Network Working Group J. Rosenberg
Request for Comments: 5627 Cisco Systems
Category: Standards Track October 2009
Obtaining and Using Globally Routable User Agent URIs (GRUUs)
in the Session Initiation Protocol (SIP)
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.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Rosenberg Standards Track [Page 1]RFC 5627 GRUU Mechanism October 2009
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RFC 5627 GRUU Mechanism October 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
3.1. Structure of GRUUs . . . . . . . . . . . . . . . . . . . . 5
3.1.1. GRUUs That Expose the Underlying AOR . . . . . . . . . 6
3.1.2. GRUUs That 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 . . . . . . . . . . . . . . . . . . . . . 9
4.1. Generating a REGISTER Request . . . . . . . . . . . . . . 9
4.2. Learning GRUUs from REGISTER Responses . . . . . . . . . . 10
4.3. Constructing a Self-Made GRUU . . . . . . . . . . . . . . 11
4.4. Using One's Own GRUUs . . . . . . . . . . . . . . . . . . 12
4.4.1. Considerations for Multiple AORs . . . . . . . . . . . 13
4.5. Dereferencing a GRUU . . . . . . . . . . . . . . . . . . . 14
4.6. Rendering GRUUs on a User Interface . . . . . . . . . . . 14
5. Registrar Behavior . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Processing a REGISTER Request . . . . . . . . . . . . . . 14
5.2. Generating a REGISTER Response . . . . . . . . . . . . . . 16
5.3. Timing Out a Registration . . . . . . . . . . . . . . . . 16
5.4. Creation of a GRUU . . . . . . . . . . . . . . . . . . . . 17
5.5. Registration Event Support . . . . . . . . . . . . . . . . 19
6. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Request Targeting . . . . . . . . . . . . . . . . . . . . 19
6.2. Record-Routing . . . . . . . . . . . . . . . . . . . . . . 21
7. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 23
9. Example Call Flow . . . . . . . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
10.1. Outside Attacks . . . . . . . . . . . . . . . . . . . . . 29
10.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . . . 30
10.3. Privacy Considerations . . . . . . . . . . . . . . . . . . 31
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11.1. Header Field Parameter . . . . . . . . . . . . . . . . . . 33
11.2. URI Parameter . . . . . . . . . . . . . . . . . . . . . . 33
11.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 33
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
13.1. Normative References . . . . . . . . . . . . . . . . . . . 34
13.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Example GRUU Construction Algorithms . . . . . . . . 37
A.1. Public GRUU . . . . . . . . . . . . . . . . . . . . . . . 37
A.2. Temporary GRUU . . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Network Design Considerations . . . . . . . . . . . . 39
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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.) that are all referenced by the
same AOR. There are a number of contexts in which it is desirable to
have an identifier that addresses a single user agent rather than the
group of user agents indicated by an AOR.
As an example, consider a blind transfer application (see RFC 5589
[19]). 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 voicemail 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 to the user agent B is currently using, and it can be
dereferenced by C's user agent. 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. 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 is updated by target refresh requests or responses.
Contact header field: The term "Contact header field", with a
capitalized C, refers to the header field that can appear in
REGISTER requests and responses, redirects, or 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. In turn,
the domain maintains the binding between the GRUU and the particular
UA instance. When a GRUU is dereferenced while 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 dereferenced 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.
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In principle, a GRUU can be constructed in any way the domain
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 that expose the underlying AOR
2. GRUUs that hide the underlying AOR
3.1.1. GRUUs That 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 to 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" URI 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" URI 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 That 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
can have any content, provided that it meets the requirements in
Sections 3.1 and 5.4, and the AOR cannot be readily determined from
the GRUU. The GRUU will have the "gr" URI 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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An example of a temporary GRUU constructed using a stateful algorithm
would be:
sip:asd887f9dfkk76690@example.com;gr
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 UAs that know they are globally reachable using their IP
address or hostname.
o Via some locally specified administrative mechanism.
A UA that wants to obtain a GRUU via its REGISTER request does so by
providing an instance ID in the "+sip.instance" Contact header field
parameter, defined in RFC 5626 [14]. For example:
Contact: <sip:callee@192.0.2.2>
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
The registrar detects this header field 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" Contact header field parameters in the
response, respectively. For example:
<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:tgruu.7hs==
jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
Note that the <allOneLine> tag is used as defined in [17].
