SIPPING J. Rosenberg
Internet-Draft Cisco Systems
Expires: August 29, 2006 G. Camarillo, Ed.
Ericsson
D. Willis
Cisco Systems
February 25, 2006
A Framework for Consent-Based Communications in the Session Initiation
Protocol (SIP)
draft-ietf-sipping-consent-framework-04.txt
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Copyright (C) The Internet Society (2006).
Abstract
The Session Initiation Protocol (SIP) supports communications across
many media types, including real-time audio, video, text, instant
messaging, and presence. In its current form, it allows session
invitations, instant messages, and other requests to be delivered
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from one party to another without requiring explicit consent of the
recipient. Without such consent, it is possible for SIP to be used
for malicious purposes, including amplification and DoS (Denial of
Service) attacks. This document identifies a framework for consent-
based communications in SIP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relays and Translations . . . . . . . . . . . . . . . . . . . 3
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Permissions at a Relay . . . . . . . . . . . . . . . . . . 6
4.2. Consenting Manipulations on a Relay's Transaction Logic . 6
4.3. Permission Servers . . . . . . . . . . . . . . . . . . . . 7
4.4. Recipients Grant Permissions . . . . . . . . . . . . . . . 8
5. Overview of Operations . . . . . . . . . . . . . . . . . . . . 8
5.1. Amplification Avoidance . . . . . . . . . . . . . . . . . 10
5.2. Subscription to the Permission Status . . . . . . . . . . 11
5.3. Request for Permission . . . . . . . . . . . . . . . . . . 11
5.4. Permission Document Structure . . . . . . . . . . . . . . 11
5.5. Permission Requested Notification . . . . . . . . . . . . 12
5.6. Permission Upload . . . . . . . . . . . . . . . . . . . . 12
5.6.1. SIP Identity . . . . . . . . . . . . . . . . . . . . . 13
5.6.2. P-Asserted-Identity . . . . . . . . . . . . . . . . . 13
5.6.3. Return Routability . . . . . . . . . . . . . . . . . . 13
5.7. Permission Granted Notification . . . . . . . . . . . . . 14
5.8. Permission Revocation . . . . . . . . . . . . . . . . . . 14
5.9. Request-contained URI Lists . . . . . . . . . . . . . . . 15
5.10. Registrations . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 24
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1. Introduction
The Session Initiation Protocol (SIP) [1] supports communications
across many media types, including real-time audio, video, text,
instant messaging and presence. This communication is established by
the transmission of various SIP requests (such as INVITE and MESSAGE
[4]) from an initiator to the recipient, with whom communication is
desired. Although a recipient of such a SIP request can reject the
request, and therefore decline the session, a SIP network will
deliver a SIP request to the recipient without their explicit
consent.
Receipt of these requests without explicit consent can cause a number
of problems in SIP networks. These include amplification and DoS
(Denial of Service) attacks. These problems are described in more
detail in a companion requirements document [17].
This specification defines a basic framework for adding consent-based
communication to SIP.
2. Definitions
Recipient URI: The request-URI of an outgoing request sent by an
entity (e.g., a user agent or a proxy). The sending of such
request may have been the result of a translation operation.
Target URI: The request-URI of an incoming request that arrives to an
entity (e.g., a proxy) that will perform a translation operation.
Translation operation: Operation by which an entity (e.g., a proxy)
translates the request URI of an incoming request (i.e., the
target URI) into one or more URIs (i.e., recipient URIs) which are
used as the request URIs of one or more outgoing requests.
3. Relays and Translations
Relays play a key role in this framework. A relay is defined as any
SIP server, be it a proxy, B2BUA (Back-to-Back User Agent), or some
hybrid, which receives a request and translates the request URI into
one or more next hop URIs to which it then delivers a request. The
request URI of the incoming request is referred to as 'target URI'
and the destination URI of the outgoing requests is referred to as
'recipient URIs', as shown in Figure 1.
