SIPPING J. Rosenberg
Internet-Draft dynamicsoft
Expires: January 6, 2005 G. Camarillo
Ericsson
D. Willis
dynamicsoft
July 8, 2004
A Framework for Consent-Based Communications in the Session
Initiation Protocol (SIP)
draft-rosenberg-sipping-consent-framework-00.txt
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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
from one party to another without requiring explicit consent of the
recipient. Without such consent, it is possible for SIP to be used
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for malicious purposes, including spam and denial-of-service attacks.
This document identifies a framework for consent-based communications
in SIP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Architecture . . . . . . . . . . . . . . . . . . . 4
4. Structure of a Permission . . . . . . . . . . . . . . . . . 4
5. Attempting Communication . . . . . . . . . . . . . . . . . . 5
6. Requesting a Permission . . . . . . . . . . . . . . . . . . 6
7. Waiting for Permissions . . . . . . . . . . . . . . . . . . 6
8. Granting a Permission . . . . . . . . . . . . . . . . . . . 7
8.1 Permission Servers . . . . . . . . . . . . . . . . . . . . 7
9. Retrying the Original Request . . . . . . . . . . . . . . . 8
10. Permission Revocation . . . . . . . . . . . . . . . . . . . 8
11. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1 Basic Flow with No Permission Server . . . . . . . . . . 8
11.2 Basic Flow with a Permission Server . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1 Normative References . . . . . . . . . . . . . . . . . . . 11
12.2 Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . 13
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1. Introduction
The Session Initiation Protocol (SIP) [1] supports communications
across many media types, including real-time audio, video, text,
instant messaging and presence. This communication is established by
the transmission of various SIP requests (such as INVITE and MESSAGE
[2]) from an initiator to the recipient, with whom communication is
desired. Although a recipient of such a SIP request can reject the
request, and therefore decline the session, a SIP network will
deliver a SIP request to the recipient without their explicit
consent.
Receipt of these requests without explicit consent can cause a number
of problems in SIP networks. These include spam and DoS (Denial of
Service) attacks. These problems are described in more detail in a
companion requirements document
[draft-rosenberg-sipping-consent-reqs-00.txt].
This specification defines a basic framework for adding consent-based
communication to SIP.
2. Relays
A central concept in this framework is that of a relay. A relay is
defined as any SIP server, be it a proxy, back-to-back user agent or
some hybrid, which receives a request and translates the request URI
into one or more next hop URIs to which it then delivers a request.
So, an essential aspect of a relay is that of translation.
When a relay receives a request, it translates the request URI into
one or more additional URIs. Or, more generally, it can create
outgoing requests to one or more additional URIs. The translation
operation is what creates the consent problem. Since the translation
operation can result in more than one URI, it is 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,
sip:user@example.com, it cannot deliver a MESSAGE request to the user
agent of that user without having access to the registration data
that maps sip:user@example.com to the UA 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.
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3. Reference Architecture
The reference architecture is shown in Figure 1. In this
architecture, a UAC wishes to send a message to a request URI
representing a resource in domain A (sip:resource@A). This request
may pass through a local outbound proxy (not shown), but eventually
arrives at a server authoritative for domain A. This server, which
acts as a relay, performs a translation operation, translating the
request URI into one or more next hop URIs, which may or may not
belong to domain A. This relay may be a proxy server of a URI-list
service, for instance.
+-------+
| |
>| UAS 1 |
+-------+ / | |
| Rules | / +-------+
| DB | /
+-------+ /
| /
V /
+-----+ +-------+ / +-------+
| | | |/ | |
| UAC |------>| Relay |-------->| UAS 2 |
| | | |\ | |
+-----+ +-------+ \ +-------+
\
\ [...]
\
\
\ +-------+
\ | |
>| UAS n |
| |
+-------+
Figure 1
4. Structure of a Permission
This framework centers on the idea that a relay will only perform a
translation if a permission is in place authorizing that translation.
As such, the notion of a permission is another key part of this
framework. A permission is an object, represented in XML, that
contains several pieces of data:
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Identity of the Sender: A URI representing the identity of the sender
for whom permissions are granted.
Identity of the Recipient: A URI representing the target of the
translation. The permission grants ability for the sender to send
requests, and for a relay receiving those requests to forward them
to this URI. This is also called the recipient URI.
Operations Permitted: A set of specific methods or qualifiers for
which the permission applies. For example, the permission may only
grant relaying for INVITE or MESSAGE, or for MESSAGE with specific
MIME types.
Signature: A digital signature over the rest of the permission,
signed by an entity that can identify itself as the recipient URI.
The signature is not always present.
Permissions are installed on a resource by resource basis. That is,
for each target URI to which a request is sent, there is a set of
permissions installed for that URI. Each permission has the content
described above.
