SIP J. Rosenberg
Internet-Draft dynamicsoft
Expires: June 4, 2004 December 5, 2003
Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in
the Session Initiation Protocol (SIP)
draft-rosenberg-sip-gruu-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on June 4, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Several applications of the Session Initiation Protocol (SIP) require
a user agent (UA) to construct and distribute a URI which can be used
by anyone on the Internet to route a call to that specific UA
instance. A URI which routes to a specific UA instance is called a
Globally Routable UA URI (GRUU). This document describes an extension
to SIP for obtaining a GRUU from a server, and for communicating a
GRUU to a peer within a dialog.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Operation . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
4. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 4
4.1 REGISTER Processing . . . . . . . . . . . . . . . . . . . . 4
4.2 Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . 5
5. Registrar Behavior . . . . . . . . . . . . . . . . . . . . . 5
6. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 7
7. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
10.1 Header Field Parameter . . . . . . . . . . . . . . . . . . . 12
10.2 URI Parameter . . . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
Normative References . . . . . . . . . . . . . . . . . . . . 12
Informative References . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Introduction
Several applications of the Session Initiation Protocol (SIP) [1]
require a user agent (UA) to construct and distribute a URI which can
be used by anyone on the Internet to route a call to that specific UA
instance. An example of such an application is call transfer, based
on the REFER method [4]. Another application is the usage of
endpoint-hosted conferences within the conferencing framework [8].
We call these URIs Globally Routable UA URIs (GRUU). Requirements [9]
have been defined which identify the capabilities that any mechanism
for obtaining and using a GRUU has to provide. This specification
describes a mechanism that meets these requirements.
2. Overview of Operation
This section is tutorial in nature, and does not specify any
normative behavior.
This extension allows a UA to obtain a GRUU, and to use a GRUU. These
two mechanisms are separate, in that a UA can obtain a GRUU in any
way it likes, and use the mechanisms in this specification to use
them. Similarly, a UA can obtain a GRUU but never use it.
A UA can obtain a GRUU by generating a normal REGISTER request, as
specified in RFC 3261 [1]. This request contains a Supported header
field with the value "gruu", indicating to the registrar that the UA
supports this extension. If the domain that the user is registering
against also supports GRUU, the REGISTER responses will contain the
"gruu" parameter in each Contact header field. This parameter
contains a GRUU which the domain guarantees will route to that
specific Contact URI. That GRUU is guaranteed to remain valid for the
duration of the registration.
Since the GRUU is a URI like any other, it can be handed out by a UA
by placing it in any header field which can contain a GRUU. A UA will
normally place the GRUU into the Contact header field of dialog
creating requests and responses it generates. However, it is
important for the UA receiving the message to know whether the
Contact URI is a GRUU or not. To make this determination, the UA
looks for the presence of the Supported header field in the request
or response. If it is present with a value of "gruu", it means that
the Contact URI is a GRUU.
When a UA uses a GRUU, it has the option of adding the "grid" URI
parameter to the GRUU. This parameter is opaque to the proxy server
handling the domain. However, when the server maps the GRUU to the
corresponding Contact URI, the server will copy the grid parameter
into the Contact URI. As a result, when the UA receives the request,
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the Request URI will contain the grid parameter it placed in the
corresponding GRUU.
3. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant implementations.
4. User Agent Behavior
User agent behavior is divided into two separate parts - REGISTER
processing, and GRUU usage.
4.1 REGISTER Processing
When a UA wishes to obtain a GRUU within the domain of its AOR, when
it generates a REGISTER request (initial or refresh), it MUST include
the Supported header field in the request. The value of that header
field MUST include "gruu" as one of the option tags. This alerts the
registrar for the domain that the UA supports the GRUU mechanism.
Besides the presence of this option tag in the Supported header
field, the REGISTER request is constructed identically to the case
where this extension was not understood. Specifically, the Contact
URI in the REGISTER request SHOULD NOT contain the gruu Contact
header field parameter. Any such parameters are ignored by the
registrar, as the UA cannot propose a GRUU for usage with the Contact
URI.
If a UA wishes to guarantee that the request is not processed unless
the domain supports and uses this extension, it MAY include a Require
header field in the request with a value that contains the "gruu"
option tag.
If the response is a 2xx, each Contact header field may contain a
"gruu" parameter. This parameter contains a SIP URI that represents a
GRUU corresponding to that Contact URI. Any requests sent to the GRUU
URI will be routed by the domain to the Contact URI. The GRUU will
not change in subsequent 2xx responses to REGISTER as long as the UA
does not let the registration expire. However, if the UA waits until
the last moment to refresh its registration, it may cause a race
condition where the registration expires while the registration is in
transit. The resulting 200 OK might then contain a different GRUU.