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+---------------+
| | recipient URI
| |---------------->
target URI | Translation |
-------------->| Operation | recipient URI
| |---------------->
| |
+---------------+
Figure 1: Translation operation
Thus, an essential aspect of a relay is that of translation. When a
relay receives a request, it translates, following its translation
logic, the request URI into one or more additional URIs. Or, more
generally, it can create outgoing requests to one or more additional
URIs. The translation operation is what creates the consent problem.
Additionally, since the translation operation can result in more than
one URI, it is also the source of amplification. Servers that do not
perform translations, such as outbound proxy servers, do not cause
amplification.
Since the translation operation is based on local policy or local
data (such as registrations), it is the vehicle by which a request is
delivered directly to an endpoint, when it would not otherwise be
possible to. In other words, if a spammer has the address of a user,
'user@example.com', it cannot deliver a MESSAGE request to the UA
(User Agent) of that user without having access to the registration
data that maps 'user@example.com' to the user agent on which that
user is present. Thus, it is the usage of this registration data,
and more generally, the translation logic, which must be authorized
in order to prevent undesired communications. (Of course, if the
spammer knows the address of the user agent, it will be able to
deliver requests directly to it.)
Figure 2 shows a relay that performs translations. The user agent
client (UAC) in the figure sends a SIP request to a URI representing
a resource in the domain 'example.com' (resource@example.com). This
request may pass through a local outbound proxy (not shown), but
eventually arrives at a server authoritative for the domain
'example.com'. This server, which acts as a relay, performs a
translation operation, translating the target URI into one or more
recipient URIs, which may or may not belong to the domain
'example.com'. This relay may be, for instance, a proxy server or a
URI-list service [18].
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+-------+
| |
>| UAS |
/ | |
/ +-------+
/
/
+-----------------------+ /
| | /
+-----+ | Relay | / +-------+
| | | |/ | |
| UAC |------>| |-------->| Proxy |
| | |+---------------------+|\ | |
+-----+ || Translation || \ +-------+
|| Logic || \
|+---------------------+| \ [...]
+-----------------------+ \
\
\ +-------+
\ | |
>| B2BUA |
| |
+-------+
Figure 2: Relay performing a translation
This framework allows potential recipients of a translation to agree
to be actual recipients by giving permission to the relay performing
the translation to send them traffic.
4. Architecture
Figure 3 shows the architectural elements of this framework.
Section 4.1 describes the role of permissions at a relay.
Section 4.2 discusses the actions taken by a relay when its
translation logic is manipulated by a client. Section 4.3 introduces
permission servers and their functionality. Section 4.4 describes
how potential recipients can grant permissions to a relay to add them
to the relay's translation logic.
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+-----------------------+ Permission +------------+
| | Request | |
+--------+ | Relay |----------->| Permission |
| | | | | Server |
| Client | | | | |
| | |+-------+ +-----------+| +------------+
+--------+ ||Transl.| |Permissions|| |
| ||Logic | | || Permission |
| |+-------+ +-----------+| Request |
| +-----------------------+ V
| ^ ^ +------------+
| Manipulation | | Permission Grant | |
+---------------+ +-------------------| Recipient |
| |
+------------+
Figure 3: Reference Architecture
4.1. Permissions at a Relay
Relays implementing this framework need to obtain and store
permissions associated to their translation logics. These
permissions indicate if a particular recipient has agreed to receive
traffic or not at any given time. Recipients that have not given
permission to the relay to send them traffic are simply ignored by
the relay when performing a translation.
Permissions are valid as long as the context where they were granted
is valid. For example, the permissions obtained by a URI-list SIP
service that distributes MESSAGE requests to a set of recipients will
be valid as long as the URI-list SIP service exists.
4.2. Consenting Manipulations on a Relay's Transaction Logic
This framework aims to ensure that any particular Relay only performs
translations towards destinations that have given permission to the
Relay to perform such a translation. Consequently, when the
translation logic of a relay is manipulated (e.g., a new recipient
URI is added), the relay needs to obtain permission from the new
recipient in order to install the new translation logic. Relays ask
recipients for permission using CONSENT [10] requests.
