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Versions: 00 01 02 03                                                   
SIPPING Working Group                                       G. Camarillo
Internet-Draft                                                  Ericsson
Expires: August 6, 2004                                 February 6, 2004

    Requirements and Framework for Session Initiation Protocol (SIP)
                          Exploder Invocation

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

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


   This document describes the need for SIP exploders 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.1 Carrying URI Lists in SIP  . . . . . . . . . . . . . . . . . .  5
   4.2 Managing Ad-Hoc URI Lists  . . . . . . . . . . . . . . . . . .  5
   4.3 Transaction State Information  . . . . . . . . . . . . . . . .  5
   4.4 Multiple REFER Targets . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Normative References . . . . . . . . . . . . . . . . . . . . .  6
       Informational References . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

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1. Introduction

   Some applications require that, at a given moment, a SIP UA 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 exploders in the
   network. The task of an exploder is to receive a request from a UA
   and send a number of similar requests to a number of destinations.
   Once the requests are sent, the exploder needs to inform the UA about
   their status. Effectively, the exploder behaves as a B2BUA.

   Note that resource lists, as described in [2], already use SIP
   exploders for SUBSCRIBE transactions. Still, the set of destinations
   needs to be preconfigured using out-of-band mechanisms (e.g., XCAP).

   The Advanced Instant Messaging Requirements for SIP  [3] also
   mentions the need for exploders 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 to invoke
   exploders in an efficient manner and a framework that meets these

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3. Requirements

   This section contains the requirements:

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   1.   The invocation mechanism MUST allow the invoker to provide a
        list of destination URIs to the exploder. This URI list MAY
        consist of one or more URIs.

   2.   It MUST be possible to send URI list "deltas" to update the list
        of URIs handled by the exploder.

   3.   The invocation mechanism MUST NOT be request specific.

   4.   The invocation mechanism SHOULD NOT require more than one RTT.

   5.   An exploder MAY provide services beyond request explosion. That
        is, exploders can be modelled as application servers. For
        example, an exploder handling INVITE requests may behave as a
        conference server and perform media mixing for all the

   6.   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.

   7.   It MUST be possible for the invoker to find out about the result
        of the operations performed by the application with the URI
        list. An invoker may, for instance, be interested in the status
        of the transactions initiated by the exploder.

   8.   It MUST be possible for the application that makes use of a list
        of URIs to convey the list of URIs to any recipients of messages
        created by the application from that list. OPEN ISSUE: do we
        really need this requirement?

   9.   Exploders MUST NOT perform any request explosion without
        authenticating the invoker.

   10.  The UA MUST be able to provide credentials to the exploder so
        that the exploder can use them to prove to the destinations that
        it is sending requests on behalf of the UA.

4. Framework

   Although Section 3 contains specific requirements for SIP exploders,
   this framework is not restricted to application servers that only
   provide request explosion services. We also deal with application
   servers that provide a particular service that includes a request
   explosion (e.g., a conference server that INVITEs several
   participants which are chosen by a user agent).

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   We need to use several SIP extensions to meet the requirements in
   Section 3. We list these extensions in the following sections and
   explain which role they play within the framework.

4.1 Carrying URI Lists in SIP

   User agents can send a list of URIs to an application server using
   the list SIP and SIPS URI parameter defined in
   (draft-camarillo-sipping-uri-list-01). The user agent adds a list
   parameter to the Request-URI of the SIP request sent to the
   application server. This parameter contains a pointer to a URI list,
   which can be carried in the SIP request itself or can be stored in an
   external server (e.g., an http URI pointing to an XCAP resource
   list). The way the application server interprets the URI list
   received in the request is service specific.

4.2 Managing Ad-Hoc URI Lists

   An application server that receives a request with a URI list (or a
   pointer to it) creates a so called ad-hoc list, whose lifetime
   depends on the service provided by the server. Services that involve
   ad-hoc lists that are valid for a period of time need to allow user
   agents to modify these lists.

   A user agent can manage ad-hoc lists at a server in two ways, as
   described in (draft-camarillo-sipping-adhoc-management-00): using SIP
   or using an external means (e.g., XCAP).

   User agents using SIP to manage ad-hoc lists send a new SIP request
   with a pointer to a new list that will substitute the old list.

   User agents using an external means to manage ad-hoc lists need to
   obtain from the server a URI that allows them to manipulate the list
   (e.g., an http URI pointing to an XCAP resource list). The server
   provides such a URI in an Associated-List-Manipulation header field
   in the response to the request that created the ad-hoc list.

4.3 Transaction State Information

   User agents may be interested in the results of the message explosion
   at the application server. That is, user agents may want to know the
   result of the transactions that the application server initiated
   towards the URIs in the URI list provided by the user agent. The
   transaction state event package defined in
   (draft-camarillo-sipping-transac-package-00) provides this
   information to the user agent subscribing to this package.

   Still, in order to subscribe to the transaction state event package,

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   the user agent needs a URI to subscribe to. The application server
   provides such a URI in an Associated-Transactions-State header field
   in the response to the request that triggered the new transactions,
   as defined in (draft-camarillo-sipping-transac-package-00).

4.4 Multiple REFER Targets

   Building REFER requests with multiple REFER targets requires special
   considerations, as described in
   (draft-camarillo-sipping-multiple-refer-00). The Refer-To header
   field carries a pointer to a URI list, and the NOTIFIES carry
   transaction state information using the transaction state event
   package. User agents may use bodies whose disposition type is
   template to describe the messages to be sent by the application

   A conferencing application is an example of an application that may
   use REFERs with multiple REFER targets. A user agent may send a REFER
   to the conferencing server so that the server BYEs a set of users.

5. Security Considerations

   Requirements related to security are considered in Section 3.

   TBD: this section should be expanded considerably.

6. Acknowledges

   Duncan Mills and Miguel A. Garcia-Martin supported the idea of 1 to n

Normative References

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

Informational References

   [2]  Roach, A., Rosenberg, J. and B. Campbell, "A Session Initiation
        Protocol (SIP) Event Notification Extension for  Resource
        Lists", draft-ietf-simple-event-list-04 (work in progress), June

   [3]  Rosenberg, J., "Advanced Instant Messaging Requirements for the
        Session Initiation Protocol  (SIP)",
        draft-rosenberg-simple-messaging-requirements-00 (work in
        progress), December 2002.

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Author's Address

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

   EMail: Gonzalo.Camarillo@ericsson.com

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