SIPPING                                                     J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                           J. van Elburg
Expires: September 9, 2009                                   C. Holmberg
                                                             F. Francois
                                                       S. Schubert (Ed.)
                                                          March 08, 2009

             Delivery of Request-URI Targets to User Agents

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on September 9, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Rosenberg, et al.       Expires September 9, 2009               [Page 1]

Internet-Draft                 Target URI                     March 2009


   When a Session Initiation Protocol (SIP) proxy receives a request
   targeted at a URI identifying a user or resource it is responsible
   for, the proxy translates the URI to a registered or configured
   contact URI of an agent representing that user or resource.  In the
   process, the original URI is removed from the request.  Numerous use
   cases have arisen which require this information to be delivered to
   the user agent.  This document describes these use cases and defines
   an extension to the History-Info header field which allows it to be
   used to support those cases.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  retarget . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Unknown Aliases  . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Unknown GRUU . . . . . . . . . . . . . . . . . . . . . . .  4
     4.3.  Limited Use Addresses  . . . . . . . . . . . . . . . . . .  5
     4.4.  Sub-Addressing . . . . . . . . . . . . . . . . . . . . . .  5
     4.5.  Service Invocation . . . . . . . . . . . . . . . . . . . .  6
     4.6.  Freephone Numbers  . . . . . . . . . . . . . . . . . . . .  6
   5.  Architectural Roots of the Problem . . . . . . . . . . . . . .  7
   6.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Detailed Semantics . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 11
     7.2.  UA Behavior  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  The difference to P-Called-Party-Id  . . . . . . . . . . . . . 12
   9.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     11.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

Rosenberg, et al.       Expires September 9, 2009               [Page 2]

Internet-Draft                 Target URI                     March 2009

1.  Introduction

   A key part of the behavior of proxy servers and B2BUA in the Session
   Initiation Protocol (SIP) [RFC3261] is that they rewrite the Request-
   URI of requests as the request moves from the User Agent Client (UAC)
   to the User Agent Server (UAS).  This is particularly true for
   requests outside of a dialog; requests within a dialog have their
   path dictated primarily by the Route header fields established by the
   Record-Routes when the dialog was initiated.

   The most basic instance of this behavior is the processing executed
   by the "home proxy" within a domain.  The home proxy is the proxy
   server within a domain which accesses the location information
   generated by REGISTER messages, and uses it to forward a request
   towards a UAC.  Based on the rules in [RFC3261], when a home proxy
   receives a SIP request, it looks up the Request-URI in the location
   database or mapping table, and translates it to the contact(s) that
   were registered by the UA or configured in the mapping table.  This
   new contact URI replaces the existing Request URI, and causes the
   request to be forwarded towards the target UA.  Consequently, the
   original contents of the Request URI are lost.

   Over the years, this practice of rewriting the Request-URI has proven
   problematic.  Section 4 describes the problems with this mechanism.
   Section 5 analyzes the architectural issues which drive these
   problems.  Section 6 overviews a mechanism to solve this problem by
   extending the History-Info header field.  Section 7 describes
   detailed procedures for user agents and proxies.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Definitions

3.1.  retarget

   A Request-URI rewrite operation is considered to be a retargeting
   operation if the entity to which the request is ultimately delivered
   could not, based on the policies of the domain of that entity, place
   the URI prior to translation in the From header field, and have an
   identity service in its domain sign it.  The inverse is not true
   however.  If an entity can legitimately claim the identity prior to
   the translation operation, it may still be a retargeting.  In this

Rosenberg, et al.       Expires September 9, 2009               [Page 3]

Internet-Draft                 Target URI                     March 2009

   case, it is a matter of domain policy about whether it is or not.