Since "last moment" is ill defined, it is RECOMMENDED that a UA be
prepared to handle a change in the GRUU during a registration.
Handling a change depends on the way in which it has been used. In
the case where it is included in the Contact URI of a dialog
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initiating request or response, the UA would update the Contact URI
with a target refresh request.
4.2 Using the GRUU
A UA first obtains a GRUU using the procedures of Section 4.1.
A UA can use the GRUU in the same way it would use any other SIP URI.
However, a UA compliant to this specification MUST use a GRUU when
populating the Contact header field of dialog-creating requests and
responses. This includes the INVITE request and its 2xx response, the
SUBSCRIBE [3] request, its 2xx response, and the NOTIFY request, and
the REFER [4] request and its 2xx response. Similarly, in those
requests and responses where the GRUU is used in the Contact header
field, the UA MUST include a Supported header field that contains the
option tag "gruu". However, it is not necessary for a UA to know
whether or not its peer in the dialog uses a GRUU before inserting
one into the Contact header field.
When placing a GRUU into the Contact header field of a request or
response, a UA MAY add the "grid" URI parameter to the GRUU. This
parameter MAY take on any value permitted by the grammar for the
parameter, but MUST NOT exceed 128 characters. When a UA sends a
request to the GRUU, the proxy for the domain that owns the GRUU will
copy this parameter from the GRUU into the Contact URI matching that
GRUU. This allows the UA to effectively manufacture an infinite
supply of GRUU, each of which differs by the value of the "grid"
parameter. When a UA receives a request that was sent to the GRUU, it
will be able to tell which GRUU was invoked by the "grid" parameter.
An implication of this behavior is that all mid-dialog requests will
be routed through intermediate proxies. There will never be direct,
UA to UA signaling. It is anticipated that this limitation will be
addressed in future specifications.
Once a UA knows that the Contact URI provided by its peer is a GRUU,
it can use it in any application or SIP extension which requires a
globally routable URI to operate. One such example is assisted call
transfer.
5. Registrar Behavior
When a registrar compliant to this specification receives a REGISTER
request, it checks for the presence of the Require header field in
the request. If present, and if it contains the "gruu" option tag,
the registrar MUST follow the procedures in the next paragraph for
inclusion of the "gruu" parameter in a 2xx response to REGISTER. If
not present, but a Supported header field was present with the "gruu"
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option tag, the registrar SHOULD follow the procedures in the next
paragraph for inclusion of the "gruu" parameter in a 2xx response to
REGISTER. If the Supported header field was not present, or it if was
present but did not contain the value "gruu", the registrar SHOULD
NOT follow the procedures of the next paragraph for inclusion of the
"gruu" parameter in a 2xx response to REGISTER.
If the register request contained any "gruu" Contact header field
parameters, these MUST be ignored by the registrar. A UA cannot
suggest or otherwise provide a GRUU to the registrar.
A GRUU is provided to a UA by including it in the "gruu" Contact
header field parameter for a particular Contact URI. The value of
this parameter is a quoted string containing the URI that is the GRUU
for the associated Contact URI. If the REGISTER request was
refreshing that Contact URI, and the registrar had provided a gruu to
the UA previously, the registrar MUST include the "gruu" Contact
header field parameter for that Contact URI, and its value MUST
contain the same URI provided previously. The result is that there is
a one to one mapping of a GRUU to a Contact URI for the duration that
the Contact is registered to the UA. Note that, should the
registration of that Contact expire, and then the UA re-registers it
at a later time, the registrar is under no obligation to use the same
GRUU for that Contact URI. The implication of these rules is that a
registrar is responsible for reliable storage of the GRUU for the
duration of a registration.
If the REGISTER request is creating a new Contact URI for a
particular address of record, and the registrar decides to provide
the UA with a gruu for that Contact URI, it constructs a new GRUU.
This specification does not mandate a particular mechanism for
construction of the GRUU. However, the GRUU MUST exhibit the
following properties:
o The domain part of the URI is an IP address present on the public
Internet, or, if it is a host name, exists in the global DNS and
corresponds to an IP address present on the public Internet.
o When a request is sent to this URI, it routes to a proxy server in
the same domain as that of the registrar.
o A proxy server in the domain can determine that the URI is a GRUU.
o When a proxy server in this domain translates this URI, the result
is equal to the Contact URI corresponding to the GRUU.
o It MUST NOT be possible, based on inspection of the URI, to
determine the associated Contact URI or Address of Record.
