SIPPING J. Rosenberg
Internet-Draft Cisco Systems
Expires: December 14, 2006 G. Camarillo, Ed.
Ericsson
D. Willis
Cisco Systems
June 12, 2006
A Framework for Consent-Based Communications in the Session Initiation
Protocol (SIP)
draft-ietf-sipping-consent-framework-05.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 and Terminology . . . . . . . . . . . . . . . . . 3
3. Relays and Translations . . . . . . . . . . . . . . . . . . . 4
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. Framework Operations . . . . . . . . . . . . . . . . . . . . . 8
5.1. Amplification Avoidance . . . . . . . . . . . . . . . . . 9
5.2. Subscription to the Permission Status . . . . . . . . . . 10
5.3. Request for Permission . . . . . . . . . . . . . . . . . . 10
5.4. Permission Document Structure . . . . . . . . . . . . . . 11
5.5. Permission Requested Notification . . . . . . . . . . . . 12
5.6. Permission Grant . . . . . . . . . . . . . . . . . . . . . 13
5.6.1. SIP Identity . . . . . . . . . . . . . . . . . . . . . 13
5.6.2. P-Asserted-Identity . . . . . . . . . . . . . . . . . 13
5.6.3. Return Routability . . . . . . . . . . . . . . . . . . 14
5.7. Permission Granted Notification . . . . . . . . . . . . . 14
5.8. Permission Revocation . . . . . . . . . . . . . . . . . . 15
5.9. Request-contained URI Lists . . . . . . . . . . . . . . . 16
5.10. Registrations . . . . . . . . . . . . . . . . . . . . . . 17
5.11. Relays Generating Traffic towards Recipients . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
The Session Initiation Protocol (SIP) [3] 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 [5]) 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 its recipients 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 [13].
This specification defines a basic framework for adding consent-based
communication to SIP.
2. Definitions and Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
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.
Any SIP server, be it a proxy, B2BUA (Back-to-Back User Agent), or
some hybrid, that receives a request, translates its Request-URI
into one or more next-hop URIs (i.e., recipient URIs), and
delivers the request to those URIs.
Target URI: The Request-URI of an incoming request that arrives to a
relay that will perform a translation operation.
Translation operation: Operation by which a relay 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.
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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, translates its Request-URI into one
or more next hop URIs, and delivers the request to those URIs. The
Request-URI of the incoming request is referred to as 'target URI'
and the destination URIs of the outgoing requests are referred to as
'recipient URIs', as shown in Figure 1.
+---------------+
| | 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. That is,
the relay 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 in the figure sends a SIP request to a URI representing a
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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 [14].
+-------+
| |
>| UA |
/ | |
/ +-------+
/
/
+-----------------------+ /
| | /
+-----+ | Relay | / +-------+
| | | |/ | |
| UA |------>| |-------->| 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 the relay performing the
translation permission 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
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how potential recipients can grant a relay permissions to add them to
the relay's translation logic.
+-----------------------+ 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 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 the relay permission 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 or until they are revoked. 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 or until the permissions are revoked.
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 the relay
permission 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 obtains permission from the new recipient in
order to install the new translation logic. Relays ask recipients
for permission using MESSAGE [5] requests.
For example, the relay hosting the URI-list service at
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'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 MESSAGE request to 'bob@example.org' asking whether or
not it is OK to perform the translation from 'friends@example.com' to
'bob@example.org'. The MESSAGE request carries in its message body a
permission document that describes the translation for which
permissions are being requested and a human readable part that also
describes the translation. If the answer is positive, the new
translation logic is installed at the relay. That is, the new
recipient URI is added.
The human-readable part is included so that user agents that do
not understand permission documents can still process the request
and display it in a sensible way to the user.
Note that the mechanism to be used to manipulate the translation
logic of a particular relay depends on the relay. Two existing
mechanisms to manipulate translation logic are XCAP [11] and REGISTER
transactions.
In any case, relays implementing this framework SHOULD have a means
to indicate that a particular recipient URI is in the states
specified in [10] (i.e., pending, waiting, error, denied, or
granted).
4.3. Permission Servers
When a MESSAGE request with a permission document arrives to the
recipient URI to which it was sent by the relay, 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 MESSAGE 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
MESSAGE requests for a user.
So, a permission server stores incoming MESSAGE requests when the
user is unavailable and delivers them when the user is available
again. Conceptually, a permission server provides a store-and-
forward message service.