4.  Problem Statement

   Several problems arise from the practice of rewriting the request

4.1.  Unknown Aliases

   SIP user agents are associated with an address-of-record (AOR).  It
   is possible for a single UA to actually have multiple AOR associated
   with it.  One common usage for this is aliases.  For example, a user
   might have an AOR of but also have the AORs and  Rather than
   registering against each of these AORs individually, the user would
   register against just one of them, and the home proxy would
   automatically accept incoming calls for any of the aliases, treating
   them identically and ultimately forwarding them towards the UA.  This
   is common practice in the Internet Multimedia Subsystem (IMS), where
   it is called implicit registrations and each alias is called a public

   It is a common requirement for a UAS, on receipt of a call, to know
   which of its aliases was used to reach it.  This knowledge can be
   used to choose ringtones to play, determine call treatment, and so
   on.  For example, a user might give out one alias to friends and
   family only, resulting in a special ring that alerts the user to the
   importance of the call.

   However, based on the procedures in [RFC3261], when an incoming call
   hits the home proxy, the request URI (which contains the alias) is
   rewritten to the registered contact(s).  Consequently, the alias that
   was used is lost, and cannot be known to the UAS.

4.2.  Unknown GRUU

   A variation on the problem in Section 4.1 occurs with Globally
   Routable User Agent URI (GRUU) [I-D.ietf-sip-gruu].  A GRUU is a URI
   assigned to a UA instance which has many of the same properties as
   the AOR, but causes requests to be routed only to that specific
   instance.  It is desirable for a UA to know whether it was reached
   because a correspondent sent a request to its GRUU or to its AOR.
   This can be used to drive differing authorization policies on whether
   the request should be accepted or rejected, for example.  However,
   like the AOR itself, the GRUU is lost in translation at the home
   proxy.  Thus, the UAS cannot know whether it was contacted via the
   GRUU or its AOR.

Rosenberg, et al.       Expires September 9, 2009               [Page 4]

Internet-Draft                 Target URI                     March 2009

4.3.  Limited Use Addresses

   A limited use address is an SIP URI that is minted on-demand, and
   passed out to a small number (usually one) remote correspondent.
   Incoming calls targeted to that limited use address are accepted as
   long as the UA still desires communications from the remote target.
   Should they no longer wish to be bothered by that remote
   correspondent, the URI is invalidated so that future requests
   targeted to it are rejected.

   Limited use addresses are used in battling voice spam [RFC5039].  The
   easiest way to provide them would be for a UA to be able to take its
   AOR, and "mint" a limited use address by appending additional
   parameters to the URI.  It could then give out the URI to a
   particular correspondent, and remember that URI locally.  When an
   incoming call arrives, the UAS would examine the parameter in the URI
   and determine whether or not the call should be accepted.
   Alternatively, the UA could push authorization rules into the
   network, so that it need not even see incoming requests that are to
   be rejected.

   This approach, especially when executed on the UA, requires that
   parameters attached to the AOR, but not used by the home proxy in
   processing the request, will survive the translation at the home
   proxy and be presented to the UA.  This will not be the case with the
   logic in RFC 3261, since the Request-URI is replaced by the
   registered contact, and any such parameters are lost.

4.4.  Sub-Addressing

   Sub-Addressing is very similar to limited use addresses.  Sub-
   addresses are addresses within a subdomain that are multiplexed into
   a single address within a parent domain.  The concept is best
   illustrated by example.  Consider a VoIP service provided to
   consumers.  A consumer obtains a single address from its provider,
   say  However, Joe is the patriarch of a
   family with four members, and would like to be able to have a
   separate identifier for each member of his family.  One way to do
   that, without requiring Joe to purchase new addresses for each member
   from the provider, is for Joe to mint additional URI by adding a
   parameter to the AOR.  For example, his wife Judy with have the URI;member=judy, and Joe himself would have the
   URI;member=joe.  The SIP server provider would
   receive requests to these URI, and ignoring the unknown parameters
   (as required by [RFC3261]) route the request to the registered
   contact, which corresponds to a SIP server in Joes home.  That
   server, in turn, can examine the URI parameters and determine which
   phone in the home to route the call to.

Rosenberg, et al.       Expires September 9, 2009               [Page 5]

Internet-Draft                 Target URI                     March 2009

   This feature is not specific to VoIP, and has existing in Integrated
   Services Digital Networking (ISDN) for some time.  It is particularly
   useful for small enterprises, in addition to families.  It is also
   similar in spirit (though not mechanism) to the ubiquitous home
   routers used by consumers, which allow multiple computers in the home
   to "hide" behind the single IP address provided by the service
   provider, by using the TCP and UDP port as a sub-address.