For example, the relay hosting the URI-list service at
'friends@example.com' performs a translation from that URI to a set
of recipient URIs. When a client (e.g., the administrator of that
URI-list service) adds 'bob@example.org' as a new recipient URI, the
relay sends a CONSENT request to 'bob@example.org' asking whether or
not it is OK to perform the translation from 'friends@example.com' to
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'bob@example.org' (CONSENT requests carry in their message bodies a
permission document that describes the translation for which
permissions are being requested). If the answer is positive, the new
translation logic is installed at the relay. That is, the new
recipient URI is added.
Note that the mechanism to be used to manipulate the translation
logic of a particular relay depends on the relay. One possible
mechanism to manipulate translation logic is XCAP [15]. Section 5.9
and Section 5.10 describe how to add recipients to a translation
using request-contained URI lists and REGISTER requests respectively.
In any case, manipulation mechanisms implementing this framework need
to have a means to indicate that a particular recipient URI is in the
'Permission Pending' state and to provide the URI where the REFER
request needs to be sent to.
4.3. Permission Servers
When a CONSENT request sent by a relay arrives to the recipient URI
to which it was sent, the receiving user can grant or deny the
permission needed to perform the translation. Nevertheless, users
are not on-line all the time and, so, sometimes are not able to
receive CONSENT requests.
This issue is also found in presence, where a user's status is
reported by a presence server instead of by the user's user agents,
which can go on and off-line. Similarly, we define permission
servers, which are a key element of this framework. Permission
servers are network elements that act as SIP user agents and handle
CONSENT requests for a user.
Permission servers inform users about new CONSENT requests using the
'grant-permission' event package [12]. Figure 4 illustrates this
point.
The user associated with the recipient URI for which the relay will
ask for permission subscribes [2] (1) to the 'grant-permission' event
package at the permission server. This event package models the
state of all pending CONSENT requests for a particular resource.
When a new CONSENT request (3) arrives to the permission server, a
NOTIFY (5) is sent to the user. This informs them that permission is
needed for a particular sender. The NOTIFY contains the permission
document received in the CONSENT request. This permission document
is a description of the translation for which permissions are being
requested.
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There is a strong similarity between the 'winfo' event template-
package [19] and the 'grant-permission' event package. Indeed,
the grant-permission package is effectively a superset of
watcherinfo. Once in place, presentities could use the grant-
permission event package for presence in addition to all other
services for which opt-in is being provided.
Relay B's Permission B
Server
| |(1) SUBSCRIBE |
| |Event: grant-permission
| |<------------------|
| |(2) 200 OK |
| |------------------>|
| |(3) NOTIFY |
| |------------------>|
| |(4) 200 OK |
| |<------------------|
|(5) CONSENT B@example |
|------------------>| |
|(6) 202 Accepted | |
|<------------------| |
| |(7) NOTIFY |
| |------------------>|
| |(8) 200 OK |
| |<------------------|
Figure 4: Permission server operation
4.4. Recipients Grant Permissions
Recipients provide relays with permissions using SIP PUBLISH
requests. These requests contain a permission document that
describes the translation for which permissions are being granted.
5. Overview of Operations
This section provides an overview of this framework using an example
of the prototypical call flow. The elements described in Section 4
(i.e., relays, translations, and permission servers) play an
essential role in this call flow.
Figure Figure 5 shows the complete process to add a recipient URI
('B@example.com') to the translation logic of a relay. The call flow
starts with user B subscribing to the permission server using the
'grant-permission' event package [12]. User B will be informed about
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the arrival of CONSENT [10] requests addressed to 'B@example.com'.
User A attempts to add 'B@example.com' as a new recipient URI to the
translation logic of the relay (5). User A uses XCAP [15] and the
XML (Extensible Markup Language) format for representing resource
lists [16] as extended by [14] to perform this addition. Since the
relay does not have permission from 'B@example.com' to perform
translations towards that URI, the relay places 'B@example.com' in
the 'Pending' state [14] and informs user A (6).