There are several mechanisms to implement store-and-forward message
services (e.g., with an instant message to email gateway). Any of
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these mechanisms can be used between a user agent and its permission
server as long as they agree on which mechanism to use. Therefore,
this framework does not make any recommendation on the interface
between user agents and their permission servers.
Note that the same store-and-forward message service can handle
all incoming MESSAGE requests for a user while this is off line,
not only those MESSAGE requests with a permission document in
their bodies.
4.4. Recipients Grant Permissions
Relays include in the permission documents they generate URIs that
can be used by the recipient of the document to grant or deny the
relay the permission described in the document. Relays always
include SIP URIs and may include HTTP [2] URIs for this purpose.
Consequently, recipients provide relays with permissions using SIP
PUBLISH requests or HTTP GET requests.
5. Framework Operations
This section specifies this consent 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 4 shows the complete process to add a recipient URI
('B@example.com') to the translation logic of a relay. User A
attempts to add 'B@example.com' as a new recipient URI to the
translation logic of the relay (1). User A uses XCAP [11] and the
XML (Extensible Markup Language) format for representing resource
lists [12] 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, as
specified in [10].
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A@example.com Relay B's Permission B@example.com
Server
|(1) Add Recipient B@example.com | |
|--------------->| | |
|(2) HTTP 202 (Accepted) | |
|<---------------| | |
| |(3) MESSAGE B@example |
| |Permission Document |
| |--------------->| |
| |(4) 202 Accepted| |
| |<---------------| |
|(5) SUBSCRIBE | | |
|Event: pending-additions | |
|--------------->| | |
|(6) 200 OK | | |
|<---------------| | |
|(7) NOTIFY | | |
|<---------------| | |
|(8) 200 OK | | |
|--------------->| | |
| | | |User B goes
| | | | on line
| | |(9) Request for |
| | | stored messages
| | |<---------------|
| | |(10) Delivery of|
| | | stored messages
| | |--------------->|
| |(11) PUBLISH uri-up |
| |Permission Document |
| |<--------------------------------|
| |(12) 200 OK | |
| |-------------------------------->|
|(13) NOTIFY | | |
|<---------------| | |
|(14) 200 OK | | |
|--------------->| | |
Figure 4: Prototypical call flow
5.1. Amplification Avoidance
Once 'B@example.com' is in the pending state, the relay needs to ask
user B for permission by sending a MESSAGE 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
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URIs to the translation logic of a relay. The relay would then send
a MESSAGE 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
MESSAGE requests towards those recipient URIs. When XCAP is used,
this requirement is met by not allowing clients to add more than one
URI per HTTP transaction.
Therefore, relays implementing this framework MUST NOT allow clients
to add more than one URI per HTTP transaction. If a client attempts
to add more than one URI in a single HTTP transaction, the XCAP
server SHOULD return an HTTP 403 (Forbidden) response. The XCAP
server SHOULD describe the reason for the refusal (i.e., only one URI
can be added at a time) in the entity, as described in [2].
5.2. Subscription to the Permission Status
Clients can use the Pending Additions SIP event package [10] to be
informed about the status of the operations they requested. That is,
the client will be informed when an operation (e.g., the addition of
a URI to the translation logic of a relay) is authorized (and thus
executed) or rejected.
OPEN ISSUE: how do clients obtain the URI to subscribe to the Pending
Additions event package, both when using XCAP and when using
REGISTERs to manipulate the translation?
In our example, after receiving the response from the server (2),
user A subscribes to the Pending Additions event package at the relay
(5). This subscription keeps user A informed about the status of the
permissions (e.g., granted or denied) the relay will obtain.
5.3. Request for Permission
Relays MUST obtain permissions from potential recipients before
adding them to their translation logics. Relays request permissions
from potential recipients using MESSAGE requests.
MESSAGE requests sent to request permissions MUST include a
permission document and SHOULD include a human-readable part in their
bodies. MESSAGE requests also carry a body part that contains the
same information as the permission document but in a human-readable
format so that user agents that do not understand permission
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documents can still process the request and display it in a sensible
way to the user.
Section 5.6 describes three methods a relay can use to authenticate
recipients giving the relay permission to perform a particular
translation. Relays that use the method consisting of a return
routability test have to send their MESSAGE requests to a SIPS URI,
as specified in Section 5.6.
In our example, on receiving the request to add User B to the
translation logic of the relay (1), the relay generates a MESSAGE
request (3) towards 'B@example.com'. This MESSAGE request carries a
permission document, which describes the translation that needs to be
authorized and carries a set of URIs to be used by the recipient to
grant or to deny the relay permission to perform that translation.
