Network Working Group J. Rosenberg

Network Working Group J. Rosenberg
Request for Comments: 5360 Cisco Systems
Category: Standards Track G. Camarillo, Ed.
Ericsson
D. Willis
Unaffiliated
October 2008

A Framework for Consent-Based Communications
in the Session Initiation Protocol (SIP)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

SIP supports communications for several services, 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 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.

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RFC 5360 Consent Framework October 2008

Table of Contents

1. Introduction ....................................................3
2. Definitions and Terminology .....................................3
3. Relays and Translations .........................................4
4. Architecture ....................................................6
4.1. Permissions at a Relay .....................................6
4.2. Consenting Manipulations on a Relay's Translation Logic ....7
4.3. Store-and-Forward Servers ..................................8
4.4. Recipients Grant Permissions ...............................9
4.5. Entities Implementing This Framework .......................9
5. Framework Operations ............................................9
5.1. Amplification Avoidance ...................................11
5.1.1. Relay's Behavior ...................................12
5.2. Subscription to the Permission Status .....................12
5.2.1. Relay's Behavior ...................................13
5.3. Request for Permission ....................................13
5.3.1. Relay's Behavior ...................................13
5.4. Permission Document Structure .............................15
5.5. Permission Requested Notification .........................16
5.6. Permission Grant ..........................................17
5.6.1. Relay's Behavior ...................................17
5.6.1.1. SIP Identity ..............................17
5.6.1.2. P-Asserted-Identity .......................17
5.6.1.3. Return Routability ........................18
5.6.1.4. SIP Digest ................................19
5.7. Permission Granted Notification ...........................19
5.8. Permission Revocation .....................................19
5.9. Request-Contained URI Lists ...............................20
5.9.1. Relay's Behavior ...................................21
5.9.2. Definition of the 470 Response Code ................21
5.9.3. Definition of the Permission-Missing Header Field ..22
5.10. Registrations ............................................22
5.11. Relays Generating Traffic towards Recipients .............25
5.11.1. Relay's Behavior ..................................25
5.11.2. Definition of the Trigger-Consent Header Field ....25
6. IANA Considerations ............................................26
6.1. Registration of the 470 Response Code .....................26
6.2. Registration of the Trigger-Consent Header Field ..........26
6.3. Registration of the Permission-Missing Header Field .......26
6.4. Registration of the target-uri Header Field Parameter .....26
7. Security Considerations ........................................27
8. Acknowledgments ................................................28
9. References .....................................................28
9.1. Normative References ......................................28
9.2. Informative References ....................................29

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1. Introduction

The Session Initiation Protocol (SIP) [RFC3261] supports
communications for several services, 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 [RFC3428]) 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
network of SIP proxy servers 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. These include amplification and DoS (Denial of Service)
attacks. These problems are described in more detail in a companion
requirements document [RFC4453].

This specification defines a basic framework for adding consent-based
communication to SIP.

2. Definitions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

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 can have been the result of a translation operation.

Relay: 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 logic: The logic that defines a translation operation at
a relay. This logic includes the translation's target and
recipient URIs.

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) that 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, that 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 the Request-URI (target URI)
into one or more additional URIs (recipient URIs). Through this
translation operation, the relay can create outgoing requests to one
or more additional recipient URIs, thus creating the consent problem.

The consent problem is created by two types of translations:
translations based on local data and translations that involve
amplifications.

Translation operations based on local policy or local data (such as
registrations) are 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,
'sip:[email protected]', it cannot deliver a MESSAGE request to the UA
(user agent) of that user without having access to the registration
data that maps 'sip:[email protected]' 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, that is expected to 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.

Translation operations that result in more than one recipient URI are
a source of amplification. Servers that do not perform translations,
such as outbound proxy servers, do not cause amplification. On the
other hand, servers that perform translations (e.g., inbound proxies

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authoritatively responsible for a SIP domain) may cause amplification
if the user can be reached at multiple endpoints (thereby resulting
in multiple recipient URIs).

Figure 2 shows a relay that performs translations. The user agent
client in the figure sends a SIP request to a URI representing a
resource in the domain 'example.com' (sip:[email protected]).
This request can 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 can (but need not) belong to the domain
'example.com'. This relay can be, for instance, a proxy server or a
URI-list service [RFC5363].

+-------+
| |
>| 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.