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
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learned from previous REGISTER responses during the lifetime of a
contact remain valid as long as (1) a contact with that instance ID
remains registered, and (2) the UA doesn't change the Call-ID in its
REGISTER request compared to previous ones for the same reg-id [14].
When the last contact for the instance expires, either through
explicit de-registration or timeout, all of the temporary GRUUs
become invalidated. Similarly, if a register refresh for a contact
(or, if RFC 5626 is being used, for a reg-id) changes the Call-ID
compared to previous register refreshes, all of the previous
temporary GRUUs are invalidated. When the user agent later creates a
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. First, 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 [1], 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.
Second, 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.
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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 (see RFC
5626 [14]) as a UA characteristic (see [7]), whose value MUST be the
instance ID that 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 [1], 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 [1], 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 temporary GRUUs
associated with that UA instance; the only valid one will be the new
one returned in that REGISTER response. When RFC 5626 is in use,
this rule applies to the reg-ids: If the Call-ID changes for the
registration refresh for a particular reg-id, the server will
invalidate all temporary GRUUs associated with that UA instance as a
whole. 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 GRUUs 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 RFC 5626 [14].
A UA utilizing GRUUs can still perform third-party registrations
and can include contacts that omit the "+sip.instance" Contact
header field parameter.
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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 can also
contain a "pub-gruu" and "temp-gruu" Contact header field parameter.
These header field 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 (that is, through refreshes), 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, 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 or reg-id, the UA MUST discard all
temporary GRUUs 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 at least one contact or reg-id
remains continuously 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 such that the REGISTER refresh transaction will either
complete or timeout prior to the expiration of the registration. For
default transaction timers, this would be at least 32 seconds prior
to expiration, assuming the registration expiration is larger than 64
seconds. If the registration expiration is less than 64 seconds, the
UA SHOULD refresh its registration halfway prior to expiration.
Note that, when [14] is in use, and the UA is utilizing multiple
flows for purposes of redundancy, the temporary GRUUs remain valid as
long as at least one flow is registered. Thus, even if the
registration of one flow expires, the temporary GRUUs learned
previously remain valid.
In cases where registrars forcefully shorten registration intervals,
the registration event package, RFC 3680 [24], is used by user agents
to learn of these changes. A user agent implementing both RFC 3680
[24] and GRUU MUST also implement the extensions to RFC 3680 [24] for
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RFC 5627 GRUU Mechanism October 2009
conveying information on GRUU, as defined in RFC 5628 [28], as these
are necessary to keep the set of temporary GRUUs synchronized between
the UA and the registrar. More generally, the utility of temporary
GRUUs depends on the UA and registrar being in sync on the set of
valid temporary GRUUs at any time. Without support of RFC 3680 [24]
and its extension for GRUU, the client will remain in sync only as
long as it always re-registers well before the registration
expiration. Besides forceful de-registrations, other events (such as
network outages, connection failures, and short refresh intervals)
can lead to potential inconsistencies in the set of valid temporary
GRUUs. For this reason, it is RECOMMENDED that a UA that utilizes
temporary GRUUs implement RFC 3680 [24] and RFC 5628 [28].
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 user and host parts of the GRUU learned by the UA in the REGISTER
response MUST be treated opaquely by the UA. That is, the UA MUST
NOT modify them in any way. A UA MUST NOT modify or remove URI
parameters it does not recognize. Furthermore, the UA MUST NOT add,
remove, or modify URI parameters relevant for receipt and processing
of request at the proxy, including the transport, lr, maddr, ttl,
user, and comp (see RFC 3486 [25]) URI parameters. The other URI
parameter defined in RFC 3261 [1], method, would not typically be
present in a GRUU delivered from a registrar, and a UA MAY add a
method URI parameter to the GRUU before handing it out to another
entity. Similarly, the URI parameters defined in RFC 4240 [26] and
RFC 4458 [27] are meant for consumption by the UA. These would not
be included in the GRUU returned by a registrar and MAY be added by a
UA wishing to provide services associated with those URI parameters.
Note, however, that should another UA dereference the GRUU, the
parameters will be lost at the proxy when the Request-URI is
translated into the registered contact, unless some other means is
provided for the attributes to be delivered to the UA. Mechanisms
for such delivery are currently the subject of future standardization
activity (see "Delivery of Request-URI Targets to User Agents" [29]).