It is important to note that the permission itself does not depend
on, or contain, the identity of a target URI (i.e., the input). As
such, if a request is sent to sip:resource1@A and to sip:resource2@A,
and for both targets, the same permission was installed, allowing
requests from the sender to be relayed to sip:resource@B, that same
permission would allow the translation to take place for both
targets.
A natural format for representing permissions appears to be the
common policy format [4]. This format is also used for presence
permissions.
5. Attempting Communication
When a UA sends a request to a target resource, the request
eventually arrives at a server that is authoritative for the domain
in the request URI. The server may require, as part of its processing
logic, the relaying of the request to one or more next hops. If such
relaying is required, the server first authenticates the sender of
the request. Such authentication can be done using the SIP identity
mechansim [5]. Once the sender is authenticated, the server checks
its permission database for that target resource. It looks for
permissions containing senders whose URI matches the identity of the
sender of the request. Of those that are found, the server checks to
see if the permitted translated URI matches the URIs to which the
server wishes to relay the request.
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If at least one of the next hops to which the server wishes to relay
have not been permitted, the server rejects the request with a 470
(Consent Needed) response. The 470 response code indicates that the
request couldn't be relayed because at least one permission was not
present. The error response can contain a body, which contains a list
of URIs for the translations for which permissions have not yet been
obtained. This is effectively an instruction for the sender to go
off, and obtain permissions from those URIs.
6. Requesting a Permission
If the attempt to communicate was rejected with a 470 (Consent
Needed) response, the client knows that it must obtain some number of
permissions in order for the communications to take place. The error
response will include a list of URIs for which permission must be
obtained. To obtain permission, the client sends a CONSENT request to
each of the URIs it learned from the body of the error response.
These URIs typically route to the relay, which will forward them on
to the destinations whose permissions have not been obtained yet. The
CONSENT request carries a Consent-Methods header field which
indicates for which methods consent is being requested.
When the CONSENT request arrives at the relay, the relay adds a
Permission header field which contains a URI that the receiver can
use to upload a permission (e.g., the receiver can use XCAP to upload
an XML-based permission document). Then, the relay forwards the
request towards its destination.
If there are several relays between the sender and the final
destination, those CONSENT requests may also fail if permissions have
not yet been obtained, in which case the process recurses.
Eventually, the client will have sent a request to all of the relays
at the leaves of the translation tree between the sender and the
final destinations.
7. Waiting for Permissions
A CONSENT request is responded with a 202 (Accepted) response, which
carries a URI in a Call-Info header field (wait-permission purpose)
where the client can SUBSCRIBE to using the wait-permission event
package. This event package models the state of the permission
granted to the client for communicating with the target URI. When a
permission is granted, the state changes, and the client receives a
NOTIFY. This NOTIFY contains the permission(s) that have been granted
for the sender.
Usage of an event package has the benefit that the client can come
back at any time and do a query SUBSCRIBE to see if permissions were
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granted, or it can wait for them to be granted, and find out when.
There is no requirement that the client use this event package to
wait. For some requests, it may not be important for the sender to
find out when permission is granted (e.g., a presence subscription).
8. Granting a Permission
On reception of a CONSENT request, if the user wishes to grant a
permission, XCAP is used, just as it is today in presence. The owner
of the target resource would use contact the URI in the Permission
header field of the CONSENT request and use XCAP to place the
permission into a document containing the list of permissions for
that target resource.
The XCAP server needs to make sure that the entity uploading the
permission document is the same as the destination of the CONSENT
request. This is done by inserting a URI in the Permission header
field of the CONSENT request which is long and random enough so that
it cannot be guessed. In addition, the CONSENT request is delivered
to the user using a SIPS URI. Then, the server inserting such a URI
relies on the SIP routing infrastructure to deliver the CONSENT
request to its proper destination.
If the SIP routing infrastructure is compromised, it could route
the CONSENT request to an attacker so that the attacker could
authorize requests addressed to a victim. Nevertheless, if the SIP
routing infrastructure gets compromised, many types of attacks
much worse than this are possible. So, relaying on the SIP routing
infrastructure seems like a sensible choice.
Using XCAP to grant permissions will require the definition of a new
application usage. We note that this usage appears to be a
generalization of the presence rules usage currently defined
[PRES-RULES].
8.1 Permission Servers
We have just described how a user agent that receives a CONSENT
request can use XCAP to grant certain permissions. Nevertheless,
users are not on-line all the time and, so, sometimes are not able to
receive CONSENT requests.
This issue is also found in presence, where a user's status is
reported by a presence server instead of by the user's user agents,
which can go on and off-line. Similarly, we define permission
servers. Permission servers are network elements that act as SIP UAs
and handle CONSENT requests for a user.