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4. Architecture

Figure 3 shows the architectural elements of this framework. The
manipulation of a relay's translation logic typically causes the
relay to send a permission request, which in turn causes the
recipient to grant or deny the relay permissions for the translation.
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 discusses store-and-forward
servers and their functionality. Section 4.4 describes how potential
recipients can grant a relay permissions to add them to the relay's
translation logic. Section 4.5 discusses which entities need to
implement this framework.

+-----------------------+ Permission +-------------+
| | Request | |
+--------+ | Relay |----------->| Store & Fwd |
| | | | | 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 logic. These permissions indicate
whether or not a particular recipient has agreed to receive traffic
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.

In principle, 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.

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Additionally, if a recipient is removed from a relay's translation
logic, the relay SHOULD delete the permissions related to that
recipient. For example, if the registration of a contact URI expires
or is otherwise terminated, the registrar deletes the permissions
related to that contact address.

It is also RECOMMENDED that relays request recipients to refresh
their permissions periodically. If a recipient fails to refresh its
permissions for a given period of time, the relay SHOULD delete the
permissions related to that recipient.

This framework does not provide any guidance for the values of the
refreshment intervals because different applications can have
different requirements to set those values. For example, a relay
dealing with recipients that do not implement this framework may
choose to use longer intervals between refreshes. The refresh
process in such recipients has to be performed manually by their
users (since the recipients do not implement this framework), and
having too short refresh intervals may become too heavy a burden
for those users.

4.2. Consenting Manipulations on a Relay's Translation 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 [RFC3428] requests.

For example, the relay hosting the URI-list service at
'sip:[email protected]' performs a translation from that target URI
to a set of recipient URIs. When a client (e.g., the administrator
of that URI-list service) adds '[email protected]' as a new recipient
URI, the relay sends a MESSAGE request to 'sip:[email protected]'
asking whether or not it is OK to perform the translation from
'sip:[email protected]' to 'sip:[email protected]'. 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.

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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 XML Configuration Access Protocol
(XCAP) [RFC4825] and REGISTER transactions.

Section 5 uses a URI-list service whose translation logic is
manipulated with XCAP as an example of a translation, in order to
specify this framework. Section 5.10 discusses how to apply this
framework to registrations, which are a different type of
translation.

In any case, relays implementing this framework SHOULD have a means
to indicate that a particular recipient URI is in the states
specified in [RFC5362] (i.e., pending, waiting, error, denied, or
granted).

4.3. Store-and-Forward 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.
However, the receiving user may not be available when the MESSAGE
request arrives, or it may have expressed preferences to block all
incoming requests for a certain time period. In such cases, a
store-and-forward server can act as a substitute for the user and
buffer the incoming MESSAGE requests, which are subsequently
delivered to the user when he or she is available again.

There are several mechanisms to implement store-and-forward message
services (e.g., with an instant message to email gateway). Any of
these mechanisms can be used between a user agent and its store-and-
forward server as long as they agree on which mechanism to use.
Therefore, this framework does not make any provision for the
interface between user agents and their store-and-forward servers.

Note that the same store-and-forward message service can handle
all incoming MESSAGE requests for a user while they are offline,
not only those MESSAGE requests with a permission document in
their bodies.

Even though store-and-forward servers perform a useful function and
they are expected to be deployed in most domains, some domains will
not deploy them from the outset. However, user agents and relays in
domains without store-and-forward servers can still use this consent
framework.

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When a relay requests permissions from an offline user agent that
does not have an associated store-and-forward server, the relay will
obtain an error response indicating that its MESSAGE request could
not be delivered. The client that attempted to add the offline user
to the relay's translation logic will be notified about the error
(e.g., using the Pending Additions event package [RFC5362]). This
client MAY attempt to add the same user at a later point, hopefully
when the user is online. Clients can discover whether or not a user
is online by using a presence service, for instance.

4.4. Recipients Grant Permissions

Permission documents generated by a relay include 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 can include HTTP [RFC2616] URIs for this purpose. Consequently,
recipients provide relays with permissions using SIP PUBLISH requests
or HTTP GET requests.

4.5. Entities Implementing This Framework

The goal of this framework is to keep relays from executing
translations towards unwilling recipients. Therefore, all relays
MUST implement this framework in order to avoid being used to perform
attacks (e.g., amplification attacks).

