UA.
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 or responses.
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. Overview of Operation
The basic idea behind a GRUU is simple. GRUUs are issued by SIP
domains and always route back to a proxy in that domain. The domain
in turn maintains the binding between the GRUU and the particular UA
instance. When a GRUU is de-referenced when sending a SIP request,
that request arrives at the proxy. It maps the GRUU to the contact
for the particular UA instance, and sends the request there.
3.1. Structure of GRUUs
A GRUU is a SIP URI that has two properties:
o It routes to a specific UA instance.
o It can be successfully de-referenced by any user agent on the
Internet, not just ones in the same domain or IP network as the UA
instance to which the GRUU points.
In principle, a GRUU can be constructed in any way the domain
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chooses, as long as it meets the criteria above. However, all GRUUs
contain the "gr" URI parameter (either with or without a value), so
that a recipient of a GRUU can tell that it has these two properties.
In practice, there are two different types of GRUUs:
1. GRUUs which expose the underlying AOR
2. GRUUs which hide the underlying AOR
3.1.1. GRUUs Which Expose the Underlying AOR
In many cases it is desirable to construct the GRUU in such a way
that the mapping to the AOR is apparent. For example, many user
agents retain call logs, which keep track of incoming and outgoing
call attempts. If the UA had made a call towards a GRUU (perhaps as
a consequence of a transfer request), the call log will contain the
GRUU. Since the call log is rendered to the user, it would be useful
to be able to present the user with the AOR instead, since the AOR is
meaningful to users as an identifier.
This type of GRUU is called a public GRUU. It is constructed by
taking the AOR, and adding the "gr" URI parameter with a value chosen
by the registrar in the domain. The value of the "gr" parameter
contains a representation of the UA instance. For instance, if the
AOR was "sip:alice@example.com", the GRUU might be:
sip:alice@example.com;gr=kjh29x97us97d
If a UA removes the "gr" parameter, the result is the AOR. Since
many systems ignore unknown parameters anyway, a public GRUU will
"look" like the AOR to those systems.
3.1.2. GRUUs Which Hide the Underlying AOR
In other cases it is desirable to construct a GRUU that obfuscates
the AOR such that it cannot be extracted by a recipient of the GRUU.
Such a GRUU is called a temporary GRUU. The most obvious reason to
do this is to protect the user's privacy. In such cases, the GRUU
may have any content provided that it meets the requirements in
Section 3.1, and the AOR cannot be readily determined from the GRUU.
The GRUU will have the "gr" parameter, either with or without a
value. In order to avoid creating excessive state in the registrar,
it is often desirable to construct cryptographically protected
"stateless" GRUUs using an algorithm like that described in Appendix
A.
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3.2. Obtaining a GRUU
A User Agent can obtain a GRUU in one of several ways:
o As part of its REGISTER transaction.
o By constructing one locally, using the IP address or hostname of
the user agent instance as the domain part of the URI. These are
called self-made GRUUs, and are only really GRUUs when constructed
by UA that know they are globally reachable using their IP address
or hostname.
o Via some locally-specified Administrative mechanism.
A UA which wants to obtain a GRUU via its REGISTER request does so by
providing an instance ID in the "+sip.instance" parameter in the
Contact header field [12]. For example:
Contact: <sip:callee@192.0.2.2>
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
The registrar detects this parameter and provides two GRUUs in the
REGISTER response. One of these is a temporary GRUU, and the other
is the public GRUU. These two GRUUs are returned in the "temp-gruu"
and "pub-gruu" parameters, respectively, in the Contact header field
of the response. For example:
Contact: <sip:callee@192.0.2.2>
;pub-gruu="sip:callee@example.com;gr=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;temp-gruu="sip:8ffkas08af7fasklzi9@example.com;gr"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
When a user agent refreshes this registration prior to its
expiration, the registrar will return back the same public GRUU, but
will create a new temporary GRUU. Despite the fact that each refresh
provides the UA with a new temporary GRUU, all of the temporary GRUUs
learned from previous REGISTER responses during the lifetime of a
contact remain valid as long as (1) that contact remains registered,
and (2) the UA doesn't change the Call-ID in its REGISTER request
compared to previous ones. When the contact expires, either through
explicit de-registration or timeout, all of the temporary GRUUs be
invalidated. Similarly, if a register refresh changes the Call-ID
compared to previous register refreshes, all of the previous
temporary GRUUs are invalidated. When the user agent later creates a
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new registration with the same instance ID, the public GRUU is the
same. The temporary GRUU will be new (as it is with refreshes), and
it will be the only valid temporary GRUU for the instance until the
next refresh, at which point a second one becomes valid too.
Consequently, temporary GRUUs "accumulate" during the lifetime of a
registration.
3.3. Using a GRUU
Once a user agent obtains GRUUs from the registrar, it uses them in
several ways. Firstly, it uses them as the contents of the Contact
header field in non-REGISTER requests and responses that it emits
(for example, an INVITE request and 200 OK response). According to
RFC 3261, the Contact header field is supposed to contain a URI that
routes to that user agent. Prior to this specification, there hasn't
been a way to really meet that requirement. The user agent would use
one of its temporary GRUUs for anonymous calls, and use its public
GRUU otherwise.