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 can be publicly reachable.
This would mean that the policy of the domain is that requests can
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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 that 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
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 is not publicly reachable, it
would need to obtain a GRUU through some other means. Typically, the
UA would be configured with a GRUU, the GRUU would be configured into
the proxy, and the proxy will be configured with a mapping from the
GRUU to the IP address (or hostname) and port of the UA.
4.4. Using One's 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:
o the INVITE request
o a 2xx or 18x response to an INVITE which contains a To tag
o the SUBSCRIBE request (see [5])
o a 2xx response to a SUBSCRIBE which contains a To tag
o the NOTIFY request
o the REFER request (see [6])
o a 2xx response to NOTIFY
o the UPDATE request
o a 2xx response to NOTIFY
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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 that has lapsed (in other words, one
obtained when registering a contact that 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
(and, if RFC 5626 [14] is in use, reg-id). When a UA wishes to
construct an anonymous request as described in RFC 3323 [15], 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 [1], 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 can have a multiplicity of AORs,
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 AORs. This AOR will typically show up in the From header field
of 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 (as
defined in [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.
Rosenberg Standards Track [Page 13]
RFC 5627 GRUU Mechanism October 2009
4.5. Dereferencing a GRUU
A GRUU is identified by the presence of the "gr" URI parameter, and
this URI parameter might or might 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 URI-handling mechanisms that
compensate for the fact that heretofore many contact URIs have not
been globally routable. Since any URI containing the "gr" URI
parameter is known to be globally routable, a UA SHOULD NOT apply
such mechanisms when a contact URI contains the "gr" URI 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 (defined in [12]) 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. Their 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 can be cases where the desired operation truly
is a preferential routing request.
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" URI 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 might
provide improved mechanisms that would allow an automaton to know
that a URI is anonymized, and therefore inappropriate to render.
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.
Rosenberg Standards Track [Page 14]RFC 5627 GRUU Mechanism October 2009
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
[1], the registrar checks whether each Contact header field in the
REGISTER message contains a "+sip.instance" header field parameter.
If present with a non-zero expiration, 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 [1]
rules.
Note that handling of a REGISTER request containing a Contact
header field with value "*" and an expiration of zero still
retains the meaning defined in RFC 3261 [1] -- all contacts, not
just those with a specific instance ID, are deleted. As described
in Section 5.4, this removes the binding of each contact to the
AOR and the binding of each contact to its GRUUs.
If the contact URI is equivalent (based on URI equivalence in RFC
3261 [1]) 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 header field 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 same AOR, instance ID, and if the contact in the REGISTER request
is registering a flow [14], reg-id. If there is at least one, the
registrar 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 GRUUs for the AOR
and instance ID. A consequence of this invalidation is that requests
addressed to those GRUUs will be rejected by the domain with a 404
from this point forward.
Rosenberg Standards Track [Page 15]RFC 5627 GRUU Mechanism October 2009
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
that 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 for different AORs and instance IDs.
If the contact contained a "temp-gruu" Contact header field
parameter, the header field 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 [1] 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 a "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 most 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" Contact header field parameter is the public GRUU.
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
Rosenberg Standards Track [Page 16]RFC 5627 GRUU Mechanism October 2009
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 (Temporarily Unavailable) response.
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 the GRUU still meets
the requirements described here.
5.4. Creation of a GRUU
This section defines additional behaviors associated with the
construction and maintenance of a GRUU that are specific to a
registrar. These rules do not apply to self-made GRUUs or GRUUs 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
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 1. 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
Rosenberg Standards Track [Page 17]RFC 5627 GRUU Mechanism October 2009
contacts whose instance ID is 1, and the second resolves only to
contacts whose instance ID is 2. There will typically be multiple
contacts for a given instance ID if a UA has crashed, rebooted, and
re-registered with the same instance ID, or is using the mechanisms
of RFC 5626 [14] to have multiple registrations for redundancy. If
the contact for instance ID 1 expires, 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 1
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 AORs or different instance IDs or
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 [1]
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 URIs are equivalent based on those rules.
Rosenberg Standards Track [Page 18]RFC 5627 GRUU Mechanism October 2009
Once a temporary GRUU is constructed, it MUST be considered valid by
the registrar until invalidated based on the rules described
previously. Once a public 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, all of its GRUUs MUST be considered
invalid as well.
