SIPPING Working Group                                       G. Camarillo
Internet-Draft                                                  Ericsson
Expires: January 5, 2005                                    July 7, 2004

    Requirements and Framework for Session Initiation Protocol (SIP)
            Uniform Resource Identifier (URI)-List Services

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document describes the need for SIP URI-List Services and
   provides requirements for their invocation. Additionaly, it defines a
   framework which includes all the SIP extensions needed to meet these

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1   Requirements for Request-Contained URI-List Services . . .  4
     3.2   General Requirements for URI-List Services . . . . . . . .  4
   4.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1   Carrying URI-Lists in SIP  . . . . . . . . . . . . . . . .  5
     4.2   Processing of URI-Lists  . . . . . . . . . . . . . . . . .  5
     4.3   Results  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
     5.1   List Integrity and Confidentiality . . . . . . . . . . . .  6
     5.2   Amplification Attacks  . . . . . . . . . . . . . . . . . .  6
     5.3   Unsolicited Requests . . . . . . . . . . . . . . . . . . .  8
     5.4   General Issues . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.1   Normative References . . . . . . . . . . . . . . . . . . . .  8
   7.2   Informational References . . . . . . . . . . . . . . . . . .  8
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10

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1.  Introduction

   Some applications require that, at a given moment, a SIP [2] UA (User
   Agent) performs a similar transaction with a number of remote UAs.
   For example, an instant messaging application that needs to send a
   particular message (e.g., "Hello folks") to n receivers needs to send
   n MESSAGE requests; one to each receiver.

   When the transacton that needs to be repeated consists of a large
   request, or the number of recipients is high, or both, the access
   network of the UA needs to carry a considerable amount of traffic.
   Completing all the transactions on a low-bandwidth access would
   require a long time. This is unacceptable for a number of

   A solution to this problem consists of introducing URI-list services
   in the network. The task of a SIP URI-list service is to receive a
   request that contains or references a URI-list and send a number of
   similar requests to the destinations in this list. Once the requests
   are sent, the URI-list service typically informs the UA about their
   status. Effectively, the URI-list service behaves as a B2BUA

   If the request references an external URI-list (e.g., the Request-URI
   is a SIP URI which is associated with a URI-list at the server), this
   URI-list is referred to as an stored URI-list. If the request
   contains the URI-list, the URI-list is referred to as a
   request-contained URI-list.

   Stored URI-lists are typically set up using out-of-band mechanisms
   (e.g., XCAP [9]). An example of a URI-list service for SUBSCRIBE
   requests that uses stored URI-lists is described in [4].

   The Advanced Instant Messaging Requirements for SIP [5] mentions the
   need for request-contained URI-list services for MESSAGE transactions

   "REQ-GROUP-3: It MUST be possible for a user to send to an ad-hoc
   group, where the identities of the recipients are carried in the
   message itself."

   The remainder of this document provides requirements for both stored
   and request-contained SIP URI-list services.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as

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   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3.  Requirements

   Section 3.1 discusses requirements that only apply to
   request-contained URI-list services and Section 3.2 discusses
   requirements that apply to both stored and request-contained URI-list

3.1  Requirements for Request-Contained URI-List Services

   1.  The URI-list service invocation mechanism MUST allow the invoker
       to provide a list of destination URIs to the URI-list service.
       This URI-list MAY consist of one or more URIs.
   2.  The mechanism to provide the URI-list to the URI-list service
       MUST NOT be request specific.
   3.  The invocation mechanism SHOULD NOT require more than one RTT
       (Round-Trip Time).

3.2  General Requirements for URI-List Services

   1.  An URI-list service MAY include services beyond sending requests
       to the URIs in the URI-list. That is, URI-list services can be
       modelled as application servers. For example, a URI-list service
       handling INVITE requests may behave as a conference server and
       perform media mixing for all the participants.
   2.  The interpretation of the meaning of the URI-list sent by the
       invoker MUST be at the discretion of the application to which the
       list is sent.
   3.  It MUST be possible for the invoker to find out about the result
       of the operations performed by the URI-list service with the
       URI-list. An invoker may, for instance, be interested in the
       status of the transactions initiated by the URI-list service.
   4.  URI-list services MUST NOT send requests to multiple destinations
       without authenticating the invoker.

4.  Framework

   Although Section 3 contains specific requirements for SIP URI-list
   services, this framework is not restricted to application servers
   that only provide request fan-out services. Per the general
   requirement number 1, we also deal with application servers that
   provide a particular service that includes a request fan-out (e.g., a
   conference server that INVITEs several participants which are chosen
   by a user agent).