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With these rules, it is possible, though not required, to construct a
GRUU without requiring the maintenance of any additional state. To do
that, the URI would be constructed in the following fashion:
user-part = "GRUU" + BASE64(E(K, (salt + Contact URI + AOR)))
Where E(K,X) represents a suitable encryption function (such as AES
with 128 bit keys) with key K applied to data block X, and the "+"
operator implies concatenation. Salt represents a random string that
prevents a client from obtaining pairs of known plaintext and
ciphertext. A good choice would be at least 128 bits of randomness in
the salt.
The benefit of this mechanism is that a server need not store
additional information on mapping a GRUU to its corresponding Contact
URI. The user part of the GRUU can itself contain the Contact URI.
Encryption is needed to prevent attacks whereby the server is sent
requests with faked GRUU, causing the server to direct requests to
any named URI. Even with encryption, the proxy should validate the
user part after decryption. In particular, the AOR should be one
managed by the proxy in that domain. Should a UA send a request with
a fake GRUU, the proxy would decrypt and then discard it because
there would be no URI or an invalid URI inside.
6. Proxy Behavior
When a proxy server receives a request, and the proxy owns the domain
in the Request URI, and the proxy is supposed to access a Location
Service in order to compute request targets (as specified in Section
16.5 of RFC 3261 [1]), the proxy MUST check if the Request URI is a
GRUU created by that domain.
If the URI is a GRUU, the proxy MUST determine if the Contact URI
associated with the GRUU is still registered to the AOR it was
registered to when the GRUU was constructed. If that AOR no longer
has any registered contacts, or if it does have registered contacts,
but none of them equal the Contact URI associated with the GRUU, the
proxy MUST generate a 404 (Not Found) response to the request.
Otherwise, the proxy MUST populate the target set with a single URI.
This URI MUST be equal to the Contact URI associated with that GRUU.
Furthermore, if the GRUU contained a "grid" URI parameter, the URI in
the target set MUST also contain the same parameter with the same
value.
A proxy MAY apply other processing to the request, such as execution
of called party features. In particular, it is RECOMMENDED that
non-routing called party features, such as call logging and
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screening, that are associated with the AOR are also applied to
requests for the GRUU.
In many cases, a proxy will record-route an initial INVITE request,
and the user agents will insert a GRUU into the Contact header field.
When this happens, a mid-dialog request will arrive at the proxy with
a Route header field that was inserted by the proxy, and a
Request-URI that represents a GRUU. Proxies follow normal processing
in this case; they will strip the Route header field, and then
process the Request URI as described above.
The procedures of RFC 3261 are then followed to proxy the request.
The request SHOULD NOT be redirected in this case. In many instances,
a GRUU is used by a UA in order to assist in the traversal of NATs,
and a redirection may prevent such a case from working.
7. Grammar
This specification defines a new Contact header field parameter,
gruu, and a new URI parameter, grid.
contact-params = c-p-q / c-p-expires / c-p-gruu
/ contact-extension
c-p-gruu = "gruu" EQUAL SWS DQUOTE SIP-URI DQUOTE
uri-parameter = transport-param / user-param / method-param
/ ttl-param / maddr-param / lr-param / grid-param
/ other-param
grid-param = "grid=" pvalue
8. Examples
The following call flow shows a basic registration and call setup,
followed by a subscription directed to the GRUU.
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Caller Proxy Callee
| |(1) REGISTER |
| |<--------------------|
| |(2) 200 OK |
| |-------------------->|
|(3) INVITE | |
|-------------------->| |
| |(4) INVITE |
| |-------------------->|
| |(5) 200 OK |
| |<--------------------|
|(6) 200 OK | |
|<--------------------| |
|(7) ACK | |
|-------------------->| |
| |(8) ACK |
| |-------------------->|
|(9) SUBSCRIBE | |
|-------------------->| |
| |(10) SUBSCRIBE |
| |-------------------->|
| |(11) 200 OK |
| |<--------------------|
|(12) 200 OK | |
|<--------------------| |
| |(13) NOTIFY |
| |<--------------------|
|(14) NOTIFY | |
|<--------------------| |
|(15) 200 OK | |
|-------------------->| |
| |(16) 200 OK |
| |-------------------->|
The Callee supports the GRUU extension. As such, its REGISTER (1)
looks like:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl
Supported: gruu
To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
CSeq: 1 REGISTER
Contact: <sip:callee@client.example.com>
Content-Length: 0
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The REGISTER response would look like:
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
CSeq: 1 REGISTER
Contact: <sip:callee@client.example.com>;gruu="sip:hha9s8d=-999a@example.com"
Content-Length: 0
Note how the Contact header field in the REGISTER response contains
the gruu parameter with the URI sip:hha9s8d=-999a@example.com. This
represents a GRUU associated with the Contact URI
sip:callee@client.example.com.