This specification does not mandate a particular mechanism for
construction of the GRUU. Example algorithms for public and
temporary GRUUs that work well 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 hostname, 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 an authoritative proxy, defined in RFC 5626 [14].
5.5. Registration Event Support
RFC 3680 [24] 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 [24] and GRUU, it MUST also support RFC 5628 [28].
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.
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 (Not Found) response; this is the same behavior a proxy would
exhibit for any other URI within the domain that is not valid.
Rosenberg Standards Track [Page 19]RFC 5627 GRUU Mechanism October 2009
If the Request-URI contains 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 [1],
except that the "gr" URI 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 used for record-
routing.).
Note that the "gr" URI parameter is retained just for the purposes of
finding the GRUU in the location service; if a match is found, the
Request-URI will be rewritten with the registered contacts, replacing
the GRUU and its "gr" URI parameter. The "gr" URI parameter is not
carried forward into the rewritten Request-URI.
If there are no registered contacts bound to the GRUU, the server
MUST return a 480 (Temporarily Unavailable) response. If there are
more than one, there are two cases:
1. The client is using RFC 5626 [14] and registering multiple
contacts for redundancy. In that case, these contacts contain
"reg-id" Contact header field parameters, and the rules described
in Section 7 of RFC 5626 [14] for selecting a single registered
contact apply.
2. The client was not using RFC 5626 [14], in which case there would
only be multiple contacts with the same instance ID if the client
had rebooted, restarted, and re-registered. In this case, these
contacts would not contain the "reg-id" Contact header field
parameter. The proxy MUST select the most recently refreshed
contact. As with RFC 5626, if a request to this target fails
with a 408 (Request Timeout) or 430 (Flow Failed) response, the
proxy SHOULD retry with the next most recently refreshed contact.
Furthermore, if the request fails with any other response, the
proxy MUST NOT retry on any other contacts for this instance.
Any caller preferences in the request (as defined in RFC 3841 [12])
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 the processing of any Path headers
stored in the registration (see RFC 3327 [3]). If the received
request has Route header field values beyond the one pointing to the
Rosenberg Standards Track [Page 20]RFC 5627 GRUU Mechanism October 2009
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 [3] 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 blacklist
and whitelist 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 might be an attempt to join it (using the
Join header field defined in RFC 3911 [21]) or replace it (using the
Replaces header field defined in RFC 3891 [22]). When the new
request is related to an existing dialog, the UA will typically make
its own authorization decisions; bypassing screening services at the
authoritative proxy might make sense, but needs to be carefully
considered by network designers, as the ability to do so 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 and forwarding functions.
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 might 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:
o an originating authoritative proxy receives a dialog-forming
request,
Rosenberg Standards Track [Page 21]RFC 5627 GRUU Mechanism October 2009
o AND the Contact header field contains a GRUU in the domain of the
proxy,
o AND that GRUU is a valid one in the domain of the proxy,
o AND that GRUU is associated with the AOR matching the
authenticated identity of the requestor (assuming such
authentication has been performed),
o AND the request contains Record-Route header fields,
then the authoritative proxy MUST record-route. If all of these
conditions are true, except that the GRUU is associated with an AOR
that did not match the authenticated identity of the requestor, it is
RECOMMENDED that the proxy reject the request with a 403 (Forbidden)
response.
If:
o a terminating authoritative proxy receives a dialog-forming
request,
o AND the Request-URI contains a URI in the location service (either
a GRUU or an AOR),
o AND the contact selected for sending the request has an instance
ID and is bound to a GRUU,
o AND the registration contain Path URI,
then the authoritative proxy MUST record-route.
If a proxy is in either the originating or terminating domains but is
not an authoritative proxy, the proxy MAY record-route.
If a proxy in the terminating domain requires mid-dialog requests to
pass through it for whatever reason (firewall traversal, accounting,
etc.), the proxy MUST still record-route, and MUST NOT assume that a
UA will utilize its GRUU in the Contact header field of its response
(which would cause mid-dialog requests to pass through the proxy
without record-routing).