   The sub-addressing feature is not currently feasible in SIP because
   of the fact that any SIP URI parameter used to convey the sub-address
   would be lost at the home proxy, due to the fact that the Request-URI
   is rewritten there.

4.5.  Service Invocation

   Several SIP specifications have been developed which make use of
   complex URIs to address services within the network rather than
   subscribers.  The URIs are complex because they contain numerous
   parameters that control the behavior of the service.  Examples of
   this include the specification which first introduced the concept,
   [RFC3087], control of network announcements and IVR with SIP URI
   [RFC4240], and control of voicemail access with SIP URI [RFC4458].

   A common problem with all of these mechanisms is that once a proxy
   has decided to rewrite the Request-URI to point to the service, it
   cannot be sure that the Request-URI will not be destroyed by a
   downstream proxy which decides to forward the request in some way,
   and does so by rewriting the Request-URI.

4.6.  Freephone Numbers

   Freephone numbers, also known as 800 or 8xx numbers in the United
   States, are telephone numbers that are free for users to call
   (although the author will note that such notions are becoming less
   important as billing models evolve, and harken back to an era where
   phone service depended on global agreement on such billing concepts).

   In the telephone network, freephone numbers are just aliases to
   actual numbers which are used for routing of the call.  In order to
   process the call in the PSTN, a switch will perform a query (using a
   protocol called TCAP), which will return either a phone number or the
   identity of a carrier which can handle the call.

   There has been recent work on allowing such PSTN translation services
   to be accessed by SIP proxy servers through IP querying mechanisms.
   ENUM, for example [RFC3761] has already been proposed as a mechanism
   for performing Local Number Portability (LNP) queries [RFC4769], and
   recently been proposed for performing calling name queries

Rosenberg, et al.       Expires September 9, 2009               [Page 6]

Internet-Draft                 Target URI                     March 2009

   [I-D.ietf-enum-cnam].  Using it for 8xx number translations is a
   logical next-step.

   Once such a translation has been performed, the call needs to be
   routed towards the target of the request.  Normally, this would
   happen by selecting a PSTN gateway which is a good route towards the
   translated number.  However, one can imagine all-IP systems where the
   8xx numbers are SIP endpoints on an IP network, in which case the
   translation of the 8xx number would actually be a SIP URI and not a
   phone number.  Assuming for the moment it is a PSTN connected entity,
   the call would be routed towards a PSTN gateway.  Proper treatment of
   the call in the PSTN (and in particular, correct reconciliation of
   billing records) requires that the call be marked with both the
   original 8xx number AND the target number for the call.  However, in
   our example here, since the translation was performed by a SIP proxy
   upstream from the gateway, the original 8xx number would have been
   lost, and the call will not interwork properly with the PSTN.

   Similar problems arise with other "special" numbers and services used
   in the PSTN, such as operator services, pay numbers (9xx numbers in
   the U.S), and short service codes such as 311.

5.  Architectural Roots of the Problem

   There is a common theme across all of the problems in Section 4, and
   this theme is the confounding of names, routes, and addresses.

   A name is a moniker for an entity which refers to it in a way which
   reveals nothing about where it is in a network.  In SIP, tel URI
   which doesn't represent the location of the entity is a name.  An
   address is an identifier for an entity which describes it by its
   location on the network.  In SIP, the SIP URI itself is a form of
   address because the host part of the URI, the only mandatory part of
   the URI besides the scheme itself, indicates the location of a SIP
   server that can be used to handle the request.  Finally, a route is a
   sequence of SIP entities (including the UA itself!) which are
   traversed in order to forward a request to an address or name.