A@example.com Relay B's Permission B@example.com
Server
| | |(1) SUBSCRIBE |
| | |Event: grant-permission
| | |<---------------|
| | |(2) 200 OK |
| | |--------------->|
| | |(3) NOTIFY |
| | |--------------->|
| | |(4) 200 OK |
| | |<---------------|
|(5) Add Recipient B@example.com | |
|--------------->| | |
|(6) Permission Pending | |
|<---------------| | |
|(7) REFER | | |
|Refer-To: B@example.com?method=CONSENT |
|--------------->| | |
|(8) 200 OK | | |
|<---------------| | |
|(9) SUBSCRIBE | | |
|Event: list-state | |
|--------------->| | |
|(10) 200 OK | | |
|<---------------| | |
|(11) NOTIFY | | |
|<---------------| | |
|(12) 200 OK | | |
|--------------->| | |
| |(13) CONSENT B@example |
| |Permission-Upload: uri-up |
| |Permission Document |
| |--------------->| |
| |(14) 202 Accepted |
| |<---------------| |
| | |(15) NOTIFY |
| | |uri-up |
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| | |Permission Document
| | |--------------->|
| | |(16) 200 OK |
| | |<---------------|
| |(17) PUBLISH uri-up |
| |Permission Document |
| |<--------------------------------|
| |(18) 200 OK | |
| |-------------------------------->|
|(19) NOTIFY | | |
|<---------------| | |
|(20) 200 OK | | |
|--------------->| | |
Figure 5: Prototypical call flow
5.1. Amplification Avoidance
Once 'B@example.com' is in the 'Permission Pending' state, the relay
needs to ask user B for permission by sending a CONSENT request to
'B@example.com'. However, the relay needs to ensure that it is not
used as an amplifier to launch amplification attacks.
In such an attack, the attacker would add a large number of recipient
URIs to the translation logic of a relay. The relay would then send
a CONSENT request to each of those URIs. The bandwidth generated by
the relay would be much higher than the bandwidth used by the
attacker to add those URIs to the translation logic of the relay.
This framework uses a credit-based authorization mechanism to avoid
the attack just described. It requires users adding new recipient
URIs to a translation to generate an amount of bandwidth that is
comparable to the bandwidth the relay will generate when sending
CONSENT requests towards those recipient URIs. This requirement is
met by having users generate REFER requests [5] towards the relay.
Each REFER request triggers the sending of a CONSENT request by the
relay.
So, the relay sends user A the URI (6) where user A needs to send a
REFER request. User A generates such a REFER request (7) and sends
it to the relay. User A uses the 'norefersub' extension [7], which
supresses the implicit subscription that is associated with REFER
transactions. This is because user A is not interested in the result
of the CONSENT transaction, but in whether or not user B will
authorize the translation by providing the requested permission.
The relay provides a URI (6) where user A can subscribe to obtain
information on whether or not user B provides the requested
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permission. User A subscribes to that URI using the 'list-state'
[14] event package (9).
5.2. Subscription to the Permission Status
After sending the REFER (7) user A subscribes to the 'list-state'
event package at the relay. This subscription keeps user A informed
about the status of the permissions (e.g., granted or denied) the
relay will request on receiving the REFER request (7).
5.3. Request for Permission
On receiving the REFER request (7), the relay generates a CONSENT
request (13) towards 'B@example.com'. This CONSENT request carries a
permission document, which describes the translation that needs to be
authorized, and a URI where to upload the permission for that
translation. User B will authorize the translation by uploading the
permission document received in the CONSENT request into this URI, as
described in Section 5.6.
When the permission document is uploaded to the URI provided by the
relay (17), the relay needs to make sure that the permission document
received was generated by user B and not by an attacker. The relay
can use three methods to authenticate the permission document: SIP
identity, P-Asserted-Identity [3], or a return routability test.
These methods are described in Section 5.6. Relays using a return
routability test to perform this authentication need to send the
CONSENT request to a SIPS URI.