The INVITE from the caller is a normal SIP INVITE. The 200 OK
generated by the callee, however, now contains a GRUU in the Contact
header field. The UA has also chosen to include a grid URI parameter
into the GRUU.
SIP/2.0 200 OK
Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKnaa8
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a
From: Caller <sip:caller@example.com>;tag=n88ah
To: Callee <sip:callee@example.com> ;tag=a0z8
Call-ID: 1j9FpLxk3uxtma7@host.example.com
CSeq: 1 INVITE
Supported: gruu
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:hha9s8d=-999a@example.com;grid=99a>
Content-Length: --
Content-Type: application/sdp
[SDP Not shown]
At some point later in the call, the caller decides to subscribe to
the dialog event package [10] at that specific UA. To do that, it
generates a SUBSCRIBE request (message 9), but directs it towards the
GRUU contained in the Contact header field.
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SUBSCRIBE sip:hha9s8d=-999a@example.com;grid=99a SIP/2.0
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:caller@example.com>;tag=kkaz-
To: Callee <sip:callee@example.com>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:bad998asd8asd0000a0@example.com>
Content-Length: 0
In this example, the caller itself supports the GRUU extension, and
is using its own GRUU to populate the Contact header field of the
SUBSCRIBE.
This request is routed to the proxy, which proceeds to perform a
location lookup on the request URI. It is translated into the Contact
URI bound to that GRUU, and then proxied there (message 10). Note how
the grid parameter is maintained.
SUBSCRIBE sip:callee@client.example.com;grid=99a SIP/2.0
Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK9555
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:caller@example.com>;tag=kkaz-
To: Callee <sip:callee@example.com>
Call-ID: faif9a@host.example.com
CSeq: 2 SUBSCRIBE
Supported: gruu
Event: dialog
Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
Contact: <sip:bad998asd8asd0000a0@example.com>
Content-Length: 0
9. Security Considerations
Since GRUUs do not reveal information about the identity of the
associated address-of-record or Contact URI, they provide routability
without identity. However, GRUUs do not provide a complete or
reliable solution for privacy. In particular, since the GRUU does not
change during the lifetime of a registration, an attacker could
correlate two calls as coming from the same source, which in and of
itself reveals information about the caller. Furthermore, GRUUs do
not address other aspects of privacy, such as the addresses used for
media transport. For a discussion of how privacy services are
provided in SIP, see RFC 3323 [7].
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It is important for a UA to be assured of the integrity of a GRUU
when it is given one in a REGISTER response. If the GRUU is tampered
with by an attacker, the result could be denial of service to the UA.
As a result, it is RECOMMENDED that a UA use the SIPS URI scheme when
registering.
10. IANA Considerations
This specification defines a new Contact header field parameter and
URI parameter.
10.1 Header Field Parameter
This specification defines a new header field parameter, as per the
registry created by [5]. The required information is as follows:
Header field in which the parameter can appear: Contact
Name of the Parameter gruu
RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
10.2 URI Parameter
This specification defines a new SIP URI parameter, as per the
registry created by [6].
Name of the Parameter grid
RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
11. Acknowledgements
The author would like to thank Rohan Mahy, Paul Kyzivat, Alan
Johnston, and Cullen Jennings for their contributions to this work.
Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[3] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[5] Camarillo, G., "The Internet Assigned Number Authority Header
Field Parameter Registry for the Session Initiation Protocol",
draft-ietf-sip-parameter-registry-00 (work in progress), August
2003.
[6] Camarillo, G., "The Internet Assigned Number Authority Universal
Resource Identifier Parameter Registry for the Session
Initiation Protocol", draft-ietf-sip-uri-parameter-reg-00 (work
in progress), August 2003.
Informative References
[7] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[8] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-00 (work in
progress), May 2003.
[9] Rosenberg, J., "Requirements for Construction and Usage of
Globally Routable User Agent (UA) URIs for the Session
Initiation Protocol (SIP)",
draft-rosenberg-sipping-gruu-reqs-01 (work in progress),
October 2003.
[10] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
Event Package for the Session Initiation Protocol (SIP",
draft-ietf-sipping-dialog-package-02 (work in progress), July
2003.
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Author's Address
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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