   SIP, unfortunately, uses the Request-URI as a mechanism for routing
   of the request in addition to using it as the mechanism for
   identifying the name or address to which the request was targeted.  A
   home proxy rewrites the Request-URI because that rewriting is the
   vehicle by which the request is forwarded to the target of the
   request.  However, this rewritten URI (the contact from the
   register), is not in any way a meaningful name or address for the UA.
   Indeed, with specifications like SIP outbound
   [I-D.ietf-sip-outbound], even the IP address within the registered

Rosenberg, et al.       Expires September 9, 2009               [Page 7]

Internet-Draft                 Target URI                     March 2009

   contact is meaningless since the flow on which the REGISTER is sent
   is used rather than the IP address.  Consequently, the home proxy is
   fundamentally replacing the *address* in the Request-URI with a
   *route* to reach that UA.  This architectural mistake is the cause of
   the problems described above.

   Interestingly, this same mistake was present in [RFC2543] for the
   handling of mid-dialog requests.  It was fixed through the loose
   routing mechanism in RFC 3261, which used Route header fields to
   identify each hop to visit for a mid-dialog request, and separated
   this from the Request-URI, which identified the ultimate target of
   the request (the remote UA), and remained unmodified in the
   processing of the request.

   Unfortunately, application of this same technique to address the
   problem at hand cannot be done in a backwards compatible manner.
   Consequently, some other means is needed to clearly identify which
   URIs are addresses, and which are routes.  To avoid confusion, we
   refer to a SIP URI that is an address for a user or resource as a
   "target" and a SIP URI that is a hop for reaching that user as a

6.  Solution Overview

   The History-Info header field, defined in [RFC4244], defines a
   mechanism by which an enumeration of the URIs traversed can be passed
   to both the UAC and UAS.  History-Info was designed to provide a
   general purpose mechanism which can support the needs of many
   applications, including diagnostics and traditional telephony
   features like voicemail.  Were a home proxy to implement History-
   Info, it would provide a means for that proxy to deliver the target
   URI to the UAS.

   Unfortunately, History-Info makes no distinction between URIs that
   are targets and URIs that are hops.  Consequently, if there were
   additional proxies downstream of the home proxy which modified the
   Request-URI in any way, the UA would have no way to know which URI in
   the list of History-Info values was actually the target.  To remedy
   that, this document defines an extension to the History-Info header
   field which indicates whether the URI is a target or not.

   When a home proxy receives a request for a user or resource for which
   it has a registration, the proxy adds two History-Info header field
   values.  The first is the incoming request URI.  Since the Request-
   URI identifies a user or resource for which it has a registration,
   the Request-URI is an AOR and thus an address for the user.  The
   proxy adds a History-Info header field parameter, "istarget", which

Rosenberg, et al.       Expires September 9, 2009               [Page 8]

Internet-Draft                 Target URI                     March 2009

   indicates this.  Next, the proxy inserts the contact URI it used in
   the outgoing Request-URI.  No "istarget" parameter is included in
   this History-Info value.

   For a UA to determine the URI target, it need only walk backwards
   through the list of HI values, and take the first one containing the
   "istarget" parameter.

   For example, consider the architecture in Figure 1.  In the example
   user A calls user B. User B is in  The call from A to B
   passes through A's outbound proxy, their home proxy, B's home proxy,
   and B's outbound proxy, prior to reaching B. B's home proxy, H-B,
   performs the translation of the R-URI to the registered contact based
   on the registration database.  Consequently, it adds two History-Info
   header fields, the first of which represents the incoming R-URI and
   includes the "istarget" parameter.

Rosenberg, et al.       Expires September 9, 2009               [Page 9]

Internet-Draft                 Target URI                     March 2009

               +-------+    +-------+   +-------+   +-------+

    //--\\     |       |    |       |   |       |   |       |   //--\\

   |  A   |--- | OB-A  |----|  H-A  |---| H-B   |---| OB-B  |--|  B   |

   |      |    |       |    |       |   |       |   |       |  |      |

    \\--//     +-------+    +-------+   +-------+   +-------+   \\--//











                               HI: <>index=1;istarget,



                       Figure 1: Target URI Example

Rosenberg, et al.       Expires September 9, 2009              [Page 10]

Internet-Draft                 Target URI                     March 2009

7.  Detailed Semantics

   The "istarget" parameter in the History-Info header field indicates
   that the URI that it parameterizes was either subject to a lookup in
   a location service created through the registration process of the UA
   or was available through configured mapping.  Furthermore, if that
   URI had an 'index' of N, the URIs with indices N.M for all M, are the
   registered contacts to that URI.