5.4. Permission Document Structure
A permission document is the XML representation of a permission. A
permission document contains several pieces of data:
Identity of the Sender: A URI representing the identity of the sender
for whom permissions are granted.
Identity of the Original Recipient: A URI representing the identity
of the original recipient, which is used as the input for the
translation operation. This is also called the target URI.
Identity of the Final Recipient: A URI representing the result of the
translation. The permission grants ability for the sender to send
requests to the target URI, and for a relay receiving those
requests to forward them to this URI. This is also called the
recipient URI.
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Signature: A digital signature over the rest of the permission,
signed by an entity that can identify itself as the recipient URI.
The signature is not always present.
Permission documents may contain wildcards. For example, a
permission document may authorize any relay to forward requests
coming from a particular sender to a particular recipient. Such a
permission document would apply to any target URI. That is, the
field containing the identity of the original recipient would match
any URI.
The format for permission documents is defined in [11].
The permission document in the CONSENT request (13) sent by the relay
contains the following values:
Identity of the Sender: Any.
Identity of the Original Recipient: friends@example.com
Identity of the Final Recipient: B@example.com
It is expected that the Sender field often contains a wildcard.
However, scenarios involving request-contained URI lists, such as the
one described in Section 5.9, may require permission documents that
apply to a specific sender. Of course, in cases where the identity
of the sender matters, it is essential that relays authenticate
senders.
5.5. Permission Requested Notification
On receiving the CONSENT request (13), B's permission server sends a
NOTIFY request (15) to user B, who had previously subscribed to the
grant-permission event package (1). This NOTIFY request contains,
the permission document, which describes the translation that needs
to be authorized, and a URI where to upload the permission for that
translation. Both the permission document and the URI to upload the
permission are copied from the CONSENT request (13) into the NOTIFY
request (15).
5.6. Permission Upload
On receiving the NOTIFY request (15), user B authorizes the
translation described in the permission document received by
uploading this permission document to the relay. User B uses a
PUBLISH request (17) to upload the permission document to the URI
received in the NOTIFY request.
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When the permission document is uploaded to the URI provided by the
relay (17), the relay needs to make sure that the permission document
received was generated by user B and not by an attacker. The relay
can use three methods to authenticate the permission document: SIP
identity, P-Asserted-Identity [3], or a return routability test.
5.6.1. SIP Identity
The SIP identity [8] mechanism can be used to authenticate the sender
of the PUBLISH request uploading the permission document. The relay
checks that the originator of the PUBLISH request is the owner of the
recipient URI in the permission document. Otherwise, the permission
document is discarded.
5.6.2. P-Asserted-Identity
The P-Asserted-Identity [3] mechanism can be used to authenticate the
sender of the PUBLISH request uploading the permission document.
However, as discussed in RFC 3325 [3], this mechanism should only be
used within networks of trusted SIP servers. That is, the use of
this mechanism is only applicable inside an administrative domain
with previously agreed-upon policies.
The relay checks that the originator of the PUBLISH request is the
owner of the recipient URI in the permission document. Otherwise,
the permission document is discarded.
5.6.3. Return Routability
SIP identity provides a good authentication mechanism for this type
of scenario. Nevertheless, SIP identity is not widely available on
the public Internet yet. That is why an authentication mechanism
that can already be used at this point is needed.
Return routability tests do not provide the same level of security as
SIP identity, but they provide a good-enough security level in
architectures where the SIP identity mechanism is not available
(e.g., the current Internet). The relay generates an unguessable URI
(e.g., with a long and random-looking user part) and places it in the
CONSENT request (13). The recipient needs to upload the permission
document to that URI.
Relays using a return routability test to perform this authentication
need to send the CONSENT request to a SIPS URI. This ensures that
attackers do not get access to the (unguessable) URI. Thus, the only
user able to upload the permission document to the (unguessable) URI
is the receiver of the CONSENT request.
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Relays can transition from return routability tests to SIP identity
by simply requiring the use of SIP identity for incoming PUBLISH
requests. That is, such a relay would reject PUBLISH requests that
did not use SIP identity.
