Internet Engineering Task Force (IETF)                       A. Johnston
Request for Comments: 6567                                         Avaya
Category: Informational                                         L. Liess
ISSN: 2070-1721                                      Deutsche Telekom AG
                                                              April 2012

Problem Statement and Requirements for
Transporting User-to-User Call Control Information in SIP

Abstract

   This document introduces the transport of call control User-to-User
   Information (UUI) using the Session Initiation Protocol (SIP) and
   develops several requirements for a new SIP mechanism.  Some SIP
   sessions are established by or related to a non-SIP application.
   This application may have information that needs to be transported
   between the SIP User Agents during session establishment.  In
   addition to interworking with the Integrated Services Digital Network
   (ISDN) UUI Service, this extension will also be used for native SIP
   endpoints requiring application UUI.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6567.

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   described in the Simplified BSD License.

   1. Overview ........................................................2
   2. Use Cases .......................................................3
      2.1. User Agent to User Agent ...................................3
      2.2. Proxy Retargeting ..........................................4
      2.3. Redirection ................................................4
      2.4. Referral ...................................................5
   3. Requirements ....................................................6
   4. Security Considerations .........................................8
   5. Acknowledgements ...............................................10
   6. Informative References .........................................10

1. Overview

   This document describes the transport of User-to-User Information
   (UUI) during SIP [RFC3261] session setup.  This section introduces
   UUI and explains how it relates to SIP.

   We define SIP UUI data as application-specific information that is
   related to a session being established using SIP.  It is assumed that
   the application is running in both endpoints in a two-party session.
   That is, the application interacts with both the User Agents in a SIP
   session.  In order to function properly, the application needs a
   small piece of information, the UUI, to be transported at the time of
   session establishment.  This information is essentially opaque data
   to SIP -- it is unrelated to SIP routing, authentication, or any
   other SIP function.  This application can be considered to be
   operating at a higher layer on the protocol stack.  As a result, SIP
   should not interpret, understand, or perform any operations on the
   UUI.  Should this not be the case, then the information being
   transported is not considered UUI, and another SIP-specific mechanism
   will be needed to transport the information (such as a new header
   field).  In particular, this mechanism creates no requirements on
   intermediaries such as proxies, Back-to-Back User Agents, and Session
   Border Controllers.

   UUI is defined this way for two reasons.  First, this definition
   supports a strict layering of protocols and data.  Providing
   information and understanding of the UUI to the transport layer (SIP
   in this case) would not provide any benefits and instead could create
   cross-layer coupling.  Second, it is neither feasible nor desirable

   for a SIP User Agent (UA) to understand the information; instead, the
   goal is for the UA to simply pass the information as efficiently as
   possible to the application that does understand the information.

   An important application is the interworking with User-to-User
   Information (UUI) in ISDN, specifically the transport of the call-
   control-related ITU-T Q.931 User-to-User Information Element (UUIE)
   [Q931] and ITU-T Q.763 User-to-User Information Parameter [Q763] data
   in SIP.  ISDN UUI is widely used in the Public Switched Telephone
   Network (PSTN) today in contact centers and call centers.  These
   applications are currently transitioning away from using ISDN for
   session establishment to using SIP.  Native SIP endpoints will need
   to implement a similar service and be able to interwork with this
   ISDN service.

   Note that the distinction between call control UUI and non-call-
   control UUI is very important.  SIP already has a mechanism for
   sending arbitrary UUI data between UAs during a session or dialog --
   the SIP INFO [RFC6086] method.  Call control UUI, in contrast, must
   be exchanged at the time of setup and needs to be carried in the
   INVITE and a few other methods and responses.  Applications that
   exchange UUI but do not have a requirement that it be transported and
   processed during call setup can simply use SIP INFO and do not need a
   new SIP extension.

   In this document, four different use case call flows are discussed.
   Next, the requirements for call control UUI transport are discussed.

2. Use Cases

   This section discusses four use cases for the transport of call
   control User-to-User Information.  These use cases will help motivate
   the requirements for SIP call control UUI.

2.1. User Agent to User Agent

   In this scenario, the originating UA includes UUI in the INVITE sent
   through a proxy to the terminating UA.  The terminating UA can use
   the UUI in any way.  If it is an ISDN gateway, it could map the UUI
   into the appropriate DSS1 [Q933] information element, QSIG [QSIG]
   information element, or ISDN User Part (ISUP) parameter.
   Alternatively, the using application might render the information to
   the user or use it during alerting or as a lookup for a screen pop.
   In this case, the proxy does not need to understand the UUI
   mechanism, but normal proxy rules should result in the UUI being
   forwarded without modification.  This call flow is shown in Figure 1.