7.1.  Proxy Behavior

   A proxy compliant to this specification SHOULD add a History-Info
   header field value to a request under the following conditions:

   o  The proxy is responsible for the domain in AOR in the Request-URI

   o  The proxy will be translating the contents of the Request-URI to
      one or more contacts either based on a location database populated
      through REGISTER requests from user agents or based on configured

   o  The R-URI exists in the location database.

   The proxy SHOULD populate the History-Info header field regardless of
   whether there is a Supported header field with value 'histinfo'.  If
   the incoming request already contains a History-Info header field,
   and the last value of that header field is identical to the Request-
   URI of the received request, the proxy MUST add a "istarget"
   attribute to that History-Info value.  If the request did not contain
   a History-Info header field, or if it did, but the last value is not
   identical to the Request-URI of the received request, the proxy MUST
   add another History-Info header field value.  The URI MUST be equal
   to the incoming Request-URI, and MUST contain a "istarget" attribute.
   The index is set as defined in [RFC4244].

   Once the proxy has translated the Request-URI into a registered
   contact or configured contact, it MUST add an additional History-Info
   header field value containing the Contact URI for each request to be
   forwarded.  The "istarget" attribute MUST NOT be present.  The index
   is set as defined in [RFC4244].

   Since the principal purpose of the "istarget" parameter is to
   indicate, to a UAS, the target URI by which it was reached, there is
   no need for the History-Info header field values to be passed outside
   of the domain which inserted them, unless there is an apparent need
   for passing on the value downstream (e.g. freephone number).

   If the proxy is actually redirecting and not forwarding the request,

Rosenberg, et al.       Expires September 9, 2009              [Page 11]

Internet-Draft                 Target URI                     March 2009

   it SHOULD include a History-Info URI in the response for the target.
   That URI, if present, MUST contain the "istarget" attribute.  It
   SHOULD NOT add a History-Info URI for the registered contact; the
   previous hop proxy will do that.  Note that, this rule violates a
   SHOULD-strength rule in Section 4.3.4 of [RFC4244].  That section
   indicates that redirections "SHOULD NOT" contain any new History-Info
   header fields, as those will be added by the upstream server.  For
   this application however, only the downstream server knows that the
   R-URI was a target, and thus the History-Info header field and the
   "istarget" attribute must be added by the downstream server.

7.2.  UA Behavior

   A UAS receiving a request, and wishing to determine the original
   target dialog, takes the values in the History-Info header field, and
   traverses through them in reverse order.  Note that, the value of the
   "index" attribute is not relevant; the traversal is in order of the
   header fields values themselves.  The UAS finds the first header
   field value containing the "istarget" parameter.  If such a value
   does not exist, the target URI cannot be reliably determined.  If it
   does exist, the URI is examined.  If the domain of the URI matches
   the domain of the UA, based on the UA's configured awareness of its
   own domain, that URI is the target URI.  If the domains do not match,
   the target URI cannot be reliably determined.  This domain check is
   present to handle cases where a request is forwarded through two
   separate domains, and the domain of the actual UAS didn't support
   this specification, but the previous domain did.  If there are more
   than one header field value containing "istarget" parameter, handling
   of second and latter value with "istarget" parameter is up to local
   policy and is outside the scope of this document.  For example, if
   freephone number was invoked, there may be two header field value
   with "istarget" parameter;one indicating the retargeting of freepone
   number to a corporate address and another indicating the retargeting
   of corporate address to a registered contact.

   NOTE: Do we want to introduce another parameter to indicated the
   difference between retargeting based on location lookup and
   configured mapping?

   Beyond this, there is no special UA processing associated with the
   "istarget" parameter.