This framework has been designed with backwards compatibility in mind
so that legacy user agents (i.e., user agents that do not implement
this framework) can act both as clients and recipients with an
acceptable level of functionality. However, it is RECOMMENDED that
user agents implement this framework, which includes supporting the
Pending Additions event package specified in [RFC5362], the format
for permission documents specified in [RFC5361], and the header
fields and response code specified in this document, in order to
achieve full functionality.

The only requirement that this framework places on store-and-forward
servers is that they need to be able to deliver encrypted and
integrity-protected messages to their user agents, as discussed in
Section 7. However, this is not a requirement specific to this
framework but a general requirement for store-and-forward servers.

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 store-and-forward servers) play an
essential role in this call flow.

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Figure 4 shows the complete process to add a recipient URI
('sip:[email protected]') to the translation logic of a relay. User A
attempts to add 'sip:[email protected]' as a new recipient URI to the
translation logic of the relay (1). User A uses XCAP [RFC4825] and
the XML (Extensible Markup Language) format for representing resource
lists [RFC4826] to perform this addition. Since the relay does not
have permission from 'sip:[email protected]' to perform translations
towards that URI, the relay places 'sip:[email protected]' in the pending
state, as specified in [RFC5362].

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[email protected] Relay B's Store & Fwd [email protected]
Server

|(1) Add Recipient | |
| sip:[email protected] | |
|--------------->| | |
|(2) HTTP 202 (Accepted) | |
|<---------------| | |
| |(3) MESSAGE sip:B@example |
| | Permission Document |
| |--------------->| |
| |(4) 202 Accepted| |
| |<---------------| |
|(5) SUBSCRIBE | | |
| Event: pending-additions | |
|--------------->| | |
|(6) 200 OK | | |
|<---------------| | |
|(7) NOTIFY | | |
|<---------------| | |
|(8) 200 OK | | |
|--------------->| | |
| | | |User B goes
| | | | online
| | |(9) Request for |
| | | stored messages
| | |<---------------|
| | |(10) Delivery of|
| | | stored messages
| | |--------------->|
| |(11) PUBLISH uri-up |
| |<--------------------------------|
| |(12) 200 OK | |
| |-------------------------------->|
|(13) NOTIFY | | |
|<---------------| | |
|(14) 200 OK | | |
|--------------->| | |

Figure 4: Prototypical Call Flow

5.1. Amplification Avoidance

Once 'sip:[email protected]' is in the pending state, the relay needs to
ask user B for permission by sending a MESSAGE request to
'sip:[email protected]'. However, the relay needs to ensure that it is
not used as an amplifier to launch amplification attacks.

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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 MESSAGE request to each of those recipient URIs. The bandwidth
generated by the relay would be much higher than the bandwidth used
by the attacker to add those recipient 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. When a REGISTER transaction is used, this
requirement is met by not allowing clients to register more than one
contact per REGISTER transaction.

5.1.1. Relay's Behavior

Relays implementing this framework MUST NOT allow clients to add more
than one recipient URI per transaction. If a client using XCAP
attempts to add more than one recipient URI in a single HTTP
transaction, the XCAP server SHOULD return an HTTP 409 (Conflict)
response. The XCAP server SHOULD describe the reason for the refusal
in an XML body using the <constraint-failure> element, as described
in [RFC4825]. If a client attempts to register more than one contact
in a single REGISTER transaction, the registrar SHOULD return a SIP
403 response and explain the reason for the refusal in its reason
phrase (e.g., maximum one contact per registration).

5.2. Subscription to the Permission Status

Clients need a way to be informed about the status of the operations
they requested. Otherwise, users can be waiting for an operation to
succeed when it has actually already failed. In particular, if the
target of the request for consent was not reachable and did not have
an associated store-and-forward server, the client needs to know to
retry the request later. The Pending Additions SIP event package
[RFC5362] is a way to provide clients with that information.

Clients can use the Pending Additions SIP event package 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 recipient URI to a relay's translation logic) is authorized (and
thus executed) or rejected. Clients use the target URI of the SIP
translation being manipulated to subscribe to the 'pending-additions'
event package.

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In our example, after receiving the response from the relay (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.

5.2.1. Relay's Behavior

Relays SHOULD support the Pending Additions SIP event package
specified in [RFC5362].

5.3. Request for Permission

A relay requests permissions from potential recipients to add them to
its translation logic using MESSAGE requests. 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
'sip:[email protected]'. 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. Since user B
is offline, the MESSAGE request will be buffered by user B's store-
and-forward server. User B will later go online and authorize the
translation by using one of those URIs, as described in Section 5.6.
The MESSAGE request also carries 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 using that URI, and not an attacker. The relay can
use any of the methods described in Section 5.6 to authenticate the
permission document.