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4.1  Carrying URI-Lists in SIP

   The requirements that relate to request-contained URI-list services
   identify the need for a request-independent mechanism to provide a
   SIP URI-list service with a URI-list in a single RTT. The mechanism
   described in (draft-ietf-sipping-uri-list-00.txt) [3] meets these
   three requirements.

   UAs (User Agents) use body parts whose disposition type is uri-list
   to transport URI-lists. The default URI-list format for SIP entities
   is the XCAP resource list format defined in [6].

4.2  Processing of URI-Lists

   According to the general requirements 1 and 2, URI-list services can
   behave as application servers. That is, taking a URI-list as an
   input, they can provide arbitrary services.

   So, the interpretation of the URI-list by the server depends on the
   service to be provided. For example, for a conference server, the
   URIs in the list may identify the initial set of participants. On the
   other hand, for a server dealing with MESSAGEs, the URIs in the list
   may identify the recipients of an instant message.

   At the SIP level, this implies that the behavior of application
   servers receiving requests with URI-lists SHOULD be specified on a
   per method basis. Examples of such specifications are
   [draft-ietf-sipping-uri-list-conferencing-00.txt] for INVITE,
   [draft-ietf-sipping-multiple-refer-00.txt] for REFER,
   [draft-ietf-sipping-uri-list-message-00.txt] for MESSAGE, and
   [draft-ietf-sipping-uri-list-subscribe-00.txt] for SUBSCRIBE.

4.3  Results

   According to requirement 6, user agents should have a way to obtain
   information about the operations performed by the application server.
   Since these operations are service specific, the way user agents are
   kept informed is also service specific. For example, a user agent
   establishing an adhoc conference with an INVITE with a URI-list may
   discover which participants were successfully brought in into the
   conference by using the conference package [8].

5.  Security Considerations

   Security plays an important role in the implementation of any
   URI-list service. By definition, a URI-list service takes one request
   in and sends a potentially large number of them out. Attackers may
   attempt to use URI-list services as traffic amplifiers to launch DoS

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   attacks. In addition, malicious users may attempt to use URI-list
   services to distribute unsolicited messages (i.e., SPAM) or to make
   unsolicited VoIP calls. This section provides guidelines to avoid
   these attacks.

5.1  List Integrity and Confidentiality

   Attackers may attempt to modify URI-lists sent from clients to
   servers. This would cause a different behavior at the server than
   expected by the client (e.g., requests being sent to different
   recipients as the ones specified by the client). To prevent this
   attack, clients SHOULD integrity protect URI-lists using mechanisms
   such as S/MIME, which can also provide URI-list confidentiality if

5.2  Amplification Attacks

   URI-list services take a request in and send a potentially large
   number of them out. Given that URI-list services are typically
   implemented on top of powerful servers with high-bandwidth access
   links, we should be careful to keep attackers from using them as
   amplification tools to launch DoS (Denial of Service) attacks.

   Attackers may attempt to send a URI-list containing URIs whose host
   parts route to the victims of the DoS attack. These victims do not
   need to be SIP nodes; they can be non-SIP endpoints or even routers.
   If this attack is successful, the result is that an attacker can
   flood with traffic a set of nodes, or a single node, without needing
   to generate a high volume of traffic itself.

      Note, in any case, that this problem is not specific to SIP
      URI-list services; it also appears in scenarios which relate to
      multihoming where a server needs to contact a set of IP addresses
      provided by a client (e.g., an SCTP [10] endpoint using HEARTBEATs
      to check the status of the IP addresses provided by its peer at
      association establishment).

   There are several measures that need to be taken to prevent this type
   of attack. The first one is keeping unauthorized users from using
   URI-list services. So, URI-list services MUST NOT perform any request
   explosion for an unauthorized user. URI-list services MUST
   authenticate users and check whether they are authorized to request
   the service before performing any request fan-out.

      Note that the risk of this attack also exists when a client uses
      stored URI-lists. Application servers MUST use authentication and
      authorization mechanisms with equivalent security properties when
      dealing with stored and request-contained URI-lists.