In addition, the UA can use the GRUU in any other place it needs to
use a URI that resolves to itself, such as a webpage.
3.4. Dereferencing a GRUU
Because a GRUU is simply a URI, a UA dereferences it in exactly the
same way as it would any other URI. However, once the request has
been routed to the appropriate proxy, the behavior is slightly
different. The proxy will map the GRUU to the AOR and determine the
set of contacts that the particular UA instance has registered. The
GRUU is then mapped to those contacts, and the request is routed
towards the UA.
4. User Agent Behavior
This section defines the normative behavior for user agents.
4.1. 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 [12] as
a UA characteristic [7], whose value MUST be the instance ID that
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identifies the UA instance being registered. Each such Contact
header field SHOULD NOT contain a "pub-gruu" or "temp-gruu" header
field. The contact URI MUST NOT be equivalent, based on the URI
equality rules in RFC 3261, to the AOR in the To header field. If
the contact URI is a GRUU, it MUST NOT be a GRUU for the AOR in the
To header field.
As in RFC 3261, the Call-ID in a REGISTER refresh SHOULD be identical
to the Call-ID used to previously register a contact. With GRUU, an
additional consideration applies. If the Call-ID changes in a
register refresh, the server will invalidate all temp-gruu associated
with that UA instance; the only valid one will be the new one
returned in that REGISTER response. Consequently, if a UA wishes its
previously obtained temporary GRUUs to remain valid, it MUST utilize
the same Call-ID in REGISTER refreshes. However, it MAY change the
Call-ID in a refresh if invalidation is the desired objective. Note
that, if any dialogs are in progress that utilize a temporary GRUU as
a remote target, and a UA performs a registration refresh with a
change in Call-ID, those temporary GRUU become invalid, and the UA
will not be reachable for subsequent mid-dialog messages.
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 [12].
A UA utilizing GRUUs can still perform third party registrations
and can include contacts which omit the "+sip.instance" Contact
header field parameter.
If a UA wishes to guarantee that the REGISTER 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 also containing the "gruu" option tag.
The use of Proxy-Require is not necessary and is NOT RECOMMENDED.
4.2. Learning GRUUs from REGISTER Responses
If the REGISTER response is a 2xx, each Contact header field that
contains the "+sip.instance" Contact header field parameter may also
contain a "pub-gruu" and "temp-gruu" Contact header field parameter.
These parameters convey the public and a temporary GRUU for the UA
instance, respectively. A UA MUST be prepared for a Contact header
field to contain just a "pub-gruu", just a "temp-gruu", neither, or
both. The temporary GRUU will be valid for the duration of the
registration, while the public GRUU persists across registrations.
The UA will receive a new temporary GRUU in each successful REGISTER
response, while the public GRUU will typically be the same. However,
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a UA MUST be prepared for the public GRUU to change from a previous
one, since the persistence property is not guaranteed with complete
certainty. If a UA changed its Call-ID in this REGISTER request
compared to a previous REGISTER request for the same contact, the UA
MUST discard all temporary GRUU learned through prior REGISTER
responses. A UA MAY retain zero, one, some, or all of the temporary
GRUUs that it is provided during the time over which its contact
remains registered. If a UA stores any temporary GRUUs for use
during its registration, it needs to be certain that the registration
does not accidentally lapse due to clock skew between the UA and
registrar. Consequently, the UA MUST refresh its registration well
in advance of expiration. In cases where registrars forcefully
shorten registration intervals, the registration event package, RFC
3860 [23] is used by user agents to learn of these changes. A user
agent implementing both RFC 3680 and GRUU MUST also implement the
extensions to RFC 3680 for conveying information on GRUU [24], as
these are necessary to keep the set of temporary GRUU synchronized
between the UA and the registrar.
A non-2xx response to the REGISTER request has no impact on any
existing GRUUs 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.
The GRUU learned by the UA in the REGISTER response MUST be treated
opaquely by the UA. That is, the UA MUST NOT modify it in any way
(including adding or removal of URI parameters).
4.3. Constructing a Self-Made GRUU
Many user agents, such as gateways to the Public Switched Telephone
Network (PSTN), conferencing servers and media servers, do not
perform registrations, and cannot obtain GRUUs through that
mechanism. These types of user agents may be publicly reachable.
This would mean that the policy of the domain is that requests can
come from anywhere on the public Internet and be delivered to the
user agent without requiring processing by intervening proxies within
the domain. Furthermore, firewall and NAT policies administered by
the domain would allow such requests into the network. When a user
agent is certain that these conditions are met, a UA MAY construct a
self-made GRUU. Of course, a user agent which does REGISTER, but for
whom these conditions are met regardless, MAY also construct a self-
made GRUU. However, usage of GRUUs obtained by the registrar is
RECOMMENDED instead.
A self-made GRUU is one whose domain part equals the IP address or
hostname of the user agent. The user part of the SIP URI is chosen
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arbitrarily by the user agent. Like all other GRUUs, the URI MUST
contain the "gr" URI parameter, with or without a value, indicating
it is a GRUU.