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
Rosenberg Standards Track [Page 22]RFC 5627 GRUU Mechanism October 2009
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 [1]. It also defines a new SIP URI
parameter ("gr") by extending the grammar for "uri-parameter" as
defined in RFC 3261 [1]. The ABNF [13] is as follows:
contact-params =/ temp-gruu / pub-gruu
temp-gruu = "temp-gruu" EQUAL quoted-string
pub-gruu = "pub-gruu" EQUAL quoted-string
uri-parameter =/ gr-param
gr-param = "gr" ["=" pvalue] ; defined in RFC 3261
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.
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 (currently, "Delivery of
Request-URI Targets to User Agents" [29]).
Rosenberg Standards Track [Page 23]RFC 5627 GRUU Mechanism October 2009
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 (currently, "Delivery of
Request-URI Targets to User Agents" [29]).
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 (currently, "Delivery of Request-URI
Targets to User Agents" [29]).
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 might 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 that 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.
9. Example Call Flow
The following call flow, shown in Figure 2, 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 RFC 4475 [17] are used to describe
the representation of long message lines.
Rosenberg Standards Track [Page 24]RFC 5627 GRUU Mechanism October 2009
Caller Proxy Callee
| |(1) REGISTER |
| |<--------------------|
| |(2) 200 OK |
| |-------------------->|
|(3) INVITE | |
|-------------------->| |
| |(4) INVITE |
| |-------------------->|
| |(5) 200 OK |
| |<--------------------|
|(6) 200 OK | |
|<--------------------| |
|(7) ACK | |
|-------------------->| |
| |(8) ACK |
| |-------------------->|
|(9) SUBSCRIBE | |
|-------------------->| |
| |(10) SUBSCRIBE |
| |-------------------->|
| |(11) 200 OK |
| |<--------------------|
|(12) 200 OK | |
|<--------------------| |
| |(13) NOTIFY |
| |<--------------------|
|(14) NOTIFY | |
|<--------------------| |
|(15) 200 OK | |
|-------------------->| |
| |(16) 200 OK |
| |-------------------->|
| | |Crashes,
| |(17) REGISTER | Reboots
| |<--------------------|
| |(18) 200 OK |
| |-------------------->|
Figure 2
Rosenberg Standards Track [Page 25]RFC 5627 GRUU Mechanism October 2009
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:tgruu.7hs==
jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr"
;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
;expires=3600
</allOneLine>
Content-Length: 0
The Contact header field in the REGISTER response contains the "pub-
gruu" Contact header field parameter with the public GRUU sip:callee@
example.com;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6, and the
"temp-gruu" header field parameter with the temporary GRUU
sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@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
GRUU.
Rosenberg Standards Track [Page 26]RFC 5627 GRUU Mechanism October 2009
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 (defined in [16]) 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 is
using its own GRUU to populate its remote target.
Rosenberg Standards Track [Page 27]RFC 5627 GRUU Mechanism October 2009
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,
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
Rosenberg Standards Track [Page 28]RFC 5627 GRUU Mechanism October 2009
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:tgruu.7hatz6cn-098shfyq193=
ajfux8fyg7ajqqe7@example.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:tgruu.7hatz6cn-098shfyq193=
ajfux8fyg7ajqqe7@example.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 Section 6.1 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 outside
attacks (where a third party is trying to attack the system) and
inside attacks (where the attacker is a valid participant in the
system but is malicious). In addition, there are privacy
considerations with using GRUUs.
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 (DoS) to the UA. As
a result, it is RECOMMENDED that a UA use the SIPS URI scheme in the
Rosenberg Standards Track [Page 29]RFC 5627 GRUU Mechanism October 2009
Request-URI when registering. Proxies and registrars MUST support
the SIPS URI and MUST support TLS. This does not represent a change
from the requirements in RFC 3261 [1].
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 another UA (called the
correspondent), which then uses them in requests that they emit.
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" URI parameter produces a URI that
is not recognized as a GRUU and is not equal to any AOR. The request
will be rejected. If the GRUU had been public, removing the "gr" URI
parameter would have produced 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 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, and from it, observe their GRUU. Once the GRUU is
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
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RFC 5627 GRUU Mechanism October 2009
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. SIP does not require outbound proxies, and this does leave a
potential area of vulnerability. However, in practice, nearly all
deployments of SIP utilize an outbound proxy, and therefore this
vulnerability is not likely to be a concern.