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   Even though the previous rule keeps unauthorized users from using
   URI-list services, authorized users may still launch attacks using a
   these services. To prevent these attacks, we introduce the concept of
   opt-in lists. That is, URI-list services should not allow a client to
   place a user (identified by his or her URI) in a URI-list unless the
   user has previously agreed to be placed in such a URI-list. So,
   URI-list services MUST NOT send a request to a destination which has
   not agreed to receive requests from the URI-list service beforehand.
   Users can agree to receive requests from a URI-list service in
   several ways, such as filling a web page, sending an email, or
   signing a contract. Additionally, users MUST be able to further
   describe the requests they are willing to receive. For example, a
   user may only want to receive requests from a particular URI-list
   service on behalf of a particular user. Effectively, these rules make
   URI-lists used by URI-list services opt-in lists.

   When a URI-list service receives a request with a URI-list from a
   client, the URI-list service checks whether all the destinations have
   agreed beforehand to receive requests from the service on behalf of
   this client. If the URI-list has permission to send requests to all
   of the targets in the request, it does so. If not, the URI-list
   service rejects the request, indicating in the rejection the set of
   targets for which it did not have permission. This allows the client
   to request permission for those targets.

   DoS amplification would still happen if the URI-list service
   automatically contacted the full set of targets for which it did not
   have permission in order to request permission. The URI-list service
   would be receiving one SIP request and sending out a number of
   authorization request messages. In order to avoid this amplification,
   the URI-list service must ensure that the client generates roughly
   the same amount of traffic towards the URI-list service as the
   service generates towards the destinations. Consequently, the
   URI-list service MUST require that clients send and individual
   authorization request for each destination.

   These individual authorization requests sent by the client may or may
   not be routed through the URI-list service. In any case, the URI-list
   service MUST be informed about the destinations' responses to these
   authorization requests in order to authorize requests towards them.
   One possible mechanism for clients to send authorization requests to
   the destinations is specified in
   [draft-rosenberg-sipping-consent-framework-00.txt], which discusses
   consent-based communications in SIP. The requirements for
   consent-based communications in SIP are discussed in

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5.3  Unsolicited Requests

   Opt-in lists should help fighting SPAMMERS. Still, if a URI-list
   service is used to send unsolicited requests to one or several
   destinations, it should be possible to track down the sender of such
   requests. To do that, URI-list services MAY provide information about
   the identity of the original sender of the request in their outgoing
   requests. URI-list services can use Authenticated Identity Bodies
   (AIB) [7] to provide this information.

5.4  General Issues

   URI-list services MAY have policies that limit the number of URIs in
   the lists they accept, as a very long list could be used in a denial
   of service attack to place a large burden on the URI-list service to
   send a large number of SIP requests.

   The general requirement number 4, which states that URI-list services
   need to authenticate their clients, and the previous rules apply to
   URI-list services in general. In addition, specifications dealing
   with individual methods MUST describe the security issues that relate
   to each particular method.

6.  Acknowledges

   Duncan Mills and Miguel A. Garcia-Martin supported the idea of 1 to n
   MESSAGEs. Jon Peterson and Dean Willis provided useful comments.

7.  References

7.1  Normative References

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

   [2]  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.

7.2  Informational References

   [3]   Camarillo, G., "Providing a Session Initiation Protocol (SIP)
         Application Server with a  List of URIs",
         draft-camarillo-sipping-uri-list-01 (work in progress),
         February 2004.

   [4]   Roach, A., Rosenberg, J. and B. Campbell, "A Session Initiation
         Protocol (SIP) Event Notification Extension for  Resource

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         Lists", draft-ietf-simple-event-list-04 (work in progress),
         June 2003.

   [5]   Rosenberg, J., "Advanced Instant Messaging Requirements for the
         Session Initiation Protocol  (SIP)",
         draft-rosenberg-simple-messaging-requirements-01 (work in
         progress), February 2004.

   [6]   Rosenberg, J., "An Extensible Markup Language (XML)
         Configuration Access Protocol (XCAP)  Usage for Presence
         Lists", draft-ietf-simple-xcap-list-usage-02 (work in
         progress), February 2004.

   [7]   Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
         draft-ietf-sip-authid-body-02 (work in progress), July 2003.

   [8]   Rosenberg, J. and H. Schulzrinne, "A Session Initiation
         Protocol (SIP) Event Package for Conference State",
         draft-ietf-sipping-conference-package-03 (work in progress),
         February 2004.

   [9]   Rosenberg, J., "The Extensible Markup Language (XML)
         Configuration Access Protocol (XCAP)",
         draft-ietf-simple-xcap-02 (work in progress), February 2004.

   [10]  Bradner, S., "A Proposal for an MOU-Based ICANN Protocol
         Support Organization", RFC 2690, September 1999.

Author's Address

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


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