            Originating UA            Proxy           Terminating UA
                   |                    |                    |
                   |   INVITE (UUI) F1  |                    |
                   |------------------->|   INVITE (UUI) F2  |
                   |      100 Trying F3 |------------------->|
                   |<-------------------|         200 OK F4  |
                   |          200 OK F5 |<-------------------|
                   |<-------------------|                    |
                   |  ACK F6            |                    |
                   |------------------->|            ACK F7  |
                   |                    |------------------->|

      Figure 1: Call Flow with UUI Exchanged between Originating and
                              Terminating UAs

2.2. Proxy Retargeting

   In this scenario, the originating UA includes UUI in the INVITE
   request sent through a proxy to the terminating UA.  The proxy
   retargets the INVITE request, changing its Request-URI to a URI that
   addresses the terminating UA.  The UUI data is then received and
   processed by the terminating UA.  This call flow is identical to
   Figure 1 except that the proxy retargets the request, i.e., changes
   the Request-URI as directed by some unspecified process.  The UUI in
   the INVITE request needs to be passed unchanged through this proxy
   retargeting operation.  Note that the contents of the UUI is not used
   by the proxy for routing, as the UUI has only end-to-end significance
   between UAs.

2.3. Redirection

   In this scenario, UUI is inserted by an application that utilizes a
   SIP Redirect Server.  The UUI is then included in the INVITE request
   sent by the originating UA to the terminating UA.  In this case, the
   originating UA does not necessarily need to support the UUI mechanism
   but does need to support the SIP redirection mechanism used to
   include the UUI data.  Two examples of UUI with redirection (transfer
   and diversion) are defined in [ANSI] and [ETSI].

   Note that this case may not precisely map to an equivalent ISDN
   service use case.  This is because there is no one-to-one mapping
   between elements in a SIP network and elements in an ISDN network.
   Also, there is not an exact one-to-one mapping between SIP call
   control and ISDN call control.  However, this should not prevent the
   usage of SIP call control UUI in these cases.  Instead, these slight
   differences between the SIP UUI mechanism and the ISDN service need
   to be carefully noted and discussed in an interworking specification.

   Figure 2 shows this scenario, with the Redirect Server inserting UUI
   that is then included in the INVITE request F4 sent to the
   terminating UA.

            Originating UA        Redirect Server      Terminating UA
                   |                    |                    |
                   |          INVITE F1 |                    |
                   |------------------->|                    |
                   | 302 Moved (UUI) F2 |                    |
                   |<-------------------|                    |
                   |            ACK F3  |                    |
                   |------------------->|                    |
                   |  INVITE (UUI) F4   |                    |
                   |---------------------------------------->|
                   |  200 OK F5                              |
                   |<----------------------------------------|
                   |  ACK F6                                 |
                   |---------------------------------------->|

    Figure 2: Call Flow with UUI Exchanged between Redirect Server and
                              Terminating UA

   A common example application of this call flow is an Automatic Call
   Distributer (ACD) in a PSTN contact center.  The originator would be
   a PSTN gateway.  The ACD would act as a Redirect Server, inserting
   UUI based on called number, calling number, time of day, and other
   information.  The resulting UUI would be passed to the agent's
   handset which acts as the terminating UA.  The UUI could be used to
   lookup information for rendering to the agent at the time of call
   answering.

   This redirection scenario and the referral scenario in the next
   section are the most important scenarios for contact center
   applications.  Incoming calls to a contact center almost always are
   redirected or referred to a final destination, sometimes multiple
   times, based on collected information and business logic.  The
   ability to pass along UUI in these call redirection scenarios is
   critical.

2.4. Referral

   In this scenario, the application uses a UA to initiate a referral,
   which causes an INVITE request to be generated between the
   originating UA and terminating UA with UUI data inserted by the
   referrer UA.  Note that this REFER method [RFC3515] could be part of
   a transfer operation, or it might be unrelated to an existing call,
   such as out-of-dialog REFER request.  In some cases, this call flow

   is used in place of the redirection call flow: the referrer
   immediately answers the call and then sends the REFER request.  This
   scenario is shown in Figure 3.