5.3.1. Relay's Behavior

Relays that implement this framework MUST obtain permissions from
potential recipients before adding them to their translation logic.
Relays request permissions from potential recipients using MESSAGE
requests.

Section 5.6 describes the methods a relay can use to authenticate
those recipients giving the relay permission to perform a particular
translation. These methods are SIP identity [RFC4474],
P-Asserted-Identity [RFC3325], a return routability test, or SIP
digest. 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.

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MESSAGE requests sent to request permissions MUST include a
permission document and SHOULD include a human-readable part in their
bodies. The human-readable part contains the same information as the
permission document (but in a human-readable format), including the
URIs to grant and deny permissions. User agents that do not
understand permission documents can still process the request and
display it in a sensible way to the user, as they would display any
other instant message. This way, even if the user agent does not
implement this framework, the (human) user will be able to manually
click on the correct URI in order to grant or deny permissions. The
following is an example of a MESSAGE request that carries a human-
readable part and a permission document, which follows the format
specified in [RFC5361], in its body. Not all header fields are shown
for simplicity reasons.

MESSAGE sip:[email protected] SIP/2.0
From: <sip:[email protected]>;tag=12345678
To: <sip:[email protected]>
Content-Type: multipart/mixed;boundary="boundary1"

--boundary1
Content-Type: text/plain

If you consent to receive traffic sent to
<sip:[email protected]>, please use one of the following
URIs: <sips:[email protected]> or
<https://example.com/grant-1awdch5Fasddfce34>. Otherwise, use one of
the following URIs: <sips:[email protected]> or
<https://example.com/deny-23rCsdfgvdT5sdfgye>.
--boundary1
Content-Type: application/auth-policy+xml

<?xml version="1.0" encoding="UTF-8"?>
<cp:ruleset
xmlns="urn:ietf:params:xml:ns:consent-rules"
xmlns:cp="urn:ietf:params:xml:ns:common-policy"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<cp:rule id="f1">
<cp:conditions>
<cp:identity>
<cp:many/>
</cp:identity>
<recipient>
<cp:one id="sip:[email protected]"/>
</recipient>
<target>
<cp:one id="sip:[email protected]"/>
</target>

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</cp:conditions>
<cp:actions>
<trans-handling
perm-uri="sips:[email protected]">
grant</trans-handling>
<trans-handling
perm-uri="https://example.com/grant-1awdch5Fasddfce34">
grant</trans-handling>
<trans-handling
perm-uri="sips:[email protected]">
deny</trans-handling>
<trans-handling
perm-uri="https://example.com/deny-23rCsdfgvdT5sdfgye">
deny</trans-handling>
</cp:actions>
<cp:transformations/>
</cp:rule>
</cp:ruleset>
--boundary1--

5.4. Permission Document Structure

A permission document is the representation (e.g., encoded in XML) 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.

URIs to Grant Permission: URIs that recipients can use to grant the
relay permission to perform the translation described in the
document. Relays MUST support the use of SIP and SIPS URIs in
permission documents and MAY support the use of HTTP and HTTPS
URIs.

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URIs to Deny Permission: URIs that recipients can use to deny the
relay permission to perform the translation described in the
document. Relays MUST support the use of SIP and SIPS URIs in
permission documents and MAY support the use of HTTP and HTTPS
URIs.

Permission documents can contain wildcards. For example, a
permission document can 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. However, the recipient URI MUST NOT be wildcarded.

Entities implementing this framework MUST support the format for
permission documents defined in [RFC5361] and MAY support other
formats.

In our example, the permission document in the MESSAGE request (3)
sent by the relay contains the following values:

Identity of the Sender: Any sender

Identity of the Original Recipient: sip:[email protected]

Identity of the Final Recipient: sip:[email protected]

URI to Grant Permission: sips:[email protected]

URI to Grant Permission: https://example.com/grant-1awdch5Fasddfce34

URI to Deny Permission: sips:[email protected]

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, can require permission documents that
apply to a specific sender. 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 store-and-forward
server stores it because user B is offline at that point. When user
B goes online, user B fetches all the requests its store-and-forward
server has stored (9).

Rosenberg, et al. Standards Track [Page 16]

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5.6. Permission Grant

A recipient 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 recipient 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. Requests to grant or deny
permissions contain an empty body.