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Permission servers inform users about new CONSENT requests using the
"grant-permission" event package. The user associated with the target
URI SUBSCRIBEs to the "grant-permission" event package at the
permission server. This event package models the state of all pending
CONSENT requests for a particular resource, for which permissions do
not yet exist. When a new CONSENT request arrives for which
permissions have not been granted, a NOTIFY is sent to the user. This
informs them that permission is needed for a particular sender. The
NOTIFY contains information on the operation which was requested.
There is a strong similarity between the watcherinfo event package
and the grant-permission event package. Indeed, the
grant-permission package is effectively a superset of watcherinfo.
Once in place, presentities could use the grant-permission event
package for presence in addition to all other services for which
opt-in is being provided.
9. Retrying the Original Request
The sender learns about permissions through the wait-permission event
package. Once it has obtained permissions for all of the resources
that were identified in the 470 (Consent Needed) response, the client
can retry the original request.
10. Permission Revocation
At any time, if a client wants to revoke any permission, it uses the
XCAP URI that received in the CONSENT message or through the
grant-permission event package. If a client lost this URI for some
reason, it would need to wait until it received a new request and
respond with a 470 (Consent Needed) response. The client would get
the URI in a new CONSENT request.
OPEN ISSUE: if we defined the Permission header field so that it can
be present in any request, and not only in CONSENT requests, the
relay could add this header field to every request directed to the
user which used SIPS.
11. Use Cases
The following use cases exhibit how the framework works.
11.1 Basic Flow with No Permission Server
A Relay B
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| MESSAGE list@relay | |
|-------------------------->| |
| 470 | |
| xyz@relay | |
|<--------------------------| |
| | |
| CONSENT xyz@relay | CONSENT B |
| Consent-methods: MESSAGE | Consent-methods: MESSAGE |
|-------------------------->| Permission: xcap-uri |
| |-------------------------->|
| 202 Accepted | |
| Call-Info: 123@relay; | 202 Accepted |
| purpose: wait-permission |<--------------------------|
|<--------------------------| |
| | |
| SUBSCRIBE 123@relay | |
|-------------------------->| |
| 200 OK | |
|<--------------------------| |
| | |
| NOTIFY (no permission) | |
|<--------------------------| |
| 200 OK | |
|-------------------------->| |
| | |
| | XCAP xcap-uri |
| | Permission Grant |
| |<--------------------------|
| | 200 OK |
| NOTIFY (permission) |-------------------------->|
|<--------------------------| |
| 200 OK | |
|-------------------------->| |
| | |
| MESSAGE list@relay | |
|-------------------------->| MESSAGE B |
| |-------------------------->|
| | |
Figure 2
Alternatively, the Call-Info header field could have been inserted by
B directly. In this case, A would SUBSCRIBE to B, instead of
subscribing to the Relay.
11.2 Basic Flow with a Permission Server
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A Relay B's Permission B
Server
| MESSAGE list@relay | | |
|-------------------------->| | |
| 470 | | |
| xyz@relay | | |
|<--------------------------| | |
| | | |
| CONSENT xyz@relay | CONSENT B | |
| Consent-methods: MESSAGE | Consent-methods: MESSAGE |
|-------------------------->| Permission: xcap-uri |
| |----------------->| |
| 202 Accepted | | |
| Call-Info: 123@relay; | 202 Accepted | |
| purpose: wait-permission |<-----------------| |
|<--------------------------| | |
| | | |
| SUBSCRIBE 123@relay | | |
|-------------------------->| | |
| 200 OK | | |
|<--------------------------| | |
| | | |
| NOTIFY (no permission) | | [B goes on-line] |
|<--------------------------| | |
| 200 OK | | |
|-------------------------->| | SUBSCRIBE |
| | | grant-permission |
| | |<------------------|
| | | 200 OK |
| | |------------------>|
| | | |
| | | NOTIFY |
| | | xcap-uri |
| | |------------------>|
| | | 200 OK |
| | |<------------------|
| | XCAP xcap-uri | |
| | Permission Grant| |
| |<-------------------------------------|
| | 200 OK | |
| NOTIFY (permission) |--------------------------------------|
|<--------------------------| | |
| 200 OK | | |
|-------------------------->| | |
| | | |
| MESSAGE list@relay | | |
|-------------------------->| | |
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| | MESSAGE B | |
| |------------------------------------->|
| | | |
Figure 3
12. References
12.1 Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D.
Gurle, "Session Initiation Protocol (SIP) Extension for Instant
Messaging", RFC 3428, December 2002.
12.2 Informative References
[3] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
in progress), January 2003.
[4] Schulzrinne, H., "Common Policy",
draft-ietf-geopriv-common-policy-00 (work in progress), February
2004.
[5] Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
draft-ietf-sip-authid-body-02 (work in progress), July 2003.
Authors' Addresses
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Dean Willis
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75028
USA
EMail: dean.willis@softarmor.com
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