If a user agent does not register, but it is not publicly reachable,
it would need to obtain a GRUU through some other means. Typically,
the UA would be configured with a GRUU, and the GRUU would also be
configured into the proxy which will receive requests targeted to the
GRUU, along with a static mapping to the IP address and port of the
UA.
4.4. Using Ones Own GRUUs
A UA 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 one of its
GRUUs as its remote target. This includes the INVITE request, its
2xx response or 18x response with a To tag, the SUBSCRIBE [5]
request, its 2xx response or with a To tag, the NOTIFY request, the
REFER [6] request, its 2xx response, and the UPDATE request and its
2xx response. The only reason not to use a GRUU would be privacy
considerations; see Section 10.3.
When using a GRUU obtained through registrations, a UA MUST have an
active registration prior to using a GRUU, and MUST use a GRUU
learned through that registration. It MUST NOT reuse a GRUU learned
through a previous registration which has lapsed (in other words, one
obtained when registering a contact which has expired). The UA MAY
use either the public or one of its temporary GRUUs provided by its
registrar. A UA MUST NOT use a temporary GRUU learned in a REGISTER
response whose Call-ID differs from the one in the most recent
REGISTER request generated by the UA for the same AOR and instance
ID. Of course, when a UA wishes to construct an anonymous request as
described in RFC 3323 [13], it SHOULD use a temporary GRUU. See
Section 10.3 for a more complete discussion on the level of privacy
afforded by temporary GRUUs.
As per RFC 3261, a UA SHOULD include a Supported header with the
option tag "gruu" in requests and responses it generates.
4.4.1. Considerations for Multiple AORs
In some SIP networks, a user agent may have a multiplicity of AOR,
either in different domains, or within the same domain. In such
cases, additional considerations apply.
When a UA sends a request, the request will be sent 'using' one of
its AOR. This AOR will typically show up in the From header field of
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the request, and credentials unique to that AOR will be used to
authenticate the request. The GRUU placed into the Contact header
field of such a request SHOULD be one that is associated with the AOR
used to send the request. In cases where the UA uses a tel URI [11]
to populate the From header field, the UA typically has a SIP AOR
that is treated as an alias for the tel URI. The GRUU associated
with that SIP AOR SHOULD be used in the Contact header field.
When a UA receives a request, the GRUU placed into the Contact header
field of a 2xx response SHOULD be the one associated with the AOR or
GRUU to which the request was most recently targeted. There are
several ways to determine the AOR or GRUU to which a request was
sent. For example, if a UA registered a different contact to each
AOR (by using a different user part of the URI), the Request-URI
(which contains that contact) will indicate the AOR.
4.5. Dereferencing a GRUU
A GRUU is identified by the presence of the "gr" URI parameter, and
this parameter may or may not have a value. A UA that wishes to send
a request to a URI that contains a GRUU knows that the request will
be delivered to a specific UA instance without further action on the
part of the requestor.
Some UAs implement non-standard mechanisms for handling URIs that
compensate for the fact that heretofore many contact URIs have not
been globally routable. Since any URI containing the "gr" parameter
is known to be globally routable, a UA SHOULD NOT apply such
mechanisms when contact URI contain the "gr" parameter.
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 [16] 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.
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4.6. Rendering GRUUs on a User Interface
When rendering a GRUU to a user through a user interface, it is
RECOMMENDED that the "gr" parameter be removed. For public GRUUs,
this will produce the AOR, as desired. For temporary GRUUs, the
resulting URI will be seemingly random. Future work may provide
improved mechanisms that would allow an automata to know that a URI
is anonymized and thus should not be rendered.
5. Registrar Behavior
5.1. Processing a REGISTER Request
A REGISTER request might contain a Require header field with the
"gruu" option tag; this indicates that the registrar has to
understand this extension in order to process the request. It does
not require the registrar to create GRUUs, however.
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 based on the rules in
the remainder of this section. Otherwise, the contact is processed
based on normal RFC 3261 rules.
If the contact URI is equivalent (based on URI equivalence in RFC
3261) to the AOR, the registrar MUST reject the request with a 403,
since this would cause a routing loop. If the contact URI is a GRUU
for the AOR in the To header field of the REGISTER request, the
registrar MUST reject the request with a 403, for the same reason.
If the contact is not a SIP URI, the REGISTER request MUST be
rejected with a 403.
Next, the registrar checks if there is already a valid public GRUU
for the AOR (present in the To header field of the REGISTER request)
and the instance ID (present as the content of the "+sip.instance"
Contact header field parameter). If there is no valid public GRUU,
the registrar SHOULD construct a public GRUU at this time according
to the procedures of Section 5.4. The public GRUU MUST be
constructed by adding the "gr" URI parameter, with a value, to the
AOR. If the contact contained a "pub-gruu" Contact header field
parameter, the parameter MUST be ignored by the registrar. A UA
cannot suggest or otherwise provide a public GRUU to the registrar.
Next, the registrar checks for any existing contacts registered to
the AOR and instance ID. If there is at least one, the registrar
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finds the one that was most recently registered, and examines the
Call-ID value associated with that registered contact. If it differs
from the one in the REGISTER request, the registrar MUST invalidate
all previously generated temporary GRUU for the AOR and instance ID.