In our example, user B obtains the permission document (10) that was
received earlier by its store-and-forward 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.

5.6.1. Relay's Behavior

Relays MUST 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 four methods to authenticate those
requests: SIP identity, P-Asserted-Identity [RFC3325], a return
routability test, or SIP digest. While return routability tests can
be used to authenticate both SIP PUBLISH and HTTP GET requests, SIP
identity, P-Asserted-Identity, and SIP digest can only be used to
authenticate SIP PUBLISH requests. SIP digest can only be used to
authenticate recipients that share a secret with the relay (e.g.,
recipients that are in the same domain as the relay).

5.6.1.1. SIP Identity

The SIP identity [RFC4474] 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.

5.6.1.2. P-Asserted-Identity

The P-Asserted-Identity [RFC3325] mechanism can also be used to
authenticate the sender of a PUBLISH request. However, as discussed
in [RFC3325], this mechanism is intended to be used only 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.

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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.

5.6.1.3. 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 better-than-nothing 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 cryptographically random 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. A part of these URIs (e.g., the user part of a SIPS
URI) 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.

Rosenberg, et al. Standards Track [Page 18]

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5.6.1.4. SIP Digest

The SIP digest mechanism can be used to authenticate the sender of a
PUBLISH request as long as that sender shares a secret with the
relay. 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.

5.7. 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, 'sip:[email protected]' has been added as a
recipient URI.

5.8. Permission Revocation

At any time, if a recipient wants to revoke any permission, it uses
the URI it received in the permission document to deny the
permissions it previously granted. If a recipient 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 Trigger-Consent header field
will have a target-uri header field parameter identifying the target
URI of the translation. The recipient needs to send a PUBLISH
request with an empty body 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 that header field, the user sends a PUBLISH request
(4) to the relay. On receiving the PUBLISH 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 [email protected]
|(1) INVITE |
| Trigger-Consent: sip:[email protected]
| ;target-uri="sip:[email protected]"
|---------------------------->|
|(2) 603 Decline |
|<----------------------------|
|(3) ACK |
|---------------------------->|
|(4) PUBLISH sip:[email protected]
|<----------------------------|
|(5) 200 OK |
|---------------------------->|
|(6) MESSAGE sip: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 recipient 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 consent framework and providing request-
contained URI-list services behave in a slightly different way than
the relays described so far. This type of relay also maintains a
list of recipient URIs for which permissions have been received.
Clients also manipulate this list using a manipulation mechanism
(e.g., XCAP). Nevertheless, this list does not represent the
recipient URIs of every translation performed by the relay. This
list just represents all the recipient URIs for which permissions
have been received -- that is, the set of URIs that will be accepted

Rosenberg, et al. Standards Track [Page 20]

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if a request containing a URI-list arrives to the relay. This set of
URIs is a superset of the recipient URIs of any particular
translation the relay performs.

5.9.1. Relay's Behavior

On receiving a request-contained URI list, the relay checks whether
or not it has permissions for all the URIs contained in the incoming
URI list. If it does, the relay performs the translation. If it
lacks permissions for one or more URIs, the relay MUST NOT perform
the translation and SHOULD return an error response.

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
field with the URIs for which the relay has no permissions.

Figure 6 shows a relay that receives a request (1) that contains URIs
for which the relay does not have permission (the INVITE carries the
recipient URIs in its message body). 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.

[email protected] Relay

|(1) INVITE |
| sip:[email protected] |
| sip:[email protected] |
|---------------------->|
|(2) 470 Consent Needed |
| Permission-Missing: sip:[email protected]
|<----------------------|
|(3) ACK |
|---------------------->|

Figure 6: INVITE with a URI List in Its Body

5.9.2. Definition of the 470 Response Code

A 470 (Consent Needed) response indicates that the request that
triggered the response contained a URI list with at least one URI for
which the relay had no permissions. A user agent server generating a
470 (Consent Needed) response SHOULD include a Permission-Missing
header field in it. This header field carries the URI or URIs for
which the relay had no permissions.

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A user agent client receiving a 470 (Consent Needed) response without
a Permission-Missing header field needs to use an alternative
mechanism (e.g., XCAP) to discover for which URI or URIs there were
no permissions.

A client receiving a 470 (Consent Needed) response uses a
manipulation mechanism (e.g., XCAP) to add those URIs to the relay's
list of URIs. The relay will obtain permissions for those URIs as
usual.