5.7. Permission Granted Notification
On receiving the PUBLISH request (17), the relay sends a NOTIFY
request (19) to inform user A that the permission for the translation
has been received that the translation logic at the relay has been
updated. That is, 'B@example.com' has been added as a recipient URI.
5.8. Permission Revocation
At any time, if a client wants to revoke any permission, it uses the
same URI as before to upload, using a PUBLISH request, a new
permission document that does not authorize the translation at the
relay any longer. If a client loses this URI for some reason, it
needs to wait until it receives a new request product of the
translation. Such a request will contain a Trigger-Consent header
field with a URI. That URI will have an escaped Refer-To header
field identifying the client (i.e., the recipient of the
translation). The client needs to send a REFER request to the URI in
the Trigger-Consent header field in order to receive a CONSENT
request from the relay. Such a CONSENT request will contain the
permission document that was uploaded to the relay at some point and
the URI where the user can upload a new permission document that does
not authorize the translation at the relay any longer.
Figure 6 shows an example of a user revoking permissions at a relay.
The user rejects an incoming INVITE (5) request, which contains a
Trigger-Consent header field. Using the URI in the that header
field, the user sends a REFER request (8) to the relay. On receiving
the REFER request (8), the relay generates a CONSENT request (8)
towards the user. Finally, the user revokes the permissions by
sending a PUBLISH request (14) to the relay.
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Relay B's Permission B@example.com
Server
| |(1) SUBSCRIBE |
| |Event: grant-permission
| |<---------------|
| |(2) 200 OK |
| |--------------->|
| |(3) NOTIFY |
| |--------------->|
| |(4) 200 OK |
| |<---------------|
|(5) INVITE | |
|Trigger-Consent: <123@relay.example.com>
| ?Refer-To=<B%40example.com>
|-------------------------------->|
|(6) 603 Decline | |
|<--------------------------------|
|(7) ACK | |
|-------------------------------->|
|(8) REFER 123@relay.example.com |
|Refer-To: B@example.com |
|<--------------------------------|
|(9) 200 OK | |
|-------------------------------->|
|(10) CONSENT B@example |
|Permission-Upload: uri-up |
|Permission Document |
|--------------->| |
|(11) 202 Accepted |
|<---------------| |
| |(12) NOTIFY |
| |uri-up |
| |Permission Document
| |--------------->|
| |(13) 200 OK |
| |<---------------|
|(14) PUBLISH uri-up |
|Permission Document Revoking Permissions
|<--------------------------------|
|(15) 200 OK | |
|-------------------------------->|
Figure 6: Permission Revocation
5.9. Request-contained URI Lists
In the scenarios described so far, a user adds recipient URIs to the
translation logic of a relay. However, the relay does not perform
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translations towards those URIs until permissions are obtained.
URI-list services using request-contained URI lists are a special
case because the selection of recipient URIs is performed at the same
time as the communication attempt. A user places a set of recipient
URIs in a request and sends it to a relay so that the relay sends a
similar request to all those recipient URIs.
This type of URI-list services maintain a list of recipient URIs from
which permission have been received. This list is manipulated in the
same way as described in Section 5 and represents the set of URIs
that will be accepted if a request containing a URI-list arrives to
the relay. Additionally, Figure 7 shows another way to add entries
to that list.
If the relay receives a request (1) that contains URIs for which the
relay does not have permission, the relay rejects the request with a
470 (Consent Needed) response (2). Such a response contains a
Trigger-Consent header field with a URI for each recipient for which
there is no permission, as shown in Figure 7. Each URI entry in the
Trigger-Consent header field contains an escaped Refer-To header
field with the URI of the recipient. The user needs to send REFER
requests to the URIs in the Trigger-Consent header field.
Additionally, the response also contains a Call-Info header field
with a URI where the user can subscribe in order to be informed on
whether or not the relay receives permission from user B. The value
of the purpose parameter for this entry is 'list-state'.