User B will authorize the translation by using one of those URIs, as
described in Section 5.6. The MESSAGE request also carry a body part
that contains the same information as the permission document but in
a human-readable format.
When User B uses one of the URIs in the permission document to grant
or deny permissions, the relay needs to make sure that it was
actually User B the one using that URI, and not an attacker. The
relay can use three methods to authenticate the permission document:
SIP identity [7], P-Asserted-Identity [4], or a return routability
test. These methods are described in Section 5.6.
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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URIs to Grant Permission: URIs that recipients can use to grant the
relay permission to perform the translation described in the
document. At least one of these URIs MUST be a SIP or SIPS URI.
HTTP and HTTPS URIs MAY also be used.
URIs to Deny Permission: URIs that recipients can use to deny the
relay permission to perform the translation described in the
document. At least one of these URIs MUST be a SIP or SIPS URI.
HTTP and HTTPS URIs MAY also be used.
Permission documents may contain wildcards. For example, a
permission document may request permission for 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.
Entities implementing this framework MUST support the format for
permission documents defined in [9].
In our example, the permission document in the MESSAGE request (3)
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
URI to Grant Permission: sips:grant-1awdch5Fasddfce34@example.com
URI to Grant Permission: https://example.com/grant-1awdch5Fasddfce34
URI to Deny Permission: sips:deny-23rCsdfgvdT5sdfgye@example.com
URI to Deny Permission: https://example.com/deny-23rCsdfgvdT5sdfgye
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, relays MUST authenticate senders.
5.5. Permission Requested Notification
On receiving the MESSAGE request (3), User B's permission server
stores it because User B is off line at that point. When User B goes
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on line, User B fetches all the requests its permission server has
stored (9).
5.6. Permission Grant
A client gives a relay permission to execute the translation
described in a permission document by sending a SIP PUBLISH or an
HTTP GET request to one of the URIs to grant permissions contained in
the document. Similarly, a client denies a relay permission to
execute the translation described in a permission document by sending
a SIP PUBLISH or an HTTP GET request to one of the URIs to deny
permissions contained in the document.
In our example, User B obtains the permission document (10) that was
received earlier by its permission server in the MESSAGE request (3).
User B authorizes the translation described in the permission
document received by sending a PUBLISH request (11) to the SIP URI to
grant permissions contained in the permission document.
Relays need to ensure that the SIP PUBLISH or the HTTP GET request
received was generated by the recipient of the translation and not by
an attacker. Relays can use three methods to authenticate those
requests: SIP identity, P-Asserted-Identity [4], or a return
routability test. While return routability tests can be used to
authenticate both SIP PUBLISH and HTTP GET requests, SIP identity and
P-Asserted-Identity can only be used to authenticate SIP PUBLISH
requests.
5.6.1. SIP Identity
The SIP identity [7] mechanism can be used to authenticate the sender
of a PUBLISH request. The relay MUST check that the originator of
the PUBLISH request is the owner of the recipient URI in the
permission document. Otherwise, the PUBLISH request SHOULD be
responded with a 401 (Unauthorized) response and MUST NOT be
processed further.
5.6.2. P-Asserted-Identity
The P-Asserted-Identity [4] mechanism can also be used to
authenticate the sender of a PUBLISH request. However, as discussed
in RFC 3325 [4], 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 MUST check that the originator of the PUBLISH request is
the owner of the recipient URI in the permission document.
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Otherwise, the PUBLISH request SHOULD be responded with a 401
(Unauthorized) response and MUST NOT be processed further.
5.6.3. Return Routability
SIP identity provides a good authentication mechanism for incoming
PUBLISH requests. 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
(i.e., with a long and random-looking user part) and places it in the
permission document in the MESSAGE request (3). The recipient needs
to send a SIP PUBLISH request or an HTTP GET request to that URI.
Any incoming request sent to that URI SHOULD be considered
authenticated by the relay.
Note that the return routability method is the only one that
allows the use of HTTP URIs in permission documents. The other
methods require the use of SIP URIs.
Relays using a return routability test to perform this authentication
MUST send the MESSAGE request with the permission document to a SIPS
URI. This ensures that attackers do not get access to the
(unguessable) URI. Thus, the only user able to use the (unguessable)
URI is the receiver of the MESSAGE request. Similarly, permission
documents sent by relays using a return routability test MUST only
contain secure URIs (i.e., SIPS and HTTPS) to grant and deny
permissions. The user part of these URIs MUST be cryptographically
random with at least 32 bits of randomness.