5.9.3. Definition of the Permission-Missing Header Field

Permission-Missing header fields carry URIs for which a relay did not
have permissions. The following is the augmented Backus-Naur Form
(BNF) [RFC5234] syntax of the Permission-Missing header field. Some
of its elements are defined in [RFC3261].

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:[email protected]

5.10. Registrations

Even though the example used to specify this framework has been a
URI-list service, this framework applies to any type of translation
(i.e., not only to URI-list services). Registrations are a different
type of translations that deserve discussion.

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
sent to the user's AoR (Address of Record) 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

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receives traffic over the same connection used for the registration
as described in [OUTBOUND]). However, this schema creates some
potential attacks that 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
'sip:[email protected]' to perform translations towards that
recipient URI, the relay places 'sip:[email protected]' in the
'pending' state. Once 'sip:[email protected]' is in the
'Permission Pending' state, the registrar needs to ask
'sip:[email protected]' for permission by sending a MESSAGE request
(3).

After receiving the response from the relay (2), user A subscribes to
the Pending Additions event package at the registrar (5). 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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[email protected] Registrar [email protected]

|(1) REGISTER | |
| Contact: sip:[email protected] |
|------------------>| |
|(2) 202 Accepted OK| |
|<------------------| |
| |(3) MESSAGE sip:[email protected]
| | 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 can 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 are only accepted between 9:00 and 11:00)
will have to use other mechanisms such as Call Processing Language
(CPL) [RFC3880].

Note that, as indicated previously, user agents using the same
connection to register and to receive traffic from the registrar,
as described in [OUTBOUND], do not need to use the mechanism
described in this section.

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A user agent being registered by a third party can be unable to use
the SIP Identity, P-Asserted-Identity, or SIP digest mechanisms to
prove to the registrar that the user agent is the owner of the URI
being registered (e.g., sip:[email protected]), which is the recipient
URI of the translation. In this case, return routability MUST be
used.

5.11. Relays Generating Traffic towards Recipients

Relays generating traffic towards recipients need to make sure that
those recipients can revoke the permissions they gave at any time.
The Trigger-Consent helps achieve this.

5.11.1. Relay's Behavior

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 a target-uri header field
parameter identifying the target URI of the translation.

On receiving a PUBLISH request addressed to the URI that a relay
previously placed in a Trigger-Consent header field, the relay SHOULD
send a MESSAGE request to the corresponding recipient URI with a
permission document. Therefore, the relay needs to be able to
correlate the URI it places in the Trigger-Consent header field with
the recipient URI of the translation.

5.11.2. Definition of the Trigger-Consent Header Field

The following is the augmented Backus-Naur Form (BNF) [RFC5234]
syntax of the Trigger-Consent header field. Some of its elements are
defined in [RFC3261].

Trigger-Consent = "Trigger-Consent" HCOLON trigger-cons-spec
*( COMMA trigger-cons-spec )
trigger-cons-spec = ( SIP-URI / SIPS-URI )
*( SEMI trigger-param )
trigger-param = target-uri / generic-param
target-uri = "target-uri" EQUAL
LDQUOT *( qdtext / quoted-pair ) RDQUOT

The target-uri header field parameter MUST contain a URI.

The following is an example of a Trigger-Consent header field:

Trigger-Consent: sip:[email protected]
;target-uri="sip:[email protected]"

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6. IANA Considerations

Per the following sections, IANA has registered a SIP response code,
two SIP header fields, and a SIP header field parameter.

6.1. Registration of the 470 Response Code

IANA has added 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: [RFC5360]

6.2. Registration of the Trigger-Consent Header Field

IANA has added the following new SIP header field to the Header
Fields subregistry under the SIP Parameters registry.

Header Name: Trigger-Consent
Compact Form: (none)
Reference: [RFC5360]

6.3. Registration of the Permission-Missing Header Field

IANA has added the following new SIP header field to the Header
Fields subregistry under the SIP Parameters registry.

Header Name: Permission-Missing
Compact Form: (none)
Reference: [RFC5360]

6.4. Registration of the target-uri Header Field Parameter

IANA has registered the 'target-uri' Trigger-Consent header field
parameter under the Header Field Parameters and Parameter Values
subregistry within the SIP Parameters registry:

Predefined
Header Field Parameter Name Values Reference
---------------------------- --------------- --------- ---------
Trigger-Consent target-uri No [RFC5360]

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7. Security Considerations

Security has been discussed throughout the whole document. However,
there are some issues that deserve special attention.