A consequence of this invalidation is that requests addressed to
those GRUU will be rejected by the domain with a 404 from this point
forward.
Next, the registrar SHOULD create a new temporary GRUU for the AOR
and instance ID with the characteristics described in Section 5.4.
The temporary GRUU construction algorithm MUST have the following two
properties:
1. The likelihood that the temporary GRUU is equal to another GRUU
which the registrar has created MUST be vanishingly small.
2. Given a pair of GRUUs, it MUST be computationally infeasible to
determine whether they were issued for the same AOR or instance
ID or different AORs and instance IDs.
If the contact contained a "temp-gruu" Contact header field
parameter, the parameter MUST be ignored by the registrar. A UA
cannot suggest or otherwise provide a temporary GRUU to the
registrar.
5.2. Generating a REGISTER Response
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, and the registrar has at
least one temporary GRUU assigned to the instance ID and AOR, the
registrar MUST add an "temp-gruu" Contact header field parameter to
that Contact header field. The value of the "temp-gruu" parameter is
a quoted string, and MUST contain the mostly recently created
temporary GRUU for that AOR and instance ID. In addition, if the
registrar has a public GRUU assigned to the instance ID and AOR (and
the client supports GRUUs), the registrar MUST add a "pub-gruu"
Contact header field parameter to that Contact header field. The
value of the "pub-gruu" parameter is the public GRUU.
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. As described in
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Section 5.4, this removes the binding of each contact to the AOR
and the binding of each contact to its GRUUs.
The registrar SHOULD NOT include the "gruu" option tag in the Require
or Supported header field of the response.
5.3. Timing Out a Registration
When a registered contact expires (either due to timeout or explicit
de-registration), its binding to the AOR is removed as usual. In
addition, its binding to its GRUUs are removed at the same time as a
consequence of the relationships described in Section 5.4
If, as a consequence of the expiration of the contact, a particular
GRUU no longer has any registered contacts bound to it, and the GRUU
is a temporary GRUU, the GRUU MUST be invalidated. This means that
all of the accumulated temporary GRUUs get invalidated once the last
contact for a given instance ID expires.
If, however, the GRUU was a public GRUU, the registrar SHOULD
continue to treat the GRUU as valid. Consequently, subsequent
requests targeted to the GRUU, prior to re-registration of a contact
to the GRUU, SHOULD return a 480. In addition, since the GRUU
remains valid, the rules in Section 5.1 will cause it to be retained
when a contact with that instance ID is once again registered to the
AOR.
These rules give a public GRUU a semi-permanent property. The
intent is that the registrar make every attempt to retain validity
of the GRUU for as long as the AOR itself is known within the
domain. The requirements for doing so are at SHOULD strength and
not MUST strength because of the difficulty in meeting a MUST
strength requirement; registrar failures could cause the set of
valid GRUUs to be lost and this specification requires the UA to
be robust against such cases. That said, it is possible for a
public GRUU to be constructed such that a registrar does not need
to retain any additional state for it, yet still meet the
requirements described here.
5.4. Creation of a GRUU
This section defines additional behaviors associated with the
construction and maintenance of a GRUU which are specific to a
registrar. These rules do not apply to self-made GRUU or GRUU not
obtained through registrations.
When a registrar creates a GRUU, it is required to maintain certain
information associated with the GRUU, regardless of whether it is a
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public or temporary GRUU. Every GRUU is associated with a single AOR
and a single instance ID. A registrar MUST be able to determine the
instance ID and AOR when presented with a GRUU. In addition, the
GRUU, like an AOR, resolves to zero or more contacts. While the AOR
resolves to all registered contacts for an AOR, a GRUU resolves only
to those contacts whose instance ID matches the one associated with
the GRUU. For this reason, a contact with an instance ID is always
bound to both a GRUU and its AOR, never just an AOR or just a GRUU.
This is shown pictorially in Figure 5. The figure shows three
contacts registered to a single AOR. One of the contacts has an
instance ID of 1, and the other two have an instance ID of 2. There
are two GRUUs for this AOR. One is associated with instance ID 1,
and the other with instance ID 2. The first GRUU resolves only to
contacts whose instance ID is one, and the second resolves only to
contacts whose instance ID is two. If the contact for instance ID 1
should expire, the AOR would resolve to two contacts, but the GRUU
associated with instance ID 1 would resolve to zero.
+----------+ +----------+ +----------+
| GRUU | | | | GRUU |
| | | AOR | | |
|Instance:1| | | |Instance:2|
+----------+ +----------+ +----------+
| / | \ / |
| / | \ / |
| / | \ / |
| / | \ / |
| / | \ / |
| / | \ / |
| / | X |
| / | / \ |
| / | / \ |
| / | / \ |
V V V V V V
+----------+ +----------+ +----------+
| Contact | | Contact | | Contact |
| | | | | |
|Instance:1| |Instance:2| |Instance:2|
+----------+ +----------+ +----------+
Figure 5
There can be multiple GRUUs with the same instance ID and AOR.
Indeed, this specification requires registrars to maintain many - one
that is public, and several that are temporary. However, if two
GRUUs are associated with different AOR or different instance IDs or
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both, the GRUUs MUST be different based on URI equality comparison.