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 (11), the relay sends a NOTIFY
request (13) to inform user A that the permission for the translation
has been received and that the translation logic at the relay has
been updated. That is, 'B@example.com' has been added as a recipient
URI.
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5.8. Permission Revocation
At any time, if a client wants to revoke any permission, it uses the
URI it received in the permission document to deny the permissions it
previously granted. If a client loses this URI for some reason, it
needs to wait until it receives a new request produced by 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 MESSAGE
request from the relay. Such a MESSAGE request will contain a
permission document with a URI to revoke the permission that was
previously granted.
Figure 5 shows an example of how a user that lost the URI to revoke
permissions at a relay can obtain a new URI using the Trigger-Consent
header field of an incoming request. The user rejects an incoming
INVITE (1) request, which contains a Trigger-Consent header field.
Using the URI in the that header field, the user sends a REFER
request (4) to the relay. On receiving the REFER request (4), the
relay generates a MESSAGE request (6) towards the user. Finally, the
user revokes the permissions by sending a PUBLISH request (8) to the
relay.
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Relay B@example.com
|(1) INVITE |
| Trigger-Consent: <123@relay.example.com>
| ?Refer-To=<B%40example.com>
|---------------------------->|
|(2) 603 Decline |
|<----------------------------|
|(3) ACK |
|---------------------------->|
|(4) REFER 123@relay.example.com
| Refer-To: B@example.com |
|<----------------------------|
|(5) 200 OK |
|---------------------------->|
|(6) MESSAGE B@example |
| Permission Document |
|---------------------------->|
|(7) 200 OK |
|<----------------------------|
|(8) PUBLISH uri-deny |
|<----------------------------|
|(9) 200 OK |
|---------------------------->|
Figure 5: 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
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.
Relays implementing this framework and providing this type of URI-
list services MUST 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.
A relay that receives a request-contained URI-list with a URI for
which the relay has no permissions SHOULD return a 470 (Consent
Needed) response. The relay SHOULD add a Permission-Missing header
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field with the URIs for which the relay has no permissions.
The following is the augmented Backus-Naur Form (BNF) [6] syntax of
the Permission-Missing header field. Some of its elements are
defined in RFC 3261 [3].
Permission-Missing = "Permission-Missing" HCOLON per-miss-spec
*( COMMA per-miss-spec )
per-miss-spec = ( name-addr / addr-spec )
*( SEMI generic-param )
The following is an example of a Permission-Missing header field:
Permission-Missing:<sip:C@example.com>
Figure 6 shows a relay that 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). That response
contains a Permission-Missing header field with the URIs for which
there was no permission.
A@example.com Relay
|(1) INVITE |
| B@example.com |
| C@example.com |
|---------------------->|
|(2) 470 Consent Needed |
| Permission-Missing: C@example.com
|<----------------------|
|(3) ACK |
|---------------------->|
Figure 6: 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
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
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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 [8]). 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 7. User
A performs a third-party registration (1) and receives a 202
(Accepted) response (2).
Since the relay does not have permission from 'a@ws123.example.com'
to perform translations towards that URI, the relay places
'a@ws123.example.com' in the 'pending' state. Once
'a@ws123.example.com' is in the 'Permission Pending' state, the
registrar needs to ask 'a@ws123.example.com' for permission by
sending a MESSAGE request (3).
After receiving the response from the server (2), user A subscribes
to the Pending Additions event package at the registrar (4). This
subscription keeps the user informed about the status of the
permissions (e.g., granted or denied) the registrar will obtain. 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 |
|------------------>| |
|(2) 202 Accepted OK| |
|<------------------| |
| |(3) MESSAGE a@ws123.example
| |Permission Document|
| |------------------>|
| |(4) 200 OK |
| |<------------------|
|(5) SUBSCRIBE | |
|Event: pending-additions |
|------------------>| |
|(6) 200 OK | |
|<------------------| |
|(7) NOTIFY | |
|<------------------| |
|(8) 200 OK | |
|------------------>| |
| |(9) PUBLISH uri-up |
| |<------------------|
| |(10) 200 OK |
| |------------------>|
|(11) NOTIFY | |
|<------------------| |
|(12) 200 OK | |
|------------------>| |
Figure 7: Registration
Permission documents generated by registrars are typically very
general. For example, in one such document a registrar may ask a
recipient for permission to forward any request from any sender to
the recipient's 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 user A should only be accepted between 9:00 and
11:00) should use other mechanisms such as CPL [15].