10.3. Privacy Considerations
RFC 3323 [15] defines mechanisms for privacy. It distinguishes
between network-provided privacy and user-provided privacy. In the
former, the user requests privacy services from the network by
including a Privacy header field in the request. In the latter, the
UA follows a basic set of guidelines for construction of its request,
so a certain level of privacy is afforded.
The guidelines in Section 4.1 of RFC 3323 [15] 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 [15]. A user
agent would gather the temporary GRUU returned in each REGISTER
response, 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 GRUU 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. This is
also true for the user-provided privacy procedures in RFC 3323
[15], since the IP address or hostname in the Contact URI provides
a similar correlator.
Rosenberg Standards Track [Page 31]RFC 5627 GRUU Mechanism October 2009
o A UA utilizing a different temporary GRUU 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)
(defined in [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 (see RFC 4730
[20]), 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 a temporary 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 GRUUs. 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 GRUUs.
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
privacy service, a correspondent will be able to determine the SIP
Rosenberg Standards Track [Page 32]RFC 5627 GRUU Mechanism October 2009
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 RFC 3968 [8]. The required information is as
follows:
Header field in which the parameter can appear: Contact
Name of the Parameter: pub-gruu
Predefined Values: none
RFC Reference: RFC 5627
Header field in which the parameter can appear: Contact
Name of the Parameter: temp-gruu
Predefined Values: none
RFC Reference: RFC 5627
11.2. URI Parameter
This specification defines one new SIP URI parameter, as per the
registry created by RFC 3969 [9].
Name of the Parameter: gr
Predefined Values: none
RFC Reference: RFC 5627
11.3. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261 [1].
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 is not expected to
process the registration unless it supports the GRUU extension.
Rosenberg Standards Track [Page 33]RFC 5627 GRUU Mechanism October 2009
12. Acknowledgments
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.
[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.
Rosenberg Standards Track [Page 34]RFC 5627 GRUU Mechanism October 2009
[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] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[13] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[14] Jennings, C., Ed. and R. Mahy, Ed., "Managing Client-Initiated
Connections in the Session Initiation Protocol (SIP)",
RFC 5626, October 2009.
13.2. Informative References
[15] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[16] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[17] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and
H. Schulzrinne, "Session Initiation Protocol (SIP) Torture Test
Messages", RFC 4475, May 2006.
[18] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-
for-IPv4) Option for Session Initiation Protocol (SIP)
Servers", RFC 3361, August 2002.
[19] Sparks, R., Johnston, A., and D. Petrie, "Session Initiation
Protocol (SIP) Call Control - Transfer", BCP 149, RFC 5589,
June 2009.
[20] Burger, E. and M. Dolly, "A Session Initiation Protocol (SIP)
Event Package for Key Press Stimulus (KPML)", RFC 4730,
November 2006.
[21] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[22] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
Rosenberg Standards Track [Page 35]RFC 5627 GRUU Mechanism October 2009
[23] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003.
[24] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[25] Camarillo, G., "Compressing the Session Initiation Protocol
(SIP)", RFC 3486, February 2003.
[26] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network Media
Services with SIP", RFC 4240, December 2005.
[27] Jennings, C., Audet, F., and J. Elwell, "Session Initiation
Protocol (SIP) URIs for Applications such as Voicemail and
Interactive Voice Response (IVR)", RFC 4458, April 2006.
[28] Kyzivat, P., "Registration Event Package Extension for Session
Initiation Protocol (SIP) Globally Routable User Agent URIs
(GRUUs)", RFC 5628, October 2009.
[29] Rosenberg, J., van Elburg, J., Holmberg, C., Audet, F., and S.
Schubert, Ed., "Delivery of Request-URI Targets to User
Agents", Work in Progress, June 2009.
Rosenberg Standards Track [Page 36]RFC 5627 GRUU Mechanism October 2009
Appendix A. Example GRUU Construction Algorithms
The mechanism for constructing a GRUU is not subject to
specification. This appendix provides an example that can be used by
a registrar to construct a public and a temporary GRUU. Of course,
others are permitted, as long as they meet the constraints defined
for a GRUU.
A.1. Public GRUU
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
that does not require additional storage that grows in size with the
number of temporary GRUUs. The following algorithm meets this goal.