A GRUU in a domain MUST NOT be equivalent, based on URI comparison,
to any AOR in a domain except for the one associated with the GRUU.
A public GRUU will always be equivalent to the AOR based on URI
equality rules. The reason is that the rules in RFC 3261 cause
URI parameters that are in one URI, but not in the other, to be
ignored for equality purposes. Since a public GRUU differs from
an AOR only by the presence of the "gr" URI parameter, the two URI
are equivalent based on those rules.
Once a temporary GRUU is constructed, it MUST be considered valid by
the registrar until invalidated based on the rules described
previously. Once a temporary GRUU is constructed, it MUST be
considered valid for the duration that the AOR itself is valid. Once
an AOR is no longer valid within a domain, any of its GRUU MUST be
considered invalid as well.
This specification does not mandate a particular mechanism for
construction of the GRUU. Several example approaches are given in
Appendix A. However, in addition to the properties described in
Section 3.1, a GRUU constructed by a registrar 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 proxy that can
access the registration data generated by the registrar. Such a
proxy is called a authoritative proxy [12].
5.5. Registration Event Support
RFC 3680 [23] defines an event package that allows a client to learn
about registration events at the registrar. This package allows
registrars to alter registrations forcefully (for example, shortening
them to force a re-registration). If a registrar is supporting RFC
3680 and GRUU, it MUST also support [24].
6. Proxy Behavior
Proxy behavior is fully defined in Section 16 of RFC 3261 [1]. GRUU
processing impacts that processing in two places -- request targeting
at the authoritative proxy and record routing.
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6.1. Request Targeting
When a proxy 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 it contains the "gr" URI
parameter but is not equivalent, based on URI comparison, to a
currently valid GRUU within the domain, it SHOULD be rejected with a
404; this is the same behavior a proxy would exhibit for any other
URI within the domain that is not valid.
If the Request-URI contains a the "gr" URI parameter and is
equivalent, based on URI comparison, to a GRUU which is currently
valid within the domain, processing proceeds as it would for any
other URI present in the location service, as defined in Section 16.5
of RFC 3261, except that the "gr" parameter is not removed as part of
the canonicalization process. This is the case for both out-of-
dialog requests targeted to the GRUU, and mid-dialog requests
targeted to the GRUU (in which case the incoming request would have a
Route header field value containing the URI that the proxy Record-
Routed with).
If there are no registered contacts bound to the GRUU, the server
MUST return a 480. If there are more than one, the rules described
in Section 7 of [12] for selecting a single registered contact apply.
Any caller preferences in the request [16] SHOULD be processed
against the contacts bound to the GRUU.
In essence, to select a registered contact, the GRUU is processed
just like it was the AOR, but with only a subset of the contacts
bound to the AOR.
Special considerations apply to processing of any Path headers stored
in the registration [3]. If the received request has Route header
field values beyond the one pointing to the authoritative proxy
itself (this will happen when the request is a mid-dialog request),
the Path URI MUST be discarded. This is permitted by RFC 3327 as a
matter of local policy; usage of GRUUs will require this policy in
order to avoid call spirals and likely call failures.
A proxy MAY apply other processing to the request, such as execution
of called party features, as it might do for requests targeted to an
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
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Join header field [20]) or replace it (using the Replaces header
field [21]). In such cases, the UA will typically make its own
authorization decisions. 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.
If the request is a mid-dialog request, a proxy SHOULD only apply
services that are meaningful for mid-dialog requests, generally
speaking. This excludes screening functions, as well as forwarding
ones.
In addition, 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.
6.2. Record-Routing
There are two distinct requirements for record-routing - in the
originating domain and in the terminating domain. These requirements
avoid unnecessary and possibly problematic spirals of requests.
If an originating authoritative proxy receives a dialog-forming
request, and the Contact header field contains a GRUU in the domain
of the proxy, and that GRUU is associated with the AOR matching the
authenticated identity of the requestor (assuming such authentication
has been performed), and the request contains Record-Route header
fields, the authoritative proxy MUST record route. If the request
contained a GRUU in the domain of the proxy, but this GRUU had an AOR
which did not match the authenticated identity of the requestor, it
is RECOMMENDED that the proxy reject the request with a 403.
If a terminating authoritative proxy receives a dialog-forming
request, and the Request-URI contains a URI in the location service
(either a GRUU or an AOR), and the contact selected for sending the
request has an instance ID and is bound to a GRUU, and the
registration contain Path URI, the authoritative proxy MUST record
route.
If a proxy in either the originating or terminating domains but is
not an authoritative proxy, the proxy MAY record route.
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Implementors should note that, if a UA uses a GRUU in its contact,
and a proxy inserted itself into the Path header field of a
registration, that proxy will be receiving mid-dialog requests
regardless of whether it record routes or not. The only
distinction is what URI the proxy will see in the topmost Route
header field of mid-dialog requests. If the proxy record-routes,
it will see that URI. If it does not, it will see the Path URI it
inserted.