Note that, as indicated previously, user agents using the same
connection to register and to receive traffic from the registrar, as
described in [8] 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
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registered (e.g., sip:user@192.0.2.1), which is the recipient URI of
the translation. In this case, return routability MUST be used.
5.11. Relays Generating Traffic towards Recipients
A relay executing a translation that involves sending a request to a
URI from which permissions were obtained previously SHOULD add a
Trigger-Consent header field to the request. The URI in the Trigger-
Consent header field MUST have an escaped Refer-To header field
identifying the recipient of the translation so that a REFER request
sent to that URI will cause a MESSAGE request to be sent to the
recipient.
On receiving a REFER request addressed to the URI a relay placed in a
Trigger-Consent header field, the relay SHOULD send a MESSAGE request
to the URI in the Refer-To header field with a permission document.
The following is the augmented Backus-Naur Form (BNF) [6] syntax of
the Trigger-Consent header field. Some of its elements are defined
in RFC 3261 [3].
Trigger-Consent = "Trigger-Consent" HCOLON trigger-cons-spec
*( COMMA trigger-cons-spec )
trigger-cons-spec = ( name-addr / addr-spec )
*( SEMI generic-param )
The following is an example of a Trigger-Consent header field:
Trigger-Consent:<sip:relay@example.com
?Refer-To=<sip:recipient%40example.net>>
6. IANA Considerations
The IANA is requested to add the following new response code to the
Methods and Response Codes subregistry under the SIP Parameters
registry.
Response Code Number: 470
Default Reason Phrase: Consent Needed
Reference: [RFCxxxx]
Note to the RFC editor: substitute xxxx with the RFC number of this
document.
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The IANA is requested to add the following new SIP header field to
the Header Fields subregistry under the SIP Parameters registry.
Header Name: Trigger-Consent
Compact Form: (none)
Reference: [RFCxxxx]
Note to the RFC editor: substitute xxxx with the RFC number of this
document.
The IANA is requested to add the following new SIP header field to
the Header Fields subregistry under the SIP Parameters registry.
Header Name: Permission-Missing
Compact Form: (none)
Reference: [RFCxxxx]
Note to the RFC editor: substitute xxxx with the RFC number of this
document.
7. Security Considerations
Security has been discussed throughout the whole document. However,
there are some issues that deserve special attention.
The specifications of mechanisms to manipulate translation logic at
relays usually stress the importance of client authentication and
authorization. Having relays authenticate and authorize clients
manipulating their translation logic keeps unauthorized clients from
adding recipients to a translation. However, this does not prevent
authorized clients to add recipients to a translation without their
consent. Additionally, some relays provide web interfaces for any
client to add new recipients to the translation (e.g., many email
mailing lists are operated in this way). In this situation, every
client is considered authorized to manipulate the translation logic
at the relay. This makes the use of this framework even more
important. Therefore, it is RECOMMENDED that relays performing
translations implement this framework.
As pointed out in Section 5.6.3, when return routability tests are
used to authenticate recipients granting or denying permissions, the
URIs used to grant or deny permissions need to be protected from
attackers. SIPS URIs provide a good tool to meet this requirement,
as described in [9].
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The information provided by the Pending Additions event package can
be sensitive. For this reason, as described in [10], relays need to
use strong means for authentication and information confidentiality.
SIPS URIs are a good mechanism to meet this requirement.
8. Acknowledges
Henning Schulzrinne, Jon Peterson, and Cullen Jennings provided
useful ideas on this document.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[3] 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.
[4] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002.
[5] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[7] 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.
[8] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-03 (work in progress), March 2006.
[9] Camarillo, G., "A Document Format for Requesting Consent",
draft-camarillo-sipping-consent-format-01 (work in progress),
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June 2006.
[10] Camarillo, G., "The Session Initiation Protocol (SIP) Pending
Additions Event Package",
draft-camarillo-sipping-pending-additions-00 (work in
progress), June 2006.
[11] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)",
draft-ietf-simple-xcap-11 (work in progress), May 2006.
[12] Rosenberg, J., "Extensible Markup Language (XML) Formats for
Representing Resource Lists",
draft-ietf-simple-xcap-list-usage-05 (work in progress),
February 2005.
[13] Rosenberg, J., "Requirements for Consent-Based Communications
in the Session Initiation Protocol (SIP)",
draft-ietf-sipping-consent-reqs-04 (work in progress),
January 2006.
[14] 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.
9.2. Informative References
[15] 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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