OPEN ISSUE: do we need to provide that URI in a Call-Info header
field (or in a new header field) or can we assume that the sender has
a relationship with the relay and will know that URI already?
The rest of the process is similar to the one described in Section 5.
Note, however, that for simplicity, Figure 7 does not show the split
between user B's permission server and user agent.
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A@example.com Relay B@example.com
|(1) INVITE | |
|B@example.com | |
|C@example.com | |
|------------------>| |
|(2) 470 Consent Needed |
|Trigger-Consent: <123@relay.example.com>
| ?Refer-To=<B%40example.com>
|Call-Info: 456@Relay;purpose=list-state
|<------------------| |
|(3) ACK | |
|------------------>| |
|(4) SUBSCRIBE 456@Relay |
|Event: list-state |
|------------------>| |
|(5) 200 OK | |
|<------------------| |
|(6) NOTIFY | |
|<------------------| |
|(7) 200 OK | |
|------------------>| |
|(8) REFER 123@Relay| |
|Refer-To: B@example.com |
|------------------>| |
|(9) 200 OK | |
|<------------------| |
| |(10) CONSENT B@example
| |Permission-Upload: uri-up-relay
| |Permission Document|
| |------------------>|
| |(11) 202 Accepted |
| |<------------------|
| |(12) PUBLISH uri-up-relay
| |Permission Document|
| |<------------------|
| |(13) 200 OK |
| |------------------>|
|(14) NOTIFY | |
|<------------------| |
|(15) 200 OK | |
|------------------>| |
Figure 7: INVITE with a URI list in its body
5.10. Registrations
Registrations are a special type of translations. The user
registering has a trust relationship with the registrar in its home
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domain. This is not the case when a user gives any type of
permissions to a relay in a different domain.
Traditionally, REGISTER transactions have performed two operations at
the same time: setting up a translation and authorizing the use of
that translation. For example, a user registering its current
contact URI is giving permission to the registrar to forward traffic
sent to the user's AoR (Address of Records) to the registered contact
URI. This works fine when the entity registering is the same as the
one that will be receiving traffic at a later point (e.g., the entity
receives traffic over the same connection used for the registration
as described in [9]). However, this schema creates some potential
attacks which relate to third-party registrations.
An attacker binds, via a registration, his or her AoR with the
contact URI of a victim. Now, the victim will receive unsolicited
traffic that was originally addressed to the attacker.
The process of authorizing a registration is shown in Figure 8. User
A performs a third-party registration (1) and receives a 200 (OK)
response (2) with a Trigger-Consent header field. This header field
contains the URI for which there is no permission in an escaped
Refer-To header field. That is, the URI the user is attempting to
register. A REFER request sent to the URI in the Trigger-Consent
header field will trigger the registrar to send a CONSENT request to
the URI being registered.
The user sends a REFER request (7) to the URI received in the
Trigger-Consent header field. In order to know whether or not the
registrar receives the permission needed, the user subscribes (3) to
the 'reg-event' event package [6], which can report consent-related
information using the extension defined in [13]. The rest of the
process is similar to the one described in Section 5.
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A@example.com Registrar a@ws123.example.com
|(1) REGISTER | |
|Contact: a@ws123.example.com |
|Supported: consent-reg |
|------------------>| |
|(2) 200 OK | |
|Required: consent-reg |
|Trigger-Consent: <123@relay.example.com>
| ?Refer-To=<a%40ws123.example.com>
|<------------------| |
|(3) SUBSCRIBE | |
|Event: reg-event | |
|------------------>| |
|(4) 200 OK | |
|<------------------| |
|(5) NOTIFY | |
|<------------------| |
|(6) 200 OK | |
|------------------>| |
|(7) REFER 123@Registrar |
|Refer-To: a@ws123.example.com |
|------------------>| |
|(8) 200 OK | |
|<------------------| |
| |(9) CONSENT a@ws123.example
| |Permission-Upload: uri-up
| |Permission Document|
| |------------------>|
| |(10) 202 Accepted |
| |<------------------|
| |(11) PUBLISH uri-up|
| |Permission Document|
| |<------------------|
| |(12) 200 OK |
| |------------------>|
|(13) NOTIFY | |
|<------------------| |
|(14) 200 OK | |
|------------------>| |
Figure 8: Registration
Permission documents used to authorize registrations are very
general. For example, one such document may authorize the registrar
to forward any request from any sender to a particular recipient URI.