             Originating UA         Referrer           Terminating UA
                   |                    |                    |
                   |  REFER (UUI) F1    |                    |
                   |<-------------------|                    |
                   |  202 Accepted F2   |                    |
                   |------------------->|                    |
                   |  INVITE (UUI) F3   |                    |
                   |---------------------------------------->|
                   | NOTIFY (100 Trying) F4                  |
                   |------------------->|                    |
                   |         200 OK F5  |                    |
                   |<-------------------|                    |
                   |  200 OK F6                              |
                   |<----------------------------------------|
                   |  ACK F7                                 |
                   |---------------------------------------->|
                   | NOTIFY (200 OK) F8 |                    |
                   |------------------->|                    |
                   |        200 OK F9   |                    |
                   |<-------------------|                    |

                 Figure 3: Call Flow with Referral and UUI

3. Requirements

   This section states the requirements for the transport of call
   control User-to-User Information (UUI).

   REQ-1: The mechanism will allow UAs to insert and receive UUI data in
   SIP call setup requests and responses.

      SIP messages covered by this include INVITE requests and end-to-
      end responses to the INVITE, i.e., 18x and 200 responses.  UUI
      data may also be inserted in 3xx responses to an INVITE.  However,
      if a 3xx response is recursed on by an intermediary proxy, the
      resulting INVITE will not contain the UUI data from the 3xx
      response.  In a scenario where a proxy forks an INVITE to multiple
      UAS who include UUI data in 3xx responses, if a 3xx response is
      the best response sent upstream by the proxy, it will contain the
      UUI data from only one 3xx response.

REQ-2: The mechanism will allow UAs to insert and receive UUI data in
SIP dialog terminating requests and responses.

      Q.931 UUI supports inclusion in release and release completion
      messages.  SIP messages covered by this include BYE and 200 OK
      responses to a BYE.

   REQ-3: The mechanism will allow UUI to be inserted and retrieved in
   SIP redirects and referrals.

      SIP messages covered by this include REFER requests and 3xx
      responses to INVITE requests.

   REQ-4: The mechanism will allow UUI to be able to survive proxy
   retargeting or redirection of the request.

      Retargeting is a common method of call routing in SIP and must not
      result in the loss of User-to-User Information.

   REQ-5: The mechanism should not require processing entities to
   dereference a URL in order to retrieve the UUI data.

      Passing a pointer or link to the UUI data will not meet the real-
      time processing considerations and would complicate interworking
      with the PSTN.

   REQ-6: The mechanism will support interworking with call-control-
   related DSS1 information elements or QSIG information elements and
   ISUP parameters.

   REQ-7: The mechanism will allow a UAC to learn that a UAS understands
   the UUI mechanism.

   REQ-8: The mechanism will allow a UAC to require that a UAS
   understands the call control UUI mechanism and have a request routed
   based on this information.  If the request cannot be routed to a UAS
   that understands the UUI mechanism, the request will fail.

      This could be useful in ensuring that a request destined for the
      PSTN is routed to a gateway that supports the UUI mechanism rather
      than an otherwise equivalent PSTN gateway that does not support
      the ISDN mechanism.  Note that support of the UUI mechanism does
      not, by itself, imply that a particular application is supported
      (see REQ-10).

   REQ-9: The mechanism will allow proxies to remove a particular
   application usage of UUI data from a request or response.

      This is a common security function provided by border elements to
      header fields such as Alert-Info or Call-Info URIs.  There is no
      requirement for UAs to be able to determine if a particular usage
      of UUI data has been removed from a request or response.

   REQ-10: The mechanism will provide the ability for a UA to discover
   which application usages of UUI another UA understands or supports.

      The creation of a registry of application usages for the UUI
      mechanism is implied by this requirement.  The ISDN service
      utilizes a field known as the protocol discriminator, which is the
      first octet of the ISDN UUI data, for this purpose.

   REQ-11: The UUI is a sequence of octets.  The solution will provide a
   mechanism of transporting at least 128 octets of user data and a one-
   octet protocol discriminator, i.e., 129 octets in total.

      There is the potential for non-ISDN services to allow UUI to be
      larger than 128 octets.  However, users of the mechanism will need
      be cognizant of the size of SIP messages and the ability of
      parsers to handle extremely large values.

   REQ-12: The recipient of UUI will be able to determine the entity
   that inserted the UUI.  It is acceptable that this is performed
   implicitly where it is known that there is only one other end UA
   involved in the dialog.  Where that does not exist, some other
   mechanism will need to be provided.  The UUI mechanism does not
   introduce stronger authorization requirements for SIP; instead, the
   mechanism needs to be able to utilize existing SIP approaches for
   request and response identity.

      This requirement comes into play during redirection, retargeting,
      and referral scenarios.

4. Security Considerations

   The security requirements for the UUI mechanism are described in this
   section.  It is important to note that UUI security is jointly
   provided at the application layer and at the SIP layer.  As such, is
   important for application users of the UUI mechanism to know the
   level of security used and deployed in their particular SIP
   environments and not to assume that a standardized (but perhaps
   rarely deployed) security mechanism is in place.