7. Grammar
This specification defines two new Contact header field parameters
("temp-gruu" and "pub-gruu") by extending the grammar for "contact-
params" as defined in RFC 3261. It also defines a new SIP URI
parameter ("gr") by extending the grammar for "uri-parameter" as
defined in RFC 3261.
contact-params =/ temp-gruu / pub-gruu
temp-gruu = "temp-gruu" EQUAL LDQUOT *(qdtext / quoted-pair )
RDQUOT
pub-gruu = "pub-gruu" EQUAL LDQUOT *(qdtext / quoted-pair )
RDQUOT
uri-parameter =/ gr-param
gr-param = "gr" ["=" pvalue] ; defined in RFC3261
The quoted strings for temp-gruu and pub-gruu MUST contain a SIP URI.
However, they are encoded like all other quoted strings and can
therefore contain quoted-pair escapes when represented this way.
8. 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.
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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. NOTE: This
requirement is not met by this specification, and is being
addressed in a separate specification.
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. NOTE: This
requirement is not met by this specification, and is being
addressed in a separate specification.
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. NOTE: This requirement is
not met by this specification, and is being addressed in a
separate specification.
REQ 7: It must be possible for a proxy to execute services and
features on behalf of a UA instance represented by a GRUU. As an
example, if a user has call blocking features, a proxy may want to
apply those call blocking features to calls made to the GRUU, in
addition to calls made to the user's AOR.
REQ 8: It must be possible for a UA in a dialog to inform its peer
of its GRUU, and for the peer to know that the URI represents a
GRUU. This is needed for the conferencing and dialog reuse
applications of GRUUs, where the URIs are transferred within a
dialog.
REQ 9: When transferring a GRUU per 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.
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9. Example Call Flow
The following call flow, shown in Figure 7, 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 [15] are used to describe
representation of long message lines.
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 |
| |-------------------->|
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Figure 7
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 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 registrar assigns a temporary and a public GRUU. 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>
;pub-gruu="sip:callee@example.com
;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;temp-gruu="sip:pig8a788@example.com;gr"
;+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 pub-gruu parameter with the public GRUU sip:callee@
example.com;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6, and the
temp-gruu parameter with the temporary GRUU
sip:pig8a788@example.com;gr. Both are valid GRUUs for the AOR and
instance ID, and both translate 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 chosen to use its public
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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, SUBSCRIBE
<allOneLine>
Contact:
<sip:callee@example.com
;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>
</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 [14] 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;gr=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6
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;gr=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6>
</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;gr=hdg7777ad7aflzig8sf7>
Content-Length: 0
In this example, the caller itself supports the GRUU extension, and
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is using its own GRUU to populate its remote target.
This request is routed to the proxy, which proceeds to perform a
location lookup on the Request-URI. It is translated into the
contact for that instance, and then proxied to that contact.
SUBSCRIBE sip:callee@192.0.2.1 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;gr=urn:uuid:f8
1d4fae-7dec-11d0-a765-00a0c91e6bf6>
</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;gr=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
<allOneLine>
Contact: <sip:callee@192.0.2.2>
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
</allOneLine>
Content-Length: 0
The registrar notices that a different contact, sip:callee@192.0.2.1,
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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 public GRUUs, but the registrar has generated a second temporary
GRUU for this AOR and instance ID combination. Both contacts are
included in the REGISTER response, and the temporary GRUU for each is
the same - the most recently created one for the instance ID and AOR.
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>
;pub-gruu="sip:callee@example.com;gr=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;temp-gruu="sip:asd98fggg7example.com;gr"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
<allOneLine>
Contact: <sip:callee@192.0.2.1>
;pub-gruu="sip:callee@example.com;gr=urn:
uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
;temp-gruu="sip:asd98fggg7example.com;gr"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=400
</allOneLine>
Content-Length: 0
There is no need for the UA to remove the stale registered contact;
the request targeting rules in [12] will cause the request to be
delivered to the most recent one.
10. Security Considerations
Attacks in SIP networks using GRUUs can be divided into inside
attacks (where the attacker is a valid participant in the system but
is malicious), and outside attacks, where a third party is trying to
attack the system. In addition, there are privacy considerations
with using GRUUs.
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10.1. Outside Attacks
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
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.
10.2. Inside Attacks
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 "gr" parameter
from the URI before using it. Consequently, consideration must be
given to the effect of such removal.
If a malicious correspondent removes the "gr" URI parameter, the
request will be routed to the authoritative proxy. If the GRUU had
been temporary, removal of the "gr" parameter produces a URI that is
not recognized as a GRUU and not equal to any AOR. The request will
be rejected. If the GRUU had been public, the resulting of removing
the "gr" parameter produces the AOR. Therefore, the request is
treated like a call to the AOR. Since it is a desired goal to allow
users to extract the AOR from the GRUU, this is not an attack and the
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call will be handled normally.
A malicious user in the system might try to use a GRUU for launching
a DoS attack against another SIP UA. To do that, it would wait for a
call from that UA, from it, observe their GRUU. Once obtained, the
UA would launch a SIP request to an entity, such as a presence
server, which will generate many requests back towards the UA.
However, the attacker will use the target's GRUU in the Contact
header field of that SUBSCRIBE request. This will cause the traffic
to be directed towards the target instead. Since the GRUU is
globally routable, such traffic is more likely to be delivered to the
target than traffic sent to its IP address. This specification helps
mitigate this attack by requiring proxies to validate that the GRUU
in the Contact of a request matches the authenticated identity of the
sender of the request. This check requires the use of an outbound
proxy.