Relays generally implement several security mechanisms that relate to
client authentication and authorization. Clients are typically
authenticated before they can manipulate a relay's translation logic.
Additionally, clients are typically also authenticated and sometimes
need to perform SPAM prevention tasks [RFC5039] when they send
traffic to a relay. It is important that relays implement these
types of security mechanisms. However, they fall out of the scope of
this framework. Even with these mechanisms in place, there is still
a need for relays to implement this framework because the use of
these mechanisms does not prevent authorized clients to add
recipients to a translation without their consent. Consequently,
relays performing translations MUST implement this framework.

Note that, as indicated previously, user agents using the same
connection to register and to receive traffic from the registrar,
as described in [OUTBOUND], do not need to use this framework.
Therefore, a registrar that did not accept third-party
registrations would not need to implement this framework.

As pointed out in Section 5.6.1.3, 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 [RFC5361]. When store-and-forward servers are used,
the interface between a user agent and its store-and-forward server
is frequently not based on SIP. In such a case, SIPS cannot be used
to secure those URIs. Implementations of store-and-forward servers
MUST provide a mechanism for delivering encrypted and integrity-
protected messages to their user agents.

The information provided by the Pending Additions event package can
be sensitive. For this reason, as described in [RFC5362], relays
need to use strong means for authentication and information
confidentiality. SIPS URIs are a good mechanism to meet this
requirement.

Permission documents can reveal sensitive information. Attackers may
attempt to modify them in order to have clients grant or deny
permissions different from the ones they think they are granting or
denying. For this reason, it is RECOMMENDED that relays use strong
means for information integrity protection and confidentiality when
sending permission documents to clients.

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The mechanism used for conveying information to clients SHOULD ensure
the integrity and confidentially of the information. In order to
achieve these, an end-to-end SIP encryption mechanism, such as
S/MIME, as described in [RFC3261], SHOULD be used.

If strong end-to-end security means (such as above) are not
available, it is RECOMMENDED that hop-by-hop security based on TLS
and SIPS URIs, as described in [RFC3261], is used.

8. Acknowledgments

Henning Schulzrinne, Jon Peterson, and Cullen Jennings provided
useful ideas on this document. Ben Campbell, AC Mahendran, Keith
Drage, and Mary Barnes performed a thorough review of this document.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2616] 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.

[RFC3261] 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.

[RFC3428] Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
Huitema, C., and D. Gurle, "Session Initiation Protocol
(SIP) Extension for Instant Messaging", RFC 3428, December
2002.

[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.

[RFC5361] Camarillo, G., "A Document Format for Requesting Consent",
RFC 5361, October 2008.

[RFC5362] Camarillo, G., "The Session Initiation Protocol (SIP)
Pending Additions Event Package", RFC 5362, October 2008.

Rosenberg, et al. Standards Track [Page 28]

RFC 5360 Consent Framework October 2008

[RFC5363] Camarillo, G. and A.B. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) URI-
List Services", RFC 5363, October 2008.

9.2. Informative References

[RFC3325] 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.

[RFC3880] Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
Language (CPL): A Language for User Control of Internet
Telephony Services", RFC 3880, October 2004.

[RFC4453] Rosenberg, J., Camarillo, G., Ed., and D. Willis,
"Requirements for Consent-Based Communications in the
Session Initiation Protocol (SIP)", RFC 4453, April 2006.

[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.

[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

[RFC4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
for Representing Resource Lists", RFC 4826, May 2007.

[RFC5039] Rosenberg, J. and C. Jennings, "The Session Initiation
Protocol (SIP) and Spam", RFC 5039, January 2008.

[OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
Work in Progress, June 2007.

Rosenberg, et al. Standards Track [Page 29]

RFC 5360 Consent Framework October 2008

Authors' Addresses

Jonathan Rosenberg
Cisco
Iselin, NJ 08830
USA

EMail: [email protected]
URI: http://www.jdrosen.net

Gonzalo Camarillo (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland

EMail: [email protected]

Dean Willis
Unaffiliated
3100 Independence Pkwy #311-164
Plano, TX 75075
USA

EMail: [email protected]

Rosenberg, et al. Standards Track [Page 30]

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Full Copyright Statement

Copyright (C) The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
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this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
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Rosenberg, et al. Standards Track [Page 31]