The registrar maintains a counter, I. This counter is 48 bits and is
initialized to zero. The counter is persistently stored, using a
backend database or other similar technique. When the registrar
creates the first temporary GRUU for a particular AOR and instance
ID, the registrar notes the current value of the counter, I_i, and
increments the counter in the database. The registrar then maps I_i
to the AOR and instance ID using the database, a persistent hashmap
or similar technology. If the registration expires such that there
are no longer any contacts with that particular instance ID bound to
the GRUU, the registrar removes the mapping. Similarly, if the
temporary GRUUs are invalidated due to a change in Call-ID, the
registrar removes the current mapping from I_i to the AOR and
instance ID, notes the current value of the counter I_j, and stores a
mapping from I_j to the AOR and instance ID. Based on these rules,
the hashmap will contain a single mapping for each AOR and instance
ID for which there is a currently valid registration.
The usage of a counter in a 48-bit space with sequential assignment
allows for a compact representation of the hashmap key, which is
important for generating GRUUs of reasonable size. The counter
starts at zero when the system is initialized. Persistent and
reliable storage of the counter is required to avoid misrouting of a
GRUU to the wrong AOR and instance ID. Similarly, persistent storage
of the hashmap is required, even through proxy and registrar
Rosenberg Standards Track [Page 37]
RFC 5627 GRUU Mechanism October 2009
restarts. If the hashmap is reset, all previous temporary GRUUs
become invalidated. This might cause dialogs in progress to fail, or
future requests towards a temporary GRUU to fail when they normally
would not. The same hashmap needs to be accessible by all proxies
and registrars that can field requests for a particular AOR and
instance ID.
The registrar maintains a pair of local symmetric keys K_e and K_a.
These are regenerated every time the counter is reset. When the
counter rolls over or is reset, the registrar remembers the old
values of K_e and K_a for a time. Like the hashmap itself, these
keys need to be shared across all proxy and registrars that can
service requests for a particular AOR and instance ID.
To generate a new temporary GRUU, the registrar generates a random
80-bit distinguisher value D. It then computes:
M = D || I_i
E = AES-ECB-Encrypt(K_e, M)
A = HMAC-SHA256-80(K_a, E)
Temp-Gruu-userpart = "tgruu." || base64(E) || base64(A)
where || denotes concatenation, and AES-ECB-Encrypt represents AES
encryption in electronic codebook mode. M will be 128 bits long,
producing a value of E that is 128 bits and A that is 80 bits. This
produces a user part which has 42 characters.
When a proxy receives a request whose user part begins with "tgruu.",
it extracts the remaining portion, and splits it into 22 characters
(E') and the remaining 14 characters (A'). It then computes A and E
by performing a base64 decode of A' and E' respectively. Next, it
computes:
Ac = HMAC-SHA256-80(K_a, E)
If the counter has rolled over or reset, this computation is
performed with the current and previous K_a. If the Ac value(s) that
are computed do not match the value of A extracted from the GRUU, the
GRUU is rejected as invalid. Next, the proxy computes:
M = AES-ECB-Decrypt(K_e, E)
If the counter has rolled over, this computation is done using the
value of K_e that goes with the value of K_a, which produced a valid
Ac in the previous HMAC validation. The leading 80 bits (the
distinguisher D) are discarded, leaving an index I_i in the hashmap.
This index is looked up. If it exists, the proxy now has the AOR and
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RFC 5627 GRUU Mechanism October 2009
instance ID corresponding to this temporary GRUU. If there is
nothing in the hashmap for the key I_i, the GRUU is no longer valid
and the request is rejected.
The usage of a 48-bit counter allows for the registrar to have as
many as a billion AORs, with 10 instances per AOR, and cycle through
10,000 Call-ID changes for each instance through the duration of a
single registration. These numbers reflect the average; the system
works fine if a particular AOR has more than 10 instances or a
particular instance cycles through more than 10,000 Call-IDs in its
registration, as long as the average meets these constraints.
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 User Agent Client (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 [23], 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.
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 [18].
4. In cases where the client discovers a local outbound proxy via a
mechanism such as DHCP, and is not implementing the service route
mechanism, 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
Rosenberg Standards Track [Page 39]RFC 5627 GRUU Mechanism October 2009
effect of the service route mechanism, but is accomplished
through static configuration.
Author's Address
Jonathan Rosenberg
Cisco Systems
Edison, NJ
US
EMail: jdrosen@cisco.com
URI: http://www.jdrosen.net
Rosenberg Standards Track [Page 40]