10.3. Privacy Considerations
RFC 3323 defines mechanisms for privacy. It distinguishes between
user-provided privacy and network-provided privacy. In the latter,
the user requests privacy services from the network by including a
Privacy header field in the request. In the former, the UA follows a
basic set of guidelines for construction of its request so let a
certain level of privacy is afforded.
The guidelines in Section 4.1 of RFC 3323 for user-provided privacy
request that a UA construct its Contact header field with a URI that
omits a user part, and utilizes the IP address or hostname of the UA.
Such recommendations are in conflict with the rules defined in this
specification, which require the usage of a GRUU in the Contact
header field.
However, the temporary GRUUs provided by the registrar can be used in
place of the Contact URI format described in RFC 3323. A user agent
would gather the temporary GRUU returned in each REGISTER responses,
and keep a small number of them cached. When it makes or receives a
call, a temporary GRUU is used to populate the Contact header field.
A UA can either elect to use the same temporary GRUU in each call, or
it can use a different temporary GRUU in each call. The choice
depends on the level of privacy desired:
o A UA utilizing the same temporary URI for each call will allow a
correspondent, based solely on investigation of the Contact header
field, to correlate calls as coming from the same UA. Note that
this is also true for the user provided privacy procedures in RFC
3323, since the IP address or hostname in the Contact URI provides
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a similar correlator.
o A UA utilizing a different temporary URI for each call will not
allow a correspondent, based solely on investigation of the
Contact header field, to correlate calls as coming from the same
UA.
o In both cases, absent network-provided privacy, IP address and
port information in the Session Description Protocol (SDP) [10]
will allow a correspondent to correlate calls as coming from the
same UA.
o In both cases, if a user makes a call, the correspondent will be
able to call back by directing requests towards the GRUU in the
Contact header field. Similarly, features such as transfer and
digit collection by network application servers [19], which depend
on a Contact with the GRUU property, will also be possible. These
kinds of inbound requests will be possible until the registration
for that UA lapses. A UA that wishes to invalidate its previous
temporary GRUU in order to limit reachability MAY do so by
generating a REGISTER refresh with a Call-ID that differs from
ones used previously. A UA SHOULD NOT forcefully expire its
registration and then re-register in order to invalidate temporary
a GRUU; this results in a brief period of unreachability and will
often produce excess load on the network. Refreshing with a new
Call-ID is more efficient and is meant as the technique for
coarse-grained control over the validity of temporary GRUU. A UA
wishing to not be disturbed by a specific call back will need to
implement manual or automated call handling procedures to reject
it. This specification does not provide the UA the ability to
manually invalidate individual temporary GRUU. If a UA insists on
not receiving any such inbound requests (including ones generated
by network applications, such as those used for collecting
digits), the UA can place a non-GRUU into the Contact header
field. However, this is NOT RECOMMENDED. Usage of a GRUU coupled
with automated call rejection features is far superior.
o As long as a temporary GRUU is used to populate the Contact header
field, a correspondent will not be able to ascertain any
information about the AOR or instance ID of the UA by inspection
of the Contact header field. However, absent a network-provided
privacy service, the IP address in the SDP can be used to
determine information about the UA, such as its geographic
location and ISP.
o In all cases, regardless of whether the UA uses a temporary or
public GRUU in the Contact, regardless of whether it utilizes GRUU
at all, and regardless of whether it invokes a network-provided
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privacy service, a correspondent will be able to determine the SIP
service provider of the UA.
11. IANA Considerations
This specification defines two new Contact header field parameters,
one SIP URI parameter, and a SIP option tag.
11.1. Header Field Parameter
This specification defines two new header field parameters, as per
the registry created by [8]. The required information is as follows:
Header field in which the parameter can appear: Contact
Name of the Parameter: pub-gruu
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
Header field in which the parameter can appear: Contact
Name of the Parameter: temp-gruu
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
11.2. URI Parameter
This specification defines one new SIP URI parameter, as per the
registry created by [9].
Name of the Parameter: gr
Predefined Values: none
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
11.3. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
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Name: gruu
Description: This option tag is used to identify the Globally
Routable User Agent URI (GRUU) extension. When used in a
Supported header, it indicates that a User Agent understands the
extension. 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.
12. Acknowledgements
The author would like to thank Eric Rescorla, Robert Sparks, Rohan
Mahy, Paul Kyzivat, Alan Johnston, Ya-Ching Tan, Dale Worley, Jeroen
van Bemmel, Vijay Gurbani, Andrew Allen, Alan Hawrylyshen, Francois
Audet, Fredrik Thulin, Dean Willis, David Hancock, Keith Drage, and
Cullen Jennings for their comments and contributions to this work.
Eric Rescorla provided the text for the introduction and the GRUU
construction algorithm in the appendix.
13. References
13.1. 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] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[6] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[7] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
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[8] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[9] 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.
[10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[11] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[12] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-07 (work in progress), January 2007.
13.2. Informative References
[13] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[14] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[15] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and
H. Schulzrinne, "Session Initiation Protocol (SIP) Torture Test
Messages", RFC 4475, May 2006.