   There are three main security models that need to be addressed by the
   UUI mechanism.  One model treats the SIP layer as untrusted and
   requires end-to-end integrity protection and/or encryption.  This
   model can be achieved by providing these security services at a layer
   above SIP.  In this case, the application integrity protects and/or
   encrypts the UUI data before passing it to the SIP layer.  This
   method has two advantages: it does not assume or rely on end-to-end
   security mechanisms in SIP, which have virtually no deployment, and
   it allows an application that understands the contents of the UUI to
   apply a proper level of security.

   The second approach is for the application to pass the UUI without
   any protection to the SIP layer and require the SIP layer to provide
   this security.  This approach is possible in theory, although its
   practical use would be extremely limited.

   The third model utilizes a trust domain and relies on perimeter
   security at the SIP layer.  This is the security model of the PSTN
   and ISDN where UUI is commonly used today.  This approach uses hop-
   by-hop security mechanisms and relies on border elements for
   filtering and application of policy.  This approach is used today in
   UUI deployments.  Within this approach, there is a requirement that
   intermediary elements can detect and remove a UUI element based on
   policy, but there is no requirement that an intermediary element be
   able to read or interpret the UUI (as the UUI contents only have end-
   to-end significance).

   The next three requirements capture the UUI security requirements.

   REQ-13: The mechanism will allow integrity protection of the UUI.

      This allows the UAS to be able to know that the UUI has not been
      modified or tampered with by intermediaries.  Note that there are
      tradeoffs between this requirement and requirement REQ-9 for
      proxies and border elements to remove UUI.  One possible way to
      satisfy both of these requirements is to utilize hop-by-hop
      protection.  This property is not guaranteed by the protocol in
      the ISDN application.

   REQ-14: The mechanism will allow end-to-end privacy of the UUI.

      Some UUI may contain private or sensitive information and may
      require different security handling from the rest of the SIP
      message.  Note that this property is not available in the ISDN
      application.

   REQ-15: The mechanism will allow both end-to-end and hop-by-hop
   security models.

      The hop-by-hop model is required by the ISDN UUI service.

5. Acknowledgements

   Thanks to Joanne McMillen, who was a co-author of earlier draft
   versions of this specification.  Thanks to Spencer Dawkins, Keith
   Drage, Dale Worley, and Vijay Gurbani for their review of earlier
   draft versions of this document.  The authors wish to thank Christer
   Holmberg, Frederique Forestie, Francois Audet, Denis Alexeitsev, Paul
   Kyzivat, Cullen Jennings, and Mahalingam Mani for their comments on
   this topic.

6. Informative References

   [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.

   [Q931]     ITU-T, "ISDN user-network interface layer 3 specification
              for basic call control", ITU-T Recommendation Q.931,
              <http://www.itu.int/rec/T-REC-Q.931-199805-I/en>.

   [Q763]     ITU-T, "Signalling System No. 7 - ISDN User Part formats
              and codes", ITU-T Recommendation Q.763,
              <http://www.itu.int/rec/T-REC-Q.763-199912-I/en>.

   [RFC6086]  Holmberg, C., Burger, E., and H. Kaplan, "Session
              Initiation Protocol (SIP) INFO Method and Package
              Framework", RFC 6086, January 2011.

   [Q933]     ITU-T, "ISDN Digital Subscriber Signalling System No. 1
              (DSS1) - Signalling specifications for frame mode switched
              and permanent virtual connection control and status
              monitoring", ITU-T Recommendation Q.933,
              <http://www.itu.int/rec/T-REC-Q.933/en>.

   [QSIG]     ECMA, "Private Integrated Services Network (PISN) -
              Circuit Mode Bearer Services -  Inter-Exchange Signalling
              Procedures and Protocol (QSIG-BC)", Standard ECMA-143,
              December 2001.

   [ANSI]     ANSI, "Telecommunications-Integrated Services Digital
              Network (ISDN)-Explicit Call Transfer Supplementary
              Service", ANSI T1.643-1995.

   [ETSI]     ETSI, "Integrated Services Digital Network (ISDN);
              Diversion supplementary services", ETSI ETS 300 207-1, Ed.
              1, 1994.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

Authors' Addresses

   Alan Johnston
   Avaya
   St. Louis, MO  63124

   EMail: alan.b.johnston@gmail.com

   Laura Liess
   Deutsche Telekom AG

   EMail: laura.liess.dt@gmail.com