This is the type of granularity that this framework intends to
provide for registrations. Users who want to define how incoming
requests are treated with a finer granularity (e.g., requests from
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user A should only be accepted between 9:00 and 11:00) should use
other mechanisms such as CPL [20].
Note that, as indicated previously, user agents using the same
connection to register and to receive traffic from the registrar, as
described in [9] do not need to use the mechanism described in this
section.
A user agent being registered by a third party may not be able to use
the SIP Identity or P-Asserted-Identity mechanisms to prove to the
registrar that the user agent is the owner of the URI being
registered (e.g., sip:user@192.0.2.1), which is the recipient URI of
the translation. In this case, return routability needs to be used.
6. IANA Considerations
This document does not require the IANA to take any actions.
7. Security Considerations
TBD.
Editor's note: we have to avoid that attackers provide permissions
for translations that apply to other users (e.g., allow everyone to
send traffic to a victim) and that attackers provide permissions for
a translation that apply to them but routes to a victim (e.g., 3rd
party registration that binds attacker@relay to victim@somewhere).
For the former we need authentication (e.g., SIP identity) and for
the latter we relay on the routing infrastructure to route CONSENTs
to the same place the traffic will be sent to once permissions are
obtained (i.e., a return routability test).
8. References
8.1. 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] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[3] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
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within Trusted Networks", RFC 3325, November 2002.
[4] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[5] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[6] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[7] Levin, O., "Suppression of Session Initiation Protocol REFER
Method Implicit Subscription",
draft-ietf-sip-refer-with-norefersub-04 (work in progress),
January 2006.
[8] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06 (work in progress), October 2005.
[9] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-01 (work in progress), October 2005.
[10] Camarillo, G., "A Document Format for Expressing Consent",
draft-camarillo-sip-consent-method-00 (work in progress),
February 2006.
[11] Camarillo, G., "A Document Format for Expressing Consent",
draft-camarillo-sipping-consent-format-00 (work in progress),
February 2006.
[12] Camarillo, G., "A Document Format for Expressing Consent",
draft-camarillo-sipping-grant-permission-00 (work in progress),
February 2006.
[13] Camarillo, G., "Session Initiation Protocol (SIP) Registration
Event Package Extension for Consent-Based Communications",
draft-camarillo-sipping-consent-reg-event-00 (work in
progress), February 2006.
[14] Camarillo, G., "The Session Initiation Protocol (SIP) List
State Event Package", draft-camarillo-sipping-list-state-00
(work in progress), February 2006.
[15] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)",
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draft-ietf-simple-xcap-08 (work in progress), October 2005.
[16] Rosenberg, J., "Extensible Markup Language (XML) Formats for
Representing Resource Lists",
draft-ietf-simple-xcap-list-usage-05 (work in progress),
February 2005.
[17] Rosenberg, J., "Requirements for Consent-Based Communications
in the Session Initiation Protocol (SIP)",
draft-ietf-sipping-consent-reqs-04 (work in progress),
January 2006.
[18] Camarillo, G. and A. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) Uniform
Resource Identifier (URI)-List Services",
draft-ietf-sipping-uri-services-05 (work in progress),
January 2006.
8.2. Informative References
[19] Rosenberg, J., "A Watcher Information Event Template-Package
for the Session Initiation Protocol (SIP)", RFC 3857,
August 2004.
[20] Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
Language (CPL): A Language for User Control of Internet
Telephony Services", RFC 3880, October 2004.
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Authors' Addresses
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Gonzalo Camarillo (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Dean Willis
Cisco Systems
2200 E. Pres. George Bush Turnpike
Richardson, TX 75082
USA
Email: dean.willis@softarmor.com
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