[16] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[17] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-
for-IPv4) Option for Session Initiation Protocol (SIP)
Servers", RFC 3361, August 2002.
[18] Sparks, R., "Session Initiation Protocol Call Control -
Transfer", draft-ietf-sipping-cc-transfer-07 (work in
progress), October 2006.
[19] Burger, E. and M. Dolly, "A Session Initiation Protocol (SIP)
Event Package for Key Press Stimulus (KPML)", RFC 4730,
November 2006.
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[20] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[21] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
[22] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003.
[23] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[24] Kyzivat, P., "Registration Event Package Extension for Session
Initiation Protocol (SIP) Globally Routable User Agent URIs
(GRUUs)", draft-ietf-sipping-gruu-reg-event-08 (work in
progress), October 2006.
[25] Rescorla, E., "How to Implement Secure (Mostly) Stateless
Tokens", draft-rescorla-stateless-tokens-01 (work in progress),
March 2007.
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. Public GRUU Algorithm
The most basic approach for constructing a public GRUU is to take the
AOR, and place the actual value of the instance ID into the contents
of the "gr" URI parameter.
A.2. Temporary GRUU
This specification requires a registrar to create a new temporary
GRUU on each registration refresh. If a registration is very long
lived, this can quickly result in hundreds or even thousands of
temporary GRUUs being created and allocated to a UA. Consequently,
it is important to have an algorithm for constructing temporary GRUUs
which does not require additional storage that grows in size with the
number of temporary GRUUs. The following algorithm meets this goal.
When the first contact with a specific instance ID is registered, the
registrar notes the wallclock time (W) and stores this as part of the
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registration. When the registrar wishes to create a new temporary
GRUU, it constructs a state token using the procedures defined in
[25]. The input to this procedure is a string S representing the
state to be stashed in the token. The registrar constructs S as:
S = (AOR || " " || instance_ID || Call-ID || W || T1 || T2 || .. || Tn)
Where || indicates concatenation, and T1, T2, and so on represent the
current expiration times (measured in absolute time) of each contact
with that instance ID. Call-ID represents the Call-ID of the most
recently registered contact for that AOR and instance ID. Usage of
the current expiration times allows a new value for S to be produced
whenever a contact for that particular instance ID is refreshed,
since a refresh always changes the expiration time.
The stateless tokens in [25] include an expiration time and sequence
number. Expiration of temporary GRUU is determined through data
within the state string S, and consequently, these fields can be set
to zero.
The GRUU is then constructed as:
user-part = "GRUU" || BASE64(T)
Where T is the token generated in [25] using string S.
This mechanism uses the user-part of the SIP URI to convey the
encrypted AOR and instance ID. The URI contains the "gr" parameter
without a value, and the domain part is the domain of the provider.
When the authoritative proxy receives a request addressed to the
GRUU, it:
o Decrypts it and verifies it as described in [25]
o Extracts the string S stashed in the token
o Checks the value of W against the current value stored for that
instance ID and AOR:
* If they match, the proxy extracts the value of Call-ID from S,
and checks it against the Call-ID value of the most recently
registered contact for that AOR and instance ID:
+ If they match, the GRUU is valid.
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+ If they don't match, the UA has invalidated this GRUU by
changing the Call-ID, and the GRUU is invalid.
* If they don't match, the GRUU was allocated in a previous
registration cycle and is no longer valid.
The benefit of this mechanism is that a registrar 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. Message integrity is needed to prevent attacks
whereby the proxy is sent requests with fake GRUUs, causing it to
direct requests to any named URI.
While this approach has many benefits, it has the drawback of
producing very long GRUUs due to the non-trivial amount of
information that is encrypted.
Appendix B. Network Design Considerations
The GRUU specification works properly based on logic implemented at
the user agents and in the authoritative proxies on both sides of a
call. Consequently, it is possible to construct network deployments
in which GRUUs will not work properly.
One important assumption made by the GRUU mechanism is that, if a
request passes through any proxies in the originating domain prior to
visiting the terminating domain, one of those proxies will be the
authoritative proxy for the UAC. Administrators of SIP networks will
need to make sure that this property is retained. There are several
ways it can be accomplished:
1. If the user agents support the service route mechanism [22], the
registrar can implement it and return a service route that points
to the authoritative proxy. This will cause requests originated
by the user agent to pass through the authoritative proxy.
2. The user agents can be configured to never use an outbound proxy,
and send requests directly to the domain of the terminating
party. This configuration is not practical in many use cases but
it is a solution to this requirement.
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3. The user agents can be configured with an outbound proxy in the
same domain as the authoritative proxy, and this outbound proxy
forwards requests to the authoritative proxy by default. This
works very well in cases where the clients are not roaming; in
such cases the outbound proxy in a visited network may be
discovered dynamically through DHCP [17].
4. In cases where the client discovers a local outbound proxy via a
mechanism such as DHCP, and is not implementing service route,
the UA can be configured to automatically add an additional Route
header field after the outbound proxy, which points to a proxy in
the home network. This has the same net effect of service route,
but is accomplished through static configuration.
Author's Address
Jonathan Rosenberg
Cisco
Edison, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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