8.  The difference to P-Called-Party-Id

   As defined in [RFC3455], if a SIP entity, which acts as registrar/
   home proxy for the terminating user, re-writes the Request-URI with
   the contact address of the registered UA it may insert a P-Called-

Rosenberg, et al.       Expires September 9, 2009              [Page 12]

Internet-Draft                 Target URI                     March 2009

   Party-ID header field with the previous value of the Request-URI.

   The last hi-entry in History-Info minted with an "istarget" attribute
   and P-Called-Party-ID header field have different semantics.  The
   last hi-entry in History-Info minted with an "istarget" attribute
   represents the current target identity, while the P-Called-Party-ID
   represents the last Request-URI value used to reach the user before
   the Request-URI value was re-written with the Contact address of the
   UAS.  In some cases the P-Called-Party-ID may be the same as the
   current target but, it may also be the last route taken (not equal to
   the current target) to deliver the request.  Therefore the P-Called-
   Party-ID can not be used in a generic SIP environment to represent
   the current target.

   3GPP has defined procedures for the usage of P-Called-Party-ID, so
   3GPP would need to continue to use the header, in addition to the new
   Target header.  However, both mechanisms can exist in parallel.

9.  Syntax

   This specification extends the syntax of hi-param in Section 4.1 of
   RFC 4244:

   hi-param = hi-index / hi-target / hi-extension

   hi-target = "istarget"

10.  Security Considerations

   The "istarget" parameter indicates that a URI was subject to
   translation by a home proxy, and consequently, acts as an explicit
   indicator that a particular URI was an AOR for a user.  This might be
   useful for attackers wishing to farm requests for targettable URIs
   for purposes of spamming.  Of course, such attackers can utilize URIs
   in History-Info even if they lack the "istarget" attribute, so
   "istarget" does not really exacerbate this.  Nonetheless, since the
   princpal application of the "istarget" parameter is delivery of a URI
   to a UAS within the same domain, History-Info values inserted solely
   for this purpose SHOULD be removed at the domain boundary.

11.  References

Rosenberg, et al.       Expires September 9, 2009              [Page 13]

Internet-Draft                 Target URI                     March 2009

11.1.  Normative References

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

   [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.

   [RFC4244]  Barnes, M., "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 4244,
              November 2005.

11.2.  Informative References

              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-15 (work in progress),
              October 2007.

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

   [RFC3087]  Campbell, B. and R. Sparks, "Control of Service Context
              using SIP Request-URI", RFC 3087, April 2001.

   [RFC4240]  Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
              Media Services with SIP", RFC 4240, December 2005.

   [RFC4458]  Jennings, C., Audet, F., and J. Elwell, "Session
              Initiation Protocol (SIP) URIs for Applications such as
              Voicemail and Interactive Voice Response (IVR)", RFC 4458,
              April 2006.

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC3455]  Garcia-Martin, M., Henrikson, E., and D. Mills, "Private
              Header (P-Header) Extensions to the Session Initiation
              Protocol (SIP) for the 3rd-Generation Partnership Project
              (3GPP)", RFC 3455, January 2003.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

Rosenberg, et al.       Expires September 9, 2009              [Page 14]

Internet-Draft                 Target URI                     March 2009

   [RFC4769]  Livingood, J. and R. Shockey, "IANA Registration for an
              Enumservice Containing Public Switched Telephone Network
              (PSTN) Signaling Information", RFC 4769, November 2006.

              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-16 (work in progress),
              October 2008.

              Shockey, R., "IANA Registration for an Enumservice Calling
              Name Delivery (CNAM)  Information and IANA Registration
              for URI type 'pstndata'", draft-ietf-enum-cnam-08 (work in
              progress), September 2008.

Authors' Addresses

   Jonathan Rosenberg
   Edison, NJ


   Hans Erik van Elburg
   Ericssonstraat 2
   Rijen  5121 ML
   The Netherlands


   Christer Holmberg
   Hirsalantie 11, Jorvas


Rosenberg, et al.       Expires September 9, 2009              [Page 15]

Internet-Draft                 Target URI                     March 2009

   Francois Audet


   Shida Schubert (editor)

   Email: shida at

Rosenberg, et al.       Expires September 9, 2009              [Page 16]